Plastics Materials 7 Episode 12 potx

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 3mm 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 628 Cellulose Plastics 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. CH,OC,H, 0 -, C H I C c I 0. C,H, I H Figure 22.5 It is essential that there be sufficient alkali present, either combined with the cellulose, or free, to neutralise the acid formed by both the main reaction and in a side reaction which involves the hydrolysis of ethyl chloride. 630 Cellulose Plastics 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 Solubility -0.5 0.8-1.3 1.4-1 .E 1.8-2.2 2.2-2.4 2.4-2.5 2.5-2.8 soluble in 4-8% sodium hydroxide soluble in water swelling in polar-non-polar solvent mixtures increasing solubility in above mixtures increasing solubility in alcohol and less polar solvents widest range of solubilities 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. 632 Cellulose Plastics 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). R(ONa),, + CH,Cl R(ONH,),, Methyl Cellulose R(ONa),, + CH, CH, - R(OCH,CH,OH),, Hydroxyethyl Cellulose R(ONa),, + ClCH,.COONa - R(OCH,COONa>,, + NaCl 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 R.ONa + CS, R.O.C.SNa II S The resultant yellow sodium cellulose xanthate is dispersed in an aqueous caustic soda solution, where some hydrolysis occurs. This process is referred to as ‘ripening’ and the solution as ‘viscose’. When the hydrolysis has proceeded sufficiently the solution it transferred to a hopper from which it emerges through a small slit on to a roller immersed in a tank of 10-15% sulphuric acid and 10-20% sodium sulphate at 35-40°C. The viscose is coagulated and by completion of the hydrolysis the cellulose is regenerated. The foil is subsequently washed, bleached, plasticised with ethylene glycol or glycerol and then dried. 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 634 Cellulose Plastics 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 (1958) 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 SI~IG, M., Water Soluble Resins, Reinhold, New York (1962) 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, Reinhold, New York (1958) KOWELL, R. M. and YOUNG, K. A. (Eds.), Modified Cellulosics, Academic Press, New York-San SxwNETr, 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) 23 Phenolic Resins 23.1 INTRODUCTION The phenolic resins may be considered to be the first polymeric products produced commercially from simple compounds of low molecular weight, i.e they were the first truly synthetic resins to be exploited. Their early development has been dealt with briefly in Chapter 1 and more fully elsewhere.' 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 subjeFo 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. 635 636 Phenolic Resins 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 + NaOH 4 @ + H,O + 2NaOH + moNa + Na$O, + H,O + H,SO, - 2aoH + 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 [...]... Asbestos felt Glass fabric 10' Ibf/in2 MPa IO3 Ibf/in2 MPa ft Ibf J 10 69 20 139 0.2-0.35 0. 27- 0. 47 1. 37 0.018 0.032 4.6 450 177 14 97 27 190 0.35-0.45 0. 47- 0.61 1.40 0.020 0.042 5.2 380 150 16 110 19 131 1.5 2.0 1.36 0.25 0.10 6.5 40 15 .7 7-15 48-104 16 110 1.o 1.35 1.6-1.8 12- 35 83-240 25 175 10.0 13.5 1.4-1 .7 0.01-0.04 0.01-0.02 4.5-5.5 - - V/O.OOl in KV/cm - 0.11 6.1 100 39.4 - of low humidity... Nigrosine dye Woodflour Mica Cotton flock Textile shreds Asbestos Electrical grade Medium shockresisting grade High shockresisting grade IO0 12. 5 3 2 4 100 14 100 12. 5 100 17 2 2 3 100 - - 120 - 40 2 3.3 3 150 - the extent of about 1% to give these somewhat intractable materials adequate flow properties Some typical formulations are given in Table 23.1 23.5.2 Compounding of Phenol-Formaldehyde Moulding... phenolic moulding materials in Western Europe was approximately: Electrical engineering, including wiring devices, and electronics Domestic appliances: pot and pan handles and tableware Automotive industry Sanitary sector (toilet seats, bathroom equipment) Closures Other 40% 33% 12% 3% 2% 10% 23.6 PHENOLIC LAMINATES There are now commercially available a large range of laminated plastics materials Resins... minimum time required to mould a blister-free flow cup under the BS 77 1 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... 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... 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: Phenol Formalin ( 37% w/w) Oxalic acid 100 parts by weight 70 parts by weight 1.5 parts by weight 644 Phenolic Resins t Figure 23.16 Diagrammatic representation of resin kettle and associated... moulding of thermosetting plastics together with the advent of fastcuring 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... the hexa (Figure 23. 17) Between 10 and 15 parts of hexa are used in typical moulding compositions The mechanism by which it cross-links novolak resins is not fully understood but it appears capable of supplying the requisite methylene bridges required for cross-linking It also functions as a promoter for the hardening reaction 6CH,O + 4NH, /N\ Figure 23. 17 Moulding Powders 641 Basic materials such as...Raw Materials 6 37 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 23.2) The cumene is then oxidised in the presence of alkali at about 130°C (Figure 27. 3) The hydroperoxide formed is decomposed in a... construction and in chemical plant Miscellaneous Applications 23 .7 659 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 At one time cast phenolic resins' were also an important class of plastics materials These are made by reacting 1 mole of phenol with about 2.25 . recently the market has continued to grow but not at the same rate as for plastics materials in general. For example, in 19 57 production of phenolic resins was of e same order as for PVC and. 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,. of formaldehyde (added as 37% w/w formalin) in the presence of some acid as catalyst. A typical charge ratio would be: Phenol 100 parts by weight Formalin ( 37% w/w) 70 parts by weight Oxalic