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151
in the particle size, the sign of the charge on the particle, the pH of the
ROH +
CH
CH
KOR
130-180
0
C
CH
2
CH
OR
+
128kJ/mole
CH
3
+
ROH
+
CH
CH
CH
2
CH
OR
2
Acidic
Mercuric
Catalyst
OR
OR
200-300
0
C
Catalyst
ROH
aqueous phase and in other details.
Being an amorphous polymer with a solubility parameter of 19.4
1/2
MPa
, it dissolves in solvents with similar solubility parameters (e.g.
benzene δ=18.8 MPa
1/2
MPa
.
1/2
, chloroform δ=19.0 MPa
1/2
, and acetone δ=20.4
2.5.2 Poly(vinyl ethers)
It is not possible to polymerize vinyl et her s by fr ee -radical-initiated
methods but, as with isobutylene polymers, it is possible to make polymers
using Friedel-Crafts type catalysts.
The poly(vinyl ethers), which were first made available in
Germany before 1940, are not of importance in the plastics industry but
have applications in adhesives, surface coatings and rubber technology. Of
the many vinyl ether polymers prepared, only those from the vinyl alkyl
ethers and some halogenated variants are of interest. Two methods of
monomer preparations may be used.
(1) The direct vinylation of alcohols by acetylene diluted with
nitrogen or methane (Reppe method):
High pressure autoclaves may be used fitted with remote control
behind safety barricades, which are necessary because of the danger of
explosions. In a typical process the autoclave is half-filled with alcohol
containing 15% potassium hydroxide or potassium alcoholate. The free
space is then thoroughly purged with oxygen-free nitrogen and the
temperature raised to 140°C. Acetylene and nitrogen are run in under
pressures of about 0.69 MPa. Conversions are usually taken to 70-80%.
(2) Preparation via acetals (Carbide and Chemicals Corporation).
A typical catalyst for the final stage would be 10% palladium
deposited on finely divided asbestos. The vinyl alkyl ethers polymerize
violently in the presence of small quantities of inorganic acids.
The following details for the commercial manufacture of poly(vinyl
methyl ether) have been made available. Agitated vinyl methyl ether at 5°C
152
is treated over a period of 30 minutes with 0.2% of catalyst solution
consistin g o f 3% B F3·2H2O in dioxane. When the reaction rises to 12°C the
reaction is moderated by brine cooling. Over the next 3-4 hours further
monomer and catalyst is added. The autoclave is then closed and the
temperature allowed to rise slowly to 100°C.
The end of the reaction is indicated by the pressure and temperature
observations. The total reaction time is of the order of 16-18 hours.
The polymer is a water-soluble viscous liquid which has found
application in the adhesive and rubber industries. One particular use has
been a heat sensitizer used in the manufacture of rubber latex dipped goods.
A number of higher poly(vinyl ether)s, in particular the ethyl and
butyl polymers, have found use as adhesives. When antioxidants are
incorporated, pressure-sensitive adhesive tapes from poly(vinyl ethyl ether)
are said to have twice the shelf life of similar tapes from natural rubber.
Copolymers of vinyl isobutyl ether with methyl acrylate and ethyl acrylate
(Acronal series) and with vinyl chloride have been commer cially marketed.
The first two products have been used as adhesives and impregnating
agents for textile, paper and leather whilst the latter (Vinoflex MP 400) has
found use in surface coatings.
2.6 Polymers Based on Derivatives of Ethylene with Complex Substituents
2.6.1 Coumarone-Indene Resins
Fractionation of coal tar naphtha (b.p. 150-200°C) yields a portion
boiling at 168-172°C consisting mainly of coumarone (benzofuran) and
indene (Figure 21) bear a strong formal resemblance to styrene and may be
polymerized. For commercial purposes the monomers are not separated but
are polymerized in situ in the crude naphtha, sulphuric acid acting as an
ionic catalyst to give polymers with a degree of polymerization of 20-25.
Coumarone
O
b.p. 168 - 172
CH
CH
0
C
Indene
CH
b.p. 182
Styrene
CH
CH
2
0
C
b.p.143
CH
CH
2
0
C
Figure 21
In one process the naphtha fraction boiling between 160 and 180°C
is washed with caustic soda to remove the acids and then with suilphuric
153
acid to remove basic constituents such as pyridine and quinoline. The
CH
2
CH
2
CH
CH C
CH
CH
2
n
CH
2
CH
naphtha is then frozen to remove naphthalene, and agitated with sulphuric
acid, then with caustic soda and finally with water. Concentrated sulphuric
acid is then run into the purified naphtha at a temperature below 0°C. The
reaction is stopped by addition of water after 5-10 minutes, any sediment is
removed, and the solution is neutralized and then washed with water.
Residual naphtha is distilled off under vacuum, leaving behind the resin,
which is run into trays for cooling.
By varying the coumarone/indene ratio and also the polymerization
conditions it is possible to obtain a range of products varying from hard and
brittle to soft and sticky resins.
Being either brittle or soft, these resins do not have the properties
for moulding or extrusion compounds. These are, however, a number of
properties which lead to these resins being used in large quantities. The
resins are chemically inert and have good electrical insulation properties.
They are compatible with a wide range of other plastics, rubbers, waxes,
drying oils and bitumens and are soluble in hydrocarbons, ketones and
esters.
The resins tend to be dark in colour and it has been suggested that
this is due to a ful venation process involving the unsaturated end group of a
polymer molecule. Hydrogenation of the polymer molecule, thus
eliminating unsaturation, helps to reduce discolouration.
Because of their wide compatibility and solubility, coumarone
resins are used considerably in the paint and varnish industry. The resins
also find application as softeners for plastics and rubbers such as PVC,
bitumens and natural rubber.
Soon after World War II the hard thermoplastic floor tile was
developed. These tiles use coumarone resins as a binder for the other
ingredients, which may contain fibrous fillers such as asbestos, inert fillers
such as china clay and softeners such as paraffin wax.
The initial mixing of these compounds is carried out in an internal
mixer; the resin melts and forms a hot dough on admi xture with the fillers.
The dough is then pigmented and banded out on a hot mill. Marbling
154
effects are produced by adding chips of another colour to the mill nip. The
+
NH
CH
CH
N
CH
CH
2
rough sheet is then cut off and calendered and the product cut into tiles.
These tiles may easily be cut when warmed, thus making laying a simple
operation. Because of the low cost of the raw materials and the relatively
simple method of manufacture, coumarone tiles were cheaper than the vinyl
tile based on vinyl chloride-vinyl acetate copolymers and have been
extensively used for both industrial and domestic flooring.
2.6.2 Poly(vinyl Carbazole)
Early in World War II there was a shortage of mica in Germany
and in the United States. A need therefore arose for a mat erial with good
electrical insulation characteristics coupled with good heat resistance. In an
attempt to meet this need poly(vinyl carbazole) was produced in both
Germany (Luvican-IG Farben) and the United States (Polectron-General
Aniline & Film Corporation). In addition to the homopolymer (Luvican
M.150) the IG Farben complex also produced styrene copolymers
(Luvicans M.125 and M.100–the numerical term corresponding to the value
of the Martens Softening point) and at one time production of vinyl
carbazole polymers reached a level of five tons a month. Because of its
brittleness and its tendency to cause an eczema-type of rash on people
handling the material, production of these polymers became very small.
However, the discovery that exposure to light could increase
substantially the electrical conductivity of this polymer, i.e. it is said to be
photoconductive, has led to important new applications, particularly in
xerography.
Vinyl carbazole is obtained by reacting carbazole, readily available
as a byproduct of coal tar distillation, with acetylene in the presence of a
catalyst and solvent un der p ress ure :
Typically, the reaction would be carried out at 140°C in white spirit
with potassium carbazole as a catalyst. Davidge has reported problems in
polymerization of N-vinyl carbazole prepared from carbazole obtained
from coal tar, attributing this to the presence of sulphur. To overcome these
problems carbazole has been prepared synthetically by reactions of
cyclohexanone with phenylhydrazine to give tetrahydrocarbazole, which is
then dehydrogenated with Raney nickel. N-Vinyl carbazole is a solid with a
155
melting point of 64-67°C.
Properties
Value
Density, g/cm3
Moisture absorption, %
1.19
<0.1
High molecular weight polymers are produced by an adiabatic bulk
polymerization process using di-tert-butyl peroxide (0.02%) and 2,2'-azobisdi-isobutyronitrile (0.01%) as initiators and pressurised with N2. Heating
to 80-90°C causes an onset of polymerization and a rapid increase in
temperature. After the maximum temperature has been reached the mass is
allowed to cool under pressure.
The polymerization in situ of monomer impregnated into rolled and
stacked condensers was at one time of commercia l import ance .
The most important properties of poly (vinyl carbazole) are:
(1) Its good photoconductivity.
(2) A high softening point.
(3) Excellent electrical insulating properties.
(4) An exceptionally high refractive index (n
D
= 1.696).
20
(5) A brittleness associated with a tendency to crystallise and
fibrillate during mechanical stressing.
Some numerical values of significant properties are given in Table
7. The polymers are, however, more brittle than polystyrene and not
suitable for applications which are to be subject to mechanical shock.
Table 7 – Some properties of poly(N-vinyl carbazole)
Tensile strength, MPa
Elongation at break, %
Vicat softening point, ºC
10
0,5
~195
Poly(vinyl carbazole) is insoluble in alcohols, esters, ethers,
ketones, carbon tetrachloride, aliphatic hydrocarbons and castor oil. It is
swollen or dissolved by such agents as aromatic and chlorinated
hydrocarbons and tetrahydrofuran.
The polymer is not easy to process and in injection moulding melt
temperatures of 300°C are employed. In order to prevent excess
embrittlement by shock cooling of the melt, mould temperatures as high as
150°C may be used. The polymer may also be compression moulded at
temperatures of 250-260°C.
The main application today for poly(vinyl carbazole) arises out of
its photoconductivity and is in electrostatic dry copying machines. The
156
polymer is applied from solution in thin film (10-15 µm) layers onto a
conductive substrate.
In order to obtain the desired photoconductive characteristics,
toughness and adherence to the substrate it is usual to incorporate additives
such as electron acceptors, plasticizers and primers. A typical electron
acceptor is 2,4,7-trinitro-fluoronone, plasticizers include benzyltetraline
and phenanthrene whilst as primers styrene-butadiene block copolymers
(30-35% styrene) and styrene-maleic anhydride copolymers (5-30% maleic
anhydride) are of use.
When an electrostatic charge is applied to a coating in the dark it is
observed to discharge to an equilibrium value. When the light source is
switched on, the conductivity is increased and discharging occurs, leading
to a negligible charge. In x erography this phenomenon is used as a means
of forming a latent electrostatic image which is then developed by a dry
method by transferring the charge onto a powder known as the toner.
The polymer may be regarded in these applications as a form of
photoresistor and is now finding other applications in this area. It has been
used in holography and in the manufacture of printing plates whilst it has
also been suggested for use in solar cells and for measuring photoelectric
resistance.
Earlier applications as a capacitor dielectric and other electrical
applications such as switch parts, cable connectors and co-axial cable
spacers are now very limited.
2.6.3 Poly(vinyl Pyrrolidone)
Poly(vinyl pyrrolidone) (PVP) was introduced by the Germans in
World War II as a blood plasma substitute. A water-soluble polymer, its
main value is due to its ability to form loose addition compounds with
many substances.
The monomer is prepared from acetylene, formaldehyde and
ammonia via but-2-yne-l,4-diol, butane-1,4-diol, γ-butyrolactone and γpyrrolidone:
157
CH
CH
+ 2HCHO
HO
CH
2
C
C
CH
2
OH
But-2-yne-1,4-diol
2H
2
HO
(CH
2)4
OH
Butane-1,4-diol
-2H
2
CH
2
CH
2
CH
2
C
O
O
ButyrolactOne
NH
3
O
C
CH
2
CH
2
CH
2
N
H
α-Pyrrolidone
CH
CH
N
CH
2
CH
2
CH
2
C
O
CH
CH
2
N-VinylPyrrolidone
Polymerization is carried out in aqueous solution to produce a
solution containing 30% polymer. The material may be marketed in this
form or spray dried to give a fine powder. Polymers may be produced with
molecular weights in the range 10000-100000 (K values 20-100) of which
products with а К value of 30-35 are the most important.
In addition to its water solubility poly(vinyl pyrrolidone) is soluble
in a very wide range of materials, including aliphatic halogenated
hydrocarbons (methylene dichloride, chloroform), many monohydric and
polyhydric alcohols (methanol, ethanol, ethylene glycol), some ketones
(acetyl acetone) and lactones (α-butyrolactone), lower aliphatic acids
(glacial acetic acid) and the nitro-paraffins. The polymer is also compatible
with a wide range of other synthetic polymers, with gums and with
plasticizers.
PVP has found several applications in the textile industry because
of its affinity for dyestuffs. Uses include dye stripping, removal of
identification tints, in the formulation of sizes and finishes and to assist in
dye-levelling operations. In the field of cosmetics PVP is used because of
its unique property of forming loose addition compounds with skin and
hair. Hair lacquers may be formulated based on 4-6% PVP in ethyl alcohol,
whilst wave sets use about 1-2% of polymer. The polymer is also said to
reduce the sting of after-shave lotion and is used in hand cream, lotions and
liquid make-up. On the continent of Europe PVP is still used as a blood
plasma substitute, the original application, and is stockpiled for emergency
use in the United States. It is not used for this purpose in Britain. Because
of its complexing action it finds miscellaneous uses in the pharmaceutical,
brewing, soap and paper industries.
Copolymers of vinyl pyrrolidone with vinyl acetate, styrene and
ethyl acrylate have been marketed by the General Aniline and Film
Corporation.
158
2.7 Polyethers
From the time that formaldehyde was first isolated by Butlerov in
1859 polymeric forms have been encountered by those handling the
material. Nevertheless it is only since the late 1950s that polymers have
been available with the requisite stability and toughness to make them
useful plastics. In this period these materials (referred to by the
manufacturers as acetal resins or polyacetals) have achieved rapid
acceptance as engineering materials competitive not only with the nylons
but also with metals and ceramics.
The first commercially available acetal resin was marketed by Du
Pont in 1959 under the trade name Delrin after the equivalent of ten mi llion
pounds had been spent in research or polymers of formaldehyde. The Du
Pont monopoly was unusually short lived as Celcon, as acetal copolymer
produced by the Celanese Corporation, became available in small quantities
in 1960. This material became commercially available in 1962 and later in
the same year Farbwerke Hoechst combined with Celanese to produce
similar products in Germany (Hostaform). In 1963 Celanese also combined
with the Dainippon Celluloid Company of Osaka, Japan and Imperial
Chemical Industries to produce acetal copolymers in Japan and Britain
respectively under the trade names Duracon and Alkon (later changed to
Kematal). In the early 1970s Ultraform GmbH (a joint venture of BASF
and Degussa) introduced a copolymer under the name Ultraform and the
Japanese company Asahi Chemical a homopolymer under the name Tenal.
By the late 1990s the main manufacturers were the American-based
Du Pont, the Japanese-based Polyplastics and the European-based Ticona
with similar plant capacities totally some 60% of the global capacity which
is of the order of 600000 t.p.a. Among at least eight plants in Asia those of
Mitsubishi Gas and Asahi were significant as was also that of BASF.
As with other so-called engineering thermoplastics, the polyacetals
are available modified with glass fibre, and may contain fire retardants, and
some grades are blended wi th PTFE. In 1982 Hoechst introduced blends of
polyacetals and polyurethanes to give materials of improved toughness yet
retaining most of the key f eatures of polyacetals, and they have since been
followed by other suppliers.
Besides being commercially referred to as polyacetal materials
poly-formaldehydes are also often known as polyoxymethylenes and are
the simplest type of a family of aliphatic polyethers.
159
Of the many other aliphatic polyethers, polythioethers and
CO +
2H
2
300-400
0
C
Catalysic
200atm
CH
3
OH
polysulphides prepared in the laboratory, some have become commercially
available.
Preparation of formaldehyde
Formaldehyde is an important chemical in the plastics industry,
being a vital intermediate in the manufacture of phenolic and amino resins.
It was also used by Reppe during World War II as an important starting
point for the preparation of a wide range of organic chemicals.
Consumption of formaldehyde in acetal resins is still a minor outlet for the
material but exceptionally pure material is required for this purpose.
The most important route for the production of formaldehyde is
from methanol, this normally being prepared by interaction of carbon
monoxide and hydrogen.
The two gases involved can be obtained by the “water-gas
reaction” which involves passing water vapour over hot coke.
H2O + C → H2 + CO
Methanol is converted into formaldehyde by catalytic vapour phase
oxidation over a metal oxide catalyst. In one variation of the process
methanol is vaporised, mixed with air and then passed over the catalyst at
300-600°C. The formaldehyde produced is absorbed in water and then fed
to a fractionating column. A 37% solution of formaldehyde in water is
removed from the bottom of the column with some methanol as a stabilizer
whilst excess methanol is taken from the top of the column and recycled.
Formaldehyde is also produced by the oxidation of light petroleum
gases, a process which also yields methanol and acetaldehyde. This process
is currently used in the Celanese Corporation plant for the production of
Celcon.
Formaldehyde is a gas with a boiling point of -21°C. It is usually
supplied as a stabilized aqueous solution (~40% formaldehyde) known as
formalin. When formalin is used as the source of the aldehyde, impurities
present generally include water, methanol, formic acid, methylal, methyl
formate and carbon dioxide. The first three of these impurities interfere
with polymerization reactions and need to be removed as much as possible.
In commercial polymerization the low polymers trioxane and
paraformaldehyde are convenient sources of formaldehyde since they can
be obtained in a greater state of purity.
160
Polymerization of formaldehyde
CH2O
O
CH
2
O
CH
2
O
CH
2
O
H2C
+
H
2
C
O
CH
2
O
CH
2
O
I
II
HO[CH
2O]n
H
CH3O[CH2O]nH
III
IV
CH
3
OH
CH
OH
n
CHO
+
CH
2
OH
CHOHCHOHCH
OH
C CH
2
OH
O
Formaldehyde will polymerize in a number of ways, as indicated in
Figure 22.
2.7.1 Acetal Resins
Figure 22
The cyclic trimer (trioxane) and tetramer are obtained by a trace of
sulphuric acid acting on hot formaldehyde vapour (I). Linear polymers with
degrees of polymerization of about 50 a nd a terminal hydroxyl group are
obtained by evaporation of aqueous solutions of formaldehyde (II). In the
presence of strong acid the average chain length may be doubled.
Evaporation of methanol solution leads to products of type (III).
In the presence of lime water more complex reactions occur,
leading to the formation of aldoses and hexoses (IV). This particular
reaction is of interest to the biochemist as it is now generally held that
optically active plant carbohydrates are obtained from carbon dioxide and
water via formaldehyde.
During the 1920s Staudinger and his collaborators prepared linear
polymers of formaldehyde in some classic researches which demonstrated
for the first time the molecular structure of high polymers. When prepared
by a solution polymerization technique, brittle, pulverisable and thermal ly
unstable products were obtained, but Staudinger also prepared polymers by
allowing the material to polymerize in bulk at -80°C. These products,
though still thermally unstable, possessed some degree of toughness.
polymerization of formaldehyde was initiated by the Du Pont Company. As
In the early 1940s an intensive research programme on the