Elastomers - Polymer Materials - Lecture Slides, Slides of Engineering Chemistry

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Classes of Polymeric Materials

Chapter 3: Elastomers

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 distinction of being stretched

200% and returned to original shape. Elastic limit isDocsity.com

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 Docsity.com

Vulcanizable Elastomeric Compounds

• Rubbers are compounded into practical elastomers
  • The rubber (elastomer) is the major component and other components are given as weight pre hundred weight rubber (phr) - Sulfur is added in less than 10 phr - Accelerators and activators with the sulfur - hexamethylene tetramine (HMTA) - zinc oxide as activators - Protective agents are used to suppress the effects of oxygen and ozone - phenyl betabaphthylamine and alkyl paraphenylene diamine (APPD) - Reinforcing filler - carbon black - silica when light colors are required - calcium carbonate, clay, kaoilin - Processing aids which reduce stiffness and cost - Plasticizers, lubricants, mineral oils, paraffin waxes,

Synthetic Rubber

• Reactive system elastomers
  • Low molecular weight monomers are reacted in a polymerization step with very little cross-linking.
  • Reaction is triggered by heat, catalyst, and mixing
    • Urethanes processed with Reaction Injection Molding (RIM)
    • Silicones processed with injection molding or extrusion
• 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 Docsity.com

Thermoplastic Elastomers

• Four types of elastomers
  • Olefinics and Styrenics
  • Polyurethanes and Polyesters
• Olefinics (TPOs are used for bumper covers on cars)
  • Produced by
    • Blending copolymers of ethylene and propylene (EPR) or ter polymer of ethylene-propylene diene (EPDM) with
    • PP in ratios that determine the stiffness of the elastomer
      • A 80/20 EPDM/PP ratio gives a soft elastomer (TPO)
• Styrenic thermoplastic elastomers (STPE)
  • Long triblock copolymer molecules with
    • an elastomeric central block (butadiene, isoprene, ethylene-butene, etc.) and
    • end blocks (styrene, etc.) which form hard segments
  • Other elastomers have varying amounts of soft and hard blocks

Commercial Elastomers

• Diene C=C double bonds and Related Elastomers
  • Polyisoprene- (C 5 H 8 )20,
    • Basic structure of natural rubber
    • Can be produced as a synthetic polymer
    • Capable of very slow crystallization
    • Tm = 28°C, Tg = -70°C for cis polyisoprene
    • Tm = 68°C, Tg = -70°C for trans polyisoprene
  • Trans is major component of gutta percha, the first plastic
  • Natural rubber was first crosslinked into highly elastic network by Charles Goodyear (vulcanization with sulfur in 1837) - Sulfur crosslinked with the unsaturations C=C
  • Natural rubber in unfilled form is widely used for products with
    • very large elastic deformations or very high resilience,
    • resistance to cold flow (low compression set) and
    • resistance to abrasion, wear, and fatigue.
  • Natural rubber does not have good intrinsic resistance to sunlight, oxygen, ozone, heat aging, oils, or fuels.

C

H

C

C

H

H H 3 H

H

[ C C ]

C

H

C

H

H H

H

[ C C ]

CH 3

Cis

Trans

Commercial Elastomers

• Polybutadiene
  • Basis for synthetic rubber as a major component in copolymers Styrene-Butadiene Rubber (SBR, NBR) or in
  • Blends with other rubbers (NR, SBR)
  • Can improve low-temperature properties, resilence, and abrasion or wear resistance - Tg = -50°C
• 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.

H H

C

H

C

H H H

[ C C ]

H H

C

H

C

H Cl H [ C C ]

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)
• 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.

Thermoplastic Elastomers

• 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

C C H

H H

n

H H

C

H

C

H H H

[ C C ]

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Ethylene-propylene rubber (EPR)

• EPR and EPDM
  • Form a noncrystallizing copolymer
    • with a low Tg.
  • The % PP and PE units determines properties
    • Tg = -60°C for PE/PP of 67/33 to 50/
  • Unsaturated polymer since PP and PE are saturated
    • Resistant to ozone, weathering, and heat aging
    • Does not allow for conventional vulcanization
  • Terpolymer with addition of small amount of third monomer (Diene D) has unsaturations referred to as EPDM - 1,4, hexadiene (HD); 5-ethylidene-2-norbornene (ENB); diclopentadiene (DCPB) feature unsaturations in a side (pendant) group - Feature excellent ozone and weathering resistance and good heat aging
  • Limitations include poor resistance to oils and fuels, poor adhesion to many substrates and reinforcements
  • Applications include exterior automotive parts (TPO is PP/EPDM), construction parts, weather strips, wire and cable insulation, hose and belt products, coated fabrics.

C C

H H

H H

n

C C

H CH 3

H H

m

C C

H CH 2

H H

CH^ m CH CH (^3)

Ethylene Related Elastomers

• Chlorosulfonated Polyethylene (CSPE)
  • Moderate random chlorination of PE (24-43%)
  • Infrequent chlorosulfonic groups (SO2Cl)
  • Sulfur content is 1-1.5%.
  • CSPE is noted for excellent weathering resistance
    • Good resistance to ozones, heat, chemicals, solvents.
    • Good electrical properties, low gas permeability, good adhesion to substrates
  • Applications include hose products, roll covers, tank linings, wire and cable covers, footwear, and building products
• Chlorinated Polyethylene (CPE)
  • Moderate random chlorination
    • Suppresses crystallinity (rubber)
    • Can be crosslinked with peroxides
    • Cl range is 36-42% versus 56.8% for PVC
  • Properties include good heat, oil, and ozone resistance
  • Used as plasticizer for PVC

C C

H H

H H

n

C

Cl

H

m

C

S

H

k Cl

O O

FluoroElastomers

• Polyvinylidene fluoride (PVDF)
  • Tg = -35°C
• Poly chloro tri fluoro ethylene (PCTFE)
  • Tg = 40°C
• Poly hexa fluoro propylene (PHFP)
  • Tg = 11°C
• Poly tetra fluoro ethylene (PTFE)
  • Tg = - 130°C
• Fluoroelastomers are produced by
  • random copolymerization that
  • suppresses the crystallinity and
  • provides a mechanism for cross linking by terpolymerization
    • Monomers include VDF, CTFE, HFP, and TFE

C C

H F

H F

n C^ C F Cl

F F

n

C C

F CF 3

F F

n

C C

F F

F F

n

FluoroElastomers

• Fluoroelastomers are expensive but have outstanding

properties

• Exceptional resistance to chemicals, especially oils, solvents
• High temperature resistance, weathering and ozone resistance.
• Good barrier properties with low permeability to gases and vapors
• Applications
• Mechanical seals, packaging, O-rings, gaskets, diaphrams, expansion joints, connectors, hose liners, roll covers, wire and cable insulation.
• Previous fluoroelastomners are referred to as
• Fluorohydrocarbon elastomers since they contain F, H, and C atoms with O sometimes
• Two other classes of elastomers include fluorinated types
• Fluorosilicone elastomers remain flexible at low temperatures
• Fluorinated polyorganophosphazenes have good fuel resistanceDocsity.com