In styrene–ethylene/butylene–styrene (SEBS) hot-melt adhesive compounding, Kraton™ G1651 HU is introduced as the linear high-molecular-weight base polymer where the formulation requires cohesive strength retention across a broad service-temperature window without sacrificing slot-die processability. The grade is selected for hot-melt adhesives rather than low-viscosity radial or diblock-rich SEBS alternatives because the higher molecular weight produces a stronger styrene-domain network and better creep resistance after the adhesive is applied to polyethylene films and corrugated substrates. Typical addition of G1651 HU in hot-melt adhesives ranges from 15 wt% to 35 wt% of the total formulation, with hydrogenated hydrocarbon tackifier at 35–55 wt%, naphthenic or white mineral oil at 10–25 wt%, and a hindered phenolic antioxidant at 0.3–1.0 wt%. The final adhesive for non-direct food-contact packaging falls under FDA 21 CFR 175.105 for adhesives, while the use of white mineral oil in the formulation is evaluated under FDA 21 CFR 178.3620(a) and the complete compound must meet REACH registration obligations for the European market. Published data for this specific formulation configuration is limited, but production-scale compounding records indicate that the viscosity and heat-resistance window is reproducible when the tackifier softening point is matched to the styrene content of G1651 HU.

Production-scale compounding is carried out on a co-rotating twin-screw extruder with an L/D 40:1–52:1 barrel length and segmented screw elements fitted with two or three kneading blocks in the melting zone. Barrel temperatures are profiled from 140 °C at feed to 190 °C at discharge, while melt temperature is maintained below 210 °C to limit styrene-domain degradation and below 230 °C to avoid measurable chain scission of the saturated ethylene/butylene midblock. The tackifier is pre-blended with oil and antioxidant before being injected into the melt phase after the polymer has passed through the first kneading block, and the screw speed is normally set at 250–400 min⁻¹ with a specific mechanical energy input of 0.12–0.18 kWh/kg. Once compounded, the adhesive is either pelletised and re-melted in a slot-die coater or fed directly to a gear pump with filtration through a 100–150 µm screen pack. In slot-die coating, the pump outlet pressure is kept above 35 bar to maintain a stable coat weight of 15–50 g/m² on silicone-coated release liner. A process conflict emerges when G1651 HU is loaded above 35 wt%: low-shear viscosity at 180 °C under ASTM D3236 rises beyond 60,000 mPa·s, and the adhesive can exhibit edge waving and web breaks at coating speeds above 250 m/min. Conversely, below 15 wt% the formulation loses heat resistance and cohesive strength, with shear adhesion failure temperature under ASTM D4498 falling below 65 °C.

Finished article types produced from these hot-melt adhesives include elastic strand attachment adhesives for baby diapers and adult incontinence briefs, case and carton sealing adhesives for corrugated packaging, and pressure-sensitive label adhesives where creep resistance at warehouse temperatures above 50 °C is required. In diaper lamination, the adhesive is applied discontinuously by a multi-bead nozzle at 160–175 °C onto polyethylene backsheet, followed immediately by elastic strands drawn at 250–350% elongation; the linear high-molecular-weight SEBS delays bond failure during elastic retraction and reduces adhesive bleed through the nonwoven layer.

What Limits Storage Stability When G1651 HU Replaces SBS in Bituminous Membranes?

In bituminous waterproofing and roofing membrane production, Kraton™ G1651 HU is evaluated as a high-molecular-weight saturated alternative to SBS when improved heat aging and UV resistance are required, but low-temperature flexibility and storage stability become binding constraints. The formulation addition ratio in polymer-modified bitumen is typically 8–16 wt% G1651 HU relative to bitumen, with optional addition of 1–3 wt% linear low-density polyethylene or Fischer-Tropsch wax to reduce phase separation during vertical storage. Compliance for the finished membrane is anchored to EN 13707:2013 for flexible sheets for waterproofing, ASTM D6164/D6164M-21 for polyester-reinforced polymer-modified bitumen sheets, and EN 1426:2015 for needle penetration. Where the membrane is used in potable-water reservoirs, additional testing under EN 13632 or local drinking-water approval is required, and the compound must not release mineral oil above the specified migration limit.

The mixing process is carried out in a vertical thermic-fluid-heated mixer at 180–200 °C with a high-shear rotor-stator head operated at 1,500–2,500 min⁻¹ for 90–150 min until the polymer particles are reduced below 10 µm as verified by fluorescence microscopy. The critical threshold is phase inversion: at addition below 8 wt%, G1651 HU forms a discontinuous polymer-rich phase, and the elastic recovery under ASTM D6084-18 remains below 40% because the bitumen continuous phase dominates. Above 12 wt%, a semi-continuous polymer network develops, but the high molecular weight of G1651 HU raises the zero-shear viscosity at 180 °C to values that may exceed 3,000 mPa·s, and pumping through in-line homogenisers into the peroxide-crosslinking calender becomes difficult. In addition, storage stability in vertical tanks at 170 °C without agitation shows a density difference between polymer-rich and bitumen-rich phases, with top-bottom softening point differences exceeding 8 °C under ASTM D36/D36M-14 after 72 h when the addition is above 14 wt% and no compatibility agent is present. This is the main operational boundary: maintain continuous low-shear mixing or add 0.5–1.0 wt% of a maleic anhydride–grafted polyolefin to reduce coalescence and stabilise the morphology.

Terminal products from this application include torch-applied bituminous waterproofing membranes for concrete decks, self-adhesive roofing underlayments, and bridge-deck bridge waterproofing sheets. In each case the polyester or glass-fibre reinforcement is impregnated with the G1651 HU-modified bitumen compound at 170–190 °C on a continuous impregnation line, and the final rolls are cooled below 35 °C before slitting to prevent blocking.

Injection-Moulded Soft-Touch Compounds Based on SEBS and Polypropylene

Compounds for soft-touch over-moulding and flexible automotive interior components are produced by melt blending Kraton™ G1651 HU with polypropylene homopolymer or random copolymer, white mineral oil, and inorganic fillers. The addition ratio of G1651 HU is 25–50 wt%, polypropylene 15–35 wt%, white mineral oil 20–40 wt%, calcium carbonate or talc 0–15 wt%, and stabiliser package 0.2–0.5 wt%. The resulting compound is tested for melt mass-flow rate under ISO 1133-1:2022 at 230 °C/2.16 kg, with target values of 8–25 g/10 min for thin-wall injection moulding. Mechanical properties are measured according to ISO 37:2017 for tensile strength, with values between 6 and 14 MPa depending on oil loading, and ISO 868 for Shore A hardness in the 40–75 A range. For automotive interior components where odour and fogging are constrained, the compound must meet VDA 270 and VDA 278 as specified by OEM material codes.

Production equipment includes a co-rotating twin-screw extruder with L/D 40:1–60:1, screw diameter typically 40–75 mm, and vacuum venting at −0.08 MPa to strip volatiles from mineral oil. The oil is injected downstream after the first melting section to preserve shear and dispersion; barrel temperatures are set from 160 °C at the feed throat to 210 °C at the die. Pelletising is performed by an underwater die-face cutter with 2.0–3.5 mm hole diameter. In injection moulding, the material is dried at 80 °C for 2–3 h only when exposed to relative humidity above 60%; otherwise moisture uptake is negligible. Mould surface temperature is maintained at 25–50 °C, and injection speed is profiled through the gate to avoid jetting and visible weld lines on textured surfaces. Published data for this specific configuration is limited, but the property window is reproducible on production-scale equipment when the oil viscosity is held between 40 and 100 cSt at 40 °C.

Finished product types include automotive soft-touch dashboards, door trim inserts, gear lever boots, cable grommets, and power-tool over-moulded grips. The high molecular weight of G1651 HU provides low compression set at 70 °C/24 h under ISO 815-1:2019, typically below 35% at Shore A 55, which is relevant for sealing lips and grips that must recover after repeated deformation. The material also exhibits stable tactile properties after short-term heat exposure at 100 °C for 500 h, with tensile retention above 85% under ISO 37:2017.

When Gamma-Sterilised SEBS Compounds Must Retain Tensile Strength After 25 kGy

Medical articles produced from Kraton™ G1651 HU are compounded without heavy-metal stabilisers and with white mineral oil or polyisobutylene as plasticiser, when required, to satisfy biocompatibility screening. The addition ratio of G1651 HU in medical-grade compounds ranges from 30 to 60 wt%, with polyethylene or polypropylene 10–30 wt%, white mineral oil 10–40 wt%, and silica or barium sulfate 0–20 wt% for radiopacity. Compliance is anchored to ISO 10993-1:2018 for biological evaluation, ISO 10993-5:2009 for cytotoxicity, ISO 10993-10:2010 for skin sensitisation, and USP <88> Class VI for plastics with prolonged patient contact. The raw material is also evaluated under 21 CFR 177.1810 where the final article is intended for repeated-use food-contact applications, though the final compound and article remain subject to end-use testing.

Downstream processing for tubing and film is typically performed on a single-screw extruder with L/D 24:1–30:1, a barrier screw with Maddock mixing section, and a screen changer with 100–150 µm mesh. Extrusion temperatures range from 180 °C to 220 °C, and the melted material is formed through an annular die for tubing or a coat-hanger slot die for film with a thickness of 0.10–0.50 mm. For injection-moulded luers and stoppers, a closed-loop injection moulding machine with 100–200 t clamp force is used, with melt temperature 200–225 °C and holding pressure 40–60 MPa. Gamma irradiation at 25–40 kGy is the common sterilisation route; G1651 HU has an aliphatic midblock, which resists crosslinking-induced embrittlement better than unsaturated block copolymers, but the molecular weight and high styrene-domain order can cause a measurable increase in tensile modulus under ISO 37:2017 after 50 kGy. Therefore the upper sterilisation dose is normally limited to 40 kGy unless post-sterilisation performance testing is repeated.

Finished product types include respiratory mask elastic straps, medical tubing, blood-pressure cuff bladders, IV bag port seals, and radiopaque catheter components. In each case the compound must pass ethylene oxide residuals testing under ISO 10993-7 if EO sterilisation is used, and leachables analysis under ISO 10993-18 where the article contains a liquid contact pathway. The saturated midblock also allows repeated autoclave exposure at 121 °C for short cycles, but the compound is not recommended for continuous steam service above 135 °C.

Heat-Aged Insulation Compounds and the Role of High-Molecular-Weight SEBS

In flexible wire and cable insulation and jacketing, Kraton™ G1651 HU is incorporated to provide high-temperature creep resistance and improved recovery after flexing. Typical compound addition is 20–40 wt% G1651 HU, 10–25 wt% polypropylene or high-density polyethylene, 30–45 wt% halogen-free flame retardant, and 3–8 wt% processing oil, with zinc borate and magnesium hydroxide as required. The compound is tested to IEC 60811-501:2012 for insulation mechanical properties after thermal ageing, UL 1581 for wire and cable insulation, and IEC 60332-1-2:2015 for flame propagation on a single vertical wire. When used in automotive thin-wall cables, ISO 6722-1:2011 class C and D heat resistance requirements may apply, specifically long-term ageing at 125 °C for 3,000 h.

Compounding is performed on a twin-screw extruder with L/D 36:1–48:1, temperature profile 160–205 °C, and flame retardants added through a side feeder to limit shear-induced particle attrition. The pelletised compound is then extruded onto copper conductors using a single-screw extruder with L/D 25:1–30:1 and a crosshead die; the melt temperature at the die is held at 190–210 °C, and the screw is cooled with water through the feed throat to prevent premature melting. Insulation thickness is controlled by a laser micrometer to 0.20–1.20 mm depending on conductor gauge, and the line speed is set at 100–800 m/min depending on cross-section. A key processing conflict is that magnesium hydroxide addition above 45 wt% reduces elongation at break below 150% under IEC 60811-501 and raises melt viscosity to a point where the crosshead die pressure exceeds 250 bar. This narrows the operating window and forces a reduction in line speed if the insulation thickness tolerance is to remain within ±0.05 mm.

Finished cable types include robotic flex cables, appliance internal wiring, automotive thin-wall wire, and photovoltaic junction-box leads. The selected G1651 HU grade provides low smoke density under IEC 61034-2 and low halogen acid gas evolution under IEC 60754-1 when used in halogen-free flame-retardant formulations. However, the compound should not be combined with free-amine stabilisers that can accelerate surface bloom and impair heat-aging performance after extrusion.

For soft elastomeric gel articles and void-filling cable compounds, Kraton™ G1651 HU is processed at high oil loadings to form a physically crosslinked network where the ethylene/butylene midblock is swollen by mineral oil and the styrene domains provide load-bearing points. The formulation addition ratio in such gel compounds is 85–250 phr of white mineral oil per 100 phr G1651 HU, with 0.1–0.5 wt% antioxidant; for extremely soft cushioning, oil loading can reach 300 phr, but syneresis and tensile strength loss become significant. The chemical compliance foundation is FDA 21 CFR 178.3620(a) for white mineral oil, REACH registration for process oil, and ISO 10993-5 when the gel contacts skin in medical cushioning applications.

Production-scale mixing is conducted in a jacketed vacuum kneader or a co-rotating twin-screw extruder with L/D 48:1, with oil injected in three stages: 30% is added before the polymer enters the first kneading block, 40% after the mid-barrel vent, and the remaining 30% downstream of the third kneading block to avoid phase separation. Barrel temperatures are held at 160–190 °C, and the vacuum port is operated at −0.095 MPa to remove air bubbles. The critical operating boundary is the oil absorption limit: above 250 phr, the gel exhibits syneresis within 72 h at 25 °C, detected as a film of process oil on the surface, and compression set under ISO 815-1:2019 can exceed 30% at 40 °C. Below 85 phr, the compound becomes too rigid for gel applications and is better classified as a thermoplastic elastomer.

Finished part types include bicycle seat paddings, shoe insoles, anti-vibration pads for industrial machinery, medical positioning cushions, and cable void-filling gels used in optical fibre closures. In cable filling, the gel is dispensed at 120–140 °C into splice closures, and the low-temperature flexibility must be maintained below −40 °C without cracking. The material should be protected from prolonged exposure to strong acids and chlorinated solvents, which can disrupt the styrene domains and increase surface tack.