The monomeric building block for polyethylene is ethylene (CH2=CH2), a gas having a boiling point of about – 148° F (-100° C) which is one of the most important HYDROCARBON products of the petrochemicals industry. If polymerization is carried out under high pressure (from 100 to 300 times atmospheric pressure) low-density polyethylene (LDPE) is obtained. Polymerization at pressures close to atmospheric pressure (up to 40 atmospheres) gives high-density polyethylene (HDPE). LDPE is prepared by bulk polymerization, whereas HDPE is produced by a precipitation polymerization process. The initiator is usually oxygen in the case of LDPE and a Ziegler – Natta catalyst (named for the inventors) in the case of HDPE. In both methods, the final step is granulation. Brand names for polyethylene include Lupolen and Lotrene. The polymerization of propylene (CH2=CHCH3) proceeds under similar conditions to the method for HDPE. The vinyl chloride polymers are prepared by polymerization of vinyl chloride (CH2=CHC1) usually by a suspension or emulsion method.
Monomeric vinyl chloride is gaseous and boils at 7° F (-14° C). The process is carried out at a pressure of about ten times atmospheric pressure in large stainless-steel or glass-lined reactors. The polyvinyl chloride (PVC) product can be recovered from suspension or emulsion by any of a number of conventional drying methods. Suspension PVC is obtained as particles about 0.004 in. (0.1 mm) in diameter, whereas the primary particles of emulsion PVC are much smaller but form larger particles by agglomeration.
Styrene polymers may be prepared by almost any of the conventional polymerization methods. In the bulk polymerization of styrene, the monomer is firstintroduced into small kettles in which the polymerization reaction begins. These kettles are water-cooled, because most heat is generated at the beginning of the reaction and it is important to prevent overheating. When the polymerization is about 25 per cent complete, the reaction mass is passed into the top end of a vertically mounted cylindrical reactor where the reaction is completed. As the reaction mixture passes through this reactor, its temperature is raised from about 230° F (110° C) to about 392°F (200° C). At this last temperature, the product is fairly fluid and it is passed to an extruder which forms it into rods. The rods are then broken into granules. In the production of toughened polystyrene, a styrene butadiene rubber is dissolved in the monomer before polymerization is started.
The starting materials for the acrylic plastics are esters of acrylic acid (CH2=CHCOOH) and methacrylic acid (CH2=C(CH3)COOH) and acrylonitrile (CH2=CHCN). Methyl methacrylate (CH2=C(CH3) COOCH3) polymerized by bulk polymerization yields acrylic glass or Plexiglas. Pure or slightly modified polyacrylonitrile is a starting material for synthetic fibers, such as Orlon and Courtelle. Acrylates can be combined with various other monomers by emulsion polymerization to give latex-type products which are mainly used for finishing textiles, as well as paper and leather.