Polymers: Structure, General Properties, and Applications

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 glass-

transition 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 5oo 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 materials

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-to-thickness 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