Elastomers Elastomers are rubbers E < 1 GPa 1. The material must be macromolecular. 2. Must be amorphous (at least at low strains). 3. Tg must be below the operating temperature. Must have low secondary forces between molecules so as to obtain the requisite flexibility. 5. A moderate degree of crosslinking must exist to establish an elastomeric network.

Polymers World usage is 15 million metric tons (1000kg) Natural rubber Natural rubber is 35% Synthetic rubber is 65%, (SBR –18%, rest is other elastomers) Natural rubber 75% goes to tires, 5% automotive mechanical parts, 10% non-automotive mechanical parts, 10% miscellaneous parts (medical and health related). Available as technically specified rubbers, visually inspected rubbers, and specialty rubbers. ASTM has 6 grades of rubber (Table I) Six grades of coagulated technically specified natural rubber which is processed and compacted into 34-kg blocks Rubber Manufacturers has further set of standards for 8 types of rubber Table II

Common Elastomers

Mechanical Behaviour of Elastomers X-linked elastomer

Natural Rubber Natural rubber in unfilled form Raw material extracted from trees Poly-cis-isoprene (40%) in water cis polyisoprene Tm = 28°C, Tg = -70°C trans polyisoprene (gutta percha) Tm = 68°C, Tg = -70°C Natural rubber in unfilled form • very large elastic deformations • very high resilience, resistance to cold flow resistance to abrasion, wear, and fatigue. Natural rubber does not have good intrinsic resistance to sunlight, oxygen, ozone, heat aging, oils, or fuels (reactive double bond). Vulcanizes with 4% sulfur

Natural Rubber Material is processed

Natural Rubber Latex is then dried, sorted and smoked

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. Additives Antioxidants, antiozonants, oil extenders to reduce cost and soften rubber, fillers, reinforcement

Vulcanizable Rubber Typical tire tread Typical shoe sole compound Natural rubber smoked sheet (100), sulfur (2.5) sulfenamide (0.5), MBTS (0.1), steric acid (3), zinc oxide (3), PNBA (2), HAF carbon black (45), and mineral oil (3) Typical shoe sole compound SBR (styrene-butadiene-rubber) (100) and clay (90) Typical electrical cable cover polychloroprene (100), kaolin (120), FEF carbon black (15) and mineral oil (12), vulcanization agent dibenzothiazyl disulphide (MTBS) dibenzothiazyl disulphide (MTBS)

Vulcanization - Sulfur and Peroxide Chemistry Curative formulations are developed by trial and error. Sulfur cures provide a wide range of properties at low cost. Peroxides provide high-temperature stability and function on saturated polymers. Sulfur Cures: applied only to unsaturated materials Peroxide Cures: can be used on most every polymer

Crosslinked Polymer Networks Vulcanization, curing and crosslinking are equivalent terms referring to the process by which individual polymer chains are transformed into a network. Most vulcanizates have an average molecular weight of about 4,000-10,000 in between crosslinks.

Elastomer Processing Compounding Banbury mixer

Elastomer Processing Preforming Molding Dipping

Natural Rubber The difficulties with natural rubber Strength Availability Bacterial breakdown Creep Residual proteins = immune response

Compression Molding Process Materials Elastomers: Thermoplastic Thermoplastic Olefin (TPO), Thermoplastic Elastomer (TPE), Thermoplastic Rubber (TPR) Thermoset rubbers Styrene Butadiene Rubber, isoprene Thermoplastic: Heat Plastic prior to molding Thermosets: Heat Mold during molding

Elastomers Styrene-Butadiene Block Copolymer Tensile Strength = 3 MPa Tensile Modulus = 130 MPa Elongation at break 550%

Oil-Resistant Elastomers Polychloroprene Polychloroprene or neoprene was the very first synthetic rubber Due to polar nature of molecule from Cl atom it has very good resistance to oils and is flame resistant (Cl gas coats surface) Used for fuel lines, hoses, gaskets, cable covers, protective boots, bridge pads, roofing materials, fabric coatings, and adhesives Tg = -65°C Slowly crystallizes & hardens below 10 °C Copolymer with 2,3-dichlorobutadiene won’t crystallize

Butyl rubber- addition polymer of isobutylene. Copolymer with a few isoprene units, Tg =-65°C Contains only a few percent double bonds from isoprene Small extent of saturation are used for vulcanization Good regularity of the polymer chain makes it possible for the elastomer to crystallize on stretching Soft polymer is usually compounded with carbon black to increase modulus

Silicones

Transfer Molding of Rubbers Transfer molding is a process by which uncured rubber compound is transferred from a holding vessel (transfer pot) to the mold cavities using a hydraulically operated piston. Transfer molding is especially conducive to multicavity designs and can produce nearly flashless parts.

Polydimethylsiloxane Silicone Rubbers Si-0 replaces C-C backbone in Chemically Inert Low conductivity Heat/cold resistant Relatively expensive X-linking increases stiffness and strength. Polydimethylsiloxane Tg = -123 °C

Calendering of Rubbers Calendering is the process for producing long runs of uniform thickness sheets of rubber either unsupported or on a fabric backing. A standard 3 or 4 roll calender with linear speed range of 2 to 10 feet/minute is typical for silicone rubber. Firm compound with good green strength and resistance to overmilling works the best for calendering.

Silicones

Vulcanization of Silicones

Thermoplastic Elastomers Five types Olefinics Fluoropolymers Styrenics Polyurethanes Polyesters Use physical cross-links to “vulcanize” the polymer

Thermoplastic Elastomers Processing involves melting of polymers, not thermoset reaction Processed by injection molding, extrusion, blow molding, film blowing, or rotational molding. Injection molded soles for footwear Advantages of thermoplastic elastomers Less expensive due to fast cycle times More complex designs are possible Wider range of properties due to copolymerization Disadvantage of thermoplastic elastomers Higher creep

Thermoplastic Elastomers Tri-block (or more) copolymers consisting of a ‘soft’ elastomeric segment and two ‘hard’ amorphous blocks. Under processing conditions, both segments are above Tg, allowing the material to flow. On cooling, separation of the phases into two domain types creates physical crosslinks between molecules. Examples include: polystyrene-block-polybutadiene-block-polystyrene segmented polyurethanes - Spandex, Lycra

Olefinic Thermoplastic Elastomers: EPDM (Ethylene-Propylene-Diene Monomer) Many of the properties of vulcanized elastomers Resiliency Elasticity More easily processed Injection molding, extrusion and other standard thermoplastic processes Highly compatible with polyolefins EPDM is crosslinked very lightly and may not be capable of being melted Ziegler-Natta Polymers 7-21 MPa Ultimate Tensile Service range: -50 °C-150 °C 100-600% elongation Diene 0-15wt%): norbornadiene, cyclopentadiene Roof liners Ground liners

Thermoplastic Elastomers: EPDM (Ethylene-Propylene-Diene Monomer)

Fluoropolymer elastomers Terpolymers Viton, Dynecon, Aflas, Kalrez, Chemraz most chemically resistant of all elastomers resistant to acids, caustics, amines, aldehydes, steam, and salt water very expensive Only available as o-rings and sheets Amorphous Viton: Hexaflouropropylene-vinylidene fluoride copolymer Use range: –40 to 200 °C   Excellent resistance to petroleum products and solvents. Very good high-temperature performance. Fluorocarbon elastomers make up the most widely used seals in the semiconductor industry. Tensile Strength 12.1 MPa, Elongation 210%

Styrene Butadiene Rubber (SBR) •Developed during WWII Germany under the name of BUNA-S. North America as GR-S,Government rubber-styrene. •Random copolymer of butadiene (67-85%) and styrene (15-33%) •Tg of typical 75/25 blend is –60°C •Not capable of crystallizing under strain and thus requires reinforcing filler, carbon black, to get good properties. •One of the least expensive rubbers and generally processes easily. •Inferior to natural rubber in mechanical properties •Superior to natural rubber in wear, heat aging, ozone resistance, and resistance to oils. •Applications include tires, footwear, wire, cable insulation, industrial rubber products, adhesives, paints (latex or emulsion) More than half of the world’s synthetic rubber is SBR World usage of SBR equals natural rubber

Oil-Resistant Elastomers NBR—Nitrile Butadiene Rubber Copolymerization of butadiene and acrylonitrile More expensive than SBR or BR Solvent resistant rubber due to nitrile C:::N Irregular chain structure will not crystallize on stretching, like SBR vulcanization is achieved with sulfur like SBR and natural rubber

Thermoplastic Elastomers: Spandex DuPont sells under the trade name Lycra hard and soft blocks in its repeat structure

Polyurethane Processing Polyurethane can be processed by Slow process: Casting or foaming, or Fast process: Reaction Injection Molding (RIM)

Polyester thermoplastics Riteflex® MT9000 series of copolyester thermoplastic elastomers (TPE) are certified for use in drug delivery systems, medical devices, pharmaceutical and other healthcare applications, as well as in repeat-use, food-contact applications

Santoprene specialty thermoplastic-elastomer resin

Mass loss