Coal is one of the world's most plentiful energy resources. Coal is one of the fastest growing forms of energy after renewable sources and its share in the global primary energy consumption increasing rapidly. Lignin is the most abundant natural raw material available on Earth in terms of solar energy storage. Lignin is a complex chemical compound, cross linked polymer that forms a large molecular structure. Lignin can be used as a green alternative to many petroleum-derived substances, such as fuels, resins, rubber additives, thermoplastic blends and pharmaceuticals. Rosin is a complex mixture of mainly resin acids and small amount of non-acidic components.
Energy markets are evolving with technological advancements supporting rapid growth in renewable energy capacity. The coal market is set to witness great boost in near future because of the rising government initiatives.
Coal is one of the main power generation sources all over the world. The factors that are favoring the market growth include rising electricity demand and rapid industrialization. Presently the global coal industry market is valued at $9.4 with CAGR of 11.21 % is poised to reach $22 billion in coming years. Asia Pacific has the larger demand and emerging as a larger supplier of Coal. The present global lignin market demand is estimated at $ 4,222.1 million and is expected to reach $6,190.5 million in future.
The Major contents of the book are coal, analysis of coal and coke, cotton, lignin and hemicelluloses, degradation of wood, CCA-treated wood, wood-polymer composites, lignocellulosic-plastic composites from recycled materials, chemical modification of wood fiber, delignification of wood with pernitric acid, rosin and rosin derivatives, polymerizable half esters of rosin. It describes the manufacturing processes and photographs of plant & machinery with supplier’s contact details.
It will be a standard reference book for professionals, entrepreneurs, those studying and researching in this important area and others interested in the field of these industries.
Coal
Coal carbonization byproducts (coal tar
and coke-oven gas) were
the main sources for the production of organic chemicals till the
Second World
War. However, the abundant availability of cheap petroleum and natural
gas
caused a major shift from coal to these raw materials. After the Second
World
War, there was a consistent increase in the consumption of
petrochemicals. In
the last 10 years, with the continuing rise in oil prices, and scarcity
and
uncertainty regarding assured supplies of natural gas and petroleum
oils,
attention towards achieving self-sufficiency in chemicals and
production of
fuels has been focussed on exploring alternative feedstocks like coal
and
biomass. In this context, there is a need for reassessing the
possibilities of
obtaining organic chemicals from coal. It is anticipated that it will
be
cheaper to make liquid and gaseous fuels from coal than from oil before
the end
of this century.
The use of coal as a source of chemicals
is at present confined
mainly to high temperature carbonization (HTC) through the recovery of
by
products. A part from the traditional coking route, concerted efforts
are being
made to obtain various chemicals by other coal conversion processes,
such as
direct hydrogenation or solvent extraction of coal followed by
cracking,
hydrotreating or hydrocracking of the products and by synthesis of
hydrocarbons
via synthesis gas produced by coal gasification.
Ethylene
Ethylene
is an important basic
building block of the petrochemical industry. The production and
recovery of
large quantities of olefins from petroleum refinery off-gases first
started
with the advent of catalytic cracking. Rapid growth in the demand for
petrochemicals necessitated the setting up of plants to produce
ethylene and
other olefin monomers by thermal cracking of a wide range of petroleum
feedstocks.
The
routes considered for the
production of ethylene from coal include: (i) Fischer-Tropsch
synthesis, (ii)
direct conversion of synthesis gas, (iii) conversion of coal-derived
methanol,
(iv) homologation of methanol to ethanol followed by dehydration, (v)
cracking
of dimethyl ether which again is derived from synthesis gas or
methanol, and
(vi) cracking of synthetic liquids produced in direct liquefaction
processes.
Fischer
– Tropsch Synthesis for
Olefins
Fischer –
Tropsch (FT) synthesis for the
production of fuels from coal has been in operation on a commercial
scale since
1955 in Sasolburg, South Africa. It is known that through proper choice
of
reactor, catalyst and operating conditions, the product pattern can be
changed
in FT synthesis. The dilute fluid bed or Kellogg type of reactor
(Synthol)
shifts the product spectrum to lower hydrocarbons. It gives an ethylene
yield
of 4.4% and also C3 and C4 olefins. Ethane produced in
the process can
also be converted to ethylene. The cost of ethylene from the recovery
process
is about 30 cents/lb compared to 18-19 cents/lb from the conventional
gas oil
cracking. In the steam cracking of FT product, the cost of ethylene is
estimated at 30 - 50cents/lb. In the Sasol 2 plant in South Africa, the
annual
production of 1.5 million tones of automotive fuels, 1,60,000 tonnes of
ethylene and 50,000 tonnes of chemicals, 1,00,000 tonnes of ammonia,
90,000
tonnes of sulphur and 2,00,000 tonnes of tar products, including tar
acids, is
envisaged.
Direct Conversion of Synthesis Gas to
Ethylene
Direct conversion of synthesis gas from
coal to ethylene and
ethanol is now attracting attention. Efforts to get ethylene by passing
synthesis gas (CO: H2 ratio,
74:24:8) in a copper tube at 550ºC resulted in 5.6% ethylene in the
product
gas, whereas when synthesis gas containing 0.6% ethylene was passed
over a CuO
(3%) – Al2O3 catalyst
at 250ºC and 20 – 40 atm pressure, the ethylene content increased to
7.6%. On
the other hand, Peters used a four-component catalyst system to convert
synthesis gas (46.4% CO, 42.5% H2 and 11.1% inert) to
ethylene. Several iron and
cobalt-based FT catalysts have been evaluated for the conversion of
synthesis
gas to C2 – C4 olefins. As the copper and
the alkali contents
were increased in iron-copper-kieselguhr-K2O
catalysts, the
product distribution shifted towards lower molecular weight
hydrocarbons.
The
activity and selectivity of Ni- or
Co-based bimetallic catalysts containing several other metals as
promoters have
been evaluated. The bimetallic catalysts give a somewhat different
product
distribution compared to Ni or Co catalysts. The addition of Re to Ni
favours
the formation of ethane (22% at 185ºC), that of Ru favours the long
chain
hydrocarbons, while addition of lr favours the formation of dimethyl
ether at
210ºC (40% at 190ºC). Co - Mn gives products containing a higher
proportion of
olefins which is higher at lower temperatures and lower H2 - CO ratios.
The
favourable olefin selectivity
obtained on the addition of basic promoters to the conventional iron
catalysts
suggests that metal interactions with basic or electron-donor sites in
zeolites
may have an important influence on selectivity. A marked increase in
the olefin
selectivity of ruthenium is observed as sodium ions in the zeolite are
replaced
by potassium or caesium ions.
The work done at Pittsburgh Energy
Research Center indicated that
a fused iron catalyst is particularly effective in giving high yields
of
olefins. Experiments were conducted in a microreactor with temperature
range
250 - 350ºC, pressure range 20 - 68 atm, synthesis gas having
composition of
1:1, 2:1 and 3:1 of H2:
CO and also mixtures containing CO2 and hourly space velocities
of 500-5000.
Ethanol from Synthesis Gas
Synthesis gas has been converted to
ethanol at 250-290ºC, 50 atm
and a space velocity of 1000-4000 hr-1 in a bench scale unit using
a special rhodium
cluster catalyst. The conversion of CO was 10% and the reaction product
contained 75-80% ethanol. The activity of the catalyst during one month
period
of operation (8 hr-a-day) remained almost constant. Work done at the
Union
Carbide Corporation showed that addition of Fe to Rh increased the
yield of
ethanol at 68 atm. On the other hand, large proportions of ethane and
dimethyl
ether were obtained with alumina-supported platinum.
Olefins from Methanol
Conversion of coal-based methanol to
ethylene and propylene
offers an attractive route to the use of coal as a basic raw material.
The
formation of C2 - C5 olefins (1.6 mole%) was
noticed by Mattox
during methanol dehydration over NaX zeolites. Similar results have
been
reported by several investigators using various catalysts. The product
distribution suggests that an a-elimination
process (H-CH2-OH
!®H2O
+ CH2)
is occurring in the methanol adsorbed on the zeolite surface.
The carbene methylene species so formed can polymerize to form simple
low
molecular weight olefins.
Methanol cannot dehydrate to form an
olefin unless recombination
of one-carbon fragments takes place. Aromatics are produced by
dimerization and
cyclization of C3,
C4 and C5 olefins.
Laboratory scale experiments were
conducted using tungstic acid
impregnated on zeolites, MgO or Al2O3.
Zeolites exchanged
with rare earths gave higher yields ranging from 16 to 30% ethylene and
29 to
30% propylene with methanol conversion of 80 – 90%. Dimethyl ether
formed in
the process can also be converted to olefins using the same catalyst.
The
reaction mechanism involves three consecutive surface reactions.
Adsorbed
methanol is dehydrogenated to formaldehyde followed by dimerization of
formaldehyde and dehydration of the dimer to give adsorbed ethylene.
The possibilities of producing ethylene
and propylene from
coal-based methanol on a laboratory scale have been explored by BASF.
In 1979,
BASF constructed a pilot plant with 15 tonnes per month
capacity at a cost
of $ 1.5-1.6 million at Ludwigshafen, West Germany to produce olefins
from
methanol. In this process, 3-4 lb of methanol is catalytically
converted
(catalyst, ZSM-5 zeolite) to give 1 lb of olefins. Methanol is first
converted
to dimethyl ether, which, in turn, gives ethylene by dehydration.
Propylene and
C4
olefins are also produced in varying amounts, depending on
the reaction
conditions.
Methanol
Homologation
The best potential approach to the
production of ethylene from
synthesis gas appears to be through its reaction with methanol to form
ethanol
and subsequent dehydration of ethanol to ethylene. Wender showed that
methanol
reacts with synthesis gas in the presence of dicobalt octacarbonyl [CO2(CO)8]
catalyst to give ethanol, but higher alcohols, including ethanol, are
relatively unreactive in this system. The reaction was carried out in
batch
autoclaves at 183 – 185oC
and 275 atm at 75% conversion to
give about 40% of ethanol in the product. About 4% propanol was formed
together
with 0.3% butanols.
The
methanol homologation reaction as
catalysed by Co2(CO)8 is a very complex one,
highly dependent on a
variety of reaction parameters and variables. The initial carbonylation
product, acetaldehyde, is obtained in high selectivity by proper ligand
modification. A selectivity of 62.3% was obtained at 200oC
and 270 atm
for 3 hr process time using a cobalt-iodine catalyst. High
iodine/cobalt ratios
lead to undesirable reaction by products and catalyst deactivation. The
active
form of Co in the iodine – promoted system is Co(CO)4–,
as indicated by high pressure IR spectroscopy.
Methanol to Acetic Acid
Liquid
phase carbonylation of methanol
has become an important industrial process which uses iodide or
carbonyl of
cobalt as a catalyst and an organic or inorganic iodide, such as methyl
iodide,
hydrogen iodide or potassium iodide, as promoter.
Nickel
carbonyl and some other nickel
compounds are also effective at low pressures of about 50 atm in the
presence
of iodide in the liquid reaction media of some organic amines of
phosphines.
In
the process developed by Monsanto,
a new iodide-promoted rhodium catalyst which exhibits high activity for
methanol carbonylation even at atmospheric pressure is used. The yield
of
acetic acid is 99% based on methanol and 90% based on carbon monoxide.
A plant
with a current annual capacity of 143 million kilograms is now
operating. About
40% of the world’s capacity for acetic acid is based on Monsanto
process.
Ethylene Glycol
The most promising route to ethylene
glycol involves direct conversion
of synthesis gas using rhodium catalyst.
High
yields of ethylene glycol were reported. The reaction takes place at
about 2000C
and 1400 - 3000 atm over a rhodium carbonyl catalyst, but comparatively
lower
pressures of the order of 400 atm may be sufficient to achieve a
reasonable
degree of conversion. Such a low pressure process using a more specific
catalyst promises to be a commercial success. In the work carried out
by Union
Carbide, a solvent having a high dielectric constant to ensure ion
separation
and/or complex the cations to prevent pairing with a anionic centers
has been
used.
Vinyl Acetate
Halcon
International has nearly
completed the development of a methanol carbonylation route to vinyl
acetate.
In this process, methanol is reacted with recycled acetic acid to make
methyl
acetate; the methyl acetate is reacted with synthesis gas to produce
ethylidene
diacetate, which is subjected to cracking to produce vinyl acetate and
acetic
acid for recycle.
Other Chemicals
Monsanto reported a method for the
alkylation of toluene with
methanol over modified zeolite catalyst to produce a mixture of
ethylbenzene
and styrene. At
present, the product
concentrations are very low (5 mol% of styrene and ethylbenzene) and
frequent
catalyst regeneration is necessary. Both BASF and Rohm and Haas have
been
investigating the base catalysed addition of formaldehyde (as acetal)
to methyl
propionate to form methyl methacrylate.
Coal Pyrolysis Processes
Coal tar is an inevitable byproduct in the
production of
metallurgical coke. Important chemicals, such as benzene, toulene and
xylenes
(BTX), can be produced from the benzole recovered from coke oven gas as
well as
from the light oil obtained by the distillation of coal tar. The other
main
constituents of high temperature coal tar are naphthalene (10%),
anthracene
(1.8%), phenanthrene (5.0%), fluorine (2.0%), fluoranthene (3.3%),
phenol
(0.4%) and pyridine (0.02%). Bulk products, such as creosotes and
pitches, are
also produced from coal tar.
With the exception of limited quantities
produced in USA through
dealkylation of aromatic petroleum fractions, naphthalene is almost
exclusively
obtained from coal tar. Methyl naphthalene and dimethyl naphthalenes
are
present in the heavier coal tar fractions (250 – 3500C).
The
dealkylation of these methyl naphthalenes can yield further quantities
of
naphthalene.
The Coalite and Chemicals Products Ltd,
UK, are processing the
tar from LTC into valuable chemicals (pure phenols, chlorinated
phenols) and
chemical intermediates for plasticizers, pharmaceuticals and adhesives.
A
phenol-free middle oil fraction was processed into diesel oil till 1965.
The pyrolysis of tars or vapours from LTC
of coal is considered
as a possible route for producing olefins. Laboratory scale
investigations were
conducted on cracking of vapour and tars obtained by LTC of Texas and
North
Dakota lignites. When Texas lignite was carbonized at 5500C
and the tar vapours were cracked at 8000C,
the yields of ethylene,
propylene and benzene were 40, 7.6 and 6 lb respectively per tonne of
lignite.
However, the process was not considered economically viable. LTC of
cannel
coals from West Virginia in the presence of superheated steam followed
by cracking
of the vapours at 800 – 9000C
gave 191-245 lb of ethylene, 20-41
lb of propylene and 21-47 lb of benzene per tonne of coal (MAF).
Acetylene
Acetylene is an important chemical
intermediate for the
production of acetaldehyde, vinyl acetate, vinyl chloride,
acrylonitrile, etc.
It is generally obtained by high temperature pyrolysis of natural gas
or
naphtha fractions in addition to the conventional calcium carbide
route. A
process to make acetylene by pyrolysing coal in an electric arc has
been
developed by Avco Everett Research Laboratory, Everett, Mass. In this
process,
pulverized coal is injected into a stabilized arc, the pyrolysis
products are
quenched and acetylene is separated from the product. Pyrolysis is
conducted in
hydrogen atmosphere, which increases the yield of acetylene. The
process has
been tested in a unit that operates at 75-100 kW producing 0.15-0.20
lb/min of
acetylene. A pilot plant to process 10 tonnes/day of coal that would
yield
3-3.35 tonnes/day of acetylene has been proposed.
Production of Chemicals by Coal
Liquefaction Processes
Several investigations have been made to
study the possibilities
of producing chemicals from coal liquefaction products. Dow Chemical
Company
studied the feasibility of producing chemical feedstocks from
coal-derived
liquids from COED, Synthoil, H-Coal and Solvent Refined Coal (SRC)
processes.
In the Exxon Donor Solvent (EDS) process,
which provides a
technically feasible route to the production of liquid products from
various
types of coal, the byproducts form a potential source of petrochemical
feedstock. The major source of ethylene and propylene is derived by
cracking
the light gas product blend containing ethane, propane and mixed
butanes.
Analysis of
Coal and Coke
Coal
is a readily combustible rock containing more than 50% by weight and
more than
70% by volume of carbonaceous material, formed from compaction or
induration of
variously altered plant remains similar to those of peaty deposits.
Differences
in the kinds of plant materials (type), in degree of metamorphosis
(rank), and
range in impurity (grade) are characteristic of the varieties of coal.
In lump
form, coal appears to be black or brownish black. The luster, fracture,
and
texture vary according to type, rank, and grade. Chemically, coal
consists
mainly of carbon, hydrogen, and oxygen, with minor amounts of nitrogen,
sulfur,
silica, alumina, iron, and trace amounts of fifty-three other elements.
The
composition varies to such an extent that Hendricks states that no two
coals
are absolutely the same. Some of the elements are inherent in the
original
plants; others have been incorporated during the coalification process;
and
others have been picked up as extraneous matter in mining, preparation,
and
transportation.
That
types, ranks, and grades of
coal are dependent on the characteristics of the original plants and
the
chemical, biological, and physical processes involved in metamorphosis.
Variations in the combination of processes and the materials processed
produce
end products varying in chemical and physical properties. Analytical
techniques
that will consistently show numerical differences between these
products provide
data with which the types, ranks, and grades can be determined.
Each
of the principal coal
producing countries in the world has a series of procedures or national
standards. The Technical Committee 27 on Solid Mineral Fuels of the
International Standards Organisation (ISO) consists of representatives
of
Australia, Austria, Belgium, Canada, Denmark, Spain, France, Germany,
India,
Italy, Netherlands, Poland, Romania, Switzerland, Czechoslovakia,
Turkey,
United Arab Republic, United Kingdom, United States, Soviet Union, and
Yugoslavia collaborating on the international standardization for coal
and
coke.
Even
though the coke was produced
and used by metallurgical plants, the specifications were maintained by
chemical and physical tests. Uniformity is very essential in the
various
Processes.
Methods
of Analysis
In
this section, the methods for
coal are discussed in detail. Testing methods for coke are the same as
for coal
except the procedures for stability, porosity, and density. More
detailed
information on the testing methods can be found in the publications of
the U.S.
Bureau of Mines, British Standards Institution, and the American
Society for
Testing and Materials.
Sampling
Preliminary
to any chemical or
physical testing of coal in the laboratory it is essential that a
representative sample be provided. The most carefully conducted
analysis is
meaningless unless the sample be truly representative of the lot. This
means
that increments are specified in number, weight, frequency, and method
of
extraction. The properly collected gross sample must be reduced in
particale
size and mass to laboratory size by a method that retains the
representativeness without gain or loss of such fleeting substances as
moisture.
Determination
of Constitution and Physical Properties
Knowledge
of the constitution and
physical properties of coal is of great value in combustion,
carbonization,
gasification, and in the utilization of coal in making nonsolid fuels
and in
chemical syntheses. A number of techniques have been developed and
other are in
development status to provide this vital information.
Functional Group Analysis
Essentially all of the
functional groups
recognized as being present in coal are associated with oxygen. Both
the
nitrogen and organic sulfur are considered to be present primarily as
parts of
heterocyclic systems since there is no satisfactory evidence for any
functional
groups containing either element.
Carboxyl groups, both free
and as salts, are
present in lignites and brown coals and may represent as much as 14wt %
of the
dry lignite. Determination of free carboxyl group generally has been
carried
out by ion exchange between coal and barium or calcium acetate,
followed by
titration of the liberated acetic acid. Methoxyl groups also are
present only
in low-rank coals. Zeisel determination of methoxyl contents of
lignites and
brown coals indicates a maximum of 2% for lowest-rank coals. The number
of
carbonyl groups also decreases with increating rank, although no
satisfctory
procedures have been developed for quantitative determination of this
functionalgroup in coal. Some oxygen is also presentin heterocyclic
rings.
Acetynlation. Several different acetylation
procedures have been
used on coal. The one which is most widely used utilizes acetic
anhydride in
pyridine
Procedure
Add a weighed sample (500-800mg) of
dried coal to 10 ml of a
1:2 mixture of acetic anhydride and pyridine in a 200-ml flask. Reflux
the
mixture for 24 hr, cool, and dilute with 100 ml of water. Filter the
mixture and
wash the residue until free of acid Boil the acetylated coal for 5hr in
a
150-ml flask with 2 g of barium hydroxide in 40 ml of water and keep on
a
steamplate overnight. Add 2 ml of 85% phosphoric acid, and distill 24
ml into a
titration flask. Titrate the acetic acid in the distillate with sodium
hydroxide. Add increments of water to the residue and distill into the
titration flask until no more acid is observed upon titration.
Trimethylsilylation. A second method for determining
the hydroxyl content of
coal involves treatment of the coal with hexamethyldisilazane, and
trimrthylchlorosilane in pyridine. This results in the conversion of
the
hydroxyl groups to trimethylsilyl ethers. Silicon analysis of the
resultant
coal ethers affords a quantitative value from which the hydroxyl
content can be
calculated. Alternatively, the trimethylsilyl ether content can be
estimated
from the extinction coefficient of the trimethylsilyl group measured at
8.0µm.
Spectroscopy
Various
spectral techniques are
now applied to the study of coal. The methods discussed here include
infrared,
ultraviolet and visible absorption, mass, nuclear magnetic resonance,
and
electreon paramagnetic resonance spectroscopy. A good general reference
is that
of Silverstein and Bassler. Although considerable coal research work is
done by
spectral methods there are as yet not many analytical applications of
spectroscopy in the coal industry.
Infrared
Spectroscopy. Infrared
spectroscopy is the most universally applicable spectral method. With
the
exception of monatomic gases, all substances have infrared spectra.
Historically, infrared has been applied principally to organic
materials, but
it is being used increasingly on inorganic substances. Of all spectral
methods
infrared has most earned the right to the title of “fingerprint method”.
Ultraviolet
and Visible Absorption Spectroscopy. The
principal use of ultraviolet and visible spectroscopy is the study of
aromatic
compounds particularly polynuclear condensed aromatic compounds.
Because of the
prolific and characteristic spectral fine structure that results from
the
electronic excitation of polynuclear condensed aromatics, ultraviolet
and
visible spectroscopy is the best spectral method for establishing
identification of these compounds. Similar aromatic compounds have
radically
different characteristic spectra- for example, anthracene and
phenanthrene,
chrysene and benz-phenanthrene, pyrene and fluoranthene. Also the
derivatives
of the parent compounds display.
Mass
Spectroscopy.
Since the first mass
spectral studies of coal a considerable amount of work has been done,
principally at the Pittsburgh Energy Research Center of the U.S. Bureau
of
Mines. Mass spectra of pyridine extracts of bituminous coal have shown
the
presence of thirteen types of aromatic compounds having molecular
weights from
78 to 400.
Research
investigations of various oils and
pitches from coal tar have been carried out. Many aromatic hydrocarbon
types
were identified along with several nitrogen and a few oxygen compounds.
Also,
average molecular weights, aromaticities, and mean size of aromatic
nuclei of
these samples were calcutated from the mass spectra of oils and pitches.
Nuclear
Magnetic and Electron Paramagnetic Resonance Spectroscopy. The application of NMR or EPR in
the coal utilization
industries is limited. One development appears to be approaching the
application
stage; the analysis of water in coal is being carried out on moving
conveyor
belts. Belt speeds are low and throughput is small as yet but the
method has
promise.
Free
radicals exist in coals in relatively large amounts and are easily
detected by
EPR. Unfortunately, no fine structure has been detected in
coal spectra it
is therefore difficult to characterize structures. However,
considerable
characterizing information is obtainable from free radical
concentrations, line
widths, and spectral g-values.
Peltrographic Analysis
Coal petrography deals with
descriptions of different
physical parts of coals that can be distinguished best by microscopic
observation. These small parts of coals look different because they are
of
different size and shape and they modify in different ways the light
that they
reflect or transmit to the eyes. Coal petrography relates these
different parts
to the plants or plant parts from which they were derived and deduces
something
about the changes that have occurred from plant to coal.
Another
physical component of coals that may observed microscopically is
inorganic
mineral matter. These mineral constituents may have been present in the
original plants, may have deposited as the peat beds formed, or
crystallized
from waters percolating through the coal bed. The principal ones are
quartz,
kaolinite, calcite, and pyrite. Traces of many mineral elements are
present.
Some attempts have been made to
separate or concentrate
different coal components on a commercial basis to take advantage of
properties
favorable to various uses. These usually depend upon differences in
resistance
to breakage or differences in density.
Thin-Section
Analysis. Petrographic
thin– section analysis is usually made on
specimens obtained from column samples cut out of a coal bed from top
to
bottom. The specimens are small blocks measuring approximately 1×
0.8×
0.5 in. The 0.8- in. dimension is in the vertical
direction of the
column or bed, so that the total of the 1×
0.8-in./faces represents a
ribbon area over the entire height of the bed, excluding the material
lost in
cutting between the blocks.
Microscope
analysis of thin coal sections is
conducted with a biological microscope equipped with binocular
eyepieces. A
pair of 15× eyepieces is used with 16 and 32 mm objectives to attain
magnifications of 60× and 150×. Anthraxylon is measured at 150×
and opaque attritus, fusain, and mineral at 60×. Translucent
attritus is determined by difference. Measurements are made by means of
a
Whipple micrometer disc inserted in one of the eyepieces. This a glass
disc on
which is centered a 7 mm square field divided into one hundred 0.7 mm
squares
with the central one subdivided into twenty-five 0.14 mm squares. The
effective
sizes of these squares under different magnification conditions are
determined
by comparison with a stage micrometer.
A
detailed study of the errors involved in this type of thin-section
analysis
showed that the percentages of components in a column sample of a coal
bed as
determined by this technique usually should not be in error by more
than 2%.
Figure
l illustrates the results of
thin-section analysis on a complete vertical column of a coal bed.
polished
surface analysis. Petrographic polished surface
analysis is usually
made on briquets of granular coal molded with a binder. The granular
sample,
usually of –20 mesh size, is representative of much larger lots of
broken coal.
A
variety of grinding and
polishing steps may be used. A detailed grinding plan would include the
use of
240, 400, and 600 grit papers in succession. Similarly, a careful
polishing
procedure might involve successive use of 3 mm alumina on chemotextile cloth, 1mm on chemotextile, and 0.05 mm on two or three layers of
cloth, the top being silk
or cotton. Some of these steps may be omitted, depending on experience
or the
desired quality of the polish. After polshing most coal briquets should
be
dried in a desiccator for a period of about 15 hr. More details or
variations
in specimen preparation procedures can be found in References 54 and 55.
Reflectance Measurement. This measurement is important
in petrographic
analysis and permits the determination of optical constants. Various
assemblages of equipment have been used for this purpose. A brief
description
of one of these will serve to illustrate the general principles of the
measurement.
It
has been found that, for
accurate reflectance measurements, the polised specimen surface must be
precisely perpendicular to the optic axis of the microscope. A commonly
used
hand-leveling press cannot be relied on to attain this positioning. A
reliable
method involves observation of focus preservation with an objective of
at least
a 60 × magnification as the specimen surface is traversed over about 1
in. in
two perpendicular directions. Mounting the specimen on a device capable
of
controlled tilt in two directions permits adjustment of the surface
plane until
the specimen remains in focus through all possible displacements.
In
this method, absolute
reflectance is obtained by reference to a standard of known refractive
index
and negligible absorption. The reference to a standard of should be in
the
range of those of coal components. Some investigators have used a
series of
glasses of different reflectances in this range, although some surface
changes
by oxidation have been encountered. Synthetic ruby and particularly
sapphire
have proven satisfactory. The standard should be cut in a form so that
reflectance from the back face is not allowed to interfere with the
measurement. Measurements on several minerals of known refractive
index, in
different media, provide an excellent means of checking the accuracy of
the
method. If the indices calculated from the reflectance measurements
agree
within an acceptable limit with the indices provided by other means,
the
operation of the equipment is satisfactory.
Determination
of Vol % of Macerals. Macerals
commonly determined are vitrinite, exinite, resinite, micrinite,
semifusinite,
and fusinite. The equipment used is essentially that described for
reflectance
measurement with the addition of devices for scanning the speciment and
for
selection of areas to be identified and summed. The additional devices
are a
Whipple ocular disc or a crosshair disc, a mechanical stage, and a
counter. The
stage maybe operated manually or electrically. In the latter case it
maybe
coupled to an electrically actuated counter. The mechanical stage stage
should
be of such type that the specimen can be advanced quickly by definite
fixed
increments in two perpendicular directions. If an electrically operted
stage is
used, increment steps in one direction across the specimen may be
actuated by
the counter switches. The counter should be capable of totalizing
counts in at
least six channels.
The
specimens for automatic scanning microscopy are prepared in a way very
similar
to those previously described for visual studies in reflected light.
However,
because there is no element of human judgment involved, the surfaces
should be
more carefully polished to attain the least possible relief, pits, and
scratches. This objective may require modifications to the grinding and
polishing steps, depending, for one factor, on the relative importance
of the
results for the softer organic components or the harder mineral,
especially
pyrite.
Determination
of Optical Constants
Optical
constants are of interest because they
are the basis of the optical descriptions of the petrographic
components and
because they are related to structural atomic arrangements in coals.
One of
the simplest methods of determining these optical constants involves
only the
microscopic measurement of reflectance in two different media. The
measurements
are made using objectives corrected for air, oil or water immersion,
preferably
the first two.
The
simple equation for
transmission is the equation given above for defining k.
However, in
practical measurements on thin sections of coals and carbons,
corrections must
be made for reflections at interfaces between the various media. The
thin
section usually is supported on a transparent substrate and the
radiation
enters the system on the substrate side. Reference light (Io) is usually measured after
transmission through the substrate. An equation governing transmission
through
a system consisting of an absorbing medium on a transparent substrate
has been
given by Born and Wolf.
Electron Microscopy
The
principal procedure involved
in the study of coal with an electron microscope is the preparation of
suitable
specimens. At first, powdered coal mounts were observed, but these
supplied
little useful information. Replicas of polished or etched coal surfaces
were
later tried with some interesting results, but these have been little
used
recently. The most productive type of specimen for electron microscopy
is the
ultrathin section, prepared as described previously. Preparation of
electron
micrographs and electron diffraction patterns of coal specimens
involves no differences
in procedure from those commonly used.
Density
The true density of a
somewhat porous material
such as coal cannot be determined in a simple way. Any satisfactory
medium for
displacement should consist of molecules small enough to penetrate the
pores
but should not react chemically with the coal. It has been found that
helium
most satisfactorily meets these requirements. An approximation for the
density
can be derived by measuring the displacement of water by the coal,
usually in
granular form. However, it is questionable whether the water molecules
can
penetrate into the minute pores of the coal sufficiently to result in
the
determination of true density.
A
blank run without a sample in a sample frame will give the scattering
due to air
and background. Air scattering is appreciable only at low angles. A few
scans
of the blank run are sufficient. The readings to be subtracted must be
normalized to the number of scans used in obtaining the intensity with
sample
in place. An idea about the instrumental broadening can be obtained by
preparing a sample of 2.5mm diamond
powder and obtaining the scattered intensity for the diamond powder
under the
same conditions. The widths of the diamond lines are largely due to
instrumental factors. If they are excessive, the observed intensities
with the
coal sample can corrected by unfolding them with those of diamond.
Consider
an x-ray beam traversing a distance
in the sample before reaching a differential element of volume dV
in the
interior of the sample and further consider the scattered beam
traversing a
distance y in the sample before leaving it. The
incident beam will
undergo a reduction in intensity by a factor exp (-mx). Where is
the linear absorption coefficient, and the scattered intensity will be
reduced
by a factor exp (-my). The over all loss of intensity
due to absorption
depends upon the scattering angle as well as the sample diffractometer
geometry, thus corrections must be made. Further, the scattered
intensity is
polarized to a different degree at each scattering angle, and
correction must
be made for polarization. Finally, the observed intensities are in
arbitrary
units, they must be converted or normalized into Thomson or atomic
units.
This form of the
Warren-Bodenstein equation also can
be modified to consider an assembly of stacks each containing different
layers.
X-Ray Fluorescence Analysis. X-ray fluorescence analysis of
elements of relatively
high atomic number is well developed. Presently the method is practical
for
element down to, perhaps, fluorine, of atomic number 9. It has not yet
been
shown to be reliable for oxygen, nitrogen, and carbon, although
research in to
these applications
is progressing well.
Therefore, fluorescence analysis may now be used for virtually all the
mineral
elements in coal, but cannot yet determine the composition of the
organic
components.
The
sample required for fluorescence analysis
is about 2g. It must be ground to an extremely fine particle size
approximately
2 mm or less, in order to
minimize and to render less variable the absorption of the fluorescence
x rays
by the mineral matter in the coal sample. This extreme pulverization
can be
accomplished in a tungsten carbide pestle by vibration for 5 min on a
device
similar to a Crescent Wig-L- Bug, available from the Crescent Dental
Manufacturing Co. Chicago. The sample is then pressed into a frame with
typical
dimensions of 4cm.× 2.5 cm ×2.5 mm, under an applied pressure of about
150Kg/cm2.
For
the analysis , a standard sample containing proportions of each of the
elements
to be analyzed similar to those to be a expected in the unknown samples
is
necessary. For coal analysis, this could be a typical coal sample or a
synthetic mixture of the various coal mineral elements in carbon
black.This
sample is kept in one of the spectrometer chambers. Reading on the
standard
sample are taken at regular intervals so that those on unknown samples
may
normalized to minimize errors due to variations in conditions.
Specification Tests
The
American National Standards
Institute, Inc. (ANSI) represents the United States at the
international
standards level and corresponds in part to the British Standards
Institute (BSI)
in the United Kingdom. The ANSI is unlike the BSI in that it does not
develop
standards but recognizes the work of some independent organization. For
coal,
the American Society for Testing and Materials has been recognized as
the
developing body. The committee for coal consists of approximately
seventy five
experts equality divided in interest as producers, consumers and
general
interest
The
tests constituting coal
analysis have been divided into three group –Proximate, ultimate, and
miscellaneous analysis. Some of the taste are empirical and strict
adherence to
the specified conditions is required in order to obtain meaningful
results.
Proximate Analysis
Proximate analysis is the
quantitative
separation of the components of a mixture. In the proximate analysis of
coal,
the amounts of moisture, volatile matter, and are determined. The sum
of the
percentages of these components is subtracted from 100 to calculate the
amount
of fixed carbon present.
Moisture. The
determination of the
moisture content of coal is probably the most important test. It is
also one of
the most difficult components to determine accurately because of its
several
forms, its fleeting nature during the collection and preparation of the
gross
sample, and its strong affinity for the coal during the last stages of
drying.
Usually, three moisture
values are required in
the analysis of coal. First, if the coal is too wet to crush and sieve
without
losing moisture, it must be air-dried, which results in the value of air-dry
moisture. The air-dried coal is then crushed and prepared for
the
laboratory where the residual moisture is
determined. This laboratory
sample is used to prepare the analysis sample which is used for most of
the
tests. The knowledge of the total moisture content
of this sample is
necessary in order to express all items on a dry basis. The total
moisture is
calculated from the air-dry and residual moisture values and is used to
convert
the various dry analyses to the as-received basis.
Residual
Moisture. Two methods are given;
the first is used for coals known to be resistant to oxidation and the
second
is applicable to coals subject to oxidation or to coals of unknown
origin.
Procedures
oxidation-resistant
coals. Transfer one of the air dried sample to a shallow,
weighed,
noncorrodible metal pen to a depth of about 10 mm, weight, and heat in
a forced
air circulation oven at 105-110oC
to constant weight. Calculate the %
of residual moisture from the loss on heating and weight of the air
–dried
sample.
other
coals. Mix the air-dried sample well and place
approximately 10 g into a
weighed capsule made of glass or noncorrodible metal with close-fitting
covers
with a diameter of 60mm, cover, and weight to the nearest mg. Place the
cover
in desiccator containing magnesium perchlorate desiccant, and the
capsule in an
oven with a minimum free space, capable of maintaining a temperature of
105-1100C
and having facilities for introducing nitrogen at a flow rate of three
oven
changes per minute. Use a nitrogen containing less than 10 ppm of
oxygen. Heat
the sample in the oven to constant weight; approximately 2 hr are
required.
Remove the capsule, cover and put on a metal plate for 10min.Transfer
to
desiccator until cooled to room temperature, then weigh. Calculate
the %
residual moisture from the loss in weight and weight of the air-dried
sample.
Total
Moisture.
The % total moisture is
calculated from the percentage of air-dry and residual moistures, as
follows:
% total moisture = % air-dry
+ % residual
Moisture of Analysis Sample. This moisture value is used to
calculate other
analyses made to a dry basis, using the analysis sample of 250mm size coal.
Volatile Matter. Volatile matter refers to loss of
weight, corrected
for moisture, that results when coal is heated in specified equipment
and under
standard conditions. It is one of the more important items in purchase
specifications of coal; it is valuable for the fuel engineer in setting
up and
maintaining burning rates, and is used with other parameters in almost
all coal
classification systems.
The test given below is
empirical and easily
performed. However, it can lead to erroneous results if all the vital
factors
are not observed strictly. If carried out properly, duplicate tests by
the same
operator and equipment should agree within 0.3% for anthracite, 0.5%
for
semianthracite and bituminous coals, 0.7% for subbituminous coals, and
1.0% for
lignite. For the test, special high-form crucibles made of platinum, 31
mm
high, with a 25.4 mm diameter at the top and 16 mm diameter at the
bottom, are
used. The crucibles are equipped with platinum covers, 10 mm high, with
a 25.5
mm diameter at the top and 25.3 mm diameter at the bottom. The total
weight of
the crucible and capsule is between 14 and 15g.
The heating of
the sample is to be performed in an electrically heated vertical tube
or muffle
furnace. The tube furnace heating zone should be approximately 38 mm ID
by 100
mm long. The crucible is suspended in the heating zone by a platinum or
nichrome wire cradle. The size of the muffle furnace should be large
enough to
insert and remove the crucible conveniently. It is essential that the
heating
zone of the furnace be maintained as close as possible to 9500 C
, but must not vary more than 200C.
Ash. Coal ash is the residue
remaining after the
combustion of coal under specified conditions. Ash should not be
confused with
mineral matter. The ash can be more than, equal to, or less then the
take place
during combustion.
The
quantity of ash in coal is of prime importance in purchase
specifications,
utilization, and classification.
Ash represents an impurity, adds
to the cost of transportation of the coal, and is costly to dispose to
after
utilization. In certain methods of utilization, ash contributes to air
pollution as well as corrosion of the burning equipment.
Fixed Carbon. The fixed carbon is calculated
by subtracting the
sum of moisture, volatile matter, and ash from 100. It consists mainly
of
carbon but, also contains small amounts of hydrogen, sulfur, nitrogen,
and
oxygen.
Ultimate Analysis
The
ultimate analysis of coal
expresses the elemental composition in terms of the determined
percentages of
carbon, hydrogen, nitrogen, sulfur, and the calculated value for
oxygen. The
five items are generally calculated on moisture and ash free basis. A
slight
more accurate value for oxygen is obtained if the analysis computed on
a
moisture and mineral-matter-free basis.
Carbon and
Hydrogen. These two elements
contribute virtually all the energy derived from coal. The carbon and
hydrogen
contents of coal are utilized in certain classification systems and
also permit
an estimate of the calorific values of coals.
All methods employ the same
basic principle of
burning the sample in oxygen in a closed system and collecting and
determining
quantitatively the combustion products. The standard U.S. method
carries out
the combustion at 800-850oC. A high temperature (1350oC) combustion
method has been used successfully in a number of European laboratories.
The oxygen to the combustion
tube flows
through the other area of the T connection by passing the above
described
pressure-safety device, and passes through a U-tube. The first arm
contains
soda-lime, the bottom contains a glass-wool plug , and the second arm
is filled
with magnesium perchlorate. A one-hole rubber stopper is used to seal
the open
end of the combustion tube. Metal or glass tubing is used to connect
this
U-tube with the tee and the combustion tube. The joints are sealed with
flexible tubing.
Repeat
this procedure using an accurately weighed sample of standard sucrose
or benzoic
acid. Several tests may have to be made before the determined values
for
hydrogen and carbon are in satisfactory agreement with the theoretical
values.
Repeat the procedure with an accurately weighed sample of coal.
Duplicate
determinations by the same operator using the same equipment and the
same
analysis sample should not differ more than 0.07% for hydrogen and
0.30% for
carbon.
Nitrogen. Nitrogen occurs almost
exclusively in the organic
matter of coal. The amount is
rather constant and in most bituminous
coals it varies from 1.0 to 1.5 % in anthracite and low-rank coals.
The
kjeldahl method is most commonly used, and consists of destructive
digestion of
the coal with a mixture of concentrated sulfuric acid, potassium
sulfate, and
mercury catalyst. The nitrogen is distilled from alkaline solution as
ammonia
into a standard acid solution and quantitatively determined
titrimetrically.
Total Sulfur. Sulfur occurs in coal as the
sulfide of iron and in
combination with the organic matter. In some weathered coals small
amounts of
sulfate sulfur may also be found. Two chemical methods are used for the
determination of total sulfur. In the method given , the coal is
oxidized in an
Eschka mixture. The resulting oxides of sulfur are retained by an
alkaline
mixture, extracted with water, precipitated high-temperature combustion
in a
closed system with oxygen, the sulfur oxides are absorbed in aqueous
hydrogen peroxide,
and the sulfuric acid formed is treated with a standard alkali. It is
particularly applicable where results are required in a very short
time.
Oxygen. The oxygen content of coal can
be used to estimate
the degree of oxidation of a weathered bituminous coal, and it is an
indicator
of the lowest rank in the bituminous group that can be used for making
a
satisfactory metallurgical coke.
Oxygen
can be determated directly by a semimicro combustion method. For most
purposes
it is estimated by subtracting from 100 the sum of the percentage
hydrogen,
carbon, nitrogen, sulfur, moisture and ash. A more accurate estimate
can be
made by converting the analysis to a mineral-matter-free basis.
Colorific
Value
The
test consists of burning a
weighed amount of the material in a closed vessel under a pressure of
oxygen.
Heat is released and absorbed in a fixed mass of water. The increase in
temperature of the water is directly related to the calorific value of
the
material. The manner of absorbing this indicates the two principal
types of
calorimeters commonly used in the isothermal calorimeter,
the
temperature of the water bath surrounding the calorimeter is maintained
at a
contained at a constant temperature. Since the temperature of the water
surrounding the combustion vessel or bomb varies, there is heat
exchange
between the two systems, and the amount of heat gained or lost must be
determined between the two systems, and the amount of heat gained or
lost must
be determined. Adiabatic calorimeters have the
facilities to adjust or
raise the temperature of the outer water bath to the same temperature
of the
water surrounding the combustion vessel.
Remove the bomb and release
the pressure at a
uniform rate in about 1min. Inspect the interior of the bomb for
unburned
carbon, and if it is present discard the results and repeat the test.
Wash the interior
of the bomb with water containing methyl orange indicator until all
acid is
removed , collect the washings and titrate with a sodium carbonate
solution
containing 20.90g/liter, 1 ml of this solution corresponds to 10.0 Btu.
If a
combustible firing wire is used, determine the weight of the unburned
wire.
Fusibility of Coal Ash
This
test gives an approximation of the
temperatures at which the ash remaining after the combustion of coal
will
sinter, melt and flow. The correlation of the laboratory and
utilization
results is only approximate, because of the homogeneity of the
laboratory test
piece and the heterogeneity of the residue or ash from utilization. The
fusibility datas are of value in the procurement of coals that most
likely will
perform satisfactorily.
The
procedure given below consists of heating
a molded trilateral pyramid of the ash in a mildly reducing or
oxidizing
atmosphere and observing the changes in shape and the temperatures at
which the
changes occur. Four such temperatures are defined below.
Behavior
on Healing
Some of the phenomena
exhibited by coal on
heating have been assigned names that refer to essentially the same
behavior;
they differ only in the rate of heating. These slight differences of
operation
become of great importance when they are used in the classification of
coals.
The various terms may be defined as follows:
Caking
refers to the behavior of the
coal when it is heated rapidly in measuring the free-swelling index and
Roga
index or as the combustion of coal. The agglomerating index refers to
the
behavior of coal when heated rapidly in the volatile matter test.
Coking refers to the behavior of the
coal when it is heated
slowly in the Audibert-Arnu dilatometer and Gray-King coke-type tests
or as in
carbonization.
Plasticity,
dilation ,
contraction, and agglutinating value are other terms also used to
describe the
behavior of coal on heating.
Below,
some of the tests used to
investigate the behavior of coal during heating are discussed more
detail.
Free-Swelling Index
or Crucible Swelling
Number. The free-swelling index is quite important in
differentiating
between groups in the classification of coals; it also gives
considerable
information on the combustion characteristics of coal. The test
consists of
heating the coal under prescribed conditions and comparing the residue
or
button with standard profiles as shown in Figure 5. Since the test is
empirical, the special apparatus is specified exactly.
Roga Index. The
Roga index is an
alternate procedure for measuring the caking properties of coals. It
requires a
special drum tumbler and an anthracite with specified properties to mix
with
the coal before carbonizing. Because of the limited use of the Roga
index and
the difficulty of getting the specific anthracite or suitable
substitute, the
details of the test will not be given.
Agglomerating Index.The
agglomerating
index is required in the ASTM classification of coals in order to
differentiate
between the anthracitic and bituminous classes and between the
bituminous and
subbituminous classes of coals.
The
test is made by examining the residue from
the platinum crucible in the standard volatile matter determination. If
the
agglomerate button shows a cell structure or swelling or supports a
500-g
weight without pulverizing, it is considered as agglomerating.
Gray-King
Coke-Type Test. This test and
the Audibert-Arnu dilatometer are used alternatively for assessing the
coking
phenomena of coal in the International Classification of Hard Coal by
Type. The
equipment for the Gray-King coke test is more easily acquired and the
technique
of operation less exacting than for the Audibert-Arnu dilatometer.
Consequently, only the procedure for the Gray-King test is given.
The
test consists of heating pulverized coal
out of contact with air in a horizontal tube retort at a uniform and
slow rate.
Plasticity of Coal Measured by
the Gieseler Plastometer. The test is
a semiquantitative measurement of the plastic property or apparent
melting of
coal on heating in the absence of air. The plasticity phenomena have
been quite
closely related to carbonization and many researchers have attempted to
relate
the Gleseler plasticity values with the quality of coke produced from a
particular coal or more often a blend of coals. The detailed
specifications of
the testing equipment and procedure of operation make this a very
empiric test.
The slightest variation in equipmental detail or deviation in operation
can
results in erroneous results.
At present, two types of
equipment are in use,
one utilizing a constant torque of 40g-in. by suspended weight on the
stirrer
imbedded in coal, and another which produces an equal torque by a
magnetic
clutch between a constant speed motor and the stirrer. For more
detailed
information on the procedures, the instruction manuals supplied with
the
equipment should be consulted.
Equilibrium
Moisture of Coal at
96-97% Relative Humidity and 300C
This test is used to restore
wet coal and
partially dried coal to the moisture content it contained in the bed.
The test
results are used to measure the degree of during and excess moisture,
and in
the classification of coals by rank. It is applicable to all coals from
metaanthracite to lignite. The measurement is carried out in a moisture
oven
maintained at 1050C and with provision in dry air
or dry nitrogen at a
rate of two oven changes per minute. Air is used only with coals known
to be
slowly oxidized.
Determination of Hardgrove
Grindability Index of Coal
The
Hardgrove grindability index
(HI) of coal is a relative measure of the ease of pulverizing. The test
consists of grinding a specially prepared sample in a laboratory mill
of
standardized design. The Hardgrove grindability machine is available
from
Babcock & Wilcox Co. Alliance, Ohio. An index is derived from a
sieve
analysis of the ground product. For comparison, a standard coal sample
with an
index of 100 is used.
Special
Constituents
In
addition to the determination of the carbon, hydrogen, nitrogen,
sulfur, and
oxygen content of the coal, sometimes investigations are carried out to
determine the amount of sulfur present in various forms, and the
chlorine and
mineral carbonate content of coal.
Froms of Sulfur. Sulfur
occurs in coal
as sulfate, pyrite, and in combination with the organic material.
Sulfate
sulfur is usually less than 0.1% except for weathered coal containing
appreciable FeS2.
The pyritic sulfur content varies considerably more than the organic
sulfur,
and is of more interest because it is the only form that can be removed
from
coal by current preparation practices.
Chlorine. A considerable amount of the
chlorine in coal is
present as sodium chloride and therefore the amount of chlorine present
has
been used as an estimate of the sodium content. Sodium is suspected of
contributing to operational difficulties caused by deposits on the
heating
surfaces Coals containing less than 0.15% chlorine are considered as
potentially trouble free. The chlorine content is used in some system
for the
estimation of the amount of mineral matter from the determined ash
content.
In
the method given, a muffle furnace, heated
to 675± 250C and
ventilated at three to four air changes per minute, is used. The coal
sample is
oxidized with an Eschka mixture, and the chlorine content retained as
chloride.
The resulting mixture is dissolved in diluted nitrate acid, the
chloride is
precipitated with silver nitrate, and the excess of silver is titrated
with
potassium thiocyanate.
Mineral
Carbonates.
Infrequently, a coal
contains mineral carbonates in sufficient quantity to affect its carbon
content, mineral matter, and classification. The carbon dioxide of the
mineral
carbonates is determined by heating the coal with dilute hydrochloric
acid,
absorbing the evolved carbon dioxide in a suitable absorbent, and
determining
the weight increase. Duplicate determinations by the same operator
using the
same equipment and sample should not differ more than 0.05%
The
second hole of the stopper in the Erlenmeyer flask is fitted with the
small end
of a coil-type condenser, 300mm long. The open top end of the condenser
is
connected with an approximately 300 mm long, 7 mm OD glass tube. The
top of a
Stetser- Norton bulb is filled with 15 mm glass-wool in the bottom and
anhydrous calcium chloride on top. The bottom connection of this bulb
is
connected with a short section of rubber tubing to the bottom
connection of a
second Stetser-Norton bulb filled two thirds full with granular pumice
impregnated with anhydrous copper sulfate; the remainder of this bulb
is filled
with anhydrous calcium chloride.
Coal
Classification
The
wide variation in chemical and physical characteristics shown by the
several
testing methods, methods, provides an opportunity to devise numerous
systems
for the classification of coals. Some of the systems have received only
limited
acceptance possibly because of little practical applicability.
Three
of the more widely known and used
systems are briefly described below. The British National
Coal Board Coal
Classification System is based on the dry,
mineral-matter-free, volatile
matter and the Gray-King coke-type test.
Cotton
Cotton
is one of the plants utilized by mankind at the dawn of civilization
archeological evidence indicates that cotton was used in the Indus
Valley, in
about 3000 BC, for making cloth. Cotton was brought from India to Egypt
by
Alexander the Great in the fourth century bc,
and from there it spread to other countries around the; Mediterranean
Sea.
Cotton was introduced from India to China during the seventh century ad, first as
a garden plant, and about
1000 ad
for use as a fiber.
In America, remains of cotton bolls dating back to about 5000 bc have
been found in Mexico, and
discoveries indicate that it was used for textile purposes about 2500 bc.
All
wild and cultivated species of cotton plants belong to the botanical
genus gossypium.
Most of the cultivated cotton belongs to four species: Gossypium
herbaceum and
G. arboreum, which probably originated in southern
Arabia or
northeastern Africa and were introduced into India, G.
hirsutum, which
originated in southern Mexico and Guatemala, and G. barbadense,
which
was found in the northern part of South America but is also cultivated
in Egypt
and Sudan.
Chemical
Properties.
Cotton fiber, like most vegetable fibers, is essentially cellulosic;
however,
the percentage of cellulose which it contains is higher than in most
other
materials and may be as high as 96% of the dry weight of the unpurified
fiber.
Methods of Analysis
Methods
related to the identification and analysis of the nature of cottons
modified
with organic reagents and organic finishings, as well as the
determination of
trace inorganic elements present either as contaminants or as
additives, are
discussed below.
Modified Cottons
As
mentioned, superficial or partial reactions of the hydroxyl groups of
cotton
cellulose produce valuable permanent finishes on cotton products. These
reactions include esterification and etherification, further reaction
with
double or multiple bonds, and replacement reactions resulting in
specifically
modified cottons.
Infrared
Spectroscopy. The
identification of the organic groups introduced into the cellulose
molecule, as
well as the determination of the degree of substitution, can be carried
out by
the use of infrared spectroscopy.
The
fact that infrared absorption spectra can be obtained without modifying
the
crystallinity of even highly crystalline native cotton because grinding
of the sample
is not required is a major advantage of this procedure. O’Connor et al.
showed
that satisfactory spectra could be obtained with very large pieces of
native
cotton cellulose, and cutting 20 mesh was recommended in order to
provide a
reproducible technique.
Two
special methods are also of interest in the infrared investigation of
modified
cotton products. The first developed by McCall, Miles, and O’Conor
utilizes the
frustrated multiple internal reflectance (FMIR) technique; the
technique of
Knight and co-workers permits the measurement of cotton fibers without
any
cutting, grinding, sifting or mixing with extraneous material.
The
technique of direct pressing into parallel fibers was demonstrated to
give
satisfactory spectra, and had the advantage that the cotton fiber was
subjected
to no treatment other than mechanical pressing and the highly oriented
manner
in which the fibers are aligned should be helpful in polarization
studies. This
technique is the only satisfactory manner in which the study of
crystallinity
by means of deuterium and infrared measurements can be extended to
include
crystallinities of short, staple cellulosic fibers of native or
modified
cottons.
Finishing Agents
There
are a great variety of subatances which are being used in the chemical
finishing of cotton fabric. Included in this list are chemicals used to
produce
water repellency, fire resistance, resistance to mildew, or protection
from
actinic radiation, lubricants, plasticizers, softeners, antibacterial
agents,
deodorants, fungicides, germicides, rot resistant agents, and resins
used to
obtain durable-press or wash-and-wear characteristics. It is almost
impossible
to establish a single coordinated procedure for the analysis of cotton
for any
chemical finishing agent; however, two systematic methods approach this
goal.
Separation and Identification
The
first systematic approach to separate and identify the chemical agents
used in
cotton fabrics was developed by Skinkle in 1946. The sample is
successively extracted
with five solvents, each selected to remove by solubilizing certain
classes of
substances. Subsequent evaporation of the solvent reveals the presence
of the
entire group. Individual components of each group are subsequently
identified
either by further separations, by application of additional solvents,
or by
applying chemical tests which are valid in the presence of some or all
of the
other substances within the specific group.
Spectroscopic
Methods
Ultraviolet,
visible, and infrared spectroscopy permit the identification and
quantitative
analysis of a number of chemical finishing agents used for cotton and
cotton
fabrics.
Ultraviolet
and Visible Spectroscopy. The
end points
in the analysis of several of the chemical finishes, after extraction
and
separation, as by the scheme described above or by one of the many
modifications of this scheme, have been achieved through the production
of a
characteristic color produced by reaction of the finishes in a single
group
with a reagent which would produce a specific color with only one
constituent
of the group. Often interfering colors, by reaction of the selected
reagent
with other materials in the group or through a reaction with the
solvent, would
necessitate further preseparations before the color test could be
applied.
Total
Formaldehyde. An
accurate quantitative method for the determination of the total
formaldehyde
content of cotton fabrics treated with a formaldehyde-containing resin
is often
of considerable aid in the elucidation of the resin reactions involved.
However, the usual procedure involving distillation with dilute acid
and
estimation of the formaldehyde in the distillate is not entirely
satisfactory,
as it is difficult to distill formaldehyde completely from solution.
Melamine-Formaldehyde
Resin.
Hirt, described a method for the determination of melamine resins used
to
impart wet strength to paper. The method, which is applicable to
extracted
melamine resin from cotton cellulose fabric, is based on the strong
ultraviolet
absorption of melamine near 235 nm.
Cationic
Surfactants. The
method of
Scott depends upon the chloroform extraction of dye salt formed
quaternary
ammonium surfactant and excess Orange II (p-(2-hydroxy-l-napthylazo)
benzenesulfonate) in aqueous solution. Advantage is taken of this last
fact to
establish a standardization factor, related to molar absorptivity,
obtained
from purified surfactants for the analysis of commercial surfactants.
Infrared
Spectroscopy. Although the infrared
absorption technique was
completely successful for the identification and quantitative
determination of
the extent of chemical modification by reactions with the active
hydroxyl
groups of cotton cellulose, its application for the analysis of the
chemical
finishes encountered several difficulties. These difficulties led to
the
development of complex methods. In one such method, differential
infrared
spectroscopy accompanied by linear scale expansion is utilized. The
frustrated
multiple internal reflectance (FMIR) technique of McCall et al. already
discussed for chemically modified cotton fabrics also be applied for
the
investigation of the cotton fabrics with chemical finishes.
Differential IR
Spectroscopy. McCall,
Miles, Tripp, and O’Connor investigated cottons with chemical finish by
comparing their spectra with those obtained from untreated cotton. This
method
is only practical if some estimate of the actual amount of the
modifying
reagent (the so-called add-on) can be made.
The
analytic
scheme starts with the conventional potassium bromide disc procedure.
If this
technique permits satisfactory detection, identification, and
quantitative
determination of the resin finish, the analysis is completed with
minimum
effort. If, however, sensitivity problems are encountered, the same
prepared
sample is remeasured using the differential and/or linear scale
expansion
techniques. If the resin finish still cannot be identified or
quantitatively
determined, resort is made to the acid hydrolysis procedure.
Inorganic Constituents
Because
it is an
agricultural product, cotton may contain a number of metallic or
nonmetallic
elements or anions. These substances may also contaminate the original
native
product during processing. Besides this, a number of elements may be
added in
small amounts as inorganic salts or oxides or as organometallic
compounds to
the cotton during processing to produce a specific property not native
to
cotton.
Chemical Methods
The
various
elements (either metallic or nonmetallic) in cotton samples may be
identified
by the chemical spot test methods developed originally by Feigl.
These
spot tests
have one major disadvantage; a separate test is required for the
identification
of each element. A second disadvantage is that spot tests do not yield
any real
quantitative data regarding the concentration of the specific element
in the
sample. For chemical methods for the quantitative determination of the
major
inorganic constituents, see Cellulose.
Spot
tests have
one advantage over spectroscopic techniques: they permit distinguishing
between
various anion groups. Thus, tests can be made to differentiate among
sulfite,
sulfate, or sulfide, between nitrate and nitrite, chloride or chlorate,
etc.
The atomic spectroscopic methods in current use permit only the
identification
(and determination) of total elemental content. Thus, sulfur is
identified and
measured as differentiation of the oxidation state of a specific
element as by
induced electron emission are not impossible, they have not been
developed to
the point where they can be said to be tools for the use of the
analytical
chemist engaged in the analysis of cotton and cellulose.
Spectroscopic Methods
Although
there
are about a dozen spectroscopic techniques which are of potential
application
to the determination of elements, both metals and nonmetals, only
three,
electronic emission, x-ray fluorescence, and atomic absorption
spectroscopy,
have been used in the analysis of cotton cellulose. It is of some
importance,
therefore, to consider the respective merits and disadvantages of each
of these
three techniques.
The
advantage of
emission spectroscopy in cotton analysis is primarily its ability to
permit,
with a single scan, a complete qualitative analysis of a considerable
portion
of the entire spectrum. If the photographic technique is being
employed, such a
spectrogram permits the operator to investigate in detail the presence
or
absence of almost all specific metals. In many large spectroscopic
laboratories, the use of emission spectroscopy is confined to such
qualitative
surveys, and x-ray fluorescence or atomic absorption is used (often
subsequent
to the qualitative survey) when quantitative values are required.
A
limitation of
x-ray fluorescence analysis is that light elements may escape detection
or
accurate measurement. However, techniques and selection of accessories
such as
x-ray tube, analyzing crystal, and detector have pushed these analyses
down to
magnesium and, with special vacuum spectrophotometers and specially
designed
analyzing crystals, such as lead stearate, analyses are being made down
to
boron, leaving very few elements which cannot be detected and
determined by
this technique. These latter determinations, between magnesium and
boron, can
be made, however, only by a significant increase in instrumental
sophistication
and expense.
Another
disadvantage of x-ray fluorescence is the sophistication and the cost
of the
initial equipment, which is higher than the cost of the equipment used
in the
other techniques.
Emission
Spectroscopy.
Emission spectroscopy consists of three more or less independent steps.
First,
the proper preparation of the sample; second, the technique for
obtaining the
raw data, the emitted atomic lines of the desired constituents; and
third, the
evaluation of these data to properly identify and quantitatively
determine the
element or elements of interest.
In
the original
paper of O’Connor and Heinzelman, describing the general method for the
spectrochemical analysis of plant products, two procedures were
recommended,
one for samples with an ash content of less than 1% (where there could
be no
matrix effect from the composition of the ash), and another for samples
with
ash content of sample over 1% (where the composition of the ash might
effect
the condition for constant matrix). In later works the second procedure
was
used, mainly because the cotton samples being analyzed were fabrics
chemically
modified by the introduction of metal-containing substances to produce
specifically desired effects. The two procedures are given below.
Procedures
ash-content
of sample less than 1%. Accurately weigh 16.67 of the
sample into a
Vycor dish with 90-mm diameter, and add 2 ml of a solution of 250 g of
magnesium nitrate hexahydrate in 1 liter of 95% ethanol. The ratio of
sample
weight to the magnesium nitrate buffer is arbitrary and can be varied
for
different samples depending upon the sensitivity required, if the ratio
actually used is considered in obtaining and using the working curves.
The
ratio suggested here will permit analysis with a sensitivity of about 1
part of
metal in 10,000,000 parts of sample for the more sensitive metals,
copper and
iron, for most types of samples.
ash content of sample
over 1%. Ash exactly 5 g of sample in a tared Vycor dish
as described in
the earlier procedure, but without addition of the magnesium nitrate
solution.
After removal from the furnace, cool the dish and reweigh to determine
the ash
content of the sample. Add a volume of a very dilute aqueous solution
of
germanium dioxide so as to introduce a quantity of the dioxide equal to
4.14%
of the ash. Grind and thoroughly mix the ash prior to analysis.
Prepare a mixture of salts
representing the major
constituents of a typical ash. A suggested composition is as follows:
65.18%
potassium carbonate, 0.95% sodium chloride, 11.02% calcium carbonate,
18.70%
magnesium carbonate, and 4.14% germanium oxide. Prepare working curves
by
adding graduated amounts of elements to portions of this salt mixture.
X-ray
Fluorescence Analysis.
When x rays strike any material, as any analytical
sample, one usually considers their penetration, as it is their
dominant
characteristic. However, if the beam of x rays is measured before and
after
striking the sample, a decrease in intensity will be found. Some of the
x rays
have been absorbed by the material, the amount, of course, depending
upon the
nature, particularly the density of the material, and the thickness of
the
sample. These absorbed x rays can be used as a successful method of
spectroscopic analysis in a manner essentially analogous to the use of
the
absorption of photons, except that absorption of x rays is an atomic
process,
and absorption by a given element is independent of the chemical
combination in
which the particular element occurs. Thus, x-ray absorption is a
technique for
elemental analysis.
An
x-ray
spectrometer consists of an x-ray tube capable of emitting an intense
x-ray
beam that impinges upon the sample to be analyzed. Fluorescent rays,
which
appear as secondary emission, are dispersed by utilizing crystal with a
known
lattice constant, frequently a bent mica crystal, to act as a
diffraction
grating. The diffracted fluorescent x rays are detected by a Geiger
counter
goniometer, and their intensities may be automatically recorded as a
function of
the foniometer angle which is proportional to the wavelength or
frequency of
the x rays.
Sections
of such
a tabulation are reproduced in Table 11 for the five elements, calcium,
chromium, potassium, manganese, and zirconium. All the lines in Table
11involve
the innermost orbit, as indicated by the letter K in column 3, although
in more
complete tables the L and occasionally M and N lines may occur. ABS
indicates
the absorption line or edge.
For
the purpose
of determination by x-ray fluorescence methods, the elements introduced
into
cotton through chemical and additive modification (which are the major
purposes
for such analyses) fall into two broad categories; those of atomic
number 22
and higher, and the lighter elements. The heavier elements are used to
impart
protection against degradation by outdoor exposure or microorganisms
(copper,
mercury, lead, cadmium, selenium, zirconium), and to impart flame
resistance
(tinbromine); they are also used in certain classes of dyes and
pigments
(chromium, iron, titanium) and for miscellaneous purposes, such as
catalysts.
Count
rates of
equivalent discs usually show a maximum difference of 2%. The
relationship of
percentage composition to counting rate is found to be linear over a
range of
0.01 to about 3% for most of the elements examined. In all cases, a
loss in
counting rate was noted at concentrations above 3% indicating self-
absorption.
This loss of counts becomes marked at concentrations of about 10% and
dilution
with pure cellulose should be employed.
Lignin
and
Hemicellulose
The
compositions
and percentages of lignin and hemicellulose very from one plant species
to
another so it is difficult to arrive at generalizations concerning
structure
and abundance of these polymers. Moreover, composition varies within a
single
plant (roots, stems, leaves), with age (heartwood versus sapwood),
stage of
growth (early wood versus late wood in annual rings) and with the
conditions
under which the plant grows. Study over many decades has elucidated the
major
structural features of wood hemicelluloses and lignin’s along with the
biochemical mechanisms for their degradation.
Most recently attention has turned to the molecular
characteristics of
these enzymes. It is beyond the scope of the present review to
recapitulate
these findings in detail. Rather, the focus will be on specialized or
recently
revealed aspects.
II.
Hemicellulose
Softwoods
(gymnosperms), hardwoods (angiosperms), and grasses (graminaceous
plants) have evolved
separately, and they contain different lignin and hemi cellulose
constituents.
Moreover their specialized tissues have varying proportions of
cellulose,
hemicellulose, lignin, pectins, proteins, and extractives. Lignin is
deposited
during maturation of cell walls, and some carbohydrates become cross
linked to
it. Because the lignin and hemicellulose constituents differ, the
crosslinks
between these polymers differ form plant to plant and from tissue to
tissue.
A. Assay
systems
Hemicellulases
are most commonly assayed by measuring the rate of reducing group
formation
under optimum conditions. A suitable polysaccharide substrate is
suspended in
buffer and mixed with an enzyme solution that is appropriately diluted
to yield
a linear response over time Alternately, several successive two-fold
dilutions
are assayed for a single fixed time (10 to 30 min), and the enzyme
titer is
calculated from the average of several successive dilutions that
exhibit a
consistent enzyme activity. This approach is necessary because at very
low
dilutions (or long assay times) the substrate is exhausted, and the
calculated
activity is not representative of the actual value.
B.
Classification
Hemicellulases
are classified according to the substrates they act upon, by the bonds
they
cleave and by their patterns of product formation, Table 1, but greater
variety
exists among the endo-xylanases and ß- glucosidases than is reflected
in this
simple classification system. One notable distinction is made between
endo-1,4-b-xylanase
(EC
3.2.1.8) and xylan 1,4-b-xylosidase
(EC 3.2.1.37). While the former produces
oligosaccharides from random cleavage of xylan , the latter acts
principally on
xylan oligosaccharides producing xylose. Some endo-xylanases appear to
have
greater specificity for straight chain substrates, and others appear to
be able
to accommodate more frequent side chains of branching.
Deacetylation
makes native xylans much less soluble in water, which is an observation
that
causes some consternation to trained organic chemists. Even though
acetylation
makes the xylan polymer more hydrophobic, it also blocks extensive
intrachain
hydrogen bonding. When the acetyl groups are removed, hydrogen bonding
leads to
xylan precipitation. Even though the substrate is less soluble,
deactylation
generally increases susceptibility of the substrate to enzyme attack.
Some
acetylesterses, however, show specificity for the native acetylated
substrate.
C.
Thermophilic
hemicellulases
Thermostable
enzymes are often of interest for biotechnological applications, and
hemicellulases are no exception. Thermophilic xylanases have been
recognized
for several years. Ristoph described an extra cellular xylanase
produced by the
thermophilic actinomycete Thermonospora that was
stable for approximately
1 month at 55 °C and could withstand up to 80°C in a 10 min assay.
Gruninger
described a highly thermos table xylanase form a thermophilic bacillus
that has a
catalytic optimum of 78° and
a half life of 15 h at 75°. Both enzymes were predominantly endo in
activity
and produced only trace quantities of xylose after long periods of
incubation.
The thermophilic fungus Thermoascus aurantiacus
produces an extra
cellular endo-xylanase that has a temperature optimum of 80°C. At that
temperature, the half life is 54 min. These characteristics are fairly
similar
to thermos table xylanases from Talaromyces emersonii,
another
thermophilic fungus that grows on straws and pulps.
D. Alkaline active xylanases
An
alkaline
active enzyme was also purified and characterized from an alkalophilic
actinomycete. Tsujibo et al. purified three endo-xylanases from the
culture
filtrate of Nocardopsis dassonvillei subsp. alba
OPC- 18. The
molecular weights were 23,000, 23,000
and 37,000 for X-1,X-11,and X-111, respectively.
E. ß- Xylosidase
Endo-xylanases
are much more common than ß-xylosidases, but the latter are necessary
in order
to produce xylose. Most ß-xylosidases are cell bound, and the enzymes
are large
relative to endo-xylanases. The ß-xylosidase of Bacillus
sterothermophilus
has a molecular weight of about 150,000 and is stable at up to 70°C Utt
described a novel bifunctional ß- xylosidase from the ruminal bacterium
Butyrivibrio
fibrisolvens was cloned in E. coli
and shown to consist of a
60,000Mr protein with dual
glycosidase activity.
F.
Mannanases
and galactanses
Mannanases
and
galactanses are described far less frequently than xylanases. It is not
known
whether this is attributable to their lower prevalence in nature or
simply
because they are sought less often. Bacterial species known to produce
mannanases include Aeromonas hydrophila, Cellulomonas
sp. and Strepromyces
olivochromogenus. Multiple endo – ß-mannanases are found in
the extra
cellular broth of Polyporous versicolor.
G. Accessory
enzymes for
hemicellulose utilization
A
number of
enzymes appear to be critical in the early steps of hemicellulose
utilization.
These include acetyl xylan esterases, ferulic and p-coumaric
esterases,a-l a
rabinofuranosidases, and a-4-O-methyl
glucuranosidases.
Acetyl
xylan
esterase was first described by Biely in several species of fungi known
to
degrade lignocellulose and most especially in A.
pullulans.
Acetyl xylan esterase was subsequently described in a number of
different
micres different microbes including (Schizophylum commune, Aspergillus
niger
and Trichoderma reesei, Rhodotorula mucilaginosa and Fibrobacter
succinogenes. Acetyl xylan esterase acts in a cooperative
manner with
endoxylanase to degrade xylans. This enzyme is not involved in breaking
LCbonds
because the acetyl esters are terminal groups.
The
ferulic acid
esterase of S. commune exhibits specificity for its
substrate, and it
has been separated form other enzymes. Borneman et al. assayed feruloyl
and p-
coumaryl esterase activities form culture filtrates of anaerobic fungi
using
dried cell walls of Bermuda grass as a substrate. The enzyme
preparations
released ferulic acid more readily than they released p-courmaric
acid
from plant cell walls. Assays using methyl ferulate or methyl p-coumarate
as substrates in place of dried cell walls showed the presence of about
five
times as much enzyme activity. McDermid employed ethyl esters of p-coumarate
and ferulate as substrates for these activities.
111.
Lignin
Lignin
is an
aromatic polymer with the substituents connected by both ether and
carbon-carbon linkages. It is composed of three principal building
blocks: p-coumaryl
alcohol (p-hydroxyphenyl propanol), coniferyl
alcohol (guaiacyl
propanol), and sinapyl alcohol (syringyl propanol)
A.
Lignin-degradingenzymes
Recent
years
have witnessed an increasing interest in the enzymatic mineralization
and
depolymerization of lignin. No dougbt this stemmed in part form the
description
of a family of peroxidase-like enzymes from active lignin-degrading
cultures
of Phanerochaete
chrysosporium.
These enymes, termed lignin peroxidases (LiP) are capable of degrading
a number
of lignin model compounds- including the b-
aryl ether model-and are closely
associated with the mineraliation of 14C-
labelled lignin to CO2. Of particular note is the
ability of enzyme
preparations from p.chrysosporium
to depolymerize methylated spruce lignin.
B. Lignin
degradation in whole cell cultures
There
are
essentially two approaches to understanding the roles of these various
enzymes
in lignin degradation. One approach is to reconstitute a lignin-
degrading
system in vitro using crude, purified, or cloned enzyme constitutents.
The
other is to regulate the cellular metabolism of lignin- degrading fungi
in vivo
so as to produce either LiP or MnP, and then to observe the cultures
for
depolymerization and mineralization.
Studies
by Perez
showed that both the appearance of lignin- degrading enzymes and
mineralization
of lignin are affected by the presence of manganese. The effects,
however, vary
with the organisms and test systems employed. In p.chrysosporium,
elevated Mn concentrations represses
LiP while inducing MnP, and in phlebia
brevispora,Mn represses
LiP while
inducing both MnP and laccase. In both cases, high rates of
mineralization
correlate with the appearance of LiP at low Mn concentrations and not
with the
appearance of MnP or laccase D. squalens produces
only MnP and laccase,
and in this organism, mineralization correlates with the appearance of
MnP.
C.
Degradation
by cell-free enzyme systems
Complete
catalytic depolymerization of lignin has not been demonstrated in vitro
with a
cell-free enzyme system. Extracellular proteins form p.chrysosporium,
Coriolis versicolor, and
phlebia radiata were reported to increase the
number of hydroxyl groups,
decrease the mean molecular weight, or otherwise chemically alter
various
lignin preparations. Kern investigated the action of crude and
partially
purified LiP form p.
chrysosporium on the molecular size distribution of 14C
– labelled lignins, and he further
used pyrolysis GC mass spectrometry to investigate changes in substrate
characteristics.
The
mechanism of
lignin depolymerization may not be the same in all organisms. Galliano
studied
lignin degradation (solubilization) by Rigidoporous
lignosu, an organism that
does not produce LiP, but rather synthesizes MnP and laccase. When the
tow
enzymes were puriried and their properties studied in vitro,neither
enzyme was able to solubilize radioactive lignins. When both enzymes
were added
to the reaction, mixture at the same time, lignin solubilization was
extensive;
the MnP and the laccase acted synergistically. In addition, glucose
oxidase
enhanced lignin solubilization by preventing repolymerization of the
radicals
formed by the two oxidative enzymes.
D. Role of
glycosides in lignin
degradation
One
of the first
enzymes implicated in lignin biodegradation was cellobiose: quinone
oxidoreductase(CBQase). This enzyme catalyzes the reduction of a
quinone and
the simultaneous oxidation of cellobiose discovered this enzyme and
proposed
that its tole might be to prevent
repolymerization of lignin during degradation. Recent
studies have not
borne this out, but the enzyme may be important monetheless. The CBQase
of P.chrysosporium
binds very tightly to microcrystalline cellulose, but such
binding does not
block its ability to oxidize cellobiose, indicating that the binding
and
catalytic sites are in two different domains.
An
essential
feature of lignin biodegradation is that degradation products resulting
form
the activity of extracellular enzymes must be taken up by the mycelium;
glycosylation by b-glucosidase
seems to be an important part of this
process. Whether or not sugars attached to lignin in the native
substrate by
nonglycosidic linkages play a similar role has not been addressed.
1V.
Lignin-carbohydrate complexes
Lignin-carbohydrate
complexes (LCCs) are heterogeneous, poorly defined structures that are
found in
many plant species. Lignin is directly or indirectly bound covalently
to
carbohydratate, and the resulting complexes present a barrier to
biological
degradation. LLCs have proven to be highly intractable materials that
are in
large part responsible for limiting the biodegradation rate of plant
materials.
A.
Fractionation of lignin and carbohydrate in wood
The existence and character of
LC bonds have long
been studied and debated. In 1956, Bjorkman published techniques for
liberating
MWL form finely ground wood. He also obtained an LCC by exhaustive
milling
and extraction.
Aqueous dioxane was used
to extract MWL, and dimethylformamide, dimethylsulfoxide, and aqueous
acetic
acid were used to extract LCC. The lignin contents of LCCs are similar
to that
of the MWLs except that the latter contain less carbohydrate. Despite
many
attempts Bjorkman could not separate carbohydratae from LCC without
resorting to
chemical degradation.
B. Isolation
of
LCCs
The
LCCs are
highly heterogenous and difficult to study. Many conventional chemical
treatment, such as alkali, readily disrupt some of the most prevalent
LC bonds.
The best approaches employ neutral solvents and purified enzyme
preparations.
Most research has focused on the isolation of water- soluble lLCCs,
because
they can be fractionated, sized, and subjected to spectroscopic study.
Larger,
insoluble LCCs exist and may even account for the bulk of LC bonds in
the
substrate, but less is lnown about them. Most evidence suggests that
the
chemical linkages are similar to those of the soluble LCCs.
C. Chemical
characteristics of LC bonds
Existence
of
chemical bonds between lignin and carbohydrate has been questioned
because of
the intimate physical intergration between the lignin and carbohydrate
constituents, the possibility of entrapment or adsorption, and lability
of many
linkages. Several different types of LC bonds have been proposed based
on
knowledge about changes in sugar composition following digestion and
about
hemicellulose structures. The proposed LCbonds include bonds to xylan,
glucomannan, cellulose, various other hemicellulosic sugars, and pectin.
D. Ferulic
and
p-coimaric ester side chains
Grass
and bamboo
lignins differ from those of hardwoods and softwoods in that they are
formed
not only from guiacyl and syringyl units, but also from p-hydroxyphenyl
units. Esterified p-coumaric acid can comprise 5%
to 10% of the total
weight of isolated lignin. The participation of p-hydroxphenyl
glycerol-b-aryl
ether
structures is of minor importance.Based on methanolysis and spectral
studies,
Shimada proposed that the majority of p-coumaric
acid molecules in
bamboo and grass lignin are ester-linked to the terminal g
carbon of the
side chain of the lignin molecule Fig.11. Ferulic acid is also present
in small
amounts. The p-coumaric ester linkages are
extremely stable as they are
not removed by methanolysis, thioglycolic acid treatment, or catalytic
hydrogenolysis.
E. Frequency
and stability of LC
bonds
The
amount of carbohydrate remaining on the lignin can measured by
sugar analysis following acid hydrolysis. Obst found 10.8% carbohydrate
in a
MWEL from loblolly pine. Afraction of this amount (11%) was removed by
dilute
alkali.
F. Residual
lignin in kraft pulp
Kraft
pulping
removes large quantities of hemicellulose and lignin and disrupts ester
linkages between lignin and carbohydrate and between hemicellulose
chains. Even
so, significant amounts of carbohydratae remain bound to the residual
lignin
after kraft
pulping. As the reaction
progresses, however, residual lignin becomes harder to remove. Lignin
remaining
in the kraft pulp cannot be removed without unacceptable large yield
losses.
to
the O-6
of galactoglucomannan. Although this structure is in accord with what
is known
about native LC bonds, the bonds in kraft pulp can be very different.
The
nature of the covalent linkages in kraft pulp have not been fully
characterized
because (1) many rearrangements occur during pulping (2) difficulties
are
encountered in isolating the residual lignin, and (3) reliable
degradation and
characterization methods are lacking. At least some changes appear to
result
from the formation of alkali- and acid- stable carbon- carbon bonds
between
lignin and carabohydratae Several rearrangements are possible.
Particularly,
primary hudroxyls of glucose and mannose can react with the a,
b
or
g
carbons of
phenylpropane units to form ether linkages. The reducing –end groups
can also
react. Glucose is the most prevalent sugar bound to residual lignins
form kraft
pulps, and it seems probable that this results form the reaction of
cellulose
with lignin during the pulping process.
This conclusion is supported, but not proven, by the observation that
the
glucose content of lignins from wood. Reactions with the b
and g
hydroxyls of
the phenyl propane are particularly acid and alkali- stable and may
ascape
detection in methylation analysis.
Minor
used
methylation analysis to determine the characteristics of
polysaccharides
attached to residual lignin in loblolly pine kraft pulps. The total
carbohydrate content of the residual MWEL was only about 8%. As
compared to the
12% abtained with MWEL from nataive wood, but methylation data
indicated that
the carbohydratae bonding was similar in kraft and native wood. The
primary O-6
position was most frequently found for hexans and the primary O-5
for arabinan. Xylan was
bonded to lignin
at O-3, with a small amount at O-2
The predominant
methylated derivatives obtained from glactose and arabinose indicated 1®4
and 1®
5
linkages,
respectively. The apparent DP ranged from 4 for xylan to almost 13 for
galactan. Because of the small differences in methylation patterns between
carbohydrates form MWELs of
pine and pine kraft pulp. Minor was not able to confirm the possible
formation
of LC bonds during pulping.
G.
Biodegradation of LCCs
Most
carbohydrate chains or side groups appear to be attached to lignin
through the
nonreducing moieties. Because exo-splitting enzymes
generally attack a
substrate form the nonreducing end of a polysaccharide, removing
substituents
progressively toward the reducing end of the molecule, complete
degradation is
not possible. Even when cargohydrates are attached to the lignin by the
O-1
hydroxyl, a single sugar residue could remain attached even after
complete
attack by exo-splitting glycosidases.
I. Residual
LC
structures after exhaustive enzymatic digestion
The
presence of
lignin, aromatic acid, and other modifications of hemicellulose clearly
retards
digestion of cellulose and hemicellulose by ruminants. Phenolic acids
associated with forage fibre are known to decrease fibre digestion when
they
are in the free state. p- Coumaric, ferulic, and
sinapic acids inhibit
the activity of rumen bacteria and anaerobic fungi.
The
influence of
LCCs on ruminant digestion was studied by examining the solubilization
of
LCCs using cell-
free hemicellulase
complexes from the rumen. LCCs from grasses of increasing maturity were
isolated and treated with cell-free rumen hemicellulases. As the lignin
content
increased, the extent of degradation declined, indicating that the
lignin
content of the LC was the overriding factor in determining its
digestibility.
2.
Solubilization of LCC by microbial activity
In
recent years,
studies on the solubilization of lignin from grasses or wood labelled
with 14C
phenylalanine have proliferated.The
bulk of the radioactive label is incorporated into the lignin rather
than into
carbohydrate or protein of the plant, but it is clear that lignin
purified from
the labelled plant tissue contains significant amounts of carbohydrate.
This
material is probably closer to the structure of native lignin than is
synthetic
lignin prepared by in vitro dehydrogenative
polymerization of coniferyl
alcohol.
In
summary,
research on the solubilization of LCC by microbial activity has
periodically
shown that cellulases, hemicellulases, esterases, and perhaps
peroxidases all
correlate with lignin solubilization. The mineralization rates and
extents
reported for streptomyces are relatively low, and the solubilized
lignin is not
extensively modified. Lignin mineralization and solubilization could,
therefore
be sttributable to two (or more) different enzymes. These studies
require
additional rigorous clarification.
V.
Enzymatic treatments of pulps
For
many
applications, residual lignin in kraft pulp must be removed by
bleaching.
Successive chlorination and alkali extraction remove the remaining
lignin to
leave a bright, strong pulp suitable for printing papers and other
consumer
products. Although chlorine bleaching solves the immediate problem of
residual
lignin, the chlorinated aromatic hydrocarbons produced in the bleaching
step
are are recalcitranat and toxic. These chlorinated products are hard to
remove
from waste streams, and trace quantities are left
in the paper, so other bleaching
processes were devised. One approach is to use hemicellulases to
facilitate
bleaching.
V1.
Conclusions
Lignin
and
hemicellulose are complex polymers occuring in plant materials. Either
polymer alone
presents a formidable challenge to microbial degradation. In native
substrates,
however, lignin and hemicellulose are intermeshed and chemically bonded
through
covalent corss-linkages. As such they are even more resistant. Covalent
lignin
– corbohydrate linkages can be divided into two types: ester linkages
through
the free carboxy terminus of uronic and aromatic acids and ether
linkages
through sugar hydroxyls.
Degradation of
Wood
Introduction
Microbial
degradation of wood
should be considered on two levels. From an organismic point of view
degradation concerns the interaction of the cell with its
microenvironment. At
this level, degradation is strongly affected by diffusion of the
degradative
agents and the uptake of the oligomeric products. From a biochemical
perspective, degradation concerns the molecular architecture of the
biopolymers, the capacities of the enzymatic catalysts to bind to them
and the
catalytic turnover rates. Lignin reactivity is limited by the
accessibility,
heterogeneity, and stability of the polymeric linkages; cellulose
reactivity is
limited mostly by accessibility and crystallinity; and hemicellulose
reactivity
is limited by substitution.
Distribution of Wall Components
The
three principal components of
wood are found throughout the wall. Cellulose is organized into laminar
crystallites which are bundled into the microfibrils. Each micro-fibril
contains regions of amorphous cellulose interspersed and intertwined
with hemicellulose,
Fig. 1. The latter is in turn crosslinked to lignin. Most of the
cellulose is
in the secondary wall layers. Its concentration depends on the relative
portions of hemicellulose and lignin.
Component Chemistries
Lignin
is a stable, highly
cross-linked aromatic macromolecule arising through dehydrogenative
free
radical polymerization of p-coumaryl, coniferyl;
and sinapyl alcohols.
Gymnosperm lignin is made up principally of coniferyl alcohol (guaiacyl
lignin)
whereas angiosperm lignin is made up from approximately equal amounts
of
coniferyl and sinapyl alcohols (syringyl/guaiacyl lignin). p-Coumaryl
alcohol is found principally as a precursor to the lignin of grasses
and in
reaction wood. Cellulose microfibrils are the most conspicuous element
of the
plant cell wall. The b-(1,4)-D-glucose
chains which make up the cellulose polymer can be arranged in both
parallel and
antiparallel crystallites; but based on what we now know about the
biosynthetic
and biodegradative mechanisms and x-ray crystallographic studies, the
parallel
arrangement is almost certainly the form found in nature.
Microstructure
and Porosity
Porosity
and microstructure are critical in determining accessibility of enzymes
to
their polyumeric substrates. The relationship between enzyme diffusion
and
substrate accessibility has been shown in the number of ways. For
example,
reducing particle size greatly increases surface area and speeds up the
enzymatic hydrolysis of cellulose. Grinding or cutting wood to open all
the
lumina to attack will expose considerably more surface than is
available in
whole wood. The total surface area reaches about 1 m2 g-1.
Once the lumina are accessible, however, relatively little
further increase in surface area occurs following further grinding,
because the
cell wall thickness is very small compared to its length. Particle size
is not
the only factor important in enzymatic depolymerization. Enzymatic
accessibility
depends on the structure of the wall itself.
Degradation
of whole wood
Microbial
degradation of whole wood occurs principally by fungi. Although
bacteria
apparently possess much of the biochemical machinery necessary for
biodegradation of the wood components, they do not form mycelia, and
hence,
cannot propagate through the tissue structure or translocate nutrients
into the
wood or from one region to another as decay progresses.
Biodegradation
of Lignin
Biodegradation
of lignin has been studied for more than 80 years with only incremental
progress over much of the time. Early assays employed the disappearance
of
acid-insoluble Klason lignin. The development of Synthetic 14C-labelled
lignins enabled rapid radiorespirometric determination of lignin
degradation
under controlled conditions, and a series of experiments using this
assay led
to improved understanding of the cultural and nutritional conditions
under
which lignin biodegradation occurs.
Other
aspects of lignin biodegradation such as the roles of oxidative species
details
of biochemical mechanisms, regulation and bacterial lignin degradation
have
been reviewed elsewhere.
Biodegradation
of Cellulose
The
biodegradation of cellulose has probably attracted more attention
world-wide
than any other single biochemical event outside the filed of medicine,
yet the
mechanism of cellulose hydrolysis is still imperfectly understood. The
cellulase complex from Trichoderma reesei has been
studied most
thoroughly, but it is not yet clear exactly how this complex works to
depolymerize cellulose. The focus here is placed on the biochemical
mechanisms
involved in hydrolysis.
When
all of the cellulases are present, Valonia crystals
undergo a
delamination to form bundles of microcrystallite fibrils. A similar
delamination has been observed with the purified bacterial cellulose
from Acetobacter
xylinum. Although no direct evidence is available, the
delamination
probably occurs between the sheets adjacent van der Waals-bonded sheets
rather
than between adjacent hydrogen bonded chains.
The
distinction between CBH I and II is not confined to T. reesei.
CBH I and
II are also found in the cellulase complex of Penicillium
pinophilum and
in Fusarium lini. In the later instance, CBH I was
found to be active
against D-xylan, whereas CBH II was not. Given the apparent importance
of the
C6 carbon in forcing the half-chair conformation during the hydrolysis
of the
glycosidic bond by lysozyme, it is apparent that substrates lacking
this
substituent may not be appropriate for some cellulases.
Biodegradation
of Hemicellulose
Because
of the diversity of hemicelluloses, it is not possible to survey here
all of
the different types known. Dekker have provided an extensive, useful
compilation of this sort. Likewise have recently reviewed the
literature on
microbial xylanases. Mannanases have been studied to a lesser extent,
hence,
the emphasis here is placed on xylanases.
The
characterization of hemicellulase mechanisms presents numerous
difficulties not
encountered with cellulase because the substrate employed is
heterogeneous and
branched. Moreover, the literature is not definitive because substrates
such as
larch arabino – glucuronoxylan commonly used for hemicellulose assays –
presumably because of their easy solubility – but they are not really
representative of the bulk of hemicellulose found in wood. Selection of
an
appropriate substrate is critical because the apparent specific
activity of a
xylanase can vary greatly depending on the xylan preparation used for
assay.
Substrate variability is not confined to the type of xylan employed,
because
different batches of xylan from the same supplier can lead to apparent
changes
of as much as two-fold. Native xylans tend to be highly acetylated.
Acetyl
groups increase colubility but decrease the rate of enxymatic
depolymerization
so the method of substrate preparation can have a profound affect on
the
apparent activity.
Many
xylanases are not able to remove glucuronic acid resudies from the
xylan chain
probably because of the 30-fold greater stability of glucuroniside
glycolytic
linkages noted earlier. By comparison, the ester linkages connecting
acetyl
groups to the xylan are exceedingly unstable and most are removed ruing
isolation of the polymer. Although synergism is observed between xylan
esterases and endoxylanases during hydrolysis of partially acetylate4d
xylan,
it seems unlikely that deacetylation would be a rate-limiting step in
the
enzymatic hydrolysis of xylan under most practical conditions because
of the
extreme lability of the ester linkages.
Applications
Technical
developments, particularly the discoveries of xylose-fermenting yeasts
and
lignin-degrading enzymes have paved the way for specific removal of
hemicellulose and lignin in order to recover cellulosic fibers and to
produce
useful chemicals from the hemicellulose and lignin fractions.
An
alternative approach to biological pulping which permits more direct
control
over the degradation activities is to employ isolated enzymes for
lignin or
hemicellulose removal. Because the lignin is cross-linked mostly into
the
hemicellulose is more readily depolymerized than lignin, it has been
the
initial target of treatment. Removing only a small portion of the
hemicellulose
might be sufficient to open up the polymer and ease solvent removal for
the
residual lignin. The principal problem in these studies has been in
obtaining
selective removal of hemicellulose without degrading the cellulose.
Selective
inhibition of cellulase activity can be realized by the addition of I
mM HgCl2.
Xylan was specifically removed from delignified cell walls leading to a
decrease in energy demand during beating. Removing less than 2% of the
initial
dry pulp weight gave better fiber bonding due to increased flexibility.
Therefore, enzymatic treatments of pulp hold the prospect of both
decreased
cost and improved fiber qualities.
CCA-Treated
Wood
Wood
treated with CCA is widely used in outdoor architectural projects such
as
decks, walkways, gazebos and retaining walls. Although this material
was
originally marketed as a maintenance-free product, most preservatives
manufacturers and end users today recognize the advantages of
protecting and
beautifying CCA-treated lumber with surface finishes. As a consequence,
it
frequently becomes a substrate for a variety of coatings products.
Unfortunately,
little published information is available on the performance of surface
finishes
over CCA-treated wood. This has led to some confusion among
preservatives
manufacturers, coatings companies, the trade, and consumers as to the
compatibility of coatings with this material. Not surprisingly, there
are a
number of misconceptions about the effects of CCA treatment on coatings
performance. This project was undertaken to help clarify some of these
issues.
All
of this suggests that surface finishes applied over CCA-treated lumber
would
perform better than those applied over untreated wood of the same
species.
The
objective of this project was to evaluate the performance of a variety
of
surface finishes over untreated and CCA-treated wood and to determine
the
extent of the chromium enhancement effect.
Wood-Polymer
Composites
Wood
is a hygroscopic material that can sorb water as a liquid or
as vapor from its surroundings. The sorbed moisture can exist in wood
as liquid
or vapor in all cavities and as water bound chemically within cell
walls.
Moisture in the cell walls affects all wood properties, but moisture in
voids
affects all wood properties, but moisture in voids affects only weight.
The
amount of moisture held in cell walls varies according the particular
piece of
wood and is a function of both the relative humidity and temperature of
the
surrounding air. The fiber saturation point is the point at which cell
walls
are empty. Wood is subject to dimensional changes when its moisture
content
fluctuates below the fiber saturation point.
One
way wood-polymer composites (WPC) are made is by saturating
wood with monomers followed by in situ polymerization
of the monomers.
Many monomers used to make WPC do not penetrate the cell walls and do
not
completely fill the voids after polymerization, leaving voids in the
wood
because of shrinkage of the polymer. Solvents can be used to help swell
wood
and aid the penetration of monomers into cell walls, resulting in
moderately
high dimensional stability. Methanol has been used to swell the wood;
after
methanol treatment, the wood is then heated.
The
objectives of this research were to identify monomers that
swell and penetrate the cell wall without the use of solvents, measure
the
extent of swelling in individual monomers, polymerize the monomer in
the wood,
and measure the volumetric swelling and moisture content of the WPC at
90%
relative humidity (RH).
Materials
and Methods
Monomers
Monomers
selected for this study were acrylates and metacrylates,
which can be polymerized using the thermal polymerization initiator
2,2´-azobnis-(2-ethylbutyronitrile). All monomers were used as received
without
further purification or removal of inhibitors added by the
manufacturers.
Wood
specimens
Maple
(Acer saccharum ), northern red oak (Quercus rubra),
and southern pine (Pinus sp.) woods were used. The
wood was cut into
specimens 2.5 by 2.5 by 0.6 cm (radial by tangential by longitudinal).
Growth
rings were oriented parallel to one side of each specimen to minimize
distortion during shrinking and swelling.
Volumetric
swelling and moisture content
The
WPC specimens and untreated control specimens were conditioned
in a humidity chamber at 27 C and 30% relative humidity (RH) for 7
days. The
conditioned specimens were weighed, dimensions measured, and volumes
calculated. The specimens were then placed in a humidity chamber 27 C
and 90%
RH. Specimens were weighed and measured after 2,4, 6, and 24 h, and 2,
3, 4,
and 7 days. The percentage of volumetric swelling based on the treated
oven-dry
volumes was calculated based on the oven-dry untreated weight.
Volumetric
swelling of WPC
specimens
The
rate of swelling of WPC specimens was slow for the first few
hours at 90% RH, compared to that of the control specimens Tables 6 to
8 After
6 h in 90% RH,
maple red oak, and
southern pine control specimens swelled 6.6, 6.1 and
7.0%, respectively. In contrast, all maple
WPC specimens swelled less than 3%, red oak WPC specimens less than
3.6%, and
the southern pine WPC specimens less than 4.0% during the same period
moisture
into the wood, which lowered the rate of swelling compared to that of
controls
during the first few
hours at 90% RH.
Moisture
content of WPC specimens
The
moisture content of WPC specimens was calculated from specimen
weight at specified time intervals in 90% RH. The rate of moisture
sorption
during the first few hours was slower than it was after 24 h in 90%RH.
After 7
days at 90% RH, the moisture content calculated on the basis of only
the wood
in the specimens (untreated
oven-dry
weight) excluding the weight of polymer, showed that many treated
specimens
sorbed more moisture than did control specimens.
Conclusions
Certain
monomers can swell
wood at temperatures above room temperatures. These monomers have low
molecular
weights and molecular structures that facilitate hydrogen-bonding. For
wood-polymer composite (WPC) specimens that displayed some volumetric
swelling
after polymerization, cell-walk bulking had the potential to reduce the
moisture-related swelling, but this potential was counteracted by the
hygroscopicity of the polymers. This attraction of moisture to the WPC
specimens resulted in some swelling that likely would not have occurred
had the
polymers not been hygroscopic.
Lignocellulosic—Plastic
Composites from Recycled Materials
Use
of the word “waste” projects a vision of material with no value or
useful
purpose. However, technology is evolving that holds promise for using
waste or
recycled wood and plastics to make an array of high-performance
products that
are, in themselves, potentially recyclable. Preliminary research at
Forest
Products Laboratory (FPL), and elsewhere indicates that recycled
plastics such
as polyethylene, polypropylene, or polyethylene terephthalate can be
combined
with wood fiber waste to make useful reinforced thermoplastic
composites.
Advantages associated with these composite products include lighter
weight and
improved acoustic, impact, and heat reformability properties—all at a
cost less
than that of comparable products made from plastics alone. In addition,
previous research has shown that composite products can possibly be
reclaimed
and recycled for the production of second-generation composites.
Thermoformable Composites as Outlets
for Waste Paper, Wood,
and Plastics
Two
general types of thermoformable composites exist, distinguished by
their very
different manufacturing processes. Both processes allow and require
differences
in composition and in the lignocellulosic component. The two processes
used to
produce thermoformable composites are melt blending and air laying or
nonwoven
mat formation.
Currently,
the primary application of the thermoformed composites, both melt
blended and
air laid, is for interior door panels and trunk liners in automobiles.
As
noted, additional large-volume, low-to-moderate cost applications are
expected
in areas such as packaging (trays, cartons), interior building panels,
and door
skins.
Recent
Research on Wood Fiber-Thermoplastic Composites
The
following is not intended to
be a comprehensive review of recent research on wood
fiber-thermoplastic
composites. Instead, we simply illustrate the effects of some important
composition and processing variables in the composite processes,
including
preliminary indications of the effects of recycled ingredients.
Composites
Made by Melt Blending.
The 1980s brought a resurgence of
research into various aspects of melt-blended composites made from
wood-based
flour or fiber in virgin thermoplastic matrices. For example, Kokta and
his
colleagues have published numerous papers in this area, emphasizing
improvements in the filler-matrix bond through coupling agents and
grafting of
polymers on cellulosic fiber surfaces.
Composites
Made by Nonwoven Mat Technologhy.
Numerous
articles and technical papers have been written and several patents
have been
issued on both the manufacture and use of nonwoven fiber webs
containing
combinations of textile and cellulosic fibers. This technology is
particularly
well-known in the consumer products industry. For example, Sciaraffa
and others
have been issued a patent for producing a nonwoven web that has both
fused spot
bonds and patterned embossments for use as a liner material for
disposable
diapers. Bither has found that polyolefin pulps can serve as effective
binders
in nonwoven products, many additional references could be cited in this
area.
Research
and Development Needs
At
the FPL and the UW, we are conducting a program aimed at developing
technology
to convert recycled wood fiber and plastics into durable products that
are
themselves recyclable, are environmentally friendly, and will remove
the raw
materials from the waste stream. This program is being conducted in
cooperation
with the U.S. Environmental Protection Agency. In support of this goal,
we have
defined a number of research and development needs.
Chemical
Modification of Wood Fiber
The
wood composite industry has an opportunity to follow this trend and
greatly
expand markets for new materials based on blends and alloys with other
materials. The research program at the Forest Products Laboratory is
focusing
on wood fiber/plastic blends and alloys in an attempt to produce
materials with
consistent, uniform, continuous, predictable, and reproducible
properties.
The
purpose of this paper, is to present some of the initial results in the
area of
wood fiber thermoplasticization. The wood fiber is a composite made up
of a
crystalline, thermoset polymer (cellulose) in a thermoplastic matrix
(lignin
and the hemicelluloses). The melting point of the thermoplastic matrix
is too
high to allow this phase to flow at temperatures that do not degrade
the wood
fiber. If the glass transition temperature of the thermoplastic matrix
of wood
is reduced through chemical modification, it is possible to plasticize
the wood
fiber allowing it to become more thermo-formable through
thermopressing,
extrusion, or injection.
Experimental
Procedure
Esterification Procedure
Modification
of the fibers using maleic (MA) and succinic (SA) anhydrides was
performed
using the following procedure. Hot xylene was saturated with each of
the
anhydrides in a reaction vessel. When reflux temperature was reached,
the aspen
fiber was added and allowed to react for different lengths of time to
give
different levels of chemical weight gain. Excess anhydride was removed
by sohxlet
extraction of the esterified fiber with xylenes for four hours. Excess xylene was removed by
evaporation and then
the fiber was oven dried overnight.
Analyses
of Esterification Products
FTIR
spectroscopy was performed using a Nicolet 6000 Spectrophotometer.
Samples of
each modification were dried, ground, and mixed with potassium bromide
in a
ratio of 1 mg; 200 mg and pressed under a vacuum to form pellets.
Absorbance
was measured over a range from 4000 cm-1 to 400 cm-1with air as a
reference.
Scanning
electron micrographs (SEM) were taken with a jeol 840 scanning electron
microscope. All samples were coated with gold. A magnification of 750x
was
used.
Acetyl
contents were determined by saponifying the acetyl groups and using
gas-liquid
chromatography (GLC) with propionic acid as a standard. The ester
content of
the MA and SA modified fibers were calculated from the acid and
saponification
values using a similar technique to that used by Matsuda.Titration of
the free
carboxyl groups in the esterified wood fiber and in ester groups
saponified
from the esterified wood fiber gave determinations of the ester content
based
on the acid and saponification values and indicated whether the
anhydride added
as a monoester or diester.
Board
Formation
Five
percent of a liquid phenol-formaldehyde (PF) dryprocess hardboard resin
(GP2341.50% aqueous solution), was sprayed onto the esterified fiber
followed
by attrition milling to distribute the resin. The fiber was hand formed
into a
mat approximately 15 by 15 cm and then pressed in a heated Carver press
at
190°C for 10 minutes to a predetermined thickness of 6 cm and specific
gravity
of 0.7. for the no resin boards, controls AA, MA, and SA modified
fibers were
hand formed into fiber mats and pressed in a heated carver press at
210°C for 5
minutes at 8.5 MPa (board pressure).
Board
Testing
Moisture
sorption
Fiberboards
were cut into 5 by 5 cm specimens and placed in separate rooms at 27°C
and 30,
65,, or 90% relative humidity. Each specimen was weighed at the end of
21 days
and the equilibrium moisture content determined. Triplicate samples
were done
and the results averaged.
Esterification
of Wood Fiber
Figure
1 shows the extent of modification achieved with the three types of
anhydrides
based on a molar addition. The reaction with AA results in the
splitting out of
byproduct acetic acid during wood esterification while the reactions
with MA
and SA result in 100% anhydride carbon skeleton added to the wood. MA
showed a
much lower reactivity than SA which has the same structure except for
the
double bond in MA. A possible explanation for the molecule into the
cell wall
as well as the instability of the reactivity MA chemical intermediate.
Moisture
Sorption of Esterified
Fiberboards
Figure
2 shows that the equilbrium moisture content (EMC) of the esterified
fiberboards is independent of the type of esterification, but is
dependent on
the molar gain of reacted ester in the wood fiber. A reduction in EMC
was seen
for each esterification as the level of modification was increased.
Rate
and Extent of Swelling of
Fiberboards in Liquid Water
Figure
4 shows the rate of fiberboard swelling in liquid water at equivalent
levels of
anhydride modification. while control boards swelled almost 25% in
thickness in
the first 20 minutes, all types of esterified boards swelled less than
10%. AA
reduced the swelling to less than half of the control specimens while
MA and SA
modifications reduced the swelling further. The reduction in swelling
can be
attributed to the bulking of the cell wall and the reduction in
hygroscopicity
of wood especially with the larger MA and SA molecules.
Delignification
of Wood with Pernitric Acid
Organic
peroxides such as peracetic and performic acids will effectively
delignify
wood and other lignocellulosics. The use of performic acid as a pulping
agent
is under active investigation. We recently found that the inorganic
peroxide,
peroxymonosulfuric acid, will readily delignify these materials.
Hydrogen
perxide is quite ineffective, however. On the basis of these findings,
it
seemed highly likely that other inorganic peroxyacids might also be
effective
in delignification. Consequently, we initiated an exploratory study on
delignification of wood with other inorganic peroxyacids.
On
the basis of this consideration, we decided to study the possibility of
delighifying wood with pernitric acid. We chose finely divided aspen
wood as
the substrate for study because it delignifies easily. Success with
aspen
delignification would suggest further pulping studies.
Generation
of pernitric acid
There
are several methods for preparing pernitric acid. We initially chose
the method
described by Kenley, Trevor, and Lan that uses only concentrated
hydrogen
peroxide and concentrated nitric acid. Several attempts at generating
pernitric
acid by this method were unsuccessful. After adding a drop of peroxide
to the
chilled nitric acid in an ice bath, a puff of yellowish-brown vapor
would occur
arter a fraction of a second. Apparently, when the drop of peroxide
mixed with
the acid, some pernitric acid formed and then immediately decomposed to
nitric
acid and oxygen.
On
the basis of the large variability of the data and the ease of
obtaining
explosive mixtures, it appeared that pernitric acid was quite unstable
and
would readily decompose, especially at higher concentration. If
decomposition
of pernitric acid in dilute aqueous was too rapid, it might possess
little
ability to function as an oxidizing agent. It would therefore lose the
ability
to degrade the lignin in wood. Because of this consideration, we
performed some
experiments on the decomposition of pernitric acid in dilute aqueous
solutions.
Decomposition
of pernitric acid
We
generated pernitric acid by adding 1.35 g of chilled (from a
refrigerator) 70%
hydrogen peroxide to 4.0 g 90% nitric acid in a vial in an ice bath.
After 60
min, the mixture was diluted with 24.6 g of distilled to water. The
solution
was then analyzed and was 0.68 m (5.4%) in
pernitric acid. The total
acidity of the solution was 1.93 m based on the input of nitric acid.
Half the
solution remained at room temperature (22°C) and the remaining half at
refrigerator temperature (6°C) with periodic analysis of each of the
two
samples.
Delignification
of aspen wood
The
generation procedure used in the decomposition work produced a 5.7%
solution of
pernitric acid. Then 25 g of this solution was added to 1.00 g (o.d.
basis) of
finely divided aspen wood (passing a 40-mesh screen, 6.0% moisture).
The
mixture was held at 22°C for 305 min. The liquor was drained from the
solid
residue. The residue was washed thoroughly with RO water, and
air-dried. The
residue was dried in a vacuum
oven
overnight at 60% and its yield was determined. It was then analyzes for
lignin
content, and its 0.5% cupriethylenediamine (CED) viscosity was
determined.
Because the residue was not completely soluble in the CED solution, it
was
delignified using the chlorite procedure given in the Experimental
section.
Rosin
and Rosin Derivatives
Rosin
is a complex mixture of mainly resin acids and small amount of
non-acidic
components. Its colour, depending on the source and method of
processing, can
vary from a water white through dark red to almost black, with a tinge
of red.
It is generally translucent and brittle at room temperature. The other
properties which are of importance in judging the quality of rosin are
acid
value, saponification value, softening point, and unsaponification
value,.
However, entire rosin is sold and purchased on the colour basis, the
lightest
colour bring the highest price. Opacity in rosin some times degrades
its value,
Sometimes rosin is upgraded to get better market price.
Composition
Resin
acids are monocarboxylic acid having typical molecular formula C20H30O2.
The structural formulas of all the known resin acid are shown in Fig.1,
which
may be divided into abietic and primaric type. The pimaric type resin
acid
differs from those of abietic type, in that they have two alkyl groups
in the
C-7 position in the place of isopropyl group. When double bond occurs
in one of
these alkyl groups, it can not conjugate with a cyclic double bond.
Consequently the double bonds in pimaric and isopimaric acid are not
conjugated and so not very reactive.
Reaction
and derivatives
The
resin acid molecule possesses two chemically reactive centers, the
double bonds
and the carboxyl gorup. Through these, many modifications in structure
and
numerous derivatives are obtainable. Abietic acid is customatily and
conveniently used to exemplify the structural and chemical reaction
products c,
constitute the bulk of commercial products. Although the molecular
weight of
pure acid is 303, for rosin, the value is usually taken as 340 due to
the presence
of neutal for calculation in involving carboxyl group.
Isomerization
Pure
resin acids of the abietic type are isomerized thermally or
by treatment with dilute mineral acids. The equilibrium mixture from
the
mineral acid isomerization of levopimaric acid was 4% palustire, 93%
abietic
and 3% meoabietic. The equilibrium mixture of a thermally isomerized
levopimaric acid was 13% palustric acid, 80% abietic acid and 7%
meobietic
acid. Methyl abietate isomerised to give the same equilibrium
distribution as
abietic acid. A major difference between the two was an extensive
disproportionation reaction that occurred with the ester.
Photosensitized
oxidation
An
alcoholic solution of levopimaric acid containing a sensitizing dye,
e.g.,
methylens blue, readily absorbs one mole of oxygen, when contacted with
air, to
produce quantitatively a peroxide, 8,12-peroxide-13,14 dihydroabietic
acid, by
1-4 addition.
Hydrogenation
Hydrogenation
is one of the more satisfactory methods for decreasing the
susceptibility of
rosin to air oxidation. Because of the structural features of the resin
acids,
however more vigorous conditions are necessary than with ordinary
olefins.
Reduction with heavy metals and mineral acids or which sodium amalgam and water,
accomplishes only
incomplete hydrogenation, even for one double bond. On the other hand
molecular
hydrogen, in the presence of noble metal catalysts, such as palladium
or
platinum, will saturate one or both double bonds, depending on the
polarity of
the solvent employes. By taking advantage of solvent effect, the
desired
selectivity can be obtained with palladium carbon catalyst.
Hydrogenless
Hydrogenation
Resin
acids can be hydrogenated by the transfer of hydrogen from sodium
formate in
the presence of water and a palladium carbon catalyst. The primary
product are
the dihydro derivatives Incorporation of a polar solvent (Methyl
tertiary butyl
in the solvent increases the reaction rate. Although methyl neoabierate
hydrogenates faster than abictatc, the initial rate of formation of the
dihydro
compounds is practically the same, same isomerization to methyl
abietate
(maximumabout 25%) and slight dehydrogenation to methyl dehydroabietate
occur.
The reaction product distribution is essentially the same as for
abietate: only
trace amounts of 13(15)-abietenate are produced.
Polymers
of vinyl esters of hydrogenated rosin
Vinyl
esters were prepared and homopolymerized in emulsion and copolymerized
with
vinyl acetate, vinyl chloride, and butadiene and thus compared to vinyl
tetrahydroabietatc (VTA) and vinyl dehydroabietate (VDA).
Prehydrogenation
One
of the double bonds of rosin acid containing conjugated unsaturation,
such as
abietic acid, and the exocyclic double bonds of dextropimaric and
isodextropimaric acids are easily reduced. The endocyclic double bonds
of
dihydroabietic acid and pimaric acids are more difficult to
hydrogenate, but
they can be reduced over a nickel catalyst, under severe conditions.
Dehydro abietic
acid, which contains a
highly hindered aromatic ring, cannot be reduced at a practical rate
over
conventional catalyst.
Dehydrogenation
The
dehydrogenation reaction is the second means of effectively modifying
rosins to
render them less susceptible to oxidation by atmosphcric oxygen. Part
of the
hydrogen that is removed is readily absorbed by the pimaric type acids,
present
in rosin, to produce the stable dihydropimaric acid. Because of the
absorption
of a part of the hydrogen, this reaction frequently has been described
as a
disproportionation. However, the appearance of only 2-3% of
dihydroabietic type
acids and the absence of tetrahydro abietic acid, show that the
disproportionation of the abietic type acid is not involved.
Thus
disproportionated rosin is chemically stable, thermoplastic resin, by
the loss
of conjugation or aromatization of the molecule. This treatment
converts its
total oxidation prone AA content to highly stabilised forms of resin
acids,
predominantly dehydroabietic acid. As a result this DPR has a much
greater
resistance to discoloration, embrittlement, and other effects of
oxidation then
do pale grades of ordinary rosin. This improved oxidation resistance,
together
with a wide compatibility with polymeric thermoplastic and elastomeric
materials, makes DPR especially suitable as an ingredient of hot melt
applied
adhesives and coatings for paper and paper boards substrates, and as a
tackifier and processing aid for rubber based adhesives and molding
compounds.
Polymerization
In
any application only reduction in acid value of rosin does not satisfy
the
performance requirements of coatings. Rosin (abietic acid) possesses
active
conjugated dienoic unsaturation, which is very much susceptible to
oxidation by
atmospheric oxygen. Thus, when exposed to air, rosin and its
derivatives
undergo discolouration to develop ultimately dark brown colour, On
oxidation
rosin becomes more brittle and friable and changes its solubility
characteristics. Enhanced performance of rosin derivatives with their
pale
colour and good colour retention properties is obtained by proper
nuclear
modifications of rosin, such that the oxidation susceptible
unsaturation in
rosin is reduced to a greater extent or completely eliminated by
definite
chemical reactions such as polymerization.
Phenolic
modification
The
phenolic modified rosin is made by adding to the hot rosin either the
phenol
formaldehyde condensate or the phenol and the formaldehyde separately.
The
phenols commonly used are bis-phenol, phenol and creasols. The phenolic
modified rosins are usually further modified by esterification with
glycerol or
pentaerythritol.
The condensation reaction is an addition reaction that takes place
between the
methylol phenol and rosin or rosin esters. These esters are used in oil
soluble
varnishes.
Salt
formation
With
unsaturated cyclic and acyclic hydrocarbons
Metal
salts of the condensates of resin with unsaturated
hydrocarbons, which salts are particularly valuable for use in
protective
coatings such as varnishes, having excellent bodying and drying
characteristics
and forming harder films of improved water resistance.
Drier
metal salts of this rosin-butadiene condensates were pepared
by heating the condensate to 235°C and then gradually adding, with
intermittent
stirring, during a period of about 40 minutes, the metal salt, while
simultaneously increasing the temperature to about 320 °C. The hot,
highly
viscous resins was then cooled to room temperature and analysed.
With
pentaerythritol and other polyhydric alcohols
Glycols
and
diethylene glycols are also used to prepare esters by heating rosin at
20-260oC
in the presence of zinc, or boric acid
catalyst. The unreacted alcohol is removed from the ester at 300oC
under reduced pressure. One of the
glycol ester is prepared by the reaction with rosin, diglycol and
acetic acid.
The PVC sandals made from these esters can fully meet the international
specifications.
Hydrogenolysis
Rosin
has been
successfully hydrogenated to produce the hydroabietyl alcohol of
commerce. The
alcohol may be produced by the hydrogenation of the methyl esters of
rosin at
300oC
and 5000 psi,
in the presence of copper chromite catalyst. With soluble film formers,
such as
casein and zein, the alcohol is used to impart improved tack, colour
and
stability. Hydro abietyl alcohol produces esters with a wide range of
physical
properties, when reacted with organic and inorganic acids.
Polyesterification
Unsaturated
polyester modified rosin (UPER) are prepared by reacting first with
dibasic
acid glycol, maleic anhydride and then with rosin.
The
mechanical
and physical properties of the castings, prepared by copolymerization
with
styrene are good to excellent when compared with accepted values of
commercial
styrenated polyester cast resins. Foams prepared from these polyesters
shows
promising use in garments as a fabric-foam laminated. This has been due
to
their ability to be flame taminated directly to the fabric without the
use of
an adhesive.
Copolyesters
(Copolyesterification)
copolysters belongs
to broad class of organic high molecular weight compounds, which are
formed by
repeated esterification reaction. They constitute one of the most
important
class of plastic material. The great scope lies in the powder coatings,
solventless convertible coatings, aqueous coatings etc. The film
properties of
these polysters have high dimensional stability against change in
temperature.
Ammonolysis
The
action of ammonia on
dehydrogenated rosin at elevated temperatures, yields a nitrile. It is
a pale
yellow, waxy solid with softening points about 65o
C,
containing 85-95% of
nitrile from which the nitrile of dehydroabietic acid can be
crystallized with
mp 87-90oC.
It
is used in the
production of Amine D, by
catalytic hydrogenation at elevated temperature and pressure. The amine
is
relatively weak base that forms salts with both mineral and organic
acids, at
room temperature, in suitable solvents or at elevated temperature.
Preparations
Dehydroabietylamine
acetate
To
a solution of 2.85 kg. of Amine D dissolved in 4.71 lit. of
toluene was added a solution of 654 g. (10.8 mole) of glacial acetic
acid in
1.56 lit. of toluene. The solution was stored at 10oC
for 2 hour. The crystalline salt was collected, washed with cold
toluene and
recrystallized from 4.23 lit. of boiling toluene. The colourless
crystals were
collected, washed several times with n-pentene and air dried to obtain
1.365
kg. (78.5%).
Methods
of preparation
Sulfuric
acid catalyzed condensation of abietic acid with formaldehyde: Abietic
acid, 10
g. (0.033 mole) of formaldehyde (as paraformaldehyde) were suspended in
50 ml.
of purified dioxane. On addition of 3 g. of conc. Sulfuric acid, the
solution
became homogeneous, and the temperature rose spontaneously to 60oC.
The darkened solution was maintained
at 60oC
for 0.5 hr. On
addition of 300 ml. of water, a cream coloured precipitate was formed
which was
thoroughly washed with water and dried in vacuum at 50oC.
Experiments
similar to that above were run using 2,3 and 5 moles
of formaldehyde per mole of abietic acid. These products showed neutral
equivalent values of 357,394, and 419, respectively. The product from
condensation of abietic acid with 2 moles of formaldehyde showed no
maximum in
the region 220-285 mµ.
Nitrogenous
intermediates
The
chlorosulphonyl isocyanate, acrylonitrile, fumaronitrile,
benzoquinone, tetracyano ethylene, and many dinitrile and diamine
adducts of
levopimaric acid are reported here. Diamines and isocynates from rosin
have
found many industrial applications, such as polyurethane, ingredients
for
adhesives and corrosion inhibitors.
The
Polymerizable Half Esters of Rosin
This
commucation is concerned
with the partial esters of the polymerizable rosin adducts which have
at least
two carboxyl group.They are some times referred to as half esters,but
in at
least one case a trivalent acid is involved .Although they are quite
well
known, but there is little information published in the literature to
coordinate their behaviour and properties.
The
specific interest of the
partial is that while maintaining acid functionality a wide gradition
of
properties from hydrophilic to hydrophobic can be obtained and also
differential solubilities in various solvents. These properties are
transmitted
to various copolymers which may be formed from them, dependent on the
amount of
partial ester included and the nature of the alcohol radical.
Experimental
All
the chemical used are of comical type.
Preparation
and properties of monomers
The
simplest
method of preparing the rosin ester is by reacting a stoichiometric
quantity of
the requisite alcohol with an adduct being by far the most utilized in
this
respect. The general object is to avoid simultaneous formation of the
diester
and free acid.
Maleic
rosin esters:
Maleic rosin is refluxed with absolute alcohols until the conversion to
monoester is complete. Rp increases rapidly with
temperature till 80OC
and is constant to 120oC.
Heating above 80oC
tends to cause diester formation. Phenol methyl isohexyl alcohol and
tert-butanol react slowly. The above experiment was carried out under
pressure.
Purification of the H rosin esters may be effected by first
neutralizing the
ester with sodium carbonate solution. In order to remove diesters, the
sodium
salt is extracted with the solvent. At 25oC,
esterificition to
the half ester in acetone is extremely rapid. Secondary alcohols fail
to
esterify. Preparations are as flows.
Polymerization
&Copolymerization
General
and
non-aqueous:
The dibasic do not, except under special and rather drastic conditions,
homopolymer, but they are normally copolymerizable although with rather
low
reactivities. The anhydride salt, especially maleic rosin,can be made
to
homopolymerize under drastic condition, but they copolymerize very
readily with
a strong tendency to alternate, e.g., with vinyl acetate or styrene.The
diesters of the dibasic adducts copolymerize readily with vinly
esters,e.g.,
vinly acetate, but the reactivity ratios are somewhat
unfavourable for monomers such as styrene.
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