Plastics Materials 7 Episode 12 potx

Although they are now approaching their centenary, phenolic resins continue to be used for a wide variety of applications, such as moulding powders, laminating resins, adhesives, binders

Cellulose Esters 627 for a variety of purposes Thin sheet is useful for high-quality display boxes whilst thicker sheet is used for spectacle frames

Triacetate film is used in the graphic arts, for greetings cards, and for specialised electrical applications such as non-conducting separators

The use of cellulose acetate for moulding and extrusion is now becoming small owing largely to the competition of the styrene polymers and polyolefins The major outlets at the present time are in the fancy goods trade as toothbrushes, combs, hair slides etc Processing provides no major problem provided care is taken to avoid overheating and the granules are dry The temperatures and pressure used vary, from 160 to 250°C and 7 to 15 ton/in2 respectively, according to grade The best injection mouldings are obtained using a warm mould

Secondary cellulose acetate has also been used for fibres and lacquers whilst cellulose triacetate fibre has been extensively marketed in Great Britain under the trade name Trice]

Biodegradable Cellulose Acetate Compounds

As a result of development work between the Battelle Institute in Frankfurt and

a German candle-making company, Aetema, biodegradable cellulose acetate compounds have been available since 1991 from the Rh6ne-Poulenc subsidiary Tubize Plastics They are marketed under the trade names Bioceta and Biocellat The system is centred round the use of an additive which acts both as a plasticiser and a biodegrading agent, causing the cellulose ester to decompose within 6-24 months

The initial use was as a blow moulded vessel for vegetable oil candles However, because of its biodegradability it is of interest for applications where paper and plastics materials are used together and which can, after use, be sent into a standard paper recycling process Instances include blister packaging (the compound is transparent up to 3 m m in thickness), envelopes with transparent windows and clothes point-of-sale packaging

Compared with more common plastics used as packaging materials, the compound does have some disadvantages, such as a high water vapour permeability and limited heat resistance, losing dimensional stability at about 70°C It is also substantially more expensive than the high-tonnage polyolefins Last but not least its biodegradability means that it must be used in applications that will have completed their function within a few months of the manufacture

of the polymer compound

22.2.3 Other Cellulose Esters

Homologues of acetic acid have been employed to make other cellulose esters and of these cellulose propionate, cellulose acetate-propionate and cellulose acetate-butyrate are produced on a commercial scale These materials have larger side chains than cellulose acetate and with equal degrees of esterification, molecular weights and incorporated plasticiser, they are slightly softer, of lower density, have slightly lower heat distortion temperatures and flow a little more easily The somewhat greater hydrocarbon nature of the polymer results in

slightly lower water-absorption values (see Table 22.2)

It should, however, be realised that some grades of cellulose acetate may be softer, be easier to process and have lower softening points than some grades of

cellulose acetate-butyrate, cellulose acetate-propionate and cellulose propionate since the properties of all four materials may be considerably modified by chain length, degree of substitution and in particular the type and amount of plasticiser

Cellulose acetate-butyrate (CAB) has been manufactured for a number of

years in the United States (Tenite Butyrate-Kodak) and in Germany (Cellidor B-Bayer)

In a typical process for manufacture on a commercial scale bleached wood pulp or cotton linters are pretreated for 12 hours with 40-50% sulphuric acid and then, after drying, with acetic acid Esterification of the treated cellulose is then carried out using a mixture of butyric acid and acetic anhydride, with a trace of sulphuric acid as catalyst Commercial products vary extensively in the acetate/ butyrate ratios employed

The lower water absorption, better flow properties and lower density of CAB compared with cellulose acetate are not in themselves clear justification for their continued use There are other completely synthetic thermoplastics which have

an even greater superiority at a lower price and do not emit the slight odour of butyric acid as does CAB Its principal virtues which enable it to compete with other materials are its toughness, excellent appearance and comparative ease of mouldability (providing the granules are dry) The material also lends itself to use

in fluidised bed dip-coating techniques, giving a coating with a hard glossy finish which can be matched only with more expensive alternatives CAB is easy to vacuum form

A number of injection mouldings have been prepared from CAB with about 19% combined acetic acid and 44% combined butyric acid Their principal end products have been for tabulator keys, automobile parts, toys and tool handles In the United States CAB has been used for telephone housings Extruded CAB piping has been extensively used in America for conveying water, oil and natural gas, while CAB sheet has been able to offer some competition to acrylic sheet for outdoor display signs

In the mid- 1950s cellulose propionate became commercially available

(Forticel-Celanese) This material is very similar in both cost and properties to CAB Like CAB it may take on an excellent finish, provided a suitable mould is used, it is less hygroscopic than cellulose acetate, and is easily moulded

As with the other esters a number of grades are available differing in the degree of esterification and in type and amount of plasticiser Thus the differences in properties between the grades are generally greater than any differences between 'medium' grades of cellulose propionate and CAB Whereas

a soft grade of the propionate may have a tensile strength of 20001bf/in2 (14MPa) and a heat distortion temperature of 51"C, a hard grade may have tensile strength as high as 6000 lbf/in2 (42 MPa) and a heat distortion temperature

of 70°C

Cellulose acetate-propionate (Tenite Propionate-Kodak) is similar to cellu-

lose propionate With the shorter side chains, cellulose propionate and cellulose acetate propionate tend to be harder, stiffer and of higher tensile strength than CAB Like CAB they are easy to vacuum form and also tend to be used for similar applications such as steering wheels, tool handles, safety goggles and blister packs

Many other cellulose esters have been prepared in the laboratory and some have reached pilot plant status Of these the only one believed to be of current

importance is cellulose caprate (decoate) According to the literature, degraded

Cellulose Ethers 629

wood pulp is activated by treating with chloroacetic acid and the product is esterified by treating with capric anhydride, capric acid and perchloric acid The material is said to be useful as optical cement.’

22.3 CELLULOSE ETHERS

By use of a modification of the well-known Williamson synthesis it is possible

to prepare a number of cellulose ethers Of these materials ethyl cellulose has found a small limited application as a moulding material and somewhat greater use for surface coatings The now obsolete benzyl cellulose was used prior to World War I1 as a moulding material whilst methyl cellulose, hyroxyethyl cellulose and sodium carboxymethyl cellulose are useful water-soluble polymers

With each of these materials the first step is the manufacture of alkali cellulose (soda cellulose) This is made by treating cellulose (either bleached wood pulp or cotton linters) with concentrated aqueous sodium hydroxide in a nickel vessel at elevated temperature After reaction excess alkali is pressed out, and the resultant

‘cake’ is then broken up and vacuum dried until the moisture content is in the range 10-25% The moisture and combined alkali contents must be carefully controlled as variations in them will lead to variations in the properties of the resultant ethers

22.3.1 Ethyl Cellulose

Ethyl cellulose is prepared by agitating the alkali cellulose with ethyl chloride in the presence of alkali at about 60°C for several hours Towards the end of the reaction the temperature is raised to about 130-140°C The total reaction time is approximately 12 hours The reaction is carried out under pressure

If the etherification were taken to completion the product would be the compound shown in Figure 22.5

Ethyl ether and ethyl alcohol which are formed as by-products are removed by distillation and the ethyl cellulose is precipitated by hot water The polymer is then carefully washed to remove sodium hydroxide and sodium chloride and dried

The properties of the ethyl cellulose will depend on:

(1) The molecular weight

( 2 ) The degree of substitution

(3) Molecular uniformity

The molecular weight may be regulated by controlled degradation of the alkali cellulose in the presence of air This can be done either before or during etherification The molecular weight of commercial grades is usually expressed indirectly as viscosity of a 5% solution in an 80:20 toluene-ethanol mixture The completely etherified material with a degree of substitution of 3 has an ethoxyl content of 54.88% This material has little strength and flexibility, is not thermoplastic, has limited compatibility and solubility and is of no commercial

value A range of commercial products are, however, available with a degree of

substitution between 2.15 and 2.60, corresponding to a range of ethoxyl contents from 43 to 50%

The ethoxyl content is controlled by the ratio of reactants and to a lesser degree

by the reaction temperature

Whereas mechanical properties are largely determined by chain length, the softening point, hardness, water absorption and solubility are rather more

determined by the degree of substitution (see Figure 22.6)

ETHOXYL CONTENT OF ETHYL CELLULOSE IN ' / D

90"

Figure 22.6 Influence of the ethoxyl content of ethyl cellulose on softening point moisture absorption

and hardness (Hercules Powder Co literature)

Typical physical properties of ethyl cellulose are compared with those of the

cellulose ethers in Table 22.2

The solubility of ethyl cellulose depends on the degree of substitution At low degrees of substitution (0.8-1.3) the replacement of some of the hydroxyl groups

by ethoxyl groups reduces the hydrogen bonding across the cellulosic chains to such an extent that the material is soluble in water Further replacement of hydroxyl groups by the less polar and more hydrocarbon ethoxyl groups

Cellulose Ethers 63 1 increases the water resistance Fully etherified ethyl cellulose is soluble only in non-polar solvents

The relationship between degree of substitution and solubility characteristics is predictable from theory and is summarised in Table 22.5

Table 22.5 Solubility of ethyl cellulose

Average number of ethoxyl

groups per glucose unit

soluble only in non-polar solvents

Ethyl cellulose is subject to oxidative degradation when exposed to sunlight and elevated temperatures It is therefore necessary to stabilise the material against degrading influences during processing or service In practice three types

of stabiliser are incorporated, an antioxidant such as the phenolic compound

2,2’-methylenebis-(4-methyl-6-tert-butylphenol), an acid acceptor such as an

epoxy resin for use where plasticisers may give rise to acidic degradation products and an ultraviolet absorber such as 2,4-dihydroxybenzophenone for outdoor use Plasticisers such as tritolyl phosphate and diamylphenol have a beneficial stabilising effect

Ethyl cellulose has never become well known in Europe and apart from one or two specific applications has not been able to capture any significant proportion

of the market held by the cellulose esters Although it has the greatest water resistance and the best electrical insulating properties amongst the cellulosics this

is of little significance since when these properties are important there are many superior non-cellulosic alternatives The principal uses for ethyl is cellulose injection mouldings are in those applications where good impact strength at low temperatures is required, such as refrigerator bases and flip lids and ice-crusher parts

Ethyl cellulose is often employed in the form of a ‘hot melt’ for strippable coatings Such strippable coatings first became prominent during World War I1 for packaging military equipment Since then they have been extensively used for protecting metal parts against corrosion and mamng during shipment and storage A typical composition consists of 25% ethyl cellulose, 60% mineral oil,

10% resins and the rest stabilisers and waxes Coating is performed by dipping the cleaned metal part into the molten compound The metal part is withdrawn and an adhering layer of the composition is allowed to harden by cooling Hot melts have also been used for casting and paper coating

The ether is also used in paint, varnish and lacquer formulations A recent

development is the use of ethyl cellulose gel lacquers These are permanent coatings applied in a similar way to the strippable coatings They have been used

in the United States for coating tool handles, door knobs and bowling pins

22.3.2 Miscellaneous Ethers

Only one other cellulose ether has been marketed for moulding and extrusion

applications, benzyl cellulose This material provides a rare example of a polymer

which although available in the past is no longer commercially marketed The material had a low softening point and was unstable to both heat and light and has thus been unable to compete with the many alternative materials now available

A number of water-soluble cellulose ethers are marketed! Methyl cellulose is

prepared by a method similar to that used for ethyl cellulose A degree of substitution of 1.6-1.8 is usual since the resultant ether is soluble in cold water but not in hot It is used as a thickening agent and emulsifier in cosmetics, as a paper size, in pharmaceuticals, in ceramics and in leather tanning operations

Hydroxyethyl cellulose, produced by reacting alkali cellulose with ethylene

oxide, is employed for similar purposes

Hydroxypropyl cellulose, like methyl cellulose, is soluble in cold water but not

in hot, precipitating above 38°C It was introduced by Hercules in 1968 (Klucel) for such uses as adhesive thickeners, binders, cosmetics and as protective colloids for suspension polymerisation The Dow company market the related

hydroxypropylmethyl cellulose (Methocel) and also produce in small quantities a hydroxyethylmethyl cellulose

Reaction of alkali cellulose with the sodium salt of chloracetic acid yields

sodium carboxmethyl cellulose, (SCMC) Commercial grades usually have a

degree of substitution between 0.50 and 0.85 The material, which appears to be physiologically inert, is very widely used Its principal application is as a soil- suspending agent in synthetic detergents It is also the basis of a well-known proprietary wallpaper adhesive Miscellaneous uses include fabric sizing and as

a surface active agent and viscosity modifier in emulsions and suspensions Purified grades of SCMC are employed in ice cream to provide a smooth texture and in a number of pharmaceutical and cosmetic products

Schematic equations for the production of fully substituted varieties of the above three ethers are given below (R represents the cellulose skeleton)

Methyl Cellulose

Hydroxyethyl Cellulose

Sodium Carboxymethyl Cellulose

22.4 REGENERATED CELLULOSE

Because of high interchain bonding, cellulose is insoluble in solvents and is incapable of flow on heating, the degradation temperature being reached before the material starts to flow It is thus somewhat intractable in its native form Cellulose, however, may be chemically treated so that the modified products may

Regenerated Cellulose 633

be dissolved and the solution may then either be cast into film or spun into fibre

By treatment of the film or fibre the cellulose derivative may be converted back (regenerated) into cellulose although the processing involves reduction in molecular weight

In the case of fibres three techniques have been employed:

(1) Dissolution of the cellulose in cuprammonium solution followed by acid coagulation of extruded fibre (‘cuprammonium rayon’-no longer of commercial importance) In this case the acid converts the cuprammonium complex back into cellulose

(2) Formation of cellulose acetate, spinning into fibre and subsequent hydrolysis into cellulose

(3) Reaction of alkali cellulose with carbon disulphide to produce a cellulose xanthate which forms a lyophilic sol with caustic soda This may be extruded into a coagulating bath containing sulphate ions which hydrolyses the xanthate back to cellulose This process is known as the viscose process and

is that used in the manufacture of rayon

By modification of the viscose process a regenerated cellulose foil may be produced which is known under the familiar trade name Cellophane

The first step in the manufacture of the foil involves the production of alkali cellulose This is then shredded and allowed to age in order that oxidation will degrade the polymer to the desired extent The alkali cellulose is then treated with carbon disulphide in xanthating chums at 20-28°C for about three hours

The xanthated cellulose contains about one xanthate group per two glucose units The reaction may be indicated schematically as

The product at this stage is ‘plain’ foil and has a high moisture vapour transmission rate Foil which is more moisture proof may be obtained by coating with pyroxylin (cellulose nitrate solution) containing dibutyl phthalate as plasticiser or with vinylidene chloride-acrylonitrile copolymers A range of foils are available differing largely in their moisture impermeability and in heat sealing characteristics

Regenerated cellulose foil has been extensively and successfully used as a wrapping material, particularly in the food and tobacco industries Like other cellulose materials it is now having to face the challenge of the completely synthetic polymers Although the foil has been able to compete in the past, the

advent of the polypropylene film in the early 1960s produced a serious competitor which led to a marked reduction in the use of the cellulosic materials

Regenerated cellulose does, however, have the advantage that it biodegrades well aerobically in composting (rather more slowly anaerobically)

22.5 VULCANISED FIBRE

This material has been known for many years, being used originally in the

making of electric lamp filaments In principle vulcanised fibre is produced by

the action of zinc chloride on absorbent paper The zinc chloride causes the cellulosic fibres to swell and be covered with a gelatinous layer Separate layers

of paper may be plied together and the zinc chloride subsequently removed to leave a regenerated cellulose laminate

The removal of zinc chloride involves an extremely lengthy procedure The plied sheets are passed through a series of progressively more dilute zinc chloride solutions and finally pure water in order to leach out the gelatinising agent This may take several months The sheets are then dried and consolidated under light pressure

The sheets may be formed to some extent by first softening in hot water or steam and then pressing in moulds at pressures of 200-500 Ibf/in2 (1.5-3SMPa) Machining, using high-speed tools, may be camed out on conventional metal-working machinery

A number of grades have been available according to the desired end use The principal applications of vulcanised fibre are in electrical insulation, luggage, protective guards and various types of materials-handling equipment The major limitations are dimensional instability caused by changes in humidity, lack of flexibility and the long processing times necessary to extract the zinc chloride

References

1 PAIST, w D., Cellulosics, Reinhold, New York ( 1 9 5 8 )

2 STANNETT, v., Cellulose Acetate Plastics, Temple Press, London (1950)

3 FORDYCE, c K., and MEYER, L w A., Ind Eng Chem., 33, 597 (1940)

4 DAVIDSON, K L., and sirric, M., Water Soluble Resins, Reinhold, New York (1962)

Bibliography

DAVIDSON, R L., and S I ~ I G , M., Water Soluble Resins, Reinhold, New York ( 1 9 6 2 )

MILES, F D., Cellulose Nitrate, Oliver and Boyd, London (1955)

OTT, G., SPURLIN, H M., and GKAFFLIN, M w., Cellulose and its Derivatives (3 vols), Interscience, New

PAIST, w D., Cellulosics, R e i n h o l d , New York (1958)

KOWELL, R M and YOUNG, K A (Eds.), Modified Cellulosics, Academic Press, New York-San

S x w N E T r , v., Cellulose Acetate Plastics, Temple Press, London (1950)

YAKSLEY, v E., FLAVELL, w., ADAMSON, P s., and PEKKINS, N G., Cellulosic Plastics, Iliffe, London

York, 2nd Edn (1954)

Francisco-London (1978)

( 1964)

Although they are now approaching their centenary, phenolic resins continue to

be used for a wide variety of applications, such as moulding powders, laminating resins, adhesives, binders, surface coatings and impregnants Until very recently the market has continued to grow but not at the same rate as for plastics materials in general For example, in 1957 production of phenolic resins was of e same order

as for PVC and for polyethylene and about twice that of polystyren 's, Today it is less than a tenth that of polyethylene and about one-third that of polysthene In the early 1990s it was estimated that production in the USA was about 1 20@000 t.p.a.,

in Western Europe 580 000 t.p.a and in Japan 380 000 t.p.a With most markets for phenolic resins being long-established but at the same time s u b j e F o increased competition from high-performance thermoplastics the overall situation had not greatly changed by the end of the 1990s

Phenolic moulding powders, which before World War I1 dominated the plastics moulding materials market, only consumed about 10% of the total phenolic resin production by the early 1990s

In recent years there have been comparatively few developments in phenolic resin technology apart from the so-called Friedel-Crafts polymers introduced in the 1960s and the polybenzoxazines announced in 1998 which are discussed briefly at the end of the chapter

Phenolic resins are also widely known as phenol-formaldehyde resins, PF resins and phenoplasts The trade name Bakelite has in the past been widely and erroneously used as a common noun and indeed is noted as such in many English dictionaries

23.2 RAW MATERIALS

The phenolics are resinous materials produced by condensation of a phenol, or mixture of phenols, with an aldehyde Phenol itself and the cresols are the most widely used phenols whilst formaldehyde and, to a much less extent, furfural are almost exclusively used as the aldehydes

635

23.2.1 Phenol

At one time the requirement for phenol (melting point 41"C), could be met by distillation of coal tar and subsequent treatment of the middle oil with caustic soda to extract the phenols Such tar acid distillation products, sometimes

containing up to 20% a-cresol, are still used in resin manufacture but the bulk of

phenol available today is obtained synthetically from benzene or other chemicals

by such processes as the sulphonation process, the Raschig process and the cumene process Synthetic phenol is a purer product and thus has the advantage

of giving rise to less variability in the condensation reactions

In the sulphonation process vaporised benzene is forced through a mist of sulphuric acid at 100-120°C and the benzene sulphonic acid formed is neutralised with soda ash to produce benzene sodium sulphonate This is fused with a 25-30% excess of caustic soda at 300-400°C The sodium phenate obtained is treated with sulphuric acid and the phenol produced is distilled with steam (Figure 23.1)

SO,H

+ 2NaOH -+ moNa + Na$O, + H,O

+ H,SO, - 2 a o H + NqSO, S0,Na

Figure 23.1

Today the sulphonation route is somewhat uneconomic and largely replaced by newer routes Processes involving chlorination, such as the Raschig process, are used on a large scale commercially A vapour phase reaction between benzene and hydrocholoric acid is carried out in the presence of catalysts such as an aluminium hydroxide-copper salt complex Monochlorobenzene is formed and this is hydrolysed to phenol with water in the presence of catalysts at about 450"C, at the same time regenerating the hydrochloric acid The phenol formed

is extracted with benzene, separated from the latter by fractional distillation and purified by vacuum distillation In recent years developments in this process have reduced the amount of by-product dichlorobenzene formed and also considerably increased the output rates

A third process, now the principal synthetic process in use in Europe, is the cumene process

In this process liquid propylene, containing some propane, is mixed with benzene and passed through a reaction tower containing phosphoric acid on kieselguhr as catalyst The reaction is exothermic and the propane present acts as

a quench medium A small quantity of water is injected into the reactor to

Raw Materials 637 maintain catalyst activity The effluent from the reactor is then passed through distillation columns The propane is partly recycled, the unreacted benzene

returned to feed and the cumene taken off (Figure 2 3 2 ) The cumene is then

oxidised in the presence of alkali at about 130°C (Figure 27.3) The

hydroperoxide formed is decomposed in a stirred vessel by addition of dilute sulphuric acid The mixture is passed to a separator and the resulting organic

layer fractionated (Figure 23.4) Some benzophenone is also produced in a side

is no by-product of reaction In all of the above processes benzene is an essential starting ingredient At one time this was obtained exclusively by distillation of coal tar but today it is commonly produced from petroleum

A route to phenol has been developed starting from cyclohexane, which is first oxidised to a mixture of cyclohexanol and cyclohexanone In one process the oxidation is carried out in the liquid phase using cobalt naphthenate as catalyst The cyclohexanone present may be converted to cyclohexanol, in this case the desired intermediate, by catalytic hydrogenation The cyclohexanol is converted to phenol

by a catalytic process using selenium or with palladium on charcoal The hydrogen produced in this process may be used in the conversion of cyclohexanone to cyclohexanol It also may be used in the conversion of benzene to cyclohexane in processes where benzene is used as the precursor of the cyclohexane

Other routes for the preparation of phenol are under development and include the Dow process based on toluene In this process a mixture of toluene, air and catalyst are reacted at moderate temperature and pressure to give benzoic acid

This is then purified and decarboxylated, in the presence of air, to phenol (Figure

2 3 5 )

Pure phenol crystallises in long colourless needles which melt at 41°C It causes severe burns on the skin and care should be taken in handling the material

Figure 23.5

Phenol is supplied commercially either in the solid (crystalline) state or as a

‘solution’ in water (water content 8-20%) Where supplied as a solid it is usually handled by heating the phenol, and the molten material is pumped into the resin kettles or into a preblending tank If the ‘solution’ is used care must be taken to avoid the phenol crystallising out

so this is normally the desired material The o-isomer is easily removed by distillation but separation of the close-boiling m- and p-isomers is difficult and so mixtures of these two isomers are used in practice

Xylenols, also obtained from coal tar, are sometimes used in oil-soluble resins

Of the six isomers (Figure 23 7) only 3,5-xylenol has the three reactive positions

Chemical Aspects 639 necessary for cross-linking and thus mixtures with a high proportion of this isomer are generally used

Other higher boiling phenolic bodies obtainable from coal tar distillates are sometimes used in the manufacture of oil-soluble resins Mention may also be made of cashew nut shell liquid which contains phenolic bodies and which is used in certain specialised applications

A few synthetic substituted phenols are also used in the manufacture of oil- soluble resins They include p-tert-butylphenol, p-tert-amylphenol, p-tert- octylphenol, p-phenylphenol and dihydroxyphenylpropane (bis-phenol A)

Furfural (see Chapter 28) is occasionally used to produce resins with good flow properties for use in moulding powders

23.3 CHEMICAL ASPECTS

Although phenolic resins have been known and widely utilised for over 60 years their detailed chemical structure remains to be established It is now known that the resins are very complex and that the various structures present will depend on the ratio of phenol to formaldehyde employed, the pH of the reaction mixture and the temperature of the reaction Phenolic resin chemistry has been discussed in detail and will be discussed only briefly here

Reaction of phenol with formaldehyde involves a condensation reaction which leads, under appropriate conditions, to a cross-linked polymer structure For commercial application it is necessary first to produce a tractable fusible low molecular weight polymer which may, when desired, be transformed into the cross-linked polymer For example, in the manufacture of a phenolic (phenol- formaldehyde, P-F) moulding a low molecular weight resin is made by condensation of phenol and formaldehyde This resin is then compounded with other ingredients, the mixture ground to a powder and the product heated under pressure in a mould On heating, the resin melts and under pressure flows in the mould At the same time further chemical reaction occurs, leading to cross- linking It is obviously desirable to process under such conditions that the required amount of flow has occurred before the resin hardens

The initial phenol-formaldehyde reaction products may be of two types,

novolaks and resols

23.3.1 Novolaks

The novolaks are prepared by reacting phenol with formaldehyde in a molar ratio

of approximately 1 :0.8 under acidic conditions Under these conditions there is

The novolak resins themselves contain no reactive methylol groups and do not form cross-linked structures on heating If, however, they are mixed with compounds capable of forming methylene bridges, e.g hexamethylenetetramine

or paraformaldehyde, they cross-link on heating to form infusible, ‘thermoset’ structures

I CH,OH

Figure 23.12

Heating of these resins will result in cross-linking via the uncondensed methylol groups or by more complex mechanisms The resols are sometimes referred to as one-stage resins since cross-linked products may be made from the initial reaction mixture solely by adjusting the pH On the other hand the novolaks are sometimes referred to as two-stage resins as here it is necessary to add some agent which will enable additional methylene bridges to be formed

23.3.3 Hardening

The novolaks and resols are soluble and fusible low molecular weight products They were referred to by Baekeland as A-stage resins On hardening, these resins pass through a rubbery stage in which they are swollen, but not dissolved, by a variety of solvents This is referred to as the B-stage Further reaction leads to rigid, insoluble, infusible, hard products known as C-stage resins When prepared from resols the B-stage resin is sometimes known as a resitol and the C-stage

product a resit The terms A-, B- and C-stage resins are also sometimes used to describe analogous states in other thermosetting resins

The mechanism of the hardening processes has been investigated by Zinke in Austria, von Euler in Sweden and Hultzsch in Germany using blocked methylol phenols so that only small isolable products would be obtained

In general their work indicates that at temperatures below 160°C cross-linking occurs by phenol methylol-phenol methylol and phenol methylol-phenol

condensations, viz Figure 23.13

CH,OH

Figure 23.13

As these condensation reactions can occur at the two ortho and the para

positions in phenol, m-cresol and 3,5-xylenol, cross-linked structures will be formed It has been pointed out by Megson' that because of steric hindrance the amount of cross-linking that can take place is much less than would involve the three reactive groups of all the phenolic molecules It is now generally considered that the amount of cross-linking that actually takes place is less than was at one time believed to be the case

Above 160°C it is believed that additional cross-linking reactions take place involving the formation and reaction of quinone methides by condensation of the

ether linkages with the phenolic hydroxyl groups (Figure 23.14)

Resin Manufacture 643 lower temperatures, are water-white in colour If, however, these castings are heated to about 180°C they darken considerably

In addition to the above possible mechanisms the possibility of reaction at m-positions should not be excluded For example, it has been shown by Koebner that 0- and p-cresols, ostensibly difunctional, can, under certain conditions, react with formaldehyde to give insoluble and infusible resins Furthermore, Megson has shown that 2,4,6-trimethylphenol, in which the two ortho- and the one para- positions are blocked, can condense with formaldehyde under strongly acidic conditions It is of interest to note that Redfarn produced an infusible resin from 3,4,5,-trimethylphenol under alkaline conditions Here the two m- and the p-positions were blocked and this experimental observation provides supplemen- tary evidence that additional functionalities are developed during reaction, for example in the formation of quinone methides

PI{

Figure 23.15 Effect of pH on the gel time of a P-F cast resin (After Apley')

The importance of the nature of the catalyst on the hardening reaction must also be stressed Strong acids will sufficiently catalyse a resol to cure thin films

at room temperature, but as the pH rises there will be a reduction in activity which passes through a minimum at about pH 7 Under alkaline conditions the rate of reaction is related to the type of catalyst and to its concentration The effect of pH value on the gelling time of a casting resin (phenol-formaldehyde ratio -1 :2.25) is shown in Figure 23.15

23.4 RESIN MANUFACTURE

Both novolaks and resols are prepared in similar equipment, shown dia- grammatically in Figure 23.16 The resin kettle may be constructed from copper, nickel or stainless steel where novolaks are being manufactured Stainless steel may also be used for resols but where colour formation is unimportant the cheaper mild steel may be used

In the manufacture of novolaks, I mole of phenol is reacted with about 0.8 mole of formaldehyde (added as 37% w/w formalin) in the presence of some acid

as catalyst A typical charge ratio would be:

Formalin (37% w/w) 70 parts by weight

t

Figure 23.16 Diagrammatic representation of resin kettle and associated equipment used for the

preparation of phenolic resins (After Whitehouse, Pritchett and Bamett2)

The reaction mixture is heated and allowed to reflux, under atmospheric pressure at about 100°C At this stage valve A is open and valve B is closed Because the reaction is strongly exothermic initially it may be necessary to use cooling water in the jacket at this stage The condensation reaction will take a number of hours, e.g 2-4 hours, since under the acidic conditions the formation

of phenol-alcohols is rather slow When the resin separates from the aqueous phase and the resin reaches the requisite degree of condensation, as indicated by refractive index measurements, the valves are changed over (Le valve A is

closed and valve B opened) and water present is distilled off

In the case of novolak resins the distillation is normally carried out without the application of vacuum Thus, as the reaction proceeds and the water is driven off, there is a rise in the temperature of the resin which may reach as high as 160°C

at the end of the reaction At these temperatures the fluid is less viscous and more easily stirred In cases where it is important to remove the volatiles present, a vacuum may be employed after the reaction has been completed, but for fast- curing systems some of the volatile matter (mainly low molecular weight phenolic bodies) may be retained

The end point may be checked by noting the extent of flow of a heated pellet down a given slope or by melting point measurements Other control tests include alcohol solubility, free phenol content and gelation time with 10% hexa

Moulding Powders 645

In the manufacture of resols a molar excess of formaldehyde (1.5-2.0: 1) is reacted with the phenol in alkaline conditions In these conditions the formation of the phenol alcohols is quite rapid and the condensation to a resol may take less than an hour A typical charge for a laboratory-scale preparation would be:

The reaction may be followed by such tests as melting point, acetone or alcohol solubility, free phenol content or loss in weight on stoving at 135°C Two classes of resol are generally distinguished, water-soluble resins prepared using caustic soda as catalyst, and spirit-soluble resins which are catalysed by addition of ammonia The water-soluble resins are usually only partially dehydrated during manufacture to give an aqueous resin solution with a solids content of about 70% The solution viscosity can critically affect the success in

a given application Water-soluble resols are used mainly for mechanical grade paper and cloth laminates and in decorative laminates

In contrast to the caustic soda-catalysed resols the spirit-soluble resins have good electrical insulation properties In order to obtain superior insulation characteristics a cresol-based resol is generally used In a typical reaction the refluxing time is about 30 minutes followed by dehydration under vacuum for periods up to 4 hours

Novolaks are most commonly used in the manufacture of moulding powders although resols may be used for special purposes such as in minimum odour grades and for improved alkali resistance The resins are generally based on phenol since they give products with the greatest mechanical strength and speed

of cure, but cresols may be used in acid-resisting compounds and phenol-cresol mixtures in cheaper compositions Xylenols are occasionally used for improved alkali resistance

The resols may be hardened by heating and/or by addition of catalysts Hardening of the novolaks may be brought about by addition of hexamethy- lenetetramine (hexa, hexamine) Because of the exothermic reaction on hardening (cure) and the accompanying shrinkage, it is necessary to incorpo- rate inert materials (fillers) to reduce the resin content Fillers are thus generally necessary to produce useful mouldings and are not incorporated simply to reduce cost Fillers may give additional benefits such as improving the shock resistance

Other ingredients may be added to prevent sticking to moulds (lubricants), to promote the curing reaction (accelerators), to improve the flow properties (plasticisers) and to colour the product (pigments)

(7) Plasticiser (not always used)

In addition to the selection of phenol used and the choice between novolak and resol there is a number of further variations possible in the resin used For example, in the manufacture of a novolak resin slight adjustment of phenol/ formaldehyde ratio will affect the size of novolak molecule produced Higher molecular weight novolaks give a stiff-flow moulding powder but the resin being

of lower reactivity, the powders have a longer cure time A second variable is the residual volatile content The greater the residual volatiles (phenolic bodies) the faster the cure Thus a fast-curing, stiff-flow resin may be obtained by using a phenol/formaldehyde ratio leading to larger molecules and leaving some of the low molecular weight constituents in the reaction mixture Yet another modification may be achieved by changing the catalyst used Thus whereas in the normal processes, using oxalic acid catalysts, the initial products are p-p- and o-p-diphenylmethanes, under other conditions it is possible to achieve products which have reacted more commonly in the ortho-position Such resins thus have the p-position free and, since this is very reactive to hexa, a fast-curing resin is obtained

Hexa is used almost universally as the hardener It is made by passing a slight excess of ammonia through a lightly stabilised aqueous solution of formalde-

hyde, concentrating the liquor formed and crystallising out the hexa (Figure

Moulding Powders 641

Basic materials such as lime or magnesium oxide increase the hardening rate

of novolak-hexa compositions and are sometimes referred to as accelerators They also function as neutralising agents for free phenols and other acidic bodies which cause sticking to, and staining of, moulds and compounding equipment Such basic substances also act as hardeners for resol-based compositions

Woodflour, a fine sawdust preferably obtained from softwoods such as pine, spruce and poplar, is the most commonly used filler Somewhat fibrous in nature,

it is not only an effective diluent for the resin to reduce exothei-m and shrinkage, but it is also cheap and improves the impact strength of the mouldings There is

a good adhesion between phenol-formaldehyde resin and the woodflour and it is possible that some chemical bonding may occur

Another commonly employed low-cost organic filler is coconut shell flour This can be incorporated into the moulding composition in large quantities and this results in cheaper mixes than when woodflour is used The mouldings also have a good finish However, coconut shell flour-filled mouldings have poor mechanical properties and hence the filler is generally used in conjunction with woodflour

For better impact strength cotton flock, chopped fabric or even twisted cord and strings may be incorporated The cotton flock-filled compounds have the greatest mouldability but the lowest shock resistance whilst the twisted cords and strings have the opposite effect Nylon fibres and fabrics are sometimes used to confer strength and flexibility and glass fibres may be used for strength and rigidity

Asbestos may be used for improved heat and chemical resistance and silica, mica and china clay for low water absorption grades Iron-free mica powder is particularly useful where the best possible electrical insulation characteristics are required but because of the poor adhesion of resin to the mica it is usually used

in conjunction with a fibrous material such as asbestos Organic fillers are commonly used in a weight ratio of 1 : 1 with the resin and mineral fillers in the ratio 1.5: 1

In some countries the extensive use of asbestos as a filler is somewhat discouraged because of the hazards associated with its use In other parts of the world moulding compositions of enhanced heat resistance have been developed

by the use of especially heat-resisting polymers used in conjunction with asbestos and other mineral fillers

Stearic acid and metal stearates such as calcium stearate are generally used as lubricants at a rate of about 1-3% on the total compound Waxes such as carnauba and ceresin or oils such as castor oil may also be used for this purpose

In order that the rate of cure of phenolic moulding compositions is sufficiently rapid to be economically attractive, curing is carried out at a temperature which leads to the formation of quinone methides and their derivatives which impart a dark colour to the resin Thus the range of pigments available is limited to blacks, browns and relatively dark blues, greens, reds and oranges

In some moulding compositions other special purpose ingredients may be incorporated For example, naphthalene, furfural and dibutyl phthalate are occasionally used as plasticisers or more strictly as flow promoters They are particularly useful where powders with a low moulding shrinkage are required

In such formulations a highly condensed resin is used so that there will be less reaction, and hence less shrinkage, during cure The plasticiser is incorporated to

100

12.5

High shock- resisting grade

100

17

2 3.3

Some typical formulations are given in Table 23.1

23.5.2 Compounding of Phenol-Formaldehyde Moulding Compositions

Although there are many variants in the process used for manufacturing moulding powders, they may conveniently be classified into dry processes and wet processes

In a typical dry process, finely ground resin is mixed with the other ingredients for about 15 minutes in a powder blender This blend is then fed on to a heated two-roll mill The resin melts and the powdery mix is fluxed into a leathery hide which bands round the front roll The temperatures chosen are such that the front roll is sufficiently hot to make the resin tacky and the real roll somewhat hotter

so that the resin will melt and be less tacky Typical temperatures are 70-100°C

for the front roll and 100-120°C for the back As some further reaction takes

place on the mill, resulting in a change of melting characteristics, the roll temperatures should be carefully selected for the resin used In some processes two mills may be used in series with different roll temperatures to allow greater flexibility in operation To achieve consistency in the end-product a fixed mixing schedule must be closely followed Milling times vary from 10 minutes down to

a straight pass through the mill

The hide from the mill is then cooled, pulverised with a hammer-mill and the resulting granules are sieved In a typical general purpose composition the granules should pass a 14 X 26 sieve For powders to be used in automatic

moulding plant fine particles are undesirable and so particles passing a 100 X 41 sieve (in a typical process) are removed In 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

Moulding Powders 649

Extrusion compounders such as the Buss KO-Kneader have been used for mixing phenolic resins It is claimed that they produce in some respects a better product and are more economical to use than mill-mixers

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

23.5.3 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 1-2 ton/in2 (15-30MPa) at 155- 170°C

In the case of transfer moulding, moulding pressures are usually somewhat higher, at 2-8 todin’ (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 increases the rate of cure As a result of these two

effects it is found that in a graph of extent of flow plotted against temperature there is a temperature of maximum flow (Figure 23.18)

vertical axis

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 2.5g 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 (Le 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 However, one useful comparative test is the minimum time required to mould a blister-free flow cup under the BS 771 test conditions 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 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:

Cylinder temperature 65-90°C

Moulding Powders 65 1

Screw back pressure

<7 (typically 1)MPa

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 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

23.5.4 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 xylenol-based resins are more resistant Provided the filler used is also resistant, phenolic mouldings are resistant to acids except 50% sulphuric acid, formic acid and oxidising 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 The mechanical properties are strongly dependent on the type of filler used and typical figures are given in Table 23.2

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 carbonisation 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

23.5.5 Applications

Since the advent of Bakelite some 90 years ago phenol-formaldehyde 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 11, in Britain at least, urea-formaldehyde

plastics have largely replaced phenol-formaldehyde 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 (see Chapter 24) 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 phenol-formaldehyde mouldings are used in fuse-box covers, distributor heads and in other applications where good electrical insulation together with good heat resistance are required

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 typewriters

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%

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-catalysed 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

Phenolic Laminates 655

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 laminates

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 a-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 23.19 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 about 145°C The evaporating solvent is recovered and the resin taken to the required degree of polymer- isation 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

Figure 23.19 Impregnation plant fitted with vertical drying oven (After Brown’)

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

Flat laminates are prepared by plying up pieces of impregnated paper and pressing in a multi-daylight press between metal plates under pressure of 1000-2000 lbf/in2 (7-14 MPa) and a temperature of 150-160°C After curing, which may take about 30 minutes for ;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 volatisation and thus reduce blistering

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 23.20 Three-roller tube winding machine (After Brown')

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 23.20 A number of other simple shapes may be prepared by laminating under low pressure using hand-clamped tools or rubber bags

23.6.1 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) The type of resin used, including the nature of the catalyst, the concentration

( 2 ) The properties of the varnish, such as the nature of the solvent and the

of methylol groups and the average molecular weight

viscosity and resin content of the varnish