Thermoplastics

Important Roles of Polymers Polymers are one of the most widely used materials these days in our daily life. It is playing a more and more important role in new industrial applications. Thermoplastics – polymers that soften when heated and regain their solid form when cooled. (reversible) THermosets – polymers can pass through only one heat cycle in which thermosets undergo a curing, or setting, reaction. (irreversible)

Polymers and Polymerization Polymer – substance composed of repeating structural units called monomers that are connected by covalent chemical bonds. homopolymer – formed with identical monomers copolymer – formed with two or more types of monomers Polymerization – the process monomers bond together forming various 3D structures. ethylene

Examples of Polymers Polyethylene (plastic bag) – ethylene (C 2 H 4 ) only Polypropylene (textile) – one hydrogen atom in ethylene is replaced by methyl (CH 3 ) Poly Vinyl Chloride (PVC) (pipe) – one hydrogen atom in ethylene is replaced by chloride (Cl) Polystyrene (packaging foam) – one hydrogen atom in ethylene is replaced by Benzene (C 6 H 6 )

Polymeric Structure There are three types of different polymeric structure Linear chain – makes polymer very flexible Branched – causes strengthening and stiffening of polymer Network – molecules chains cross-link and form a network, which makes polymer can not be remelted

Classifying Thermoplastics Classifying thermoplastics is difficult with all variety existed and new kinds being developed Polyolefins – polyethylene (PE), Polypropylene (PP), Polybutylene, Polymethylpentene (PMP), Ethylene-Vinyl Alcohol (EVOH) Styrenic plastics – Polystyrene (PS), Styrene-Acrylonitrile (SAN), ABS plastics Vinyl plastics – Poly Vinyl Chloride (PVC), Polyvinylidene Chloride (PVdC) Acrylics – Polymethyl Methacrylate (PMMA) Engineering thermoplastics – Acetals, Fluoroplastics, Polyamides (Nylons), Polycarbonate,

Thermoplastic Processing Most thermoplastic processing route includes melting, molding and cooling three steps. And, there are four major processing methods Extrusion Injection molding Blow Molding Rotational Molding

Properties of Thermoplastics A tensile test can be performed on thermoplastics in the same way similar to metallic materials.

Properties of Thermoplastics Stress-Strain behavior for plastics is different from that of metals in that their properties are temperature and time dependent. Time – Viscoelasticity Creep Stress relaxation Temperature – Glass transition

Viscoelasticity of Thermoplastics Plastics have properties lie in between of metal and viscous fluids. They show a tensile property of combination of the two. Strain time relationship of ideal elastic solid Strain time relationship of viscous fluid Strain time relationship of a thermoplastic Creep – continuous deformation of thermoplastics at a constant stress or load over time Stress Relaxation – thermoplastics deformed and maintained at a given

Glass Transition of Thermoplastics Thermoplastics have two distinctly different types of mechanical behavior. Rubber Glassy The change of thermoplastics from rubber state to glassy state is known as glass transition. The temperature at which such a transition occurs is called glass transition temperature. rubber duck in liquid nitrogen

Applications Polyolefins – most widely used plastics due to low cost & mass production. Used for packaging film, wire coating, piping, house hold container, fibrics and carpets, diapers, medical devices. Styrenic plastics – Typically for consumer products & packaging due to easiness to process and low cost. Used for injection molding, housewares, shelves, medical devices. Vinyl plastics – Versatile and can be modified to suit various applications. Used for building (PVC), food film and medical packaging materials. Acrylics – Good sunlight & weather resistance. Outdoor equipments. Engineering thermoplastics – More expensive with superior properties. Used for reinforced fibers, chemical resistance (PETE), Nylons, headlight housing etc.

Recycling Plastics taking 20% of volume of landfill Non-degradable Thermoplastics can be re-melted and processed Recycling in Japan