GB402067A - Improvements in or relating to the manufacture of rubber or ebonite and articles composed thereof - Google Patents

Abstract

In a process for the manufacture of cellular rubber or ebonite, or articles thereof, from aqueous rubber dispersions such as latex, the dispersion is gasified during coagulation by effecting fermentation of material in the mass with consequent evolution of gas, which is preferably insoluble or slightly soluble in the dispersion. The gasification may be continued after coagulation is completed. The cellular coagulum may be heat treated to rupture the cell walls, or vulcanise the mass, or both, and the parent dispersion may have vulcanising agents incorporated in it for this purpose. Suitable fermentable substances include glucose, cane-sugar, beet-sugar or molasses, and the ferment may be a saccharomyce of yeast or its enzymes. The acidity or alkalinity of the dispersion should be adjusted to suit the ferment used, e.g. ammoniapreserved latex is treated to remove ammonia. A known coagulant, in an amount insufficient to effect immediate coagulation and incapable of killing the ferment, may be added to the dispersion e.g. magnesium sulphate. In this case, ammonium sulphate should be added to prevent precipitation of the albumens in the serum of the latex by the magnesium salt. Other fillers, dyestuffs, antioxidants and substances common in the industry may be added, avoiding the use of those bodies which would reduce the action of the ferment. When using sugars as the fermentable material, the presence of sulphur in the parent dispersion causes the formation of hydrogen sulphide, almost exclusively when the mixture designed for the production of ebonite. Small quantities of hydrogen sulphide may be removed by the addition of zinc oxide. Finely divided inert substances e.g. ground glass, pumice stone or siliceous material may be added to promote the formation of fine bubbles, in amounts varying between 4 and 20 per cent. of the dry rubber content of the dispersion. In the absence of finely divided solid matter, intercommunication of the cells is produced by immersing the fermented mass in boiling water, or by subjecting it to a vacuum, either at ordinary or increased temperatures, vulcanisation being performed if necessary. When the final products are elastic rubber, they tend to shrink during the drying of the coagulum. This may be prevented by preventing the outer surfaces from contracting, for example, the fermented mass, preferably prior to vulcanisation, is placed in a mould having network walls which have been previously coated with rubber solution or similar adhesive. The mould is then heated to vulcanise the rubber, e.g. by immersion in boiling water, and then cooled and dried and the mass detached. The temperatures used in the fermentation process range from 25 DEG -36 DEG C., and the time required to attain maximum volume depends upon the contents of the dispersion, especially the amount of sulphur present. The actual degree of volume increase may be controlled by selecting a mould of the required final volume, having an exit for displaced air and fermentation gases, and treating in it a sufficient amount of the parent dispersion to occupy the whole of the mould when fermented. According to examples (1) a mixture of 100 gms. ammonia-free latex concentrated to 50 per cent. dry rubber content, 10 c.c. of a 50 per cent. aqueous solution of sugar containing 2 gms. of yeast, 1.5 gms. each of crystallised magnesium sulphate and ammonium sulphate in 10 c.c. water, and 15 c.c. of a 5 per cent. aqueous solution of glue having suspended in it 1 gm. sulphur, 0.5 gm. zinc dimethyldithio-carbamate, 0.2 gms. zinc oxide and 2 gms. ground pumice stone was fermented in a suitable mould at 30 DEG C. for 6 hours. The mass was transferred to a network mould coated with rubber solution, enclosed in a suitable container and immersed in boiling water for 1 1/2 hours for vulcanisation. The cage was then left in cold water, dried and the mass removed. (2). A mixture of 100 gms. ammonia-free latex concentrated to 50 per cent. dry rubber content, 15 c.c. of a 50 per cent. aqueous sugar solution containing 1 gm. yeast, 4 gms. crystallised magnesium sulphate and 4 gms. ammonium sulphate in 35 c.c. of water and 20 c.c. of a 5 per cent. aqueous glue solution containing 17 gms. sulphur was fermented for 6 hours at 30 DEG C. The mould was then heated in an autoclave for 8 hours at 3 atmospheres pressure (about 143 DEG C.), cooled and dried. The final product was a spongy ebonite.