**4.1 Influence of mixing dump temperature in silica-DPNR compound**

The properties of silica-filled compounds show a strong dependence on mixing temperature, since the silanization reaction and silica dispersion are key parameters in this system [25]. The optimal mixing conditions for silica-filled NR compounds with TESPT as a coupling agent are required to obtain high reinforcement in NR [56]. The influence of the dump temperature of the first mixing step on the vulcanization behavior of the DPNR-silica compounds as compared to the NR-silica compounds is illustrated in **Figure 3**. Dump temperature is the final temperature reached when the compound is discharged from the internal mixer. The compound formulation is described in **Table 10**. The vulcanization curves and behavior of the NR-silica compounds can be divided into three groups according to dump temperature. The first group encompasses the compounds with low dump temperatures (below 150°C), the middle group is represented by the compounds with moderate dump temperatures around 150–155°C, and the third group is the compounds with high dump temperatures (>155°C). At low dump temperature region, both NR-silica and DPNR-silica compounds display pronounced silica flocculation as evidenced by initial torque rise, high maximum torques, and long scorch times. At

#### **Figure 3.**

*Vulcanization behavior at 150°C of (a) the NR compounds and (b) the DPNR compounds, mixed till different dump temperatures (DT) in the first mixing stage.*

**53**

higher dump temperature [56, 57].

compound.

**Table 10.**

*Silica-Reinforced Deproteinized Natural Rubber DOI: http://dx.doi.org/10.5772/intechopen.85678*

*Basic compounding formulation used for NR and DPNR.*

moderate dump temperature region, no appearance of flocculation is observed for NR-silica and DPNR-silica compounds but at lower maximum torques and shorter scorch times compared to those of the compounds with low dump temperature. The maximum torque of the DPNR compound mixed at moderate temperature (around 150°C) is comparable with the NR compound at higher dump temperature region; both NR- and DPNR-silica compounds display a lower maximum torque and no sign of flocculation as the silanization reaction increases and results in a reduction of silica-silica interaction. Nonetheless, the decrease in maximum torque for the DPNR compound at higher dump temperature is much less compared to the NR-silica compound. The low-protein content in DPNR may attribute to better silanization in the compound which gives more filler-to-rubber linkages. Noticeably in the vulcanization curves for these high dump temperatures, there is clear indication of reversion for NR-silica compound. This reversion sign in vulcanization curve has also been reported in NR compound in the presence of TESPT at temperature higher than 150°C where the behavior is totally different as compared to SBR compound [56]. The DPNR-silica compounds show no sign of reversion at high dump temperature as seen in NR compounds. This suggests that a different network structure in the DPNR contributes to the thermal resistance of the cured

**Ingredients Amount (phr)** Natural rubber (vary as stated in text) 100 Silica Ultrasil 7005 55 Silane TESPT 5 Zinc oxide 2.5 Stearic acid 1 TDAE oil 8 TMQ 2 Sulfur 1.4 CBS 1.7 DPG 2

A comparison of filler-filler interaction in DPNR-silica-TESPT compounds with NR-silica-TESPT as a function of mixing temperature is shown in **Figure 4**. A common way to measure the filler-filler interaction is calculating the Payne effect from the strain sweep of the dynamic testing or different in the storage modulus of the low or 0.56% and high strain or 100% while maintaining the frequency and temperature at constant value. Both NR- and DPNR-silica-TESPT show the decreasing trend of Payne effect with increasing dump temperature. This trend is due to the increase of silanization reaction with increasing temperature which results in the efficiency of the silane to hydrophobize the surface of the silica. High Payne effect of NR- and DPNR-silica compounds at low dump temperature is in agreement with the sign of flocculation observed in vulcanization curve. The results are also in agreement with other works reported on compound comprising TESPT showed a reduction of flocculation of silica when mixed at

**Figure 5** illustrates the effect of mixing temperature on Wolff's filler structure parameter, *αf*. Wolff's filler structure parameter, *αf*, is determined from the ratio

*Silica-Reinforced Deproteinized Natural Rubber DOI: http://dx.doi.org/10.5772/intechopen.85678*


#### **Table 10.**

*Silicon Materials*

silanization to be completed to obtain high reinforcement [7]. The silanization is influenced by the temperature as it is a chemical reaction. Hence, mixing temperature is the parameter of paramount importance in mixing silica and rubber in the presence of coupling agent such as TESPT [55, 56]. The temperature has a more dominant effect than time in the silica-TESPT reaction [23]. In order to achieve a sufficient degree of silanization, the temperature during mixing should be between 150 and 160°C. However, above 160°C either the coupling agent starts to prematurely react with the rubber matrix or the TESPT starts to donate sulfur; both result in pre-scorch of the compound. A mixing time of at least 10 minutes at 150°C is necessary to ensure complete coupling of the silica and the silane and that the reaction between the silica and the silane takes place primarily during the first mixing step [50]. In an investigation of cure characteristics of NR and TESPT in comparison with SBR, it clearly shows that NR starts to react with TESPT at a temperature of 120°C but SBR only reacts with TESPT at a higher temperature of 150°C [56].

**4.1 Influence of mixing dump temperature in silica-DPNR compound**

The properties of silica-filled compounds show a strong dependence on mixing temperature, since the silanization reaction and silica dispersion are key parameters in this system [25]. The optimal mixing conditions for silica-filled NR compounds with TESPT as a coupling agent are required to obtain high reinforcement in NR [56]. The influence of the dump temperature of the first mixing step on the vulcanization behavior of the DPNR-silica compounds as compared to the NR-silica compounds is illustrated in **Figure 3**. Dump temperature is the final temperature reached when the compound is discharged from the internal mixer. The compound formulation is described in **Table 10**. The vulcanization curves and behavior of the NR-silica compounds can be divided into three groups according to dump temperature. The first group encompasses the compounds with low dump temperatures (below 150°C), the middle group is represented by the compounds with moderate dump temperatures around 150–155°C, and the third group is the compounds with high dump temperatures (>155°C). At low dump temperature region, both NR-silica and DPNR-silica compounds display pronounced silica flocculation as evidenced by initial torque rise, high maximum torques, and long scorch times. At

*Vulcanization behavior at 150°C of (a) the NR compounds and (b) the DPNR compounds, mixed till different* 

**52**

**Figure 3.**

*dump temperatures (DT) in the first mixing stage.*

*Basic compounding formulation used for NR and DPNR.*

moderate dump temperature region, no appearance of flocculation is observed for NR-silica and DPNR-silica compounds but at lower maximum torques and shorter scorch times compared to those of the compounds with low dump temperature. The maximum torque of the DPNR compound mixed at moderate temperature (around 150°C) is comparable with the NR compound at higher dump temperature region; both NR- and DPNR-silica compounds display a lower maximum torque and no sign of flocculation as the silanization reaction increases and results in a reduction of silica-silica interaction. Nonetheless, the decrease in maximum torque for the DPNR compound at higher dump temperature is much less compared to the NR-silica compound. The low-protein content in DPNR may attribute to better silanization in the compound which gives more filler-to-rubber linkages. Noticeably in the vulcanization curves for these high dump temperatures, there is clear indication of reversion for NR-silica compound. This reversion sign in vulcanization curve has also been reported in NR compound in the presence of TESPT at temperature higher than 150°C where the behavior is totally different as compared to SBR compound [56]. The DPNR-silica compounds show no sign of reversion at high dump temperature as seen in NR compounds. This suggests that a different network structure in the DPNR contributes to the thermal resistance of the cured compound.

A comparison of filler-filler interaction in DPNR-silica-TESPT compounds with NR-silica-TESPT as a function of mixing temperature is shown in **Figure 4**. A common way to measure the filler-filler interaction is calculating the Payne effect from the strain sweep of the dynamic testing or different in the storage modulus of the low or 0.56% and high strain or 100% while maintaining the frequency and temperature at constant value. Both NR- and DPNR-silica-TESPT show the decreasing trend of Payne effect with increasing dump temperature. This trend is due to the increase of silanization reaction with increasing temperature which results in the efficiency of the silane to hydrophobize the surface of the silica. High Payne effect of NR- and DPNR-silica compounds at low dump temperature is in agreement with the sign of flocculation observed in vulcanization curve. The results are also in agreement with other works reported on compound comprising TESPT showed a reduction of flocculation of silica when mixed at higher dump temperature [56, 57].

**Figure 5** illustrates the effect of mixing temperature on Wolff's filler structure parameter, *αf*. Wolff's filler structure parameter, *αf*, is determined from the ratio

**Figure 4.**

*Payne effect of silica compounds with silane TESPT as a function of dump temperature (*□*) NR and (*●*) DPNR.*

**Figure 5.** *Effect of dump temperature on Wolff's filler structure parameter, αf of silica-reinforced compounds: (*□*) NR and (*●*) DPNR.*

**55**

**Figure 6.**

*at varying dump temperature: (*□*) NR and (*●*) DPNR.*

*Silica-Reinforced Deproteinized Natural Rubber DOI: http://dx.doi.org/10.5772/intechopen.85678*

pounds and that of the unfilled gum [24]:

*Do*

nm<sup>2</sup>

*max* − *D<sup>o</sup>*

to polymer by weight.

\_\_\_\_\_\_\_\_\_

type of filler and rubber network in the two compounds.

nia atmosphere which is referred to as the chemically BRC [59, 60].

between the increase in rheometer torque during vulcanization of the filled com-

where *Dmax* − *Dmin* is the maximum change in torque for the filled rubber,

As observed earlier with the Payne effect, *αf* is reduced with increasing dump temperature for the DPNR and NR compounds. Better hydrophobation leads to a decrease in silica-silica interaction and consequently results in reduced *αf*. The DPNR compound shows a higher *αf* than the NR compound, indicating a different

The filler-to-rubber interaction of silica-filled DPNR can be assessed from bound rubber content (BRC). Bound rubber is the layer of rubber polymer that covers the filler surface and unextracted by the solvent during swelling process. The bound rubber content is a combination of tightly bound rubber skin and loosely bound rubber shell [58]. These combination or referred to as total BRC can be measured using the bound rubber measurement in normal atmosphere. The tightly bound rubber skin can be measured from bound rubber measurement in an ammo-

The chemically and physically bound rubber content of silica-filled DPNR as compared to silica-filled NR is illustrated in **Figure 6**. The chemically bound rubber of both silica-filled NR and DPNR increases with increasing dump temperature up to 150°C. The increase of chemically bound rubber as a function of dump temperature for NR-silica-TESPT system has been reported [56, 25]. This can be explained by the higher rate of silanization with increasing mixing temperature. However, at 150°C, there is saturation in the amount of TESPT which has reacted and the surface of silica covered. Precipitated silica has about four to five silanol groups per

. Hence, there is no increase in chemically bound rubber for compounds mixed

till above 150°C dump temperature. Above 150°C, the chemically bound rubber slowly decreases for DPNR and NR. In comparison, DPNR has more chemically

*Comparison of (a) chemically and (b) physically bound rubber content of silica compound containing TESPT* 

*<sup>o</sup>* − 1 = α*<sup>f</sup>*

*min* is torque difference in unfilled rubber, and mf/mp is the ratio of filler

*mf* \_\_\_

*mp* (1)

*D*max − *D*min *D*max *<sup>o</sup>* − *D*min *Silicon Materials*

**Figure 4.**

*DPNR.*

*Payne effect of silica compounds with silane TESPT as a function of dump temperature (*□*) NR and (*●*)* 

*Effect of dump temperature on Wolff's filler structure parameter, αf of silica-reinforced compounds: (*□*) NR* 

**54**

**Figure 5.**

*and (*●*) DPNR.*

between the increase in rheometer torque during vulcanization of the filled compounds and that of the unfilled gum [24]:

$$\frac{D\_{\text{max}} - D\_{\text{min}}}{D\_{\text{max}}^o - D\_{\text{min}}^o} - \mathbf{1} = \mathbf{c}\_f \frac{m\_f}{m\_p} \tag{1}$$

where *Dmax* − *Dmin* is the maximum change in torque for the filled rubber, *Do max* − *D<sup>o</sup> min* is torque difference in unfilled rubber, and mf/mp is the ratio of filler to polymer by weight.

As observed earlier with the Payne effect, *αf* is reduced with increasing dump temperature for the DPNR and NR compounds. Better hydrophobation leads to a decrease in silica-silica interaction and consequently results in reduced *αf*. The DPNR compound shows a higher *αf* than the NR compound, indicating a different type of filler and rubber network in the two compounds.

The filler-to-rubber interaction of silica-filled DPNR can be assessed from bound rubber content (BRC). Bound rubber is the layer of rubber polymer that covers the filler surface and unextracted by the solvent during swelling process. The bound rubber content is a combination of tightly bound rubber skin and loosely bound rubber shell [58]. These combination or referred to as total BRC can be measured using the bound rubber measurement in normal atmosphere. The tightly bound rubber skin can be measured from bound rubber measurement in an ammonia atmosphere which is referred to as the chemically BRC [59, 60].

The chemically and physically bound rubber content of silica-filled DPNR as compared to silica-filled NR is illustrated in **Figure 6**. The chemically bound rubber of both silica-filled NR and DPNR increases with increasing dump temperature up to 150°C. The increase of chemically bound rubber as a function of dump temperature for NR-silica-TESPT system has been reported [56, 25]. This can be explained by the higher rate of silanization with increasing mixing temperature. However, at 150°C, there is saturation in the amount of TESPT which has reacted and the surface of silica covered. Precipitated silica has about four to five silanol groups per nm<sup>2</sup> . Hence, there is no increase in chemically bound rubber for compounds mixed till above 150°C dump temperature. Above 150°C, the chemically bound rubber slowly decreases for DPNR and NR. In comparison, DPNR has more chemically

#### **Figure 6.**

*Comparison of (a) chemically and (b) physically bound rubber content of silica compound containing TESPT at varying dump temperature: (*□*) NR and (*●*) DPNR.*

bound rubber than NR, particularly at high dump temperature. In **Figure 6**(**b**), the small increase in the physically bound rubber of the DPNR-silica and NR-silica compounds containing TESPT at higher dump temperature can be explained by the saturation of silica-TESPT coupling. Additional interactions above 150°C between the non-hydrophobized silica surfaces and rubber are physical of nature.

The effect of dump temperature on the physical properties of silica vulcanizates is depicted in **Figure 7**. At dump temperatures above 150°C, NR vulcanizate shows a clear reduction in tensile strength and is in good agreement with the occurrence of reversion and decrease in the maximum torque observed in the vulcanization curve. This shows that mixing temperature is of importance for the NR-silica compounds. This observation is also seen in the case of synthetic equivalent of NR, IR [61]. However, this reduction in tensile strength at high dump temperature is not seen for the low-protein DPNR-silica vulcanizates. The elongation at break for the NR and DPNR vulcanizates reduces slightly with increasing dump temperature.

A decrease in both moduli at 300% elongation (M300) and 100% elongation (M100) at higher dump temperatures is seen for the NR vulcanizates. In contrast, DPNR vulcanizates exhibit higher moduli with the increasing mixing dump temperature. The reduction in the tensile properties of NR at high mixing temperature is in good agreement with the occurrence of reversion and decrease in the maximum torque observed in their vulcanization curves. In addition, it has been reported that the reinforcement index (M300/M100) and tear resistance of NR-silica-TESPT vulcanizate is improved with increasing dump temperature to an optimum point

**Figure 7.**

*Physical properties of silica vulcanizates: (*□*) NR and (*●*) DPNR; (a) tensile strength, (b) elongation at break, (c) modulus at 100% elongation (M100), (d) modulus at 300% elongation (M300).*

**57**

**Figure 8.**

better retention of the properties.

**4.2 Influence of silane coupling agents**

categorized into the following groups [25]:

*Silica-Reinforced Deproteinized Natural Rubber DOI: http://dx.doi.org/10.5772/intechopen.85678*

of 135–150°C [56]. In contrast, reversion is not seen in the vulcanization curve of DPNR, and the effect of dump temperature on torque difference is also smaller. Another possible reason for the better properties achieved for DPNR is the difference in the naturally occurring networking as compared to NR. The network structure of purified DPNR has been proposed is associated with phospholipids linking both terminal end groups of the rubber chain via hydrogen bonding and ionic linkages,

*The effect of dump temperature on tan delta at 60°C of silica compound: (*□*) NR and (*●*) DPNR.*

Commonly, the loss tan δ at 60°C of a cured compound is employed as indication for the rolling resistance of tires made thereof. The lower the tan δ at 60°C, the lower the rolling resistance expected in real tire performance. **Figure 8** illustrates indications of rolling resistance of the silica-filled vulcanizates. Both NR and DPNR vulcanizates show a strong decrease in tan δ at 60°C with increasing dump temperature. This must obviously be the result of more coupling of silica to the rubber with greater silanization efficiency at high temperatures. The DPNR vulcanizates exhibit the lowest tan δ at 60°C at high dump temperature. This actually relates well with the higher chemically bound rubber content of DPNR compared to the NR compound. In general, DPNR shows better mechanical and dynamic properties as compared to NR. The decrease in the mechanical properties of NR vulcanizates mixed at high dump temperature correlates well with the reversion sign in vulcanization curve and decrease in the maximum torque. Interestingly, the DPNR-silica compounds show no sign of reversion at high dump temperature as seen in NR compounds and

Organofunctional silanes used for sulfur-cured rubber compounds can be

1.Di- and polysulfidic silanes: [(RO3)▬Si▬(CH2)3▬S]2▬Sx

while the protein bonds are released because of deproteinization [43].

*Silica-Reinforced Deproteinized Natural Rubber DOI: http://dx.doi.org/10.5772/intechopen.85678*

*Silicon Materials*

bound rubber than NR, particularly at high dump temperature. In **Figure 6**(**b**), the small increase in the physically bound rubber of the DPNR-silica and NR-silica compounds containing TESPT at higher dump temperature can be explained by the saturation of silica-TESPT coupling. Additional interactions above 150°C between

The effect of dump temperature on the physical properties of silica vulcanizates is depicted in **Figure 7**. At dump temperatures above 150°C, NR vulcanizate shows a clear reduction in tensile strength and is in good agreement with the occurrence of reversion and decrease in the maximum torque observed in the vulcanization curve. This shows that mixing temperature is of importance for the NR-silica compounds. This observation is also seen in the case of synthetic equivalent of NR, IR [61]. However, this reduction in tensile strength at high dump temperature is not seen for the low-protein DPNR-silica vulcanizates. The elongation at break for the NR and DPNR vulcanizates

A decrease in both moduli at 300% elongation (M300) and 100% elongation (M100) at higher dump temperatures is seen for the NR vulcanizates. In contrast, DPNR vulcanizates exhibit higher moduli with the increasing mixing dump temperature. The reduction in the tensile properties of NR at high mixing temperature is in good agreement with the occurrence of reversion and decrease in the maximum torque observed in their vulcanization curves. In addition, it has been reported that the reinforcement index (M300/M100) and tear resistance of NR-silica-TESPT vulcanizate is improved with increasing dump temperature to an optimum point

*Physical properties of silica vulcanizates: (*□*) NR and (*●*) DPNR; (a) tensile strength, (b) elongation at break,* 

*(c) modulus at 100% elongation (M100), (d) modulus at 300% elongation (M300).*

the non-hydrophobized silica surfaces and rubber are physical of nature.

reduces slightly with increasing dump temperature.

**56**

**Figure 7.**

**Figure 8.** *The effect of dump temperature on tan delta at 60°C of silica compound: (*□*) NR and (*●*) DPNR.*

of 135–150°C [56]. In contrast, reversion is not seen in the vulcanization curve of DPNR, and the effect of dump temperature on torque difference is also smaller. Another possible reason for the better properties achieved for DPNR is the difference in the naturally occurring networking as compared to NR. The network structure of purified DPNR has been proposed is associated with phospholipids linking both terminal end groups of the rubber chain via hydrogen bonding and ionic linkages, while the protein bonds are released because of deproteinization [43].

Commonly, the loss tan δ at 60°C of a cured compound is employed as indication for the rolling resistance of tires made thereof. The lower the tan δ at 60°C, the lower the rolling resistance expected in real tire performance. **Figure 8** illustrates indications of rolling resistance of the silica-filled vulcanizates. Both NR and DPNR vulcanizates show a strong decrease in tan δ at 60°C with increasing dump temperature. This must obviously be the result of more coupling of silica to the rubber with greater silanization efficiency at high temperatures. The DPNR vulcanizates exhibit the lowest tan δ at 60°C at high dump temperature. This actually relates well with the higher chemically bound rubber content of DPNR compared to the NR compound.

In general, DPNR shows better mechanical and dynamic properties as compared to NR. The decrease in the mechanical properties of NR vulcanizates mixed at high dump temperature correlates well with the reversion sign in vulcanization curve and decrease in the maximum torque. Interestingly, the DPNR-silica compounds show no sign of reversion at high dump temperature as seen in NR compounds and better retention of the properties.

## **4.2 Influence of silane coupling agents**

Organofunctional silanes used for sulfur-cured rubber compounds can be categorized into the following groups [25]:

1.Di- and polysulfidic silanes: [(RO3)▬Si▬(CH2)3▬S]2▬Sx

2.Mercaptosilanes: [(RO3)▬Si▬(CH2)3▬S]2▬SH

3.Blocked mercaptosilanes: [(RO3)▬Si▬(CH2)3▬S]2▬S▬B

where R = CH3 or C2H5 or other groups, B = CN or C7H15C〓O, and x = 0–8.

The comparison of reinforcing effect of different types of silane in DPNR-silica compound is given in **Table 11**. Silane Si-69 is an organosilane TESPT from Evonik. X50s is a blend of silane Si69 and N330 black in the ratio 1:1 by weight, and it is in solid form. All types of silanes give high reinforcement of silica in DPNR especially TESPT with excellent tensile properties and better abrasion resistance.

The commonly and effectively used silane coupling agent in rubber system is TESPT. The polysulfidic silanes like TESPT contribute additional sulfur to the compound unlike the other type of silanes. The influence of sulfur ranks in polysulfidic silane is compared between silica-DPNR and silica-NR. Bis-triethoxysilylpropyldisulfide (TESPD) has an average sulfur rank of 2.2, while TESPT has an average sulfur rank of 3.83. The compound is prepared based on the earlier formulation (**Table 10**), and 4.4 phr of TESPD is used as the equivalent moles to 5 phr TESPT in


*\*Rubber formulation: rubber 100, Zeosil 1165 55, ZnO 3, StA 2, TMQ 2, TDAE 8, DPG 1.1, CBS 1.5, S 1.5, cured at 150°C for 10 min.*

#### **Table 11.**

*Physical properties of DPNR-silica with different type of silanes used.*

**59**

**Table 12.**

**Figure 10.**

**Compounds TS** 

*1.4, cured at 150°C for 7 min.*

*Comparison of bound rubber content between TESPT and TESPD compounds.*

**EB (%) M100** 

*Comparison of physical properties between DPNR-TESPT and DPNR-TESPD vulcanizates\*.*

DPNR-silica-TESPT 29.7 460 3.5 18.1 63 0.077 NR-silica-TESPT 28.8 520 2.4 14.1 61 0.094 DPNR-silica-TESPD 29.4 520 3.8 14.9 70 0.139 NR-silica-TESPD 29.5 550 2.6 13.3 64 0.145 *\*Rubber formulation: rubber 100, Ultrasil7005 55, silane 5, ZnO 2.5, StA 1, TMQ 2, TDAE 8, DPG 2, CBS 1.7, S* 

**(MPa)**

**M300 (MPa)** **Hardness (shore A)**

**tan δ at 60°C**

**(MPa)**

of silica, ɤ<sup>s</sup>

*Silica-Reinforced Deproteinized Natural Rubber DOI: http://dx.doi.org/10.5772/intechopen.85678*

ethoxy functionality. The TESPD and TESPT were compared on an equimolar basis with correction for the missing sulfur in the final mill mixing stage. The optimal loading of TESPT is at approximately 9.0% wt relative to the amount of silica [62]. In **Figure 9**, the Payne effect between NR and DPNR compounds is compared. DPNR compounds show slightly lower Payne effect compared to NR either with TESPT or TESPD. For both NR and DPNR compounds, the Payne effect of compound with TESPD is higher than those with TESPT. In this case, the reactivity of TESPT toward silica is higher in hydrophobizing silica surface than TESPD in both rubbers. The silanization silica with TESPD occurs at lower rate than TESPT. Similar observation has been reported where the TESPT-based NR compound shows lower filler-filler interaction and better silica dispersion than those of the TESPD compounds [63]. The comparison of BRC between NR- and DPNR-silica compounds both with and without silanes is shown in **Figure 10**. The use of silanes TESPT and TESPD in NR- and DPNR-silica compounds results in almost totally chemically BRC formation. The silanization of silica with TESPD or TESPT has successfully hydrophobize silica surface through silica-TESPT and silica-TESPD couplings. The silica-silane coupling is translated into reduction of the specific component of the surface energy

sp, and gives rise in the interaction of rubber and filler. DPNR-TESPT shows slightly higher chemically BRC than NR-TESPT-silica compound. In comparison, both DPNR and NR compounds with TESPT have higher BRC than compound with TESPD. This corresponds well with the lower Payne effect of the DPNR-silica compounds with TESPT as compared to with TESPD. For both DPNR- and NR-silica compounds without silane, only physically BRC, are formed as no chemically BRC is obtained after ammonia treatment. It is interesting to see that NR-silica without silane has higher physically BRC compared to DPNR-silica without silane. This relates back to the interaction of proteins with silica that provide filler-rubber

**Figure 9.**

*Comparison of Payne effect between TESPT and TESPD compound.*

## *Silica-Reinforced Deproteinized Natural Rubber DOI: http://dx.doi.org/10.5772/intechopen.85678*

*Silicon Materials*

2.Mercaptosilanes: [(RO3)▬Si▬(CH2)3▬S]2▬SH

3.Blocked mercaptosilanes: [(RO3)▬Si▬(CH2)3▬S]2▬S▬B

TESPT with excellent tensile properties and better abrasion resistance.

where R = CH3 or C2H5 or other groups, B = CN or C7H15C〓O, and x = 0–8. The comparison of reinforcing effect of different types of silane in DPNR-silica compound is given in **Table 11**. Silane Si-69 is an organosilane TESPT from Evonik. X50s is a blend of silane Si69 and N330 black in the ratio 1:1 by weight, and it is in solid form. All types of silanes give high reinforcement of silica in DPNR especially

The commonly and effectively used silane coupling agent in rubber system is TESPT. The polysulfidic silanes like TESPT contribute additional sulfur to the compound unlike the other type of silanes. The influence of sulfur ranks in polysulfidic silane is compared between silica-DPNR and silica-NR. Bis-triethoxysilylpropyldisulfide (TESPD) has an average sulfur rank of 2.2, while TESPT has an average sulfur rank of 3.83. The compound is prepared based on the earlier formulation (**Table 10**), and 4.4 phr of TESPD is used as the equivalent moles to 5 phr TESPT in

**Properties Polysulfidic silane Mercaptosilane Block** 

Silane Si-69 X50S Si 363 NXT phr 5 10 6.2 5 Tensile strength, MPa 31 31 28 30 Elongation at break, % 560 550 540 600 M100, MPa 2.8 2.9 2.9 2.3 M300, MPa 13.3 14.2 13.1 10.2 Hardness, IRHD 67 69 65 65 Resilience, % 67 68 74 64 Abrasion resistance index (ARI), % 120 123 121 108 *\*Rubber formulation: rubber 100, Zeosil 1165 55, ZnO 3, StA 2, TMQ 2, TDAE 8, DPG 1.1, CBS 1.5, S 1.5, cured at* 

**mercaptosilane**

**58**

**Figure 9.**

*150°C for 10 min.*

*Physical properties of DPNR-silica with different type of silanes used.*

*Comparison of Payne effect between TESPT and TESPD compound.*

**Table 11.**

ethoxy functionality. The TESPD and TESPT were compared on an equimolar basis with correction for the missing sulfur in the final mill mixing stage. The optimal loading of TESPT is at approximately 9.0% wt relative to the amount of silica [62]. In **Figure 9**, the Payne effect between NR and DPNR compounds is compared. DPNR compounds show slightly lower Payne effect compared to NR either with TESPT or TESPD. For both NR and DPNR compounds, the Payne effect of compound with TESPD is higher than those with TESPT. In this case, the reactivity of TESPT toward silica is higher in hydrophobizing silica surface than TESPD in both rubbers. The silanization silica with TESPD occurs at lower rate than TESPT. Similar observation has been reported where the TESPT-based NR compound shows lower filler-filler interaction and better silica dispersion than those of the TESPD compounds [63].

The comparison of BRC between NR- and DPNR-silica compounds both with and without silanes is shown in **Figure 10**. The use of silanes TESPT and TESPD in NR- and DPNR-silica compounds results in almost totally chemically BRC formation. The silanization of silica with TESPD or TESPT has successfully hydrophobize silica surface through silica-TESPT and silica-TESPD couplings. The silica-silane coupling is translated into reduction of the specific component of the surface energy of silica, ɤ<sup>s</sup> sp, and gives rise in the interaction of rubber and filler. DPNR-TESPT shows slightly higher chemically BRC than NR-TESPT-silica compound. In comparison, both DPNR and NR compounds with TESPT have higher BRC than compound with TESPD. This corresponds well with the lower Payne effect of the DPNR-silica compounds with TESPT as compared to with TESPD. For both DPNR- and NR-silica compounds without silane, only physically BRC, are formed as no chemically BRC is obtained after ammonia treatment. It is interesting to see that NR-silica without silane has higher physically BRC compared to DPNR-silica without silane. This relates back to the interaction of proteins with silica that provide filler-rubber

#### **Figure 10.** *Comparison of bound rubber content between TESPT and TESPD compounds.*


#### **Table 12.**

*Comparison of physical properties between DPNR-TESPT and DPNR-TESPD vulcanizates\*.*

network in the NR-silica [64]. With reduced protein present in the DPNR, less network are formed in comparison to NR and this is reflected with lower BRC.

The comparison of physical properties between DPNR-silica-TESPT and DPNR-silica-TESPD vulcanizates is shown in **Table 12**. The tensile strength for all vulcanizates is not affected by the type of silane. The tensile properties of DPNR vulcanizate are comparable to NR vulcanizates. The indication of rolling resistance of the tread compounds can be observed from tan δ at 60°C from temperature sweep using dynamic mechanical testing. It is obvious that the use of TESPT gives lower tan δ at 60°C for DPNR-silica and NR-silica compounds as when compared to those using TESPD. A study on the role of different functionalities in silane on silica-filled NR has shown that the TESPT gives superior efficiency than TESPD due to the effect of sulfur donation by TESPT [65]. In addition, the DPNR vulcanizates generally give lower tan δ at 60°C than the NR vulcanizates which indicate an improvement in rolling resistance of tread compound made from DPNR.
