Kraton™ MD4224 KU is supplied as a linear styrene-ethylene/butylene-styrene (SEBS) triblock copolymer in dense pellet form. The product is an unfilled, unplasticized hydrogenated styrenic block copolymer with a saturated ethylene/butylene midblock. Differential scanning calorimetry on the neat polymer typically shows an ethylene/butylene-domain glass transition below -40°C and a styrenic endblock glass transition in the region of 95°C. The morphology depends on phase separation of polystyrene endblock domains that act as thermally reversible crosslinks. This thermoplastic character permits processing on conventional extrusion and injection-moulding equipment without a vulcanization step and without a crosslinking agent.

The grade is differentiated from general-purpose SEBS by its packaging for medical-device use, its narrower lot-to-lot rheological profile, and its low gel specification. Lot-to-lot variance in melt flow rate observed on production compounding lines is reduced relative to standard SEBS grades, which permits tighter control of tubing wall thickness when using a single-screw extruder with a barrier screw and a melt pump. In practice, polypropylene-based compounds with 20 wt% to 30 wt% MD4224 KU generate stable melt pressure at the die when barrel zones are held between 190°C and 220°C.

Melt Filtration and Rheological Boundaries Across Extrusion and Injection Moulding

A production-scale twin-screw compounding line with an L/D ratio of 30:1 to 40:1 is suitable for dispersing this material into isotactic polypropylene. The extruder should be equipped with melt filtering at 100 mesh to 250 mesh depending on gel sensitivity. Processing temperatures below 180°C produce incomplete melting of the styrenic endblock phase, and melt temperatures above 260°C can promote endblock relaxation and gel formation. At a melt temperature of 230°C, the viscosity profile is shear-thinning; injection-moulding conditions with a screw speed of 80 rpm to 150 rpm and back pressures of 5 bar to 10 bar are commonly used for small medical components. Published data for the exact capillary rheometry constants at very high shear rates associated with thin-wall injection moulding is limited, but the melt flow rate provides a comparative end-use control point.

Pre-drying is not required for the neat pellets when stored in sealed packaging at ambient conditions. At a relative humidity above 60%, surface moisture on pellets can introduce measurable splay in translucent moulded parts and is controlled by drying at 60°C to 70°C for 2 h to 4 h in a desiccant dryer. Incompatibility with amine-based antistatic additives has been observed because residual amines can disrupt the styrenic cleavage and lead to yellowing above 200°C.

In the unfilled state, the material exhibits a Shore A hardness of approximately 68 when measured after 30 s according to ASTM D2240. Tensile specimens die-cut from compression-moulded plaques produce tensile strength values near 27.5 MPa, a 100% secant modulus of 2.8 MPa, and an elongation at break of 650% when tested according to ASTM D412. These properties reflect the high restoring force of the polystyrene endblock network. In comparison with Kraton™ G series SEBS polymers, MD4224 KU maintains a lower diblock content and a lower solvent extractables profile, which is relevant for final-device biological evaluation under ISO 10993.

These values are typical for neat, unfilled material and are not batch-release specifications; the certificate of analysis for each lot may include narrower acceptance criteria. The specific gravity of 0.91 g/cm³ is measured by ASTM D792. Bound styrene content, measured by ASTM D5775, is typically 31 wt%.

What Distinguishes MD4224 KU from Standard Non-Medical SEBS Grades?

Three molecular and physical characteristics separate MD4224 KU from general-purpose SEBS grades. First, the diblock content is controlled at a lower level, which raises tensile strength and reduces the viscous-flow fraction that contributes to creep in load-bearing parts. Second, the stabilizer package is selected for thermal processing without the use of alkylphenol ethoxylate or phthalate plasticizers, removing a significant source of extractables that must be quantified under ISO 10993-12. Third, the material is produced and packaged under conditions that reduce foreign-particle contamination for medical-device applications.

In contrast to oil-extended SEBS compounds, MD4224 KU is supplied neat, so it does not introduce mineral-oil migration into medical surfaces. Compounds formulated with medical-white mineral oil show opacity and hardness reductions that scale with oil addition; at 20 phr oil, the Shore A hardness typically declines to approximately 50, while the flexibility of the saturated midblock improves. The absence of an oil carrier is an advantage in applications where extraction by lipid-containing fluids is a concern.

When the Device Requires Repeated Steam Sterilization Rather than Gamma Irradiation

Steam-sterilization exposure at 121°C for 30 min or 134°C for 5 min produces little irreversible property change in soft components formulated from MD4224 KU, provided the residual stress from injection moulding is minimized by gas counter-pressure or by post-mould annealing at 80°C for 2 h. Gamma irradiation at doses up to 50 kGy is reported to cause measurable reductions in elongation and slight yellowing; the magnitude is dependent on the presence of antioxidants and on the coexistence of polypropylene in the final compound. Electron-beam sterilization at similar doses produces faster oxidation of the polypropylene phase; published data for this specific neat resin under electron-beam at 70 kGy is limited and must be confirmed on the finished device.

Applications reported for this grade include extruded medical tubing, overwrap film, respiratory circuit components, syringe seals, blood-contact valve components, and moulded ergonomic soft-touch features on hand-held diagnostic devices. In tubing applications, the material is commonly used in blends with polypropylene at loadings of 30 wt% to 50 wt% and with a medical-white oil addition up to 20 phr. The blend is then extruded into single-lumen or multi-lumen tubing with an outer diameter of 3 mm to 8 mm and a wall thickness of 0.5 mm to 1.5 mm.

The Regulatory Limits are Met Only After Extractables Subtraction

Conformity statements from the producer for the unfilled product typically reference 21 CFR 177.1810, 21 CFR 170.199, USP Class VI, ISO 10993-5, ISO 10993-10, ISO 10993-11, EU Regulation 10/2011, and REACH. The finished-device manufacturer is responsible for extractables testing because processing temperatures, coextruded layers, colorants, and sterilization can shift the extraction profile. Published data for extractables from a neat resin plaque is not a substitute for exhaustive migration testing on the finished device under the contact conditions specified in ISO 10993-18 and ISO 10993-12.

Chemical resistance of the saturated midblock to polar solvents is superior to unsaturated styrenic block copolymers. The material is attacked by aromatic hydrocarbons and chlorinated solvents; ketone and ester extractants can swell the styrenic domains. For this reason, the material should not be used in contact with liquid formulations containing high concentrations of benzene, toluene, or methyl ethyl ketone. Alcohol-water mixtures such as 70% isopropanol produce negligible property loss in short-term exposures, but long-term recovery of the polymer network should be verified on the finished device.

Naphthenic or paraffinic medical-white mineral oils are preferred for viscosity control; aromatic oils are not compatible with the saturated midblock and will produce exudation. Oil absorption is slower than in unsaturated SBS copolymers because the saturated midblock has no double bonds to solvate. A two-stage screw configuration with a distributive mixing section and oil injection downstream of the melting zone is used to avoid torque spikes when compounding oil into the polymer. In such coextruded structures, the melt-temperature split between the outer polypropylene layer and the inner MD4224 KU—typically 10°C to 20°C—should be validated by pyrometer measurements across the die.