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Styrenic Thermoplastic Elastomer
Chapter 65
Styrenic Thermoplastic Elastomer Category: Thermoplastic, elastomer. General Properties: Kraton® D and G polymers are styrenic block copolymers (SBCs) based on the feedstocks styrene, butadiene, and isoprene. Kraton® D polymers have polybutadiene or polyisoprene midblocks that provide ultimate elasticity and flexibility. Kraton® G polymers are SBCs with a fully saturated midblock. They are elastic and flexible with the additional benefits of enhanced oxidation and weather resistance.[186] Laporte AlphaGary Evoprene® G material is a styrene-ethylene-butylene-styrene block copolymer with a fully saturated midblock (i.e., no double bonds) giving excellent ozone and UV resistance.
Weathering Properties Kraton® D series rubbers are unsaturated and are therefore susceptible to attack by oxygen, ozone, and UV radiation, especially when stressed. Degradation is evidenced by surface crazing and hardening, or major cracking. Stabilizers should be added to the formulation to protect the material throughout processing and throughout its service life.[187] Kraton® D rubbers contain an unsaturated rubber midblock which is similar to natural rubber or SBR in its resistance to degradation. The two types of rubber midblocks—polybutadiene and polyisoprene— in Kraton® D series rubbers behave differently when attacked by oxygen, ozone, or UV radiation. Polybutadiene (Kraton® SBS rubbers) tends to cross-link with film, becoming hard and brittle. Polyisoprene (Kraton® SIS rubbers) tends to undergo chain scission, whereby films become soft and tacky. Blends of the two types show less change on aging than either type alone.[187] Kraton® G Polymer compounds exhibit good ozone resistance and can withstand prolonged outdoor exposure applications.[186] They contain a low level of phenolic antioxidant. Kraton® G rubbers have a saturated, olefin rubber type centerblock and
thus have good resistance to degradation. When properly formulated, they have the ability to withstand 4000 hours of weatherometer exposure with minimal change in properties. They will also pass ozone chamber testing without cracking or degradation of properties. Although the stability of these polymers is much better than that of the Kraton® D series rubbers, it is good practice to include additional stabilizers.[187] Kraton® G 1000 and G 4000 polymers can offer enhanced oxidation and weather resistance.[186] The results of bothASTM 1171 andASTM D518 ozone testing indicate that Kraton® rubber exhibits outstanding resistance to ozone.[176] All Evoprene® G compounds have excellent UV resistance when pigmented black, but some grades require extra stabilization when used in light or natural colors. Evoprene® G shows no cracks when ozone tested (100 pphm/200 hours/20% strain) and excellent UV resistance (1500 hours @ 40◦ C QUV (black)).[188]
Weathering Properties: Stabilization In choosing a stabilizer package for Kraton® materials, it should be noted that the rubber network is more susceptible to attack than the polystyrene domains. Thus it is best to use stabilizers that associate primarily with the rubber network.[187] One or more stabilizers should be added during compounding, at a level of about 0.5 phr. Ciba Tinuvin® P has been found to be effective at this level. A combination of 0.3 phr each of BASF Uvinul® 400 and Ciba Tinuvin® n 326 is particularly effective for most products, although it can cause discoloration in white stocks. With opaque products, even without UV stabilizers, the addition of up to five parts of a reflective filler such as titanium dioxide or a light-absorbing filler such as carbon black gives excellent protection.[187]
314
The Effects of UV Light and Weather on Plastics and Elastomers
Weathering Properties by Material Supplier Trade Name Table 65-1. Ozone Resistance of Kraton® Styrenic Thermoplastic Elastomer
Styrenic Thermoplastic Elastomer Category: Thermoplastic, elastomer. General Properties: Kraton® D and G polymers are styrenic block copolymers (SBCs) based on the feedstocks styrene, butadiene, and isoprene. Kraton® D polymers have polybutadiene or polyisoprene midblocks that provide ultimate elasticity and flexibility. Kraton® G polymers are SBCs with a fully saturated midblock. They are elastic and flexible with the additional benefits of enhanced oxidation and weather resistance.[186] Laporte AlphaGary Evoprene® G material is a styrene-ethylene-butylene-styrene block copolymer with a fully saturated midblock (i.e., no double bonds) giving excellent ozone and UV resistance.
Weathering Properties Kraton® D series rubbers are unsaturated and are therefore susceptible to attack by oxygen, ozone, and UV radiation, especially when stressed. Degradation is evidenced by surface crazing and hardening, or major cracking. Stabilizers should be added to the formulation to protect the material throughout processing and throughout its service life.[187] Kraton® D rubbers contain an unsaturated rubber midblock which is similar to natural rubber or SBR in its resistance to degradation. The two types of rubber midblocks—polybutadiene and polyisoprene— in Kraton® D series rubbers behave differently when attacked by oxygen, ozone, or UV radiation. Polybutadiene (Kraton® SBS rubbers) tends to cross-link with film, becoming hard and brittle. Polyisoprene (Kraton® SIS rubbers) tends to undergo chain scission, whereby films become soft and tacky. Blends of the two types show less change on aging than either type alone.[187] Kraton® G Polymer compounds exhibit good ozone resistance and can withstand prolonged outdoor exposure applications.[186] They contain a low level of phenolic antioxidant. Kraton® G rubbers have a saturated, olefin rubber type centerblock and
thus have good resistance to degradation. When properly formulated, they have the ability to withstand 4000 hours of weatherometer exposure with minimal change in properties. They will also pass ozone chamber testing without cracking or degradation of properties. Although the stability of these polymers is much better than that of the Kraton® D series rubbers, it is good practice to include additional stabilizers.[187] Kraton® G 1000 and G 4000 polymers can offer enhanced oxidation and weather resistance.[186] The results of bothASTM 1171 andASTM D518 ozone testing indicate that Kraton® rubber exhibits outstanding resistance to ozone.[176] All Evoprene® G compounds have excellent UV resistance when pigmented black, but some grades require extra stabilization when used in light or natural colors. Evoprene® G shows no cracks when ozone tested (100 pphm/200 hours/20% strain) and excellent UV resistance (1500 hours @ 40◦ C QUV (black)).[188]
Weathering Properties: Stabilization In choosing a stabilizer package for Kraton® materials, it should be noted that the rubber network is more susceptible to attack than the polystyrene domains. Thus it is best to use stabilizers that associate primarily with the rubber network.[187] One or more stabilizers should be added during compounding, at a level of about 0.5 phr. Ciba Tinuvin® P has been found to be effective at this level. A combination of 0.3 phr each of BASF Uvinul® 400 and Ciba Tinuvin® n 326 is particularly effective for most products, although it can cause discoloration in white stocks. With opaque products, even without UV stabilizers, the addition of up to five parts of a reflective filler such as titanium dioxide or a light-absorbing filler such as carbon black gives excellent protection.[187]
314
The Effects of UV Light and Weather on Plastics and Elastomers
Weathering Properties by Material Supplier Trade Name Table 65-1. Ozone Resistance of Kraton® Styrenic Thermoplastic Elastomer