Effects of cellulose nanocrystals on the vulcanization of natural rubber/cellulose nanocrystals nanocomposite and corresponding regulating strategies

Vulcanization is a vital process in rubber processing, it endows rubber with valuable physical and mechanical properties, making rubber a widely used engineering material. In addition to vulcanization agent, reinforcing fillers play a non-ignorable influence on the vulcanization of rubber nanocomposites. Herein, the effects of cellulose nanocrystals (CNCs) on the vulcanization of natural rubber (NR)/CNCs nanocomposite was studied. It was found that even though the addition of CNCs can effectively improve the dispersion of ZnO in NR matrix, the vulcanization of NR was inhibited. This may be attributed to the CNCs' adsorption of vulcanizing agents (DM, ZnO) and the acidic chemical environment on the surface of CNCs. In order to improve the vulcanization properties of NR/CNCs nanocomposite, tetramethyldithiochloram (TMTD) and triethanolamine (TEOA) were used as a combination accelerator and curing activator, respectively, and polyethylene glycol (PEG) was introduced to screen hydroxyl groups on the surface of CNCs to prohibit the CNCs' adsorption of vulcanizing agents. The results indicate that TMTD and TEOA effectively improved the vulcanization rate of NR/CNCs nanocomposite and increased the crosslink density by an order of magnitude. Subsequently, the tensile strength, tear strength, and so forth. of NR/CNCs nanocomposite were significantly improved. However, PEG hardly help to improve the vulcanization properties of NR/CNCs nanocomposite. In addition, the control samples without CNCs were prepared and characterized, the comparation between NR and NR/CNCs nanocomposite shows that the synergistic effect of crosslink density and CNCs' reinforcement more effectively improve mechanical properties of NR. This work not only elucidates the inhibiting mechanisms of CNCs on the vulcanization of NR, but also provides practical strategies for improving the vulcanization and properties of NR/CNCs nanocomposite. It may accelerate the application of CNCs as rubber reinforcing filler.     

水溶性高分子弱凝胶体系凝胶化过程的Monte Carlo模拟

为研究弱凝胶的形成过程,并把高分子弱凝胶用于三次采油,采用三维Monte Carlo模拟了高分子溶液凝胶化过程. 模拟预测了凝胶化开始的时间,得到了凝胶化过程中分子量分布的演化规律和胶团生长的三维图像. 发现生成溶胶与凝胶团的歧化过程,初始聚合物的浓度对能否形成凝胶至关重要,低于临界浓度不能形成凝胶. 模拟了凝胶化速度和聚合物浓度以及交联剂浓度的关系,并与粘度随凝胶化时间变化的实验结果进行比较, 结果表明, 聚合物浓度较高时,浓度对交联反应的影响减弱,这一趋势与实验结果相一致.     

Influence of gel content on the physical properties of unfilled and carbon black filled natural rubber vulcanizates

Recently, gel content has been considered as a standard property for evaluating commercial grade natural rubber (NR). In this study, NR containing various amounts of gel was prepared by accelerated storage hardening as a model to clarify the influence of gel content on the physical properties of both unfilled and carbon black filled vulcanizates. Furthermore, the NR samples were investigated to determine the effect of gel fraction on Mooney viscosity and the structure of the gel after mastication. The results revealed that Mooney viscosity was related to the percentage of gel fraction that has been proven to be the result of interactions between proteins and phospholipids at chain ends. After mastication, although the gel fraction of NR can be decomposed to ∼0% w/w, the interactions of proteins and phospholipids at the chain ends still existed, corresponding to the gel content of the raw rubber. In the case of unfilled vulcanizates, the gel content showed no effect on cure characteristics, crosslink density and ultimate tensile strength, whereas the upturn of stress occurred at a smaller strain when the gel content increased. However, in the case of carbon black filled vulcanizates, the gel content played a dominant role in the carbon black dispersion, which was poorer when gel content increased, contributing to a decrease of crosslink density and ultimate tensile strength.     

Influence concentration of modifying agent on properties of natural rubber/organoclay nanocomposites

Sodium-montmorillonite (Na-MMT) nanoclay was modified with different concentrations of octadecylamine organic modifying agent at 0.5, 1.0 and 1.5 times the CEC of Na-MMT. Influence of concentration of modifying agent on properties of the organoclays and natural rubber/organoclay nanocomposites was investigated. It was found that the optimum concentration of modifying agent was 1.5 times the CEC of Na-MMT. That is, at this concentration, larger d-spacing of organoclay particles and higher degree of clay dispersion in natural rubber matrix were observed. Larger interlayer d-spacing also caused enhancement of the mechanical properties of the NR/organoclay nanocomposites. Additionally, the NR/organoclay nanocomposites with higher concentration of modifying agent exhibited faster curing reaction with higher crosslink density. Furthermore, the organoclays with larger d-spacing and higher degree of dispersion in the natural rubber matrix exhibited enhancement of the mechanical and dynamic properties and thermal stability of natural rubber/organoclay nanocomposites.     

Thermal analysis and mechanical methods applied to studying properties of SBR compounds containing ionic liquids

The goal of this work was to study new systems for activating sulphur vulcanisation of butadiene-styrene elastomer (SBR) using thermal analysis techniques complementarily with rheological and mechanical properties measurements. New activating systems consist of nanosized zinc oxide and ionic liquids (ILs) – benzalkonium or alkylammonium salts. In this article, we discuss the effect of zinc oxide and ILs on the cure characteristics, vulcanisation kinetics, crosslink density, mechanical properties, damping behaviour and thermal stability as well as resistance to thermooxidative and UV ageing of SBR.Nanosized zinc oxide with ILs reduced the vulcanisation time and temperature of rubber compounds. Their application results in increased vulcanisate crosslink density as well as the improvement of mechanical properties such as tensile strength and vibration damping ability. The influence of ionic liquids on SBR properties depends mainly on the anion present in the molecule. The most active ILs seem to be those with the lactate anion.     

Cure and dynamic mechanical properties in peroxide-cured isoprene rubber: effects of stearic acid and amine-based antioxidant

Peroxide-cured isoprene rubber with various amounts of stearic acid and N-isopropyl-N^′-phenyl-p-phenylenediamine (i-PPD) was prepared. Processability, cure and dynamic mechanical properties of compounds were investigated. It has been found that stearic acid affects only the uncured compounds in terms of processability by acting as a lubricant, and plays no significant role in curing and dynamic mechanical properties. By contrast, i-PPD as an amine-based antioxidant shows pronounced effect on processability, cure and dynamic mechanical properties. The addition of i-PPD up to 2 phr could increase Mooney viscosity due to a thermal stabilization during high shear mixing. Exceeding this concentration, i-PPD shows plasticizing effect leading to a decrease in Mooney viscosity. Additionally, it has been shown that the addition of i-PPD reduces the crosslink density affecting remarkably the dynamic mechanical properties of compounds.     

A model study on effect of glucose on the basic characteristics and physical properties of natural rubber

Glucose at various concentrations was incorporated into sugar free purified natural rubber (PNR) latex to model the effect of carbohydrate on the basic characteristics and physical properties of natural rubber (NR). PNR samples treated with various concentrations of glucose were characterized for the basic properties of unvulcanized NR, i.e., gel content, molecular weight distribution and Mooney viscosity to evaluate the effect of sugar on these parameters. In addition, the effect of glucose on the physical properties of vulcanizates derived using sulfur and peroxide vulcanization was investigated. Glucose was shown to affect the viscosity of unvulcanized NR and the discoloration of vulcanized NR. Moreover, glucose was found to have a strong effect on crosslink density, as well as tensile and dynamic properties of sulfur vulcanizates, while those properties of peroxide vulcanizates was not much affected by glucose.     

Multilayer graphene/chlorine‐isobutene‐isoprene rubber nanocomposites: the effect of dispersion

Multilayer graphene (MLG) is composed of approximately 10 sheets of graphene. It is a promising nanofiller just starting to become commercially available. The dispersion of the nanofiller is essential to exploit the properties of the nanocomposites and is dependent on the preparation method. In this study, direct incorporation of 3 parts per hundred of rubber (phr) MLG into chlorine‐isobutene‐isoprene rubber (CIIR) on a two‐roll mill did not result in substantial enhancement of the material properties. In contrast, by pre‐mixing the MLG (3 phr) with CIIR using an ultrasonically assisted solution mixing procedure followed by two‐roll milling, the properties (rheological, curing, and mechanical) were improved substantially compared with the MLG/CIIR nanocomposites mixed only on the mill. The Young's moduli of the nanocomposites mixed in solution increased by 38%. The CIIR/MLG nanocomposites produced via solution showed superior durability against weathering exposure. Copyright © 2016 John Wiley & Sons, Ltd.     

Studies of vulcanization with tetramethylthiuram disulfide and sulfur vulcanization accelerated by tetramethylthiuram disulfide of styrene–butadiene rubber in presence and absence of dicumyl peroxide

The salient features of nonelemental sulfur vulcanization by tetramethylthiuram disulfide (TMTD) and elemental sulfur vulcanization promoted by TMTD both in presence and absence of ZnO and stearic acid have been studied. In stock containing TMTD, a higher rate constant value for dicumyl peroxide (DCP) decomposition was observed. TMTD decreases the crosslinking density due to DCP depending on its concentration. An entirely radical mechanism has been advanced in the absence of ZnO. ZnO or ZnO–stearic acid seems to alter the entire course of reaction. The rate of crosslinking increases in the presence of ZnO or ZnO–stearic acid. Moreover, crosslinks are formed additively (further supported from the activation energy data), and mixed crosslink formation has been confirmed by the methyl iodide test of the vulcanizates. Stearic acid has no effect on crosslink formation. An ionic chain mechanism has been postulated in the presence of ZnO, as suggested by British authors.     

Influence of alkaline treatment and acetone extraction of natural rubber matrix on properties of carbon black filled natural rubber vulcanizates

In this study, air dried sheet (ADS) showed higher molecular weight, proteins, lipids and more gel content than the low protein natural rubber (LPNR) from alkaline treatment or acetone-extracted natural rubber (AENR). After removal of proteins and lipids, LPNR and AENR had shorter scorch and cure times among the rubber compounds observed. This is due to higher content of free fatty acids, glycerides and sodium salts of fatty acid that might act as cure activators with sulfur curing. Furthermore, a finer dispersion of CB was found in ADS, due to chemical interactions of CB with proteins and lipids at terminal ends of NR molecules. Also, high molecular weight and gel in ADS could induce a higher reinforcing index (α) resulting to superior mechanical, dynamic, thermo-mechanical, hardness and crosslink density. In contrast, CB agglomerates occurred in LPNR and AENR with low molecular weight and reduced proteins and lipids, causing inferior mechanical, dynamic, thermo-mechanical, along with loss of stiffness and crosslink density.     

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.     

Investigation into morphology, microstructure and properties of SBR/EPDM/ORGANO montmorillonite nanocomposites

Rubber compounds based on styrene-butadiene rubber/ethylene propylene diene monomer blends of different compositions (60/40, 70/30, 80/20, 90/10, 100/0) reinforced with 1 wt%, 3 wt%, 5 wt% and 7 wt% organoclay (Cloisite 20A) were prepared on a two roll mill via a vulcanization process and characterized by several techniques. Results of X-ray diffraction showed expansion of the inter-gallery distance, and transmission electron microscopy (TEM) micrographs confirmed that the prepared nanocomposite samples have intercalated and partially exfoliated structures. Cure characteristics showed that, organoclay not only accelerates the vulcanization reaction, but also gives rise to a marked increase of the torque, indicating crosslink density of the prepared compounds increases at the presence of organoclay. Mechanical properties of samples received markedly increase by clay loading due to the good interaction established between nanoclay particles and polymer matrix as it was evidenced by SEM photomicrographs. At the same time, rheological properties showed that addition of nanoclay could improve storage modulus as well as complex viscosity of SBR/EPDM samples. The results of ozone test revealed that the ozone resistance of samples significantly increases as nanoclay or EPDM content increases.     

PREPARATION OF HIGH-TEMPERATURE VULCANIZED SILICONE RUBBER OF EXCELLENT MECHANICAL AND OPTICAL PROPERTIES USING HYDROPHOBIC NANO SILICA SOL AS REINFORCEMENT

Hydrophobic nano silica sol(HNSS)was incorporated into polyvinylmethylsiloxane to prepare reinforced high- temperature vulcanized(HTV)silicone rubber.HTV silicone rubber filled with 40 phr HNSS showed excellent mechanical and optical properties:the tensile strength reached 11.7 MPa and the optical transmittance was higher than 90%.Possible reasons for reinforcement and transparency were discussed on the basis of the bound rubber percentage,total crosslink density,and SEM analysis.Our work suggests that H...     

Changes in the Morphology and Size of Carbon Black Agglomerates upon Mechanical Activation

The effect that mechanical activation performed for 5–30 min in an AGO-2 planetary mill with the centripetal acceleration of the milling bodies of 300 m s^–2 exerts on the size and morphology of agglomerates of P399 carbon black and on the physicomechanical and conducting properties of rubber stocks prepared by blending SKMS-10 ARK rubber and P399 carbon black was studied.     

Effect of nature and extent of crosslinking on swelling and mechanical behavior of styrene–butadiene rubber membranes

The influence of crosslink type and crosslink density on the swelling and mechanical behavior of styrene–butadiene rubber (SBR) membranes were studied in four aliphatic hydrocarbons. To vary the crosslink type and crosslink density, SBR was vulcanized by four different vulcanizing systems viz conventional, efficient, peroxide and a mixture of sulfur and peroxide. SBR vulcanizates having mono or disulfidic crosslinks (efficient system) exhibited the highest solvent uptake whereas those with C–C bonds (peroxide system) showed the lowest. SBR crosslinked by the mixed system showed superior mechanical properties in the unswollen, swollen and deswollen conditions. Arrhenius and thermodynamic parameters were evaluated from the diffusion data. Finally, a comparison between theoretical and experimental diffusion results was carried out.     

Studies of conventional sulfur vulcanization of SBR rubber: Analysing the reaction products from thermal degradation of the accelerator by means of MCC-IMS technique

A combination of knowledge on curing process of rubber mixes with novel methods of chemical analysis, a new unconventional approach to analysis of rubber vulcanization is presented in this study. Six SBR samples containing various N-tert-butyl-2-benzothiazole sulfenamide (TBBS) accelerator: sulfur ratios (within) the range of conventional (CV) sulfur vulcanization system were studied using multi-capillary column ion mobility spectrometry (MCC-IMS) technique. For these analysis, calibration/dilution curves were established. Moreover, data from MCC-IMS were correlated with other parameters of the rubber vulcanizates – their crosslink density and structure as well as their tensile strength and modulus at elongation. For such comparison, one of the reaction products from thermal decomposition of TBBS, benzothiazole was selected. Furthermore, the concentration of benzothiazole released during the vulcanization process corresponded well with the crosslink density of the rubber vulcanizates studied. It was even possible to calculate the crosslink density from the concentration of benzothiazole determined by MCC-IMS, using Boltzmann fitting curve. The presented results could be an important contribution in understanding the mechanisms occurring during rubber vulcanization, demonstrating a new approach to testing and evaluation of the process.     

Investigation of aging behavior and mechanism of nitrile-butadiene rubber (NBR) in the accelerated thermal aging environment

Nitrile-butadiene rubber (NBR) was exposed to an accelerated thermal aging environment produced by an air-circulating oven for different time periods. NBR aging was evaluated by morphology, crosslink density, mechanical properties, chemical changes and thermal stability. The results showed that the surface damage of NBR turned severe and inhomogeneous, and the aging degree was most serious on the edge region of voids. Crosslinking reactions mainly occurred in the aging process. The tensile strength increased with increase in crosslink density up to a maximum value and thereafter decreased with further increase in crosslink density. X-ray Photoelectron Spectroscopy (XPS) and Pyrolysis Gas Chromatography-Mass Spectrometry (Py-GC/MS) analysis demonstrated that hydroxyl groups were formed and the additives migrated from inner to surface of NBR samples. In addition, the thermogravimetric analysis (TGA) indicated that the thermal stability of NBR did not significantly change in the accelerated thermal aging environment.     

Effect of Ground Rubber Powder on Properties of Natural Rubber

Ground rubber powder (GRP) with three different sizes was incorporated into nature rubber matrix with different loading. Cure characteristics, swelling behaviour, crosslink density, tensile fractured surface, and mechanical properties have been studied. Based on the cure characteristics, it is evident that the processability of the rubber compounds has not changed obviously with the different GRP loading. The introduction of GRP in virgin rubber leads to the increase in swelling degree and the decrease in crosslink density. Tensile strength, hardness and abrasion resistant deteriorate with the increase of GRP loading, but the tear resistance gets better. If the ground rubber particles are smaller, the properties are more similar to the virgin rubber. Because of the phase separation of the GRP and matrix, the properties get worse with the bigger ground rubber powder.     

An environmental friendly approach for recycling of post‐vulcanized fluoroelastomer scraps through high‐shear mechanical milling

An environmental friendly approach for partial de‐crosslinking of post‐vulcanized fluoroelastomer (FKM) scraps through high‐shear mechanical milling has been developed for recycling of the FKM. The method not only overcomes the expensive use and recovery of organic solvents but also gives rise to reclaimed rubbers with superior mechanical properties. After 32 cycles of milling, the gel fraction of FKM decreased from its original 97.8% to 79.7%. The appearance of the –CF₂‐associated peaks C1s spectra after mechanical milling confirmed the partial de‐crosslinking of FKM. The structure change of FKM sol part before and after mechanical milling was also investigated by Fourier transform infrared (FTIR) analysis and gel permeation chromatography (GPC) measurements. The reclaimed FKM exhibited excellent mechanical and thermal properties, indicating a strong potential for future applications. The tensile strength of FKM re‐vulcanizates is 6.6 MPa, retaining about 84% properties of virgin FKM vulcanizates (7.9 MPa), and the elongation at break was increased from 337.1% to 368.7%. Copyright © 2010 John Wiley & Sons, Ltd.     

Thermoplastic elastomers based on waste rubber and expanded polystyrene: Role of devulcanization and ionizing radiation

Waste rubber (WR) powder as a vulcanized one was introduced into a two-roll mill in the presence of various ratios of curatives to develop sheets of devulcanized waste rubber (DWR). Thermoplastic elastomers (TPE) were prepared by blending either WR or the obtained DWR with the waste of expanded polystyrene (EPS) at a fixed ratio of 70 rubber/30 EPS in the absence or presence of gamma irradiation at 100?kGy. Maleic anhydride (MA) 1?wt% with respect to rubber was used as coupling agent. The mechanical parameters of the blends: tensile properties, rheology, hardness (Shore D), and abrasion resistance were studied.