Rubber Network Research Articles

Recyclability of Photoinduced Cross-Linked EPM Rubber with Anthracene-Grafted Groups: Problems and Their Solutions.

In this paper, we present the formation of reversible covalently cross-linked networks in ethylene propylene rubber with grafted anthracene groups (EPM-g-AN) based on the principles of photoinduced anthracene dimerization. First, an industrial-grade EPM rubber grafted with maleic anhydride functional groups (EPM-g-MA) was modified with 9-anthracenemethanol. By irradiating EPM-g-AN with UV light (365 nm), the anthracene moieties dimerize via [4 + 4]cycloaddition, forming a covalent network. The network cleavage proceeds at high temperatures (>170 °C), even if with considerable (chemical) degradation. Furthermore, one of the degradation routes has been identified by 1H NMR to occur via the ester bond cleavage releasing 9-anthracenemethanol. Nevertheless, the reversibility of cross-linking has been achieved by performing the reverse reaction in decalin. The UV–vis spectroscopy clearly shows that the de-cross-linking process in these conditions is due to the anthracene dimer cleavage. Although the recovery in mechanical properties upon recycling is yet to be optimized, the disclosed results pave the way toward the use of anthracene chemistry in thermally reversible networks with possible industrial perspective applications.

Influence of γ-irradiation on mechanical behaviors of poly methyl-vinyl silicone rubber foams at different temperatures

In this study, the radiation dose effect of γ rays on the temperature-dependent mechanical behaviors of poly methyl-vinyl silicone rubber foams is investigated by uniaxial compression tests. After exposure to a wide radiation dose range from 0 to 500 kGy at room temperature, silicone rubber foams were tested at 10, 40, 70, and 100 °C. The test results show that temperature increase causes linear hardening for both unirradiated and irradiated samples due to the predominating entropy-related deformation of silicone rubber matrix. Moreover, increase of irradiation dose leads to linear hardening as a result of dominating cross-linking reactions of rubber networks. A stress hardening factor is introduced to characterize the effect of irradiation dose on temperature-related hardening behavior, which is defined as the ratio between the compressive stresses of irradiated and unirradiated samples for a given compression strain. The approximated constant stress hardening factor for each radiation dose condition implies that the irradiation dose does not affect the temperature-related hardening behavior. Based on the test findings, the Ogden Hyperfoam model was extended by correlating the initial shear modulus-related model parameters with radiation dose and temperature. The hardening exponent-related model parameters have been shown to be independent of radiation dose and temperature, and the Poisson's ratio-related model parameters are approximated to zero for the high porosity of the material. The prediction performance of the extended model is demonstrated by comparing the fitted model responses with test data over the studied irradiation doses and temperature range. The results are expected to be helpful for the safety evaluation of nuclear engineering structures.

Vulcanizate Structures of SBR Compounds with Silica and Carbon Black Binary Filler Systems at Different Curing Temperatures.

The tire industry has shown an increasing demand for the reduction in rolling resistance. Efforts have been made to improve the viscoelastic properties of tire compounds and reduce the weight of tires through optimization of the vulcanizate structure, which has become extremely complex. In this study, vulcanizates using carbon black and silica as binary fillers were prepared at various curing temperatures. Vulcanizate structures with respect to curing temperature were classified according to the chemical crosslink density by sulfur, carbon black bound rubber (i.e., physical crosslink due to carbon black), and silica-silane–rubber network. All properties exhibited a decreasing trend under the application of high curing temperatures, and the decrease in the crosslink density per unit content of filler with an increase in curing temperature was shown to be greater in carbon black than in silica. Mechanical and viscoelastic properties were also measured to evaluate the impact that the compound variates have on tire tread performance. These results serve as a guideline for determining the content and filler type and for setting the cure condition during the design of actual compound formulations to increase the crosslink density of rubber while retaining the necessary mechanical and viscoelastic properties for practical application.

Preparation of intrinsic antibacterial silicone rubber with matrix tethering quaternary ammonium salt groups

The immobilization of the antibacterial agent in the matrix is crucial to endow materials with excellent antibacterial activity. In this paper, a strategy was presented to prevent the antibacterial agent leaching out from the cross-linking network of silicone rubber via construction of the matrix tethering quaternary ammonium salt (QAS) groups by covalent bond. A series of hydroxyl-terminated polysiloxanes with pendent QAS groups were prepared and characterized. The polysiloxanes were subsequently used as a matrix for preparing an intrinsic antibacterial silicone rubber. SEM analysis indicated a high compatibility of the formulations of silicone rubber. The silicone rubber exhibits excellent antibacterial activity against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). Most importantly, the silicone rubber exhibits prominent durability of antibacterial activity even after staying in the natural environment for 3 months. Moreover, the hydrophilicity of silicone rubber could be adjusted by the content of QAS groups.

Experimental investigation of the nonlinear quasi-static and dynamic mechanical behaviour of novel PA6/XHNBR thermoplastic vulcanizates: Linking mechanical nonlinearities to microstructural features

Driven by the need to design environmentally friendly and sustainable polymeric materials that achieve superior mechanical properties, this work is centred on the relationship between the microstructure and the nonlinear quasi-static and dynamic mechanical behaviour of novel polyamide 6 (PA6)/carboxylated hydrogenated acrylonitrile butadiene rubber (XHNBR) thermoplastic vulcanizates (TPVs). In particular, with the aim to contribute to the further understanding of the physical mechanisms involved in the complex nonlinear mechanical behaviour of TPV compounds, the impact of different microstructural features of the PA6 matrix and the rubber network on the mechanical behaviour has been investigated. Interestingly, stronger rubber network interactions and so better elastic recovery capacity of the rubber network can hinder the growth of voids more efficiently. The observed micromechanical deformation processes are interpreted in terms of the role of the yielding, buckling and bending of thin PA6 ligaments at low strain levels and the rupture of covalent and non-covalent bonds presented in the rubber network at high strain levels.

Effects of recycled carbon-based materials on tyre

This study examined the effect of recycled carbon-based material (rCBM), obtained by chemical degradation of end-of-life tires, on the mixture of tire tread (rubber composite) by comparing it with commercial carbon black (CB-N550). The new chemical degradation method was defined and the new rCBM was obtained. This newly method is more advantageous than other methods (devulcanization and pyrolysis). The rheometeric properties, mechanical properties, hardness, morphological properties, functional structures, thermal degradation behaviours of the rubber composites prepared were determined. The total number of bonds decreased as the amount of rCBM increased in the rubber composite mixture. As the vulcanized rubber network of the rCBM-filled rubber decreases, the cross-linking density of the rCBM-filled rubber was also reduced. As rCBM has a high sulfur value, the degree of chemical cross-linking of rCBM was reduced. In conclusion, rCBM will not be used in place of commercial carbon black but may be used by mixing with commercial carbon black depending on the desired properties of the end products.

Accelerated Aging Behaviors and Mechanism of Fluoroelastomer in Lubricating Oil Medium

The aging behaviors and mechanism of fluoroelastomer (FKM) under lubricating oil (FKM-O) and air (FKM-A, as a comparison) at elevated temperatures were studied from both physical and chemical viewpoints. The obvious changes of mechanical and swelling performances indicate that the coupling effect of lubricating oil and temperature causes more serious deterioration of FKM-O compared to that of FKM-A. Meanwhile, much stronger temperature dependence of both bulk properties and micro-structures for FKM-O is found. Three-stage physical diffusion process is defined in FKM-O due to the competition between oil diffusion and elastic retraction of network. FTIR results reveal that the dehydrofluorination reaction causes the fracture of C-F bonds and produces a large number of C-C bonds in the backbone. The coupling effect of oil medium and high temperature could accelerate the scission of C-C bonds and generate a series of fragments with different molecular sizes. The TGA results, crosslinking density Ve, and glass transition temperature Tg derived from different measurements coherently demonstrate the network destruction in the initial stage and the simultaneous reconstruction occurring at the final stage. The newly formed local network induced by reconstruction cannot compensate the break of the original rubber network and thus only provides lower tensile strength and thermal stability.

VULCANIZATE STRUCTURES AND MECHANICAL PROPERTIES OF RUBBER COMPOUNDS WITH SILICA AND CARBON BLACK BINARY FILLER SYSTEMS

ABSTRACTThe physical properties of rubber compounds are mainly determined by the filler dispersion within the rubber matrix, filler–rubber interaction, and chemical crosslink structure caused by sulfur. Carbon black or silica is typically used as a reinforcing filler in tire tread compounds; however, binary filler systems comprising the two types of filler are also currently being used to complement each other. This study used binary filler systems to manufacture vulcanizates and classified the vulcanizate structures as chemical crosslinks caused by sulfur, physical crosslinks caused by carbon black (carbon black–bound rubber), and silica–silane–rubber networks caused by silica and silane. The effect of each vulcanizate structure on the physical properties was also calculated. In the proposed binary filler system, silica chemically bonds with rubber molecules, unlike carbon black. Therefore, the crosslink density per unit of silica content was 19% higher than that of carbon black, in which rubber molecules were physically adsorbed on the surface. Tensile properties affected by 1 unit of crosslinking density for each filler were calculated, and silica was found to contribute more in the low-elongation range, whereas carbon black contributed more in the high-elongation range. Regarding tan δ at 60 °C and abrasion resistance per unit crosslink density of filler, carbon black made a greater contribution than silica, whereas silica had a greater contribution to wet traction and snow traction.

Strain-Induced Crystallization in Natural Rubber: Flory’s Theory Revisited

In this paper, some aspects of the physical mechanisms involved in strain-induced crystallization (SIC) in cross-linked natural rubber networks are discussed. The theory of SIC as developed by Flory is considered within a somehow innovative perspective, in analogy to the liquid–gas phase transformation. A simple lever rule is proposed to relate the crystallinity index to the local and global draw ratios. Equilibrium properties are considered first. Some simple experiments are discussed within this framework to enlighten some fundamental aspects of SIC. As the order parameter for SIC is the draw ratio of the amorphous phase, the importance of directly measuring this parameter is emphasized. Special emphasis is put on the relaxation of the remaining amorphous fraction that accompanies SIC. The question of crystallization kinetics underlies most aspects of SIC. We show that the actual time dependence of the crystalline content may be related to the mechanism of strain relaxation in a quite simple manner using the lever rule mentioned above. Crystallization kinetics is also fundamental to explain the hysteretic behavior observed in dynamical conditions. Similarities and differences with static SIC are discussed. We transpose the classical theory of nucleation to the case of SIC, and we discuss the kinetics within this framework.

Entanglement Locking in the Unique Elasticity of Polydimethylsiloxane Rubbers

AbstractThe viscoelasticity behavior of the polydimethylsiloxane (PDMS) rubber blends of high molecular weight polymers modified by low molecular weight agents is studied in compression mode by stress–strain curves, the creep and step‐strain, and dynamic‐mechanical experiments. The strain spectra highlight the dynamics of active chains at high frequencies and of dangling ends in the low frequencies field. At low deformation, the blending agent enhances the elastic properties by increasing the density of the active chains in the rubber network, in agreement with the classical theories of elasticity. At high deformation, in analogy with the rheological studies on similar liquid blends, the Entanglement Locking model is proposed: the short chains of the blending agent are adsorbed on entanglement sites of long dangling chains, giving effective crosslinks via high entropy dynamics. In this way, the long‐term entanglement locking enhances the density of active chains and elastic behavior. At high deformation, the entanglement locking model enriches the rubber elasticity theories according to the tube network model and Mooney–Rivlin equation. The models herein and the possibility they offer for improving rubber viscoelasticity are valuable for the development of polymer physics and technology.

Strain and filler ratio transitions from chains network to filler network damage in EPDM during single and cyclic loadings

Chains and filler network damage were investigated during single and multiple cycles on a series of vulcanized EPDM containing various filler contents. In both series of experiments, a strain and a filler ratio transitions for damage mechanisms were identified. For low filler content (≤40 phr), damage mechanisms mostly occur in the elastically active rubber network consistent with the chains network alteration theory associated with irreversible chains scission/bond breakage. For high filler content (>40 phr), a strain transition occurs with damage initially located in the elastically active rubber network, but subsequently localizes in the vicinity of the filler-filler network. This is ascribed to filler re-aggregation with strain, improving its load-bearing capacity, that may release the immobilized rubber formed by the chains that are occluded and bonded to fillers. During cyclic experiments, such reversible release involving loss of weak physical bonds and chains slippage yields in a progressive cavity closing with cyclic accumulation that prevents further irreversible damage of the elastically active rubber network.

Flexible Liquid Sensor Based on Rubber/Graphite Composite

Hydrocarbon liquids such as gasoline, diesel, jet fuel, and solvents are hazardous materials derived from petroleum. These materials can diffuse the rubber network structures and cause swelling in these polymers. Rubber materials containing conductive fillers exhibit a high conductivity at the higher percolation threshold. As direct effect of solvent on the polymer, swelling in the rubber composite alters the structure of the filler network and subsequently the composite properties change significantly. This Process can be considered as a signal for solvent or hydrocarbon fuel detector system and used conductive rubber composite as a flexible sensor. In this study, the nitrile/graphite composite samples were prepared containing different amounts of graphite particles. These samples have the ability to measure electrical resistance. The electrical resistance of rubber/graphite samples decreases with increasing content of graphite particles. The increase in the electrical resistance of the specimens was measured by using conductive composite swelling in toluene solvent and it was observed that all specimens eventually became electrical insulation. Samples with concentrations of 60, 70, and 80 phr are conductive after recovery and solvent removal. While their conductivity is less than that of the virgin samples. The incremental trend of electrical resistance against the conductive composite swelling was measured in toluene solvent and it was observed that all samples were eventually converted to electrical insulation. But there is little difference between the third swelling process and the second one. This phenomenon has occurred for all three samples, which reveals a good similarity with the Mullins effect.

Scaling laws of Mullins effect in nitrile butadiene rubber nanocomposites

Rubber nanocomposites experiencing cyclic deformation undoubtedly exhibit Mullins effect whose underlining mechanisms are not yet clear. Herein this effect in nitrile butadiene rubber nanocomposites is systematically investigated for revealing the influences of pre-strain interval, loading and unloading velocities, temperature, filler type and content, as well as crosslinking agent. The results show that the recovery hysteresis energy and accumulative softening energy of the nanocomposites can be superposed onto master curves as a function of microscopic strain of the rubber phase, revealing that both involving the viscoelastic deformation of the rubber phase. Especially the recovery hysteresis highly depending on temperature and loading and unloading velocities is connected to the viscoelasticity of nonideally crosslinked rubber network in the nanocomposites. On the other hand, the accumulative softening energy loss comes from recovery retardation of rubber chains and is somewhat sensitive to the filler, temperature and crosslinking agent. The investigation would be instructive to clarify the physical origin of Mullins effect to produce low dissipation rubber nanocomposites.

The Effect of Palmitic Acid Loadings on the Shape Memory Cycle of Shape Memory Natural Rubber

This article investigates the effect of palmitic acid loadings on the shape memory cycle testing of shape memory natural rubber (SMNR) fabricated via prevulcanisation method. By incorporating the palmitic acid, the strain-induce crystal (SIC) formation between the SMNR molecules possess the ability of demonstating the shape memory effect (SME) above the room tempeature. To analyse the SIC formation on prevulcanised SMNR, the shape memory cycling tests were investigated as a function of palmatic acid loadings under strain control mode. From the observation of TGA results, various palmitic acid contents of SMNR were fabricated under prevulcanisation method but the palmitic acid loading is different from the palmitic acid amount used in the mixing process. Furthermore, it was also found that shape fixity of the SMNR developed can be improved by increasing the palmitic acid loadings but the shape fixity results starts to decline with increasing number of shape memory cycle due to the bleeding of palmatic acid. It indicates that the palmitic acid content has signficant influence on the SIC formation. On the other hand,the specimens still remain the elastic property which reform back to orginal length upon recovery process even after repeatative shape memory test. It is believed that the material properties of SMNR still were dominated by the natural rubber network during the recovery process.

Giving a Second Opportunity to Tire Waste: An Alternative Path for the Development of Sustainable Self-Healing Styrene-Butadiene Rubber Compounds Overcoming the Magic Triangle of Tires.

Current regulations demand tires with long lifetime and reduced fuel consumption without sacrificing car safety. However, tire technology still needs to reach a suitable balance between these three indicators. Here, we address them by developing a self-healing tire compound using styrene–butadiene rubber (SBR) as the matrix and reclaimed tire waste as the sustainable filler. The addition of ground tire rubber (GTR) to the matrix simultaneously improved the rolling resistance and maintained both wet grip and healing ability. We provide an in-depth analysis of the healing behavior of the material at a scale close to the relevant molecular processes through a systematic dynamic-mechanical and dielectric analysis. We found that SBR and SBR/GTR compounds show a complete recovery of stiffness and relaxation dynamics after being damaged by cyclic deformation, resulting in a heterogeneous repaired rubber network. This new development could well overcome the so-called magic triangle of tires, which is certainly one of the key objectives of the tire industry.

Influence of selective distribution of SiO2 nanoparticles on shape memory behavior of co-continuous PLA/NR/SiO2 TPVs

As a kind of commercial thermoplastic elastomers, thermoplastic vulcanizates (TPVs) have been widely used in many fields. In this work, a shape memory polymer (SMP) was fabricated based on co-continuous polylactide (PLA)/natural rubber (NR)/silica (SiO2) TPVs. Selective distribution of SiO2 nanoparticles was achieved with the impact of thermodynamic factor and different processing methods, and the influence on shape memory behavior was investigated as well. The results demonstrated that the co-continuous structure functioned actively in both shape fixity and recovery. The PLA phase played the role of “switch” to control the shape memory behavior, while the deformation of continuous rubber network stored sufficient resilience and successfully drove the shape recovery process. Excellent shape fixing ratio (~100%) and shape recovery ratio (>90%) were obtained when SiO2 nanoparticles were distributed in NR phase. The addition of SiO2 nanoparticles reinforced the rubber phase and the recovery ratio was effectively improved.

Effects of tension fatigue on the structure and properties of carbon black filled-SBR and SBR/TPI blends

Abstract The aim of this study was to explore the impact of tension fatigue on the structure and properties of filled SBR and SBR/TPI blends. The effect of tension fatigue on the dynamic properties of carbon black-filled styrene-butadiene rubber (SBR) and SBR/trans-1,4-polyisoprene (SBR/TPI) blend vulcanizates were investigated by dynamic mechanical analysis (DMA). The Mooney-Rivlin analysis of tensile stress-strain data is used for the determination of a rubber network crosslink density. The fatigue fracture surface of SBR/TPI vulcanizates was observed with a scanning electron microscopy (SEM). The crystallinity of TPI in carbon black-filled SBR/TPI (80/20) was characterized by X-ray diffraction (XRD). The results showed that the incorporation of TPI into SBR vulcanizates influences the fatigue properties of the blend vulcanizates. The blend vulcanizates showed optimum fatigue properties with 20 phr TPI. With increasing fatigue cycles, the tensile properties and crosslink density of SBR vulcanizates were decreased substantially. Compared with that of SBR vulcanizates, the tensile properties and crosslink density of SBR/TPI (80/20) blend vulcanizates changed little with the increase in fatigue cycles, and tan δ and E′ decreased gradually with the fatigue cycles. There was a sharp decrease in the E′ and tan δ curve in the temperature range of 40 ~ 60°C. The XRD diffraction peak corresponding to 3.9 Å broadened when the fatigue cycles were increased to 1 million times, and a new peak with inter-planar spacing at 7.6 and 4.7 Å appeared.

Recyclable and self-healing rubber composites based on thermorevesible dynamic covalent bonding

Commercial ethylene propylene diene monomer (EPDM) rubber grafted with maleic anhydride (EPDM-g-MA) was thermoreversibly crosslinked by silane modified silica. EPDM-g-MA was first modified with furfurylamine to obtain furan functionalized EPDM (EPDM-g-FA) which was then crosslinked with 3-methacryloxypropyltrimethoxysilane (as electron-poor agent) modified silica via a Diels-Alder reaction. The as-formed rubber network could be broken at high temperature and reconstructed by thermal annealing, which were proven by differential scanning calorimetric analysis and solubility testing. The mechanical strength of the resulting EPDM/silica composites could be tailored by the amount of modified silica and were superior to the previously reported EPDM rubber crosslinked by low molecular organic agents. More importantly, the rubber composites showed good thermal reprocessability and self-healing behavior, by which the crosslinked composites could be recycled to use with comparable mechanical property as the original composites.

Using cellulose nanocrystals as sustainable additive to enhance mechanical and shape memory properties of PLA/ENR thermoplastic vulcanizates

Cellulose nanocrystals (CNC) show great potential in polymers due to their sustainability and excellent mechanical properties, etc. Herein, we used CNC as sustainable additive to improve mechanical and shape memory properties of polylactide (PLA)/epoxidized natural rubber (ENR) thermoplastic vulcanizates (TPVs). A method of combining latex mixing and melt mixing was used to achieve homogeneous dispersion of CNC. With only 5 phr (relative to ENR) CNC, tensile strength, Young’s modulus and dynamic storage modulus at 20 ℃ increased by 22 %, 64 % and 78 % compared with unfilled sample. The shape recovery behavior also improved with the incorporation of CNC, which increased from 87.4 % of unfilled TPV to 91.7 % of sample with 5 phr CNC. Moreover, the decline of Rr in the subsequent cycles also reduced with incorporation of CNC. These results indicate that the enhanced cross-linked rubber network and increased crystallinity of PLA contributed to the enhanced properties.

Diffusion coefficient of solute in heterogeneous and macroporous hydrogels and its correlation with the effective crosslinking density

The diffusion coefficient, Dg, of a solute in hydrogels is a key parameter in their applications. Dg has been correlated with the volume fraction, φp, of polymer in gel. In this study, the effective crosslinking density, νe, was determined from the rubber network theory using compression tests, and was applied as a model parameter for the characterization of the polymer networks and gel structures. It was subsequently examined to estimate Dg for various types of gels, including heterogeneous and macroporous gels. The Dg values of NaCl and trisodium citrate were determined with a permeability test using the gel membrane. The diffusion of the solute in all the tested gels was accurately represented by both conventional (Dg/D0 = exp(-kφpn)) and novel (Dg/D0 = exp(-aνeb)) models.