Vulcanization Of Elastomers Research Articles

The Vulcanization of Elastomers. 16. The Vulcanization of Natural Rubber with Sulfur in the Presence of Dithiocarbamates (I)

Abstract The kinetics of vulcanization of natural rubber with sulfur (initial concentration: 1 mole S8 in 10 kilograms of mixture) was investigated in the presence of varying amounts of zinc diethyldithiocarbamate (ZnDEDC). The following results were obtained. 1. At all temperatures and, depending upon the dithiocarbamate content of the mixture, over practically the entire, or at least the greater, extent of vulcanization, sulfur decrease is a zero order process. 2. The rate of sulfur decrease is, as long as it proceeds according to zero order, independent of the molar ratio S8:ZnDEDC. From the slope of the log rate versus 1/T plot is derived an activation energy of 29.2 kcal. 3. During vulcanization the dithiocarbamate content declines as long as the sulfur decrease follows the zero order. With decreasing starting content of dithiocarbamate, the stock finally uses up all the zinc dithiocarbamate during vulcanization. In other cases, a constant limiting value of dithiocarbamate decrease appears. 4. When zinc dithiocarbamate and zinc oxide are both present, sulfur decrease again takes place according to zero order. Under these conditions the process will use up less dithiocarbamate and shortly after the onset of vulcanization a constant final value of dithiocarbamate appears. 5. After the initial lag, crosslinking, measured by reciprocal equilibrium swell in benzene, proceeds according to zero order over the major extent of vulcanization. The temperature function of relative rates of crosslinking furnishes an activation energy similar to that for the rate of sulfur decrease. 6. Reciprocal equilibrium swell deviates from zero order time dependence as soon as the dithiocarbamate decrease reaches its final value, and the sulfur decrease also experiences a considerable lag. 7. It is shown that the limiting value of equilibrium swell increases as the starting content of dithiocarbamate decreases and also that reversion sets in at higher temperatures. When the molar ratio of mixing S8:ZnDEDC amounts to 1 or to 0.5, the limiting value of the swelling index is reached independent of temperature, i.e., the degrees of crosslinking are the same for these two ratios. The investigations will be continued.

The Vulcanization of Elastomers. 17. The Role of the Oxide in Vulcanization with Thiuram Compounds. II

Abstract The reaction of tetraethylthiuram disulfide with zinc oxide, in which significant amounts of zinc dithiocarbamate are formed, was investigated quantitatively. The decrease in thiuram disulfide and increase in dithiocarbamate were followed by conductometric titration with HCl (dithiocarbamate) and with CuSO4 in the presence of hydroquinone (thiuram disulfide) as well as by a spectrophotometric method. Both methods gave as the limiting value for the dithiocarbamate formation, 90 mole %, calculated on the initial thiuram disulfide. This value is independent of the reaction temperature. Judging from this evidence on dithiocarbamate formation, it seems clear that some condition holds here other than that which holds in the case of a thiuram vulcanization. In the latter case, the limiting value for dithiocarbamate formed, also independent of temperature, is 66 mole % based on the initial thiuram disulfide. In incompletely reacted mixtures of ZnO and thiuram disulfide, the spectrophotometric method indicates higher dithiocarbamate content than does the conductometric method of analysis. This shows that spectrophotometry shows at least one (unknown) compound which conductometry does not detect. The spectrophotometric analyses of thiuram vulcanizates are in agreement with the results of conductometric titrations of the vulcanizate extracts; both methods indicate 66 mole % as being the limiting value for dithiocarbamate formation; likewise, the same kinetic relations are found by the two methods. The use of the spectrophotometric method was carefully checked with control tests.

The Vulcanization of Elastomers. 13. The Vulcanization of Natural Rubber with Sulfur in the Presence of Mercaptobenzothiazole. II

Abstract The vulcanization of natural rubber with sulfur, using mercaptobenzothiazole as accelerator in the presence of zinc stearate, was investigated. The results were as follows: 1) When natural rubber is cured with sulfur in the presence of zinc oxide and mercaptobenzothiazole, as well as zinc stearate, one observes, with the first order diminution of sulfur concentration, an induction period that grows longer as the cure temperature falls. 2) For the disappearance of sulfur there is calculated, from the temperature dependence of the rate constants, an activation energy of 19.5 kcal/mole. This value is considerably smaller than that found for the mercaptobenzothiazole-accelerated sulfur cure when no zinc stearate is present. 3) The percentage loss in mercaptobenzothiazole during vulcanization is, in the presence of zinc stearate, independent of the temperature; there is a reaction, independent of the amount of stearate added of 5 molecules of S8 for each molecule of mercaptobenzothiazole. 4) The diminution in accelerator can also be explained as a first order reaction, and it becomes evident that disappearance of accelerator and sulfur are equivalent-rate processes. 5) The crosslinking, measured by the reciprocal equilibrium swelling, goes according to the first order, yet sulfur disappearance and the crosslinking reaction are not equal rate processes, which was always true in the absence of zinc stearate. In each case the crosslinking rate constants are greater than those for the decrease in sulfur. 6) For the activation energy of network formation, we calculated, from the temperature dependence of the rate constants of the reciprocal equilibrium swelling, 20.5 kcal/mole. 7) When the stearate content is constant, there is a linear relationship between kSI, the rate constant for sulfur diminution, and the given amount of mercaptobenzothiazole, up to an initial molar ratio of mercaptobenzothiazole: stearate = 4:1. Increases in accelerator proportion beyond this initial ratio cause no further rise in the rate constants. 8) The rate constants of the crosslinking reaction also increase with increasing proportion of mercaptobenzothiazole (stearate portion remaining constant), until the initial molar ratio of mercaptobenzothiazole:zinc stearate = 4:1 is reached. Nevertheless, the relationship is not linear. 9) There is a linear relationship between the amount of zinc stearate in the vulcanizate and the maximum sulfur-loss rate constants kSI (max). 10) The maximum rate constants of the network forming reaction kQI (max) are proportional to the square root of the stearate content. 11) The number of crosslinkage points formed is independent of the quantity of mercaptobenzothiazole, when the stearate content is constant. 12) The number of crosslinking points formed increases, however, along with increasing stearate content, and is proportional to the square root of the stearate content. The investigations will be continued.

The Vulcanization of Elastomers. X. The Vulcanization of Natural Rubber with Thiuram Disulfides (V)

Abstract The kinetics of crosslinking natural rubber during vulcanization with tetramethyl and tetraethylthiuram disulfide was investigated. The following results were derived: 1. The increase of crosslinking during vulcanization which was measured by the change of reciprocal swelling is first order over a wide range of reaction time and temperature. 2. During vulcanization, crosslinking and dithiocarbamate formation are reactions which proceed with identical rate. From this we had to conclude that the formation of dithiocarbamate is the immediate cause of the crosslink formation. 3. The concentration of the thiuram disulfide decreases much faster with respect to vulcanization time than crosslinking increases. In other words, the thiuram decrease can only be considered a reaction which precedes crosslinking.

The Vulcanization of Elastomers. XI. The Vulcanization of Natural Rubber with Sulfur in the Presence of Mercaptobenzothiazole. I

Abstract 1. The decrease of free sulfur occurs according to the first order law during the vulcanization of natural rubber accelerated by mercaptobenzothiazole in the presence of zinc oxide. The activating energy for this reaction amounts to 30.5 kcal./mole. 2. If zinc benzothiazolylmercaptide is used as an accelerator, one obtains the same rate constants for the sulfur decrease as in the presence of mercaptobenzothiazole. These seem to be equivalent as regards their effectiveness of acceleration. 3. A kinetic analysis of the reciprocal swelling, which represents a measure of network formation, indicates that the reaction is first order. Sulfur decrease and reciprocal swelling prove to be equal processes as regards rate. This is true where vulcanization is accelerated with mercaptobenzothiazole or with the zinc salt. 4. During vulcanization there occurs a decrease of accelerator concentration. This is dependent upon the temperature and is tied in with the combination sulfur with rubber. 5. If the quantity of the accelerator added is changed, the rate constants for sulfur decrease and for reciprocal swelling do not change, provided that a minimum quantity of accelerator is present. 6. In vulcanization accelerated with zinc benzothiazolylmercaptide, zinc oxide being absent, sulfur decrease again occurs according to the first order law but considerably faster, without thereby changing the activation energy. These investigations are being continued and the results will be discussed in detail in relation to other published contributions in this field.

The Vulcanization of Elastomers. VII. The Vulcanization of Natural Rubber with Thiuram Disulfides. Part IV

Abstract The present paper deals with the kinetics of the thiuram vulcanization at varying thiuram disulfide concentrations as well as with the influence of the zinc oxide content upon vulcanization at constant temperature (120° C). It was found : 1. The limiting value of dithiocarbamate formation of 66 mole per cent relative to the amount of thiuram disulfide used is—within wide limits—independent of the initial thiuram disulfide concentration. 2. At constant thiuram disulfide concentration the limiting value of dithiocarbamate formation is unchanged when the zinc oxide content is successively increased. 3. Changing the zinc oxide content while the thiuram disulfide concentration is kept constant exerts an influence on the kinetics of the decrease of thiuram as well as on the increase of dithiocarbamate. The rate constants of both reactions increase. 4. It is concluded from these results that in all probability vulcanization is initiated on the surface of the zinc oxide crystals. 5. Even in stocks containing as much as 40 g. zinc oxide per hundred g. of compounds, the limiting value of dithiocarbamate formation remains unchanged. This was verified by the analysis of vulcanizates after extremely long cure times. The average limiting value of two experiments after 24 hours of cure was 65 mole per cent which corresponds to a two-thirds reaction within the experimental error.

Studies of the Vulcanization of Elastomers. VIII. The Vulcanization of Synthetic Rubbers with Thiuram Disulfides (1)

Abstract The vulcanization of Buna S-3 by tetraethylthiuram disulfide is reported. A quantitative study of the transformation reactions led to the following results. (1) Using tetraethylthiuram disulfide as representative of the class, it was found that two thirds of it undergoes transformation to zinc dithiocarbamate during the vulcanization of Buna S-3, independent of the temperature. (2) Both the decrease of thiuram disulfide and increase of dithiocarbamate are first-order reactions within a wide range of the transformation. (3) The rate constants of the decrease of thiuram and of the increase of dithiocarbamate are a little less than a tenth power greater than for the vulcanization of natural rubber. This is possibly explainable by the different structure of the allyl units in Buna S-3. (4) The activation energies of the decrease of thiuram as well as the increase of dithiocarbamate are nearly the same and are greater than for the vulcanization of natural rubber.

Abrasion Resistance of GR-S Vulcanizates

Abstract A method for determining the abrasion resistance properties of reinforced vulcanizates was required for evaluating new GR-S type elastomers and carbon blacks developed for use in automobile tire treads. In spite of the fact that past experiences revealed that most laboratory machines are unsatisfactory for testing GR-S vulcanizates, a program for evaluating several machines was undertaken. A Lambourn abrader similar to the one recently adopted by the British Standards Institution for testing elastomer vulcanizates appeared to be most promising, for which reason efforts were expended on constructing a similar machine. The abrader that was constructed and operated to simulate wear brought about by a tire slipping on road surfaces embodied the following four modifications of the British machine: 1. All parts and accessories were either obtained or fabricated in the United States. 2. The electrical eddy current type of brake used in the original Lambourn machine was replaced with a hydraulic, gear pump braking system. 3. Intermittent talcing of the test wheel during operation was specified. 4. A revolution counter equipped with a friction drive wheel, one foot in circumference, was installed to ride on the test wheel for the purpose of conveniently determining test run distances. The conditions for testing vulcanizates were standardized to obtain abrasion loss values that could be expressed in terms of loss in cubic centimeters per 3000 feet of running at 24 per cent slip. Adjustments were made in calculating abrasion ratings from volume loss values to bring them more in line with reported service test ratings.

Retraction Test for Serviceability of Elastomers at Low Temperatures

Abstract Elastomer vulcanizates progressively stiffen as the temperature is lowered. Additional stiffening, due to crystallization, may occur as exposure to low temperatures is prolonged. The available methods of testing the low temperature flexibility of rubber and rubberlike materials do not reveal the losses in flexibility caused by crystallization except by using prolonged storage at low temperatures. A retraction test employing large deformations, which greatly increases the rate of crystallization, has been developed. This test rapidly gives a temperature index correlating with the stiffness of elastomer vulcanizates after storage at low temperatures, and can be used to measure the merit for low temperature applications of both crystallizable and noncrystallizable elastomers. This test in conjunction with conventional (room temperature) tests has been used successfully to study the low temperature performance of Hevea, GR-S, Paracril, and polybutadiene vulcanizates along with vulcanizates of many experimental elastomers. Correlation of results with cold compression set and hardness after low temperature storage has been excellent and substantiates the usefulness of the test.

Retraction Test for Serviceability of Elastomers at Low Temperatures

Abstract Elastomer vulcanizates progressively stiffen as the temperature is lowered. Additional stiffening, due to crystallization, may occur as exposure to low temperatures is prolonged. The available methods of testing the low temperature flexibility of rubber and rubberlike materials do not reveal the losses in flexibility caused by crystallization except by using prolonged storage at low temperatures. A retraction test employing large deformations, which greatly increases the rate of crystallization, has been developed. This test rapidly gives a temperature index correlating with the stiffness of elastomer vulcanizates after storage at low temperatures, and can be used to measure the merit for low temperature applications of both crystallizable and noncrystallizable elastomers. This test in conjunction with conventional (room temperature) tests has been used successfully to study the low temperature performance of Hevea, GR-S, Paracril, and polybutadiene vulcanizates along with vulcanizates of many ex...

State of Cure of Neoprene Vulcanizates. Measurement by the T-50 Test

Abstract Generally the physical properties of elastomer vulcanizates are considered to depend on their states of cure. Although many tests for measuring the state of cure have been developed, none are completely satisfactory. Some of them fail to distinguish between the physical or chemical effects of compounding ingredients, others are too laborious to be of practical value. The T-50 test developed by Gibbons, Gerke, and Tingey for natural rubber vulcanizates eliminates some of these difficulties and further reduces the number of tests required for production control. The T-50 test is based on the retraction of a stretched and frozen specimen with an increase in temperature. The temperature at which the sample retracts to 50 per cent of its initial frozen elongation is termed the T-50 value. From a physical standpoint the test is sensitive, precise, and easily performed. From a chemical standpoint the original workers as well as Vila showed that, for any specific natural rubber composition, the T-50 value correlates with the amount of combined sulfur. Since the state of cure of a rubber vulcanizate is frequently discussed in terms of combined sulfur, the T-50 test offers a correlation of the state of cure with combined sulfur without the necessity for involved chemical analyses.