Dynasol Calprene® SEBS CH6174 is a linear styrene-ethylene/butylene-styrene triblock copolymer supplied as a free-flowing pellet. The midblock is fully hydrogenated, which removes the main-chain unsaturation present in styrene-butadiene-styrene block copolymers and shifts the thermal-oxidative stability of the polymer to a higher level. Manufacturer-published typical property data include a bound styrene content of 31%, a specific gravity of 0.91 determined by ASTM D792, a Shore A hardness of 65 at 10 s determined by ASTM D2240, tensile strength of 25 MPa at 500 mm/min determined by ASTM D638-14, and elongation at break of 500% determined by ASTM D638-14. The grade is employed in oil-extended elastomeric compounds, polypropylene modification, soft-touch grips, films, medical device components, and extruded profiles in which low-strain hysteresis, reprocessability, and melt strength define the processing window.

Composition, Morphology, and Hardness Response

The polystyrene end-blocks form glassy domains with a glass transition near 95°C, while the ethylene-butylene midblock has a glass transition near -60°C. At room temperature the styrene domains act as load-bearing physical crosslinks, and the EB phase provides elastic recovery. The 31% styrene content places CH6174 in the medium-hardness SEBS range. Hardness on compression-moulded plaques is 65 Shore A after 10 s; oil extension with 150–200 phr of paraffinic oil can reduce hardness to 20–30 Shore A, and polypropylene addition can raise hardness above 90 Shore A. The neat tensile strength of 25 MPa and elongation at break of 500% reflect the domain-reinforced network; these values decrease as the EB phase is swollen with process oil.

Compounding of CH6174 on a corotating twin-screw extruder is typically performed on equipment with a screw diameter of 25–75 mm and an L/D ratio of 36:1–48:1. Barrel set-point temperatures from 190°C to 220°C are used, with the feed throat water-cooled below 50°C to prevent pellet bridging. Paraffinic or naphthenic process oil is injected downstream after the primary melting zone; early oil injection lubricates the solid-conveying zone and reduces shear heat input. A screw speed of 250–350 rpm is typical for dispersive mixing, but melt temperature should remain below 240°C because SEBS chain scission accelerates with thermal residence time. Pre-drying at 60–80°C for 2 h is recommended if pellets have been stored above 60% relative humidity; surface moisture can produce roughness and voids in extruded profiles. On injection molding equipment, a general-purpose screw with a compression ratio of 2.5:1–3:1 is adequate, whereas compression ratios above 4:1 can generate excessive shear heating.

Polypropylene is incorporated at 10–30 phr to elevate hardness and upper service temperature. At 30 phr polypropylene, oil-filled compounds typically reach 75–85 Shore A and show improved tensile retention after air aging at 100°C for 168 h. Above 50 phr polypropylene, the compound transitions toward a continuous PP phase, and elastomeric recovery declines. Filled compounds containing 20–40 phr calcium carbonate or talc can be produced without coupling agents at moderate filler loadings, although the high melt viscosity of CH6174 assists dispersion. Notched Izod impact values measured by ASTM D256 on filled and unfilled compounds should be verified on the final article because impact behavior depends on the ratio of EB phase to PP phase.

Capillary rheometry of CH6174 shows shear-thinning behavior typical of SEBS; the apparent viscosity at 200°C and 100 s⁻¹ is often in the range of 500–1500 Pa·s, while at 1000 s⁻¹ the apparent viscosity falls by about one order of magnitude. These values are compound-dependent and must be measured on the specific formulation because oils, polypropylene, and fillers shift the viscosity curve. The melt-flow index alone does not provide enough information for die design; a full shear-rate sweep per ISO 11443 is required for extrusion tooling.

What Limits Melt Flow and Injection Molding Throughput with CH6174?

The melt-flow index of CH6174 is approximately 2.0 g/10 min at 230°C/5.0 kg per ISO 1133-1:2022. This value places the grade at the lower-flow end of medium-MFI SEBS materials and controls injection molding throughput. Thin-wall parts with flow-length-to-thickness ratios greater than 200:1 may require melt temperatures of 220–240°C, injection velocities above 60 mm/s, and holding pressures of 40–70 MPa to fill without jetting or sink marks. Published data for this specific configuration is limited, and machine qualification trials are required. The lower MFI is less restrictive in profile extrusion and blow molding, where higher melt strength maintains parison stability. In hot-runner injection molds, the total thermal residence time should not exceed 10 min at 220°C, and stagnant zones should be minimized because styrene domain reordering can alter melt viscoelasticity.

Substitution of CH6174 for a higher-MFI SEBS such as Calprene H6144 changes the melt pressure, oil absorption, and filler dispersion. Table 2 compares the two grades on typical manufacturer-reported properties. In a 40:1 L/D extruder, a 70/30 SEBS/PP compound with 100 phr paraffinic oil may show a melt pressure increase of 15–25% at equivalent screw speed when CH6174 is substituted for the higher-MFI grade; the die pressure rating and thrust bearing load must be reviewed. Dispersion of calcium carbonate above 30 phr may require a downstream feed port and a distributive mixing section rather than additional kneading energy, because severe shear heating can push the melt above the 240°C limit.

When CH6174 Replaces Higher-MFI SEBS in Oil-Extended Compounds

Formulations that substitute CH6174 for a higher-flow SEBS require adjustment of oil type and feed point. The higher melt viscosity of CH6174 permits higher paraffinic oil loadings without complete loss of melt strength; however, it also reduces the efficiency of low-pressure injection molding. In oil-extended compounds, 100–150 phr of paraffinic oil typically reduces hardness to 30–45 Shore A and lowers tensile strength below 15 MPa. A paraffinic oil with a kinematic viscosity of 40–100 cSt at 40°C is preferred; naphthenic oils provide similar softening but increase surface tack. Aromatic oils are incompatible at high loading because they swell the polystyrene domains, reduce upper service temperature, and migrate from the final part. Compression set measured by ASTM D395-18 method B after 22 h at 70°C should be evaluated on the final compounded article, because oil mobility and polypropylene content change the EB-phase modulus and the recovery of the styrene network.

Adhesion to polypropylene substrates in overmolding is controlled by surface energy and interdiffusion. Untreated polypropylene can show peel forces below 2 N/mm when tested by ASTM D3330; corona, flame, or plasma treatment raising the surface energy above 40 mN/m is generally required for a bonded interface. The overmolding melt temperature should be kept at 200–230°C to promote interfacial entanglement without causing PP deformation. In medical device components, validation per ISO 10993-1 is carried out on the final compounded overmolded article, not on the neat SEBS.

Compared with SBS block copolymers of equivalent styrene content, CH6174 has higher melt viscosity, lower low-temperature flexibility, and better retention of properties after heat aging and UV exposure. The absence of main-chain unsaturation reduces the rate of oxidative crosslinking and discoloration in air-oven aging; however, the saturated midblock also reduces the efficiency of peroxide curing unless a co-agent is present. Compared with thermoplastic vulcanizates, CH6174 is a non-crosslinked block copolymer, so compression set at elevated temperature is generally higher and must be managed with polypropylene modification or end-block targeting.

Regulatory Position and Material Incompatibilities

The raw polymer is not a finished food-contact article. In the United States, styrene block copolymers may be referenced under 21 CFR 177.1810; final formulations must be tested for extractives and migration under conditions of use. In the European Union, food-contact compliance is governed by Regulation (EU) No 10/2011 with overall migration testing per EN 1186-1 and specific migration limits for additives. Medical applications require biocompatibility testing per ISO 10993-1 on the final compounded material; the neat polymer does not establish cytotoxicity, sensitization, or hemocompatibility. The material is incompatible with high levels of aromatic process oils, strong oxidizing agents, and halogenated solvents at elevated temperature. Continuous service above 100°C is not generally recommended without polypropylene modification or end-block crosslinking, because the styrene domains lose physical crosslink strength near their glass transition.