Carbon Black-filled Natural Rubber Research Articles

Dynamic Fracture of Natural Rubber

Abstract This paper illustrates how the fracture energy of a tensile strip made of unfilled and 25 phr carbon black-filled natural rubber varies with far-field strain rate in the range 0.01–71 s−1. Quasistatic and dynamic fracture tests were performed at room temperature with an electromechanical INSTRON machine, a servo-hydraulic MTS machine, and Charpy tensile apparatus, respectively. It was found that the fracture energy of the unfilled natural rubber did not vary significantly over the range of sample strain rate, but there was significant variation in the fracture energy of the 25 phr carbon black-filled natural rubber from 0.01 to 71 s−1 sample strain rate. The fracture energy of the 25 phr carbon black-filled natural rubber at a sample strain rate of 0.1 s−1 was about three times greater than it was at the 10 s−1 sample strain rate. While the carbon black fillers increased the fracture energy of natural rubber by about 200% at quasistatic sample strain rates (0.01–0.1 s−1) and at 71 s−1, the carbon black fillers did nothing to improve the fracture energy of natural rubber at sample strain rates between 5 and 29 s−1. In this strain rate range, the fracture energy of 25 phr carbon black-filled natural rubber was almost the same as that in the unfilled natural rubber. The variation in the fracture energy with far-field strain rate was due to changes in the material behavior of natural rubber at high strain rates. Finite element analysis using a high-strain-rate constitutive equation for the 25 phr carbon black rubber specimen was used to calculate the fracture energy of the specimen at a sample strain rate of 55 s−1, and good agreement was found between the test and finite element results.

Fracture properties of silica/carbon black-filled natural rubber vulcanizates

Crack growth property of natural rubber (NR) vulcanizate with varying silica/carbon black content was examined. Tensile specimen with edge cut was used for estimating fracture properties. All filled NR specimens showed critical cut-size (Ccr), which is related to abrupt decrease in tensile strength. Carbon black-filled NR, S0 (Si/N330=0/50) has higher tensile strength than equivalently loaded silica-filled NR vulcanizates, S5 (Si/N330=50/0). When the precut size of specimen was less than critical cut-size, tensile strength of S1 (Si/N330=10/40) composition was the highest and that of S5 was the lowest. The critical cut-size passes through a maximum for S2 (Si/N330=20/30) and then decreases gradually with silica loading. An interesting result was that silica and carbon black-blended compounds gave higher critical cut size than the all-carbon black compounds, S0. The inherent flaw size decreased from 246 μm for S0 to 80 μm for S5 as the silica content increased.

The Behavior of Carbon Black-Filled Natural Rubber under High Strain Rates

Abstract Tensile tests were conducted to obtain the deformation and failure characteristics of unfilled natural rubber (NR) and natural rubber with 25, 50, and 75 phr of N550 carbon black filler under quasistatic and dynamic loading conditions. The quasistatic tests were performed on an electromechanical INSTRON machine, while the dynamic test data were obtained from tensile impact experiments using a Charpy impact apparatus. In general, the modulus of the stress-extension ratio curves increases with increasing strain rate up to about 407, 367, 346, and 360 s−1 for unfilled, and 25, 50, and 75 phr for filled NR, respectively. Above these strain rates, the unfilled and filled natural rubber stress-extension ratio curves remained unchanged. The modulus increased with increasing strain rate because there was little time for stress relaxation. Above a critical strain rate, no change in modulus was observed because the time of the experiment was short compared to the lowest characteristic relaxation time of the material. Dynamic stress-extension ratio curves did not have the very sharp upturn at break, which is observed from strain-induced crystallization in natural rubber under quasistatic loading. Strain-induced crystallization appeared to be suppressed at high rates of loading. In fact, the highest dynamic tensile strength for the 25- and 50-phr carbon black-filled natural rubbers was smaller than those under quasistatic loading, while the highest dynamic tensile strength of the 75-phr carbon black-filled NR was greater than that in the static test. This indicated that high amounts of carbon black fillers will impede strain-induced crystallization in natural rubber.

The nonlinear viscoelastic response of carbon black-filled natural rubbers

Three series of tensile relaxation tests are performed on natural rubber filled with various amounts of carbon black. The elongation ratio varies in the range from λ=2.0 to 3.5. Constitutive equations are derived for the nonlinear viscoelastic behavior of filled elastomers. Applying a homogenization method, we model a particle-reinforced rubber as a transient network of macromolecules bridged by junctions (physical and chemical cross-links, entanglements and filler clusters). The network is assumed to be strongly heterogeneous at the meso-level: it consists of passive regions, where rearrangement of chains is prevented by surrounding macromolecules and filler particles, and active domains, where active chains separate from temporary nodes and dangling chains merge with the network as they are thermally agitated. The rate of rearrangement obeys the Eyring equation, where different active meso-domains are characterized by different activation energies. Stress–strain relations for a particle-reinforced elastomer are derived by using the laws of thermodynamics. Adjustable parameters in the constitutive equations are found by fitting experimental data. It is demonstrated that the filler content strongly affects the rearrangement process: the attempt rate for separation of strands from temporary nodes increases with elongation ratio at low fractions of carbon black (below the percolation threshold) and decreases with λ at high concentrations of filler.

The influence of pre-loading and thermal recovery on the viscoelastic response of filled elastomers

Experimental data are reported in tensile relaxation tests on carbon black-filled natural rubber at strains up to 200%. Constitutive equations are derived for the time-dependent response of a particle-reinforced elastomer at finite strains. Adjustable parameters in the model are found by fitting observations. The effects on mechanical pre-loading and thermal recovery are analyzed on the material constants.

Finite Viscoelasticity of Particle-Reinforced Elastomers: the Effect of Filler Content

Constitutive equations are derived for the time-dependent behavior of particle-reinforced rubbers at isothermal loading with finite strains. An elastomer is thought of as a network of macromolecules bridged by junctions that slip with respect to the bulk material under straining. A filled rubber is treated as an ensemble of meso-regions where sliding of junctions occurs at different rates. Two types of domains are distinguished: passive, where slippage of junctions is prevented by intermolecular links, and active, where the sliding process is not affected by interchain forces. Activation of passive meso-regions under loading is modelled as mechanically-induced breakage of van der Waals links between strands. Stress–strain relations are developed using the laws of thermodynamics. For tensile relaxation tests, these equations are determined by four adjustable parameters that are found by fitting experimental data on gum rubber, unfilled rubber, and carbon black-filled natural rubber at longitudinal strains in the range from 100 to 250%. It is revealed that the rate of sliding and the concentration of active meso-regions are noticeably affected by stretching, whereas the distribution of potential energies for sliding of junctions is independent of strains, but is severely influenced by the filler content.

Binary Cure Systems of 1,6-Bis(N,N'-dibenzylthiocarbamoyldithio)-hexane and Benzothiazole Sulfenamides in Carbon Black-filled Natural Rubber Compounds

Binary cure system is composed of different two cure accelerators, which can cause a synergy effect to delay the scorch time and to increase the cure rate. In this study, binary cure systems between 1,6-bis(N,N'-dibenzylthiocarbamoyldithio)-hexane (DBTH) and benzothiazole sulfenamides were investigated using carbon black-filled natural rubber compounds. N-Cyclohexyl-2-benzothiazole sulfenamide (CBS), N-tert-butyl-2-benzothiazole sulfenamide (TBBS), and 2-(morpholinothio) benzothiazole (MOR) were employed as benzothiazole sulfenamides. The binary cure systems show scorch safty at high temperature. The binary cure systems have faster cure rate and better reversion resistance than the single cure system of the benzothiazole sulfenamides. DBTH is found to be more effective to decrease the viscosity of a compound than the benzothiazole sulfenamides. Physical properties of the vulcanizates with the binary cure system are better than those of the vulcanizates with the single one.

Strain and anisotropic effects on migration behaviors of antiozonants in carbon black-filled natural rubber vulcanizates

The influence of strain and anisotropy on the migration behavior of antiozonants in carbon black-filled natural rubber vulcanizates was studied at constant temperatures of 50–90 °C and outdoors. N-Phenyl- N′-isopropyl- p-phenylenediamine (IPPD) and N-phenyl- N′-(1,3-dimethylbutyl)- p-phenylenediamine (HPPD) were employed as antiozonants. Migration rates of the antiozonants in the strained vulcanizates are faster than in the non-strained ones. The migration rates in the vulcanizate strained parallel to the milling direction are slightly slower than those in the vulcanizate strained perpendicularly to the milling direction at high temperatures.

Continuous ultrasonic devulcanization of carbon black-filled NR vulcanizates

The continuous ultrasonic devulcanization of natural rubber (NR) filled with various concentrations of carbon black (CB) indicated a minimum of crosslink density and gel fraction at an intermediate amplitude, which is independent of CB content. An attempt was made to improve the efficiency of devulcanization by use of various chemicals (1,3 Diphenylguanidine, 2-Mercaptobenzothiazole, Thianaphthene). However, these experiments did not indicate any improvement in comparison with devulcanization without chemicals. An idea of adding fresh CB into devulcanized compound, which has been shown to improve mechanical properties in the case of styrene–butadiene rubber (SBR), was tested in the present study for CB filled NR compound. The obtained result indicated that an addition of fresh CB to devulcanized CB-filled NR did not lead to an improvement in mechanical properties upon revulcanization. The revulcanization recipe was optimized to improve the mechanical properties of revulcanized CB-filled NR vulcanizates. It was found that CB-filled NR upon revulcanization retained its strain-induced cystallizability with the tensile strength and elongation at break at about 50 and 70% level of the virgin vulcanizates. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 2340–2348, 2001

Transport of benzene and methyl-substituted benzenes through carbon black-filled epoxidized natural rubber

Sorption and diffusion of benzene and methyl-substituted benzenes were investigated through epoxidized natural rubber (ENR) reinforced with four types of carbon black: superabrasion furnace (SAF), intermediate superabrasion furnace (ISAF), high-abrasion furnace (HAF), and semireinforcing furnace (SRF). Kraus equation has been used to investigate the extent of reinforcement for the different types of carbon black used in the experiments. Effect of the type and concentration of the carbon black on solvent uptake and mechanism of diffusion were studied in detail. The rate constant for diffusion of the solvents in epoxidized natural rubber vulcanizate based on different carbon black type, and loading was investigated. Diffusion constant was found to decrease with increase in the degree of reinforcement. The interaction constant values were experimentally determined. The sorption data were used to determine the activation energy for the diffusion process and the enthalpy and entropy of the sorption process. The experimental results were compared with theoretical predictions. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 415–427, 1999

Effect of the Environment on the Wear of Carbon Black-Filled Natural Rubbers?

Abstract In this study, the effect of the environment on the wear of natural rubber (NR) vulcanizates containing three kinds of carbon black, namely, HAF(N330), ISAF(N220), and SAF(N110), were examined. The friction coefficient and wear rate of each carbon black-filled NR tended to decrease with decreasing ambient air pressure. In air, the wear rates of NR vulcanizates containing ISAF and SAF were lower than NR containing HAF. No difference in the former two vulcanizates could be observed. However, at a reduced air pressure of 66 Pa, the wear rates of NR with ISAF and SAF were 22% and 7%, respectively, when compared with that of NR with HAF. With a reduction in the size of carbon black, a large difference in the wear rates could be seen in vacuum.

Method for Estimating the Chemical Crosslink Densities of Cured Natural Rubber and Styrene-Butadiene Rubber

Abstract Chemical crosslink densities of gum and carbon black-filled natural rubber (NR) and styrene-butadiene rubber (SBR) were estimated by using a newly developed rheometer. The rheometer is the Rubber Process Analyzer (RPA 2000) which is designed specifically to measure dynamic properties such as shear storage modulus G′ and shear loss modulus G″ in cured and uncured rubber. It was found that the differences between the G′ values of dicumyl peroxide-cured NR and those of uncured samples yielded estimates of the crosslink densities which were nearly the same as the values inferred by chemical analysis. For TMTD-cured SBR, the same procedure yielded estimates of chemical crosslinks very close to those estimated by a tensile stress-strain method and by NMR. In addition, accelerated sulfur-cured natural rubber was also investigated. The agreement between the crosslink densities of these stocks determined from G′ values and from a solvent-swelling method was very good.

An investigation on the failure envelopes of carbon black-filled natural rubber and styrene-butadiene rubber vulcanizates

An investigation on the failure envelopes of carbon black-filled natural rubber and styrene-butadiene rubber vulcanizates

Note on Conductivity and Modulus of Carbon-Black-Loaded Rubbers

Abstract Electrical conductivity and dynamic shear modulus measurements were made simultaneously on carbon black-filled natural rubbers. G′, the in-phase shear modulus, and the electrical conductivity, G, were shown to vary in an approximately similar way as the amplitude of dynamic oscillation was increased. Both G′ and C altered sigmoidally from a high value at low amplitudes of oscillation to very much reduced values at high amplitudes of oscillation. Recovery curves for conductivity were shown to be parallel with respect to the logarithm of the time, even though the initial deforming amplitudes varied widely.

Dynamic properties of natural rubber containing heat-treated carbon blacks

The dynamic properties of a series of carbon black-filled natural rubber vulcanizates have been studied for their amplitude dependence. The HAF blacks studied possessed low, normal, or high structure, and one series of the high structure blacks was subjected to heat treatment up to 2700°C. The effect of heat treatment of the black was to increase the low amplitude dynamic shear modulus and hysteresis but to reduce the high strain tensile modulus. These effects were studied with respect to the dispersion of the black.

A note on the conductivity and modulus of carbon black‐loaded rubbers

AbstractElectrical conductivity and dynamic shear modulus measurements were made simultaneously on carbon black‐filled natural rubbers. G′ the in‐phase shear modulus, and the electrical conductivity, C, were shown to vary in an approximately similar way as the amplitude of dynamic oscillation was increased. Both G′ and C altered sigmoidally from a high value at low amplitudes of oscillation to very much reduced values at high amplitudes of oscillation. Recovery curves for conductivity were shown to be parallel with respect to the logarithm of the time (in minutes), even though the initial deforming amplitudes varied widely.

Swelling and Network (Crosslink) Density of Carbon Black-Filled Natural Rubber Vulcanizates

Abstract The method cited by W. Kuhn and coworkers for the determination of network densities on the basis of a thermal analysis of swollen samples has been applied to unfilled and carbon black-filled natural rubber vulcanizates. The results with unfilled rubber on the basis of the new method are in the expected correlation with the measurements of the degree of swelling and the modulus of elasticity. From the results with carbon black-filled samples it is probable that an inhomogeneously crosslinked system is formed under the influence of the filler. It is concluded that there are regions of high network density adjacent to regions of low density. Consequently, the degree of swelling, referred solely to the rubber portion, is to be considered a mean value in the case of filled vulcanizates from which the network density cannot be deduced without additional measurement.