lecture 4
thermosetting plastics
long-chain molecules in a polymer are cross-linked in a three dimensional arrangement
they are one giant molecule with strong covalent bonds
curing(cross-linked)reaction, unlike that of thermoplastics, is irreversible
polymerization process for thermosets takes place in two stages
1- in chemical plant, molecules are partially polymerized onto linear chain
2- cross-linking is completed under heat and pressure during the molding and shaping of the part
additives in plastics
to impact certain specific properties, polymers are compounded with additives
plasticizers are added to polymers to impart flexibility and softness by lowering glass-transition temperature
:stabilizers: ultraviolet radiation and oxygen which weaken and break the primary bonds which cause polymer degrades and becomes stiffer and brittle
fillers improve strength, toughness, hardness, abrasion resistance, dimensional stability
colorants added to get wide variety colors. organic(dyes) or inorganic(pigments)
flame retardants added to reduce the flammability of polymers
lubricants added to reduce friction during subsequent processing into final products. also they prevent parts from sticking to the molds
thermoplastics vs thermosets
thermoplastic
can be reversible cooled & heated, i.e recycled
heat until soft, shaped as desired then cool
thermosets
when heated forms molecular network cross linking (chemical reaction)
degrades (not melt) when heated
prepolymer molded into desired shape, then (curing/crosslinking) chemical reaction occurs
examples: (polyethylenes, polypropylenes, polystyrenes, polyvinyl (pvc), polyesters)
examples: (epoxies, silicones, phenolics, polyesters)
elastomers (rubbers)
biodegradable plastics
biodegradability means that microbial species in the environment will degrade a portion of polymeric material under proper environmental conditions
there are three different biodegradable plastics
lactic-based system
fermentation of sugar
starch-based system
biodegradability will divert attention from recycling plastics and conservation of material and energy
elastomer is capable of recovering substantially in shape and size after the load has been removed
rubber is capable recovering from large deformation quickly
hardness of elastomers increases with the cross-linking of the molecular chains (e.g vulcanization of rubber with sulfur)
hysteresis loss in stretching and compression gives the rubber the capacity to dissipate energy, damping vibration and absorb shock load
natural rubber
base for natural rubber is latex
has good resistance to abrasion and fatigue, and high friction, low resistance to oil, heat, ozone and sunlight
synthetic rubber
have better resistance to heat, gasoline, and chemicals, higher range of useful temperatures