Kraton Kraton™G SEBS-Comp. G2705 GU-N is supplied as a linear styrene-ethylene/butylene-styrene block copolymer pellet for compounding and polymer modification. The ethylene/butylene midblock is produced by selective hydrogenation of the butadiene-derived elastomer segment, removing main-chain unsaturation and altering oxidative degradation mechanisms relative to SBS. The grade carries a nominal bound styrene content of 30 wt% and a typical specific gravity of 0.91; exact values are lot-specific and are reported on the certificate of analysis. The product is not a ready-to-mould finished elastomer but a high-styrene SEBS base polymer intended for formulation with polyolefins, paraffinic oils, resins, and mineral fillers.

The SEBS triblock architecture consists of two terminal polystyrene blocks and a central ethylene/butylene segment. When cooled from melt, the polystyrene blocks aggregate into glassy domains with characteristic sizes from 10 nm to 30 nm, while the ethylene/butylene midblock forms a continuous elastomer phase. The domain structure provides physical crosslinks that are thermally reversible, allowing reprocessing at temperatures above the polystyrene glass transition. Because the styrene content is 30 wt%, the equilibrium morphology is dominated by spherical or short-cylinder polystyrene domains in an ethylene/butylene matrix; higher styrene contents would shift the domain geometry toward lamellar structures. This morphology controls the balance of stiffness, tensile set, and elastic recovery observed in unfilled compression-moulded specimens.

Where Does G2705 GU-N Sit in the SEBS Viscosity and Service-Temperature Ladder?

Within the Kraton G-series, G2705 GU-N is differentiated from higher-molecular-weight grades such as G1650 and G1652 by its melt and solution rheology; the specific solution viscosity value should be verified against the current technical data sheet because published data for this specific configuration is limited. The 30 wt% styrene content places it in the higher-styrene segment of the SEBS range, above G1657 at 13 wt% styrene. This composition increases the volume fraction of dispersed polystyrene domains, raising room-temperature Shore hardness and tensile set while reducing low-temperature flexibility relative to low-styrene grades. The ethylene/butylene midblock contributes a sub-ambient glass transition near −45 °C, and the polystyrene domains soften near 95 °C, defining the upper limit for load-bearing applications. In injection-moulded or compression-moulded parts, G2705 GU-N is selected when a higher modulus and hardness are required without the addition of crystalline polyolefin.

A co-rotating twin-screw extruder with L/D ratio between 40:1 and 48:1 is commonly used for compounding this grade with paraffinic process oil and polypropylene homopolymer. Barrel temperatures from 180 °C to 230 °C are typical, with the feed zone held below 120 °C to prevent pellet agglomeration in the hopper. Moisture content above 0.05 wt% can cause surging, surface splay, and reduced melt stability; pellets conditioned at relative humidity above 60% should be pre-dried in a desiccant dryer at 70–75 °C for 2–3 h. Screw configurations for filled systems should include kneading blocks early in the melt zone for dispersion of calcium carbonate or talc, followed by reverse elements to control fill length. Excessive shear heating above 250 °C can initiate chain scission of the ethylene/butylene midblock, producing an increase in melt flow rate and a loss of tensile properties.

When SBS Is Replaced by SEBS in Thermally Loaded Compounds

Substitution of SBS with G2705 GU-N in adhesive, sealant, and moulded compounds alters long-term aging behaviour because the saturated midblock lacks the double bonds responsible for SBS oxidative crosslinking and embrittlement. In forced-air aging at 70 °C and 100 °C, SEBS formulations typically show lower carbonyl-index development than SBS controls when measured by FTIR; retention depends on the stabiliser package, oil type, and filler surface chemistry. The improved resistance to chain scission allows G2705 GU-N to be processed at melt temperatures 20–30 °C higher than conventional SBS compounds, but it does not eliminate the need for UV stabilisers in outdoor applications. Hindered-amine light stabilisers and benzotriazole UV absorbers are added at 0.1–0.5 wt% for UV exposure, and exposure testing according to ASTM G154 Cycle 1 is used to rank formulation stability. Organotin mercaptide thermal stabilisers used in PVC should be avoided in SEBS systems due to phase incompatibility.

Oil Extension, Filler Loading, and Hardness Depletion in G2705 GU-N Systems

Paraffinic white mineral oils are chosen for G2705 GU-N because the ethylene/butylene midblock absorbs aliphatic oil without large-scale phase separation. Naphthenic oils can be used at lower addition levels; aromatic oils are avoided because they preferentially solvate the polystyrene end-blocks and depress the upper service temperature. A starting range of 30–150 phr oil is common in soft-touch compounds, with Shore A hardness typically falling from the neat SEBS range of 60–70 to below 5 at high oil loadings. Above 150 phr oil, tensile strength and elastic recovery decline steeply, and the compound may exhibit surface oil exudation. Calcium carbonate and talc at 5–30 wt% reduce cost and increase stiffness; filler loadings above 40 wt% can create melt-pressure fluctuations in single-screw extrusion and require twin-screw dispersion. Polypropylene addition at 20–40 wt% improves processability and raises the softening point; above 50 wt% polypropylene, the continuous phase inverts to a thermoplastic matrix and the elastic recovery of the SEBS phase is largely lost. Shore hardness is measured according to ISO 868 or ASTM D2240 after a 10 s delay, while tensile set is evaluated at 100% strain according to ISO 2285.

When Residence Time Exceeds 10 Minutes in High-Shear Processing

In high-shear compounding, G2705 GU-N degradation is most frequently detected as an increase in melt flow rate, a drop in solution viscosity, and the appearance of a low-molecular-weight volatile fraction. The onset of chain scission is accelerated by oxygen ingress through open feed ports and by barrel temperatures above 230 °C. Processors should monitor melt temperature with an infrared probe at the die and maintain specific mechanical energy below 0.25 kWh/kg for unfilled systems; filled systems may exceed this value because of the additional work required for filler dispersion. A nitrogen blanket on the hopper and a closed vent stack reduce oxidative degradation. For quality assurance, a 10 wt% solution in toluene at 25 °C is used to track lot-to-lot viscosity; a deviation greater than ±10% from the reference lot indicates a change in molecular weight or contamination.

Determining Regulatory Limits for General-Purpose SEBS in Converted Articles

The GU-N suffix identifies a general-purpose supply form and does not by itself establish food-contact, medical, or pharmaceutical compliance. SEBS polymers of this type are often evaluated under FDA 21 CFR 177.1810 for styrene block copolymers when used in contact with food, and under EU Regulation 10/2011 for plastics intended for food contact; however, the finished compound must be tested by the converter because oils, resins, and pigments affect overall migration. REACH registration for the base polymer is maintained according to EU EC 1907/2006. RoHS Directive 2011/65/EU compliance is generally limited to lead, mercury, cadmium, hexavalent chromium, PBB, and PBDE below threshold limits; halogen-free declarations require verification of flame-retardant additives rather than the hydrocarbon base polymer. For medical applications, ISO 10993-5 cytotoxicity evaluation is performed on the final sterilised article, not on the raw SEBS granulate.

Hot-melt and solvent-borne adhesives formulated with G2705 GU-N typically use tackifying resins that are compatible with the ethylene/butylene midblock, such as rosin esters, hydrogenated hydrocarbon resins, and polyterpene resins. The weight ratio of tackifier to SEBS is commonly 0.5:1 to 3:1, and the addition of paraffinic oil at 20–80 phr reduces viscosity. Formulations are coated at processing temperatures between 150 °C and 180 °C using slot-die or roll coaters; solvent-borne systems are prepared at solids levels limited by solution viscosity. The saturated midblock provides improved thermal stability during hot-melt aging at 177 °C compared with SIS or SBS alternatives, but prolonged residence times above 200 °C can degrade tackifier resins and should be avoided. ASTM D4498 and ASTM D3654 are used to measure hot-melt open time and shear adhesion failure temperature.

Differentiation from G1650, G1652, and G1657 Requires Rebalanced Oil and Polypropylene Levels

G1650 and G1652 are higher-molecular-weight SEBS grades with the same nominal polystyrene content of about 30 wt%; G1650 produces higher solution and melt viscosity, while G1652 offers lower viscosity and easier processing. G2705 GU-N is positioned for applications requiring further melt-flow improvement and more rapid wetting of fillers and polypropylene. G1657 has a bound styrene content of 13 wt%, yielding lower Shore hardness and modulus, higher elongation, and a softer feel than G2705 GU-N. In adhesive formulations, solution viscosity determines coating behaviour; a lower-viscosity grade such as G2705 GU-N permits higher solids at a given coating viscosity, but shear strength can be lower than with G1650. The exact ratio of styrene block to ethylene/butylene block governs the balance between tensile strength and elastic recovery, so reformulation from G1652 to G2705 GU-N should include rebalancing of oil and polypropylene levels according to ISO 37 and ISO 868.

In footwear sidewall compounds, G2705 GU-N is typically blended with polypropylene homopolymer, paraffinic oil, and silica or calcium carbonate. The compound is injection-moulded at clamp-force settings sufficient to maintain a cavity pressure of 30–60 MPa, with mould temperatures below 40 °C to prevent ejection distortion. The resulting parts show elastic recovery values above 90% when tested at 100% strain after 10 min relaxation, provided the formulation remains below the phase-inversion limit. Published data for this specific configuration is limited, so compounding trials with a designed experiment are required to establish the exact processing window.