Vulcanization Of Elastomers Research Articles

Studies on reducing the surface friction of elastomers : U.S. Naval Research Laboratory, 1963, 14 pp. (Order no. AD 405–838 from O.T.S., Department of Commerce, Washington, D.C., 20230, price 50 cents.)

Abstract : Methods for reducing the surface friction of elastomeric vulcanizates were investigated, and frictional data were obtained on polytetrafluoro ethylene coatings sprayed from aqueous dispersions onto elastomeric vulcanizates. Compounding polytetrafluoroethylene (TFE) resins with neoprene and silicone elastomers produced vulcanizates having inferior physical properties with no decrease in surface friction. Polytetra fluoroethylene coatings on elastomer vulcanizates provided surfaces having low coefficients of friction, which adhered sufficiently well for many applications although they are not as durable as similar coatings on metal. Lower values for the coefficient of sliding friction between metal/elastomer surfaces were obtained by coating the elastomer surface with TFE than by coating the metal surface. The surface friction al characteristics of TFE coated vulcanizates were essentially independent of the hardness of the vulcanizate, in marked contrast to the behavior of uncoated elastomers. The frictional behavior of coated elastomers was virtually independent of the thickness of the elastomer substrate.

Vulcanization of Elastomers. 38. the Reactivity of Synthetic Rubbers. I

Abstract Investigations relating to the development of other than conventional procedures for vulcanizing synthetic rubbers, especially cis-1,4-polybutadiene, are reported. The results were as follows: 1. cis-Polybutadiene and trans-polybutadiene containing 2% and more of 1,2 addition can be crosslinked to a certain extent by means of dibenzothiazolyl- or dibenzimidazolyldisulfide. 2. cis-1,4-Polyisoprene undergoes a much lower degree of crosslinking with the above disulfides. 3. The physical properties of vulcanizates are considerably improved if the molecules of the added disulfides contain groups which react with isocyanates. When diisocyanates are incorporated, supplementary crosslinking sites are formed. 4. The crosslinking of cis- and trans-polybutadiene via DBIS and Desmodur 15, in the absence and presence of black is described in more detail. The usual physical properties are determined and compared with those obtained when sulfur is used as the vulcanizing agent.

New Vulcanizing Systems for Ethylene-Propylene Elastomers

Abstract The absence of chemical functionality in ethylene-propylene elastomers precludes crosslinking with conventional rubber vulcanizing agents. It is known that such elastomers can be vulcanized with combinations of sulfur and certain organic peroxides, but the resulting vulcanizates leave something to be desired from the standpoint of tear strength, odor, and peroxide cost. A chemical study of the reaction between isoparaffins, sulfur, and organic peroxides gave results showing that some important alterations must be made of previous views that have been advanced for the vulcanization of saturated hydrocarbon elastomers by peroxides alone. A new reaction scheme is briefly reviewed. In the course of this study, three new classes of substances have been discovered to be effective vulcanizing agents for ethylene-propylene elastomers, and for other rubbers also. The outstanding characteristics which these new agents produce in ethylene-propylene elastomer vulcanizates are excellent tear strength and no objectionable odor. Data are presented on some properties of vulcanizates prepared with these agents.

Influence of Vulcanization on Adhesion to Non-Vulcanizing Polymers

Abstract According to contemporary ideas the mechanisms of adhesion and co-vulcanization of rubbers have much in common. The phenomenon of co-vulcanization, like that of adhesion of polymers, includes the diffusion of macromolecules and of segments of them, which is particularly intensive in the initial period of vulcanization, when the crosslinks have not yet formed. The difference between covulcanization and adhesion lies solely in the fact that the process of interpenetration of the molecular chains in the former case is sooner or later broken off by the vulcanization process being superimposed upon it, securing the macromolecules or portions of them by strong chemical bonds. This leads to the formation of an adhesion joint considerably superior in strength to a joint in which the molecules diffusing through are attached only by weak intermolecular bonds. It is important to ascertain what is the influence of the vulcanization of elastomers upon their adhesion to polymers not capable of forming chemical bonds with them. This case is of immense practical importance, since unvulcanized rubbers are very often used in combination with vegetable and synthetic fibers which are not capable of vulcanizing. As adhesives we selected elastomers of varying molecular structure, polarity and degree of molecular mobility: butadiene-acrylonitrile rubber (SKN-26), butadiene-styrene rubber (SKS-30A), butadiene-styrene oil-extended rubber (SKS-30ARM-15) and butyl rubber. Using these rubbers we prepared stocks both containing and not containing vulcanizing groups (in the latter case the components of the vulcanizing groups were replaced by an equal quantity of inert filler—whiting). The amount of vulcanizing group was set at the optimum for each rubber. The substrates used were films containing no plasticizers—cellophane (cellulose hydrate) and Perfol' PK-4 (polycaprolactam). The adhesive was applied to the substrate by coating on a calender at the optimum conditions for each elastomer. The specimens produced in this way were plied up and subjected to vulcanization. Vulcanization was carried out in a Berstorff apparatus for continuous vulcanization at 143° C and a specific pressure ≈ 0.85 kg/sq cm. The time of cure was varied between 0 and 88 min.

Vulcanization of Elastomers. 30. Kinetics of the Decrease of Sulfur Concentration during Vulcanization

Abstract The present discussion deals with the explanation and critical analysis of the kinetics of the decrease in concentration of sulfur in accelerated and unaccelerated vulcanizations of natural and synthetic rubber. A marked distinction is made between the time law and the dependence on concentration of the rate of decrease of sulfur, since there is lack of agreement between them, both in accelerated and unaccelerated vulcanizations, which indicates the complex nature of vulcanization. In unaccelerated vulcanization, the sulfur concentration drops, at all temperatures and independent of the concentration, according to a time law with exponent n=0.6. On the other hand, the dependence on concentration of the initial rate of sulfur decrease proceeds according to a first order reaction. These relations make it seem probable that sulfur decrease involves a process which is catalyzed by reaction products. In accelerated vulcanization, the reduction of concentration of sulfur also is expressed by a time law with the exponent n<1, and this too is independent of temperature and concentration of reactants (sulfur and accelerator). However, the concentration dependence of the rate of sulfur decrease cannot be expressed by a power law, so that we find no order of reaction in the usual sense. We attempt to explain the characteristic trend of the initial rate of sulfur decrease with sulfur concentration at constant accelerator content, or with accelerator concentration at constant sulfur content, by assuming that the vulcanization takes place through intermediate compounds. The pertinent theoretical conceptions are disclosed and critically examined and the attempt is made to show the extent to which they may be brought into accord with the experimental data.

Relation of Structure to Properties in Polyurethanes. Effect of Branching

Abstract Attaching short N-arylurethane branches to a high molecular weight, linear polyether urethane introduces interchain forces—very likely arising from hydrogen bonding—which increase as the side chains become more polar. These forces are sufficiently strong to increase the bulk viscosity of the raw polymers and, with the more highly polar groups, are strong enough to impart an appreciable elastic memory. At slightly elevated temperatures (80° C) these forces are reduced considerably, which is to be expected if they arise through hydrogen bonding. However they are either not strong enough or are too limited in number to compete with the covalent crosslinks in the elastomer vulcanizates. Thus no significant alteration is observed in the vulcanizate properties which are customarily measured. The marked effect of these short aryl side chains to reduce crystallization tendencies can most likely be attributed to the bulkiness. of the pendant aryl moiety, which prevents—or slows down—the ordering of the polytetramethyleneether chains into a crystal lattice. Long chain branches influence dynamic properties considerably. Present evidence indicates that optimum dynamic properties—as measured by Yerzley resilience—are obtained when “loose ends” are absent.

Relation of structure to properties in polyurethanes: Effect of branching

Abstract Attaching short N-arylurethane branches to a high molecular weight, linear polyether urethane introduces interchain forces—very likely arising from hydrogen bonding—which increase as the side chains become more polar. These forces are sufficiently strong to increase the bulk viscosity of the raw polymers and, with the more highly polar groups, are strong enough to impart an appreciable elastic memory. At slightly elevated temperatures (80° C) these forces are reduced considerably, which is to be expected if they arise through hydrogen bonding. However they are either not strong enough or are too limited in number to compete with the covalent crosslinks in the elastomer vulcanizates. Thus no significant alteration is observed in the vulcanizate properties which are customarily measured. The marked effect of these short aryl side chains to reduce crystallization tendencies can most likely be attributed to the bulkiness. of the pendant aryl moiety, which prevents—or slows down—the ordering of the po...

Vulcanization of Elastomers. 28. Vulcanization of Natural and Synthetic Rubbers with Sulfur in the Absence of Accelerators. I

Abstract The decrease of sulfur concentration in the reaction of sulfur with natural rubber was studied. The following was found : 1. Sulfur decrease follows the 0.6th order law independent of temperature as long as the sulfur is dissolved in the gum (temperature!). From the rate-temperature relation, an activation energy of 35 kcal/mole is calculated. 2. In an investigation of the kinetics of sulfur concentration at constant temperature but increasing starting concentration, the following two cases can be differentiated : a) As long as the sulfur is soluble in the gum, sulfur decrease still follows the 0.6th order, possibly due to autocatalysis ; the linear relationship between starting rate and starting concentration shows that the process is 1st order with respect to the concentration; this may be the consequence of a thermal, rate determining cleavage of the S8 ring. b) As the sulfur at higher concentration is only incompletely soluble in the gum, conversion curves with points of inflection are found ; this becomes more pronounced at higher concentration. At the start of the reaction one finds an autocatalytic sulfur decrease basically of zero order; in the latter part of the reaction after passing the point of inflection a 0.6th order is observed, as in the range of complete sulfur solubility. 3. A discrepancy between the time law and the concentration-rate relationship is found in pure as well as accelerated sulfur vulcanization ; these conditions are compared and discussed.

Vulcanization of Elastomers. 25. Natural Rubber and Synthetic Elastomers with Sulfur and Sulfenamides. I

Abstract The sulfur vulcanization of Perbunan N 2818 (acrylonitrile-butadiene) was investigated in the presence of N-cyclohexyl-2-benzothiazolylsulfenamide (CZ) at different temperatures and various concentrations of reactants. The following were found : 1. The decreasing sulfur concentration vs. time plot followed the 0.8th order under all experimental conditions and an activation energy of 28.2 kcal/mole was calculated. 2. When using the cyclohexylammonium salt of MBT as accelerator, sulfur decrease proceeds at the same rate as with CZ. 3. It was concluded from 1 and 2 that even in the presence of CZ, the ammonium salt was the actual accelerator, which forms during the scorch or induction period through reaction of CZ with the rubber. 4. In view of the discrepancy found in the relation of reaction order with respect to time and concentration of reactants, the formation of a reactive intermediate is postulated; the analogy between the kinetics of sulfenamide accelerated sulfur vulcanization, and those accelerated with MBT as well as DPG is pointed out. 5. In connection with 3 the dependence of starting rate as well as rate constant of 0.8th order of the decreasing sulfur concentration on the molar ratio of CZ/S8 and on the sulfur starting concentration is discussed.

Vulcanization of Elastomers. 26. Vulcanization of Natural and Synthetic Rubbers by Sulfur in the Presence of Organic Bases

Abstract In connection with earlier research work, we have carefully studied the decrease of sulfur during the vulcanization of Perbunan N 2818 in the presence of DPG, at various temperatures and with varied molar concentrations of sulfur and accelerator. We found : 1. The sulfur concentration decreases at all temperatures according to a time law of order 0.8. An activation energy of about 28 kcal/mole is derived from the temperature dependence of the rate constant. 2. The reaction order is independent of the initial concentration of reactants. 3. A discrepancy is found in the order of the reaction with respect to time and with respect to concentration, and it is clearly shown that the dependence of the initial rate on the initial concentration can not be expressed by a single power law. 4. The attempt is made to explain the discrepancy by postulating catalysis by an intermediate compound. The consequences from this consideration are compared with the experimental results, and, on the whole, agreement is found between theory and experiment.

A modified helical spring stress relaxometer for automatic and manual operation

AbstractA relatively simple and inexpensive automatic and self‐recording apparatus giving good accuracy has been developed for measurement of chemical stress relaxation in elastomer vulcanizates. The apparatus is based on a previously described helical spring relaxometer intended for manual operation. Several improvements applicable to this manually controlled instrument are also described.

Vulcanization of Elastomers. 23. The Chemical Kinetics of Vulcanization Reactions and The Physical Properties of The Vulcanizates. I

Abstract As a complement to earlier investigations, and in order to examine more closely the connection between the chemical kinetics and the changes with vulcanization time of the physical properties in the case of vulcanization reactions, we used thiuram vulcanizations as an example, and concerned ourselves with the dependence of stress values (moduli) at different degrees of elongation and different vulcanization temperatures. We found: 1. Stress values attain a limiting value, dependent on the degree of elongation, but independent of the vulcanization temperature at constant elongation. 2. The rise in stress values with the vulcanization time is characterized by an initial delay, which, however, is practically nonexistent at higher temperatures. 3. The kinetics of the increase in stress values with vulcanization time are both qualitatively and quantitatively in accord with the dependence of the reciprocal equilibrium swelling on the vulcanization time; both processes, after a retardation, go according to the first order law and at the same rate. 4. From the temperature dependence of the rate constants of reciprocal equilibrium swelling, as well as of the increase in stress, an activation energy of 22 kcal/mole can be calculated, in good agreement with the activation energy of dithiocarbamate formation in thiuram vulcanizations.

Vulcanization of Elastomers. 24. The Effect of Fillers on the Course of Vulcanization Reactions. I

Abstract The effect of Aerosil on the chemical reactions and the kinetics observed in thiuram vulcanizations was subjected to closer scrutiny. Results now available show: 1. Aerosil does not alter the chemical mechanism of thiuram vulcanizations. It also has no qualitative effect on the kinetics of the various reactions involved. Thiuram disappearance and dithiocarbamate formation over by far the greatest range of conversions are first order reactions. Nevertheless, the limiting value of dithiocarbamate formation is somewhat higher than in the absence of Aerosil. 2. Quantitatively speaking, essential differences are involved. The rate constants for both thiuram loss and dithiocarbamate formation rise considerably when Aerosil is used; in both cases there is a dependence on the Aerosil content of the rubber compound. Yet the activation energies of thiuram disappearance and dithiocarbamate formation are practically the same as in Aerosil-free thiuram vulcanizations, amounting to about 21 kcal/mole. 3. Measurements of the limiting equilibrium swelling reveal that adding Aerosil does not result in additional crosslinking, so that in thiuram vulcanizations the catalytic action displayed by silica gel is the outstanding feature.

Vulcanization of Elastomers. 22. Thermal Vulcanization of Synthetic Rubbers. I

Abstract In this work, the kinetics of the thermal vulcanization of Perbunan were studied with and without additives. The following results were obtained : 1. The pure thermal vulcanization of Perbunan is a very slow process which obeys a second order rate law. A limiting value of the crosslinking (reciprocal limit of equilibrium swelling) is reached, which limit is independent of the temperature. The activation energy is 23.3 kcal/mole. 2. The thermal vulcanization can be inhibited by hydroquinone but not by benzoquinone. 3. The thermal vulcanization of Perbunan can be considerably accelerated by MBTS, and other materials, and the reaction also follows a second order course. The activation starts suddenly after the expiration of an induction period, which decreases with increase in temperature. The activation energy is about 27 kcal/mole. 4. In a thermal vulcanization accelerated with MBTS, a portion of the MBTS is changed over into MBT ; the amount changed is independent of temperature. Perbunan takes up MBTS in the form of molecular fragments, in stable combination. 5. The reduction in MBTS (which falls to zero) and the increase in MBT follow a first order reaction and have the same activation energy which is also identical with the energy of activation of the accelerated crosslinking. The formation of MBT is the slower of the two reactions. 6. The rate constants for the decrease in MBTS and for the increase in MBT are independent of the starting amount of MBTS, and hence we consider that this is a unimolecular process (homolysis). 7. The rate constant for the second order crosslinking reaction increases with the square root of the initial benzothiazolyl disulfide content. 8. It is indicated that the above data must be explained, with the aid of experience in the realm of polymerization kinetics. The investigations are being continued.

Vulcanization of Elastomers. 21. Vulcanization of Natural Rubber with Thiuram Disulfides. VI

Abstract Bound sulfur in a pure thiuram vulcanizate increases relatively rapidly at first at all temperatures, reaches a poorly defined maximum at about 27 to 30%, independent of temperature, and then recedes slightly to reach a limiting value of 25% also independent of temperature, based on the original thiuram disulfide. The rise in sulfur content at the start points to a temperature-independent limiting value of 33%. It is shown that the combination of sulfur in this region initially follows a first order reaction, and goes at the same rate as the reduction in concentration of thiuram disulfide. It can be seen from the above that sulfur may be combined in thiuram vulcanization without simultaneous crosslinking. The dithiocarbamate formation increases rapidly in the region of longer vulcanization times, after the maximum in bound sulfur has been reached, without further combination of sulfur with the vulcanizate. The rate constants for thiuram decrease, for dithiocarbamate increase and for sulfur combination were calculated. The temperature dependence of each of these reactions has practically the same activation energy, 23 kcal/mole. The bound sulfur content of the vulcanizates in pure thiuram vulcanizations is no criterion of the state of vulcanization.

Vulcanization of Elastomers. 19. The Function of Oxides in Thiuram Vulcanization

Abstract This article is a continuation of our discussion of the role of oxides in thiuram vulcanization. The following additional results were obtained: The reaction of TETD and ZnO gives off gaseous products and is pressure dependent. By taking this into account, both the conductometric and the spectrophotometric methods of analysis furnish identical results for the DEDC yields as a function of reaction times. The reaction of TETD with Bi2O3 produces in the presence or absence of NR (independent of the temperature) approximately 82 mole per cent BiDEDC based on the initial amount of TETD. However, no vulcanization takes place. The reaction between TETD and TeO2 produces in the presence or absenee of NR (independent of the temperature) a maximum yield of Te-II-DEDC of 60 to 70 mole per cent based on the initial amount of TETD. Such a yield of DEDC is also observed during a TETD vulcanization with ZnO. Nevertheless, no normal thiuram vulcanization takes place when TETD, TeO2 and NR are heated together. When Te-IV-DEDC is dissolved in various organic solvents, it undergoes disproportionation and equimolar amounts of Te-II-DEDC and TETD are formed. The limiting value of the Te-II-DEDC formation of 60 to 70 mole per cent during the reaction of TETD with TeO5 in the presence or absence of NR can be explained by a primary formation of Te-IV-DEDC in a maximum yield of 80 mole per cent followed by a disproportionation of the Te-IV-DEDC (the same as with a corresponding reaction in the presence of Bi2O3). Of course, it is assumed that the metal oxides during their reaction with TETD give maximum yields of 80 to 90 mole per cent DEDC based on the initial amount of TETD. Furthermore, the experiments showed that thiuram vulcanization obviously is due to the reaction of TETD with ZnO in which NR participates with formation of crosslinks. This concept is supported by the identical activation energies of the TETD decrease and DEDC increase during the reaction of ZnO with TETD in the presence and absence of NR.

The Vulcanization of Elastomers. 20. Sulfur Vulcanization Accelerated with Thiuram Compounds. I

Abstract The present paper deals with the results of an orientating, quantitative investigation of sulfur vulcanization accelerated by thiuram disulfide, with tetramethylthiuram disulfide as the representative example. It was found: In the sulfur cure of natural rubber with tetramethylthiuram disulfide at different TMTD:S ratios, the rates of TMTD decrease and dithiocarbamate formation increase with increasing sulfur concentration, the TMTD content being kept constant. The rates practically do not change any further when the compounds contain 6 gram atoms of sulfur per mole of thiuram disulfide. The peak value of dithiocarbamate formation increases with the increase of sulfur concentration and reaches a constant end value of about 90 mole per cent based on the amount of original thiuram disulfide, when the stocks contain 4 gram atoms of sulfur per mole thiuram disulfide. This end value is identical to the end value of dithiocarbamate formation in the reaction of thiuram disulfide with zinc oxide (in the absence of rubber). The crosslinking, as measured by the change of reciprocal equilibrium swelling per time unit is also a reaction whose rate increases with the sulfur concentration to the point where the compounds contain 6 gram atoms of sulfur per mole of thiuram disulfide. The optimum degrees of crosslinking are roughly proportional to the sulfur concentration; at high sulfur levels the vulcanizates tend to revert. As in the pure TMTD vulcanization, the TMTD decrease as well as the dithiocarbamate formation are always first order reactions. The reversion at higher sulfur levels as well as the complicated course of the increase of combined sulfur during vulcanization render all but impossible an accurate determination of the reaction order for the crosslinking at higher sulfur levels. Nevertheless, in vulcanizations with 1 mole TMTD per 1 or 2 gram atoms of sulfur the crosslinking is a first order reaction.

Vulcanization of Elastomers. 12. Perbunan with Thiuram Monosulfide and Sulfur. I

Abstract The vulcanization of Perbunan 2818 by tetramethylthiuram monsulfide plus sulfur (1 mole monosulfide per gram-atom S) was thoroughly studied. The following results were shown: The limiting value for dithiocarbamate formation is 66 mole per cent of the initial thiuram monosulfide, indicating a two-thirds transformation. The limiting value is practically independent of temperature. The formation of dithiocarbamate can be described as a reaction of the first order. The formation of dithiocarbamate is characterized by an induction period which grows longer with lowering of the temperature, and at 100° C it amounts to about 100 minutes. The rate constants for dithiocarbamate formation were calculated, and it was shown that they were practically the same as those for the vulcanization of Perbunan with tetramethylthiuram disulfide. The activation energies as derived from the temperature dependence of the rate constants for dithiocarbamate formation in the vulcanization of Perbunan by thiuram monosulfide plus sulfur on the one hand and with thiuram disulfide on the other, are only very slightly different and are practically the same as the activation energy for dithiocarbamate formation during the vulcanization of natural rubber with thiuram monosulfide plus sulfur. The results were thoroughly discussed in light of the present conceptions of the course of thiuram vulcanizations.

Vulcanization of Elastomers. 14. Role of the Oxide in Vulcanization with Compounds of the Thiuram Series. I

Abstract The present paper is concerned with the influence of TETD concentration and of zinc oxide content upon the kinetics of the vulcanization of natural rubber and Perbunan 2818 with TETD. The following results were obtained for vulcanization at 120° C. 1. The velocity constants of TETD decrease and ZnDEDC formation are linear functions of the zinc oxide content at constant TETD concentrations. 2. If TETD and zinc oxide are present in constant molar mixing ratios, then the velocity constant of TETD decrease is independent of the amount of the mixture while that of ZnDEDC formation rises linearly with zinc oxide addition. 3. The velocity constant of TETD decrease diminishes with increasing TETD concentration when the zinc oxide content of the mixture is held constant. Under these conditions the velocity constant of ZnDEDC formation remains unchanged. 4. Everything shows that the velocity constant of ZnDEDC formation depends only on the zinc oxide content of the mixture while that of TETD decrease is dependent on the molar ratio of zinc oxide and TETD. 5. The relationships found are expressed mathematically, and the practical significance of the relationships is pointed out.

Vulcanization of Elastomers. 18. Vulcanization of Natural Rubber with Sulfur in the Presence of Mercaptobenzothiazole. III

Abstract The present work is concerned with the influence of the concentration of sulfur and the zinc oxide content on the kinetics of the vulcanization of natural rubber with sulfur in the presence of mercaptobenzothiazole, zinc stearate and zinc oxide. The following results were obtained. 1. The velocity constant of the sulfur decrease, kSI, is inversely proportional to the initial concentration of sulfur. 2. The velocity constant of the change with time of the reciprocal equilibrium swelling, kQI, is inversely proportional to the square root of the initial concentration of sulfur. 3. The limiting value of the reciprocal equilibrium swelling, 1/Q∞ (degree of crosslinking), at constant stearate content, is proportional to the square root of the initial concentration of sulfur. 4. The amount of accelerator consumed during vulcanization increases with increase in the initial amount of sulfur. The decrease in the accelerator is a first order reaction having approximately the same rate as the decrease of sulfur. 5. The velocity constants of the sulfur decrease and the change with time of the reciprocal equilibrium swelling are practically independent of the amount of zinc oxide. Other situations exist, of course, for compounds without zinc oxide. 6. Empirical equations were derived to express the dependence of the constants kSI, kQI and 1/Q∞ on the composition of the mixture.