Rubber–silica nanocomposites obtained by in situ sol–gel method: particle shape influence on the filler–filler and filler–rubber interactions

Abstract

Silica-natural rubber composites were prepared by in situ sol-gel synthesis of silica nanoparticles functionalized with alkylthiol or alkylpolysulfide. The functionalizing groups were linked to silica particles by hydrolysis and polycondensation of a mixture of tetraethoxysilane (TEOS) with a suitable amount of (3-mercaptopropyl) trimethoxysilane (TMSPM), bis (3-triethoxysilylpropyl) disulfide (TESPD) or bis (3-triethoxysilylpropyl) tetrasulfide (TESPT). Particles from TEOS are spherical, instead those from TESPD, TESPT and TMSPM have irregular anisotropic shapes. This is due to the presence of isotropic or anisotropic interactions among the particle base units. Silica particles synthesized in the presence of TMSPM can also undergo condensation of the alkylthiol chains with the silanol groups, thus giving rise to a strong preferential direction for the anisotropic shape. Predominant filler-filler interactions and easy self-assembly were detected in particles from TEOS while those from TESPD and TESPT showed high filler-rubber interactions. Both filler-rubber and strong filler-filler interactions are present in silica particles synthesized by TMSPM. The dynamic mechanical properties of the composites, tested with stress-strain measurements, show that the storage modulus increases by increasing the filler-filler interaction, and it is maximum when also the filler-rubber interaction occurs. Strong silica-rubber interaction favors the silica dispersion in rubber, while it makes the filler network less compact and lowers the storage modulus.

Compatibilization of natural rubber/nitrile rubber blends by sol–gel nano-silica generated by in situ method

“…It is evident that r 50% , r 100% and r 200% of the composites become higher upon silica incorporation as expected. Furthermore, there is consistent and steady rise of the moduli as silica content increases in the composites (2)(3)(4)(5). This is in accordance with decreased equilibrium swelling ratio values in the series (Table 3).…”

“…One interesting point is noted here regarding peak height reduction [36,37]. The tan d peak height decreases consistently in NR phase as silica content increases for the filled composites (2)(3)(4)(5), while that in NBR phase shows inconsistent variation within small range. Thus, for these composites, in situ silica reinforces the former phase of the blend more strongly.…”

“…It is well established in literature that silane can link with the silica via its hydroxyl group that is resulted upon hydrolysis of ethoxy group present at one end. On the other hand, the sulfur bridges present at the other side can be broken and connect to rubber chains during vulcanization (both NR and NBR) [4,[36][37][38][39][40][41]. In addition to this, processing conditions might also influence the silica distribution.…”

“…Then, all the crosslinking ingredients (ZnO, stearic acid, MBTS and sulfur) were added stepwise and mixed for 10 min to obtain homogenous rubber mixture. This process was followed for all the in situ silica-filled composites (2)(3)(4)(5)(6)(7). Unfilled sample (1) was prepared by dissolving 12 g of NR and 18 g of NBR in toluene and set to stirring for homogenous mixing.…”

“…There are many reports on in situ silicafilled elastomer composites where mostly single rubber system such as natural rubber (NR) [1][2][3][4][5][6][7], styrene butadiene rubber (SBR) [8][9][10][11], butadiene rubber (BR) [12], chloroprene rubber (CR) [13,14] and nitrile rubber (NBR) [15][16][17][18][19][20][21] is focused. Studies on the rubber blends, filled with in situ silica, are rather scanty in literature.…”