LCY Chemical Globalprene® SEBS 7720 is a linear styrene-ethylene/butylene-styrene block copolymer supplied as neat pellets, not oil-extended. Nominal polystyrene content is 30–32 wt%. Specific gravity by ISO 1183-1:2019 is 0.90–0.92. Melt mass-flow rate measured by ISO 1133-1:2022 at 230 °C under 5 kg load is 2–5 g/10 min. Compression-molded test sheets exhibit Shore A hardness of 60–68 under ISO 48-4:2018. Tensile strength determined by ISO 37:2017 is 25–32 MPa, with elongation at break of 400–600% and stress at 300% elongation of 2.5–4.0 MPa. These values position the grade as a mid-flow, general-purpose SEBS for compounds requiring controlled hardness without supplier-level oil extension.

How does the saturated midblock alter thermo-oxidative resistance relative to SBS?

Hydrogenation of the butadiene midblock to an ethylene/butylene sequence removes the main-chain unsaturation that limits conventional styrene-butadiene-styrene block copolymers. In circulating-air aging at 120 °C for 168 h, unstabilized SBS compounds often lose more than 50% of initial tensile strength, whereas SEBS 7720 compounds containing 0.1–0.3 wt% hindered phenolic antioxidant and 0.1–0.2 wt% phosphite processing stabilizer typically retain above 80%. The saturated midblock also raises the onset of thermal chain scission in thermogravimetric analysis by approximately 30–50 °C compared with SBS, depending on heating rate. This does not confer UV stability to the aromatic styrene domains; outdoor compounds require a benzotriazole or hindered amine light stabilizer package.

The linear triblock architecture of SEBS 7720 produces lower melt elasticity than radial SEBS of comparable molecular weight, reducing die swell in profile extrusion and improving gauge control in calendered sheet. The styrene content of 30–32 wt% is sufficient to form physical crosslinks at room temperature while maintaining Shore A hardness below 70. Grades with styrene content below 20 wt% typically develop higher tack and lower cohesive strength, while grades above 35 wt% require elevated processing temperatures and transition toward Shore D hardness.

Compounding of SEBS 7720 on co-rotating twin-screw extruders with L/D ≥ 40:1 generally uses barrel settings from 170 °C to 220 °C and a die temperature of 200–210 °C. The polymer is pseudoplastic, and apparent viscosity decreases with increasing shear rate. Polypropylene homopolymer or random copolymer can be dispersed as a hard phase modifier. Paraffinic white mineral oil may be added up to 50–100 phr depending on target hardness, but oil should be injected after the initial melting zone to maintain high local shear for distributive mixing. Injection molding of SEBS 7720 compounds uses melt temperatures of 190–230 °C and mold temperatures of 20–50 °C. Pre-drying at 80 °C for 2–3 h in a desiccant dryer is recommended when pellets have been stored at relative humidity above 60%. Although the saturated midblock has low equilibrium moisture uptake, surface moisture can produce splay in molded parts.

Compounding Rheology at L/D 40:1

In a 75 mm co-rotating twin-screw extruder processing a 35 wt% SEBS 7720, 20 wt% PP homopolymer, and 45 wt% white mineral oil formulation, die pressure typically ranges from 4–8 MPa at screw speeds of 250–350 rpm. Specific mechanical energy input for this formulation commonly falls between 0.20 kWh/kg and 0.28 kWh/kg. Increasing the PP fraction above 30 wt% raises melt viscosity in the early mixing zones and can produce torque-limited operation on extruders below 60 mm screw diameter. The processing window narrows when total oil loading exceeds 60 phr because phase separation may cause oil migration and unstable pelletization. A barrel set-point deviation of ±5 °C near the feed throat can shift melting onset and increase screw barrel wear. Zones above 230 °C should be avoided for residence times over 5 min to limit viscosity reduction from chain scission.

Feed-throat temperature control is a further boundary condition. If the feed section exceeds 50 °C, softened pellets may prematurely form a melt film on the screw root and bridge the feed throat. This failure mode can reduce feed rate by 10–20% and raise melt temperature by 5–10 °C, shifting final compound hardness because of incidental shear heating. Feed-throat jackets maintained at 30–40 °C prevent this condition in high-throughput compounding lines.

When higher flow is required for thin-wall injection molding

SEBS 7720 is not the highest-flow SEBS in the Globalprene range. For wall sections below 1 mm, a melt mass-flow rate above 10 g/10 min is often required to prevent short shots. Addition of 10–20 wt% controlled-rheology polypropylene with melt flow rate of 25–35 g/10 min under ISO 1133-1:2022 at 230 °C/2.16 kg reduces compound viscosity and increases modulus, but elongation at break decreases by 15–30% relative to neat SEBS 7720. Paraffinic oil addition above 60 phr improves flow but can cause tackiness and oil bleed-out. The choice between PP modification and blending with a high-flow SEBS depends on whether the final part must retain Shore A hardness below 60 or compression set below 35% at 70 °C.

Steam sterilization at 121 °C for 30 min is a common requirement for medical elastomers. Compounds based on SEBS 7720 with 0.2–0.3 wt% high-molecular-weight hindered phenolic antioxidant and no more than 30 phr oil have shown tensile strength retention above 90% after 1–3 autoclave cycles in screening studies. Repeated cycles beyond 5 can reduce elongation at break by 5–10%, depending on oil type and antioxidant loading. Medical device manufacturers must qualify final compounds under ISO 10993-5 for cytotoxicity and ISO 10993-10 for irritation and sensitization, because additive packages and processing residuals are formulation-specific.

Mechanical Property Benchmarks for Neat Compression-Molded Sheets

The following nominal ranges are suitable for initial formulation screening. Final part properties depend on molding thermal history, orientation, and additive package.

Where the grade is less suitable than maleic anhydride-grafted SEBS

SEBS 7720 is non-functionalized and exhibits low spontaneous adhesion to polar substrates such as polyamide 6, polybutylene terephthalate, aluminum, and glass. Overmolding onto engineering thermoplastics requires either a tie layer, plasma or corona surface activation, or replacement with a maleic anhydride-grafted SEBS. Corona treatment raises surface energy, but adhesion decay can occur within 24–72 h unless the part is immediately bonded or coated. In adhesive and sealant formulations, tackifier compatibility follows the aliphatic midblock. Hydrogenated hydrocarbon resins produce clear blends, whereas highly aromatic C9 resins may phase-separate and reduce peel adhesion. Toluene and cyclohexane swell the styrene domains and are suitable for solution coating; ketones, esters, and alcohols are poor solvents at room temperature.

Oil-extended SEBS grades containing 30–50% supplier-added paraffinic oil reduce compounding steps and produce lower hardness without high-melt-temperature oil injection. However, neat SEBS 7720 provides wider formulation latitude because the compounder controls oil type, oil addition point, and hard phase ratio. For applications requiring hardness below 30 Shore A, an oil-extended grade may be preferable. SEBS 7720 with more than 100 phr oil becomes difficult to pelletize and usually exhibits compression set at 70 °C above 60%.

For flexible PVC replacement and soft-touch applications, SEBS 7720 compounds can meet the physical requirements of consumer goods when formulated with polypropylene and mineral oil. The product does not contain ortho-phthalate plasticizers. Regulatory statements such as EU REACH registration, RoHS screening under IEC 62321, and food-contact compliance are grade- and lot-specific. Processors should request current documentation from LCY Chemical for the intended production site. Shelf life for neat pellets stored below 30 °C and 50% relative humidity is typically 24 months from the certificate of analysis date.