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171
Applications of the acetal polymers and copolymers
There has been a steady increase in demand for polyacetals over the
years, with global nameplate capacity increasing from about 260000 t.p.a.
at the beginning of the 1980s to about 600000 t.p.a. at the end of the
century. In the same period consumption has risen from about 140000 t.p.a.
to about 480000 t.p.a. In terms of nameplate capacity the acetal copolymers
dominate the homopolymers by a ratio of the order of 3:1.
About 95% of the polymers are processed by injection moulding.
Virtually all the remainder is extruded into sheet and rods for subsequent
machining into finished parts.
The acetal resins may best be considered as engineering materials.
They therefore become competitive with a number of plastics materials,
nylon in particular, and with metals.
Because of their light weight, ability to be moulded into intricate
shapes in one piece, low coefficients of friction and absence of slip-stick
behaviour, acetal resins find use as bearings.
The lowest coefficients of friction and wear are obtained with
acetal resin against steel. With other metals, in particular with aluminium,
greater wear and higher friction occur. From the design point of view it is
not generally desirable to use acetal to acetal in bearings because of the
tendency to heat build-up, except with very light loads. Where the use of a
non-metallic material is desirable it is found that better results are obtained
using acetal and nylon in conjunction rather than either on its own.
Acetal resins find a number of applications in gears, where they
come into competition with the nylons. Acetal gears are superior to those
made from nylon in fatigue resistance, dimensional stability and stiffness,
whereas nylon gears in conditions of average humidity have greater
resistance to impact fatigue and abrasion.
Amongst the many other applications for acetal resins should be
mentioned links in conveyor belts, moul ded sprockets and chains, blower
wheels, cams, fan blades, check valves, pump i mpellers, carburettor bodies,
blow-moulded aerosol containers and plumbing components such as valve
stems and shower heads.
It may therefore be seen that acetal resins are primarily engineering
materials being used to repl ace metals because of such desirable properties
as low weight, corrosion resistance, resistance to fatigue, ease of fabrication
and low coefficient of friction. Because of their comparatively high cost
they cannot be considered as being general purpose thermoplastics
alongside polyethylene, polypropylene, PVC and polystyrene.
172
In the late 1990s it was estimated that in Western Europe usage
breakdown was as follows:
Transportation 35.7%
Industrial uses 13.6%
Consumer products 12.0%
Appliances / power tools 9.3%
Electrical / electronics 8.6%
Plumbing, hardware 2.9%
Other 17.9%
In North America somewhat less is used in transportation
applications and rather more in plumbing.
Whilst usage of polyacetals is substantially less, in tonnage terms,
than that of the major polymers such as polyethylene, polypropylene, PVC
and polystyrene, it is perhaps worth noting that they are used in a very large
number of applications. This, however, commonly is in the form of small
mouldings. For instance, there was an example quoted in the 1970s of a
small Italian car that contained some 450 components made from
polyacetals, weighing only one kilogram in total.
2.7.2 Miscellaneous Aldehyde Polymers
A large number of polymers from aldehydes have been reported in
the literature but, apart from those polymers already described, they are not
of commercial importance.
With the exception of formaldehyde, the aldehydes may polymerize
to give varying molecular configurations and, depending on the stereoregulating influence of the catalysts, either amorphous rubber or crystalline
polymers may be obtained. It may, however, be mentioned that in such
cases as iso-butyraldehyde, n-heptaldehyde and chlorinated acetaldehydes
the steric hindrance of side groups allows polymerization to proceed only
when the molecules are in certain configurations. In these cases a degree of
stereo-regularity may be imposed.
Care has to be taken in the polymerization of aldehyde polymers in
order to achieve reproducible results. It is also difficult to stabilize most of
the products since thermodynamics frequently favour depolymerization at
temperatures a little above or at room temperature.
In the immediate future it is unlikely that any of these polymers
will attain commercial significance. Hopes that polyhaloaldehydes such as
polychloral might be of some use because of their good acid stability have
not been realised. This is because polymers prepared to date have poor
173
alkali and thermal resistance, decomposing without melting. Chloral-
CH
2
CH
3
CHO
ZnCl
2
HO
CH
2
OH
Conc.
H
2
SO
4
CH
2
CH
2
O
CH
2
CH
2
O
OH
CH
2
CH
2
OH
-H
2
O
CH
2
CH
2
O
CH
2
CH
2
O
CH
2
CH
2
O
Property
Value
Tensile strength, MPa
Brittle temperature, ºC
13-22
-50
dichloracetaldehyde copolymers have also proved similarly disappointing.
2.7.3 Polyethers from Glycols and Alky lene Ox ides
If ethylene glycol is subjected to vigorous dehydrating conditions,
simple molecules such as dioxan and acetaldehyde may be prepared:
Under appropriate conditions it is possible to obtain linear
polymers, the poly(ethylene oxides), from either glycol or oxide:
Controlled polymerization of ethylene oxide under alkaline
conditions will produce a range of polymers marketed under the trade name
Carbowax. These have molecular weights in the range 1500-20000 and are
greases or waxes according to their degree of polymerization. Lower
molecular weight polymers have also been prepared, of which carbitol,
НО·СН2·СН2·О·СН2·СН2·ОН, may be considered as the limiting dimer.
In 1958 the Union Carbide Corporation introduced high molecular
weight, highly crystalline ethylene oxide polymers under the trade name
Polyox. Although similar in appearance to polyethylene they are miscible
with water in all proportions at room temperature.
Unlike the lower molecular weight poly(ethylene oxides) these
materials are tough and extensible, owing to their high molecular weight
and their crystallinity. Typical mechanical properties of the polymer are
given in Table 9.
Table 9 – Some properties of a medium-high molecular weight ethylene
oxide polymer
Yield strength, MPa
Ultimate elong at io n, %
Shore A hardness
Melting point, ºC
7-11
700-1200
99
66
174
The tensile strength will depend to a large extent on the rate of
extension and the relative humidity. There is a severe drop in tensile values
as the relative humidity exceeds 80%.
Unlike other water-soluble resins the poly(ethylene oxides) may be
injection moulded, extruded and calendered without difficulty. The
viscosity is highly dependent on shear rate and to a lesser extent on
temperature. Processing temperatures in the range 90-130°C may be used
for polymers with an intrinsic viscosity of about 2.5. (The intrinsic
viscosity is used as a measure of molecular weight.)
The polymers are of interest as water-soluble packaging films for a
wide variety of domestic and industrial materials. (Additional advantages
of the poly(ethylene oxides) are that they remain dry to the feel at high
humidities and may be heat sealed.) The materials are also of use in a
number of solution application such as textile sizes and thickening agents.
As a water-soluble film they are competitive with poly(vinyl alcohol)
whereas in their solution applications they meet competition from many
longer established natural and synthetic water-soluble polymers.
Elastomeric polyethers
The flexible backbone of an aliphatic polyether chain can lead to
polymers with a low glass transition temperature. Providing crystallization
can be inhibited by either copolymerization or by the polymers having a
substantially atactic structure, many of these materials are rubbery.
Incorporation of chlorine atoms into the structure will give materials of
good hydrocarbon oil resistance and, providing such attachments are not
too close to the chain backbone, the glass transition temper ature will not be
greatly raised.
This approach was used in the development of the epichlorhydrin
rubbers which became commercially available around 1965 from Goodrich
(Hydrin) and Hercules (Herclor). Both homopolymers of epichlorhydrin
(Hydrin 100, Herclor H) and copolymers of epichlorhydrin with ethylene
oxide (Hydrin 200, Herclor C) became available. (In 1986 Hercules sold
their interest in these materials to Goodrich, who in turn later sold this to
Nippon Zeon).
Initiation of polymerization is said to be effected by zinc diethylwater and aluminium trialkyl-water-acetyl acetone systems to give the
structures indicated in Figure 24.
175
CH
2
CH
CH2Cl
O
CH2CH
CH2Cl
O
CH2CH
2
O
Figure 24
The polymers are amorphous with brittle points (quite closely
related to the Tg) of about -15°C and -40°C respectively.
Vulcanization can be effected by diamines, polyamines and lead
compounds such as lead oxides and basic lead phosphite. The
homopolymer vulcanizate is similar to butyl rubber in such characteristics
as low air permeability, low resilience, excellent ozone resistance, good
heat resistance and good weathering resistance. In addition the
polyepichlorohydrins have good flame resistance. The copolymers have
more resilience and lower brittle points but air impermeability and oil
resistance are not so good. The inclusion of allyl glycidyl ether in the
polymerization recipe produces a sulphur-curable elastomer primarily of
interest because of its better resistance to “sour gas” than conventional
epichlorhydrin rubbers.
Epichlorhydrin rubbers, whilst being speciality materials, have a
useful combination of properties which leads to their use in many
applications such as gaskets, oil-f ield components, fuel pump diaphragms,
oil seals, fuel and hydraulic hose and printing rollers.
Copolymers of propylene oxide with a cure site monomer ( usually
allyl glycidyl ether used to a level of about 10% of the total monomer) were
first described in 1963 and first marketed by General Tire and Rubber as
Dynagen X P -139 in the 1960s. This material was then marketed as Parel by
Hercules until 1986 when they sold their interest in the material to
Goodrich. (As with the epichlorhydrin rubbers, this interest was l ater sold
on to Nippon Zeon). This material has a strong structural resemblance to
the epichlorhydrin rubbers, the absence of the chlorine atoms reducing oil
and flame resistance but improving low-temperature flexibility. Although
sulphur-cured, the rubbers have very good heat resistance and an operating
range of -60°C to +150°C. They are similar to natural rubber in exhibiting
high resilience and excellent flex life, but in addition show excellent lowtemperature properties together with good heat resistance, good ozone
resistance and moderate oil resistanc e.
A number of other polyethers derived from polyfunctional hydroxy
compounds or alkylene oxides are important intermediates in the
manufacture of polyurethanes.
176
2.7.4 Oxetane Polymers
CH
2
CH
2
C
n
R
R
1
CH
2
OH
OH
HO
C
CH
2
CH
2
CH
2
OH
Pentaerytheritol
Cl
CH
2
C
CH
2
Cl
CH
2
OH
CH
2
Cl
NaOH
Cl
CH
2
C
CH
2
Cl
CH
2
CH
2
O
In the early 1950s novel polyethers were prepared in the
laboratories of the Hercules Powder Company and of Imperial Chemical
Industries Limited from oxacyclobutane derivatives. One such polyether
that from 3,3-dichloromethyl-1-oxacyclobutane, was marketed by the first
named company in 1959 under the trade name of Penton. Commercial
manufacture of this material had, however, ceased by the end of 1972.
The polymers are of the general form
The chloromethyl derivatives may be prepared from pentaerythritol
via the trichlor ide or tr ichloride monoacetate:
The monomer, 3,3-dichloromethyl-1-oxacyclobutane, has the
following characteristics:
Boiling point 83°C at 11 mmHg
Melting point 18.75°C
Density (25°C) 1.2951 g/cm3
Refractive index (20°C) 1.4858
The patent literature indicates that polymerization may be carried
out in the range -80 to +25°C using boron trifluoride or its ethereate as
catalyst.
Solvents mentioned include hexane, benzene, liquid sulphur
dioxide, chloroform, methylene dichloride and ethyl bromide. Where
chlorinated solvents are employed the polyme r is separated by addition of
methanol , filtered, washed with methanol and the product dried in vacuo at
60°C.
The commercial polymer was said to have a number average
molecular weight of 250000-350000. Because of its regular structure it is
capable of crystallization.
177
O
CH
2
C
CH2Cl
CH2Cl
CH
2
O
CH
2
C
CH2Cl
CH2Cl
CH
2
Two crystalline forms have been observed. One is formed by slow
Property
Value
Tensile strength (23ºC), MPa
41
cooling from the melt and the other by slow heating of the amorphous
polymer. The properties of the commercial products were therefore to some
extent dependent on their heat history. Glass transition temperatures
observed range from 7 to 32°C and depend on the time scale of the me thod
of measurement.
Typical properties of Penton are given in Table 10.
Consideration of the figures given in Table 10 shows that the
physical properties of the chlorinated polyethers are not particularly
outstanding when compared with other plastics materials. On the other
hand, apart from a somewhat low impact strength, these figures reveal no
particular limitation.
Table 10 – Some typical properties of Penton
Modulus of elasticity (23ºC), MPa
Flexural strength (23ºC), MPa
Thermal conductivity, W/mK
Flammability
The principal applications of these plastics arose from their very
good chemical resistance, as they are resistant to mineral acids, strong
alkalis and most common solvents. They were, however, not recommended
for use in conjunction with oxidising acids such as fuming nitric acid,
fuming sulphuric acid or chlorosulphonic acid, with fluorine or with some
chlorinated solvents, particularly at elevated temperatures.
It was claimed that the maximum continuous operating temperature
in most chemical environments was 120°C and even 140-150°C in some
instances.
The major chemical applications were in the form of pipe and tank
linings and injection moulded valve and pump parts. Coatings could be
applied to metals by means of fluidised bed, water suspension and organic
dispersion techniques.
1100
900
0.13
self extinguishing
178
In 1975 the Hercules company announced the preparation of a
CH CH
O
CH
2
Cl
CH2Cl
polymer very similar in structure to the discontinued Penton. The new
polymer is poly-(1,2-dichloromethyl)ethylene oxide:
It may be prepared in two stereo-regular forms, cis- and trans-. The
cis--polymer, which crystallises in zig-zag form, has a Tm of 235°C, whilst
the trans-polymer, which crystallizes in helical form, melts at the much
lower temperature of 145°C. Tensile strengths of both forms are reportedly
similar to that of Penton whilst the tensile modulus of 2300 MPa is about
twice as high. Unfortunately the material is rather brittle with an impact
strength only about half that of polystyrene although this may be improved
by orientation.
2.8 Polyurethanes and Polyisocyanurates
The reaction of an isocyanate and an alcohol results in the
formation of an urethane
R·NCO + HOR1 → R·NH·COOR1
By the same reaction polyhydroxy materials will react with
polyisocyanates to yield polyurethanes. For example, the reaction between
1,4-butanediol and hexamethylenedi-isocyanate is shown below:
HO (CH2)
OH
6
O
OOCNH (CH2)6NHCOO
(CH2)
4
OCN
(CH2)
NCO HO(CH
6
(CH
2)4
OH
2)4
O
This particular polymer is a fibre-forming material (Perlon U).
Although in many respects this reaction resembles the formation of
polyesters and polyamides it is not a condensation reaction but involves a
transfer of hydrogen atoms and thus may be considered as an example of
rearrangement polymerization.
Although the first polyurethanes were similar to that shown above,
several polymers currently used contain many linkages in addition to the
urethane group. Because of this the term polyurethane is now generally
extended to cover all the complex reaction products of isocyanates and
polyhydroxy compounds (the latter frequently known in this context as
polyols).
Commercial development of the polyurethanes arose from the work
179
of German chemists attempting to circumvent the Du Pont patents on nylon
66. O.Bayer and his team of chemists were able to produce fibre-forming
polymers by reacting aliphatic di-i socyanates and aliphatic diols (glycols).
Subsequent work resulted in the production of useful products by using
polymeric hydroxyl-containing compounds such as polyesters to give
rubbers, foams, coatings and adhesives.
Whilst initial development was primarily in the fields of fibres and
rubbers, the presence of polyurethanes at about sixth position in the
production league tables is largely due to the widespread use of foam
materials. By 1980 global consumption was of the order of 3·106 tonnes per
annum.
Since 1980, partly due to the maturity of markets and partly due to
the advent of legislation aimed at reducing fire risks, particularly in
furniture, annual growth rates have not always been positive. Furthermore
statistics for polyurethane production and consumption are somewhat
unreliable. Nevertheless it was estimated in one trade journal that in 1998
global polyurethane production was as high as 5·1 06 t.p.a. with Europe and
North America each consuming about one-third of global output, Latin
America 10% and the Pacific Rim 25%. Particularly noteworthy was the
rapidly growing Chinese demand for polyurethanes from about 100000
t.p.a. to 500000 t.p.a. during the course of the 1990s.
The market is dominated by flexible foam applications (43% in the
United States) and rigid and semi-rigid foam (29%). Cast elastomers (4%)
and RIM elastomers (3%) have only specialised outlets. The remaining
sizeable 21% of the market cover such diverse uses as thermoplastic
rubbers, surface coatings, adhesives, sealants and synthetic leathers.
2.8.1 Fibres and Crystalline moulding Compounds
The initial research on polyurethanes was directed towards the
preparation of fi bre-forming polymers. Many polyhydroxy compounds and
many di-isocyanates were used.
Di-isocyanates and glycols are capable of hydrogen bonding and in
both types the greater the distance between amide or urethane links the
lower the melting point, provided there is an even number of carbon atoms
between the characteristic groupings. Both polyamides and polyurethanes
with an odd number of methylene groups in the repeating unit have lower
melting points than the polymer with one more, intermediate, carbon atom
(i.e. an even number of carbon atoms).
Although 4,4-polyurethane has the highest melting point this
180
material was not produced commercially because of the difficulty of
obtaining tetramethlyene di-isocyanate with the desired degree of purity.
The polymer with the next highest melting point, 6,4-polyurethane, the
reaction product of hexamethylene di-isocyanate and butane-1,4-diol , was
thus chosen for commercial production by German chemists during World
War II because of the availablility of the isocyanate.
The polymer may be prepared by running the isocyanate into the
glycol while the temperature is raised slowly to near 200°C. The reaction is
exothermic and carried out under a blanket of nitrogen. The polymers
produced have a molecular weight of 10000-15000 and after filtration may
be melt spun into fibres.
Compared with nylon 66 fibres, the polyurethane fibres (known as
Perlon U) have a tensile strength at the higher end of the range quoted for
nylon 66, they are less prone to discolouration in air, are more resistant to
acid conditions and they have a lower moisture absorption. On the debit
side they are less easy to dye, are hard, wiry and harsh to handle and have
too low a softening point for many applications. They are currently of little
importance but have found some use in bristles, filler cloths, sieves and a
few other miscellaneous applications.
The linear polyurethanes used to make fibres can also be used as
thermoplastics and may be processed by injection moulding and extrusion
techniques. A number of grades are available varying in hardness, softening
point, water absorption and other properties. That with the highest melting
point is based on 6,4-polyurethane but those with lower melting points are
copolymers in which about 10-15% of the butanediol-1,4-diol is replaced
by another diol such as hexamethylene glycol or methylhexamethylene
glycol. The processing characteristics are very similar to the nylons, in
particular the low melt viscosity requires the use of nylon-type injection
nozzles. The polymers start to decompose at about 220°C and care should
be taken to prevent overheating.
The properties of the polyurethane moulding compositions are also
very similar to nylon 66. The greatest difference in properties is in water
absorption, the 6,4-polyurethane absorbing only about 1/6 of that of nylon
66 under comparable conditions. This results in better dimensional stability
and a good retention of electrical insulation properties in conditions of high
humidity. Resistance to sulphuric acid is somewhat bettter than with nylon
66 but both types of polymer are dissolved by phenols and formic acid.
There is little call for these thermoplastics materials (marketed as
Durethan U, Farbenfabriken Bayer) since they are about twice the price of