Synthetic plastic thermosetting polymers

In practice, synthetic polymers are sometimes divided into two classes, thermosetting and thermo-plastic. Those polymers which in their original condition will flow and can be moulded by heat and pressme, but which in their finished or cured gtate cannot be re-softened or moulded are known as thermo setting (examples phenol formaldehyde or urea formaldehyde polymer). Thermoplastic polymers can be resoftened and remoulded by heat (examples ethylene polymers and polymers of acrylic esters). 

The first synthetic polymers were produced as by-products of various organic reactions and were regarded as unwanted contaminants. Thus the first preparations of many of the polymers now regarded as essential to our modern lifestyle were thrown away in disgust. One chemist who refused to be defeated by the "tarry" products obtained when he reacted phenol with formaldehyde was the Belgian-American chemist Leo H. Baekeland (1863-1944). Baekeland s work resulted in the first completely synthetic plastic (cal led Bakelite), a substance that when molded to a certain shape under high pressure and temperature cannot be softened again or dissolved. Bakelite is a thermoset polymer. In contrast, cellulose nitrate is a thermoplastic polymer that is, it can be remelted after it has been molded. 

The first completely synthetic plastic material was made from the condensation of phenol and formaldehyde in the presence of a catalyst. The production of this material was perfected by Leo Hendrik Baekland (1863-1944), a Belgian chemist working in the United States, and it was marketed from 1909 under the name Bakelite. Bakelite is a highly crosslinked three-dimensional thermosetting polymer, and in the 1920s and 1930s a number of similar materials were developed such as urea formaldehyde and melamine formaldehyde. 

In the following data acquisition, the same 163 standard polymer samples used in the former edition were adopted as a set of representative ones utilized in versatile fields, which include representative synthetic polymers [a) polyolefins (homopolymers) (001— 007), b) vinyl polymers with ethylene units (copolymers) (008—015), c) vinyl polymers with styrene units (016—028), d) vinyl polymers with styrene derivatives (029—035), e) acrylate-type polymers (036—049), f) chlorine-containing vinyl polymers (050-059), g) fluorine-containing vinyl polymen (060—066), h) the other vinyl polymers (067—070), i) diene-type elastomers (071—081), j) polyamides (082-090), k) polyacetals and polyethers (091—095), 1) thermosetting polymers (096—106), m) polyimides and polyamide-type engineering plastics (107—114), n) polyesters (115—126), o) the other engineering plastics with phenylene skeletons (127—138), p) sificone polymers (139—143), and q) polyurethanes (144—147)] along with some natural polymers [r) cellulose-type polymers (148-155) and s) the other some natural polymers (156-163)]. 

Commonly accepted practice restricts the term to plastics that serve engineering purposes and can be processed and reprocessed by injection and extmsion methods. This excludes the so-called specialty plastics, eg, fluorocarbon polymers and infusible film products such as Kapton and Updex polyimide film, and thermosets including phenoHcs, epoxies, urea—formaldehydes, and sdicones, some of which have been termed engineering plastics by other authors (4) (see Elastol rs, synthetic-fluorocarbon elastol rs Eluorine compounds, organic-tdtrafluoroethylenecopolyt rs with ethylene Phenolic resins Epoxy resins Amino resins and plastics). 

This chapter discusses synthetic polymers based primarily on monomers produced from petroleum chemicals. The first section covers the synthesis of thermoplastics and engineering resins. The second part reviews thermosetting plastics and their uses. The third part discusses the chemistry of synthetic rubbers, including a brief review on thermoplastic elastomers, which are generally not used for tire production but to make other rubber products. The last section addresses synthetic fibers. 

The broadest classification for plastics is the old thermoplastic and thermosetting . Examples of the former group are polyethylene, polystyrene, and poly-(methyl methacrylate) examples of the latter are urea-formaldehyde condensation polymers, powder coatings based on polyesters, epoxy resins, and vulcanized synthetic elastomers. 

The prepolymer concept, per se, is not new. In fact, the oldest synthetic polymer—Baekeland s phenolic resin—was made through an intermediate, soluble prepolymer. The major success of the newer systems is due to the use of well-defined prepolymer units and more versatile cross-linking mechanisms. The older thermoset plastics were randomly built up from their base monomers and therefore control of their growth and structure was difficult and limited. 

Synthetic polymers in general can be classified (1) by thermal behavior, i.e., thermoplastic and thermosetting (2) by chemical nature, i.e., amino, alkyd, acrylic, vinyl, phenolic, cellulosic, epoxy, urethane, siloxane, etc. and (3) by molecular structure, i.e., atactic, stereospecific, linear, cross-linked, block, graft, ladder, etc. Copolymers are products made by combining two or more polymers in one reaction (styrene-butadiene). See cross-linking block polymer epitaxy homopolymer plastics. 

Plasticizers today are used in commercially important amounts in about 35 different polymers with poly(vinyl chloride) (PVC) taking the largest share. Beyond the starting point in cellulosics, plasticizers have become highly important in poly(vinyl butyral), acrylic polymers (especially for surface coatings) and poly(vinyl acetate) (coatings and adhesives) and even in thermosets such as epoxies. Plasticizers are also used in crystalline polymers such as nylon, polypropylene, and poly(vinylidene chloride). Traditionally, extenders for rubber and synthetic elastomers were considered outside the field of plasticizers yet many such... 

Polymers can be classified in many ways, such as by source, method of synthesis, structural shape, thermal processing behavior, and end use of polymers. Some of these classifications have already been considered in earlier sections. Thus, polymers have been classified as natural and synthetic according to source, as condensation and addition (or step and chain) according to the method of synthesis or polymerization mechanism, and as linear, branched, and network according to the structural shape of polymer molecules. According to the thermal processing behavior, polymers are classified as thermoplastics and thermosets, while according to the end use it is convenient to classify polymers as plastics, fibers, and elastomers (Rudin, 1982). 

This year s U.S. production of thermoplastics, thermosets, and synthetic rubber is expected to be 29 billion pounds. About 80% of this is based on only a few common monomers. To improve performance, the polymer industry rarely changes to a new, probably more expensive polymer, but instead it shifts from mere homopolymers to copolymers, polyblends, or composites. These three types of multicomponent polymer systems are closely inter-related. They are intended to toughen brittle polymers with elastomers, to reinforce rubbers with active fillers, or to strengthen or stiffen plastics with fibers or minerals. 

---