Heat History Sensitivity Limits in SBR 1502 Footwear Sole Compounds

Heat history in SBR 1502 footwear sole compounds is the cumulative thermal exposure experienced by the raw polymer during drying, storage, mixing, mill sheeting, extrusion, injection molding, and any rework of unvulcanized or vulcanized scrap. The emulsion-polymerized cold SBR grade designated SBR 1502 is specified under ISO 2322:2009 with a bound styrene content of 22.5% to 24.5%, a Mooney viscosity ML 1+4 at 100°C of 47 MU to 57 MU, and a volatile-matter content not exceeding 0.75%; these boundaries define the raw-polymer state before compounding, but they do not define the heat history sensitivity that becomes evident after the addition of sulfur cure systems, zinc oxide, stearic acid, fillers, and processing oils. In a production-scale intermeshing mixer with a net chamber volume in the 45 L to 270 L range, the practical dump temperature after masterbatch mixing routinely reaches 130°C to 150°C, and the subsequent exposure of the same stock to a two-roll mill at 50°C to 60°C for sheeting does not erase the thermal damage already imposed. The central technical problem is that SBR 1502 contains residual fatty acid soaps and small amounts of unsaturated polymer chain ends; under heat, these species participate in both chain-scission and branching reactions, so the net Mooney viscosity response can be an increase, a decrease, or an increase followed by a decrease depending on oxygen availability, antioxidant type, and mastication energy. Compounded footwear sole stock that is discharged from an internal mixer at 155°C may exhibit a higher apparent viscosity than the same formulation discharged at 140°C, even when the final curatives are added only on a cooler second-pass mill. The practical consequence is that incoming SBR 1502 lots with Mooney values near the upper end of the 47 MU to 57 MU range can enter the mixing cycle already carrying more gel fraction than lower-Mooney lots, and the mixing time required to reach a uniform state is not directly transferable across raw-polymer lots under identical machine settings.

Footwear sole production relies on SBR 1502 because the polymer provides an economical balance of flex resistance, abrasion resistance, and filler tolerance when compounded with precipitated silica, clay, ground calcium carbonate, naphthenic oil, zinc oxide, stearic acid, sulfur, and sulfenamide accelerators. The heat history limits are not governed solely by the raw polymer specification; they are governed by the interaction between the cure system and the partially vulcanized or oxidized structures formed during processing. At an injection molding machine processing a direct-formed sole unit with a clamp force of 400 t, a screw diameter of 65 mm, and an L/D ratio of 20:1, the barrel zones are typically held between 80°C and 95°C while the mold is held at 160°C to 170°C. The residence time in the barrel, normally 4 min to 8 min, is sufficient to consume part of the accelerator system if the compound has already been overheated during mixing. The resulting loss in scorch safety is measurable as a reduction in the time to one unit of torque rise above minimum torque in a rotorless curemeter at 160°C. Published data for this specific injection-molded configuration is limited, but the directional loss of processing safety with repeated heat exposure is consistently observed across sulfenamide-accelerated SBR compounds.

How does accumulated heat history shift Mooney viscosity and cure rheometry in SBR 1502 sole compounds?

Mooney viscosity remains the most practical production-floor measurement for heat history accumulation because it responds to both chain scission and crosslink formation. The standard method ISO 289-1:2018 uses a shearing-disc viscometer at 100°C with a preheat time of 1 min and a rotor speed of 2 min⁻¹, and the ML 1+4 value is recorded after 4 min of rotation. The equivalent ASTM D1646-19a method adds stress relaxation and pre-vulcanization determinations that are useful when heat-treated SBR 1502 stock is suspected of having developed microgel. A compounded SBR 1502 sole formulation with a target ML 1+4 at 100°C of 55 MU to 65 MU may show an increase of 8 MU to 12 MU after one additional hot-mill pass at 90°C for 10 min, whereas prolonged mastication at 50°C to 60°C can reduce the same value by 6 MU to 10 MU through mechanical chain scission. The exact direction depends on whether the compound is in a carbon-black-filled system, which protects against some oxidative chain scission, or a silica-filled system, where acidic silanol groups and residual moisture can accelerate both hydrolysis and oxidation. In rotorless curemeter testing according to ISO 6502-2:2018 or ASTM D5289-19a, the minimum torque ML is a low-strain elastic response that correlates with compound Mooney viscosity, while the maximum torque MH reflects the crosslink density developed under the selected cure conditions. Accumulated heat history often lowers ML and MH simultaneously when chain scission dominates, but heat history above 150°C in sulfur-containing SBR 1502 can produce a distinct pre-vulcanization shoulder in the cure curve, lowering the scorch time ts2 and causing the compound to be rejected for subsequent injection molding.

The interpretation of heat history effects requires separation of uncured compound storage effects from mixing and processing effects. Storage at 40°C to 50°C for 14 days in a warehouse may not change the ML 1+4 value beyond the reproducibility of ±3 MU reported under ISO 289-1:2018, but the same period at 70°C can produce changes in cure rheometry that are outside the normal batch-to-batch range. The rotorless curemeter test at 160°C is preferred because it captures both the early scorch response and the reversion behavior after t90. A compound aged at 70°C for 168 h may show a reduction in ts2 from 2.4 min to 1.6 min, while the t90 remains nearly unchanged or shortens by 0.3 min to 0.6 min; this pattern indicates partial accelerator decomposition without complete cure system consumption. When the cure system is partially consumed, the MH value drops by 1.0 dN·m to 2.5 dN·m, and the tensile properties after vulcanization measured under ASTM D412-16(2021) show a loss of elongation at break of 60% to 120% from the reference value. The use of hot air ageing according to ASTM D573-04(2019) at 70°C for 168 h is therefore not only a finished-product durability test; it is also an indirect probe of premature heat history in uncured stock because the retained tensile strength and elongation are affected by the same degradation pathways.

Internal mixer dump temperature and rotor speed boundaries

Internal mixing of SBR 1502 footwear sole compounds is performed either in tangential-rotor mixers of the Banbury type or in intermeshing-rotor mixers, with the latter providing more efficient cooling and more reproducible energy input per unit mass. In a production-scale intermeshing mixer with a net chamber volume of 45 L to 270 L, a fill factor of 0.72 to 0.78, and a rotor speed of 35 min⁻¹ to 50 min⁻¹, the specific energy input for masterbatch mixing of silica-filled SBR 1502 sole stock commonly falls between 0.35 kWh/kg and 0.50 kWh/kg. The upper boundary of the dump temperature is set by the scorch response of the cure system and by the tendency of SBR 1502 to form microgel at temperatures above 145°C to 150°C. A dump temperature of 155°C is not automatically catastrophic, but it narrows the downstream processing window because the compound will cool more slowly in a large batch and the residual heat can accelerate the reaction between sulfur, accelerator, and zinc oxide even when the curatives are added in a second pass. Process audits on intermeshing mixers used for silica-filled SBR 1502 sole compounds show that increasing the specific energy input from 0.38 kWh/kg to 0.46 kWh/kg can raise the ML 1+4 at 100°C by 8 MU to 12 MU due to microgel formation, while energy inputs above 0.50 kWh/kg can produce the opposite effect through mechanical chain scission. These observations are specific to the mixer geometry and rotor speed combination; published data for the exact configuration may be limited, but the directional behavior is consistent with known SBR degradation chemistry.

The rotor speed boundary is equally important because the relationship between rotor speed and stock temperature is nonlinear at high fill factors. Increasing the rotor speed from 35 min⁻¹ to 45 min⁻¹ can shorten the masterbatch mixing time by 20 s to 40 s, but the peak stock temperature may rise by 8°C to 15°C depending on ram pressure and cooling water temperature. When the cooling water inlet temperature is 20°C, the heat transfer coefficient is sufficient to hold the stock below 145°C for a controlled mixing sequence; if the plant cooling water rises to 30°C, the same sequence can exceed 155°C and the heat history limit must be re-evaluated. Batch-to-batch variance therefore cannot be attributed only to raw SBR 1502 lot differences; the seasonal variation in cooling water temperature can alter the heat history of nominally identical batches. In a tangential-rotor Banbury of 270 L net chamber volume, the temperature probe may read 10°C to 20°C lower than the true localized stock temperature near the rotor tips because the thermocouple is exposed to the chamber wall and is influenced by the cooling jacket. For this reason, a dump temperature limit of 150°C measured at the wall is often interpreted as a practical upper boundary, while the actual local stock temperature may be closer to 160°C. This measurement bias must be considered when transferring a validated mixing sequence from one mixer to another.

Two-roll mill sheeting after internal mixing does not remove heat history; it merely redistributes the heat and imposes additional mechanical work. A production two-roll mill with a front roll diameter of 550 mm, a roll face length of 1500 mm, and a friction ratio of 1.15:1 is typically operated at a front roll temperature of 50°C to 60°C and a rear roll temperature of 45°C to 55°C. The stock is sheeted to a thickness of 8 mm to 12 mm, and the residence time on the mill can range from 5 min to 15 min depending on the operator and the need for blending. During this period, the compound remains above 50°C, and oxygen is continuously introduced by the rolling bank. Repeated mill passes can reduce Mooney viscosity by 4 MU to 8 MU when the stock is already at the higher end of the specification, and the reduction is more pronounced in sulfur-free masterbatch than in final mixed stock containing curatives. The mill sheeting step is often the point at which accidental heat history is generated because an operator may leave a hot batch on a slow-rotating mill while attending to another mixing cycle, effectively extending the thermal exposure by 10 min to 20 min. This type of production-scale failure mode is detected only if the final compound Mooney viscosity or the MDR scorch time is measured before the stock is released to the injection molding cell.

The dump temperature ceiling: bounded by scorch safety, not by chain scission alone

For sulfenamide-accelerated sulfur cures in SBR 1502 sole compounds, the practical dump temperature ceiling is set by the loss of scorch safety and not solely by the onset of thermal-oxidative chain scission. The accelerator N-cyclohexyl-2-benzothiazolesulfenamide is commonly used because it provides delayed action at processing temperatures and rapid cure at vulcanization temperatures; however, its decomposition products are heat-sensitive and can accumulate during prolonged mixing above 145°C. In an MDR cure curve at 160°C, a reference compound with a target ts2 of 2.0 min to 2.5 min may drop to 1.2 min to 1.5 min after a single over-temperature dump at 160°C. At a mold temperature of 165°C, the same compound may still be processable, but the margin between the injection barrel residence time and the onset of scorch becomes too narrow for stable production. The upper dump temperature boundary of 150°C is therefore maintained not because the SBR 1502 polymer instantly degrades above that value, but because the cure system begins to react prematurely and because the heat transfer limitations in large batches create nonuniform cure-system consumption. In compounds containing tetramethylthiuram disulfide as a secondary accelerator, the sensitivity is even greater; the thiuram component can begin to donate sulfur at temperatures above 135°C to 140°C, reducing the scorch time more rapidly than a pure sulfenamide system.

Thermal degradation in SBR 1502 under mixing conditions follows two competing pathways. The first is oxidative chain scission, which is promoted by oxygen in the mixer headspace and by residual peroxide species from emulsion polymerization. The second is radical addition across the butadiene double bond, producing branching and eventually gel formation. The balance between these pathways is influenced by the bound styrene content of 22.5% to 24.5%, because styrene units interrupt the polybutadiene sequence and reduce the propagation of oxidative scission relative to high-cis polybutadiene. The presence of phenolic antioxidants such as 2,6-di-tert-butyl-4-methylphenol at 0.5 phr to 1.0 phr delays oxidative chain scission, but it does not prevent thermal crosslinking. Consequently, heat history in SBR 1502 frequently produces an initial increase in Mooney viscosity due to branching and microgel, followed by a decrease if mastication continues long enough to break the branched structures. In a production internal mixer, the transition between these regimes can occur within 30 s to 60 s at 155°C, making the exact dump temperature and mixing time critical. The use of a power integrator or energy controller is recommended because the specific energy input provides a more reproducible heat history record than wall temperature alone.

When SBR 1502 sole scrap is reworked through injection molding

Rework of uncured SBR 1502 sole scrap introduces one of the most severe heat history challenges in footwear manufacturing. Unvulcanized edge trim, runner scrap, or rejected preforms are accumulated at ambient temperature for 8 h to 24 h before being blended back into fresh compound. If the scrap has already passed through an injection barrel once, its cure system has been partially activated, and the addition of fresh curatives cannot fully restore the original scorch safety. A direct injection molding machine operating with a barrel temperature profile of 80°C/85°C/90°C/95°C and a mold temperature of 165°C can be used to process a fresh SBR 1502 sole compound with a barrel residence time of 6 min to 10 min, but when 15 wt% to 20 wt% of reworked scrap is blended into the feed, the effective residence time of the scrap fraction exceeds the nominal barrel residence time and the scorch time of the blend decreases disproportionately. The resulting flow marks, short shots, or partially scorched regions in the sole are difficult to detect by hardness testing alone because the local crosslink density in the scorched zones is not uniform. Measurement of the minimum torque ML from a rotorless curemeter at 160°C is more useful than visual inspection for detecting the presence of heat-damaged scrap; a blend containing excessively reworked scrap may show a double-step cure curve or a high low-strain elastic modulus.

The injection molding cell should therefore establish a maximum barrel residence time based on the specific compound’s MDR scorch time at the barrel temperature. If the fresh compound has an MDR ts2 of 2.2 min at 160°C, the safe barrel residence time is not directly equal to ts2 because the barrel temperature is lower and the shear conditions differ. A practical production limit is to hold the barrel residence time below 50% of the MDR ts2 measured at the barrel temperature, but this requires the MDR test to be run at 90°C or 95°C rather than at 160°C. Running a cure curve at 95°C for 60 min is less convenient than the standard 160°C test, but it provides an estimate of the time available before the compound begins to scorch in the injection barrel. When the compound contains 2.0 phr to 3.0 phr of sulfur and 1.5 phr to 2.0 phr of N-cyclohexyl-2-benzothiazolesulfenamide, the ts2 at 95°C may be in the range of 18 min to 25 min. The barrel residence time limit is then set at 8 min to 12 min, depending on the safety margin required for automated production.

In tropical warehousing of SBR 1502 sole compounds, the storage environment can contribute thermal history even when the stock is never processed above 50°C. Containers exposed to direct sunlight can reach internal air temperatures above 60°C, and black rubber compound stacked near the container roof can reach surface temperatures of 70°C to 80°C. The effect of this storage exposure is often underestimated because the duration is long but the temperature is perceived as moderate. For a sulfur-cured SBR 1502 compound stored at 60°C for 7 days, the MDR ts2 at 160°C may decrease by 25% to 35%, while the ML 1+4 at 100°C may increase by 5 MU to 10 MU. The increase in Mooney viscosity reflects slow crosslinking and gel formation in the solid state, and the decrease in scorch time reflects partial decomposition of the delayed-action accelerator. When the same compound is stored at 23°C for 7 days, the changes are often within the normal testing variation of ±3 MU and ±0.2 min. The operational boundary is therefore not a single temperature; it is a time–temperature integral. A compound stored for 24 h at 70°C may have a heat history equivalent to weeks at 40°C, and the safe re-use of such stock requires re-testing before it is blended into production.

Determining storage heat history limits in sulfenamide-cured SBR 1502 sole compounds

Storage heat history limits for sulfenamide-cured SBR 1502 sole compounds are established by comparing the cure characteristics of a retained reference sample with the cure characteristics of the stocked compound after a defined ageing interval. The reference sample is typically wrapped in polyethylene to exclude oxygen, stored at 23°C, and tested within 24 h of mixing. The stocked compound is sampled from the center of a bale or slab, because the surface layer may be more oxidized than the bulk. Testing according to ISO 6502-2:2018 at 160°C provides the minimum torque, maximum torque, ts1, and t90 values. A permissible storage limit may be defined as a ts1 decrease of no more than 20% relative to the reference, or a scorch time no lower than 1.5 min at 160°C. The specific numerical limit depends on the downstream process; a compound intended for open mill sheeting can tolerate a shorter scorch time than a compound intended for injection molding because the open mill provides a cooler surface and continuous exposure to air. Silica-filled SBR 1502 sole compounds are more sensitive to moisture uptake during storage than carbon-black-filled compounds; if the moisture content exceeds 0.5% by mass, pre-drying at 80°C for 2 h is recommended before direct injection molding to avoid porosity in the sole.

Accelerated storage tests at 70°C for 168 h are often used as a conservative upper boundary, but they must not be interpreted as a linear predictor of ambient shelf life. The decomposition kinetics of sulfenamide accelerators have an apparent activation energy in the range of 80 kJ/mol to 100 kJ/mol; as a consequence, the rate of scorch-time loss at 70°C is roughly an order of magnitude higher than at 40°C. A compound stored at 70°C for 7 days may show the same MDR ts2 reduction as 6 weeks to 10 weeks at 40°C, but the exact equivalence depends on the antioxidant package, the sulfur level, and the presence of residual moisture. The use of a pre-vulcanization inhibitor at 0.2 phr to 0.4 phr can extend the processing window, but it also alters the cure rate at the mold temperature and may increase the t90. The addition of a pre-vulcanization inhibitor is not a substitute for controlling storage temperature; it is a compensating measure that must be validated by producing a full cure curve and vulcanized physical test data under ASTM D412-16(2021) and ASTM D2240-15(2021).

Under high-sulfur low-accelerator conditions in hot runner sole molds

High-sulfur low-accelerator formulations are occasionally used in SBR 1502 footwear sole compounds to reduce accelerator cost and to produce a high initial crosslink density, but the heat history sensitivity of such systems is strongly nonlinear. A formulation containing sulfur at 3.5 phr to 4.0 phr and N-cyclohexyl-2-benzothiazolesulfenamide at 0.8 phr to 1.2 phr may have an acceptable MDR ts2 of 2.0 min to 2.5 min at 160°C when freshly mixed, but the same compound after a single additional hot-mill pass at 90°C for 10 min can exhibit a ts2 below 1.2 min. The reason is that the sulfur concentration is high enough to participate in heat-induced vulcanization even without rapid accelerator decomposition, and the low accelerator level provides little delayed-action reserve. In hot runner sole molds, the runner system retains a portion of the compound at 160°C to 170°C for multiple cycles, and the material in the hot runner can re-circulate or remain stagnant. The compound at the runner wall may reach the beginning of reversion while the cavity fill is still occurring, producing a distinct discoloration line and a loss of surface definition. Mold temperature control must therefore be tighter for high-sulfur systems; a mold temperature variation of ±5°C can shift the t90 by 25% to 35%, which is sufficient to move the process from a slightly undercured state to a reverted state in the thickest sole section.

The reversion behavior of SBR 1502 soles under prolonged heat is measured by continuing the rotorless curemeter test beyond t90 for 30 min to 60 min at the intended mold temperature. A stable sulfur-cured SBR 1502 network may retain 90% to 95% of its maximum torque after 30 min at 160°C, while a high-sulfur low-accelerator system may lose 10% to 20% of its maximum torque within the same period. The loss of torque corresponds to polysulfidic crosslink shortening and liberation of sulfur-containing by-products, and it is accompanied by a reduction in tensile strength and elongation at break when the vulcanizate is tested under ASTM D412-16(2021). Blown sole defects occur when volatile decomposition products nucleate pores in the rubber before the mold pressure is released. The use of adequate mold clamping force, typically above 250 t for multi-cavity sole molds, and the maintenance of a short injection time below 3 s are required to prevent the molten front from stalling in the cavity. If the injection time exceeds 5 s, the material near the gate can begin to cure before the cavity is fully packed, and the resulting pressure drop allows volatile by-products to expand into pores. This failure mode is often incorrectly attributed to moisture alone; in heat-sensitive SBR 1502 compounds, it can be produced by heat history alone even when the moisture content is below 0.3%.