Unfilled Rubber Research Articles

Effect of strains and temperatures on the stress relaxation of unfilled natural rubber

Effect of strains and temperatures on the stress relaxation of unfilled natural rubber

Changes in natural rubber mechanical behavior during oxidation: Relationship with oxygen consumption

This paper deals with the oxidation of rubber and more specifically with the possibility of setting up relationships between chemical degradation and its impact on mechanical properties for the lifetime prediction of polymers when the degradation process does not follow a single linear Arrhenian behavior. In this study, oxidation of unfilled natural rubber is first characterized at the molecular scale using oxygen consumption measurements for temperatures ranging from 115 °C to 21 °C. The results clearly show that oxidation does not follow a single linear Arrhenian behavior. Oxidation is then characterized at two other scales: the macromolecular scale using swelling measurements, and the macroscopic scale using tensile tests. Based on these findings, the relationship between the amount of oxygen consumed by the rubber and its crosslink density is first proposed within the temperature range of 115 °C to 85 °C. Finally, the relationships between the mechanical properties (Young modulus, elongation, and stress at break) and crosslink density of rubber are discussed.

Multifunctionality of MWCNT and TiC hybrid filler silicone composite for energy harvesting and strain sensing

AbstractThe flexible silicone elastomer‐conductive composite was developed with enhanced energy harvesting and strain sensing. A hybrid nanofiller system with multi‐walled carbon nanotubes (MWCNTs) and titanium (IV) carbide (TiC) was used as reinforcing agents for the elastomer. The 4 MWCNT +5 TiC composite has 1.34 MPa tensile strength, 136.85% higher than unfilled silicone rubber (SR) matrix. The unfilled sample has a 1.558 MPa compressive modulus, while 4 MWCNT +5 TiC has 3.87 MPa. The machine and finger presses were utilized to test the energy harvesting of the composite. With greater cyclic stability, the 4 MWCNT +5 TiC sample performed exceptionally well in the machine press in the output voltage range of around 120 mV. Among the thumb presses, 2 MWCNT +5 TiC yielded the best results, with a voltage of about 100 mV. The 3 MWCNT +5 TiC sample produced 9044% more energy than the single filler 3 phr MWCNT sample from our previous work. Higher MWCNT content enhanced composite stiffness, making finger pressing harder and decreasing energy output. The strain sensing test on the 4 MWCNT +5 TiC composite showed high gauge factors (GF = 7.532 and 23.944) and good linearity (R2 = 0.909 and 0.939) over a strain range of Δε = 0%–14% and 14%–22%. Silicone composites' mechanical, energy harvesting, and strain sensing performance improved greatly with the MWCNT and TiC hybrid fillers. These composites could be used in self‐powered wearable electronics, machine or structural deformation monitoring, and human motion sensing.Highlights A stretchable device was developed with improved energy harvesting and strain sensing. Rubber composites were fabricated from MWCNTs, TiC hybrid fillers, and SR matrix. The energy harvesting was 9044% higher for hybrid filler compared to MWCNT as the only filler. Robust strain sensing properties with a gauge factor of 23.9, linearity of 0.91, and relaxation time of 0.1 s. The final composites are useful for self‐powered wearable electronics, health monitoring, and human motion sensing.

MWCNT-reinforced silicone's dual response to human and mechanical motion: Exploring strain sensing behavior and practical applications

This study investigates the human motion sensing capabilities of silicone rubber (SR) composites reinforced with multi-walled carbon nanotubes (MWCNTs). It focuses on assessing the responsiveness of the composites to both compressive and tensile deformations, replicating motions from machines to human movements. The analysis underscores the enhanced sensitivity of the 3 parts per hundred (phr) compressive sample, with gauge factors of 7.5 and 14.44 at distinct linear ranges under 30% strain. During forefinger and thumb pressing, the relative change in resistance was 160 and 350, respectively. Additionally, the study examines the 5 phr sample, revealing a significant 346.67% increase in relative resistance changes during the initial cyclic loading phase under 30% tensile deformation compared to the compression sample. The mechanical properties are also evaluated, showing the 5 phr compressive sample possessing a compressive modulus 180.09% higher than the modulus of the unfilled specimen (1.246 MPa), along with a hysteresis loss of 37.24 kJ. Furthermore, the tensile modulus of the 5 phr specimen exceeds that of the unfilled rubber by 218.84%. The developed composites hold promise for creating sensors capable of tracking athletes' movements in real time, offering immediate performance feedback and post-processing analysis. Moreover, these materials could contribute to the design of rehabilitation devices aimed at assisting individuals with hand injuries in regaining their agility and mobility.

Reprocessable and Recyclable Self-Healing Natural Rubber/Carbon Black Composite Based on Metal Thiolate Ionic Network

Reprocessable and recyclable self-healing rubber composites were fabricated by mixing natural rubber (NR) with carbon black (CB) filler in the presence of zinc thiolate (ZT) to form the ionic association in the rubber system. This work investigated and compared the unfilled and natural rubber filled with 5phr of carbon black. The recycling process was repeated three times, and the mechanical performance was measured each time. Tensile strength was increased by more than 430% for unfilled rubber and 520% for NR/5CB composites after the third recycling process. Tear strength was also increased with the number of the recycling process. According to a welding test ability, the developed materials showed potential for repair. Scanning electron micrographs revealed that as the recycling number increased, the white spot of ZT responsible for generating the ionic network reduced as more ZT was converted into Zn2+ salt bonding.

Strain-induced crystallisation of reinforced elastomers using surface calorimetry

The crystallinity of rubbers which are amenable to crystallisation upon stretching is classically measured using X-ray diffraction (XRD) techniques. Recently, an alternative method for evaluating the crystallinity in unfilled natural rubber (NR) during stretching was proposed. This method is based on surface calorimetry principles (Le Cam, 2018) and was validated by XRD measurements in Le Cam et al. (2018). For reinforced rubber composites, the calorimetric response is not only due to the elastic couplings and the crystallisation/melting process, but also due to the intrinsic dissipation and additional viscoelastic dissipation resulting from the presence of fillers. This makes estimating the crystallinity of carbon black (CB) reinforced rubber compounds from calorimetric techniques typically more challenging. In this paper, however, the calorimetric approach for measuring crystallinity for CB reinforced rubber compounds was investigated. Crystallinity measurements were performed using both an unfilled and three CB reinforced NR compounds. A comparative study was performed using the XRD technique on the same four compounds. The results confirmed the appropriateness of the calorimetric technique for determining the crystallinity in the unfilled NR. The combination of the XRD and the calorimetric techniques allowed the different heat sources under strain to be identified and therefore, the thermal energy associated with crystallisation to be determined. This was done initially using one of the CB reinforced NR compounds and validated using the two other CB reinforced NR compounds. The results show reasonable correlation between the crystallinity measured using the novel calorimetric technique and the classical XRD technique for CB reinforced NR compounds. The development of this methodology has potentially significant impact. The calorimetric approach does not require the measurement of the mechanical response during testing. This is useful for measuring crystallinity gradients from infrared thermography coupled with full kinematic field measurements, typically at crack tips, where estimating the local mechanical response is very difficult or inaccurate.

ПИРИТНЫЕ ОГАРКИ КАК НАПОЛНИТЕЛЬ КОМПОЗИЦИОННОГО МАТЕРИАЛА НА ОСНОВЕ БУТАДИЕН-СТИРОЛЬНОГО КАУЧУКА

Currently, recycling waste from various types of production, as well as finding the possibility of reusing it, is a very relevant topic. In the modern world, rubbers make a tremendous contribution to the development of the industrial sector. In order to study the possibility of using pyrite cinders as a filler for a rubber mixture based on styrene-butadiene rubber, the elastic-strength and rheological characteristics of the finished composite mixtures were studied, and the contact angle of wetting was determined to assess the interaction of the compositions with other materials. It was established that there was no influence of pyrite cinders on the nature of the surface of the rubber mixture. The dependence of the storage modulus on the deformation amplitude for different types of fillers has been established, which makes it possible to evaluate the qualitative and quantitative distribution of the filler in the polymer and the interaction of the polymer and the filler. It was revealed that pyrite cinders and chalk have a weak interaction with the polymer, from which it follows that they behave as a non-reinforcing filler, but their introduction into the polymer matrix does not deteriorate the elastic-strength properties of the rubber. The dependence of stress on deformation for mixtures with different types of fillers has been established. It was established that in terms of technological characteristics, no deviations in the material from the generally accepted patterns of introducing fillers into the polymer matrix were found. The values remain at the level of unfilled rubber and rubber filled with technical chalk. The general complex of properties of the composition based on styrene-butadiene rubber with pyrite cinders allows it to be recommended, after modification, for use in the production technology of rubber products.

Styrene–butadiene rubber‐based nanocomposites toughened by carbon nanotubes for wide and linear electromechanical sensing applications

AbstractThe contribution from specific interactions between rubber and filler sometimes plays a crucial role in the mechanical reinforcement and electromechanical sensitivities of rubber composites. This research explores the physical, mechanical, and electromechanical properties of styrene‐butadiene rubber (SBR) composites reinforced with carbon nanotubes (CNTs). Mechanical, thermodynamic, and scanning electron microscopy studies reveal that CNTs primarily enhance the mechanical properties through physical interactions with the rubber. Notably, π‐stacking interactions between the aromatic π‐electrons in SBR and the delocalized π‐electrons in CNTs contribute to significant improvements in mechanical strength. For instance, adding 4 phr (per hundred grams of rubber) of CNTs results in an impressive 871% increase in fracture toughness compared to unfilled rubber. These composites also exhibit remarkable stress and strain improvements even with small amounts of CNTs, making them suitable for robust strain sensing applications. Specifically, the composite containing 3 phr of CNTs demonstrates superior strain sensitivity (2.87 < GF < 48.62), a wide detection range (0% < Δε < 187%), and excellent linearity (0.927 < R2 < 0.995). Lower CNT content enhances sensitivities but reduces overall linearity, while higher CNT content improves linearity but decreases sensitivity. Moreover, these composites perform exceptionally well under compressive forces, even at a 0.5 N applied force. Given their excellent response to mechanical forces and strain, these composites hold promise for applications like electronic skin, strain sensors, and energy‐harvesting devices.Highlights Carbon nanotubes (CNTs) were used as reinforcing fillers in SBR composites. CNTs significantly improve physical, mechanical, and electrical properties. Primarily, π‐stacking may be involved in enhancing these properties. The composites exhibit strong strain and force‐sensing capabilities. These composites could be valuable for wearable electronics in the future.

Characterization and transport properties of ceramic filler incorporated natural rubber composites

AbstractCrystalline glass–ceramic fillers were prepared from calcium carbonate, silica, alumina, and calcium fluoride by heating and subsequent quenching in cold water. The fillers were incorporated into natural rubber (1,4‐cis‐polyisoprene) and the filled rubber composites were crosslinked with sulfur in the presence of different rubber additives. The unfilled and filled rubber composites were characterized. The transport properties of benzene, toluene, and p‐xylene (BTX) through the rubber composites were studied in terms of sorption, diffusion, permeation, and mass transfer coefficients. The effect of the ceramic fillers on the mechanical, thermal and transport properties were studied. The sorption data at different temperatures were used for calculating activation energy of diffusion, permeation, free energy, and enthalpy of sorption. The BTX remained in the liquid state within the composite matrix as evident from negative ΔS. The diffusion coefficient (D) and mass transfer coefficient (kmtc) of BTX decreased with the increase in filler loading. Accordingly, for the transport of BTX the unfilled rubber showed a D (D × 107 cm2/s) and mass transfer coefficient (kmtc × 104 cm/s) of 5.67/3.97/2.96 and 7.71/7.08/7.04, respectively which decreased to 5.06/2.95/2.57 and 7.53/6.95/6.90, respectively for the composite containing 50 wt.% ceramic filler.

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Fabrication of High-Performance Natural Rubber Composites with Enhanced Filler-Rubber Interactions by Stearic Acid-Modified Diatomaceous Earth and Carbon Nanotubes for Mechanical and Energy Harvesting Applications.

Mechanical robustness and high energy efficiency of composite materials are immensely important in modern stretchable, self-powered electronic devices. However, the availability of these materials and their toxicities are challenging factors. This paper presents the mechanical and energy-harvesting performances of low-cost natural rubber composites made of stearic acid-modified diatomaceous earth (mDE) and carbon nanotubes (CNTs). The obtained mechanical properties were significantly better than those of unfilled rubber. Compared to pristine diatomaceous earth, mDE has higher reinforcing efficiencies in terms of mechanical properties because of the effective chemical surface modification by stearic acid and enhanced filler-rubber interactions. The addition of a small amount of CNT as a component in the hybrid filler systems not only improves the mechanical properties but also improves the electrical properties of the rubber composites and has electromechanical sensitivity. For example, the fracture toughness of unfilled rubber (9.74 MJ/m3) can be enhanced by approximately 484% in a composite (56.86 MJ/m3) with 40 phr (per hundred grams of rubber) hybrid filler, whereas the composite showed electrical conductivity. At a similar mechanical load, the energy-harvesting efficiency of the composite containing 57 phr mDE and 3 phr CNT hybrid filler was nearly double that of the only 3 phr CNT-containing composite. The higher energy-harvesting efficiency of the mDE-filled conductive composites may be due to their increased dielectric behaviour. Because of their bio-based materials, rubber composites made by mDE can be considered eco-friendly composites for mechanical and energy harvesting applications and suitable electronic health monitoring devices.

Combined Sulfur and Peroxide Vulcanization of Filled and Unfilled EPDM-Based Rubber Compounds.

The sulfur curing system, peroxide curing system and their combinations were applied for the cross-linking of unfilled and carbon black-filled rubber formulations based on ethylene-propylenediene-monomer rubber. The results demonstrated that the type of curing system influenced the course and shape of curing isotherms. This resulted in the change of curing kinetics of rubber compounds. The cross-link density of materials cured with combined vulcanization systems was lower than that for vulcanizates cured with the peroxide or sulfur system. Good correlation between the cross-link density as well as the structure of the formed cross-links and physical-mechanical characteristics of the cured materials was established. Both filled and unfilled vulcanizates cured with combined vulcanization systems exhibited a higher tensile strength and elongation at break when compared to their equivalents vulcanized in the presence of the peroxide or sulfur curing system. It can be stated that by proper combination of vulcanization systems, it is possible to modify the tensile behavior of vulcanizates in a targeted manner. On the other side, dynamical-mechanical properties were found not be significantly influenced by the curing system composition.

A Modified Constitutive Model for Isotropic Hyperelastic Polymeric Materials and Its Parameter Identification.

Given the importance of hyperelastic constitutive models in the design of engineering components, researchers have been developing the improved and new constitutive models in search of a more accurate and even universal performance. Here, a modified hyperelastic constitutive model based on the Yeoh model is proposed to improve its prediction performance for multiaxial deformation of hyperelastic polymeric materials while retaining the advantages of the original Yeoh model. The modified constitutive model has one more correction term than the original model. The specific form of the correction term is a composite function based on a power function represented by the principal stretches, which is derived from the corresponding residual strain energy when the Yeoh model predicts the equibiaxial mode of deformation. In addition, a parameter identification method based on the cyclic genetic-pattern search algorithm is introduced to accurately obtain the parameters of the constitutive model. By applying the modified model to the experimental datasets of various rubber or rubber-like materials (including natural unfilled or filled rubber, silicone rubber, extremely soft hydrogel and human brain cortex tissue), it is confirmed that the modified model not only possesses a significantly improved ability to predict multiaxial deformation, but also has a wider range of material applicability. Meanwhile, the advantages of the modified model over most existing models in the literatures are also demonstrated. For example, when characterizing human brain tissue, which is difficult for most existing models in the literature, the modified model has comparable predictive accuracy with the third-order Ogden model, while maintaining convexity in the corresponding deformation domain. Moreover, the effective prediction ability of the modified model for untested equi-biaxial deformation of different materials has also been confirmed using only the data of uniaxial tension and pure shear from various datasets. The effective prediction for the untested equibiaxial deformation makes it more suitable for the practice situation where the equibiaxial deformation of certain polymeric materials is unavailable. Finally, compared with other parameter identification methods, the introduced parameter identification method significantly improves the predicted accuracy of the constitutive models; meanwhile, the uniform convergence of introduced parameter identification method is also better.

Synergistically toughened silicone rubber nanocomposites using carbon nanotubes and molybdenum disulfide for stretchable strain sensors

Stretchable rubber nanocomposites have been investigated for detection in piezoresistive strain sensors. Tensile toughness must be improved while maintaining stretchability to realize high-performance, robust, stretchable nanocomposites. We investigate a stretchable and synergistically toughened silicone nanocomposite-based, piezoresistive strain sensor using multiwalled carbon nanotubes (MWCNTs) and molybdenum disulfide (MoS2). One-dimensional MWCNTs mechanically reinforce the silicone rubber matrix, improving tensile strength by 95% but decreasing fracture strain by 27% at 5 phr, providing an electrical percolation network. Adding two-dimensional nanolayers of MoS2 as a single filler increases fracture strain from 205% (unfilled) to 320% (5 phr MoS2), possibly due to cure retardation in the silicone rubber, preventing over-curing. Silicone nanocomposites exhibit improved tensile toughness at 8.46 kJ/m3 for 5-phr MWCNT-MoS2 hybrids due to the synergistic effects of the hybrid filler, a 125% improvement over unfilled rubber. Nanocomposites maintain a low elastic modulus (<0.45 MPa) suitable for stretchable sensors. The nanocomposite utility is demonstrated as body-attachable, piezoresistive strain sensing with a gauge factor of ∼25.95 (100%<Δε<155%), stretchability of up to 155%, and durability of over 5000 cycles. Toughened nanocomposites with MWCNT and MoS2 hybrid fillers could be useful for applications requiring robust stretchability and sensitivity.

Competition between Strain-Induced Crystallization and Cavitation at the Crack Tip of Unfilled and Carbon Black-Filled Natural Rubber

Competition between Strain-Induced Crystallization and Cavitation at the Crack Tip of Unfilled and Carbon Black-Filled Natural Rubber

Development of Sustainable, Mechanically Strong, and Self-Healing Bio-Thermoplastic Elastomers Reinforced with Alginates.

New bio-thermoplastic elastomer composites with self-healing capacities based on epoxidized natural rubber and polycaprolactone blends reinforced with alginates were developed. This group of salts act as natural reinforcing fillers, increasing the tensile strength of the unfilled rubber from 5.6 MPa to 11.5 MPa without affecting the elongation at break (~1000% strain). In addition, the presence of ionic interactions and hydrogen bonds between the components provides the material with a thermally assisted self-healing capacity, as it is able to restore its catastrophic damages and recover diverse mechanical properties up to ~100%. With the results of this research, an important and definitive step is planned toward the circularity of elastomeric materials.

Effect of black sand nanoparticles on physical-mechanical properties of butyl rubber compounds

A novel type of silica namely black sand (BS) with silane coupling agent was chosen as reinforcing filler in butyl rubber (IIR). BS was obtained from the beach area of Rosetta in Egypt. The filler was characterized by Fourier-transform infrared (FTIR), X-ray diffraction (XRD), and transmission electron microscope (TEM). The XRD analysis showed that BS is composed mainly of Albite (Sodium Aluminium Silicate) up to 64% approximately. Novel nanocomposites were prepared via a two-roll mill by incorporating different concentrations of BS nanoparticles (5,10, 15, 20,30 and 40 phr) in IIR. The effects of filler loading on shape factor, curing characteristics, and mechanical and dynamic properties of IIR vulcanizates were investigated. The modulus of IIR vulcanizates was used to predict the shape factor of BS aggregation in the polymer. The data indicated that cure time decreased with the increase in filler loading, and the change in scorch time is less. It was found that the shape factor is independent of the concentration. With an increase in the filler loading, tensile and elongation of the composites were significantly increased, especially at 30 phr. The bound rubber content increased with increasing BS content up to 30 phr exhibiting the rubber-filler interaction. The reinforcing factor, R (related to the difference in tan δ peak height at the glass transition temperature (Tg) for the filled and unfilled rubbers), also demonstrated that the BS-IIR interaction was stronger than for unfilled rubber. Since 30 phr black sand loaded IIR composite showed superior physical-mechanical properties. In general, the results indicated that the black sand filler was an effective reinforcement material for the butyl rubber.

In situ–produced polyurethane-based thermoplastic elastomers: the effect of the acrylonitrile content of the rubber phase

We prepared thermoplastic dynamic vulcanizates (TDVs) with an in situ–produced thermoplastic polyurethane (TPU) matrix. We used unfilled acrylonitrile butadiene rubber (NBR) with different acrylonitrile contents and investigated the effect of the polarity of the rubber phase on the properties of the TDV samples. The results show that the acrylonitrile (AN) content of the rubbers affects the tensile strength and modulus slightly but the strain at break of the TDVs only negligibly. The results of TDVs are in between those of TPU and unfilled NBRs, demonstrating the feasibility of the TPU/NBR combination.

First evidence and characterization of strain-induced crystallization heterogeneity in natural rubber under homogeneous strain states

This paper provides the first evidence of strain-induced crystallization heterogeneities at the macroscopic scale, accompanied with strong memory effects during uniaxial cyclic tensile loadings in unfilled natural rubber. Specimens with two different sulphur amounts, 1.2 phr and 1.6 phr, and two different geometries, small and large widths, have been considered. The crystallinity has first been evaluated in the middle of each specimen thanks to a quantitative surface calorimetry approach based on infrared thermography. The calorimetric approach was then used to simultaneously access the crystallinity at different points of the tested specimens and to investigate spatial variations in the crystallinity along a profile at the middle of each specimen. SIC was found to occur heterogeneously along the profile, i.e. at the macroscopic scale, whatever the geometry and the sulphur amount are. Moreover, crystallization and melting initiate in the same zones during load-unload cycles. Several families in terms of heat sources due to SIC have been identified.

Unfilled Natural Rubber Compounds Containing Bio-Oil Cured with Different Curing Systems: A Comparative Study

This study focuses on the properties of unfilled natural rubber compounds containing bio-oils cured with a peroxide curing system and then discusses the comparisons to those cured using the sulfur system from our previous work. Two types of bio-oils, i.e., palm oil and soybean oil, were used, and distillate aromatic extract (DAE)-based petroleum oil was employed as a reference. The bio-oils caused no significant change in the vulcanization of rubber compounds cured using peroxide. However, the compounds containing bio-oils and cured with sulfur showed a faster vulcanization than the ones with DAE. The bio-oils strongly affected the crosslink density of rubber compounds in both curing systems. The use of bio-oils caused a low crosslink density due to the possible implication of curing agents to bio-oil molecules. The properties of rubber compounds dependent on the different levels of crosslink density were also investigated. The results revealed that when the crosslink density increased, the modulus, tensile strength, and hardness of the rubber compounds increased and the elongation at break and compression set decreased. The use of bio-oils in the rubber compounds cured with different curing systems gave low modulus at 300% strain, tensile strength, and hardness but high elongation at break and compression set when compared to the ones with DAE. However, no significant change was observed for the compression set of the rubber compounds cured using sulfur. With the presence of bio-oils, the properties of rubber compounds cured with sulfur system deteriorated less than those of the ones cured with peroxide.