The specification of 60 to 70 Shore A in solid footwear soling compounds is controlled by durometer indentation under ASTM D2240-15(2021) and ISO 48-4:2018, with a type A indentor, a minimum specimen thickness of 6.0 mm, a conditioning interval of at least 16 h at 23 ± 2 °C and 50 ± 5 % RH, and a reading taken after a dwell time of 3 s unless otherwise specified. Solid soling compounds differ from microcellular midsoles in that the hardness is derived from the rubber or thermoplastic matrix, filler network, plasticizer volume fraction, and crosslink density rather than gas-blown cell wall geometry. In production, the 60 to 70 Shore A band is selected for commodity footwear and occupational outsoles because it balances the tensile strength of vulcanized SBR/NR compounds, typically 12 to 18 MPa according to ISO 37:2017, with an abrasion loss measured on ISO 4649:2017 of approximately 140 to 220 mm³, while maintaining adequate flex crack resistance. The indentation test itself is sensitive to surface roughness, backing thickness, and the time between moulding and testing; plant laboratories therefore condition plaques on rigid glass plates and report median values from five readings to reduce operator bias.

Common polymer bases for the 60 to 70 Shore A solid soling window include SBR/NR blends at 70/30 to 50/50 ratios, NBR/PVC blends for oil-resistant outsoles, TPE-s block copolymers based on SBS or SEBS, and plasticized PVC. Hardness in these systems is not an intrinsic polymer constant; SBR with 35 phr N330 carbon black and 5 phr paraffinic oil may read 62 Shore A, whereas the same polymer with 70 phr N330 and no oil may reach 78 Shore A in the cured state. This sensitivity to formulation variables is the primary reason why Shore A is used as a batch-release criterion rather than a predictor of field performance. In addition to formulation, mixing shear history and dispersion quality alter hardness through filler agglomeration and bound rubber formation; an inadequately dispersed compound can exhibit a 2 to 4 Shore A point increase at the surface due to carbon black aggregate migration during curing.

How does filler–plasticizer balance keep Shore A inside the 60–70 window in cured rubber outsoles?

In a vulcanized SBR/NR outsole compound, the 60 to 70 Shore A window is reached by a compensation between reinforcing filler and mineral oil or ester plasticizer rather than by single-ingredient addition. Carbon black grades such as N330, N339, and N550 contribute different reinforcement and hardness increments; at equal loading, N339 generally raises Shore A by 3 to 5 points relative to N550 because of higher surface area and structure, measured by iodine adsorption in the range 82 to 90 g/kg and DBP absorption in the range 110 to 130 × 10^-5 m³/kg. A representative compound with 70 phr SBR 1502, 30 phr SMR 20 natural rubber, 50 phr N330, 10 phr paraffinic oil, 5 phr zinc oxide, 2 phr stearic acid, 1.8 phr sulfur, and 1.2 phr n-tert-butyl-2-benzothiazyl sulfenamide (TBBS) will typically read 66 Shore A after curing. Raising oil from 10 phr to 15 phr lowers hardness by approximately 4 Shore A points, while reducing N330 from 50 phr to 40 phr lowers hardness by a further 3 to 5 points; the combined changes move the compound to 58 Shore A and simultaneously reduce tensile strength by 15 to 20 %, as measured by ASTM D412-16(2021).

Plasticizer selection for Shore A control is constrained by migration kinetics and cure compatibility. Paraffinic oils are preferred in SBR/NR because they have lower aniline point and slower diffusion than aromatic oils, but they also reduce vulcanization rate and crosslink density if added before carbon black; the resulting compound may show longer t90 in a moving die rheometer at 160 °C, from 5.5 min to 7.0 min, while tensile modulus at 300 % elongation, measured by ISO 37:2017, declines from 6.5 MPa to 4.8 MPa. When a compound must meet a Shore A target at the upper boundary of 70, formulators often replace part of the oil with high-styrene resin, which contributes 1 Shore A point per 2 to 3 phr but narrows processing safety by increasing compound viscosity and heat generation in the final mixing pass.

Because plasticized PVC and NBR/PVC blends occupy a significant portion of the solid soling market where oil resistance and low cost are required, hardness control in these systems is governed by plasticizer solvency, gelation, and post-processing migration. A typical PVC outsole compound at 62 Shore A contains suspension PVC with K value 65 to 68, 70 to 85 phr di-isononyl phthalate or dioctyl terephthalate, 3 to 5 phr epoxidized soybean oil, 2 to 4 phr barium-zinc stabilizer, and 0.3 to 0.8 phr stearic acid lubricant. Hardness is measured after the injection-moulded plaque has been conditioned for 24 h at 23 °C because plasticizer diffusion to the surface continues for several days; a part that reads 64 Shore A immediately after moulding may decrease to 60 Shore A after 7 days if the plasticizer has limited compatibility. The solvency parameter of DINP relative to PVC is lower than that of di-octyl phthalate, so replacement of DOP with DINP commonly requires an additional 3 to 5 phr plasticizer to maintain equivalent hardness, but this adjustment alters tensile strength and abrasion loss tested under ISO 37:2017 and ISO 4649:2017.

NBR/PVC blends for solid soling are compounded at NBR/PVC ratios from 30/70 to 50/50, with NBR grades of 33 % acrylonitrile content providing the best balance of oil resistance and flexibility. In laboratory evaluation, increasing NBR from 30 phr to 50 phr in a PVC-based soling compound raises Shore A hardness by 2 to 3 points because the rubber phase occludes plasticizer and increases blend modulus; simultaneously, DIN 53516 abrasion loss declines from approximately 190 mm³ to 160 mm³, but melt viscosity during injection moulding rises by 20 to 35 %. The processing conflict at Shore A 60 to 70 arises because plasticizer levels required for the target hardness reduce melt strength and can create screw slip on a single-screw extruder with L/D 24:1; this is usually mitigated by using a screw with a barrier mixing section and maintaining barrel zone temperatures at 140 to 160 °C.

When SBS and SEBS block copolymers are injection moulded into unit soles

When SBS block copolymers are selected for solid footwear soling in the 60 to 70 Shore A range, hardness is primarily controlled by the styrene content of the block copolymer, the oil extension level, and the polystyrene endblock morphology. Commercial TPE-s compounds for direct injection moulding often use SBS with 30 to 33 wt% styrene, oil extension from 50 to 120 phr paraffinic or naphthenic oil, and polystyrene or polyolefin fillers at 0 to 20 phr; a compound with 100 phr SBS, 80 phr paraffinic oil, and 10 phr general-purpose polystyrene reads approximately 64 Shore A under ISO 48-4:2018. Because SBS is phase-separated, hardness develops as the polystyrene endblocks within the butadiene midblock matrix form glassy domains; oil partitions preferentially into the rubber phase, reducing hardness but leaving the endblock domains intact, which preserves tensile strength and compression set far better than plasticizer addition to a homogeneous polymer. The processing window for these materials on a horizontal injection moulding machine with a 40 mm screw and 20:1 L/D is narrow: melt temperatures are typically maintained between 160 °C and 190 °C, with barrel temperatures rising from 150 °C in the feed zone to 180 °C at the nozzle. Exceeding 200 °C initiates oxidative chain scission in the butadiene midblock, observed as a 3 to 5 Shore A point drop and a sharp increase in melt flow index measured at 190 °C/5 kg under ISO 1133-1:2022; falling below 150 °C produces short shots and weld-line brittleness because the viscosity is too high for the runner geometry.

Production-scale experience with SBS soling has shown that batch-to-batch hardness variation is often traceable to oil absorption capacity and the mixing order in the compounding extruder. When paraffinic oil is introduced too early in a co-rotating twin-screw extruder with L/D 44:1, the torque drops and the dispersed polystyrene phase loses domain uniformity; the resulting pellets may read 62 Shore A in one lot and 68 Shore A in another after moulding. To stabilize hardness at 65 Shore A, the oil should be split across two injection ports, with 30 % added after the melt seal and 70 % downstream beyond the first mixing zone. In addition, hygroscopic additives such as chalk or silica require pre-drying at 80 °C for 3 h when ambient relative humidity exceeds 60 %; otherwise steam volatilization during injection creates surface splay and local hardness depressions at the gate area.

Compounding of vulcanized rubber soling at production scale begins with a masterbatch of SBR/NR, carbon black, oil, zinc oxide, and stearic acid discharged from a tangential internal mixer with net chamber volumes from 35 L to 270 L. The dump temperature for a 60 Shore A compound is typically held between 125 °C and 145 °C to limit thermal degradation and premature scorch; higher-Shore-A compounds with 60 to 70 phr carbon black may reach 150 °C if the ram pressure and rotor speed are not reduced. After masterbatch cooling, sulfur and accelerators are added on a 550 mm × 1500 mm two-roll mill at a nip of 6 to 10 mm and a batch temperature below 90 °C, because TBBS and sulfur begin to form crosslinks above 110 °C and the scorch time ts2 at 121 °C measured by ASTM D1646-19 can drop below 5 min. A typical curing package of 1.6 to 2.0 phr sulfur and 1.0 to 1.5 phr TBBS gives a t90 at 160 °C of 5.0 to 6.5 min and a maximum moving die rheometer torque of 7.5 to 9.5 dNm under ASTM D5289-21; lower torque corresponds to lower crosslink density and can reduce Shore A by 2 to 3 points even without formulation changes.

Hardness after compression moulding is influenced by cure time, mould temperature, and pressure. On a single-station hydraulic press with 400 mm × 400 mm platens and 14 MPa ram pressure, a 3 mm outsole slab cured at 160 °C for t90 plus 2 min typically reaches the target 65 Shore A, while the same slab cured at t90 without the extra 2 min can be 1 to 2 Shore A points lower because sulfur crosslinking is incomplete and residual free sulfur acts as a temporary plasticizer. Overcure beyond 8 min at 160 °C leads to reversion in natural rubber phases, especially when the NR content exceeds 50 phr; reversion reduces tensile strength from 16 MPa to 11 MPa and decreases Shore A by 2 to 4 points in the centre of the slab. Compression moulding of higher-viscosity compounds with Mooney viscosity above 85 MU requires prewarming to 70 °C and increased pressure to 18 MPa to avoid trapped air at the mould periphery; otherwise hardness readings near the edge can be 3 Shore A points higher than at the centre due to local overcure and higher density.

Abrasion, flex fatigue, and wet-slip trade-offs across the durometer band

Comparative wear data for solid soling compounds in the 60 to 70 Shore A band show that abrasion loss measured by DIN 53516:2012 decreases as hardness rises, but flex crack initiation resistance measured by ASTM D813-20 or ISO 132:2017 is not monotonic. A 58 Shore A SBR/NR outsole may lose 210 mm³ in abrasion and pass 100,000 flex cycles at 25 °C, while a 70 Shore A compound with higher carbon black loading may lose 150 mm³ but initiate cut growth after 30,000 cycles when strain amplitude is 10 mm. The wet-slip performance, quantified by SATRA TM144:2021 on a wet quarry tile, generally improves as hardness moves from 55 to 65 Shore A because the soling surface deforms sufficiently to achieve a larger contact area, but above 70 Shore A the coefficient of friction may plateau or decline because the compound can no longer micro-roughen against the floor; published data for specific acceptance limits in the 60 to 70 band is limited, and end-use specifications typically require coefficient of friction values between 0.40 and 0.65 depending on the footwear category and heel geometry.

The selection of 60 to 70 Shore A for occupational outsoles also interacts with penetration resistance and energy absorption. EN ISO 20345:2021 does not set a hardness pass/fail clause for solid outsoles, but its requirements for abrasion resistance and sole flexibility are evaluated by laboratory tests referenced in the standard; a compound with hardness below 60 Shore A often fails the abrasion loss threshold after prolonged wear on rough concrete, while hardness above 70 Shore A can increase the risk of heel strike discomfort and reduce slip resistance on smooth wet surfaces. Flex fatigue of NBR/PVC soling at 65 Shore A measured under ISO 17707:2018 is affected by the rubber phase domain size; when the NBR phase is poorly dispersed in PVC, flex crack propagation occurs after 40,000 cycles at a 45° flex angle, but a well-mixed blend from a twin-screw line with L/D 44:1 and an intensifier screw can reach 150,000 cycles without visible crack growth.

Injection moulding of solid TPR outsoles with Shore A 60 to 70 compounds on horizontal hydraulic machines with clamp forces from 1,500 kN to 3,500 kN produces characteristic defects when hardness falls at the lower end of the band. A 60 Shore A SBS compound with high oil content may show gate blush, surface smear, and excessive shrinkage of 1.2 to 1.8 % after demoulding because the melt has lower viscosity and the part cools with lower frozen-in stress; by comparison, a 68 Shore A compound with less oil exhibits shrinkage of 0.8 to 1.2 % and a glossier surface. Mould temperature significantly influences hardness at the gate region: with a mould temperature of 25 °C, rapid solidification can produce a 62 Shore A reading at the gate and 65 Shore A at the toe, while a mould temperature of 45 °C reduces this gradient to less than 2 Shore A points. Holding pressure is set at 40 to 60 % of injection pressure and maintained for 6 to 10 s; insufficient holding pressure yields sink marks on the tread surface and lower density in the thick lug areas, causing local hardness reductions of 3 to 5 Shore A points.

To maintain batch-to-batch consistency in TPR soling, processors monitor melt flow index per ISO 1133-1:2022, pellet moisture, and the temperature gradient across the injection barrel. A shift in melt flow index from 15 g/10 min to 25 g/10 min at 190 °C/5 kg is sufficient to move a nominal 65 Shore A compound to 61 Shore A after moulding, especially when regrind is blended above 20 wt%. On production lines, the use of 20 % regrind is standard but increases variability because the regrind has undergone a first heat history and contains sheared polystyrene domains; when regrind content rises above 30 %, tensile strength measured by ASTM D412-16(2021) can fall by 10 to 15 % and the hardness distribution across a 50-cavity outsole mould widens from ±1 Shore A to ±3 Shore A.

Thermal degradation pathways emerge above 70 Shore A in vulcanized soling compounds

Thermal degradation and oxidative embrittlement in vulcanized SBR/NR soling compounds become measurable when Shore A hardness exceeds 70 because the same formulation changes that raise hardness—higher carbon black loading, lower oil content, and increased crosslink density—also increase heat generation during service and reduce the mobility of antioxidant systems. At 70 Shore A, a compound containing 60 to 70 phr N330 and 0 to 5 phr oil exhibits a heat build-up temperature of 28 to 35 °C in a Goodrich flexometer at 2.5 mm stroke and 25 °C ambient; at 76 Shore A, the same test can produce a heat build-up of 45 °C and a loss of tensile strength after 24 h air aging at 100 °C of 35 to 50 %, measured by ASTM D573-21. Because footwear soling compounds are flexed repeatedly, the combination of thermal softening and oxidative crosslinking creates a hardness drift of +3 to +5 Shore A after 70 h of laboratory flexing at 50 °C; this drift can take an outsole from 68 Shore A to 73 Shore A and alter slip resistance and flexibility.

Limitations at the 60 to 70 Shore A boundary are therefore defined by antioxidant package and filler selection. A compound at 65 Shore A with 2 phr polymerized 2,2,4-trimethyl-1,2-dihydroquinoline and 1 phr N-isopropyl-N-phenyl-p-phenylenediamine retains 80 % of original tensile strength after 168 h air aging at 70 °C per ISO 188:2011, whereas the same formulation at 72 Shore A may retain only 60 % because the higher filler surface area adsorbs antioxidants and reduces their effective concentration in the rubber matrix. Oil-exuded surfaces, particularly in NBR/PVC and SBS soling at Shore A 60 to 65, may also exhibit hardness increase after solvent extraction of the plasticizer; an outsole exposed to repeated contact with oil or cleaning agents can lose 2 to 4 Shore A or gain 2 to 5 Shore A depending on whether the agent extracts plasticizer or swells the polymer phase, as evaluated by ASTM D471-16(2021) and ISO 1817:2015. Published data for long-term hardness drift of bio-based plasticizer compounds in solid soling applications is limited, and qualified formulations should be tested under the specific end-use oil and temperature conditions rather than interpolated from generic compatibility tables.