Monomer and Polymer

Polymers - plastics - are very large molecules made from many smaller molecules called monomers. Alkenes are able to act as monomers because they contain a double bond. They can join end-to-end in a reaction called addition polymerisation. The polymers they form are called addition polymers. In general: A lot of monomers → a polymer molecule

Some small molecules can join together to make very long molecules called polymers. This process is called polymerisation.

Thermosetting plastics do not melt but break down and char on heating. In these plastics, the long chains are also linked together sideways by carbon-carbon bonds: the material is said to be cross-linked. polymers - end to end thermosetting -end to end and sideways

Polymers such as polyethylene, polypropylene and polystyrene soften and eventually melt at temperatures in excess of 100-150°C Such materials are called thermoplastics.

1 Toxic and corrosive gases. • smoke • burning tar • exotherms • catylists • flammable solvents • dusts

If plastics only contain the elements carbon and hydrogen, or carbon, hydrogen and oxygen, the main toxic gas to be expected is carbon monoxide (CO) which is formed when all organic materials are burned in quantity. CARBON MONOXIDE Many plastics contain nitrogen in addition to carbon, hydrogen and oxygen The fire products from these will contain nitrogen-containing species such as organic nitriles, hydrogen cyanide and NO2. All of these are toxic. Intense, well ventilated burning will convert most of the original nitrogen into NO2. NITOGEN DIOXIDE HYDROGE CYANIDE Chlorine is present in polyvinyl chloride in PVC fires, almost all the chlorine goes to form hydrogen chloride (HC1) gas in the fire gases. HC1 is both toxic and corrosive, having a very sharp smell and forming a corrosive solution with water (hydrochloric acid HYDROGEN CHLORIDE PTFE (poly-tetrafluoroethylene - ‘Teflon’) and some related materials, such as ‘Kel-F’ and some synthetic rubbers, sometimes known as “vitons’. contain fluorine

Fires involving materials which contain the aromatic (benzene) ring structure will tend to produce large quantities of smoke. These include polyurethanes, phenol-formaldehyde resins, polystyrene , polyesters, epoxy resins and polycarbonates. large quantities of thick black smoke causing smoke logging and smoke filled escape routes

Thermoplastics melt on heating and so in a fire may form burning droplets which could help the fire to spread.

if blocks of polyurethane foam are stored before the exothermic curing process is complete, self-heating may occur, leading to spontaneous combustion POLYURETHANE

Various types of catalyst are used in polymerisation processes including acids, alkalis, complex organometallic compounds and organic peroxides Acids and alkalis present well known hazards. An organo-metallic catalyst may be in the form of a slurry in flammable solvents: some of these compounds, such as aluminium triethyl, react violently with water. Organic peroxides are oxidising agents and, therefore, present a considerable fire risk. Under some conditions such materials can be explosive.

Flammable liquids, such as acetone, methyl ethyl ketone, toluene, industrial alcohol and methyl alcohol are widely used as solvents in various processes and also as cleaning fluids, so may be present in fires on industrial premises

Some processes produce fine plastic dusts. These may present an explosion hazard if dispersed as a suspension in air

Many plastics are described as “self-extinguishing”. While PVC and phenol-formaldehyde resins are naturally so, others may be made self-extinguishing by chemical changes in the polymer molecule or by the use of special additives. The term ‘self-extinguishing’ means that, while a flame may (or may not) cause the material to burn, it will not continue to burn if the applied flame is removed. However, the term refers to the performance of plastics in a specific small scale test. In a fire situation, it will burn if surrounded by other burning materials, or perhaps on its own if a large enough area of the plastic has been ignited to produce flames which are self-sustaining

Monomers by definition are reactive compounds capable of polymerisation. Some, like ethylene, do not polymerise very easily and need exactly the right conditions of temperature and pressure, perhaps with a catalyst. Others, like styrene, may polymerise by accident, due to the presence of impurities, water, heat or other causes, and when this happens a great deal of heat may be given out. Some monomers have to be transported with a polymerisation inhibitor added to prevent the process occurring spontaneousl As these monomers are mostly poor conductors of heat, the heat cannot get away easily, temperatures may rise and a fire may result monomers are flammable, and some are toxic.