How do chemicals and additives modify the properties of Plastics?
Significant advances have been made in the modification of the properties of plastics materials through the incorporation of chemicals and additives to improve performance in different applications. The possibilities are limitless. The following outlines some of the things that are being done;-
Plasticisers usually high boiling point, low volatility esters are incorporated with polymers by heating and mixing to increase flexibility by inserting themselves between the polymer chains and thus reducing the attractions between chains. They are essential in cellulose plastics and include triphenyl phosphate, a common ingredient to reduce flammability. The esters are usually diethyl and dimethoxyl glycol phthalates but in PVC, which accounts for the largest proportion of plasticiser used, phthalate esters of much higher molecular weight are employed because the plasticiser proportion is higher and low volatility is imperative. It is also important that flexibility be maintained at low temperatures. The compound di-2-ethylhexyl phthalate (DOP) is used, and for maximum low temperature flexibility the adipate as well as the azelate are employed. These three plasticisers have the advantage of being food safe in vinyl wrapping film, whereas of the phosphates, only octyl diphenyl phosphate may be used.
Another class of plasticiser, either a complete polyester made from a glycol or an epoxy compound, usually of soya oil, is known as a permanent plasticiser because of its non-volatility; the epoxy types are used with other plasticisers in vinyl compounds and also function as stabilisers.
Stabilisers and Antioxidants Polymers are subject, in varying degrees, to degradation by heat, light and oxidation and can be protected by substances that only arrest but do not entirely prevent these processes. PVC in particular requires stabilisation since the manipulation temperatures are high enough to initiate decomposition, but polyolefins and polystyrene are also improved by suitable additives. In PVC the main problem is the emission of hydrochloric acid; this can be reduces by adding lead salts or soaps such as lead stearate. Though these are inexpensive, they cause opacity and are toxic; they are widely used for insulation compounds. Where a degree of transparency is required with freedom from stain, barium- cadmium salts, with or without zinc, are employed. With silicate and asbestos fillers, epoxy resins and phosphites are used, but for rigid transparent PVC processed at high temperature, a different class of stabiliser, that of alkyl compounds of tin, is used.
After 1950, a series of dibutyltin derivatives, such as dilaurate and maleate, were recognized as excellent though expensive stabilisers fir high clarity PVC, a property becoming more important with the increasing use of blown PVC bottles. These stabilisers are now replaced by dioctyltin derivatives containing maleate groups or mercapto- acetates, which are less toxic and cheaper. For plastisols, barium, cadmium, and zinc compounds, together with organic derivatives are preferred. Non toxic stabilisers also include calcium zinc compounds with phosphates and epoxies. Formulations are widely varied according to processing conditions, resin characteristics, and end uses.
Antioxidants fall into five classes: phenol derivatives, amines, esters, organic phosphites, and miscellaneous – the choice depending on the polymer to be protected and the end use. LDPE is an important example of a polymer that must be protected by antioxidants. It is characterised by side chains, and each junction is vulnerable to oxidation, which is dealt with by phenols alone or in conjunction with sulphur containing esters or phosphites. Oxidation caused by light calls for ultraviolet absorbers, the most satisfactory of which is carbon black, the antioxidants in this case being phenols and naphthols containing sulphur.
Because of its structure, polypropylene is extremely susceptible to oxidation, a reaction that becomes important because the substance is handled at high temperatures. Antioxidants similar to those used for LDPE are effective, especially phenols and sulphur compounds, such as distearyl thiodipropionate, used together.
Polystyrene is relatively stable to oxidation but sensitive to light degradation. High impact polystyrene, however, because of its rubber content, requires the use of phenols. Most other polymers are less subject to oxidative degradation, but phosphite esters are often used, especially with polyurethanes, which are likely to discolour.
As already indicated, ultraviolet light combines with oxidation to promote degradation, and combinations of absorbers and antioxidants are common, the ultraviolet absorber being increasingly important as plastics are used in outdoor applications. The two classes of absorbers are screening agents and excited-state quenchers. Excited-state quenchers stabilise the polymer by removing energy from the molecule that has already absorbed ultraviolet light. Screening agents are the most widely used, absorbing most of the incident ultraviolet light and converting it to heat. The most effective of these are the benzophenones, especially their ethers and benzotriazoles. Ultraviolet absorbers also work by converting shortwaves into harmless longer light waves through fluorescence. Protection by fluorescent dyes is particularly applicable to films. Of the excited-state quenchers, nickel compounds are the most effective, especially organo-nickel complexes.
Organic peroxides and hydroperoxides are derivatives of hydrogen peroxide and split readily into free radicals (group of atoms that act as a unit in a chemical reaction) required in addition polymerisation, over a temperature range determined by decomposition. Room temperature splitting can be brought about by activators, such as cobaltous ion or tertiary amine.
Flame Retardants Where nonflammability is especially important, all polymers except rigid PVC and fluorocarbons require flame retardant additives, even though some, such as nylons or polycarbonates, are self extinguishing. The most important flame retardants are bromine, chlorine, antimony, and phosphorus compounds. Antimony compounds can be used only with opaque materials, however. Additives vary with the polymer; various phosphates are used with plasticised PVC but with polyurethanes the phosphates must be brominated, and the same applies to polystyrene foam, where highly brominated phosphates or hydrocarbons are incorporated in the beads as supplied for moulding. An alternative method is to build bromine containing units into the polymer chains, as can be done in the case of polyesters, polyurethane foam and epoxy resins.
Fillers & reinforcements These fall into five main categories: mineral and synthetic inorganic powders, carbon blacks, cellulosics, metal powders, and microspheres. Reinforcements include cotton and asbestos flocks; glass fibres, chopped or in the form of rovings, mats or monofilaments; carbon fibres; and mineral whiskers. A great variety of fillers especially suitable for different polymers and end uses is available- e.g. wood flour, first used for phenolics, which is cheap and reduces mould shrinkage; asbestos flock (tiny fibres of asbestos), which greatly increases impact strength; various fillers that improve polyurethanes; and aluminium, magnesium, and titanium oxides, which give added stiffness to composites. Cotton flock is essential in urea- formaldehyde resin mouldings, and many synthetic fibres are used with different polymers; the great bulk of reinforcement, however, is glass fibre of less than 25 microns in diameter and available as glass cloth; the cloth can be chopped for incorporation in mouldings or used as mats or as a filament for a thin surface mat or for a winding. Carbon fibres made by heating acrylonitrile fibres have enormous tensile strength, and plastics bonded with then can replace metals. Sapphire fibres and single crystal whiskers also provide powerful reinforcement.
Colourants are inorganic or mineral pigments, organic pigments (either toners, insoluble organic salts or metal complexes of dyes), or are lakes, dyes deposited on alumina. Uniform dispersion and standardization of colour are usually accomplished by predispersion in a resin at high temperatures or, especially in the case of epoxy resins, polyesters, and urethane systems by dispersion in the plasticiser, monomer or solvent. Dyes must be used for transparent plastics, either dissolved in solvent or in such fine dispersions, as in the case of basic dyes, that no particles are visible.
Lubricants Plastic materials such as nylon are available in self lubricating grades. These grades are made by the incorporation of materials which are themselves well known for their lubricating properties such as graphite, molybdenum disulphide and in some instances, fluorocarbon polymers.
Foaming Almost all plastics materials can be made in cellular or foam form. Products such as polyurethane, polystyrene and urea formaldehyde in low density foam form have been used in large quantities for many years. The foam forms of plastics are produced when gas, introduced or generated within the plastics, expands and produces a cellular structure. If a mixture of gas and plastics is injection moulded, the gas will expand as the hot plastic enters the mould and a moulding with a dense, cellular core and an integral skin will be produced. The resultant mouldings, which are termed structural foams, offer improved strength weight ratios and rigidity and allow greater density freedom for large structural parts. Finished products can produce the texture and feel of wood and are often seen in furniture applications.