Natural Rubber Networks Research Articles

The different reinforcing behavior between siliceous earth nanoparticles and silica on natural rubber

The reinforcing behaviors of siliceous earth (SE) nanoparticles and silica on natural rubber (NR) were compared. SE employed in this work was formed by volcanic eruptions 100 million years ago and mainly consists of SiO2 (∼80%), Al2O3 (∼14%), and K2O (∼4%) and is composed of low quartz and illite in crystal structure and with a special flaky and laminated morphology. The effects of SE slurry with different filler loading of 30, 40, 50 per hundred parts of rubber (phr), and 40 phr silica(S-40) were investigated. Mechanical properties were measured, and the results show that 300% modulus, elongation at break, tensile, and tear strength of SE/NR are all higher than those of S-40. Combination results of tensile properties and rubber process analyzer (RPA), the rubber–filler interaction parameter (I) was calculated and the results show the value of I increase slightly with SE loading, while obviously higher than that of S-40. But the results of dynamic mechanical analysis and bound rubber content show that S-40 has obviously lower tan δ and higher bound rubber content, which indicate that there exists stronger rubber–filler interaction between silica and NR. The seemingly conflicting results imply that the effect of the two fillers on the NR network structure should be different. The special morphology of SE can help them to distribute more evenly in the NR matrix, making it less likely to form particle networks and providing very strong filler–rubber interaction, while showing higher average rubber–filler interaction and results in better mechanical properties of SE/NR composites.

Processing rheological behavior of functionalized silica/natural rubber composites

The filler networks of natural rubber (NR) silica-filled coupling agents KH560, Si69, and NXT were studied using a rubber processing analyzer (RPA). The effects of different coupling agents on the rheological behavior of silica-filled NR were investigated. The results showed that the three types of coupling agents could control the surface properties of silica, diminish the silica network, reduce the storage modulus and loss modulus, weaken the Payne effect, and improve processability. A comprehensive modification effect was observed for the coupling agent NXT. For different modified silica loading scenarios, the rheological properties of natural rubber composites filled with silica modified by the silane coupling agent NXT were assessed. The Payne effects were more obvious with the increased loading of fillers, while G′ and G″ nonlinearly rose with the increase of frequency. Finally, the platform effect appeared, demonstrating the shear thinning phenomenon. The best compatibility and minimum extent of agglomeration of silica was obtained from the composite filled with 50 phr silica.

Study on mechanical and electrical properties of cellulose nanofibrils/graphene-modified natural rubber

Abstract To improve the mechanical and electrical properties of natural rubber (NR), we used cellulose nanofibrils (CNF) as biotemplates, dispersing conductive graphene composites at 10% and 7.5% and coated graphenes (GNS) between natural rubber molecules on the CNF surface by solution blending. The conductive CNF-GNS/NR nanocomposite material was prepared by dispersing and overlapping CNF and GNS in NR to form a three-dimensional multi-layered network structure composite. The product was characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), tensile testing, and conductivity testing. Results show that the combination of GNS and CNF did not clearly change the GNS chain, did not destroy the GNS structure, and maintained GNS conductivity. When 10% GNS was added, the draw ratio of the CNF-GNS/NR composite was 208%, 56% higher than that of pure NR. The tensile strength increased by 340% compared with pure NR and was 79% higher than that of CNS without CNF. The percolation threshold of CNF-GNS/NR was between 5 and 7.5 phr, lower than the 7.5–10 phr permeation threshold of GNS-NR. When 7.5% GNS was added, the conductivity of CNF-GNS/NR was 3.80 × 10−2 S/m. Compared to the conductivity of 1.25 × 10−6 S/m GNS-NR, the improvement was approximately 4 orders of magnitude. The percolation threshold of CNF-GNS/NR composites was significantly lower than that of composites without CNF. Thus, the addition of CNF can reduce the percolation threshold of composites. Using CNF as a biological template to construct a multi-level three-dimensional conductive network in NR can improve the conductive properties of NR composites substantially.

Research on architecture and composition of natural network in natural rubber

Though the superior properties of natural rubber (NR) have been attributed to its special network architecture, the currently accepted model “naturally occurring network” is far from describing its authentic network structure. In this paper, we focused on the composition of the chain entanglements in the network structure of unvulcanized NR. By using synchrotron wide-angle X-ray diffraction (WAXD), the evolution of its strain-induced crystallization (SIC) behaviors was real-time traced, and the stress-strain behaviors at various strain rates and temperatures were also tested. The results demonstrated that the entanglements can act as crosslinking points to increase the network chain density and lead to the easier SIC behavior. By applying the tube model to analyze the stress-strain curves of unvulcanized NR, we found that the contribution of the entanglement network to the stress is about an order of magnitude larger than that of the network formed by two terminal groups. Via double-quantum nuclear magnetic resonance (1H-DQ NMR), we further detected two types of entanglement in NR and deproteinized NR (DPNR), i.e. transiently trapped entanglements (TTEs) and permanently trapped entanglements (PTEs), among which TTEs are the main composition of the entanglement network. Moreover, the network formed by two terminals causes more PTEs in NR, while due to lacking the constraint of proteins, more TTEs exist in DPNR. Based on the research, we proposed a novel network model for unvulcanized NR.

Preparation and Characterizations of Zeolite – Filled Natural Rubber/Poly(vinyl alcohol) Semi–IPN Membranes

Semi–interpenetrating polymer network (semi–IPN) of natural rubber (NR) and crosslinked poly(vinyl alcohol) (PVA) was composited with zeolite 4A to form a mixed matrix (MM) membrane. PVA was crosslinked with sulfosuccinic acid (SSA) in the immediate presence of NR and zeolite. FT–IR spectroscopy confirmed the crosslinking of PVA in the MM membrane. Swelling measurements were carried out in both water and absolute ethanol. The degrees of swelling in both water and ethanol increased with increasing PVA content in the membranes, however they were found to decline upon increasing zeolite content more than 10 %w/w. The mechanical properties of the MM membranes were investigated by tensile testing. It was observed that the modulus increased with increasing zeolite content in membranes but the elongation at break was found to decrease. The thermal degradation of the MM membranes was studies using thermal gravimetry (TG). The MM membranes showed higher thermal stability compared with NR/PVA membrane due to the incorporation of zeolite. The dispersion of zeolite particles in the MM membrane was observed by scanning electron microscopy (SEM) analysis which showed a well dispersing of zeolite particles in the semi–IPN matrix. The sorption selectivities of MM membranes were enhanced with zeolite content especially, at low water concentration of ethanol–water mixtures. However the sorption selectivities of MM membranes decreased when water content in the liquid mixtures increased.

Solvents for thermoporosimetry analysis of natural rubber networks

ABSTRACTThermoporosimetry is known as an analytical method to investigate network structures in swollen elastomers. Despite this interesting potential capability, only few investigations are published in literature which apply different solvents on differently crosslinked natural rubber samples. The choice of the solvent is critical as it strongly influences the separation of confined and bulk solvent transition signal, the confined solvent signal intensity and the peak width representing the pore size resolution. No critical comparison has been done in these investigations regarding the solvent choice related to peak separation, pore size resolution or signal intensity. Furthermore, no critical assessment is available relating thermoporosimetry results to solvent parameters to identify an optimal solvent regarding the mentioned criteria. This argument motivated the present investigation to identify the most appropriate solvents for the analysis of natural rubber compounds by thermoporosimetry. Different types of solvents (aliphatic, aromatic, non‐polar, and polar) were selected and benchmarked against each other. It was concluded that n‐heptane was identified as the best solvent for these investigations. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43998.

Mechanical and thermal properties of carbon black reinforced natural rubber/polyvinyl alcohol fully‐interpenetrating polymer networks

Nano composite based on fully interpenetrating polymer network (IPN) of natural rubber and polyvinyl alcohol was prepared by reinforcing carbon black (CB) into the IPN matrix. As a common crosslinking agent for both the constituent polymers, glutaraldehyde was used to prepare the fully IPN. Effects of weight fraction of CB on the physical properties of the composites were investigated. A remarkable improvement was observed in the mechanical strength and the thermal stability of the blend by the incorporation of CB. Maximum tensile strength was observed for the nano composite with 0.8% of CB. Further addition of CB resulted in decrease of tensile properties due to the formation of agglomerates, as evident from scanning electron microscope images. J. VINYL ADDIT. TECHNOL., 24:E21–E29, 2018. © 2016 Society of Plastics Engineers

Self-stabilized polyaniline@graphene aqueous colloids for the construction of assembled conductive network in rubber matrix and its chemical sensing application

Facile dispersion and construction of graphene-based 3D conductive network in polymer matrix are highly attractive for the preparation of conductive polymer composites (CPCs) and flexible electronic devices. In this study, a simple and effective strategy for the fabrication of graphene-based assembled conductive network in natural rubber (NR) matrix is reported. Specifically, polyaniline@graphene nanohybrids were prepared by in situ reduction of graphene oxide using aniline as both reducing and stabilizing agents. The resulted self-stabilized polyaniline@graphene nanohybrids could well disperse in aqueous suspension by electrostatic repulsion. When homogenized with NR through latex assembly technique, they could be located in the interstitial space between the NR latex microspheres and organized into a continuous 3D hierarchical network. The as-prepared CPCs with this 3D conductive network exhibited a very low percolation threshold (3-fold lower than that of the conventional NR/graphene blends), enhanced electrical conductivity (up to 6 orders of magnitude improvement) and meanwhile excellent chemical sensing properties. This work provides a feasible approach to stabilize graphene nanosheets in aqueous media without the use of stabilizer or complicated surface modification and extend their applications in the preparation of CPCs as chemical sensing materials.

Tailoring percolating conductive networks of natural rubber composites for flexible strain sensors via a cellulose nanocrystal templated assembly.

Conductive polymer composites (CPCs) just above the percolation threshold exhibit a unique strain-reversible electric response upon application of tensile strain, which can be used to prepare strain sensors. However, it is difficult to balance the electric conductivity which is fundamental to a stable output signal and the strain sensing sensitivity due to the relatively dense conductive pathways of the traditional CPCs. Constructing a "brittle" but effective conductive network structure in CPCs is the essential foundation of a desirable sensing material. Here, we demonstrate for the first time that highly flexible, stretchable, sensitive, and reversible strain sensors can be fabricated by a facile latex assembly approach, in which nontoxic, sustainable and biodegradable cellulose nanocrystals played a key role in tailoring the percolating network of conductive natural rubber (NR)/carbon nanotube (CNT) composites. The resulting nanocomposites with a continuous 3D conductive structure exhibited a very low electrical conductivity percolation threshold (4-fold lower than that of the conventional NR/CNT composites), high resistivity and sensitivity (gauge factor ≈ 43.5) and meanwhile good reproducibility of up to 100% strain. The proposed materials and principles in this study open up a novel practical approach to design high performance flexible sensors for a broad range of multifunctional applications.

Strikingly improved toughness of nonpolar rubber by incorporating sacrificial network at small fraction

ABSTRACTIt poses a huge challenge to create nonpolar rubber with high fracture toughness. In the present letter, inspired by the concept of sacrificial bonding associated with many biological materials, we propose that a small fraction of additional sacrificial network can strikingly improve the fracture toughness of nonpolar rubbers. As a proof of concept, we created the additional “fragile” epoxidized natural rubber (ENR) network in commercially available SBR rubber in a facile process. With addition of only 10 phr ENR, the SBR/ENR double network (DN) exhibits a fracture energy nearly fourfold higher than that for the neat SBR. The formation of DN formation and the correlation between the high toughness and presence of the second brittle network have been fully discussed. This is the first time sacrificial networks are created in diene‐based rubber towards high toughness elastomers in a facile and efficient way. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 781–786

Unveiling Reinforcement and Toughening Mechanism of Filler Network in Natural Rubber with Synchrotron Radiation X-ray Nano-Computed Tomography

Double network structure constructed with filler network of carbon black and molecular network of natural rubber possesses excellent toughness and strength. However, due to lack of proper in situ imaging techniques to detect the structural evolutions under loading, the reinforcement mechanism of filler network is still under debate. Here in situ synchrotron radiation X-ray nano-computed tomography with high spatial resolution (100 nm) is employed to study structural evolution of carbon black in a large volume of natural rubber matrix. For the first time, strain-induced deformation, destruction, and reconstruction of filler network are directly observed under cyclic loading. Combining mechanical test, the reinforcing and toughening effect of filler network is quantitatively assigned to three mechanisms, namely elastic deformation, destruction, and friction of filler network. Elastic deformation mainly occurs at low strain for energy storage, while network destruction plays the dominant role at larger strai...

Formation and stability of carbon nanotube network in natural rubber: Effect of non-rubber components

The formation and stability of carbon nanotube (CNT) network in natural rubber (NR), deproteinized NR (DPNR) and polyisoprene (IR) compound were investigated by means of the method of the online measured electrical conductance in a whole process from processing and rolling over pressing/cross-linking to post-stretching. The kinetics of CNT flocculation was described and explained by taking into consideration the depletion force considered as driving force and the thickness of the bound rubber layer considered as hindering factor. The presence of linked phospholipids in NR and DPNR improves the rubber-filler interaction of CNTs through the cation-π bonding that hinders the filler flocculation. The absence of the cation-π bonding in CNT/IR compound and the related thin layer of bound rubber are the reason for the strong tendency of flocculation of CNTs in IR even at room temperature. The effect of pressing time, temperature and cross-linking reaction as well as mechanical deformation on the formation and stability of CNT network in NR compounds was also investigated and discussed by taking into consideration the role of the linked phospholipids.

A new paradigm for the molecular basis of rubber elasticity

The molecular basis for rubber elasticity is arguably the oldest and one of the most important questions in the field of polymer physics. The theoretical investigation of rubber elasticity began in earnest almost a century ago with the development of analytic thermodynamic models, based on simple, highly-symmetric configurations of so-called Gaussian chains, i.e. polymer chains that obey Markov statistics. Numerous theories have been proposed over the past 90 years based on the ansatz that the elastic force for individual network chains arises from the entropy change associated with the distribution of end-to-end distances of a free polymer chain. There are serious conceptual objections to this assumption and others, such as the assumption that all network nodes undergo a simple volume-preserving linear motion and that all of the network chains have the same length. Recently, a new paradigm for elasticity in rubber networks has been proposed that is based on mechanisms that originate at the molecular level. Using conventional statistical mechanics analyses, Quantum Chemistry, and Molecular Dynamics simulations, the fundamental entropic and enthalpic chain extension forces for polyisoprene (natural rubber) have been determined, along with estimates for the basic force constants. Concurrently, the complex morphology of natural rubber networks (the joint probability density distributions that relate the chain end-to-end distance to its contour length) has also been captured in a numerical model (EPnet). When molecular chain forces are merged with the network structure in this model, it is possible to study the mechanical response to tensile and compressive strains of a representative volume element of a polymer network. As strain is imposed on a network, pathways of connected taut chains, that completely span the network along strain axis, emerge. Although these chains represent only a few percent of the total, they account for nearly all of the elastic stress at high strain. Here we provide a brief review of previous elasticity theories and their deficiencies, and present a new paradigm with an emphasis on experimental comparisons.

Microscopic Study of Chain Deformation and Orientation in Uniaxially Strained Polymer Networks: NMR Results versus Different Network Models

The complex pathway of transmitted stress and strain within elastic networks affects the conformations of single network chains in a complex and inhomogeneous manner. For a novel experimental assessment, we investigated moderately to highly cross-linked vulcanized natural rubber networks with proton multiple-quantum NMR and studied the correlation between macroscopic uniaxial elongation and local effects of strain and/or confinement on the level of elastically active subchains. Our method and the data treatment allow for a separate analysis of chain deformation and orientation, which in turn allows for critical comparisons of our experimental results with predictions from various network models. Specifically, comparisons with the affine fixed-junction model, the phantom model, and different tube models allowed us to draw conclusions on the presence of possibly tube-related local confinement effects. As none of the models alone was able to provide a comprehensive, quantitative description of all experimental data, we performed numerical calculations inspired by earlier simulation results, which match the data and support the presence of two distinct populations of network chains that are either more or less stretched as well as oriented.

Homogenization of natural rubber network induced by nanoclay

ABSTRACTVulcanization gives birth to the nonuniformity of rubber network, the identification of which is the basis of improvement of performance of rubber products. We established a visco‐hyperelastic constitutive equation to reveal the quantitative distribution of the inhomogeneous network phases according to a three‐phase model. The cross‐linked network was assumed to be composed of the cross‐linking cluster, the low network chain density domain and the fluid‐like mass. The incorporation of clay with high specific surface area induced the effective uniformity of network structure by decreasing the content of the cross‐linking cluster and increasing that of the low network chain density domain. These structural variations were responsible for excellent mechanical properties and strong strain‐induced crystallization ability probed by the in situ synchrotron wide‐angle X‐ray diffraction. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40324.

Sulfur-Cured Natural Rubber Elastomer Networks: Correlating Cross-Link Density, Chain Orientation, and Mechanical Response by Combined Techniques

We present a combination of independent techniques in order to characterize vulcanized natural rubber elastomer networks. We combine solid state proton multiple-quantum NMR, equilibrium swelling, mechanical experiments, and in-situ tensile X-ray scattering measurements, all of them giving access to the segmental orientation effects in relation to the cross-linking of the systems. By means of the combination of these techniques, we investigate a set of unfilled natural rubber networks with different levels of cross-linking. The relevance of this work is the application of this approach in order to study the reinforcement effect in filled elastomers with nanoparticles in a following work.

The effects of endlinking network and entanglement to stress–strain relation and strain-induced crystallization of un-vulcanized and vulcanized natural rubber

Simultaneous synchrotron X-ray and stress-strain measurement of un-vulcanized natural rubber (NR) revealed that an upturn of stress at large strains and strain induced crystallization (SIC) are caused by endlinking pseudo network and entanglements. Higher pseudo network density of un-vulcanized NR shows higher modulus and larger fraction of SIC. At faster stretching speed, un-vulcanized NR shows higher stress and larger fraction of SIC. On the other hand, vulcanized NR show almost the same stress-strain relation and smaller fraction of SIC at faster stretching speed. Those different behaviors are governed by entanglements, endlinking pseudo networks in un-vulcanized NR and chemical bond networks, small amount of entanglements and endlinking networks in vulcanized NR.

Thermal aging on mechanical properties and crosslinked network of natural rubber/zinc Dimethacrylate composites

AbstractThis article dealt with the relationship between mechanical properties and crosslinked networks of natural rubber (NR) reinforced by zinc dimethacrylate (ZDMA) after thermal aging. After thermal aging at the present experimental conditions, the covalent crosslink density showed a decrease all the time, whereas the ionic crosslink density was stable at 80°C but decreased at a higher temperature. The decrease in the total crosslink density after aging indicates the degradation of the crosslinked network. However, an experimental phenomenon observed was that the tensile strength and tear strength increased in a certain degree after aging at 80°C or at a 100°C for a short time. In addition, the thermal stability of the NR/ZDMA composite was evaluated by thermal gravimetric analysis. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

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.

Dynamics near the filler surface in natural rubber-silica nanocomposites

We study the effect of in situ synthesized 10 nm silica nanoparticles on the glass transition and dynamics of natural rubber networks using differential scanning calorimetry, broadband dielectric relaxation spectroscopy and thermally stimulated depolarization currents. Even in the absence of specific polymer-filler interactions, polymer segments within a few nanometers of the filler particles exhibit relaxation times up to 2–3 orders of magnitude slower and reduced heat capacity increment at the glass transition compared to bulk natural rubber. These effects are only observed when the nanoparticles are uniformly distributed in the polymer matrix.