CHAPTER 1
TYPES OF POLYMERS AND THEIR USES
Synthetic resins, in which plastics are also included, vary widely in their chemical composition and in their physical properties. The number of synthetic resins which can be made is vast; relatively few, however, have achieved commercial importance. Some of the polymers that have achieved commercial importance and their uses are tabulated in Table 1 and some of their important physical properties are listed in Appendix 2.
Well over 90% of all synthetic resins made today comprise no more than 20 different types, although there are certain variations to be found within each type. Synthetic resins are familiar to most people as plastics, but they have other uses, such as in the manufacture of surface coatings, glues, synthetic textile fibres, etc. The rapid growth of the synthetic resin industry has to a large extent, been made possible by the fact that ample supplies of necessary raw materials have become available from petroleum.
The synthetic resins may be divided into two classes, known respectively as 'thermosetting' and 'thermoplastic' resins, each class differing in its behaviour on being heated. The former do not soften; the la tter soften but regain their rigidity on cooling. Both types are composed of large molecules, known as macromolecules, but the difference in thermal behaviour is due to differences in internal structure.
The larger molecules of the thermoplastics have a long-chain structure. with little branching. They do not link with each other chemically, although they may intertwine and form a cohesive mass with properties ranging from those of hard solids to those of soft pliable materials, in certain cases resembling rubber. On being heated, the chain molecules can move more freely relative to each other, so that, without melting, the material softens and can flow under pressure and be moulded to any shape. On cooling, the moulded articles regain rigidi ty. Some resins require the addition of liquid plasticizers to improve the flow of the plastic material in the mould. In such cases the moulded articles are usually softer and more flexible than the products made from the unplasticized resins.
The macromolecules of the thermosetting resins are often strongly• branched chains and are chemically joined by crosslinks, thus forming a complex network. On heating, there is less possibility of free movement, so that the material remains rigid.
Production of these resins also falls into two groups since there are, generally, two main types of chemical reaction by which they are made. These are polycondensation reactions and polymerization reactions.
1
T. R. Crompton, Practical Polymer Analysis © Plenum Press, New York 1993 '" TABLE1
Polycondensation types Phenol formaldehyde (a) Unfilled (a) Adhesives, laminates, pulp mouldings, particle board. (b) Woodflour/cotton (b) Bottle tops, electrical parts, fuse boxes, meter cases, heat• flock filled resistant close-tolerance mouldings, toilet seats, restricted in colours obtainable, coloured ashtrays. Urea formaldehyde Cellulose filled As for, cellulose filled melrunine formaldehyde but suitable for dinner ware. U/F resins are used for similar applications to those shmm under unfilled phenol formaldehyde, white electric plugs. Helamine formaldehyde (a) Unfilled (a) Usually occurs in laminate form as surfacing for tables etc. (b) Alpha-cellulose (b) Noted for durability, hardness and good electrical proper• ties, suitable for appliance housings, dinnerware, closures, writing equipment, clock housings, knobs, handles, lighting fixtures, appliances, instruction panels. Polyesters (a) Resin (a) Unreinforced resin for buttons, surface coatings, embedding and potting and nut locking. Filled resin for imitation marble, flooring, pipe joints, mortars and body stoppers. (b) Dough moulding (b) Protective housings, connectors, cowls, guards and ducts. compound Components often replace metal, offering non-corrosion, durability, good electrical performance and high strength. (c) Sheet moulding (c) Outlets in the electrical, building, motor engineering and compound furniture industries that compete on a cost basis with die castings and sheet metal fabrications owing to ease of moulding complicated shapes and short moulding cycles. Expoxides Chemically resistant paints, adhesives, tools, PVC stabilizers, electrical insulation, chemical- and wear-resistant jointless flooring, road coatings, cements, laminates, powder coatings, stopping compounds, repair kits, printed circuits, filament wound tanks and pipes, pressure vessels. (") Nylon (a) Type 6 (a) Moulded mechanical parts, gear wheels, bushings, sliding parts for storm windows, automobile and refrigerator door ~ o-l closures, mixer valves, switch housings, grommets, cable clamps, t'J :: w ~ TABLE1 continued Styrene acrylonitrile Cups, tumblers, trays and general table, kitchen ware, toothbrush Acrylonitrile/butadiene/ handles, refrigerator components, radio knobs and scales, lenses, styrene cosmetic items, hi-fi covers and cases, packaging. Shoe heels, telephone handsets, housing for consumer durables, food containers, luggage, refrigerator liners, safety helmets, radio cabinets, tote boxes, car facia panels, instrument clusters, boat hulls, furniture. Vinyl polymers (a) Rigid polyvinyl (a) Extrusion of piping, profiles and sheet in applications chloride requiring chemical inertness and scuff-resistance combined with light weight. Plastisols for toys, leathercloth etc. Plastic guttering, high density bottles, formed packaging trays, moulded containers. (b) Rigid vinyl (b) Similar applications to those for PVC but widely used in the chloride/acetate manufacture of calendared sheet used for toys, novelties, wall coverings, displays, templates, etc. Gramophone records. (c) Rubber modified (c) Same as for PVC. PVC. Polyacetals Load-bearing mechanical parts, small pressure vessels, aerosol containers business machine parts, appliances, automobile, engineering and industrial products. Polycarbonates Business machine parts, camera components, electrical apparatus, sterilisable ware, draughtsman's instruments, lamp covers, safety helmets, tail-lights, de luxe housewares, engineering and industrial components, sterilisable transparent feeding bottles for babies. Ionomers Shoe heel tips, tool handles, hammer and mallet heads, bottles, skin packaging, coating, toys, shoe soles, shoe stiffeners, meat packaging, flexible packaging, packaging of wine and fruit juices. Polyphenylene oxide TV set components, valve bases, switches, housings, parts for C"l domestic appliances, meter cases, machine housings, computer and S; "0 camera parts, automotive grilles, ducts, light housing, ...., ttl instrumentation parts. : ()l 6 CHAPTER 1 In polycondensation reactions, two or more chemicals are brought together and a reaction between them is initiated by using heat or a catalyst or both. The reaction proceeds with the elimination of water and the molecules are joined by chemical bonds to form macromolecules, either long-chain or crosslinked structures of the thermoplastic or thermosetting types, respectively. Many resins obtained by polycondensation are the thermosetting type. In the manufacture of these resins the chemical reactions are arrested at an intermediate stage in which the resins are temporarily thermoplastic; they are set in their final shape by the application of heat and pressure. At this stage the interlinking of the molecules takes place. Important thermosetting synthetic resins made by polycondensation, using petroleum chemicals as raw materials, include the phenol formaldehyde ('Bakelite'), urea-formaldehyde, alkyd- and epoxy- types. Resins with long-chain macromolecules obtained by polycondensation, have thermoplastic properties. Polyesters ('Terylene') and polyamides (nylon) are examples of polycondensations. The synthetic fibre 'Terylene' (known as 'Dacron' in the U.S.A.) is a polyester formed by the reaction of ethylene glycol with terephthalic acid; the terephthalic acid is obtained from para-xylene by oxidation. CDDH polyconden - I [- CH, - CH,OOC - ~ +@ sat ion CDDH Nylon type fibres (polyamides) are manufactured from adipic acid, which can be made from either cyclohexane or phenol the adipic acid is condensed with hexa-methylene diamine, which is a derivative of adipic acid. COOH ~OH hydrogeneration oxidation I (CH Z)4 10 I CDOH phenol cyclohexanol adipic acid COOH NH Z I I polycondensation (CHZ)4 + (CH Z)6 I I COOH NH adipic hexamethylene acid diamine Resins produced by polymerization reactions, known technically as high polymers, are rapidly increasing in number and in importance as compared with the polycondensation resins. High polymers are usually made by joining together into long chains a number of molecules which have the same kind of reactive points or groupings in their structure. These individual molecules are usually olefins or other compounds with double bonds, and are called 'monomers'. The molecule of the polymer often contains hundreds of monomer units. The manufacture of high polymers therefore takes place in two stages; first, the production of the monomer, or repeating chemical unit; TYPES OF POLYMERS AND THEIR USES 7 and second, the polymerization to a resin. Thus, if we take the preparation of polyvinyl chloride as an example we have: 1st stage CH 7 = CH 2 + C1 2- CHll - CH 7Cl- CH 2 = CHCl + HCl ethylene dichloroetnane vinyr chloride monomer Znd stage polymerization CH 2 = CH 2 + CH 2 = CHCl + CH 2 = CHel - CH CHCr - CH - CHCl - CH 2 - CHCl - 2 polyvinyf chloride In some cases it is possible to form polymers from two or even three monomers which may differ from one another in chemical form and yet be capable of linking end-to-end to form mixed monomer chains. These are known as I copolymers I, and such polymers form the basis of the most important types of synthetic rubber. Further examples of polymerizations: Styrene butadiene copolymer CH = CH 2 + CH2 = CH - CH = CH 2 -- - ~H - CH2 - CH2 - CH = CH - CHZ- Ph Ph Styrene butadiene Styrene - butadiene copolymer Polymethylmethacrylat~ COoMel nCH Z = C + COOMe-• CH - ~ - I I Z Me ~e I n Buna N rubber CH Z = CHCN + CH Z = CH - CH = CH2 ~ CH2 CH CH 2 - CH = CH - CH2 - CN Vinyl chloride - vinylidine chloride copolymer CH 2 = CH - Cl + CH? = CC1 2 ~ CH 2 - CC1 2 - CH 2 - CHCl - vinyl chloride vinylidine chloride Polyacrylonitrile CH - CH nl"CH 2 = ~H1 - r - 2 I -l _ CN~ l CN n Polyvlnyl acetate nC:-1 2 = CH -- CH 2 - CH - DOC CH 3 ooCCH3 n 8 CHAPTER 1 Polyurethanes DCN -@- NCO + HD DH I I Me CH 2 - CH2 -- eg toluene eg ethylene dissocyanate glycol -1(5\ NH(COOCH 2 - CH 200CNH - @ Me~ Me polyurethane The number of forms in which polymers are encountered in practice is immense. The polymer might exist as a solid, or, a liquid, either in the neat form or as an aqueous or solvent emulsion such a polyvinyl acetate or as a gum, or adhesive. Solid polymers might be rigid or flexible. Examples of rigid polymers are polyolefins, polystyrene and unplasticized PVC. Examples of flexible polymers are rubbers, elastomers and plasticized PVC. The polymer might be virtually 100% polymeric or might contain inert fillers to impart desired properties. Examples of these range from additions of a few percent of substances such as talc, zinc oxide or titanium dioxide to materials where a major proportion of the sample is a filler, ego the old style automotive battery cases which contain only 10-20% of a polymeric constituent the remainder being coal dust. CompOSite polymeriC materials containing high percentages of non-polymeric materials are used extensi vely in the fabrication and engineering industries. The sample might be in the form of a polymer impregnated paper or textile such as is used in the packaging or clothing industries. Polymers may occur as a constituent with other materials in, for examples, paints, varnish and glass fibre reinforced plastics. Polymers can be arbitrarily divided into three types, i) those completely soluble in organic solvents, (eg. conventional polystyrene), ii) those which do not completely dissolve in organic solvents but which do swell sufficiently to enable soluble additives to be removed prior to further analysis (Chapter 3), (eg. low density polyethylene and polypropylene or cross linked styrene-butadiene copolymer or acryloni trile-butadiene-styrene terpolymer in which the crosslinked gel component is completely insoluble and the uncrosslinked material is soluble) and iii) polymers which are completely or almost completely insoluble in organic solvents, (ego polytetrafluoroethylene, some silicones and some epoxy resins). The simplest type of polymer sample to handle is, of course, the type of polymer for which a good organic solvent exists and which contains no fillers and little or no additives. In these instances a fractional precipitation or a fractional extraction procedure using organic solvents and non-solvents such as described in Chapter 3 will suffice to separate and purify the gross polymer and gross additive fractions preparatory to further analysis of each fraction. Partially soluble polymers of type ii mentioned above can also often be handled by these procedures. Thus if low density polyethylene is refluxed with toluene or xylene, although the majority of the polymer does not dissolve it does soften and disperse in the organic solvent leaving the additives in solution. Filtration or centrifuging of the mixture gives a pure polymer extract and a solvent extract containing addi ti ves. In the case of high impact polystyrene, TYPES OF POLYMERS AND THEIR USES 9 i. e. styrene-butadiene latices contact with chloroform or toluene dissol ves the uncrosslinked copolymer and additives, leaving the undissolved gel which can be separated by filtration or centrifuging. Treatment of the chloroform or toluene phase with absolute ethanol precipitates the uncross linked copolymer leaving the additives in solution. Thus three cross fractions are obtained, crosslinked copolymer, uncrosslinked copolymer and gross additives each of which is amenable to further examination by various techniques. In some cases, eg. acryloni trile-butadiene-styrene terpolymers, extremely powerful sol vent systems such as boiling chlorobenzene are required to soften the polymer. The separation of gross polymer from additives and fillers is important both from the pOints of view of obtaining fractions of these for further examination and also from the point of view of obtaining uncrosscontaminated fractions of each to facilitate these identifications. This applies particularly in the case of polymeric materials containing high proportions of non-polymeric constituents. Thus, heavily plasticized PVC might contain up to 60% of a plasticizer which it is essential to separate from the PVC, usually by solvent extraction, prior to attempted identification of the polymer or the plasticizer. In the case of a polymer containing percentage levels of inorganic metal oxides, solution of the polymer in an organic solvent followed by extraction with an aqueous acid might suffice to separate the polymer from the metallic constituent. Polymers for which no solvent can be found are a different problem and present difficulties especially if appreciable amounts of fillers or additives are present. Direct spectroscopic examination by infra-red or NMR spectroscopy of thin cast films of the polymer will often yield some information, as will pyrolysis or photolytic techniques, especially if the latter is coupled to gas chromatography and/or mass spectrometry. Pyrolytic or photolytic techniques (Chapter 4.4.2, 4.4.3) operate by providing information regarding the polymer degradation products formed under these conditions, the nature of which will often yield information on the structure of the original polymer. Much information is provide throughout this book on preparing samples for analysis especially in the case of the more commonly used polymers. In the case of the less commonly used polymers, especially the wide range of copolymers now being produced and particularly in the case of highly filled materials the analyst may have to exhibi t considerable ingenuity in devising methods of preliminary work-up of the sample prior to the commencement of analysis. The general principals illustrated in this chapter and throughout the book will help in this respect.