Plastics can be broadly classified as standard plastics, commodity plastics, engineering plastics, and high-performance plastics. Important classes of plastics exist under each of these categories. For example, polyethylene, polystyrene, polypropylene, and polyvinyl chloride can all be categorized as commodity plastics. Some important types of plastic are touched upon in this article along with their properties and uses.
Types of Commodity Plastics
Commodity plastics are the plastics which are inexpensive and do not possess any exceptional mechanical properties. Such plastics are produced in large volumes for a wide spectrum of applications such as food packaging and storage, household products, and carry bags. Some common types of commodity plastics are listed in this subsection.
Polyethylene, also known as polyethene or just polythene, is a very common plastic whose monomer is ethylene (a chemical compound with the formula C2H4). The generalized chemical formula of this plastic can be written as (C2H4)n. The density of polyethylene can range from 0.88 grams per cubic centimetre to 0.96 grams per cubic centimetre. The melting point of this plastic is relatively low, ranging from 115 degrees Celsius to 135 degrees Celsius. Therefore, it can be said that polyethylene is not very resistant to heat. The primary application of polyethylene is in packaging products such as plastic films, plastic bags, plastic bottles, and geomembranes. It can also be noted that there are several types of polyethylene, such as low-density polyethylene, high-density polyethylene, and polyethylene terephthalate.
Polypropylene (sometimes referred to as polypropene and abbreviated to PP) is a non-polar, partially crystalline polymer whose monomer is propylene (a chemical compound with the formula C3H6). The generalized chemical formula of polypropylene can be written as (C3H6)n. It is important to note that polypropylene is a thermoplastic polymer which becomes moldable when heated to certain temperatures. The melting point of this plastic usually ranges from 130 degrees Celsius to 171 degrees Celsius. The density of amorphous polypropylene is known to be 0.855 grams per cubic centimetre whereas that of the crystalline variant is roughly equal to 0.95 grams per cubic centimetre.
Polyvinyl chloride (often abbreviated to PVC) is a synthetic polymer whose monomer is vinyl chloride (a chemical compound with the formula CH2=CHCl or C2H3Cl). Over 40 million tons of this plastic are produced annually across the globe. The two basic forms of PVC are rigid PVC (or RPVC) and flexible PVC. Rigid PVC is primarily used in the construction industry for piping, windows, and doors. This form of PVC is also used in the manufacture of bank cards, packaging, bottles, and food-covering sheets. Flexible PVC is known to have applications in the insulation of electrical cables, flooring, plumbing, and imitation leather. The melting point of PVC is known to range from 100 degrees Celsius to 260 degrees Celsius.
Types of Engineering Plastics
The plastics whose mechanical properties and/or thermal properties are much better than those of commodity plastics are categorized as engineering plastics. Such plastics are usually more expensive than commodity plastics and are, therefore, generally used for low-volume applications. Most engineering plastics are known to be thermoplastic rather than thermosets. Some common types of engineering plastics are listed in this subsection.
Acrylonitrile Butadiene Styrene
Acrylonitrile butadiene styrene (often abbreviated to ABS) is a thermoplastic synthetic polymer which is made up of three different monomers. They are:
- Acrylonitrile – an organic compound with the formula CH2CHCN or C3H3N.
- Styrene – an organic derivative of benzene with the chemical formula C8H8.
- 1,3-Butadiene – an organic compound with the chemical formula (CH2=CH)2 or C4H6.
The chemical formula of acrylonitrile butadiene styrene can be written as (C8H8.C4H6.C3H3N)n. The density of this engineering plastic usually ranges from 1.06 to 1.08 grams per cubic centimetre. One of the most important applications of ABS is in drain-waste-vent pipe systems or DWV pipe systems. This plastic is also used in some musical instruments, inhalers, automotive bumper bars, and the heads of golf clubs.
Nylon 6 is a synthetic plastic with the chemical formula (C6H11NO)n. Unlike other nylons, this polymer is prepared from a ring-opening polymerization reaction rather than a condensation polymerization reaction. The fibres of nylon 6 are known to be extremely tough. They possess very high tensile strength and are also lustrous and elastic to some extent. Nylon 6 fibres are known to be highly resistant to chemicals (such as alkalis and acids). They are also resistant towards abrasion and wrinkling. In fact, nylon 6 is considered to be wrinkleproof.
Because of its many desirable properties, the nylon 6 plastic is widely used in many industries such as the aircraft industry, the automotive industry, the electronics industry, and the clothing industry.
Nylon 6,6 is a polyamide whose chemical formula is given by (C12H22N2O2)n. This plastic is generally prepared from the polymerization reaction between adipic acid and hexamethylenediamine. The density of this engineering plastic is approximately 1.314 grams per millilitre. Nylon 6,6 is known to have a melting point of 264 degrees Celsius. The key properties exhibited by this polymer are listed below.
- Nylon 6,6 is known to have very high tensile strength.
- Nylon 6,6 is also quite rigid.
- This plastic exhibits good stability when exposed to heat.
- It is also resilient towards some chemicals.
The primary application of nylon 6,6 is in synthetic fibres used in textiles. However, this synthetic plastic is also used in airbags, carpet fibres, automobiles, ball bearing cages, pipes, zip ties, hoses, conveyor belts, and some other machine parts. It can be noted that both the monomers of nylon 6,6 (adipic acid and hexamethylenediamine) each contain a total of 6 carbon atoms, which is the reason why the suffix ‘6,6’ is attached in the name of this plastic.
Types of High-Performance Plastics
High-performance plastics are the plastics that meet much higher requirements than engineering plastics and, by extension, commodity plastics. High-performance plastics are usually quite expensive and are, therefore, used in relatively smaller amounts than commodity plastics and engineering plastics. High-performance plastics are known for their chemical resistance towards acids and alkalis, their mechanical properties (such as tensile strength, rigidity, and elasticity), and their temperature stability. Some common examples of high-performance plastics include polytetrafluoroethylene (or Teflon), phenol formaldehyde resins, polyepoxides (or epoxy resins), polymethyl methacrylate, melamine formaldehyde, and silicone.
Polytetrafluoroethylene (often abbreviated to PTFE, also known as Teflon) is a synthetic plastic that is entirely made up of fluorine and carbon. The chemical formula of PTFE is (C2F4)n. It is a fluorocarbon solid with a very high molecular weight. This plastic cannot be wet by water or substances that contain water (due to the mitigated London dispersion forces in fluorocarbons as a consequence of the high electronegativity of fluorine). The primary application of this polymer is in non-stick coatings in cookware such as vessels and frying pans. This polymer is also used as a lubricant in certain types of machinery. PTFE is also used as a graft material in certain surgeries.
The density of PTFE under standard conditions is roughly 2200 kilograms per cubic metre. The melting point of this polymer is approximately equal to 327 degrees Celsius.
Phenol Formaldehyde Resins
Phenol formaldehyde resins, also known as phenolic resins and abbreviated to PF, are synthetic plastics that are prepared from the chemical reactions between phenols or substituted phenols with formaldehyde. These polymers are often employed in the construction of countertops in laboratories. They are also used as adhesives and coatings.
Phenolic resins are usually prepared via step-growth polymerization reactions. These reactions are typically catalyzed by either an acid or a base. It is important to note that this reaction is both pH-dependent and temperature-dependent (because the concentration of the reactive form of formaldehyde is dependent on pH and temperature; formaldehyde exists in a dynamic equilibrium of methylene glycol oligomers when it is placed in a solution).
Silicones, also known as polysiloxanes, are the synthetic polymers made up of the monomer siloxane (general formula: -R2-Si-O-Si-R2– where ‘R’ denotes an organic group). Typically, silicones have colourless appearances and oily or rubbery textures. This class of polymers is widely employed in the production of lubricants and adhesives. Silicones are also used in medicine, electrical insulation, thermal insulation, and cooking utensils. The key properties of silicones are listed below.
- Silicones have very low thermal conductivities.
- The chemical reactivities of these polymers are also very low.
- Silicones have very low toxicity towards human beings.
- They are resistant to ozone, ultraviolet radiation, and oxygen.
- They are electrical insulators.
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