PRODUCTION OF PLASTIC MATERIALS

1.0 Introduction

Before the advent of synthetic plastics, most forms of construction and production were dependent upon the use of natural materials such as metal, wood, rubber, tar and various minerals. These materials are still very much in evidence today, but in many instances they have been replaced by the new wonder materials called PLASTICS. Through the magic of the chemist, we now have plastics that can meet almost every engineering requirement.

The first man-made plastic was nitrocellulose. It was developed by chemist in both America and Europe who had been trying to find new uses for cellulose, the basic substances for wood and cotton. They finally succeeded in synthesizing the new chemical by treating the cotton fibres with nitric acid. In 1970, a young painter named John Wesley Hyatt and his brother Isaiah added camphor to nitrocellulose to form the first commercially successful plastic, which they called celluloid. The new materials was used to make combs, bashes, photographic film, and early automobile windows and safety glass. But celluloid has two inherent weaknesses that have severely curtailed its use in modern times.

• It is highly flammable.
• in the form of clear film, it yellows upon aging.

The second plastic to appear on the market was called Bakelite. It was developed by Dr Leo Baekland in 1909. Dr Baekeland had been searching for a synthetic coating material to replace shellacs. He began experimenting with two chemicals, phenol and formaldehyde, the two was combined with catalyst to form the new material. Bakelite was also used for moulding, casting, adhesive bonching and electrical insulation.

What Are Plastics?

Most plastics are man-made materials with the ability to flow, take shape and solidify. They are made from chemicals extracted from substances such as petroleum, coal, air, water and agricultural by-products. The production sequence starts with the manufacturer of raw materials who combines the various chemicals to form meltable solids and syrupy liquids called resins.

Unlike metals, plastics are light in weight, pleasant to touch and easy to form .complicated shapes are easily reproduced by molding , casting .because they are do not allow electricity to pass through them , they are used extensively in dielectric application . they are also poor conducts of temperature which accounts for their use as pot handles ,electric iron handles refrigerator housing and thermos bottles.

Plastics are very versatile materials .some have great optical clarity ,and most can be colored in a wide range of colures shapes . one plastics ,acrylic is even clears than glass e.g. lens .because the dyes for colored plastics are mixed with the raw materials , the completed p need never be painted . plastics surfaces are also free from the atmospheric corrosion that is prevalent with some metals .the light weight of plastics is still another advantage over metals, like polypropylens, are lighter than water and therefore float on water.

The ability of plastics to be foamed has led to a completely new concept of lightweight construction. Rigid urethane foams are already fericling wide use in furniture and other structural applications. The strength of a rigid urethane foam panel weighing 816/ cup is something to behold, especially when one considers the fact that water weighs 62.4 u per cuft. Another extraordinary development is light weight structure is the combining of plastics binders and adhesive with honey comb and farcing materials to form panels with out standing weight – to- strength ratios. Honey comb application have in waded such basic fields as transportation, architecture, illumination air craft and gaicted missiles, electronics , materials handling home appliances and instrumentation . Indeed, both the engineer and the architect are in constant search for materials that will improve , economy , weight – to – strength ratio and ease of fabrication with no sacrifice in performance.

Many plastics can be coated with a thin metal film to change their appearance or meet a functional requirement bright, glossy , chromium – coated plastics are especially popular. They are difficult to distinguish from solid metal unless lifted by hand, at which time the much lighter plastics is very discernable . pumping fixtures and toys are among the more popular applications of metallized plastics.

An important breakthrough in surgery has been the use of plastics as implantations in the human body strong, tough, Ron allergic plastics are finding increased use as replacements for effective blood vessels and other damaged parts of the body . The softer silicone rubbers are also used for implantation , but mostly for cosmetics purpose.

Plastics products are now being produced more quickly and in larger volumes , thanks to the tremulous stickles made in injection , compression and transfer molding .improved mechanization of automatic and semiautomatic machines plus improved molding materials have brought about shorter cure cycles and a reduction in handling operations.

Many plastics can be used in specialized chemical resistant application . with few exception (such as the fluorocarbons) , most plastics materials are attacked by one or more chemicals .By careful materials selection, light weight, unbreakable plastics can be used to replace glass and stainless steel as chemical containers . In many instances, plastics have replaced stainless steel as industrial clouting used to carry away chemical vapor.

Plastics have replaced paper in many packaging applications plastics-foam packing materials provide more protection for boxed products than crushed paper does. Many product, particularly delicate electronics equipment , are packed in foamed plastics inserts that got the shape of the item exactly. Plastics wraps have many uses. They preserve foods longer than paper wraps can.

Plastics have also replaced wood and stone in many applications. Furniture makers use plastics to produce cabinet doors and table tops that look like wood but are easier to clean and do not warp . Plastics have also replaced wood in boat hulls, boats are stronger than wooden ones and require less maintenance.

The textile industry used plastics to replaced such natural fibers as cotton , silk and wood. Plastic fibers have such qualities as strength, curability and resistance to stains and wrinkling .plastics are also used to create synthetic leathers

Types of Plastics

There are two main groups of plastics:

1. Thermoplastics
2. Thermo set

Thermoplastics are materials that remain permanently feasible. this means that they will melt when exposed to sufficient heat . For each thermoplastic, there is a specific temperature at which the material will start to distort. this is known as the heat distortion point . Most raw materials used in the production of thermoplastics products are melt able solids in the form of granules and powders such as those used for injection molding and extrusion . they are simply heated and melted into specific shapes though thermoplastics melt readily , they do have a high melt viscosity and require pressure in addition to heat to force them into molds. The shape of the mold being produced and resolidification takes place when the mold is cooled below the heat distortion point of the materials.

Thermo set raw materials are supplied in an uncured or partially cured state and are fully cured during the fabrication operation by the action of a catalyst or other curing agent .all require a form of heat to initiate the chemical change . The heat can come from an oven ,press , lamb or in some cases , from just the curing agent alone as it reacts with the resume . once cured , thermo sets becomes permanently infusible which means they cannot be melted down like thermoplastics and are used primarily in reinforced plastics . inherently, they are somewhat brittle but are combined with reinforcements such as fiber glass to form composites of great strength , some even surpassing metals in a weight – to strength ratio . when a thermo set is heated , it undergoes a chemical reaction called cross- linking , which binds its polymer chains together . this reaction is similar to the hardening of an egg when it is boiled . Product made from thermosetting plastics include pot handles , trays for sterilizing medical instruments and parts for airplanes and spacecraft . Manufactures and prefer to use thermo sets for such products as plastic seats on buses instead of thermoplastics because some thermoplastics tend to lose their shape when exposed to constant pressure over a long period of

Thermoplastics are those plastics formed by addition polymerization which is capable of repeated by softened by heat , remolded and hardened by cooling some typical thermoplastics are a acrylic (repex), acryonitrile (nylon), polyethylene (polyethylene), poly-propylene, polyvinyl Acetate (PVA), polystyrene

Thermo set are plastic formed by condensation polymerization which cannot be remolded by heating some typical thermo sets are bakelite , epoxy , melamine polyester ,polyurethane , phenolics amnions alkycls. Thermoplastics differs from thermo sets in that thermoplastics do not cure or set under heat but thermo sets do.