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SIS thermoplastic elastomer is a styrenic block copolymer consisting of polystyrene terminal blocks and an unsaturated polyisoprene midblock. The block sequence is normally obtained by anionic polymerisation, which yields a narrow molecular weight distribution with polydispersity index 1.01–1.10 when measured by gel permeation chromatography per ISO 13885; total number-average molecular weight ranges from 80,000 g/mol to 180,000 g/mol across commercial grades. The polystyrene domains exhibit a glass transition temperature near 95 °C and the polyisoprene matrix near −60 °C as determined by differential scanning calorimetry per ISO 11357-2. This thermodynamic incompatibility produces physically crosslinked networks that do not require sulphur vulcanisation; the polystyrene domains soften and disrupt above the order-disorder transition, typically 120–150 °C, allowing melt processing. Typical styrene content is 14–30 wt%, and diblock content can range from 0% to 50%, the latter reducing tensile strength while increasing pressure-sensitive tack. Mechanical properties measured on compression-moulded sheets per ISO 37 type 2 dumbbells include tensile strength 10–25 MPa and elongation at break 500–1200%; hardness per ISO 868 spans Shore A 20–90. Specific gravity per ISO 1183-1 is 0.92–0.96, and moisture absorption per ISO 62 is below 0.1%. The midblock vinyl content is usually 5–12 mol%; higher vinyl levels raise the soft-phase glass transition and reduce low-temperature flexibility.

What Limits the Oxidative Stability of Unsaturated Midblock Sequences?

The unsaturated polyisoprene midblock of SIS contains cis-1,4, trans-1,4, and 3,4-addition units that are susceptible to radical oxidation, chain scission, and crosslinking during melt processing or long-term service. Unstabilised SIS exhibits an oxidative induction time measured by differential scanning calorimetry per ISO 11357-6 or ASTM D3895 that is typically below 10 min at 190 °C, whereas stabilised formulations containing a hindered phenolic primary antioxidant at 0.1–0.3 wt% and a phosphite secondary antioxidant at 0.1–0.2 wt% can exceed 30 min. Ultraviolet exposure per ASTM G154 cycle 1 or xenon-arc testing per ISO 4892-2 causes surface chalking, gloss loss, and embrittlement of unpigmented SIS films; published data for specific formulations are limited, but ultraviolet stabilisers at 0.2–0.5 wt% are normally required for outdoor exposure. The presence of double bonds also means that certain amine-based antidegradants should be avoided because they can stain substrates and produce viscosity instability in acidic tackifier blends. Formulations intended for long-term thermal stability should limit processing temperature to 200 °C and use nitrogen blanketing during hot-melt holding.

Capillary rheometry of SIS compounds according to ISO 11443 shows pronounced shear thinning between 100 s⁻¹ and 1000 s⁻¹. Melt flow rate measured by ISO 1133-1 procedure A at 200 °C with 5 kg load ranges from 3 g/10 min to 60 g/10 min depending on total molecular weight and styrene content; hot-melt adhesive grades typically occupy the higher end, while high-tensile elastic film grades occupy the lower end. Apparent viscosity at 180 °C and 100 s⁻¹ for adhesive compounds falls between 50 Pa·s and 500 Pa·s. Continuous melters, gear pumps, and slot-die systems are operated at 150–180 °C for hot-melt adhesives, while extrusion film lines require 190–220 °C to achieve adequate melt strength and nonwoven penetration. Co-rotating twin-screw extruders used for compounding SIS with tackifiers and oils typically have L/D ratios of 30:1 to 44:1 and screw speeds of 200–400 min⁻¹. Moisture absorption is sufficiently low that pre-drying is not mandatory for dry pellets, but pellets stored at relative humidity above 60% require dehumidified drying at 60 °C for 2 h to prevent surface splay in cast film or slot-die coating.

Pressure-Sensitive Adhesive Formulation Variables and Tackifier Loading Boundaries

Hot-melt pressure-sensitive adhesives formulated with SIS depend on selective tackifier association with the polyisoprene midblock and endblock reinforcement for shear resistance. C5 aliphatic hydrocarbon resins, hydrogenated C5 resins, rosin esters, and terpene phenolic resins are commonly used; aromatic-modified resins associate more strongly with the polystyrene domains and can raise cohesive strength but reduce tack. The typical formulation window consists of SIS 20–40 wt%, tackifier 40–60 wt%, naphthenic or paraffinic oil 0–20 wt%, and antioxidant 0.5–1.2 wt%. Tackifier loading above 60 wt% dilutes the elastomer network, lowers shear adhesion failure temperature, and can produce adhesive transfer in paper label applications; loading below 40 wt% reduces wet-out on low-energy surfaces such as polyethylene. The following table summarises the formulation space and the test methods used for quality control.

Slot-die coating heads with adjustable lip gaps of 0.1–0.5 mm deliver melted SIS adhesive onto release liners at line speeds from 50 m/min to 300 m/min. Reservoir temperature is normally held at 150–170 °C with a nitrogen blanket at 0.2–0.5 bar positive pressure; recirculation is maintained through 100 mesh screen packs to remove gel particles. Coating weight is verified gravimetrically per ASTM D6463 or equivalent, and peel adhesion is conditioned for 24 h at 23 °C and 50% relative humidity before testing per ASTM D3330.

In solvent-borne contact adhesives, SIS is dissolved in toluene or cyclohexane at solids contents of 15–25 wt%; Brookfield viscosity measured per ASTM D2196 ranges from 500 mPa·s to 5000 mPa·s. Open time after application to leather or styrene-butadiene rubber soles is 3–10 min at 23 °C and 50% relative humidity, and bonding requires nip pressure of 0.3–0.8 MPa for 10–30 s. The unsaturated midblock provides faster solvent release and tack development than hydrogenated SEBS, but resistance to plasticizer migration from plasticised polyvinyl chloride is lower; testing per ASTM D5402 or internal peel retention under load is used to verify bond durability when such substrates are involved.

When Hot-Melt Tanks Exceed 175 °C, Residence Time Governs Viscosity Drift

In continuous hot-melt coating, the temperature setpoint creates a process conflict between viscosity reduction and oxidative degradation. At 175 °C, a typical SIS tape adhesive may have a Brookfield viscosity of 8,000–15,000 mPa·s per ASTM D3236; raising the setpoint to 190 °C lowers viscosity by approximately 30–40%, permitting higher line speeds or thinner coating weights. However, production-scale monitoring of melter reservoirs has documented viscosity drift exceeding 10% after 8 h at 175 °C and after less than 4 h at 190 °C in partially stabilised formulations. Gel retention on a 25 µm screen can increase from <0.1% at 175 °C to 0.5–1.0% at 190 °C over the same residence time, causing coating streaks and die lip build-up. The practical processing window for high-molecular-weight SIS adhesive grades is therefore typically ±5 °C around the recommended setpoint when using unblanketed melters; with nitrogen blanketing, the window can be widened by 5–10 °C. Dead-volume corners in rectangular melters must be eliminated or minimised because char particles can detach and create film defects. Start-up and shutdown protocols include purging with mineral oil and reducing setpoint to 130 °C during production pauses longer than 30 min.

In polymer-modified bitumen, SIS is dispersed at 3–8 wt% into paving-grade bitumen using a high-shear rotor-stator mill at 180–190 °C for 2–4 h. Elastic recovery per EN 13398 improves to 60–90%, ring-and-ball softening point per EN 1427 rises by 10–25 °C, and low-temperature flexibility per EN 13587 improves compared with unmodified bitumen. The main limitation is the unsaturated midblock, which degrades during hot storage at 180 °C under air if the antioxidant concentration is below 0.2 wt%; this produces viscosity build, loss of ductility, and possible polymer phase separation. Some asphalt modification operations select SEBS or SIS/SEBS blends where prolonged high-temperature storage or high ultraviolet exposure is expected. Published field data for specific SIS grades in bitumen are limited; comparative laboratory studies under EN 14023 document the required performance envelopes but do not replace job-site validation.

Medical Adhesives Must Satisfy ISO 10993 Before Skin Contact

SIS-based adhesives intended for skin contact require biocompatibility evaluation on the finished device according to ISO 10993-5 for cytotoxicity and ISO 10993-10 for irritation and sensitisation. Certain SIS-containing hot-melt adhesives are formulated to comply with 21 CFR 175.105 for indirect food contact when used in packaging closures, but compliance depends on the complete raw material set and residual monomer levels. Repeated-use rubber articles can fall under 21 CFR 177.2600 with extraction limits for total nonvolatile extractives; each finished adhesive must be tested under the intended food type and use condition. Gamma sterilisation at 25–50 kGy can induce crosslinking, yellowing, and stiffening because of the unsaturated midblock; published data for specific SIS grades under ISO 11137 sterilisation doses is limited, so dose mapping on the finished device is required. Ethylene oxide sterilisation at 55 °C and 30–70% relative humidity is generally less damaging to the polyisoprene midblock but requires volatile organic compound and ethylene oxide residue testing per ISO 10993-7. Amine-based additives are avoided in medical SIS adhesives because of potential cytotoxicity and discoloration. The following compliance matrix lists the commonly referenced frameworks for SIS-containing adhesives.

Extrusion lamination of SIS compounds onto polypropylene nonwovens produces elastic side panels, waistbands, and fastening tabs for hygiene articles. Film thickness is typically 50–150 µm, and melt temperature is maintained at 190–220 °C to achieve bond strength to nonwoven without excessive penetration. Elastic recovery after 300% elongation measured according to a modified ASTM D882 procedure is commonly 80–95%; tension set at 200% strain is 10–25%. Higher melt temperatures reduce viscosity and improve fibre penetration but accelerate degradation if residence time exceeds 5 min. Recycled edge trim from SIS elastic film is normally limited to 10–20 wt% because repeated heat history reduces extensibility and increases gel content. Scrap reclamation requires grinders with chilled blades to prevent sticking and screening through 300 µm mesh before re-extrusion.

Compounding Faults on Co-Rotating Twin-Screw Extruders with L/D 40:1

When SIS is compounded with tackifiers, oils, and fillers on a co-rotating twin-screw extruder with L/D of 40:1, several manufacturing faults can be traced to specific energy input. Undispersed tackifier particles resulting from specific mechanical energy below 0.12 kWh/kg cause fisheyes in cast film and slot-die coating. Conversely, specific mechanical energy above 0.25 kWh/kg can generate local melt temperatures above 220 °C, leading to gel formation from polyisoprene crosslinking and discoloration. Screw configurations with forward kneading blocks staggered at 30° and 60° before a liquid side feed permit faster wetting of solid tackifier while limiting excessive viscous heating. Die pressure fluctuations greater than ±2 bar indicate unmelted domains or inconsistent feed. Vacuum venting at −0.8 bar downstream removes residual moisture and low-molecular-weight volatiles, but insufficient vacuum allows surface defects. Production-scale batch-to-batch variance is controlled by melt flow rate per ISO 1133-1 and by Fourier transform infrared spectroscopy per ISO 4650 to verify styrene and isoprene content before release.