3 Plastics 3.1 Basic concepts You may have briefly covered the basics of what polymers and plastics are in an earlier subject. However, it will have only been brief, if at all, so some revision (or first contact) is appropriate. What is a polymer? A polymer is a very large molecule, which has been made up by the connection of many small molecules, called monomers. Typical polymers can have formula weights from 10,000 to over 1,000,000. Are polymers and plastics the same thing? No. It is true to say that all plastics are polymers, but the reverse is not the case. Plastics are the name given to the large group of synthetic polymers made first in the 20th century, such as polythene, Nylon and Bakelite. There are many other polymers, which are naturally occurring, such as cellulose, starch and protein. Why are plastics called plastic? Plastic is a mechanical property of a material that, when stretched or other deformed, does not return to its original shape. It is the opposite of elastic. Many so‐called plastics are plastic. However, some are elastic and other are not deformable at all, so the name plastic is somewhat misleading. What happens to plastics when you heat them? Some plastics – the truly plastic ones – melt, or at least, soften on heating and can be reshaped before cooling into their new shape. These are known as thermoplastic. Examples include polythene, PVC, polystyrene and Nylon. However, many other plastics are not affected by heat after their original manufacture. The original monomers react on heating and set into their final shape. These are known as thermoset. Examples include Bakelite, phenol‐formaldehyde and melamine. Between these two extremes are the elastomers, which are truly elastic. They cannot soften to the same extent as thermoplastics, but have some movement. These include natural and synthetic rubbers. Are plastics pure compounds? No. While it is true to say that a particular plastic has only one type of compound, the long chains that make up the material are of varying lengths. Thus polythene, which is made up of long chains of CH2 units, will have many different individual molecules of varying sizes, all large. Plastics are characterised by an average chain length or average formula weight. What causes the differences in properties between different plastics? Polymers are still organic compounds, with various functional groups and different formula weights. The factors that affect small molecules affect polymers as well: 
polarity of functional groups – these affect the strength of intermolecular forces and therefore the melting/softening points (polymers do not truly melt, they only soften) and the ability of a solvent to dissolve/penetrate it (polymers are too big to dissolve, but solvents can break up some of the intermolecular forces (known as solvent penetration); they can be dispersed in certain solvents (like disperses like) 
molecular weight – the larger molecule, the more energy required to melt and dissolve it; polymer are very large 
intermolecular forces – the stronger the “bonds” between chains, the harder and higher melting will be the plastic; nylon is made up of amide linkages between alkane/aromatic groups, and these can form quite string inter‐chain hydrogen bonds, making nylon stronger 3. Plastics
and harder to melt than polythene which is like a very large alkane; thermoset plastics are the extreme because they only feature covalent bonds not only in the chains but between the chains by crosslinking (see Figure 3.1); elastomers are many fewer covalent cross links than the thermosets, and therefore have limited flexibility and will return to their original shape as the covalent bonds return to their original orientation (a) (b) (c) FIGURE 3.1 The molecular difference between (a) thermosets (b) thermoplastics (c) elastomers (heavy lines indicate covalent bonds, light lines weaker intermolecular forces) Do manufacturing methods make a difference to the properties of plastics? Yes. The plastic in laboratory wash bottles and Gladwrap are the same – polythene. The former is called high density polythene (HDPE) and the latter low‐density polythene (LDPE). The difference is in the way that the chains are made. We will look at this in some detail later in this chapter. Do plastics contain only one monomer type? Some do, some don’t. Plastics with one monomer type only are called homopolymers, those with two or three monomers are called copolymers. What are the monomers in some common plastics? Plastic Monomer(s) Polythene Polyvinyl chloride (PVC) Polystyrene Nylon ethene – CH2=CH2 chloroethene (vinyl chloride) ‐ CH2=CHCl phenylethene (styrene) ‐ CH2=CHC6H5 example only – there many different nylons, based on differences in the carbon chains between H2N‐(CH2)6‐NH2 HOOC‐(CH2)4‐COOH Polyethyleneterephthalate HOCH2CH2OH (PET) 1,4‐benzenedicarboxylic acid (terephthalic acid) HOOC
CIP COOH
Thermoset or Thermoplastic TP TP TP TP TP 3.2 3. Plastics
CIP 3.3 3. Plastics
Bakelite methanal (formaldehyde) H2C=O
phenol OH
Melamine methanal melamine NH2
TS N
N
TS NH2
N
NH2
How do the monomers join together? You should have noticed in the structures of the monomers in the table above that there seemed to be two quite distinct types of molecules: one type based on ethene and the others with polar functional groups such as amines, acids and alkanols. These two types of monomers react in different ways: 
addition – alkene monomers join together by the second bond in one molecule going to form the link to the next one and so on; substituent groups like Cl or benzene then are attached to the chain; addition polymers have a purely carbon skeleton CLASS EXERCISE 3.1 Show how the chain of PVC is built. 
condensation – as you should know from basic organic functional group reactions, acids and amines form amides (see Nylon) and acids and alkanols form esters (see PET); the reactions generate water, hence the name; condensation polymers have other atoms (eg N, O) as well as carbon in the skeleton; in the case of Bakelite (see Figure 3.2) and melamine, the reaction is not via an ester or amide link, but another reaction with which you are not familiar CLASS EXERCISE 3.2 Show how the chain of nylon is built. CIP 3.4 3. Plastics
OH
OH
OH
+
OH
CH2OH
CH2O
OH
CH2
further linkages between
CH2 and phenol rings
FIGURE 3.2 Chain growth in Bakelite 
polyaddition – this is a somewhat rarer form of reaction; it involves a transfer of a hydrogen atom from one group to another and no loss of water; polyurethanes are formed in this way; as shown below in Figure 3.3 CH3
CH3
NCO
NHCOO(CH2)4OH
+ HO(CH2)4OH
NCO
NHCOO(CH2)4OH
FIGURE 3.3 Chain growth in polyurethane 3.2 Plastics manufacture processes There are a variety of basic processes by which the plastics material – especially thermoplastics – are made: bulk – this involves no more than the pure monomer mixed with an initiator to start the 
polymerisation process; one of the major problems here is the generation of large quantities of heat from the exothermic process 
solution– the monomer is dissolved in a suitable solvent, which provides a dilution of the generated heat 
suspension – the monomer is present as small particles in a liquid in which it is not soluble (eg styrene in water); stirring is maintained to ensure separation 
emulsion – the monomer is dispersed by use of a soap or detergent into small droplets (micelles) Each of these methods produces a plastic, which may be chemically similar, but is very different in its physical properties. CIP 3.5 3. Plastics
Additives in plastics The basic polymer material is rarely used in a “pure” state. A range of additives are used to enhance desirable properties, and reduce the effects of undesirable ones. These include: plasticisers – improve flexibility; most commonly esters of phthalic acid (1,2‐

benzenedicarboxylic acid) 
antioxidants – retard reaction with oxygen and consequent breakdown; polypropylene cannot be used outdoors without antioxidants; most are phenol derivatives and are oxidised instead of the plastic 
heat stabilisers – protect against heat‐induced decomposition ultraviolet stabilisers – many plastics are degraded by ultraviolet radiation; stabilisers 
preferentially absorb the UV 
flame retardants – synthetic fibres, such as nylon, are very flammable unless an additive is used 
colorants – not always used just to make the plastic look better; rubber is white – imagine how dirty your car tyres would get if they weren’t black 
fillers – inert powders, principally to improve strength, eg carbon black in tyres reinforcements – as for fillers except non‐powder material , eg glass fibres in fibreglass 
curing agents – improve the formation of the plastic; the use of sulfur in rubber to “vulcanise” 
the product is the most important example – it makes the rubber much harder 
antistatic agents – many plastics build up electrical charge – for example, synthetic fibres in carpets; 
foaming agents – allow the formation of foam plastic by vaporising, or reacting to form gases, during processing; the bubbles of gas are trapped inside the plastic 
compatibilisers – like detergents, allowing incompatible materials in a plastic to blend properly 
antibacterials – few plastics are actually attacked by micro‐organisms, but can allow growth of moulds etc on their surface 3.4 Manufacture of plastic products Thermoplastics The basic plastic material (called a resin) is likely to be powdered or granulated into small pellets for easy transport. The manufacturer of the plastic product is then required to reshape the plastic into the final form. There are many forms of processing available, depending on the type of plastic and the final shape, including: 
blow moulding injection moulding 
calendering 
extrusion 
foaming 

laminating thermoforming 
Thermosets This type of plastic is, of course, fixed in shape once it has polymerised, so it must be supplied to the plastic product manufacturer in a non‐polymerised form. Often the monomers (eg phenol, methanal) are toxic and difficult to handle, so it is often not appropriate to supply the makings in this form. CIP 3.6 3. Plastics
A pre‐polymer is made where an intermediate number of monomer units are joined together, but because one (usually the “joiner” like methanal) has been deliberately lowered in concentration, the process stops well short of full polymerisation. The pre‐polymer can be mixed with more of the other monomer by the manufacturer of the product to make the final plastic. Compression moulding is the most common method for making the basic shape of a thermoset plastic product. The pre‐polymer mixture is added to the bottom half of a mould, the top half put in place, and heat and pressure applied to cause the final polymerisation process. Thermoset plastics, once cured, are hard enough to be machined like metals and timber, unlike the softer thermoplastics. 3.5 Plastics properties Beyond the basic division of plastics into thermoplastic, thermoset and elastomer, there are many and varied properties – physical, chemical, thermal, electrical and mechanical – that dictate the various uses to which the material can be put. CLASS EXERCISE 3.3 The table below lists some of the more important properties that influence their applications. Those important in the use of a plastic as electrical insulation are ticked. Complete the table for plastics to be used in drink bottles. Property Electrical Insulation
Drink Bottle  Density CIP Hardness  Strength  Flexibility  Heat stability  Light stability Abrasion resistance Tear resistance Electrical resistance  Impact resistance  Heat distortion  Toxicity Water resistance Solvent penetration Chemical resistance Flammability  Appearance 3.7 3. Plastics
CASE STUDY – The manufacture of polyethylene History In 1933, the research department of ICI in England was investigating the effect of high pressure on chemical reactions. What they discovered was polyethylene. Commercial production began six years later. Significant use of PE did not begin until after World War 2, but once the technology was established, the growth in production was spectacular. By 1980, the world production was approaching 100 million tonnes annually. Structure Polyethylene consists of long chains of CH2 units, formed by the addition polymerisation of ethene (old name ethylene). There are three basic forms of polyethylene, based on the level and length of the side‐chain branching, which is an inevitable, but controllable part of the polymerisation process. These three forms, which at the simplest level, differ in density, are known as: 
low density (LDPE) 
linear low density (LLDPE) 
high density (HDPE) They are manufactured in three different ways, using different temperatures, pressures and catalysts. Low density polyethylene (LDPE) Manufacture ‐ this was the first type of PE available, and it was made by a modification of the discovery at ICI in 1933: high pressures (up to 2000 atm) and moderate temperatures (around 200C); bulk reaction of ethene Properties – very flexible; relatively low melting with heat degradation occurring with or without oxygen present; extremely good electrical insulation properties; damaged by various solvents such as hydrocarbons and alkanols; resistant to dilute acids and alkalies, but will e oxidised by concentrated oxidising acids Additives – the most common antioxidants, because PE’s ready reaction with air. Processing – mostly by extrusion Applications – more than 50% ends up as film (eg gladwrap), as insulation for electrical cables Recyclability – limited Linear low density polyethylene (LLDPE) Manufacture – low pressure (10‐40 atm) reaction, either in gas phase or in hexane solution Properties – stronger and harder, but less flexible than LDPE; otherwise similar Additives – antioxidant Processing – extrusion & injection moulding Applications – film, improved strength allows more load bearing uses such as pipes Recyclability – not usually High density polyethylene (HDPE) Manufacture – low pressures (20 atm) and temperatures (100C) in hydrocarbon solution, and a catalyst are used to produce the low level of branching; the catalysts are either chromium (VI) or a complex organometallic catalyst discovered by Ziegler and Natta, for which they received a Nobel Prize Properties – much stronger and harder, because of greater crystallinity, allowed by very little chain branching (think of stacking trees before and after the branches are removed) Additives – antioxidant CIP 3.8 3. Plastics
Processing – blow moulding, injection moulding, extrusion Applications – moulded parts such as toys, seats and bottles, shopping bags Recyclability – one of the two most commonly recycled plastics in the form of milk and juice bottles What You Need To Be Able To Do 
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define important terminology explain the differences in properties between plastics list manufacturing methods for plastics list manufacturing methods for plastic items list common plastics additives explain the issues associated with plastics recycling CIP 3.9