Transparency of Rubber Compounds Containing Magnesium Carbonate

Abstract

Abstract 1. Basic magnesium carbonates of the type 5MgO·4CO2·xH2O will produce transparent vulcanized rubber. 2. Neutral carbonates of the magnesite type result in stocks of poor light transmission, even at low loading. 3. The basic carbonates, although of extremely fine particle size, are definitely crystalline in structure, as revealed by x-ray analysis. 4. The sample of magnesium carbonate producing the best transparency was a Japanese variety. When 25 per cent by weight was compounded in rubber, an over-all transmission of 45 per cent was produced. None of the domestic varieties gave higher than 36 per cent, and only two samples out of the eighteen examined produced this value. 5. The best Japanese carbonate (sample 3) and the best domestic carbonate (sample 9) possessed the same structure— that of a basic magnesium carbonate of the general form type 5MgO·4CO2·xH2O. 6. The Japanese carbonate, corresponding to the formula type 5MgO·4CO2·6H2O, gave higher total light transmissions at low filler concentrations than the best domestic carbonate, corresponding to the formula 5MgO·4CO2·9H2O. 7. Mixes containing between 25 and 40 per cent by weight of basic magnesium carbonate passed the greatest amount of undeviated light. 8. The amount of undeviated light passed by samples compounded with good Japanese magnesium carbonate and good domestic carbonate was essentially the same between 25 and 40 per cent filler—about 22 per cent. 9. The tensile strength of compounds containing domestic carbonate was higher in the 25 per cent filler range than that of mixtures containing Japanese carbonate.