NR Compounds Research Articles

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.

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.

Use of Amine Terminated Liquid Natural Rubber as a Plasticiser in Filled NR and NBR Compounds

Abstract Amine Terminated Liquid Natural Rubber (ATNR) was used as a plasticiser in filled NR and NBR compounds replacing oil/DOP. The scorch time and cure time were found to be lowered when ATNR was used as the plasticiser. ATNR was found to improve the mechanical properties like tensile strength, tear strength and modulus of the vulcanizates. The ageing resistance of the vulcanizates containing ATNR was superior compared to the vulcanizates containing oil/DOP.

Effect of High Crystalline trans-Polyoctenylene (TOR) on Different Cure Systems of NR Compounds

With a view to improve degradation properties of natural rubber on prolonged cure at moderately high temperature, a study is made using a co- vulcanizable processing aid known as trans-polyoctenylene (TOR) on natural rubber filled compounds. Rheometer results show that reversion resistance im proves at all curing temperatures in the presence of TOR-more so with EV (efficient vulcanization) and semi-EV systems compared to CV (conventional vulcanization) systems. Cure rate diminishes and activation energy increases with incorporation of TOR in natural rubber irrespective of the curing system. Modulus and hardness retention also are much better for TOR filled com pounds under anaerobic conditions. Tensile strength, however, drops unlike modulus and hardness. Heat buildup and loss tangent (tanδ) results reveal less heat development in TOR filled compounds. Retention of Vr (volume fraction of rubber) value indicates a similar trend like modulus and hardness, showing preservation of network structure. Such an improvement of reversion prop erties under anaerobic ageing conditions may be attributed to simultaneous crosslink formation on TOR phase in addition to natural rubber phase which has been explained through DSC studies.

Comparison of two distinct phenolic antioxidants in γ‐sterilized NR compounds used as rubber pistons in syringes for single use

AbstractThe effects of γ‐irradiation and aging on pharmaceutical NR compounds have been studied mainly by DSC. The extent of aging was measured quantitatively by enthalpy changes and peak temperature shifts of the primary oxidation exotherm. Quantitative thermal evaluations were supplemented by extraction tests and mechanical testing. The NR compound formula has been kept constant and only the type and concentration of antioxidant was varied. Two distinct phenolic antioxidants, BHT and Wingstay‐L at four different concentration levels, have been employed for this purpose. The most effective antioxidant package for a given elastomeric compound, subjected to γ‐irradiation and aging, giving the lowest changes in physical properties, is evaluated. © 1992 John Wiley & Sons, Inc.

ＮＲ‐，ＳＢＲ‐，ＩＩＲ‐，ＥＰＲ‐カーボンブラックＨＡＦ系の混練機構

Morphological changes of carbon black agglomerates were observed for NR-, SBR-, and IIR-HAF-process oil compounds during the mixing in an internal mixer, using an optical microscope of a transmission method. For the convinience of observation, the recipe was designated as containing only a lesser amount of HAF, 10phr. Those observations were carried out over the minimum mixing times. The changes of the NR compounds are very similar to those of the EPR compounds shown in our previous articles. It suggests the onion peeling mechanism might control the dispersion stage of the NR mixing. Although some agglomerates indicate the existence of that mechanism, it was not evidenced whether or not the mechanism was dominant in the other rubber systems. The process oil influences little on the mechanism as understood by the investigation on the oilless systems of NR and EPR.

The Resolution of Elastomer Blend Properties by Stress-Strain Modelling. An Extension of the Model to Carbon-Black-Loaded Elastomers

Abstract The unique stress-strain behavior of a carbon-black-loaded elastomer is due to the presence of a rigid, particulate phase, but also to the interaction of the elastomer chains with the filler. It is postulated that this interaction takes the form of adsorption on the filler-particle surface, which results in trapped entanglements. Upon deformation, the trapped chains are aligned parallel to the axis of stress. Thus, a practical stress-strain relationship could be developed which is capable to model the stress-strain behavior of compounds over the full range of extensions up to break. The analysis of a highly prestrained carbon-black-loaded NR compound in which the entanglement effect had been mechanically destroyed, demonstrated that the “sea-island” (SIP) coupling arrangement is most suitable for accounting for the interaction effect of the elastomer and carbon black. For moderately prestrained carbon-black-loaded NR and BR compounds a good fit of theory to experiment is obtained for a combination of the SIP coupling arrangement and the specially derived stress-strain relationship. Thus, a practical method is available for describing the deformation of carbon-black-loaded elastomers and for the modelling of carbon-black-loaded elastomer blends.

Detection of the Molecular Orientation in Nonvulcanized and Vulcanized Natural Rubber Compounds by the Low-Temperature X-Ray Method

Abstract The low-temperature x-ray method can detect the orientation of NR molecules in both vulcanizate samples and nonvulcanizate samples of NR compounds. The presence of carbon black is important for causing the orientation. NR molecules orient in the direction of stretch-relaxation in the case of uniaxially fatigued vulcanizates or in the direction of shear in the case of nonvulcanizates prepared by various processing methods. The orientation of NR molecules in nonvulcanizates usually disappears when vulcanized, but a fairly large extent of molecular orientation remains, even after vulcanization, if there exist crosslinks in the oriented nonvulcanizate and the sample is prevented from shrinkage.

Effect of Brown Vulcanized Vegetable Oil on Ozone Resistance, Aging, and Flow Properties of Rubber Compounds

Abstract The addition of accelerator to vegetable oils during the preparation of brown V.V.O. results in a decrease of the free sulfur and the acetone extract values of the obtained V.V.O. In addition, the reaction time necessary to achieve the gel state is reduced. NR vulcanizates containing brown V.V.O. based on soybean oil are superior in retention of their tensile strength and 100% modulus after aging to those containing V.V.O. based on boiled linseed oil and dehydrated castor oil. The heat resistance of NR vulcanizates which contain brown V.V.O. increases as the amounts of the free sulfur and the acetone extract in the V.V.O. decrease. Addition of small amounts of brown V.V.O. to NR and SBR compounds helps to maintain the tensile strength after aging. However, the addition of high concentrations of the V.V.O. reduces the aging properties. The addition of brown V.V.O. based on soybean oil and TMTD to NR compounds used for the manufacture of printer rolls and rubber hoses enhances the ozone resistance of the rubber vulcanizates. Furthermore, the addition of this type of V.V.O. to NR and SBR compounds is advantageous because of: (1) delay of the onset of melt fracture, (2) reduction of extrudate swell, (3) smoother extrudate surfaces, (4) dimensional stability of NR and SBR extrudates.

ChemInform Abstract: Biomimetic Oxidation of Hydrocarbons and Drugs by Metalloporphyrinic Systems

Most biological oxidations of exogenous compounds, such as drugs and endogenous compounds, especially when the hydroxylation of an inert C-H bond is involved, are catalyzed by cytochrome P-450-dependent monooxygenases. These enzymes have the unique ability to transfer an oxygen atom from oxygen atom donors including O2 into almost any organic substrates including alkanes in a highly selective manner. Based on the knowledge of the mechanisms of substrate oxidation by these cytochromes, chemical model systems using metalloporphyrin catalysts have been built. In particular, a very efficient system for hydrocarbon oxidation by H2O2 in mild conditions is described. It is also shown that Fe- and Mn-porphyrins catalyze the transfer of the NR nitrene moiety of PhI=NR compounds into hydrocarbons leading to direct aziridination of alkenes and amidation of alkanes with high yields.

Biomimetic Oxidation of Hydrocarbons and Drugs by Metalloporphyrinic Systems

AbstractMost biological oxidations of exogenous compounds, such as drugs and endogenous compounds, especially when the hydroxylation of an inert C‐H bond is involved, are catalyzed by cytochrome P‐450‐dependent monooxygenases. These enzymes have the unique ability to transfer an oxygen atom from oxygen atom donors including O2 into almost any organic substrates including alkanes in a highly selective manner. Based on the knowledge of the mechanisms of substrate oxidation by these cytochromes, chemical model systems using metalloporphyrin catalysts have been built. In particular, a very efficient system for hydrocarbon oxidation by H2O2 in mild conditions is described. It is also shown that Fe‐ and Mn‐porphyrins catalyze the transfer of the NR nitrene moiety of PhI=NR compounds into hydrocarbons leading to direct aziridination of alkenes and amidation of alkanes with high yields.

Time-Dependent Tearing of Carbon Black-Filled and Strain Crystallizing Vulcanizates

Abstract 1. Unstable tearing can be minimized and tear-tip diameter held reasonably constant for carbon black-filled and strain-crystallizing vulcanizates using a constrained trouser tear testpiece first suggested by Gent and Henry. In the absence of these complicating features, the tear energy has been shown to be thermo-rheologically simple, readily lending itself to time-temperature superposition over a broad range of temperatures and rates. 2. Shift factors used to superpose tear energy data at different temperatures for both filled and unfilled SBR also superposed small deformation stress-relaxation data. This result did not hold for the NR formulations tested, indicating that the temperature dependency of the principal viscoelastic process is different for these properties. Evidence of this was observed in the thermal expansion coefficient, a, derived from the WLF coefficients, which increased by a factor of 10 for tear energy data. It is concluded that even with the tear path constrained, strain-induced crystallization still occurs near the tear tip for the NR formulations. 3. Temperature-reduced tear energy master curves for both filled and unfilled vulcanizates showed the same general tear behavior when referenced to Tg. At high rates or low temperatures, the master curves coincide, approaching a characteristic tear energy of approximately 50 kJ/m2, regardless of formulation. Only at lower rates did the different nature of the rubbers become apparent. All the master curves showed evidence of a tear energy plateau between −15 and −10 decades in rate, with the NR compounds showing the lowest slopes in this region. 4. Addition of filler increases the tear energy by approximately 20–40% over most of the master curves. This supports a previous conclusion of Gent and Henry that the major effect of reinforcing filler is to dissipate energy over a larger volume of material without significantly increasing inherent strength. Addition of filler also tended to mask the plateau in the tear energy master curve and decrease both the free volume at Tg and the thermal expansion coefficient of the material.

Natural Rubber Compounds for Truck Tires

Abstract An 80 NR/20 BR truck tread compound containing a semi-EV cure system and modified with a 6.0 phr level of stearic acid has been shown to exhibit excellent resistance to reversion when compared to a similar compound containing a normal 2.0 phr level of stearic acid. Improvements in the retention of laboratory abrasion resistance, heat generation, and most physical properties have been identified on test pieces subjected to typical truck retread overcure conditions. In highway fleet testing trials of 1100 × 22.5 truck retreads fitted to both third and fourth drive axles of tipper trucks, the modified compound displayed a 42% improvement in treadwear performance over the normal compound in the lower severity third axle positions while performance in the higher severity fourth axle positions was inferior by 20%. In comparison to a 55 SBR/45 BR truck tread, both NR compounds displayed superior wear performance on the fourth axles while some further adjustments of the modified compound are required to match the synthetic compound on the third axles. The reversal of wear performances for all compounds between third and fourth axles is due to the different abrasion mechanisms encountered. Laboratory abrasion rankings do not correlate with wear performances of compounds on the fourth drive axle of trucks, but they do correlate with wear performances on third drive axles. Despite the reversion characteristics of the normal semi-EV compound, no significant adverse effect on treadwear performance was evident at the start of tire life. The low heat generation of the modified compound in laboratory tests is confirmed in actual tire testing. Advantages in rolling resistance characteristics are also evident for the modified compound. Current studies at MRPRA suggest that further modifications of cure system design, in combination with the optimization of NR/BR ratios and mixing methods, will potentially provide NR dominant truck tread compounds which will exhibit superior wear performance in both the higher and lower abrasion severities encountered in heavy-duty truck tire service conditions.

Stress Relaxation Measurement as a High-Speed Method for Predicting the Long-Term Behavior of Vulcanized Rubber

Abstract Intermittent stress relaxation is well suited for quantifying aging processes in rubber. For NR, the effects of different antioxidants can be measured by the rates at which the modulus falls. In synthetic rubbers (SBR, NBR, and CR), it can be measured through the modulus increases caused by oxygen crosslinking. Equal-value times, e.g., t0.75 for NR and t1.25 for SBR, NBR, and CR, are the times taken by the modulus to decrease or increase to a given percentage of its original level. They are better measures of aging than the modulus change after a given time. For NR, it has been shown that stress relaxation measurements at elevated temperatures and in pure oxygen correlate with the results of conventional aging methods. Thus the testing times can be drastically reduced—from up to 28 days to 1–8 hours. Discontinuous stress relaxation measurements are considerably more accurate and selective than conventional oxygen aging. For the repeatability of a single equal-value time measurement, we obtained a coefficient of variation (s/x) of 5–10% for stress relaxation measurement and about ±20% for oxygen aging. The degree of selectivity for oxygen aging and stress relaxation was found to be 3.6 and 18, respectively. The correlation between the results of stress relaxation measurements and those of the conventional oxygen and hot air aging tests was investigated for a large number of NR compounds and for some SBR, NBR, and CR compounds. The correlation is not very good, but it must be remembered that the equal-value times determined according to the two methods show relatively pronounced variability. With the conventional aging methods, the results also depend on what quantity is measured, e.g., the change in tensile strength or hardness, and on what equal-value time is chosen, e.g., the time taken by the property concerned to decrease to 90 or 75% of its original value. The conclusion of the investigations described above is that intermittent modulus measurement is an interesting high-speed method for the assessment of aging behavior. It will acquire a firm position in the arsenal of rubber testing methods, but will probably supplement, and not completely replace, the aging methods currently used.

Electron Microscopical Examination of Worn Tire Treads and Tread Debris

Abstract Tire wear is an important phenomenon, and the industry needs to know as much as possible about the mechanism of rubber abrasion that results in wear. A wealth of information can be obtained by studying tread surfaces and debris particles using the scanning electron microscope, energy dispersive x-ray analysis, and transmission electron microscopy of ultrathin sections. Such an examination was conducted on the worn tread surface of an SBR compound containing 75 phr N285 carbon black and 45 phr oil. A liquid-like layer composed of an intimate mixture of degraded rubber and road silt could be seen on the worn tread surfaces. The thickness of this layer is variable but generally in the micron and submicron region, except where it had been rolled up and was about to be thrown off as a debris particle. A general examination of NR and BR tread compounds revealed a similar appearance. Several collections of rubber debris particles were also examined and found to have the same composition of degraded rubber and road silt that was observed on the tread surface. These observations are in agreement with the results of Boonstra, Heckman, and Kabaya with respect to the analysis of road silt materials on tread surfaces. Since the silt is within the thin layer of degraded rubber, it cannot be washed off the tread surface with soap and water. It can be removed with benzene swabbing. In addition, this report provides visual documentation of tire wear that serves to support previous speculations. These results emphasize the importance of considering the abrasion of tire treads as a three-bodied system: the tread compound, the road surface and the layer between the two, which is composed of an intimate mixture of degraded rubber and road silt. Previous references to the presence of road silt particles on worn tread surfaces implied their incidental nature. This study shows that road silt penetrates into and becomes a part of the substructure of worn tread surfaces. Furthermore, the internal morphology of this substructure is identical to the morphology of the tread debris thrown off from the tread surface.

Preparation of silica by acid dissolution of sepiolite and study of its reinforcing effect in elastomers

AbstractThe preparation of amorphous silica by acid treatment of a natural magnesium silicate and its use as reinforcing filler for NR and SBR compounds are studied. Limited to the treatment with nitric acid, the rate constants at different temperatures and acid concentrations and the activation energy are calculated, as a function of percentage of extracted magnesium.The resulting material is basically an amorphous silica (92.5% SiO2), as shown by X‐ray analysis, that keeps the fibrous morphology of the original mineral. According to a preliminary technological study the resulting silica is a suitable reinforcing filler for general purpose rubbers; a comparison between the experimental results and those obtained with a commercial fine particle precipitated silica was made. Using a SBR compound the effect of silane coupling agents on the new silica was studied.

Factors Which Influence Autohesion of Elastomers

Abstract The following conclusions can be drawn from our studies: (1) An optimum molecular weight (Mn=2095) exists for t-octylphenol-formaldehyde resin to give maximum autohesion to rubber compounds. A critical level of the phenolic resin is required in a given rubber compound to make the tackifying action effective. In addition, the presence of polar groups in a resin molecule is a structural requirement in order for the resin to provide tackiness to rubber compounds. (2) The autohesion of a rubber compound decreases as the loading of processing oil increases from 5 phr to 25 phr. This behavior is independent of the type of oil. (3) The autohesion of a rubber compound decreases with increasing carbon black level, surface area, and structure. (4) Autohesion maximizes at an optimum molecular weight of rubber. (5) Humidity has a pronounced effect on the autohesion of aged SBR compounds, whereas very little effect on that of aged NR compounds. (6) A dual diffusion-adsorption mechanism is operative in autohesion.

A Surface Study of Ozone Attack and Antiozonant Protection of Carbon Black Loaded Natural Rubber Compounds

Abstract This work reports a direct surface analysis of ozonization of carbon black loaded NR compounds by ATR and SEM. On ozonization of raw NR or NR containing carbon black, a thick layer builds up consisting of ozonides and carbonyl compounds. It is suggested that this layer is composed principally of the reaction products of ozone with unsaturated fatty acids and unsaturated fatty acid esters found in NR. On ozonization of cured and uncured carbon black loaded NR compounds containing curatives, the NR is attacked causing a decrease of rubber on the surface and a corresponding increase in the surface concentration of carbon black. The thick ozonide/carbonyl layer did not form, probably due to the coordination of the fatty acids and esters with ZnO, which was added with the curatives. On ozonization of carbon black-loaded NR compounds containing N,N′-dioctyl-p-phenylenediamine antiozonant, a continuous film is seen by SEM which has an ATR spectrum essentially identical to ozonized antiozonant. There is no evidence of ozonized NR products, or of complex reaction products between antiozonant and ozonized NR. These results are consistent with a dual scavenger and protective layer mechanism for antiozonant protection.

A Fatigue Resistant Polychloroprene Compound for High Temperature Dynamic Applications

Abstract A polychloroprene material has been developed which can replace the natural rubber materials traditionally used in dynamic, fatigue-producing applications, when these applications require high temperature stability. An optimized compound based on a mercaptan modified polychloroprene with large particle high structure carbon black, silica filler and rapeseed oil has superior compression set, improved heat aged fatigue life and comparable dynamic properties to current NR compounds. This CR compound was designed to have good fatigue life while keeping compression set to a minimum. A positive correlation between tear resistance and fatigue life has been demonstrated for a series of CR compounds.