Natural rubber cover compounds specified for heavy-duty conveyor belt covers are formulated around NR for its strain-crystallizing tear resistance and low heat build-up. When solution styrene-butadiene rubber is introduced at loadings between 5 phr and 30 phr as a partial replacement for NR, the ISO 4649:2017 Method B abrasion index of the vulcanizate responds to competing changes in crosslink density, filler phase distribution, and energy dissipation under the rotating drum. The cover compound is typically mixed on an intermeshing internal mixer of 270 L net chamber volume, with a fill factor of 0.68–0.72, followed by dump at 145 °C–155 °C and final sheeting on a two-roll mill at 55 °C ± 5 °C. The ISO 4649:2017 method expresses the abrasion index relative to a reference rubber and therefore captures relative volume loss under a standardized abrasive path rather than an absolute field wear prediction. The reference compound used with the abrader is tested alternately with the unknown to reduce abrasive sheet drift, and the index is calculated from the volume loss of the reference relative to that of the test compound, with values above 100 indicating better abrasion resistance. In a typical SSBR-modified NR cover compound, the index response is not linear with SSBR loading because polymer phase morphology, filler migration, and vulcanization kinetics change simultaneously. The selection of SSBR architecture, especially styrene content and vinyl content, affects the glass transition temperature and therefore the viscoelastic energy release rate at the abraded surface, which is the primary physical basis for the ISO 4649:2017 index change.

When SSBR Replaces Portions of NR in a Conveyor Belt Cover Compound

At loadings of 10–30 phr, styrene content and vinyl content of the SSBR govern the glass transition temperature and the resulting abrasion behavior under ISO 4649:2017. A solution SSBR with 25 wt% styrene and 50 wt% vinyl has a glass transition temperature near -35 °C, whereas natural rubber has a glass transition temperature near -65 °C. This difference raises the compound’s hysteresis at service temperatures and can reduce or improve abrasion depending on the severity of the abrasive contact. The solubility parameter of natural rubber is approximately 16.7 MPa^1/2, while an SSBR with 25 wt% styrene is closer to 17.0 MPa^1/2. Although this difference is small, dynamic mechanical analysis of vulcanized blends often shows two loss tangent maxima when the SSBR content exceeds 15 phr, indicating phase separation. Sulfur and primary sulfenamide accelerators partition preferentially into the NR domain because of the higher allylic hydrogen density in cis-1,4-polyisoprene, producing a crosslink density gradient detectable by equilibrium swelling in toluene per ISO 1817:2015. If the cure system is not rebalanced, the SSBR phase remains undercrosslinked, and the ISO 4649:2017 abrasion index may not improve despite the higher styrene content. This cure-rate mismatch is partially compensated by increasing the accelerator dosage by 0.1–0.3 phr for each 10 phr SSBR addition, but the adjustment must be verified on an oscillating disc rheometer at 160 °C according to ISO 6502:2016. The phase morphology also depends on the mixing sequence, because the lower viscosity NR phase can encapsulate filler before SSBR is uniformly distributed.

Mixing parameters determine whether SSBR distributes as a dispersed phase or forms co-continuous domains. In a production-scale intermeshing internal mixer, the order of addition and rotor speed control the phase morphology. If SSBR is added simultaneously with NR and carbon black, the filler localizes preferentially in the NR phase because of the higher unsaturation of cis-1,4-polyisoprene. With a 1.5 L laboratory internal mixer fitted with tangential rotors and a ram pressure of 0.5–0.7 MPa, a single-pass mix of 4.5 min at 45 rpm typically yields a Mooney viscosity ML(1+4) 100 °C of 72–88 MU when the SSBR loading is 20 phr. The resulting compound sheet is cooled to below 40 °C before curatives are added on a two-roll mill. Premature scorch is avoided by maintaining mill surface temperature at 60 °C and by adding sulfur in the final 30 s of mixing. The time from mill sheet to cure press should not exceed 24 h at 25 °C and 50% relative humidity to avoid moisture absorption on silica-filled compounds. Batch-to-batch variation in Mooney viscosity on a 270 L production mixer is typically controlled to ±3 MU through strict carbon black addition sequence, while unacceptable black dispersion appears as undispersed agglomerates larger than 10 µm in phase-contrast light microscopy. All mixing procedures should follow ISO 2393:2014 for test compound preparation when laboratory scale is used.

What Limits the Practical SSBR Loading Window in NR Cover Compounds?

Three interrelated limits define the upper SSBR level. First, green tack required for conveyor belt cover splicing declines as SSBR loading exceeds 25 phr; a Polyken probe tack test on uncured plied strips may show a drop from 2.5 N/mm to below 1.5 N/mm depending on tackifier level, but published data for the exact NR/SSBR cover compound configuration is limited. Second, viscosity increase from high-Mooney SSBR can shift Mooney viscosity above 100 MU, causing extrusion die swell greater than 35% on a 120 mm cold-feed pin extruder. Third, low-temperature flexibility deteriorates because SSBR raises the compound’s glass transition temperature; this is quantified by dynamic mechanical analysis as a shift in loss tangent peak temperature from -55 °C to -45 °C when SSBR loading reaches 30 phr. The practical upper loading in NR cover compounds is normally 30–40 phr unless process oil is increased by 5–10 phr and the cure system is adjusted. Compounds with SSBR loadings above 40 phr may need an additional silane coupling agent to maintain the abrasion index; however, the silane must be added in the first mixing pass and the dump temperature held below 160 °C to prevent premature sulfur release from the polysulfidic silane. In addition, the use of a high-vinyl SSBR with more than 62% vinyl should be avoided in compounds with a high sulfur-to-accelerator ratio because the resulting high crosslink density in the SSBR phase creates a brittle interphase that peels away from the NR matrix under the abrasive drum.

Vulcanization kinetics in NR/SSBR blends are complex because the cure rate of NR is inherently higher than that of SSBR. The difference arises from the higher concentration of reactive allylic hydrogen atoms in the cis-1,4-polyisoprene repeat unit and the lower solubility of sulfur in the styrene-butadiene phase. An oscillating disc rheometer trace at 160 °C typically shows a lower minimum torque and a longer t90 in the SSBR-modified compound compared with the all-NR control. This kinetic gap can be reduced by using a semi-EV cure system with sulfur at 1.5–2.0 phr and a sulfenamide accelerator at 1.2–1.8 phr. The tensile strength and elongation at break measured according to ISO 37:2017 are often reduced by 5–15% at 30 phr SSBR loading, but the ISO 4649:2017 abrasion index may improve if the crosslink density in the SSBR phase is sufficient. A cure mismatch can be observed by swelling the vulcanizate in toluene for 72 h at 23 °C according to ISO 1817:2015 and examining the swollen gel; a broad gel fraction distribution indicates non-uniform crosslinking. The t90 from the rheometer should be used to set the press cure time rather than relying on the all-NR control, because the slower SSBR phase governs the final crosslink density and the abrasion index.

Thermal oxidative aging and crosslink density redistribution in NR/SSBR cover compounds

After aging for 168 h at 100 °C in air per ISO 188:2011, the retained elongation at break of NR-rich compounds can be lower than that of SSBR-rich compounds because the NR phase undergoes chain scission at allylic sites while the styrene-butadiene phase crosslinks further. This aging divergence affects the ISO 4649:2017 result if aged specimens are tested; the abrasion index may appear to improve on aged SSBR-rich test pieces because of increased modulus, but the field wear trend can reverse under dynamic strain. The phenomenon is measurable through the change in tensile strength at break according to ISO 37:2017 and hardness change according to ISO 48-4:2018, with hardness increases of 3–6 Shore A commonly reported in SSBR-containing vulcanizates after 168 h at 100 °C. The use of 2,2,4-trimethyl-1,2-dihydroquinoline at 1–2 phr in the NR phase and a paraphenylenediamine antiozonant at 1–2 phr is standard practice to mitigate oxidative hardening. The SSBR phase, however, may retain more of its original elongation at break because the aromatic rings and saturated butadiene segments are less prone to allylic oxidation. This phase-specific aging response means that the selection of an SSBR with 15 wt% styrene rather than 25 wt% styrene can narrow the aging gap but sacrifices some abrasion index according to ISO 4649:2017. In applications requiring continuous exposure to 100 °C, the SSBR content should not exceed 20 phr unless the cover compound is recompounded with a higher antioxidant loading and a sulfur donor cure system.

The ISO 4649:2017 test does not distinguish between macro-tearing and micro-abrasive loss. An NR cover compound that exhibits high tensile strength and high elongation at break according to ISO 37:2017 can still produce a lower abrasion index than an SSBR-modified compound if the NR phase undergoes mechano-oxidative chain scission on the abrasive drum surface. The equipment used for this comparison is a rotating cylindrical drum abrader with a nominal drum diameter of 150 mm and a 60-grit aluminium oxide abrasive sheet. The test piece traverses the drum with lateral movement to minimize ridging, and the mass loss is converted to volume loss using the vulcanizate density determined according to ISO 2781:2018. Abrasion index values above 100 indicate lower volume loss than the reference compound, while values below 100 indicate higher volume loss. Because the test uses a reference rubber, the absolute index is only meaningful when the reference compound is from the same batch as specified by the standard. The standard also requires conditioning at 23 °C ± 2 °C and 50% ± 10% relative humidity for at least 16 h before testing. In SSBR-modified NR compounds, the abrasive sheet must be changed at the frequency specified in ISO 4649:2017 because the sticky smear produced by NR can load the abrasive cloth and artificially raise the measured volume loss.

Carbon black and silica partition differently between NR and SSBR phases. Carbon black N234 or N330 tends to localize in the NR phase during the first mixing stages, while precipitated silica with silane coupling remains partly in the SSBR phase if the silane is added in the first pass. The resulting filler distribution is visible in scanning electron microscopy as contrast differences between the two polymer phases. A filler mismatch produces a bimodal abrasion index ranking after ISO 4649:2017. When the SSBR phase is underfilled, the abraded surface develops thumbnail patterns at the phase boundaries; when the NR phase is overfilled, heat build-up under the rotating drum leads to surface tack and material transfer to the abrasive sheet. The balance is improved by splitting the carbon black addition between the first and second mixing passes and by using an organosilane at 4–8 phr based on silica loading. This mixing strategy is evaluated on a 1.5 L internal mixer and then scaled to a 270 L intermeshing mixer by holding constant the ram pressure and tip speed rather than rotor speed. The resulting compound batch is then passed through a dump extruder at 90 °C to produce a uniform sheet, but the dump temperature must remain below the silane reaction onset to avoid premature coupling. Published data for this exact NR/SSBR/silica configuration is limited; however, the above operational constraints are consistent with ISO 2393:2014 and common internal mixer scale-up rules.

ISO 4649:2017 abrasion index values on NR/SSBR blends have an intra-laboratory repeatability that is influenced by the abrasive sheet batch, the reference compound storage, and the cutting of the test piece. Test pieces cut from the edge of a compression-molded sheet may show higher wear than those from the center if the cure state is nonuniform. A statistically meaningful comparison requires at least five specimens per compound, and the median volume loss is used rather than the mean when the coefficient of variation exceeds 10%. If the density of the vulcanizate is measured by immersion in ethanol according to ISO 2781:2018, bubbles attached to the specimen must be removed; otherwise the volume loss is underreported. The method also specifies that the abrasive cloth be replaced after a fixed number of runs or when the measured volume loss of the reference compound changes by more than 10% from the initial value. These procedural controls are essential when SSBR loading is being varied over 0–30 phr, because the resulting abrasion index differences can be smaller than the test repeatability if the reference rubber is not consistently conditioned.

Compounds containing SSBR and silica should be stored below 60% relative humidity before mixing, and drying of silica at 105 °C for 2 h is recommended if the moisture content exceeds 0.5 wt% to avoid scorch during mixing. The combination of SSBR with amine-based antioxidants and high-sulfur curing systems can lead to premature crosslinking during storage; therefore, antiozonant selection should be limited to paraphenylenediamine types at 1–2 phr and the use of hexamethylenetetramine-based donors should be avoided unless proven through rheometer cure checks. In transfer molding of conveyor belt cover splices, the cure time should be derived from the t90 value of the SSBR-containing compound rather than the all-NR control, because the slower SSBR phase governs the final crosslink density. When a 270 L intermeshing mixer is used, the dump temperature of the masterbatch should not exceed 160 °C for a sulfur-silane system, and the final curatives should be added only after the compound has cooled to below 45 °C on a two-roll mill. The lower unsaturation of the SSBR phase also reduces the solubility of zinc stearate, so zinc oxide at 3–5 phr and stearic acid at 2–3 phr may be required to maintain a uniform cure state. These conditions define the operational boundary for SSBR loading in an NR cover compound tested under ISO 4649:2017.