Functionalized SSBR Grades

Low Rolling Resistance Tread Compounds for Passenger Radial Tires

In passenger car radial tread formulations, silane-functionalized SSBR grades are compounded with precipitated silica at loadings between 70 phr and 95 phr to reduce rolling resistance while maintaining wet grip. Mixing is performed in an internal mixer with intermeshing rotors, typically a 4-wing H rotor configuration, at a fill factor of 0.70 to 0.75. The silanization reaction between the alkoxysilane functionality of the functionalized SSBR and the silanol groups on silica requires a dump temperature controlled within 145°C to 155°C. Below 140°C the coupling reaction remains incomplete, producing higher Payne effect and elevated tan δ at 60°C. Above 160°C the polymer-bound silane begins to undergo secondary condensation, which raises Mooney viscosity and reduces extrudate surface quality. Two-stage mixing is mandatory for this compound class. The first non-productive stage incorporates silica, silane coupling agent, zinc oxide, stearic acid, and 6 PPD antioxidant at a rotor speed of 40 to 50 rpm for 180 to 240 seconds. The second non-productive stage remills the batch at 35 to 40 rpm without curatives to complete silanization. Sulfur, CBS accelerator, and DPG are added in the final productive stage at a maximum dump temperature of 100°C to avoid scorch. Vulcanization kinetics follow a reversion-resistant cure system; typical cure rheometer data at 160°C show t90 of 8 to 12 minutes with a maximum torque MH of 18 to 22 dN·m depending on silica surface area and silane dosage. Rolling resistance prediction from dynamic mechanical thermal analysis uses tan δ at 60°C, 10 Hz, and 1% strain; functionalized SSBR compounds with 80 phr of 175 m²/g silica typically achieve tan δ values between 0.090 and 0.120. The Payne effect, measured as the difference in storage modulus between 0.1% and 10% strain at 100°C, is kept below 0.8 MPa in adequately silanized batches. Extrusion of the tread profile through a pin barrel extruder with a 150 mm screw diameter and an L/D ratio of 16:1 requires a head pressure between 90 and 120 bar. The die swell at 40°C is controlled between 15% and 25% to maintain dimensional stability during the continuous tread extrusion line.

What Causes Continuous Phase Inversion in Silica-Reinforced SSBR for Winter Tire Treads?

High-cis polybutadiene is blended with functionalized SSBR in winter tire treads to improve low-temperature flexibility and abrasion resistance. The blend ratio varies from 60/40 to 75/25 of SSBR to BR, with the functionalized SSBR phase acting as the continuous matrix when its Mooney viscosity ML 1+4 at 100°C remains below 65 MU. Phase inversion is prevented by controlling the viscosity ratio between the two polymers; the BR component is selected with a Mooney viscosity at least 10 MU lower than the SSBR grade at the same test temperature. Mixing follows a sequential polymer blending protocol in a tangential internal mixer with a 1.5 L chamber volume. The two polymers are first masticated together for 60 seconds at 30 rpm before silica addition. Silica is added in two splits, with the first 60% introduced with half the silane and the second 40% introduced with the remainder after an additional 60 seconds of mixing. This split addition prevents over-lubrication of the mixer walls and maintains shear stress above 1.0 MPa during the silanization window. The glass transition temperature of the functionalized SSBR chosen for winter traction is typically between -30°C and -20°C, while the BR component has a Tg near -100°C. The resulting dynamic mechanical spectrum shows two loss peaks, but the functionalized SSBR peak dominates the temperature range from -25°C to 0°C, which corresponds to wet and cold road braking frequencies. Tan δ at 0°C is maintained between 0.300 and 0.450, while tan δ at -20°C remains above 0.250. Silane coupling efficiency is monitored by the bound rubber content after the second non-productive mixing stage; values between 40% and 55% are considered acceptable for silica loadings of 85 phr. Inadequate coupling produces a bound rubber content below 30% and increases tan δ at 60°C above 0.160, which negates the rolling resistance benefit of the functionalized polymer. The extruded winter tread sheet passes through a calender at a line speed of 25 to 35 m/min with a roll temperature of 80°C to 90°C. Tread porosity is controlled below 2% by maintaining a minimum head pressure of 85 bar and a minimum screw speed of 25 rpm during extrusion.

Damping Layer Compounds with Controlled Crosslink Density in Engine Mounts

Functionalized SSBR is used in engine mount damping compounds when high vibration isolation over a broad temperature range is required, typically blended with EPDM at a ratio of 30/70 or 40/60 of SSBR to EPDM. The functionalized SSBR provides improved fatigue resistance and tear strength, while the EPDM phase contributes heat and ozone resistance. The cure system is based on a semi-EV formulation with sulfur as the crosslinking agent at 0.8 to 1.5 phr, TBBS as the primary accelerator at 1.2 to 2.0 phr, and TMTD at 0.2 to 0.5 phr as a secondary accelerator. This cure system produces a predominantly mono- and disulfidic crosslink network, which is necessary for low compression set and improved dynamic fatigue performance. Compression set is tested according to ASTM D395 Method B after 22 hours at 100°C; values below 20% are achieved with the semi-EV cure in compounds containing 40 phr functionalized SSBR. Dynamic stiffness and damping are measured on double-shear test specimens according to ISO 10846. The loss factor at 15 Hz and 25°C is maintained between 0.12 and 0.18, while the dynamic-to-static stiffness ratio remains below 1.6 at 100 Hz. Carbon black N550 is added at 20 to 40 phr in combination with 20 to 30 phr of a naphthenic oil to tune damping without sacrificing low-temperature flexibility. Mixing is carried out in a two-stage cycle; the first stage incorporates all dry ingredients at a dump temperature of 150°C, and the second stage adds sulfur and accelerators at a maximum dump temperature of 95°C. The functionalized SSBR component must be pre-masticated for 90 seconds before adding EPDM to prevent uneven phase distribution and localized high viscosity zones. The bound antioxidant system uses TMQ at 1.0 phr and 6 PPD at 1.5 phr, as the functionalized SSBR phase is susceptible to oxidative degradation at sustained service temperatures above 90°C. Fatigue life in shear, tested at 2 Hz to 100% shear strain, shows crack initiation after 200,000 to 400,000 cycles, and the crack growth rate remains below 0.5 mm per 100,000 cycles in compounds with adequate coupling agent dispersion.

High-hardness functionalized SSBR compounds are selected for automotive serpentine synchronous belt backings where dynamic crack growth resistance under repeated flexing is the primary performance criterion. The compound uses a silica/carbon black N234 dual filler system at a total filler loading of 65 to 85 phr. The silica fraction is limited to 25 to 35 phr to maintain adhesion to the treated polyester tension member without sacrificing modulus. A mercaptosilane coupling agent is used at 4 to 6 wt% of the silica loading. The mixed compound reaches a Shore A hardness of 75 to 85 after vulcanization, with tensile strength above 18 MPa measured according to ISO 37. Tear strength, measured with the trouser tear specimen of ISO 34-1, exceeds 35 N/mm. The functionalized SSBR improves fatigue resistance under constant energy input; belt flex testing at 100°C and 3000 cycles per minute shows no visible cracking after 200 hours. The compound is processed through a roller-head extruder with a 90 mm screw and a 10:1 L/D ratio. The sheet is calendered to a thickness tolerance of ±0.2 mm at a roller temperature of 60°C to 70°C. To achieve this tolerance, the compound requires a Mooney viscosity ML 1+4 at 100°C between 50 and 65 MU. Lower viscosity grades cause edge waviness and thickness variation, while higher viscosity grades generate excessive heat in the calendar nip and scorch risk.

When Functionalized SSBR Replaces Natural Rubber in Vibration Isolation Bushings

In rubber-to-metal bonded bushings, functionalized SSBR is used as a partial replacement for natural rubber at levels from 30 to 50 phr to improve low-temperature damping and reduce dynamic stiffening with frequency. The bonding system is based on a two-coat adhesive, typically a silane-based primer and a chlorinated rubber cover cement compatible with sulfur-cured diene elastomers. The brass or zinc-phosphated steel insert is degreased and shot-blasted to a surface roughness Ra of 2.5 to 5.0 µm before adhesive application. The compound contains 20 to 30 phr of carbon black N330 and 10 to 20 phr of a precipitated silica with a CTAB surface area of 150 to 180 m²/g. Silane coupling agent is required only when the silica content exceeds 15 phr; below this level, the silica functions primarily as an adhesion promotor and tear strength modifier. The cure system is a conventional sulfur system with 2.0 to 2.5 phr sulfur, 0.8 to 1.2 phr CBS, and 0.3 to 0.5 phr DPG. Vulcanization is performed in a compression molding press at 160°C for 8 to 12 minutes under a clamp force of 150 to 250 tonnes for multi-cavity tools. The demolding force is reduced by applying a semi-permanent release agent to the tool surface, but the rubber-to-metal insert must remain free of release agent to avoid bond failure. Bond strength is tested by the cone pull-out method according to ASTM D429 Method A; the rubber must show 100% cohesive failure with no adhesive interfacial failure. The functionalized SSBR reduces compression set after 72 hours at 70°C to below 15%, compared with values above 25% for equivalent natural rubber formulations. However, the low elongation at break of the functionalized SSBR phase, typically 350% to 450%, limits its use in bushings subjected to strains above 150%. The operational temperature window is -30°C to 80°C; above 80°C the thermo-oxidative degradation of the diene backbone reduces the service life to fewer than 3,000 hours.

Conveyor Belt Cover Stock for Cold-Room Sorting Lines

Functionalized SSBR is utilized in cover compounds for conveyor belts operating in cold-storage environments at temperatures down to -35°C. The compound design uses a functionalized SSBR with a vinyl content between 25% and 35% and a styrene content of 20% to 25% to shift the glass transition region into the service temperature range without excessive stiffening. The compound is reinforced with carbon black N220 at 45 to 60 phr and plasticized with a low-temperature ester plasticizer, typically dioctyl sebacate, at 8 to 12 phr. The ester plasticizer raises the low-temperature flexibility but reduces the dynamic modulus and tensile strength; therefore, the plasticizer content is capped at 12 phr. Tensile strength after vulcanization is tested according to ISO 37 and must exceed 15 MPa. The low-temperature resistance is evaluated by the Gehman torsion test according to ISO 1432; the T10 value, at which the relative modulus reaches 10 times the room temperature value, is below -40°C. Abrasion resistance measured according to ISO 4649 Method A yields a relative volume loss below 120 mm³ when tested against a non-standard reference compound. The cover stock is applied to a polyester/nylon EP fabric carcass through a triple-roll calender at a line speed of 10 to 20 m/min. The calender bowl temperature is maintained at 65°C to 80°C. The compound must be pre-heated on a two-roll mill to a bank diameter of 15 to 25 mm to ensure uniform feed. The cured belt is tested for cold resistance by impact after conditioning at -35°C for 24 hours; no cracking is observed when a 2 kg steel ball is dropped from 1.5 m onto the cover surface. The functionalized SSBR cover stock is not recommended for belts in contact with oils or greases, as the polar functional groups increase swelling in hydrocarbon oils above 50°C. For oil-resistant cold-room belts, the functionalized SSBR is blended with NBR at ratios of 50/50 to 70/30, but this blend raises the low-temperature flexibility limit to -25°C.

Shoe Sole Compounds with Wet Slip Resistance and Flex Fatigue Performance

In direct-on injection molding for safety footwear soles, functionalized SSBR is used as the primary elastomer at 70 to 90 phr, blended with BR or EPDM for improved flex life and ozone resistance. The compound is filled with precipitated silica at 20 to 40 phr and a hydrocarbon resin at 5 to 10 phr to achieve wet slip resistance without excessive hardness. The functionalized SSBR disperses the silica with lower mixing energy than non-functionalized grades, but a silane coupling agent is still added at 2 to 4 phr to reduce filler-filler networking and improve tear resistance. The sole compound is designed for a hardness of 55 to 65 Shore A, measured according to ISO 868. The coefficient of friction on wet ceramic tile is measured according to ASTM F2913; the functionalized SSBR compound with 30 phr silica achieves a wet coefficient between 0.45 and 0.60. The Ross flex test according to ASTM D1052 uses a 2.5 mm pierced sample flexed at 100 cycles per minute; crack growth after 150,000 cycles remains below 4 mm. The injection molding process uses a screw plasticating unit with an L/D ratio of 20:1 and a compression ratio of 2.5:1. Barrel temperature zones are set from 60°C in the feed zone to 85°C at the nozzle, and the mold temperature is held at 170°C to 190°C. Injection pressure is maintained between 80 and 120 MPa, with a holding pressure of 40 to 60 MPa for 5 to 10 seconds. The functionalized SSBR compound must contain a processing aid such as fatty acid ester at 1 to 2 phr to prevent sticking to the mold and to reduce cycle time. The sole component is co-molded with a polyurethane midsole in some designs; adhesion between the SSBR outsole and PU midsole requires a halogenated primer applied to the SSBR surface after buffing and cleaning. Without the primer, bond strength is below 2 N/mm in peel testing. The functionalized SSBR sole compound is not suitable for direct contact with PVC, because the functional groups and plasticizer migration can cause discoloration and surface tack.

Functionalized SSBR is incorporated into rubber track pads for compact construction equipment where cut growth resistance and heat build-up control determine replacement intervals. The compound contains 50 to 70 phr of functionalized SSBR and 30 to 50 phr of BR. The filler system uses carbon black N234 at 50 to 70 phr, with silica limited to 10 phr or less because higher silica loadings increase the DeMattia cut growth rate under high severity flexing. The vulcanization package is based on a conventional sulfur system with 1.8 to 2.5 phr sulfur and 0.8 to 1.2 phr TBBS. The heat build-up measured by the Goodrich flexometer according to ISO 4666-3 remains below 30°C after 25 minutes at 1 MPa static load and 4.45 mm stroke. The cut growth resistance is tested according to ISO 132 using a pierced strip specimen; the functionalized SSBR compound shows cut growth below 10 mm after 100,000 cycles. The track pad is compression molded in a hydraulic press at 150°C under 20 MPa for 15 to 25 minutes depending on pad thickness. The functionalized SSBR compound requires storage in a temperature-controlled area at 25°C or lower, because the functional groups can absorb moisture and generate microporosity in thick sections during vulcanization. The shelf life of the uncured compound is limited to 4 weeks in unlined cardboard packaging. For track pads exposed to continuous temperatures above 80°C, the functionalized SSBR phase is protected by adding 2.0 phr of TMQ and 1.0 phr of 6 PPD. The use of a bimodal SSBR grade with one high-molecular-weight component improves green strength, but reduces flow into the deep lugs of the mold; therefore, the mold design must include a minimum radius of 3 mm at the lug base to prevent non-fill defects.

High-Damping Flooring Underlayments and Impact Sound Insulation Systems

Functionalized SSBR is used in filled flooring underlayment sheets where static load recovery and impact sound reduction in floating floor systems are specified. The compound is loaded with 150 to 250 phr of ground calcium carbonate with a median particle size of 5 to 10 µm and plasticized with 30 to 50 phr of a paraffinic process oil. The high filler loading reduces cost and increases density to 1.6 to 1.9 g/cm³, which improves impact sound insulation in the frequency range from 100 Hz to 3,150 Hz according to ISO 10140-3. The dynamic stiffness per unit area is tested according to ISO 9052-1; the functionalized SSBR underlayment achieves an apparent dynamic stiffness between 10 and 30 MN/m³ at a load of 2 kPa. The compound is calendered into sheets of 2 to 5 mm thickness, with a roller temperature of 50°C to 70°C and a line speed of 10 to 20 m/min. The calendered sheet is vulcanized in a hot air tunnel at 180°C for 5 to 10 minutes. The functionalized SSBR improves tear resistance of the highly filled compound; trousers tear strength exceeds 8 N/mm, whereas equivalent EPDM-based compounds show tear strength below 5 N/mm at the same filler loading. The compression set after 24 hours at room temperature under a static load of 7 kPa remains below 5% because the filler network resists permanent deformation. The compound must be protected from moisture before processing; upstream storage at relative humidity above 60% requires pre-drying of the filler and polymer for at least 2 hours at 80°C to avoid water vapor porosity. The underlayment is not recommended for direct outdoor exposure, as the diene backbone degrades under UV radiation, causing surface cracking and loss of tensile strength. For indoor floating wood floors with underfloor heating, the functionalized SSBR sheet is used below a 0.2 mm polyethylene vapor barrier film; the service temperature must not exceed 50°C at the heating pipe level to prevent plasticizer migration and hardening.

In automotive interior floor mats, functionalized SSBR is incorporated into thermoplastic elastomer compounds based on polypropylene at elastomer loadings of 40 to 60 wt%. The functionalized SSBR phase is dynamically vulcanized in a co-rotating twin-screw extruder with an L/D ratio of 48:1 and a screw diameter of 50 mm. The peroxide cure system is based on 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane at 0.2 to 0.5 phr, with triallyl cyanurate as co-agent at 0.2 to 0.4 phr. The vulcanization reaction is completed in the second half of the extruder barrel after the polypropylene melting zone; the residence time in the dynamic vulcanization zone is 20 to 40 seconds at a melt temperature of 200°C to 220°C. The resulting thermoplastic vulcanizate has a Shore A hardness of 55 to 75 and a compression set below 35% after 22 hours at 70°C according to ASTM D395. The functionalized SSBR improves the low-temperature flexibility of the TPV, maintaining a brittle point below -40°C. The compound also shows improved paint adhesion after flame or plasma surface activation compared with non-functionalized SSBR or EPDM-based TPVs. Injection molding of floor mats uses a mold with a grained surface and a draft angle of at least 3° to prevent sticking of the soft functionalized SSBR phase. Mold temperature is set between 20°C and 40°C, and cycle times range from 40 to 70 seconds for 2 mm thick mats. The sheet or mat may be overmolded with a glass-fiber reinforced polypropylene carrier; adhesion between the TPV and the carrier is achieved by mechanical interlocking into a patterned insert surface. Fogging characteristics are tested according to ISO 6452; the functionalized SSBR compound must show a reflectometric fogging value above 80% to meet automotive interior specifications. The compound must avoid amine-based heat stabilizers, because residual amines accelerate the dehydrochlorination of any PVC components in contact with the mat and can also interfere with peroxide crosslinking during dynamic vulcanization.