Rubber Network Research Articles

Effect of Proteins on the Vulcanized Natural Rubber Crosslinking Network Structure and Mechanical Properties.

Proteins are important factors affecting the properties of natural rubber. Therefore, investigating the effect of free and bonded proteins on the structure and mechanical properties of the vulcanized crosslinking network of natural rubber would provide a theoretical basis for the production of high mechanical resistance natural rubber. Herein, natural rubbers with different protein contents and types were prepared by high-speed centrifugation. And, the effects on their network structure, vulcanization, tensile strength, tearing strength and dynamic mechanical properties were investigated. The results showed that the reduction in protein content led to the decrement in the entanglement networks, crosslinking density and tensile and tear strengths of the vulcanized natural rubber. Moreover, the bonded proteins had an obvious influence on the vulcanization process, while free proteins played an important role in the crosslink densities. These results reveal that both bonded and free proteins are involved in the vulcanization process and the construction of the vulcanized crosslinking network structure of natural rubber, which enhances the mechanical properties such as the modulus and tensile strength of vulcanized natural rubber.

Raw rubber network of acid-free natural rubber and its application with solution-polymerized styrene butadiene rubber in green tires

Natural raw rubber was prepared by flash evaporation process (VNR), its raw rubber network was compared with that of acid-coagulated rubber (STR20), and the effects of the ratios of VNR and STR20 to SSBR on the comprehensive performance of green tire composites were investigated. The results showed that compared with 80SSBR composites, the tensile strength and tensile product coefficient of 80VNR composites were improved by 23 % and 94 %, and the Payne effect, compression heat generation, and rolling resistance were reduced by 39 %, 9 %, and 5 %, respectively. Compared with 40STR20/40SSBR composites, the tensile product coefficient, wear resistance, and wet skid resistance of 40VNR/40SSBR composites are increased by 11 %, 5 %, and 5 %, respectively, and the Payne effect, compression heat generation, and rolling resistance were reduced by 46 %, 11 %, and 5 %, respectively. This study increased the amount of natural rubber in the green tire formulations, improving the performance of composites while achieving sustainability.

Biomimetic design of polyisoprene rubber with terminal synergistic effects leading to mechanical properties and creep resistance boost

The enhancement of polyisoprene rubber performance through biomimetic simulation of the terminal effect of natural rubber has been explored. So far, the presence of synergistic effects between different terminal groups and the challenge of controlling the relaxation behavior of non-covalent bonds while preserving their advantages remain open questions. In this study, we successfully prepared natural rubber biomimetic polymers with different functionalized terminals. Specifically, we constructed terminal-olefin cross-linking points at one end of the polymer chains, while dissociable oligopeptide assemblies were formed at the other end. Investigation of the viscoelastic behavior of the polymers and the structure of the rubber network confirmed the existence of synergistic effects between different terminal groups. The terminal-olefin cross-linking points effectively anchored the rubber network and stabilized the non-covalent bonds. Simultaneously, the non-covalent bonds formed by the oligopeptides acted as weak bonds, dissipating energy and improving the dimensional stability and mechanical properties of the vulcanized rubber.

Influences of non-rubber components on the molecular network and viscoelasticity of natural rubber gum

The molecular network of natural rubber (NR) gum is determined by the interparticle diffusion when NR latex coagulates. However, the roles played by the non-rubber components (NRC) species on the heterogeneous network structure and on the viscoelasticity of NR gum have not been throughly investigated. Here, the molecular relaxation was studied by linear rheology. In addition, the inter- and intra-cycle nonlinearities under large amplitude oscillatory shear (LAOS) were studied for gums with extracted and added NRC. It was found that the high molecular weight free protein restricts the interdiffusion and improves the entanglement density, while the added lipid induces homogeneous distribution of NRC. The physical associations coming from free proteins increase the segment friction and retard the terminal relaxation. Onset of the strain softening was found to be induced by entangled network density. Both the elastic and viscous LAOS nonlinearities can be regulated by adjusting the NRC species. This work sheds light on the contributions of major NRC species on the network structure and nonlinear viscoelasticity of NR gum.

Mechanism of the green coagulation processes on the construction of raw rubber network structure

AbstractAcid coagulation is the most traditional latex coagulation technology, but this coagulation process leads to delayed vulcanization, corrosiveness, and environmental pollution. Different coagulation processes significantly impact the raw rubber network structure, leading to differences in the properties of both raw rubber and rubber vulcanizates. Raw rubber was prepared by three acid‐free coagulation processes: freeze, microwave, and flash drying. The network density, molecular chain flexibility, molecular weight, processing fluidity, and plasticity retention of the raw rubber were characterized, and the vulcanization characteristics, viscoelasticity, mechanical properties, and dynamic mechanical properties of the cured rubber composites were investigated. Raw rubbers prepared by microwave drying and flash drying had higher crosslink density, more flexible molecular chains, larger molecular weight, and wider molecular weight distribution, thereby increasing the crosslink density of rubber vulcanizates. The crosslink density of the raw rubber prepared by microwave drying versus the acid coagulated raw rubber increased by 51% to 105.71 mol/m3, the tensile strength increased by 16% to reach 28.12 MPa, and the elastic modulus and rolling resistance under dynamic stress increased. This paper provides a new idea for analyzing the relationship between the raw rubber network structure and the properties of vulcanized rubber.Highlights Acid‐free raw rubbers are prepared by freeze, microwave, and flash drying. Coagulation processes significantly impact the raw rubber network structures. Effects of raw rubber network structures on properties are investigated. Crosslink density, plasticity retention, and tensile strength are improved.

Influence of axial pressure on the Payne effect of natural rubber vulcanizates

Rubber nanocomposites are key materials for aircraft or automobile tires, which are often subjected to the combined effect of pressure and shear during service. However, the effect of pressure on their strain softening under large amplitude oscillatory shear is not yet clarified. Herein a strain-controlled rotational rheometer is used to investigate the influence of axial pressure on the Payne effect of vulcanized natural rubber composites. Under high axial pressure, the Payne effect of rubber vulcanizates is remarkably suppressed, mainly attributed to the increasing intermolecular local friction hindering wall slip and nonlinear stretching of the rubber network. Furthermore, low temperature, high frequency and high filler fraction may weaken the suppression behavior. The investigation may reveal the role of axial pressure in the strain softening of vulcanized rubber.

Applicability of Kroon network theory for biaxial mechanics in rubber through multiscale approach

In this paper, we address the problem of the multiscale prediction of the multiaxial hyperelastic behavior of rubber. Our goal is to test the predictive capabilities of a hyperelastic model grounded in the principles of continuum mechanics, bridging the gap between continuum-level mechanics and molecular-level descriptions. This constitutive model posits that macroscopic behavior results from the non-affine deformation of the molecular network, influenced by topological constraints. We employ molecular dynamics methods to measure the mechanical behavior of a rubber network structure under various deformation modes. The study includes an analysis of the molecular network deformation during the straining process, shedding light on the deformation mechanisms at the molecular level. By examining how the total network stress in our specifically designed molecular structure responds to biaxial strain ratios, we are able to assess the effectiveness of the hyperelastic constitutive model.

Revealing the Effect of the Length of Chains on Rubber Mechanical Properties by the Two‐Component Quantum Mechanical Gaussian Model

AbstractThe special mechanical properties of natural rubber originate from the stretching of the network chains (i.e., cis‐1,4‐polyisoprene chains) in the rubber network. The chains with various lengths exhibit various relative extensions, leading to various normalized energy contributions to the mechanical properties of rubber. However, the stretching behaviors of cis‐1,4‐polyisoprene chains with different lengths are not studied clearly. Here, the stretching behavior, which is the basis for analyzing the normalized energy contribution, is obtained by the modified freely jointed chain model. Next, using a new statistical model, i.e., the two‐component quantum‐mechanics Gaussian (TCQMG) model, the relative extension–length relationship of network chains is determined. Combining the above results, the energy contributions of network chains with different lengths are analyzed. The results show that long and short chains show different stretching states. The following can be explained by these results: short chains play an important role in the mechanical properties of rubber. As the network chain density increases, the average length of the chains decreases, leading to the variation of the normalized extensions. It is found that the variation of the normalized extensions is responsible for both the effects of the network chain density and network chain length on the mechanical properties of rubber.

Construct of heterogeneous crosslinked networks and the reversible bonds for TPV toward the combination of excellent reinforcement and extensibility

Introducing heterogeneous network or reversible bonds is one kind of useful non-filling methods to improve the properties of elastomers. Perhaps the combination of the two methods could have a better effect on the elastomer. However, there are few reports about combining these two methods employing to improve the performance of thermoplastic vulcanizates (TPVs). Herein, a TPV based on PA6/NBR, with both heterogeneous network and reversible bond, has been prepared by melt blending. For this TPV, the deformation of short chains for heterogeneous rubber network could alleviate the stress concentration of PA6 ligament during the stretching process. And the reversible bonds have been successfully generated between PA6 and NBR interface to improve the compatibility between PA6 and NBR, which could make the stress transfer from the matrix to the heterogeneous rubber network better. In addition, the reversible dissociation/association of reversible bonds could avoid the premature destruction of the matrix. Compared with commercial TPVs based on PA6, this TPV has reached the higher strength and elongation at break. Thus, a TPV based on PA6/NBR combining excellent reinforcement and extensibility have been realized.

Dual Network Co-Crosslinked HNBR Composites with Enhanced Tribological Properties under Water Lubrication

Water-lubricated bearings play a critical role in underwater propulsion systems but are often prone to failure due to mechanical wear and vibration, especially under high loads and prolonged friction. In response to this issue, our study introduces a novel approach: a dual network co-crosslinking strategy utilizing hydrogenated nitrile butadiene rubber (HNBR). This strategy connects the rubber network with the epoxy network through epoxidized Eucommia ulmoides gum. A comprehensive analysis was conducted to assess the resulting composite’s damping, tribological, and mechanical properties. The results show that the material has excellent mechanical, damping, and tribological properties relative to pure HNBR, with a 65.9% increase in the damping temperature domain, a 78.5% increase in tensile strength, a low coefficient of friction of 0.022, and a high resistance to abrasion of 3.87 × 10−6 mm3/Nm. The successful synthesis of HNBR-based composites via the dual network co-crosslinking strategy underscores their potential as a practical solution for improving the reliability and prolonging the service life of water-lubricated bearings.

Syntheses and Characterization of Cephalexin-Chitosan/Natural Rubber network polymer for Slow-Release Fertilizer.

The increase in the world's population has forced the agricultural sector to increase the production of crops to meet food needs. The spread of nutrient deficiencies in the soil led to economic losses, a reduction in nutritional quality, and total grain quantity for humans and livestock. Coating fertilizers with polymers is one of the most important and effective aspects of slow-release fertilizers in the soil. aim this study, the reaction between chitosan and expired cephalexin as a grafted polymer was done and vulcanized with natural rubber in the presence of tetra methyl thiuram disulfide as catalyst to cover the urea fertilizer. The grafted polymer (chitosan/expired cephalexin compound) was prepared by the Schiff base method. The expired medicinal compound of cephalexin is rich in carbon, nitrogen, sulfur, and hydrogen atoms which improved the diffusion of the fertilizer .So the reaction of the validity of the reaction between chitosan and expired Cephalexin was confirmed by infrared (IR) and it was confirmed the presence of new absorption band due to the Azomethine group. The swelling, water retention, slow release of urea, and biodegradation of the NR/chitosan/expired cephalexin compounds were characterized. The rheometer test was carried out under the pressure of 250 bar and temperature of 170°C, The results showed that the Slow release of urea was increased with the increase in the concentration of cephalexin. The highest release percentage in soil media was 2.67 g/l for the C2 blend. The highest swelling ratio was 189.49 % for the C4 blend while the highest water retention was 75.67% for the C2 blend. It was also shown a good biodegradability, where the highest concentration of cephalexin with chitosan gave the highest rate of biodegradation to be 14.61% for C4. The homogeneity of the blends was studied by FE-SEM. It was concluded that the C1-C4 blends were the best for slow-release fertilizer application.

In-situ decorating diene-rubber with carbamyl maleamic handles as sacrificial units toward network reinforcement

The decoration of sacrificial units (SUs) into rubber network has gained momentum as a non-filling alternative toward efficient network reinforcement. However, most established methods, including the copolymerization with SUs-containing monomers or the post-modification of rubber backbone, still suffer from the multi-step and time-consuming backbone preparation with poor designability. In this work, we proposed a facile and practical strategy to achieve efficient network reinforcement through in-situ decorating isoprene rubber (IR) network with N-carbamyl maleamic acid (NCMA) as SUs. Model compound vulcanization experiment demonstrated that during sulfur vulcanization, the poly/disulfides in active sulfurating agents or crosslinking precursors can cleave into reactive sulfur radicals, which subsequently proceeded radical addition reaction with the unsaturated NCMA. The decorated carbamyl maleamic handles can spontaneously form multiple hydrogen-bonding linkages to serve as SUs. Upon deformation, the reversible detachment/reattachment of the SUs will smooth out the local stress concentration, but also facilitate the orientation of network chains. Therefore, as the NCMA loading increases, the onset of strain-induced crystallization and the overall crystallinity of the obtained IR network are simultaneously promoted, giving rise to a gradual increase in the tensile modulus (∼80 %) of the system. By virtue of the rich variety, commercial availability and facile structural tunability of maleamic acid-derivatives, we envisage that the present study can open up a novel strategy to achieve in-situ decoration of rubber with various functional handles toward efficient network reinforcement and functionalization.

Ultra-Low Loading of Ultra-Small Fe3 O4 Nanoparticles on Nonmodified CNTs to Improve Green EMI Shielding Capability of Rubber Composites.

From a material design perspective, the incorporation of Fe3 O4 @carbon nanotube (Fe3 O4 @CNT) hybrids is an effective approach for reconciling the contradictions of high shielding and low reflection coefficients, enabling the fabrication of green shielding materials and reducing the secondary electromagnetic wave pollution. However, the installation of Fe3 O4 nanoparticles on nonmodified and nondestructive CNT walls remains a formidable challenge. Herein, a novel strategy for fabricating the above-mentioned Fe3 O4 @CNTs and subsequently assembling segregated Fe3 O4 @CNT networks in natural rubber (NR) matrices is proposed. The advanced and unique structure, magnetism, and lossless conductivity endow the as-obtained Fe3 O4 @CNT/NR with a shielding effectiveness (SE) of 63.8dB and a low reflection coefficient of 0.24, which indicates a prominent green-shielding capability that surpasses those of previously reported green-shielding materials. Moreover, the specific SE reaches 531dB cm-1 , exceeding that of those of previously reported carbon/polymer composites. Meanwhile, the outstanding conductivity enables the composite to reach a saturation temperature of ≈95°C at a driving voltage of 1.5V with long-term stability. Therefore, the as-fabricated Fe3 O4 @CNT/rubber composites represent an important development in green-shielding materials that are applied in cold environment.

Effect of crosslinking agent dosage on the morphology and properties of thermoplastic vulcanizates based on hydrogenated acrylonitrile butadiene rubber and thermoplastic polyester elastomer

This work aims to develop heat- and oil-resistant thermoplastic vulcanizates (TPVs) based on hydrogenated acrylonitrile butadiene rubber (HNBR) and thermoplastic polyester elastomer (TPEE) using dynamic vulcanization and following a masterbatch procedure. It focuses on the effect of the cross-linking agent dosage on the morphology and properties of HNBR/TPEE-TPVs. An increase in the peroxide dosage yields a higher cross-linking degree of the HNBR, a smaller size and a more uniform distribution of the HNBR domains, and a stronger rubber network. Consequently, the elasticity and high-temperature oil resistance of the TPV increases, but its flowability in the molten state, tensile properties, and heat resistance decrease. It turns out that a peroxide dosage of 1.5 phr is preferred for the preparation of HNBR/TPEE-TPV under the specified processing conditions to achieve a more comprehensive TPV performance.

Effect of dynamic vulcanization time on the performance of thermoplastic vulcanizates based on hydrogenated acrylonitrile butadiene rubber/thermoplastic polyester elastomer

AbstractThermoplastic vulcanizates (TPVs) are obtained by dynamic vulcanization (DV). Time is a crucial processing parameter. This work aims to develop heat‐ and oil‐resistant TPVs with hydrogenated acrylonitrile butadiene rubber (HNBR) and thermoplastic polyester elastomer (TPEE). It focuses on the effect of DV time on the morphology and properties of the resulting TPVs. The investigation demonstrates that as the DV time increases from 2 to 6 min, the rubber network progressively reinforces and the HNBR domain size decreases under more adequate shearing action, resulting in a more homogeneous morphology. This leads to a significant improvement in tensile properties, elasticity, rheological properties, thermal stability, and oil resistance of the TPV. A further extension in DV time (over 8 min) results in a slight deterioration in elasticity and thermal stability of the TPV because of excessive degradation of the TPEE. Accordingly, an appropriate DV time for HNBR/TPEE‐TPV preparation is between 6 and 8 min under the specified DV conditions.

SULFUR VULCANIZATION OF LOW- VERSUS HIGH-UNSATURATED RUBBERS (IIR AND EPDM VERSUS NR AND BR): PART II—NETWORK STRUCTURE AND TENSILE PROPERTIES

ABSTRACTSulfur vulcanization is the most common crosslinking technology for unsaturated rubbers. To enhance our generic understanding of the structure–property relationships for sulfur-vulcanized rubber networks, we have studied two low-unsaturated rubbers, IIR and EPDM, and two high-unsaturated rubbers, NR and BR, at varying levels of sulfur curatives. In the first part of this series, the rheometer torque maximum and the compression set as a function of the temperature were discussed. In this second part, the effects of the level of the rubber unsaturation, the density of the trapped entanglements, and the chemical crosslink density on the network structure and tensile properties are discussed quantitatively. Our results reveal that the networks of the vulcanized BR and EPDM consist mainly of trapped entanglements. For EPDM and IIR, all unsaturation can be fully converted to sulfur crosslinks. The tensile strength at break (TS) of vulcanized EPDM and BR is independent of the permanent network density, which is the sum of the chemical crosslink density and the trapped entanglements. The TS shows a pronounced maximum versus the permanent crosslink density for vulcanized IIR and NR, due to the absence of the reinforcing effect of strain-induced crystallization (SIC) at low crosslink densities and the suppression of SIC at high crosslink densities. The elongation at break decreases with increasing network density, following a power-law relation. Mooney–Rivlin analysis of the stress–strain curves confirms our findings of the network structure as obtained from rheometry. However, an unexpected, curved course of the second Mooney–Rivlin parameter as a function of the varying sulfur content is observed for the EPDM samples, indicating that vulcanized EPDM has a different, entanglement-dominated network structure in contrast to IIR, NR, and BR.

X-ray 3D imaging–based microunderstanding of granular mixtures: Stiffness enhancement by adding small fractions of soft particles

This research focuses on performing ultrasound propagation measurements and micro-X-ray computed tomography (µXRCT) imaging on prestressed granular packings prepared with biphasic mixtures of monodisperse glass and rubber particles at different compositions/fractions. Ultrasound experiments employing piezoelectric transducers, mounted in an oedometric cell (complementing earlier triaxial cell experiments), are used to excite and detect longitudinal ultrasound waves through randomly prepared mixtures of monodisperse stiff/soft particles. While the fraction of the soft particles is increasing linearly from zero, the effective macroscopic stiffness of the granular packings transits nonlinearly and nonmonotonically toward the soft limit, remarkably via an interesting stiffer regime for small rubber fractions between 0.1 ≲ ν ≲ 0.2. The contact network of dense packings, as accessed from µXRCT, plays a key role in understanding this phenomenon, considering the structure of the network, the chain length, the grain contacts, and the particle coordination. While the maximum stiffness is due to surprisingly shortened chains, the sudden drop in elastic stiffness of the mixture packings, at ν ≈ 0.4, is associated with chains of particles that include both glass and rubber particles (soft chains); for ν ≲ 0.3, the dominant chains include only glass particles (hard chains). At the drop, ν ≈ 0.4, the coordination number of glass and rubber networks is approximately four and three, respectively, i.e., neither of the networks are jammed, and the chains need to include particles from another species to propagate information.

In Situ Modification to Reinforce Isoprene Rubber by Sacrificial Bonds

Functionalization and reinforcement of polymers by introducing dynamic sacrificial bonds have attracted increasing attention. In this contribution, the in situ modification of isoprene rubber by the formation of sacrificial bonds during vulcanization has been investigated. The sacrificial bonds were formed by in situ introduction of hydrogen bonds and metal–ligand motifs, which became part of the vulcanized isoprene rubber network by reaction of the former with the vulcanizing agent. The introduction of three-component modifiers (nadic anhydride (NA), 2-aminopyrimidine (AP), and ZnCl2) and the molar ratio of these components can greatly affect the tensile strength and modulus of the vulcanized rubber. Under the external load, the sacrificial bonds preferentially rupture prior to the covalent network and facilitate rubber chain orientation, resulting in high fracture energy. In addition, the dynamic behavior of the sacrificial bonds and the hysteresis behavior of the vulcanized rubbers were studied.

Time-Resolved Extensional Rheo-NMR Spectroscopy for Investigating Polymer Nanocomposites under Deformation

Understanding the microstructure change of polymer nanocomposites (PNCs) under elongation deformation at the molecular level is the key to coupling structure-property relationships of PNCs. In this study, we developed our recently proposed in situ extensional rheology NMR device, Rheo-spin NMR, which can simultaneously obtain both the macroscopic stress-strain curves and the microscopic molecular information with the total sample weight of ∼6 mg. This enables us to conduct a detailed investigation of the evolution of the interfacial layer and polymer matrix in nonlinear elongational strain softening behaviors. A quantitative method is established for in situ analysis of (1) the fraction of the interfacial layer and (2) the network strand orientation distribution of the polymer matrix based on the molecular stress function model under active deformation. The results show that for the current highly filled silicone nanocomposite system, the influence of the interfacial layer fraction on mechanical property change during small amplitude deformation is quite minor, while the main role is reflected in rubber network strand reorientation. The Rheo-spin NMR device and the established analysis method are expected to facilitate the understanding of the reinforcement mechanism of PNC, which can be further applied to understand the deformation mechanism of other systems, i.e., glassy and semicrystalline polymers and the vascular tissues.

Biobased epoxidized natural rubber/sodium carboxymethyl cellulose composites with enhanced strength and healing ability

Conventional vulcanized rubbers cause a non-negligible waste of resources due to the formation of 3D irreversible covalently cross-linked networks. The introduction of reversible covalent bonds, such as reversible disulfide bonds, into the rubber network, is an available solution to the above problem. However, the mechanical properties of rubber with only reversible disulfide bonds cannot meet most practical applications. In this paper, a strengthened bio-based epoxidized natural rubber (ENR) composite reinforced by sodium carboxymethyl cellulose (SCMC) was prepared. SCMC forms a mass of hydrogen bonds between its hydroxyl groups and the hydrophilic groups of ENR chain, which gives the ENR/2,2′-Dithiodibenzoic acid (DTSA)/SCMC composites an enhanced mechanical performance. With 20 phr SCMC, the tensile strength of the composite increases from 3.0 to 10.4 MPa, which is almost 3.5 times that of the ENR/DTSA composite without SCMC. Simultaneously, DTSA covalently cross-linked ENR with the introduction of reversible disulfide bonds, which enables the cross-linked network to rearrange its topology at low temperatures and thus endows the ENR/DTSA/SCMC composites with healing properties. The ENR/DTSA/SCMC-10 composite has a considerable healing efficiency of about 96 % after healing at 80 °C for 12 h.