Rubber Network Research Articles

Mechanism of the foaming agent‐assisted microwave drying process on the construction of natural raw rubber network and cross‐linking network

AbstractThe drying process of natural rubber latex significantly affects the structure of the raw rubber network and vulcanizate crosslinking network, resulting in various anti‐aging performances. In the present study, a microwave generator was used as an efficient source of clean energy; potassium oleate was introduced as a foaming agent to increase the porosity and water loss channel of the latex system. Aiming at dehydrating and drying natural rubber latex efficiently, an aging resistant rubber composite was prepared. Meanwhile, the mechanism of the foaming agent‐assisted microwave drying process on the raw rubber network and the cross‐linking network was studied. The experimental results show that the prepared rubber using by this process has higher plastic retention and fluidity. Moreover, it contains more non‐rubber components (e.g. protein and acetone extract) and better network structure of raw rubber and vulcanized rubber. It is found that applying this process increases the tensile product by 13.5% and the retention rate of the tensile product after aging by 15.3 times. This process is important for the development of the rubber industry in the direction of green environmental protection, energy conservation, and high efficiency.

Influence of liquid polybutadiene on crosslinking network and mechanical properties of SSBR/BR

AbstractThe treated distillate aromatic extract (TDAE) is widely used in tread compounds. However, polycyclic aromatic hydrocarbons (PAHs) contained in TDAE is serious harm to the environment and human. In this paper, liquid polybutadiene (LPB) of different brands was used to replace TDAE to plasticize solution polymerized styrene‐butadiene rubber/cis‐1,4‐polybutadiene rubber (SSBR/BR). The effects of liquid polybutadiene instead of TDAE on crosslinking network and mechanical properties of matrix rubber were studied. LPB could participate in vulcanization reaction (different from TDAE). The results showed that LPB effectively reduced the critical strain amplitude of Payne effect (ameliorated filler dispersion) compared with TDAE plasticizing SSBR/BR. The degree of crosslinking decreased slightly, but there was no significant difference in rolling resistance and wet skid resistance at the same sulfur content. Changing the sulfur content, the wet skid resistance of tread and tensile strength of SSBR/BR vulcanizate plasticized by LPB were significantly improved, and the effect of LPB with high 1,2‐vinyl content was better. In a word, LPB as a new environmental protection plasticizer can be used to effectively replace the TDAE plasticizing SSBR/BR.

Influence of Sulfur-Curing Conditions on the Dynamics and Crosslinking of Rubber Networks: A Time-Domain NMR Study.

The characterization of the structural and dynamic properties of rubber networks is of fundamental importance in rubber science and technology to design materials with optimized mechanical properties. In this work, natural and isoprene rubber networks obtained by curing at three different temperatures (140, 150, and 170 °C) and three different sulfur contents (1, 2, and 3 phr) in the presence of a 3 phr accelerator were studied using a combination of low-field time-domain NMR (TD-NMR) techniques, including 1H multiple-quantum experiments for the measurement of residual dipolar couplings (Dres), the application of the Carr–Purcell–Meiboom–Gill pulse sequence for the measurement of the transverse magnetization decay and the extraction of 1H T2 relaxation times, and the use of field cycling NMR relaxometry for the determination of T1 relaxation times. The microscopic properties determined by TD-NMR experiments were discussed in comparison with the macroscopic properties obtained using equilibrium swelling, moving die rheometer, and calorimetric techniques. The obtained correlations between NMR observables, crosslink density values, maximum torque values, and glass transition temperatures provided insights into the effects of the vulcanization temperature and accelerator/sulfur ratio on the structure of the polymer networks, as well as on the effects of crosslinking on the segmental dynamics of elastomers. Dres and T2 were found to show linear correlations with the crosslink density determined by equilibrium swelling, while T1 depends on the local dynamics of polymer segments related to the glass transition, which is also affected by chemical modifications of the polymer chains occurring during vulcanization.

Shape‐Memory Semicrystalline Polymeric Materials Based on Various Rubbers

AbstractA simple strategy is presented for the fabrication of shape‐memory materials containing commercial rubbers including natural rubber, cis‐polybutadiene, and styrene‐butadiene rubber. Dissolution of the rubbers in n‐octadecyl acrylate (C18A) monomer followed by UV polymerization at 30 °C leads to the formation of interconnected interpenetrating polymer networks (c‐IPNs) possessing crystalline domains. They exhibit melting (Tm) and crystallization temperatures (Tcry) between 45–50 and 35–40 °C, respectively, that can be tuned by the amount and the type of the rubbers. All c‐IPNs exhibit a significant temperature sensitivity in their viscoelastic and mechanical properties when the temperature is changed between below and above Tm and Tcry. The morphology of c‐IPNs consists of amorphous nanoparticles of around 64 nm diameter composed of interconnected noncrystalline poly(C18A) (PC18A) and rubber networks, surrounded by crystalline PC18A segments. c‐IPNs exhibit tunable mechanical properties, for example, their Young's modulus and toughness can be varied between 8.3–73 MPa, and 1.9–23 MJ·m−3, respectively, by changing the amount and type of the rubber. Because of the coexistence of chemical cross‐links and crystalline domains acting as the netpoints and switching segments, respectively, c‐IPNs exhibit an efficient shape‐memory function as demonstrated by their potential application as a robotic gripper.

Design of High-Performance Rubber Materials for Sports: Challenge by Control Technology of Rubber Networks

Design of High-Performance Rubber Materials for Sports: Challenge by Control Technology of Rubber Networks

Enhancement of thermal energy absorption/storage performance of paraffin wax (PW) phase change material by means of chemically synthesized Ethylene Propylene Diene Monomer (EPDM) rubber network

Phase change materials (PCMs) are kind of energy storage systems utilized for thermal energy storage (TES) by virtue of high fusion latent heat property. In this research, Paraffin wax (PW) PCM and Ethylene-Propylene-Diene-Monomer (EPDM) were Vulcanized together by using various Benzoyl Peroxide contents to determine EPDM rubber network characteristics required for low leakage and high thermal performance. PCM systems were developed using vulcanized EPDM impregnation in high temperature molten PW. It is depicted that PW PCM system containing 5 wt.% Benzoyl Peroxide for EPDM curing has resulted in significantly higher thermal characteristics, while leakage was merely 0.81%, which is negligible and achieved along with PW shape stability in rubber network. Time -temperature performance of optimum PCM denotes that PW PCM presence could decrease rate of temperature change significantly. Furthermore, for adequate PCM system, more than 60% of inflow heat flux was absorbed.

Unravelling the effect of healing conditions and vulcanizing additives on the healing performance of rubber networks

Self-healing rubbers have steadily been growing during the last decades. Various strategies have been studied to transform the vulcanized rubber into a reversible cross-linked network with multiple repair cycles. However, a detailed description on how the chemical constituents of a rubber formulation affect the healing properties of the vulcanizate is unknown and has not been reported until now. In this research, the individual effects of both the curing agent (sulfur and peroxide) and the activating complex (zinc oxide and stearic acid) have been assessed on the healing capability of a styrene-butadiene rubber (SBR) compound. The analysis of different healing conditions revealed that pristine materials should be subjected to the same thermal treatment as the healed samples to obtain scientifically sound values of the healing efficiency. Results also showed that each component affected the healing at different time scales, with the activating complex being responsible for the initial physical healing stage. Meanwhile, sulfur would act at later stages as dynamic healing moiety, enabling disulfide exchange reactions. In conclusion, the mechanical recovery of a rubber network can be potentially tailored depending on the relation between the chemical components of the rubber formulation.

A unified mechanical based approach to fracture properties estimates of rubbers subjected to aging

In this work, the influence of aging on the mechanical properties at break of rubber materials are examined. When subjected to physical-chemical aging, the structure of the rubber network is deeply modified through two main mechanisms: crosslinking and chain scission. These aging mechanisms act both on the mechanical behaviour and on the fracture properties of the rubber materials.The goal of this work is to propose a predictive mechanical tool able to give estimates of these mechanical properties. To address this issue, an hyperelastic constitutive model based upon the energy limiter approach, was coupled with physical-chemical parameters in order to capture the whole mechanical behaviour of the damaged materials beyond the fracture. That allows fracture stress and strain estimates.A wide set of materials and experimental data extracted from the literature or obtained in our team were selected based upon the aging mechanism they can exhibit.We found that the elastically active chains concentrations and the swelling rate can be used as relevant indicators of damage either for crosslinking or chain scission process.These parameters are then introduced as damage parameters in the mechanical modelling. The proposed approach leads to very satisfactory predictions, either to capture the whole mechanical behaviour in uniaxial tension or in terms of stress and strain at break estimates.

Pickering emulsion template method induced highly oriented functionalized graphene oxide in PDMS composites to achieve high gas barrier properties

Constructing oriented layered filler networks in methyl vinyl silicone rubber (PDMS) composites is an effective strategy to enhance their gas barrier properties. Herein, GO was first functionalized by sodium dodecyl benzene sulfonate (SDBS) and cetyltrimethylammonium bromide (CTAB) to form amphiphilic GO (SCGO). Furthermore, SCGO and PDMS were mixed to form the water-in-oil type SCGO/PDMS Pickering emulsions. A porous PDMS masterbatch contained hollow SCGO shells was further obtained through freeze-drying the SCGO/PDMS Pickering emulsions. Followed by hot pressing, these hollow shells composed of SCGO sheets collapsed to form the oriented filler networks in the PDMS composites. When the content of SCGO was 3 phr, the gas barrier properties of SCGO/PDMS composites increased 85.7% relative to the pure PDMS. Meanwhile, the gas barrier properties of the SCGO/PDMS composites with oriented filler networks were 20–30% higher than those of the SCGO/PDMS composites with random filler networks at the same filler content.

Conductive, self-healing and recyclable electrodes for dielectric elastomer generator with high energy density

Dielectric elastomer generator (DEG), consisting of highly deformable dielectric elastomer (DE) film sandwiched between two compliant electrodes, can convert mechanical energy into electrical energy. To achieve high energy harvesting performance and practical durability, electrode material with high stability of electrical conductivity (EC) and self-healing/recycling capability is required. In this study, we designed and prepared conductive rubber electrode with high EC, high stability of EC at high strain and high durability during cyclic stretching, self-healing and recycling ability for DEG by synthesizing a hydrogen bond crosslinked supramolecular network of silicone rubber (SiR-SN) followed by introducing a commercially available, highly conductive carbon black (CB) and carbon grease (CG) into SiR-SN matrix. The addition of CG composed of silicone oligomer (SO) and CB can induce the partial phase separation of polar SiR-SN and the formation of segregated conductive network, leading to the significantly enhanced EC and stability of EC at high strain. The presence of SO chains also increases the chain mobility, leading to the high durability, self-healing ability and recyclability of the electrode. Compared with CG electrode commonly used in laboratory, the energy loss at 3 kV and 400% strain of DEG using the 5 phr CB/60 phr CG/SiR-SN electrode is reduced by 83.5%, and the harvested energy density increases by 73.1%, which reaches 23 mJ/cm3, much higher than that reported in previous studies. Meanwhile, this electrode shows high self-healing efficiency of conductivity (92% at 60 °C for 4 h), and can be recycled for 5 times without negative effect on performance.

Relationship between macromolecular network and fatigue properties of unfilled polychloroprene rubber

Fatigue life of unfilled polychloroprene rubber is characterized in fully relaxing (R = 0) and non-relaxing conditions (R = 0.2) for more than 15 network structures. Network changes are induced by thermal oxidation ageing of the rubber and characterized especially in terms of crosslink density. For the first time, in the case of unfilled elastomers, we show that an increase in crosslink density leads to a decrease in fatigue life, which cannot be attributed to large changes in strain-induced crystallization processes. The results obtained here are used to determine the relationship between network structure and fatigue properties. In fully relaxing conditions, it is possible to predict fatigue lifetime by considering crosslink density using a theoretical energetic approach. Nevertheless, the same approach does not work in non-relaxing conditions. In the latter case, a new empirical relationship is proposed to link fatigue life to the crosslink density of rubber.

Quantitatively predicting the mechanical behavior of elastomers via fully atomistic molecular dynamics simulation

Employing computer simulation to accurately and quantitatively predict the mechanical properties of polymeric materials is a long-standing challenge. Herein, we consider two methods to simulate the mechanical properties based on a fully atomistic model of crosslinked styrene-butadiene rubber (SBR) network via molecular dynamics simulation. In the first method (stepwise deformation and relaxation), by keeping the tensile velocity constant and regulating the simulation time that each deformation period takes, sufficient relaxation occurs in the consecutive deformation, which enables each deformation period followed by a corresponding relaxation period. The engineering stress-strain curve finally obtained by this stepwise deformation and relaxation method exhibits similar features of elastomers compared with the experimental results. The second method divides the whole deformation process into several discontinuous parts. Sufficient relaxation also occurs in several independent SBR systems at the specific strain and the relaxed stress plateau is indicated to represent the stress that SBR system exhibits at specific strain after allowing the system to relax. Compared with the traditional uniaxial tensile method (continuous deformation in constant strain rate) and the first method we adopt, the second method could also characterize the mechanical behavior of elastomers and improve the computational capability on the level of a large-scale system. Compared with the excessively large values obtained by traditional method, the elastic modulus, the modulus at 100 %, 300 % and 500 % of the full atomistic SBR are calculated as 38.5 MPa, 12.0 MPa, 25.1 MPa and 83.1 MPa, respectively, which exhibits more realistic mechanical behavior and merely exceeds around one order of magnitude than the experimental results. These methods are believed to serve as a good basis for further accurately simulating and predicting the mechanical properties of elastomers.

Design of PLA/ENR thermoplastic vulcanizates with balanced stiffness‐toughness based on rubber reinforcement and selective distribution of modified silica

AbstractElastomer‐toughened PLA has incubated plenty outstanding achievements. Principally, increasing elastomer content will immolate vast rigidity in exchange for toughness. In this work, silane coupler KH550‐treated SiO2 and ENR were used to synergistically enhance and toughen PLA via dynamic vulcanization. It is found that the PLA/ENR/SiO2 TPV with balanced stiffness and toughness exhibited a bi‐continuous phase structure. Thereinto, the hydrophobic SiO2 nanoparticles were regulated to uniformly disperse in the ENR phase, which played an essential role in reinforcing the rubber and bracing the symmetrical rubber network, so as to toughen the PLA with retaining high stiffness at an acceptable plastic‐to‐rubber ratio. By incorporating nano‐SiO2 modified with 10 phr KH550, the TPV could obtain an impact strength of 23.7 kJ/m2 at 0°C while maintain a tensile strength of 42.1 MPa, which were 1.62‐fold and 1.03‐fold of those before modification, respectively. The synergistic enhancing and toughening mechanism of nano‐SiO2 and ENR was revealed through a series of test and analysis, which furnished a succinct and effective proposition of fabricating high‐performance PLA materials.

A Self-Healing System Based on Ester Crosslinks for Carbon Black-Filled Rubber Compounds

Carbon black-reinforced rubber compounds based on the blends of natural rubber (NR) and butadiene rubber (BR) for tire sidewall applications were formulated to investigate the self-healing efficacy of a modifier called EMZ. This modifier is based on epoxidized natural rubber (ENR) modified with hydrolyzed maleic anhydride (HMA) as the ester crosslinking agent plus zinc acetate dihydrate (ZAD) as the transesterification catalyst. The influence of EMZ modifier content in sidewall compounds on processing characteristics, reinforcement, mechanical and fatigue properties, as well as property retentions, was investigated. Increasing the content of EMZ, the dump temperatures and Mooney viscosities of the compounds slightly increase, attributed to the presence of extra polymer networks and filler–rubber interactions. The bound rubber content and Payne effect show a good correction that essentially supports that the EMZ modifier gives enhanced filler–rubber interaction and reduced filler–filler interaction, reflecting the improved homogeneity of the composites. This is the key contribution to a better flex cracking resistance and a high fatigue-to-failure resistance when utilizing the EMZ modifier. To validate the property retentions, molecular damages were introduced to vulcanizates using a tensile stress–strain cyclic test following the Mullins effect concept. The property retentions are significantly enhanced with increasing EMZ content because the EMZ self-healing modifier provides reversible or dynamic ester linkages that potentially enable a bond-interchange mechanism of the crosslinks, leading to the intermolecular reparation of the rubber network.

Rubber-reinforced rubbers toward the combination of high reinforcement and low energy loss

Reinforcement is a fundamental issue in rubber science and engineering. Traditionally, rubber reinforcement relies on the incorporation of rigid particles into rubber network or creation of the rubber network with dual-crosslink. However, all these methods inevitably sacrifice the unique elasticity of rubber, which is a shackle toward some crucial applications such as energy-saving tires. In this contribution, a novel strategy of rubber-reinforced rubber (3R) with the combination of significant reinforcement and high elasticity is proposed. With experimental/simulating integrated method, a brand-new reinforcing mechanism for rubber is disclosed, called soft matrix-induced large extensionability of hard rubber (“SMILE-HR”). The soft matrix endows the highly-crosslinked rubber particles (HPs) with the capacity of large extensionablity, so the molecular chains in HPs can be highly oriented and thus obtain high strength during stretching, which are primarily responsible for the dramatic reinforcement of 3R. This skillful combination of these two elastic constituents conquers the stubborn contradiction between reinforcement and elasticity, which is very promising for the engineering rubbers served in dynamic environment.

Electro-interpenetration as tool for high strain trilayer conducting polymer actuator

Electrochemical actuators based on electronic conducting polymers can convert electrical energy into mechanical work. The amplitude of the electromechanical response is related on volumetric charge density of conducting polymer electrodes and strain-to-charge ratio, an empiric electromechanical coupling parameter. In this work we describe the elaboration of high performance trilayer bending actuators by the electro-interpenetration of a conducting polymer, i.e. poly(3,4-ethylenedioxythiophene) (PEDOT), within a host membrane. The host membrane is first elaborated as an interpenetrating polymer networks combining nitrile butadiene rubber network and poly(ethylene oxide) network. An oxidative chemical prepolymerization of EDOT allows turning the host membrane into conductive substrate suitable for subsequent electropolymerization. The electropolymerization of EDOT from the chemically polymerized layer allows controlled interpenetration of PEDOT underneath the surface. A significant increase of local concentration of PEDOT in the electrodes, compared to those obtained from oxidative polymerization lead to a significant increase of the charge density. The polymerization conditions do not have significant impact on sign and absolute value of strain-to-charge ratio, but it can be improved by the nature of the electrolyte. The combination of these two effects leads to an actuator with up to 15% of strain difference.

Alleviating Molecular-Scale Damages in Silica-Reinforced Natural Rubber Compounds by a Self-Healing Modifier.

The property retentions of silica-reinforced natural rubber vulcanizates with various contents of a self-healing modifier called EMZ, which is based on epoxidized natural rubber (ENR) modified with hydrolyzed maleic anhydride (HMA) as an ester crosslinking agent plus zinc acetate dihydrate (ZAD) as a transesterification catalyst, were investigated. To validate its self-healing efficiency, the molecular-scale damages were introduced to vulcanizates using a tensile stress–strain cyclic test following the Mullins effect concept. The processing characteristics, reinforcing indicators, and physicomechanical and viscoelastic properties of the compounds were evaluated to identify the influences of plausible interactions in the system. Overall results demonstrate that the property retentions are significantly enhanced with increasing EMZ content at elevated treatment temperatures, because the EMZ modifier potentially contributes to reversible linkages leading to the intermolecular reparation of rubber network. Furthermore, a thermally annealing treatment of the damaged vulcanizates at a high temperature, e.g., 120 °C, substantially enhances the property recovery degree, most likely due to an impact of the transesterification reaction of the ester crosslinks adjacent to the molecular damages. This reaction can enable bond interchanges of the ester crosslinks, resulting in the feasibly exchanged positions of the ester crosslinks between the broken rubber molecules and, thus, achievable self-reparation of the damages.

Effect of the Strain Rate on Damage in Filled EPDM during Single and Cyclic Loadings.

The effect of the strain rate on damage in carbon black filled Ethylene Propylene Diene Monomer rubber (EPDM)stretched during single and multiple uniaxial loading is investigated. This has been performed by analyzing the stress–strain response, the evolution of damage by Digital Image Correlation (DIC), the associated dissipative heat source by InfraRed thermography (IR), and the chains network damage by swelling. The strain rates were selected to cover the transition from quasi-static to medium strain rate conditions. In single loading conditions, the increase of the strain rate yields in a preferential damage of the filler network while the rubber network is preserved. Such damage is accompanied by a stress softening and an adiabatic heat source rise. Conversely, increasing the strain rate in cyclic loading conditions yields in a filler network accommodation and a high self-heating whose combined effect is proposed as a possible cause of the ability of filled EPDM to limit damage by reducing cavities opening during loading, and favoring cavities closing upon unloading.

Unusual low temperature relaxation behavior of crosslinked acrylonitrile-butadiene co-polymer

Conventional crosslinking of rubbers (using sulphur and peroxide vulcanization) usually leads to covalent rubber network junctions. The present article reports about a new unconventional crosslinking of acrylonitrile butadiene copolymer (NBR) by zinc chloride (ZnCl2). This crosslinked NBR displayed very interesting relaxation properties below the glass transition temperature. In this work, detailed chemical mechanistic pathways were suggested and correlated with the low-temperature relaxation dynamics of the crosslinked material. Fourier-transform infrared spectroscopy analysis clearly indicated a coordination effect between zinc and nitrile groups. At lower temperatures, beside the glass transition, another relaxation process (β relaxation) was detected. Interestingly, a hydrolyzed crosslinked sample showed even two other additional relaxation processes. The origin of such relaxations was critically discussed from the viewpoints of modification of the polymer chains as well as low molecular weight fragments of the polymer.

The Role of Non-Rubber Components on Molecular Network of Natural Rubber during Accelerated Storage.

Though the non-rubber components have long been recognized to be a vital factor affecting the network of natural rubber (NR), the authentic role of non-rubber components on the network during accelerated storage has not been fully illuminated. This work attempts to clarify the impact of non-rubber components on the network for NR during accelerated storage. A natural network model for NR was proposed based on the gel content, crosslinking density, and the non-rubber components distribution for NR before and after centrifugation. Furthermore, the effect of non-rubber components on the network was investigated during accelerated storage. The results show that terminal crosslinking induced by non-rubber components and entanglements are primary factors affecting the network formation during accelerated storage. By applying the tube model to analyze the stress-strain curves of NR, we found that the contribution of the entanglements to the network formation is larger than that of terminal crosslinking during accelerated storage. The work highlights the role of non-rubber components on the network during accelerated storage, which is essential for understanding the storage hardening mechanism of NR.