Rubber–Filler Interactions and Network Structure in Relation to Stress–Strain Behavior of Vulcanized, Carbon Black Filled EPDM

Vegetable derived-oil facilitating carbon black migration from waste tire rubbers and its reinforcement effect

Three dimensional chemically cross-linked polymer networks present a great challenge for recycling and reutilization of waste tire rubber. In this work, the covalently cross-linked networks of ground tire rubber (GTR) were degraded heterogeneously under 150 °C due to the synergistic effects of the soybean oil and controlled oxidation. The degradation mechanism was discussed using Horikx theory and Fourier transformation infrared spectroscopy (FTIR). The results showed that the structural evolution of sol and gel parts, which indicated that the sols consisted of degraded GTR chains with low molecular weight, while the gels were mainly composed of bound rubber coated carbon black, which are separated from the cross-linked network of GTR in a high degradation degree. The degraded GTR compound demonstrated an excellent reinforcing effect on solution styrene-butadiene rubber (SSBR), due to the presence of core-shell structured carbon black. This work provide an efficient and economic approach to degrade GTR and transform it into useful products.

Rubber-Filler Interactions in Polyisoprene Filled with In Situ Generated Silica: A Solid State NMR Study

In this paper we used high- and low-resolution solid state Nuclear Magnetic Resonance (NMR) techniques to investigate a series of polyisoprene samples filled with silica generated in situ from tetraethoxysilane by sol-gel process. In particular, ¹H spin-lattice and spin-spin relaxation times allowed us to get insights into the dynamic properties of both the polymer bulk and the bound rubber, and to obtain a comparative estimate of the amount of bound rubber in samples prepared with different compositions and sol-gel reaction times. In all samples, three fractions with different mobility could be distinguished by ¹H T₂ and ascribed to loosely bound rubber, polymer bulk, and free chain ends. The amount of bound rubber was found to be dependent on sample preparation, and it resulted maximum in the sample showing the best dispersion of silica domains in the rubber matrix. The interpretation of the loosely bound rubber in terms of "glassy" behaviour was discussed, also on the basis of ¹H T₁ and T1ρ data.

NONLINEAR VISCOELASTIC DISSIPATION IN VULCANIZATES CONTAINING CARBON BLACK AND SILANIZED SILICA HYBRID FILLERS

Different mechanisms of large-strain viscoelastic dissipation in vulcanizates containing carbon black or silanized silica and their synergistic effects in hybrid filler tread vulcanizates were investigated. In addition, the pure effects of fillers were examined while the degree of chemical cross-linking in the rubber matrix was similar in the vulcanizates. The results revealed that nonlinear viscoelastic loss modulus under strain-controlled dynamic tests, as a measure of dissipation mechanism, is high in both carbon black and silanized silica tread vulcanizates. However, a synergistic effect in reducing loss modulus was observed in the hybrid filler vulcanizates. Conversely, storage modulus in the vulcanizates containing more silanized silica is distinguishably higher and results in a lower loss factor representing load-controlled cyclic deformation of vulcanizates, such as in the heat buildup test. Both loss factor and heat build-up reduced nonlinearly as the amount of silanized silica increased in the vulcanizates, verifying the synergistic effect of hybrid carbon black–silanized silica filler in reducing dissipative mechanisms in large-strain dynamic loadings. This feature is highly favorable for the tire industry, where the lowering of viscoelastic dissipation in tread vulcanizates is of great importance.

Molecular insight into the Mullins effect: irreversible disentanglement of polymer chains revealed by molecular dynamics simulations

The debate regarding the possible molecular origins of the Mullins effect has been ongoing since its discovery. Molecular dynamics (MD) simulations were carried out to elucidate the underlying mechanism of the Mullins effect. For the first time, the key characteristics associated with the Mullins effect, including (a) the majority of stress softening occurring in the first stretch, (b) continuous softening with stress increase, (c) a permanent set, and (d) recovery with heat treatment, are captured by molecular modeling. It is discovered that the irreversible disentanglement of polymer chains is physically sufficient to interpret these key characteristics, providing molecular evidence for this long-controversial issue. Our results also reveal that filled polymers exhibit three distinct regimes, i.e., the polymer matrix, the interface, and the filler. When subjected to external strain, the polymer matrix suffers from excess deformation, indicating strong heterogeneity within the filled polymer, which offers molecular insight for the formulation of physics-based constitutive relations for filled polymers.

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.

DO WE NEED VERY STIFF FILLERS?

The influence of reinforcing fillers on the stretching of a rubber matrix is analyzed. It is shown that a filler stiffness higher than a critical stiffness does not further enhance the stiffness of the reinforced elastomer. The stiffer filler induces a higher stress–strain concentration and causes filler–rubber dissociation or chain scission at a lower macroscopic strain. Reducing filler stiffness can reduce the stress–strain concentration and therefore delay rubber chain scission or dissociation from the filler surface. Accordingly, the toughness of the reinforced elastomer could be improved. A simple material model is developed to predict the maximum macroscopic strain without bond scission in a reinforced elastomer. It is shown that reduced filler stiffness is beneficial for cases with (i) reduced bond strength, (ii) increased rubber matrix stiffness, and (iii) increased application strain of the reinforced elastomer. The model can be used to design the appropriate filler stiffness to balance trade-offs of stiffness and toughness of reinforced elastomers.

SILICA GRAFTED WITH EPOXIDIZED LIQUID POLYBUTADIENES: ITS BEHAVIOR AS FILLER FOR TIRE TREAD COMPOUNDS

Achieving high polymer-filler interaction and lowering the energy consumption required to disperse precipitated silica in a rubber matrix have been the main motivations behind the recent interest in silica-coating processes. A simple, low-cost, and environmentally friendly process is available to graft epoxidized liquid polybutadienes onto a silica surface. The polymer-grafted silicas are applied as reinforcing fillers in typical car tire tread compounds. The processability, cure kinetics, and properties of the vulcanizates are greatly affected by the degree of epoxidation, molar mass, and microstructure of the epoxidized polybutadienes used for coating the silica surface. The dynamic-mechanical behavior is superior to that of the reference compound, using bis-[triethoxysilylpropyl] tetrasulfide as a coupling agent, in stiffness and hysteresis at low temperatures, which is indicative of superior performance in wet grip and emergency maneuvers (hard-handling). Thus, the use of this reinforcement system for high-performance car tires, for which safety features should be prioritized, is promising.

REVIEW OF THE RECLAIMING OF RUBBER WASTE AND RECENT WORK ON THE RECYCLING OF ETHYLENE–PROPYLENE–DIENE RUBBER WASTE

Rubbers do not decompose easily, and therefore, disposal of rubber waste is a serious environmental concern. Raw material costs, diminishing natural resources, and the growing awareness of environmental issues and sustainability have made rubber recycling a major area of concern. Reclaiming and recycling rubber waste is a major scientific and technological challenge facing rubber scientists today. This article reviews a number of important areas related to the reclaiming, characterizing, testing, and recycling of rubber waste. These include chemical and microbial devulcanization with particular emphasis on main chain scission and kinetics of chemical devulcanization reactions; the cutting-edge techniques for reclaiming devulcanized rubber waste by the action of large shearing forces, heat, and chemical agents: and analytical techniques and methods for characterizing composition and testing of devulcanized rubber waste, respectively. In addition, some aspects of the recycling of devulcanized ethylene–propylene–diene rubber (EPDM) waste will be reported. EPDM is used extensively in automotive components worldwide, and recycling the rubber at the end of its useful service life is of major importance to manufacturers of automotive components.

Nanostructures and Dynamics of Macromolecules Bound to Attractive Filler Surfaces

We report in situ nanostructures and dynamics of polybutadiene (PB) chains bound to carbon black (CB) fillers (the so-called "bound polymer layer (BPL)") in a good solvent. The BPL on the CB fillers was extracted by solvent leaching of a CB-filled PB compound and subsequently dispersed in deuterated toluene to label the BPL for small-angle neutron scattering and neutron spin echo techniques. The results demonstrate that the BPL is composed of two regions regardless of molecular weights of PB: the inner unswollen region of ≈ 0.5 nm thick and outer swollen region where the polymer chains display a parabolic profile with a diffuse tail. In addition, the results show that the dynamics of the swollen bound chains can be explained by the so-called "breathing mode" and is generalized with the thickness of the swollen BPL.

COMBINED DIELECTRIC AND MECHANICAL INVESTIGATION OF FILLER NETWORK PERCOLATION BEHAVIOR, FILLER–FILLER CONTACT, AND FILLER–POLYMER INTERACTION ON CARBON BLACK–FILLED HYDROGENATED ACRYLONITRILE–BUTADIENE RUBBER

The filler network percolation behavior, filler–filler contact, and filler–polymer interaction of carbon black (CB)–filled hydrogenated acrylonitrile–butadiene rubber were investigated with the combination of dielectric and mechanical properties, by means of changing CB concentration, CB-specific surface area, cycling deformation for vulcanizates, and rotation speed of rotors in the internal mixer during the mixing process. A dielectric relaxation spectrometer was used to determine the percolation threshold of the CB network, which was made up of filler–filler contact, combined with the percolation theory. Dynamic mechanical analysis and a universal testing machine were employed to observe the filler–filler contact and filler–polymer interaction. The filler–filler bonds were determined by analysis of the Arrhenius plot on the basis of temperature sweep. The filler–polymer interaction and filler–filler contact were determined by hysteresis loss dependence and analysis of the reinforcement factor on the basis of strain sweep and quasi-static stress–strain behavior. The model of variable network density proposed by Maier and Goritz was applied to explain the contribution of stable and unstable bonds, which constituted the filler–polymer interaction, to storage modulus. The results showed that the percolation threshold of the filler network increased with an increase in rotation speed and a decrease in CB-specific surface area, which indicated that the filler–filler contact was affected by increasing rotation speed to some degree. Cycling deformation did not obviously affect the percolation threshold. This means that the filler–filler contact was not obviously affected by cycling deformation. However, the filler–polymer interaction was weakened by this procedure, according to careful investigation for the variation of some mechanical properties. We found new interesting correlations between the macroscopic mechanical properties and CB concentration. The molecular nature of these findings was discussed, combined with the overlap model and traditional molecular slip theory.

EPDM RECYCLED RUBBER POWDER CHARACTERIZATION: THERMAL AND THERMOGRAVIMETRIC ANALYSIS

Investigations on ethylene–propylene–diene rubber (EPDM) with varying cross-link densities and carbon black contents, as well as commercial EPDM waste ground rubber (WGR), have been performed to improve characterization. The aim is to provide additional quality-control methods in the field of rubber recycling, as WGR is transformed and reused in the form of rubber mats mainly for playground, agricultural, sport, and automobile applications. Inductively coupled plasma optical emission spectrometry, atomic absorption spectrometry, and elemental (CHNS) analyses were used to determine the content of various chemical elements. Thermogravimetric analysis (TGA) was used to determine the amount of carbon black and inorganic material in the sample. Mass loss at 400 °C was related to cross-link density. TGA coupled with mass spectrometry showed that this mass loss corresponds to the loss of SO2 as cross-links are destroyed during heating. Melt point and glass transition temperatures determined by differential scanning calorimetry are mainly proportional to the ethylene/propylene weight ratio.

Alumina–clay nanoscale hybrid filler assembling in cross-linked polyethylene based nanocomposites: mechanics and thermal properties

Herein, investigation on XLPE–Al₂O₃–clay ternary hybrid systems of Al₂O₃ and clay in 1 : 1 and 2 : 1 ratios, binary systems of XLPE–clay and XLPE–Al₂O₃ nanocomposites, with special reference to the hybrid filler effect and the superior microstructural development in ternary systems is conducted.