bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation Analysis of Curing Agents
Authors: Morita E.
Published: February 2011

The linear correlation of the pKa to the σ* constants of the substituted phenylthio group indicates the validity of the σ* being equal to the sum of the Hammett's constant of these substituent groups and the σ* of the phenyl group (0.6). The inhibitory activity of the compound LSR depends on the reactivity of BtSSR with BtSH and the general stability of the mixed disulfides which form during the curing process. For BtSSR as accelerators, the Hammett's rule applies to scorch delay time versus σ* constant as the variable with negative slope in the LFER. In benzothiazole-2-sulfenamides, the 13C NMR chemical shift of the carbon at the 2-position of the benzothiazolyl group (C8) is consistently greater than the chemical shift at the carbon adjacent to the amino nitrogen (CN) in the same molecule. It indicates that the electronic distribution at the S-N bond is consistently more positive at the S atom and more negative at the amino nitrogen. The 13C NMR chemical shift at C8 is inversely proportional to the σ* constant of the amino substituents. A wider range of amino substituents is applicable for the correlation analysis with 13C chemical shifts than σ* constants as the variable. When benzothiazole-2-sulfenamides are used as accelerators, two linear relationships with slopes of opposite signs are obtained for the N-substituted phenyl and the N-alkyl sulfenamides, respectively, in the relationship of the scorch delay to the 13C8 chemical shift or the σ* constant. Longer scorch delay was obtained with the pertinent electron-withdrawing phenyl or the sterically hindered alkyl substituents. The more basic amino derivatives give a faster acceleration rate and a higher crosslink efficiency. A significant linear correlation was obtained for scorch delay versus Hammett's σ constant of the phenyl substituents of the N,N′-dithiobisformanilides as vulcanizing agents.
Investigation of Abrasion of Nitrile Rubber
Authors: Zhang S. W.
Published: February 2011

Abrasion of NBR appears as a dry (particulate) wear, similar to unfilled BR, NR, and SBR, and filled BR. The abraded surface seems textured on two scales: that of the ridges of the abrasion pattern and that of small particles of debris. The formation of macroridges on filled and unfilled NBR appears to involve two distinct ridges one formed after the other. First, fine and tightly spaced primary ridges, followed by rough and sparsely spaced secondary ridges. The abrasion pattern on unfilled NBR is relatively stable once steady state has been reached and the pattern has been fully developed. As for the filled NBR, both the scale and roughness of the secondary ridges are uneven, even if steady state has been reached completely. Wear rates of filled and unfilled NBR have been found to be proportional to an exponent n of the applied frictional work. The unsteady state rates of wear were also somewhat dependent on the number of revolutions of the test wheel. Both the proportionality coefficient k and exponent n are dependent upon the composition and also upon the wear state. The exponent n for unfilled NBR is greater than that for filled NBR, and for the unsteady state it is greater than that for steady state. Reversals in the relative rates of wear between filled and unfilled NBR are also observed at different severities of abrasion. This phenomenon perhaps reflects a competition between two different physical processes: crack growth and rupture of tongue tip. The dominant process may be crack growth at low severities of wear and rupture of the tongue tip at high severities of wear. Therefore, the fact that, at high severities, carbon black markedly reduces the rates of abrasion might be attributed to the tensile strength gain of rubber compounds due to reinforcement by carbon black.
Mechanisms of Antioxidant Action: The Antioxidant Activity of “Spin-Traps” in Rubber
Authors: Nethsinghe L. P. , Scott G.
Published: February 2011

A number of nitrones containing a partially hindered phenolic group are shown to be effective antidegradants for sulfur vulcanized rubbers. They exhibit a high level of antiozonant and antifatigue activity, but unlike commercial antiozonants they do not cause discoloration of light colored formulations. The new antidegradants are less effective than conventional additives as thermal antioxidants, but their performance as antifatigue agents and antioxidants is augmented by their use in combination with nonstaining antioxidants. The mechanism of action of the phenolic nitrones is believed to involve their oxidation to stable phenoxyl/nitroxyl radicals.
Determination of p-Phenylenediamine Antiozonant in Polychloroprene Rubber
Authors: Potter Noel M. , Mehta Ram K. S. , Wyzgoski Michael G.
Published: February 2011

Elastomers susceptible to ozone cracking can be protected by the addition of antiozonants. For polychloroprene (CR) rubber, a mixture of diaryl-p-phenyl-enediamines, Wingstay 100 (WS 100), is one of the most effective antiozonants. Preliminary studies in our laboratory have shown that the effectiveness of WS 100 can be improved even further by the use of an organobentonite clay. To provide more insight into the way in which this antiozonant provides resistance to ozone cracking, a technique for its analysis was required. High performance liquid chromatography (HPLC) is an attractive technique for analyzing WS 100, as well as for a wide variety of additives in polymers. This study describes a newly developed method for the determination of WS 100 in CR. The method is based on acetonitrile extraction of the antiozonant from the rubber, followed by quantitative analysis using HPLC. With this method, subtle differences in concentrations of WS 100 in CR, caused by changes in compounding, mixing sequence, curing conditions, and grease extraction, have been measured.
Processability Testing of Injection Molding Rubber Compounds
Authors: Sezna John A. , DiMauro P. J.
Published: February 2011

A simple model of the injection molding process has been constructed using data from a capillary rheometer (MPT) and the Oscillating Disk Rheometer (ODR). For an NR and an SBR compound, the model had an excellent correlation with injection molding trials. The model successfully predicted the effects of adjusting injection pressure, mold temperature, and barrel temperature on injection times and scorch conditions. Such a model enables an injection molder to predict the effect of adjusting molding conditions, optimize his process for a given mold and compound, and control processability of his compounds batch-to-batch.
Correlation between End Correction and Extrudate Swell for Some Raw Elastomers and Black-Filled Rubber Compounds
Authors: Kannabiran R.
Published: February 2011

A good linear correlation between end pressure and recoverable energy has been shown to exist for some raw elastomers and black-filled rubber compounds. The recoverable energy calculated from extrudate swell is almost a constant fraction of exceess energy given by end pressure for the materials studied in the given entrance geometry with the exception of NR.
Mechanisms of Antiozonant Protection: Antiozonant-Rubber Reactions during Ozone Exposure
Authors: Lattimer R. P. , Layer R. W. , Rhee C. K.
Published: February 2011

Rubber (IR or BR) vulcanizatcs containing DOPPD antiozonant have been aged under ozone, and the soluble constituents have been removed by acetone extraction. Small amounts of nitrogen from the antiozonant remain in the rubber after extraction; this nitrogen is presumed to be chemically attached to the polymer network. Some of this “unextractable nitrogen” may be due to reaction of ozonized rubber with antiozonant, but other explanations are possible. The amount of unextractable nitrogen in the vulcanizate before ozone aging is about the same, within experimental error, as the amount remaining after static ozone aging. Model compound experiments (with cis-9-tricosene or squalene) show that chemical reactions can take place between the ozonized olefins (or unsaturated rubber) and DOPPD. The chemical structure for a model reaction product has been determined. Despite these findings, it seems unlikely that these types of reactions will play a significant role in the overall protection of rubber vulcanizates from ozone attack. That is, these products can form only after the rubber starts to become ozonized, and it has been shown that ozone attack on the rubber does not occur until after the antiozonant is nearly completely consumed. The results of this work are consistent with previous studies in our laboratory that indicate a combined “scavenger-protective film” mechanism is principally responsible for antiozonant protection. There is no definitive evidence to date to show that ozonized rubber reacts with antiozonant during ozone aging of rubber vulcanizates.
Thermal and Age-Resistance Properties of NR Gum Vulcanizates in EV Systems
Authors: Datta Rabindra Nath , Das Mathur Mohan , Basu Dipak Kumar , Chaudhuri Ajit Kumar
Published: February 2011

A comparative study of the cure behavior of thiocarbamyl sulfenamides both in the presence of MBTS and thiazole sulfenamides has been made at 140 and 170°C. It is observed that most of the NR vulcanizates obtained at 170°C are capable of retaining a considerable portion of torque, modulus, and tensile values obtained at 140°C. This provides information regarding the heat resistance of these vulcanizates. It is observed that ZDC's up to a certain concentration increases the thermal resistance of the NR vulcanizates, the maximum effect being produced by ZBD and ZPD. ZOD and ZPD above a critical concentration, impart a deleterious effect to NR gum vulcanizates where a drop in the R∞ value is observed. The age resistance property of the various combinations of thiocarbamyl sulfenamides plus MBTS and thiazole sulfenamides plus thiocarbamyl sulfenamides has been discussed. It is evident from the study that the former combination generally provides higher physical properties of the vulcanizates and better age-resistance than those obtained with the latter combination. Vulcanizates obtained with the CTOS-MBTS combination in the presence of ZDC's provide notable thermal as well as age-resistance.
Mechanisms of Antioxidant Action: The Antifatigue Mechanism of Nitrosamines
Authors: Dweik H. S. , Scott G.
Published: February 2011

1. The present studies have shown that N-nitrosamines derived from the more active diarylamine antifatigue agents are considerably more effective as antifatigue agents than the parent amines when they are predispersed into rubber on an open mill, followed by conventional vulcanization. 2. Some N-nitrosamines are effective retarders of premature vulcanization. 3. All the nitrosamines examined with the exception of nitroso diphenylamine are effective heat stabilizers for rubber in an air oven test. 4. The nitrosamines are partially converted to the parent amine during vulcanization, but other products are also formed which contribute to the antidegradant activity.
Mechanisms of Antioxidant Action: Complementary Chain-Breaking Mechanisms in the Mechanostabilization of Rubbers
Authors: Nethsinghe L. P. , Scott G.
Published: February 2011

It is clear that aliphatic nitroxyls cannot function effectively in the catalytic CB-A/CB-D cycle (Scheme 1) during mechanodegradation in a sulfur vulcanizate, although they have been shown to be effective mechano-chemical inhibitors in rubbers before vulcanization. Aliphatic nitroxyls appear to be more effective when they contain an additional CB-D function.
Elastomers from Polymeric Chelates
Authors: Iwamoto Nancy , Eichinger B. E. , Andersen N. H.
Published: February 2011

In the previous work, we demonstrated that gels may be formed from polymeric acetylacetonate chelates. Now it has been shown that useful elastomers can be prepared from these derivatized polymers as well. Stress-strain measurements have been performed on these Pd(II) crosslinked SIR elastomers, and the results demonstrate that the materials are useful for physical testing. Because of unforeseen complications, we do not yet have a perfectly ideal system to solve fully the problems mentioned in the Introduction. The observations reported here are more than sufficient to encourage us to pursue this research vigorously. The difficult synthesis of ACAC-endcapped PDMS that has been achieved provides a viable route to model networks. What is required now is that we focus attention on the inorganic chemistry in an effort to better understand such things as the kinetics of ligand exchange reactions and the structure of ACAC complexes in low polarity media.
End Group Effects in Polymer Modification of Calcium Carbonate Fillers
Authors: Nakatsuka Takuo , Kawasaki Hitoshi , Yamashita Shinzo
Published: February 2011

Two types of polymer, e.g., polybutadiene and poly(oxy-2-butenyleneoxyfumaroyl), having four different end groups, were prepared and used to treat ground and precipitated calcium carbonate fillers. The coupling effects of the polymer modifiers were evaluated from the physical properties of the filler-loaded vulcanizates of sulfur-cured SBR and peroxide-cured EPDM. The phosphate-terminated polymer was found to be an excellent surface modifier of the ground calcium carbonate filler. For the precipitated calcium carbonate filler of superfine grade, higher coupling effects were attained by the phosphates having relatively small alkyl groups. The modification mechanism is also discussed by taking into account the results of infrared spectroscopy, x-ray analysis, gel permeation chromatography, scanning electron micrography, and BET analysis.
Polymerization of Butadiene and Isoprene with Lanthanide Catalysts; Characterization and Properties of Homopolymers and Copolymers
Authors: Hsieh H. L. , Yeh H. C.
Published: February 2011

The lanthanide polymerization catalysts represent a significant achievement in the synthesis of stereospecific high-cis cis-polybutadiene, high-cis cis-polyisoprene, and high cis random and block copolymers of butadiene with isoprene. In contrast to the conventional 3d transition metal catalysts, changes in lanthanide metals, halogens, and ligands do not significantly affect the cis-1,4 content of the polymer. But, the catalyst activity is greatly dependent on the choice of lanthanide elements, counterions, and donor ligands. Energy efficient processes are possible by performing the polymerizations in low boiling aliphatic hydrocarbon solvents with high solids content and possibly, in the gas phase. In addition, the described lanthanide catalysts are very effective for producing high cis polymers with a broad range of molecular weights.
Model Vulcanization of EPDM Compounds—Part III. Influence of Vulcanization System on the Vulcanization Products from Ethylidene Norbornane
Authors: van den Berg J. H. M. , Duynstee E. F. J. , Maas P. J. D.
Published: February 2011

The vulcanization of ENBH (C9H14), a model for vulcanization of EPDM containing ENB, was studied (140°, 1 h) using four different vulcanization systems (conventional, sulfur free, low-sulfur and high-sulfur). The influence of the system on the total yield of crosslinked products (C9H13—Sn—C9H13) is considerable, low-sulfur giving the lowest and high-sulfur giving the highest yield. The influence of the system on the composition of the crosslinked products as determined by HPLC is surprisingly small. By using 1H-NMR it was shown that formation of monosulfides is almost negligible under our conditions with all four vulcanization systems.
Epoxidized Natural Rubber
Authors: Baker C. S. L. , Gelling I. R. , Newell R.
Published: February 2011

When natural rubber is epoxidized under carefully controlled conditions, it can be converted to a totally new polymer with some properties more akin to speciality rubbers and some properties that appear to be advantageous for tire treads. Three levels of epoxidation have been extensively evaluated. They were 50, 25 and 10 mole% epoxidized NR referred to as ENR-50, ENR-25 and ENR-10 respectively. ENR-50 has been found to undergo strain crystallization like NR, but with oil resistance similar to a medium acrylonitrile NBR and gas permeability similar to butyl rubber. It is a highly damping rubber with a very low room-temperature resilience. ENR-50 and ENR-25 both exhibit good wet grip characteristics and have been examined as tire tread materials. In particular, ENR-25 compounds containing silica or silica/black have been found to give lower rolling resistance than NR and better wet traction than OESBR, so providing an ideal combination of these two properties for tire treads. Unfortunately, wear data is as yet incomplete, but it is anticipated, from truck tire experience, that the black/silica compounds will present no problems. ENR-25 and ENR-50 exhibit this unique feature with silica of reinforcement equivalent to black without the use of a coupling reagent. Thus, these rubbers have potential of providing white or colored vulcanizates with properties previously associated only with black-filled compounds, so extending applicational areas, or even resulting in colored tires or other automotive parts. ENR-10 provides a damping grade of NR when lower resiliences are required. Alternatively, blends of ENR-25 or -50 with NR may be used. Many engineering applications are calling for reduced resilience, and this new form of NR can give precisely this.
Recycled Rubber Crumb as a Toughener of Polystyrene
Authors: Pittolo M. , Burford R. P.
Published: February 2011

Our preliminary investigations indicate that recycled rubber crumb may be used as a toughening agent for brittle thermoplastic such as polystyrene. The toughness of the resultant materials was found to increase with increasing rubber-to-matrix adhesion and decreasing particle size. Although crazing is expected to play some role in the toughening mechanism, the absence of a yield point and the type of fracture morphology indicate it is only a minor factor It is suggested that the main contribution is from a crack bridging mechanism. The increased energy to break results from the rupture of the rubber particles. Further studies in which crumb with known tear strength and tensile properties is used as a filler are currently being carried out. It is hoped that this will lead to a clearer understanding of the toughening mechanism.
Viscoelastic Properties of a Class of Carboxylated Rubbers: The Carboxylato-Telechelic Polydienes
Authors: Jéróme R. , Broze G.
Published: February 2011

Halato telechelic polymers are well-characterized ion-containing polymers the investigation of which gives the scientist a clear insight into the role played by the ions in the modification of the base polymer properties. The end-neutralization of very short length, α,É-dicarboxylic acid polydienes by alkaline earth alcoholates triggers off the crosslinking of the rubber. In a nonpolar environment, the metal carboxylate bonds constitute ion pairs which associate into separate domains responsible for the vulcanization and the reinforcement of the rubber. For the very first time, the relaxation of these ionic domains has been observed and thoroughly investigated. Its mechanism obeys an Arrhenius-type of temperature dependence, whereas the activation energy strongly depends on the ion pair features. When entangled acid-ended polydienes are end-neutralized, an efficient crosslinking is again reported, whereas the relaxation of ionic domains is no longer observed. It means that the ion pair association is extended enough to stabilize a network of entangled chains, but that the size of the aggregates is too small to be accurately detected. In the series of alkaline earth metals, the higher the ionic radius, the less interacting the ion pairs. Nevertheless, the strength of the formed network increases with the ionic radius, which means that the functionality of the crosslinks, i.e., the mean size of the ionic aggregates, increases too. The extent of the ion pair association is therefore largely independent of their anchoring at the ends of macromolecules and is mainly related to the geometry of the metal carboxylates and to their mutual interaction.
Sulfur Vulcanization and Oxidative Aging of Epoxidized Natural Rubber
Authors: Gelling I. R. , Morrison N. J.
Published: February 2011

1. Vulcanization by sulfur alone is faster and more efficient for ENR than for NR because isolated double bonds react more rapidly than contiguous double bonds. This vulcanization may be accelerated by sodium carbonate, which also protects the cured rubber against the subsequent effect of oxidative aging, and the scorch time may be increased by the addition of CTP. 2. The CBS-accelerated sulfur vulcanization of ENR is essentially similar to that of NR, although ENR reacts with MBT formed during the process. 3. The poor aging of sulfur vulcanizates of ENR is due to acid-catalyzed ring-opening reactions of the epoxide groups with the formation of ether crosslinks. The acids are produced by the thermal decomposition of oxidized sulfides. 4. The addition of a suitable base confers excellent resistance to oxidative aging upon conventional, semi-efficient, and efficient vulcanizates of ENR.
A Review of Recent Developments in the Mechanisms of Antifatigue Agents
Authors: Scott Gerald
Published: February 2011

In spite of the remarkable advances that have been made in the engineering design of tires during the past two decades, the basic formulations used in vulcanization and protection during service have essentially remained unchanged. This is to a large extent due to two major factors: 1. The versatility of the traditional accelerated sulfur curing system which provides the necessary combination of resilience and strength with good resistance to cyclical stress. 2. The development of diarylamine antidegradants which confer a high level of thermal-oxidative and mechano-oxidative (fatigue) resistance to the rubber. Both of these developments have occurred slowly with small incremental improvements and, probably because of their success, relatively little fundamental work has been published which addresses the questions of why a polysulfide network is so resistant to fatigue and why N-sec-alkyl-N′-phenyl-p-phenylenediamines are so much more successful as antidegradants than other classes of antioxidant. It is becoming evident, however, that if tires are to withstand the increasingly demanding conditions to which they are subjected in service, much more attention must be paid to the material design as opposed to the structural design of tires. Nowhere is this more evident than in aircraft tires. Recent studies have shown that the tires of heavily laden wide-bodied aircraft reach temperatures in excess of 70°C at the relatively modest speed of 32 km/h (20 mph). Similarly, in heavy duty truck tires, temperatures over 100°C are not abnormal in the shoulder region. This leads to extensive restructurization of the fatigue resistant polysulfide network, particularly in the shoulder of the tire, to give a much weaker mono-disulfide structure. It is no coincidence then that failure normally occurs in this region. The practice of multiple retreading exacerbates the change in chemical decomposition of the rubber. It is well known to the rubber scientist that extending the vulcanization process also leads to restructurization of the rubber network from polysulfide to mono- and disulfide. This is shown typically for a HAF-black tire formulation at 140°C in Figure 1. Antidegradants have virtually no effect on anaerobic restructurization (see Figure 2), and indeed, the established antifatigue agent, IPPD (I), actually accelerates the loss of polysulfide crosslinks from the vulcanizate at 140°C. During fatiguing, on the other hand, IPPD effectively retards restructurization (see Figure 3), whereas a typical bisphenol, nonstaining antidegradant, II, has much less effect.
Vulcanization Chemistry
Authors: Morita E. , Sullivan A. B. , Coran A. Y.
Published: February 2011

Prevulcanization inhibitors based on triazines can be prepared. In each case the triazine ring is amino-substituted such that one or two of the amino hydrogen atoms per amino-group are alkylthio- or arylthio-substituted. The new triazine-derived prevulcanization inhibitors are extremely active in accelerated sulfur vulcanization of diene rubbers (e.g., NR or SBR). Differing accelerators or accelerator systems may require different triazine-derived prevulcanization inhibitors for best results. Provided that the reactivity of the inhibitor is sufficiently near the ideal, its weight-basis activity is greatest when its equivalent weight per S—N bond is least. Overall, hexakis(cyclohexylthio)melamine and especially hexakis(isopropylthio)melamine are the most effective premature vulcanization inhibitors tested, on an equal-weight basis.
Analysis of Rubber, Resin, and Moisture Content of Guayule by Near Infrared Reflectance Spectroscopy
Authors: Black L. T. , Hamerstrand G. E. , Kwolek W. F.
Published: February 2011

Although the overall error associated with the NIR analysis of guayule may be slightly higher than that of some other methods, the trade-off between the increased error and the very rapid aquistion of data by NIR is quite positive.
Carbon Black in NR/BR Blends for Truck Tires
Authors: Hess W. M. , Vegvari P. C. , Swor R. A.
Published: February 2011

A series of ten commercial tread-grade carbon blacks were evaluated in a 60/40 NR/BR truck tire tread formulation. A number of important physical properties and performance criteria were assessed in terms of carbon black surface area and DBPA. Significant response equations were obtained for viscosity, bound rubber, resilience, heat buildup, tear strength, and dynamic properties. Dynamic modulus showed a much greater dependence on DBPA in comparison to previous studies on SBR/BR compounds. In a second designed experiment, a single carbon black (N299) was studied as a function of the NR/BR ratio and the amount of carbon black added to the BR phase. The BR black loading was varied at 30, 60, and 90 phr using separate masterbatches which were blended with NR-black masterbatches to give the same final composition for all of the compounds. Properties such as resilience, heat buildup, fatigue life, and tear strength were all improved in the direction of higher loadings of carbon black in the NR phase. A high loading of black in the BR phase caused low bound-rubber development and poor dispersion. This was found to be related to the viscosity ratio of the separate masterbatches. NR to BR viscosity ratios of about 1 to 3 produced good dispersion and high bound rubber. When the BR masterbatch viscosity was two to three times higher than the NR masterbatch, however, dispersion and bound-rubber development dropped sharply at the same total mixing energy. Low hysteresis properties were found to be most dependent on high bound-rubber development, with polymer phase distribution having a relatively minor influence. In contrast, tear strength and fatigue life reached their maximum levels when the NR was the more continuous polymer phase. High bound rubber also appears to enhance tear strength and fatigue life by improving the microdispersion of the carbon black.
Effect of Pressure and Shear on Compaction of Powdered Rubber with Carbon Black
Authors: Nakajima N. , Kumler P. R. , Harrell E. R.
Published: February 2011

During the incorporation stage of mixing carbon black with elastomer, the mixture becomes one solid mass, i.e., compacted. In this process, the elastomer must be deformed to conform with the topology of carbon black. This work attempts to elucidate the mechanism of compaction. In particular, the effects of static pressure and shear on compaction have been examined. A Sieglaff-McKelvey capillary rheometer was used for the higher pressure range and a Rheometrics mechanical spectrometer for the lower pressure range. A mixture of powdered rubber and carbon black was used. With static pressure alone, the compaction was slow and ineffective because the mixture forms a bridge structure. The compacted material was friable. With the application of shear, the bridge structure was easily broken, the compaction was attained immediately, and the compacted material was “fused” together. However, the application of shear was effective only when there was no slip between the rotor surface and the elastomer compound.
Observations on the Tearing of Elastomers
Authors: Stacer R. G. , Yanyo L. C. , Kelley F. N.
Published: February 2011

1. Frequency distributions of stick-slip tear arrest and initiation energies have different standard deviations; initiation values show greater variance. Tear arrest frequency distributions are symmetrical and have been described by a normal distribution. The tear-initiation distribution is skewed and has been modeled using both the extreme value and Weibull distributions. As a result, it is concluded that two separate mechanisms are responsible for the creation of the force minima and maxima observed in stick-slip tearing. Consequently, data reduction methods which average these values to estimate tear energy are inappropriate. 2. The direction of skewness of the tear-initiation energies indicates a distribution of the largest extreme. This has been contrasted with constant-rate tensile-rupture data which are distributed according to the smallest extreme. As a result, it is concluded that the process responsible for tear initiation is not a weakest link mechanism, but, rather, depends on a process which structurally inhibits tear propagation. A mechanism of anisotropic tear-tip reinforcement caused by large regional deformation has been discussed as a possible explanation. 3. Tear energies calculated from tear-initiation forces have been shown to be dependent upon the energy stored in simple tensile extension and the volume of material available to be extended. This type of behavior shows that the tear-initiation energy is perhaps a reasonable indicator of strength but does not meet the criterion for an inherent material property. Tear-arrest values have been shown to be independent of these features and are considered unbiased estimators of inherent material tear energies. 4. It has been shown that for experimental purposes, stick-slip tearing can be eliminated and tear-tip diameter can be held constant using a constrained trouser-tear test. This is in agreement with earlier work by Gent and Henry. It has also been shown that the constrained tear energy thus measured is an inherent feature of stick-slip tearing results, requiring only additional data reduction to extract. Consequently, the constrained tear energy can be estimated from stick-slip tearing, and conversely, the magnitude of stick-slip tearing is predictable from a constrained tear energy and simple tensile extension data.
Heat Transfer and Property Development in Liquid Silicone Rubber Molding
Authors: Macosko Christopher W. , Lee L. James
Published: February 2011

This study has used a heat transfer model to describe the cure of liquid silicone rubbers. The model shows that cure always starts from the wall and will continue during cooling. Thick parts may not be totally cured upon demolding, although the material near the wall has reached a desirable mechanical strength. Since silicone rubbers gel fast at high temperature, mold filling problems may occur for the thin and long molds, especially at high mold temperatures.
Failure of Rubber by Abrasion
Authors: Pulford Carl T. R.
Published: February 2011

This short review presents the landmark discoveries and ideas in rubber abrasion that have brought the field to where it is today. First, the important features of rubber abrasion are reviewed as background for a physical model for the abrasion of rubber. The model, due to Thomas, is described in detail, since it clearly shows the connection between the failure of rubber by abrasive wear and the appropriate rubber fracture properties. The implications of the model for improved abrasion resistance are also discussed. Then, physical principles are applied to the failure of rubber by abrasion in actual products, such as tires. The tire wear model of Schallamach and Turner is described, together with its success in explaining several features of tire wear.
Rubber Enters the Polymer Age
Authors: Morton Maurice
Published: February 2011

I hope that this review of the highlights of my research and educational activities over the past 40 years has demonstrated how polymer science has taken over the fairly “ancient” art of rubber technology; and how both the methods of research used in and the career training required for man's utilization of this unique material are now firmly established as part of the science of macromolecules.
The Action of Antiozonants in Rubber
Authors: Brück D. , Königshofen H. , Ruetz L.
Published: February 2011

Rubbers with olefinic double bonds, and rubber goods based on them, are destroyed by ozone. Crack formation can be retarded by adding antiozonants. The mechanism of the antiozonant effect has not yet been fully explained. The mechanism by which bis-(1,2,3,6-tetrahydrobenzaldehyde)-pentaerythrityl acetal (AFS), a nonstaining antiozonant, hinders the ozonation of polychloroprene has been described. AFS competes with the double bonds in the polymer for the available ozone. Additionally, the resulting ozonization products prevent the ozone from penetrating into the polymer matrix. This mechanism was corroborated by carrying out kinetic measurements of the competing reactions in solution and in the rubber. The findings were confirmed by studying model substances with basic structures identical to AFS but with modified active groups.
Internal Mixing: A Practical Investigation of the Flow and Temperature Profiles during a Mixing Cycle
Authors: Freakley P. K. , Patel S. R.
Published: February 2011

From the results of mixing trials with a highly instrumented BR Banbury and biconical rotor rheometry of mixed batches, a detailed analysis of flow and mixing characteristics in the region of a rotor wing has been undertaken. An ‘angled spreader blade’ analogy of the rotor wing is proposed as being a viable basis for mathematical modelling. A one-dimensional flow analysis is used, in which power-law flow behavior and isothermal conditions are assumed. Dispersive mixing, which depends on the stress levels generated during mixing, is shown to occur throughout the entire mass of material swept in front of the rotor wing and not simply at the rotor tip. In addition, the stress levels depend more strongly on batch temperature than on rotor speed. High rotor speeds tend to lead to reduced stress levels as a result of the associated rapid rise in batch temperature, although choosing an appropriate fill factor can minimize temperature rise by promoting efficient heat transfer to the cooling water. During each rotor revolution, the rotor wing collects a mass of material from the reservoir between the rotors. This mass of material is then progressively reduced by leakage flow under the rotor tip and flow around the end of the wing, until the revolution is completed by the return of a residue to the reservoir. The flow around the end of the rotor is shown to be consistently greater than the leakage flow, although the ratio can be influenced by both fill factor and rotor speed. At high rotor speeds and low fill factors, it appears that material is retained in the regions of the side frames of the mixer and may give batch inhomogeneity through poor distribution mixing.
Mixing of Carbon Black with Rubber. II. Mechanism of Carbon Black Incorporation
Authors: Cotten George R.
Published: February 2011

The present study leads to postulating the following mechanism for carbon black incorporation. During the first stage of incorporation, carbon black agglomerates become encapsulated by the polymer. At this stage, the interstices within these agglomerates are still filled with air, giving a very weak, crumbly composite. The rubber becomes forced into these interstices during the second stage of incorporation. As the rubber is being forced through the narrow channels between the aggregates, bound rubber is being formed. The immobilized layer of the polymer tends to reduce the effective cross section of the channels through which more rubber must pass before reaching the inner part of the agglomerates. Thus, as the activity of carbon black increases, e.g., higher bound rubber, the incorporation time increases. At a given carbon black activity, the effective thickness of the immobilized polymer layer would increase with increasing molecular weight of polymer, but be independent of the oil loading. This agrees with experimental observation that the incorporation time increases with molecular weight of the polymer, while oil loading has no effect other than that associated with the lowering of bound rubber content. The average size of channels through which rubber must pass depends also on the carbon black morphology. The size of these channels would be expected to increase with increasing structure of carbon black, and (to a lesser extent) with increasing particle size. These parameters have the expected influence on the incorporation time; since the incorporation time decreases with increasing DBPA value and decreasing tint.
Dynamic Property and Fatigue Crack Propagation Research on Tire Sidewall and Model Compounds
Authors: Young D. G.
Published: February 2011

Research was conducted to define appropriate compound loading conditions and energy parameters required to properly control and analyze fatigue crack propagation experiments for tire sidewall applications. The effects of strain level, pulse frequency, overall cycle frequency, sample thickness, and oven temperature were screened, and strain level was shown to be the dominant variable in the region of interest. Designed experiments further confirmed that frequency (i.e., strain rate) effects upon strain energy are small at normal rates of tire deformation (equivalent to 40 Hz). However, at typical laboratory test frequencies (≤5 Hz), strain rate effects on strain energy are large, and the differences vs. results under tire conditions depend heavily on polymer type as well as test temperature. Thus, the use of strain level, strain rate, and temperature conditions which simulate the tire service environment are critical to give representative results in laboratory testing. A constitutive equation was defined which provides an excellent model for strain energy in pure (or simple) shear as a function of the principal extension ratio (i.e., strain level) at constant frequency. Therefore, computer modeling of such experiments appears straightforward using an on-line minicomputer. Fatigue crack propagation studies showed major effects of pure-shear sample thickness, processing prior to molding, different types of reference compounds, and different polymer types. Halobutyl compounds and halobutyl/EPDM/NR blends were shown to provide superior FCP resistance at a given strain or strain energy level. These results were consistent with earlier tire and laboratory data.
Defect Accumulation in Rubber
Authors: Roland C. M. , Smith C. R.
Published: February 2011

It has been demonstrated that the material adjacent to the fracture plane in cured rubber specimens can scatter intense x-ray radiation at small angles. This phenomenon is associated with the presence in the material of voids of very small size. These microvoids exist only adjacent to the fracture surface and are the result of the loading imposed to create the macroscopic fracture. The significance of these microscopic defects is their influence on the manner in which externally applied loads produce fracture of rubber; that is, they provide a link between the experimentally observed fracture behavior of rubbers and the fundamental processes governing this behavior. To cite a few examples, it has been observed that the critical value of tearing energy required for catastrophic failure of a rubber specimen differs for a static versus dynamic mode of deformation. It can be recognized that these differences are related, not only to rate effects, but also to the differences in the state of the fractured material directly in front of the crack tip. It has also been noted that the manner in which one introduces a precut into a rubber test specimen can influence the ensuing strength measurement by affecting the nature of the initial fracture plane. It should be understood that the test result can be affected not only by such differences in the geometry of the precut, but as well by the effect the precutting can have on the condition of the unfractured material adjacent to the precut. Currently, our efforts are being directed towards modification of the SAXS instrument to permit measurements to be made at lower values of scattering angle. It is hoped that this will allow quantification of the size and abundance of the microcracks. Such information can provide insight into the significance of defect accumulation in the development of tear resistance in elastomers.
Rubber-Thermoplastic Compositions. Part IX. Blends of Dissimilar Rubbers and Plastics with Technological Compatibilization
Authors: Coran A. Y. , Patel R. , Williams-Headd D.
Published: February 2011

From this work, one can conclude that compositions which have excellent mechanical properties can be prepared by melt-mixing thermoplastic vulcanizates. (A thermoplastic vulcanizate is a composition containing vulcanized rubber particles dispersed in a thermoplastic. Such a composition is usually prepared by vulcanizing the rubber during its melt-mixing with a thermoplastic.) The excellent mixed TPV compositions can be obtained even though the rubbers and plastics are mutually grossly incompatible with respect to thermodynamic considerations. In such cases, however, it appears to be necessary that a compatibilizing agent be present in the mixture to promote the interaction between the thermoplastic materials. Block copolymers whose molecules contain blocks common to each of the thermoplastic blend components are good technological compatibilizing agents (e.g., polypropylene-nylon block copolymers to compatibilize mixtures containing polypropylene and nylon). Compatibilizing block copolymers can form in situ during melt mixing. This appears to be the case when one of the thermoplastic blend components is functionalized to chemically link with another thermoplastic component of the mixed TPV composition. The rubber associated with one thermoplastic can differ greatly from the rubber associated with another thermoplastic component of the mixed TPV blend. Thus, a composition which has good mechanical properties can contain both differing thermoplastics and differing rubbers. As a result, the possible combinations of components for TPV compositions has been greatly expanded.
Relationship between the Cohesive Strength and the Tack of Elastomers: Part II, Contact Time Effects
Authors: Hamed G. R. , Shieh C-H.
Published: February 2011

The self-tack (also called autohesion or simply tack) of an elastomer is its ability to resist separation from another piece of the same elastomer compound with which it has been brought into contact for a short time under a light pressure. Briefly, the events that take place in the interfacial region to form the tack bond are: 1) the attainment of intimate molecular contact of the polymeric chains from each surface; this requires viscous flow of the material near the surfaces and displacement of any impurity that may prevent contact, and, 2) interdiffusion of chains across the (temporary) interface; shorter chains will diffuse more rapidly. If the two contacted elastomeric layers are uncrosslinked, then the interface between them will eventually disappear and the strength of the junction (tack bond) will become identical to the cohesive strength of the material. This process has been referred to as “healing”. It is instructive to define relative tack as the measured value of tack (given, e.g., by the force to peel apart two contacted elastomeric strips) divided by the cohesive strength of the material determined in the same test geometry. Relative tack can have a value between zero and one. When the value is near zero, then it is inferred that the degree of interfacial contact and/or interdiffusion is quite incomplete. On the other hand, when the value approaches unity, the tack bond behaves as though the junction were completely healed. A very important feature of relative tack is that, even for a given time and pressure of contacting, its value is not unique, but depends markedly upon the test conditions selected to pull the bond apart. This interesting behavior was shown in a previous publication. Specifically, relative tack is rate dependent such that it may have a low value at some test rates (indicating only partial healing), but be equal to one at other test speeds—in spite of incomplete interfacial fusion. In this earlier investigation, measurements were carried out on specimens that possessed only a single extent of healing, i.e., the contacting conditions (time and pressure) were held constant. In this paper (Part II), studies of the rate and temperature dependence of bond strength are extended to include tack bonds possessing varying degrees of healing. This was simply achieved by varying the contact time before measuring tack.
Auger Electron Spectroscopic Analysis of Dispersants in Rubber
Authors: Lin Sin-Shong
Published: February 2011

Auger electron spectroscopy is a versatile tool in studying the chemical constituents of dispersions in rubber. This technique combined with the SAM analyses produces two dimensional images which are useful in the elucidation of aggregations and dispersion states of the rubber additives. Especially, particle inclusions and agglomerations can be studied together with the topographic features of the complementary SED image. Additional information concerning the binding states of elastomers and additives, aggregations of excessive crosslinkages, unsaturated double bonds, and large hard segments of polymers can be imaged with the aid of chemical reactants. The success of the present AES technique depends on the proper preparation of sample surfaces. A conductive surface is an essential factor in the analyses of additive dispersions. The heating of the surface by the electron beam, as well as the alternation of the surface topology by electron specimen interactions, should also be considered in the correct interpretation of the observed images. Artifacts associated with the analysis should be carefully examined so that they might reveal information about the composition and heterogeneity.