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Файл:Plastics technology. Часть 2. Учебное пособие.pdf
51
addition to being more suitable for automatic moulding machines these
powders are also more dust-free and thus more pleasant to use. For ease of
pelleting, however, a proportion of “fines” is valuable.
For the manufacture of medium-shock-resisting grades the preblend
of resin, filler and other ingredients does not readily form a hide on the mill
rolls. In this case the composition is preblended in an internal mixer before
passing on to the mills.
High-shock grades cannot be processed on mills or other intensive
mixers without destroying the essential fibrous structure of the filler. In
these cases a wet process is used in which the resin is dissolved in a
suitable solvent, such as industrial methylated spirits, and blended with the
filler and other ingredients in a dough mixer. The resulting wet mix is then
laid out on trays and dried in an oven.
Processing characteristics
As it is a thermosetting material, the bulk of phenol-formaldehyde
moulding compositions has in the past been largely processed on
compression and transfer moulding plant, with a very small amount being
extruded. The injection moulding process as modified for thermosetting
plastic is now being used significantly but still on a smaller scale than the
traditional processes.
Moulding compositions are available in a number of forms, largely
determined by the nature of the fillers used. Thus mineral-filled and
woodflour-filled grades are generally powders whilst fibre-filled grades
may be of a soft-lumpy texture. Fabric-filled grades are sold in the form of
shredded impregnated “rag”. The powder grades are available in differing
granulations. Very fine grades are preferred where there is a limited flow in
moulds and where a high-gloss finish is required. Fine powders are,
however, dusty and a compromise may be sought. For mouldings in which
extensive flow will occur, comparatively coarse (and thus dust-free)
powders can be used and a reasonable finish still obtained. For the best
pelleting properties it would appear that some “fines” are desirable for good
packing whilst “fines” are generally undesirable in powders employed in
automatic compression moulding.
Since the resins cure with evolution of volatiles, compression
moulding is carried out using moulding pressures of 15-30 MPa at 155170°C. In the case of transfer moulding, moulding pressures are usually
somewhat higher, at 30-120 MPa. As with other thermosetting materials an
increase in temperature has two effects. Firstly, it reduces the viscosity of
the molten resin and, secondly, it increa ses the rate of cure.
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There is no entirely satisfactory way of measuring flow. In the BS
2782 flow cup test an amount of moulding powder is added to the mould to
provide between 2 and 2.5 g of flash. The press is closed at a fixed initial
rate and at a fixed temperature and pressure. The time between the onset of
recorded pressure and the cessation of flash (i.e. the time at which the
mould has closed) is noted. This time is thus the time required to move a
given mass of material a fixed distance and is thus a measure of viscosity. It
is not a measure of the time available for flow. This property, or rather the
more important “length of flow” or extent of flow, must be measured by
some other device such as the flow disc or by the Rossi-Peakes flow test,
neither of which are entirely satisfactory. Cup flow times are normally of
the order of 10-25 seconds if measured by the BS specification. Moulding
powders are frequently classified as being of “stiff flow” if the cup flow
time exceeds 20 seconds, “medium flow” for times of 13-19 seconds and
‘soft flow” or “free flow” if under 12 seconds.
The bulk factor (i.e. ratio of the density of the moulding to the
apparent powder density) of powder is usually about 2-3 but the high-shock
grades may have bulk factors of 10-14 when loose, and still as high as 4-6
when packed in the mould. Powder grades are quite easy to pellet, but this
is difficult with the fabric-filled grades.
Phenol-formaldehyde moulding compositions may be preheated by
high-frequency methods without difficulty. Preheating, by this or other
techniques, will reduce cure time, shrinkage and required moulding
pressures. Furthermore, preheating will enhance the ease of flow, with
consequent reduction in mould wear and danger of damage to inserts.
Moulding shrinkage of general purpose grades is in the order of
0.005-0.08 in/in. Highly loaded mineral-filled grades have a lower
shrinkage whilst certain grades based on modified resins, e.g. acid-resistant
and minimum odour grades, may have somewhat higher shrinkage values.
Cure times will depend on the type of moulding powder used, the
moulding temperature, the degree of preheating employed and, most
important, on the end-use envisaged for the moulding. The time required to
give the best electrical insulation properties may not coincide with the time
required, say, for greatest hardness. For general purpose material this is
normally about 60 seconds but may be over twice this time with special
purpose grades.
One of the disadvantages of thermosetting plastics which existed
for many years was that whilst the common moulding processes for
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thermoplastics were easily automated this was much more difficult with
thermoset compression moulding. With the development of the
reciprocating single-screw injection moulding machines, equipment
became available which facilitated the adoption of injection moulding to
thermosets. In this adapted process the thermosetting granules are carefully
heated in the barrel so that they soften but do not cross-link before entering
the mould cavity. The moulds are, however, heated to curing temperatures
so that once the mould is filled cure is as fast as possible consistent with
obtaining the best balance of properties in the end-product.
As a result of these considerations, typical injection moulding
conditions are:
Melt temperature 110-140°C
Cylinder temperature 65-90°C
Nozzle temperature 85-120°C
Mould temperature 165 -195 °C
Injection pressure 85-250 MPa
Screw back pressure <7 (typically l) MPa
Screw speed 65-85 rev/min
Curing time 15-80 s
In order to obtain a good control of cylinder temperature, a fluid
heat transfer system is desirable. Such fluid may be heated in an adjacent
temperature controller or perhaps more commonly be circulated in channels
which are built in between electrical heaters and the barrel chamber.
Special temperature-controlled nozzles are employed to avoid setting up
either by cooling or cross-linking whilst moulds are usually electrically
heated. Many machines are now available which may be changed from
thermoplastics to thermosetting moulding and vice versa by a change of the
nozzle end-cap and change of screw. For thermosetting plastics screws
often have a low compression ratio and are water cooled.
There is a slowly resolving but intensive controversy over the
relative merits of compression, transfer and injection moulding. Compared
with compression methods both injection and transfer moulding are
advantageous in that they are more easily automated, mouldings are flash
free and have a good surface finish, it is easier to mould thick and/or void
free sections and it is possible to increase cure rates by frictional heat. It is
probably also true that in all these instances injection moulding has a slight
advantage over transfer. Injection moulding can be very fast and claim has
been made that sometimes cycles may be reduced to one-sixth of the
compression moulding time. Pelleting and preheating are also unnecessary.
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Yet another advantage is that the thermoplastics moulder may, by small
machine changes, be able to handle a range of materials without the
purchase of compression presses. The increased versatility of the machines
can also give greater flexibility in planning and potentially increase the
loading factor of the equipment.
There are, however, disadvantages to the injection moulding
process. Injection moulding machines are very much more expensive than
compression presses and with the larger sizes injection machines may be
several times the price of compression machines of similar mould size
capacities. There may also be possible technical disadvantages. If not
moulded carefully the mouldings may exhibit inferior and anisotropic
mechanical properties, particularly with thin-walled mouldings. The
dimensional stability on heating may be worse and the shrinkage more
variable than occurs with compression moulding. The selection between
compression and injection moulding must therefore be made with care,
with perhaps a tendency for injection moulding to be preferred with fairly
small, thick-section long-run mouldings.
Injection moulding compositions have a number of requirements
with regard to granule flow and cure characteristics not always met by
conventional formulations. For example, granules should be free-flowing
(i.e. of a narrow particle size distribution and not too irregular in shape).
There are also certain requirements in terms of viscosity.
The viscosity should quickly reach a suitable value on heating in
the barrel. It should not be too high since it may be difficult to fill the
mould. At the same time it should not be so low that little heat is generated
by friction. At the injection melt temperature of 100-130°C the compound
should have a good stability but should cure rapidly at the high curing
temperatures as exist within the mould.
Properties of phenolic mouldings
Since the polymer in phenolic mouldings is cross-linked and highly
interlocked, phenolic mouldings are hard, heat-resistant insoluble materials.
The chemical resistance of the mouldings depends on the type of
filler and resin used. Simple phenol-formaldehyde materials are readily
attacked by aqueous sodium hydroxide solution but cresol- and xylenolbased resins are more resistant. Provided the filler used is also resistant,
phenolic mouldings are resistant to acids except 50% sulphuric acid, formic
acid and oxidizing acids. The resins are stable up to 200°C. Some recently
developed grades of moulding compounds are claimed to be capable of
exposure to 300°C for short periods.
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The mechanical properties are strongly dependent on the type of
filler used.
As the mouldings are polar, the electrical insulation properties are
not outstanding but are adequate for many purposes. At 100°C a typical
woodflour-phenolic moulding has a dielectric constant of 18 and a power
factor of 0.7 at 800 Hz.
One disadvantage of phenolics compared with the aminoplastics
and the alkyd resins is their poor tracking resistance under conditions of
high humidity. This means that phenolics have a tendency to form a
conductive path through carbonization along a surface between two metal
electrodes at differing potential. Whether tracking will occur depends on
the separation of the electrodes, the humidity of the atmosphere, the
potential difference and the presence and nature of surface contaminants.
For many applications the poor tracking resistance is not a serious problem
and the wide use of phenolic laminates and mouldings for electrical
insulation applications is evidence of this.
Applications
Since the advent of Bakelite some 90 years ago phenolformaldehyde moulding compositions have been used for a great variety of
purposes. Perhaps the most well-known applications are in domestic plugs
and switches. It should, however, be pointed out that since World War II, in
Britain at least, urea-formaldehyde plastics have largely replaced phenolformaldehyde for these purposes because of their better anti-tracking
properties and wider colour range. There are, nevertheless, many
applications where the phenolics have proved quite adequate and continue
to be used as insulators. In general it may be said that the phenolics have
better heat and moisture resistance than the urea-formaldehyde mouldings.
Phenol-formaldehyde mouldings have also found many other applications
in the electrical industry, in some instances where high electrical insulation
properties are not so important. These include instrument cases, knobs,
handles and telephones. In some of these applications they have now been
replaced by urea-formaldehydes, melamine-formaldehydes, alkyds or the
newer thermoplastics because of the need for bright colours or in some
cases in an attempt to produce tougher products. In the car industry phenolformaldehyde mouldings are used in fuse-box covers, distributor heads and
in other applications where good electrical insulation together with good
heat resistance are required.
56
The newer improved heat-resistant grades are finding use in
saucepan handles, saucepan lid knobs, lamp housings, cooker handles,
welding tongs and electric iron parts.
Because of its hardness and ability to be electroplated, together
with good dimensional stability, phenolic mouldings are used in the
manufacture of “golf ball” heads for typewr iters.
Phenol-formaldehyde mouldings continue to be used in many
industrial applications where heat resistance, low cost and adequate shock
resistance (varying of course with the type of powder used) are important
features. Bottle caps and closures also continue to be made from phenolics
in large quantities. For some applications minimum odour grades based on
resols are used. The development of automatic compression presses and
machines suitable for the injection moulding of thermosetting plastics
together with the advent of fast-curing grades has stimulated the use of
phenol-formaldehydes for many small applications in spite of the
competition from the major thermoplastics.
Today the phenol-formaldehyde moulding compositions do not
have the eminent position they held until about 1950. In some important
applications they have been replaced by other materials, thermosetting and
thermoplastic, whilst they have in the past two decades found use in few
new outlets. However, the general increase in standards of living for much
of this period has increased the sales of many products which use phenolics
and consequently the overall use of phenol-formaldehyde moulding
powders has been well maintained.
Recent estimates suggest that in the early 1990s the percentage
breakdown of consumption of phenolic moulding materials in Western
Europe was approximately:
Electrical engineering, including wiring devices, and electronics 40%
Domestic appliances: pot and pan handles and tableware 33%
Automotive industry 12%
Sanitary sector (toilet seats, bathroom equipment) 3%
Closures 2%
Other 10%
Phenolic laminates
There are now commercially available a large range of laminated
plastics materials. Resins used include the phenolics, the aminoplastics,
polyesters, epoxies, silicones and the furane resins, whilst reinforcements
may be of paper, cotton fibre, other organic fibres, asbestos, carbon fibre or
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glass fibre. Of these the phenolics were the first to achieve commercial
significance and they are still of considerable importance.
One-stage resins (resols) in which there are sufficient methylol
groups to enable cross-linking to occur without the need for formaldehyde
donors are invariably used. Resins based on phenol, or phenol-cresol
mixtures, are used in fabric laminates where the greatest mechanical
strength is required, whereas cresylic acid (m-cresol content 50-55%) is
generally used for electrical grade laminating resins because of the better
electrical properties which result. Caustic soda is commonly used as the
catalyst for mechanical laminates but is not used in electrical laminates
because it affects the electrical insulation properties adversely, and
ammonia is the usual catalyst in this instance.
For laminating, the ammonia-catalyzed resins are usually dissolved
in industrial methylated spirits (IMS) or, less commonly, isopropyl alcohol.
Resins which have a high hydroxymethyl content (i.e. made by using a high
ratio of formaldehyde to phenol) and in which caustic soda is used as the
catalyst are water-soluble and the aqueous solutions are useful where a high
degree of impregnation is desirable. They are commonly used in
mechanical and decorative lam inate s.
The reinforcement may be a paper or a fabric. Many different
papers are used, being selected according to the end-use of the laminate.
For example, the Kraft papers are strong and produce laminates of high
mechanical strength, the relatively non-porous sulphite wood pulp papers
are used for electrical tubes whilst cotton paper and α-cellulose paper,
which are highly absorbent and of good colour, are used in conjunction
with phenolic resins. They include cotton, linen, rayon, glass fabrics and
asbestos mat cloth.
Although certain solventless processes have been used the resin is
usually applied to the reinforcement by passing the latter through a varnish
(40-50% solids content) of the resin in solvent. To ensure consistency of
impregnation it is important to control the solids content, the viscosity and
the specific gravity of the resin. At the same time the thickness, absorbency
and density of the reinforcement, or base material, should also be kept
within narrow limits. Figure 22 shows a typical arrangement for applying
the resin to the reinforcement. The reinforcement is led into a tank of
varnish and the resulting wet base is led through pressure rollers to squeeze
out the excess varnish. The coated base material is then passed through
either a vertical or horizontal drying oven. In a typical arrangement the
temperature at the inlet end of the oven is at about 50-90°C and at the outlet
58
about 145°C. The evaporating solvent is recovered and the resin taken to
the required degree of polymerization before emerging from the oven. The
oven temperature must thus be dependent on the curing characteristics of
the resin, the length of the oven and the coating rate employed. The
impregnated paper is commonly checked for resin content and degree of
cure. Control of degree of cure is important, as the resin must have precise
flow properties. If the viscosity is too high it will not flow sufficiently to
consolidate the resin; conversely if it is too low the resin will spew out and
leave a dry and inferior laminate. The degree of cure is perhaps most
conveniently assessed by the practical test of preparing a small laminate in
the laboratory by pressing at some controlled temperature and pressure. The
weight of resin which spews out of the laminate is thus inversely related to
the degree of cure, whilst more directly it will give an assessment of the
laminating behaviour of the paper.
Figure 22 - Impregnation plant fitted with vertical drying oven.
Flat laminates are prepared by plying up pieces of impregnated
paper and pressing in a multi-daylight press between metal plates under
pressure of 7-14 MPa and a temperature of 150-160°C. After curing, which
may take about 30 minutes for ¼ in thick sheet, the platens are partially
cooled before removal of the laminates in order to reduce blistering and
warping. Where the impregnated paper has a high volatile content it may
also be necessary to heat the press after loading in order to control the rate
of volatization and thus reduce blistering.
59
By the use of carefully tailored pieces of impregnated
reinforcement, it is possible to produce laminated mouldings. Such
mouldings are tough and have a high mechanical strength but take
considerably longer to cure than corresponding products prepared from
moulding powders.
Figure 23 - Three-roller tube winding machine.
Tubes and bushings are prepared by winding coated or impregnated
paper around a mandrel and in pressure contact with heated rollers. A
typical three-roller tube winding machine is shown in Figure 23. A number
of other simple shapes may be prepared by laminating under low pressure
using hand-clamped tools or rubber bags.
The properties of phenolic laminates
The properties of a phenolic laminate will obviously depend on a
great many factors. Of these the following are perhaps the most important:
(1) T he type of resin used, including the nature of the catalyst, the
concentration of methylol groups and the average molecular weight.
(2) T he properties of the varnish, such as the nature of the solvent
and the viscosity and resin content of the varnish.
(3) T he type of reinforcement. In the case of fabric reinforcement,
factors such as cloth weight and crimp will have a large effect on
mechanical properties.
(4) Moulding conditions, i.e. moulding pressure, temperature and
time.
In the manufacture of a laminate for electrical insulation, paper,
which is the best dielectric, is normally selected as the base reinforcement.
An “electrical” grade of paper is in fact a better dielectric than the resin and
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thus in conditions of low humidity the resin content of the laminate can be
quite low, particularly if the surfaces of the laminate are protected with an
insulating varnish. For humid conditions a high resin content is used since
this will lead to laminates with low water absorption, an essential property
for a good insulator. Tubular laminates normally show superior insulation
properties to the flat sheets since they are cured layer-by-layer, which
allows water of condensation to escape during manufacture. Cresylic-based
resins are usually used for such laminates in conjunction with ammonia as
catalyst in order to achieve the best dielectric properties.
Applications of phenolic laminates
Phenolic resin-paper laminates are extensively used for highvoltage insulation applications. Laminates from other reinforcements are
less suitable for this purpose but may be used for low-voltage applications.
Phenolic laminates are of value not only because of their good insulation
properties but also because of their good strength, high rigidity and
machinability. Sheet, tubular and moulded laminates are all employed.
Cotton fabric laminates are used in the manufacture of gear wheels
which are quiet running and which withstand shock loading. Since the
laminates have a lower strength than steel, gear wheels made from them
should be used at lower working stresses and designed with a greater face
width for load transmission as compared with a similar gear made from
steel. W ater-lubricated bearings from phenolic-cotton or phenolic-asbestos
laminates are used as bearings for steel rolling mills called to sustain
bearing loads as high as 21MPa.
Although phenolic resins are too dark for use in the surface layers
of decorative laminates these resins are employed in impregnating the core
paper. In these cases a melamine-formaldehyde resin is used for
impregnating the top decorative layer. Phenolic laminates have also been
used in aircraft construction and in chemical plant.
Miscellaneous applications
Although the two most well-known applications of phenolic resins
are in mouldings and laminates they are also used in a very large number of
other applications.
The selection of the phenol-formaldehyde ratio of about 1:2.25 is a
compromise in balancing mechanical properties (for which phenolformaldehyde ratios of about 1:2.5 are most suitable) and curing rates (for
which the optimum phenol-formaldehyde ratio is about 1:1.75). The use of