Damping Properties of Blends Based on EVM

The mechanical and damping properties of blends of ethylene-vinyl acetate rubber (VA content >40% wt) (EVM)/ethylene-propylene-diene copolymer (EPDM) and EVM/nitrile butadiene rubber (NBR), both with 1.4 phr BIPB (bis (tert-butyl peroxy isopropyl) benzene) as curing agent, were investigated by dynamic mechanical analysis (DMA). The effect of added polyvinyl chloride (PVC), amido donor N-cyclohexyl-2-benzothiazole sulfonamide (CZ), and dicumyl peroxide (DCP) as a substitute curing agent, on the damping and mechanical properties of both rubber blends were studied. The results showed that in EVM/EPDM/PVC blends, EPDM was immiscible with EVM and could not expand the damping range of EVM at low temperature. PVC was miscible with EVM and dramatically improved the damping property of EVM at high temperature while keeping good mechanical performance. In EVM/NBR/PVC blends, PVC was partially miscible with EVM/NBR blends and remarkably widened the effective damping temperature range (EDTR) from 41.1°C for EVM/NBR to 62.4°C. Curing agents BIPB and DCP had a similar influence on EVM/EPDM blends. DCP, however, dramatically raised the height of tan δ peak of EVM/NBR = 80/20 and expanded its EDTR to 64.9°C. CZ had no obvious influence on the EVM/EPDM blends cured with BIPB. However, a small content of CZ enlarged the tan δ peak of EVM/NBR = 80/20 in both height and width, but at the cost of a deterioration of mechanical performance.     

Dynamic Mechanical Properties of Aged Filled Rubbers

Abstract

It is well known that the macromolecular structure and the microstructure of the fillers play an important role in the mechanical properties of filled rubbers. This paper focuses on the dependence of the complex modulus of aged natural rubber vulcanizates on the filler network and polymer structure. Dynamic mechanical analysis (DMA) at 30°C, 50°C, and 70°C on the aged rubbers with/without prestrain showed the Payne effect, i.e., a storage modulus drop with increasing amplitude, and the appearance of a loss tangent maximum at strain of about a few percent. The storage modulus increased with the aging time at 70°C, 24 < 72 < 240 hr, in the case of nonprestrain. When the prestrain was applied, strain‐induced crystallization was generated that enhanced the storage modulus. As time passed, the prestrain relaxed and the crystalline structures began to disappear. After 72 hr, the crystalline structures had almost disappeared, and they had only a weak effect. Consequently, there existed a higher modulus for an aging time of 24 hr than 72 hr at testing temperatures of 30°C and 50°C. It was concluded that the storage modulus was determined by the postvulcanization, strain‐induced crystallization, aging, and relaxing time.     

Small-angle X-ray scattering from styrene-butadiene-styrene block copolymers

Abstract

A new type of time-dependent and strain-history-dependent viscoelasticity was discovered in semidilute polymer solutions. Dynamic moduli G′ and G″ of 20% and 10% nitrile butadiene rubber (NBR) solutions were recorded as a function of time while oscillatory shear deformations were maintained. The moduli decrease with time was observed only at lower frequencies. The time dependence of G′ was more pronounced than that of G″. At a higher temperature, the time dependence was extended toward higher frequencies also, and the time dependence became stronger. Lowering the concentration of solution gave a similar effect as increasing temperature. After the cessation of oscillations, a slow recovery was observed. The recovery was somewhat faster at the higher temperature. The time-dependent moduli and their recovery were explained by the change and recovery of structures associated with long branches and gels in the NBR. The structure change occurred at higher frequencies also, but it was not observed during the application of oscillation. Only in subsequent measurements at lower frequencies could the structure change be detected. Thus, the change may be regarded as strain history dependent. The mechanism of the structural change was explained with either the entanglement or osmotic pressure models, depending on concentration.     

Simulation of Electrical Resistivity of Carbon Black Filled Rubber under Elongation

It has been known that the carbon black (CB) network is responsible for the electrical and mechanical behaviors of filled rubber. Due to the complexity involved in the filled rubber in relation to the conductive mechanism of the CB network, there has been little work concerned with simulation of the electrical behavior at large strains. Based upon an infinite circuit model, the electrical resistivity of CB filled rubber under elongation is simulated. For CB (N330) filled natural rubber with volume fraction of 27.5%, the simulated electrical resistivity increases with elongation at small stains, corresponding to the breakup of the agglomerates. The reduction in resistivity at larger strains corresponds to the decrease of the junction width, which results in a decrease of the contact resistance. Good agreement is found between the simulations and the experimental data available in the literature. The simulated results confirm the effects of the breakdown of the CB network and the alignment of CB aggregates under strain on the electrical resistivity.     

Investigation of Carbon Black Network in Natural Rubber with Different Bound Rubber Contents

A new method was applied to modify the surface activity of virginal carbon black (VCB). LA‐57, one kind of hindered amine light stabilizer, was adsorbed onto the carbon black surface through a strong shear force induced by the screws of a HAAKE internal mixer. The modified carbon black (MCB) was characterized by FT‐IR and thermogravimetric analysis (TGA). The bound rubber content of the natural rubber (NR) compounded with MCB and VCB varied with the fraction of LA‐57 on the MCB surface. The nonlinear effect at small strains, generally referred as the Payne effect, was investigated in the rubber compounds based on the different bound rubber contents. The NR compound containing the lowest bound rubber content had an obvious Payne effect. Based on the bound rubber content, the types of filler network varied from direct contact mode to the joint rubber shell mechanism.     

Crystalline Morphology and Nonisothermal Crystallization Behaviors of iPP/PcBR Blends

Isotactic polypropylene/poly(cis‐butadiene) rubber (iPP/PcBR) blends were prepared by melt mixing. The influence of PcBR content on crystalline morphology and nonisothermal crystallization behaviors of iPP was investigated by polarized optical microscopy (POM), small angle light scattering (SALS), and differential scanning calorimetry (DSC). The POM showed that an increase of PcBR ranging from 10 vol% to 40 vol% led to less perfection of spherulites, vaguer boundaries between spherulites, and smaller spherulite size, which was quantitatively validated by SALS. The presence of PcBR also remarkably affected the nonisothermal crystallization behaviors of iPP. An addition of PcBR caused higher crystallization peak temperature and a faster crystallization rate, meaning a heterogeneous nucleation effect of PcBR upon crystallization of iPP. For the same sample, the crystallization peak temperature moved to lower temperature and the crystallization rate increased as the cooling rate increased. The Ozawa and combined Avrami and Ozawa equations were used to describe the nonisothermal crystallization process of iPP and blends. The combined Avrami and Ozawa equation was more appropriate for the crystallization of the blends. Crystallization activation energy of iPP and blends was calculated by the Kissinger equation; the result showed that crystallization activation energy decreased as the content of PcBR increased from 30 vol% to 40 vol%.     

Properties of Natural Rubber Vulcanizates/Nanosilica Composites Prepared Based on the Method of In-situ Generation and Coagulation

Using the characteristics of silica sol dispersing well in water and easy formation of silica gel when the silica sol is heated, by mixing a system of concentrated natural rubber latex and silica sol, the silica sol can in-situ generate SiO₂ particles when heated. After coagulation of the mixed system, natural rubber/nanosilica composites C(NR/nSiO₂) were obtained. The composites C(NR/nSiO₂) and their vulcanizates were studied using a rubber processing analyzer (RPA), dynamic mechanical analysis (DMA), and scanning electron microscopy (SEM). The influence of silica contents on the C(NR/nSiO₂) vulcanizates mechanical properties, cross-linking degree, Payne effect, dissipation factor (tanδ), and the particle size and dispersion of SiO₂ in NR were investigated. The results obtained were compared with the NR/SiO₂ composites based on traditional dry mixing of bale natural rubber and precipitated silica (white carbon black). The results showed that when using a sulfur curing system with a silica coupling agent (Si69) in C(NR/nSiO₂), the vulcanizate had better mechanical properties, higher wet resistance, and lower rolling resistance than those without Si69. In the composites C(NR/nSiO₂) and their vulcanizates, the SiO₂ particles’ average grain diameter was 60 nm, and the good-dispersion of the in-situ generated SiO₂ in the rubber matrix were a significant contribution to the satisfactory properties of C(NR/nSiO₂) composites and their vulcanizates.     

Relaxation processes in the glassy state: Molecular aspects

Molecular relaxations in the glassy state of amorphous polymers are discussed, and related to the energy requirements of the various possible molecular motions involved under two broad headings: those occurring in groups pendant to the main chain and those occurring in the main chain units themselves. A variety of motions can result in relaxations in polymers with alkane side chains, whereas only rotation (or partial rotation) is possible for pendant phenyl rings. Relaxations in polymers with cycloalkane rings in the side chain can originate within the ring itself or from motion of the ring as a unit. Restricted movements of various types of backbone chain are discussed, and possible alternatives to the postulated crank-shaft motions in polymethylene chains are presented.     

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.     

Performance and Mechanism of Epoxy Resin for Reinforcement of Styrene–Butadiene Rubber

Styrene–butadiene rubber (SBR) vulcanizates reinforced by epoxy resin (EP) have been synthesized by an in-situ vulcanization and curing process. The influences of synthetic parameters, such as the contents of EP, carbon black, and types of compatilizers, on the microstructures, vulcanization, and mechanical properties of SBR have been investigated. It was found that EP in SBR exists in the form of a fibrillar interpenetrating network, which is important for the enhancement of mechanical properties of SBR. The experimental results showed that when the percentage of EP was in the range of 10–20%, the composite materials had the best comprehensive performance. In comparison with pure SBR, the tear strength and the tensile stress at 300% elongation of SBR-EP composite were increased significantly. The method can be applicable for other rubber vulcanizates to improve their mechanical properties.