Anil K. Sircar

Analysis of elastomer vulcanizate composition by TG-DTG techniques

Abstract Two common problems and probable solutions in the TG-DTG analysis of unknown elastomer vulcanizates have been discussed. The first problem concerns the difficulty in quantitatively determi...

Thermal Conductivity of Elastomer Vulcanizates by Differential Scanning Calorimetry

Abstract A du Pont 990 differential scanning calorimeter was modified to measure thermal conductivity of elastomer vulcanizates. The method is quick and reproducible and is recommended for scouting research of polymeric materials. The thermal conductivity λ values obtained by this method of some known materials compare favorably with those in the literature. Carbon black increases λ values in all elastomers. Particle size and structure of carbon black seem to affect λ values erratically, although the trend is towards higher λ values with larger particle size and higher structure. A linear relationship of λ vs. volume concentration of carbon blacks and other fillers is observed, except at a very high concentration. The extrapolated values of λ for a filler differ in different elastomeric media. Noncompatible elastomer blends increase or decrease λ values according to the nature and proportion of elastomers. The effect of the manner of addition of carbon black does not seem to affect the λ value of compatib...

Electrical Properties of Stretched Carbon Black Loaded Vulcanizates

Abstract To study the function of the carbon network, electrical conductivities and dielectric constants were measured on carbon black filled vulcanizates through the full range of elongation. Data were obtained for a variety of elastomers and carbon blacks, at different loading levels and covering the commercial particle size and structure ranges. The influence of the temperature and strain rate was investigated. In most of the reinforcing blacks there is a pronounced fall in conductivity and a minor decrease in dielectric constant in the first area of elongation, corresponding to the elimination of the transient structure. This is followed by an area of steady increase, caused by alignment of persistent carbon chains. At still higher elongations a sharp decrease occurs, due to destruction of carbon-elastomer bonds. The first two effects are very pronounced in high structure blacks but negligible in very low structure reinforcing blacks. The third effect leads to hysteresis and irreversible changes in th...

Studies on polyelectrolytes: VI. Gum agar

Measurements of viscosity with varying concentration of solute, with and without added electrolyte, and at different pHs of solutions of agar acid and its sodium salt are reported. A connected explanation of the observations has been formulated on the assumption of the polyelectrolytic character of the molecule. A tentative explanation has been put forward for the phenomenon of the swelling of agar gum in water at different pHs.

Studies on polyelectrolytes. VII. Gum tragacanth

In two previous communications (1, 2) it has been shown tha t the free acids and their sodium salts from both arabic and agar gums behave as polyelectrolytes. In the present paper the results of measurements on the free acid and its sodium salt of gum tragacanth are reported. The method of preparation of free acid and its sodium salt was exactly the same as described in the case of gum arabic (1). Removal of calcium from the gum was effected in a way described for gum agar (2). The equivalent weight of free t ragacanth acid was 550. The molecular weight of the free acid and its sodium salt was in the range of 80,000 to 100,000, as determined osmometrically in the presence of HC1 and NaC1, respectively. On an average there are about 180 dissociable groups per molecule. As will be evident from Fig. 1, the viscosity measurements with solutions of free acid in water showed a rapid rise in reduced viscosity with dilution. In the presence of HC1, ~p/c vs. c curve showed a maximum which shifted to higher concentrations of gum acid with increase in the free HC1 concentration and ultimately became linear at sufficiently high concentration of free acid. All these clearly indicate the polyelectrolyte nature of the free acid and the linear character of the molecule. When, however, viscosity measurements were extended to the sodium salt of the gum acid, certain peculiarities were noted (Fig. 2). The reduced viscosity diminished with diminishing concentration of sodium salt, cont ra ry to what has been noticed in the case of gum acid. The ~p/C vs. c curve was not linear as with neutral polymers. Addition of sodium chloride caused an over-all lowering of the viscosity of the solution at all concentrations measured and the ~p/c vs. c curves became linear. At any particular concentration, however, the reduced viscosity of the sodium salt is higher than that of the free acid. Since reduced viscosity is a measure of the hydrodynamic volume unit of the dissolved molecules, the sodium salt must have a more extended structure than the free acid, or in other words, the COOH groups are less dissociated than the COONa groups. Measurements of pH of the solution of sodium salt showed that the

Reaction of Mercaptobenzothiazole with Rubber during Mastication

Abstract Mercaptobenzothiazole (MBT), determined amperometrically in the acetone extract from an unvulcanized rubber-sulfur-MBT mix shows a loss of about 13 per cent of MBT. The results improve slightly when mixing is carried out at a low temperature (10° C) or in an inert atmosphere. After exploring the various possibilities, e.g., oxidation of MBT to MBTS or reaction with rubber it was concluded that main loss of MBT is due to a side reaction with nonrubber ingredients, extractable with acetone. Total MBT recovered when mixed with acetone extracted pale crepe or with synthetic cis-polyisoprene is about 98 per cent. In the absence of sulfur no appreciable reaction takes place even when MBT is heated with rubber at 145° C for 5 hours.

Studies of the Hard Rubber Reaction. I. Heat of Reaction

Abstract A method for the determination of the heat of the hard-rubber reaction by the application of differential thermal analysis is reported. The heat of reaction was determined with stocks containing different rubber/sulfur ratios and also with a 68/32 stock, preheated to contain different amounts of combined sulfur. Heat evolution is observed first with samples containing about 7 per cent sulfur and therefrom the amount of heat evolved shows a nearly linear increase up to 30 per cent sulfur. With increasing combined sulfur in the 68/32 stock, the quantity of exothermic heat gradually diminishes; so also does the temperature of initiation, i.e., the temperature at which heat evolution appears to begin. Initiation of the exothermic reaction appears to be a function of composition and temperature of the mass. An increase in the rate of reaction was observed when the composition reached 0.5 g-atom of sulfur per isoprene unit. An endothermic dehydrogenation reaction is observed at the end of the hard-rubb...

Determination of Thiazole Type of Rubber Accelerators by Amperometric Titration

Abstract The method of amperometric titration has been applied for the quantitative determination of mercaptobenzothiazole, zinc benzothiazolylmercaptide and dibenzothiazolyl disulfide, the latter two being known to constitute the decomposition products of the first during vulcanization reaction. It has been found that mercaptobenzothiazole as it is, or formed by a suitable reaction from the other two accelerators mentioned above, can be quantitatively estimated amperometrically. The total amount of mercaptobenzothiazole present in a mixture of two or all three of the above-mentioned accelerators and also the total amount of mercaptobenzothiazole liberated in a mixture of two or three of them can be quantitatively estimated.