While a great deal of furniture is still glued with animal adhesives, synthetic resin glues are extensively used in practically every manufacturing industry, and offer unparalleled advantages in certain classes of work where ease of working, rapid setting, great strength, permanence and water-resistant properties are called for. A considerable advantage for small craftsmen is their tolerance of low temperatures, for they can be satisfactorily worked in relatively cold conditions (50° F [10° C]), whereas animal glues require heated conditions for satisfactory results. Conversely, all synthetic resin glues are thermosetting, and respond to heat which greatly accelerates the setting-rate.
The type of synthetic resin glue most commonly used in furniture-making is urea formaldehyde (UF), which has adequate strength, durability and water-resistance for all types of indoor-work, will set at normal room temperatures, and is relatively inexpensive. Phenol formaldehyde (PF) is used where great strength, durability and water resistance is necessary, but heat is required for the set and the glue-line is dark coloured, and therefore it is confined to heavy constructional work and plywood manufacture including 'exterior' quality ply. Resorcinol formaldehyde (RF) has outstanding durability under the severest conditions and is therefore used mainly for exterior work. Here again a dark glue-line is formed which is sometimes used for its decorative possibilities (archery bow laminations). Melamine formaldehyde (MF) is more expensive, and is usually employed as a fortifier of other glues and for the top coating of decorative laminated plastics (formica, etc.). Polyvinyl acetate emulsion (PVA) is ready mixed, requires no accelerator and is extensively used for gluing or strengthening a variety of substances, but it lacks the great strength of straight synthetics, and tends to creep from thick joints under humid conditions. (Many so-called PVA adhesives are no longer composed of polyvinyl acetate and a better term is, therefore, emulsion glue. They are thermoplastic, with high-tensile strength but little inherent strength in thick films and with little if any penetration, therefore they should not be used in tropical climates, for gap-filling work or for dowelling. They are, however, useful for close-contact work with moderate pressure, and as they do not become brittle with age they can be used for laying plastic laminates. Bearing in mind their limitations they are in all probability the most popular type or glue for the small user). Epoxy resin glues require no volatile solvent, and are, therefore, capable of bonding non-porous materials, metals, glass, china, etc. A comparison of the various types is listed on p. 79.
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