Dynasol Calprene® SEBS CH6120 P is a linear styrene-ethylene/butylene-styrene block copolymer with a nominal polystyrene content of 30% by mass and a fully hydrogenated aliphatic midblock. The grade is manufactured in porous pellet form; the “P” designation identifies that morphology, which increases available surface area for oil absorption and permits shorter mixing cycles in dry-blend and continuous compounding operations. The polymer is supplied as a neat thermoplastic styrenic elastomer without extender oil, allowing compounders to select the paraffinic, naphthenic, or hydrocarbon resin package required by the end-use specification.

What Distinguishes the Hydrogenated Midblock from Conventional SBS?

In an unsaturated SBS block copolymer, the butadiene-derived midblock contains residual double bonds that can participate in chain scission, crosslinking, and autoxidation during high-temperature processing or prolonged UV exposure. In CH6120 P, the midblock is saturated ethylene/butylene; no olefinic sites remain available for radical-initiated degradation under typical melt-processing shear. This architectural change raises the polymer’s resistance to viscosity drift in twin-screw extrusion and allows continuous service in formulations where an SBS grade would require antioxidant packages exceeding 0.3 phr. The styrene end-blocks in CH6120 P remain phase-separated at room temperature, forming a physical crosslink network that is reversible above the polystyrene glass transition near 95°C. At ambient temperature, the network produces elastomeric recovery without vulcanisation; no sulfur or peroxide crosslinking package is required, and rework scrap can be re-extruded.

On a production-scale co-rotating twin-screw extruder with 44:1 L/D ratio and segmented screw elements, CH6120 P is typically fed in the first barrel, while paraffinic oil is injected after the first kneading block to prevent slip at the feed throat. Barrel temperatures from feed to die are set at 180°C, 190°C, 200°C, 200°C, 205°C, 205°C, and 210°C, with screw speed 280–350 rpm and melt temperature measured by an immersion thermocouple in the die adapter not exceeding 235°C. A side-stuffer is used for mineral fillers when filler loading exceeds 20 wt%; at screw speeds above 350 rpm, the porous pellets rapidly absorb oil, but localised oil pooling can occur if injection pressure drops below 2 MPa. Segmented screw configuration for CH6120 P generally places two forward kneading blocks before the oil injection point to melt the polymer and build pressure, followed by distributive mixing elements after injection. At 44:1 L/D, a neutral conveying segment in the final 8 L/D allows venting of residual volatiles without excessive die pressure. Operators should monitor die pressure at 3–5 MPa; lower values may indicate incomplete fill of the screw, while values above 7 MPa at 300 rpm suggest filler agglomerates or under-melting in the first kneading zone. A gear pump is recommended for pelletising compounds above 65 Shore A because it reduces melt temperature variation by 3–5°C compared with open discharge.

Neat-Resin Property Envelope and Test Methodology

The property values listed below are representative for compression-moulded or injection-moulded specimens prepared according to the cited methods; individual lot values may differ within manufacturing specification limits. The 2.0 g/10 min melt flow rate at 230°C under 5 kg load places CH6120 P in the low-flow, high-cohesive-strength end of the Dynasol SEBS range, which is appropriate for fibre-reinforced compounding and thick-wall profiles rather than spiral-flow-sensitive thin-wall injection moulding.

Injection moulding of CH6120 P neat or lightly compounded grades requires adequate plasticising capacity because the polymer’s viscosity under shear remains higher than that of general-purpose SEBS. Barrel temperatures between 190°C and 220°C with nozzle temperature not above 225°C prevent thermal degradation of the styrene block; mould temperature is held at 30–50°C for rapid skin formation and release. A clamp force of 6–8 kN/cm² of projected cavity area is typical for soft-touch overmoulding when CH6120 P is compounded to 45 Shore A. Pre-drying is not normally required for opaque mouldings, but hygroscopic fillers introduced with the polymer require drying of the compound at 80°C for 2 h when ambient relative humidity exceeds 60%. Gate dimensions below 1.0 mm can generate shear heating sufficient to exceed 235°C and should be avoided unless the formulation contains a process stabiliser package.

Why Does Porous Pellet Morphology Influence Oil Absorption and Mixing Behaviour?

The porous pellet structure of CH6120 P is not cosmetic; it changes the mass-transfer pathway during oil extension. Dense SEBS pellets absorb paraffinic oil from the surface inward over several hours, leading to oil-rich shells and undispersed cores in short mixer residence times. A porous pellet presents interconnected void volume, so low-viscosity oil penetrates by capillary action before the pellet melts. This allows dry-blend batch mixing to reach visual uniformity in ribbon blenders at 60–80°C in 15–30 min, depending on oil viscosity and total liquid loading. When oil is added above 100 phr, staged injection is still recommended because a single injection in a twin-screw extruder can produce die pressure fluctuations greater than 2.5 MPa. Field data from compounding lines with 40:1 L/D extruders indicate that porous pellets reduce torque peaks during oil wetting by 10–20% relative to dense pellets of the same melt flow rate; however, published data for this specific configuration is limited and mixer geometry strongly affects the result.

An oil-extended compound based on 100 phr CH6120 P, 110 phr of hydrogenated paraffinic oil with kinematic viscosity 40 mm²/s at 40°C, and 15 phr of untreated fumed silica can reach a Shore A hardness of 18–22 with tensile strength of 6–8 MPa and elongation at break above 900% when tested according to ISO 37:2017 on 2 mm dumbbell specimens. The same formulation with an unsaturated SBS of equivalent styrene content typically fails to retain tensile strength after 168 h of air oven ageing at 100°C, whereas the CH6120 P-based compound commonly retains more than 80% of original elongation. In low-hardness sealing profiles, the saturated midblock also reduces migration of low-molecular-weight tackifiers to the surface because the rubber phase has a narrower molecular-weight distribution and no unsaturated oxidation products.

When High-Flow SEBS Grades Cannot Provide Sufficient Cohesive Strength

When a moulded part requires spiral flow lengths above 30 cm at 2 mm wall thickness, high-flow SEBS grades with melt flow rates above 20 g/10 min are usually selected. CH6120 P occupies a different segment: its melt flow rate of approximately 2.0 g/10 min indicates a higher molar mass, which correlates with higher tensile strength and better retention of cohesive strength after oil dilution. This makes the grade suitable for thick-wall automotive sealing profiles, conveyor skirts, and impact-absorbing mats where demoulding temperatures above 50°C and long-term compression recovery are more critical than filling speed. Compared with a high-flow SEBS grade of the same styrene content, CH6120 P can tolerate roughly 20–30% higher oil loading before Shore A hardness drops below 15 because the entanglement network persists at high extender content. The trade-off is narrower processing latitude in thin-wall injection moulding and longer plasticising time; screw rotation times shorter than 8 s on machines below 80 t clamp force may produce unmelted pellets if the thermocouple set point is below 210°C.

Unlike oil-extended SEBS compounds sold at 50 Shore A, CH6120 P is a neat polymer. This distinction matters for formulators that need to control extender chemistry; the grade contains no pre-blended oil that would impose a single paraffinic hydrocarbon profile. Oil-extended products can reduce mixing time, but they fix the compound hardness and restrict the choice of oil compatibility with the intended adhesive or overmoulding substrate. CH6120 P provides the same base elastomer as the high-hardness intermediate but leaves the oil and filler selection under compounder control.

When polypropylene is used as a hard-phase modifier, CH6120 P is pre-blended with polypropylene homopolymer at ratios between 10/90 and 70/30 SEBS/PP. Addition of 30 phr CH6120 P to an impact copolymer typically raises Izod notched impact strength at −30°C by 8–12 kJ/m², depending on the base PP; however, published data for specific PP grades is limited. The P pellet form allows direct dry blending with PP pellets, but segregation can occur if pellet sizes differ by more than 1.5 mm; multi-component feeding is preferred.

Hot-melt pressure-sensitive adhesive formulations use CH6120 P at 15–25 phr with tackifying resins and low-aromatic process oils. The high cohesive strength of the polymer helps maintain loop tack and peel after thermal ageing, while the hydrogenated midblock reduces odour and colour development compared with SBS in similar formulations. Mixing is performed in a sigma-blade kneader at 170–190°C under nitrogen; after 45 min of mixing, the adhesive should be filtered through a 200 μm screen to detect undispersed polymer. Adhesive formulations with ester tackifiers above 40 phr can soften the styrene phase and reduce shear adhesion failure temperature; formulators compensate by increasing CH6120 P content to 25 phr or replacing part of the ester tackifier with a C5 aliphatic resin.

Compliance and Regulatory Boundary Conditions

CH6120 P is part of the Dynasol styrenic block copolymer portfolio, and regulatory compliance is formulation-dependent. The neat polymer may be assessed under the European REACH regulation and for resistance to specific migration when used in food-contact compounds. Users must verify the final compound against the target jurisdiction and specific food or medical contact standard; no statement here substitutes for a written compliance certificate for the purchased lot. The polymer does not require vulcanising agents, and compounding with peroxide levels above 0.5 phr is not recommended because residual peroxide can cause chain scission in the ethylene/butylene midblock and reduce tensile strength.

For profile extrusion of 50 Shore A seals, a single-screw extruder with 24:1 L/D and barrier screw is run at 180–200°C. Output stability is improved by pre-compounding CH6120 P with oil to a hardness of 60 Shore A before final letdown, because direct oil addition at the hopper can reduce solids conveying and produce porosity in the extrudate. A vacuum vent at −0.08 MPa is required when moisture or low-molecular-weight oil is present.

Acceptance testing on incoming lots should include melt flow rate and hardness on a control compound, not only visual pellet colour. Batch-to-batch variation in SEBS of this class is low, but oil grade changes can shift Shore A hardness by 3–5 points at equal phr. The grade should be stored in a dry location below 40°C and protected from UV; lengthy storage above 35°C can cause pellet blocking in pallet stacks. When processing over 230°C for more than 10 min residence time, purge with a neutral polymer and avoid combination with amine-based antiozonants that may cause yellowing in hydrogenated midblock grades.