Unitl about myself

 

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79 
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 formal-
dehyde 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.