Utilization of biosilica for energy-saving tire compounds: Enhancing performance and efficiency

2.1 Materials

SSBR Buna VSL 5025-0 HM and BR Nd-BR CB24 were purchased from Lanxess, Germany. SSBR/BR blend was strengthened by HD silica (Zeosil 1165 MP, Solvay, China), biosilica (Nano Powdering 550, Agrotech Industries, Indonesia), and Carbon Black N330 (OCI, Korea). TESPT (JHS69, Jianghan Fine Chemical Co. Ltd, China) was used to enhance the rubber–filler interaction. Naphthenic oil (Nytex 840, Nynas, Sweden) was used as a plasticizer. All compounds used N-cyclohexyl-2-benzothiazole sulfenamide (CBS OCP, Kemai Chemical Co. Ltd, China), diphenyl guanidine (DPG, Kemai Chemical Co. Ltd, China), and Tetrabenzylthiuram disulfide (Rhenogran TBzTd-70, Rhein Chemie Rheinau Mannheim GmbH, German) as accelerator. The compounds were protected against oxygen and ozone by using 2,2,4-trimethtyl-1,2-dihydroquinoline (TMQ Vulkanox HS/LG, Rhein Chemie Rheinau Mannheim GmbH, German), p-phenylenediamine (6PPD, Sirantox Vulkanox 4020, Lanxess, Deutschland GmbH, German), and wax (Antilux 654, Lanxess, Deutschland GmbH, German). Stearic acid (Aflux 52, Rhein Chemie Rheinau Mannheim GmbH, German) and zinc oxide (rubber activator, Lanxess, Deutschland GmbH, German) were used as an activator, and sulfur (Midas SP-325, Miwon Chemicals Co., Ltd, Korea) was used as a vulcanizing agent.

2.2 Characterization of HD silica and biosilica

The characterization of silica and biosilica involved a comprehensive examination of their SSA and surface chemistry. The multipoint Brunauer–Emmet–Teller (BET) nitrogen surface area of HD silica and biosilica was determined according to ASTM D1993-18 standards. The BET test was performed using Quantachrome NOVA 4200e (Boynton Beach, Florida, USA). Subsequently, functional groups on HD silica and biosilica were investigated using attenuated total reflectance FTIR (ATR-FTIR) Nicolet iS5 (Thermo Scientific, USA). The samples HD silica and biosilica were dried at 105°C in an oven for 2 h and put in a desiccator before being characterized. The morphology of HD silica and biosilica were characterized using an Environmental scanning electron microscope (ESEM; Quattro, Thermo Fisher, USA). The samples without coating were directly tested in ESEM.

2.3 Preparation of passenger tread tire compound

Passenger car tread tire compound was prepared using the formulation shown in Table 1. The rubber compound was designed with different ratios between HD silica and biosilica while maintaining an equivalent number of total fillers of 80 parts per hundred rubber (phr). The ratios of HD silica/biosilica were varied at 80:0 (HD100), 60:20 (B25), 40:40 (B50), 20:60 (B75), and 0:80 (B100), respectively.

The mixing process was carried out using a Banbury mixer (David Bridge Co. Ltd, Manchester, England), following a three-stage mixing procedure as shown in Table 2. In the first stage, rubber materials were added with half of the silica, silane coupling agent, oil, activator, and protective agent. The mixer was operated at a fill factor of 70% and chamber temperature at 60°C with a rotor speed of 80 rpm. After all the materials were added into the Banbury mixer, the rotor speed was increased to 100 rpm, resulting in a chamber temperature rise to attain a dump temperature of 150°C, and this facilitated the execution of the silanization reaction (3). Subsequently, the dump temperature was measured and rubber compound was sheeted out using two roll mills to homogenous filler distribution and stored for 24 h at room temperature before being used for the next mixing stage.

In the second step, cold compound was re-mixed in the Banbury mixer. Initially, the chamber’s temperature was calibrated to 60°C, accompanied by a rotor speed of 100 rpm, and a fill factor set at 70%. Subsequently, The silanization reaction was facilitated by the inclusion of DPG as an activator (11), and mixing persisted until the temperature reached 150°C. In the last mixing stage, accelerators and sulfur were added into the Banbury mixer with an initial temperature of 50°C and a fill factor of 65%. The compound was homogenized and sheeted out by using two roll mills, then stored at room temperature for 24 h.

2.4 Characterization of tread tire rubber compound

Filler–filler interaction which indicated the change in storage modulus (G′) at different strains of rubber compound without accelerator and the vulcanizing agent was analyzed by Rubber Processing Analyzer (RPA Elite, TA Instrument, USA). RPA was operated at 100°C, a frequency of 0.5 Hz under varying strains from 0.56% to 100%. The Payne effect was determined from G′ at low and high strains, i.e., at 0.56% and 100%, respectively. The results from the first sweep were recorded.

Both the overall and chemically bound rubber contents were subjected to analysis. The physical linkages between rubber and silica were cleaved using ammonia, to determine chemical-bound rubber content (12). A total of 0.2 g rubber compound without curatives was cut into small pieces, put into a metal cage, and immersed in toluene with and without ammonia for 168 h. Furthermore, the samples were dried at 105°C for 24 h. The bound rubber contents were determined by the weights of samples before and after extraction using Eq. 1 as follows (12):

where m is the weight of the sample after extraction, m _s is the weight of filler in the sample, and m _r is the weight of rubber in the sample.

The cure characteristics of tread tire compound with curatives agent were analyzed by using moving die rheometer (MDR 2000, Alpha Technology, USA) at 150°C to determine min torque (M _L), max torque (M _H), scorch time (t _s), and optimum curing time (t ₉₀).

2.5 Morphology of tread tire vulcanizate

The dispersion of HD silica and biosilica in a rubber blend matrix was performed using ESEM. The cryogenic fractured surface of rubber vulcanizate was used as the testing specimens. The rubber vulcanizates HD100 and B100 were tested to show the influence of biosilica.

2.6 Characterization of tread tire rubber vulcanizate

The compound experienced were vulcanized, reaching the respective optimum cure time (t ₉₀). This process was carried out using a laboratory compression press at 150°C and 100 kg·cm⁻² in order to obtain vulcanized samples. To determine the apparent crosslink density, approximately 0.2 g of each vulcanized sample was immersed in 50 mL of aromatic solvent (toluene) for 72 h at room temperature. After immersion in toluene, the vulcanized sample was removed, blotted quickly with filter paper, weighed, and finally dried at room temperature for 24 h. The swelling value (Q) was identified as the number of grams of toluene absorption in rubber per gram of rubber hydrocarbon.

The formula weight is defined as the total weight of all compounding materials including rubber based on 100 phr. Apparent crosslink density was measured as reciprocal swelling value, 1/Q (13).

Tensile properties were tested using Universal Testing Machine (UTM; MTS, USA) operating at a crosshead speed of 500 mm·min⁻¹ at room temperature according to ISO 37. The type 2 dumbbell specimens were prepared by die-cut of 2 mm thick vulcanized rubber. Three specimens were tested and the median value was recorded as the testing results.

Young’s modulus was measured using UTM (MTS, USA) following the procedure outlined in ASTM E111-17. The measurement was carried out at room temperature, while the crosshead speed was set at 100 mm·min⁻¹. The specimens in accordance with dumbbell-shaped ISO 37 Type 2 were strained to 100% and then Young’s modulus was calculated from the slope of the stress–strain curve at low strain (<100%). Three specimens were tested and the median value was recorded as the testing results. In addition, to evaluate the hardness of vulcanized rubber as per ISO 48−4, Shore A Frank Durometer was used.

Uniaxial cyclic testing was performed using UTM (MTS, USA) at a constant crosshead speed of 100 mm·min⁻¹ at room temperature. The testing protocol employed dumbbell-shaped samples that were extracted from 2 mm thick plates, adhering to the specifications outlined in ISO 37 Type 2. The specimens were strained to 100% and four cycles were applied to obtain the hysteresis loop that was calculated by the ratio of areas under the loading and unloading curve, using Eq. 3 (14).

where H is the hysteresis losses, and S _load and S _unload are areas under loading and unloading curves, respectively. In addition, the softening stress was determined from the maximum stress value at the end of the loading path.

2.7 Magic triangle properties of tread tire rubber

The DIN volume loss was used to determine the abrasion resistance of rubber vulcanizate, performed by DIN Abrader according to the guidelines of ASTM D5963. Subsequently, three specimens were tested and the median value was recorded as the DIN volume loss for each sample.

The dynamic mechanical properties of rubber vulcanizate were characterized using Dynamic Mechanical Analyzer (DMA; DMA7100, Toshiba, Japan). The vulcanized rubber samples were cut into 20 mm × 9 mm × 2 mm specimens and tested in tension mode under temperature sweep between −80°C and 80°C at the frequency of 10 Hz and 0.1% strain. The heating rate during the test was determined at 2°C‧min⁻¹. The tan δ at 0°C and 60°C was recorded indicating wet grip and rolling resistance of the tread tire, respectively.

3.1 HD silica and biosilica properties

Silica serves as a filler primarily used to reinforce the rubber matrix. The properties of the reinforcing filler, such as surface area, structure, and surface activity parameters, significantly impact the mechanical and dynamic properties of elastomers (15). The surface area of silica is determined by using BET nitrogen adsorption, which indicated the total surface area including porosity (15). The multipoint BET surface area and pore volume of HD silica are slightly higher than biosilica as shown in Table 3. Furthermore, the Spectra ATR-FTIR and ESEM images of HD silica and biosilica are shown in Figures 1 and 2, respectively.

Figure 1

ATR-FTIR spectra of HD silica and biosilica.

Figure 2

ESEM images (1,000× magnification) of HD silica and biosilica.

3.2 Properties of tread tire rubber compound

The investigation into the filler–filler interaction in silica-filled rubber, referred to as the “Payne effect,” was conducted by observing the variation in storage modulus (G′) at different strain amplitudes. Figure 3 shows the strain dependence of G′ at 100°C of rubber-HD silica compared to rubber-biosilica with different ratios of HD silica to biosilica.

Figure 3

Storage modulus (G′) against strain of SSBR/BR-silica with different ratio of HD silica:biosilica compared to HD silica-filled SSBR/BR blend vulcanizate.

The analysis of bound rubber content was used to determine the physical and chemical adsorption of silica onto SSBR/BR rubber blend, respectively. The total bound rubber indicates the physical and chemical interaction between rubber and reinforce filler. While the chemically bound rubber content was determined after applying ammonia treatment to cleavage the physical bonding between rubber and filler (12). Figure 4 shows the total physical and chemical bound rubber content of HD silica, biosilica, and HD silica/biosilica blend.

Figure 4

Total, physically, and chemically bound rubber contents of SSBR/BR-silica with different ratios of HD silica:biosilica compared to HD silica-filled SSBR/BR blend vulcanizate.

The cure characteristics of HD silica, biosilica, and different ratios of HD silica/biosilica-filled rubber blend are shown in Figure 5.

Figure 5

Cure torques (a), scorch time, cure time, and cure rate index (b) of SSBR/BR-silica with different ratios of HD silica:biosilica compared to HD silica-filled SSBR/BR blend vulcanizate.

3.3 Morphology of tread tire vulcanizate

The dispersion of HD silica and biosilica in tread tire vulcanizate captured by ESEM are shown in Figure 6.

Figure 6

ESEM images (20,000× magnification) of biosilica compared to HD silica-filled SSBR/BR blend.

3.4 Mechanical properties of tread tire rubber

The apparent crosslink density of HD silica, biosilica, and HD silica/biosilica blend filled in rubber blend vulcanizate is shown in Figure 7.

Figure 7

Apparent crosslink density of SSBR/BR-silica with different ratio of HD silica:biosilica compared to HD silica-filled SSBR/BR blend vulcanizate.

Figure 8 shows the modulus elasticity and hardness of HD silica, biosilica, and HD silica/biosilica blend filled in tread tire vulcanizate.

Figure 8

Modulus elasticity and hardness of SSBR/BR-silica with different ratios of HD silica:biosilica compared to HD silica-filled SSBR/BR blend vulcanizate.

The tensile properties of biosilica and different ratios HD silica/biosilica filled rubber blend in terms of modulus 50% and 100% strain, tensile strength, and elongation at break of HD silica are shown in Figure 9.

Figure 9

Stress–strain curve of SSBR/BR-silica with different ratios of HD silica:biosilica compared to HD silica-filled SSBR/BR blend vulcanizate.

The reinforcement phenomenon of silica-filled rubber was analyzed using stress softening (σ _soft), and behavior was determined by using the uniaxial cyclic test. Figure 10 shows the loading–unloading stress–strain curve for four cycles for HD silica, biosilica, and its blend filled in the rubber matrix. The stress softening (σ _soft) values and hysteresis losses (H) are tabulated in Table 4.

Figure 10

Four loading–unloading cycles of HD silica and biosilica-filled SSBR/BR blend vulcanizate.

3.5 Magic triangle properties of tread tire rubber

The incorporation of silica-silane-filled rubber enhances the characteristics of tread tire, leading to a reduction in rolling resistance, improved grip during braking, and maintained abrasion resistance (2). The abrasion resistance of tread rubber vulcanizate was determined using a DIN abrader. The DIN volume loss values are shown in Figure 11.

Figure 11

DIN volume loss of SSBR/BR-silica with different ratios of HD silica:biosilica compared to HD silica-filled SSBR/BR blend vulcanizate.

Loss tangent or tan δ as a function of the temperature of HD silica and biosilica with various ratios of biosilica-filled SSBR/BR blend are shown in Figure 12.

Figure 12

tan δ values against temperature of SSBR/BR-silica with different ratios of HD silica:biosilica compared to HD silica-filled SSBR/BR blend vulcanizate.

The wet grip and rolling resistance of tread tire vulcanizate could be predicted by DMA. The wet grip of tread tire vulcanizate was evaluated from tan δ value at 0°C. Meanwhile, rolling resistance was indicated from tan δ value at 60°C. Table 5 shows the wet grip and rolling resistance of tread tire vulcanizate with different HD silica:biosilica ratios.

4.1 HD silica and biosilica properties

As shown in Table 3, the elevation in BET surface area is directly attributed to the corresponding augmentation in silica’s pore volume (16). Higher BET surface area and pore volume contribute to better incorporation of silica in rubber matrix (5).

The result of ATR-FTIR analysis shows striking similarities between the spectra of HD silica and biosilica. The highest peak is shown at a wavenumber of 1,081 cm⁻¹ for HD silica and 1,089 cm⁻¹ for biosilica assigned symmetric stretching vibration of Si–O on Si–O–Si, respectively. In addition, the spectra ATR-FTIR of HD silica and biosilica show symmetric stretching vibration of Si–O on Si–O–Si at wavenumber 799 cm⁻¹ (6,10). These particular peaks are considered as the main peaks for silica materials (17). However, some silica materials show different peak at wavenumber 963 cm⁻¹ which indicates asymmetric stretching vibration of Si–O on Si–OH (10,17). Interestingly, ATR-FTIR spectra of both silica do not show O–H stretching and O–H bending at a broad peak of 3,200−3,600 and 1,650 cm⁻¹, respectively. This indicates the absence of water molecules in silica particles (18). During preparation, before use in the rubber compound, HD silica and biosilica were dried at 105°C for 4 h. Consequently, any water molecules present within the silica particles evaporated during this drying procedure.

The morphology of both silica shows irregular shapes. However, the morphology of HD silica shows the presence of certain spherical agglomerations. The size of agglomerate biosilica particles is smaller compared to HD silica. The presence of silanol groups on silica surfaces as shown in the ATR-FTIR result (Figure 1) leads to the formation of aggregates or agglomerates between silica particles due to hydrogen bonding interaction (1).

4.2 Properties of tread tire rubber compound

Figure 3 shows the Payne effect value derived from the calculation of the difference between G′ (storage modulus) at 100% strain and G′ at 0.56% strain. The Payne effect of HD silica-filled rubber blend is higher compared to biosilica-filled rubber blend uncured compound. The addition of biosilica into tread tire compound decreased the Payne effect and G′ at low strain value because a higher BET surface area of HD silica can promote the aggregation of silica particles (16). Furthermore, the incorporation of biosilica serves to decrease the interactions between fillers within tread tire compound. This observation suggests that TESPT (tetrasulfide silane) demonstrates effective interaction with silica surfaces, thereby enhancing the hydrophobic characteristics of the silica component, and providing better compatibility between silica and SSBR/BR blend. In this research, the same amount of TESPT was used in tread tire compound formulation. However, it is important to note that the amount of TESPT depended on the SSA of silica (19). Therefore, biosilica compound contains slightly higher TESPT which can enhance rubber–filler interaction and reduce filler–filler interaction of silica.

HD silica-filled rubber blend compound shows slightly higher physically bound rubber content. Meanwhile, the addition of biosilica leads to a reduction in the physical interaction between rubber and filler. This phenomenon is confirmed by the Payne effect result as shown in Figure 3. The physical interaction between rubber and silica is not only due to the interaction between silica particles but also the physical adsorption of rubber onto silica particles in the presence of a silane coupling agent. HD silica with higher BET surface area and pore volume facilitates the absorption of TESPT onto the silica surface. Furthermore, rubber chains are adsorbed to the silica surfaces to form stable and unstable interactions between rubber and silica (20).

The chemically bound rubber content of biosilica is higher than HD silica-filled rubber blend compound. However, when examining rubber compound with varying ratios of HD silica and biosilica, the chemically bound rubber content appears to be relatively comparable across different formulations. The chemically bound rubber content denotes chemical interaction among rubber, TESPT, and silica. Lower agglomeration of biosilica as shown in the Payne effect value (Figure 3) of biosilica-filled rubber blend supports better interaction among rubber, TESPT, and silica. Moreover, a slightly higher amount of TESPT based on the SSA in biosilica leads to an initiated silanization reaction and increases chemical-bound rubber content.

Among the various rubber blends, those containing HD silica exhibit the most notable values for minimum, maximum, and torque differences. However, the incorporation of biosilica into tread tire compound results in a reduction in M _H (maximum torque), M _L (minimum torque), and the torque difference (M _H–M _L). In general, a decrease in M _L leads to a reduction in viscosity and indicates better processing due to an increased degree of silanization reaction between rubber and silica as a consequence of better Payne effect as shown in Figure 3. The decreasing M _H and torque difference (M _H−M _L) due to the decreasing Payne effect value and increasing chemical bound rubber are shown in Figure 4.

The scorch time of HD silica, biosilica, and HD silica/biosilica blend are comparable. Meanwhile, distinct variations become evident in terms of the optimal cure time and the cure rate index, and lower optimum time and higher cure rate index indicate better productivity. It can be seen that the cure rate index of HD silica-filled rubber blend is lower than biosilica in tread tire compound. The presence of acid silanol groups causes the adsorption of base accelerators on silica surfaces, and as a result, it inhibits the vulcanization reaction (21). As previously mentioned, the amount of TESPT depends on the SSA of silica. Therefore, higher SSA increase TESPT doses in rubber compound. This research is in line with the work reported by Lolage et al. (5), the compound containing rice husk silica showed a lower cure time and higher cure rate index.

4.3 Morphology of tread tire vulcanizate

As shown in Figure 6, distinct aggregates and agglomerates of HD silica are prominently showcased. The formation of aggregates and agglomerates of HD silica due to the interaction between HD silica is due to the hydrogen bonding between silanol groups on the HD silica surfaces. This mechanism contributes to the formation of a filler network, leading to an augmentation of the Payne effect value, as corroborated by the observations presented in Figure 3. On the other hand, the dispersion of biosilica in the SSBR/BR blend matrix is better than HD100 tread tire vulcanizate samples as shown in Figure 6. This result is in line with the Payne effect results (Figure 3).

4.4 Mechanical properties of tread tire rubber

The observed crosslink density shows a slight reduction as the proportion of biosilica in tread tire vulcanizate increases. This apparent crosslink density was assessed through the swelling technique in toluene. The filler containing rubber vulcanizate retards the adsorption of toluene, and the higher filler-filler interaction tends to increase trapped rubber (1). Therefore, some rubber molecules are unable to come in contact with toluene, leading to a decrease in swelling value. However, the addition of biosilica decreases the Payne effect as shown in Figure 3. The reduction of the Payne effect induces more contact between rubber and toluene, and this affects the increasing swelling value and decreasing apparent crosslink density, respectively.

Modulus elasticity (Young’s modulus) and hardness of HD silica-filled rubber blend vulcanizate show the highest values. The hardness of rubber is correlated to Young’s modulus by theoretical and empirical equations (22). The relationship is determined at low strain using UTM. At this condition, the reinforcement effect is due to the formation of a filler network (23). Therefore, the increase in Young’s modulus and hardness resulted from the increase in the Payne effect value of high-dispersion silica within tread tire compound. On the contrary, the addition of biosilica tends to decrease the Payne effect, Young’s modulus, and hardness of tread tire, respectively.

As shown in Figure 9, the HD silica-filled rubber (HD100) curve has higher stress compared to biosilica-filled rubber at the same strain. Furthermore, more biosilica loading in rubber matrix tends to reduce stress. However, the biosilica-filled SSBR/BR blend (B100) shows the opposite direction since it has the highest stress than other HD silica/biosilica-filled rubber blend. This enhanced performance can be attributed to the prevalence of rubber–filler interaction, particularly evident through bound rubber, during low-strain conditions of either 50% or 100%. This dominance surpasses the filler–filler (Payne effect) or rubber–rubber (crosslink density) interactions. Therefore, stress at a low strain of biosilica-filled SSBR/BR blend is more influenced by rubber–filler interaction.

The incorporation of biosilica into the rubber matrix leads to a slight reduction in tensile strength while resulting in an increase in elongation at break. This indicates that at high strain, filler networks break down and rearrange. Stress is mainly contributed by chemical rubber–filler interaction and rubber network (23). The lower the filler network, the larger the inter-aggregate distance of biosilica leading to the mobility of the chains, which is not restricted by filler. As a consequence, it resulted in increasing elongation at break but decreased tensile strength properties.

The occurrence of stress-softening behavior (Mullins and Tobin effect) is evident in all vulcanizate samples. Stress softening and hysteresis loss are more pronounced at the first to second stretching, and further softening is very weak. This phenomenon is due to many aspects, i.e., the breaking of filler networks, agglomerates, aggregates, rubber–filler linkages, or slippage of the elastomer chains at the filler surface (24). During the first hysteresis area, filler network and/or rubber–filler linkages breakdown caused the formation of dissipated energy. The higher Payne effect value of HD silica in tread tire compound is accompanied by increased hysteresis losses. The addition of biosilica suppressed the filler network and caused the decrease in the hysteresis losses.

4.5 Magic triangle properties of tread tire rubber

The lower DIN volume loss values result in better abrasion resistance. The abrasion resistance of HD silica is comparable to B50, B75, and B100 tread tire vulcanizate, and highly influenced by filler–rubber interaction and crosslink density (25). The addition of biosilica led to enhance chemically bonded rubber and a reduced silica network due to the better interaction between TESPT and silanol groups on silica surfaces.

The tan δ at the peak of HD silica-filled SSBR/BR blend vulcanizate is lower than biosilica-filled SSBR/BR blend vulcanizate. This indicates that the filler network of the HD silica-based compound is stronger, consistent with its elevated Payne effect value (Figure 3). In the glassy regions, the main contribution of energy dissipation is the rubber matrix. The higher filler network leads to entrapping the rubber chains in the filler networks. Therefore, the ratio of rubber in the compound will reduce since the filler network cannot be easily broken causing the decrease in the tan δ at peak (1,26). The presence of biosilica decreases the filler network as shown in Figure 3. Moreover, increasing the chain mobility of rubber and loose rubber chains contribute to energy loss resulting in increased tan δ at the peak.

The improved wet grip of tread tire is accompanied by a higher tan δ value at 0°C. It is important to note that the inclusion of biosilica in tread tire compound enhances wet grip. The addition of 100% biosilica (B100) to substitute HD silica results in the highest tan δ value at 0°C. This phenomenon describes that better dispersion of silica leads to an increase in the chain mobility of rubber and results in increasing tan δ at 0°C. The lower value of tan δ at 60°C shows better rolling resistance. The filler network will break down due to the increase in temperature, and the main contribution of energy dissipation is affected by the rubber network and chemical interaction between rubber and filler. For the HD 100 and B25 tread tire vulcanizates with higher Payne effect values, the filler network is broken during testing. As the temperature rises, the rubber chains previously trapped within filler clusters are released, leading to an elevated tan δ value at 60°C, which indicates an unfavorable impact on rolling resistance. Meanwhile, with an increased quantity of biosilica (B50, B75, and B100), the tan δ value at 60°C decreases, signifying an improvement in rolling resistance. This is due to the increasing filler dispersion and chemically bound rubber of biosilica-filled SSBR/BR blend as shown in Figures 3 and 4, respectively. Therefore, based on the performance of tread tire vulcanizate testing, HD silica can be substituted by biosilica for a minimum ratio of HD silica: biosilica (50:50) to enhance wet grip and rolling resistance while maintaining the abrasion resistance.