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Файл:Plastics technology. Часть 2. Учебное пособие.pdf
131
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Polymerization of aliphatic polyamides
The fibre-forming polyamides are produced commercially by
reacting diamines with dibasic acids, by self-condensation of an amino acid
or by opening of a lactam ring. Whatever method is chosen it is important
that there should be an equivalence in the number of amine and acid groups
for polymers of the highest molecular weight to be obtained. In the case of
the amino acids and lactams this is ensured by the use of pure monomer but
when diamines and dibasic acids are used it is necessary to form a salt to
ensure such an equivalence. Small quantities of monofunctional
compounds are often used to regulate molecular weight.
Nylons 46, 66, 69, 610 and 612
The nylon 66 salt is prepared by reacting the
hexamethylenediamine and adipic acid in boiling methanol, the
comparatively insoluble salt (melting point 190-191°C) precipitating out.
A 60% aqueous solution of the salt is then run into a stainless steel
autoclave together with a trace of acetic acid to limit the molecular weight
(9000-15000). The vessel is sealed and purged with oxygen-free nitrogen
and the temperature raised to about 220°C. A pressure of 1.7 MPa is
developed. After 1-2 hours the temperature is raised to 270-280°C and
steam bled off to maintain the pressure at 1.7 MPa. The pressure is then
reduced to atmospheric for one hour, after which the polymer is extruded
by oxygen-free nitrogen on to a water-cooled casting wheel, to form a
ribbon which is subsequently disintegrated. Nylon 610 is prepared from the
appropriate salt (melting point 170°C) by a similar technique. Nylon 612
132
uses decane-1, 10-dicarboxylic acid. Azelaic acid is used for nylon 69.
Nylon 46, introduced in the late 1970s as Stanyl by DSM, is prepared by
reacting 1,4-diaminobutane with adipic acid.
Nylon 6
Both batch and continuous processes have been used for the
manufacture of nylon 6. In a typical batch process the caprolactam, water
(which acts as a catalyst) and a molecular weight regulator, e.g. acetic acid,
are charged into the vessel and reacted under a nitrogen blanket at 250°C
for about 12 hours. The product consists of about 90% high polymer and
10% low molecular weight material such as the monomer. In order to
achieve the best physical properties the low molecular weight materials
may be removed by leaching and/or vacuum distillation. In the continuous
process the react ants are maintained in reservoirs which continuously feed
reaction columns kept at a temperature of about 250°C.
The polymerization casting of nylon 6 in situ in the mould has
been developed in recent years. Anionic polymerisation is normally
employed; a typical system uses as a catalyst 0.1-1 mol.% of acetic
caprolactam and 0.15-0.50 mol.% of the sodium salt of caprolactam. The
reaction temperature initially is normally between 140 and 180°C but
during polymerization this rises by about 50°C. Mouldings up to one ton in
weight are claimed to have been produced by these casting techniques.
Reaction injection moulding techniques, developed primarily for
polyurethanes, have also been adapted for nylon 6 in what must be
considered as a variation of the polymerization casting tech n iq u e.
Nylon 11
This polymer may be prepared by stirring the molten ω-
aminoundecanoic acid at about 220°C. The reaction may be followed by
measurements of the electrical conductivity of the melt and the intrinsic
viscosity of solutions in m-cresol. During condensation 0.4-0.6% of a 12membered ring lactam may be formed by intramolecular condensation but
this is not normally removed since its presence has little effect on the
properties of the polymer.
Nylon 12
The opening of the caprolactam ring for nylon 6 involves an
equilibrium reaction which is easily catalyzed by water. In the case of
nylon 12 from dodecanelactam, higher temperatures, i.e. above 260°C, are
133
necessary for opening the ring structures but since in this case the
condensation is not an equilibrium reaction the process will yield almost
100% of high polymer.
Nylon 7
The ω-aminoenanthic acid is polymerized in an aqueous solution
under nitrogen at 14-15 atm pressure at 250-260°C. The process takes
several hours.
Structure and properties of aliphatic polyamides
Aliphatic polyamides such as nylons 46, 66, 6, 610 and 11 are
linear polymers and thus thermoplastic. They contain polar –CONH–
groups spaced out at regular intervals so that the polymers crystallise with
a high intermolecular attraction. These polymer chains also have aliphatic
chain segments which give a measure of flexibility in the amorphous
region. Thus the combination of high interchain attraction in the crystalline
zones and flexibility in the amorphous zones leads to polymers which are
tough above their apparent glass transition temperatures.
The high intermolecular attraction leads to polymers of high
melting point. However, above the melting point the melt viscosity is low
because of the polymer flexibility at such high temperatures, which are
usually more than 200°C above the Tg, and the relatively low molecular
weight.
Because of the high cohesive energy density and their crystalline
state the polymers are soluble only in a few liquids of similar high
solubility parameter and which are capable of specific interaction with the
polymers.
The electrical insulation properties are quite good at room
temperature in dry conditions and at low frequencies. Because of the polar
structure they are not good insulators for high-frequency work and since
they absorb water they are also generally unsuitable under humid
conditions.
There are a number of structural variables which can considerably
affect the properties of the aliphatic polyamides:
(1) The distance between the repeating –CONH– groups. It is the
presence of the –CONH– groups which causes the aliphatic polyamides to
differ from polyethylene, and the higher their concentration the greater the
difference. As a rule, the higher the amide group concentration, i.e. the
shorter the distance between –CONH– gr oups, the high er the: (a) density;
134
(b) forces required to mechanically separate the polymer molecules and
hence the higher the tensile strength, rigidity, hardness and resistance to
creep; (c) the Tm and heat deflection temperature; (d) resistance to
hydrocarbons; (e) water absorption. Nylon 11 has twice the distance
between amide groups of that in nylon 6, and consequently is intermediate
in properties between nylon 6 and polyethylene.
(2) The number of methylene groups in the intermediates. It has
been observed that polymers from intermediates with an even number of
methylene groups have higher melting points than similar polymers with an
odd number of methylene groups. These differences are due to the
differences in the crystal structure of polymers with odd and even
methylene groups which develop in order that oxygen atoms in one
molecule are adjacent to amino groups of a second molecule. Hydrogen
bonds with an NH–O distance of 2.8 Å are produced and are the reason for
the high strength and the high melting points of polyamides such as nylon
6, 66 and 7. The crystal structures of the polyamides differ according to the
type of polymer and in some cases, such as with nylon 66, two crystal
forms co-exist in the same mass of polymer.
(3) The molecular weight. Specific types of nylon, e.g. 66, are
frequently available in forms differing in molecular weight. The main
difference between such grades is in melt viscosity, the more viscous
grades being more suitable for processing by extrusion techniques.
(4) N-Substitution. Replacement of the hydrogen atom in the –
CONH– group by such groups as ~CH3 and ~CH2OCH3 will cause a
reduction in the interchain attraction and a consequent decrease in
softening point. Rubbery products may be obtained from methoxymethyl
nylons.
(5) Copolymerization. Except in those rare cases where monomer
segments are isomorphous, copolymerization, as usual, leads to less
crystalline and frequently amorphous materials. As might be expected,
these materials are tough, leather-like, flexible and, when unfilled,
reasonably transparent.
The properties of the nylons are considerably affected by the
amount of crystallization. Whereas in some polymers, e.g. the polyacetals
and PCTFE, processing conditions have only a minor influence on
crystallinity, in the case of the nylons the crystallinity of a given polymer
may vary by as much as 40%. Thus a moulding of nylon 6, slowly cooled
and subsequently annealed, may be 50-60% crystalline, while rapidly
cooled thin-wall mouldings may be only 10% crystalline.
135
As with other crystalline polymers, properties are dependent not
only on total percentage crystallinity but also on the size of morphological
structures such as spherulites. According to the method of processing,
different morphological structures will be produced. Slowly cooled melts
may form spherulites, rapidly cooled polymers may form only fine
aggregates. It follows that in an injection moulding the morphological form
of rapidly cooled surface layers may be quite different from that of the
slower cooled centres. Smaller spherulites can be obtained by the use of
nucleating agents and this can give a more uniform structure in an injection
moulding. Several years ago it was found that seeding the polymer with
about 0.1% of a fine silica gave a polymer of greater tensile strength,
hardness and abrasion resistance but with some reduction in impact
strength and elongation at break. Subsequent developments using
phosphorus compounds as nucleating agents were found to give profoundly
shortened moulding cycles, in a typical instance down from 30 to 4
seconds. It was also found that overnucleation tended to give a crystalline
surface layer with some undesirable properties and the current aim is to
produce a polymer which on moulding is of a two-phase structure, the bulk
consisting of a union crystal structure with a very thin near-amorphous
surface layer.
The suppliers of nylon 46 have laid particular emphasis on the fact
that this polymer, with its highly symmetrical chain structure, leads to both
a high level of crystallinity and a high rate of nucleation. In turn the high
nucleation rate leads to a fine crystalline structure which in this case is
claimed to lead to a higher impact strength (dry as moulded) than with
nylons 6 and 66.
As is commonly the case with crystalline polymers the glass
transition temperature is of only secondary significance with the aliphatic
polyamide homopolymers. There is even considerable uncertainty as to the
numerical values. Rigorously dried polymers appear to have Tg of about
50°C, these figures dropping towards 0°C as water is absorbed. At room
temperature nylon 66 containing the usual amounts of absorbed water
appears to be slightly above the Tg and cryst allization may occur only very
slowly. This can lead to after-shrinkage effects which may occur for
periods up to two years. With nylon 6 the effect is less marked. The aftershrinkage process may be accelerated by annealing the samples at an
elevated temperature, typically that which corresponds to the maximum
crystallization rate for that polymer.
The greater the degree of crystallinity the less the water absorption
136
and hence the less will be the effect of humidity on the properties of the
Property
46
66 6 610
612
11
12
66/610
(35:65)
66/61
Tensile stress:
Elongatio n at break,
polymer. The degree of crystallinity also has an effect on electrical and
mechanical properties. In p articular high crystallinity leads to high abrasion
resistance.
General properties of the nylons
Typical mechanical properties of some commercial grades of nylon
are given in Table 8.
Table 8 – Mechanical properties of typical polyamides
- at yield, MPa
- at break, MPa
%
Tension modul us,
MPa
Rockwell hardness
Specific gravity
The figures given in the table are obtained on mouldings relatively
free from orientation and tested under closely controlled conditions of
temperature, testing rate, and humidity. Changes in these conditions or the
use of additives may profoundly affect these properties.
Laboratory tests and experience during use have demonstrated that
the nylons have extremely good abrasion resistance. This may be further
improved by addition of external lubricants and by processing under
conditions which develop a highly crystalline hard surface e.g. by use of
hot injection moulds and by annealing in a non-oxidising fluid at an
elevated temperatu re (15 0 -200°C for nylon 66).
The glass transition temperatures of the nylons appear to be below
room temperature so that the materials have a measure of flexibility in spite
of their high crystallinity under general conditions of service. The polymers
have fairly sharply defined melting points and above this temperature the
homopolymers have low melt viscosities. Some thermal properties of the
nylons are given in Table 9.
(dry)
100
80
30
3000
R 123
1.18
80
80100
3000
R 118
1.14
76
-
100-
200
2800
R 112
1.13
55
-
100-
150
2100
R 111
1.09
60
60
100-
250
-
R 114
1.07
38
52
30-
300
1400
R 108
1.04
45
54
200
1400
R 107
1.02
38
-
>200
1400
-
1.08
0/6
40:30
:30
51
300
1400
R 83
1.09
-
137
Table 9 – Some thermal properties of unfilled nylons
Property
46
66 6 610
612
11
12
66/610/6
30)
Melting point, °C
295
264
215
215
210
185
175
160
Coefficient of
linear expansion,
-5
cm/cm·°C
10
(dry)
9.2
10
9.5
15
9
15
12
(40:30:
-
The nylons are reasonably good electrical insulators at low
temperatures and under conditions of low humidity but the insulation
properties deteriorate as humidity and temperature increase.
Nylons 46, 6, 66, 610, 11 and 12 are polar crystalline materials
with exceptionally good resistance to hydrocarbons. Esters, alkyl halides,
and glycols have little effect. Alcohols generally have some swelling action
and may in fact dissolve some copolymers (e.g. nylon 66/610/6). There are
few solvents for the nylons, of which the most common are formic acid,
glacial acetic acid, phenols and cresols.
Mineral acids attack the nylons but the rate of attack depends on
the type of nylon and the nature and concentration of the acid. Nitric acid is
generally active at all concentrations. The nylons have very good resistance
to alkalis at room temperature. Resistance to all chemicals is more limited
at elevated temperatures.
The nylons are hygroscopic. The absorbed water has a plasticizing
effect and thus will cause a reduction in tensile strength and modulus, and
an increase in impact strength. The presence of absorbed water also results
in a deterioration of electrical properties.
As may be expected from its structure, nylon 46 has a higher level
of water absorption (about 14% in water at 23 °C) than any of the other
commercial nylons. For this reason there can be large differences in
properties of dry, as moulded, samples, compared to samples conditioned to
more common atmospheric conditions. Some of these differences are
emphasised in Table 10.
When in service indoors or otherwise protected from sunlight the
nylons show no appreciable change of properties on aging at room
temperature. Care should be taken in the use of the polymers when exposed
to direct sunlight, particularly in film and filament applications, where
embrittlement is liable to occur. Some improvement may be achieved if
stabilized compounds are used. Continuous exposure to air at temperatures
138
above 60°C will also cause surface discolouration and a lower impact
Property
Dry
Conditioned
Impact strength (Izod Notched kJ/m2)
Rockwell hardness
10
R 123
40
R 107
strength of mouldings. The useful life of a moulding in service at 100°C
will be of t he orde r of only four to six weeks. If the moulding is immersed
in oil, or otherwise shielded from oxygen, a considerably longer life-time
may be expected. Heat-stabilized grades have markedly improved
resistance.
Table 10 – Effect of conditioning on selected mechanical properties of
nylon 46 (Conditioning according to ISO 1110 accelerated method;
70°C/62% RH. All measurements at 23°C)
Tensile yield strength, MPa
Tensile strength (break), MPa
Elongation at break, %
Tensile modulus, MPa
Flexural modulus, MPa
Additives
The major nylon moulding materials are each available in a number
of grades. These may differ in molecular weight but they may also di ff er in
the nature of additives which may be present.
Additives used in nylon can be grouped as follows:
(1) Heat stabilizers.
(2) Light stabilizers.
(3) Plasticizers.
(4) Lubricants.
(5) Reinforcing fillers.
(6) Pigments.
(7) Fungicides.
(8) Nucleating agents.
(9) Flame retarders.
(10) Impact modifiers.
Amongst heat stabilizers are copper salts, phosphoric acid esters,
phenyl-β-naphthylamine, mercaptobenzothiazole and
mercaptobenzimidazole. Of these, copper salts in conjunction with halides
have been found particularly effective, and some automotive specifications
require the use of copper for heat stabilization. Light stabilizers include
100
80
30
3000
3200
40
65
280
1000
1000
139
carbon black and various phenolic materials.
CH
3
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O
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3
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5
O
O
Plasticizers are comparatively uncommon but plasticized grades
are supplied by some manufacturers. Plasticizers lower the melting point
and improve toughness and flexibility, particularly at low temperatures. An
example of a plasticizer used commercially in Santicizer 8, a blend of o-
and p-toluene ethyl sulphonamide:
Self-lubricating grades are of particular value in some gear and
bearing applications. One commercial nylon compound incorporates 0.20%
molybdenum disulphide and 1% of graphite whilst many other commercial
compounds contain only one of these two lubricants. Lubricants may also
be used to enhance flow and mould release. Materials used are usually of
low molecular weight, contain a hydrocarbon component and an amide
component, and are typified by ethylene bis(stearamide).
In addition to the nucleating agents many other materials have been
found to be effective. Whilst the nylons may be self-nucleating, particularly
if there is some unmelted crystal structure, seeding with higher melting
point polymers can be effective. Thus nylon 66 and poly(ethylene
terephthalate) are reported to be especially attractive for nylon 6.
As with many other plastics materials there have been substantial
efforts to improve the resistance of nylons to burning. Halogen compounds
synergized by zinc oxide or zinc borate have been used whilst compounds
containing red phosphorus exhibit very good electrical insulation properties
as well as improved flame resistance. They are, however, dark in colour.
Furthermore grades of nylons for electrical and electronic applications need
to be phosphorus and halogen-free, and melamine cyanurate is used as a
flame retardant in this instance with unreinforced grades. With glass-filled
grades magnesium hydroxide may be used for this purpose but it is required
in substantial quantities and is less effective than typical halogen and
phosphorus-containing additives.
Glass-reinforced nylon compounds have become available in recent
years.
There has also been some interest in polymers containing
particulate mineral fillers.
Whilst the aliphatic nylons are generally classified as being impact
140
resistant, they are affected by stress concentrators like sharp corners which
may lead to brittle failures. Incorporation of rubbers which are not soluble
in the nylons and hence form dispersions of rubber droplets in the
polyamide matrix but which nevertheless can have some interaction
between rubber and polyamide can be most effective. Materials described
in the literature include the ethylene-propylene rubbers, ionomers,
polyurethanes, acrylates and methacrylates, ABS polymers and polyamides
from dimer acid.
Glass-filled nylons
There are a number of properties in which the thermoplastics show
up to a disadvantage when compared with metals. These include:
(1) Low rigidity and tensile strength.
(2) Dimensional instability due to a high temperature coefficient of
expansion and a high water absorption.
(3) Low impact strength to fracture.
(4) Low maximum service temperature.
(5) Low creep resistance.
(6) Low hardness and scratch resistance.
In an attempt to minimize these disadvantages glass-filled varieties
of a number of thermoplastics have been successfully introduced. Of these
the glass-filled nylons form the most important group and these in turn can
be subgrouped into glass-fibre-filled grades and glass-bead-filled varieties.
The glass-fibre-filled types can be obtained in two ways.
One route (the “long-glass” process) involves passing continuous
lengths of glass fibre (as rovings) through a polymer melt or solution to
produce a glass-reinforced strand that is chopped into pellets about 0.3 cm
dia 0.6-1.2 cm in length. In this case the fibres will be parallel to the “long”
axis of the pellet. The alternative route involves blending a mixture of resin
and glass fibres about 0.6 cm length in an extruder. From 20% to 40% glass
is used, usually of the electrical grade (E-grade) and with diameter of about
0.001 cm. Before blending with the nylon the glass fibres are often treated
with a lubricant to improve mechanical handling of the roving, a coupling
agent such as a silane to improve the resin-glass bond and some p oly(vi nyl
acetate) resin to hold the filaments together as a strand.
Such reinforcement leads to a substantial increase in tensile
strength, modulus, hardness, creep resistance and a sharply reduced
coefficient of expansion.
The glass-fibre nylons have a resistance to creep at least three