Natural rubber nanocomposites based on hybrid fillers of carbon nanotubes and graphene

Abstract

The influence of graphene (GP)-filled rubber nanocomposites was investigated as a function of rubber type: unmodified natural rubber (NR) and epoxidized natural rubber with 25 mol% epoxides (ENR-25) with various GP loadings at 1, 3, 5, and 7 phr. The results showed that ENR/GP filled with 5 phr had superior properties compared to NR/GP, including higher storage modulus, initial relaxation modulus, bound rubber, mechanical properties, degree of reinforcement, as well as electrical properties. This might be due to the polar epoxide rings in ENR molecules reacting with the hydroxyl and carbonyl groups on the GP surfaces, forming permanent linkages among them. Therefore, GP 5 phr was selected to prepare the ENR-GP/CNTs hybrid nanocomposite. Hereafter, the ENR nanocomposite filled with GP 5 phr was filled with varying loadings of carbon nanotubes (CNTs) at 1, 3, 5, and 7 phr. It was found that the incorporation of CNTs could develop new hybrid rubber nanocomposites. That is, CNTs enhanced dispersion of GP within the ENR-25 matrix, attributed to ?-? interactions and Van der Waals forces between GP and CNTs, resulting in higher crosslink density and bound rubber content, suggesting the formation of a three-dimensional filler network with strong interactions. For this reason, mechanical, thermo-mechanical, and dynamic mechanical properties were improved, including increased modulus, hardness, storage modulus, and glass transition temperature (Tg), along with a decreased coefficient of reinforcement (C-factor). This indicates effective reinforcement of CNT-GP in the ENR matrix. Also, the electrical conductivity with a low percolation threshold at 2.34 phr of CNTs transforms the composites into conductive materials. Additionally, the dicarboxylic acid (DA) as a crosslinker for unconventional GP-filled ENR nanocomposite activated by 1,2-dimethylimidazole (DMI) was studied. The results indicated that the DMI activation improved the curing rate faster than without DMI. However, the incorporation of GP exhibited reduced scorch and cure times. It also showed enhanced mechanical properties and improved relaxation moduli, indicating higher thermo-mechanical properties. These improvements might be attributed to increased crosslink density, owing to the formation of a three-dimensional filler network with strong ENR/GP interactions throughout, facilitated by ionic interaction between DA and DMI. Furthermore, significant changes were observed in the electrical properties, with a low percolation threshold at 1.25 phr of GP. In addition, GP-filled ENR nanocomposites cured with DA and sulfur were compared. It was found that the cure characteristics showed evident differences between DA and sulfur, with sulfur-cured ENR-25 exhibiting a reversion curve due to the breakdown of polysulfidic bonds. Furthermore, DA-cured samples revealed a higher storage modulus at low strain, indicating better filler-filler and rubber-filler interactions corresponding to bound rubber content. They also showed the highest electrical conductivity due to the higher polarity of the DA and the conductive paths in the ionic state of the imidazolium system within the ENR molecular network.

??????????????????????????????????????? (GP) ?????????????????????????????????????????? ???????????????????????? ?????? ??????????????????????????? (NR) ??????????????????????????????????????????? 25 ?????????????? (ENR-25) ?????????????????????? GP ??? 1, 3, 5 ??? 7 phr ???????????????????????????????????????????????????????????????????????? GP ?????? 5 phr (ENR/GP5) ??????????????????????????????????????????????????????? GP (NR/GP) ???? ?????????????? ??????????????????????????? ?????????????????????????????????? ??????????? ???????????????? ?????????????????????????????? ??????????????????????????????????????????????? ENR ????????????????????????????????????????????????? GP ?????????????????????????????? ????????????????????? GP ??? 5 phr ???????????????????????????????????????????????????????????????????????? (hybrid fillers) ??????? GP ????????????????? (carbon nanotubes, CNTs) ???????????? CNTs ??? 1, 3, 5 ??? 7 phr ??????????? CNTs ???????????????????????????????????????????????????????????????? ??????? CNTs ???????????????????????? GP ???????????????? ENR ??????????????????????????? ?-? ?????????????????????????? GP ??? CNTs ??????????????????????????????????????????????????????????????????????????????? ???????????????????????????????????????????????????????????????????????? ?????????????????????? ????????????????? ????????????????? ????????????????????????????? ?????????????????????????????????????????? (Tg) ???????????? ????????????????????????????????????? (C-factor) ??????? ???????????????????????????????????????????????????????? (CNT/GP) ????????????? ENR ?????????????????????????????????????? (Percolation threshold concentration) ??? CNTs ????????? 2.34 phr ????????????????????????????????????????????????????????? ??????????????????????????????????????? ????????????????? (DA) ????????????????????????? ENR ???????????? GP ????????????????????????????? 1,2-???????????????? (DMI) ???????????????????????????????? DMI ?????????????????????????????????????????? DMI ????????????????????????????? GP ??????????????????????????? ???????????????????????????? (Scorch time) ??????????????? (Cure time) ??? ???? ????????????????????????? ??????????????????????????????? ???????????????????????????????????????????????????? ??????????????????????????????????????????????????? ENR ??? GP ????? ??????????????????????????????????????? DA ??? DMI ????????????????????????????????????????????????????????????? ????????????????????????????????? GP ????????? 1.25 phr ???????????????????????????????????????????????????? ENR ??????? GP ??????????????? DA ?????????? ????????????????????????????????????????????????????? DA ?????????? ?????????????????????? ??????????????????????????????????????????????????????????????? (reversion) ?????????????????????????????????????? ???????????????????????????? ENR ??????????????? DA ??????? Payne effect ?????????????????????????? ?????????????????????????????????????????????????????? ??????????????????? ???????????????????????????? ENR ??????????????? DA ???????????????????????????????????????? DA ????????????????????????????????????????????????????????????????? DMI ??? DA ???????????????????????? ENR