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91
of glass filaments) about 2 in long bound together by a resinous binder.
This type of mat is used extensively in glass-reinforced polyester
structures.
(3) Needle mat. This is similar to chopped strand mat except that
the mat is held together by a loose stitching rather than a binder.
(4) Preforms. Preformed shapes may be made by depositing glass
fibres on to a preform mould. The fibres are then held together by spraying
them with a binder.
Other types of glass structures used include rovings, yarns, tapes,
rovings fabrics and surfacing mats.
The major process today for laminates production is the hand layup technique in which resin is stippled and rolled into the glass mat (or
cloth) by hand. Moulds are easy to fabricate and large structures my be
made at little cost.
For mass production purposes matched metal moulding techniques
are employed. Here the preform or mat is placed in a heated mould and the
resin poured on. The press is closed and light pressure applied. Curing
schedules are usually about three minutes at 120°C. It is possible to
produce laminates using less resin with pressure moulding than with hand
lay-up techniques and this results in better mechanical properties.
A number of techniques intermediate between these two extreme
processes also exist involving vacuum bags, vacuum impregnation, rubber
plungers and other devices. In addition there are such diverse processes as
filament winding, cold moulding, e.g. the Resinject process, and extrusion
techniques using glass filaments,
Inert fillers are sometimes mixed with the resin in an effort to
reduce cost. However, many fillers increase the viscosity to such an extent
that with hand lay-up methods much more of the resin-filler mix is
required to impregnate the mat. Since greater difficulty in working may
also prolong processing time and there is invariably a marked drop in
mechanical properties care must be taken before making a decision
whether or not to employ fillers.
There is one particular type of filler whose value can be in no
doubt. This is the so-called thixotropic filler exemplified by certain fine
silicas and silicates which appear to increase the viscosity of the resin on
standing. These are useful in minimising drainage of resins from vertical
and near-vertical surfaces during hand lay -up operations.
Some typical properties of polyester-glass laminates are given in
Table 3. From these figures it will be seen that laminates can have very
92
high tensile strengths. On the other hand some laminates made by hand
Fine
Specific gravity
1.4-1.5
1.5-1.8
~2.0
2.19
lay-up processes may have mechanical properties not very different from
those of thermoplastics such as the polyacetals and unplasticized PVC.
Table 3 - Some typical properties of polyester-glass laminates
Tensile strength, MPa
Flexural strength, MPa
Flexural modulus, MPa
Dielectric constant, 10
Hz
Water absorption, %
complicated equipment.
metals.
panels.
desired.
particularly those that are available in sheet form.
epoxide and furane resins for general purpose laminates and account for at
least 95% of the low-pressure laminates produced. The epoxide resins find
specialized uses for chemical, elec tric al and heat -resistant applications and
for optimum mechanical properties.
for roofing and building insulation and accounts for about one-third of the
resin produced. For the greatest transparency it is important that the
refractive indices of glass, cured resins and binder be identical. For this
reason the glass fibre and resin suppliers provide raw materials which are
Press
formed
mat
laminate
124-173
138-190
4150
3.2-4.5
0.2-0.8
square
woven
cloth
laminate
210-310
267-380
6890
3.6-4.2
0.2-0.8
Rod
from
rovings
1030
1100
45500
-
-
Property
laminate
6
Hand
lay-up
mat
55-117
69-138
3440
3.2-4.5
0.2-0.8
The most desirable features of polyester-glass laminates are:
(1) They can be used to construct large mouldings without
(2) Good strength and rigidity although much less dense than most
(3) They can be used to make large, tough, low-density, translucent
(4) They can be used to make the materials fire retardant where
(5) Superior heat resistance to most rigid thermoplastics,
Because of their favourable price, po lyest ers are prefe rred to
The largest single outlet for polyester-glass laminates is in sheeting
93
specially made to approximate to these requirements. This outlet is now
CH
2
CH2OH
O
CH
2
CH
2
OH
+
CH
2
CH CH
2
O
CO
OH
2
CH
2
CH
2
O
CO
O CH
2
CH
2
O
CH
2
CH
2
CH
2
CH
2
O
CO
O
Diethylene Glycol
Allyl Acid Carbonate
being challenged by rigid PVC sheeting, which is much cheaper than fireretardant polyester laminate s.
Polyester resins have been widely accepted in the manufacture of
boat hulls, including minesweepers. Such hulls are competitive in price
with those built from traditional materials and are easier to maintain and
repair.
The third major outlet is in land transport, where the ability to form
large structures has been used in the building of sports car bodies, in lorry
cabs, in panelling for lorries, particularly translucent roofing panels, and in
public transport vehicles. In such applications the number of mouldings
required is quite small. The polyester-glass structures are less suitable for
large-quantity production since in these circumstances the equipment
requirements rise steeply and it eventually becomes more economical to
use the more traditional stamped metal shapings.
Aircraft radomes, ducting, spinners and other parts are often
prepared from polyester resins in conjunction with glass cloth or mat. The
principal virtue here is the high strength/weight ratio possible, particularly
when glass cloth is used. Land, sea and air transport applications account
for almost half the polyester resin produced.
Other applications include such diverse items as chemical plant,
stacking chairs, swimming pools, trays and sports equipment.
Allyl resins
A number of useful resins have been prepared from allyl
compounds, i.e. derivatives of allyl alcohol CH2 = CH CH2OH. One of
these, diethylene glycol bisallyl carbonate, was one of the first polyestertype materials to be developed for laminating and casting. It was
introduced in about 1941 by the Pittsburgh Plate Glass Company as
Allymer CR39 and was produced by the reaction shown in Figure 27. It
could be cured with benzoyl peroxide at 80°C. It is used today for
spectacle lenses.
Figure 27
94
Diallyl phthalate has also been used as a laminating resin but
because of its higher price it has been largely replaced by the glycolsaturated acid-unsaturated acid polyesters.
Other allyl compounds described in the literature include diallyl
carbonate, diallyl isophthalate and diallyl benzene phosphonate.
1.4.2 Polyester Moulding Compositions
Although phenolic and amino moulding powders remain by far the
most important of the thermosetting moulding compositions a number of
new materials have been introduced over the last 30 years based on
polyester, epoxy and silicone resins.
Five classes of polyester compound may be recognized:
(1) Dough moulding compounds (DMC).
(2) Sheet moulding compounds (SMC).
(3) Alkyd moulding compositions, sometimes referred to as
“polyester alkyds”.
(4) Diallyl phthalate compounds.
(5) Diallyl isophthalate compounds.
The dough moulding compounds were originally developed in an
attempt to combine the mechanical properties of polyester-glass laminates
with the speed of cure of conventional moulding powder. In spite of their
somewhat high cost they have now established themselves in a number of
applications where a mechanically strong electrical insulant is required.
Dough moulding compositions, also known as bulk moulding
compounds, are prepared by blending resin, powdered mineral filler,
reinforcing fibre, pigment and lubricant in a dough mixer, usually of the Zblade type. The resins are similar to conventional laminating resins, a fairly
rigid type being preferred so that cured mouldings may be extracted from
the mould at 160°C without undue distortion. Organic peroxides such as
benzoyl peroxide and tertiary butyl perbenzoate are commonly used as
“catalysts”. The choice of “catalyst” will influence cure conditions and will
also be a factor in whether or not surface cracks appear on the mouldings.
Mineral fillers such as calcium carbonate are employed not only to reduce
costs but to reduce shrinkage and to aid the flow since an incorrect
viscosity may lead to such faults as fibre bunching and resin-starved areas.
Although glass fibre (E type) is most commonly employed as the
reinforcing fibre, sisal is used in cheaper compositions. Stearic acid or a
metal stearate are the usual lubricants.
In common with all polyester moulding compositions the dough
95
moulding compounds cure without evolution of volatiles and thus pressures
as low as 1.4MPa, but normally about 6.9 MPa, may be used. The material,
of putty-like consistency, is first preformed into a ball shape and loaded
into the mould of a fast-acting press in such a way that there should be a
minimum of weld lines and undesirable fibre alignment. Temperatures in
the range 110-170°C may be employed and at the higher temperatures cure
times of less than one minute are possible.
Dough or bulk moulding compounds can suffer from a number of
disadvantages of which the most important are:
(1) Problems of easy metering and handling of the materials before
loading into the mould.
(2) Tendency of thick sections to crack.
(3) Warping, difficulty of moulding to close tolerances and wavy
or fibre-patterned surfaces or faults arising from the high shrinkage during
cure.
(4) Difficulties in moulding large structural parts with no control
on fibre orientation.
Manufacture of traditional dough moulding compounds involves
intensive shear and hence extensive damage to fibres so that strengths
obtained with GRP laminates are seldom realized. This problem is largely
avoided with the sheet moulding compounds, which were introduced in
about 1967 and by 1972 were being produced at the rate of about 20000
tonnes per year. Resin, lubricant, filler thickening agents and curing
systems are blended together and then coated on to two polyethylene films.
Chopped glass rovings are then fed between the resin layers, which are
subsequently sandwiched together and compacted as indicated in Figure
28. For moulding, blanks may easily be cut to the appropriate weight and
shape. There appears to be no reason why this system should not be
extended to allow predetermined fibre orientation or to superimpose
oriented continuous filament on the chopped randomly oriented fibres
where this is desirable. Low-profile resins are often used with these
compounds whose main applications are in car parts, baths and doors.
The “polyester” alkyd moulding compositions are also based on a
resin similar to those used for laminating. They are prepared by blending the
resin with cellulose pulp, mineral filler, lubricants, pigments and peroxide
curing agents on hot rolls until thoroughly mixed and of the desired flow
properties. The resultant hide is removed, cooled, crushed and ground.
96
Figure 28 - Outline of machine for preparing sheet moulding compounds
COO
COO
CH
2
CH
CH
2
CH
2
CH
CH
2
(SMC)
On heating with a peroxide, diallyl phthalate will polymerize and
eventually cross-link because of the presence of two double bonds:
This monomer has been used as the basis of a laminating resin and
as a reactive diluent in polyester laminating resins, but at the present time its
principal value is in moulding compositions. It is possible to heat the
monomer under carefully controlled conditions to give a soluble and stable
partial polymer in the form of a white powder. The powder may then be
blended with fillers, peroxide catalysts and other ingredients in the same
manner as the polyester alkyds to form a moulding powder. Similar
materials may be obtained from dially l iso p hthalate.
The diallyl phthalate (DAP) resins compare favourably with the
phenolic resins in their electrical insulation characteristics under conditions
of dry and wet heat. The diallyl isophthalate (DAIP) compositions are more
expensive but have better heat resistance and are claimed to be capable of
withstanding temperatures as high as 220°C for long periods. Both the DAP
and the DAIP materials are superior to the phenolics in their tracking
resistance and in their availability in a wide range of colours. They do,
however, tend to show a higher shrinkage on cure and in cases where this
97
may be important, e.g. thin walls round inserts, it may be necessary to
employ epoxide moulding compositions.
The “polyester alkyd” resins are lower in cost than the DAP resins
but are weaker mechanically, have a lower resistance to cracking round
inserts and do not maintain their electrical properties so well under severe
humid conditions. Fast-curing grades are available which will cure in as
little as 20 seconds.
1.4.3 Poly(ethylene terephthalate) Moulding Materials
In 1997 it was estimated that global production of PET was about
16.7·106 t.p.a., of which 12 million tonnes was used in textiles, 2 million
tonnes for audio and video film (with a small quantity for technical
mouldings) and 3 million tonnes for packaging, particularly bottles. The
tremendous growth in the bottles market from zero in the late 1970s to 1.5
million tonnes in the USA alone in 1998 is, in consumption terms, one of
the most spectacular examples of growth in plastics materials in recent
times.
Because of its rather high transition temperature (of about 80°C)
only a limited amount of crystallization can occur during cooling after
injection moulding poly(ethylene terephthalate). Such mouldings are
transparent and amorphous and are of little value. When they are heated
above 80°C crystallization can occur and they show considerable
distortion, shrinkage and clouding, Injection moulding is also difficult
because of the sensitivity of melts to traces of moisture.
For this reason the idea of moulding poly(ethylene terephthalate)
was for many years not a technical proposition. However, developments
with nucleating agents in the early 1960s led to the production in 1966 of
semi-finished products such as rod and pipe by a continuous casting
process developed by Glanzstoff-AG of Obernbung/Main. These materials
had a high hardness, creep resistance and rigidity, with a water absorption
similar to that of acetal resins, but have a disadvantage of sensitivity to hot
water and alkaline solutions.
In the same year AKZO introduced a poly(ethylene terephthalate)
moulding and extrusion material known as Arnite PETP. This material was
one of several introduced in the mid-1960s as engineering thermoplastics;
others include the polysulphones, phenoxies and poly(phenylene oxide).
The principal properties claimed for Arnite PETP are its high gloss, hard
scratch resistance surface and a high rigidity. Some of its properties are
given in Table 4.
98
Table 4 – Some properties of poly(ethylene terephthalate) moulding
Property
Value
Specific gravity
1.37-1.38
material (“Arnite”) (ASTM Test Methods unless otherwise stated.)
Moisture absorption in water
24 h at 23°C, %
2 h at 100°C, %
Vicat softening point, °C
Tensile strength
at yield, MPa
at break, MPa
Dielectric constant, 10
material and because of the free-flowing nature of the melt, restricted
nozzles should be used and a back flow valve fitted to screw injection
moulding machines. Cylinder temperatures are about 260°C and mould
temperatures as high as 140°C to promote a controlled crystallization.
Because of this high temperature it is generally recommended that the
mould be thermally insulated from the locking mechanism and other
machine parts.
according to the moulding conditions two quite dissimilar products, one
amorphous, the other transparent, may be obtained, this being a
consequence of having a Tg of about 80°C. For both types, however, there
are certain common points to be observed. As with other polymers that are
slightly hygroscopic and which require high melt temperatures the granules
must be thoroughly dry, particular care being necessary with reworked
material. In addition, because of the low melt viscosity injection, moulding
screws should be fitted with back flow valves and the barrel nozzles should
have shut-off valves. Melt temperatures are of the order of 260°C.
terephthalate) as a moulding material remained at a low level for many
years. In the 1970s it became recognized that reinforcement of the polymer
with glass fibre had an even greater influence on modulus and rigidity than
with other engineering plastics. For example, at 23°C and 50% RH the
flexural modulus of unfilled crystalline poly(ethylene terephthalate) is
slightly less than that of a polyacetal. On the other hand, at a glass-fibre
0.02
0.10
261
71.5
6
Hz
52.9
3.37
The moulding of Arnite PETP must be carried out with dried
An interesting feature of poly(ethylene terephthalate) is that
In spite of the introduction of Arnite PETP the use of poly(ethylene
99
loading of 30% the modulus of the polyester is some 10% higher (11000
MPa c.f. 10000 MPa). At 50% fibre loading the modulus is as high as
15000 MPa.
By the late 1980s it was estimated that 90% of crystalline PET
moulding materials were glass filled. Their major use was in electrical and
electronic applications. Thin, complex sections such as transformer
bobbins may be formed easily because of the ease of flow of the polymer
even when fibre filled. These materials have also been used for the
housings and components for toasters, coffee machines, industrial plugs
and sockets, car heater housings and water meter housings. Tougher grades
are used for car grilles and fuel filler flaps. Amorphous grades are used
mainly for bottles.
Towards the end of the 1970s Du Pont introduced Rynite. This is a
poly(ethylene terephthalate) nucleated with an ionomer, containing a
plasticizer (thought to be n-pentyl glycol dibenzoate) and only available in
glass-fibre-filled form (at 30, 45, and 55% filler levels). Although Tg is
slightly reduced, due to the presence of the plasticizer, to about 55-60°C
the polymer is very rigid, exceeding that of a polysulphone. It is less water
sensitive than an unfilled polymer. Apart from its poor tracking resistance,
a common feature of many highly aromatic polymers, its electrical
properties are generally good whilst, as with the Arnite-type materials, fireretarding grades are now available.
In the late 1970s the benefits of biaxial stretching of poly(ethylene
terephthalate) were extended from sheet film to bottle manufacture. As a
result important new markets were opened. For some years the plastics
industry had made great efforts to secure part of the market for the
packaging of carbonated beverages. In the early 1970s it seemed that this
hope would be fulfilled by the use of the nitrile resins but toxicity problems
largely associated with residual acrylonitrile made this impossible.
Fortunately the recognition that nitrile resins could no longer be considered
for this market coincided with the development of techniques for bottle
blowing poly(ethylene terephthalate). In 1978 estimates for USA
consumption of poly(ethylene terephthalate) for bottles were in the range
68000-86000 tonnes. By 1998 the corresponding figure was 1430000
tonnes. This involves special polymer grades and copolymers with
isophthalic acid or cyclohexanedimethanol are being increasingly used to
improve clarity, toughness and barrier properties. Whilst the USA market
has been dominated by the carbonated beverage market the process has
been extended, particularly in Europe, to produce bottles for other purposes
100
such as fruit juice concentrates and sauces. Wide-necked jars, for coffee
and for other materials, also made their appearance.
Success in bottle blowing involves first the production of a
substantially amorphous parison by injecting into a cold mould. The
parison is then withdrawn from the mould, heated (for example by infrared
heaters) and subjected to a stretch-blow process that biaxially stretches the
parison, giving a thin-wall containers of high strength and toughness
combined with a low permeability to oxygen and carbon dioxide. Further
reductions in gas permeability may be achieved using multi-layer parison
extrudates. For example, in Britain PET bottles coated with vinylidene
chloride-based copolymers are used for packaging beer. There has also
been some interest in poly-m-xylylene adipamide and, more particularly,
ethylene-vinyl alcohol copolymers as barrier materials.
As with many other plastics materials being manufactured in a
large number of countries statistics for capacity and usage are subject to
considerable uncertainty. One estimate was that in 1997 capacity for
making “container” grades was about 6000000 t.p.a. with consumption at
about 4000000 t.p.a.
Other estimates placed the film and bottle market to be of a
similar size in Japan while globally the bottle market was about 20% of
the total. Together with other data this suggests that the fibre and filament
market absorbs about 72% of PET capacity, containers about 19%, film
about 7% and mouldings 2%. Considerable quantities of PET bottles are,
however, recycled into fibres for use, for example, in outdoor clothing.
1.4.4 Polycarbonates
Reaction of polyhydroxy compounds with polybasic acids gives
rise to condensation polymers containing ester (–COO–) groups. Because
of the presence of these groups such polycondensates are known as
polyesters and find use in such diverse applications as fibres, surface
coatings, plasticizers, rubbers and laminating resins.
By reaction of polyhydroxy compounds with a carbonic acid
derivative, a series of related polymers may be produced with carbonate (–
OCOO–) linkages, the polymers being referred to as polycarbonates.
Carbonic acid, CO(OH)2, itself does not exist in the free state but by means
of ester exchange (Figure29) (I) and phosgenation techniques (II) it is
possible to produce useful products.