高分子材料与应用（英文版） Chapter 6 Rubbers.pptVIP

Product Design Considerations - Draft Draft is usually unnecessary for molded parts of rubber, because its flexibility allows it to deform for removal from the mold Shallow undercuts, although undesirable, are possible with rubber molded parts for the same reason The low stiffness and high elasticity of the material permits removal from the mold Introduction for major rubbers Natural Rubber Raw material extracted from trees Natural Rubber Latex is then dried, sorted and smoked Types of natural rubber 1. RSS (烟胶片) 2. Crepes（绉胶片） 3. Air dried sheets (风干胶片) 4. Granular or crumb rubber（颗粒胶） Natural Rubber Hevea Rubber (cis-polyisoprenene) Gutta percha or Balatta (trans-polyisoprene) Natural Rubber A material from latex, in plant sap In rubber repeating unit has 5 carbons and Z stereochemistry of all C=C Gutta-Percha is natural material with E in all C=C Looks as if it is the head-to-tail polymer of isoprene (2-methyl-1,3-butadiene) Processing of NR Natural Rubber The difficulties with natural rubber Strength Availability Bacterial breakdown Creep Properties of NR Creep Solved by Goodyear in the 1800s Discovered that the polymer could be crosslinked (cured or vulcanized) by heating with sulphur Sulphur attacks this double bond - As many as 8 sulphur atoms might be in the bridge between molecules Properties of TPI Applications of NR Tires Belts Tubes Medical application Daily life commodity Synthetic Polyisoprene or Isoprene Rubber Need Supply of natural rubber disrupted during WWI and WWII Used in tires for bicycles and early cars Used the Ziegler-Natta catalyst system to improve properties The trans- or cis- nature of the rubber could be controlled up to 90% in either direction Chloroprene Rubber Chemical polymerization of isoprene does not produce rubber (stereochemistry is not controlled) Synthetic alternatives include neoprene, polymer of 2-chloro-1,3-butadiene This resists weathering better than rubber Butadiene Rubber (BR) How is this polymer different fro