Kraton™ MD6959 HU is a linear styrene-ethylene/butylene-styrene block copolymer in the Kraton MD series, intended for applications requiring low-extractable, sterilizable elastomeric behavior. The grade is supplied without intentionally added processing oil, and supplier literature describes a styrene content near 13 wt%; the remaining block is a hydrogenated ethylene/butylene sequence. The molecular weight is high relative to general-purpose SEBS injection-molding grades, which increases melt viscosity and melt strength. In its unfilled state, the density is approximately 0.90 g/cm³ when measured by ISO 1183-1:2019, and neat compression-molded specimens typically show Shore A hardness in the range of 45–50 at a 10 s reading under ASTM D2240-15. Tensile properties on compression-molded films tested according to ISO 37:2017 are generally reported as tensile strength below 5 MPa and elongation at break above 500%; these values are representative rather than lot-release specifications. The low styrene content reduces the room-temperature modulus compared with SEBS grades containing approximately 30 wt% styrene, while the high molecular weight improves cohesive strength and creep resistance.

The property balance originates from a two-phase morphology in which polystyrene end-block domains act as thermoreversible physical crosslinks. At melt temperatures above the polystyrene glass transition, the crosslinks disengage and permit thermoplastic flow. At service temperatures below approximately 60°C under continuous load, the domains remain glassy and provide recovery after deformation. Because the midblock is hydrogenated, oxidative susceptibility is lower than that of unsaturated SBS, but the polymer is not intrinsically stabilized against prolonged thermal-oxidative aging. Compounds therefore require an added antioxidant system, typically a hindered phenol and phosphite combination at 0.05–0.2 wt% based on total compound, when processing above 200°C or when shelf-life requirements exceed 24 months.

What Limits the Compounding Window for Kraton MD6959 HU?

The primary processing limit is thermal degradation of the ethylene/butylene midblock. Melt temperatures above 240°C for extended periods promote chain scission, gel formation, and yellowing; above 260°C the degradation rate accelerates rapidly. Barrel profiles on a co-rotating twin-screw extruder with L/D ratio of 44:1 or greater are therefore set from 180°C in the feed zone to 220°C at the die, with melt temperature controlled by an infrared sensor at the strand die. Feeds containing more than 0.05 wt% moisture, determined by Karl Fischer titration, should be pre-dried at 70–80°C for 2–4 h in a desiccant dryer with a dew point below -40°C. High molecular weight generates high melt viscosity; screw configuration should include right-handed conveying elements, two or three kneading block sections, and vacuum venting at 600–700 mbar absolute to remove volatile impurities. Specific mechanical energy input in the range of 0.18–0.25 kWh/kg at screw speeds of 300–500 rpm is typical for a 40 mm laboratory twin-screw extruder; production-scale machines require higher torque because of scale-dependent shear heating. Melt pressure ahead of the screen changer is typically maintained between 30 bar and 80 bar. Underwater pelletizing with die-face cutting is preferred over strand pelletizing when melt strength is insufficient for stable strand haul-off.

When blending with polypropylene, the polypropylene phase requires a barrel profile that reaches 190–230°C, while the SEBS phase remains shear-sensitive. The recommended mixing strategy is to feed the polypropylene and SEBS together in the feed throat or to side-feed the SEBS after polypropylene has melted, depending on the rheological ratio. Side-feeding lowers the total residence time of the SEBS and reduces thermal damage. If oil extension is required, the oil is injected into the melt after the first kneading section using a heated metering pump; the injection point must be downstream of a melt seal to prevent backflow. High-viscosity SEBS can cause torque spikes if introduced into cold polypropylene; the screw should therefore be started with polypropylene until a stable melt stream is established. Capillary rheometry per ISO 11443:2021 is used to establish the shear rate dependence of viscosity before injection molding. Melt flow rate measured at 230°C under 5.0 kg by ISO 1133-1:2022 is typically below 5 g/10 min; the exact value is lot-dependent. Processors should use a high-torque extruder with screw speeds that maintain melt temperature below the degradation limit, because low screw speeds increase residence time and can generate gel particles.

Compounds softer than the neat polymer are prepared with paraffinic white mineral oil having kinematic viscosity between 50 mm²/s and 100 mm²/s at 40°C by ISO 3104. A loading of 20–60 phr lowers Shore A hardness by roughly 10–15 points per 20 phr oil and reduces tensile strength and 100% modulus. Oil absorption is improved by pre-blending pellets with oil in a heated low-shear mixer at 60–80°C for 20–40 min before extrusion. The oil must be free of aromatic fractions; aromatic oils solvate the polystyrene domains and reduce the upper service temperature. Talc or calcium carbonate at 5–20 phr can be added to reduce tack and increase modulus, but filler particle size below 5 µm is preferred to maintain clarity and surface finish. Haze and light transmittance are measured on 2 mm plaques by ASTM D1003-13; the neat polymer exhibits high clarity, but oil and filler selection controls the final optical properties.

For medical applications, compounds based on MD6959 HU are screened for cytotoxicity using ISO 10993-5:2009, for acute systemic injection using USP 88 Class VI, for hemolysis using ISO 10993-4:2017, and for intracutaneous irritation using ISO 10993-10:2010. These are component-level biological reactivity tests, not evidence of compliance for a finished device; ISO 10993-1:2018 requires a toxicological risk assessment on the final formulation and sterilization process. The hydrogenated midblock tolerates gamma irradiation at 25–50 kGy, but crosslinking or chain scission can shift tensile properties after aging. Ethylene oxide sterilization according to ISO 11135:2014 is also common, provided residues are desorbed below acceptance limits in ISO 10993-7:2008. Steam autoclaving at 121°C for 30 min can cause distortion of thin-wall parts because of creep at elevated temperature, so fixturing is mandatory if dimensional stability is specified.

Injection Molding Control Ranges for Thin-Wall Components

Unfilled and lightly oil-extended MD6959 HU compounds are processed in injection molding machines with clamp force requirements of approximately 3–5 kN/cm² of projected area. Melt temperature measured at the nozzle is maintained between 190°C and 230°C. Mold temperature is set between 20°C and 40°C to maximize clarity and minimize sink marks. Injection velocity from 50 mm/s to 150 mm/s is used; excessive shear above 100,000 s⁻¹ in small gates can produce melt fracture and surface haze. Holding pressure is typically 30–60 MPa for a duration sufficient to seal the gate, usually 3–8 s. Because the high molecular weight reduces flow length, wall thickness below 0.8 mm requires high melt temperature and gates located to produce a uniform flow front. Hot-runner systems should use externally heated manifolds with no dead spots, because stagnation of SEBS at high temperature accelerates gel formation. Cold runners should have generous diameters of 1.5–3.0 mm and vent depths of 0.02–0.04 mm to avoid burn marks. Production-scale experience indicates that batch-to-batch variation in melt viscosity can shift die pressure or fill time by 10–15%; lot certificates should be reviewed before setting injection speed and holding pressure.

MD6959 HU differs from Kraton G1650 and Kraton G1651 in styrene content and molecular weight. G1650 and G1651 contain approximately 30 wt% styrene and produce higher tensile strength and hardness, but lower softness and higher modulus in thick sections. MD6959 HU, with low styrene content, is softer and retains elastomeric recovery at lower modulus. Compared with Kraton G1657, which also has low styrene content, MD6959 HU has a higher molecular weight and lower diblock content; the lower diblock fraction reduces the soluble fraction that can migrate from the compound. This distinction matters in medical and pharmaceutical packaging where total extractables are limited. The grade does not contain maleic anhydride functionality, unlike Kraton FG series products, so adhesion to polyamide, polyester, or polycarbonate is not intrinsically improved. In overmolding applications requiring polar-substrate adhesion, a tie-layer or a functionalized SEBS is required. Unlike SBS grades, the hydrogenated midblock provides better resistance to UV and thermal oxidation, but the nonpolar surface still requires corona, plasma, or solvent-based primer for bonding to polyurethane or epoxy adhesives.

When High-Clarity Polypropylene Compounds Require a Low-Extractable SEBS Modifier

MD6959 HU is used as an impact modifier in polypropylene homopolymer and random copolymer at addition levels of 10–30 phr. The modifier is dispersed on a twin-screw extruder with a barrel profile of 190–230°C and then injection-molded into test specimens. Impact resistance is evaluated with notched Izod per ISO 180/A at 23°C and at -20°C; the low-temperature response is sensitive to modifier particle size, matrix crystallinity, and interphase thickness. For high-clarity systems, the refractive index mismatch between the SEBS phase and the polypropylene matrix should be minimized by selecting a polypropylene grade with a refractive index close to 1.50, or by limiting the modifier particle size below 1 µm. Haze values measured on 1 mm plaques by ASTM D1003-13 remain below 20% only when the SEBS domains are well-dispersed and the polypropylene clarity is high. If the compound is intended for food-contact or medical packaging, migration testing under FDA 21 CFR sections 177.2600 or 177.1520, or European Regulation (EU) No 10/2011, is required on the final formulation. Published data for this specific grade in every polypropylene matrix configuration is limited, so screening under production conditions is required.