Uses of thermoplastics Essay Example

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8Uses of Thermoplastics

Uses of Thermoplastics

Abstract

In recent times, polymeric materials have gained wide acceptance in engineering applications, even to the extent of replacing metals in some of the high-performance applications. The materials fall into two broad categories, namely thermosets and thermoplastics. This research explores various engineering aspects related to thermoplastic materials, including their manufacture, recycling, properties, fabrication, and applications.

Uses of Thermoplastics

In recent times, polymeric materials have gained wide acceptance in engineering applications, even to the extent of replacing metals in some of the high-performance applications. The materials fall into two broad categories, namely thermosets and thermoplastics. The latter represents polymeric materials that comprise of linear or lightly branched networks, which can slip past each other under the influence of force and temperatures. In addition, thermoplastics can be homopolymers or copolymers, where the latter represents with identical base units (monomers) and the former, those with more than one monomer. There is a wide range of thermoplastics, with the commonly used ones being polyethylene Terephthalate (PET), High-Density Polyethylene (HDPE), Polyvinyl Chloride (PVC), Low-Density Polyethylene (LDPE), and Polystyrene (PS), among others. Manufacturers use polymerization process to make the materials, which renders themselves recyclable after usage. One of the reasons for their increased use lies on their properties (mechanical, electrical, and thermal), which make them easily and cost effectively mouldable or extruded to form to make various engineering and household components. This research explores various engineering aspects related to thermoplastic materials, including their manufacture, recycling, properties, fabrication, and applications.

Manufacture and Recycling

Thermoplastics are produced through polymerization processes – addition (chain growth) or condensation. Used plastic components or production wastes can be recycled by subjecting them to polymerization.

Addition Polymerization

Addition polymerization is commonly used to produce thermoplastics for household components, such as food storage containers. It occurs through four main mechanisms: cationic, anionic, free radical, and coordination polymerization.

According to Peacock and Calhoun (2006), the first three mechanisms follow four distinct stages, namely initiation, propagation, chain transfer, and termination. In initiation, the initiator forms an active spot on the base unit, which reacts with another unit or monomer to add the monomer residue to the end of the polymer chain and form another active spot during propagation phase. These two processes make the chain to develop until the chain reached the required form and size, deactivating the active end during termination stage. Besides the initiation and propagation processes, the active end can alternatively react through chain transfer in which the site moves to another base molecule leaving a terminated unit and creating a new active site. For instance, in formation of PE, ethylene (C2H4) serves as the monomer. In the first stage of polymerization, the initiator (suitable combination of heat, catalysts, and pressure) breaks the double bond joining the two carbon atoms and substituted with a single covalent bond. This makes the carbon atoms become active sites, which then attract additional ethylene monomers to either end for chain growth during propagation. The chain grows until the carbon atoms become deactivated, therefore terming the chain to form PE.

In coordination polymerization, the monomer forms a coordinated structure with catalysts, facilitating reorganization of the electrons making the monomer bond (Peacock & Calhoun, 2006, p.30). The catalysts activate the resulting structure to attract more repeat units. The procedure repeats itself until the chain produces the required thermoplastic. Some of the thermoplastics produced through chain growth polymerization include PVC, LDPE, HDPE, and others.

Condensation Polymerization

It involves reaction of one or more saturated comonomers with active site units, such as hydroxyls, amines, and carboxyls. Besides production of the required plastic, the process also produces water (as by-product), which requires continuous removal to sustain chain growth. The process renders itself suitable for manufacture of PET, nylon (PA), and polycarbonate (PC), among others.

Fabrication

Thermoplastics are mainly fabricated using moulding processes although fabricators also use of other methods such as extrusion and pressing. Many of the components (commodity and engineering) made out of plastics are fabricated through injection moulding. According to Mikos, Ferreira, Gomes, and Lorenzo (2010), the process involves subjecting the thermoplastic materials to controlled pressure and temperature. Viscous and frictional energy from the materials and energy from heating elements with the mould transform the material’s properties. The design or shape of the moulding equipment is chosen to reflect the shape of the component being manufactured. There are various factors that must be considered when carrying out the process. Such factors, among others, include the type of thermoplastic, temperature profile of the mould, injection pressure, and velocity profile of the mould. Injection moulding accounts for over 30 percent of thermoplastic-based components. It is widely used because it renders itself suitable for manufacturing components with complicated shapes and close tolerance levels cost-effectively.

Properties and Applications

All thermoplastic materials are exceedingly receptive to temperature variations, with semi-crystalline ones showing clearly defined melting points and amorphous ones, gradual softening range of melting temperatures. These thermal properties influence other characteristics of the materials, including chemical, mechanical, and electrical. The materials find a wide range of applications, both in engineering and commodity industries. As demonstrated by the following table, properties of the plastics differ from one material to the other and usually determine usage of the materials.

Table: Characteristics and applications of most common thermoplastics

Thermoplastic

Properties

  • Dimensionally stable and tough over considerable range of temperature

  • Easily fabricated

  • Resistance to major solvents

  • Easily deteriorates with extended light exposure

  • Commodity packaging products such as bottles and bags

  • Plastic household containers

  • Withstands major solvents

  • Dimensionally stable

  • Tough, hard, and flexible

  • Commodity containers

  • Gutters and pipes

  • Electrical insulator

  • Degrades with extended outdoor exposure

  • Withstands major solvents except water

  • Abrasion resistant

  • Flexible, strong, and flexible

  • Good lubrication

  • Engineering components such as bearings, cams, shock absorbers, gears, and valves

  • Commodity products such as packaging products for medical devices, textiles, and others

  • Good transparency

  • Solvent resistance

  • Hard, brittle, and good impact resistance

  • Toys, electrical components, ropes, television castes and others

  • Stiff and hard

  • Good sterilization

  • Tubes and bottles

  • Good chemical resistance

  • Electrical resistance

  • Flexible and tough

  • Toys, bags, and bottles

Source: BBC, 2011; Essentially Education, 2011

Conclusion

Thermoplastic polymers comprise of linear or lightly branched networks, which can slip past each other under the influence of force and temperatures. They are produced through polymerization processes – addition (chain growth) or condensation. Many of the components (commodity and engineering) made out of plastics are fabricated through injection moulding. All thermoplastic materials are exceedingly receptive to temperature variations, with semi-crystalline ones showing clearly defined melting points and amorphous ones, gradual softening range of melting temperatures. The materials find a wide range of applications, both in engineering and commodity industries.

References

BBC. (2011). Design & technology: Materials. Retrieved from http://www.bbc.co.uk/schools/gcsebitesize/design/resistantmaterials/materialsmaterialsrev5.shtml

Essentially Education. (2011). Chemistry – Material choices – Physical properties and uses of important thermoplastic polymers. Retrieved from http://essentiallyeducation.co.uk/chemistry/thermoplastic-polymers.html

Mikos, W.L., Ferreira, J.C., Gomes, F.G., & Lorenzo, R.M. (2010). A combined multi-agent and case-based reasoning approach to support collaborative nonconformance problem solving in the thermoplastic injection moulding process. International Journal of Computer Integrated Manufacturing, 23 (2), 177-194.

Peacock, A.J. & Calhoun, A.R. (2006). Polymer chemistry: Properties and applications. Munich: Hanser Verlag.