Thursday, October 2, 2008

The Plastic Age

We currently live in the 'Plastics Age' with a total consumption of about 182 million tonnes of polymers per year. Of this, polypropylene is 24 t
o 25 per cent and all polyethylenes, about 40 per cent. This industry has grown phenomenally in the last century. Its growth within this period will be of interest to many.

Human race has always progressed by using raw materials around. Growth from 'Stone Age' to 'Plastics Age' has been very rapid. Use of plastics within the last century and in particular, after World War II has increased exponentially.

Plastic is not a uniformly defined term. Its origin is from the Greek word 'Plasticos', which means 'to form or mould'.
Plastics can broadly be defined or described as materials composed essentially of very large molecules (called macromolecules or polymers) which may be natural, semi natural (modified natural) or synthesised from small molecules, termed as monomers.

At times, plastics and polymers are used as synonyms in the colloquial world. Some prefer to differentiate the two by defining plastics as made by compounding polymer and additives. Polyvinyl chloride is a polymer, but when compounded with a plasticiser and the thermal stabiliser, it may be termed as a plastic.

Some polymers may not need any additives and these can be used as such in their virgin form. It is preferable to use the word polymer instead of plastic, as the word polymer denotes big or giant molecules. 'Poly' means many and 'mer' indicates substance.

Classification of Polymers

Polymers can be classified based on their end uses or origin of raw materials.

Classification based on End-uses

This type of classification also highlights different market and key properties depending upon specific use. Thus, adhesives need totally different properties from materials used as consumer goods like packaging films, ice cream cups, paints or moulded furniture.

Thermoset Polymers: Thermosets were the early polymers made synthetically. The first totally synthetic polymer, Phenol, Formaldehyde was, in fact, thermoset in nature, its moulding powder, polyurethane foams used in sofa seats, automobile seats or the 'setting adhesives' are examples of thermoset resins. Thermoset materials 'set' when heated (thermal effect). One can imagine boiling of an egg. The yolk becomes hard or it is 'set'. Once it is set, it cannot be reprocessed again. It does not become liquid again.

Thermoplastic Polymers: Thermoplastic materials melt when heated and can be shaped as desired. They can be reheated and melted several times. Polyethylene, polypropylene, polystyrene, nylon, polyester and PVC are examples of thermoplastic polymers. Today, production of thermoplastic polymer is several times that of thermoset polymers.

Rubber: Rubber can be natural or synthetic, such as styrene-butadiene-styrene block copolymer, nitrile rubber, silicone rubber or isoprene. The high impact is the key property for rubber.

Fibers: Polymers for fibers need different properties as compared to rubber or thermoplastics used in furniture or in milk packaging. Fibers have textile and non-textile applications. For textile applications, dyeability, washability, lightness and comfort on feel are major properties; while that for non-textile use, solvent resistance or tenacity may be of importance. Polyester and nylon are commonly used in textile applications, whereas polypropylene has properties suitable for non textile use.

Adhesives: These polymers must have a good adhesion between different substrates and good bonding strength. It should be easy to apply on the surface. Poly vinyl alcohol, epoxy polymers or cyno acrylic polymers are examples of adhesives.

Paints and Inks: These polymers should have a film forming ability, good flow properties and sufficient tack to the surface on which they are applied. They should be able to incorporate large quantities of pigments. Inks on paper or plastics have different properties. Alkyd polymers, polyesters, epoxy polymers or amino polymers are common examples.

Classification of Polymers based on Origin of Raw Materials

Polymers can be classified based upon their origin, as:

Natural Polymers: These occur as macromolecules or polymers in their natural form. Cellulose, starch, sugar, lignin etc. have plant origin. Other materials such as shellac, wool etc. have animal origin. All these materials are natural polymers. They exist in nature. It is interesting to know that different starches behave differently due to difference in their spatial structure although their chemical formulae are the same.

Semi-Natural or Modified Natural Polymers: Most natural polymers cannot be used in their virgin or native form, but can be chemically modified. Thus, cellulose when treated chemically (acetylated) yields cellulose acetate, which can be spun into fibre, extruded or injection moulded. Carboxy methyl cellulose, CMC, dissolves in water but cellulose itself does not. Thus, cellulose acetate or CMC can be classified as semi-natural polymers.

Synthetic Polymers: These are made from monomers through a chemical reaction, called polymerisation. Polyethylene, polypropylene, nylon, polyvinyl chloride, polyester etc. are examples of synthetic polymers. Although synthetic polymers outnumber natural polymers, the total quantum of natural polymers is far greater than synthetic polymers.

(Source: Prof. D.D. Kale, Ex-HOD, Polymer Department, UICT, Mumbai)

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