Rubber is the most fascinating material known to mankind both on account of its range of applications in every day life, defence and civilian purposes and its behaviour under the most diverse conditions of applications. For rubber processing and compounding certain chemicals are required. 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. The book covers processes of different rubber chemicals like waxes, synthetic organic chemicals, amines, silicones resins, releasing agents, stabilizers, solvents and many more. Having in view of importance of chemicals in rubber industry, we have brought out this book which will help a lot 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
waxes mineral waxes such as montan and ozokerite; and synthetic waxes
including polyethylene and Fischer-Tropsch wax made from non-wax 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 dewaxing 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 n-alkanes
that are their principal components. However, paraffin waxes do undergo
a number of chemical reactions, including formation of adducts,
cracking reactions, and free-radical substitution reactions. The n-alkane
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 Linde’s type 5A, Linde
Molecular Sieves, Union Carbide Corp. The thermal cracking of paraffin
wax is a commercial process for making a-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,
m-toluidine, 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, N-ethyl-N-methylpropylamine.
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,
2-butanediamine, CH3CH2CH(NH2) CH2NH2. Althought 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 hydrogens
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 clearcut. 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 (C1—C3) resemble that of ammonia; they
become fishy in the C4—C7 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 (C1—C5) 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 C1—C4 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 1-4.
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
so-called meta-directing substituents on the aryl group decrease the
basicity, whereas ortho-para-directing 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. Tertiary amines react with alkyl halides
to form compounds similar to ammonium salts but processing four alkyl
groups attached to nitrogen, thus called quaternary ammonium
salts, R4NÃ… XÃ… . Besides forming salts with
protonic acids, amines interact with Lewis acids to provide acid-base
complexes. An example is the BF3.H2NC2H5 complex that is used as a latent
catalyst for epoxy resins. 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 N—H 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, 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.â€
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
800-1000°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 by-product 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 250-350
atmospheres, and temperatures in the range of 300-400°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 hexamethylenetetra-mine, the latter being formed by condensation
with ammonia.
Oxo Chemicals. The oxo chemicals are
compounds-primarily C4 and higher alcohols-made by the
so-called “oxoâ€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 conditionsis 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 n-butanol,
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 dry-cleaning 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
fluorohydro-carbons.
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 kw-h 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 coke-oven gas, from sodium and calcium cyanides, and
by the decomposition of formamide.
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 by-product 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
acid-proof 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
120-300 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
ethanolamines, 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 alkyl-phenol-ethylene oxide derivatives,
(2) fatty acid-alkanolamine condensates, (3) tall oil-ethylene 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 low-sudsing
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 acid-gas (H2S, CO2) removal, It is to a lesser
extent, a chemical intermediate for compounds such as ethylene imine.
Diethanolamine’s major end use is in detergents, but it is also
utilized in textiles and as a gas-purification 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 oraganic 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 di-ethylene
glycol, triethylene glycol, etc, as by-products.
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 Dowanolâ,
Cellosolveâ, and Polysolveâ, 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 crosslinked
network, produced by hydrolysis of appropriated mixtures of primarily
di- and trichlorosilanes (D-T resins); and those presumably having a
knot-and-chain structure, produced by combining a co-hydrolyzate of
mono- and tetrachlorosilanes with a hydrolyzate of dichlorosilanes
(M-Q-D 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, mono-phenyl, 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 produces 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 crosslinked
bifilar molecule, or ladder polymer, results. These ladder polymers are
thermosplastics 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 batchwise 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 M-Q-D resins is
generally similar to that outlined above for D-T 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 crosslinking 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 sheetlike
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 crosslinking 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 10,000lb/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
M-Q-D polymers are outstanding in their ability to adhere to almost any
solid, including polymers with poor wettability such as
polytetrafluoroethylene. 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 non-flammability, 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 canbe used: anionic, cationic, and nonionic. In
most cases, a water-in-oil 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 10-70% silicone, but are usually diluted
to 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 de-emulsification. 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,
flow-stable 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 nonload-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, fluoroalkyl, 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
slicone-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
alkoxy-terminated branched silicone, containing an average of one
trifunctional siloxy group and three alkoxy groups per molecule. This
is combined with a hydroxy-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 finisthed 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 M-Q-D 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 R‘Si(OR)3. Carbon-functional groups, which
have been successfully utilized, include vinyl, aminoalkyl,
acrylatoalkyl, glycidoxyalkyl, and variations of these. The alkoxy
substituents are generally methoxy, ethoxy, or b-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, eg.
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 by-product formation are effective since the
presence of residual by-products (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 1-3%, 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 crosslinking of unsaturated linear polyester
resins, such as those used to impregnate glass fibers, has resulted in
glass-fiber-reinforced plastics of greatly improved light stability, as
compared to the similar system compounded with styrene monomer used as
the sole crosslinking 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 an “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 “stir-in†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 polycomponent 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, ie, 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(4-methyl-1-pentene), 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 p-phenylenediamine, 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.
Ultraviolet-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 2900-3900A, with sharp cut-off
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 aluded 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 1,1,2-tris (2-methyl-4-hydroxy-5-text-butylphenyl)butane,
which refers to a condensate of 3-methyl-6-tert-butylphenol
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 polytetrafluoroethylene 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
freeradical 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 an a,b-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 an
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 re-used.
After use in some phases of the explosives, petroleum, and
dye industry it is often recovered in a form unsuitable for re-use 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 superphosphate (a mixture of monocalcium
phosphate and calcium sulfate) or crude (“wet processâ€) 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.â€
In 1827 Gay-Lussac, and in 1859 Glover, changed the
circulation of gases in the plant by adding towers which are now known
as Gay-Lussac 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 pyrite-burning contact plant
using the Herreshoff 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 1940’s 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.â€
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
over-all effect is that the sulfur dioxide is oxidized to sulfur
trioxide, which combines with water vapor to form sulfuric acid (2SO2 +
O2 + 2H2O ® 2H2SO4). 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 Gay-Lussac tower as nitrosylsulfuric acid. The solution of
nitrosylsulfuric acid (nitrose) from the Gay-Lussac tower is pumped to
the denitration (Glover) tower where heat releases the nitrogen oxides
for re-use in the cycle. In
the Glover tower the denitrated sulfuric acid is concentrated to 60°Be. Part of this acid is
returned to the Gay-Lussac 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 Gay-Lussac towers. Here
the nitrogen oxides in the gas are recovered by absorption in a
counter-current 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 Leonard-Monsanto,
or a Wellman-Lord plant. In
Europe and other parts of the world Lurgi Gesellschaft fur Chemie and
Huttenwesen mbH, Chemiebau Dr. A. Zieren GmbH & Co. KG, and
Simon-Carves 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, .
Dithiocarba-mates 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
physico-chemical 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 ethylene-diaminetetraacetate
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 1:3 acetic
acid. Hilton used
an ethanol-phosphoric acid mixture.
More recently, Roth used a pyridine-phosphoric 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 so-called
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 sulfur-containing 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 starch-iodine 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 0.1-0.3 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 ice-water bath. Equip the reaction flask with a 50 ml
additional funnel and a small magnetic stirring bar. Charge
approximately 10 ml of 34% tetrasodium ethylene-diaminetetraacetate
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,
95-100ºC, 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 500-ml
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 have 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
Dickinson-Viles’ 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. Cullen investigated this matter using the
copper(II) dithiocarbamate complex of diethanolamine instead of
diethylamine. Two
copper complexes are formed, namely the 1:1 chelate R2NCS2Cu+ and the 1:2 chelate (R2NCSS)2Cu.
The former has a maximum at 380 nm while that of the
latter is at 435 nm. The
amount of each form is controlled by the molar ratio of Cu2+ to CS2. The
1:1 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 1:2 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 20-160g
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 (85-90ºC). 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 3-4 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 8.0-ml 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 x-ray diffraction methods.
Zijp determined amine hydrochlorides obtained in a similar
manner by paper chromatography.
Ethylenediamine and other diamines
not sufficiently volatile to be remived by stewam 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 water-soluble 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 pyridine-isopropyl 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)
dimethyldithio-carbamate, 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 have devised
infrared methods for determining iron dimethyldithiocarbamate, zinc
ethylene bisdithiocarbamate, and tetramethylthiuram disulfide in
mixtures of various non-thiocarbamate 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 a-amino
acids, ie, a-(dithiocarboxyamino)carboxylic acids, have been separated
satisfactorily by paper chromatography by Jensovsky and by Zahradnik. The most suitable
developing solvent found by Jensovsky was 3:7 2 N ammonium
hydroxide-propanol, using Whatman No. 4 paper.
The spots were located by spraying with ammoniacal silver
nitrate solution. Zahradnik
and Kobrle used borax-impregnated Whatman No. 1 paper: the developing
solvent was 70:5:25 propanol-25% 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 n-hexane 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 120:33:57
butanol-ethanol-water as the developing solvent.
Again the spots were detected using
the iodine-azide reagent or Grote’s 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% acrylonitrile-butadiene
copolymer in 2:1 benzene-acetone 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
products-ethylenethiuram mono-sulfide, ethylenethiourea, and
sulfur–have been separated by thin-layer chromatography on silica gel
DF-5 and silica gel G plates. Three
solvent systems were utilized: (a) immobile: 5% formamide in acetone;
mobile: chloroform; (b) 120:33:57 butanol-methanol-water; (c) immobile:
5% paraffin oil in ethyl ether; mobile: dimethylformamide. Iodine-azide, zincon
(2-carboxy-2’-hydroxy-5’-sulfoformazyl), dithizone, and potassium
ferricyanide-ferric 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 suger-mineral 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 12-20 per cent sugar solution is normally used along with mineral
supplements. The
duration of shallow pan fermentation is 7-10 days at 26-28ºC. Submerged
fermentation periods are shorter but yields are less.
On shallow pans, yields on the sugar used may be 90-95 per
cent, while the submerged process normally runs 75-78 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 by-product of the citric fermentation
process.
Recently,
it has been shown that certain strains of Candida (a
yeast) can produce citric and isocitric acid from n-paraffins 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 130,000,000 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 glucose-salt 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
surface-active agents, or in the manufacture of synthetic organic
chemical compounds. Its esters may be polymerized.
Itaconic
acid may be produced by either a shallow-pan or a deep-tank
fermentation process by growing Aspergillus terreus
or A. itaconicus on lactose, glucose-, or
molasses-salt media. Fermentation
of solutions of 20-25 per cent glucose gives yields equivalent to 50-70
per cent based on the sugar consumed.
Kojic
Acid
Kojic acid was first discovered in
Japan in 1907 by Saito; it was a by-product 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 flavor-enhancing agents. Chemically the –CH2OH group is oxidized to –COOH (comenic acid) which is
removed by pyrolysis (pyromeconic acid).
The 1,4-pyronenucleus is reactive at the 5-position 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. L-Sorbose
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 100-200 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 30-35ºC, where about 2.5 per cent inoculum is
added. The tank is
aerated and sometimes stirred. Yields
of 80-90 per cent of the sugar used are commonly obatained in 20-30
hours.
The only commercial use of
L-sorbose 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 2-keto-L-gluconic 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.
2-Ketogluconic
Acid
2-Ketogluconic
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.
D-Gluconic acid is an intermediate in the oxidation of
glucose to 2-ketogluconic acid. 2-ketogluconic
acid is structurally related to both gluconic acid and glucosone, and
may be derived from both by oxidation.
The 2-ketogluconic acid is recovered as the calcium salt. The principle use of
2-ketogluconic acid is as an intermediate in the preparation of D-arabo
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 has 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 RÂ3 may be a sulfornic or sulfuric
ester (these groups may have many forms), that over 1,620,000
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 contain
a reactive hydrogen; the most important hydrophobes are given in Table
1. As discussed
under hydrophil-hydrophobe balance, these adducts 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 low-sudsing
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. Alkylene oxides such as ethylene,
propylene, isobutylene, or 1,2-epoxybutane can be polymerized alone to
form homopolymers, or alternatively, they may be copolymerized as
“block-type†products: An–A–B–Bm, where A represents propylene
oxide and B ethylene oxide, the polyoxypropylene thus becoming the
hydrophobe which is solubilized by addition of ethylene oxide. 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
polyoxiypropylene-polyoxyethylene compounds can approximate 10,000,
molecular weights of the ethylenediamine products can approach 27,000.
New
1:1 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 dimethyl-formamide (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 90-95ºC at 80-100 mm of mercury
for 9-12 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, 1-2 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 turbulent-film
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 inner-ester
sorbitol anhydrides may be further reacted with ethylene glycol.
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