New-Generation SEBS Has Processing Advantages
Kraton Polymers has launched a new styrene-ethylene-butadiene-styrene (SEBS) elastomer family that delivers better flow, lower warpage, and greater adhesion to other thermoplastics. New Kraton A resins are said to open avenues for replacing flexible PVC, TPU, and silicone elastomers.
Kraton A polymers draw on a new chemistry that intermingles a specially controlled distribution of styrene with the ethylene-butadiene mid-block of SEBS without loss of critical elastomeric properties. Relative to conventional SEBS, the new resins elevate stiffness, improve processability, shrink more isotropically (MD vs. TD), and adhere more strongly when overmolded onto many engineering thermoplastics.
Two initial grades, RP6936 (39% styrene) and RP6935 (58% styrene), are available for compounding, molding, and extrusion. The first is a 65 Shore A grade that can be compounded to provide improved flow and clarity similar to PVC of equal hardness. RP6935 has 85 Shore A hardness, higher molecular weight, and superior heat resistance.
Dale Handlin, senior staff researcher, says several compounders are experimenting with Kraton A, including GLS Corp., Kraiburg TPE, Multibase, and Teknor Apex Co.’s TPE Div.
Better flow, less warpage
Kraton A reportedly delivers higher initial stiffness and toughness levels than are typical for conventional SEBS, such as Kraton G. In addition, the new SEBS has easier flow than comparable Kraton G grades, as revealed by lower shear at a given viscosity and reduced molded-in stresses, both of which increase formulation latitude. One practical effect is that mold filling and reproduction of detail are superior with Kraton A, making it suitable for complex parts.
Lower molded-in stress makes Kraton A grades more consistent and balanced in shrinkage behavior than conventional SEBS. Isotropic shrinkage is said to correlate with the ratio between tensile strengths in ma chine and transverse directions. A ratio of 1 indicates equal strength in both directions. RP6935 has an isotropic ratio close to one versus 2.5 for a comparable Kraton G (see graph). The iso tropic behavior of Kraton A grades is beneficial in large, flat parts where war page is especially noticeable.
Finally, the new chemistry imparts a higher polarity to Kraton A grades than Kraton G equivalents. As a result, Kraton A grades have higher adhesion when overmolded onto various substrates, especially styrenic resins like PS, HIPS, and PPO/PS alloys.
One target for Kraton A is flexible PVC in toys, packaging, medical parts, sports gear, and construction products. “Kraton A molds a lot like PVC,” says Handlin. Kraton A can simply be dropped into existing PVC molding and (in some cases) compounding equipment. The hardnesses of Kraton A grades are comparable to those of many PVC compounds. The higher cost/lb of SEBS versus PVC puts the focus on applications where health concerns have been raised about PVC, such as medical parts and soft toys.
Garret Davies, Kraton Polymer’s v.p. for the Americas, says there is also potential for replacing liquid silicones in baby-bottle caps, computer keyboard under-pads, and medical tubing. There is also potential competition with TPUs used as compatibilizers or for overmolding in soft-touch parts.
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