Plastics

Covalent bond In a covalent bond the outer electrons of some atoms are close enough to overlap and are shared between the nuclei, forming a covalent bond. Each pair of electrons is called a covalent bond. Covalent bonds are strong bonds of primary bonds (as are metallic and ionic bonds).

Secondary bonds Secondary bonds are weak forces of attraction between molecules

Thermoplastic bonds Thermoplastics are linear chain molecules with weak secondary bonds between the chains.

Deformation Deformation of thermoplastics can occur in two ways: 1.Elastic, in which initially coiled chains are stretched and the material returns to its original size and shape when the load is removed 2.Plastic, when at higher loads the secondary bonds between the chains weaken and allow the molecular chains to slide over each other, and the material does not return to its original size and shape when the load is removed.

The reversible effect of temperature on a thermoplastic, with reference to orientation of the polymer chains, is that the increase in temperature causes plastic deformation. When thermoplastic polymers are heated they become flexible. There are no cross- links and the molecules can slide over each other. Thermosetting polymers do not soften when heated because molecules are cross- linked together and remain rigid.

The reversible effect of temperature on a thermoplastic contributes to the ease of recycling of thermoplastics. For example, polyester fleece can be made using plastic bottles, which are melted into polyester fibres and woven into polyester fleece, which can then be used to create ‘hoodies’. Recycling thermoplastics

Back in 1993, the outdoor clothing company Patagonia became the first to use fleece made out of recycled plastic bottles. It now offers more than 40 different products made from recycled plastic fleece, including jackets, hats, vests and socks. To turn plastic into fleece, used bottles are collected through bottle banks or kerbside collection schemes. Any bottles made from PET (a type of plastic) are chopped up into flakes, cleaned, dried and melted. The melted plastic is then squeezed out into long strands of fibre, which are spun into yarn and woven to make the fleece material. On average, each fleece garment made by Patagonia uses 25 two-litre drinks bottles. Every 150 fleeces made of plastic bottles saves a barrel of oil (42 gallons) and avoids about half a ton of toxic air pollution.

Thermosets Thermosets are linear chain molecules with strong primary bonds between adjacent polymer chains. This gives thermosets a rigid 3D structure.

Once a thermoset has been moulded into a product it cannot be reformed. There is a non-reversable effect of temperature on a thermoset.

Polypropene & Polyethene Properties and uses: Polypropylene or polypropene (PP) is a thermoplastic polymer, made by the chemical industry and used in a wide variety of applications, including packaging, textiles (e.g. ropes, thermal underwear and carpets), stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes. Polyethylene or polythene is the most widely used plastic, with an annual production of approximately 80 million metric tons. Its primary use is within packaging (notably the plastic shopping bag).

Polyurethane & Urea-formaldehyde Properties and uses: Polyurethanes are widely used in high resiliency flexible foam seating, rigid foam insulation panels, microcellular foam seals and gaskets, durable electrometric wheels and tires, automotive suspension bushings, electrical potting compounds, high performance adhesives and sealants, Spandex fibres, seals, gaskets, carpet underlay, and hard plastic parts. Urea-formaldehyde, also known as urea-methanol is a non-transparent thermosetting resin or plastic. These resins are used in adhesives, finishes, MDF, and moulded objects.

The disposal of plastics Disposing of plastics can be very difficult, whether it is a thermoplastic or a thermoset. For example, PVC. Polyvinyl Chloride (PVC) is a thermoplastic which is widely used as a structural material in windows and for guttering and drainpipes. However, disposing of PVC is problematic.

Despite the wide range of recycled plastics applications, the actual tonnage of waste plastic which is returned to the material cycle is relatively small. Currently, recycled plastics are rarely used in food packaging - the biggest single market for plastics - because of concerns about food safety. A method of addressing this problem is by enclosing the recycled plastic between layers of virgin plastic to ensure the packaging conforms to hygiene standards. These multi- layered containers are now being used in some drinks bottles, but recycling cannot eliminate the colours from plastics so they cannot be used in transparent or light coloured applications. Another constraint on the use of recycled plastics is that, to be economically viable, plastic processors require large quantities of recycled plastics, manufactured to tightly controlled specification at a competitive price in comparison to that of virgin polymer. This is a challenging task, particularly in view of the diversity of sources of waste plastics, the wide range of polymers used and the high potential for contamination of plastics waste.