Instant Notes: 22. Polymers: Important Polymers

22.6 Important Polymers

Polyethylene is cheap and is the largest volume thermoplastic polymer.

High-density polyethylene (HDPE) is characterized by a higher crystallinity and higher melting temperature than LDPE due to the absence of branching.
Low-density polyethylene (LDPE) (also called as high-pressure polyethylene) is more highly branched (both short and long branches) than high-density polyethylene (also called as low-pressure polyethylene) and is therefore lower in crystallinity (40–60% vs. 70–90%) and density (0.91–0.93 g/cm\(^3\) vs. 0.94–0.96 g/cm\(^3\)). Branching increases the specific volume and thus reduces the density of the polymer.
Linear low density polyethylene (LLDPE) has properties between HDPE and LDPE. It has fewer branches, higher density, and higher crystallinity than LDPE. LLDPE is produced like the high-density polymer at low pressures.
Because LDPE is flexible and transparent, it is mainly used to produce film and sheets. HDPE is important for producing bottles and hollow objects.

Polypropylene was not developed until the 1950s when Ziegler and Natta invented coordination catalysts.
The structural difference between polyethylene and polypropylene is the methyl group in the propylene unit. Its presence makes a difference because it makes possible three different polymer structures: Isotactic—with all methyl groups in the same plane makes the best plastic; syndiotactic—in which the methyl groups alternate in the same plane; and atactic—with the methyl groups randomly arranged. Atactic polypropylene is an amorphous rubbery material whereas isotactic is hard and strong.
Usually, isotactic polypropylene is preferred for applications and hence produced on an industrial scale.

It is widely used as a water-soluble packaging film and suspension agent and as an insolubilized fiber.
It is made by linking units of vinyl acetate (\(\ce{CH2CHOCOCH3}\)) and subsequently subjecting the polyvinyl acetate polymer to alcoholysis with ethanol or methanol.

Expanded polystyrene foams, which are produced by polymerizing styrene with a volatile solvent such as pentane, have low densities. They are used extensively in insulation and flotation (life jackets).
Styrene copolymers are very important commercially.
Styrene-acrylonitrile (SAN): is stiffer and has better chemical and heat resistance than the homopolymer. However, it is not as clear as polystyrene, and it is used in articles that do not require optical clarity, such as appliances and houseware materials.
A copolymer containing 25% styrene and 75% butadiene is known as SBR and is used for making tires, hoses, rubber-coated fabrics, and adhesives.
ABS, a copolymer of 50% styrene, 20% butadiene, and 30% acrylonitrile, is a plastic that is both tough and hard. It is used to manufacture luggage, high quality plastic pipe and fittings, telephones, appliance housings, etc.

It is commonly abbreviated as PET.
PET is produced by esterifying terephthalic acid [\(\ce{C6H4(COOH)2}\)] and ethylene glycol (or 1,4-butanediol) or, more commonly, by the transesterification of dimethyl terephthalate and ethylene glycol.
The majority of the world’s PET production is for synthetic fibers (in excess of 60%), with bottle production accounting for around 30% of global demand.
In the context of textile applications, PET is referred to by its common name, polyester, whereas the acronym PET is generally used in relation to packaging.
Polyester fibers: Dacron or Terylene are well-known brand names.
Polyester makes up about 18% of world polymer production and is the third-most-produced polymer; polyethylene (PE) and polypropylene (PP) are first and second, respectively.

Polymethyl 2-methylpropenoate often known as polymethyl methacrylate or PMMA is better known as Lucite, Perspex and Altuglas (when in sheet form) and as Diakon (when in powder form).
It is a clear, colourless polymer, commonly called acrylic glass or simply acrylic. PMMA exhibits very good optical properties—light transmissivity is comparable to that of glass.

They are resistant to high temperature, and ultra-stable; used for microwave cookwares.

Nylon 6,6 (Nylon 66): It is obtained by polymerization reaction of adipic acid and hexamethylene diamine. The number 6,6 is because of each of the raw material chain contains 6 carbon atoms.
Nylon 6: It is a homopolymer of caprolactum. It has similar properties to nylon 66, but posses better abrasion resistance.

Polycarbonates are condensation polymers made from phosgene and bisphenol A. They have high impact strength and are used in glazing, helmets, and appliance casings.
These materials, designed to replace metals and glass in applications demanding strength and temperature resistance, offer advantages of light weight, low cost, and ease of fabrication.
These are special variety of polyester in which a derivative of carbonic acid is substituted for adipic, phthalic, or other acid, and a diphenol is substituted for glycols.

Epoxy polymers are used mainly as adhesives, surface coatings, and in combination with glass fibers or cloth, as lightweight, rigid structural materials.
Many different epoxy prepolymers are available. (A prepolymer is a low-molecular-weight polymer that is reacted to increase its molecular size. Such reactions are usually carried out during or after a process that shapes the material into desired form). The most widely used epoxy prepolymer is made by condensing epichlorohydrin and bisphenol A.

Urea resins (urea formaldehyde polymers) are formed by the reaction of urea with formaldehyde. Urea-formaldehyde resins have large use as a plywood adhesive.
Melamine-formaldehyde resins are used as dinnerware and for extra-hard surfaces (in the trade name Formica\(^\circledR\) ). The melamine is synthesized by condensation of urea molecules.