Kraton™ G SEBS-Comp. G7720

Kraton™ G SEBS-Comp. G7720 is a pelletized hydrogenated styrenic block copolymer compound supplied as a thermoplastic elastomer for conversion by injection molding, extrusion, and overmolding. The product designation places it within the G series of Kraton polymers, indicating a saturated midblock architecture. In chemical terms, the condensed structure comprises hard polystyrene end-blocks and a soft ethylene/butylene midblock. The absence of main-chain unsaturation in the midblock distinguishes G7720 from unhydrogenated styrene-butadiene-styrene materials and is the principal structural basis for improved retention of mechanical properties after exposure to heat, ozone, and ultraviolet radiation as assessed by ASTM D1148 or equivalent weathering protocols. The bulk morphology is phase-separated: polystyrene domains act as thermoreversible physical crosslinks below the order–disorder transition, eliminating the need for sulfur or peroxide vulcanization. At elevated processing temperatures, the styrene domains soften and the compound flows under shear; upon cooling, the network reforms. This characteristic permits regrind reuse in production environments when the material is clean, dry, and not degraded by previous heat history. Quantitative specifications are subject to the version-controlled supplier data document; if a numerical property range is not reproduced here, the omission prevents the transfer of non-authoritative values.

No supplier data sheet should be used to set screw recovery stroke without first conditioning the pellets to the plant environment. On co-rotating twin-screw extruders with barrel length-to-diameter ratios between 32:1 and 52:1, pellet melting is governed less by barrel wall conduction than by viscous shear heating. If screw speed is raised while the feed throat remains below ambient temperature, the material can generate localized melt temperatures that exceed the barrel setpoint. For unfilled SEBS compounds, melt temperatures in the 180 °C to 230 °C range are typical. Processing above 250 °C shortens the allowable residence time and increases the probability of odor generation and yellowing. Shot-to-shot consistency in injection molding improves when the nozzle, manifold, and gate zones are controlled within ±5 °C; a wider spread produces part-to-part variation in fill and surface gloss because viscosity shifts disproportionately near the styrene domain dissolution region.

Cold pellets transferred into a molding bay with relative humidity above 60 % can develop surface condensation, which may create splay or surface roughness. A dehumidifying dryer set to 70 °C for 2 h is adequate when condensation is observed. Pre-drying in an oven above 85 °C is not recommended because pellet fusion and hopper bridging can occur. When processing G7720 on reciprocating-screw injection machines, screw compression ratios in the 2.2:1 to 2.8:1 range are commonly used for unfilled SEBS compounds. Deep metering sections without sufficient back pressure may result in incomplete homogenization of the polystyrene domains. Field observations from production lines with 25 mm to 35 mm diameter screws indicate that a back pressure of 0.5 MPa to 1.5 MPa improves homogeneity, but higher back pressure increases cycle time and melt temperature. The melt-flow curve should be evaluated by capillary rheometry under ISO 11443 or ASTM D3835, not inferred from a single melt-flow-rate condition, because shear thinning behavior can vary with lot.

What Limits Hot Runner Pressure Stability During G7720 Injection Molding?

Hot runner manifold thermal uniformity is the principal processing variable in multi-drop injection tools. In unfilled SEBS compounds, viscosity is strongly temperature-dependent and shear-thinning; therefore, inter-zone temperature deviation greater than ±5 °C produces measurable differences in melt pressure across the runner. On a 150-ton hydraulic injection molding machine with a 25 mm diameter screw, gate pressure variation can exceed 15 % when the nozzle body temperature is not independently controlled. The resulting fill imbalance is often misdiagnosed as a mold venting defect. Hot runner tips should be selected with minimal dead spots because SEBS compounds can degrade after long residence in stagnant molten regions. A gradual increase in melt pressure at constant screw speed is an early indication of tip blockage or degradation residue at the gate. Purging with a commercial purging compound and then running a clean feedstock is preferred over raising barrel temperature to force the obstruction through the gate.

Injection speed should be set to avoid shear-induced surface melt fracture. Unfilled SEBS compounds can exhibit melt fracture at excessive linear gate velocities, seen as matte or wavy surface bands originating at the gate and propagating toward the end of fill. Gate geometry, not barrel setpoint, is usually the root cause when surface defects occur only at the gate. The use of valve-gated hot drops reduces drool but increases residence time in the manifold; the thermal load on G7720 is therefore higher than in cold-runner tools. If hot runner shut-off valve pins remain closed for extended cycles, the material in the manifold may undergo viscosity shift and color change. Published data for this specific configuration is limited; production validation should include consecutive-shift trials with purging intervals not exceeding 15 min of static melt residence at 230 °C.

When G7720 Replaces Vulcanized Rubber in Soft-Grip Overmolding

Two-shot overmolding onto polypropylene or other olefin substrates involves mold temperatures between 20 °C and 40 °C for many SEBS compounds. When the substrate surface is not preheated, the thermoplastic elastomer must retain enough heat to form a coherent mechanical bond before crystallization or solidification of the skin. Field reports from production lines with 80-ton to 180-ton toggle machines indicate that poor venting at the end of fill produces gas marks that are frequently mistaken for chemical incompatibility. Vent depths of 0.02 mm to 0.05 mm in the steel near the last area to fill improve reproducibility. Unlike vulcanized rubber, G7720 does not require a secondary oven cure; overall cycle time is controlled by cooling the part below the polystyrene domain re-solidification temperature before ejection. If ejection is attempted while the part is still above that temperature, the part can distort and the soft-grip thickness may vary.

Adhesion between G7720 and a polypropylene substrate is dominated by surface preparation, melt temperature, and the cooling rate at the interface. The bond is not primarily chemical; it depends on intimate melt contact and mechanical interlocking. Therefore, mold release agents should be excluded from the substrate cavity before the second shot. Surface contamination from low molecular weight amides or silicones can reduce peel performance. Where quantitative peel data are required, a tensile peel test based on ASTM D903 or equivalent fixture design is used, but a universal overmolded TPE peel standard does not exist. Compression set under clamping load should be evaluated according to ASTM D395 Method B when the grip is exposed to sustained mechanical pressure. Unlike sulfur-cured rubber, the SEBS network is thermoreversible, so long-term creep under continuous load must be considered separately.

Medical device sealing surfaces require cytotoxicity, hemolysis, and systemic toxicity data before material selection. The supplier may hold biocompatibility files; however, the finished device manufacturer is responsible for validation under ISO 10993-5, ISO 10993-10, and ISO 10993-23 after sterilization and aging. When G7720 is considered for respiratory masks, syringe grommets, or hand-held diagnostic housings, the engineering review should include the effect of gamma irradiation or ethylene oxide sterilization on tensile properties. For gamma sterilization, doses above 25 kGy can cause crosslinking or chain scission depending on the dose rate and atmosphere; the compound should be tested at the upper dose limit of the device validation protocol. Dimensional stability after sterilization is more reliably assessed according to ASTM D5994 or internal part measurement after thermal conditioning. Food-contact suitability must be confirmed under FDA 21 CFR or EU 10/2011 for the finished article, not inferred from polymer class alone.

Comparative Position Against Unhydrogenated SBS and High-Molecular-Weight SEBS

Unhydrogenated SBS grades possess midblock double bonds that are susceptible to oxidative crosslinking and embrittlement. SEBS compounds such as G7720 carry a saturated midblock and generally exhibit lower gloss loss and elongation loss in accelerated UV weathering conducted under ASTM D1148. This does not imply unlimited resistance to ultraviolet radiation; external applications still require UV stabilizer packages and should be validated by natural weathering exposure. High-molecular-weight neat SEBS polymers usually require high melt temperatures or high shear to process, and they may be extended with paraffinic oil to reduce hardness. A compounded SEBS grade such as G7720 is formulated to reduce melt viscosity relative to a high-molecular-weight neat polymer. The actual magnitude is measured by capillary rheometry under ISO 11443 or ASTM D3835, not inferred from melt flow rate alone.

Compared with highly oil-extended SEBS compounds, G7720 is positioned toward higher hardness and lower elongation. It is not a universal substitute for all SEBS grades; selection must consider target Shore hardness, compression set, creep resistance, and regulatory constraints. For parts requiring low compression set at elevated temperature, a crosslinkable thermoplastic vulcanizate or a dedicated higher-styrene SEBS grade may be more appropriate. Conversely, for overmolding onto polypropylene where low stress whitening and softness are required, G7720 offers a useful balance. Comparisons involving oil migration or fogging should be made under ISO 6452 or automotive internal standards because the compound type and oil content can influence condensable volatile release.

Material handling constraints include avoiding open flame and high-shear conditions with residence time exceeding 15 min at 230 °C. When G7720 is compounded with polyolefins, addition of incompatible plasticizers or amorphous low-molecular-weight oils can reduce heat aging. Do not combine the compound with amine-containing additives that may interfere with the thermal stabilizer package; compatibility should be verified by oven aging according to ASTM D573. The product is supplied as pellets and should be stored in a cool, dry area away from direct sunlight. Opened containers should be covered to reduce dust pickup because surface contamination can create visible defects in thin-wall applications. Compliance with RoHS Directive 2011/65/EU and REACH Regulation 1907/2006 should be confirmed from the supplier’s current regulatory statement for the specific grade and delivery form.