Influence of Crosslink Density on Mechanical Properties of Natural Rubber Vulcanizates

Crosslink density is an important structural parameter for cured rubber. Natural rubber (NR) vulcanizates with different crosslink densities were obtained through using different sulfur and accelerator amounts and different accelerator types. The crosslink density was characterized by an ¹ H-NMR technique and its influence on mechanical properties, such as Shore A hardness, 300% modulus, tensile strength, and elongation at break, of NR vulcanizates was investigated. The results showed that both the sulfur amount and the accelerator type and amount had an influence on the crosslink density of the NR networks. The relationship between total crosslink density and mechanical properties was also studied. The results, by changing either the sulfur or the accelerator amount, showed that tensile strength of NR vulcanizates reached maximum value when the total crosslink density was around 13.5 × 10^?5 mol/cm³, equivalently the average molecular weight of the intercrosslink chains (Mc) was around 7000 g/mol. The maximum value of tensile strength came from the balance between contributions of crosslink joints and stretch-induced orientation and/or crystallization of intercrosslink chains. The study on influence of total crosslink density on Shore A hardness and 300% modulus of NR vulcanizates showed that they both increased linearly with the crosslink density, the slopes were 2.7 ～ 3.0 cm³/10^?5 mol and 0.27 ～ 0.31 MPa cm³/10^?5 mol for Shore A hardness and 300% modulus, respectively, whether the crosslink density was varied by sulfur or accelerator.     

Dynamic Rebound Behavior of Silica/Natural Rubber Composites: Fly Ash Particles and Precipitated Silica

This article investigated the elastic response of natural rubber (NR) compounds filled with silica from fly ash particles (FASi) and commercial precipitated silica (PSi), through a dynamic rebound test. The effects of silica content and initial drop‐height on the height and number of rebounds, dynamic stiffness, and the energy loss were of interest. The results suggested that the unfilled NR vulcanizates exhibited a greater elastic response than the FASi and PSi‐filled vulcanized composites. For given silica contents, the NR compounds with FASi had better elastic response than those with PSi, where the elastic response decreased with an increase in silica content. The greater the silica contents, the higher the dynamic stiffness of the composites. The initial drop‐height had no effects on the elastic response change for the unfilled NR compound, but resulted in an increase in the energy loss for the silica‐filled NR composites. The differences in the elastic responses for the NR compounds filled with silica from FASi and PSi were associated with the differences in crosslink density and the filler–filler interaction influenced by content of bis(3‐triethoxysilylpropyl) tetrasulfane (designated as Si69) used.     

Comparison of Mechanical Properties and Rheological Behavior of Trans-Polyisoprene/Polypropylene and Ethylene Propylene Diene Rubber/Polypropylene Thermoplastic Vulcanizates

The degree of dynamic vulcanization, mechanical properties, rheological behavior, and the ageing-resistant performance of trans 1,4-polyisoprene (TPI)/polypropylene (PP) and ethylene propylene diene rubber (EPDM)/PP thermoplastic vulcanizates with a blend ratio of 60/40 were investigated comparatively. The results showed that TPI had fully dynamically vulcanized when mixed with PP in the Hakke mixer chamber (175°C, 60 rpm) while EPDM had only partly dynamically vulcanized due to its saturated main chain backbone. With increased sulfur content, the torque at the end of the curing curves of the two thermoplastic vulcanizates (TPVs) increased in the curing characteristics measuring process as the degree of crosslinking increased. Comparing the two blends, TPI/PP-TPVs were possessed of a better mobility, a little lower tensile strength and tear strength, a little higher 100% modulus and hardness, and much lower elongation at break. EPDM/PP-TPVs had better ageing-resistant characteristics due to EPDM's saturated main chain backbone.     

Preparation of Water-suspensible Carbon Nanoparticles and Their Influence on the Properties of Epoxidized Natural Rubber Latex

A new kind of water-suspensible carbon nanoparticle was prepared by oxidizing carbon black (CB) using aqueous ammonium persulfate (APS) as an oxidant. The obtained water-suspensible carbon nanoparticles (APS-CBs) were still homogeneously distributed in water three weeks after sonication. Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) were employed to characterize the changes of the oxygen containing functional groups on the surfaces of the APS-CBs. Furthermore, a high performance of an epoxidized natural rubber latex (ENRL)/APS-CBs composite was obtained by a liquid latex blending method. The carbon nanoparticles were homogeneously distributed in the matrix. In comparison with the ENRL/pristine CBs composites, the APS-CBs improved the mechanical properties of the ENRL/APS-CBs composites. The dynamic rheological properties were also studied.     

Effect of Wood Flour on the Curing Behavior,Mechanical Properties,and Water Absorption of Natural Rubber/Wood Flour Composites

The preparation of natural rubber/wood flour (NR/WF) composites and the influence of WF content, modification, and particle size on the vulcanizing behavior, mechanical properties, and water absorption of NR/WF composites are described. Results show that the addition of WF into NR delayed the scorching time and vulcanizing time of NR. The appropriate WF contents can improve the mechanical properties of NR. However, the overloading of WF destroys the mechanical properties of NR. The addition of WF increased the water absorption of NR. The silicone couple agents that were used to modify the WF had little effect on the water absorption of NR/WF composites. Decreasing the WF particle size enhanced the water absorption of NR/WF composites because the water-absorbing surface area increased with decreasing WF particle size. The water absorption of sisal-fiber-filled NR-based composites was larger than that of the WF-filled NR-based composites. A useful equation, w=ktⁿ , was inferred from the water absorption results to calculate the water absorption (w) of the NR/WF composites as a function of time (t), where k was a constant concerning the compounds’ character that was primarily determined by the WF's character and n was the power of time that was related to the NR's inherent character, such as cross-linking density, and primarily determined the water absorption rate.     

受阻酚AO-80/丁腈橡胶复合材料的压力-体积-温度关系研究

 研究制备了具有高阻尼性能的受阻酚AO-80/丁腈橡胶复合材料样品,采用动态粘弹性测试得到了损耗因子tan δ与温度T的关系曲线。分析表明,随着受阻酚含量的增加,tan δ峰的位置向高温方向移动,tan δ峰的峰值也逐渐提高;利用pvT高压膨胀计对样品进行了测试,采用Tait方程研究了其压力-体积-温度（pvT）性能,对此计算出Tait方程中的相应参数。结果表明,由Tait方程计算所得的pvT理论关系与定性实验结果一致,具有较好的规律性,即对AO-80/丁腈橡胶复合体系b₁值随AO-80量的增加而逐渐降低,AO-80量越大,该体系T_g时的绝对比容越小;b₂随AO-80量的增加而逐渐增大,AO-80量越大,该体系随温度膨胀越显著;b₃、b₄均为常数,即B为常数,复合材料的比容只与压力存在一定关联;同时用Tait方程计算所得的值与实验值的平均残差均小于0.02,吻合度较好;结果表明,Tait方程可以较好地描述该体系的pvT关系,为定量研究阻尼材料的pvT性能提供了理论指导。     

Mechanical and Flame Retardant Properties and Microstructure of Expandable Graphite/Silicone Rubber Composites

Flame-retardant expandable graphite (EG)/silicone rubber (SR) composites were prepared using nano-CaCO₃ particles as reinforcement filler. In addition to mechanical measurements, limited oxygen index (LOI), UL-94 and cone calorimeter tests (CCT), the thermal properties were tested by thermogravimetric analysis (TGA). The results showed that the content and particle size of the EG both had large effects on the flammability and mechanical properties of the EG/SR blends. The composites that contained 25 phr EG (50–80 mu) had excellent LOI values, 47–48, and achieved the UL-94 V-0 level while the pure SR sample had the LOI value of 25 and achieved the UL-94 V-2 level. The data obtained from the CCT indicated that the addition of EG decreased remarkably the heat release rate, smoke emission, and mass loss rate of the composites. SEM microphotographs of the EG/SR composites before and after combustion demonstrated that EG underwent a large volume expansion, and the multiporous char structure blocked heat transfer and protected the substrate from fire.     

Synthesis and Characterization of ZnO Nanoparticles and Their Natural Rubber Composites

Abstract

Nanoparticles of zinc oxide were synthesized by a solution combustion method. The average size of these particles was analyzed by using X-ray diffraction. Composites of natural rubber and the ZnO nanoparticles were prepared by a latex blending method. The matrix phase was cured by using the crosslinking agent, pentane-1,5-diylidenediamine. Effects of crosslinking and incorporation of nanoparticles on the tensile and solvent transport properties of the natural rubber were studied in detail. The nature of the dispersion of the nanoparticles was analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). It was observed from the tensile studies that the addition of the curing agent and the ZnO nanoparticles increased the stability considerably. Incorporation of the nanoparticles also considerably increased the solvent resistance of the cured natural rubber. We suggest the addition of ZnO nanoparticles at a low loading level provided better properties compared to other reinforcements, such as carbon black and nano-clay.     

Thermoplastic Elastomers Based on Natural Rubber/Polypropylene Blends: Effect of Blend Ratios and Dynamic Vulcanization on Rheological,Thermal, Mechanical,and Morphological Properties

Thermoplastic elastomers (TPEs) based on natural rubber (NR)/polypropylene (PP) with different blend ratios were prepared and studied. The TPEs were obtained by dynamic vulcanization of NR/PP using a sulfur (S)/N-tert-butyl-2-benzothiazolesulphenamide (TBBS) and tetramethylthiuram disulphide (TMTD) curative system during processing in the melt state in an internal mixer equipped with cam rotors. Rheological, thermal, mechanical, dynamic, and morphological properties of the TPEs prepared were investigated. Based on this study a mechanism for the NR crosslinking was proposed where the sulfur vulcanization occurs through radical substitution in the forms of polysulfide bridges. The dynamic vulcanization process increases the stiffness of the NR phase in the TPEs and modifies the rheological and thermal behavior of the system compared to the behavior of the basic material PP. The crosslinked NR particles restrict the spherulitic growth and the regular arrangement of the spherulites of PP phase, decreasing the crystallinity degree. On the other hand, a reduction of mobility of the chain segments was also observed and, consequently, an increase of the T_g values. NR/PP TPEs with high content of NR showed superior mechanical performance compared to the uncrosslinked NR/PP blends in terms of tensile strength, Young's modulus and hardness. An increase of approximately 320% in Young's modulus values was obtained for the NR70/PP30 TPE compared to NR70/PP30. Morphological studies revealed the formation of large aggregates of NR domains in NR/PP TPEs which increased in size with an increase of the rubber content.     

Interactions Between an Ionic Liquid and Silica,Silica and Silica,and Rubber and Silica and Their Effects on the Properties of Styrene-Butadiene Rubber Composites

An investigation of the effect of an ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate (BMI), on the properties of silica reinforced styrene-butadiene rubber (SBR), aimed to correlate the interactions between the ionic liquid and silica, silica and silica, and silica and rubber with the macro-properties and microstructure of SBR and SBR/silica vulcanizates is described. The interaction between the ionic liquid and silica was characterized by differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR), the interaction between silica and silica was characterized by a rubber processing analyzer (RPA), and the interaction between rubber and silica was characterized by the bound rubber content. The FTIR analysis revealed that BMI can react with the hydroxyl groups on the surface of silica, improving the compatibility between the rubber and silica. The RPA and bound rubber testing indicated that the interactions between silica and silica particles were weakened and the interaction between silica and rubber increased with the incorporation of BMI into the SBR rubber. The bound rubber content showed a maximum with a BMI content of 3 phr. At the same time, the dispersion of silica in SBR was improved with the incorporation of BMI. With the increase of BMI content, the curing rate was greatly improved and the crosslink density increased. BMI also increased the tensile strength and abrasion resistance of the SBR vulcanizates. Most important, the BMI significantly improved the dynamic properties of the rubber composites, especially the wet-skid resistance and rolling resistance. However, excessive BMI (beyond 3 phr) acted as a plasticizer and was detrimental to the mechanical properties, resulting in a decrease of tensile strength and abrasion resistance.     

Comparison of Two Recent Non-Gaussian-Type Tube Models: Analysis of Mechanical Data on Unfilled Natural and Styrene-Butadiene Rubber Vulcanizates

Literature stress-strain data of filler-unfilled networks, observed in uni and biaxial deformation modes, were comparatively characterized using Hoei's finite extensibility tube model and Meissner and Matějka's primary finite extensibility tube model excluding their empirical correction term for the finite extensibility. As a result of this analysis, the former model was found to be able to more advantageously describe the stress-strain behaviors over the entire range of strain without needing the empirical correction term.     

Styrene Butadiene Rubber/Epoxidized Natural Rubber (SBR/ENR50) Nanocomposites Containing Nanoclay and Carbon Black as Fillers for Application in Tire-Tread Compounds

Nanocomposite vulcunizates based on a SBR/ENR50 (50/50%wt) rubber blend containing nanoclay (5 or 10 phr) with and without carbon black (CB 20 phr) were prepared by melt blending in an internal mixer. The compound containing 35 phr carbon black (only) was prepared as a reference sample. Microstructure of nanocomposite samples was investigated by using X-ray diffraction (XRD), melt rheo-mechanical spectroscopy (RMS), and scanning electron microscopy (SEM). The XRD patterns revealed that the distance between the clay layers were increased by adding CB to the nanocomposite samples; they caused better diffusion of chains between the layers and resulted in an intercalated structure. The RMS results also indicated the formation of the filler-filler networks. SEM images of fracture surfaces showed the presence of much roughness in the samples containing both nanoclay and CB compared to the other samples. The results obtained from application of the Flory–Rhener equation showed a high crosslink density for the sample with 10 phr nanoclay and 20 phr CB. Dynamic mechanical behavior, mechanical properties, and abrasion resistance of the nanocomposites were evaluated. The results indicated that the sample containing 10 phr nanoclay and 20 phr CB had an increased dynamic elastic modulus, reduced maximum loss factor (tanδ)_max,, and an improved tensile strength and abrasion resistance compared to the reference sample. Also, this sample showed the lowest maximum loss factor, at 50–60°C, so it can be a candidate for tire-tread application.     

Study on the Reinforcing Effect of Milled Carbon Fibers in a Natural Rubber Based Composite

Milled carbon fibers (MCF) have been tested at 2, 4, and 6 phr in a standard natural rubber compound with 45 phr of N375 carbon black. A dramatic increase in the low elongation moduli was observed even with only 2 phr of MCF. The presence of MCF confers anisotropic properties to the rubber compounds that can be measured by an anisotropic factor σ, defined as the ratio between the modulus parallel to the MCF prevalent direction over the modulus orthogonal to the MCF prevalent direction. It has been shown that the presence of MCF is able to reduce the mechanical hysteresis and also the compression set of the natural rubber compound. However, the tear strength properties are affected negatively. The present study demonstrates the feasibility and the advantages derived by the utilization of the carbon fibers as extra reinforcing filler in rubber compounds.     

A Comparative Study of the Dynamic-Mechanical Properties of Styrene Butadiene Rubber/Epoxidized Natural Rubber Dual Filler Nanocomposites Cured by Sulfur or Electron Beam Irradiation

The properties of (50/50?wt%) styrene butadiene rubber/epoxidized (50%) natural rubber (SBR/ENR50) blends containing nanoclay (NC, 5 or 10phr) without and with carbon black (CB 20phr) cured by sulfur or by electron beam (EB) irradiation (50 and 100kGy), were compared. A sulfur cured compound containing 35phr CB was prepared as a reference sample. Dynamic mechanical thermal analysis (DMTA) indicated that the sulfur cured sample containing 10phr NC and 20phr CB and the 100kGy irradiated sample with 5phr NC and 20phr CB had higher crosslink density, storage modulus, and tensile strength, and less loss factor and loss modulus, compared to the reference sample. Scanning electron microscopy (SEM) images of cryo- fractured surfaces confirmed the DMA and crosslink density results. We suggest a light weight 100kGy irradiated sample containing the lowest amount of NC and 20phr CB with a uniform distribution of the –C–C– bonds crosslinks, for high thermal stability applications and also for passenger cars tire treads, for its ice grip and wet skid properties especially for icy and wet roads, with improvements of 23% and 20%, respectively as compared to the reference sample.     

The Effect of Epoxidized Natural Rubber and Two Kinds of Organoclay upon Molecular Interaction,Structure and Mechanical Properties of (Styrene-Butadiene Rubber/Acrylonitrile-Butadiene Rubber/Organoclay) Nanocomposites

Epoxidized natural rubber (ENR50) and two different kinds of organoclay (C30B and C15A) were used in blends of styrene-butadiene rubber (SBR) and acrylonitrile butadiene rubber (NBR) and their effects upon interaction between phases, morphology, and mechanical properties of the blends were investigated. The compounds were characterized by means of Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and differential scanning calorimetry (DSC). The obtained results showed formation of hydrogen bonding between the compounds ingredients due to incorporation of C30B, especially in presence of ENR. AFM and FE-SEM analysis revealed good dispersion of the nanoparticles in the polymer matrix upon addition of ENR as well as better dispersion of C30B than C15A in the NBR phase. XRD results showed a greater expansion of the silicate layers by simultaneous use of organoclay and ENR Incorporation of organoclay alone or in combination with ENR in the blends caused shifting of the SBR T_g toward the NBR T_g. The tensile properties of the blends showed improvement by using nanoparticles in the presence of ENR.     

Synthesis of a New Nanosilica-Based Antioxidant and Its Influence on the Anti-Oxidation Performance of Natural Rubber

Nanosilica was modified with a silane coupling agent 3-Glycidoxypro- pyltrimethoxysilane (KH-560) and then reacted with p-aminodiphenylamine (RT) to obtain a new nanosilica-based antioxidant. Fourier transform infrared and thermogravimetric (TGA) analysis measurements confirmed the successful grafting of RT to the nanosilica surface. Scanning electron microscope analysis showed that the nanosilica-based antioxidant could be homogeneously dispersed in a natural rubber matrix. Differential scanning calorimetry and TGA were used to measure the thermal oxidative behavior of rubber vulcanizates with different types of fillers. It was found that the thermal oxidative stability of rubber vulcanizates with the nanosilica-based antioxidant was improved to a greater extent than that with other fillers examined.     

Preparation and Properties of Natural Rubber (NR)/Polycaprolactone (PCL) Bio-Based Shape Memory Polymer Blends

Shape memory NR/PCL bio-based blends, where NR served as the reversible phase and PCL served as the switching phase, were prepared using a melt blending process. Peroxide, besides its role as a cross-linking agent to NR, was used to enhance the compatibility between NR and PCL, which was confirmed via Fourier transform infrared spectra and scanning electron microscopy analyses. With increasing peroxide content, the tensile strength increased steadily with decreasing PCL contents, up to 12.3 ± 0.8 MPa for the NR/PCL (70/30) blend at 2 phr peroxide. The shape fixing ratio decreased slightly with increasing peroxide content, especially for high NR content. The values of the shape recovery ratio were high, up to 100%, regardless of PCL content. The recovery stress was found to reach its maximum value around 60°C. In order to consider both fixing ratio (R_f) and recovery ratio (R_r) together, an additional denotation of shape memory index, R_fR_r, coupling both ratios by the product of the fixing ratio and recovery ratio, is proposed. The shape memory index (R_fR_r) reached values close to 100%, at the NR/PCL composition of 50/50 and 70/30; in particular, the highest value was for the 50/50 case regardless of peroxide concentration. The shape recovery ratio and fixing ratio remained largely unchanged even after 30 cycles of deformation and recovery processes for a typical NR/PCL/DCP (30/70/0.5) blend which, thus, outperformed all known shape memory blends in the literature.     

Mullins Effect and its Reversibility for Zinc Dimethacrylate-Reinforced Thermoplastic Vulcanizates Based on a Ethylene-Vinyl Acetate Copolymer/Nitrile-Butadiene Rubber Blend

The Mullins effect during uniaxial compression tests, together with its reversibility, of ethylene-vinyl acetate copolymer (EVA)/nitrile-butadiene rubber (NBR)/zinc dimethacrylate (ZDMA) thermoplastic vulcanizates (TPVs) were investigated systematically. The experimental results indicated that the Mullins effect could be observed during the cyclic uniaxial compression tests and was accompanied with stress softening, residual deformation accumulation and hysteresis. The reversibility of the Mullins effect showed a temperature-dependent feature and was enhanced with increasing temperature.     

基于表面生成法的粗糙基底上半透明涂层的光散射特性

针对光在半透明涂层和粗糙基底构成的两层粗糙面的散射过程,采用表面生成法,考虑入射遮蔽以及多次散射效应,构建表面散射的蒙特卡罗模型,得到粗糙基底上半透明涂层的反射光的能量分布,研究涂层厚度、表面粗糙度、入射光波长等对双向反射分布函数(Bidirectional Reflectance Distribution Function,BRDF)的影响.结果表明,BRDF受表面的粗糙程度影响较大,其峰值随表面均方根斜率的增加线性递减;半透明涂层厚度影响BRDF峰值的大小,厚度为十分之一波长时,BRDF达最小值;近红外光入射时镜反射方向附近BRDF的局部分布曲线相对于可见光入射时有展宽趋势,且方向半球反射率相对于可见光入射时更大.     

Effect of Silica and Silicone Oil on the Mechanical and Thermal Properties of Silicone Rubber

The effect of three types of silicas with varied loading and the loading of hydroxyl terminated silicone oil on the mechanical and thermal properties of silicone rubbers (SRs) were investigated. Mechanical properties were affected by the silica loading because of the interaction between fillers and polymer and the filler dispersion. Fumed silica filled SRs showed higher tanδ, tensile strength, and elongation at break compared to those containing two types of precipitated silicas. With increasing silicone oil loading, the tensile strength, tear strength, hardness, and tanδ of SRs first increased and then decreased.