Material Properties - Plastic Theory


The term 'Plastic' encompasses a wide range of synthetic or semi-synthetic natural compounds that are malleable and can be molded into solid objects. They primarily consist of petrochemicals extracted from beneath the earths surface as Petroleum from crude oil by a process of fractional distillation. Crude oil is a mixture of compounds known as 'hydrocarbons' (Hydrogen & Carbon) which can be separated by their individual boiling points within a heated column

Small hydrocarbons with less carbon atoms have a lower melting points and are tapped off at the top of the column as gases where it is coolest. Larger hydrocarbons with a greater portion of carbon are solid and due to their lower melting point are tapped off at the bottom. Hydrocarbons with around 5 - 10 carbon atoms are liquid and ideal for fuel due to the smaller molecules making them more volatile

Plastics,developed from petroleum, are made up of polymers. These are long chains of molecules that consist of many repetitions of a basic molecule called a monomer

Monomers of hydrocarbons are joined to form polymers using a chemical process known as 'Addition Polymerisation'. In this process two different types of polymer bonds can be formed. Polymers with straight chains of unconnected monomers are known as thermoforming plastics. They are characterised by their ability to reformed by heating and cooling. Polymers which form cross-link bonds between the monomor chains are known as thermosetting. Once heated and formed they cannot be reformed by heat
Additives can be joined with the polymers during or after manufacture to enhance various properties of the resulting plastic; 
  • Antioxidants: for plastic processing and outside application where weathering resistance is needed
  • Colorants: for colored plastic parts
  • Foaming agents: for expanded polystyrene cups and building board and for polyurethane carpet underlay
  • Plasticizers: used in wire insulation, flooring, gutters, and some films
  • Lubricants: used for making fibers
  • Anti-stats: to reduce dust collection by static electricity attraction
  • Antimicrobials: used for shower curtains and wall coverings
  • Flame retardants: to improve the safety of wire and cable coverings 
Thermoplastics are therefore ideal for recycling as they can be melted down and reformed into new products. Each plastic has it's own identification symbol embossed somewhere on the material which groups it into 7 categories defined by the  'Society of the Plastics Industry' (SPI). It is known as the Resin Identification Code (RIC). This is not required by law and changes depending on the country the item is sold in. The symbols are simply a quick way to identify plastic types for the recycling industry and each council in the UK will collect different categories depending on the recycling plants available

1) Polyester fibers, soft drink bottles.  2) Plastic bottles, plastic bags, trash cans, oil cans, imitation wood. 3) Window frames, chemical bottles, flooring, plumbing pipes. 4) Plastic bags, buckets, soap bottles, milk bottles. 5) Bumpers, car interior trim, industrial fibers, carry-out beverage cups.​ 6) Toys, flower pots, food coolers, carry-out food containers. 7) Polycarbonate (PC), polyamide (PA), styrene acrylonitrile (SAN), acrylic plastics/polyacrylonitrile (PAN), bioplastics
​Following polymerisation pellets,granules or powder (PVC) is produced ready for processing. Profile Extrusion is a common manufacturing method in the production of sheet plastic. Much like Injection molding, material pellets are fed into the hopper and moved along an archimedes screw within a heated chamber. This process is know as extrusion and results in a molton plastic at the end. Unlike injection molding the plastic is then forced through a shaped die  at high pressure that is in the shape (profile) of the required sheet

The sheet is sized within a vacuum chamber to the required thickness using air pressure and cooled in a bath of water to set. Rollers and formers are often also used at this stage to ensure a consistent sheet thickness. Once cooled the sheet moves on to processing where it is soften and cut to size to suit the application required

Alternatively plastic can becast into a mould at a molten state and left to cool. This produces sheets with greater structural strength due to the directional weakness created by the extrusion process

< Hydrogen

< Carbon

Thermoplastic - long chain, unconnected

Thermosetting - cross-link bonded chains


Thermoforming Plastics

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