Kraton™ MD6951 M is a linear styrene-ethylene/butylene-styrene block copolymer in which the bound styrene content is 42% by weight. The elastomeric midblock is saturated ethylene/butylene, while the hard domains are glassy polystyrene; on cooling from melt processing, the polystyrene domains phase-separate into physical cross-links and form a network without vulcanization. The saturated midblock removes the main-chain double bonds present in styrene-butadiene-styrene analogues and is the primary reason for improved oxidative stability in this SEBS class. Supplier-published typical properties for the neat resin list a specific gravity of 0.91 (ASTM D792), hardness of 71 Shore A (ASTM D2240), tensile strength at break of approximately 33 MPa (ASTM D638), and elongation at break of approximately 880% (ASTM D638). The grade belongs to the MD series, which Kraton positions for medical and regulated applications; however, the raw resin itself is not a finished medical device and regulatory conformance must be established on the final compounded article through lot-specific certificates and biological testing. Melt processing is possible on conventional single-screw extruders, twin-screw compounding lines, and injection molding machines configured for polyolefin-grade thermoplastic elastomers.

What Distinguishes MD6951 M from Lower-Styrene SEBS and Thermoplastic Polyurethanes?

Compared with a 30% styrene SEBS representative such as Kraton G1652, the 42% styrene fraction of MD6951 M increases the volume fraction of glassy endblock domains. This raises the published 100% tensile modulus to approximately 2.8 MPa and the 300% modulus to approximately 4.4 MPa (ASTM D638). The penalty is a reduction in low-strain elastomeric recovery and a higher magnitude of stress relaxation at fixed extension because the relaxation spectrum is modified by the additional glassy phase. Against thermoplastic polyurethane, MD6951 M has a specific gravity of 0.91 instead of the 1.10–1.25 range typical for polyester or polyether TPU, and it does not require the aggressive drying that moisture-sensitive TPU requires. But the ultimate tensile strength of MD6951 M, approximately 33 MPa, is below the 40–70 MPa range frequently reported for engineering TPU grades; similarly, abrasion resistance and tear strength are lower. In comparison with a random polypropylene copolymer, MD6951 M provides much lower flexural modulus and superior low-temperature ductility; the ethylene/butylene segment retains elastomeric response well below −40 °C, whereas polypropylene homopolymer can exhibit brittle failure near 0 °C depending on molecular weight and cooling history. The glassy polystyrene domains, however, soften near 90–100 °C, and this limits continuous load-bearing service to temperatures below the glass transition of the endblock phase.

Mechanical, Physical, and Rheological Property Ranges

Published data for MD6951 M are generated on plaques conditioned at 23 ± 2 °C and 50 ± 10% relative humidity. The following table consolidates typical values from supplier literature and does not represent a lot-specific certificate of analysis.

The mechanical response of MD6951 M is controlled by the microphase-separated morphology. The polystyrene endblock volume fraction of 0.38–0.42 is sufficient to form discontinuous spherical or short cylindrical domains in the ethylene/butylene matrix, depending on annealing conditions. Dynamic mechanical thermal analysis of similar high-styrene SEBS materials shows a glass transition of the ethylene/butylene phase near −55 °C and an endblock glass transition near 90–100 °C. The plateau modulus between these transitions is a function of the styrene content and domain morphology; the 42% styrene grade therefore has a higher plateau modulus than a 30% styrene SEBS. Tensile set after cyclic loading increases when the material is strained into the strain-softening region beyond the yield point of the glassy polystyrene domains. In injection moulded components, shear-induced orientation can produce anisotropic shrinkage and anisotropic mechanical response; moulded plaques should be conditioned and tested parallel and perpendicular to flow using ASTM D638 or ISO 527. The supplier-published value of tensile strength 33 MPa is a guide; moulded parts with knit lines or high residual orientation may exhibit lower values. Published DMTA data specific to MD6951 M are limited, so comparative measurements are taken on the actual formulation under ISO 6721-1.

Chemical exposure data for MD6951 M follow general SEBS behaviour. The saturated ethylene/butylene midblock gives better resistance to oxidative aging and weather ageing than SBS, but the material is not recommended for continuous contact with aromatic hydrocarbons, chlorinated solvents, ketones, or strong acids because those fluids solvate the polystyrene domains or degrade the midblock. In applications requiring light contact with aliphatic hydrocarbons, dilute aqueous acids, and bases, the material is generally more dimensionally stable than polybutadiene-based SBS; however, published data for MD6951 M in specific solvent systems are limited, so immersion testing should follow ISO 1817 and ASTM D471 on the finished compound. In medical cleaning and repeated sterilization, lipid emulsions and alcohol-based disinfectants can extract low-molecular-weight additives and stress-crack the styrenic phase if the part is under strain; therefore the additive package and moulded-in stress must be evaluated rather than assuming resistance from the base polymer alone.

When Low Extractables and Biological Reactivity Requirements Govern Material Selection

MD6951 M is commonly specified for tubing, drip chambers, medicinal film, syringe plungers, and other device components where softness, clarity, and regulatory compatibility are required. The raw grade is not, by itself, a medical device; the finished article is qualified under ISO 10993-5 for cytotoxicity and, where contact is longer or internal, under USP <88> Class VI biological reactivity protocols. In food-contact packaging, formulations based on styrene block copolymers can be considered under 21 CFR 177.1810 when the finished article meets the end-use limitations of the regulation. To preserve low extractables, converters select stabilizer packages with low migration potential and avoid low-molecular-weight lubricants unless they are shown to remain within the extraction limits of the relevant monograph. Gamma sterilization at 25–50 kGy is commonly applied to SEBS-based medical devices; the saturated midblock is significantly less prone to radiation-induced chain scission than butadiene-based styrenic block copolymers. The specific colour shift after gamma irradiation is formulation-dependent and must be measured on the finished device material according to the device qualification plan.

On a 25:1 L/D single-screw extruder equipped with a barrier screw and strand pelletizing head, MD6951 M is typically run with barrel setpoints of 150–170 °C at the feed section, 170–190 °C in compression, 190–210 °C in metering, and 210–230 °C at the die. The melt temperature measured with an immersion thermocouple should be maintained below 230 °C. Above 240 °C, extended residence time produces yellowing, gel particles, and a downward drift in melt viscosity, even though the midblock is saturated; the degradation is primarily in the styrenic domains and the antioxidant package. MD6951 M is not hygroscopic, but surface moisture on pellets stored at relative humidity greater than 60% can create splay during extrusion. Pellets are therefore pre-dried at 65–75 °C for 2–4 h when condensation is observed. For injection molding, general-purpose polyolefin screws with 20:1 to 24:1 L/D and compression ratio of 2.5:1 to 3.0:1 are accepted; back pressure of 0.5–1.0 MPa and injection speeds of 30–80 mm/s are typical for thin-wall medical parts, but published data for specific mould geometries using MD6951 M are limited and melt temperature at the nozzle should be directly measured.

Thin-walled tubing extruded from MD6951 M is often designed with an inner diameter tolerance of ±0.05 mm and wall thickness below 0.8 mm. The extrusion line uses a vacuum sizing tank, a puller, and an on-line diameter gauge. Surface roughness is controlled by keeping melt temperature at the die below 220 °C and by avoiding moisture condensation. Coextruded structures with polyethylene or polypropylene can improve clamp resistance and reduce tack in automated assembly. Film formulations containing MD6951 M are used in medical pouches and as monolayer or coextruded sealing layers; the sealing window is typically above 100 °C because the styrenic domains must soften sufficiently for heat seal formation. Specific seal strength is influenced by dwell time, pressure, and surface contamination; standard test methods include ASTM F88 for seal strength and ASTM F1929 for dye penetration. Published seal-strength data for MD6951 M in these structures are limited and should be generated on the commercial line.

The Saturated Midblock Limits Oxidation but Polystyrene Domains Limit Upper Service Temperature

In high-shear twin-screw compounding on a 40:1 L/D co-rotating extruder, viscous dissipation can raise the melt temperature above the barrel setpoint. Because the 42% styrene content increases viscosity relative to 30% styrene SEBS, the maximum screw speed before the melt exceeds 230 °C is lower under identical barrel settings. Screw configurations with short kneading blocks and forward conveying elements of pitch equal to the screw diameter are used to narrow the residence time distribution. Oil extension, if required, is introduced downstream because early injection leads to feed slippage and unstable throughput. In coextruded medical film with polypropylene, the higher styrene content increases the refractive index difference between the SEBS and the polypropylene phase; haze levels are therefore checked on the actual cast film structure using ASTM D1003, and published comparative haze data for MD6951 M are limited. The grade is not suitable for continuously load-bearing service above the styrenic domain glass transition region of 90–100 °C. During autoclave sterilization at 121 °C, parts should not be clamped or under stress because the polystyrene domains soften; the material should be re-evaluated for dimensional recovery before returning to load-bearing service. Peristaltic pump tubing made from MD6951 M is evaluated for tension set under ISO 6943 or ASTM D412; the 42% styrene level generally increases tension set relative to low-styrene SEBS, so dynamic applications should be tested at the specific strain and temperature.