Asahi Kasei Tuftec™ M1981 is a styrene-ethylene/butylene-styrene block copolymer supplied in pellet form. The grade is produced by selective hydrogenation of a styrene-butadiene-styrene triblock precursor, giving a saturated ethylene/butylene mid-block that contributes thermal and oxidative resistance relative to unsaturated styrenic block copolymers. In downstream operations, M1981 is melt-compoundable on co-rotating twin-screw extruders and injection-moldable on standard reciprocating-screw machines. The hard polystyrene end-blocks form reversible physical crosslinks at service temperature, eliminating the need for sulfur or peroxide curing. Product selection for M1981 should be verified against the manufacturer’s current certificate of analysis, because lot-specific melt flow rate, hardness, tensile properties, and additive content can vary within the production specification.

How Does the Molecular Architecture of Tuftec™ M1981 Affect Processing?

M1981’s melt behavior is governed by segregation of polystyrene end-blocks into glassy domains dispersed in a continuous ethylene/butylene phase. The polystyrene domain glass transition is typically above 90 °C, while the ethylene/butylene phase glass transition lies below −50 °C. These transitions bracket the service window of the elastomer and determine the temperature range in which recovery from applied strain occurs. The microphase-separated morphology supplies elastomeric recovery, but it also imposes shear-history dependence during mold filling and extrusion. When the melt is heated above the order-disorder transition, the domain structure is lost and viscosity decreases; on cooling, the domains re-form and mechanical integrity returns. In M1981, the segment molecular weights and styrene/ethylene-butylene ratio are selected to balance processability with tensile strength and recovery. Users should obtain grade-specific typical properties from Asahi Kasei; where a lot is intended for thin-wall injection molding, a shear-viscosity trace generated by capillary rheometry per ISO 11443 is normally more informative than a single melt flow rate value.

Processing temperatures for M1981 should remain within 180–230 °C for most melt operations. Excursions above 250 °C are avoided because the polystyrene domains can depolymerize and the saturated mid-block can undergo chain scission, producing a non-recoverable loss in tensile strength and a measurable increase in yellowness index. Residence time in the barrel should be kept below 10 minutes at upper melt temperatures. Pre-drying at 60–80 °C for 2–4 h in a desiccant dryer with a dew point of −40 °C is recommended when packaging has been exposed to relative humidity above 60%. Absorbed moisture can cause splay, surface roughness, reduced melt strength, and loss of tensile properties in molded or extruded articles. These conditions represent common practice for SEBS grades; the M1981 lot-specific datasheet may impose narrower limits for critical medical or food-contact applications.

In the production of elastic films and flexible sheets, M1981 is often blended with polypropylene homopolymer or polyethylene resin to adjust permanent set, block resistance, and process economy. On a 40:1 L/D co-rotating twin-screw extruder, SEBS pellets and polyolefin are typically fed into the first barrel, with the screw configuration using two or three kneading-block zones followed by distributive mixing elements. Specific mechanical energy input in such compounding is usually controlled between 0.10 and 0.25 kWh/kg; higher energy inputs may reduce melt viscosity but can also increase melt temperature and gel formation. Film specimens are conditioned for 40 h at 23 °C and 50% relative humidity before tensile and hysteresis testing according to ASTM D882 or ISO 527-3. The elastic recovery of formulated films is evaluated by cyclic loading; a 100% engineering strain is applied for five cycles, and permanent set is reported after the fifth cycle. Direct comparison with unsaturated SIS or SBS grades is not appropriate because mid-block saturation alters the heat and ultraviolet aging response.

For injection-molded soft-touch and grip applications, M1981 is commonly compounded with a polyolefin matrix and a process oil. The compound is injection molded with clamp force calculated from projected area and cavity pressure, commonly using 80–120 Mg clamp force for multicavity family tools. Short shots are frequently caused by insufficient melt temperature or premature solidification in thin sections, not solely by polymer molecular weight. Adjusting the barrel profile to 190 °C in the rear zone, 200 °C in the middle zones, and 205 °C at the nozzle, with mold temperature of 30–50 °C, often stabilizes fill. Hold pressure is set at 30–50 MPa hydraulic pressure, and screw back pressure is maintained at 0.3–0.7 MPa to reduce air entrapment without excessive shear heating. A shut-off nozzle is recommended to prevent drool. These settings are industrial starting points for SEBS compounds and should be adjusted for oil loading, filler content, and article geometry.

Rheological Boundaries in High-Shear Compounding

M1981 and its compounds are non-Newtonian; apparent viscosity decreases with increasing shear rate. In twin-screw extrusion, melt pumping elements generate wall shear rates between 100 s⁻¹ and 1000 s⁻¹, while injection mold filling can exceed 10 000 s⁻¹ in gates and thin walls. At low shear rates below 10 s⁻¹, the storage modulus of the phase-separated structure influences die swell and melt strength. Capillary rheometry per ISO 11443 is used to generate shear-viscosity curves at 200 °C, 220 °C, and 240 °C. Bagley and Rabinowitsch corrections are applied to obtain true wall shear rate. The viscosity-shear rate data can be fitted to a Carreau-Yasuda model to provide inputs for injection-molding simulation. M1981-specific simulation parameters should be obtained from the manufacturer or from measured lot rheology; generic SEBS parameters can underpredict pressure drop in hot-runner manifolds by 10–20%, leading to short shots or excessive shear heating.

Thermal stability during compounding is characterized by a time sweep in a rotational rheometer at 220 °C under nitrogen. A viscosity loss greater than 15% over 30 minutes indicates degradation; a loss greater than 30% indicates significant chain scission. When process oil is added, oil extension reduces plateau modulus and shifts the order-disorder transition to lower temperatures. Oil loading above 30 phr can reduce the continuous-use temperature and increase surface tack. M1981 is therefore evaluated in the target formulation rather than as a neat polymer; a grade with higher mid-block molecular weight may be more suitable for high-oil formulations, while a lower-viscosity grade may be preferred for thin-wall molding.

Direct substitution of M1981 into formulations designed for other Tuftec grades or for SEBS from other producers is not recommended without rheological and mechanical revalidation. Differences are observed in styrene content, molecular weight distribution, melt flow rate, hardness, and the presence or absence of reactive functionality. The M1981 datasheet should be checked for maleic anhydride content if adhesion to polar substrates is required; it should not be assumed to replicate the adhesion behavior of maleic anhydride-modified Tuftec M1911 or M1943 unless the manufacturer’s datasheet confirms the presence of reactive functional groups. Adhesion to polar substrates should be tested according to ISO 36 or ASTM D429. In overmolding onto rigid polypropylene, melt interdiffusion and frictional locking may be sufficient without reactive adhesion. In overmolding onto polyamide or polyester, adhesive failure can occur unless a polar-modified grade or a tie-layer is used. Selection between M1981 and other Tuftec products therefore depends on end-use adhesion, oil compatibility, hardness, and processing requirements, not solely on Shore hardness or melt flow rate.

When Melt Viscosity Reduction Is Required for Thin-Wall or High-Speed Molding

Thin-wall articles with flow length-to-thickness ratios above 150:1 require lower melt viscosity than standard SEBS compounds. In such cases, M1981 may be processed at the upper end of its recommended melt temperature range, but gas formation from thermal degradation becomes the limiting constraint. A nitrogen blanket on the hopper and vent vacuum of −0.08 MPa on the extruder reduce oxidative defects. If the compound contains a peroxide masterbatch, the temperature window contracts because premature crosslinking can occur above 190 °C. In these cases, barrel temperatures should be lowered and screw speed increased to maintain output while limiting residence time. Cooling of the feed throat to 30–40 °C prevents pellet bridging. These operating conditions are especially relevant when M1981 is used as a modifier in polypropylene compounds intended for automotive interior parts or appliance housings where warpage and surface haze are tightly controlled.

For high-speed molding, cavity pressure sensors and in-mold pressure curves are used to monitor consistency. A cavity pressure integral change greater than 10% from an approved reference lot can indicate lot-to-lot viscosity variation, moisture, or screw wear. The molding machine should be equipped with a closed-loop hydraulic or electric drive system capable of maintaining injection speed within ±2% of set value. If M1981 is compounded with high levels of filler, screw wear becomes significant; bimetallic barrels and hardened screw surfaces are recommended when filler loading exceeds 20 wt%. Without such measures, barrel clearance increases and melt homogeneity degrades, producing visible surface defects and reduced elongation at break.

Specification Verification for Incoming Resin

Incoming quality control for M1981 is performed on dried pellets. Lot-to-lot consistency is monitored by melt flow rate, density, hardness, tensile stress, elongation, and residual moisture. The following standard methods generate certificate-of-analysis data; numerical limits are provided by Asahi Kasei and may vary by production site and intended application.

When supplied in sealed original packaging, M1981 should be stored in a dry, covered area at temperatures below 40 °C. Bags exposed to ambient humidity for more than 24 h should be re-dried before processing. Each incoming lot is routinely checked for color, odor, gel content, and melt flow rate; a deviation in melt flow rate greater than ±15% from the approved reference lot may indicate contamination or thermal history and should trigger a full rheological and mechanical revalidation. This approach protects production consistency without relying on a single numerical index.