3.6 Plastic



3.6 Plastic

There is a huge variety of plastic types and properties, which leads to an associated large variety of end uses for plastic.  Again, unless you are reading this in an open field (and even then) you will likely be in the vicinity of a plastic item that started its life as a hydrocarbon.

The first plastics were made from natural fibres such as tar or tree sap, but after the first world war as oil become widely available worldwide, the production of ethylene from this oil became the base of plastics.  The fact that plastics come from oil is significant when considering the issues of peak oil as described in the previous chapter.  It is also interesting to consider that if such long lasting and effective products could be made from oil, why are we simply burning it in vehicles for transportation?

Plastic has a variety of beneficial properties such as:  electrical and thermal resistance, corrosion resistance, resistance to humidity, glossy shiny finish and chemical resistance.  However the main beneficial property is that of being able to be injection moulded into intricate and detailed shapes, unlike steel which must be pressed.  This makes it incredibly versatile and fundamentally, at current energy and oil prices, extremely cheap to manufacture.

This versatility and low price has meant that plastic is primarily used for one off uses, such as packaging for products.  Other longer term uses can be found in the construction industry (in pipes, electrical fittings and fixtures) and the automobile industry. 

The use of plastic has exploded over the last century, it's production doubles every 15 years and CO2 emissions from plastic are set to double from 2005 to 2050.  As a result plastic waste has become an immense issue to deal with, especially in developing countries where the plastic industry and trade of plastic goods has increased at a much faster rate than the waste management schemes for this type of material. 

Recycling of plastic is possible, but only once it is separated from other forms of waste and cleaned, and then separated into the different forms/types of plastic.  Due to the nature of plastic being in large quantities of small units in packaging it often enters the waste stream dirty and mixed with other waste.  Plastic takes hundreds of years to biodegrade, and this fact is becoming evident with large collections of plastics building up in the seas and countrysides of the world.

The full issues of waste will be considered in the next section, but again the important point is that an engineer, when considering materials for a design must understand the implications of the use of materials such as plastic.

Having considered the main four materials used by society we will now examine the problems caused by their manufacture and disposal.