Polymers Structure General Properties and Applications Polymer Plastics

Polymers: Structure, General Properties, and Applications

Polymer • Plastics meaning can be molded and shaped Most products have a component made of some type of polymer.

generally characterized by: • Low density. • Low strength and stiffness. • Low electrical and thermal conductivity. • Good resistance to chemicals. • High coefficient of thermal expansion.

(cont’d) • Many colors and transparencies • Low in cost. • Can be machined, cast, formed, and joined • Minimal surface-operations required advantage over metals

Structure of Plastics • Plastics are essentially a series of linked or cross-linked molecules • Poly-mer, meaning many-units (mers).

Monomer: • basic building block of polymers

Polymerization: • Chemical reaction where monomers are linked to form larger molecules.

Bonding • Determines overall strength of a polymer

Classification of Polymers • Thermoplastics • Thermosets • Elastomers

Thermoplastics • Can be reshaped after heated to glass temperature, returning to original hardness and strength after cooled.

• Repeated heating/cooling leads to degradation (thermal aging). • Can undergo large uniform deformations in tension before fracture, which enables the forming of complex shapes.

• Like metals, when deformed, molecules align in the direction of the elongation, becoming stronger along the stretched direction, yet weaker in the transverse direction (anisotropic behavior). • Deformation leads to strength in the direction of elongation

Thermosets • Curing reaction is irreversible, shape is permanently set, thus the term thermosetting plastic.

Behavior of Thermosets • Generally, thermosetting plastics have better mechanical, thermal, and chemical properties, electrical resistance, and dimensional stability than thermoplastics.

• If temperature is increased sufficiently, thermosetting polymers burn, degrade, and blacken.

Elastomers (Rubber) • Amorphous polymers (low glasstransition temperature). • Ability to undergo large elastic deformation without rupture, recovering original form and dimension after loads have been removed, even after large deformations.

• Elastomers are Cross-linked at elevated temperatures (vulcanization), cannot be reshaped.

Types of Elastomers

Natural rubber • Latex base, sap obtained from tropical tree bark, good resistance to abrasion and fatigue, low resistance to oil, heat, ozone, sunlight.

Synthetic rubber Further developed natural rubber • improved resistance to heat, gasoline, and other chemicals. •

Additives • Used to impart specific properties to polymers

Fillers • Reduce cost, improve properties • fine sawdust, silica powder, clay, cellulose, glass, asbestos

Plasticizers • impart flexibility and softness by lowering the glass transition temperature.

Antioxidants • Used to prevent or slow degradation from ultraviolet radiation and oxygen.

Colorants

Flame Retardants • Chlorine, bromine, phosphorus

Lubricants • Reduce friction during processing • Linseed oil, waxes, soaps

Traditional Thermoplastic Processing Techniques

Extrusion • Screw extruders • Feed section • Melt section – friction and heating elements • Metering or pumping section – pressure buildup at the die • Water and air cooling often used after the extrusion

Extrusion Processes • Plastic tubes and pipes • Use of a spider die • Coextrusion • Simultaneous extrusion of 2 or more polymers • Plastic coating electrical wire • Electrical cables and paper clips • Polymer sheets and films • Flat extrusion die (coathanger die) • Sheet = thicker than. 5 mm

Extrusion Processes Cont. • • • Thin film • Film = thinner than. 5 mm • Blown film process Plastic films • Such as teflon • Produced by shaving the circumference of a solid plastic billet • Much like veneers from logs • Called “skiving” Pellets • Used as the raw material for many thermoplastics processing techniques • Small diameter is extruded and chopped into pellets

Injection Molding • Use of hydraulic plunger or rotating / reciprocating screw (extruder)

250 Ton Machine

Molds • complex, several cavities • “multicomponent injection molding – various colors and shapes (rear light covers – autos) • Insert molding – metal hardware – (auto elect components • Overmolding – process of making hinges and ball & socket joints in one operation • Use of different materials that won’t bond • Use of cooling inserts between the material to avoid bonding called “ice cold molding”

Blow Molding • Modified extrusion and injection molding • Extrusion blow molding Tube (preform) is extruded into mold cavity • clamped in mold cavity with a larger diameter • blown outward with hot air blast to fill mold cavity • Drums up to 5 oo gal. •

Extrusion Blow Molding

Injection Blow molding • Parison is injection molded (may be stored for later use) Mold opens – parison loaded and mold closed Hot air blown into parison and expands to parison to mold walls • Beverage bottles, shampoo, etc. • • • Multilayer blow molding Coextruding tubes (parisons) for multilayer structures • Packaging for food and beverages • Odor, taste, aroma protection, scuff resistance, capabilty for printing

Injection Blow Molding

3 Station Injection Blow Molding

Rotational Molding • Large hollow parts • Premeasured powdered polymer placed inside warm mold • Mold is heated and rotated in oven • Catalyst is added for thermosets • Inserts may be molded integrally • Trash cans, large hollow toys, carrying cases, footballs

Thermoforming • Forming thermoplastic extruded sheets or films over a mold • Use of heat and pressure • Sheet is clamped • Heated to sag point • Forced against mold by vacuum or air pressure • Packaging trays for cookies & candy • Advertising signs, refrigerator liners, shower stalls • No parts with holes

Casting • Thermoplastics and thermosets cast into rigid or flexible molds • Typcical parts are gears, bearings, wheels, lenses, etc. • Potting and encapsulation Casting the polymer around an electrical component to embed it in plastic (coils or transformers) • Potting: housing becomes part of final product • Encapsulation: component is coated with a layer of polymer, surrounding it completely • Both of these processes - the plastic serves as a dielectric (nonconductor) •

Foam Molding • Raw material is expandable polystyrene beads • Polystyrene beads obtained by polymerization of styrene monomers are placed in a mold • Exposed to heat (steam) • Beads expand as much as 50 times original size to fill mold cavity • Thermoplastics & thermosets can be used but thermoplastics are in liquid form so the process is much like RIM • Products: styrofoam cups, food containers, insulating blocks, packaging

Foam Molding Cont. • Structural foam molding Thermoplastics are mixed with a blowing agent (inert - nitrogen) • Injection molded into cold molds • Rapid cooling against the cold mold produces a rigid skin and a cellular core • Products: furniture components, computer and business machine housings, construction trim and moldings •

Composite Materials • Combination of two or more chemically distinct and insoluble phases whose properties and structural performance are superior to those of the constituents acting independently.

Examples • addition of straw (reinforcement) to clay (matrix) for making mud huts and bricks for structural usage. • Concrete reinforced with re-bar or wire mesh

Consist of a matrix and reinforcement combined

Categories • Polymer matrix composites • Thermoplastics • Thermosets • Metal matrix composites (MMC) • Ceramic matrix composites

Types of Polymer Matrix materials • • Polyester Epoxy Phenolics Carbon

Functions of the matrix material • Support the fibers in place • Protects the fibers • Transfer stresses

Reinforcements • Give composites their directional qualities • Very anisotropic • Fibers very strong and stiff in tension • Molecules oriented in long direction

Types of Reinforcements • Glass • Carbon • Aramid (Kevlar)

Reinforcement Forms • Fibers (dia usually less than. 0004” ) • Particles • Whiskers (tiny needlelike single crystals grown)

Also classify fibers as: • Continuous • Discontinuous

Fiber forms • Chopped • Yarns and Roving – twisted and untwisted strands of fibers • Woven fabric • Mats

Other Composites

Metal Matrix Composites • high resistance to elevated temperatures. • high ductility and toughness • Aluminum, aluminum lithium, magnesium, titanium

Ceramic Matrix Composites • resist high temperatures and corrosive environments.

Honeycomb Structures • core of honeycomb, or other corrugated shape, bonded (by adhesives to thin outer skins) • High strength to weight ratio • Core carries compressive loads, faces carry the bending loads

Applications of composites • Aircraft / Aerospace • Sporting goods • Construction • Transportation • Marine • Biomedical – Artificial limbs

Composites Processing Techniques • Thermosetting Polymers

Composites Raw Material Forms • Prepregs • Fibers pre-coated in resin and rolled into sheet or tape. • Sheet Molding Compounds (SMC) • Chopped fibers deposited on layer of resin, then pressed on rollers.

Reinforced Plastics (Cont. ) • Bulk molding compounds (BMC) • Similar to SMC but in billets. Formed like dough • Thick molding compounds (TMC) • Combines characteristics of BMC (cheep) with SMC (strength) and injection molding. Electrical components.

TMC

Molding techniques

Contact Molding • Parts with high surface area-tothickness ratios (swimming pools, tubs, shower units), • usually hand layup or spray- up

Vacuum-Bag Molding • Vacuum used to form shape of plastic (prepregs) to mold

Automated Tape lamination • Multiaxis CNC Machines

Compression molding • Also called matched die molding • open mold process • material placed between molds & pressure applied

Resin Transfer Molding • Resin mixed with a catalyst is forced through a piston pump into a mold cavity with fiber reinforcements.

Reaction injection molding (Thermosets) • Two reactive resin components are mixed and then injected into a closed mold • Typical parts Auto bumpers, fenders, steering wheels, instrument panels, water skis, etc. • Parts up to approx 110 lbs •

Filament Winding • Resin and fibers combined and wrapped around mandrel. pipes, tanks.

Pultrusion • Resin and fibers pulled through a thermosetting polymer bath and a heated die. • Polyester and glass reinforcements are the most commonly used materials. • Production of long shapes with various continuous profiles rods, tubing, ladders, golf clubs, handrails.

Pulforming • After resin and fabric (fibers) are pulled through a polymer bath, product is clamped by two dies and then cured. • Non constant cross sections.

Metal Matrix Composites • Liquid Phase Processing • Casting of liquid matrix and solid reinforcement. • Solid phase processes • Fibers and powder metals tungsten carbide in cobalt matrix for tools.

Ceramic Matrix Composites • Slurry Infiltration • Fiber preform is hot pressed and impregnated with a matrix