Unitl about myself

2. MANUFACTURING OF PLASTICS 
The manufacture of plastic and plastic products involves procuring the raw materials, synthesizing the basic 
polymer, compounding the polymer into a material useful for fabrication, and moulding or shaping the plastic into its 
final form. 
Raw Materials 
Originally, most plastics were made from resins derived from vegetable matter, such as cellulose (from cotton), 
oils (from seeds), starch derivatives, or coal. Casein (from milk) was among the nonvegetable materials used. 
Although the production of nylon was originally based on coal, air, and water, and nylon 11 is still based on oil from 
castor beans, most plastics today are derived from petrochemicals. These oil-based raw materials are relatively widely 
available and inexpensive. However, because the world supply of oil is limited, other sources of raw materials, such as 
coal gasification, are being explored. 
Synthesizing the Polymer 
The first stage in manufacturing plastic is polymerization. As noted, the two basic polymerization methods are 
condensation and addition reactions. These methods may be carried out in various ways. In bulk polymerization, the 
pure monomer alone is polymerized, generally either in the gaseous or liquid phase, although a few solid-state 
polymerizations are also used. In solution polymerization, an emulsion is formed and then coagulated. In interfacial 
polymerization, the monomers are dissolved in two immiscible liquids, and the polymerization occurs at the interface of 
the two liquids. 
Additives 
Chemical additives are often used in plastics to produce some desired characteristic. For instance, antioxidants 
protect a polymer from chemical degradation by oxygen or ozone; similarly, ultraviolet stabilizers protect against 
weathering. Plasticizers make a polymer more flexible, lubricants reduce problems with friction, and pigments add 
colour. Among other additives are flame retardants and antistatics. 
Many plastics are manufactured as composites. This involves a system where reinforcing material (usually 
fibres made of glass or carbon) is added to a plastic resin matrix. Composites have strength and stability comparable 
to that of metals but generally with less weight. Plastic foams, which are composites of plastic and gas, offer bulk with 
low weight. 
Shaping and Finishing 
The techniques used for shaping and finishing plastics depend on three factors: time, temperature, and flow 
(also known as deformation). Many of the processes are cyclic in nature, although some fall into the categories of 
continuous or semicontinuous operation. 
One of the most widely used operations is that of extrusion. An extruder is a device that pumps a plastic through 
a desired die or shape. Extrusion products, such as pipes, have a regularly shaped cross section. The extruder itself 
also serves as the means to carry out other operations, such as blow moulding and injection moulding. In extrusion 
blow moulding, the extruder fills the mould with a tube, which is then cut off and clamped to form a hollow shape called 
a parison. The hot, molten parison is then blown like a balloon and forced against the walls of the mould to form the 
desired shape. In injection moulding, one or more extruders are used with reciprocating screws that move forwards to 
inject the melt and then retract to take on new molten material to continue the process. In injection blow moulding, 
which is used in making bottles for carbonated drinks, the parison is first injection moulded and then reheated and 
blown. 
In compression moulding, pressure forces the plastic into a given shape. Another process, transfer moulding, is 
a hybrid of injection and compression moulding: the molten plastic is forced by a ram into a mould. Other finishing 
processes include calendering, in which plastic sheets are formed, and sheet forming, in which the plastic sheets are 
formed into a desired shape. Some plastics, particularly those with very high temperature resistance, require special 
fabrication procedures. For example, polytetrafluoroethene (Teflon) has such a high melt viscosity that it is first 
pressed into shape and then sintered
—exposed to extremely high temperatures that bond it into a cohesive mass 
without melting it. Some polyamides are produced by a similar process. 
Uses 
Plastics have an ever-widening range of uses in both the industrial and consumer sectors. 
Packaging 
The packaging industry is a leading user of plastics. Much LDPE (low-density polyethene) is marketed in rolls of 
clear-plastic wrap. High-density polyethene (HPDE) is used for some thicker plastic films, such as those used for 
plastic waste bags and containers. Other packaging plastics include polypropene, polystyrene, polyvinyl chloride 
(PVC), and polyvinylidene chloride. Polyvinylidene chloride is used primarily for its barrier properties, which can keep 
gases such as oxygen from passing into or out of a package. Similarly, polypropene is an effective barrier against 
water vapour. Polypropene is also often used in housewares and as a fibre for carpeting and rope. 
Construction 
The building industry is a major consumer of plastics, including many of the packaging plastics mentioned 
above. HDPE is used for pipes, as is PVC. PVC is also used in sheets for building materials and similar items. Many 
plastics are used to insulate cables and wires, and polystyrene in the form of foam serves as insulation for walls, roofs, 
and other areas. Other plastic products are roofing, door and window frames, mouldings, and hardware. 
Other Uses 
Many other industries, especially motor manufacturing, also depend on plastics. Tough engineering plastics are 
found in vehicle components like fuel lines, fuel pumps, and electronic devices. Plastics are also used for interior 
panelling, seats, and trim. Many car bodies are made of fibreglass-reinforced plastic. 

---

77 
Among the other uses of plastic are housings for business machines, electronic devices, small appliances, and 
tools. Consumer goods range from sports equipment to luggage and toys 

---

78