Non‑functionalized SSBR Grades

What Limits Silica Dispersion in Non‑functionalized SSBR Tread Compounds?

In high‑performance passenger car and sport utility vehicle tire tread compounds, non‑functionalized solution styrene‑butadiene rubber is introduced at 60–85 phr of the hydrocarbon rubber phase, with the remaining elastomer typically being high‑cis BR at 15–40 phr. Because the absence of terminal alkoxysilane or amino groups reduces hydrogen bonding between polymer chain ends and precipitated silica surfaces, bound rubber formation after the first internal mixer pass is consistently lower than for functionalized grades; compounders compensate by raising the silane coupling agent addition to 6–10 phr of bis(triethoxysilylpropyl)tetrasulfide or disulfide on a filler‑weight basis. The filler system generally includes precipitated silica at 70–100 phr with carbon black N234 or N330 at 5–15 phr for electrical conductivity and processing stability, while TDAE or treated naphthenic oil is present at 10–37.5 phr depending on whether the selected SSBR grade is oil‑extended. Curatives include zinc oxide at 2–4 phr, stearic acid at 1–2 phr, sulfur at 1.5–2.5 phr, and a mixed sulfenamide/guanidine accelerator system at 1.5–3.0 phr to maintain a stable cure plateau at 155–170 °C. Mixing is performed in an intermeshing internal mixer with a 270–320 L net chamber volume and a fill factor of 0.70–0.75; the silica masterbatch drop temperature is held between 140 °C and 150 °C to drive the silanization reaction without pre‑scorch, while the final stage with curatives is discharged below 110 °C. Tread profiles are formed by a pin‑barrel extruder with an L/D ratio of 10:1–16:1, then cut to length and cured in a bladder press under nitrogen at 155–170 °C for 10–18 min. Regulatory compliance for the finished tire includes Regulation (EU) 2020/740 label classes, UN ECE R117.02 for rolling resistance, wet grip, and rolling sound, and measurement methods ISO 28580:2018, ISO 23671:2021, and ISO 13325:2019. The resultant tread compounds are used for UHP, touring, and electric vehicle tire treads in size ranges including 195/65R15 through 245/45R19. A process boundary is the high vinyl content of certain SSBR grades; above 65 wt% vinyl, the glass transition temperature can exceed −35 °C, and the processing window narrows to approximately ±5 °C during extruder barrel temperature control, with scorch risk rising if the extruder head temperature exceeds 120 °C.

In tire sidewall compounds for radial passenger and commercial tires, non‑functionalized SSBR is employed at 20–35 phr alongside natural rubber at 30–45 phr and high‑cis BR at 25–40 phr, a blend that shifts compound dynamic stiffness without requiring functionalized chain ends. The carbon black system is dominated by N550 and N660 at 45–60 phr, with an aromatic or TDAE oil loading of 5–10 phr; antiozonant 6PPD and antioxidant TMQ are present at 2–4 phr and 0.5–1.5 phr respectively. The cure package comprises sulfur at 1.5–2.2 phr and CBS or TBBS at 0.8–1.5 phr, producing a state of cure consistent with a reversion‑resistant plateau at 150–160 °C cure temperature. Production‑scale mixing normally uses a two‑stage cycle in a tangential internal mixer: the first stage discharges at 135–150 °C after carbon black incorporation, and the second stage adds curatives below 100 °C. Following mixing, the sidewall compound is extruded through a single‑screw extruder with a 12:1–18:1 L/D ratio into flat profiles of 6–10 mm thickness, then calendered onto carcass components before curing in a segmented bladder press at 150–160 °C. Relevant test methods include ASTM D412‑16 tensile properties, ASTM D624‑00(2020) tear strength, ASTM D1149‑18 ozone resistance under 50 pphm and 40 °C, and ASTM D4482‑11(2021) fatigue crack growth, with acceptance limits defined by each tire manufacturer due to the absence of a single harmonized sidewall standard. The resulting sidewall and shoulder insert components are fitted to radial passenger car, light truck, and truck bus tires; reduced SSBR loadings above 35 phr are avoided in sidewalls where cyclic flex fatigue under 10–30% local strain can initiate crack growth, because the higher vinyl fraction increases heat build‑up under repeated deformation compared with BR‑rich control compounds.

Underground Mining Conveyor Cover Compounds with Carbon Black Reinforcement

Underground mining conveyor belt cover compounds based on non‑functionalized SSBR are formulated with 30–60 phr SSBR and 40–70 phr natural rubber to balance cut resistance, tear strength, and black‑filled abrasion resistance. Carbon black N220 is added at 40–60 phr, plasticized by TDAE oil at 5–10 phr, and protected with TMQ and 6PPD at a total antioxidant loading of 1.5–3.0 phr. Zinc oxide at 3–5 phr and stearic acid at 1–2 phr are used as activators; sulfur at 1.8–2.5 phr with TBBS at 0.9–1.6 phr provides a balance between scorch safety during calendering and modulus development during continuous press curing. The mixing operation is conducted as a two‑stage internal mixer cycle, with the first stage discharged at 140–160 °C and the final stage discharged below 105 °C, followed by a two‑roll mill warm‑up at 70–80 °C to feed a calender. Cover sheets of 3–8 mm gauge are calendered onto polyester/nylon or steel breaker fabric treated with RFL dip, and the entire belt is vulcanized in a continuous hot press at 140–160 °C for 25–45 min depending on belt thickness. The finished belt is tested under ISO 14890:2013 safety requirements, ISO 4649:2021 rotary drum abrasion, ISO 34‑1:2022 tear strength, and ISO 15236‑1:2016 for steel cord conveyor belts, while flame‑resistant grades are assessed under ISO 340:2022 or EN 14973:2018 depending on the market. End products include abrasion‑resistant cover rubber for underground coal and aggregate belts with widths from 800 mm to 2200 mm, where the SSBR fraction reduces groove wear under wet abrasive ore handling relative to high‑BR control compounds, but comparative wear data must be validated through production‑scale trials rather than a single laboratory pass value. The upper SSBR addition is limited to 60 phr because higher loadings reduce green strength required for open‑mill handling and can raise calender shrinkage beyond 3% after cooling.

During compression molding of vulcanized footwear outsoles for safety boots and work shoes, non‑functionalized SSBR is introduced at 25–50 phr in a blend with standard Malaysian natural rubber at 30–50 phr and high‑cis BR at 20–40 phr to modify wet‑slip performance while retaining low‑temperature flexion. Silica is used at 30–55 phr in combination with carbon black N330 at 10–20 phr, and the silane coupling agent is added at 2–5 phr relative to silica to avoid excessive compound viscosity; polyethylene glycol is added at 0.5–1.5 phr to reduce silica adsorption of the accelerator system. The cure system uses sulfur at 1.8–2.4 phr, MBTS at 1.0–1.8 phr, and TMTD at 0.2–0.6 phr, giving a compression set below 30% after 24 h at 70 °C when tested under ASTM D395‑18 method B. Mixing is completed in a 75–160 L internal mixer at a dump temperature of 110–130 °C, followed by sheeting on a two‑roll mill and curing in a multi‑platen compression press at 145–155 °C for 8–15 min, or by injection molding in a 200–350 tonne clamp force machine with barrel temperatures of 80–95 °C and mold temperatures of 150–165 °C. Compliance requirements include ISO 20345:2021 for safety footwear, ISO 20871:2018 outsole abrasion resistance, ISO 868:2003 Shore A hardness, and ASTM D2240‑15(2021) durometer verification; slip resistance is evaluated under EN ISO 13287:2019 or ASTM F2913‑19. Finished components include direct‑vulcanized unit soles, Blake‑stitched sole sheets, and replaceable sole layers for slip‑rated work boots, outdoor walking shoes, and oil‑free leather footwear; published data for non‑functionalized SSBR‑specific outsole abrasion under ceramic‑grit protocols is limited, so formulation approval is generally based on internal rotary drum abrasion screening combined with production‑scale wear trials rather than a single harmonized pass value.

When SSBR Replaces Emulsion SBR in High‑Loss Anti‑Vibration Mounts

When non‑functionalized SSBR replaces emulsion SBR in high‑loss anti‑vibration mounts for stationary machinery and engine cradle applications, the blend is frequently specified at 40–70 phr SSBR with the balance being natural rubber or BR depending on the required dynamic stiffness ratio. High vinyl SSBR grades with vinyl contents of 50–65 wt% are selected because the loss factor tan δ at 60 °C can be maintained between 0.18 and 0.30, although non‑functionalized chain ends provide less filler networking than terminal‑modified types. Carbon black N330 or N550 is loaded at 30–60 phr, with process oil at 5–12 phr, zinc oxide at 3–5 phr, stearic acid at 1–2 phr, sulfur at 1.5–2.5 phr, and CBS/TMTD accelerator at 0.8–2.0 phr. The compound is mixed in a two‑stage cycle in a tangential internal mixer, with the masterbatch discharged at 140–150 °C and the final stage below 100 °C; the stock is then calendered or preformed into blanks and compression‑molded with bonded steel or aluminum inserts. Cure is carried out at 150–165 °C for 10–25 min, depending on insert thickness and mold thermal conductivity. Dynamic characterization is performed under ISO 10846‑1:2008 for dynamic stiffness and damping in the frequency range 20–200 Hz, while static and fatigue validation follows ISO 7743:2017 compression properties and ASTM D5992‑96(2018) dynamic properties of rubber isolators. The resulting anti‑vibration mounts, suspension bushings, generator set isolators, and rail pad segments are intended for dry, non‑oil‑exposed installations; contact with mineral oil or diesel fuel is not recommended because the SSBR/NR blend lacks the oil resistance of NBR or CR and can swell beyond 15% by volume when immersed in IRM 903 oil under ASTM D471‑16a for 70 h at 100 °C. A further operational boundary is the low‑service‑temperature limit: high vinyl SSBR compounds can exhibit a glass transition near −30 °C to −25 °C, so installation below −25 °C requires low‑vinyl SSBR grades with a demonstrated brittle point below −45 °C.

Precure tread rubber for retreaded commercial vehicle tires is a separate downstream segment in which non‑functionalized SSBR is used at 40–70 phr in combination with BR at 20–40 phr and NR at 10–30 phr, depending on traction, abrasion, and compound cost targets. The filler package consists of carbon black N234 or N220 at 50–70 phr, with TDAE oil at 5–15 phr, zinc oxide at 2–4 phr, stearic acid at 1–2 phr, sulfur at 1.5–2.5 phr, and TBBS/CBS accelerator at 0.9–1.8 phr. Mixing is performed in an internal mixer with the first non‑productive stage dropped at 140–155 °C and the productive stage dropped below 100 °C; the compound is then extruded through a roller‑head extruder with an 8:1–12:1 L/D ratio into a hot strip of 3–6 mm thickness, embossed with a tread pattern, and precured in a continuous press or autoclave at 150–165 °C for 15–35 min. Product compliance is governed by UN ECE R108 for retreaded passenger car tyres and UN ECE R109 for retreaded commercial vehicle tyres, with abrasion measured under ISO 4649:2021, hardness under ASTM D2240‑15(2021), and tensile properties under ASTM D412‑16. The cured strips are applied to buffed tire casings with a cushion gum and vulcanized in an enveloped curing chamber at 95–125 °C for 3–6 h to form retreaded truck and bus tires with rim diameters from 17.5 in to 24.5 in. The main processing constraint is adhesion to the buffed casing: non‑functionalized SSBR with high vinyl content exhibits higher shrinkage and lower green tack than NR‑rich cushion gum layers, so the precure tread strip must be cooled below 40 °C before buffing and stored under controlled relative humidity below 60% to prevent condensation‑induced adhesion loss during application.