PLASTICS Plastics are synthetic materials which are not found on the earth naturally. They are chemical combination of various ingredients, most derived from petroleum. The ingredients are liquid in the manufacturing state, but when put together under heat and pressure, they are capable of formed into various shapes. The finished product is solid, resistant to pressure, moisture, and corrosion.

The term plastics is often used interchangeably with polymers, that means a substance with large molecules made by joining many small molecules of one or several substances under heat and pressure. Therefore different names of plastics indicate different chemical group of polymers.

HISTORY In the human history, the first synthetic compound containing several thousand molecules was found in 1833 by Swedish chemist J.J. Berzelius. But it is not up until the beginning century that the plastics chemistry was fully understood. It is generally considered that 1970’s is the era of bronze age for the plastics development. Many developments have come in the later period.

Plastics are widely used in everyday living – at home, at work, during our leisurely activities and sports, and even at the frontiers of space. In modern life plastics are replacing more traditional materials like steel and wood. The volume of plastics consumed in the U.S.A has exceeded the volume of metals consumed in 1983.

INGREDIENTS OF PLASTICS The raw materials of plastics are the various resins. Resin is a gum-like solid or semisolid substance used in making many types of plastics. Plastics products are often made from resins that are processed and made solid. Both natural (cellulose) and synthetic resin materials are compounds of a serious of molecules bonded together with some resins having several thousand bounded molecules.

Resins are classified into three groups according to their sources: Natural Sources of resins include animals, vegetables, and minerals. Mostly they are obtained from trees. Modified Natural Sources of Resins include cellulose and protein. Synthetic Sources are the most important sources of resins for plastics. Agriculture, petroleum, and coal are the three main sources of chemicals for synthetic plastic materials. The most important one of these three is petroleum. (nylon)

TYPES OF PLASTICS A type of plastics may be known by several names, a generic or chemical name and one or more trade names given to it by manufacturers. Acrylic is a generic name, Plexiglas and Lucite are trade names for acrylics. The character of a particular plastics can vary greatly according to the way it is processed. Nylon, for example, appears/to be totally different when knitted into stocking, woven into carpet fibre or made into solid glide or roller.

TYPES OF PLASTICS There are two main families of plastics, and they differ in their basic qualities: 1-     THERMOPLASTICS 2-     THERMOSETTING PLASTICS

THERMOPLASTICS This is a kind of plastics that is soft and mouldable when heated, and becomes stiff and solid when cooled. Thermoplastics objects are made by moulding, rolling, or extruding the heated material. Familiar groups of thermoplastics include acrylics (transparent and clear, coloured or opaque), polystyrenes (much used for everyday household items) any vinyl (common as floor tiles and as alternatives to leather in upholstery).

Other groups: cellulosics (clothing, tooth brushes, handles of various tools, films, sheets in packaging and display industries etc.) fluoroplastics (Teflon) (cable insulation, laminates for printed circuits, pads for many structural elements) polycarbonates (pc) (windshields of motorcars and other vehicles, skylight shields in building construction, light fixtures, coffee pots, hair dryers and housing appliances etc.) Thermoplastics Polyesters (photograph film, video tape, computer and magnetic tapes, beverage bottles, electrical switches and sporting goods)

Polyolefins: Polypropylene (dishes, appliance parts, dishwasher components, containers, rot proof ropes, outdoor carpeting, chair seats and backs etc.) Polyvinyls / Polyvinyl Chloride (PVC): The most used and commonly thought of vinyl. (floor coverings, pipes, window frames, wood finishing for window / door frames, all sorts of architectural mouldings, seals, gutter, exterior siding, garden hose, movable partitions etc.)

THERMOSETTING PLASTICS These are made from a liquid resin and a second liquid called a catalyst that when combined and subjected to heat harden, and become solid. Once formed, objects of thermosetting plastics cannot be softened or melted. They can be processed by rolling, moulding, or foaming to make sheets, objects, of complex forms.

The melamins are a strong thermosetting plastic. Phenolics (bakelite is a well known trade name) are weaker but inexpensive and good insulator. Glass fibres are embedded in polyester to make a hybrid, high-strength material usually called the trade name Fibreglass.

Plastic laminates are made from layers of special paper bonded together and surfaced with melamine plastic to make a thin sheet material of great strength and excellent resistance to scratching or other damage. Plastic laminates are generally bonded to an under structure of plywood or particle board.

FORMS OF PLASTICS moulding / sheet moulding compounds Adhesives (silicon) Dimensional or profile forms (exterior siding of solid vinyl with wood –grain appearance, window – door frames etc. Filaments and Fibres (Carpets) Coatings Cellular or Foamed Plastics Composites (laminate, sandwich)

Laminates are composed of layers of resin – impregnated materials Laminates are composed of layers of resin – impregnated materials. Formica, Micarta and Texolite are trade names for a familiar laminate form. Sandwiches are defined as combination of alternating dissimilar materials assembled and intimately fixed in relationship to each other. They have core and facing materials. As a rule, honeycomb, waffle and cellular sandwiches are isotropic, with excellent thermal, acoustical and strength-to-weight ratios. Wall / roof panels, prefabricated home panels with polyurethane foam and aluminium.

ADVANTAGES AND DISADVANTAGES OF PLASTICS 1- Corrosion and chemical resistance 2- Good thermal and electrical insulating properties 3- May be made isotropic or unisotropic (isotropic: same values in all directions, unisotropic: different values in all directions) 4- Good strength – to – mass ratio 5- Light (mass) weight 6- Ease of processing 7- Available in a variety of forms 8- Capable of being foamed and made flexible 9- Available as transparent, translucent, or opaque 10- Available in wide range of colours.

ADVANTAGES AND DISADVANTAGES OF PLASTICS 1-     Dimensional instability 2-     Limited useful thermal range 3-     Fragility (may break, crack, or scratch easily) 4-     Flammability (many burn easily) 5-     Absorb moisture 6-     Non-degradability (some do not decompose) 7-     Subject to attack by chemicals (deteriorate) 8-     Odours or chemical fumes in processing 9- Difficulty of repair (thermosets) 10- Cost (vary by family)