Rubber Solutions Research Articles

The Photovulcanization of Butyl and SKB (Polybutadiene) Rubbers

Abstract It is known that the mechanical properties of raw and vulcanized rubbers change markedly under the action of light in which not only oxidation processes but also photovulcanization processes in which sulfur is involved play an essential part. In view of the wide interest shown lately in questions of radiation vulcanization of polymers, as well as the possibility of reducing the photodegradation effect upon the mechanical properties of some polymers, we made studies of the photovulcanization of butyl rubber (degradation by irradiation) and SKB (polybutadiene) rubber (structure formation by irradiation). The study was made on films 200µ thick of purified butyl rubber with a molecular weight of about 42,000 and an unsaturation of about 1%. Both ordinary (twice crystallized) sulfur and radioactive S35 were used. The films obtained from a solution of rubber plus sulfur on cellophane were irradiated with a PRK-7 lamp through a Pyrex glass light filter and distilled water at a distance of 25 cm. Then determinations were made of the absorption spectrum and the static modulus 30 minutes and 24 hours after applying the load. The amount of free sulfur remaining in samples extracted previously was determined from the intensity of the S35 radiation. SKB not containing PBNA was reprecipitated from a benzene solution. When films of SKB (200µ thick) were irradiated, the light filters UFS-1, ZhS-4 and OS-5 were used. After the irradiation of the SKB films their static modulus was determined 30 minutes after the application of the load. When we irradiated butyl rubber containing 0.25 to 2.5% sulfur, we observed a distinct photovulcanization optimum (see Figure 1) in the modulus as a function of irradiation time like that which occurs in thermal vulcanization, and an optimum of the photovulcanization rate as a function of sulfur concentration.

The Structure of Gels. X. Globulization of Rubber under the Action of Nonsolvents

Abstract 1. Addition of nonsolvents to rubber solutions causes globulization of the macromolecules, similar to that occurring on addition of active substances (sulfur monochloride, halogens, etc.) to rubber solutions. 2. The difference between these processes lies in the fact that the coiling of the rubber molecues under the action of methyl alcohol is very rapid. 3. The globular structures in rubbers, produced by addition of nonsolvents to the solutions, in contrast to the stable globules formed in halogenation and vulcanization of rubber solution, are easily broken down and the molecules assume the linear form characteristic of the original rubber.

The Mechanism of Vulcanization and the Action of Accelerators

Abstract Vulcanization of rubber is due to the formation of chemical interlinks between molecular chains of rubber. A number of investigators maintain that formation of these bonds is due to reactions of radical character. In the present paper data are presented which were obtained during the study of reactions in which elementary sulfur is liberated at room temperature. As a prototype of such reaction is the interaction of hydrogen sulfide and SO2 which in rubber causes the socalled Peachey vulcanization. The usual views on the mechanism of this process are that the activity of sulfur liberated in statu nascendi is high enough to enable it to react with rubber and to create the spatial structure of the vulcanizate. However, this is an error. We have shown that pure sodium-butadiene rubber, heated to 80° in nitrogen atmosphere, does not vulcanize by the Peachey procedure, e.g., it does not become insoluble and its modulus of elasticity does not reach finite values. Consequently, the reaction causing the vulcanizing effect has a more complex character. To elucidate the mechanism of vulcanization we have studied the reaction of benzothiazolyl disulfide (MBTS) with hydrogen sulfide. In a hydrocarbon medium these compounds react at room temperature forming quantitatively elementary sulfur and mercaptobenzothiazole (MBT). Kinetics of this reaction are shown in Figure 1. If this reaction is carried out in a 10% solution of sodium-butadiene rubber, then the sulfur adds to the rubber, but vulcanization as characterized by formation of a spatial structure does not occur. The rubber solution is not gelatinized. An analogous phenomenon is observed during interaction of benzoyl peroxide with hydrogen sulfide. Sulfur liberated in this reaction also does not cause crosslinking (vulcanization of rubber).

Effect of Mercaptobenzothiazole on Rubber Structure during Oxidation, Heating and Milling

Abstract 1. Mercaptobenzothiazole (MBT) accelerates the oxidation of natural rubber and considerably increases the rate of its oxidative degradation. 2. When benzene solutions of natural rubber are heated in an inert gas atmosphere at 100°, no irreversible changes occur in the rubber whether in presence or in absence of MBT. 3. Plasticization of natural rubber in air at 100–150° is intensified in presence of MBT, which is caused mainly by the accelerating action of the latter on the oxidative degradation of rubber. 4. Mercaptobenzothiazole assists plasticization of rubber at low temperatures in an inert gas atmosphere. MBT is consumed in the process, owing to its interaction with the polymeric radicals formed by mechanical rupture of the rubber molecules. 5. The amount of MBT reacting with the rubber increases with increasing contents of 1, 4 structure in the rubber molecules. 6. It is shown that different fractions of the masticated product contain different amounts of MBT, indicating that MBT reacts preferentially with the end valencies of the radicals formed by cleavage of diallyl bonds. 7. Reaction schemes for the interaction of MBT with rubber in mechanical and oxidative processes are suggested.

Structural Modifications Induced in Rubber by Neutron Irradiation of Rubber Solutions

Structural Modifications Induced in Rubber by Neutron Irradiation of Rubber Solutions

Experimental studies of peripheral arterial embolism in dogs

1. 1. Using a specially prepared rubber compound solution, experimental embolism of several arteries was produced. The solution was prepared by dissolving raw rubber in such media as ether, petroleum benzin, benzol and paraffin. For x-ray demonstration, lead monoxide was added. 2. 2. Intra-arterial administration of this prepared rubber solution into the artery concerned produced complete occlusion of the vessel in all cases. 3. 3. Experimental embolism was produced in the arteries of the brain, heart, pancreas, kidney, liver, portal vein, stomach, intestine and leg, and resulted in experimental encephalomalacia, blindness, coronary occlusion, pancreatic necrosis, hydronephrosis, infarct of the kidney, ascites, enteric hepatitis, portal hypertension, gastric ulcer, and perforation of the stomach and intestine. 4. 4. The present studies were undertaken not only to produce embolism of various arteries of the body but also to observe these organs from clinical, pathological and electrophysiological aspects.

Hydrogenation of Butadiene Rubber Solutions. II. Influence of the Solvent with Palladium-on-Calcium Carbonate Catalyst

Abstract Experiments with allyl alcohol demonstrated the influence of the nature of the solvent on the speed and degree of hydrogenation of an unsaturated organic compound on palladium-on-calcium carbonate catalyst. Butadiene rubber was hydrogenated on palladium-on-calcium carbonate in 2,2-dimethyldioxane-1,5 (DMD), and specimens of hydrorubber insoluble in that solvent were obtained. The same relation between degree of hydrogenation and properties was found for hydrorubber obtained with palladium-on-calcium carbonate catalyst in 2,2-dimethyldioxane-1,5 as had been established for hydrorubber obtained in heptane with the same catalyst : fall in relative viscosity of benzene solutions, lowering of refractive index, and fall in the temperature of vitrification of the rubber with increasing degree of hydrogenation. The selectivity of hydrogenation of the outer double bonds of butadiene rubber when hydrogenated in 2,2-dimethyldioxane is more clearly manifested than in the case of hydrogenation in heptane.

The Mutual Solubility of Polymers. II. The Viscosity of Mixed Rubbers and the Behavior of Solutions of These

Abstract 1. Mixtures of rubbers and raw rubber stocks prepared from a combination of two rubbers possess anomalous properties in a number of cases, revealed in the appearance of a maximum in the curve of the viscosity of such a system in relation to the proportions of the rubbers. 2. A mixture of solutions of different rubbers equal in concentration in a common solvent separates into two phases in the majority of cases, thus demonstrating the limited mutual solubility of the rubbers. This separation does not take place if the solutions are dilute enough so that the molecules of the two solvents enter into practically no reaction with one another. For a given pair of polymers, the minimum concentration at which the separation still takes place depends on the nature of the solvent. 3. In agreement with present-day theoretical opinion on the solubility of polymers, the results obtained demonstrate the decisive part played by interactions between the chain molecules, i.e., by the heat of mixing; and the insignificant part played by changes of entropy in the process of mutual solution of two polymers possessing chain molecules of equal size and flexibility. 4. The study of mixed polymer solutions is a valuable source of information on the mutual solubility of polymers. The experimentation carried out on rubber solutions confirms the mutual insolubility of a number of pairs of rubbers which was observed earlier in the study of the mechanical properties of vulcanizates prepared from mixed rubbers.

Anomalous behavior of polymer solutions II. Viscosity behavior of rubber solutions at high dilutions

Studies in the viscosity behavior of dilute solutions of crepe and deproteinized natural rubber in different solvents have shown that carbon tetrachloride and other chlorinated solvents are not suitable for viscosity determinations of rubber. Data are presented to show that the anomalous increase of viscosity at very high dilutions cannot be explained by Öhrn's adsorption hypothesis but could be attributed to the extension and disentangling of polymer chains at higher dilutions. The anomaly is also shown to disappear if the solutions are shaken for a minimum period of time before viscosity determinations.

Influence of Molecular Shape on the Tensile Strength of Vulcanizates

Abstract A study of the mechanism of the formation of albuminous gels has shown that the mechanical strength of films depends on the shape of the chain molecules. Films prepared from globular casein have a lower tensile strength than those prepared from fibrillar casein. It would be natural to assume that the mechanical properties of rubberlike polymers and vulcanizates depend on the shape of the molecular chains. In order to verify this hypothesis, vulcanized films prepared from solutions of butyl rubber in various solvents were examined, assuming that the degree of twisting depends on the nature of the solvent. Benzene and carbon tetrachloride, in which the degree of twisting of the butyl rubber chains is quite different, were chosen as solvents. The tensile strengths given in Table I are the mean statistical data from 25 specimens. Data of the change in relative viscosity of solutions of butyl rubber in benzene and in carbon tetrachloride as functions of the temperature and concentration are shown in Figure 1. It is seen that the relative viscosity of benzene solutions is lower than that of carbon tetrachloride solutions of equal concentration. An increase in the relative viscosity of benzene solutions of butyl rubber is observed with an increase in temperature, while a decrease is observed in carbon tetrachloride solutions. These changes of relative viscosity with an increase in temperature are less pronounced in solutions with concentrations up to 0.3 g per 100 cc, where the polymer forms true molecular solutions, and are more pronounced in concentrated solutions.

Hydrogenation of Butadiene Rubber

Abstract 1. Solutions of butadiene rubber were hydrogenated at atmospheric pressure and room temperature on the following catalysts: palladium on calcium carbonate; palladium on nickel, platinum black from platinum dioxide, platinized carbon activated with doses of chloroplatinous acid, and Raney nickel. 2. It was established that when butadiene rubber is hydrogenated on these catal sts the activity of the catalyst decreases, evidently owing to its surface being obstructed with products of hydrogenation. It was consequently impossible to study the behavior of the double bonds in butadiene rubber on the basis of the curves of the rate of hydrogenation. 3. Specimens of butadiene rubber were prepared at various degrees of hydrogenation on all these catalysts and the following quantities were measured : total unsaturation, proportion of external and internal double bonds, index of refraction, relative viscosity of 0.08 per cent benzene solutions, and vitrification temperature. 4. It was established that when butadiene rubber is hydrogenated on the catalysts studied the external double bonds of the rubber are hydrogenated more rapidly than are the internal bonds. 5. It was established that an increase in the degree of hydrogenation of butadiene rubber leads to a decrease in the index of refraction, relative viscosity of benzene solutions, and vitrification temperature. 6. The decrease in the index of refraction of rubber during hydrogenation shows the absence of cyclization and can be explained by the disappearance of the double bonds of the rubber molecule. 7. It was shown that the decrease in viscosity of benzene solutions of butadiene rubber with increase in the degree of hydrogenation is due to changes in the shape of the rubber molecule during hydrogenation.

Polymer solutions. III. Viscosity behaviour of dilute solutions of natural rubber in mixed solvents

AbstractThe viscosity behaviour of natural rubber in two solvents and solvent‐nonsolvent mixtures has been examined at 30°C. The data supports the earlier view that the intrinsic viscosity of a flexible polymer is independent of solvent composition at the precipitation point. It is also shown that HUGGINS' constant k′ has a unique value at the precipitation point and its dependence on solvent composition is discussed.

Proton Magnetic Resonance Studies on Natural Rubber. II. Line Shape and T1 Measurements

Proton magnetic resonance absorption measurements have been made on samples of uncured Hevea, balata, and Coral rubber as well as on vulcanized samples with up to 9% combined sulfur. The motions of the chain segments were studied by observing line shapes and spin-lattice relaxation times at temperatures between —190 and 150°C. Several types of data show different molecular mobilities for vulcanized samples with a given combined sulfur but prepared by curing mixes with different fractions of raw sulfur for different times. Line width studies of the uncured Hevea sample as a function of thermal history indicate that the molecular mobility decreases with increasing crystallinity. Chemical shifts were resolved in high resolution studies of solutions of uncured Hevea, balata, and Coral rubber. The T1 measurements, including observations at 20 and 28 Mc/sec on uncured Hevea and balata, exhibit systematic deviations from the BPP theory. The significance of the deviations is discussed.

Viscosity of Natural Rubber Solutions at Very Low Rates of Shear

Abstract The shear dependence of viscosity of benzene solutions of natural rubber was studied at rates of shear from about 500 down to less than 1 sec.−1. Measurements involved following the change of pressure head with time of the various solutions flowing in a capillary, U-tube viscometer. Curvature in the plots of the logarithm of pressure head versus time indicated non-Newtonian flow. From such curves, reduced viscosity data over the above-mentioned shear range were readily derived. As a check, data over the range 100–500 sec.−1 were also obtained with a five-bulb viscometer of the Krigbaum-Flory type, and these data overlapped those obtained with the U tube. The reduced viscosity increased very sharply with decrease in gradient, making extrapolation to the viscosity axis quite unreliable. However, a theoretical relation proposed by Bueche fitted the composite data rather well. This work furnished a nice technique for determining the zero shear reduced viscosity (ηsp/c)0 without the necessity of performing an uncertain extrapolation: evaluate the parameters of the Bueche formula which best satisfies the experimental data over a fairly wide range of shear rates, and then calculate (ηsp/c)0 directly.

Interaction of Rubber and Carbon Tetrachloride

Abstract Carbon tetrachloride has been used for studies on the solution properties of natural rubber. It has been tacitly assumed that no chemical reaction between the two takes place; but carbon tetrachloride has been found to show a negative heat of mixing with natural rubber. There is absorption of heat (0.37–1.36 cal./g.) with benzene, dichloroethylene and toluene, while there is evolution of heat (2–3 cal./g.) with carbon tetrachloride and chloroform. There is considerable evolution of heat on mixing cellulose triacetate with tetrachloroethane and the solvent is found to be completely bound with the polymer. It appears therefore that similar combination between rubber and carbon tetrachloride may occur, when these are brought together. An indication of chemical interaction of carbon tetrachloride and rubber has been found in an examination of the spectra of carbon tetrachloride solutions of natural rubber. Chemical analysis of rubber, aged in solution at the laboratory temperature, showed that chlorine was incorporated into the rubber. In the present investigation a progressive detailed study of the system of deproteinized rubber-carbon tetrachloride has been carried out from the instant the two are brought in contact till either a homogeneous solution (in darkness only) or a gel is obtained (visible radiations).

Determination of the Molecular Size and the Molecular Form of Natural Rubber by the Measurement of Light Scattering. I. Development of the Method and Orienting Measurements

Abstract An experimental method of preparing and purifying natural rubber solutions in the absence of oxygen with an ultracentrifuge is presented. The solutions are suitable for optical measurements. Light scattering is measured as a function of concentration, angle, wave length and temperature. By determining molecular weight, diameter of the coiled molecule, and thermodynamic quantities (Δμ1, Δh1, Δs1) in this way it is possible to get information on branching of the rubber molecules. The method was used for cyclohexene solutions of a carefully tapped crepe rubber. The molecular weight was 1.3⋅106 and the end to end distance of the coil 1750 A˚. The heat of dilution was positive (endothermic) the B-value being nearly as high as that found in benzene solution by osmotic measurements. Comparing these results with measurements on unbranched polymethacrylate and polyisobutylene one can conclude that these rubber molecules are not or only slightly branched.

Shear Dependence of Viscosity of Natural Rubber Solutions

Abstract In an earlier paper viscosity measurements on dilute Alfin polyisoprene solutions at quite low shear rates were reported. It was shown that the limiting slope of the intrinsic viscosity vs. rate of shear plot at low shear rates, (Δ[η]/ΔD)D→0, is proportional to the square of the zero shear intrinsic viscosity [η]02. This relationship suggested the possibility of using the slope of the (Δ[η]/ΔD)D→0 vs. [η]02 plot as a molecular weight-independent parameter expressing the magnitude of the shear dependence of intrinsic viscosity, and thus as a possible means for studying polymer microstructure. It was thought that polymers having different structures would give rise to linear plots of the latter type, but with different slopes. To test this view, it was deemed desirable to extend viscosity measurements to another polyisoprene system, namely, natural rubber. This note describes the results obtained from viscosity measurements on Hevea fractions, and shows the comparative shear effects for both the natural as and the predominantly trans Alfin polyisoprenes.

VISCOSITY OF NATURAL RUBBER SOLUTIONS AT VERY LOW RATES OF SHEAR

The shear dependence of viscosity of benzene solutions of natural rubber was studied at rates of shear from about 500 down to less than 1 sec.−1. Measurements involved following the change of pressure head with time of the various solutions flowing in a capillary, U-tube viscometer. Curvature in the plots of the logarithm of pressure head versus time indicated non-Newtonian flow. From such curves, reduced viscosity data over the above-mentioned shear range were readily derived. As a check, data over the range 100–500 sec.−1 were also obtained with a five-bulb viscometer of the Krigbaum–Flory type, and these data overlapped those obtained with the U tube. The reduced viscosity increased very sharply with decrease in gradient, making extrapolation to the viscosity axis quite unreliable. However, a theoretical relation proposed by Bueche fitted the composite data rather well. This work furnished a nice technique for determining the zero shear reduced viscosity (ηap/c)0 without the necessity of performing an uncertain extrapolation: evaluate the parameters of the Bueche formula which best satisfies the experimental data over a fairly wide range of shear rates, and then calculate (ηap/c)0 directly.

Shear Dependence of Viscosity of Natural Rubber Solutions

Abstract In an earlier paper viscosity measurements on dilute Alfin polyisoprene solutions at quite low shear rates were reported. It was shown that the limiting slope of the intrinsic viscosity vs. rate of shear plot at low shear rates, (Δ[η]/ΔD)D→0, is proportional to the square of the zero shear intrinsic viscosity [η]02. This relationship suggested the possibility of using the slope of the (Δ[η]/ΔD)D→0 vs. [η]02 plot as a molecular weight-independent parameter expressing the magnitude of the shear dependence of intrinsic viscosity, and thus as a possible means for studying polymer microstructure. It was thought that polymers having different structures would give rise to linear plots of the latter type, but with different slopes. To test this view, it was deemed desirable to extend viscosity measurements to another polyisoprene system, namely, natural rubber. This note describes the results obtained from viscosity measurements on Hevea fractions, and shows the comparative shear effects for both the na...

Protective Action of Antioxidants of the Phenolic Type on the Aging of Natural Rubber

Abstract Oxidation of natural rubber in solution by atmospheric oxygen at 100° C in the presence of antioxidants and iron palmitate was studied. On the basis of the relative viscosity of the oxidized rubber solutions, the following conclusions can be drawn. 1. Addition of 1 per cent of certain antioxidants (pyrogallol, pyrocatechol, hydroquinone, 2-naphthol, resorcinol, Neozone D, and Oxynone) retards the oxidation of rubber in solution. However, this quality of antioxidant is not sufficient to stabilize the solution. 2. Comparison of the effectiveness of antioxidants (according to the data of relative viscosity) showed that the best antioxidant is pyrogallol, followed by pyrocatechol, hydroquinone, and Oxynone. The weakest are Neozone D and especially resorcinol. 3. Pyrogallol A, which is not shown in the table, does not protect a 1 per cent rubber solution from deterioration, showing a relative viscosity in solution of 4.08 at the beginning of the experiment and 1.40 after 7 hours, which is equal to that of the unprotected rubber solution. 4. The cresol and xylenol fractions of peat phenols were also used as antioxidants. The addition of 1 per cent of peat fractions to a rubber solution retards the drop in relative viscosity of oxidized solutions, but less than does the standard antioxidant, Neozone D. 5. The structure viscosity of solutions prepared from natural-rubber films which were stored 400 days, both with and without antioxidants, was measured. 6. Antioxidants of the phenol type protect rubber during storage in the form of films in darkness in air; in this case the best antioxidant is pyrogallol, and the next best pyrocatechol. 2-Naphthol is more active than hydroquinone. 7. When rubber is stored for a long time in air in darkness in the presence of certain antioxidants of the phenol type, its solubility decreases considerably; this shows the polymerizing action of some phenols on rubber. 8. Pyrogallol has a particularly strong polymerizing effect on rubber films. After 40 days of storage in air, these films dissolve in benzene in 25–30 days; if they are stored longer, their solubility is still less.