Kraton™ G1654 HU is a linear styrene-ethylene/butylene-styrene block copolymer supplied by Kraton Corporation under the designation Kraton Kraton™ G SEBS G1654 HU. The polymer contains a polystyrene block content of 31 wt% and a diblock content below 1 wt%. Unlike unsaturated styrene-butadiene-styrene block copolymers, the saturated ethylene/butylene midblock has no backbone carbon-carbon double bonds, which reduces oxidative chain scission during high-temperature melt processing and outdoor exposure. Specific gravity is 0.91 g/cm³ under ISO 2781. Melt flow rate is typically reported below 1 g/10 min at 230 °C/5 kg using ISO 1133-1:2022, indicating a high-molecular-weight architecture with pronounced melt elasticity. The grade is supplied as a clear pellet or powder and is used in hot-melt adhesives, sealants, compounded thermoplastics, bitumen modification, oil gels, and high-cohesion elastomeric formulations.

The saturated midblock also changes solvent compatibility and thermal processing behavior relative to unsaturated SBS and lower-molecular-weight SEBS grades. In solvent-based coating systems, G1654 HU dissolves in aliphatic and cycloaliphatic solvents such as cyclohexane or toluene at solids contents of 20–30 wt%; solution viscosity is shear-sensitive and rises sharply as concentration increases. In melt processing, the absence of backbone unsaturation permits continuous operation at barrel temperatures from 180 °C to 230 °C, but the low melt flow demands sufficient shear energy. The polymer should be protected from prolonged exposure above 300 °C because thermal degradation of the styrene domains accelerates beyond that threshold.

What Distinguishes G1654 HU from Lower-Molecular-Weight SEBS Grades in Compounding?

On co-rotating twin-screw extruders with 40:1 L/D, G1654 HU typically requires screw speeds between 300 rpm and 600 rpm and melt temperatures of 220–240 °C to generate sufficient shear heating for homogeneous pellet formation. Lower-molecular-weight G1652 HU can be processed at lower torque and higher melt flow, but the resulting compound has lower cohesive strength and lower creep resistance under sustained load. G1650 HU, by contrast, has an even higher melt viscosity and is selected when maximum tensile and fatigue resistance justify reduced throughput or when formulations can accommodate higher oil dilution to lower viscosity. On production lines, torque-limited operation can occur before the set barrel temperature profile stabilizes; feed-throttle adjustments and side-stuffing of fillers are used to control motor load.

When G1654 HU is blended into polypropylene at 10–25 wt%, the mixed-phase morphology improves low-temperature impact resistance and reduces post-molding shrinkage. Impact performance should be verified using ISO 180 or ASTM D256; published data for this specific configuration is limited. The 31 wt% styrene domains provide physical crosslinks that are thermally reversible, so reprocessing is possible, but the upper service temperature is governed by the glass transition of the polystyrene domains, which is typically near 95 °C. Prolonged loading near or above that threshold leads to creep and loss of dimensional stability.

Compared with unsaturated SBS grades, G1654 HU cannot be sulfur-vulcanized directly because the midblock is fully saturated. Crosslinking, when required, is normally accomplished through peroxide or radiation. This difference is significant in sealant and adhesive applications because peroxide curing adds process complexity but can improve compression set and solvent resistance when a thermoset-like response is needed.

For hot-melt pressure-sensitive adhesives, starting formulations commonly combine 20–35 wt% G1654 HU with 40–60 wt% hydrogenated hydrocarbon tackifier and up to 30 wt% white mineral oil. The high molecular weight of G1654 HU allows higher oil loading while retaining cohesive strength, but oil compatibility must be confirmed because aromatic oils migrate into the polystyrene domains and lower the upper service temperature. Loop tack is screened using ASTM D6195, shear adhesion failure temperature using ASTM D4498, and peel adhesion using ASTM D3330. Quantitative values for G1654 HU in a fully formulated adhesive are grade-specific and depend on tackifier softening point and oil aromaticity; published data for this specific configuration is limited.

In sealant formulations, fumed silica at 5–15 phr can be dispersed to raise tensile modulus and reduce slump. Tensile strength and elongation are measured on compression-molded or solvent-cast sheets under ISO 37:2017 or ASTM D638-14. Hardness is measured after 15 s using ISO 868 or ASTM D2240-15. The saturated midblock resists oxidation in outdoor sealing applications, but the formulation should be stabilized with a hindered phenolic antioxidant and, for UV exposure, a benzotriazole or hindered amine light stabilizer because the presence of aliphatic process oil can reduce UV stability if unprotected.

Stress-Strain Response and Solvent Resistance Under ISO 37 and ASTM D638 Conditions

Because G1654 HU is a physically crosslinked elastomer, its stress-strain response is nonlinear and strongly rate-dependent. Under ISO 37:2017, test specimens cut from compression-molded sheets typically show high elongation before break and a pronounced strain-induced stiffening of the styrene domains; tensile modulus at 300% strain is a more reproducible quality-control indicator than ultimate tensile strength because ultimate values are sensitive to microvoids and molding orientation. Solvent resistance is highest for aliphatic hydrocarbons and lowest for aromatic solvents, esters, and ketones that swell the styrene phase. In immersion testing, mineral oil uptake is generally low, while toluene or methyl ethyl ketone exposure causes rapid softening and dimensional instability.

Hardness and compression set depend on the ratio of styrene domains to rubber midblock. The 31 wt% styrene content gives a Shore A hardness range that is slightly higher than typical low-styrene SEBS grades and lower than polyurethane or polyamide elastomers. Compression set testing under ASTM D395-18 Method B at 70 °C for 22 h can be used to compare crosslinked or oil-modified compounds; neat G1654 HU does not have the thermoset network required for very low compression set at elevated temperature and should be compared against other thermoplastic elastomers with that limitation in mind.

The low diblock content below 1 wt% is a relevant specification indicator because diblock chains contribute to flow but do not participate in the elastomeric network. Grades with higher diblock content may exhibit lower melt viscosity but also lower tensile strength and greater surface tack. In automated feeding, dusting agents on the pellet or powder can affect loss-in-weight hopper consistency; the producer may supply dusted or low-dust forms depending on regional product configuration. For sensitive applications, incoming material should be checked for particle size distribution and moisture content before railcar or silo transfer.

When High Oil Loadings Are Required in Hot-Melt PSA Formulation

High oil loadings are used to reduce melt viscosity and raise tack, but they move the formulation toward phase separation and oil bleed. G1654 HU tolerates higher aliphatic oil addition than lower-molecular-weight SEBS grades because its longer midblock chains can accommodate more plasticizer before the styrene network weakens. Starting points of 30 wt% oil are common; above 40 wt%, oil migration to the substrate and reduced shear holding power become significant. Formulators should measure oil exudation by placing compounded film between absorbent sheets under a 500 g load at 40 °C for 24 h and record the mass change; the limit should be defined for the finished adhesive. No single oil compatibility test is prescribed by the producer, so internal specifications are typically validated using ASTM D3654 shear adhesion.

Melt viscosity reduction in high-oil formulations is controlled by oil type. White mineral oil and hydrogenated aliphatic oils are preferred because they partition mainly into the ethylene/butylene midblock. Naphthenic oils show intermediate compatibility, while highly aromatic oils solvate the polystyrene domains and reduce the effective service temperature. The same principle applies to tackifier selection: aliphatic and hydrogenated hydrocarbon tackifiers are preferred, while highly polar or high-aromatic tackifiers can disturb the phase network and reduce cohesive strength.

In bitumen modification for roofing membranes and paving compounds, addition levels of 2–6 wt% G1654 HU are used to widen the service-temperature window. Softening point is tracked with ASTM D36 and low-temperature flexibility with EN 13587. Storage stability of polymer-modified bitumen is highly dependent on mixing temperature, which is usually maintained between 170 °C and 190 °C; phase separation can occur if the blend is left unstirred below the melting point of the styrene domains. The saturated midblock provides better oxidation resistance during hot storage than SBS-modified bitumen, but processing viscosity is higher because of the grade’s molecular weight.

Regulatory status is not a single intrinsic property. The polymer may be used in food-contact applications only when the finished formulation meets 21 CFR 177.1810 and any applicable migration limits under EU 10/2011. REACH registration and RoHS compliance are relevant for articles sold in the European Union; formulators are responsible for validating final article compliance because additives, fillers, and processing aids can alter extractive profiles.

For medical device formulations, ISO 10993-5 and USP Class VI testing are performed on the finished device or compound; the raw polymer alone does not confer medical certification. If high-purity processing is required, avoid calcium stearate or other metal soaps that can affect clarity and adhesion. Pre-drying is usually unnecessary unless surface moisture exceeds 0.5 wt% after exposure to RH > 60%; in that case, drying at 60–70 °C for 2–4 h in a desiccant dryer is applied.

For injection molding of soft-touch articles, clamp force and part geometry should be evaluated together because the low melt flow of G1654 HU can produce short shots in thin sections below 1.0 mm unless gate size and melt temperature are optimized. On production-scale injection molding machines, melt temperatures of 220–250 °C and moderate injection velocities are typically required; holding pressure is maintained until gate freeze to minimize sink marks. The polymer is generally compatible with polypropylene and polyethylene in co-molding, but adhesion to polar substrates such as polycarbonate or polyamide is limited without a functionalized tie layer.

For twin-screw compounding lines with a 40:1 L/D ratio and moderate shear screw elements, barrel temperatures are typically set between 180 °C and 230 °C; die-head temperature is maintained at 210–230 °C to balance back-pressure and melt fracture. Throughput is limited by torque rather than melting capacity, and pelletizing can become unstable if the die temperature drops below the high-shear melt transition. Side-feed ports are used for mineral fillers or silica to avoid excessive screw wear, and the extruder should be purged with a lower-viscosity olefin carrier before shutdown to prevent residual G1654 HU from degrading during extended idle periods.