3.3 Filler–rubber interaction
The chemically bound rubber contents of the silica-filled NR significantly increase while the physically bound rubber contents slightly decrease by increasing the ENR contents and mole% of epoxide on ENR, as shown in Fig. 6. When only NR is used, the addition of TESPT coupling agent leads to a remarkably high chemically bound rubber content, compared to the ones without. The addition of ENR into the NR compounds without TESPT clearly improves the bound rubber content, i.e. filler–rubber interaction. These results prove that ENR can generate chemical interaction and/or reaction in silica-filled NR, according to the mechanism as proposed in Scheme 1. The use of ENR-51 in the range of 7.5–15 phr results in an optimum chemically bound rubber content. This is in accordance with the Payne effect: Fig. 3(a) and flocculation rate constant results: Fig. 3(b), attributed to the compatibilization effect of ENR. For the NR compound without neither ENR nor TESPT, the predominance of chemically bound rubber is seen in Fig. 6. This can be mainly attributed to the nature of NR itself which has the terminal groups associated with proteins and phospholipids [40] that can possibly interact with the silanol groups of the silica surface via hydrogen bonding. The interactions between silica and proteins present in NR has been clearly demonstrated recently [41].