Transparent Rubber Compounds. The Effect of Chemical Composition of Magnesium Carbonate Fillers

Abstract

Abstract From this investigation the following conclusions may be drawn. 1. The x-ray results of Bixby and Hauser have been substantiated by chemical analysis and by microscopic examination. The magnesium carbonate productive of highest light transmitting properties is of the type: 5MgO.4CO2.xH2O. The normal carbonate, MgCO3, gives very low transmissions. 2. The best Japanese carbonate studied (sample No. 3) is pure 5MgO.4CO2.xH2O. 3. A domestic carbonate (sample No. 20), which is also pure 5MgO.4CO2.xH2O, is commercially available and produces transparency in rubber compounds equal to that obtained with the Japanese product. 4. Domestic carbonates in general contain more carbon dioxide than is required by a 5MgO.4CO2.xH2O carbonate, and are probably mixtures of this material and of the normal carbonate, MgCO3. 5. Light transmitting properties fall rapidly as the proportion of normal carbonate, MgCO3, rises. 6. Carbonates containing less than enough carbon dioxide to provide a 5MgO.4CO2.xH2O carbonate are probably mixtures of this material and hydrated magnesium oxide, MgO.H2O. 7. In preparing basic carbonates for use in producing high light-transmitting rubber, it is better to produce a material with slightly less carbon dioxide than necessary for a 5MgO.4CO2.xH2O carbonate, rather than more. 8. Particle size is an important factor influencing light transmission, especially when the normal carbonate, MgCO3, is present. Generally speaking, especially in the size ranges encountered in these carbonates, a finely divided MgCO3 will offer greater hindrance to the passage of light than will a larger size material.