4. Resins

Resins that cannot be softened by heating include the phenolics, furan resins, aminoplastics, alkyds, allyls, epoxy resins, polyurethanes, some polyesters, and silicones.

Phenolics or phenol-aldehydes

The important commercial phenolic resin Bakelite is based on phenol and formaldehyde. The two processes in general use are the one-step process producing resol res­ins (the first stage in the formation of a phenolic resin) that are either liquid or brittle, soluble, fusible solids, from more than one molecule of formaldehyde per phe­nol molecule; and the two-step process, using an excess of phenol to produce novolacs, resins that have no reac­tive methylol groups and must be mixed with an alde­hyde to undergo further reaction.

Resol resins thermoset on heating and are used for adhesives. Novolacs require a further source of formal­dehyde in the form of hexamethylenetetramine to pro­duce molding powders. Both resins are run out from the reaction vessel, after removal of water by distillation, and ground up, then compounded on heated rolls with fillers that vary from wood flour to mica; for strength and heat resistance fibrous asbestos is used as a filler (hexamethylenetetramine is also added at this stage in the case of the two-step resin). Final grinding produces the molding powders, which on further heat treatment will yield the typical thermoset resin.

Phenolic moldings are resistant to heat, chemicals, and moisture and are preferred for wet-dry applications as in washing machines. Their stability to heat and low heat conductivity suit them for use in appliance parts, and their electrical insulation qualities qualify them for electric fittings such as switches, plugs, and distributor caps; resistance to hydraulic fluids has led to their use in automotive parts. All these applications have been made more economical by the development of injection molding and extrusion methods. Complex phenols are used in manufacture of brake linings.

Furan resins

Furfural is a five-membered ring compound (i.e., the basic molecule has a ring shape and contains five atoms) of four carbon atoms and one oxygen atom, carrying the aldehyde group, — CHO; it reacts like formaldehyde with phenols in the presence of an acid catalyst to give a rigid polymer with high chemical resistance, used for coatings in industry. It can be prepared in semiliquid form with a low viscosity and remarkable penetrating power when applied to porous forms such as foundry sand cores or graphite blocks, being in this respect superior to other liquid resins.

Aminoplastics

Urea resins are made by the condensation in aqueous solution of formaldehyde and urea in the presence of ammonia as an alkaline catalyst, giving a colourless so­lution to which cellulose filler is added to yield a molding powder upon drying, which when heated in a mold gives a water-white (transparent) molding unless previously coloured by pigment.

The filler confers considerable strength, so that thin sections such as in cups and tumblers can be molded. Very large quantities of urea-formaldehyde resin are used in kitchen and bathroom hardware details, and electric ap­pliance housings and fittings.

Melamine behaves in the same way as urea, but the prod­uct is more moisture resistant, harder and stronger, lead­ing to wide use for plates and food containers. Melamine moldings are glossy and harder than any other plastic and retain a dust-free surface. Solutions of the thermoplastic forms of urea-formaldehyde resins are widely used as bonding agents for plywood and wood-fibre products.

Alkyds

Alkyds are polyesters, generally of phthalic acid (with two acid groups) and glycerol, a triol — i. e., an alcohol with three hydroxyl groups. The solid resins are molded at high speed under low pressure, cured quickly, and are used where insulating properties, strength, and dimen­sional stability over a wide range of voltage, frequency, temperature, and humidity are required, as in vacuum-tube bases and automotive ignition parts and with glass-fibre reinforcement for switch gear and housings for portable tools.

Polyesters of unsaturated alcohols

The resins known as DAP and DAIP, are crossliked allyl esters of phthalic and isophthalic acid, respectively. They are notable for maintaining rigidity and excellent electrical properties at temperatures up to 230 С, prорerties also manifested by allylic resin-impregnated glass cloth, used in aircraft and missile parts. Other advan­tages are good storage life and absence of gas evolution during polymerization. The resin allyl diglycol carbon­ate, optically clear and colourless, is used for making cast objects; fully cured castings are more heat and abrasion resistant than other cast resins.

Epoxy resins

Epoxy resins have outstanding mechanical and elec­trical properties, dimensional stability, resistance to heat and chemicals, and adhesion to other materials. They are used for casting, encapsulation, protective coat­ings, and adhesives, and for reinforced moldings and laminates of the highest quality. Popular adhesives (epoxy glues) contain the resin components and the cur­ing agent, usually an amine or an anhydride, in separate packages. The two are mixed just before use.

Polyurethanes

Formed by the reaction between diisocyanates and polyols (multihydroxy compounds), polyurethanes are among the most versatile of plastics, ranging from rigid to elastic forms. Their major use is for foams, with prop­erties varying from good flexibility to high rigidity. Thermoplastic polyurethanes that can be extruded as sheet and film of extreme toughness can also be made.

Polyesters of unsaturated acids

Certain esters can be polymerized to resin and are used on a very large scale in glass-fibre-reinforced plastics.

Unsaturated acid (usually maleic acid in the form of its anhydride) is first polymerized to a relatively short polymer chain by condensation with a dihydric alcohol such as propylene glycol, the chain length being deter­mined by the relative quantities of the two ingredients The resulting condensation polymer is then diluted with a monomer such as styrene and an initiator for addition polymerization added. This mixture is quite stable at room temperature over a long period. Frequently, a silicone compound is added to promote adhesion to glass fi­bres, and wax to protect the surface from oxygen inhibi­tion of polymerization. Glass-fibre materials are impreg­nated with the syrup and polymerization is brought about by raising the temperature. Alternatively, the polymeri­zation can be carried out at room temperature by addition of a polymerization accelerator to the syrup immediately before impregnation. After an induction period, which can be controlled, polymerization takes place, with rapid in­crease in temperature, to give a glass-fibre-reinforced cross-linked polymer, which is effectively a thermoset type of plastic and very resistant to heat. The properties of the resin are frequently varied by replacing part of the unsaturated maleic anhydride by anhydrides of satu­rated acids.

Silicones

Silicon, unlike carbon, does not form double bonds or long silicon chains. It does, however, form long chains with oxygen such as in siloxanes with hydrocarbon groups attached to the silicon; these result in a wide range of oils, greases, and rubbers.

Produced through a series of reactions involving re­placement of certain atoms in the chain, silicon resins, or silicones, can be used for high- and low-pressure lami­nation, with glass-fibre reinforcement and with mineral or short glass-fibre fillers, or for molding powders. The outstanding characteristic of these products is high di­electric strength (that is, they are good insulators at high voltages) with low dissipation over a wide temperature and humidity range. Silicones are not distorted by heat up to 400 С. They are also physiologically inert and there­fore valuable for prostheses (artificial body parts).