Rubber Chemicals are essential additives for the manufacture and quality improvement of rubber products such as automobile tires, rubber hoses, and quake absorbing rubbers. For rubber processing and compounding certain chemicals are required which are known as rubber chemicals. The primary requirement of adding different compounding ingredients to develop the different grades of rubber compounds to meet various service needs at an economic price and to provide certain desired physical properties to a considerable extent. Some of the examples of rubber chemicals are waxes, amines, thiazoles, silicone resins, alcohol, sulphuric acids, dithiocarbamates, phosphoric acid etc. They are mostly applicable for white and coloured rubber. They are generally used in rubber tubing, conveyor belt cover balloons, hot water bottles injection bottle caps, footwear related items etc. Indian rubber chemical industry has high growth potential triggered by increased consumption and steady growth in tyre and rubber industries. The speciality chemicals industry in India is projected to grow at 15-17 % per year to reach $ 80-100 billion by 2020. The demand for rubber chemicals is on the rise. All major manufacturers have raised the prices of their products substantially. Massive investment is expected to flow into the rubber chemicals manufacturing sector in India in the coming years from both domestic and global players.
The book covers different types, physical and chemical properties, applications of different rubber chemicals like waxes, synthetic organic chemicals, amines, silicones resins, releasing agents, stabilizers, solvents and many more. Some of the fundamentals of the book are synthetic hydrocarbon waxes, uses of amines in polymers, synthetic organic chemicals, analysis of specific anti-degradants, stabilization of halogenated polymers, anaerobic fermentations, the manufacture of sulfuric acid, analysis of dithiocarbamate esters, sodium hyposulfite (hydrosulfite), citric acid, gluconic acid, acetic acid, itaconic acid, kojic acid etc.
Rubber chemicals have a huge potential growth in future and considering the importance of the chemical we have brought out this book which will be an invaluable resource to rubber chemical manufacturers, technocrats, researchers, consultants and new entrepreneurs.
WAXES
Waxes
may be either natural or synthetic and of petroleum mineral vegetable
or animal
origin. They are generally smooth glossy lustrous and relatively firm
solids at
room temperature and are fusible when warmed. Originally the term wax
referred
to beeswax but now has the broader meaning of all materials that have
wax like
properties. Waxes include various types of chemical composition such as
paraffin hydrocarbons fatty esters acids alcohols and ketones. The
utilization
of waxes in polymers and of polymer additives in wax is based on the
improvement in performance or properties conferred by the components in
the
blend.
Waxes are discussed in this article
in the following
four classes Petroleum waxes covering paraffin and microcrystalline wax
neutral
waxes covering paraffin and microcrystalline wax neutral waxes
including plant
insect and animal wax mineral waxes such as montan and ozokerite and
synthetic
waxes including polyethylene and Fischer Tropsch wax made from nonwax
raw
materials. The subject of waxes is covered in texts by Warth (1) by
Bennett (2)
and by Guthrie (3) which are quite thorough although not recent. See
also
HYDROCARBON RESINS.
Petroleum
Waxes
Petroleum waxes
paraffin and microcrystalline waxes
essentially are saturated hydrocarbon mixtures obtained by the refining
of
crude waxes from petroleum. These petroleum waxes comprise by far the
largest
amount of all the different kinds of wax used in the United States and
in the
world (4). Paraffin waxes and microcrystalline waxes differ
sufficiently from
each other in hydrocarbon compositions physical properties and
the crystal form of
the solid so that there are marked differences in their functional
properties
and uses in polymers and other industrial formulations.
Paraffin
Waxes
Paraffin
waxes are solid firm
materials that are basically mixtures of saturated straight chain
hydrocarbons
obtained from refining waxy distillates derived from paraffinic crude
oils.
Fully refined paraffin wax is usually obtained by deoiling crude scale
wax
which is a soft paraffin wax intermediate containing up to 50% oil.
Scale wax
is principally made from slack wax obtained from wax bearing crude (5).
The old
pressing and sweating processes and the newer dew axing and deoiling
technology
have been described by Nelson (6) and by Tuttle (7). Typical petroleum
test
properties of most commercial paraffin wax scale wax and slack wax fall
in the
range given in Table 1.
Paraffin waxes
show the same general lack of chemical
reactivity as the nalkanes that are their principal
components. However
paraffin waxes do undergo a number of chemical reactions including
formation of
adducts cracking reactions and freeradical substitution reactions. The nalkane
components in paraffin wax can be reacted with urea to give crystalline
clathrated
adducts. Also normal paraffins can be separated from hydrocarbon by use
of
molecular sieves such as Lindes type 5A Linde Molecular Sieves Union
Carbide
Corp. The thermal cracking of paraffin wax is a commercial process for
making olefins.
Paraffin wax can
be chlorinated to introduce various
percentages of chlorine up to about 70% which corresponds to an average
of twenty
two chlorine atoms per molecule of hydrogen chloride at 150°C.
Fluorinated paraffin
wax has been made by fluorination of paraffin wax particles coated with
sodium
fluoride.
The
liquid phase oxidation of
paraffin wax as well as the chlorination reaction has been known and
investigated for more than a century. It has been used in Germany as a
commercial source of fatty acids. The crude oxidized product also
contains
esters alcohols aldehydes ketones lactones hydroxy acids and
unioxidized
alkanes.
Natural
Waxes
Vegetables Waxes
Vegetable waxes
are obtained from
the coating on leaves stems grasses fruits and barks of various plants
and
trees. These waxes are mixtures of esters of fatty acids and high
molecular weight
alcohols and unsaponifiable materials. Properties of a number of the
number of
the more commercial vegetable waxes are listed in Table 6.
The most
important commercial
vegetable waxes are carnauba candelilla ouricuri and Japan. These waxes
are
used to a great extent either by themselves or in conjunction with the
others
in the formulation of resin wax polishes (eg liquid polishes for wood
floor
waxes shoe pastes). The value of these waxes in polishes lies in the
fact that
they produce polishes with very durable luster and hardness. Carnauba
is most
preferred but candelilla ouricuri and Japan wax have been used as
substitutes. These
waxes are generally used in conjunction with resins such as acrylics
polystyrene and poly (vinyl chloride).
AMINES
Amines are named in a number of
different ways. The
different nomenclature systems in use are outlined below.
Trivial names such as aniline
mtoluidine can be
used.
Radical or
common names can also be used in this system the suffix amine is used
together
with the appropriate hydrocabon radical or radicals. Aliphatic amines
possessing only one amino group are usually named in this manner for
example
butylamine diethylamine. Mixed amines are named by choosing the larges
radical
present for the parent compound. Thus CH3CH2CH2N(CH3)C2H5 is
named as ethylmethylpropylamine or more
accurately NethylNmethylpropylamine.
The
IUPAC system which follows the rules laid down by
the International
Union of Pure and Applied Chemistry (formerly the International Union
of
Chemistry) is particularly used for polyamines. The name of the diamine
triamine etc is derived from the name of the hydrocarbon having the
longest
straight (unbranched) chain attached to the amino groups by adding the
suffix
diamine triamine etc. The positions of the NH2 groups
are indicated by the lowest possible
numbers which are normally placed before the name as in 1
2butanediamine CH3CH2CH(NH2) CH2NH2. Although the practice is
generally discouraged diamines are
frequently named as for example propylenediamine. Linear polyamines
containing
recurring amino groups in the chain may be named like
triethylenetetramine H2N(CH2)2NH(CH2)2NH(CH2)2NH2.
Physical
Properties
The simple
amines are derivatives of ammonia in which one or more of the hydrogen
is
replaced by an alkyl or an aryl group. The trivalent nitrogen might
also be
part of a saturated or aromatic heterocyclic system. Since the carbon
containing
portion generally constitutes the largest segment of the molecule in
most of
the amines the physical properties of these materials generally vary as
they do
within any homologous series of compounds that is the melting points
and
boiling points show a generally increasing progession with higher
moleculare
weights. For equivalent molecular weight compounds however other
structural
features such as whether the compounds are primary secondary or
tertiary amines
or whether other functional groups are also present have a decided
influence on
the physical properties. Primary amines generally have higher boiling
points
than secondary and secondary higher than tertiary in a series of
equivalent molecular
weights aliphatic amines. The relationship
between melting points is not quite. As clear cut the
branched chain
amines have greater volatility than the corresponding straight chain
compounds.
The densities of the aliphatic unsubstituted amines are less than one.
The
odors of the lower primary amines (C1C3)
resemble that of ammonia they become fishy in the C4C7 range and then decrease
intensity with
increasing molecular weight. The water miscibility of the aliphatic
amines is
related to their hydrocarbon composition. The lower primary amines (C1C5) are
miscible with water as well as
with alcohol and ether but as the formula weight increases (above C6) the
solubility in water decreases. The secondary and tertiary
aliphatic amines are soluble in alcohol and ether but have limited
water
solubility (only R2NH
in which R is C1C4 and (CH3)3N are
miscible with water). The lower amines form stable
hydrates making purification difficult whereas the hydrates of the
higher
amines (C10 and
higher) are unstable. The introduction of additional amino groups
raises the
boiling points of the aliphatic amines the corresponding alkanolamines
have
still higher boiling points. The alicyclic amines have physical
properties that
are not too different from the corresponding secondary or tertiary
aliphatic
amines but with slightly higher boiling points compared to the
equivalent molecular
weight compounds. The aromatic amines are high boiling oily liquids or
crystalline solids the liquids are unstable on exposure to light and
air and
become dark colored. The unsubstituted monoamines (except aniline) are
only
slightly soluble in water but the diamines are more soluble. All the
aromatic
amines are soluble in organic solvents. The simple aromatic diamines
such as
the phenylene diamines are colorless crystalline solids that turn brown
in air.
Some of the physical properties of representative amines and amine
derivatives
of various classes are shown in Table 14.
Chemical
Properties
The amines
and
amine derivatives under consideration in this article are alkyl aryl
acyl and
sulfonyl derivatives of ammonia as well as heterocyclic compounds such
as
piperazines pyridines and pyrroles. These compounds all contain the
trivalent
nitrogen atom and their chemical reactions generally involve their
behavior as
nucleophilic reagents. The degree of reactivity they possess is
dependent upon
how the various substituents attached to the nitrogen influence the
electron
density associated with the nonbonded pair of electrons at this site .
This
electron density which may be referred to as basicity is related to the
electron donating or withdrawing influence of the attached groups. As
might be
expected alkylamines are stronger bases than ammonia and secondary
amines more
basic than primary the aryl acyl and sulfonyl compounds are more weakly
basic
than primary the aryl acyl and sulfonyls compounds are more weakly
basic in
that general order. A qualitative order of basicity dependent on the
groups
attached to the nitrogen is indicated by the following sequence
In the case of arylamines the
socalled metadirecting
substituents on the aryl group decrease the basicity whereas
orthoparadirecting
substituents increase the base strength. These generalities are
actually an
over simplification of the facts since steric factors as well as the
type of
group have a decided effect on the basicity or reactivity of the amines
as
nucleophilic reagents.
The
basic character of the amines
is responsible for their ability to react with acids to form salts.
Salt
formation is a highly distinctive property of the amines. Amine salts
derived from
mineral acids are analogous to ammonium salts and can be formed both in
aqueous
solution and under anhydrous conditions. For example passing anhydrous
hydrochloric acid gas into an ether solution of the amine results in a
white
precipitate of the insoluble amine salt.
Amine
salts with halogen acids often possess organic acids form salts of low
stability. Stronger carboxylic acids such as oxalic acid and other
strongly
acidic organic structures such as picric acid give stable salts that
are less
water soluble than the mineral acid salts and have characteristic
melting
points. The stable organic acid salts are useful for identification
purposes.
The complexes are generally less stable than the salts of protonic
acids and
they can usually be decomposed into their respective components with
heat. The
stability is dependent upon the Lewis acid and base strengths of the
materials
and is also highly dependent upon steric factors.
The substituents that affect the
basicity of the
nitrogen atoms of the amines also influence the acidity of the hydrogen
atoms
attached directly to the nitrogen the groups that decrease basicity
also
increase acidity as shown by the higher reactivity of the hydrogen
atoms with
metals. All NH compounds react with Grignard reagents. Active metals
such as
sodium also displace the hydrogen atoms of amines. For example aniline
forms
sodium anilide although the reaction must be catalzed copper being most
effective (eq. 1). Phthalimide reacts with potassium hydroxide in
ethanolic
solution to give potassium phthalimide (eq.2).
As
active hydrogen compounds amines (primary and secondary) react with
activated
double bonds in the Michael addition reaction. The cyanoethylation
reaction
with acrylonitrile represents a special case of this. Both hydrogens of
a
primary amine may react (eq.3).
Amines
generally react as active hydrogen compounds with carbonyl compounds
adding to
the carbonyl group (eq.4).
This
reaction between a secondary amine and formaldehyde in the presence of
another
active hydrogen compound constitutes the Mannich reaction (eq.5).
Primary and secondary amines as
well as ammonia
react with alkylene oxides to provide hydroxyalkylated derivatives.
Ethylene
oxide reacts with ammonia to give mono di or triethanolamine depending
on the
ratio of reactants (eq.8). The reaction between epoxides and amines
forms the
basis for the hardening of epoxy resins.
Acylation
of
primary and secondary amines to form amides is commonly accomplished by
the
reactions of the amine with an acid halide or with a carboxylic acid
anhydride.
Ammonia
reacts readily with esters to provide the corresponding amides. A
method for
preparing amides that is significant in polymer chemistry particularly
in
preparing polyamides is heating the ammonium salts of the carboxylic
acid
(eq.9).
Aqueous ammonia is corrosive to
copper alloys and
galvanized surfaces. Aluminium alloys can be used for strong ammonia
liquor
solutions but their use for weak solutions is not advisable. Mercury
should
never be used in contact with ammonia as explosive chemical compounds
can
result.
Ethanolamines. The ethanolamines due to their
bifunctional
nature are highly reactive compounds. Most reagents attack the amine
group
preferentially but reaction with the hydroxyl group can be accomplished
in many
instances. Some of the characteristic reactions of the ethanolamines
are
illustrated by the following examples.
SYNTHETIC ORGANIC CHEMICALS
Synthetic
organic chemicals can be defined as derivative products of naturally
occurring
materials (petroleum natural gas and coal) which have undergone at
least one
chemical reaction such as oxidation hydrogenation halogenation
sulfonation and
alkylation.
The volume
of
synthetic organic chemicals increased form 17 billion pounds in 1949 to
more
than 130 billion pounds in 1969. The production for the past two
decades is
shown in Fig. 1. Much of this phenomenal growth has been due to the
replacement
of natural organic chemicals. Since this replacement is now essentially
complete future growth for synthetic materials will be dictated by the
expansion of present markets and development of new organic chemical
end uses.
More than
2500
organic chemical products are derived principally from petrochemical
sources.
These are commercially produced form five logical starting points.
Consequently
this chapter has been subdivided into five major raw material
classifications
methane ethylene propylene C4 and higher aliphatics and aromatics.
CHEMICALS DERIVED FROM METHANE
It has been
stated that every synthetic organic chemical listed in Beilstein can be
made in
some way or other starting with methane. This section however deals
only with
the relatively small number which can be made economically and which
are useful
enough to warrant large volume production. A diagram of the principal
materials
covered is shown in Fig. 1.
Synthesis
Gas
The most important route for the
conversion of
methane to petrochemicals is via either hydrogen or a mixture of
hydrogen and
carbon monoxide. This latter material is known as synthetic gas.
Two
important methods are presently used to produce the gas mixture from
methane.
The first is the methane steam reaction where methane and steam at
about 900°C
are passed through a tubular reactor packed with a promoted iron oxide
catalyst.
The
second commercial method involves the partial combustion of methane to
provide
the heat and steam needed for the conversion. Thus the reaction can be
considered to take place in at least two steps.
The
process is usually run with nickel catalysts in the temperature range
8001000°C.
The
main outlets in the chemical industry for the gas mixtures obtained by
the
reforming of methane are in the manufacture of ammonia the methyl
alcohols
synthesis and in the Fischer Tropsch and Oxo reactions.
Ammonia. Ammonia derived from
petroleum and natural gas sources accounts for more than 97 percent of
the
almost 26 billion pounds produced annually. Consequently if can be
termed the
number one petrochemical in volume. About three fourths of the ammonia
produced
goes directly into fertilizer uses and the rest is used to produce such
chemicals as ammonium nitrate ammonium sulfate caprolactam nitric acid
urea
acrylonitrile and organic amines.
Methanol. Before 1926 all American
methanol was obtained commercially as a byproduct of the wood
distillation
process (wood alcohol). That year however marked the first appearance
of German
synthetic methanol. Presently only a negligible amount of the four
billion
pounds produced comes from wood.
The
methyl alcohol synthesis is well known. It resembles the synthesis of
ammonia
in that the catalysts operate only at high temperature levels and
conversion
and equilibrium are greatly assisted by high-pressure operation. The
industrial
reaction conditions are pressures of 250350 atmospheres and
temperatures in the
range of 300400°C. The catalysts employed are based on zinc oxide which
is
mixed with other oxides to provide temperature resistance. Variations
between
synthetic methanol plants are quite similar to those between synthetic
ammonia
plants. In fact many ammonia operations are designed so that methanol
could
also be produced in them.
Methanol production has generally
paralleled that of
its largest end-use formaldehyde. However this is no longer completely
true
because fairly large quantities of formaldehyde now come from the
hydrocarbon
oxidation process. Other major uses of methanol are as solvents
inhibitors and
in the synthesis of methyl amines methyl chloride and methyl
methacrylate.
Formaldehyde. Formaldehyde may be made
from methanol either by catalytic vapor phase oxidation
It can also be produced directly
from natural gas
methane and other aliphatic hydrocarbons but this process yields
mixture of
various oxygenated materials.
Since both gaseous and liquid
formaldehyde readily
polymerize at room temperature it is not available in the pure form. It
is sold
instead as a 37 percent solution in water or in the polymeric form as
paraformaldehyde (HO(CH2O)nH where n
is between 8 and 50] or as trioxane [CH2O3]. The largest end use of
formaldehyde is in the field of synthetic
resins either as a homopolymer or as a copolymer with phenol urea or
melamine.
It is also reacted with acetaldehyde to produce pentaerythritol [C(CH2OH)4] which finds use in
polyester resins. Two smaller volume uses are in urea formaldehyde
fertilizers
and hexamethylenetetramine the latter being formed by condensation with
ammonia.
Oxo Chemicals. The oxo chemicals are
compoundsprimarily C4 and
higher alcohols made by the
so called ox process. This process is a method of reacting olefins with
carbon
monoxide and hydrogen to produce aldehydes containing one more carbon
atom than
the olefin these in turn are converted into alcohols. The earliest
reaction
studied used ethylene to produce both an aldehyde and a ketone. Thus
the name
oxo which was adapted from the German oxierung meaning ketonization.
However
even though other names such as hydroformylation would much more
accurately
describe the process the term oxo appears too deeply entrenched to be
replaced.
A flow sheet of a typical
process is shown in Figure 3. The steps involved in the reaction are
The cobalt catalyst used under
these conditions is
in the form of dicobalt octacarbonyl and cobalthydrocarbonyl.
At the present time the plant
capacity in the United
States amounts to more than 700 million pounds per year of oxo
chemicals. These
include such products as nbutanol isobutanol
propionaldehyde
butyraldehydes butyronitriles isooctyl alcohol decyl alcohol and
tridecyl
alcohol.
Chlorinated
Methanes
The
chlorination of methane can be carried out either thermally or
photochemically
to produce methyl chloride (CH3Cl)
methylene chloride (CH2Cl2)
chloroform (CHCl3)
and carbon tetrachloride (CCl4). If
only a
particular chlorinated material is desired other methods such as the
chlorination
of
CS2 or
the
reaction of methanol with HCl are generally used. These are fairly
large volume
chemicals with the production of carbon tetrachloride alone estimated
at almost
a billion pounds.
Although
largely replaced by other solvents in the drycleaning field carbon
tetrachloride has shown considerable growth as a raw material in the
manufacture of chlorofluorohydrobons. Next in volume is methyl chloride
which
is used to make silicones and tetramethyl lead. Methylene chloride
finds use in
paint removes solvents and aerosols. Chloroform is a
raw material for fluorohydrocarbons.
Acetylene
Acetylene is made commercially in
two ways from
calcium carbide or from hydrocarbons. The choice of method is
determined mainly
by the fact that acetylene cannot be shipped easily so large users must
be at
or near the point of origin. The carbide plant in turn must be near a
cheap
source of electric power since each pound of carbide requires about 1.5
kwh of
electricity.
Acetylene has long been a valuable
building block in
the chemical industry. Major consumers of the more than a billion
pounds
produced are the manufacture of vinyl chloride neoprene vinyl acetate
acrylic
acid and esters and chlorinated ethylene. Growth in the demand for
acetylene
has been small in recent years however due to competition from cheaper
raw
materials. Ethylene is now preferred over acetylene as the starting
material
for vinyl chloride and vinyl acetate propylene has completely
supplanted it for
acrylonitrile and is making inroads into the acrylates and neoprene can
now be
made from butadiene.
Vinyl
Chloride.
Less than 20 percent of the vinyl chloride (or about 700 million
pounds) now
comes from the addition of hydrogen chloride to acetylene. The process
involves
a mercuric chloride catalysts and temperatures around 200°C. All vinyl
chloride
is used to make plastics the most important of which are the
homopolymer (PVC)
and copolymers with vinylidene chloride or vinyl acetate.
Vinyl
Acetate.
Vinyl acetate can be produced by combining acetylene and glacial acetic
acid.
This is a catalytic reaction (zinc or mercury compounds) and it may be
carried
our either in the liquid or vapor phase.
Approximately 800 million pounds
are produced
annually all of which is utilized in the polymeric form. Polyvinyl
acetate
(PVA) can be found in films and latex paints. It also can be used to
produce
polyvinyl alcohol (a water soluble polymer) polyvinyl butyral (for
safety glass)
polyvinyl formal and various copolymers.
Acrylates and Methacrylates. The acrylates are esters
of acrylic acid (CH2 =
CHCOOR) with the R generally ranging from
methyl to ethylhexyl. The main method of preparation involves reacting
a
mixture of acetylene hydrogen chloride nickel carbonyl carbon monoxide
and the
appropriate alcohol. About 80 percent of the carbonyl group in the
product
ester is derived from the carbon monoxide and the remainder from the
nickel
compound. Other methods involve ethylene cyanohydrin ketene the
esterification
of acrylic acid or the oxidation of propylene to acrolein.
The most important products are
ethyl acrylate and
butyl acrylate. They are used in making emulsion polymers for latex
paints and
textiles.
Methyl
Methacrylate.
Methyl methacrylate is formed from acetone cyanohydrin in a two step
process.
While
this is the major process in operation there have been reports of a
route
involving the oxidation of isobutylene to methacrylic acid.
Production
of methyl methacrylate now totals more than 450 million pounds. The
largest
part of this goes into cast sheet where the clarity and resistance of
poly (methyl
methacrylate) are desirable. Other uses are in surface coating resins
and
molding powders.
Hydrogen
Cyamide.
Hydrogen cyanide is prepared as shown in Fig. 3 by passing a mixture of
air
ammonia and natural gas over a platinum catalyst. The converter is
operated at
a temperature of about 1800°F and care must be taken to minimize the
decomposition of the ammonia and methane as well as the oxidation of
methane to
carbon monoxide and hydrogen. The effluent gases are called washed with
dilute
sulfuric acid and then passed through a column where the hydrogen
cyanide is
absorbed in water. This is concentrated by distillation and an
inhibitor is
added to prevent polymerization. Although all new plants follow the
methane
ammonia route HCN can also be produced from cokeoven gas from sodium
and
calcium cyanides and by the decomposition of form amide.
Because
of the safety problems most production is captive to avoid the need for
shipment. About one third of the HCN goes into the production of
acetone
cyanohydrin while almost another third is used to make adiponitrile.
Other uses
are for chelating agents and sodium cyanide.
The
advent of the propylene ammonia process for acrylonitrile has had an
interesting effect on this material. Ten years ago acrylonitrile
manufacture
was a major consumer of HCN. Now almost 28 percent of our HCN is
produced as a
byproduct in acrylonitrile manufacture.
Carbon
Disulfide
Carbon
disulfide is made by the catalytic reaction of methane and sulfur
vapor.
Production is about 800 million pounds with the largest portion going
to the
manufacture of rayon and cellophane. The other major use is production
of
carbon tetrachloride.
CHEMICALS
DERIVED FROM ETHYLENE
Ethylene
far surpasses all other hydrocarbons both in volume and in diversity of
commercial use. In the whole field of petrochemicals it is exceeded in
tonnage
only by synthetic
ammonia. Consumption
of ethylene has grown remarkably in just the last 30 years. In 1940 300
million
pounds were produced mostly for ethanol and ethylene oxide. The wartime
demand
for styrene and the postwar impact of polyethylene aided in causing
this figure
to swell to almost 5 billion pounds in 1960. a boom in polyethylene use
and
strong growth in ethylene dichloride and ethylene oxide expanded
ethylene
production to over 16 billion pounds in 1968 and over 18 billion pounds
in
1970. The major consumers of ethylene in 1969 are shown in Table 1.
Polyethylene
Polyethylene
has shown a spectacular growth accounting for only 4 percent of total
ethylene
consumption in 1950 and almost 25 percent ten years later. In 1961
polyethylene
surpassed ethylene oxide as the principle ethylene consumer. In 1969
polyethylene and ethylene copolymer manufacture consumed nearly 40 per
cent of
all ethylene produced that year.
Three types of processes are used
to produce
polyethylene. The high pressure process yields a product of low
pressure
process yields either high density polymer via the Ziegler process or
low or
medium density polymers if the recent Phillips low pressure process is
used. Low
density polyethylene and ethylene copolymers accounted for about 70
percent of
polyethylene produced in 1969.
Ethylene
Oxide
Ethylene
oxide was discovered in 1859 by Wurtz who named it because of certain
analogies
with inorganic oxides. The method which he used was what is today known
as the
chlorohydrin process. He considered that direct oxidation was an
impossibility
and stated flatly that ethylene oxide cannot be made by the direct
combination
of ethylene and oxygen. It took almost eighty years to disprove this
statement.
A
flow sheet of this process is
shown in the first part of Fig. 4. Ethylene chlorine and water are fed
into the
bottom of a large acidproof brick lined tower at somewhere below 50°C.
The
water and chlorine form hypochlorous acid which then reacts rapidly
with
ethylene. The dilute solution emerging from the tower contains about 5
percent
chlorohydrin. The major side reaction is the formation of ethylene
dichloride.
The solution next passes to a hydrolyzer where the chlorohydrin is
treated with
either slaked lime or caustic soda to produce the oxide. The crude
ethylene
oxide contains about 10 percent ethylene dichloride which is removed by
distillation. This process accounted for about 10 percent of the
ethylene oxide
capacity in 1969 but many former ethylene oxide chlorohydrin plants are
now
used for the production of propylene oxide.
The
most important process involves the direct oxidation of ethylene with
air in
the presence of a silver catalyst.
A
number of processes are available and a typical one is shown in Figure
5.
Ethylene compressed air and recycle gases are fed to a tubular reactor
containing a silver catalyst. The oxygen and ethylene concentrations
are
maintained at a low level to avoid explosion hazards. The reaction
temperature
is 250 300°C with a pressure of 120300 psi. Two competing side
reactions which
must be minimized are the total combustion of ethylene to carbon
dioxide and
the isomerization of ethylene oxide to acetaldehyde. Some ethylene
oxide direct
oxidation plants use purified oxygen instead of air as the oxidizing
agent.
Ethylene
oxide is the most
important of the olefin oxides. While it can be used directly as a
fumigant for
foodstuffs (usually mixed with carbon dioxide) it finds its chief
outlet as a
chemical intermediate. It owes its value to a combination of two types
of
reactivity it can combine with chemicals containing replaceable
hydrogen and it
can polymerize to give a polyethenoxy chain. The first is typified in
the
formation of ethanol amines while the second occurs in the synthesis of
the
polyglycols and higher glycol ethers.
The
estimated production for 1969 of ethylene oxide derived materials and
the
amount of ethylene oxide they consume is shown in Table 2.
Ethylene
oxide is expected to maintain its present growth trend for at least the
next
few years. Ethylene glycol will remain the largest end use.
Ethylene
Glycols. Ethylene
glycol can be prepared directly by the hydrolysis of chlorohydrin but
the
indirect hydrolysis via ethylene oxide is the preferred method. This is
shown
in the second part of Figure 6. The feed stream consists of ethylene
oxide
(either from the chlorohydrin or direct oxidation process) and water.
This
mixture is fed under pressure into the reactor vessel at about 100°C.
Polyethylene glycols are produced
by passing
ethylene oxide into a small amount of a low molecular weight glycol
using a
sodium or caustic soda catalyst. The molecular weight of liquid
polyglycol
products ranges from 200 to 1000. They are used as plasticizers
dispersants
lubricants and humectants. Above a molecular weight of about 1000 the
polyglycols are waxy solids suitable for use as softening agents in
ointments
and cosmetics and as lubricants.
Surfactants. In 1969 nonionics made up
approximately 25 percent of all synthetic detergents produced in the
U.S. or
nearly one billion pounds. In the years between 1960 and 1969
production of
ethoxylated nonionics doubled. Behind this rise are several
characteristics
most nonionics are liquid they are low sudsing and they can built into
readily
biodegradable surfactants.
There are many nonionic surfactants
but four classes
account form more than 80 percent of the production. These are (1) the
alkylphenolethylene oxide derivatives (2) fatty acid alkanolamine
condensates
(3) tall oilethylene oxide adducts and (4) the fatty acid ethylene
oxide
adducts. The alkanolamine condensates are foam stabilizers in various
detergent
formulations. Tall oil adducts find use in household detergents chiefly
automatic washer products because of their lowsudsing properties. The
fatty
alcohol adducts are used mainly as light duty detergents though some
tridecyl
adducts are utilized as foam stabilizers. The trend has been away from
the
benzenoid ethers because of their biodegradability limitations.
Ethanolamines. Ethanolamines are manufactured by
reacting
ethylene oxide and ammonia. The relative amounts of the three amines
will
depend primarily on the ammonia to oxide feed ratio.
The products from the reaction are
separated by distillation.
During the last few years each of the amines has in turn been in the
greatest
demand so processing flexibility must be maintained.
Monoethanolamine is used primarily
in detergents and
as an absorbent for acidgas (H2S CO2) removal it is to a lesser extent
a chemical intermediate for
compounds such as ethylene imine. Diethanolamines major end use is in
detergents but it is also utilized in textiles and as a gaspurification
agent.
Most of the triethanolamine goes into the production of cosmetics and
textile
specialities.
Isopropanolamines derived from
propylene oxide and
ammonia are competitive with the ethanolamines and both are unique in
that they
are organic compounds and yet strongly alkaline.
Glycol
Ethers.
In
the same way that water reacts with one or more molecules of ethylene
oxide
alcohols react to give monoethers of ethylene glycol producing
monoethers of
diethylene glycol triethylene glycol etc as byproducts.
Since their commercial introduction
in 1926 glycol
ethers have become valuable as industrial solvents and chemical
intermediates.
Because glycol monoethers contain a OCH2CH2OH group they resemble a
combination of ether and ethyl alcohol in
solvent properties. The most common alcohols used are menthanol ethanol
and
butanol. Principal uses for the glycol ethers are as solvents for
paints and
lacquers as intermediates in the production of plasticizers and as
ingredients
in brake fluid formulations. The most common trade names are Dowanol
Cellosolve
and Polysolve Condensation of the monoethers produces glycol diethers
which are
also useful as solvents.
SILICONE RESINS
Silicone resin polymers differ from
fluids and gums
in containing a significant proportion of silicon atoms with only one
or with
no organic substituent groups. The high degree of latent cross linking
in these
polymers causes the formation of harder less elastic matrixes when the
resins
are completely cured and necessitates handling many resins in solution
to
permit easy application and to prevent premature cure. The glass
transition
temperature of cured commercial silicone resins range up to 200°C in
contrast
to typical silicone rubber glass transition temperatures near 60°C.
There are two broad groups of
silicone resins those
probably having a continuous cross linked network produced by
hydrolysis of
appropriated mixtures of primarily di and trichlorosilanes (DT resins)
and
those presumably having a knotandchain structure produced by combining
a
cohydrolyzate of mono and tetrachlorosilanes with a hydrolyzate of
dichlorosilanes (MQD resins). Since a great many specialized variations
of
composition processing technology application and curing techniques
exist only
general outline of the major methods and applications is given here.
Manufacture
The first step in preparing any
silicone resin
consists of formulating an appropriate blend of organochlorosilanes.
Monomethyl
dimethyl monophenyl diphenyl methylphenyl monovinyl and
methylvinylchlorosilanes together with silicon tetrachloride have been
most
widely used. Prediction of properties of the finished resin as a
function of
composition frequently fails since processing and cure have
considerable
influence on the final molecular configuration and related
characteristics.
However some generalizations can be made (1) Trifunctional siloxy units
generally produce harder less flexible resins which are frequently
immiscible
with organic polymers. (2) Difunctional siloxy units increase softness
and
flexibility. (3) Phenylsiloxanes are generally more miscible with
organic
polymers than are methylsiloxanes and produce resins which are less
brittle and
have superior thermal resistance. The chlorsilane blend may be mixed
with inert
solvents which serve both to modify the rate of hydrolysis and to
provide a
diluent for the hydrolyzed resin. The most commonly used solvents are
mineral
spirits esters such as butyl acetate chlorinated hydrocarbons toluene
and
xylene.
Resin hydrolysis is complicated by
a number of
factors not encountered in processing silicone fluid. The first is the
tendency
of the hydrolyzate to gel and become insoluble if the proportion of
trifunctional or tetrafunctional chlorosilanes is too high if the
solvent level
is too low or if conditions are not carefully controlled to prevent
excessive
silanol condensation. One means of minimizing gel formation is the
addition of
modifiers usually low molecular weight alcohols which can react with
chlorosilanes to yield less easily condensed alkoxysilanes rather than
silanols. The final conversion of these alkoxysilanes to siloxanes can
then be
accomplished at a regulated rate during subsequent processing.
A
second complicating factor is the difference in the hydrolysis rates of
various
chlorosilanes. In general the rate of hydrolysis under given conditions
increases with increasing functionality and decreases with increasing
molecular
weight of the organic substituent groups. Conditions must be balanced
to
promote the incorporation of all the hydrolyzate products in the
average resin
molecule if the desired final properties are to be obtained. This can
be
achieved through proper selection of solvent through intensive
agitation and in
some cases through sequential addition of the chlorosilanes to be
hydrolyzed.
The hydrolysis process for resins
is not understood
in detail and the products generally represent a statistical
distribution of
constituent groups and of molecular weights. It is probable that
tetrameric
ring structures as in the case of fluid hydrolyzates are the most
common
intermediate configurations. A number of these structures have been
characterized. In one well defined ease a resin product of known
structure is
obtained. When phenyltrichlorosilane is hydrolyzed and the hydrolyzate
is
condensed with base in bulk or in selected solvents a regularly cross
linked
bifilar molecule or ladder polymer results. These ladder polymers are
thermoplastics
of high intrinsic viscosity and of sufficiently high softening point
and
physical toughness to have possible use as self supporting films and as
high
temperature coatings. The chemistry of resin hydrolysis and
polymerization has
been discussed in an earlier section where typical intermediate and
final
hydrolyzate structures may be found.
In practice hydrolysis is commonly
accomplished by
one of two techniques. In the first a considerable excess of water is
used for
hydrolysis and the chlorosilane solvent mixture is fed in at a
controlled rate.
Evolved hydrogen chloride dissolves in the aqueous phase and is later
separated. In the second a near stoichiometric quantity of water is
used and
the order of addition may vary. The hydrogen chloride escapes as a gas
from the
hydrolysis system and may be recovered in an absorption tower. In both
cases
the resin hydrolyzate enters the solvent phase but its average
molecular weight
and structure and the proportion of silanol groups generated for
further
condensation differ considerably with the hydrolysis conditions used.
Water
input rate of feed hydrolysis temperature proportion and type of
solvent and
intensity of agitation can affect molecular structure of the
hydrolyzate and
properties of the finished resin. After hydrolysis the aqueous layer if
any is
withdrawn and the resin is washed. When chlorinated solvents are used
the resin
layer has a higher specific gravity than the aqueous layer and in batch
equipment
must be withdrawn and recycled to the hydrolysis vessel. When other
solvents
are used it is frequently necessary to use a salt solution for washing
to
permit ready separation of the phases.
Resin hydrolysis may be carried out
either batch
wise or continuously. Continuously processed resin have in theory at
least the
advantage of greater uniformity since each increment of hydrolyzate
experiences
a uniform processing history the molecular weight distribution should
therefore
be narrower for the continuous product. As in the case of batch
hydrolysis of
silicon fluids the standard equipment for batch hydrolysis of resins is
an
agitated jacketed kettle. Auxiliary equipment includes a feed tank and
metering
system. Continuous processing is generally done in an agitated
multistage
contactor with provision for escape of gaseous hydrogen chloride if
necessary.
For both types of hydrolyzers glass lined or other acid resistant
construction
is required.
Following
hydrolysis any residual acid may be removed by stripping out a small
amount of
solvent or the acid may be left in to serve as a catalyst for further
condensation of silanol groups if this step is required. The purpose of
the
partial condensation or bodying is to generate larger molecules with
lower
residual silanol concentration so that a rapid cure to a completely
cross
linked resin can be obtained after the solvent has been removed. If the
resin
hydrolyzate has been washed and neutralized special condensation
catalysts such
as metal soaps or acid treated clays may be added to promote bodying.
Some
solvent may be stripped off to permit attaining the higher temperature
necessary for rapid condensation or to promote intermolecular silanol
reactions. The end point is determined by viscosity measurement and is
influenced
by the concentration of resin solids in the solution. At the completion
of the
bodying step the temperature is reduced by quenching with additional
solvent.
After addition of curing catalyst filtration and blending to the
desired final
solvent content the resin solutions are drummed. For most bodying and
finishing
operations stainless steel or even carbon steel equipment may be used
provided
that the system in kept dry.
Processing of MQD resins is
generally similar to
that outlined above for DT resins except that the separate hydrolyzates
are
blended and equilibrated in a process similar to that described in
manufacture
of silicone fluids. As an alternative to the use of siliconterachloride
as a
starting tetrafunctional material processes have been designed based on
the
conversion of sodium silicate to a silicic acid solution which then
reacts with
chlorosilanes.
In addition to the usual types of
resins used in
solution solventless resins in solid form have been offered for sale.
These are
made by careful removal of solvent at relatively low temperatures using
equipment such as a vacuum drum dryer. Such resins can be compounded
with glass
flock or other filters to produce silicone molding compounds.
Cure
Two general types of cure are
applied to silicone
resins. The more common involves the removal of solvent from the resin
solution
with concurrent condensation of some silanols this is followed by
catalyzed
condensation of the relatively small proportion of silanol groups
remaining at
temperatures above 100°C. The catalyst may be a metal soap such as tin
octoate
either left from the bodying operation or added specifically as a cure
promotor
or it may be a member of one of several classes of amines which are
effective.
The second type of cross linking reaction is that used in heat curing
silicone
rubber involving a free radical reaction to form intermolecular
ethylene
bridges. This type of cure is generally more difficult to control.
Typically
application of a silicone resin involves dipping a part or a sheet like
material in the resin solution draining or scraping off excess solution
allowing the solvent to evaporate and then curing in batch or
continuous ovens
such as cloth coating towers. The finished coating is hard and
relatively
insoluble in the solvents used for the uncured polymer. Further cross
linking
and hardening of the resin may occur on continued exposure to elevated
operating temperature during normal use of the substrate or part.
Properties and Uses
The earliest uses of silicone
resins were in
electrical equipment operating at higher temperatures than were
previously
permissible. Such motors generators and transformers still constitute a
major
market for these coatings which have made possible equipment rated for
continuous service at 220°C. In hermetically sealed systems although
the high
thermal stability of silicones is particularly valuable gradual loss of
volatiles from the resins may lead to deposition of a nonconducting
silicone
layer on contacts and on commutator brushes. The rapid brush wear
encountered
under these conditions has been partially overcome through a special
brush
design for this class of electrical equipment.
Silicone resins are also used to
coat or impregnate
glass cloth mica paper asbestos paper and similar materials to form
high temperature
electrical insulating constructions. For these uses relatively soft
flexible
resins are preferable. Self supporting substrates such as glass cloth
can be
coated by dipping. When the resin is to serve as a binder it can be
sprayed onto
a layer of flakes or fibers the solvent allowed to evaporate and the
mass
compacted with rollers before being cured at high temperature. The
composite
strips so formed may be split to produce tapes for winding around
irregularly
shaped components of electrical machinery or cut into shapes to serve
as
inserts and spacers.
An
extension of impregnation techniques involves stacking a number of
glass cloth
layers impregnated with partially cured resin and curing the assembly
to form a
laminated structure. Cure of the laminate may be carried out in a
heated
hydraulic press at pressures up to 10000lb in.2 to
form dense rigid boards having thicknesses
up to several inches. These laminates are used principally as
electronic
circuit mounting boards where freedom from distortion low moisture
absorption
dimensional stability and constancy of electrical properties are
essential.
Alternatively the glass cloth plies may be placed in layers over the
surface of
a model or template. The assembly is then enclosed in a flexible heat
resistant
bag and cured in a steam chamber. Relatively easily cured resins are
necessary
for this kind of fabrication.
Most silicone resins like unfilled
silicone
elastomers are relatively weak and hence are not used where physical
strength
is the main criterion of performance. There are however applications in
electrical machinery and in electronic encapsulation where the need for
the
thermal and dimensional stability of silicones dictates the use of
silicone
moldings. When used as molding resins silicones are generally
reinforced with
fibrous or particulate fillers to improve toughness. Silicone resin
powders may
also serve as binders for ceramic frits or oxide powders in the molding
and
firing of ceramic parts requiring precise control of dimensions.
Silicone resins have long been used
as paint
vehicles for extreme high temperature service. When pigmented with
aluminum
flake they provide durable protection for metal smokestacks and similar
equipment operating at temperatures as high as about 550°C. More
recently
silicone resins have been used to improve the weather durability and
gloss
retention of maintenance paints by blending or reacting with alkyd
acrylic and
other conventional paint bases. If a copolymer is to be formed with for
example
an alkyd resin the silicone must have a relatively high proportion of
reactive
silanol or alkoxy groups which can combine with alkoxy or carboxy
groups on the
alkyd under conditions attainable in normal alkyd processing. Whether
reacted
or blended the silicone resin must have a sufficient content of phenyl
or moderate
length alkyl groups to confer ready compatibility with organic polymer.
Silicone
containing paints are accepted in manufacture of metal siding for
buildings
where the durability against weathering that can be achieved is matched
only by
fluorocarbon polymers. Application of the paint to the metal takes
place while
the metal is still in the form of a flat strip. Curing conditions can
be
controlled in a continuous coating line to provide optimum adhesion and
flexibility of the paint layer so that subsequent coiling slitting and
bending
to form the siding strips can be accomplished without chipping or
cracking the
coatings.
Applications of resins as release
agents include
those in which a degree of permanence despite temperature cycling is
desired as
in bake ware and those involving extremely high temperatures such as
glass
molding and metal casting. Silicone resin coated pans for baking bread
can be
used without greasing and permit the loaves to be discharged merely by
inverting the pan. After several hundred baking cycles the pans must be
cleaned
and recoated. Recently silicone coated bake ware for home use has been
introduced.
Like other types of silicone
polymers the resins wet
most substrates well before cross linking and can be used as bonding
agents for
high temperature assemblies. The MQD polymers are outstanding in their
ability
to adhere to almost any solid including polymers with poor wettability
such as
poly tetra fluoro ethylene. They will also adhere to many wet
substrates. For
these reasons they have been used as pressure sensitive adhesives for
high temperature
tapes with glass cloth or silicone rubber backing. At ordinary
temperatures
their resistance to environmental factors makes them useful in tapes
for semi
permanent positioning and assembly of industrial components. And since
they
retain tackiness at temperatures as low as 40°C they can be used in low
temperature tape application such as field repairs of electrical
systems in
winter. For some purposes the pressure sensitive adhesives can be cured
with
peroxides at temperatures over 125°C to provide the properties
generally
associated with cross linked silicones.
Surfactants
and Specialtics
Emulsions
Emulsions of silicone fluids and a
few resins are
available commercially for use primarily as antifoaming agents and as
release
agents. The features which make the use of emulsions desirable include
nonflammability compatibility with aqueous systems case of dilution and
effectiveness of these highly dispersed forms of silicones is
applications
where surface properties are important.
Silicone emulsions are generally
made from standard
fluids emulsifying agents water and in some cases finely divided solids
which
apparently act as carriers for the silicone increasing the exposed
silicone
interfacial area and consequently the effectiveness of the emulsion as
a
surfactant. All classes of emulsifiers can be used anionic cationic and
nonionic. In most cases a waterinoil dispersion is prepared first by
passing a
mixture of silicone fluid emulsifier some water and solid dispersant
(if used)
through a high shear blending device such as a colloid mill or
homogenizer. The
resulting paste is then dispersed in a larger amount of water with
vigorous
agitation. The final product is a silicone in water emulsion even
though the
silicone fluid may constitute up to 70% of the product composition.
Most
silicone emulsions as solid contain 1070% silicone but are usually
diluted too
much lower concentrations before use.
Commercial silicone emulsions are
pourable systems
of low to moderate viscosity with good shelf stability and resistance
to phase
separation. They are susceptible in most cases to breaking at high
(80°C) or
low temperatures and must be protected from freezing. Extremes of
hydrogen ion
concentration will also cause deemulsification. Certain emulsions for
use in
polishes are intentionally designed to break down under shear so that
good
gloss is obtained without excessive polishing effort. Other
applications of emulsions
antifoaming agents and release agents have already been described in
the
section on Silicone Fluids.
Greases
and Compounds
Compounds of silicone fluids with
particulate solids
and other thickening agents to achieve a grease like consistency are
used in
many cases where noncuring flowstable silicone containing compositions
are
required. When used primarily as lubricants these compositions are
called
greases otherwise the more general term compound applies.
The most commonly used thickening
agents in silicone
compounds are various forms of silica although clays and other finely
divided
solids may be used. In greases the thickening agents are generally
soaps. Other
ingredients include organic thickeners diluents and additives to
improve
properties ranging from oxidative stability to radiation resistance.
Compounding is usually carried out in a dough mixer or kettle with high
shear
agitation. Thorough wetting of the thickening agent may be promoted by
selective addition of the components to the mixer or by mixing at
elevated
temperature. In some cases the compound is passed through a paint mill
or
homogenizer after mixing to insure that filler agglomerates are broken
down.
The finished product is packaged in jars cans or collapsible tubes.
Silicone
compounds are used principally in electrical applications as potting
materials
or insulating coatings with excellent corona resistance. Greases are
used in a
number of non load bearing applications such as laboratory glassware
joints and
electric clock motors where chemical inertness or resistance to aging
are
important. High temperature greases are used in oven conveyors and
similar
systems. Although the relatively poor lubricating characteristics of
dimethyl
silicones have kept them from wide use in heavy duty greases
applications are
gradually increasing in the automotive and aerospace industries for
products
based on chlorophenyl fluoro alkyl and long chain alkyl silicones.
Thermal and
oxidative resistance relative invariance of properties over a range of
temperature
resistance to aging and weathering and resistance to hydrocarbon
solvents are
among the more important reasons for selection of one of these products.
Surfactants
One of the most successful
specialized applications
of silicone materials has been the use of silicone organic block
copolymers to
control the structure of organic polymer foams primarily polyurethans.
These
copolymers added at about 1% to the foam formulation serve to control
cell size
and uniformity to produce even textured foams with reproducible
physical
properties. As the proportion of surfactant is decrease wild cells of
unusually
large size appear followed at lower silicone levels by general
coarseness and a
tendency of the foam to split mechanically. With no surfactant many
commercial
foam formulations collapse completely. Polyester vinyl synthetic rubber
and
other polymeric foams have also been improved through the use of
silicone
surfactants.
Among the various chemical types of
silicone
surfactant one of the most widely used in based on an alkoxyterminated
branched
silicone containing an average of one tri functional siloxy group and
three
alkoxy groups per molecule. This is combined with a hydroxyl terminated
polyether in an ester exchange reaction using a suitable catalyst. The
displaced
low molecular weight alcohol is continuously distilled away yielding a
molecule
with a silicone segment attached on the average to three long polyether
chains.
Much other linkage can be employed to join the silicone and organic
portion of
the molecule carboxylic acid ester carbamate thioester direct silicon
carbon
linkages and others have been patented or disclosed in the technical
literature.
The function of these additives is
complex and has
been the subject of a number of articles. Whether the silicone
copolymers
promote nucleation of gas bubbles or serve merely to stabilize bubbles
introduced mechanically during the mixing of the polyurethane reactants
the
surfactants are essential to maintaining the integrity of the cell
walls during
the expansion of the foam. In the case of flexible foams intended for
use in
mattresses and seating a relatively low foam density and high
resiliency are of
paramount importance. To achieve high resiliency the foam must have a
significant proportion of open cells to permit ready deformability
while
maintaining sufficient mechanical strength. This requires a fine
balance
between the rate of gas generation and the rate of polymerization of
the
urethan so that a controlled proportion of cell walls rupture just
before the
foam becomes set. Thus the type and proportion of amine catalyst
promoting the
foaming reaction the tin catalyst promoting polymer formation and the
surfactant can all have an effect on the ultimate density and
resiliency of the
foam.
The sensitivity of the foam forming
process to the
surfactant level is attributable to the tendency of the surfactant to
concentrate at the polymer gas interfaces with the polyether oriented
inward
toward the film and the silicone portion extending into the gas phase.
The low
surface tension of the surfactant tends to stabilize the cell walls and
to
induce flow of the still plastic polyurethan to maintain uniform
surface
energy. This ability is a function of the solubility of the polyether
portion
of the surfactant copolymer in the urethan phase and of the plasticity
of the
bulk material. If prepolymer is used as a starting ingredient for the
foam
polydimethylsiloxane surfactants perform effectively as foam promoters.
With
monomeric starting materials silicone homopolymer (with no polyether
content)
act as defoaming agents.
In the case of rigid foams for
thermal insultion low
thermal conductivity must be obtained by forming a high proportion of
closed
cells in a low density foam. In this case the cell walls are strong
enough to prevent
shrinkage of the foam when the cured structure cools. In general
somewhat
different types of silicone copolymers are required for rigid foams
than those
found most effective in flexible foams. In furniture molding higher
foam
densities approximating those of hard woods are desired. Here too
although the
density of the finished foam is primarily a function of the formulation
used
the type of concentration of silicone surfactant must be carefully
selected to
achieve the desired result.
Silicones have also been
incorporated in organic
polymers intended for molding of solid plastic parts. Improved surface
texture
and easy release from the mold are among the benefits obtained.
Primers
and Adhesion Promoters
Although several types of silicones
notably MQD resins
and one part RTV compounds exhibit good adhesion to a variety of
substrates
without priming best results in achieving adherence to other materials
are
frequently obtained by using specialized primers. Few generalizations
can be
made concerning the utility of various kinds of primers in specific
applications and an empirical approach to adhesion problems is
frequently
necessary. Primers are often based on silicone resins with appropriate
additives to improve wetting of the substrate or to achieve effective
bonding
to reactive surface groups. The primers may serve to shield sensitive
curing
catalysts in the silicone polymer systems from inhibitors in the
substrate
materials or simply provide a well anchored compatible base to which
the bulk
silicone resin or rubber can become attached.
One class of primer which has
attained wide use in glass
reinforced plastics as adhesion promoters between resin and glass
fibers is the
family of carbon functional trialkoxysilanes. These are derivatives of
trichlorosilane with the general formula RSi(OR)3.
Carbonfunctional groups which have been successfully utilized include
vinyl
aminoalkyl acrylatoalkyl glycidoxyalkyl and variations of these. The
alkoxy
substituents are generally methoxy ethoxy or methoxyethoxy
used to impart water solubility.
The trialkoxysilanes are usually applied to the glass fibers from
dilute
aqueous solution directly after the fibers are blown and collected.
Partial or
complete hydrolysis of the alkoxy groups takes place in the bath
forming silanols
which can interact with similar reactive sites on the glass surface
bonding the
silane chemically to the glass through siloxy linkages. The carbon
functional
group is not attached to the glass surface and is available to provide
bonding
to the resin. The most likely mechanism of bonding involves
solubilization of
the organic group in thermoplastic matrixes whereas actual
copolymerization of
the carbon functional group with the bulk polymer appears to take place
in
thermoset polymers. Other factors including polarity of the carbon
functional
group critical surface tension of the treated fiber surface and ability
to form
hydrogenbonds may also be significant in the performance of these
bonding
agents. The coating is generally much thicker than a monolayer and
additional
silanes or their hydrolysis products are held by capillary action
between the
individual fibers. The benefits of the silane treatment are seen in
improved
reinforcement of the glass fiber plastic matrix as measured by flexural
strength of the composite and are especially apparent in retention of
physical
properties after exposure to water. The treatment prevents absorption
of water
by capillary action along the fiber polymer interfaces and the
consequent loss
of reinforcing action of the fibers. A notable current application is
in the
treatment of glass cord used in automobile tire construction.
STABILIZERS
Stabilizations as discussed in this
article denotes
the treatments or manipulations to which polymers are subjected in an
effort to
control or adjust effectively the deteriorative physicochemical
reactions at
work during the manufacture compounding processing and subsequent life
of the
polymer.
Since the goal of stabilization is
to maintain
insofar as is possible the original characteristics of the polymer it
follows
that the subject is of interest to all concerned with the various
manners in
which polymers are employed e.g. as in plastics elastomers foams
textile fibers
coatings and adhesives.
Effective polymer stabilization
involves the simultaneous
control of numerous degradative forces and mechanisms which may be at
work on
the polymer system at any given time.
Maintenance and control of color rheological
characteristics mechanical
properties electrical properties chemical resistance biological
resistance
thermal and optical properties and resistance to long term aging and
weathering
are all aspects of polymer stabilization. This article is concerned
mainly with
the effective use of certain additive to achieve a close control and
regulation
of properties of a composite polymer matrix or system.
Specifically the stabilization of halogenated
polymers in general and poly (vinyl chloride) in particular will be
discussed.
The section on stabilization of
polyolefins
discusses how the antioxidants and other special organic inhibitors are
applied
to the total stabilization of a hydrocarbon polymer.
By way of illustration the polyolefins were
chosen to be generally representative of the type of polymer
deterioration one
might expect to encounter with hydrocarbon resins and the discussion in
this
section illustrates the means one might take in combating these
phenomena. Additional
information can also be found in
articles on the specific polymer see for example ETHYLENE POLYMERS
PROPYLENE
POLYMERS STYRENE POLYMERS etc. for
information relation to the degradation of other nonhydrocarbon polymer
systems.
Methods
Polymer
stability can be achieved effectively by pursuing either of two
possible lines
of attack preventive stabilization or arrestive stabilization. Better
results
however are usually attained by a combination of the two approaches. In preventive
stabilization methods
such as closer control and regulation of the polymerization reaction to
reduce
or eliminate byproduct formation are effective since the presence of
residual
byproducts (including structural aberrations of the desired polymer)
may exert
a deteriorative influence on the properties of the newly produced
polymer. Better
regulation may be achieved by (a)
closer control of monomer purity and the purity of all of the other
materials
employed in the polymerization reaction – water organic solvents
buffers
suspending aids emulsifiers catalysts and initiators chain transfer
agents etc
(b) cleaner polymerizations such as polymerization in bulk rather than
in
emulsion solution or suspension initiation of polymerization by
nonochemical
means eg radiation initiation more painstaking workup of the polymers
after
reaction termination to remove or inactivate potentially troublesome
catalyst
residues and other polymerization reaction additives and combinations
of any or
all of the preceding. The
realization of
these objectives is slowly being attained as polymer manufacturers
realize that
the manner in which they adjust conditions in the polymerization kettle
will
ultimately determine the inherent stability of the polymer and that
process
changes which may be slightly more costly to incorporate at this point
may
ultimately be the less expensive.
Another
form of preventive stabilization occasionally practiced is through
copolymerization with a minor amount of a second monomer in an effort
to build
in blocks to the progressive chain disruptive forces that are
encountered eg in
the unzippering of hydrogen chloride from a poly(vinyl chloride)
backbone. The
inclusion of very small quantities of
ethylene or propylene on the order of 13% in a vinyl chloride
polymerization
has resulted in copolymer of greatly improved that stability as
compared to a
vinyl chloride homopolymer of equivalent intrinside viscosity yet with
very
little change in all of the other properties characteristic of poly
(vinyl
chloride). In a
similar manner the
partial replacement of styrene monomer with methyl methacrylate in the
cross
linking of unsaturated linear polyester resins such as those used to
impregnate
glass fibers has resulted in glass fiber rein forced plastics of
greatly
improved light stability as compared to the similar system compounded
with
styrene monomer used as the sole cross linking agent.
Cases such as the above two
examples in which
copolymerization effects improvements in stability are admittedly the
exception
rather than the rule. They
are found to
occur in those relatively rare instances where the resultant copolymer
is a
true block copolymer rather than a random or graft copolymer.
The concept of arrestive
stabilization
denotes the remedial treatment of polymer systems within which some
albeit
perhaps only incipient degradation has already been initiated. Arrestive stabilization
can be carried out
effectively by removal or inactivation of the degradative source to
prevent
further more serious deterioration from occurring or else it can be
accomplished by introducing reactive sites or structures which will
combine
with a repair the polymer by altering the degradation to a less
objectionable
form. A third means
of arrestive
stabilization is through the introduction of reactive species which
will
compete with the normal series of degradation reactions for the active
susceptible
sites on the polymer and will replace those groups or ligands with
others
of greater inherent
stability. All of
these remedial stabilization mechanisms will be dealt with a greater
length in
the discussion of poly(vinyl chloride) stabilization.
At present two techniques have been
developed for
achieving stability by arrestive means.
One of these is achieving stability by arrestive means.
One of these is
through copolymerization with monomeric moieties which are themselves
capable
of functioning as stabilizers through any one of the three modes
described
above. In
effectiveness this approach
has been only partially successful and is still open to considerable
improvement. It does however have the singular characteristic of
permanence
because the stabilizing group is made in integral part of the polymer
backbone. Whether
or not this
characteristic is an advantageous one is a question still open to much
debate. Much of the
answer resides in
the nature of the particular stabilization mechanism being introduced
into the
system through copolymerization and in the end use for which the
polymer
composition was developed.
By far the most effective and
widely employed means
of arresting polymer degradation is through the incorporation of chemical
additives. For
purposes of this
discussion an additive is defined as a substance which is mechanically
dispersed or dissolved (usually with the aid of heat) in the polymer
system
requiring stabilization. It
almost
always is a minor constituent of the total system usually representing
10% or
less by weight and most often constituting an even smaller amount eg on
the
order of 0.5% 3.0%
of the system by
weight. The
additive(s) maybe low molecular
weight simple chemical substances or may themselves be highly polymeric
and
polyfunctional in nature.
Traditionally
the additive approach is among the oldest of the various stabilization
techniques and is still the route most preferred by resin manufacturers
compounders processors and captive users who have the power of choice. The reason for this can be
summed up as
simplicity versatility and economy.
The
literature is replete with examples that show the favourable results
obtained
with a stabilizing additive in comparison with the same stabilizing
group
introduced by copolymerization. Although
the copolymerization method will certainly continue to be pursued and
may
eventually be developed to the point where it will surpass the additive
compounding technique for general applications the abandonment of
stabilization
through the comparatively simpler stirin method is not yet in sight.
An examination of Table 1 will
serve to illustrate
the current commercial importance within the United States of just
three
classes of additive stabilizers antioxidants ultraviolet inhibitors and
proprietary poly component mixed stabilizer systems primarily designed
for use
in poly (vinyl chloride).
These statistics pertain to the use
of the additives
in plastics applications only applications involving rubbers synthetic
fibers
paints and organic coatings are excluded although these markets also
account
for the consumption of considerable volumes of stabilizers. The rubber industry alone
for example in 1963
consumed over 96 million lb of antioxidants in the United States
without
including antiozonants and other inhibitors.
With respect to all stabilizers it is further estimated
that the total
worldwide consumption (excluding U.S.S.R. and other Eastern European
countries
for which figures are not available) is roughly 2.2 times the market
for the
U.S. alone. Although
some of the U.S.
production is used for export purposes the amount is not considered
significant.
Stabilization
of Polyolefin
Resins
Within each of these five classes
of material the
number and variety of commercially acceptable materials have been
somewhat
limited due to additional considerations chief among which is the
factor of
compatibility. Olefin
polymers are
inherently similar in structure to aliphatic hydrocarbons and therefore
similar
also in solubility characteristics i.e. they are hydrophobic and
oleophilic. Certain
otherwise extremely
effective additives are often ruled out for use with olefin polymers
because
they are too polar for optimum solubility in and retention by the
polymer. Although
the situation is not so restrictive
in the same of olefin copolymers and in polymers containing long and/or
branched pendant groups eg poly (4methyl1pentene) materials such as
polyethylene and polypropylene still present problems in certain
demanding
applications.
The toxicological characteristics
of the additive
may limit many otherwise useful material from applications in which the
health
safety factor is important. Although
this situation applies to all stabilizers regardless of polymer
application it
is especially important with olefin plastics since these materials have
come to
dominate the packaging industry.
The effect of the additive color on
the formulation
is another consideration which tends to rule out many otherwise highly
desirable materials. Alkyl
or aryl
derivatives of pphenylenediamine for example are extremely effective
antioxidants for polyethylene (use as they also are in rubber and other
hydrocarbons) but they are scarcely used today other than in dark
colored
stocks because of their staining nature.
Ultra
violet radiation absorbers have special requirements of their own which
apply
not only to polyolefins but wherever these materials are used (1) high
absorption in the ultraviolet region of 29003900A with sharp cutoff to
near
zero absorption above 4000A (absence of visible color) (2) ability to
transform
absorbed energy into inactive energy without the formation of color (3)
stability against eventual self destruction due to absorption of energy
(4)
compatibility (solubility) with the substrate (5) nonmigration.
Table
2 lists the materials most frequently employed in the stabilization of
polyolefin resins. For
the most part one
selected member from each class is used in admixture with a material
from each
of the other groups excepting that metal deactivators and ultraviolet
absorbers
are generally used only when special circumstances warrant their
inclusion eg.
polyethylene insulation over copper wire or polypropylene filaments for
indoor
outdoor carpeting.
The
degree to which olefin polymers are capable of resisting oxidative
degradation
in any given environment is determined by such obvious factors as the
chemical
composition of the particular polyolefin the polymerization reaction
conditions
and the degree of effectiveness and the concentration of the protective
agent(s) included in the resinous compound.
In addition there are certain less obvious factors which
can affect the
degree of stability of the system.
Chief
among these are the influences contributed by the other formulation
constituents including colorants lubricants mineral fillers etc. With respect to stability
their inclusion or
omission can be good or bad depending upon their own chemistry. From a practical point of
view achieving
optimum stability in a formulation entails a study and recognition of
the
effect contributed by other formulation constituents.
Synergism
and Antagonism. Some stabilizers behave
synergistically that is their combined effect
is greater than the sum of their effect when each is used alone. For example a particular
antioxidant tested
with three different ultraviolet radiation absorbers in a polyolefin
may in the
first case yield an outdoor durability improved several fold over that
of the
antioxidant used alone substantially unaffected in second and poorer in
the
third. If in the
first case the
performance of the antioxidant is also improved the materials perform
synergistically. This
unanticipated
benefit is clearly attributable to some complementary relationship
existing
between the two materials. Examples
of
true synergistic activity among stabilizer components are comparatively
rare
whereas reverse synergism or antagonism occurs rather frequently. Two examples of synergism
which have been
studied extensively are the synergism between hindered phenolic
antioxidants on
the one hand and various sulfur bearing organic materials particularly
sulfides
(thioethers) on the other and the apparent synergism between certain
grades of
carbon black and sulfer containing antioxidants.
The impetus behind the investigation into the
carbon black/sulfer synergism originally resulted from an exploration
for novel
antioxidants which might effectively replace amines and/or phenols
owing to the
fact the latter two groups or primary antioxidants were known to lose
most or
all of their effectiveness in the presence of carbon black. The latter illustrates a
typical example of
antagonism.
Effects of Formulation Constituents. Compared to the volume of published
literature
concerning the elucidation of structure and mechanisms of degradation
of olefin
polymers relatively little has been said concerning the nature of
formulation
additives and constituents and their effects on the stability
characteristics
of the formulation. It
is common
practice not only in the United States but elsewhere for the producers
of
polyolefins to compound their polymers into forms suitable for direct
processing into final compositions and shapes eg extrusion and molding
compounds rotational molding powders.
In
the compounded form the polymer blend contains the colorants lubricants
various
stabilizing agents and whatever other special purpose additive(s) may
be
required to modify the properties of the composition for the particular
end
application. The
accumulated body of
knowledge concerning the evaluation and selection of these agents is
regarded
by the polymer producers as proprietary information and this accounts
for their
reluctance to publish information of this type.
However some findings have recently come to light mainly
through the
laboratory evaluations of additive producers and large volume users of
olefin
polymers who are in a position to specify formulations to the resin
producers.
The
effects of carbon black have been alluded to previously. Effects reported on
various pigments as well
as carbon black in polyethylene wire and cable formulations were also
reported
in 1957 carbon black at 2% concentration extended the outdoor life from
one to
twenty years. Similar
studies have been
conducted more recently.
Studies of the role of stearate
processing aids upon
the melt flow stability of polypropylene particularly in the presence
of copper
show that the stearates of calcium aluminum zinc and cadmium as well as
free
stearic acid adversely affect the rheology of the compositions at
elevated
temperatures and concluded that the use of stearate processing aids
should thus
be a voided if maintenance of properties and stability are of prime
consideration in the fabricated article.
The specific effects of various
stabilizing agents
have been described with respect to a host of different properties in
various olefin
systems. The
performance of 112tris
(2methyl4hydroxy5textbutylphenyl)butane which refers
to a condensate of
3methyl6tertbutylphenol and crotonaldehyde has been
compared with a
variety of other hindered phenols alone and in the presence of dilauryl
thiodipropionate
in polypropylens polyethylene and a copolymer of ethylene and vinyl
acetate.
The effects of antioxidants and ultraviolet absorbers in polypropylene
outdoor
weathering and extraction resistance of the additives heat aging
stability and
light stability of polyethylene extraction resistance and additive loss
through
volatilization have been discussed.
Stabilization
of Halogenated Polymers
From an industrial point of view
the only
halogenated polymers which require some sort of stabilization to be
commercially useful are those containing chlorine.
Fluorinated polymers such as poly tetra fluoro
ethylene or poly (vinyl fluoride) ordinarily require no remedial or
corrective
treatments.
The following discussion deals for
the most part
with the degradation and stabilization of poly (vinyl chloride) since
this
polymer has been studied in greater depth than any of the other
chlorine
containing polymers. The
knowledge which
has been gained from studies related to poly (vinyl chloride) has also
been applied
with some success to other material similar to structure to poly (vinyl
chloride) and which generally degrade in a like manner.
Thus the polymer industries have been able to
develop and use successfully such materials as poly (vinylidene
chloride) chlorinated
polyethylene chlorinated polypropylene copolymers of vinyl chloride
with such
other monomers as vinyl acetate vinyl alkyl ethers maleic anhydride and
its
esters acrylonitrile ethylene and propylene.
A post chlorinated poly(vinyl chloride) series of
compounds designed and
used primarily in applications at temperatures above which poly(vinyl
chloride)
is not usable (eg in hot water pipes) has been successfully
commercialized. The
stabilization of
each of these materials follows the general precepts of poly (vinyl
chloride)
stabilization and also takes into account the different types of
formulations
and processing conditions to which these diverse chlorinated polymers
are
exposed.
Structure and Degradation of Poly
(vinyl Chloride). The polymerization of vinyl
chloride monomer in common with other vinyl monomers proceeds by a free
radical
mechanism involving the usual steps of initiation propagation and
termination. Poly(vinyl
chloride) is
formed in a regular head to tail manner(I) although it had been
previously
suspected that the structure was random
containing both
head to head and tail to tail units in addition to head to tail
segments.
Some chain branching (radical
transfer to polymer)
occurs during the propagation steps leading to two possible structures
(3 and
4) at the branch points both of which are believed to be present in
commercially prepared poly(vinyl chloride) whether by suspension (as is
most
common) emulsion mass or solution polymerization.
Termination is responsible for a
number of different
end group structures as well as certain irregular groups found within
poly(vinyl chloride) molecules which have been terminated by coupling. When polymerization of two
growing radical
chains is terminated by direct coupling the resultant polymer molecule
has an
initiator fragment at each end and a dichloro structure somewhere
within the
chain at the point of coupling (eq.1).
In addition to coupling termination
also occurs by
disproportionation and by chain transfer to monomer polymer initiator
solvent
or to any of the many other ingredients usually present in a
polymerization
reaction. Termination by disproportionation is illustrated by equation
2.
SULFURIC ACID
Sulfuric acid a strong acid is oily
viscous water
white nonvolatile liquid. It
absorbs
water from the atmosphere. A drop of it on the skin causes a severe
burn. It is made in
large volume by the chemical
industry. It is used as a solvent a dehydrating agent a reagent in
chemical
reactions or processes an acid a catalyst an absorbent etc. The concentrated acid is
usually stored in
steel tanks. The
dilute acid may be
stored in lead lined or plastic tanks.
It is used in very dilute concentrations and as strong
fuming acid. It
is often recovered and reused. After
use
in some phases of the explosives petroleum and dye industry it is often
recovered in a form unsuitable for reuse in that industry but suitable
for use
in another industry. It
is a versatile
useful acid and has been called the work horse of the chemical industry.
USES OF SULFURIC ACID
Sulfuric acid is one of the most
widely used of all
manufactured chemicals and its rate of production has long been a
reliable
index of the total chemical production and the Industrial activity of a
nation. For 1969 in
the United States
the average per capita consumption of sulfur was 103.8 pounds of which
most
went into the manufacture of sulfuric acid. In the world the per capita
consumption in 1969 was about 26 pounds.
The sulfuric acid consuming
industries in the United
States are listed in Table 1. Of these the largest consumer is the
fertilizer
industry which treats phosphate rock with sulfuric acid to produce
super
phosphate (a mixture of mono calcium phosphate and calcium sulfate) or
crude
(wet process) phosphoric acid. There is hardly an article of commerce
which has
not come into contact with sulfuric acid at one time or another during
its
manufacture or in the manufacture of its components.
The consumption of acid in the various
industries is undergoing constant change.
Progress demands that manufacturers strive to decrease the
consumption
of acid per unit product manufactured.
Progress also is continually turning up new uses.
Kinds of Acid
Sulfuric acid is marketed in the
United States as a
large tonnage product. It is made in numerous grades and strengths and
shipments are made in both packaged containers and in bulk. It is produced in grades
of exacting purity
for one in storage batteries and for the rayon dye and pharmaceutical
industries. It is produced to less exacting purity specifications for
use in
the steel heavy chemicals and fertilizer industries.
Originally sulfuric acid was marketed in four
grades known as chamber acid 50° Beaume (Bé) tower acid 60° Bé oil of
vitriol
66° Bé and fuming acid. At
present it is
marketed in the strengths listed in Tables 2 and 3.
The Manufacture of Sulfuric
Acid
History.
Sulfuric acid is formed in nature by the oxidation and
chemical
decomposition of naturally occurring sulfur and sulfur containing
compounds. It
is formed by the weathering of coal brasses or iron disulfide discarded
on
refuse dumps at coal mines. It
is formed
by bacteria in hot sulfur springs. It is formed in the atmosphere by
the
oxidation of sulfur dioxide emitted form the combustion of coal oil and
other
substances. It is formed by chemical decomposition resulting from
geological
changes.
In ancient times sulfuric acid was
probably made by
distilling niter (potassium nitrate) and green vitriol (ferrous sulfate
heptahydrate). Weathered
iron pyrites
were usually the source of the green vitriol.
About 1740 the acid was made in England by burning sulfur
in the
presence of saltpeter (potassium nitrate) in a glass balloon flask. The vapors united with
water to form acid
which condensed on the walls of the flask.
In 1746 the glass balloon flask was replaced by a large
lead lined box
or chamber giving rise to the name chamber process.
In 1827 GayLussac and in 1859 Glover changed
the circulation of gases in the plant by adding towers which are now
known as
GayLussac and Glover towers. These
permit the recovery from the exit gases of nitrogen oxides which are
essential
to the economic production of chamber acid. Today most sulfuric acid
made in
the United States is produced by the contact process based on
scientific
technology developed about 1900 and thereafter.
BASF built a successfully operating
contact plant in
the United States in 1898. General
Chemical erected a pyriteburning contact plant using the Herre shoff
furnace in
the United States in 1900.
Development of the Sulfuric Acid
Industry in the
United States
The manufacture of sulfuric acid
has been a basic
industry in the United States for many years. It has been made by two
well established
methods the chamber process and the contact process.
Initially the production of acid was
concentrated on the Eastern seaboard. After the Civil War the industry
spread
to the west and into the South. Between
1899 and 1904 the number of acid manufacturers increased rapidly in
Ohio and
Illinois and just before the turn of the century there was great
activity in
the South as a result of the discovery of phosphate deposits in South
Carolina
and Florida and the development of the phosphate fertilizer industry.
Over the years the number of
contact plants has increased while the number of chamber plants has
decreased. During
the mid 1940s the
number of chamber plants was approximately equal to the number of
contact
plants. Today the
contact plants are in
the majority and the trend is toward contact plants of larger and
larger
capacity. Contact
plants producing 2000 tons of acid a day in a single
train are not unusual and they may become larger in the future.
Consumers confronted with the
task of disposing of waste or spent acid find it advantageous to
arrange with
an independent producer to exchange the waste acid for fresh acid. Methods have been
developed which permit such
producers to reprocess the waste acid and obtain a product of virgin
quality. Also they
can operate centrally
located plants of large size which can produce acid at a much lower
cost than
can be realized in the small plant needed by the average user. The end use of sulfuric
acid more than any
other factor determines the location of sulfuric acid plants. Data on the production of
acid in the United
States are listed in Table 4.
The Chamber Process
for Making Sulfuric Acid
The chamber process for making
sulfuric acid at first glance appears to be a rather simple process
requiring
simple equipment. The
reactions involved
however are not simple and even today there is disagreement among
experts as to
just what does take place in the chambers.
All agree however that the oxidation of sulfur dioxide to
sulfuric acid
in the chambers is not directly effected by oxygen but that
intermediate
compounds involving nitrogen oxides are formed and that the reaction is
really
a cyclic process involving the alternate formation and decomposition of
the
intermediate compounds. Many
operators
say that operation of a chamber plant is more an art than a science.
In the chamber process chemical reactions take
place between
sulfur dioxide oxygen nitrogen oxides and water vapor.
A series of intermediate compounds are formed
which decompose to yield sulfuric acid and nitrogen oxides. The overall effect is that
the sulfur dioxide
is oxidized to sulfur trioxide which combines with water vapor to form
sulfuric
acid. The nitrogen
dioxide acts as the
oxidant and is reduced to nitric oxide which must be continually
reoxidized by
oxygen in the air. When
all the sulfur
dioxide has been consumed the nitrogen oxides appear as equal moles of
nitric
oxide and nitrogen dioxide in which ratio they are absorbed in sulfuric
acid in
the GayLussac tower as nitrosylsulfuric acid.
The solution of nitrosylsulfuric acid (nitrose) from the
GayLussac tower
is pumped to the denitration (Glover) tower where heat releases the
nitrogen
oxides for reuse in the cycle. In
the
Glover tower the denitrated sulfuric acid is concentrated to 60°Be. Part of this acid is
returned to the
GayLussac tower for recovery of the nitrogen oxides from the exit gases. The balance is available
for use or sale.
If elemental sulfur is burned a substantially clean
gas
containing 8 to 11 per cent sulfur dioxide by volume is formed. If sulfide ores or other
sulfur bearing
materials are burned a gas containing about 7 per cent sulfur dioxide
is
produced. This gas
is usually
contaminated with varying amounts of dust metallic fumes water and
other
gaseous impurities which must be removed at least in part. The sulfur dioxide gas of
suitable purity is
then conducted to the Glover tower of the chamber plant where it meets
a
countercurrent flow of sulfuric acid 50° to 54° Be containing
nitrosylsulfuric
acid. The hot gas concentrates the acid to 60° Be and decomposes the
nitrosylsulfuric acid releasing the oxides of nitrogen.
The gas leaving the Glover tower and
containing sulfur dioxide nitrogen oxides of nitrogen water and an
excess of
oxygen then enters the lead chambers.
The concentration of sulfur dioxide gas in the
system must
be controlled to allow an excess of oxygen throughout the system. Nitrogen oxides obtained
by adding nitric
acid by the decomposition of sodium nitrate reacted with sulfuric acid
or in
more recent plants by burning ammonia are added to the chambers. A fine spray of water is
also added. The
sulfuric acid produced condenses on the
walls of the lead chambers. The unreacted gas flow to the GayLussac
towers.
Here the nitrogen oxides in the gas are recovered by absorption in a
countercurrent
flow of 60° Be acid to form nitrosylsulfuric acid.
This acid containing the nitrosylsulfuric
acid is then pumped to the Glover tower and the cycle repeated. Up to 50 per cent of all
the acid produced in
the plant is formed in the Glover tower.
The acid produced is drawn from the pans in the
bottom of
the chambers or from the Glover tower.
That produced in the first and intermediate chambers
usually contains
from 63.66 to 68.13 per cent sulfuric acid (52° to 54° Be). That which is produced in
the last chamber is
weaker and contains about 59.32 per cent sulfuric acid (48° Be). That which flows from the
Glover Tower
contains from 75 to 85 per cent sulfuric acid.
Chamber acid may contain small
amounts of impurities such as oxides of nitrogen arsenic and selenium
and
sulfates of iron copper zinc mercury lead and antimony depending in
part on the
kind of sulfur bearing material used in making the sulfur dioxide gas
charged
to the plant. In
some applications as in
the manufacture of fertilizers these impurities are not harmful. In other they are harmful
and must be
removed.
The Contact Process
The basic features of the contact process for
making
sulfuric acid as practiced today were described in a patent issued in
England
in 1831. It
disclosed that if sulfur
dioxide mixed with oxygen or air is passed over heated platinum the
sulfur
dioxide is rapidly converted to sulfur trioxide which can be dissolved
in water
to make sulfuric acid. The
practical
application of this disclosure however was delayed.
An understanding of the complex reactions occurring
in the gas phase over the catalyst required the development of that
branch of
physical chemistry known as chemical kinetics and also the development
of that
branch of engineering known as chemical engineering.
A demand for acid stronger than that which
could be produced readily by the chamber process stimulated this
development.
The success of this process for making sulfuric acid led to the
development of
other catalytic processes for making many of the synthetic chemicals
known
today.
The heart of the contact sulfuric acid plant is the
converter in which sulfur dioxide is converted catalytically to sulfur
trioxide. Over the
course of the years a
variety of catalysts have been used including platinum and the oxides
of iron
chromium copper manganese titanium vanadium and other metals. The first
catalyst used was platinum. It
proved to be extremely sensitive to poisons such as arsenic compounds
present
in small amounts in some sources of sulfur dioxide.
The successful development of the contact process
depended in part on the recognition of the existence of catalytic
poisons and
in devising methods for their removal.
Platinum and iron catalysts were the main catalysts used
prior to World
War I. At present
vanadium catalysts in
various forms combined with promoters are generally used.
A number of different plant designs have been
developed for
the efficient production of sulfuric acid by the contact process. These in the United States
are often referred
to by the name of the builder or designer e.g. a Chemical Construction
(Chemico) a LeonardMonsanto or a WellmanLord plant.
In Europe and other parts of the world Lurgi
Gesellschaft fur Chemie and Huttenwesen mbH Chemiebau Dr. A. Zieren
GmbH &
Co. KG and SimonCarves Chemical Engineering Ltd. Are noted for
designing and
building contact sulfuric acid plants.
Contact plants are also classified according to the
material used in the
production of the sulfur dioxide charged to the plant e.g. sulfur
hydrogen
sulfide gypsum iron pyrites smelter gas or spent and sludge acids.
Dithiocarbamates
Dithiocarbamates
are commercially important class of substances derived from amines and
carbon
disulfide. They
possess the common
functionality and are formally derivatives of dithiocarbamic acid. Dithiocarbamates of
commercial importance may
be divided into three categories dithiocarbamate salts neutral
dithiocarbamate
esters and thiuram disulfides.
These
compounds are widely used
in analytical chemistry as agricultural fungicides and in rubber
technology. Despite
a certain
commonality of application the chemistry both preparative and
analytical of
each class is best considered individually.
DIOTHIOCARBAMIC
ACID SALTS
Metal salts
and,
or chelates of dithiocarbamates are perhaps the most important
dithiocarbamates
in a commercial sense. They
are utilized
mainly as pesticides and as accelerators in rubber vulcanization. Some use has been made of
the strong metal
binding properties of dithiocarbamates to effect separation
identification and
determination of various metal ions.
Dithiocarbamate salts are also used in the preparation of
thiuram
disulfides and dithiocarbamate esters.
If
carbon disulfide is added to
an ethanolic solution of ammonia colorless crystals of ammonium
dithiocarbamate
separate after a time. Careful
acidification of the salt permits isolation of the free thiocarbamic
acid. The latter is
however extremely unstable and
upon heating reverts back to ammonia and carbon disulfide
These
reactions
hold also for primary and secondary amines and this is the method by
which
almost all dithiocarbamate salts are prepared
One mole of
alkali may be substituted for part of the amine in which case the metal
dithiocarbamate salt is obtained
Acidification
also liberates the unstable free acids which easily disintegrate into
carbon
disulfide and amine. Ethylenediamine
or
similar aliphatic diamines yield mixtures of monomeric and polymeric
salts or
bisdithiocarbamate salts depending on the amount of carbon disulfide
available
at the instant of reaction
A
great many heavy metal
dithiocarbamates are known. They
are
often highly colored and monionic in character being in fact strongly
chelated. Preparation
is effected simply
by the addition of a solution of the heavy metal as the chloride or
sulfate to
a solution of an ammonium or alkali metal salt of the dithiocarbamine
acid
preferably with exclusion of oxygen.
The
heavy metal salts are sparingly soluble in water being more soluble in
organic
solvents such as chloroform carbon tetrachloride or ethyl ether. They can be crystallized
from solvents such
as benzene and petroleum ether.
A
great number of metal
dithiocarbamate salts have been prepared and references to a
substantial number
are listed by Thorn. The
salts vary
widely from completely ionic salts to covalent chelates. Various physicochemical
properties of these
materials have been summarized. Some
physical property data for metal dithiocarbamates derived from simple
dialkylamines are shown in Table 1. Some of commercial dithiocarbamates
along
with their more frequently encountered trade designations are listed in
Table
2.
Analysis
of Dithiocarbamate
Salts
A variety of methods have
been devised for the
detection and determination of dithiocarbamate salts the approach found
most
useful involves decomposing the dithiocarbamates to the amine and
carbon
disulfide with hot mineral acids.
Acid
Hydrolysis. In
general dithiocarbamates are smoothly
decomposed by hot dilute mineral acid to yield carbon disulfide and the
corresponding amine either of which may be determined.
Decomposition
by
route A is said to be quantitative when boiling dilute sulfuric acid is
used
and yields the anticipated two moles of carbon disulfide per mole of
ethylene
bisdithiocarbamate. Route
B is a slower
reaction favored by the use of cooler acid. If this reaction occurs 50%
of the
available sulfur which would otherwise have been evolved as carbon
disulfide is
fixed as ethylenethiourea plus hydrogen sulfide and a low result is
obtained.
The
conditions
for the acid digestion have been investigated by a number of workers
mostly
with a particular compound or type of compound in mind and with
attention being
paid to acid concentration. Generally water soluble dithiocarbamates
decompose
rapidly in sulfuric acid and only a short digestion time is required. Digestion problems and
consequent erratic
results are most often encountered in the analysis of water insoluble
heavy
metal dithiocarbamates or in the analysis of pesticide formulations in
which
copper salts (copper oxychloride) are present.
Lowen first
suggested that the analysis of manganese ethylenebisdithiocarbamate
could be
markedly improved by the addition of ethanol to the digestion mixture. Subsequently Levitsky and
Lowen used a 34%
tetrasodium ethylenediaminetetraacetate solution.
This complexing agent has also been
recommended by Pease and its use has subsequently been quite generally
accepted. Rosenthal
et al. have used 85%
phosphoric acid to disperse and digest dithiocarbamates. Ethylenethiourea was found
to interfere but
it could be removed from the sample zinc ethylenebisdithiocarbamate by
a
preliminary washing with 13 acetic acid.
Hilton used an ethanolphosphoric acid mixture. More recently Roth used a
pyridinephosphoric
acid mixture for the digestion. Although
this mixture is claimed to be generally applicable to dithiocarbamate
and
thiuram analysis collaborative studies indicate that with respect to
dithiocarbamates at least it is less reliable than the socalled Clarke
modified
by the addition of tetrasodium EDTA.
Gardner
employ
zinc sulfide to tie up copper. If
the
zinc sulfide is finely divided results obtained when phosphoric acid is
used
for decomposition are similar to those obtained in the absence of
copper. Bonnet has
attempted to remove interfering
copper by dissolving it in ammonium hydroxide.
The method is not entirely satisfactory and correction
factors must be
used. Del Re and
Fontana decompose
formulations containing copper oxychloride by digestion with anhydrous
copper
sulfate in an aprotic medium of dimethyl sulfoxide and carbon
tetrachloride. Other
methods such as
decomposition with ferricyanic acid or sulfuric acid plus potassium
ferricyanide in aqueous solution have also been suggested as a means of
overcoming the copper problem.
Xanthate
Method. This is the
most common
technique of measuring the carbon disulfide formed in the acid
decomposition of
dithiocarbamates. The
carbon disulfide
is collected in alcoholic potassium hydroxide forming the xanthate and
then
determined iodimetrically.
Hydrogen sulfide formed by acid
digestion of other
sulfurcontaining materials interferes.
To eliminate errors due to sulfide a scrubber solution of
lead acetate
or a cadmium salt is placed between the decomposition vessel and the
carbon
disulfide absorber. Hydrogen
cyanide if
formed in the hydrolysis mixture also produces high results. Carloni has recommended
the use of a silver
nitrate absorber in addition to the lead acetate scrubber to eliminate
this
interference. Sulfide ion produced by hydrolysis of the xanthate is
also a
source of error
Any
sulfide formed will consume two equivalents of iodine per mole as
opposed to
the one equivalent required by the xanthate.
Matuszak
has pointed out that for best results the methanolic potassium
hydroxide
solution must be fresh and that the xanthate solution should be cooled
to 0°C
and neutralized to the phenolphthalein end point with acetic acid
before
titrating with aqueous iodine in order to increase the permanency of
the
starchiodine titration. Cooling has an additional advantage in that it
prevents
oxidation of xanthate to dixanthogen. The latter reaction can also be
avoided
by using nitrogen or helium instead of air to sweep the carbon
disulfide
forward. Roth
titrate the methyl
xanthate solution with methanolic iodine.
The
apparatus assembly is shown in Figure 1.
It consists of a digestion flask and a reflux condenser to
the top of
which are attached one or two hydrogen sulfide scavenge traps followed
by the
methanolic potassium hydroxide scrubber for collecting carbon disulfide. Air is aspirated through
the system to
entrain the gaseous products through the scrubber train. The following generalized
procedure taken
from methods of Bontoyn is suitable for most commercial dithiocarbamate
salts
including manganese and zinc ethylenebisdithiocarbamates.
Procedure
Weigh not more than 5 g of
the sample containing 0.10.3 g of dithiocarbamate and place in a dry
reaction
flask. Assemble the
apparatus as shown
in Figure 1. Charge each lead acetate trap with about 20 ml of 10% lead
acetate
solution. Charge the alkaline scrubber with about 50 ml of 2 N
methanolic
potassium hydroxide solutions and cool with an icewater bath. Equip the
reaction flask with a 50 ml additional funnel and a small magnetic
stirring
bar. Charge approximately 10 ml of 34% tetrasodium
ethylenediaminetetraacetate
solution through the addition funnel.
(For dithiocarbamates other than manganese or zinc
ethylenebisdithiocarbamate EDTA need not be used.
With compounds such as iron (III)
dimethyldithiocarbamate having some water repellency addition of a
trace of
wetting agent is permissible). Stir the sample with the EDTA solution
for 1
min. Commence
careful aspiration of air
through the reaction flask and then add 50 ml of hot 95100ºC 4N
sulfuric acid
rapidly through the addition funnel.
Begin heating the flask to initiate boiling. As the reaction proceeds
adjust the system so
that rates of boiling and aspiration are almost in equilibrium
producing only a
very slow rate of bubbling through the methanolic potassium hydroxide
solution
(approximately 50 ml air/min). Continue
a brisk efflux for 1.5 hr (a lesser time is required for water soluble
or more
easily digested dithiocarbamates).
Disconnect the cold potassium
hydroxide absorber and quantitatively transfer the contents to a 500ml
flask. Add one or
two drops of
phenolphthalein indicator just neutralize with acetic acid added from a
buret
and then add three drops in excess.
Next
with continuous stirring titrate the solution immediately with
standardized 0.1
N iodine solution to the starch end point.
Determine the solution blank by titrating a corresponding
mixture of
methanolic potassium hydroxide solution water phenolphthalein and
acetic acid.
Spectrophotometric
Methods. Hilton has determined carbon
disulfide in dimethylamine using the thiocarbamic acid ultraviolet
maximum at
287 nm. Xanthate
salts also absorb
strongly in the ultraviolet and hence can be determined by ultraviolet
spectroscopy rather than by iodimetry.
Nevertheless
the most popular
spectrophotometric method is based on absorption of the carbon
disulfide in an
ethanolic solution of copper acetate containing triethanolamine and
diethylamine. This
solution is known as
the Viles or the DickinsonViles reagent.
This method is best suited for determining small amounts
of carbon
disulfide such as obtained from dithiocarbamate residues in food crops.
Its
application for this purpose was first published by Clarke et al. and
by
Lowen. Subsequent
refinements have been
made by Pease.
The
wavelength of maximum absorption of the copper (II)
diethyldithiocarbamate
chelate is 425 nm. Measurement
at 440 nm
435 nm 430 nm 400 nm and 380 nm has also been recommended. The 11
complex is
favored by high copper concentration which results in a high base line
due to
excess blue Cu2.
Cullen therefore recommends limiting the
copper iron concentration to promote 12 complex formation and use of
the 435 nm
maximum. He
observed that a Cu to CS2 ratio
of 0.5 was optimum but
within the limits 0.3 and 2.5 at least 85% of the maximum absorption
could
still be attained.
The
following procedure is applicable to residues of dithiocarbamate
fungicides
such as iron (III) and zinc (II) dimethyldithiocarbamate and disodium
manganese
(II) and zinc(II) ethylenebisdithiocarbamate.
It is advisable to standardize the method with untreated
crop samples to
which known amounts of fungicides have been added.
Recoveries ranging from 0.1 to 7.0 ppm have
been demonstrated. The apparatus shown in Figure 2 is similar to that
used in
the xanthate method but modified for the larger sample size required. A lead acetate trap is
used to remove
hydrogen sulfide.
Procedure
Prepare Viles
reagent by dissolving 0.05 g of copper (II) acette in 25 ml of water
and then
adding 975 ml of ethanol 1 ml of diethylamine and 20 ml of
triethanolamine. Dissolve
30 g of
neutral lead acetate Pb(OAc)2.3 H2O in water and dilute to 100 ml.
Dice the crop
sample or otherwise subdivide it into ¼ in. or smaller cubes and weigh
a
representative amount containing 20160g of dihiocarbamate. Transfer the
sample
to the digestion flask. Add 200 ml of water or for manganese or zinc
ethylenebisdithiocarbamate 200 ml of N disodium EDTA solution. Place 10
ml of
lead acetate solution in the first absorption tower and 12.5 ml of
Viles
reagent in the second tower and assemble the apparatus as shown in
Figure 2
leaving the vacuum source disconnected. Heat the contents of the
reaction flask
to just short of boiling (8590ºC). Apply gentle vacuum then cautiously
add 40
ml of boiling 10 N sulfuric acid through the dropping funnel. For
manganese or
zinc ethylenebisdithiocarbamate use 60 ml of hot acid.
Reflux the mixture for 45 min.
After digestion
disconnect the apparatus and drain the contents of the Viles reagent
trap into
a 25 ml volumetric flask. Wash
the
column with several 34 ml portion of ethanol and add the washings to
the
volumetric flask. Dilute to volume with ethanol and mix thoroughly. Determine the absorbance
at 380 nm vs a
reference solution prepared by diluting 12.5 ml of Viles reagent to 25
ml with
ethanol.
Prepare
standard solutions as follows. Dissolve
0.04 g of iron or zinc dimethyldithiocarbamate in 100 ml of chloroform
and
dilute a 5 ml aliquot to 100 ml chloroform. For disodium
ethylenebisidithiocarbamate use water as the solvent and for manganese
or zinc
ethylenebisdithiocarbamate use N tertasodium EDTA solution. Use 1.0 2.0 3.0 5.0 and
8.0ml a liquots of
the appropriate standard solution for preparation of the calibratrion
curve and
carry through the method as given for the samples. Remove chloroform
solvent by
evaporation at room temperature under a stream of nitrogen before acid
decomposition.
Gas Chromatography.
Bighi
have studied the acid decomposition of pure and commercial
dithiocarbamates by
gas chromatography. Gaseous products were swept from the digestion
flask by a
slow stream of helium passed through two drying traps containing
concentrated
sulfuric acid and hydrogen sulfide and carbon disulfide were condensed
in a
liquid air trap of special design.
Carbon disulfide was determined using the following
operating conditions
with a retention time of about 4 min.
Hydrogen sulfide plus carbon
disulfide are determined
on a column packed with 25% triceresyl phosphate on Celite C 30/60 mesh
22 at
20ºC. The retention times are about 5 min for hydrogen sulfide and 45
min for
carbon disulfide. Less
than 5g of carbon
disulfide can be determined.
Determination of Amine Acid decomposition of a
dithiocarbamate salt in addition to producing carbon disulfide
liberates the
amine portion of the molecule which remains in the aqueous phase as
nonvolatile
amine salt. As a
practical matter only
the steam volatile lower aliphatic amines are conveniently measured
quantitatively. The
aqueous residue is
made basic amines are distilled and determined by titration with acid.
Alternatively the amine may be treated with carbon disulfide and Cu
(II) ion to
produce the copper dithiocarbamate which can then be measured
colorimetrically.
Brock and Louth have identified
accelerators and
antioxidants in compounded rubber products by decomposing the
dithiocarbamate
with N hydrochloric acid making the solution alkaline and identifying
the
distilled amine as its hydrochloride by xray diffraction methods. Zijp determined amine
hydrochlorides obtained
in a similar manner by paper chromatography.
Ethylenediamine and other diamines
not sufficiently
volatile to be removed by steam distillation can be isolated by vacuum
distillation. Bighi
nd Penzo have
determined ethylenediamine in a dithiocarbamate residue by a
chlorimetric
measurement of its copper compled at 550 nm.
In
certain cases particularly with the watersoluble dithiocarbamates
decomposition
in the presence of a known excess of acid followed by back titration
with base
constitutes a simple method of assay.
When the acid decomposed residue is titrated with alkali
the
neutralization proceeds stepwise. At first the excess of strong acid is
titrated
a second step involves neutralization of the protonized amine. The last pH change
corresponds to addition of
excess base. When
dithiocarbamate salts
of weak bases are titrated an additional neutralization takes place.
Critchfield
has devised an ingenious method for determining primary and secondary
amines in
the presence of tertiary amines. In
this
method an excess of carbon disulfide is caused to react with the
primary or
secondary amine in an essentially nonaqueous medium such as isopropyl
alcohol
or pyridineisopropyl alcohol mixture.
The dithiocarbamine acid formed in the reaction is then
titrated with
sodium hydroxide solution using phenolphthalein potentiometric
indiction.
Spectroscopy.
Data
on the ultraviolet absorption of various simple
dithiocarbamate salts are summarized in Table 3.
For water soluble dithiocarbamates the
ultraviolet absorption at or about 285 nm can be measured as was done
by Bode
in his study on the stability of sodium diethyldithiocarbamate at
various pH
values. Kress
determined zinc
diethyldithiocarbmate by its absorption at 262 nm in ethyl ether
solution.
Many
of the heavy metal salts of dithiocarbamic acids absorb in the visible
region. Copper (II)
dimethyldithiocarbamate for example has an absorption band at 425 nm. Ultraviolet spectra and
simultaneous
determination of the copper nickel and cobalt salts of
diethyldithiocarbamic
acid in carbon tetrachloride have been recorded by Chilton. Morrison detected the
presence of dithiocarbamates
in rubber by a color reaction with copper chelate. A colorimetric
method for
dithiocarbamate residues which obviates degradation of the sample and
distillation has been proposed by Kerssen. The residue on glass plates
or
leaves is removed with a detergent and phosphate buffer and copper
sulfate are
added to the washings. The
copper salt
is extracted into an organic solvent and estimated colorimetrically.
The
infrared spectra of many dithiocarbamates have been investigated by
Chatt. Nevertheless
little use of infrared
spectroscopy has been made in the analysis of dithiocarbamates although
the
method has been of considerable use in determining the structure of the
various
metal coordination compounds. Fisher
has
devised infrared methods for determining iron dimethyldithiocarbamate
zinc ethylene
bisdithiocarbamate and tetramethylthiuram disulfide in mixtures of
various
nonthiocarbamate pesticides.
Liquid
Chromatography. Various chromatographic
techniques have proved useful in separating and in some cases
identifying
dithiocarbamate salts. Paper
chromatography has been most frequently employed.
Salts of the dithiocarbamic acids derived
from amino acids ie (dithiocarboxyamino) carboxylic acids have been
separated
satisfactorily by paper chromatography by Jensovsky and by Zahradnik. The most suitable
developing solvent found by
Jensovsky was 37 2 N ammonium hydroxidepropanol using Whatman No. 4
paper. The spots
were located by spraying with
ammoniacal silver nitrate solution.
Zahradnik and Kobrle used boraximpregnated Whatman No. 1
paper the
developing solvent was 70525 propanol25% ammonium hydroxide 0.05 M
sodium
tetraborate.
McKinley
and Magarvey resolved iron and zinc dimethyldithiocarbamate disodium
manganese
and zinc ethylenebisdithiocarbamate and tetramethylthuiram disulfide
into two
groups on fiber glass paper impregnated with formamide.
The fungcides derived from dimethylamine
moved readily with the solvent systems used whereas the
dithiocarbamates
derived from ethylenediamine remained relatively immobile. Chloroform petroleum ether
and a mixture of nhexane
and chloroform were used as mobile phases.
The compounds were detected by spraying with 3% aqueous
sodium azide
solution subjecting the papers to iodine vapor and immediately spraying
with 1%
soluble starch solution.
Disodium
ethylenebisdithiocarbamate and its air oxidation products
ethylenethiuram
monosulfide and ethylenethiourea have been separated by Thorn and
Ludwig on
Whatman No. 1 paper using 1203357 butanolethanolwater as the developing
solvent. Again the
spots were detected
using the
iodineazide reagent or Grotes
reagent a solution of sodium nitroprusside reduced with hydroxylamine
and
oxidized with bromine. Some
simple
dithiocarbamates and thiuram sulfides have been separated by Lu using
Whatman
No. 2 paper impregnated with 2% acrylonitrilebutadiene copolymer in 21
benzeneacetone as stationary phase.
Various organic solvents and water were used as the mobile
phase in the
ascending method and dilute acidified sulfate was used to detect the
spots. Paper
chromatographic techniques
have been employed by Zijp to identify dithiocarbamate rubber
accelerators and
antioxidants. Here
however the
dithiocarbamic acid derivatives themselves were not subjected to
chromatographic separation but rather the amine hydrochlorides obtained
by acid
decomposition of these compounds.
Manganese
and zinc ethylene bisdithiocarbamates and their degradation
productsethylenethiuram monosulfide ethylenethiourea and sulfur–have
been
separated by thinlayer chromatography on silica gel DF5 and silica gel
G
plates. Three
solvent systems were
utilized (a) immobile 5% formamide in acetone mobile chloroform (b)
1203357
butanolmethanolwater (c) immobile 5% paraffin oil in ethyl ether mobile
dimethylformamide. Iodineazide
zincon
(2carboxy2hydroxy5sulfoformazyl) dithizone and potassium
ferricyanideferric
chloride reagents were employed as detecting reagents.
Organic Acids
The microbiological production of
organic acids
represents one of the earlier areas of fermentation necessary for the
accumulation of information which made possible the large scale
production of
antibiotics and other microbial products of more recent date.
Citric
Acid
Citric acid is one of the
most important organic acids used in foods beverages and
pharmaceuticals. During
the past few years it has also become
important as an organic intermediate.
Citric Acid
Citric acid was first isolated from
lemon juice in
1784 by Scheele. In
1917 Currie of the
U.S. Department of Agriculture found citric acid could be produced
microbiologically by using Aspergillus niger grown
on a sugermineral
salts solution. Since then many other microorganisms have been shown to
produce
the citric acid however A. Niger has always given the best result in
industrial
production. Citric
acid may be fermented
either by using shallow pans or by employing a submerged or deep
fermentation
process with aeration.
Sucrose
in the form of cane or beet molasses is the principal source of sugar.
A 1220
per cent sugar solution is normally used along with mineral supplements. The duration of shallow
pan fermentation is
710 days at 2628ºC. Submerged fermentation periods are shorter but
yields are
less. On shallow
pans yields on the
sugar used may be 9095 per cent while the submerged process normally
runs 7578
per cent. The
citric acid is recovered
as the calcium salt and treated with sulfuric acid to precipitate
calcium
sulfate which is removed. Citric
acid
crystallizes upon concentrating the resulting solution.
Some oxalic acid is recovered as a byproduct
of the citric fermentation process.
Recently
it has been shown that certain strains of Candida (a
yeast) can produce
citric and isocitric acid from nparaffins or carbohydrates. The impact of this
research development may
affect the future production method.
Current
production of citric acid in the U.S. is approximately 130000000 lbs as
it
continues to be the leading food acidulant.
Gluconic
Acid
Gluconic
Acid is produced by the oxidation of the aldehyde grouping of glucose.
Gluconic
acid may be prepared from glucose by oxidation with a hypochlorite
solution by
electrolysis of a solution of sugar containing a measured amount of
bromide or
by fermentation of glucose by molds or bacteria.
The latter method is not preferred form an
economic standpoint. The
most important
microorganisms used are Aspergillus niger and Acetobacter
suboxydans grown
on a glucosesalt solution in deep tank fermentation.
Yields as high as 90 per cent on the sugar
consumed have been reported. Gluconic
acid is marketed in the form of several crystalline metal salts 50 per
cent
aqueous acid and the deltalactone.
Calcium gluconate is frequently used as a nutritional
calcium source
because of its solubility. The
sequestering properties of sodium gluconate particularly for Ca and
heavy metal
ions in strong caustic solution make it useful in cleaning operations.
Acetic
Acid
Acetic acid in the form of vinegar
(by law 5 per
cent acetic acid) is a widely used food adjunct. Vinegar is produced by
the
oxidation of ethanol by bacteria of the Acetobacter genus. In the food industry many
vinegar types are
classified on the basis of the source of alcohol.
Most vinegar is made from apple cider a yeast
converts the sugar to ethanol and the acetification is accomplished by Acetobacter
aceti strains.
Ethanol may also be converted into
acetic acid by
catalytic oxidation at high temperatures but synthetic acid cannot be
used in
foods.
Taconic
Acid
Itaconic acid is an unsaturated
dibasic acid which
may be used for the preparation of resins or surfaceactive agents or in
the manufacture
of synthetic organic chemical compounds. Its esters may be polymerized.
Itaconic
acid may be produced by either a shallowpan or a deeptank fermentation
process
by growing Aspergillus terreus or A. itaconicus
on lactose
glucose or molassessalt media.
Fermentation of solutions of 2025 per cent glucose gives
yields
equivalent to 5070 per cent based on the sugar consumed.
Kojic
Acid
Kojic acid was first
discovered in Japan in 1907 by Saito it was a byproduct of the
fermentation of
steamed rice by Aspergillus oryzae. Various other
investigators have
found that numerous species of Aspergillus and some Acetobacter
bacterial
strains produce kojic acid. In
1955 it
was first produced on a commercial scale by a fermentation process.
Kojic acid
is an acid weaker than carbonic. It
is
reactive at every position and forms a number of products.
One
of its major uses has been for the manufacture of maltol and ethyl
maltol
widely used in foods as flavorenhancing agents.
Chemically the CH2OH group is oxidized to COOH
(comenic acid) which is removed by pyrolysis (pyromeconic acid). The 14pyronenucleus is
reactive at the
5position with formaldehyde or acetaldehyde and the reduction of the
respective
aldehydes give maltol or ethyl maltol.
OTHER
KETOGENIC FERMENTATIONS
Sorbose
Sorbose
fermentation the first bacterial ketogenic fermentation discovered is
one of
the simplest. LSorbose
is produced from
the polyhydric alcohol sorbitol by the action of several species of
bacteria of
the genus Acetobacter. Sorbitol is made by the
catalytic hydrogenation
of glucose. The
most commonly used
microorganism is Acetobacter suboxydans.
Since this organism is very sensitive to nickel
ions it is important
that the medium and fermentor be free of nickel.
The medium normally consists of 100200 grams
per liter sorbitol 2.5 grams per liter corn steep liquor and antifoam
such as
soybean oil. The medium is sterilized and cooled to 3035ºC where about
2.5 per
cent inoculum is added. The
tank is aerated
and sometimes stirred. Yields
of 8090
per cent of the sugar used are commonly obtained in 2030 hours.
The only commercial use of
Lsorbose is in the manufacture of ascorbic acid (vitamin C). The chemical steps in the
conversion of
sorbose to ascorbic acid involve the preparation of the diacetone
derivative
which is then oxidized the acetone groups are removed and the resultant
2ketoLgluconic acid is isomerized to the enediol with ring closure.
This
concept of using combined microbiological and chemical conversions has
recently
been applied with commercial success to the preparation of new steroid
drugs.
2Ketogluconic
Acid
2Ketogluconic
acid may be produced by a bacterial fermentation involving various
strains of Acetobacter
and Pseudomonas. Selcted strains of Pesudomonas
Fluorescens have
been reported as giving the highest
yields (up to 70 per cent) when glucose or gluconate is used in the
medium is
highly aerated processes. DGluconic
acid
is an intermediate in the oxidation of glucose to 2ketogluconic acid. 2ketogluconic acid is
structurally related to
both gluconic acid and glucosone and may be derived from both by
oxidation. The
2ketogluconic acid is
recovered as the calcium salt. The
principle use of 2ketogluconic acid is as an intermediate in the
preparation of
Darabo greater than four or five units fails to enhance activity. An extensive treatement of
anionic
surfactants based upon nonionic materials can be found elsewhere.
Taurate Surfactants. In 1930 both H.T. Bohme A.G.
and I.G. Farbenindustrie recognized that the weakness of soap was
centered at
the carboxyl linkage the former chose the alkyl sulfate route the
latter esters
of fatty acids. Subsequently
because the
fatty esters were too unstable for many purposes fatty amides which
were taurine
derivatives were developed. Igepon
T
(oleyl methyl taurate) was one of the first widely used surfactants
which still
have many applications.
It
is of considerable interest that in the patent covering compounds of
this type
in which R1 R2 and
R3 are
branched or straight chain aliphatic cycloaliphatic
or aromatic hydrocarbon groups or heterocyclic rings and in which R3 may
be a sulfornic or sulfuric ester (these
groups may have many forms) that over 1620000 variations are possible. Obviously only relatively
few of this number
have been synthesized and of these fewer still have been merchandized.
Nonionic
Surfactants
As the name indicates these
products are not ionic
in nature and in contrast to sulfates sulfonates or phosphates their
solubility
generally depends on hydrogen bonding through a multiplicity of oxygen
groups
in the molecule. The
most widely
manufactured products are ethylene oxide adducts although mixed
ethylene and
propylene or butylenes oxide compounds are or can be produced.
Ethylene Oxide Adducts. One main requirement of the
nonionic hydrophobe used for ethylene oxide addition is that it
contains reactive
hydrogen the most important hydrophobes are given in Table 1. As discussed under
hydrophilhydrophobe
balance these adduct are not single compounds but represent mixtures
approximating
Poisson distribution. Production
of
ethylene oxide adducts is not difficult but because of corrosion and
explosion
hazards the equipment is necessarily expensive and handling and storing
raw
materials and products requires considerable capital.
Operating costs are largely dependent on the
volume produced short chain ethylene oxide adducts can be produced in
much
shorter cycles than the more generally used longer chain products.
Shick has edited a very thorough
volume concerning
nonionic surfactants. Tall oil adducts probably comprised the largest
single
group of nonionics largely because of their relatively low cost. However they have been
replaced by other
nonionics for controlled lowsudsing detergents.
The currently most used adducts are those derived from
primary or
secondary alcohols and from alkylphenols.
Use of the fatty alcohol adducts as sulfates is discussed
under Anionic
Surfactants.
Polymeric
Nonionics. Optimum polypropylene glycol
molecular weights appear to lie between 800 and 2500 g. It is obvious
that a
large number of compounds varying marketdly in characteristics can be
produced
many of these have already been investigated.
If instead of polypropylene glycol the central portion of
the molecule
becomes ethylenediamine block polymers having four hydrophilic tails
can be
made by reaction with ethylene oxide.
Where the molecular weights of the
polyoxiypropylenepolyoxyethylene
compounds can approximate 10000 molecular weights of the
ethylenediamine
products can approach 27000.
New
11 types have been developed having over 90 per cent crude amide
content
achieved by an ester interchange of 1 mole DEA with 1 mole of fatty
acid methyl
ester under special synthesis conditions.
The importance of these compounds lies in their detergent
and foaming
ability and in the fact that they act as foam boosters and stabilizers
for
dodecylbenzene sulfonates. In
addition
they are compatible with both anoinic and cationic surfactants are
emollients
can affect the viscosity of liquid detergents and are corrosion
inhibitors.
Sugar
Surfactants. Sugar is a desirably priced
raw material on which a process of surfactant preparation has been
based. The product
is a sucrose fatty acid monoester
(the monosterate for example) the eleven oxygen atoms of sucrose
contributing
about the same hydrophilic effect as a polyoxyethylene with the same
number of
oxygen atoms.
The
process is typified by the following run
Three
moles of sucrose one mole of methyl stearate and 0.1 mole of potassium
carbonate (catalyst) are dissolved in dimethylformamide (or dimethyl
sulfoxide). Potassium
carbonate is a
preferred catalyst because unlike a more alkaline catalyst (e.g. sodium
methoxide)
it will not take part in undesirable side reactions at high
temperatures.
The
reaction mixture is agitated heated at 9095ºC at 80100 mm of mercury
for 912
hours. The methyl
stearate reacts with
the sucrose to give a sucrose monostearate and methanol. The later is
stripped
off.
After
the solvent is distilled off and the product dried it contains about 45
per
cent monostearate 12 per cent potassium carbonate and about 54 per cent
unconverted sugar (because of the large excess used).
The product can be used for many jobs as
is. More likely
however the economics of
the situation will dictate that the sugar be recovered by further
purification
of the product.
The
sugar can be removed by adding toluene as a solvent.
Conversions
of over 90 per cent are claimed. An
excess of sucrose produces the monoester best for detergent products
while an
excess of nonsugar ester yields the diester product which is superior
for food
applications. Raw
material costs are a
controlling factor due to variation in world supplies.
Purification is also difficult.
The flow sheet is shown in Fig. 2.
In
the equipment shown in Figure 3 the reactants the solvent and the
catalyst are
placed in the reaction vessel (5) in which the conversion from nonsugar
ester
to sucrose monoester is carried out.
The
product alcohol and part of the solvent are striped from the system in
a
turbulentfilm evaporator (1). The product alcohol and solvent are
fractionated
in the packed reflux tower (2). The
solvent is returned to the system through (4) while the alcohol is
condensed in
(3) and collected in vessels (7).
Recovery
of the sucrose ester from the slurry containing unreacted sugar can be
accomplished using xylene as the ester solvent since filtration is
unsuccessful
due to the sugar particle size. The
xylene is then recovered by steam distillation and the sugar ester
remains. Important
to economic operation
is minimization of diester formation (which is effected by controlling
the
water content during alcoholysis) recycling of unreacted sugar and
recovery of
DMF and xylene. Several
licenses have
been granted for this process.
Sorbitol
Compounds. Sorbital may be produced by
the hydrogenation of sugars such as glucose.
Then this hexahydric alcohol may be reacted with ethylene
glycol and the
reaction product esterified to varying degrees with lipohilic fatty
acids. Or sorbitol
may be partially esterified with
fatty acids and then these innerester sorbitol anhydrides may be
further
reacted with ethylene glycol.
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