**4.3 Morphology of silica-reinforced DPNR**

The dispersion morphology of silica-DPNR vulcanizates as compared to silica-NR vulcanizates by AFM morphology in the absence of silane coupling agent is illustrated in **Figure 11** [58]. The size of the silica aggregates in DPNR without silane is bigger than in the NR vulcanizate as seen from the height image at 1 × 1 μm. The phase image of DPNR-silica without silane shows smaller silica aggregates of 100nm size as dispersed in the matrix. The size of the silica aggregates in the DPNR vulcanizate is almost comparable to that in NR, although they seem to be closer together.

An improved micro-dispersion of silica in DPNR and NR vulcanizates with the use of TESPT is shown in AFM height images in **Figure 12** as compared to morphology without coupling agent. Primary particles of silica in the size of 50 nm are clearly visible in both DPNR and NR in addition to silica aggregates of approximately 100nm. The difference between DPNR and NR can be observed from the phase image. The distances between the silica aggregates of size 50–100 nm are

#### **Figure 11.**

*Micro-dispersion of silica in NR and DPNR vulcanizates in the absence of silane coupling agent (a) NR-silica (b) DPNR-silica.*

**61**

**Figure 13.**

*coupling agent.*

**Figure 12.**

*(b) DPNR-silica.*

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

visualization using TEM [68].

clearly visible in the NR-silica-TESPT vulcanizate. However, in DPNR-silica-TESPT, the distance between the aggregates looks a little smaller, indicating a somewhat better micro-dispersion. Besides, there is an intermediate region between the silica

Attempting to analyze the morphology of filler-to-rubber interactions in silica compounds at high loading, which in this study is 55 phr of silica, is difficult as the silica aggregates are very close together. In order to gain insight into the filler-torubber interaction, TEM network visualization was carried out where the vulcanizate was swollen in styrene, styrene polymerized, staining the rubber network and

TEM network visualizations of silica-filled NR and silica-filled DPNR vulcanizates without silane coupling agent are depicted in **Figure 13**. Silica aggregates of around 50–100 nm size can be seen as dark particles throughout the DPNR and NR vulcanizates. The rubber network can be visualized after the staining process, and the region is identified with mesh structure. Likewise, the region of

*Micro-dispersion of silica in NR and DPNR vulcanizates in the presence of silane coupling agent (a) NR-silica*

*Comparison of TEM network visualization micrographs of silica-filled NR and DPNR vulcanizates without* 

and rubber phases, suggesting the bound rubber layer [65–67].

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

*Silicon Materials*

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.

The dispersion morphology of silica-DPNR vulcanizates as compared to silica-NR vulcanizates by AFM morphology in the absence of silane coupling agent is illustrated in **Figure 11** [58]. The size of the silica aggregates in DPNR without silane is bigger than in the NR vulcanizate as seen from the height image at 1 × 1 μm. The phase image of DPNR-silica without silane shows smaller silica aggregates of 100nm size as dispersed in the matrix. The size of the silica aggregates in the DPNR vulcanizate is almost comparable to that in NR, although they seem to be closer together. An improved micro-dispersion of silica in DPNR and NR vulcanizates with the use of TESPT is shown in AFM height images in **Figure 12** as compared to morphology without coupling agent. Primary particles of silica in the size of 50 nm are clearly visible in both DPNR and NR in addition to silica aggregates of approximately 100nm. The difference between DPNR and NR can be observed from the phase image. The distances between the silica aggregates of size 50–100 nm are

*Micro-dispersion of silica in NR and DPNR vulcanizates in the absence of silane coupling agent (a) NR-silica*

**4.3 Morphology of silica-reinforced DPNR**

**60**

**Figure 11.**

*(b) DPNR-silica.*

clearly visible in the NR-silica-TESPT vulcanizate. However, in DPNR-silica-TESPT, the distance between the aggregates looks a little smaller, indicating a somewhat better micro-dispersion. Besides, there is an intermediate region between the silica and rubber phases, suggesting the bound rubber layer [65–67].

Attempting to analyze the morphology of filler-to-rubber interactions in silica compounds at high loading, which in this study is 55 phr of silica, is difficult as the silica aggregates are very close together. In order to gain insight into the filler-torubber interaction, TEM network visualization was carried out where the vulcanizate was swollen in styrene, styrene polymerized, staining the rubber network and visualization using TEM [68].

TEM network visualizations of silica-filled NR and silica-filled DPNR vulcanizates without silane coupling agent are depicted in **Figure 13**. Silica aggregates of around 50–100 nm size can be seen as dark particles throughout the DPNR and NR vulcanizates. The rubber network can be visualized after the staining process, and the region is identified with mesh structure. Likewise, the region of

#### **Figure 12.**

*Micro-dispersion of silica in NR and DPNR vulcanizates in the presence of silane coupling agent (a) NR-silica (b) DPNR-silica.*

#### **Figure 13.**

*Comparison of TEM network visualization micrographs of silica-filled NR and DPNR vulcanizates without coupling agent.*

**Figure 14.**

*Comparison of TEM network visualization micrographs of silica-filled (a) NR and (b) DPNR vulcanizates with silane coupling agent, TESPT.*

polystyrene is the unstained part. Some silica aggregates in silica-filled NR are surrounded by voids or vacuoles, and some have connecting network strands to the NR network. This indicates there exist some bondings of silica to the rubber networks. This observation is totally different with the network visualization of the silica-filled DPNR. The silica aggregates in the DPNR system without silane have clear vacuoles surrounding them [62]. The vacuoles are formed through polymerization of styrene in the gap between the silica aggregates and the rubber chain. The formation of such vacuoles is due to a weak interface between silica particles and rubber chains [66]. The weak filler and rubber interaction in the system without silane is derived from the silica characteristic of surface energy with low dispersive component, ɤsd, which results in less adsorption of rubber chains to the surface of the silica. There are less vacuoles present in the NR vulcanizate as compared to the DPNR vulcanizate without silane, which suggests higher fillerto-rubber interactions in the former.

A comparison of the TEM network visualization between NR and DPNR vulcanizates with TESPT coupling agent included is shown in **Figure 14**. The presence of TESPT results in strong attachment of rubber chain to the surface of silica aggregates. No sign of vacuoles present in the system after network visualization. This is due to the establishment of chemical bonding between silica-TESPT-rubber in the compound during silanization and vulcanization. In addition, the size of silica aggregates in both NR and DPNR with TESPT is smaller than those without TESPT. This relates to the results of low Payne effect and high chemically BRC for NR and DPNR reinforced with silica and silane system. In addition, the vulcanizates with silanes exhibit denser rubber network compared to those without silane. This is in agreement with the results of cross-link density where the silica-filled NR and DPNR with TESPT have higher cross-link density compared to those without silane due to sulfur released from TESPT [62].
