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Файл:Plastics technology. Часть 2. Учебное пособие.pdf
141
times as great as unfilled polymers. In the case of impact strength the
Property
Nylon 66 +
Nylon 66 +
Tensile strength, MPa
Melting point, °C
200
263
230
263
situation is complex since unfilled nylons tend to break showing tough
fracture whereas the filled polymers break with a brittle fracture. On the
other hand the glass-filled polymers are less notch sensitive and in some
tests and service conditions the glass-filled nylons may prove the more
satisfactory.
As with other crystalline polymers, the incorporation of glass fibres
narrows the gap between the heat deflection temperatures and the
crystalline melting point.
Whilst most glass-fibre-reinforced polyamides have been of the
short-glass type, there has been a recent resurgence of interest in the longglass types. The Verton materials introduced by ICI are claimed to have
better impact resistance and higher stiffness than short-glass compounds,
together with better impact resistance retention at low temperatures and
stiffness retention at higher temperatures. A comparison of some properties
of long- and short- fibre materials is given in Table 11.
Table 11 – Comparison of short- and long-fibre glass-reinforced nylon 66
(50% fibre loading)
Elongation, %
Flexural modulus, GPa
Deflection temperature (1.8 MPa), °C
Nylons filled with glass beads were also introduced in the late
1960s. Grades filled with 40% of glass spheres have compressive strength
some eight-fold higher than those of unfilled nylons as well as showing
good improvement in heat distortion temperature, tensile strength and
modulus. Compared with glass-fibre, filled grades they are easy to process,
with low melt viscosity, uniform and predictable shrinkage and minimum
warpage. They are also more isotropic in their mechanical properties.
Both fibres and sphere fillers tend to improve self-extinguishing
characteristics.
50% short
fibre
3
12
250
50% long
fibre
4
15.8
261
142
Comparison of nylons 6 and 66 in glass-filled compositions
The presence of glass-fibre fillers can to some extent mask the
differences between nylons 6 and 66. For example, an advantage of
unfilled nylon 66 in injection moulding is that the high Tm leads to a high
solidification temperature and shorter cycle times. However, in glass-filled
grades the more rapid cooling and crystallization can lead to a poorer
surface finish than obtained with corresponding nylon 6 compounds. It is
also considered that abrasive wear on screws is gre ater w ith nylon 6.
Water absorption decreases with increasing glass-fibre content at
about the same rate with both nylons 66 and 6, and since nylon 6 has an
intrinsically higher water absorption than nylon 66 the glass-filled grades
also have higher levels at similar glass-fibre loadings.
Mechanical properties of freshly injected compositions are similar
for the two nylons but, after conditioning, differences arise largely due to
the plasticising effect of the moisture present. Thus for tensile and flexural
yield stress, tensile strength and modulus of elasticity, nylon 66 gives
slightly higher figures. Yield elongation and elongation at break are greater
with nylon 6. Izod impact strengths are similar, with nylon 6 giving
marginally higher values.
The above comments refer to comparisons between the two
compositions at the same glass-fibre level. If, however, comparison is made
between a nylon 66 composition with a glass content of x% and a nylon 6
compound with a glass content of (x + 5)%, then the differences in
mechanical properties become very small. At the same time the nylon 6
material will have slightly easier processing characteristics and surface
quality.
Whilst nylon 66 has the higher Tm, the long-term heat resistance of
typical copper-stabilized nylon 6 is somewhat superior in such properties as
impact strength and bending strength compared to nylon 66. However, it is
frequently the case that nylon 66 has better resistance to chemicals at
elevated temperatures.
Processing of the nylons
In the processing of nylons consideration should be given to the
following points:
(1) The tendency of the material to absorb water.
(2) The high melting point of the homopolymers.
(3) The low melt viscosity of the homopolymers.
(4) The tendency of the material to oxidise at high temperatures
where oxygen is present.
143
(5) The crystallinity of the solid polymer and hence the extensive
shrinkage during cooling.
The above features are particularly marked with nylons 46, 6, 66
and 610 and less marked with nylons 11 and 12. Providing they are dry the
copolymers may be processed in much the same way as conventional
thermoplastics.
In the injection moulding of nylon 66, for example, it is necessary
that the granules be dry. The polymer is normally supplied in sealed
containers but should be used within an hour of opening. If reworked
polymer is being used, or granules have become otherwise damp, the
polymer should be dried in an oven at about 70-90°C. Too high a
temperature will oxidise the surface of the granules and result in inferior
mouldings.
Injection moulding cylinders should be free from dead spots and a
temperature gradient along the cylinder is desirable.
Because of the crystallization that occurs on cooling from the melt
the polymers show a higher moulding shrinkage than that generally
observed with amorphous polymers. With average moulding conditions
this is about 0.018 cm/cm with nylon 66 but by increasing the injection
pressure and the injection time the shrinkage may be halved. This is
because a high initial mould cavity pressure is developed and a large part
of the crystallization process will be complete before the cavity pressure
has dropped to zero. The shrinkage will also be affected by the melt
temperature, the mould temperature, the injection speed and the design of
the mould as well as by the type of nylon used.
The nylons, nylon 66 in particular, may also exhibit a certain
amount of after-shrinkage. Further dimensional changes may occur as a
result of moulding stresses being relieved by the plasticizing effect of
absorbed water. It is consequently often useful to anneal mouldings in a
non-oxidising oil for about 20 minutes at a temperature 20°C higher than
the maximum service temperature. Where this is not known a temperature
of 170°C is suitable for nylon 66, with somewhat lower temperatures for
the other nylons.
The particular features of the nylons should also be taken into
account in extrusion. Dry granules must be used unless a devolatilising
extruder is employed. Because of the sharp melting point it is found
appropriate to use a screw with a very short compression zone. Polymers of
the lowest melt viscosity are to be avoided since they are difficult to
144
handle. Provision should be made to initiate cooling immediately the
extrudate leaves the die.
The polymerization casting process has been adapted to reaction
injection moulding (RIM), a process originally developed for
polyurethanes. In this process the reacting ingredients are mixed together
by impingement of jets of the materials in a small mixing chamber adjacent
to the mould cavity into which the reacting material is then injected.
Because of the low injection pressures much lower locking forces are
possible than in conventional injection moulding, making the process
attractive for large area mouldings.
Unlike polyurethane-RIM processes, nylon-RIM reactions are
endothermic and require temperatures of 130-140°C. In contrast to the
polyurethane-RIM systems, this enables thick wall parts to be made. Cycle
times of 2-3 minutes are comparable to those for polyurethane-RIM. In th e
development stage, current work is concerned with reducing moulding
times and optimizing moulding conditions.
Applications
Consumption of poly amide plastics in the late 1990s has been
estimated at about 1300000 t.p.a. Western Europe and the United States
each have about 42% of this market and Japan about 16%. This i s pr ob abl y
about 20% of the total production of polyamide materials and virtually all
of the rest of polyamide production goes into fibres where the market is
shared very roughly equally between nylons 6 and 66. The large-scale
production of these materials enables them to be available at a substantially
lower price than the other nylons, which do not have the benefit of the
economies of scale. Hence the other nylons are usually restricted to
applications where nylons 6 and 66 are unsuitable.
The nylons have found steadily increasing application as plastics
materials for speciality purposes where their toughness, rigidity, abrasion
resistance, good hydrocarbon resistance and reasonable heat resistance are
important. Because of their high cost they have not become general
purpose materials such as polyethylene and polystyrene, which are about a
third of the price of the nylons.
The largest applications of the homopol ymers (nylon 6, 66, 610, 11
and 12) have been in mechanical engineering. Well-known applications
include gears, cams, bearings, bushes and valve seats. In addition to the
advantageous properties cited above, nylon moving parts may be
frequently operated without lubrication, are silent running and may often
145
be moulded in one piece when previously a metal part required assembling
of several parts, or alternatively, extensive machining with consequent
waste of material.
In recent years the nylons have met increased competition from
acetal resins, the latter being superior in fatigue endurance, creep resistance
and water resistance. Under average conditions of humidity the nylons are
superior in impact toughness and abrasion resistance. When a nylon is
considered appropriate it is necessary to consider the relative importance of
mechanical properties, water resistance and ease of processing. For the best
mechanical properties nylon 66 would be considered but this material is
probably the most difficult to process and has a high water absorption
value. Nylon 6 is easier to process but has slightly inferior mechanical
properties and an even higher water absorption. Nylons 11 and 12 have the
lowest water absorption, and are easy to process, but there is some loss in
mechanical properties.
Sterilizable mouldings have found application in medicine and
pharmacy. Because of their durability, nylon hair combs have found wide
acceptance in spite of their higher cost.
Nylon film has been used increasingly for packaging applications
for foodstuffs and pharmaceutical products.
Although the nylons are not generally considered as outstanding
electrical insulators, their toughness and, to some extent, their temperature
resistance, have led to applications in coil formers and terminal blocks.
Nylon monofilaments have found application in brush tufting,
wigs, surgical sutures, sports equipment, braiding and outdoor upholstery.
Nylons 610 and 11 have found extensive application in these fields because
of their flexibility but nylon 66 is also used for brush tufting less than
0.0035 in. in diameter. Nylon 66/610 copolymer is used in the manufacture
of a monofilament for angling purposes.
Nylon 11 is also used in powder form in spraying and fluidised bed
dipping to produce chemical-resistant coatings. Although more expensive
than the polyolefin and PVC powders, it is of interest because of its
hardness, abrasion resistance and petrol resistance.
Amongst the products made by polymerization casting are
propellers for small marine craft, conveyor buckets used in the mining
industry, liners for coal washing equipment and main drive gears for use in
the textile and papermaking industries.
There is persisting interest in nylon-RIM materials as alternatives
to polyurethane-RIM. Advantages of the nylon materials are the better
146
shelf life and lower viscosity of the reaction components, ability to mould
thick-walled articles, absence of a need for mould lubrication and the
ability to avoid using isocyanates with their associated hazards. The main
disadvantages of nylon-RIM are the need to have heated storage tanks and
elevated temperature reactions, difficulties in catalyst handling and the
high water absorption of the product. Possible markets include exterior car
body components and appliance and business machine components.
The glass-reinforced nylon plastics are now of substantial
importance and take about 30-40% of t he UK market. The rigidity, cre ep
resistance, low coefficient of friction and high heat deflection temperature
have enabled these materials to replace metals in many applications.
Furthermore their good low-frequency electrical insulation properties and
non-magnetic characteristics may also be utilized. For these reasons glassfibre-filled grades are widely used in housings and casings, in domestic
appliances, and in car components, including radiator parts. They are also
extensively used in the telecommunications field for relay coil formers and
tag blocks. Glass-bead-filled nylons have been used in bobbins.
Carbon-fibre-reinforced nylon 6 and nylon 6/12 mixtures have
been offered commercially and found use in aerospace and tennis racket
applications.
1.5.1 Polyamides of Enhanced Solubility
Polyamides such as nylon 6, nylon 66, nylon 610, nylon 11 and
nylon 12 exhibit properties which are largely due to their high molecular
order and the high degree of interchain attraction which is a result of their
ability to undergo hydrogen bonding.
It is, however, possible to produce polymers of radically different
properties by the following modifications of the molecular structure.
(1) Repla cement of some or all of the –CONH– hydrogens by alkyl
or alkoxy-alkyl groups to reduce hydrogen bonding which results in softer,
lower melting point and even rubber polymers (TV-substitution).
(2) Use of acids or amines containing large bulky side groups
which prevent close packing of the molecules.
(3) Use of trifunctional acids or amines to give branched structures.
(4) Copolymerization to give irregular structures.
(5) Reduction in molecular weight.
The techniques of N-alkylation may be effected by the use of N-
alkylated or N,N’-dialkylated diamines, or by the use of an ω-N-
alkylaminocarboxylic acid of type R1NHRCOOH. The polymers thus have
repeating units of the general form.
147
(C
H
2
)nCON
R
NH
CO
+ CH2O + ROH
N
CO
CH2OR or
N(CH2O)2R
CO
Such N-alkyl compounds are not known to be of any current
application although fibres from a partially N-alkylated derivative of
nylon 610 have been described.
Treatment of a nylon with formaldehyde leads to the formation of
N-methylol groups but the polymers are unstable. If, however, the nylon is
dissolved in the solvent such as 90% formic acid and then treated with
formaldehyde and an alcohol in the presence of an acidic catalyst such as
phosphoric acid a process of alkoxymethylation occurs:
Methylmethoxy nylons are commercially available in which about
33% of the –NH– groups have been substituted.
Such materials are soluble in the lower aliphatic alcohols,
e.g.ethanol, and in phenols. They also absorb up to 21% of moisture when
immersed in water. If this material is heated with 2% citric acid at elevated
temperatures, typically for 20 minutes at 120°C, cross-linking will take
place:
This material finds a limited application in films and coatings
which require good abrasion and flexing resistance.
In the early 1950s a new class of poly amides became available
differing from the nylons in that they contained bulky side groups, had a
somewhat irregular structure and were of low molecular weight (2000-
5000). They are marketed under such trade names as “Versamids” and
“Beckamides”.
A typical example of this class of polymer may be obtained by
reacting ethylenediamine and “dimer fatty acid”, a material of inexact
structure obtained by fractionating heat-polymerized unsaturated fatty oils
148
and esters. An idealized structure for this acid is shown in Figure 39. These
HOOC(CH
2)7
CH
CH
2
CH
CH
(CH
2)7
COOH
CH3(CH2)5CH
CH
CH
CH
CH3(CH2)
5
materials are dark coloured, ranging from viscous liquids to brittle resins
and with varying solubility.
Figure 39
They have found use as hardeners-cum-flexibilizers for epoxy
resins and are of interest in the production of thixotropic paints and
adhesives. Related higher molecular weight materials are tough and
flexible and find use as hot melt adhesives (Versalons).
A number of copolymers such as nylon 66/610/6 are available.
Such a copolymer has an irregular structure and thus interchain bonding
and crystallization are limited. As a consequence the copolymer is soluble
in alcohols and many other common polar solvents.
1.5.2 Other Aliphatic Polyamides
Although less than a dozen aliphatic polyamide types together with
a few miscellaneous copolymers have become available commercially, a
very large number have been prepared and investigated. Of the many
diamine-dibasic acid combinations those based on intermediates with less
than four carbon atoms are unsuitable either because of the tendency to
form ring structures or because the melting points are too high for melt
spinning (important in fibre production). The many nylons based on amines
and acids with 6-10 carbon atoms might also be of interest as fibres and
plastics but are not yet attractive commercially because of the costs of
synthesis. Similar remarks must also apply to nylons 8, 9 and 10.
Polyamides have also been produced from intermediates with
lateral side groups. The effect of such groups is similar to that of N-
substitution in that there is a decrease in intermolecular cohesion and
reduction in the ability of the molecules to pack in a crystal lattice. In some
cases the polymers are still fibre-forming but they have much lower
melting points. For example the polymer from 12-aminostearic acid
(Figure 40) is fibre-forming but has a low melting point (109°C) and a low
moisture-absorbing capacity.
149
NH
3
CH(CH
2
)
10
COOH
C
6H13
Figure 40
Name
R
Glycine
Lysine
H
(СH2)4NH2
One particular type of polyamide produced from intermediates
containing lateral side groups are the poly-(α-amino acids). The α-amino
acids have the structure shown in Figure 18.23 (I) and give polymers of the
type shown in Figure 41 (II). The proteins may be considered as a special
class of such polymers in that they are long chain molecules containing the
residues of some 25-30 amino acids arranged in a highly specific way in the
molecular chain. Table 12 gives the structures of some of the α-amino acids
that are produced by breakdown of proteins.
H
CONH
C
H2N
H
C
COOH
R
(I)
(II)
R
Figure 41
Table 12 - The structures of some of the α-amino acids
Alanine
Phenylalanine
Cysteine
Glutamine
Glutamic acid
Leucine
CH3
CH
CH
(CH
(CH
(CH
2C6H5
SH
2
CONH2
2)2
COOH
2)2
CH(CH3)
2)2
Where R ≠ H the amino acids may incorporate in either a D- or Lconfiguration and so it is possible for configurational polymers to be
produced. There do not, however, show the same mechanical properties as
the configurational homopolymers, which are more regular in structure.
Currently, α-amino acids are prepared by several routes such as by
the fermentation of glucose, by enzyme action on several substances and by
the hydrolysis of proteins. Many methods for synthesising the polymers are
known, of which the polymerization of N-carboxyanhydrides is of
150
particular interest, as it yield-products of high molecular weight:
COCH
N CO
O
R
NHCOC
H
R
+ CO
2
N
CO
CO
These polymers, typical of polyamides with fewer than four main
chain carbon atoms in the repeating unit, decompose before melting and
have to be processed from solution. Several of the polymers may, however,
be spun into fibres. Over thirty years ago Courtaulds produced silk-like
fibres on an experimental commercial scale from poly-(L-alanine) and from
poly-(α-methyl-L-glutamate). The latter material is also said to be in use as
a “synthetic leather” in Japan. The Japanese have also shown interest in
poly-(L-glutamic acid) for the manufacture of silk-like fibres.
Other polyamides produced experimentally include polymers wit h
active lateral groups (hydroxy, keto groups etc.), polymers with
heteroatoms (sulphur and oxygen) in the polyamide-forming intermediates,
polymers with tertiary amino groups in the main chain and polymers with
unsaturation in the main chain. There does not, however, appear to have
been any serious attempt to develop unsaturated polyamide analogues to
the polyester laminating resins.
1.5.3 Polyimides
The polyimides have the characteristic functional group below and
are thus closely related to the polyamides.
However, the branched nature of the functional group facilitates
the production of polymers with a backbone that consists predominantly of
ring structures and hence high softening points. Furthermore, many of the
structures exhibit a high level of thermal stability so that the polymers have
become of some importance in applications involving service at higher
temperatures than had been hitherto achieved with plastics materials.
The first commercial materials were introduced by Du Pont in the
early 1960s when they marketed a range of products obtained by
condensing pyromellitic dianhydride with aromatic amines, particularly di(4-aminophenyl) ether. These included a coating resin (Pyre ML) film
(originally H-film, later named Kapton) and in machinable block form
(Vespel). In spite of their high price these materials have found established