Formulation adjustments across 40 phr to 80 phr of SBR 1712 change the dry rubber hydrocarbon balance in a conveyor belt cover compound because SBR 1712 is supplied as an oil-extended cold-polymerized styrene-butadiene copolymer containing 37.5 phr of highly aromatic extender oil per 100 phr of base elastomer. At a 40 phr charge of the commercial bale, the compound receives 29.1 phr of dry SBR and 10.9 phr of extender oil; at 80 phr, it receives 58.2 phr of dry SBR and 21.8 phr of extender oil. If the remaining dry rubber is natural rubber held to a constant total dry hydrocarbon of 100 phr, the NR content falls from 70.9 phr to 41.8 phr across the gradient. This compositional shift matters because tear strength in a black-filled conveyor belt cover is strongly influenced by the NR phase, which undergoes strain-induced crystallization at the crack tip and blunts notch propagation. Tear strength is quantified according to ISO 34-1:2022 Method B or ASTM D624 Die C by recording the maximum force per unit thickness in a trouser or nicked-angle test piece cut from a press-cured slab prepared according to ASTM D3182. In general-service textile conveyor belt specifications such as ISO 14890:2013, tear strength is not always a direct acceptance value, but purchaser material specifications frequently call for a minimum trouser tear of 30 N/mm to 40 N/mm at 23 °C after standard cure, and lower limits are imposed for covers exposed to hot, abrasive lump materials.

On a production internal mixer with a 1.5:1 rotor length-to-diameter ratio and 0.6 MPa ram pressure, the higher SBR 1712 charge lengthens the black incorporation phase because the aromatic extender oil competes with carbon black for the filler surface. A 270-L intermeshing mixer processing an 80 phr SBR 1712 cover compound may require a dump temperature of 145 °C and an additional 30 s to 90 s of mixing compared with a 40 phr charge to reach the same dispersion index. The dispersion index is assessed on a torn or cut surface by reflected light microscopy in accordance with ASTM D2663, and values below 90% in the undispersed-area rating are associated with more frequent tear-initiation sites at agglomerates. The reduced green strength of the higher-oil batch is observed on a 610 mm four-roll inclined Z calender, where the unvulcanized cover sheet may show width contraction and sagging when the SBR 1712 level exceeds 65 phr. This calender instability narrows the processing window to approximately ±5 °C in roll temperature because the oil phase reduces sheet nerve and the compound becomes sensitive to roll-release behavior. Batch-to-batch variance in SBR 1712 oil content determined by ASTM D5774 is normally controlled within ±0.5 phr by the supplier, but variations at the maximum allowable end of the specification can shift the tear strength by 3 N/mm to 5 N/mm because the effective NR content is unchanged while the black-to-oil interaction alters the filler network.

What Limits Tear Strength in a 40-phr SBR 1712 Cover Compared with an 80-phr Mix?

The dominant limiting factor is the loss of natural-rubber strain crystallization as the dry SBR fraction increases. Natural rubber at 70.9 phr of the dry polymer forms a continuous phase capable of stress-induced orientation ahead of a propagating cut, whereas the SBR phase dissipates energy primarily through viscoelastic deformation. When the SBR 1712 bale loading is raised to 80 phr, the natural rubber content falls to 41.8 phr and the crack-tip process zone becomes less effective at blunting the tear front. In tensile testing according to ISO 37:2017, the tensile strength may remain above 20 MPa because the carbon black reinforcement masks the polymer deficiency under uniform extension, but tear strength measured by ASTM D624 Die C can drop by 15% to 30% relative to the 40 phr compound for an N330-reinforced cover at 60 Shore A hardness. This divergence between tensile and tear behavior is a characteristic of high-SBR conveyor covers and must be checked by routine quality-control testing of each mixed batch. Specific published data for the exact 40 phr to 80 phr SBR 1712 belt-cover gradient are limited, but the trend is consistent with the known tear weakness of non-crystallizing SBR versus NR and with compound screening data for abrasion-resistant covers.

Carbon black selection and oil redistribution control the tear strength limits of the SBR-rich formulations more than the dry SBR content alone. An intermediate super abrasion furnace black such as N220 with a statistical thickness surface area near 110 m²/g increases tear strength at low strain rates, but the high surface area also raises mixing viscosity and can produce micro-dispersed agglomerates that act as stress concentrators. A high-structure N330 black with a dibutyl phthalate absorption of 102 cm³/100 g is often preferred for conveyor covers because the high structure provides lower compound viscosity and adequate reinforcement at 55 phr to 65 phr loading. The extender oil contributed by SBR 1712 migrates during storage and vulcanization; if the total free oil exceeds 25 phr, the tear strength measured at 70 °C may fall below 25 N/mm because the black-to-black filler network is diluted and the compound becomes more rubbery in the service temperature range of hot conveyed materials. A dynamic mechanical analyzer operating in strain sweep from 0.1% to 10% double strain amplitude according to ASTM D6601 shows a lower Payne effect for the 80 phr SBR 1712 compound, indicating less filler networking and lower energy dissipation at the crack tip.

Carbon Black, Oil Adjustment, and the SBR Phase Domain at 60 phr Loading

At 60 phr SBR 1712, the dry SBR content is 43.6 phr and the extender oil contribution is 16.4 phr, leaving 56.4 phr of natural rubber in a normalized 100 phr dry-rubber formulation. This midpoint loading often represents the practical upper boundary for a single-stage mixing cycle with a sulfur donor cure system because the oil volume fraction approaches the threshold where the filler network becomes discontinuous and the unreinforced SBR phase becomes visible as a separate microdomain in transmission electron micrographs. Tear strength in this region is strongly influenced by the degree of phase continuity, which depends on the mixing sequence, the carbon black addition split, and the storage time before calendering. If the carbon black is added too late, the black partitions preferentially into the NR phase and the SBR domains remain under-reinforced, producing trouser tear values that can be 10 N/mm to 15 N/mm lower than a fully phase-mixed control. A two-stage mix procedure with a 150 °C first pass and a 110 °C second pass produces better phase mixing, but the extra heat history can reduce scorch safety measured by ISO 6502-3:2018 from 4.2 min to 3.1 min at 135 °C. The production processing window is therefore bounded by dispersion quality on one side and scorch safety on the other.

Thermal oxidative aging is more severe in the high SBR 1712 case because the aromatic extender oil contains condensed-ring structures that can form colored oxidation products and accelerate surface hardening. After 168 h at 70 °C in forced air according to ISO 188:2011, the tear strength of a 40 phr SBR 1712 cover compound often retains 70% to 85% of its original value, while the 80 phr SBR 1712 compound may retain only 55% to 70% because the greater oil fraction softens the aged surface layer and the lower NR content reduces strain-induced healing at the crack tip. At a service temperature above 60 °C, trouser tear measurements on aged specimens conditioned for 24 h at the test temperature according to ISO 23529:2016 provide a more realistic indication of tear resistance than room-temperature acceptance checks. Conveyor covers handling hot clinker or sinter are frequently specified with a minimum 25 N/mm hot tear value under these conditions, and the 80 phr SBR 1712 formulation may fall below that boundary when the total binder content is not compensated with higher carbon black or a semi-reinforcing silica.

Adjustment of the cure system is required because the aromatic extender oil contains a small fraction of acidic residues that can consume zinc oxide and slow the vulcanization kinetics. A conventional sulfur cure with 1.5 phr to 2.0 phr sulfur, 0.8 phr TBBS, and 0.2 phr CTP is usually adequate for the 40 phr SBR 1712 compound, but the 80 phr compound may need an additional 0.3 phr TBBS or a secondary accelerator such as DPG to maintain a t90 below 8 min at 150 °C measured by ISO 6502-3:2018. The cure-state distribution in a thick cover is non-uniform because the low thermal diffusivity of an oil-extended compound reduces the heat transfer rate from the press platen to the center of a 12 mm cover section. The state of cure at the center is estimated by the difference in moving die rheometer torque between surface and center specimens after progressive cure, and the tear strength of an overcured outer layer can be 5 N/mm to 10 N/mm lower than the optimum cure plateau. For this reason, conveyor belt manufacturers limit the sulfur level and select a longer flat cure rather than a higher temperature to avoid excessive reversion in the NR-rich 40 phr formulation.

When SBR 1712 Replaces Natural Rubber in a DIN 22102 Cover Compound

When SBR 1712 is used to replace a portion of natural rubber in a cover compound nominated to DIN 22102-1 for textile conveyor belts, the reduction in green tack and raw tensile strength becomes the controlling process conflict before the vulcanized tear limit is reached. At an 80 phr SBR 1712 loading, the unvulcanized cover sheet may show a raw tensile strength below 1.5 MPa at 500% elongation, which complicates transfer from the calender to the building table and promotes ply separation at the cover-to-carcass interface under 0.2 MPa consolidation pressure. Adhesion values between the cover and an RFL-treated polyester-nylon carcass are determined according to ISO 36:2020, and the high SBR 1712 cover may require an added resorcinol-formaldehyde donor system or a low cobalt adhesion promoter to maintain greater than 5 N/mm peel strength after 4 h at 150 °C cure. The tear strength of the finished belt cover is also influenced by the fabric geometry, because a heavy square-woven fabric can initiate tears at the cover-to-carcass interface when the belt troughs over a 20° idler angle. In this configuration, the 40 phr SBR 1712 cover with higher NR exhibits better cut-growth resistance under repeated flexing, while the 80 phr SBR 1712 cover may show a smaller trouser tear value but improved abrasion resistance under ISO 4649:2017 because the higher styrene content increases stiffness in sliding contact. The selection of the SBR 1712 level is therefore a compromise between tear, adhesion, and abrasion.

Operational boundaries for compounds containing 40 phr to 80 phr SBR 1712 include pre-drying of non-black fillers at 80 °C for 2 h when the ambient relative humidity exceeds 60%, because moisture adsorption onto silica or zinc oxide surfaces can reduce the dispersion index and create steam porosity in thick belt covers. The aromatic extender oil is incompatible with paraffinic wax bloom at loadings above 1.5 phr, and the resulting surface exudation can reduce cover-to-carcass tack before curing. Avoid combination with amine-based antioxidants and phenylenediamine antiozonants at the upper end of their solubility limits, because the interaction with aromatic extender oil can produce staining and increase methanol-extractable residues, which is unacceptable for belts carrying food-contact materials under FDA 21 CFR 177.2600 or EU 10/2011. When SBR 1712 is blended with chloroprene rubber in flame-resistant covers, the sulfur-cure system must be replaced with a metal oxide system, and the tear strength response differs because the chloride-containing phase changes the crack-tip polarity. The exact tear strength limit for each formulation must be verified on a production-representative 1.5 m wide continuous press, because laboratory press data can overestimate tear strength by up to 15% relative to belt edges where the cure pressure and temperature history are less uniform.