SBR Compounds Research Articles

Styrene butadiene rubber-clay nanocomposites using a latex method: morphology and mechanical properties

In this study, octadecylamine modified MMT (C18-MMT) filled SBR nanocomposites were manufactured using a latex method and a compounding method. Cure characteristics and mechanical properties of SBR compounds filled with C18-MMT, Cloisite 15A, carbon black and Na-MMT were also evaluated. By using the latex method, the number of layers of the silicates in the SBR matrix reduced from the original 14–15 layers to 1–4 layers. This was due to the presence of octadecyl ammonium ions which reduced the number of layers of the re-aggregated silicates during the process of co-coagulation. The SBR/C18-MMT nanocomposites using the latex method showed the highest oscillating disc rheometer (ODR) torques, tensile strength, modulus and tear energy. These increased mechanical properties can be attributed to the excellent reinforcing effect of the silicates well dispersed in the rubber matrix rather than the effect of the increase in the degree of crosslinking. Without alkyl ammonium ions in the latex method, the level of dispersion of silicates in the SBR matrix was very poor. The SBR/C18-MMT nanocomposites using the compounding method were found to have a lower degree of modulus, tensile strength and tear energy due to the low level of the dispersion of silicates than the SBR/C18-MMT nanocomposites using the latex method.

A test method to measure fatigue crack growth rate of rubbery materials

Crack growth characteristics of rubbery materials are an important factor determining the strength and durability of the materials. It is necessary to define the means of measuring the fatigue crack growth rate of rubbery materials. In this paper, a test method is introduced using a newly designed test machine. Measurements were made in order to characterize the fatigue crack growth behavior under repeated load expressed as a crack growth rate, i.e. crack length per cycle, as a function of the tearing energy, determined by a broad range of the strain energy density in a test piece of pure shear geometry. Measurements were also made to observe the effects of test temperature and loading frequency. A high speed CCD camera is utilized to allow in situ measurements following the path of the crack growth simultaneously. The power law dependency between crack growth rate and tearing energy was confirmed by the new test methodology. Comparison of the fatigue crack growth behavior between NR and SBR rubber compounds was made, and the exponent α values of the NR compound were found to be, in general, lower than those of the SBR compound, indicating that the NR compound is more resistant to fatigue crack growth than the SBR compound.

Effect of a fatty amine on processing and physical properties of SBR compounds filled with silane–silica particles

AbstractThe use of fatty amines, obtained from natural fatty acids, in rubber compounds to improve silica dispersion was evaluated. Fatty amines can interact with the free silanol groups of the bis(3‐triethoxysilyl‐propyl) tetrasulfide (TESPT) silane‐modified silica forming an amine‐modified silica complex, which reduces the hydrophilic nature of the silica surface, minimizing the silica–silica interactions, and reducing the formation of secondary structures of silica. The effect of the addition of different amines and/or polyethylenglycol to SBR compounds loaded with silane‐modified silica was evaluated. Finally, incorporation of fatty amines improved the processability and traction properties of rubber compounds loaded with modified silica. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3222–3229, 2006

Effect of oleyl amine on SBR compounds filled with silane modified silica

The effect of oleyl amine on processing and physical properties of SBR compounds filled with silane–silica particles has been evaluated. Two different types of silane molecules have been used as coupling agents to minimize the silica–silica interactions and reduce the formation of secondary structures of silica, in addition with an increment in the filler–rubber interactions. Significant differences between those compounds have been found according with the silane structure. However, in both cases, the dynamic, rheological, and mechanical properties of the filled rubber compounds were improved after the incorporation of oleyl amine molecules. This fact could be related with the capacity of the amine molecules to interact with the free silanol groups of silane-modified silica forming an amine-modified silica complex, which reduces the hydrophilic nature of the silica surface. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1806–1814, 2007

Experimental Study and Simulation of Interface Development and Penetration in Two-Component Transfer Molding of Rubber Compounds

Abstract Simulation and experimental studies of pressure-controlled sequential transfer molding of two SBR rubber compounds under isothermal condition have been carried out to obtain a two-layered sandwich structure. One SBR compound, intended for the skin, is first laid up in the cavity; and another SBR compound, intended for the core, is used for transfer. The SBR is subjected to pressure in order to penetrate into the skin and push the layup to fully fill the cavity. The obtained moldings have an encapsulated skin/core sandwich structure. Two material combinations with different viscosity ratios have been studied. The rheological interaction of skin/core components and its effect on the penetration behavior and interface shape have been investigated. The influence of processing conditions, such as the volume fraction transferred and pressure, is elucidated. From the experiment and simulation it is found that the penetration and the interface development are significantly affected by the rheological properties of the compounds and the volume fraction transferred. At the same time, the pressure imposed during transfer molding is found to have little effect on the interface development at a constant volume fraction transferred.

Interface evolution and penetration behavior during two‐component transfer molding. Part II: Simulation and experiment

AbstractSimulation and experimental study of the pressure‐controlled sequential sandwich transfer molding of two SBR rubber compounds under isothermal condition have been carried out to obtain a two‐layered sandwich structure. One SBR compound, which is intended for the skin material, is first laid up in the cavity. Then, another SBR compound, intended for the core material, is transferred to penetrate into the skin material and to push the lay‐up to fully fill the cavity, resulting in an encapsulated skin/core sandwich structure. Two cases involving different material combinations with different viscosity ratios have been studied. The rheological interaction of the skin/core components and its effect on the penetration behavior and interface shape have been investigated. The influence of processing conditions, such as the volume fraction transferred and pressure, is discussed. The penetration and encapsulation behavior, and the interface development are found to be significantly affected by the rheological properties of the compounds and the volume fraction transferred. However, at a constant volume fraction transferred, the pressure imposed during transfer molding is found to have a little effect on the interface development. These experimental findings are in good agreement with the present predictions based on the model and simulation proposed in Part I of this study. Polym. Eng. Sci. 44:697–713, 2004. © 2004 Society of Plastics Engineers.

Filler‐polymer interactions of styrene and butadiene units in silica‐filled styrene–butadiene rubber compounds

AbstractStyrene–butadiene rubber (SBR) is a copolymer of styrene and butadiene, and the butadiene unit is composed of cis‐1,4‐, trans‐1,4‐, and 1,2‐components. Filler‐polymer interactions of each component of SBR in silica‐filled SBR compounds were examined by microstructure analysis of the bound and unbound rubbers. The composition ratio of butadiene and styrene units (butadiene/styrene) of the bound rubber was higher than that of the compounded rubber. Of the butadiene units, the 1,2‐component of the bound rubber was more abundant than the cis‐1,4‐ and trans‐1,4‐components. The filler‐polymer interaction of the butadiene unit with silica was stronger than that of the styrene unit, and the interaction of the 1,2‐component was stronger as compared with the others. The butadiene–styrene ratio of the bound rubber of the compounds containing the silane coupling agent was lower than for the compounds without the silane. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 577–584, 2004

Parameter Dependence and Prediction of Fatigue Properties of Elastomer Products

Abstract Fatigue tests on ethylene propylene (EPDM) and styrene-butadiene (SBR) rubber reveal physical behavior that is not seen in conventional linear elastic solids. Uniaxial cyclical tests, using cylindrical dumbbell specimens, with the same minimum stress of zero (Smin=0) and varying stress amplitude (sa), predictably gave decreased fatigue life with increased stress amplitude and hence maximum stress (Smax). However, tensile uniaxial cyclic tests where Smin was increased in successive tests while alternating stress (sa) remained constant, produced longer fatigue lives for higher values of Smax. EPDM and SBR compounds were chosen for the tests because they do not strain crystallize during deformation. Consequently, this phenomenon has no influence. The results show that Smax can not be used as criterion to predict fatigue life of elastomers. Preliminary evaluation of recorded data of stress vs. strain gave evidence that energies control the fatigue life rather than stress and strain. Experimental results on filled and unfilled rubber materials are evaluated and discussed as well as the consequences on predictions of component properties.

Fracture behavior of NR and SBR vulcanizates filled with ground rubber having uniform particle size

AbstractThe use of recycled rubber including ground scrap vulcanizates in rubber compounds was studied. When ground rubber was incorporated into rubber compounds, the physical properties, especially the tensile strength, were deteriorated compared to the virgin rubber compound. Also, incorporating ground rubber caused a change of the cure behavior via migration of sulfur or an accelerator between the virgin rubber matrix and the ground rubber vulcanizate. In this study, the fracture behavior and abrasion property of carbon black‐filled SBR and NR compounds containing ground rubber vulcanizate were investigated. Also, the effect of the particle size or loading volume of ground rubber powder on those properties was studied. Four different sizes, 420–600, 177–250, 125–150, and 75–88 μm, of ambient ground rubber powder recycled from waste tire were selected and used in the compounding. The loading amounts of ground rubber powder were 10, 30, and 50 phr. The flex crack growth of SBR‐ and NR‐based compounds was altered by the addition of ground rubber particles. More delayed crack growth was observed with an increasing loading volume and decreasing particle size of the ground rubber powders, and this behavior was more prominent in SBR than in NR compounds. Tangent delta, a direct measure of internal energy dissipation, increased with an increasing loading volume of the ground rubber particles. The abrasion rate of ground rubber‐filled compounds was more dependent on the size of the abrasion pattern than on the loading level or particle size of the ground rubber powders. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2491–2500, 2002

Weld-Line Strength of Rubber in Injection Molding: Effect of Injection Factors and Compound Characteristics

Abstract The effect of injection molding system to weld-line strength in rubber O-rings was studied using a V-shape two-stage REP injection machine. Two types of injection molds were designed and built, a standard dumbbell mold with double gates and a circular cross-section O-ring mold. Several formulations of carbon black filled NR and SBR compounds were used and vulcanization temperature was either 180 or 200 °C throughout. The results show that mold cavity pressure, compound viscosity and compound scorch time are important variables for the weld-line strength of the products. The shot volume change had no direct effect on strength, but mold cavity pressure was an important factor; unfilled shot volume gave low cavity pressure thus lowering the weld-line strength of the O-rings. The compounds having 45 or lower Mooney viscosity, ML(1 + 4)120 °C, had the same weld area strength as that of the other regions of the O-ring, but the high viscosity compounds produced low weld-line strength. Only the compounds with Mooney scorch time shorter than 10 minutes gave low weld-line strength.

The Friction and Wear of Rubber Part 1: Effects of Dynamically Changing Slip Direction and the Damage Orientation Distribution Function

Abstract In Part 1, results of an experimental and analytical study are offered which examined the effects of a dynamically changing slip direction on a rubber surface's friction and wear performance and on the properties of an industrial abrasive. For a filled SBR compound, it was found that a dynamically changing slip direction had a small effect on the friction/traction performance, but a substantial beneficial effect on the surface's wear performance. The abrasive's ability to generate wear was found to be strongly dependent on the accumulation of side slip over the life of the abrasive. Conceptualization of the Damage Orientation Distribution Function is offered to describe the statistical nature of the oriented damage generated on a slipping rubber surface. The experimental results are shown to be in excellent agreement with model predictions based on several simple assumptions regarding the effects that changing slip orientation has on the response of the Distribution Function.

Cure Kinetics for Silane Coupled Silica Filled SBR Compounds

Abstract Vulcanization kinetics for silane-modified, silica-filled SBR has been investigated using the cure meter and swelling measurements to determine the crosslink density. The silane modifier was Si-69, bis(3-triethoxy-silylpropyl)tetrasulfane. To simulate the induction and curing periods as a function of temperature and compounding recipe, a three component kinetic model for curing was developed. The kinetic model consists of (a) curing in the bulk elastomer phase, (b) curing at the silica surface and (c) cross-phase curing between the bulk components and the silica surface. In order to describe the partitioning of reagents between the bulk and surface domains, the parameter reactive surface thickness,θ, was introduced into the kinetic model. An average value of 24.7 nm for θ was calculated from the kinetic data and the silica particle surface area. Physically significant model kinetic parameters were extracted from the experimental data using a cure meter at different temperatures for (a) bulk curing (no filler) using the model of Ding and Leonov; (b) curing at the silica surface with Si-69 (no sulfur); and (c) whole compound recipe. Crosslink density values determined from the shear modulus were in good agreement with results from equilibrium swelling measurements. Using experimental data from the cure meter studies at 160 and 180 °C, it was possible to calculate (predict) the cure meter data at 140 °C for several different recipes.

Influence of some additives on state‐of‐mix, rheological, tensile, and dynamic mechanical properties in SBR compounds

AbstractThe influence of some additives (Flexon processing oil, Struktol WB 16 slipping agent, and Struktol NS 60 homogenizing resin) on state‐of‐mix, rheological, tensile, and dynamic mechanical properties were investigated. It was found that extrudate swell of the rubber compounds with either processing oil or NS 60 is governed mainly by the state‐of‐mix, but by wall slip in the case of compounds with WB 16. As for the dynamic mechanical properties, the plasticizing effect is the major factor controlling the properties in the case of processing oil, while the degree of crosslink and dilution effect are the main factors in the case of NS 60. Both degree of crosslink and state‐of‐mix are responsible for the dynamic mechanical properties in the case of WB 16. In addition, it was found that the tensile properties are controlled mainly by the plasticizing effect in the case of processing oil, but by the degree of crosslink in the cases of WB 16 and NS 60. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 2474–2482, 2001

Polyisoprene, poly(styrene-cobutadiene) and their blends. III. Tensile properties of tetramethylthiuram disulfide/sulfur and 2-bisbenzothiazole-2,2'- disulfide/sulfur compounds

Polyisoprene (IR), poly(styrene-cobutadiene) (SBR) and IR/SBR blends were vulcanized with tetramethylthiuram disulfide (TMTD)/sulfur and 2-bisbenzothiazole-2,2′-disulfide (MBTS)/sulfur formulations and their tensile properties were determined. MBTS vulcanized IR has inferior tensile properties to TMTD vulcanizates. This is attributed in part to main chain modification in MBTS vulcanizates decreasing the ability of chains to crystallize or to align as effective load-bearing chains under stress. A similar discrepancy is not found in SBR compounds that cannot stress-crystallize. Polybutadiene, which readily crystallizes on cooling, is used to demonstrate differences in the effect of MBTS and TMTD on the ability of chains in the vulcanizates to align. These differences are confirmed by X-ray diffraction studies of stressed IR vulcanizates. The addition of zinc stearate reduces main chain modification, promotes crystallization, and improves tensile properties. Blends have inferior properties to IR, and tests involving the pulling apart of laminates and analysis of the tear surfaces are used to illustrate that failure does not occur in adhesion, but within the IR phase close to the interface. It is argued that diffusion of curatives from SBR to the faster curing IR phase, leads to the development of a layer of highly crosslinked material in IR close to the phase boundary. Failure occurs in this layer and may be attributed to a decrease in the number of effective load-bearing chains in this region or to the shorter chains in this layer becoming taut. Less diffusion of the accelerator occurs with MBTS than with TMTD, leading to a less highly crosslinked IR zone close to the interface. Consequently, higher loads are required to initiate failure. Failure in blends is likewise considered to initiate in the highly crosslinked region in the IR phase close to the phase boundary with SBR. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2143–2149, 1999

An Approach to Chemorheology of a Filled SBR Compound

Abstract Three steps in complex chemorheological studies are necessary for complete rheological characterization of filled rubbers during cross-linking reaction. They include: (i) kinetic studies of cross-linking reaction, (ii) rheological studies of green rubber compounds, and (iii) correlation between the rheological parameters and the degree of cure. Basic experiments in the steps (i)–(iii) and their modeling are presented in this paper on the example of a filled SBR compound with sulfur accelerated vulcanization. The approach provides a unique possibility to trace therheological properties of rubber compounds from the green to completely cured states.

Injection Molding of a Natural Rubber Compound: Simulation and Experimental Studies

Abstract Experimental and theoretical results are obtained for the cavity filling of a natural rubber (NR) compound. The shear rate and temperature dependent viscosity function based on the modified Cross model is fitted to experimentally measured viscosity data. A nonisothermal vulcanization model with nonisothermal induction time is fitted to the nonisothermal curing kinetic data obtained by differential scanning calorimetry (DSC). These data are utilized in the simulation of the cavity filling and curing stages. The pressure traces at various locations in the mold during cavity filling are obtained at different inlet and mold temperatures. The predicted results for the pressure traces are in good agreement with those measured in the cavity, but significantly lower than those measured in the nozzle. Evolution of the state of cure in the post-filling stage is predicted and compared with experimental data measured in moldings obtained at various cycle times. The measured states of cure in the moldings are in good agreement with their predicted values. Based on earlier flow studies of SBR compounds, it is shown that the discrepancy between the experimental and predicted pressure traces in the nozzle for the NR compound is due to high local pressure drops arising during the flow in juncture regions of the delivery system and the possibility of flow-induced crystallization. The importance of taking into account the entry pressure losses, viscoelastic effects and flow-induced crystallization in the injection molding process is strongly emphasized. The complexity of the issues related to this subject is discussed.

Injection Moulding of Rubber Compounds1

Abstract The aim of this study is to show the relevant physical parameters related to rubber injection moulding, and especially compressibility, vulcanisation and wall slippage. Three materials are studied: an SBR compound, without and with a lubricant, and an EPDM compound. The viscosity is determined by capillary rheometry. The modified SBR exhibits wall slippage. Rheological vulcanisation kinetics are determined with a Moving Die Rheometer. Precise moulding experiments are carried out with an injection moulding machine and a mould equipped with pressure and temperature transducers. At low flow rate, the influence of the curing reaction on the pressure appears clearly especially for the fast curing EPDM. The conditions in which surface scorch defects appear are determined. The SBR with lubricant needs a lower pressure to fill the cavity than without lubircant. A model of mould filling is developed. The material is purely viscous, with a state of cure dependence of the viscosity. A Norton friction law is introduced to take into account a possible slippage at the cavity walls. A compressible calculation in the injection chamber allows a realistic evaluation of the flow rate at the entry of the cavity. The comparison with the experimental data confirms the importance of the compressibility, and the influence of the vulcanisation on the viscosity at low flow rate. In moulding conditions for which vulcanisation is not activated during the filling stage, despite uncertainties on the friction law parameters appropriate to the cavity wall roughness, the agreement with experiments is better when wall slippage is taken into account.

Abrasion of Rubber by a Blade Abrader: Effect of Blade Sharpness and Test Temperature for Selected Compounds

Abstract Apparatus has been constructed to abrade the flat surface of a rubber disk by rotating it against a stationary knife-blade scraper at constant frictional torque. The ratio of worn surface area to sample volume is relatively large, giving high sensitivity of measurement. Rates of abrasion were measured for several rubber compounds over a range of frictional torque from 0.05 to 1.2 Nm, corresponding to frictional work input from 150 to 3,600 J/m2 per rotation. For some materials, notably SBR compounds, the rate of abrasion decreased markedly as the experiment continued, due to rapid wear and blunting of the blade. Polybutadiene compounds caused much slower wear. Different blades were worn away at different rates, a zirconia blade showing the least wear. Also different elastomers showed different sensitivities to blade sharpness, the rate of abrasion of a high-styrene (48%) SBR compound being reduced the most with a blunted blade whereas the abrasion rate of polybutadiene (BR) was much less affected. Measurements were carried out with blades of known sharpness for several representative rubber compounds at various fnctional torques and at two temperatures, 25°C and 100°C. The high-styrene SBR compound showed the lowest rate of abrasion Unusually rapid abrasion was observed for SBR and BR compounds when the frictional work input exceeded a critical value, close to the tear ( fracture) energy at the same temperature. Also, periodic fluctuations in rate of abrasion were observed for rapidly-aging compounds at high temperatures. This is attributed to repeated formation and removal of a weakened surface layer.

Evaluation of Chain Scission during Mixing of Filled Compounds

Abstract Whether chain scission takes place during mixing of black-filled compounds has been a debatable subject which is yet unsettled. The uncertainty originates from the material system which contains gel. A direct method to evaluate scission is to quantify the change in the number of chain ends, since two chain ends will be newly created on each event of scission. It requires determination of an average molecular weight of the linear components which constitute the gel-containing system. The degree of polymerization denned in the Charlesby-Pinner theory, derived for crosslinking study by means of sol-gel analysis, could be conveniently used for the purpose. The theory, however, cannot be directly applied for black-filled compounds because the composite structure in the compounds does not allow one to satisfy the assumptions made in the theory, i.e., an equal chance of crosslinking for every reactive site. A new technique is developed so that the assumption is satisfied and the sol-gel analysis can be carried out even with black-filled compounds. This technique is applied for numbers of compounds, with the same formulation but mixed under various conditions. A NR and an SBR formulations are used here. The results clearly indicate that chain scission takes place during mixing of both NR and SBR compounds.

Injection Molding of Rubber Compound with Rheology Affected by Vulcanization: Part II. Modeling and Experiment

Abstract Experimental and theoretical results are reported on the effect of vulcanization on viscosity and yield stress during cavity filling with an SBR compound. Rheological and vulcanization models were utilized in simulation of the cavity filling process. Numerical schemes was based on the hybrid approach of control volume finite-element and finite-difference techniques. The pressure profiles at various locations along the flow path in the mold during cavity filling were obtained by conducting the experiments at various mold and inlet temperatures and mold injection speeds. It was found that the predicted pressure profiles followed the trend determined in the experiments. Comparing two versions of the modified Cross viscosity models — with and without the effect of cure on viscosity — it appears that use of a viscosity model which accounts for the cure-effect improves the accuracy, and thus the usefulness, of the cavity pressure prediction models. Evolution of the state-of-cure in the postfilling process are predicted and compared with the state-of-cure in moldings. The predicted results on the development of the state-of-cure during the postfilling stage show good agreement with the experimental data. The rubber compound exhibits yield behavior during cavity filling at very low injection speeds. The modified Cross model was further extended to include the yield stress and was implemented in the simulation program. In the case of low injection speed, the results showed a significant improvement in the prediction of cavity pressure, compared with the earlier reported predictions.