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MFGT 104Materials and Quality

Chap 5: Polymers and Elastomers

Professor Joe Greene

CSU, CHICO

MFGT 104

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Polymers and Elastomers

• Objectives– Define the common terms associated with polymers and elastomers– List the common properties of a variety of polymer and elastomer

families.– List the various manufacturing processes associated with polymers

and elastomers.– Describe the various properties used to designate or specify various

polymers and elastomers

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Definition of Plastics

All Materials

Gases SimpleLiquids

Solids

Metals Polymers(polymeric molecules)

Ceramics

ThermosetsHeat Setting

ThermoplasticsHeat Forming

Elastomers

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Early Modified Natural Materials

• Natural Rubber (gum rubber_ natural latex)– Little industrial significance compared to gutta percha

– Melted in hot temperatures and cracked in cold temperatures

– Vulcanization of rubber with sulfur (1839) by Goodyear

– Small amounts of sulfur = rubber; Large amounts = ebonite (hard)

– Civil War required large amounts of rubber

– Today- Rubber is a multi Billion $ business

• Celluloid (substitute for horn, ivory, tortoise shell)– Conversion of cotton into nitrocellulose (sulfuric acid and nitric acid): highly

nitrated is explosive, moderately nitrated is useful

– Pyroxylin is soluble in several solvents, used as a finish.

– Solid Pyroxylin called celluloid (powdered Pyroxylinwith pulverized gum camphor) heated and molded for Billiard balls.

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Automotive Applications ofPlastics and Composites

Composite Intensive Vehicles

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Automotive Plastics and Composites UseAutomotive Plastics and Composites Use

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Major Plastic Materials1994

• LDPE 6.4 M metric tons

• HDPE 5.3 M metric tons

• PVC 5.1 M metric tons

• PP 4.4 M metric tons

• PS 2.7 M metric tons

• PU 1.7 M metric tons

• PET 1.6 M metric tons

• Phenolic 1.5 M metric tons

Total 28.6 M metric tons (82% of market)

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Recycling of Plastics

• State and Federal Legislation

• PET bottle recycling

• Codes for plastics– 1 PET

– 2 HDPE

– 3 Vinyl/PVC

– 4 LDPE

– 5 PP

– 6 PS

– 7 Other

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Polymer Materials

• Polymer materials are made up of “many” (poly) repeating “units”(mers).

• Polymers are mostly based in carbon, oxygen, and hydrogen. Some have Si, F, Cl, S

• Polymers are considered a bowl of spaghetti or a bag of worms in constant motion.

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Polymer Materials• Polymers are classified broadly as

– thermoplastic: form after heat. Over and over again.– thermoset: set after heat. Only sets once. Can’t reform

• Thermoplastics– crystalline: polymers arranged in a regular order– amorphous: polymers arranges randomly like coil

• Thermosets– low molecular weight monomers that crosslink and polymerize to for polymer network

• Elastomers – Can be either thermoplastic and thermoset

• Thermoset elastomers: natural and synthetic rubbers• Thermoplastic elastomers: plastics that mimic rubber (EPDM, TPO, TPE)

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Macromolecules

• Simplest Plastic- Polyethylene (PE). – Saturated Hydrocarbon– approx. 1000 Carbon atoms in backbone (chain)– Length: ~0.0025 mm (if stretched out)– Comparison: Typing paper- 0.076 mm (30 PE on end)– Repeating unit- Mer– Degree of Polymerization= n

• DP= 500 is PE (14,000amu’s)

• DP= 9 is Paraffin wax (252 amu’s)

C C

H H

H H

n

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• Amorphous- Molecular structure is incapable of forming regular order (crystallizing) with molecules or portions of molecules regularly stacked in crystal-like fashion.

• A - morphous (with-out shape)

• Molecular arrangement is randomly twisted, kinked, and coiled

States of Thermoplastic Polymers

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Amorphous Materials

• PVC Amorphous

• PS Amorphous

• Acrylics Amorphous

• ABS Amorphous

• Polycarbonate Amorphous

• Phenoxy Amorphous

• PPO Amorphous

• SAN Amorphous

• Polyacrylates Amorphous

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• Crystalline- Molecular structure forms regular order (crystals) with molecules or portions of molecules regularly stacked in crystal-like fashion.

• Very high crystallinity is rarely achieved in bulk polymers

• Most crystalline polymers are semi-crystalline because regions are crystalline and regions are amorphous

• Molecular arrangement is arranged in a ordered state

States of Thermoplastic Polymers

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Crystalline polymers (PE, PP, PBT, PA,etc)

• Molecular Chains are in an order

• Crystalline structure forms from a single point, “spherulite”. Spherulites meet to form crystal.

• Time dependent process and consumes energy.

• Most polymers are semi-crystalline with regions of crystals surrounded by amorphous areas.

Start Intermediate Fully Grown Spherulites

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Crystalline Materials

• LDPECrystalline

• HDPE Crystalline

• PP Crystalline

• PET Crystalline

• PBT Crystalline

• Polyamides Crystalline

• PMO Crystalline

• PEEKCrystalline

• PPS Crystalline

• PTFE Crystalline

• LCP (Kevlar) Crystalline

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Carbon Chain Polymers

• Homopolymers– Simplest plastic containing one basic structure

– If X = H then Polyethylene

– If X = Cl the PVC

– If X = CH3 then PP

– If X = Benzene Ring The PSC C

H X

H H

n

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Homopolymers

• Table 3-2 Plastics Involving Single Substitutions

X Position Material Name AbbreviationH Polyethylene PECl Polyvinyl chloride PVCMethyl group Polypropylene PPBenzene ring Polystyrene PSCN Polyacrylonitrile PANOOCCH3 Polyvinyl acetate PvaCOH Polyvinyl alcohol PVACOOCH3 Polymethyl acrylate PMAF Polyvinyl fluoride PVF

Note:Methyl Group is:

|H – C – H | H

Benzene ring is:

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Carbon with Other Elements in Backbone

• Many backbones include O, N, S, or benzene rings in the main chain.

• Most are homopolymers with some copolymers and fewer terpolymers.

e.g. Polycarbonate

m

O C

CH2

CH2

C

O

O

n

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Thermosets• Thermosets are polymers that undergo a chemical reaction

during the polymerization.

• Thermosetting reaction is not reversible under heat.

• Epoxy– Standard epoxy is based on bisphenol A and epichlorohydrin. – Properties include good adhesion to many substrates, low

shrinkage, high electrical resistivity, good corrosion resistance, and thermal.

– Processing is achieved without generation of volatiles.

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Polyester and Polyurethane• Polyester– Thermoset reaction between a difunctional acid (or anhydride) and a difunctional alcohol (glycol) – Heat or radiation can trigger the cross linking reaction– Accelerators (or promoters) speed up the reaction.

– Condensation Reaction results in CO2 and H2O.

– Monomer required to polymerize, e.g., Styrene at 30% to 50% in commercial polyester systems

• Polurethane– Reaction between isocyanate and alcohol (polyol). Condensation Reaction results in CO2 and H2O.

– Crosslinking occurs between isocyanate groups (-NCO) and the polyol’s hydroxyl end-groups (-OH)– Thermoplastic PU (TPU) have some crosslinking, but purely by physical means. These bonds can

be broken reversibly by raising the material’s temperature, as in molding or extrusion.

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Polyurethane Chemistry• Reaction between isocyanate and alcohol (polyol).• Crosslinking occurs between isocyanate groups (-NCO) and the polyol’s hydroxyl

end-groups (-OH)• Thermoplastic PU (TPU) have some crosslinking, but purely by physical means.

These bonds can be broken reversibly by raising the material’s temperature, as in molding or extrusion.

• Ratio between the two give a range of properties between a flexible foam (some crosslinking) to a rigid urethane (high degree of crosslinking).

• In PUR foams density can range from 1 lb/ft3 to 70 lb/ft3.• Foams are produced by chemical blowing agents.• Catalyst are used to initiate reaction.• RIM process is used to produce fenders and bumper covers

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Applications for Thermosets• Epoxy

– Protective coatings: maintenance coatings for industrial and marine, tank linings, industrial floorings, beer and beverage can coatings, food cans, appliance primers, hospital and laboratory furniture.

– Bonding and adhesives: Automotive and aircraft industries adhesive to metals and composites.

– Molding, casting and tooling: Molding compounds in electrical and electronic industries, casting resins, potting resins. Prototype and master model tools.

– Laminating and composites: Binders in fiber reinforced laminates and composites. Laminates are used in printed wiring boards. Composite applications include filament winding (high performance pipes in oil fields, pressure vessels, tank and rocket motor housings), pultrusion, casting, and molding (graphite composites for aerospace applications)

– Building and construction: Flooring (seamless, self-leveling, or epoxy terrazzo floors), repair of bridges and roads with glass and carbon fiber wraps, concrete crack repair, coat reinforcing bars, binders for patios, swimming pool decks, and soil around oil-well drills.

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Applications for Thermosets• Polyester– Boat hulls, shower stalls, electrical components, appliances

– Recreation vehicles, automotive body panels, floor pans; SMC

– Soft tooling, patterns

– Cultured marble, buttons, corrosion resistant tanks and parts,

– Corrugated and flat paneling, simulated wood furniture, bowling balls, polymer concrete, and coatings

• Polyurethane– Rigid foams: (MDI) Laminated board stock, Moldings, Bun, Foam in place insulation,

sprayed foam, packaging – Semi-flexible foam: (MDI and TDI) Moldings, Integral-skin moldings

– Flexible foam:(TDI) Moldings, integral skin molding, carpet underlay

– Packaging: (TDI) Furniture cushioning

– Microcellular foam: (MDI) RIM parts, shoe soles

– Non-foam cast elastomers

– Coatings, binders, thermoplastic elastomers, sealants, paints

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Elastomers• Elastomers are rubber like polymers that are thermoset or

thermoplastic– butyl rubber: natural rubber– thermoset: polyurethane, silicone– thermoplastic: thermoplastic urethanes (TPU), thermoplastic

elastomers (TPE), thermoplastic olefins (TPO), thermoplastic rubbers (TPR)

• Elastomers exhibit more elastic properties versus plastics which plastically deform and have a lower elastic limit.

• Rubbers have the distiction of being strenched 200% and returned to original shape. Elastic limit is 200%

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Rubbers• Rubbers have the distinction of being stretched 200% and

returned to original shape. Elastic limit is 200%• Natural rubber (isoprene) is produced from gum resin of certain

trees and plants that grow in southeast Asia, Ceylon, Liberia, and the Congo.– The sap is an emulsion containing 40% water & 60% rubber particles

• Vulcanization occurs with the addition of sulfur (4%).– Sulfur produces cross-links to make the rubber stiffer and harder.

– The cross-linkages reduce the slippage between chains and results in higher elasticity.

– Some of the double covalent bonds between molecules are broken, allowing the sulfur atoms to form cross-links.

– Soft rubber has 4% sulfur and is 10% cross-linked.– Hard rubber (ebonite) has 45% sulfur and is highly cross-linked.

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Rubber Additives and Modifiers• Fillers can comprise half of the volume of the rubber

– Silica and carbon black.– Reduce cost of material.– Increase tensile strength and modulus.– Improve abrasion resistance.– Improve tear resistance.– Improve resistance to light and weathering.– Example,

• tires produced from Latex contains 30% carbon black which improves the body and abrasion resistance in tires.

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Synthetic Rubber• Isoprene- basic structure of rubber molecule

• Butadiene- basis for synthetic rubber (SBR) Styrene-Butadiene Rubber co-polymer

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Synthetic Rubber• Polychloroprene- neoprene rubber, closest sythetic to

natural rubber. Used for fuel lines, hoses, gaskets.

• Butyl rubber- addition polymer of isobutylene.

• Thiokol- ethylene dichloride polymerized with sodium polysulfide. Sulfur makes thiokol rubber self vulcanizing.

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Thermoplastic Elastomers• Thermoplastic Elastomers result from copolymerization of

two or more monomers.– One monomer is used to provide the hard, crystalline features,

whereas the other monomer produces the soft, amorphous features.– Combined these form a thermoplastic material that exhibits

properties similar to the hard, vulcanized elastomers.

• Thermoplastic Urethanes (TPU) were the first Thermoplastic Elastomer (TPE) used for seals gaskets, etc.

• Other TPEs– Copolyester for hydraulic hoses, couplings, and cable insulation.

– Styrene copolymers are less expensive than TPU with lower strength

– Styrene-butadiene (SBR) for medical products, tubing, packaging, etc.

– Olefins (TPO) for tubing, seals, gaskets, electrical, and automotive.

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Processing of Polymers

• Thermoplastics– injection molding, extrusion, blow molding, thermoforming,

rotational molding, compression molding

• Thermosets– compression molding, reaction injection molding, resin transfer

molding, casting, hand layup, etc.

• Elastomers– compression molding, extrusion, injection molding, casting.

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Injection Molding Process and Cycle Time

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Extruder Equipment• Exit zone- die

– die imparts shape on the material, e.g., rod, tube, sheet, channel– exit material is called extrudate– extrudate swells at end of die due to normal forces from the polymer flow, called die swell

• Cooling zone– water bath or air cooled to lower the temperature below Tg

• Auxiliary equipment – puller– rollers for proper thickness– Wind-up or cut off

Die Swell

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Compression Molding Process• Materials

•Thermosets: Polyester, Vinyl ester, or Epoxy resins with glass fiber •Sheet Molding Compound (SMC), Bulk Molding Compound (BMC)

•Thermoplastics: Polypropylene, polyester, or others with glass fibers•Glass Mat Thermoplastic (GMT), thermoplastic BMC

•Elastomers: Thermoplastic or Thermoset rubbers•Thermoplastic Olefin (TPO), Thermoplastic Elastomer (TPE), Thermoplastic Rubber (TPR)•Thermoset Styrene Butidiene Rubber

Thermoplastic:Heat Plastic

prior to molding

Thermosets:Heat Mold

during molding

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Resin Transfer Molding ProcessRef: MSU Tutorial- http://islnotes.cps.msu.edu/trp/liquid/rtm/

•Materials•Thermosets: Polyester, Vinyl ester, or Epoxy resins with glass fiber