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.

5. INDUSTRIAL PLASTICS:

RIGID AND FLEXIBLE FOAMS

Rigid polyurethane foams in sandwich forms have wide applications as building components. They are also the best insulants known today and so have wide application in refrigeration and in buildings, where they are applied in fitted slab form or are foamed into cavities at the building site. They can also be applied by spraying about six millimetres thickness with each pass of the spray gun. The ability to spray a foaming mixture through a single nozzle is a great advantage in application.

A very important use of rigid foam is for furniture parts to reproduce wood structures; these can be injection molded. Polyurethane foam can be screwed and nailed with a retention about equal to white pine lumber.

A major advance in the manufacture of sandwich structures is a new method of injection molding, in which a large machine is used to produce moldings up to 1.2 metres square. Moldings of great strength and any desired surface are obtained.

Flexible foams

Flexible foams, usually polyurethane, are made in slab form up to 2.4 metres in width and as much as 1.5 metres high; these are then cut to required shapes or sizes or are molded. The molded foams may be hot molded.

This involves filling heated aluminum castings and gives a product having high resistance to compression, as for automobile seats; or they may be cold molded, a process used particularly for semi-flexible foams with high load-bearing properties. Used almost exclusively by the automobile industry for crash pads, armrests, and dashboard covers, the process involves machine mixing the ingredients and pouring them into aluminum molds lined with vinyl or acrylo-nitrile-butadiene-styrene skins, which become the cover material for the part.

Polystyrene foams are made in a wide range of densities, from expandable beads, either by extrusion through slot-shaped openings to 40 times the original volume to form boards directly or by foaming in steam chests to form large billets. Using small beads in stainless steel molds, cups can be molded with thin sections.

Thin sheet for packaging can also be made by the tube extrusion technique. Though packaging is a major use for forms made in closed molds, the largest use is for building panels; they can be plastered directly.

Acrylonitrile-butadiene-styrene can be expanded from pellets and is particularly suitable for wood-grain effects and for the production of heavy sections.

Expanded vinyls can be made from plastisols for flooring or textile linings by calendering with a blowing agent and laminating to a fabric base, and by injection molding for insulation and such articles as shoe soles. An improved material is now obtained from cross-linked polyvinyl chloride and competes with polyester in glass reinforced plastic.

6. BASIC PRINCIPLES OF WELDING

A weld can be defined as a coalescence of metals produced by heating to a suitable temperature with or without the application of pressure, and with or without the use of a filler material.

In fusion welding a heat source generates sufficient heat to create and maintain a molten pool of metal of the required size. The heat may be supplied by electricity or by a gas flame. Electric resistance welding can be considered fusion welding because some molten metal is formed.

Solid-phase processes produce welds without melting the base material and without the addition of a filler metal. Pressure is always employed, and generally some heat is provided. Frictional heat is developed in ultrasonic and friction joining, and furnace heating is usually employed in diffusion bonding.

The electric arc used in welding is a high-current, low-voltage discharge generally in the range 10-2,000 amperes at 10-50 volts. An arc column is complex but, broadly speaking, consists of a cathode that emits electrons, a gas plasma for current conduction, and an anode region that becomes comparatively hotter than the cathode due to electron bombardment. Therefore, the electrode, if consumable, is made positive and, if non-consumable, is made negative. A direct current (dc) arc is usually used, but alternating current (ac) arcs can be employed.

Total energy input in all welding processes exceeds that which is required to produce a joint, because not all the heat generated can be effectively utilized. Efficiencies vary from 60 to 90 percent, depending on the process; some special processes deviate widely from this figure. Heat is lost by conduction through the base metal and by radiation to the surroundings.

Most metals, when heated, react with the atmosphere or other nearby metals. These reactions can be extremely detrimental to the properties of a welded joint. Most metals, for example, rapidly oxidise when molten. A layer of oxide can prevent proper bonding of the metal. Molten-metal droplets coated with oxide become entrapped in the weld and make the joint brittle. Some valuable materials added for specific properties react so quickly on exposure to the air that the metal deposited does not have the same composition as it had initially. These problems have led to the use of fluxes and inert atmospheres.

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