Rubber Network Research Articles

Structure–property relationships of fatty acid swollen, crosslinked natural rubber shape memory polymers

ABSTRACTThis article investigates shape memory polymers (SMPs) fabricated by swelling sulfur crosslinked natural rubber with four different molten fatty acids: lauric, myristic, palmitic, and stearic acid. As inexpensive additives, they allow commodity natural rubber to be directly converted to SMPs. The shape memory properties are investigated as a function of wt% fatty acid, the choice of fatty acid, and the applied load during shape memory programming. It is found that increasing the wt% acid improves the shape fixity up to ca. 97% at ≥50 wt% fatty acid, at which point the recovery starts to decline with increasing wt% acid due to network failure during shape programming. The shape fixity is found to depend on the yield stress and modulus of the fatty acid network, which both increase with increasing wt% acid. The choice of fatty acid also varies the trigger temperature for shape memory, which scales with the melting point of the fatty acid. Serendipitously, it is found that alignment of the fatty acid crystals during programming produces stiffer networks whose modulus increase with applied load, which counterbalances the higher elastic energy stored in the rubber network to produce lower sensitivity of the shape fixity to the applied load. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2018,56, 673–687

Thermally Reversible Cross-linkers To Facilitate the Improved Reprocessability of Poly(butyl methanol methacrylate) Rubber with Excellent Thermal and Mechanical Properties

This work takes advantage of the unique thermally reversible dissociation/association feature of C–ON bonds in alkoxyamine (Diol) to facilitate the reprocessability of poly(butyl methanol methacrylate) (PBMM) rubber with excellent thermal and mechanical properties. Isocyanate groups were incorporated into the PBMM backbone and then reacted with hydroxyl groups in Diol to obtain cross-linked PBMM rubber. Although its cross-linking degree is slightly lower than the conventional cross-linked rubber by an irreversible cross-linker (Dieg), it shows a higher tensile strength and elongation at break. Its decomposition temperature is about 50 °C higher than PBMM cross-linked by Dieg. Furthermore, the dissociation of C–ON bond at high temperature can break the cross-linking network of PBMM rubber to form a flowable and reprocessable melt and show an excellent ability of reshaping and healing; the association of thermally reversible C–ON bond can completely recover the cross-linking network and mechanical propertie...

Dominant formation of disulfidic linkages in the sulfur cross-linking reaction of isoprene rubber by using zinc stearate as an activator.

A linear combination fitting in sulfur K-edge X-ray absorption near edge structure (S-XANES) measurements reveals each fraction of monosulfidic, disulfidic and polysulfidic linkages in solvent extracted sulfur cross-linked isoprene rubbers. The sulfidic linkage of a disulfidic type is found for the first time to be dominant when zinc stearate and N-(1,3-benzothiazol-2-ylsulfanyl)cyclohexanamine are used as the activator and accelerator, respectively, for the sulfur cross-linking reaction at 140 °C. The presence of the bridging bidentate zinc/stearate complex as an intermediate for the sulfur cross-linking reaction is supposed to induce the generation of abundant disulfidic linkages in the rubber networks. This unexpected observation is of use for the material design of high performance rubber products with anti-aging and thermal stabilities. S-XANES is a powerful tool that was used to reveal the characteristics of the sulfur cross-linking of rubber. These results will contribute to the development of rubber science and technology.

Thermoplastic–elastomer composition based on an interpenetrating polymeric network of styrene butadiene rubber–poly(methyl methacrylate) as an efficient vibrational damper

A new series of interpenetrating polymer networks (IPNs) and semi-IPNs based on styrene butadiene rubber (SBR) and poly[methyl methacrylate] (PMMA) have been synthesized by sequential polymerization.

Detecting structural orientation in isoprene rubber/multiwall carbon nanotube nanocomposites at different scales during uniaxial deformation

AbstractEnhanced strain‐induced crystallization (SIC) behavior in isoprene rubber/multiwall carbon nanotube (IR/MWCNT) nanocomposites was analyzed in terms of structural orientation during uniaxial deformation. In situ synchrotron wide‐angle X‐ray diffraction and small‐angle X‐ray scattering (SAXS) reveal the molecular orientation in IR/MWCNT composites at different scales. The inclusion of MWCNTs leads to a decrease in the molecular orientation at small strain due to the promotion of SIC. Meanwhile, the presence of MWCNTs induces a large‐scale orientation within the vulcanized rubber network based on SAXS results. Considering the heterogeneous nature of the vulcanized network, the nucleation process during SIC is discussed from the viewpoint of thermodynamics. The oriented large‐scale structure in IR/MWCNT composites is composed of local rubber chains stretched up MWCNTs, from which the additional nuclei are induced. By forming a bound rubber layer around MWCNTs through attractive interactions, MWCNTs can amplify the local strain of rubber segments and form a highly oriented large‐scale structure, but without altering the overall molecular orientation level. The evolution of detailed structural orientation in MWCNT‐filled rubber composites during deformation is revealed for the first time. © 2017 Society of Chemical Industry

Effect of zinc oxide induced metal-ligand crosslink on the mechanical properties in the ethylene acrylic elastomer (AEM)

The metal-ligand complex (sacrificial bonds) that exist in the mussels, bone and silk can dissipate energy and reinforce mechanical strength. Inspired by the high performance of metal-ligand complex, we introduced a kind of metal-ligand crosslink into the rubber network. In this work, ethylene acrylic elastomer (AEM) was filled with ZnO, and was cured by the amine cure system. The metal-ligand crosslink structure was verified by FTIR and XPS, and the tensile strength and dynamic properties were examined with a universal testing machine and rubber process analyzer (RPA). The results showed that coordination crosslink formed in the AEM matrix and exhibited outstanding tensile strength and unexpected dynamic properties due to the coordination bonds between Zn2+ and amine group.

Non―Gaussian-Type Tube-Based Entropy Model for Elastomeric Networks with Polymer Phase Influenced by Filler Loading: Data Analysis for Lightly to Highly Carbon-Black Filled Styrene-Butadiene Rubber Networks

ABSTRACTRecently the author proposed a new, non-Gaussian, tube-based entropy model for filled elastic networks, characterized by a filler-environmental parameter that was proposed to represent the extent of filler-agglomeration; it was specifically introduced for the polymer phase entropically affected by the presence of very irregularly structured fillers. In this report, the stress-strain relations derived from the above model were used together with the traditional and latest mechanisms of amplification of the strain due to the presence of a filler: namely, literature tensile data for styrene-butadiene rubber networks with a series of carbon-black loadings were systematically analyzed to understand the filler-loading dependence of the above parameter. As a consequence, it is proposed that this parameter is linked, from the static-mechanical standpoint, to a critical filler-loading coupled to the filler-networking. In addition, prior to commencing the examinations of the high-loading networks' tensile data, the data-fitting performance of the underlying elastic molecular model for non-filled networks was also examined using the unfilled and low-loading networks' tensile curves.

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.

A thermo-viscoelastic-damage constitutive model for cyclically loaded rubbers. Part I: Model formulation and numerical examples

Cyclically loaded rubbers exhibit a complex history-dependent response characterized by fatigue-induced stress-softening and hysteresis along with dissipative heating. The coupling between these different inelastic effects usually appearing together is far from being fully established. In this Part I of two-part paper, we present a new thermo-viscoelastic-damage approach, in accordance with the thermodynamic principles, for the prediction of this set of inelastic fatigue phenomena. An interpretation of the underlying physical mechanisms is proposed in which two types of dissipative network rearrangements are considered, i.e. recoverable rearrangements inducing viscoelasticity and unrecoverable rearrangements inducing damage. The recoverable viscoelastic rearrangements are assumed to be induced by the move of entangled and non-entangled free chains superimposed on a purely elastic perfect rubber network. Each population of free chains is considered to be the main source of one aspect of the history-dependent mechanical cyclic features, i.e. stress-softening and hysteresis, respectively. The thermo-mechanical coupling is defined by postulating the existence of a free energy in which two internal state variables are introduced to account for the two types of dissipative network rearrangements. Network thermal kinetics, induced by the dissipative heating, as well as network damage kinetics, induced by the fatigue damage, are defined and used to alter the cyclically loaded perfect rubber network. The proposed constitutive model is implemented into a finite element program and a parametric study is presented via numerical applications on rubber structures in order to analyze the effects of key model parameters on the rubber inelastic fatigue response. A focus is especially made on the respective influence of temperature, viscoelasticity and damage on the rubber softening.

Liquefaction of ground tire rubber at low temperature

Low-temperature liquefaction has been investigated as a novel method for recycling ground tire rubber (GTR) into liquid using an environmentally benign process. The liquefaction was carried out at different temperatures (140, 160 and 180 °C) over variable time ranges (2–24 h) by blending the GTR with aromatic oil in a range from 0 to 100 parts per hundred rubber (phr). The liquefied GTR was separated into sol (the soluble fraction of rubber which can be extracted with toluene) and gel fractions (the solid fraction obtained after extraction) to evaluate the reclaiming efficiency. It was discovered that the percentage of the sol fraction increased with time, swelling ratio and temperature. Liquefied rubber was obtained with a high sol fraction (68.34 wt%) at 140 °C. Simultaneously, separation of nano-sized carbon black from the rubber networks occurred. The separation of carbon black from the network is the result of significant damage to the cross-linked-network that occurs throughout the liquefaction process. During liquefaction, a competitive reaction between main chain scission and cross-link bond breakage takes place.

Thermo-mechanical recycling of rubber: Relationship between material properties and specific mechanical energy

The studied and optimized devulcanization process is known as “High Shear Mixing” (HSM) recycling process. In the process the rubber is sheared between two metallic cones with special geometries. One cone is static as the other one rotates simultaneously and applies pressure to the material.Among the different parameters that are controlled and/or measured during the process two are highlighted for their importance: the temperature of the rubber, and the specific mechanical energy consumed during the process. It is shown that the energy consumed by the rotor can be correlated to the degree of devulcanization of the rubber which is measured by means of physicochemical analyses. An optimal state of surface activation on the treated rubber is also described. A physical model of the rubber network evolution along the HSM treatment is proposed.

A novel interpenetrating polymer network of natural rubber/regenerated cellulose made by simple co-precipitation

A novel interpenetrating polymer network (IPN) structure consisting of natural rubber (NR) and cellulose was successfully prepared using a simple co-precipitation process. Cellulose was first dissolved in the aqueous alkali/urea system, and then it was mixed with NR latex. The mixed NR/cellulose solution was co-precipitated with ethanol. During the fast co-precipitation process, both rubber network and cellulose network were formed and interpenetrated simultaneously. Scanning electron microscopy (SEM) clearly demonstrated that the two networks could be interlaced to form a full-IPN structure. This simple method provides a new insight into the fabrication of novel IPNs from cellulose and broadens the potential applications of cellulose in rubber compounds.

NEW INSIGHTS INTO RUBBER NETWORK STRUCTURE BY A COMBINATION OF EXPERIMENTAL TECHNIQUES

ABSTRACTRobust quantitative cross-link density characterization becomes necessary for the complete understanding of the structure and optimization of final properties of rubber compounds for industrial applications. A combination of different experimental techniques have been used to establish the quantitative consistency on the correlations between the results obtained by the individual methods within a reliable unique (physically based) platform reclined on the concept of rubber elasticity that considers the impact of entanglements in technical rubbers. The contribution of cross-links and elastically active entanglements to mechanical properties has been quantified by the analysis of uniaxial stress–strain measurements by means of the extended tube model of rubber elasticity. In a complementary manner, rubber network structure has also been investigated by state-of-the-art multiple-quantum low-field NMR experiments and classical T1 and T2 relaxation measurements. In addition, equilibrium swelling data were analyzed by the classical phantom and Flory–Rehner limits as well as by applying the theoretical approach proposed by Helmis, Heinrich, and Straube that takes into account topological constraints during swelling. Correlations among these complementary techniques have been reported, and the interpretation of the obtained differences is addressed. The baseline study focuses on unfilled NR, setting the basis for the investigation of unfilled SBR matrices and filled rubbers.

Devulcanization of Scrap Tire Rubber with Supercritical CO2: A Study of the Effects of Process Parameters on the Properties of Devulcanized Rubber

Abstract A continuous extrusion devulcanization process has been developed by using supercritical CO2. Experiments have been carried out on an industrial scale twin screw extruder to study the effects of processing conditions on the properties of devulcanized rubber. Using a fractional factorial design in three levels, the effects of process temperature, screw speed, and feed rate on the sol fraction, degree of devulcanization, Mooney viscosity, and tensile properties of devulcanized rubber have been investigated. The significance of the effects of the processing conditions decreases in the following order: flow rate, screw speed, and process temperature. Similar trends are noted for different responses with the exception of tensile properties. The observed relationship between the sol fraction and the degree of crosslinking in the rubber samples before and after devulcanization points to a highly selective disruption of the sulfur crosslinks in the rubber network during devulcanization rather than a random network scission. Mooney viscosity has been found to be in excellent agreement with the network structure properties, sol fraction, and degree of devulcanization.

Quality Characterization of Dried Crumb Rubber

Rubber research on how the rubber quality being affected by drying process is rather limited in the public domain. This paper aims to summarize the findings of the quality characterization of rubber based on total rubber network and thermo-oxidative phenomenon based on three different drying strategies (vacuum drying, hot air drying and infrared drying). In this study, vacuum drying has the highest drying rate and internal mass transfer mechanism comparing to hot air drying and infrared drying in a fixed drying condition. The quality test showed that rubber dried at temperature between 65°C to 90°C could ensure preservation of rubber strength as in total rubber network and rubber’s plasticity. There is strong correlation between the drying temperature and rubber quality, where a balance between the drying temperature and the drying time is necessarily to maximize the quality of the dried crumb rubber.

A Combined Experimental and Molecular Simulation Study of Factors Influencing the Selection of Antioxidants in Butadiene Rubber.

For the selection of antioxidants, internal factors were proposed by analyzing the thermal-oxidative aging process, which consisted of the following two inseparable steps: (1) the physical process of oxygen (O2) entering the rubber network and (2) the complex chemical process of O2 reacting with the rubber network. Antioxidants 2246, 6PPD, and MB, examples of amines, phenols, and heterocycles, respectively, were chosen to study these factors influencing the selection of antioxidants for the thermal-oxidative aging of butadiene rubber (BR). Through thermogravimetric analysis coupled with Fourier transform infrared spectroscopy and kinetic analysis by the Flynn-Wall-Ozawa method, the dissociation reaction of BR was identified to be the rate-determining step for the thermal-oxidative aging of BR. Meanwhile, the decisive positions of the dissociation reactions for the three antioxidants in improving the thermal-oxidative stability of BR were also identified. Therefore, the internal factors were subdivided into five items (i.e., the free energy of reaction for the dissociation of antioxidant, the mole ratio of active radicals or hydroperoxides that could react with the same mass of antioxidant, the solubility and mobility of the antioxidant in BR, and the permeability of O2). Combined with molecular dynamics simulations and quantum mechanics simulations, the five internal factors were clarified and quantified over the entire usable temperature range of BR. To clarify the relative importance of each factor in the selection of antioxidants, we identified the time-dependent tensile strength and elongation at break as the only responses for the first and second gray relational analyses. The relative importance of the five internal factors was evaluated and ranked in terms of gray relational grade. The two analyses were consistent and showed that, in the selection of antioxidants, we should give priority to the free energy of the dissociation reaction and the permeability of O2.

Evaluation of structural change during fast transformation process of cross-linked NR into liquid NR by light pyrolysis

The presence of cross-linked networks in rubber creates a tremendous problem for recycling and reusing of waste rubber. Fast transformation of cross-linked natural rubber (NR) into liquid natural rubber was carried out by light pyrolysis in temperature range from 240 to 300 °C in variable time (in range: 1–30 min). The transformation efficiency was evaluated by measuring the sol fraction and the cross-link density of the gel fraction. Over 80 wt% sol fraction was obtained at 280 °C in 15 min and 100 wt% sol fraction was obtained at 300 °C in 3 min. The chemical structure of sol fraction was characterized by gel permeation chromatography, nuclear magnetic resonance and infrared spectroscopy. The molecular weight of sol fraction in cross-linked NR processed at 280 °C and 300 °C decreased when gel fraction is over 50%. The nuclear magnetic resonance results showed that mono-sulfidic linkages occurred mainly in the sol fraction of liquid NR obtained at 300 °C. Isomerization from cis to trans form of isoprene unit in NR increased with extending pyrolysis time.

Enhanced mechanical properties of styrene–butadiene rubber with low content of bacterial cellulose nanowhiskers

AbstractStyrene–butadiene rubber (SBR) nano‐composites were prepared through mixing SBR latex with bacterial cellulose whiskers (BCWs). BCWs were extracted from the residual bacterial cellulose (BC) that was treated with acid hydrolysis. A rod‐like nanofiber structure of BCWs was observed by transmission electron microscopy and atomic force microscopy. BCWs maintained the crystal structure in BCWs/SBR nano‐composites as confirmed by X‐ray diffraction patterns. The dispersion of BCWs in SBR matrix was further detected with field emission scan electron microscope. BCWs acted as a multifunctional physical cross‐linker in the SBR matrix, the combination of BCW reinforcement and increased effective cross‐linking density of the rubber network with incorporating BCWs would cause a more effective transfer of stresses from matrix to filler. Compared with neat SBR, the tensile strength and tear strength of SBR/BCW nano‐composite with 2.5 phr (parts per hundred rubber) BCWs were increased by 319% and 182%, respectively.

Enabling Design of Advanced Elastomer with Bioinspired Metal-Oxygen Coordination.

It poses a huge challenge to expand the application gallery of rubbers into advanced smart materials and achieve the reinforcement simultaneously. In the present work, inspired by the metal-ligand complexations of mussel byssus, ferric ion was introduced into an oxygen-abundant rubber network to create additional metal-oxygen coordination cross-links. Such complexation has been revealed to be highly efficient in enhancing the strength and toughness of the rubbers. Significantly, such complexation also enables the functionalization of the rubber into highly damping or excellent multishape memory materials. We envision that the present work offers an efficient yet facile way of creating advanced elastomers based on industrially available diene-based rubber.

Influence of ground tire rubber devulcanization conditions on properties of its thermoplastic vulcanizate blends with copolyester

Devulcanized rubber (DR) was prepared from used truck tires via thermo-chemical devulcanization. Effects of devulcanizing reaction time and temperature were studied, and the DR contained higher sol fraction but had lower crosslink density than un-devulcanized rubber (UDR). Furthermore, the soluble fraction (sol fraction) increased with devulcanizing time and temperature. Devulcanizing for 4min at 180°C was chosen as the most efficient alternative. Then, thermoplastic vulcanizates (TPVs) from blends of copolyester (COPE) with two alternative types of recycled rubber (i.e., DR and UDR) were prepared and their mechanical, morphological, rheological, thermal and swelling properties were investigated. It was found that the dynamically cured DR/COPE blends exhibited higher mechanical and thermal resistance than the dynamically cured UDR/COPE blends. Furthermore, the complex viscosity and storage modulus of the dynamically cured DR/COPE blends decreased with devulcanizing time, due to breakdown of the rubber network and the carbon main chains in rubber molecules. In addition, phase separated micron-sized vulcanized rubber domains were finely dispersed in the COPE matrix, and their size correlated well with the mechanical strength and other related properties. However, the rubber domains agglomerated with excessive devulcanization, causing inferior mechanical strength.