As plastics are classified into two groups, according to their behaviour with respect to heating, the resins or binders are also broadly divided into the following two groups: (1) Thermo-Plastic Resins (2) Thermo-Setting Resins.
(1) Thermo-Plastic Resins:
Following are the commonly used thermo-plastic resins:
(i) Alkyd
(ii) Cellulose
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(iii) Cumarone-indene
(iv) Methyl methacrylate
(v) Styrene
(vi) Vinyl.
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(i) Alkyd:
These resins are made from glycerin and phthalic anhydride. Their chemical composition is adjusted by drying oils. They cool slowly and possess electric properties. They are used for preparing thin films of plastics.
(ii) Cellulose:
These resins are derived from various cellulose compounds such as cellulose acetate, cellulose nitrate, cellulose esters, etc. The plastics made from cellulose are as clear as glass. They are tough and strong. They possess excellent electrical properties. It is also possible to obtain all types of colour effects with such plastics.
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(iii) Cumarone-Indene:
These resins become soft over a narrow range of temperature. Their electrical properties do not change with humidity. They are brittle and are used for floor tiles, rubber manufacture, etc.
(iv) Methyl Methacrylate:
This resin is also known as the acrylic. It is derived from coal petroleum and water by a complicated process. As a matter of fact, the arrival of acrylic has opened new paths in the fields of building and architecture. As compared to the traditional building materials, the acrylic has a unique combination of properties which have facilitated its introduction in many applications typical of glass.
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The advantages of acrylic can be enumerated as follows:
(a) Breakage Resistance:
The acrylic has 10 to 17 times greater breakage resistance than glass of equivalent thickness. When broken by an impact, it merely cracks and does not shatter away like glass. Its application proves useful for the window breakage problems in schools, institutions, factories, etc. Due to its lower brittleness, the acrylic sheets can be used in thickness less than that of glass.
(b) Chemical Resistance:
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The acrylic is generally unaffected by most household detergents, cleaning agents and solutions of inorganic acids, alkalies and aliphatic hydrocarbons. However the acrylic should not come in contact with chlorinated and aromatic hydrocarbons, esters and ketones.
(c) Clarity:
The colourless acrylic sheets are as clear as the crystal. Its light transmission rate of 93% makes the acrylic the clearest plastic material and its sheets used for windows and skylights allow optimum entry of natural light into the interiors.
(d) Easy Formability:
The acrylic being a thermo-plastic has the ease of formability. It is possible to form beautiful curved surfaces of generous radius at room temperature. The flawless panels of various shapes like flat, square, spherical and circular can be thermoformed with a high degree of safety.
(e) Finishing:
The acrylic is available in transparent, translucent and opaque colours in variety of patterns. It can be readily turned, milled, drilled with tools normally used for soft metals and requires no special precautions. The parts or pieces of acrylic can be joined by cementing, hot air or gas welding, ultrasonic assembly, spin welding, snap fitting or other simple methods.
(f) Saving of Energy:
The acrylic is a better insulator than glass. In fact, the ordinary glass conducts heat more than four times faster than the acrylic. This makes the acrylic an ideal energy saving material.
The use of properly designed and tinted acrylic sheets in the air-conditioned rooms helps to save electricity. It can also be used to capture energy. For instance, the heat losses through windows due to conduction and air filtration can often be reduced by over 50%.
(g) Thermal Expansion:
The acrylic like most thermoplastics has a large coefficient of thermal expansion. It is therefore necessary to make adequate provisions for expansion and contraction of the acrylic especially when used in very large parts.
(h) Weather Resistance:
The weathering agencies like sunlight, heat, low temperature and smog have no ill-effect on the colour, light transmittance or physical properties of the acrylic fittings. The outstanding weather resistance has made the acrylic the finest construction material for the fittings which are exposed to the open. The acrylic has also the ability to cut out harmful ultra-violet radiation from the light passing through it.
(i) Weight:
It is light in weight and in fact, it weighs only about 46% as much as ordinary glass. It makes the acrylic easy to handle and to mount on more simple structures.
The acrylic sheets are used for skylights, north-lights, door panels, window glazing, sun screens, furniture articles, bath tubs, wash basins, partition decorative walls, lighting fixtures, security mirrors, telephone stands, bank cash counters, portable aquariums, enclosures for display of priceless works of art and swimming pools, lighting fixtures, etc.
(v) Styrene:
This resin is produced from ethylene which is made from petroleum. This resin is light in weight. It transmits ultra-violet waves of light. It possesses very high electric resistance. It is not easily attacked by chemicals. Its water absorption is low. The plastics prepared from this resin are widely used as insulators at radio frequencies in wireless and television industry.
(vi) Vinyl:
There are several groups of this type of resin. They are produced by passing acetylene gas through acetic acid or dry hydrogen chloride. The process is carried out under controlled light, heat and pressure. The plastics prepared from this resin are odourless, non-toxic, transparent and colourless.
The vinyl chloride possesses high resilience and it is used for wire and cable coatings. The polyethylene is a vinyl resin which is tough and flexible and is used for cable covering. The polyvinyl butyral is another variety of vinyl resin which is used in safety glass.
(2) Thermo Setting Resins:
Following are the commonly used thermo-setting resins:
(i) Casein
(ii) Melamine-formaldehyde
(iii) Phenol formaldehyde
(iv) Phenol furfuraldehyde
(v) Urea formaldehyde
(i) Casein:
The casein is a phosphor protein and it is derived by the precipitation of milk with acids. It absorbs moisture and is not very strong. But it is easily workable and it possesses bright attractive appearance. The plastics prepared from this resin are used for buckles, buttons, etc.
(ii) Melamine-Formaldehyde:
The melamine is obtained from calcium carbide. The formaldehyde is prepared synthetically from methane which is the simplest hydrocarbon. The melamine, when reacted with formaldehyde, forms this resin. It is highly resistant to the water and possesses excellent resistance to the electrical arcs.
The plastics made from this resin are used for electrical insulators, glass- reinforced plastics, preparing papers having high strength in wet condition, decorative laminates in light colours for wall and ceiling linings, etc.
(iii) Phenol Formaldehyde:
The phenol is carbolic acid. It is extracted from coal tar or prepared from benzene. When reacted with formaldehyde, it forms this resin. It is highly resistant to heat. It possesses excellent mechanical and electrical properties. It is also not easily attacked by water, dilute mineral acids, organic acids, oil, mild alkalies, common solvents, etc.
The plastics prepared from this resin are used for paints, varnishes, preparation of laminated products, electrical fittings, w.c. seats, water-resistant adhesives for plywood, etc.
(iv) Phenol Furfuraldehyde:
The furfuraldehyde vapours are formed by digesting husks or rice, shells of oat or ground-nut, etc. with sulphuric acid in the presence of a catalyst. These vapours, when reacted with phenol, form this resin. It is dark in colour and resists very high temperatures.
(v) Urea Formaldehyde:
The urea is prepared from calcium cyanamide or by heating under pressure, a mixture of liquid carbon dioxide and liquid ammonia. The urea, when reacted with formaldehyde, forms this resin. It possesses excellent electrical properties.
It is not easily attacked by dilute acids and alkalies, oil, chemicals, water, etc. The plastics made from this resin are widely used for making adhesives for wood and wood products, lighting fixtures such as lamps, reflectors, etc.