Polyamides (nylon) are the oldest and largest volume engineering polymers. Nylon was introduced in 1938 by DuPont as the world's first synthetic fiber, and by 1941 the company had introduced the first injection moldable grades. There are many types of thermoplastic polyamides commercially available. Commonly used products are designated as nylon 6; 6,6; 6,9; 6,12; 11; and 12, with the nomenclature designating the number of carbon atoms that separate the repeating amide group. Two basic reactions are used to synthesize polyamide engineering polymers: (1) polycondensation of a dibasic acid and a diamine or (2) polymerization of an amino acid or lactam. The most widely used nylon polymers are semicrystalline products with molecular weights of 10,000-40,000 and chemical structures in which amide linkages connect aliphatic chain segments. Nylons provide a mix of properties. Generally, the polyamide analogs exhibit good chemical resistance and low moisture absorption at the expense of heat resistance, impact properties in wet environments, and stiffness. All polyamides are hygroscopic to some extent. Water acts as a plasticizer in polyamides, reducing most mechanical and electrical properties while improving toughness and elongation. This problem, perhaps the major shortcoming of the nylons, is a function of the concentration of the amide groups. Water actually replaces the amide-amide hydrogen bonds with an amide-water hydrogen bond. Nylon 6 and nylon 6,6 provide very good mechanical and thermal properties in their dry-as-molded state, but are most susceptible to deterioration due to moisture absorption. They also provide many desirable properties to fulfill end-use requirements and account for the major share of the polyamide resins sold. Although property differences do exist between the materials, they are similar enough that the selection of 6 or 6,6 for an application is largely a matter of customer preference for either product. Production economics are presented for both batch and continuous processes for nylon 6 and nylon 6,6. For the continuous polymerization of nylon 6 and 6,6, a two-stage process was selected since this process can produce over 95 percent of the nylon base resins required by engineering thermoplastic applications, avoiding the need for solid stating. The facilities modeled consisted of two identical 30,000 metric ton lines. A 30,000 ton line is representative of nylon 6 polymerization lines presently being installed in the industry. A 30,000 ton line was also selected for nylon 6,6 although this size presently represents the upper limit in scale. Nylon 6,6 is more sensitive to thermal oxidation and degradation than nylon 6, and this serves to limit the size of reactor vessels. A total site containing 60,000 tons of nylon polymerization capacity was selected so as to generate some economies in utilities, infrastructure, material handling, and overheads. To complete the analysis, batch nylon 6 and 6,6 economics for 60,000 ton plants were also developed. These were developed for a hypothetical facility containing three lines, each with 4 autoclaves (thus sharing the downstream extrusion equipment). In practice, such a plant would probably not be built, as a facility that size would almost certainly utilize continuous polymerization lines for at least part of its production. From the economics developed for the four cases it is clear that nylon production via the batch process is at an economic disadvantage compared to resin made by the corresponding continuous process. The impact of this analysis is evident in the industry, since batch nylon production is practiced predominantly to make short production runs or by producers with old, depreciated batch equipment. Recent new nylon 6 or 6,6 polymerization capacity uses continuous lines partly for economic reasons, but also partly due to the problems in maintaining consistent product quality inherent in batch processes. | 
