Elastomeric (Rubber) Material Professor Joe Greene CSU, CHICO

Elastomer Definition Polymers are classified as either thermoplastic or thermoset A special group of polymers can be grouped based upon a physical property rather than chemical nature Elastomers can be stretched over and over again Elastomers have the distiction of being strenched 200% and returned to original shape. Elastic limit is 200% Figure 9.1 Strain Stress Elastic Region Elastic Limit Yield Strength Ultimate Strength Plastic Rupture

Elastomers Elastomers are rubber like polymers that are thermoset or thermoplastic Thermoset: butyl rubber (natural rubber), isoprene rubber, Butadiene Rubber, Styrene Butadiene rubber, 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.

Thermoset Elastomers Natural is produced from gum resin of certain trees and plants that grow in South America (Amazon)and Asia The sap is an emulsion or latex containing 40% water & 60% rubber particles. Crude natural rubber was made into shoes, balls, coatings, and used to rub out pencil markings, hence name rubber. Crude rubber has poor thermal properties. Needs vulcanization Crude natural rubber was composed of cis-polyisoprene. (Flexible) Trans polyisoprene has different properties. Trans is harder and is called gutta percha, used for golf ball covers or shoe soles Figure 9.4 Cis polyisoprene Trans polyisoprene C CH3 H C CH3 H n n

Thermoset Elastomers Vulcanization (Greek God of fire Vulcan) occurs with the addition of sulfur (4%). Goodyear discovered the use of sulfur and heat for rubber 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. No longer and Elastomer because elongation drops below 200% Used for combs, insulation, and bowling balls

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.

Synthetic Rubber Synthetic Isoprene- basic structure of rubber Mixture of cis and trans polyisoprene Catalyst can achieve between 10% cis and 90% cis More expensive than natural rubber Butadiene- basis for synthetic rubber (BR) Butadiene rubber- no methyl group on chain Advantages are lower cost than isoprene, better low temp flexibility and compatible with other polymers, adhesion to metals Disadvantage is poor tensile strength, tear resistance and tack Butyl Rubber Neoprene Rubber Butadiene Rubber C CH3 H C Cl H n C H n n

Synthetic Rubber Butadiene- basis for synthetic rubber (SBR) Styrene-Butadiene Rubber co-polymer Copolymer is an improved butadiene rubber. Styrene stiffens the rubber. Buna-S or GSR. Used for tires, footwear, wire insulation, adhesives, gaskets, etc. Drawbackis poor oil resistance and oxidation and UV problems Used for making ABS Butyl rubber or polybutylene Advantages are high damping capability, low gas permeability, and low UV Poor compatibility with other rubbers. Used for vibration pads, inner tubes. Styrene Butadiene Rubber Butyl Rubber C H k m n CH2 C CH3 H

Synthetic Rubber Polychloroprene- neoprene rubber, Closest sythetic to natural rubber. Used for fuel lines, hoses, gaskets. Good oil resistance and good stiffness and strength Thiokol- ethylene dichloride polymerized with sodium polysulfide. Sulfur makes thiokol rubber self vulcanizing. Neoprene Rubber C Cl H n

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

Copolymers of Polyethylene Ethylene-ethyl acrylate (EEA) Properties range from rubbery to tough ethylene-like properties Applications include hot melt adhesives, shrink wrap, produce bags, bag-in-box products, and wire coating. Ethylene-methyl acrylate (EMA) Produced by addition of methyl acrylate monomer (40% by weight)with ethylene gas Tough, thermally stable olefin with good elastomeric characteristics. Applications include food packaging, disposable medical gloves, heat-sealable layers, and coating for composite packaging

Copolymers of Polyethylene Ethylene-Vinyl Acetate (EVA) Repeating groups is ethylene with an acetate functional Part of the pendent group are highly polar. Vinyl acetate reduces crystallinity and increases chemical reactivity because of high regions of polarity. Result:flexible polymer that bonds well to other materials Excellent adhesive (Elmers Glue) Other applications include flexible packaging, shrink wrap, auto bumper pads, flexible toys, and tubing C H O C = O C H n m

Copolymers of Polyethylene Ethylene-Propylene (EPM) Ethylene and propylene are copolymerized in random manner and causes a delay in the crystallization. Thus, the copolymer is rubbery at room temp because the Tg is between HDPE (-110C) and PP (-20C). Ethylene and propylene can be copolymerized with small amounts of a monomer containing 2 C=C double bounds (dienes) Results in a ter polymer, EPDM, or thermoplastic rubber, TPO C H n CH3 m

Mechanical Properties of PE Blends

Processing Properties of PE Blends

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.

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

Injection Molding Process and Cycle Time

Extruder Equipment Exit zone- die Auxiliary equipment Cooling zone 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