Plastics Materials 7 Episode 7 docx

Compounding Ingredients 327 of the samples are then bound on to cards giving data on the processing and heating conditions. These are then available for visual comparison of colour forming. For technical reports it is common to give the colour a numerical rating. Thus water-white samples can be given a number 0, a pale yellow 1, an orange 2, a brown 3 and a black sample 4 and these numbers can be tabulated in the report. It is, however, common experience that such a technique is far less effective in imparting information than the use of well-displayed samples. In some laboratories samples are heated for prolonged periods in a press at a suitably elevated temperature. Such results may frequently fail to correlate with oven-heated samples since oxygen is largely excluded from the samples. As already indicated, the measurement of dehydrochlorination rates is not a practical way of assessing the effect of a stabiliser. Thus the congo red test sometimes specified in standards, in which a piece of congo red paper is held in a test tube above a quantity of heated PVC and the time taken for the paper to turn blue due to the evolution of a certain amount of hydrogen chloride, cannot be considered as being of much value. Many stabilisers are also useful in improving the resistance of PVC to weathering, particularly against degradation by ultraviolent radiation. This is an important consideration in building applications and other uses which involve outdoor exposure. The efficiency of stabilisers in improving resistance of PVC compounds to degradation is best measured by lengthy outdoor weathering tests. Accelerated weathering tests using xenon lamps or carbon arcs have not proved to be reliable even for purposes of comparison. The most important class of stabilisers are the lead compounds which form lead chloride on reaction with hydrogen chloride evolved during decomposition. As a class the lead compounds give rise to products of varying opacity, are toxic and turn black in the presence of certain sulphur-containing compounds but are good heat stabilisers. Of these materials basic lead carbonate (white lead) has been, and probably still is, the most important stabiliser for PVC. It may be considered as typical of the lead compounds and has a low weight cost. It is appreciated that weight cost is not, however, the best criterion to be considered in assessing the economics of a stabiliser. Far more relevant is the cost required to stabilise the material to an acceptable level for the processing and service conditions involved. One additional disadvantage of the lead carbonate is that it may decompose with the evolution of carbon dioxide at the higher range of processing conditions, thus leading to a porous product. For this reason tribasic lead sulphate, a good heat stabiliser which gives polymer compounds with better electrical insulation properties than lead carbonate, has increased in popularity in recent years at the expense of white lead. Its weight cost is somewhat higher than that of lead carbonate but less than most other stabilisers. This material is used widely in rigid compounds, in electrical insulation compounds and in general purpose formulations. Other lead stabilisers are of much specific applications. Dibasic lead phosphite gives compounds of good light stability but because of its higher cost compared with the sulphate and the carbonate its use is now restricted. In spite of its even greater weight cost dibasic lead phthalate finds a variety of specialised applications. Because it is an excellent heat stabiliser it is used in heat-resistant insulation compounds (for example in 105°C wire). It is also used in high-fidelity gramophone records, in PVC coatings for steel which contain polymerisable plasticisers and in expanded PVC formulations which use azodicarbonamide as 328 Vinyl Chloride Polymers a blowing agent. In this latter application the phthalate stabiliser also acts as a ‘kicker’ to accelerate the decomposition of the blowing agent. Normal and dibasic lead stearate have a stabilising effect but their main uses are as lubricants (see section 12.5.4). Lead silicate is sometimes used in leathercloth formulations but is today of little importance. Other lead compounds now of negligible importance are coprecipitated lead orthosilicates and lead salicylate. The use of lead compounds as stabilisers has been subject to regulation and legislation arising from their toxicity. Whilst legislation varies from country to country, lead stabilisers are not generally allowed in food packaging compounds but in most countries are allowed for use, under certain conditions, in pipes for conveying drinking water. Where lead stabilisers continue to be used there has commonly been a reduction in the level of use in a particular compound. The resulting lower level of protection may cause problems in the use of scrap and in general polymer re-work operations. Whilst lead compounds have been, and still are, the most important class of stabiliser for PVC the metallic soaps or salts have steadily increased in their importance and they are now widely used. At one time a wide range of metal stearates, ricinoleates, palmitates and octoates were offered as possible stabilisers and the efficiency of many of them has been examined. Today only the compounds of cadmium, barium, calcium and zinc are prominent as PVC stabilisers. The most important of these are cadmium-barium systems. These first became significant when it was discovered that stabilisers often behaved synergistically. Of the many stabilising systems investigated, cadmium-barium systems gave considerable promise. The first of these systems to be used successfully were based on cadmium octoate in conjunction with barium ricinoleate. Alone the cadmium salt gave good initial colour but tumed black after a relatively short heating period. The use of the barium soap in conjunction with the cadmium salt extended this period. The addition of antioxidants such as trisnonyl phenyl phosphite was found to greatly increase the heat stability whilst the further addition of epoxidised oils gave even better results. It was, however, found that on exposure to light an interaction took place between the ricinoleate and the epoxidised oil, with the formation of products incompatible with the PVC. These products exuded and caused tackiness of the compound and problems in calendering. Replacement of the octoate and ricinoleate with laurates avoided the undesirable interaction but instead led to plate-out, difficulties in heat sealing and printing and compounds yellowish in colour and lacking in clarity. However, the laurates continue to find some limited use in PVC compounding. Somewhat better results have been obtained with octoates and benzoates but these still lead to some plate-out. The use of liquid cadmium-barium phenates has today largely resolved the problem of plate-out whilst the addition of a trace of a zinc salt helps to improve the colour. Greater clarity may often be obtained by the addition of a trace of stearic acid or stearyl alcohol. Thus a modem so- called cadmium-barium stabilising system may contain a large number of components. A typical ‘packaged’ stabiliser could have the following composition: Cadmium-barium phenate 2-3 parts Epoxidised oils 3-5 parts Stearic acid 0.5-1 part Trisnonyl phenyl phosphite 1 part Zinc octoate 0.5 parts Compounding Ingredients 329 It appears that the zinc salt functions by preferentially reacting with sulphur to form white zinc sulphide rather than coloured cadmium sulphide and thus helps to reduce colour in the compound. The use of cadmium stabilisers, as with the lead compounds, gives rise to some concern because of possible toxicity and environmental problems. This has led to large efforts to produce non-toxic systems. Mixtures of calcium and zinc soaps, sometimes in conjunction with epoxidised oils, have been used for many years but these soap-based materials are both less powerful than the Cd-Ba complexes and also fail to give glass-clear products. Calcium/zinc non-soap liquid compounds, and to some extent, strontium/zinc compounds have found increasing use in recent years in efforts to cope with this problem. For flooring compositions, magnesium-barium, calcium-barium and copper-barium com- pounds are sometimes used in conjunction with pentaerythritol. The latter material has the function of chelating iron present in the asbestos and thus reducing colour formation. Another group of stabilisers are the organo-tin compounds. These materials found early applications because of their resistance to sulphur and because they can yield crystal-clear compounds. The older organo-tin compounds such as dibutyltin dilaurate, however, give only limited heat stability and problems may arise with high processing temperatures. Dibutyltin maleate imparts somewhat greater heat resistance. The availability of a number of sulphur-containing organo-tin compounds, such as dibutyltin di-iso-octylthioglycollate, which impart excellent heat resistance and clarity, has to some extent increased the scope of organo-tin compounds. They are, however, more expensive in terms of weight cost. It should be noted that the sulphur-containing organo-tin compounds should not be used where lead derivatives are present in the PVC compound since cross-staining will occur to form black lead sulphide. Such lead compounds could be present as added stabiliser or even because the polymer on manufacture was washed with water fed through lead pipes. The butyltins generally show a level of toxicity that prevents them being used in application in contact with foodstuffs. On the other hand the octyltin materials such as dioctyltin dilaurate and dioctyltin octylthioglycollate are much better in this respect and many of them meet stringent requirements for use in contact with foodstuffs. The low toxicity, excellent stabilising performance and improving relative price situation has led to considerable growth in the organo-tin market during the 1970s and their status has changed from special purpose to that of general purpose stabilisers. Furthermore, additions to this class of material have been made, including the ester tins, characterised by low odour, volatility and toxicity, and the methyl tins which, with their high metal content, may be used in lower amounts than the more common organo-tins to achieve comparable efficiency. Mention has already been made of epoxide stabilisers. They are of two classes and are rarely used alone. The first class are the epoxidised oils, which are commonly employed in conjunction with the cadmium-barium systems. The second class are the conventional bis-phenol A epoxide resins (see Chapter 22). Although rarely employed alone, used in conjunction with a trace of zinc octoate (2 parts resin, 0.1 part octoate) compounds may be produced with very good heat stability. There has been a revival of interest in recent years in antimony mercaptides as alternatives to the organo-tin stabilisers. This stems from the low level of toxicity and the strong synergism with calcium stearate. However, compared to the 330 Vinyl Chloride Polymers organo-tins they have lower resistance to sulphur staining and to ultraviolet radiation, particularly with transparent sheet. A further class of stabilisers are the amines, such as diphenylurea and 2-phenylindole. These materials are effective with certain emulsion polymers but rather ineffective with many other polymers. There is somewhat more interest in the aminocrotonates of the general formula CH, , C=CH-CO- OR / NH, Figure 12.19 many of which are approved for food packaging applications in Western Europe. They are used mainly in UPVC compounds for packaging film and blow moulded containers. In addition to stabilisers, antioxidants and ultra-violent absorbers may also be added to PVC compounds. Amongst antioxidants, trisnonyl phenyl phosphite, mentioned previously, is interesting in that it appears to have additional functions such as a solubiliser or chelator for PVC insoluble metal chlorides formed by reaction of PVC degradation products with metal stabilisers. Since oxidation is both a degradation reaction in its own right and may also accelerate the rate of dehydrochlorination, the use of antioxidants can be beneficial. In addition to the phenyl phosphites, hindered phenols such as octadecyl 3-(3,5-di-tert-butyl- 4-hydroxypheny1)propionate and 2,4,6-tris (2,5-di-tert-butyl-4-hydroxybenzyl)- 1,3,5-trimethylbenzene may be used. Low levels of ultraviolet absorbers (typically 0.2-0.8 pphr [parts per hundred resin]) can also be useful in preventing initiation of degradation mechanisms. Modified benzophenones and benzotriazoles are in widest commercial use. 12.5.2 Plasticisers The tonnage of plasticisers consumed each year exceeds the annual tonnage consumption of most plastics materials. Only PVC, the polyolefins, the styrene polymers, the aminoplastics and, possibly, the phenolics are used in large quantity. As explained in Chapter 5, these materials are essentially non-volatile solvents for PVC. Because of their molecular size they have a very low rate of diffusion into PVC at room temperature but at temperatures of about 150°C molecular mixing can occur in a short period to give products of flexibility varying according to the type and amount of plasticiser added. All PVC plasticisers have a solubility parameter similar to that of PVC. It appears that differences between liquids in their plasticising behaviour is due to differences in the degree of interaction between polymer and plasticiser. Thus such phosphates as tritolyl phosphate, which have a high degree of interaction, gel rapidly with polymer, are more difficult to extract with solvents and give compounds with the highest brittle point. Liquids such as dioctyl adipate, with the lowest interaction with polymer, have the converse effect whilst the phthalates, which are intermediate in their degree of interaction, are the best all- round materials. Compounding Ingredients 33 1 Phthalates prepared from alcohols with about eight carbon atoms are by far the most important class and probably constitute about 75% of plasticisers used. There are a number of materials which are very similar in their effect on PVC compounds but for economic reasons di-iso-octyl phthalate (DIOP), di- 2-ethylhexyl phthalate (DEHP or DOP) and the phthalate ester of the C7-C9 oxo-alcohol, often known unofficially as dialphanyl phthalate (DAP), are most commonly used. (The term dialphanyl arises from the IC1 trade name for the C7-C9 alcohols-Alphanol 79.) As mentioned in the previous paragraph these materials give the best all-round plasticising properties. DIOP has somewhat less odour whilst DAP has the greatest heat stability. Because of its slightly lower plasticising efficiency, an economically desirable feature when the volume cost of a plasticiser is less than that of polymer, dinonyl phthalate (DNP) may also be an economic proposition. Its gelation rate with PVC is marginally less than with DIOP, DAP and DEHP. In spite of their high volatility and water extractability, dibutyl phthalate and di-isobutyl phthalate continue to be used in PVC. They are efficient plasticisers and their limitations are of greatest significance in thin sheet. Until comparatively recently the bulk of general purpose phthalate plasticisers have been based on the branched alcohols because of low cost of such raw material. Suitable linear alcohols at comparative prices have become available from petroleum refineries and good all-round plasticisers are produced with the additional advantage of conferring good low-temperature flexibility and high room temperature resistance to plasticised PVC compounds. A typical material (Pliabrac 810) is prepared from a blend of straight chain octyl and decyl alcohols. Certain higher phthalates are also available. For example, ditridecyl phthalate and di-isodecyl phthalate are used in high-temperature cable insulation, the former having the better high-temperature properties. Because of its greater hydrocarbon nature than DIOP, di-isodecyl phthalate has lower water extract- ability and is used, for example, with epoxidised oils in baby-pants. Developments in the USA have led to the availability of terephthalate plasticisers, for example dioctyl terephthalate (DOTP). Whilst these materials are very similar to the corresponding o-phthalate esters they are generally less volatile and are best compared with o-phthalates with one or more carbon atom in the alkyl chain. As with the linear dialkyl phthalates the terephthalates show good fogging resistance. This is a phenomenon in which new cars on storage fields awaiting delivery develop misting on the windows due, apparently, to the volatility of additives in PVC compounds used with the car. In the 1950s phosphate plasticisers had an importance comparable with the phthalates. However, during the 1960s the development of the petrochemicals industry resulted in the phthalate plasticisers becoming available at much lower prices than obtained for the phosphates such as tritolyl phosphate (TTP) for which the cresols were obtained from coal tar. During this period trixylyl phosphate (TXP) tended to replace TTP because of its lower price structure. Because of their high price phosphates tend to be limited to products where good flame resistance is required, such as in insulation and mine belting. Other advantages of these materials are their high compatibility with PVC and good solvent resistance. On the debit side they are toxic and give products with a high cold flex temperature. The development of natural gas as a fuel source in the UK has led to reductions in tar acid supplies and this has prompted the petrochemicals industry to make I oomomooooo 0000000000 ?19?9-??p?". 5 omommommoo" Nr??rnr?r?p?'"?p? Pooooooo-0 g > WWviWO NNO -NNNNP-m I I I I I I IPP?l Compounding Ingredients 333 available synthetic alkylated phenols such as the isopropyl phenols. Tri- isopropylphenol phosphates are more price stable than the older phosphates, but have otherwise similar properties. Some problems are, however, said to arise with PVC pastes based on these materials because of their high pseudoplasticity and thixotropy leading to draining difficulties in dipping operations. For some applications it is important to have a compound with good low- temperature resistance, i.e. with a low cold flex temperature. For these purposes aliphatic esters are of great value. They have a lower interaction with PVC and thus are incorporated with greater difficulty and extracted with greater facility. For many years the sebacates such as dibutyl sebacate (DBS) and dioctyl sebacate (DOS) were used where good low-temperature properties are required. Today they have been largely replaced by cheaper esters of similar effect in PVC derived from mixed acids produced by the petrochemical industry. The most important of these mixed acids are the AGS acids (a mixture of adipic, glutaric and succinic acids). These are esterified with octyl, nonyl and decyl alcohols to give plasticisers now generally referred to as nylonates but occasionally as sugludates. The sebacate, adipate and sugludate-type plasticisers can also be used to give compounds of high resilience. Esters based on trimellitic anhydride, the trimellitates, have become very popular primary plasticisers for use at high temperatures or where a high level of resistance to aqueous extraction is required. Because of their frequency of use at elevated temperatures, they are usually supplied commercially containing an antioxidant. A number of other special purpose plasticisers are also available. The epoxidised oils and related materials are good plasticisers and very good light stabilisers and are often used in small quantities in PVC compounds. Polymeric plasticisers such as polypropylene adipate, polypropylene sebacate and similar products capped with lauric acid end groups are used where non-volatility and good hydrocarbon resistance is important. They are, however, rather expensive and are rather difficult to flux with PVC. Solid ethylene-vinyl acetate modified polymers have also recently been offered as polymeric plasticisers (e.g. Elvaloy by Du Pont). These are claimed to be true plasticisers and are non-volatile, non- migratory and, unlike most PVC plasticisers, have a high resistance to biodegradation. Certain esters of citric acid, such as acetyl tributyl citrate, find an outlet where minimum toxicity is of importance. The development of PVC as a metal-finishing material has led to the need for a good PVC-metal adhesive. For some purposes it is found convenient to incorporate the adhesive component into the PVC compound. Esters based on allyl alcohol, such as diallyl phthalate and various polyunsaturated acrylates, have provided useful in improving the adhesion and may be considered as polymerisable plasticisers. In PVC pastes they can be made to cross-link by the action of peroxides or perbenzoates simultaneously with the fluxing of the PVC. When the paste is spread on to metal the ‘cured’ coating can have a high degree of adhesion. The high adhesion of these rather complex compounds has led to their development as metal-to-metal adhesives used, for example, in car manufacture. Metal coatings may also be produced from plasticised powders containing polymerisable plasticisers by means of fluidised bed or electrostatic spraying techniques. Many other liquids have been found to be effective plasticisers for PVC but are of limited commercial value, at least in Britain. The effect of plasticisers on the properties of PVC is illustrated in Figure 12.20 (a-e). 334 PLASTICISER CONTENT IN % (a) Figure 12.20. See page 336 for key 335 0 FILLER CONTENT IN '/a BY WEIGHT (d 1 Figure 12.20. See page 336 for key 336 Vinyl Chloride Polymers PLZSTICISER CONTENT IN '/a Figure 12.20. Effect of change of plasticiser on the properties of polyvinyl chloride compounds.'! (a) Tensile strength. (b) Cold flex temperature. (c) BS softness number. (d) Elongation at break (e) 100% modulus. (The Distillers Company Ltd.) 12.5.3 Extenders A number of materials exist which are not in themselves plasticisers for PVC because of their very limited compatibility with the polymer, but in conjunction with a true plasticiser a mixture is achieved which has a reasonable compatibility. Commercial extenders, as these materials are called, are cheaper than plasticisers and can often be used to replace up to a third of the plasticiser without serious adverse effects on the properties of the compound. Three commonly employed types of extender are: (1) Chlorinated paraffin waxes. (2) Chlorinated liquid paraffinic fractions. (3) Oil extracts. The solubility parameters of these extenders are generally somewhat lower than that of PVC. They are thus tolerated in only small amounts when conventional plasticisers of low solubility parameter, e.g. the sebacates, are used but in greater amounts when phosphates such as tritolyl phosphate are employed. 12.5.4 Lubricants In plasticised PVC the main function of a lubricant is to prevent sticking of the compound to processing equipment. This is brought about by selecting a material [...]... (ft Ibf/in of notch) I Butadiene-2-vinylpridine (30 :70 ) Chlorinated polyethylene (44% chlorine) Butadiene-styrene-acrylonitrile ( 67: 17: 16) Butadiene-2-vinylpyridine (40:60) Butadiene-methyl methacrylate (35:65) Butadiene-methyl acrylate Butadiene-methyl isopropenyl ketone Butadiene-diethyl fumarate 19.3 19.2 18 .7 18 .7 18.2 17. 9 17. 4 17. 2 3 4.4 17. 4 10.0 2.8 3.3 15.9 15.0 The anomalous effect of the... L (Eds), Developments in PVC Production and Processing-I, Applied Science, London (1 977 ) SARVETNICK, H A SARVETNICK, H A Reviews and NEUNDORF, u.,Kunstofe, 77 , 954-9 (19 87) Kunstoffe 80, (lo), 1 070 -5 (1990) A N D WERNER, R Kunstoffe, 86(10), 1 474 -76 (1996) R I R K N E R , ti FRANK, A,, REHM, T ... G A R., Vinyl and Allied Polymers, Vol I1 Vinyl Chloride and Vinyl Acetate Polymers, IIiffe, London (1 972 ) Mulficomponent Polymer Systems (Advances in Chemistry Series No 99) American Chemical Society, Washington (1 971 ) NASS, L I., Encyclopaedia ofPVC Marcel Dekker, New York; Vol (1 976 ); Vol 2 (1 977 ) OWEN, E D (Ed.), Degradation of Stabilisation of’PVC,Elsevier Applied Science, London-New York ( 1984)... Acetate Polymers, Iliffe, London (1 972 ) 15 Technical Trade Literature, British Geon Ltd., London 16 JACOBSON, u., Brit, Plastics, 34, 328 (1961) 17 RRAMFITT, J E., and HEAPS, I M Advances in P V C Compounding and Processing (Ed KAUFMAN, M.), Maclaren, London, p 41 (1962) 18 SKINNER, s J., AND BOLAM, s E., Rubber and Plasfics Age, 37, 169 (1956) 19 WELDON, L H P., Trans Plastics Inst., 24, 303 (1956) 20... 1 970 s it has been largely replaced by various fabric materials which have, to this writer at least, proved very durable and preferable in terms of comfort 358 Vinyl Chloride Polymers Table 12.6 Consumption breakdown of PVC for Western Europe 19 97 (source Modern Plustics January 1998) ‘000t % Unplasticised pvc Pipes and conduit Profiles Film and sheet Bottles Injection mouldings Other Total UPVC 1 475 ... Van Nostrand-Reinhold, New York (1 972 ) SEDLACEK B (Ed.), Polyvinyl Chloride: Its Formation and Properties (IUPAC Symposium), Butterworths London (1 971 ) TII'OW, w v., PVC Technology, 4th edition, Elsevier Applied Science Publishers, London, New York ( 1984) WHELAN, A., and CRAR, J L (Eds), Developments in PVC Production and Processing-I, Applied Science, London (1 977 ) SARVETNICK, H A SARVETNICK, H A... 6 Vinyl chloride-acetate copolymer (IS0 No 60) Diabasic lead stearate Dibasic lead phthalate Lamp black 100 0 .75 0 .75 2 Formula 7 is a leathercloth formulation for use in spreading techniques There are many possible formulations and that given is for a product with a soft dry feel Formula 7 Paste-making polymer (IS0 No 125-140) DIOP Extender Ba-Cd system Epoxidised oil Pigment 100 50 20 2 3 as required... crystalline, materials polymerised at -50% are about 65% syndiotactic and 20% crystalline The Tg also increases steadily with decrease in polymerisation temperature, with a value of about 105°C for PVC polymerised at -50°C Melting occurs over a range and it has been estimated than the T,,, for 100% syndiotactic material would be about 273 °C It is interesting to note that these crystalline materials do... 80 to 87 C This is, however, sufficient for the material to be of interest in applications involving short-term exposure to elevated temperature, e.g filling hot foodstuffs into containers Such a copolymer has been introduced by Hoechst (Hostalit LP HT 5060) References 1 REGNAULT, v., Ann., 15, 63 (1835); Ann Chim., 11, 59 358 (1835) 2 REBOUL, N E., Jahresber., 304 (1 872 ) 3 German Patent, 278 , 249... 2 REBOUL, N E., Jahresber., 304 (1 872 ) 3 German Patent, 278 , 249 (E Klatte) 4 BAUMANN, E., Ann., 163, 312 (1 872 ) 5 OSTROMISLENSKY, I., Chem Zentral., 1 1980 (1912) 6 U.S Patent, 2, 188, 396 7 European Chem News, 4 (89), 35 (1963) 8 Brifish Parent, 842, 690 9 SMALL, P A,, J Appl Chem., 3, 71 (1953) 10 MARVEL, c s., SAMPLE, J H., and ROY, M F., J Am Chem SOC., 61, 3241 (1939) 11 FLORY, P J., J Am Chem . Butadiene-methyl isopropenyl ketone Butadiene-diethyl fumarate I 19.3 19.2 18 .7 18 .7 18.2 17. 9 17. 4 17. 2 3 4.4 17. 4 10.0 2.8 3.3 15.9 15.0 The anomalous effect of the last two rubbers. most plastics materials. Only PVC, the polyolefins, the styrene polymers, the aminoplastics and, possibly, the phenolics are used in large quantity. As explained in Chapter 5, these materials. (ft Ibf/in of notch) Butadiene-2-vinylpridine (30 :70 ) Chlorinated polyethylene (44% chlorine) Butadiene-styrene-acrylonitrile ( 67: 17: 16) Butadiene-2-vinylpyridine (40:60) Butadiene-methyl