ABSTRACT
In Chapter 3 it is stated that “the basic structures and characteristics of polymers and plastics are attributable to five main factors such as arrangement, chemical structure, degree of polymerization (size), form, and polymerization method [1,2].” This infers that the understanding of the behavior and characteristics of polymers and plastics requires basic and fundamental understanding of polymerization methods. This chapter presents a brief discussion of the elements of the major polymerization methods, their advantages and disadvantages, the types of resins or polymers that they are used for, and the rationale for this choice. There are five major plastics resins manufacturing (polymerization) methods:
1. Bulk polymerization 2. Solution polymerization 3. Emulsion polymerization 4. Suspension polymerization 5. Gas Phase polymerization
A given polymer can be made via different methods; the method chosen depends on several factors:
1. Chemistry of reaction of monomer(s) involved with respect to toxicity, flammability, and odor
2. Heat of polymerization removal capacity of system 3. Cost-effectiveness
Monomers are chemicals, and the toxicity concerns with chemicals apply to monomeric materials. Methyl methacrylate (MMA, monomer for PMMA, polymethyl methacrylate) and other esters of acrylic acid find wide usage in the making of transparent and glass-like, nonbreakable bottles, containers, etc. The good adhesive properties of these monomers make them popular in
dentistry as denture, filling, and coating materials for teeth. Other medical applications of these esters include usage as cement for anchoring of prosthetic devices to bone [3-5]. Acrylic esters are volatile materials, and can be toxic when inhaled. It is customary for production (dental, medical facilities, and others) and polymerization plants to take safety and precautionary measures to protect workers and the work environment from toxic, volatile materials. Polyacrylamides are very popular additives for water treatment, paper manufacturing, and textile production. The monomeric material, acrylamide [6-9], is widely used in the making of acrylamide gel; acrylamide gel finds very good use in electrophoresis, a very popular protein separation technique. Monomeric acrylamide is a suspected carcinogen in humans and a known carcinogen in laboratory animals. Acrylamide is toxic to the nervous system. Polyacrylamide is considered safe but contact with monomeric acrylamide can occur via leach out, and in polymerization plants and usage units. Polyacrylonitrile [–CH2-CHCN-]n, the popular precursor for carbon fiber, has cyanide in its composition; cyanide is a known poisonous material. Polyacrylonitrile [10,11] finds other uses in the making of polymeric copolymers such as ABS, SAN, etc., and in barrier film food packaging. It is worthy of note that vinyl chloride monomer (VCM), the monomer for the making of polyvinyl chloride (PVC), a popular food packaging material and the third most utilized plastics resin (polyethylene and polypropylene are numbers one and two, respectively), is considered a carcinogen. Removal of residual monomer and containment of leachable plasticizers used for the making of PVC products are major [12,13] concerns of the plastics industry and the Food and Drug Administration (FDA). Residual monomers occur in polymerization reactions primarily due to the fact that these reactions oftentimes do not attain complete conversion. Some of the reaction methods such as suspension, solution, and bulk polymerization methods do not achieve complete conversion. Also, the process could be intentionally terminated prior to completion in an effort to achieve specified polymer properties.
