When Ningbo Changhong SEBS CH4320H is compounded with polypropylene homopolymer, paraffinic process oil, and calcium carbonate for automotive interior mat and weatherseal formulations, the first processing boundary is not peak barrel temperature but the oil absorption capacity of the ethylene-butylene midblock. The SEBS triblock relies on polystyrene endblocks to form physical crosslinks; once the oil fraction exceeds the oil-retention threshold of the midblock, the compound exhibits plate-out on the pelletizer, tacky strand rupture, and surface exudation after 14 days at 23 °C. A practical screening window for production compounding on a co-rotating twin-screw extruder with L/D 44–52 and segmented screws is to feed CH4320H and PP at the main throat, inject paraffinic oil through a barrel-side feed port at 60–70% of barrel length, and add calcium carbonate downstream to prevent the filler from blinding the oil-wetted pellets. Melt temperatures measured at the die are maintained between 190 °C and 220 °C. Above 240 °C, the polystyrene endblocks begin to lose domain integrity, producing a measurable drop in melt viscosity and surface sharkskin on extruded strips; below 180 °C, rotational torque rises sharply because the polystyrene domains remain only partially softened. The residence time should be held between 40 s and 90 s, with a vacuum vent of −0.08 MPa to strip low-volatile oil fractions. Pelletizing stability depends on strand temperature at the water quench: a strand entering at 160–170 °C prevents both internal porosity and brittle fracture at the feed rolls.

In formulations where PP is added at 20–35 phr and oil at 50–70 phr, the hardness falls from a neat CH4320H plateau of approximately 75–85 Shore A to a soft-touch range of 45–65 Shore A. Hardness is measured according to ASTM D2240-15e1 after 15 s dwell. The PP phase contributes additional load-bearing capacity above the polystyrene glass transition, shifting the onset of thermal softening from about 70 °C for a plasticized SEBS to 105–115 °C for a filled PP/SEBS blend. Tensile properties are determined on ASTM D412-16 Die C specimens at 500 mm/min; production compounds in this window commonly fall between 6 MPa and 12 MPa tensile strength, with elongation at break between 350% and 600%. When the same compound is injection moulded, short-shot defects appear when the mould cavity thickness drops below 1.0 mm and the flow length-to-thickness ratio exceeds 180:1. Mould filling is optimized by using a nozzle temperature of 200–220 °C and a cold runner diameter of 6–8 mm, with venting slots of 0.02–0.03 mm. Ejection is performed at 40–50 °C, and the tool requires large-diameter ejector pins because the soft compound deforms under small ejector pressure. A polypropylene level of 25 phr reduces pellet blocking in bulk bags at 35 °C and permits automatic feeding without anti-blocking dust.

Does the 70 °C Compression Set Limit in EPDM/CH4320H Hybrid Building Profiles Require Partial Peroxide Crosslinking?

The substitution of EPDM into CH4320H-based building-profile formulations is driven by the need to reduce compression set in glazing gaskets and expansion joints, but the two networks are fundamentally dissimilar. CH4320H is physically crosslinked by polystyrene domains, while EPDM requires a covalent sulfur or peroxide network to develop elastic recovery. When EPDM is added at 20–40 phr without a shared co-continuous PP phase, the hybrid compound can develop two-phase morphology in which the EPDM domains soften at 70 °C and the SEBS continuous phase creeps under constant deflection. Compression set is measured according to ASTM D395-18 Method B at 70 °C for 22 h; acceptable building-profile compounds typically require ≤55%, while interior matting may tolerate ≤70%. If compression set exceeds these thresholds, a practical corrective step is to add PP homopolymer at 20–35 phr rather than pursue partial peroxide crosslinking, because the saturated midblock of CH4320H and the unsaturated EPDM domain co-crosslink poorly, and the peroxide also attacks the polystyrene endblock, lowering heat resistance. Extrusion of such profiles runs on a single-screw vented extruder with L/D 28–32, die temperature 190–205 °C, and calibrator vacuum −0.05 MPa. The melt strength of the hybrid is lower than that of a filled EPDM compound, so the coextruded rigid backing must support the profile until the water-spray cooling section brings the surface temperature below 50 °C.

For exterior weather resistance, dark and black profiles are tested under ISO 4892-2 xenon-arc exposure for 1000 h with a target ΔE of ≤3; the formulation requires a UV absorber at 0.20–0.30 phr and a hindered amine light stabilizer at 0.10–0.20 phr. Titanium dioxide at 4–8 phr is used in white profiles, but its catalytic surface can accelerate oxidation if moisture is trapped during pellet drying. The critical limitation for this hybrid is that SEBS-based compounds do not develop true thermoset compression set below 35% without a post-extrusion crosslinking unit. Published data for the specific CH4320H/EPDM configuration is limited, so end-use qualification must be performed on the exact extruded profile.

Low-smoke zero-halogen sheath compounds for flexible power cords shift the formulation risk from flame retardancy to melt fracture. CH4320H is typically loaded with aluminium trihydroxide or magnesium dihydrate at 120–160 phr to pass single-cable vertical flame testing, but these fillers raise the low-shear melt viscosity and produce a sharp die-swell increase at the insulation head. The cable compounder must keep melt temperature below 200–210 °C because aluminium trihydroxide begins endothermic dehydration between 180 °C and 200 °C; the resulting water release can foam the melt and collapse the insulation wall. A maleic anhydride grafted SEBS compatibilizer at 5–10 phr improves filler dispersion and raises the elongation at break from around 80% in uncompatibilized mineral-filled compounds to 125–180% when tested according to ASTM D412-16 Die C at 500 mm/min. The extrusion tooling uses a single-layer crosshead with a compression ratio of 2.5:1 to 3.5:1 and a die land length of 6–10 mm; a too-short die land produces orange peel on the sheath surface, while a too-long land builds head pressure above 250 bar. In production, melt fracture is reduced by raising the head temperature to 205 °C and lowering the screw speed to maintain a melt residence time below 90 s.

The final sheath must pass IEC 60332-1-2 vertical flame propagation, IEC 60754-1 halogen acid gas content, ISO 4589-2 oxygen index of ≥28%, and ISO 5659-2 smoke density. The specific CH4320H lot must be selected for a melt flow rate high enough to tolerate the filler without exceeding the thermal limit. If the compounder exceeds 210 °C barrel temperature, the aluminium trihydroxide decomposes progressively and the resulting batch-to-batch variability in sheath thickness may exceed ±0.15 mm on a 0.5 mm wall. Before extrusion, the pellets are dried at 80 °C for 2–3 h in a desiccant dryer with a dew point of −40 °C; at relative humidity above 60%, surface moisture produces voids at line speeds above 150 m/min. During strand pelletizing, the filled strand should enter the water quench at 160–170 °C; a hotter strand creates internal porosity, while a colder strand produces brittle fracture at the pelletizer feed rolls.

When CH4320H Is Overmoulded onto Glass-Filled Nylon in Two-Shot Moulding, Insert Skin Temperature Determines Failure Mode

In two-shot sequential moulding, the first shot is PA66 GF25 with a melt temperature of 285–300 °C and a mould temperature of 80–100 °C. The second shot, a soft-touch CH4320H compound, bonds to the nylon insert through polymer interdiffusion rather than chemical reaction. The controlling variable is the insert skin temperature at the moment the SEBS melt front makes contact. If the PA66 GF25 surface has cooled below 100 °C, the SEBS melt front freezes rapidly and forms a sharp interface that peels away under loading. Production trials therefore maintain the first-shot insert at 110–130 °C when the mould opens and use a short transfer time of 8–15 s before the second shot. The CH4320H melt temperature is set at 230–250 °C, not because the SEBS requires this for flow alone, but because the higher melt enthalpy delays flow-front solidification against the insert. The nylon must be dried to ≤0.10% moisture according to ISO 15512; otherwise escaping steam vents through the second-shot cavity and creates pitting at the interface. Similarly, external mould release agents on the PA66 insert must be eliminated, because silicone or wax films block interdiffusion even at correct temperatures. The mould is fitted with a heated core and insulated gate bushing; mould temperature measured at the cavity surface is maintained at 60–80 °C, while the core is 10–15 °C hotter to compensate for the lower thermal conductivity of the SEBS layer.

The second-shot cushion is controlled at 3–6 mm, and decompression after screw retraction is limited to 2–4 mm to prevent air marks without pulling melt away from the gate. Adhesion failures are classified after 90° peel at 50 mm/min: cohesive tearing within the SEBS layer is acceptable, while interfacial separation is not. No single ISO or ASTM standard fully covers two-shot TPE-to-nylon peel with a rigid first shot; production audits therefore photograph the failure surface at 40× and record the normalized peel force per linear millimetre for internal SPC. When the insert is run too cold, the process produces an apparently intact part that fails later under thermal cycling from −40 °C to 85 °C; the crack propagates along the interface because the frozen layer lacks perpendicular polymer chains. The corrective action is to raise insert surface temperature, not to increase second-shot melt temperature beyond 250 °C, because excessive SEBS melt temperature accelerates endblock damage and may create degradation gas at the vent. If the nylon first shot is coated with an alkyd or polyurethane paint before overmoulding, the SEBS does not bond to the coating and the part relies on mechanical anchoring only.

Hot-Melt Adhesive Processing Window at Low Rotary Viscosity

Hot-melt pressure-sensitive adhesives using CH4320H differ from SIS-based systems because the hydrogenated midblock has lower residual unsaturation than SIS, but requires higher processing temperatures to dissolve uniformly. In a sigma-blade mixer heated to 160–170 °C, CH4320H is masticated with a fully hydrogenated C5 or C9 tackifier at 60–100 phr and a paraffinic or naphthenic oil at 10–30 phr. The mixer is blanketed with nitrogen and run at 30–40 rpm; the dissolution endpoint is judged by a clear melt with no visible gel domains under a draw-down plate. At 80 phr tackifier and 20 phr oil, the ring-and-ball softening point measured per ASTM E28 typically falls between 85 °C and 115 °C, while Brookfield RVT viscosity at 180 °C with spindle 27 lies between 5000 mPa·s and 30000 mPa·s. If the hot-melt pump pressure exceeds 80 bar at the slot-die coating head, the formulation has too little oil or too high molecular weight; if the adhesive oozes from the die before contact, the tackifier content has reduced the high-temperature cohesion too far. Thermal shear stability is evaluated by holding the melt at 180 °C for 72 h under nitrogen; a viscosity change of ≤15% is expected for a stabilized system containing 0.2 phr hindered phenol and 0.2 phr phosphite.

Poorly stabilized CH4320H hot melts can form gel specks that block a 200 μm slot gap and produce streaks in label coating. The operational boundary for this segment is narrow: the melt must remain between 150 °C and 190 °C for no longer than 96 h; beyond this window, aromatic tackifiers can disrupt the endblock order and lower cohesive strength. This application segment generally requires no filler, so density remains below 1.0 g/cm³ and ignition risk in the ducting must be managed by routine condensate removal. Published data for the specific CH4320H hot-melt configuration is limited, so coating trials must map pump pressure, line speed, and slot gap against adhesive coat weight rather than relying on generic SEBS viscosity curves.

In constrained-layer damping pads and anti-vibration matting, CH4320H is compounded with high-density fillers such as barium sulfate at 200–400 phr to lift the mass per unit area to 1.40–1.80 g/cm³ without eliminating the midblock viscoelastic loss. The filler is preheated to 80 °C and added in an internal mixer with tangential rotors at a fill factor of 0.70; the drop temperature is held at 150 °C to avoid premature oil evaporation. The mixed stock is calendered to 1.5–3.0 mm, and the resulting sheet is laminated to a thin aluminium or steel constraining layer. Damping performance is assessed by dynamic mechanical analysis at 1 Hz in three-point bending; the loss tangent of plasticized SEBS typically peaks between −40 °C and 0 °C, while PP homopolymer addition shifts the maximum to a broader −10 °C to 40 °C plateau. Production failure modes are dominated by filler agglomeration at the feed throat and screw bed compaction; one corrective measure is oil pre-absorption onto the barium sulfate before the main mixing cycle. The damping pad is tested for compression under a static load of 50 kg/m² for 7 days at 60 °C, and the residual deformation is reported per ISO 815-1; values above 45% after load removal indicate that the polystyrene endblocks have yielded and the part will lose clamping force. This application is less sensitive to melt-flow control than to filler dispersion, so the incoming CH4320H lot should be checked for pellet hardness and oil absorption rate before batching.

Medical device compounders evaluating CH4320H for tubing, cushioning grips, or vial stoppers must separate polymer safety from formulation safety. The hydrogenated midblock removes polybutadiene unsaturation and thereby reduces autoxidation potential, but the raw material lot must still be audited for residual styrene, heavy metals, and process oil type. Cytotoxicity is evaluated according to ISO 10993-5, hemocompatibility according to ISO 10993-4, and biological reactivity according to USP <88> Class VI; these tests are performed on the fully formulated article, not on the neat SEBS pellets.

Steam autoclave sterilization at 121 °C for 30 min is close to the softening region of the polystyrene endblocks, and compounds with plasticizer above 60 phr may show dimensional distortion or surface dewetting after repeated cycles. Gamma sterilization at 25–40 kGy can induce crosslinking or chain scission in the EB midblock depending on the additive package; only lot-specific irradiation studies can define the upper dose. Food-contact sealing articles require the final compound to be evaluated under 21 CFR 177.1810 and, for the EU market, under Regulation (EU) No 10/2011 migration testing; the neat CH4320H resin does not automatically confer food-contact compliance. The operational boundary is strict: if the supplier has not certified the specific CH4320H lot for medical or food use, the converter cannot use a generic SEBS safety data sheet as a substitute for end-use qualification. Published data for this specific grade in medical configurations is limited; conversion trials must include extraction testing and confirmation that the selected process oil meets the relevant pharmacopoeial chapters.