In medical fluid-transfer components, Kraton G1651 H is dry-blended with a USP Class VI paraffinic white oil, a fractional-melt polypropylene homopolymer, and a hindered phenolic/phosphite stabilizer package before twin-screw melt compounding. The linear triblock architecture contains approximately 30 wt% styrene and 70 wt% hydrogenated ethylene/butylene midblock. This saturated midblock reduces the oxidative instability observed in unsaturated SBS grades during gamma irradiation and ethylene oxide sterilization. Compounding is performed on a co-rotating twin-screw extruder with a 40:1 L/D ratio. Barrel temperatures are held between 180 °C and 210 °C. Oil is injected after the first kneading block to prevent screw slip. Polypropylene is added at 10–30 phr to raise hardness and reduce tensile set. White oil loading is adjusted from 60–120 phr to target Shore A hardness values between 20 and 80. The melt is pelletized by underwater or strand pelletizing before injection molding or extrusion.

The compound is processed into tubing segments, oxygen mask harnesses, respiratory bellows, and non-woven stretch ear loops. The high molecular weight of G1651 H produces higher melt viscosity than low-vinyl acetate EVA or low-molecular-weight SEBS. That viscosity helps retain physical properties after aggressive oil extension. Tensile strength in medical tubing compounds typically ranges from 5 MPa to 18 MPa when measured according to ISO 37:2017 on type 5A dumbbell specimens. Elongation at break remains above 400 % in most oil-extended formulations. Sterilization exposure is a critical processing boundary. Autoclave cycles at 121 °C and 103 kPa pressure for 30 min may cause softening in high-oil formulations exceeding 80 phr. Gamma irradiation at 25–50 kGy generally produces less chain scission in SEBS than in polyisoprene-containing thermoplastic elastomers, but yellowing remains measurable. Low-phenol stabilizer packages and tocopherol-based co-stabilizers are used when color stability after irradiation is required. Compounds used in respiratory circuits may be coextruded with a polypropylene outer layer for connector bonding. The PP layer also reduces surface tack and allows ultrasonic welding to rigid manifolds.

Compliance evidence for medical device compounds relies on a matrix of biological and migration tests. The compound must also meet ISO 10993-1 risk-based evaluation for patient contact duration. When the compound is used for indirect food contact, the relevant migration limit is evaluated under FDA 21 CFR 177.1810 or European Framework Regulation (EC) No 1935/2004. USP Class VI testing uses three extraction media: sodium chloride injection, vegetable oil, and water for injection. Extractions are performed at 121 °C for 1 h for plastics. Cytotoxicity is evaluated by agarose overlay or MTT assay. Heavy metals are controlled to 1 µg/mL for lead, 0.5 µg/mL for cadmium, and 0.2 µg/mL for mercury. All values are guide maxima based on pharmacopeial limit tests. The high-molecular-weight SEBS reduces oil extraction into lipid-based media when compared with low-viscosity SEBS of similar styrene content. That reduction is important for long-dwell infusion components.

What Happens When Unmodified G1651 H Meets Polycarbonate in Two-Shot Molding?

Unmodified G1651 H does not develop structural adhesion to polycarbonate, ABS, or PC/ABS blends. The hydrogenated midblock is nonpolar, and the styrene end blocks lack sufficient reactive sites for covalent bonding to aromatic substrates. A functionalized SEBS modifier is therefore required. Typical loading is 5–20 wt% of maleic anhydride-grafted SEBS or a maleated olefinic coupling resin. The functionalized polymer migrates to the substrate interface during co-molding. It provides acid or anhydride groups for hydrogen bonding and dispersive interaction with carbonate groups. Peel force data from insert-molded plaques show cohesive failure within the soft TPE layer rather than adhesive failure when the modifier content exceeds 10 wt%. Below 5 wt%, mixed-mode failures occur. The soft-touch layer is compounded from G1651 H, paraffinic oil, PP, and the adhesion promoter. Hardness is set between 35 Shore A and 65 Shore A for hand tools and consumer electronic housings. Tensile properties are measured according to ASTM D638-14. Hardness is measured with ASTM D2240 after 15 s dwell.

Oil content is kept below 80 phr to limit exudation at the substrate surface. Exuded oil forms a weak boundary layer that reduces peel resistance and causes cosmetic contamination of the rigid substrate. Pre-drying is mandatory when relative humidity exceeds 60 %. Pellets are dried at 70 °C for 2–4 h to achieve moisture content below 0.05 % before injection molding. The melt temperature window is 190–220 °C. Above 230 °C, the styrenic end blocks begin to lose physical crosslink density. Above 250 °C, gel formation and yellowing can occur. Mold temperature is maintained at 20–60 °C to control shrinkage and peel initiation stress. In two-shot molding, the rigid PC or PC/ABS substrate is shot first. The SEBS-based TPE is shot in the second barrel. The tool is typically a rotating platen or core-back mold. Clamp force must be sufficient to prevent flash at low TPE viscosity. A typical G1651 H-based grade has a melt flow rate below 1 g/10 min at 230 °C and 5 kg, which may require high injection pressure unless oil loading is raised.

Adding oil increases flow length but reduces tensile modulus. A balance is established by capillary rheometry at 200 °C. Shear viscosity at 1000 s⁻¹ commonly falls between 100 Pa·s and 300 Pa·s for soft-touch grades. Sink marks over ribs and bosses are reduced by holding pressure of 30–60 MPa. The final component is tested for surface roughness, peel resistance, and Shore A hardness after 24 h conditioning at 23 °C and 50 % relative humidity. Venting depth is kept below 0.02 mm at the closing line to avoid flash without trapping gas. Gate size for soft-touch TPE is typically 0.8–1.5 mm wall thickness. Cold runner systems are favored over hot runners because the material has a narrow thermal degradation window. The use of G1651 H without a functionalized modifier is limited to mechanical interlock designs, where the TPE fills undercuts, holes, or perforations in the rigid substrate.

For hot melt pressure sensitive adhesive formulations, Kraton G1651 H functions as the styrenic endblock network that provides cohesive strength at elevated temperature. The high molecular weight of the grade produces melt viscosity values that are difficult to handle in standard slot-die coaters without dilution. Formulators therefore operate with a binary SBC system. G1651 H contributes high-temperature shear resistance and oil immobilization. A lower molecular weight SEBS or SEPS contributes processability. The SBC phase is dissolved in a molten matrix of hydrogenated C9 tackifying resin and naphthenic or paraffinic process oil. Typical adhesive formulations contain 20–30 wt% total SBC, with G1651 H representing 30–70 wt% of that fraction. Tackifier loading is 40–55 wt%. Oil loading is 15–30 wt%. A stabilizer package of hindered phenol and phosphite is added at 1 wt%. Blending is carried out in a sigma-blade mixer or a vertical mixer equipped with high-torque helical dispersers. A batch temperature of 170–190 °C is maintained under nitrogen. The pre-melt is held until all SBC particles dissolve and the coil temperature stabilizes. Overheating above 200 °C accelerates thermal scission of the styrene blocks. This reduces solution viscosity and shear adhesion failure temperature.

Brookfield viscosity at 180 °C is measured according to ASTM D3236. For slot-die application, viscosity is typically adjusted below 150,000 mPa·s. For screen printing or bead application, formulations can tolerate higher viscosity. Open time is controlled by the ratio of tackifier to oil. High G1651 H levels extend open time slightly because the network slows melt flow after deposition. Adhesion to corrugated board, polyolefin tape backing, and coated paperboard is tested by 180° peel and loop tack procedures under ASTM D6195 and ASTM D903. Performance limits appear in high-temperature shear tests. SAFT values improve when styrene content in the total formulation remains above 8 wt%. Below this level, cohesive failure occurs at 60–70 °C. Higher G1651 H addition can push failure temperature above 80 °C. However, cold-tack performance drops when the high-MW polymer content is too high because the adhesive does not flow sufficiently at low pressure. For freezer-grade tapes, a lower-MW SEBS is used as the primary SBC. G1651 H is used as a minor rheology modifier at 5–10 wt% of total adhesive.

The saturated midblock gives better oxidative stability during continuous coating than SIS or SBS alternatives. This reduces viscosity drift over an 8 h production run. It also reduces formation of gel specks in the coating die. Residual volatiles are controlled below 0.5 wt% to meet low-VOC PSA specifications. In assembly adhesives for automotive interiors, the compound may include a hydrocarbon resin with a softening point above 100 °C to improve heat resistance. The adhesive must pass fogging tests under DIN 75201-B or equivalent automotive OEM criteria. Volatile condensation is measured on glass plates after a specified thermal cycle. High-purity process oils are required to avoid fogging failures. The high molecular weight of G1651 H reduces migration of low-molecular-weight adhesive components after bonding. That property is used in pressure sensitive adhesives for electronic display lamination, where dimensional stability and low-outgassing are critical.

When Oil Loading Exceeds 300 phr in G1651 H Gel Compounds

Oil-extended SEBS gels are produced by shearing Kraton G1651 H with high-viscosity paraffinic or white mineral oil until a three-dimensional endblock network forms. The grade is selected because its high molecular weight resists phase separation at oil loadings from 200 phr to 500 phr. At 300 phr oil, the compound is still a coherent solid with measurable tensile strength and low surface tack. Below 100 phr oil, the material is too stiff for cushioned medical pads. Above 600 phr, oil bleed becomes difficult to control. The final hardness ranges from 10 Shore OO to 50 Shore OO when measured by ASTM D2240 with a Type 000 durometer. Tensile strength ranges from 0.3 MPa to 2.5 MPa. Elongation at break exceeds 800 % across most oil loadings. Processing is carried out in a heated sigma-blade mixer or a planetary mixer with scraping blades. The mixer temperature is maintained at 120–150 °C. The polymer is added first and allowed to form a melt. Oil is introduced in 3–5 increments. Each increment must be fully absorbed before the next is added. If oil is added too quickly, a phase inversion occurs. The batch loses wall adhesion and the mixer blades rotate without dispersion. The resulting gel shows soft, oily patches and lower tensile strength.

A typical 200 L batch at 400 phr oil requires 60–90 min total mixing time. The batch is then discharged and cast into blocks or extruded through a short-barrel single-screw extruder into continuous sheet. Cooling is performed on a chilled conveyor at 10–20 °C to freeze the physical network. The crosslinking is reversible with reheating, which allows reprocessing of trim scrap. Oil bleed is the critical quality defect. It is measured by compressing a 50 mm square specimen at 70 °C between absorbent filter paper and a 5 kg weight for 24 h. Weight loss exceeding 2 wt% indicates insufficient oil retention. High-molecular-weight G1651 H reduces oil bleed compared with low-viscosity SEBS because the longer midblock chains entangle more extensively. The gel is used in anti-decubitus seating and orthopedic positioning pads. The material is tested for compression set according to ISO 815-1:2019 or ASTM D395 Method B. At 23 °C after 24 h, compression set is normally below 30 %. At 70 °C, it may increase to 60–80 % depending on oil loading. The product also passes ISO 10993-5 when a medical grade white oil and high-purity polymer are used.

Seating gel manufacturers often use a two-layer construction. A high-oil gel core is covered with a thin SEBS skin that reduces surface tack and improves cleanability. The skin is coextruded at 150–180 °C. The core is cast at lower temperature to avoid deformation. In wheelchair cushioning, the gel must maintain thickness under repeated load cycles. Dynamic fatigue testing is performed with an indenter cycling at 20–50 N for 100,000 cycles. The acceptance criterion is less than 10 % permanent thickness loss. G1651 H provides better fatigue resistance than lower-molecular-weight SEBS at equal oil content. However, the material is not suitable for sustained load-bearing applications above 60 °C because the endblock network softens. In direct sunlight on dark fabric, cushion surface temperatures can exceed this boundary. Product design must include ventilation or reflective covering.

Where asphalt roofing membranes must retain low-temperature flexibility without sacrificing summer slump resistance, Kraton G1651 H is blended into oxidized bitumen as a high-molecular-weight modifier. The SEBS polymer is added at 6–12 wt%. Mixing occurs in a high-shear rotor-stator mill or a horizontal plow mixer at 180–200 °C. A hold time of 60–120 min is required for complete dissolution. The saturated ethylene/butylene midblock provides better storage stability and oxidative resistance than unsaturated SBS. The modified bitumen shows an increase in ring-and-ball softening point measured by ASTM D36. The penetration at 25 °C measured by ASTM D5 decreases by 20–40 % relative to unmodified bitumen. Rotational viscosity at 180 °C measured by ASTM D4402 remains processable for membrane coating if the polymer loading is below 12 wt%. The high viscosity of G1651 H creates a shear-thinning melt. This helps during coating onto polyester or glass fiber carriers. The compound is applied by a heated knife coater or calendar.

In cold climates, the modified membrane retains flexibility to approximately −20 °C. The low-temperature flexibility is evaluated by bending a specimen after conditioning for 2 h at the target temperature. The failure criteria are cracks visible at 5× magnification. Ageing performance is evaluated in a rolling thin film oven according to ASTM D2872. Retention of softening point after ageing above 80 % indicates adequate dispersion. Poor dispersion is detected by large gel particles in fluorescent microscopy of the bitumen film. These particles indicate polymer-rich domains that did not fully break during high-shear mixing. Roofing compounds formulated with G1651 H are used in self-adhesive modified bitumen membranes. The SEBS network contributes elastic recovery after membrane deformation. This is measured by repeated extension cycles at 25 % strain. The residual strain after 10 cycles is lower than for oxidized bitumen without polymer. Exact values depend on asphalt source, polymer loading, and filler content. Published data for specific commercial roofing formulations is limited. Laboratory batch comparisons are required because bitumen aromaticity varies by refinery stream. Paraffinic bitumen can show poor polymer compatibility. Aromatic-rich bitumen disperses the styrene end blocks more effectively. The compound must be protected from prolonged heating above 200 °C because endblock network degradation reduces elasticity.

Halogen-Free Cable Jacketing Compounds and Their Hot-Set Boundaries

Kraton G1651 H is used as a base polymer in halogen-free flame-retardant cable jacket compounds. The saturated SEBS backbone is combined with an ethylene-octene polyolefin elastomer, polypropylene, and a metal hydrate filler system. Magnesium hydroxide and alumina trihydrate are used at total loadings of 40–65 wt%. The filler loading is adjusted to meet oxygen index targets above 30 % under ASTM D2863. High filler levels cause embrittlement in low-molecular-weight SBC compounds. The high molecular weight and high melt strength of G1651 H help the compound retain tensile properties after filler addition. Typical tensile strength at 50 wt% filler loading ranges from 8 MPa to 14 MPa. Elongation at break remains above 200 % in well-dispersed compounds. Compounding is performed on a co-rotating twin-screw extruder with a side feeder for the mineral filler. The polymer is melted before filler addition. Barrel temperatures are 170–210 °C. The filler is side-fed at the 6 to 8 barrel section to minimize viscous heating. A downstream atmospheric vent and vacuum vent remove moisture released from ATH above 180 °C.

The compound is pelletized by underwater pelletizing at 12–18 °C water temperature. Strand pelletizing is used only for low-filler formulations because strands become brittle at high filler loadings. The pellets are dried to below 0.1 % moisture before injection molding or extrusion into cable jackets. The jacket is extruded at 170–200 °C. The cable core is preheated to 40–60 °C to minimize freeze-off on the tip and die. Hot-set performance is a key boundary. The jacket must withstand a static load at specified temperature without elongation beyond the standard limit. The test is conducted according to IEC 60811-2-1 or equivalent national cable standards. A dumbbell specimen is hung in an oven at 200 °C with a stress of 0.2 MPa for 15 min. The load elongation is measured. After removal of the load and a 5 min recovery, the permanent set is measured. Compounds with insufficient crosslinking fail by excessive elongation or sample break. SEBS is not chemically crosslinked, so hot-set resistance depends on filler network and polypropylene crystalline domains. High G1651 H content improves room-temperature tear strength but may not improve hot-set beyond 120 °C.

For higher temperature cables, a reactive graft or peroxide-cured system is used. The SEBS grade is selected for jacket flexibility at −40 °C. The saturated midblock resists oxidation in service. Jackets are tested for halogen acid gas absence according to IEC 60754-1 and IEC 60754-2. Smoke density is evaluated by IEC 61034-2. The maximum light transmittance loss is specified by the cable product standard, commonly 60 % or less. Magnesium hydroxide particle size is typically 1–3 µm. Silane surface treatment improves filler dispersion and reduces viscosity build during compounding. Untreated filler can cause water absorption and surface roughness. The use of G1651 H in cable jackets is bounded by high shear heat generation. If the melt temperature exceeds 230 °C, the SEBS endblock network degrades and the jacket loses tensile strength. Extruder screw speed and barrel cooling must be controlled precisely. Published data for specific custom cable formulations is limited, so pilot line validation is required before production scale-up.