Non-rubber Components Research Articles

Mussel-inspired, fully biobased, mechanically robust, and room temperature healable supramolecular elastomer composites for sustainable strain sensors

The sustainable development of strain sensors necessitates the use of mechanically robust and self-healing elastomers derived from biobased feedstocks to ensure durability, extend service life, and reduce reliance on fossil resources. Herein, we report mussel-inspired, fully biobased, mechanically robust, room-temperature healable supramolecular elastomer composites (SECs) fabricated from natural rubber (NR), polydopamine (PDA), and cellulose nanofiber (CNF). Hydrogen bonds were formed among the hydroxyl and secondary amine groups of PDA, hydroxyl groups of CNF, and the non-rubber components of NR, resulting in room-temperature self-healing and mechanically reinforced NR/PDA/CNF SECs. The SECs achieved a tensile strength of up to 9.5 MPa and recovered over 80 % of its strength after self-healing at room temperature for 3 h. The robust adhesion properties of PDA within the SECs enabled the surface coating of the SECs with conductive carbon nanotubes (CNT) to fabricate flexible strain sensors without additional adhesives through a simple dip-coating technique. These flexible strain sensors exhibited excellent structural stability, high sensitivity, and reliability, making them suitable for human motion signal detection. This study underscores the potential of biobased and self-healing elastomers in enhancing the performance and sustainability of flexible strain sensors, contributing significantly to the development of sustainable and durable sensing technologies.

Unveiling the Hidden Networks: AFM Insights into Pre‐Vulcanized Hevea Latex and Its Profound Impact on Latex Film Mechanical Properties

AbstractNatural rubber (NR) films with different natural networks—concentrated NR (CNR), deproteinized NR (DPNR), and small rubber particles (SRP)—are investigated to explore the relationship between network structure and film properties using atomic force microscopy (AFM) in PeakForce Quantitative Nanomechanics (QNM) mode. Nitrogen content, gel content, and particle size distribution analyses reveal distinct network topologies in each latex type. Mechanical testing shows variations in tensile strength and crosslink density. AFM analysis provides insights into the crosslink network structures within the pre‐vulcanized latex film. It is found that DPNR and CNR films have a uniform distribution of crosslink networks, with DPNR exhibiting higher Young's modulus values. In contrast, SRP shows varying Young's modulus values, suggesting poor coalescence arising from a harder particle surface and a softer rubber core in an inhomogeneous network structure intrinsic to the non‐rubber components (NRCs) make‐up of SRP latex. This study highlights the pivotal role of natural network structures formed by NRCs in determining the ultimate properties of latex films, which has significant implications for the rubber industry, particularly in the production of latex‐dipped products, medical devices, and bioengineering applications.

Performance enhancement of silica filled natural rubber nanocomposites using organic deep eutectic solvent

Silica is an important filler of “green tires” while its dispersion in the aid of silane coupling agents emits volatile organic compounds during rubber compounding and its invariably agglomeration in nonpolar rubber matrices enhances strain softening. Herein a highly active deep eutectic solvent (DES), synthesized using stearic acid as hydrogen bond donor and tetrabutylammonium chloride as hydrogen bond acceptor, is used to tailor reinforcement and softening behaviors and to replace the silane coupling agents for preparing volatile organic compounds-free nanocomposites. The results show that DES can regulate the crosslinking network structure of rubber matrix and accelerate the vulcanization by reacting with non-rubber components in natural rubber (NR) and by improving the proportion of disulfidic linkage. Furthermore, DES is able to improve the dispersion of silica, crosslinking density of NR and the interfacial interaction between silica and NR, and slow down the thermo-oxidative aging behavior. It could also weaken the damping and softening accompanying Mullins effect for the nanocomposites vulcanizates at high strains. In comparison with silane, DES endows the nanocomposites with superior vulcanization and mechanical properties, providing guides to mediate the reinforcement and strain softening behaviors and manufacture high-performance “green tires” in an energy-efficient approach.

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.

Protein‐Mediated ZnO Synthesis for Self‐Healing in Natural Rubber

AbstractNatural rubber (NR) consists of mainly cis‐1,4‐polyisoprene with proteins as notable nonrubber components that significantly influence its properties. A relatively unexplored aspect is the role of these proteins in reducing and stabilizing metal ions to form metallic particles. Here, the NR/ZnO composite is successfully prepared by in situ generation of ZnO in the NR latex for the first time. With the aid of the proteins, the amount of ZnO is slightly higher than that formed in the saponified NR latex (SPNR), where some proteins are removed. Importantly, the noncovalent bonds between the proteins and/or amino acids with the ZnO formed in the NR latex can provide the self‐healing behavior of the NR/ZnO composite. However, heating at ≈40 °C is still necessary for reducing the healing time and increasing the self‐healing efficiency of the composite. Nevertheless, greater noncovalent bonds within NR/ZnO hinder rubber molecular chain interdiffusion and rearrangement essential for self‐healing behavior in uncross‐linked rubber, resulting in lower healing efficiency compared to SPNR/ZnO. This study underscores the significance of proteins and/or amino acids on NR's properties, despite their minor presence. Without sophisticated NR modification, this is a very simple and low‐cost technique to achieve the NR/ZnO composite with self‐healing behavior.

Preparation of deep eutectic solvents based on metal ions and their influences on reinforcement and strain softening behaviors of silica filled natural rubber nanocomposites

Deep eutectic solvents (DESs) are used as green additives to reduce zinc oxide (ZnO) activator usages and prepare volatile organic compounds-free nanocomposites. Herein highly active DESs containing different metal ions (Zn2+, Sn2+, Mg2+ and Fe3+) are introduced into silica filled natural rubber (NR) nanocomposites for regulating the network structure of rubber matrix and tailoring elasticity, strength and softening behaviors of the nanocomposites. The results show that DESs react with proteins in NR, which introduces hydrogen bonding between DESs and non-rubber components in the crosslinked network. DES can improve dispersion of silica and enhance the interfacial interaction between silica and NR. In comparison with NR nanocomposites containing 2 phr (part per hundred parts of rubber) ZnO, the partial replacement of ZnO with DESs can not only improve vulcanization rate, tensile strength and crosslinking density but also weaken the Mullins effect by reducing dissipation and softening energy densities. Normalization of tensile deformation energy density reveals that stress and softening behaviors are related to crosslinking density of NR and strain amplification effect induced by silica. The investigation is enlightening for mediating the reinforcement and strain softening behaviors and paving the way for manufacture high-performance NR nanocomposites in an energy-saving efficient and ecofriendly approach.

Effect of deep eutectic solvents on vulcanization and rheological behaviors of rubber vulcanizates

Deep eutectic solvents (DESs) are able to promote vulcanization of rubbers while the mechanisms remain unclear. Herein 4-methyl-5,7-dihydroxycoumarin (DHMC) as hydrogen bond donor (HBD) was used to synthesize DES by reaction with tetrabutylammonium bromide (TBAB) as hydrogen bond acceptor (HBA). The effects of DHMC, TBAB and DES on vulcanization of natural rubber (NR) and isoprene rubber (IR) were studied. It is shown that DES could participate in crosslinking network and adjust crosslinking density (vc) of the vulcanizates through physical and chemical interactions with NR non-rubber components, zinc oxide (ZnO) activator and N-cyclohexyl-2-benzothiazolesulfenamide (CZ) accelerator. Furthermore, DES influences linear and nonlinear rheological responses and mechanical behaviors of NR vulcanizates. The NR vulcanizates cured at 120 °C with 0.1–0.3 phr (part per hundred parts of rubber) DES exhibit higher tensile strengths and lower vc in comparison with the control one cured at 140 °C, which is assigned to the more homogenous crosslinking network in the former. This work provides an effective way to modulate the crosslinking network to produce high-strength vulcanizates at lowered vulcanization temperatures and ZnO amount, which is beneficial for the preparation of rubber products with high strengths and low hysteresis energies.

Modulus self-recovery phenomenon after shearing of natural rubber based on supramolecular network aggregation structure

Natural rubber (NR) and isoprene rubber (IR) exhibit a similar shear-thinning phenomenon, in which NR has the inverse ‘S′ shape stress-strain curve and IR has the yield stress-strain curve. Different from IR, NR exhibits the modulus self-recovery phenomenon after shearing. The experimental data showed that the 300% modulus of NR could recover 41.84% after it was sheared 5 times and stored for 30 days. The tensile strength would even recover completely. The modulus self-recovery efficiency (MSRE) would decrease with increasing shearing times. A mechanism model of the structure self-healing efficiency (SSHE) was proposed based on the supramolecular network aggregation structure (SNAS) formed by the weak valence bond of the non-rubber components (NRCs) at the terminal group of the NR macromolecular chains. The mechanical shearing force would break the SNAS of NR and the short molecular chains would fall off from the SNAS. The dropped shorted molecular chains of NR would re-enter the SNAS under the hydrogen-bond interaction generated by the NRCs after storing for a while, increasing the molecular weight and improving the mechanical properties. The 300% modulus would recover partly and the tensile strength might exceed the initial value if the NR was sheared slightly. On the contrary, as the same molecular structure without NRCs, IR did not show any modulus self-recovery phenomenon after shearing. Except it was confirmed by the increased molecular weight and gel content after shearing and storing, this mechanism model could be confirmed by the de-structured transesterification natural rubber (TENR) which did not exhibit the modulus self-recovery phenomenon anymore after removing the NRCs, such as proteins and phospholipids.

Influence of proteins and phospholipids on strain softening behaviors of natural rubber

In natural rubber (NR), the non-rubber components such as protein and phospholipids strongly influence the Payne effect and Mullins effect while the mechanisms are not yet clarified so far. Herein deproteinized NR (DPNR) and transesterified DPNR (TEDPNR) are prepared and rheological behaviors are investigated. It is shown that the non-rubber components greatly retard the terminal flow behavior. In comparison with NR, DPNR and TEDPNR exhibit significant hysteresis and worse recoverability under large amplitude shear. During cyclic tensile deformation, the DPNR vulcanizate behaves similar to the NR one, while the TEDPNR one demonstrates higher deformation energy normalized with theoretical shear modulus and dissipation ratio. It is suggested that both the protein and phospholipids influence the Payne effect of NR gum by forming aggregates and creating intermolecular association. On the other hand, the phospholipids rather than protein strongly influence the Mullins effect by lowering their crosslinking density and associating the rubbery chains. This investigation would be enlightening for understanding the mechanisms of the Payne effect and Mullins effect of the NR materials.

Octylamine regulating the mechanical robustness of natural rubber by involving in the construction of crosslinking network

The formation of three dimensional network structure is critical in determining mechanical properties of natural rubber (NR). Consequently, it is vital to regulate crosslinking network of NR by controlling vulcanization process. Inspired by our previous studies on contribution of non-rubber components (NRCs) to the excellent properties of NR, we find octylamine in NRCs decreases the activation energy (Ea) of vulcanization from 82.73 kJ/mol to 44.34 kJ/mol, thereby reducing vulcanization time from 18.67 min to 2.71 min. From microscopic perspective, octylamine tends to coordinate with zinc ions to improve dispersion of ZnO in NR. And octylamine promotes ring-opening reaction of S8 to favor formation of polysulfide intermediates.Therefore, the incorporation of octylamine remarkably improves vulcanization efficiency, which contributes to the formation of a more homogeneous network with higher crosslinking density, enhancing remarkably the strength and toughness of NR. As a result, the tensile strength and fracture energy of samples are as high as 31.15 MPa and 68.88 kJ/m2, respectively. In addition, even with a 60 % reduction in ZnO content, the NR samples still maintain high vulcanization efficiency and excellent mechanical properties after the addition of octylamine, which provides a green and feasible way to alleviate the environmental pollution caused by ZnO.

Non-targeted lipidomics analysis of lipid distribution and variation of Eucommia ulmoides oliver by ultra-high performance liquid chromatography

Eucommia ulmoides Oliver (EU) is an important natural polymer source to relieve the serious shortage of natural rubber (NR), and the lipids of EU as potential non-rubber component have inherently a strong intramolecular connection to rubber biosynthesis. In this paper, Non-target lipidomics analysis platform based on ultra-high performance liquid chromatography high-resolution mass spectrometry (UHPLC-HRMS) system was used to determine the lipids in EU. The orthogonal partial least square discriminant analysis (OPLS-DA) and a statistical analysis approach were used to compare the differences of lipids in EU. Total 6 categories, 26 classes and 785 species of lipids were identified in leaves, seed kernels, seed cases and barks of EU. The contents of EU lipids with time in female and male leaves were compared. The lipids contents in female EU leaves were more abundant than that in male EU leaves, and the content of most phospholipids in the female and male leaves both increased obviously after September. The content of lipids between seed cases and leaves were compared, and further screened for different glycerophospholipids between them. There were 53 phospholipids in the seed cases and fresh leaves samples of EU that had significantly different contents. Therefore, this study provides a theoretical basis for the identification of lipids in different EU organs and also helps further explore the relationship between lipids with rubber in EU.

Insect-trapping glues made of natural rubber: Effects of nonrubber components and rubber molar mass on performance

An easy-to-prepare glue was produced from natural rubber (NR) latex and environmentally benign chemicals for use in control of insect pests in greenhouses. The effects of non-rubber components and the molar mass (MM) of NR in the latex and the glue on adhesion performance of the glues are reported. Glues were produced using five latex types: commercial high-ammonia concentrated latex (HA), fresh latex (FL), skim latex (SK), urea deproteinized latex (UD), and saponified latex (SP). The latexes’ characteristics (e.g., protein content, phospholipid content, gel content, and molar mass of NR chains) and the characteristics of the resulting glues (i.e., drying time, molar mass of NR chains, and adhesion performance) were evaluated by principal component analysis (PCA). The NR glues made of HA and SP latex types were found to have an adhesion performance comparable to commercial insect trapping glues. The HA latex was natural, more economical, and required less processing than the SP latex. PCA demonstrated that the adhesion performance of the glues correlated strongly to the gel content and the molar mass of the NR chains in the glue and the latex. Increased values of all these variables improved the cohesive strength of the glue. The HA glue was tested in a greenhouse to confirm its effectiveness: it showed an insect capturing efficacy of ∼68% of a commercial glue. The lower performance was caused by an absence of vulcanization, leading to creep behavior and degradation of the rubber chains in the hot environment of the greenhouse.

Deproteinization of Natural Rubber Latex and Its Pale-Colored Thin Films

The non-rubber components present in natural rubber latex can contribute to the dark color of dried films and may cause allergic reactions. This project aimed to develop light-color rubber films with minimal protein contamination. Various additives were incorporated, and a leaching procedure was implemented to address this issue. The evaluation focused on protein content, color changes, and swelling properties of thin natural rubber films. Texapon N70 proved effective as both a latex stabilizer and leaching agent, while Uniphen P-23 served as a preservative. The combined use of these additives facilitated the removal of soluble serum through appropriate incubation, leaching, and centrifugation processes. The introduction of additional centrifugation cycles improved deproteinization and color reduction; however, it led to a loss of rubber mass and an increase in manufacturing costs. Increasing the amount of Texapon N70 and introducing alkali potassium hydroxide (KOH) further enhanced the efficiency of deproteinization and color reduction. The optimal conditions determined in this investigation were as follows: 0.5% w/w Texapon N70, 0.5% w/w KOH, 1% w/w Uniphen P-23, a 60-min incubation period, and a single leaching cycle with distilled water. These conditions resulted in a 90.57 ± 1.20% decrease in protein contamination and a color change (ΔE) of 433.69 ± 20.23. This successful condition can be replicated and scaled up for further applications.

Research of strain induced crystallization and tensile properties of vulcanized natural rubber based on crosslink densities

Natural rubber (NR) has excellent mechanical properties than synthetic rubber for its strain induced crystallization (SIC) property. It is known that crosslink density has significant influences on SIC, but mechanism of SIC enhanced tensile properties based on crosslink densities has not investigated clearly. In this study, vulcanizates of NR and NR devoid of non-rubber components, characterized by varying crosslink densities, are prepared. Mechanical and crystallization properties and the structure evolution of the systems occurring during stretching are analyzed by conducting tensile tests and using the in-situ (WAXD) technique. Results reveal that the crosslink density significantly affects the mechanical properties and SIC of the NR materials. The maximum tensile strength is recorded when the crosslink density is 1.84 × 10−4 mol/cm3. The structural evolution of the NR vulcanizates characterized by varying crosslink densities is reported, and the strengthening mechanism associated with the systems is proposed. The stretching process associated with the NR vulcanizates can be divided into three stages, namely stages I, II, and III, based on the crystallization process. The NR vulcanizates characterized by low crosslink densities are primarily reinforced by a series of crystallites in stage II of stretching. Stress upturn is realized following the formation of a large number of crystallite networks in stage III. NR vulcanizates characterized by high crosslink densities form parallel reinforcements with the surrounding network post crystallization in stage II, and this can be attributed to the presence of a large number of crosslink chains in the system. Stress upturn can be realized even in the presence of a small number of crystallite networks in stage III. Furthermore, the results indicate that the process of crystallization significantly affects the elongation at break. The influence of crystallization on the elongation at break has different results divided by a critical crosslink density 0.75 × 10−4 mol/cm3. When the crosslink density is lower than the critical crosslink density, the ideal elongation at break is smaller than real elongation at break. While that is higher than the critical crosslink density, the ideal elongation at break is higher than the real. This is because crystallization in stages II and III determine the final elongation at break.

Comparison of methods for determining the gel content in natural rubber and the effect of gel content on rheological behavior

Natural rubber, given its natural origin, is not composed exclusively of polyisoprene chains, but also of so-called non-rubber components such as proteins or phospholipids. Such compounds are responsible for gel formation - insoluble phase if natural rubber (NR) is in a proper solvent. Gel is one of the causes why the behavior of NR is non-standard during processing. The gel content is determined by various methods, while it is not examined in detail which method has a stronger relation to the rheological behavior of rubber. Three different methods (filtration, dissolution in toluene for three different times and Soxhlet extraction) used for determining the gel content of NR are compared in this paper. Analytical (ash content, molar mass measurement, …) and rheological (Mooney viscosity, rubber process analyzer measurements, …) measurements are performed and their results are correlated with the gel content values.

Effect of non-rubber components on the wear behavior of vulcanized natural rubber nanocomposites

Effect of non-rubber components on the wear behavior of vulcanized natural rubber nanocomposites

A new testing strategy based on the wetting concept for characterizing rubber-filler interaction in rubber compounds and its application to the study of the influence of epoxy groups and non-rubber components on rubber-filler interaction in natural rubber compounds

A new testing strategy based on the wetting concept for characterizing rubber-filler interaction in rubber compounds and its application to the study of the influence of epoxy groups and non-rubber components on rubber-filler interaction in natural rubber compounds

The role of non-rubber components acting as endogenous antioxidants on thermal-oxidative aging behavior of natural rubber

Natural rubber (NR) with complex compositions has more excellent thermo-oxidative resistance properties compared to synthetic high cis-1,4-polyisoprene (IR), although synthetic IR owns similar molecular chain structure to NR. Compared with synthetic IR, the existence of non-rubber components is an obvious characteristic of NR. In this study, we pay attention to the non-rubber components acting as endogenous antioxidants, causing significant impact on thermo-oxidative resistance of NR. After removal of non-rubber components, the retention of tensile strength of NR exhibits a 2.7-fold decrease (from 84.29% to 31.29%) after 24 h of aging at 100 °C. The effect of non-rubber components on improving thermo-oxidative resistance of NR can be verified and mimicked through additional acetone extract or δ-tocopherol. The results show that the retention of tensile strength of purified NR (PNR) increases from 3.06% to 64.31% and retention of elongation at break of PNR increases from 60.61% to 92.59% after 48 h of aging after adding 0.6% δ-tocopherol, which can be comparable to or even exceed some traditional antioxidants. Furthermore, the antioxidative mechanism of endogenous antioxidant is explored. This work reveals another characteristic of non-rubber components and provides a rational design for the new biomimetic rubber with unique compositions and excellent properties similar to NR.

Influence of Non-Rubber Components on the Properties of Unvulcanized Natural Rubber from Different Clones.

Natural rubber from different Hevea braziliensis clones, namely RRIM600, RRIT251, PB235 and BPM24, exhibit unique properties. The influences of the various fresh natural rubber latex and cream concentrated latex on the non-rubber components related properties were studied. It was found that the fresh natural rubber latex exhibited differences in their particle size, which was attributed to the non-rubber and unique signature of clones which affect various properties. Meanwhile, the cream concentrated latex showed the protein contents, surface tension, and color of creamed latex to be lower than the fresh natural latex. However, TSC, DRC, viscosity, particle size and green strength of concentrated latex were found to be higher than the fresh natural latex. This is attributed to the incorporation of HEC molecules. Also, the rubber particle size distribution in the RRIM600 clone exhibited a large particle size and uniform distribution, showing good mechanical properties when compared to the other clones. Furthermore, the increased green strength in the RRIM600 clone can be attributed to the crystallization of the chain on straining and chain entanglement. These experimental results may provide benefits for manufacturing rubber products, which can be selected from a suitable clone.

Deciphering the discoloration in the manufacturing of natural rubber

Many naturally occurring color components, especially non-rubber components, are present in natural rubber (NR) latex, limiting some NR applications, particularly for light-colored NR products in high demand. Non-rubber components, like lipids and proteins, can be removed from NR to reduce its yellow color. This study looked at the factors that influence the yellow index (YI) in NR and NR latex discoloration techniques. Proteins were a crucial factor directly determining the YI of NR. Thus, increasing the number of centrifugation washings, speed, and time was used to remove proteins to minimize the YI of NR. Percentage of total solid content (% TSC) of field NR latex, the blend ratio between cream rubber and skim latex, or bottom fraction from fresh-field NR latex collected from centrifugation process, were also important factors. The addition of sodium metabisulphites as a polyphenol oxidase (PPO)-reducing agent reduced the YI significantly. The oxidative breakdown of endogenous non-rubber components at high temperatures was also confirmed to serve the increase in YI value due to air-drying conditions of NR.