Monopoly: The Materials Edition

POLYMERS

THERMOPLASTIC

Raw material

Thermoplastics are a family of polymers (also known as thermosoftening polymers) that soften when they are heated and then harden again on cooling. All polymers (meaning many units or many mers) consist of long chain molecules made up of many thousands of repeating units or monomers (meaning single unit or single mer).

Celluloid is generally considered to be the first thermoplastic, invented in the 1850s. It was used as a replacement for animal products such as ivory and horn as well as for photographic film. It is no longer widely used as it is highly flammable but in the last 160 years, many new thermoplastics have been developed for an enormous range of uses.

Most polymers consist of carbon atoms along the backbone of the chain with other elements attached. The simplest polymer is poly(ethene) which consists of a chain of carbon atoms with hydrogen atoms attached. The C-C and C-H covalent bonds are strong but the bonds between the chains are due to weak van der Waals forces so low density poly(ethene) has a low melting temperature of 105-115 °C.

Ethene PVC monomer

Exchanging one of the hydrogen atoms in the ethene monomer for a chlorine atom produces vinyl chloride. This can be polymerised to produce poly(vinyl chloride) or PVC which is a very commonly used thermoplastic. Chlorine has a high electronegativity and pulls the electrons in the C-Cl bond towards it resulting in a slight negative charge. This polarisation of the molecule means that the polymer chains are bonded together by electrostatic forces which take more energy to break, such that PVC has a melting point of about 160 °C.

This simple example illustrates how different polymer chemistries can affect the properties of the material. Other factors such as the number of monomers in the chain, if the chain is straight or branched and the use of filler materials can be used to create polymers with a range of properties.

Uses

Plastic Bags

Plastic shopping bags are made from low density poly(ethene)(LDPE). It is chosen because it is light, tough, water resistant, easy to process and very cheap. It also has good resistance to many chemicals and so is used in a wide range of packaging and containers.

plastic bag

It is estimated that 500 billion plastic bags are used worldwide each year; 8 billion of those were given out in the UK in 2011. Waste plastic bags are responsible for killing thousands of marine animals and millions of birds each year and may take up to 1000 years to degrade. In recent years retailers have been encouraging customers to reuse bags and make use of bags made from more sustainable materials. Some countries such as Ireland have introduced a tax on single use plastic bags, others have banned very thin bags or even banned plastic shopping bags altogether.

 

Plastic Chairs

Plastic chairs are often made from polypropylene (poly(propene) or PP)as it is semi-rigid, tough and has good fatigue resistance. It also offers good heat and chemical resistance. PP was first manufactured in the 1950s and is very versatile. It is suitable for the majority of

propene molecule

thermoplastic manufacturing processes and can be made into fibres, films foams and mouldings. It soon found many applications such as flexible and rigid packaging, car interiors and bumpers, toys and homeware.

PP is very similar to poly(ethene) but the presence of a methyl (CH3) group on alternate carbon atoms along the chain increases the stiffness of the chain and decreases the crystallinity of the polymer.

Piping

plastic pipes

Plastic pipework is used to carry all kinds of fluids including drinking and waste water, chemicals, foodstuffs and compressed air. Poly(vinyl chloride) or PVC is the third most widely produced polymer (after PE and PP) and about half of it is used in pipe manufacture. It has good chemical stability and is lightweight, low cost and low maintenance.

 

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