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161
a consequence of this work polymers, both tough and adequately stable to
processing conditions, were prepared and eventually marketed (Delrin).
In order to manufacture such polymers, it is first necessary to
produce a very pure form of formaldehyde. This is typically produced from
an alkali-precipitated low molecular weight polyformaldehyde which has
been carefuly washed with distilled water and dried for several hours under
vacuum at about 80°C. The dried polymer is then pyrolyzed by heating at
150-160°C, and the resultant formaldehyde passed through a number of
cold traps (typically four) at -15°C. Some prepolymerization occurs in
these traps and removes undesirable impurities from the monomer. The
monomer is then introduced into the polymerization vessel over a rapidly
stirred and carefully dried inert medium such as heptane. A number of
polymerization initiators have been cited in the literature and include Lewis
acids, amines, phosphines, arsines and stibines. A typical initiator is
triphenylphosphine used to the extent of 20 ppm based on the inert
medium. A polymer stabilizer such as diphenylamine ma y also be present
to a concentration of 100 ppm. Polymerization is carried out until a 20%
solids content is obtained. The polymer is then isolated by filtration,
washed in turn with heptane and pure acetone and then dried in a vacuum
oven at 80°C. Control of molecular weight may be made by adding traces
of water, which is an effective chain transfer agent. It is because of this
particular property of water that it is necessary to work under conditions
where the water content is carefully contro lled .
Polymers produced by methods as described above have thermal
stabilities many times greater than those obtained by the earlier bulk and
solution methods of Staudinger. Staudinger had, however, shown that the
diacetates of low molecular weight polyoxymethylenes (I)
(polyformaldehydes) were more stable than the simple polyoxymethylene
glycols (II):
CH3COO·[CH2O]
n<20CH2
OOC·CH3 (I)
HO·[CH2O]
n<20CH2
OH (II)
Staudinger also found that diacetates of polyoxymethylenes with a
degree of polymerization of about 50 were less stable. Truly high molecular
weight polyoxymethylenes (degree of polymerisation - 1000) were not
esterified by Staudinger; this was effected by the Du Pont research team
and was found to improve the thermal stability of the polymer substantially.
The esterification reaction may be carried out with a number of
162
different anhydrides but acetic anhydride is preferred. The reaction is
catalyzed by amines and the soluble salts of the alkali metals. The presence
of free acid has an adverse effect on the esterification reaction, the presence
of hydrogen ions causing depolymerization by an unzipping mechanism.
Reaction temperatures may be in the range of 130-200°C. Sodium acetate is
a particularly effective catalyst. Esterification at 139°C, the boiling point of
acetic anhydride, in the presence of 0.01% sodium acetate (based on the
anhydride) is substantially complete within 5 minutes. In the absence of
such a catalyst the percentage esterification is of the order of only 35%
after 15 minutes.
The following extract is taken from an example in British Patent
770.717 to the Du Pont Company as an illustration of a typical method of
esterification:
“Into a reaction vessel there is placed 500 g of a high molecular
weight formaldehyde polymer, 4 litres of acetic anhydride and 1.6 g of
anhydrous sodium acetate. The mixture is stirred and heated to 160°C.
Nitrogen gas at 12 to 15 p.s.i. gauge pressure is maintained in the space
above the reaction mixture during the heating period to prevent boiling.
The polymer is completely dissolved in the reaction mixture at this
temperature. The mixture is allowed to cool slowly with stirring and the
polymer precipitates from the solution at about 133°C, the total time in
solution being about 90 minutes. The acetylated polymer is removed by
filtration and washed on the filter with 3 litres of acetone. It is then reslurried in 3 litres of water using high speed agitation and the slurry is
filtered again. The water washing is repeated two more times. It is then
washed with 3 litres of acetone and 3 litres of acetone containing 2.0 g of
beta-conidendrol. The product is then dried in a vacuum oven at 67°C.”
The beta-conidendrol is incorporated as an antioxidant and is
frequently referred to in the patent literature, as is also di-β-naphthyl-pphenylenediamine for this purpose. It is claimed that in the example given
above the degradation rate at 222°C is only 0.09% per minute compared
with typical values of 0.6-0.8% for unesterified polymer.
An alternative approach to the production of thermally stable
polyoxymethylenes was made by chemists of the Celanese Corporation of
America and the commercial products were marketed as Celcon. Hostaform
and Duracon. The principle of thermal stability in this case is the
copolymerization of formaldehyde with a second monomer which is a
cyclic ether of the general form shown in Figure 23 (I).
163
R1CR
2
O
(R
3)n
R
1
CR
2
CH
2
O
CH
2
CH
2
CH
2
O
O
CH
2
I II
III
Figure 23
It is stated in the basic patent that ethylene oxide (II) and 1,3dioxolane (III) are the preferred materials. By the occasional incorporation
of molecules containing two successive methylene groups the tendency of
the molecules to unzip is markedly reduced.
In one example 25.0 g of cyclohexane were added to 25.0g of
trioxane (a cyclic trimer of formaldehyde) and cooled to -70°C; 0.03 ml of
dioxolane were added together with 0.10 ml boron fluoride ethereate (stated
in the basic patent to be the preferred catalyst). The tube was then sealed
and immersed in a water bath at 66-68°C for 4 hours. After washing the
product the polymer was dried and a 20% yield obtained. On heating this
sample for 2 hours at 225°C there was weight loss of 27.8%. Experiments
were also carried out using 0.25 parts and 1.25 parts of dioxolane, but in
these cases there was a higher weight loss and, in addition, a lower melting
point.
In another example, trioxane and dioxolane were blended in such a
ratio as “to provide one oxyethylene group for each 8.45 oxymethylene
groups”. The boron trifluoride ethereate comprised 0.089% by weight of
the mixture which was then heated in a tube in a bath of boiling water for
2.16 hours. A polymer was produced in a yield of 42.5% by weight, it had a
melting point of 158-163°C and a degradation rate at 222°C of only 0.06%
by weight per minute.
The stability of the copolymers may be enhanced by alkaline
hydrolysis following polymerization to remove oxymet hyl end-groups and
replace them with the more stable oxyethyl groups.
It is to be noted that there appear to be four processes which lead to
degradation of polyacetals.
(1) Stepwise thermal- or base-catalysed hydrolytic
depolymerization initiated from the hemi-formal chain end with the
evolution of formaldehyde. The main reasons for end-capping and
copolymerization mechanisms described above are carried out in order to
minimize this re ac t ion .
(2) Oxidative attack at random along the chain leading to chain
164
scission and subsequent depolymerization. Initial chain scission is reduced
CH
2
CH
2
CH
2
CH
2
CH
2
Polyethylene
CH
2
CH
2
CH
2
O
O
Polyeacetal
by the use of antioxidants and in recent formulations hindered phenols
seemed to be preferred. It is reported that 2,2'-methylenebis-(4-methyl-6-tbutylphenol) is present in Celcon and 4,4'-butylidenebis-(3-methyl-6-tbutylphenol) in Derlin. The copolymerization helps to reduce the rate of
depolymerization where initiation of depolymerization is not completely
prevented.
(3) Acid-catalyzed cleavage of the acetal linkage. Traces of acid
may arise as residues from polymerization and end-capping stages and
through oxidation of formaldehyde to formic acid. Acid acceptors are
believed to be used in commercial practice and epoxides, nitrogencontaining compounds and basic salts are all quoted in the patent literature.
Polyacetals should not be processed in equipment which has been used for
processing PVC unless the equipment has been cleaned because of the
serious effect of trace HC1 at elevated temperatures.
(4) Thermal depolymerization through scission of С–О bonds can
occur catastrophically above 270°C and care must be taken not to exceed
this temperature during processing.
Structure and properties of acetal resins
It is difficult to resist a comparison between the structure and
properties of acetal polymers and those of polyethylene.
Both polymers are linear with a flexible chain backbone and are
thus both thermoplastic. Both the structures shown are regular and since
there is no question of tacticity arising both polymers are capable of
crystallization. In the case of both materials polymerization conditions may
lead to structures which slightly impede crystallization; with the
polyethylenes this is due to a branching mechanism, whilst with the
polyacetals this may be due to copolymerization.
The acetal polymer molecules have a shorter backbone (–С–О)–
bond and they pack more closely together than those of polyethylene. The
resultant polymer is thus harder and has a higher melting point (175°C for
the homopolymer). The position of the glass transition is a subject of
debate since at least two transitions in addition to the melting point are
discernible. The true glass transition is usually associated with the
165
temperature at which movement of segments of about 50-150 backbone
atoms becomes relatively easy, in the amorphous zone. Unfortunately (from
the sense of trying to identify Tg) there are not a large number of such
segments in a highly crystalline polymer and the Tg only has secondary
effects. It is possible that a transition about -13°C is of the true Tg type.
Another transition at about -73°C is more prominent but is believed by the
author to be associated with the mobility of much smaller units via, for
example, the Schatzki crackshaft effect.
As is typical for crystalline polymers incapable of specific
interactions with liquids, there are no solvents at room temperature but
liquids which have a similar solubility parameter (δ=22.4 MPa
1/2
) will
cause a measure of swelling, principally in the amorphous region.
At room temperature there is only a small decrease in free energy
on conversion of monomer to polymer. At higher temperatures the
magnit ude of the free energy change decreases and becomes zero at 127°C;
above this temperature the thermodynamics indicate that depolymerization
will take place. Thus it is absolutely vital to stabilize the polyacetal resin
both internally and externally to form a polymer which is sufficiently stable
for processing at the desired elevated temperatures.
The backbone bonds are polar but the structure is balanced and the
polymer is quite a good dielectric. Reported data on resistivity indicate only
moderate values presumably because of ionic fragments, impurities and
additives.
Both the molecular and fine structure of the Du Pont
polyoxymethylenes have been investigated and reported. The repeating unit
of the polymer is –CH2O– and the end groups of the unmodified polymer
are either acetate or methoxyl (derived from methanol which is present in
trace in the formaldehyde during polymerization). The number average
molecular weights of these polymers are normally in the range of 20000-
110000. Values for molecular weight determined by end-group analysis
and by osmotic methods show close agreement. This agreement, together
with the fact that no structures which could be possible branch points in the
molecule have been discovered, indicates that the polymers are
substantially linear.
Commercial polymers are made so that they possess terminal end
groups for enhanced stability. Differences in the nature of these end groups
have been claimed to be the main reason for the higher hot water and alkali
resistance of the coplymers over the homopolymers.
The acetal polymers exhibit a high crystallinity. The percentage
166
crystallinity will depend on the quench temperature and will range from
Property
Acetal
homopolymer
Acetal
copolymer
Specific gravity
1.425
1.410
about 77%, when quenched at 0°C, to about 80% when quenched at 160°C.
The greater the percentage crystallinity the higher the yield point and
tensile modulus. It has also been shown that by raising the quench
temperature the spherul ite size is increased and that this greatly decreases
the impact toughness.
Properties of acetal resins
The principal features of acetal resins leading to commercial
application may be summarised as follows:
(1) Stiffness.
(2) Fatigue endurance.
(3) Resistance to creep.
(4) Low coefficient of friction (with equal dynamic and static
coefficients).
(5) Good appearance.
Although in many respects acetal resins are simi lar to the nylons,
they may be considered to be superior to them in their fatigue endurance,
creep resistance, stiffness and water resistance. The nylons (except under
dry conditions) are superior in impact toughness and abrasion resistance.
Some mechanical and thermal properties of acetal polymers are
listed in Table 8. The value quoted are those supplied by the manufacturers.
Table 8 – Some mechanical and thermal properties of acetal polymers
Tensile strength (23ºC), MPa
Flexural modulus (23ºC), MPa
Vicat softening point, ºC
Crystalline melting point, ºC
have little value. Like the nylons, which are also widely used for loadbearing light engineering applications, the polyacetals exhibit a small but
finite creep under load. It is thus necessary to consider mechanical
properties under those main headings.
70
2800
185
175
58
2500
162
163
It must, however, be stressed that for design purposes such data
(1) Short-term failure – in particular impact resistance.
167
(2) Long-term deformation.
(3) Long-term failure.
The acetal resins show superior creep resistance to the nylons but
are inferior in this respect, to the polycarbonates. It is to be noted, however,
that limitations in the load-bearing properties of the polycarbonates restrict
their use in engineering applications. Another property of importance in
engineering is abrasion resistance – a property that is extremely difficult to
assess. Results obtained f rom various tests indicate that the acetal polymers
are superior to most plastics and die cast aluminium, but inferior to nylon
66.
The electrical insulation properties of the acetal resins may be
described as good but not particularly outstanding. There are available
alternative materials which are better insulators and are also less expensive.
There are, however, applications where impact toughness and rigidity are
required in addition to good electrical insulation characteristics, and in
these instances acetal resins would be considered.
Acetal homopolymer resins show outstanding resistance to organic
solvents, no effective solvent having yet been found for temperatures below
70°C. Above this temperature some phenolic materials such as the
chlorophenols are effective. Stress cracking has not been encountered in
organic solvents. Swelling occurs with solvents of similar solubility
parameter to that of the polymer (δ=22.4 MPa
1/2
).
The resistance of these polymers to inorganic reagents is not,
however, so outstanding and they should not be used in strong acids, strong
alkalis or oxidising agents. Staining resistance is generally good although
hot coffee will cause staining. Acetal copolymer resins are somewhat more
resistant to hot alkalis but resistance to acids is still comparatively poor.
There do not appear to be any toxic or dermatitic hazards under normal
conditions of use with either homopolymers or copolymers. Water does not
cause any significant degrading hydrolysis of the polymer but may swell it
or permeate through it.
The polyacetals have a good record of performance in uses
involving hot air and hot water. Plumbing components have been used for
hot water service in the range 60-80°C and applications in hot air well in
excess of 90°C.
Prolonged exposure to ultraviolet light will induce surface chalking
and reduce the molecular weight, leading to gradual embrittlement. As with
the polyolefins it is found that the incorporation of a small amount of welldispersed carbon black increases resistance of ultraviolet degradation.
168
Amongst miscellaneous properties it may be noted that the resins do not
appear to be attacked by fungi, rodents and insects. The polymer burns
slowly with a soot-free flam e.
The homopolymer and the trioxane-based copolymers are generally
similar in properties. The copolymer has better thermal stability, better
hydrolytic stability at elevated temperatures, easier mouldability and better
alkali resistance. The homopolymer has slightly better mechanical
properties, e.g. higher tensile strength, higher flexural modulus and greater
surface hardness. As may be expected, the homopolymer has a slightly
higher crystalline melting point.
Processing
Acetal resins may be processed without difficulty on conventional
injection moulding, blow moulding and extrusion equipment. The main
points to be considered are:
(1) Overheating leads to the production of formaldehyde gas and if
this is produced in sufficient quantities within the confines of an injection
cylinder or extruder barrel the gas pressure may become sufficiently high
that there is a risk of damage or injury. The time for which acetal resin may
be heated at any given temperature will vary from grade to grade according
to the method and degree of stabilization. A typical copolymer may be kept
in an extruder barrel for 110 min at 190°C before serious discolouration
occurs. Dead spots must be carefully avoided.
(2) Although less hygroscopic than the nylons, acetal resins must
be stored in a dry place.
(3) With most homopolymers and copolymers the apparent
viscosity is less dependent on temperat ure and shear stress than that of the
polyolefins, thus simplifying die design. On the other hand the melt has a
low elasticity and strength and this requires that extruded sections be
supported and brought below the melting point as soon as possible
consistent with obtaining a satisfactory crystalline texture.
The lack of melt strength leads to particular problems with blow
moulding because of the extensive drawing down of the parison under
gravity. To overcome these problems copolymers have become available
with slightly branched molecules which have a greater melt elasticity and
tenacity. Such materials, which also have more stress-dependent viscosities,
are not only of specific value in blow moulding but extradates generally are
easier to handle.
(4) The high crystallinity which develops on cooling results in a
169
shrinkage of about 0.020 cm/cm. Because of the low glass transition
n
M
n
M
temperature, crystallization can take place quite rapidly at room
temperatures and after-shrinkage is usually complete within 48 hours of
moulding or extrusion. In processing operations injection moulds, blow
moulding moulds and sizing dies should be ke pt at about 80-120°C in order
to obtain the best results.
(5) Because of the low glass transition temperature it is not
possible to make clear film, stable at room temperature, by quenching.
Some improvement in clarity may be obtained by cold rolling as this tends
to dispose the crystal structure into layers.
Both homopolymers and copolymers are available in a range of
molecular weights (
20000-100000). The materials are normally
characterized by the melt flow index using basically the same test as
employed for polyethylene. For general purpose work polymers with an
MFI of about 9 are employed but high-precision work and complex
mouldings polymers with MFIs as high as 27 (viz. lower molecular weight
polymers) may be used. For extrusion and thick-walled mouldings a
polymer with MFI about 2.5 (
45000) is often employed although for
extrusion blow moulding the special polymers used have MFIs of about
1.0.
Additives
The acetal polymers are probably never supplied for use without
any additives being incorporated. Antioxidants of the phenol alkane type
are present in both homopolymers and copolymer s. Acid acceptors are also
believed to be widely used to absorb traces of acidic materials which attack
the acetal linkage. Epoxides, nitrogen compounds and basic salts have been
successfully employed.
Since acetal resins are degraded by ultra violet light, additives may
be included to improve the resistance of the polymer. Carbon black is
effective but as in the case of polyethyl ene it must be well dispersed in the
polymer. The finer the particle size the better the ultra violet stability of the
polymer but the poorer the heat stability. About 1.5% is generally
recommended. For white compounds and those with pastel colours titanium
dioxide is as good in polyacetals as most transparent ultraviolet absorbers,
such as the benzophenone derivatives and other materials. Such ultraviolet
absorbers may be used for compounds that are neither black, white nor
pastel shade in colour.
170
In order to reduce the coefficient of friction in bearing applications
small amounts of molybdenum disulphide are incorporated although in
quantity this material may cause problems through acidic impurities.
Blends of polyacetal and PTFE (20-25% PTFE) have a very low coefficient
of friction (as low as 0.02) and may be operated under much more severe
conditions than is possible with unfilled materials.
Lower cost alternatives to PTFE-modified polymers have also been
introduced for low-friction applications. These materials use graphite and
“chemical lubricants” of undisclosed composition.
Glass-filled polyacetals (20-30% glass fibre) are available when
such properties as high creep resistance, enhanced stiffness and low
coefficient of expansion are important. Properties of the filled compound
depend considerably on the glass-fibre dimensions, the nature of surface
coatings on the glass and the goodness of mixing. The main difficulties
with these compounds arise from their inferior impact strength and the
anisotropic nature of mouldings. Grades are also available in which the
glass is available in ballotini (i.e. small bead) form and as ballotini-fibre
mixtures.
Acetal-polyurethane alloys
Blends or alloys of polyacetals with polyurethane elastomers were
first introduced by Hoechst in 1982, who were then followed by other
manufacturers. The key features of these materials are their improved
toughness with little change in other important properties. There are two
aspects with respect to the impact toughness:
(1) A high strength under impact.
(2) Good elastic recovery.
Where the polyurethane comprises <30% of the blend, the
polyurethane remains in discrete droplets within the polyacetal matrix. In
this range the particle size and particle size distribution of the elastomer
particles are of importance. Where the elastomer component is in excess of
30%, interpenetrating polymer networks exist in the sense that there are
two interpenetrating continuous phases (as opposed to two cross-linked
interpenetrating polymer systems).
Charpy notched impact strengths in excess of 55 kJ /m2 have been
reported for alloys with homopolymers and in excess of 20 kJ/m2 for
copolymers.