Polymer materials history and technology

Many purely natural polymers were traditionally used to manufacture small objects by moulding and pressing. Horn and turtleshell are quite thermoplastic and were die-pressed into complex shapes such as decorative snuff boxes. When used as thin veneers they could be made to conform to mouldings or turned pilasters by gluing under heated cauls.

Most natural resins are also thermoplastic. Shellac was used to polish turned wood articles in India and Europe by simply rubbing solid sticks of the material on the spinning stock until it melted by the heat of friction. This use apparently pre-dates its use as a solvent varnish. Some of the most extensively employed early thermoplastic materials were various compositions used to imitate carved detail on picture frames in architectural interiors. This craft saw its heyday during the late eighteenth and early nineteenth centuries when 'composition' ornament makers used mixtures typically consisting of rosin, linseed oil, animal glue and whiting to mould-press elaborate sculptural detail.

Ornament made from 'composition' was popularized in Britain by the Adam brothers in the last quarter of the eighteenth century. Their fine and repetitive style of decoration was difficult and expensive to reproduce by the traditional method of carving in wood so they introduced a way of manufacturing their designs more cheaply from composition pressed into reverse-carved moulds. This method of ornamentation was used extensively throughout the nineteenth-century for picture frames, furniture and interior decoration.

Gutta-percha, a natural latex tapped from trees native to Malaysia, was first brought to European notice in 1843. It was used to manufacture picture frames, book covers and various other decorative and useful objects during the Victorian era. Although chemically identical to natural 'India' rubber, it was rigid at room temperature and mouldable at around 90 °C. In fact, natural rubber and gutta percha are the cis and trans isomers respectively of polyisoprene. Imitation woods made from ground coconut shell or other wood dusts mixed with gutta percha could be mould formed or worked by standard woodworking techniques.

In 1839, Charles Goodyear patented a process by which the much more ubiquitous natural rubber could be made hard by crosslinking the polymer with sulphur. This ebony-like material was variously called ebonite, vulcanite and hard rubber. Linoleum, patented in 1860, combined the toughness of a crosslinked polymer (partially oxidized linseed oil) with the thermoplasticity of a natural resin (pine rosin). The mixture of these polymers and various fillers produced a tough, attractive and wear-resistant floor covering.

Bois-durci, an imitation ebony, was patented in 1855 and used to produce small decorative plaques for the furniture trade until the 1880s. Made from sawdust and albumen derived from blood or eggs, the protein binder was denatured and crosslinked during the hot moulding operation and so could be described as an early thermosetting resin. Likewise casein, the protein present in milk and cheese, was used as the basis for a mouldable composition patented in Germany in the late 1890s. Casein continues to be used world-wide by button manufacturers.

Because many natural polymers were highly modified to achieve different properties, the designation of the first synthetic plastic is rather arbitrary. However, this distinction is often given to the cellulose nitrates. In 1846 an explosive known as gun-cotton was made by reacting a mixture of nitric and sulfuric acids with cotton-wool. Experiments on this material showed it to be thermoplastic, but highly unstable and flammable. The extensive efforts of Alexander Parkes in England led eventually to a thermoplastic resin composed of less highly nitrated cellulose combined with various oils, fillers and pigments, and called Parkesine. Moulded objects were made from this material between 1855 and 1868 but suffered from fragility. In 1869, the Hyatt brothers of Albany, New York patented a synthetic ivory named Celluloid made from cellulose nitrate plasti-cized with camphor which was an immediate and lasting success. The manufacturing technologies developed to convert Celluloid and its English equivalent Xylonite into a wide range of useful and decorative objects formed the basis of a plastics industry which grew and flourished as further discoveries were made.

A thermosetting, crosslinked resin synthesized from phenol and formaldehyde was patented by Leo Baekeland in 1907 as Bakelite. Bakelite could be produced in any colour (that allowed for its natural yellow tint), was very hard and rigid and found extensive use in early radio cabinets, automobile components and other high-tech applications.

In 1922, a great impetus was given to the synthesis of further plastic compounds by the work of Hermann Staudinger. In the process of synthesizing rubber, Staudinger identified and described the essential linear structure of polymers. A more sophisticated understanding of polymer structure gave rise after 1930 to a rapidly growing list of 'plastics' arrived at by deliberate chemical engineering rather than trial-and-error or chance discovery. After the Second World War, furniture made entirely of plastics became increasingly popular (e.g. see Katz, 1984). Clear acrylic sheet (Perspex®, Plexiglas®) made from poly (methyl methacry-late) was extensively employed but was soon found to be brittle and easily scratched. A summary of the main synthetic polymer materials is given in Table 4.1.

Perhaps the category of plastics most important to furniture history and manufacture are the composites or laminates. Composites are com binations of dissimilar materials that show properties different from and often superior to the individual ingredients (Gordon, 1976). This is a very old concept and in many ways describes all of the materials discussed so far in that they are almost never pure substances. An early example of structural composites showing high strength combined with low weight were the papier mâché panels used to form furniture (see section 5.4). Gypsum plaster and coarse weave jute or hemp layered into moulds or built up on wooden armatures were extensively employed for the realization of complex architectural interiors and even entire exhibition buildings. Such 'fibrous plaster' or 'staff filled the functions of what is today commonly called 'fibre-glass'.

Fibre-glass is a rather misleading term for resin and glass-fibre composites more properly called glass reinforced plastics or GRPs. Such composites use glass fibre in the form of cord, cloth or randomly oriented felt-like matt which is impregnated with thermosetting resins such as phenolics, epoxies and polyesters. They are very strong, relatively light in weight and have been used to manufacture furniture since the 1940s.

Paper-based composite laminates have also found extensive use in furniture as surface coverings. Laminates such as Formica®, based on paper and melamine resin, have been used to copy every variety of wood and stone.

Synthetic polymers had a revolutionary effect on the textile industries and synthetic fibres were quickly brought into use in upholstery. Uses of polymers in upholstery are described in Chapter 3. Polymer foams can be thought of as composites of resins and gas bubbles. They can be made either soft and flexible or hard and rigid depending on resin composition and bubble size. An extremely readable review of the technology of fibres, plastics and rubbers is given by Kaufman (1968). Kovaly (1970), Buttrey (1976) and Seymour and Mark (1990) provide further information on the utilization of plastics in furniture and as industrial finishes.

Polymers that become soft or fluid when heated have had various applications in moulding and casting. Historically they have been used to produce rigid moulds. Decorative plasterers have used mixtures of waxes, resins and fillers to produce moulds for repeat pat-

Table 4.1 Review of some important synthetic polymers

Polymer group, names Date Description and use Simple identification and trademarks introduced and country

Acrylic PMMA:

Poly(methyl methacrylate) 1934 UK (PMMA) Oroglas (Röhm & Haas - USA), Perspex (ICI - GB), Plexiglas (Röhm GmbH-D)

Acrylonitrile-butadiene rubber (NBR)

Nitrile rubber

Acrylonitrile-butadiene- 1952 USA styrene (ABS)

Butyl rubber (BUTYL)

Cellulose acetate CN: 1870

butyrate (CAB) USA

Uvex (Eastman Chemical CA: 1905

Products Co., USA) Germany

Cellulosic esters cellulose nitrate (CN) cellulose acetate (CA)

Epoxy (EP) 1947 USA

Amorphous, thermoplastic, carbon chain addition polymer. PMMA is usually produced by 'casting' from monomer to form clear or opaque coloured sheets or rods with good outdoor durability. Brittle unless toughened but cuts cleanly. Used as 'unbreakable' substitute for glass, for mouldings and light fittings. Various acrylics used as adhesives and as solution/dispersion type coatings. Widely used in conservation as consolidants and coatings

Rubber formed by emulsion polymerization of acrylonitrile and butadiene. Used for oil resistant properties

Amorphous thermoplastic mixture of styrene with acrylonitrile and nitrile rubber. Chemical resistant and tough with generally good impact resistance though toughness somewhat variable. Normally in opaque colours. Can be formed, moulded, extruded and calendared. Cuts cleanly

Formed by ionic polymerization of isobutylene with a small proportion of isoprene. Mainly used in inner tubes for tyres. Also used as adhesive

Thermoplastics formed by chemical modification of the natural polymer, cellulose, present in cotton and wood. Used with a plasticizer. CA: Moderately tough (notch brittle) with good impact resistance but poor outdoor durability. Films used in photography and for packaging are highly transparent and protective but tear easily. Found in textile fibres as cellulose acetate and triacetate. Some use as adhesive. CN used in solution type coatings

Thermoplastic. Similar to cellulose acetate but with improved outdoor durability. May develop smell of rancid butter on ageing. Cuts cleanly. Also used in simple solution coatings

Amorphous, thermosetting network polymer. Basic functional unit formed by reaction of epichlorhydrin with diphenylol propane can be polymerized in situ with a variety of crosslinking agents, the main ones being amines. Used for surface coatings and adhesives and some mouldings

Burns noisily with blue-based yellow flame, little smoke, acrid fruity smell. Continues to burn after removal from flame. Not affected by cyclohexanone or by aromatic solvents

Burns with blue-based yellow flame and black smoke with faint odour of marigolds. Continues to burn after removal from flame. Softened by cyclohexane but not by aromatic hydrocarbon solvents

CA: Burns rapidly with blue-based orange flame and smell of burning paper. Sputters and drips while burning. Continues to burn after removal from flame. Softened by cyclohexane but not aromatic hydrocarbons

Burns with yellow flame and smell of rancid butter. Continues to burn after removal from flame. Softened by cyclohexane but not by aromatic hydrocarbons

Melamine formaldehyde 1938 (MF) Germany

Phenol formaldehyde 1909 USA (PF)

e.g. Bakelite, phenolic

Poly acetal (POM)

Delrin (Du Pont - USA) Hostaform (Hoechst AG-D)

Hard and tough, amorphous, thermosetting network polymer formed by polycondensation of melamine and formaldehyde. The resin is always mixed with one or more fillers to make it suitable for particular applications. Used to make decorative laminates and mouldings, mainly in opaque colours, that are resistant to scratching and chemicals. Chips and powders when cut. Also used as adhesive and, with or without alkyd, in reactive type coatings

Amorphous, network, thermoset formed by polycondensation of phenol or cresol and formaldehyde. The resin is almost always mixed with fillers to suit particular applications. Good heating and insulating properties. Used in mouldings, paints, adhesives and laminates. Chips and powders when cut

Crystalline thermoplastic. Moderately tough (notch brittle) and rigid with good solvent and heat resistance. Good electrical properties. Cuts cleanly

Hard to ignite. Burns with pale yellow flame with bluish edge and fishy smell. Not softened by cyclohexane or aromatic hydrocarbons

Burns with yellow flame and 'phenolic' smell with evidence of formaldehyde. Self-extinguishing on removal from flame. Not softened by cyclohexane or aromatic hydrocarbons

Difficult to ignite but burns with very pale blue lame and acrid smoke with odour of formaldehyde. Continues to burn after removal from flame. Bubbles and becomes clear when molten. Not affected by cyclohexane or aromatic solvents

Polyamides (PA)

All types of Nylon. Commercially available varieties include 6, 66, 610, 7, 11, 12 and 66/610.

Polybutadiene (BR)

1939 USA Crystalline, heterochain thermoplastic formed by polycondensation of dibasic acids and diamino compounds or condensation of amino acids with themselves. Light, tough when wet, moderately tough (notch brittle) when dry chemically resistant and durable. Cuts easily. Used in textiles and in mouldings

Hydrocarbon rubber, formed by ionic polymerization of butadiene. Used in tyre treads

Burns with blue flame with yellow tip producing drips and strings of molten polymer and smell of burning vegetation. Continues to burn after removal from flame. Not affected by cyclohexane or aromatic solvents

Polycarbonate (PC) 1959 W

Lexan (General Electric Co Germany/ USA) USA

Makrolon (Bayer AG-D) Tuflak (Rohm & Haas — USA

Hard, rigid thermoplastic with good impact resistance and high softening point formed by action of phosgene on diphenyol propane. Available in transparent and opaque forms. Cuts cleanly

Difficult to ignite. Burns with spluttery orange flame, black smoke and phenolic smell. Bubbles and chars when burning and continues to burn after removal from flame. Not affected by cyclohexane or aromatic solvents

Polychloroprene (CR)

Polyester UP: 1946

linear polyesters include USA Hostaphan (Hoechst AG-D), Melinex (ICI - GB), Mylar (Du Pont - USA)

Rubber formed by addition polymerization of chloroprene. Used where non-flammability and good weathering properties required. Also used as adhesive

Polyesters, heterochain condensation polymers formed from polyfunctional acids and polyfunctional alcohols, including linear polyesters, unsaturated polyesters (UP) and alkyds. Linear

Linear polyesters burn rapidly with blue-based yellow flame and very faint odour, continuing to burn after removal from flame. Not

Polymer group, names Date Description and use Simple identification and trademarks introduced and country

Polyolefins

Low density polyethylene (LDPE)

High density polyethylene (HDPE)

Polypropylene (PP)

Polystyrene (PS)

polyesters are thermoplastic, alkyds and UPs are thermosetting. Linear polyesters have excellent clarity, dimensional stability and chemical resistance. They are used to make textile fibres such as Terylene, Dacron and Tergal and high quality films such as Melinex and Mylar. Films make metallic sound when shaken and are difficult to tear. Unsaturated polyesters (UP) are used in glass-reinforced plastics mouldings. They are usually crosslinked with styrene but other unsaturated compounds may be used. Alkyd resins are used in (reactive type) paints

LDPE: Thermoplastic, crystalline, stereoregular

1939 UK hydrocarbon chain homopolymers formed HDPE: by addition. LDPE is formed by high

1955 W. pressure free radical polymerization whilst Germany low pressure ionic polymerization of PP: 1957 monomer is used for the others. Highly Italy chemical resistant, light and strong with waxy feel. Float on water. Cut cleanly. HDPE and most PPs are moderately tough (notch brittle) but some PPs are very tough with high impact resistance. Polypropylene has greater rigidity and surface hardness than PE. Used for all kinds of mouldings and films, also as adhesives and fibres. Film stretches before tearing

1930 Thermoplastic hydrocarbon addition

Germany polymer formed mainly by bulk polymerization of styrene. Most commercial PS is hard, brittle amorphous atactic homopolymer used for mouldings, films and foams. PS alone cuts cleanly is readily moulded or formed but shatters easily (makes metallic sound when shaken) has poor resistance to solvents and poor outdoor durability. It is copolymerized with a variety of other materials to make high impact plastics, dispersion type coatings and rubbers (see ABS, SBR, SAN)

affected by cyclohexane or aromatic solvents

Burns with blue-based yellow flame and smell of candle wax. Becomes clear when molten and continues to burn on removal from flame. Not affected by cyclohexane or aromatic solvents

Burns with spluttery orange flame and dense black sooty smoke with faint odour of marigolds. Continues to burn after removal of flame. Softened by cyclohexane and aromatic solvents

Polytetrafluoroethylene 1943 USA (PTFE)

Teflon, Fluon

Thermoplastic, crystalline, carbon chain, addition polymer formed by emulsion polymerization of tetrafluoroethylene. Used where heat resistance, chemical resistance or low friction are required. Copolymers of TFE with other fluoro compounds are used as heat resistant rubbers

Polyurethanes (PUR)

Poly(vinyl acetate) (PVAC)

1943 Polyurethanes, amorphous heterochain

Germany polymers, are formed from reaction of low molecular weight polyesters or polyethers with isocyanates and are classified accordingly into two main groups of polyethers and polyesters. Polyurethanes may be thermoplastics, rubbers or thermosets. They have been used as flexible foams in upholstery, rigid foams for insulation, solid elastomers and surface coatings

Thermoplastic carbon chain addition polymers formed principally by emulsion polymerization of vinyl acetate. Principally used as emulsion paints and adhesives. Poly(vinyl alcohol), poly(vinyl formal), and poly(vinyl butyral) are chemical derivatives of PVAC and also find applications in conservation treatments as adhesives and consolidants and coating media

Burns with blue-based yellow flame and acrid smell. Continues to burn on removal from flame. Not affected by cyclohexane or aromatic solvents

Poly(vinyl chloride) (PVC)

e.g. 'Cobex' (Storey Bros. & Co - GB), 'Darvic' (ICI - GB), 'Genotherm' (Hoechst AG-D)

Polyvinyl fluoride (PVF)

1933 Thermoplastic carbon chain addition

Germany/ polymer formed mainly by suspension USA and emulsion polymerization of vinyl chloride. PVC is used in plasticized form and as copolymer with vinyl acetate and with vinylidene chloride. It is available in clear and opaque colours in limp, flexible, rigid and self adhesive forms used for mouldings of all sorts, textiles (e.g. leather cloth) and sheetings, and in dispersion coatings. Films are difficult to tear. Cuts cleanly

Thermoplastic. Carbon chain addition polymer. Unaffected by wide variety of chemicals, solvents and staining agents and highly resistant to weathering. Normally used in thin sheets in both clear and pigmented forms. Cuts easily. Films difficult to tear

Difficult to ignite, burns with green tinged orange flame, black sooty smoke and acrid acidic smell. Self-extinguishing on removal from flame. Softened by cyclohexane but not by aromatic hydrocarbons

Burns noisily with yellow flame and black smoke with acrid, acidic smell. Shrinks on burning. Continues to burn after removal of flame. Not affected by cyclohexane or aromatic solvents

Silicone (SI)

Styrene acrylonitrile (SAN)

1943 USA Heterochain polymers formed by polycondensation of polyfunctional silanols can exist as thermoplastics, rubbers and thermosets depending on starting materials and polymerizing conditions. They are used for laminates, rubbers, mould making materials, adhesives and other applications where good performance is required over a wide range of conditions

Thermoplastic similar to polystyrene but harder with better chemical resistance. Suitable for outdoor use. Cuts cleanly

Burns with orange flame, black, sooty smoke, and odour of marigolds. Continues to burn after removal from flame. Softened by cyclohexane but not by aromatic hydrocarbon solvents

Styrene butadiene (SBR)

Rubber

Polymer group, names and trademarks

Date introduced and country

Description and use

Simple identification

Urea formaldehyde (UF) 1926 UK

Very hard, amorphous, chemically resistant thermoset formed by polycondensation of urea and formaldehyde. The resin is always mixed with one or more fillers to make it suitable for particular applications. Used for adhesives and mouldings, and, with or without alkyds in reaction type coatings., Normally available in dark, opaque colours. Chips and powders when cut

Difficult to ignite, burns with pale yellow flame with bluish edges and fishy smell. Self-extinguishing on removal from flame. Not affected by cyclohexane or aromatic hydrocarbon solvents

Source: Compiled from information in Hall, 1981, Kaufmann, 1968 and others cited in Chapter 4

terns. Many nineteenth-century moulds for composition ornaments were made of pitch mixtures enclosed in a strong frame and squeezed, while hot and pliable, over an oiled carving. Gutta percha was also used as a mould material. Modern synthetic resins can also be used in this way (see section 4.7.6). Dental impression compounds have been used in picture frame restoration to make small squeeze moulds for the replacement of lost composition ornament. Commercial mould-makers and sculptors have used poly(vinyl chloride) 'hot melts' extensively for large flexible moulds.

Wood Working for Amateur Craftsman

Wood Working for Amateur Craftsman

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