Dependence of mechanical and aging properties of chloroprene rubber on silica and ethylene thiourea loadings

The interaction between precipitated silica and chloroprene rubber (CR) was investigated using a nuclear magnetic resonance (NMR) technique. The results reveal that the silanol groups on silica surface could chemically react with CR. Crosslinking of CR is therefore possible in the presence of silica at high temperature. The effects of silica and ethylene thiourea (ETU) loadings on properties of the silica-filled CR were thereafter investigated. With increasing silica loading, the compound viscosity increases considerably due to the dilution effect. As silica could act as a curative for CR, increasing silica loading results in both faster cure rate and increased crosslink density. The optimum tensile strength is found at approximately 30 phr of silica loading. The results also show that silica loading has little effect on most aging properties, except the relative modulus in which it increases rapidly with increasing silica loading due to the post curing effect. Similar to the effect of silica loading, the compound viscosity, cure rate and crosslink density are all increased with increasing ETU loading. The tensile strength is, on the other hand, slightly affected by ETU loading. Exception is found at high loading where the tensile strength drops noticeably. Interestingly, aging resistance of the vulcanizate is found to improve with the addition of ETU. Explanation is given by the hindrance capability of ETU to post curing.     

Anisotropic studies of multi-wall carbon nanotube (MWCNT)-filled natural rubber (NR) and nitrile rubber (NBR) blends

50/50 NR/NBR blends with various MWCNT loadings were prepared by mixing with MWCNT/NR masterbatches on a two-roll mill and sheeted off at the smallest nip gap. Then, the effect of milling direction, machine direction (MD) and transverse direction (TD), on the mechanical and electrical properties of the blends was elucidated. Dichroic ratio and SEM results confirmed that most of the MWCNTs were aligned along MD when MWCNT was less than 4 phr, and the number of agglomerates increased when MWCNT was more than 4 phr. Additionally, anisotropic properties were clearly observed when 4 phr MWCNT was loaded. At 4 phr MWCNT, 100% modulus and tensile strength in the MD were about 1.5 and 1.3 times higher than those in the TD, respectively. Moreover, electrical conductivity in the MD was superior to that in the TD by about 3 orders of magnitude. Results from dynamic mechanical tests also showed that the maximum tan δ in the MD sample was lower than that in the corresponding TD sample. In addition, the storage modulus at 30 °C for the MD sample containing 4 phr MWCNT was 1.15 higher than that of the corresponding TD sample. This stronger reinforcement efficiency resulted from the combination of the greater alignment and dispersion of most MWCNTs in the MD sample.     

Properties and recyclability of thermoplastic elastomer prepared from natural rubber powder (NRP) and high density polyethylene (HDPE)

Preparation of thermoplastic natural rubber (TPNR) was carried out by blending high density polyethylene (HDPE) with natural rubber powder (NRP) obtained from spray drying of pre-vulcanized natural rubber latex. The blend ratio of NRP/HDPE was varied and the properties and recyclability of the TPNRs were investigated. The results reveal that, due to flow restriction of the crosslinked NRP, viscosity of the TPNR increases continuously with increasing NRP content. As expected, when NRP content is increased, properties of the TPNR are more rubber-like, as clearly observed from the stress–strain curve characteristics, the reductions in modulus and hardness and, most importantly, the significant improvement of permanent set. Although phase size of the dispersed NRP is relatively large, the tensile strength tends to increase gradually with increasing NRP content. The phenomenon of strain-induced crystallization is proposed to explain the results. It is also found that recycling has a noticeably detrimental influence on most properties of the TPNRs such as tensile strength, tear strength, hardness and tension set. This is thought to be a consequence of thermal degradation of the NRP during the recycling process.     

Influence of accelerator type on properties of NR/EPDM blends

The effect of accelerator type on processability and mechanical properties of 60/40 natural rubber/ethylene propylene diene monomer (NR/EPDM) blend was investigated. Three groups of commercial accelerators were selected, i.e., sulfenamide group (Santocure-TBBS), thiuram group (Perkacit-TMTD) and mercapto group (Perkacit-MBT and Perkacit-MBTS). The results reveal that the accelerator type not only affects the cure characteristics, but also has great influence on compound viscosity. Among the accelerators studied, TBBS gives the best processing safety together with a relatively high state of cure. In addition, TBBS also provides good cure compatibility between the NR and EPDM phases, giving rise to superior mechanical properties. Although TMTD could give a high state of cure, it causes severe cure incompatibility, leading to poor tensile strength. Due to their lower reactivity, the two mercapto accelerators give a relatively low state of cure. Therefore, the vulcanizates obtained possess low modulus and hardness as well as degree of elasticity. Surprisingly, it is found that the tensile strength of the MBTS-cured blend is relatively high. Good cure compatibility given by MBTS could be used to explain the results.     

Comparison of reinforcing efficiency between Si-69 and Si-264 in a conventional vulcanization system

Silica has long been recognized as a reinforcing filler, especially for light colored products. The degree of reinforcement is noticeably increased when silica is used in combination with silane coupling agent. Therefore, various types of silane coupling agents are now commercially available. In the present study, two types of silane coupling agents, e.g., bis-(3-triethoxysilylpropyl) tetrasulfane (Si-69) and 3-thiocyanatopropyl triethoxy silane (Si-264) were selected for comparison of their reinforcing efficiency in a conventional vulcanization (CV) system. The results reveal that the addition of silane coupling agent not only improves compound processability, but also enhances the mechanical properties of the rubber vulcanizates. Compared with Si-69, Si-264 gives rubber compounds with better processability due to its greater ability to promote filler dis-agglomeration during mixing. In addition, Si-264 also imparts a greater degree of reinforcement. This might be attributed to the combined effects of better rubber–filler interaction, better filler dispersion and higher state of cure which are obtained when Si-69 is replaced with Si-264. The dynamic properties of the rubber vulcanizates are also improved with the presence of silane coupling agent. In this aspect, Si-69 performs better than Si-264 as it provides rubber vulcanizates with lower heat build-up.     

Bio-based epoxidized natural rubber/chitosan/cellulose nanocrystal composites for enhancing mechanical properties,self-healing behavior and triboelectric nanogenerator performance

Cellulose - A new biocomposite was prepared from epoxidized natural rubber (ENR) reinforced with chitosan (CHI) and cellulose nanocrystals (CNCs) without using conventional crosslinking agents. The...     

Roles of silane coupling agents on properties of silica-filled polychloroprene

Silane coupling agent is used extensively to improve reinforcing efficiency of silica. Recently, many types of silane coupling agents have been developed and their roles on reinforcing improvement have been studied in many types of rubbers. In the present study, roles of the two widely used silane coupling agents, i.e., bis-(3-triethoxysilylpropyl) tetrasulfane (Si-69) and 3-thiocyanatopropyl triethoxy silane (Si-264) were studied in silica-filled polychloroprene (CR). The results reveal that the presence of Si-69 and Si-264 improves significantly the processability of the rubber compounds. The improvement is more pronounced for Si-264 due to its lower molecular weight and, thus, viscosity. Surprisingly, the results show that Si-69 and Si-264 affect cure characteristics differently, i.e., Si-69 somewhat retards cure while Si-264 accelerates cure. However, both Si-69 and Si-264 result in an increased crosslink density of the vulcanizates. The presence of silane coupling agent also enhances the mechanical properties of the vulcanizates due to the combined effects of better filler dispersion, better rubber-filler interaction and increased crosslink density. A thorough look at the results also reveals that the property enhancement is obvious only at low silane loading (approximately 1.5 phr). Further increase of silane loading generally has little influence on properties of the vulcanizates. In the case of Si-264, excessive use of silane could impair certain properties such as modulus and hardness due to the plasticizing effect. Compared with Si-264, Si-69 imparts the rubber vulcanizates with poorer aging resistance. Explanation goes to the sulfur contribution effect of Si-69.