Synthetic Plastics

James Richard Fromm

Giant molecules called polymers are made up by the linkage of simpler molecules (monomers) by a polymerization reaction into essentially endless chain structures. Polymers occur naturally, but the majority which are used commercially are manufactured from simple monomers. We turn our attention now from an Introduction to Polymers to one of the most important groups of them, the synthetic plastics.

The basic starting material for modern synthetic plastics is ethene (ethylene), H2C=CH2. Ethene is itself a monomer which on heating (100-400oC) under high pressure (1000 atm) gives polyethylene:

2nH2C=CH2 rarrow.gif (63 bytes) -[CH2-CH2-CH2-CH2]n-.

Polyethylene is a tough solid, somewhat soft and very resistant to chemical attack. It can be blown into molds to make a wide variety of containers and other shapes.

Propene, CH3CH=CH2, also polymerizes under conditions similar to those of ethene to yield polypropylene:

2nCH3CH=CH2 rarrow.gif (63 bytes) -[CH2-CH(CH3)-CH2-CH(CH3)]n-.

The properties of polypropylene are similar to those of polyethylene. Polypropylene can be molded into film, sheets, and pipes as well as containers and other shapes which are manufactured by injection of polypropylene into molds. About half of the polypropylene used in the world, some 2.63 MMT in 1987, is used as molded shapes in households, automobiles, or packages. The largest single use is as textiles, mostly industrial fabrics or disposable nonwoven materials. Polypropylene fibers make excellent ropes and are replacing natural jute fibers for this purpose.

Polyethylene is the polymer manufactured in largest tonnage at the present time; polypropylene is perhaps fourth in tonnage. The second-ranking polymer is polyvinyl chloride (PVC). Polyvinyl chloride is the polymer of chloroethene or vinyl chloride, H2C=CHCl. Chloroethene is produced by dehydrochlorination of ethylene dichloride made from ethene or by chlorination of ethyne. The polymerization reaction is:

2nH2C=CHCl rarrow.gif (63 bytes) -[CH2-CHCl-CH2-CHCl]n-

In terms of tonnage, polystyrene is the third-ranking polymer. Polystyrene is the polymer of styrene, C6H5-CH=CH, which is also known as vinyl benzene. Styrene is made primarily by dehydrogenation of ethylbenzene. About 80% of it goes into plastics-oriented polymers, primarily the homopolymer polystyrene. The polymerization reaction is:

2nC6H5-CH=CH2 rarrow.gif (63 bytes) -[CH2-CH(C6H5)-CH2-CH(C6H5)]n-.

Unlike the other polymers mentioned above, polystyrene gives a clear castable polymer. Other plastics, such as acrylonitrile-butadiene-styrene (ABS) resins, use styrene as a comonomer and account for about 10% of its use; styrene-butadiene rubber (SBR) accounts for the remaining 10%.

Teflon (polytetrafluoroethylene) is similar to polyethylene in structure. The polymerization reaction is:

2nF2C=CF2 rarrow.gif (63 bytes) -[CF2-CF2-CF2-CF2]n-

Teflon owes its chemical resistance to the C-F bonds which are highly resistant to chemical attack.

Many other synthetic polymers are known, such as polymethylmethacrylate (Plexiglas, Lucite), but they are generally produced in smaller quantities for particular uses.

One type of polymer used as a foam for insulation and padding, and to a lesser extent in paint, is the polyurethanes made by reaction of toluene diisocyanate with polyfunctional alcohols or polyols. (Polyols are alcohols which contain multiple hydroxyl groups.) Toluene diisocyanate is made from toluene by nitration, followed by reduction to the diamine, reaction with phosgene, purification, and recovery of coproduct HCl:

(CH3)C6H5 + 2HNO3 rarrow.gif (63 bytes) (CH3)C6H3(NO3)2

(CH3)C6H3(NO2)2 + reducing agent rarrow.gif (63 bytes) (CH3)C6H3(NH2)2

2COCl2 + (CH3)C6H3(NH2)2 rarrow.gif (63 bytes) (CH3)C6H3(NCO)2 + 4HCl

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Copyright 1997 James R. Fromm