Showing papers in "Rubber Chemistry and Technology in 1978"

Effect of Carbon Black on Dynamic Properties of Rubber Vulcanizates

Abstract: The term dynamic properties as applied to elastomers refers to the response to periodic or transient forces which do not cause failure or appreciable fatigue (permanent change of properties) during the investigation. Generally this is limited to vulcanizates subjected to deformations not exceeding about 25%; and generally the dynamic properties are measured after several cycles or (in a transient experiment such as resilience) after several preconditioning transients, so that the Mullins effect or difference between first and second strain cycles is not of consequence. Thus, dynamic properties represent the viscoelastic properties of vulcanizates at deformations below about 25%, after reaching a pseudo-equilibrium state. The dynamic properties of rubber are altered tremendously by the addition of a filler. The scope of this article is restricted to the dynamic properties of rubber vulcanizates with carbon black as a filler. The effect covered in this article are important in designing rubber comp...

Synthesis and Dilute-Solution Behavior of Model Star-Branched Polymers

Abstract: The occurrence of polymers branched in a random fashion is common. Chain transfer reactions can cause short- and long-chain branching in polymerizations such as the high-pressure polymerization of ethylene. Branching can also be introduced intentionally by the use of a polyfunctional monomer in end-linking polymerizations. Similar branching can be produced in addition polymerizations by the use of a small amount of difunctional monomer, e.g., divinylbenzene. There also has been much interest in graft polymerization by which long chain branches can be introduced onto a backbone, which is often a different polymer from the branches. The properties of branched polymers can be quite different from those of linear polymers of the same molecular weight. For example, bulk viscosities as well as concentrated and dilute solution viscosities can be lower for branched polymers than for a linear material of equivalent molecular weight. As an example, the melt processing behavior of polymers can be manipulate...

Reinforcement of Elastomers by Carbon Black

Abstract: The reinforcement of elastomers by carbon black is governed by the morphology of the black and its physical and chemical interactions with the polymer. The latter are strongly affected by ...

Structural Change Accompanied by Plastic-to-Rubber Transition of SBS Block Copolymers

Abstract: It was shown that some block copolymers and their blends with corresponding homopolymers exhibit stress softening. When the specimens are stretched beyond the yield point, they become rubbery and exhibit high elasticity and large recoverable deformation, as a result of a breakup of their original rigid structure. Moreover, the deformed specimens heal, in that the properties of the original undeformed specimens are recovered upon removal of applied stress. This effect was attributed to the reformation of the original microdomain structure. In this work, we investigated an SBS block copolymer and a blend of the copolymer with homopolystyrene in regard to the structural change accompaning the plastic-to-rubber transition and the healing process. Electron microscopy and small angle X-ray scattering (SAXS) were used.

Mechanism of Rubber-To-Brass Adhesion : Effect of Rubber Composition on the Adhesion

Abstract: A new adhesion model for the bonding of sulfur-vulcanizable rubber compounds to brass has been presented and discussed. The adhesion is explained on the basis of an autocatalytic effect of a thin film of cuprous sulfide formed on the brass surface by the combined action of sulfur and vulcanization accelerator. This film transfers sulfur atoms to rubber molecules, which results in a high rubber surface polarity. This surface bonds to brass by physical interaction. Although minute amounts of cuprous sulfide are a prerequisite, an excess of this compound is detrimental to the adhesion, as thick films are brittle and nonadherent to the brass substrate. Consequently, all parameters which affect the reactivity of a brass surface for cuprous sulfide formation, have an effect on the adhesion. These parameters are brass copper content, brass surface composition, and rubber compound formulation. The action of the various individual rubber ingredients on the reactions at the interface can be simulated by me...

Strength of Elastomers. A Perspective

Abstract: The strength and extensibility of an elastomer depend on its overall viscoelastic properties, as reflected in the time and temperature dependence of stress-strain curves, and also on those discrete processes, including crack formation and growth, that culminate in high-speed crack propagation. The discrete processes determine the lifetime of a specimen; the viscoelastic characteristics affect the dependence of stress on deformation. The interplay between these effects causes strength and extensibility to depend strongly on test conditions. An elastomeric network composed solely of highly mobile chains is very weak indeed and fractures at a low elongation. This characteristic differs diametrically from that expected of an idealized network of mobile chains. If such a network were stretched, stress concentrations and unbalanced forces at the molecular level, which can result from short chains, entanglements, and network imperfections, would be vitiated rapidly by stress-biased segmental diffusion, ...

Tear Strength of Stretched Rubber

Abstract: Highly stretched rubber sheets can be split apart quite easily, indicating that the originally isotropic material becomes “fibrous” in character—i.e., much weaker for a tear running in the direction of extension than for one running at right angles to it. This anisotropy of strength is shown most clearly by elastomers which crystallize on stretching, for example, natural rubber compounds; it is not so evident in rubber compounds which do not undergo strain-induced crystallization. Also, it is apparently more pronounced for compounds containing reinforcing fillers, notably carbon black, which stiffen and strengthen elastomers. Indeed, carbon-black-filled natural rubber compounds show anisotropy of strength even after the extension is removed. The test sheet then regains its original dimensions more or less completely, but it remains softer for subsequent deformations in the direction of pre-stretching, and weaker for a tear running in this direction. Although unfilled natural rubber compounds and ...

Cord—Rubber Tire Composites: Theory and Applications

Abstract: The stiffness and strength behavior of the constituent cord and rubber components of the pneumatic tire have been studied in detail by investigators in the industry since the early days of tire production. For example, the effect of twist on the stress-strain properties of cotton tire cord and the reinforcing effect of carbon black on the modulus of natural rubber were well known phenomena many decades ago. Similarly, differences between the elastic and viscoelastic behavior of cord and rubber have been well documented since the 1940's. On the other hand, the material properties (specifically, the elastic constants) of the cord-rubber composite system that comprises the tire are not as well known and have only begun to receive serious attention in the last decade. These tire elastic properties, whether they be referred to as stiffnesses, compliances or moduli, are anisotropic—i.e., they vary with direction. It is the properties of the anisotropic cord-rubber composite that primarily control the o...

Monomer Sequence Distribution in Ethylene-Propylene Rubber Measured by 13C NMR. 3. Use of Reaction Probability Model

Abstract: One can measure r1r2 for an ethylene—propylene rubber by determining composition and χ, the ratio of contiguous to isolated propylene units. Our previous determination of χ depended on measuring methine carbon resonances in a 13C NMR spectrum. The χ method based solely on methine peak area measurements is inaccurate. This is not because the χ method is theoretically unsound, but because it is impossible to accurately extract the methine areas, necessary to calculate χ from the complex 13C NMR spectra of ethylene—propylene rubbers. The presence of inverted propylene produces ambiguity in a portion of the methine region. This means the methine areas can be used only to calculate a maximum and minimum boundary for r1r2. Another difficulty in using only methine areas to determine χ arises at propylene levels less than 35 wt%, because the methine areas for contiguous propylene triads become very small and are overlapped by strong resonances from long methylene sequences. We have developed a reaction p...

Effect of Strain-Induced Crystallization on the Elastomeric Properties of cis-1,4-Polybutadiene Networks

Abstract: Polymer networks, when studied at very high elongations, frequency show anomalous stress-strain isotherms in that they exhibit values of the modulus or “reduced force” [ƒ*] which increase markedly with increasing elongation. Such isotherms depart appreciably from the form predicted by the molecular theories of rubberlike elasticity and from the Mooney—Rivlin representation adopted from phenomenological arguments as well. For this reason, the interpretation of the increase in [ƒ*] at high elongations has been of great interest for a considerable period of time. For several decades now, this behavior has generally been attributed to the limited extensibility of the network chains. Critical examination of various published results pertinent to this question, in conjunction with more definitive experimental studies reported recently, however, support the alternative suggestion that such atypical isotherms are due to strain-induced crystallization. One experiment particularly relevant to this issue is...

Effect of Carbon Black Particle Size Distribution on Electrical Conductivity

Abstract: Wide particle size distribution (PSD) of carbon black increases electrical conductivity, consistent with the industrial practice of manufacturing conductive grades. An explanation has been offered based on the lower linear average diameter of wider-distribution blacks. Consequently, for the same weight, there is a larger number of particles in the broad-PSD black. The resulting agglomerates are therefore more numerous, although somewhat reduced in dimension because of higher packing density. The larger number of agglomerates results in lower average gap width, which accounts for the increased electrical conductivity.

Scrap Tire Disposal Procedures

Abstract: Many procedures and processes for the disposal of scrap tires are available Because of the complexity of the whole disposal problem—eg, collection logistics, availability and location o

Characterization of Tire Emissions Using an Indoor Test Facility

Abstract: A tire emission facility has been built in which several tire-wear modes can be simulated while the gaseous, airborne particulate, and sedimentary particulate emissions are collected. The facility was shown to produce tire temperatures and wear rates typical of operating tires. This approach to tire-emission research has many advantages over previous attempts to study tire emissions. The emission rate of gases and airborne particulate matter was nearly independent of wear rate and accounted for only 1–20% of the total emissions. The balance of the emissions were large particles which would be expected to settle on or very close to roadways. Chemical analysis of these large particles showed that approximately 30% of the SBR is unvulcanized, compared to only 1–2% in the tread. The total organic content of the particles was unchanged. This degradation of the rubber leads to enhanced oxygen absorption rates. However, the full significance of the increased rate of oxidation is not known.

Vulcanization of a Fluoroelastomer Derived from Tetrafluoroethylene and Propylene

Abstract: An effective vulcanization recipe for tetrafluoroethylene-propylene elastomer was searched for and the following results were obtained: (1) The C2F4-C3H6 copolymer is susceptible only to peroxide and is not crosslinked at all by other reagents. (2) The peroxide vulcanization is greatly accelerated by some coagents, and triallyl isocyanurate was found to be the best cure promoter. (3) The vulcanizate obtained by using a combination of α,α′-bis-(t-butyl-peroxy)-p-diisopropylbenzene with triallyl isocyanurate as curative exhibits good mechanical properties together with excellent heat and chemical resistance. (4) The mechanism of the vulcanization reactions seems to involve a chain reaction which greatly increases the efficiency of the vulcanization reaction by the peroxide.

13C NMR Characterization of Polychloroprene Microstructure II. Relaxation, NOE, and Quantitative Measurements

Abstract: Spin relaxation, NOE, and quantitative measurements have been performed on chloroprene polymers prepared by free radical techniques in the range −150 to +90°C. 13C NMR has proved to be an ...

Influence of Carbon Black Surface Properties and Morphology on Hysteresis of Rubber Vulcanizates

Abstract: A linear relation is found between tan δ and a loading-interfacial-area parameter, ϕψ, for NR vulcanisates with six untreated carbon blacks at several loadings. In IIR, a better linear rel...

Interpretation of Deviations from NEO-Hookean Elasticity by a Two-Network Model with Crosslinks and Trapped Entanglements

Abstract: Deviations from neo-Hookean elasticity in crosslinked rubbers, as measured by the ratio ψ=C2/(C1+C2) where C1 and C2 are the Mooney-Rivlin coefficients, have been found to be represented by a single function of fN, which is the ratio of network strands terminated by trapped entanglements to the total number of strands terminated by either trapped entanglements or crosslinks. It is assumed that the structure can be represented by a two-network model in which the crosslink network is neo-Hookean and the network of trapped entanglements is Mooney-Rivlin. The ratio fN is calculated from literature data for C1 and C2 and entanglement spacing derived from data on the uncrosslinked rubbers together with the Langley theory for the probability of entanglement trapping. Results are obtained for five polymers with different entanglement spacings over considerable ranges of degree of crosslinking and molecular weight before crosslinking, both of which influence the extent of entanglement trapping. The calcul...

Total Thermal Analysis of NBR Vulcanizates

Abstract: NBR vulcanizates of different nitrile content can be identified by total thermal analysis. Vulcanizate composition analysis is complicated because of formation of carbon from NBR. The difficulty can be obviated by identifying the specific type of NBR by Tg and making allowances for the known amount of carbon formation for that specific type.

Cohesive Strength in Guayule Rubber and Its Improvement through Chemical Promotion

Abstract: Green (cohesive) strength of Guayule rubber (GR) was determined and compared to that of natural rubber (NR). In the case of GR gum compounds the green strength was shown to be inferior to that of NR gum compounds; however, in the case of black compounds GR green strength was similar to that of NR. Green strength was very much improved in both rubbers by the addition of MNNA (N-(2-methyl-2-nitropropyl)-4-nitroso aniline) for gum and black compounds. MNNA also acted as an accelerator and thus reduced the difference in vulcanization rate between NR and GR. When using MNNA, a reduction in energy consumption during mastication was also noted. Finally, MNNA notably increased the tensile modulus and decreased the elongation at break for GR vulcanizates, while maintaining an almost constant tensile strength.

Adhesion of Elastomers, with Special Reference to Triblock Copolymers

Abstract: Several physical and chemical factors affect the strength of adhesion (as measured by a peeling method) for elastomers adhering to rigid substrates. These factors include: the thickness of the elastomer layer (and of an adhering backing, if present); the rate of detachment and the test temperature; and the presence of chemical bonds between elastomer and the substrate. Adhesion of thermoplastic elastomers, i.e., SBS triblock copolymers, applied as hot melts, is much stronger than for the corresponding random copolymers, crosslinked in situ. These materials also show higher cohesive (tear) strength. Both effects probably arise from the same cause: an energy-dissipation process which operates at large stresses and prevents brittle fracture. This process is thought to be plastic yielding of the polystyrene domains.

The Mechanical Stability of Natural Rubber Latexes

Abstract: From the information presented in this paper and from the cited literature, it is clear that the mechanical stability of natural latex can be influenced by a large number of factors. The arguments presented here suggest that the effects of most of these factors are explicable in terms of classical colloidal stability theory. It is suggested that the ionic strength of the aqueous phase of natural latex is a key feature in determining stability because of its effects on the electrical double layers around the latex particles. Increases in ionic strength reduce stability because of compression of the electrical double layer, such compression permitting closer approach of adjacent particles, and consequently, an increase in the attractive forces between them. Other phenomena are explained by postulating adsorption of anions at the particle surface where they have the effect of increasing the potential of the surface and therefore the repulsive force between adjacent particles—the attractive force bei...

Thermal rearrangements of unsaturated polymers

Abstract: Thermal anaerobic uncatalyzed cyclizations and cis-trans isomerizations observed in unsaturated hydrocarbon polymers are surveyed. Three main types of cyclizations are described. Type I is a radical reaction which is caused by chain rupture and gives rise to six-membered rings; this reaction occurs during pyrolysis of polymers with double bonds in a 1,5- or 1,6-diene configuration. Type II is a (2 + 2) thermal cycloaddition of double bonds in certain polymers with a 1,6-diene structure; bicycloheptane structures result. Type III is an intramolecular ene reaction. Many polymers containing a double bond linking CH units display thermal cis-trans isomerization. The common activation energy is approximately 130 kJ/mol, and the initial rate constants are lower, for homologous polymers, the greater the separation of the carbon-carbon double bonds.

Influence of Carbon Black on Processability of Rubber Stocks III. Extensional Viscosity

Abstract: A new instrument for measurement of extensional viscosity was designed and employed in the evaluation of SBR compounds containing carbon black and other non-black fillers. It was shown that carbon black filled rubbers generally do not reach steady-state viscosity ; the stress continues to increase with increasing strain up to the point of rupture, due to rapid orientation of molecular chains during extensional flow. The data was evaluated in terms of the Denn-Marrucci equation, and the effect of carbon black on the two adjustable parameters was found. It was found that higher carbon black loading or structure increases only the viscosity coefficient, i.e., it changes the relative level of the flow curve without changing its shape. On the other hand, an increase in surface area increases both the viscosity coefficient and the relaxation time, i.e., it increases the upward curvature of the flow curve. A very good correlation was observed between Mooney viscosity values and calculated viscosity coef...

Unsolved Problems in Polymer Degradation

Abstract: This review has highlighted some of the major problems in polymer degradation. It will be apparent that the degradation of commercial rubbers and plastics is exceedingly complex, since processing and small quantities of impurities or additives can interact and totally change the behavior of the substrate. It is not sufficient to examine the degradation of pure materials. The pure polymer may be studied as a base line but in order to obtain the full story it must also be studied in presence of known impurities or additives added singly and also in combination. This systematic approach is laborious and seldom used. The need for more realistic tests is now being recognized. Outdoor exposure combines the effects of uv light, heat, rain, ozone, nitrogen dioxide, and sulfur dioxide. A laboratory test should take all such elements into account. If a part is to be exposed to oil and air in the presence of heat, as it may be in an automobile, laboratory tests must examine the combined effects of these. Th...

Advances in Nitrile Rubber Technology

Abstract: Considerable advances have been made in NBR technology and still more are possible. The heat resistance of NBR has been raised so that it will with-stand 150°C in air for short periods, instead of 120°C. There is potential in the polymer backbone for still greater stability. It has been demonstrated that the ozone resistance of NBR can be improved by blending with EPDM, but the optimum blend has not yet been developed. The use of PVC to improve the ozone resistance of NBR is now much better understood. Crosslinked NBR has been developed as a non-migratory plasticizer which improves embossing characteristics of PVC and PVC-ABS blends. This, in turn, led to the production of NBR powders which may be used to speed up compounding on conventional equipment and permit rubber to be compounded in equipment generally used for fabricating plastics. Powder black masterbatches of NBR are now being evaluated in industry. Another new product form is liquid NBR, generally with reactive end groups. This is alrea...

Analysis of Fatigue Process of Rubber Vulcanizates

Abstract: The relationship between energy-input and hysteresis energy-loss during repeated deformation was analyzed, with gum and filled-rubber vuloanizates. It was recognized that the hysteresis energy-loss decreases more quickly with repeated deformation than the energy-input does. After a number of cycles both the energy-input and the energy-loss approach constant values. When these values are plotted against strain, curves similar in shape are obtained, regardless of the type of rubber. This is because the network chain is well relaxed. A group of the linear relationships between log W and log H was found with respect to N and λ. Examining the parameters, g1, g2, f1 and f2 as functions of N and λ, simple expressions, (4) and (5), were obtained for both the first deformation and after many cycles. At the latter state the hysteresis ratio tends to be constant in the wide range of λ. Finally, the relation between W and H at fatigue break is expressed with the same form of equation proposed by Grosch for t...

Measurement of Carbon Black Dispersion in Rubber by Surface Analysis

Abstract: A stylus-type surface roughness tester has been applied to the study of carbon black dispersion in a number of different rubber systems. The method is based on a freshly cut rubber surface which is tracked by the stylus to provide a roughness trace on a strip chart. Carbon black agglomerates deflect the cut path because of their higher hardness relative to the surrounding matrix. Thus, surface roughness diminishes at increasing levels of dispersion. A quantitative index of dispersion quality may be derived from the frequency and average height of the roughness peaks. The method offers a distinct advantage over previous methods in its ability to provide precise dispersion ratings over a very wide range of rubber processing levels from the masterbatch to the final product. The technique is applicable to the analysis of unvulcanized rubber compounds and could be utilized as a factory quality control procedure. Direct interfacing of the surface analyzer to a programmable calculator would provide quan...

Injection Molding of Rubber

Abstract: The outline of a strategy for the injection molding of a safe mix is described. Safe accelerators, safe curing systems, and mixes are suggested with machine conditions which should yield short curing times for a wide range of products. Comparisons have been made of the vulcanizate properties of injection and compression moldings prepared from the same mix in the same mold and press at the same temperature. Results have shown that, although anisotropy is more pronounced in injection moldings, the mean of values for modulus along and at 90° to radial mold flow lines of an optimum modulus cure are the same (within experimental error) as the mean of values for modulus along and at 90° to mill flow lines in a compression molding also cured to its optimum modulus. Injection moldings are similar to compression moldings in volume swelling and hardness but they have marginally higher rebound resilience and marginally lower compression set. The effect of an increase in mold temperature is to give a valuabl...

Differentiation of Natural and Synthetic Polyisoprene by Combined Derivative Thermogravimetry and Gas Chromatography

Abstract: A major objective of this study was to develop a general method by which NR and IR can be differentiated and quantitatively determined in unknown vulcanizates. From the above data, it is apparent that none of the thermal analysis methods—DTG, DSC or DTG-GC—can give an unequivocal answer alone, unless the type and loading of carbon black, the sulfur level and the accelerator system of the unknown compounds are known. The general type of carbon black can be determined using a recently published DTG method, and the loading is readily determined by DTG or an ashing experiment. Determination of the exact accelerator system remains a problem. The limitations of each method can be summarized: (1) The DTG thermogram will differentiate NR and IR provided the sample is vulcanized and contains carbon black. (2) The DSC will differentiate NR and IR on the basis of the observed exothermic heat, provided the sample is vulcanized. (3) In DTG—GC, the appearance of dimethylethylbenzene in the distillate is indica...