Tham khảo tài liệu 'handbook of materials for product design part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Natural and Synthetic Rubbers Properties of Polymers Both rubbers and plastics are in the family of polymers a term from the Greek meaning many units. Table shows how properties change as an increasing number n of repeating units CH2-CH2 are joined to form a high-molecular-weight MW polymer. TABLE Properties of Hydrocarbons - CH2-Ch2 n- Chemical name n MW Appearance Melting point C Boiling point C Ethane 1 30 gas -183 -89 Butane 2 58 gas -138 0 Hexane 3 86 liquid -95 68 Decane 5 142 liquid -30 174 Eicosane 10 282 grease 38 343 Low-MW PE 25 700 grease 92 dec. Low-MW PE 75 2 100 wax 106 dec. High-MW PE 7 500 210 000 solid 120 dec. As the number of units joined together and the molecular weight increase the melting and boiling points increase and the products go from gases to liquids to waxes. Only at sufficiently high molecular weight is the polymer capable of high strength to make useful loadbearing parts. High-molecular-weight polyethylene PE is used to make milk jugs. The regularity of its structure allows adjacent chain segments to align in perfect order to form crystals which are the source of opacity or cloudiness in articles made from PE. To become rubbery and recover from large deformation the amount of crystallinity must be controlled. One approach to controlling crystallinity is to add a differently shaped co-monomer such as propylene. Ethylene propylene co- and ter-polymers EPM and EPDM respectively are rubber polymers used in weatherstrip door seals and the white sidewalls of tires. The optional third monomer in EPDM is a diene that allows crosslinking by sulfur cure systems. EPM copolymers must be crosslinked with peroxides see Fig. . Many rubbers are based on diene monomers in which only one of the two double bonds polymerizes. The double bond remaining in the poly- Chapter 6 Part 1 Figure Structure of polyethylene plastic and ethylene propylene rubber. mer prevents free rotation of the polymer chain and minimizes the .
