(b) A tank containing 120 gallons of latex would need 10 gallons of the 1 per cent. acid solution.
It is assumed that all estates, not only in the preparation of sheet rubber, but also in the making of crepe rubber, always employ the system of standardising latex in order to obtain uniformity. They are ill-advised if they do not follow this practice; but in case average undiluted latex is treated in coagulation, the quantity of acetic acid to be used should be calculated from the ratio 1:1,000.
If the acid solution is to be employed in 1 per cent. strength, one-tenth of the volume of latex to be treated will indicate the required quantity of solution necessary for complete coagulation unless anti-coagulants have been used, when the quantity must be increased as experience directs. It will be recognised, of course, that undiluted latex may only be used in any case for the preparation of crepe rubber; or in some exceptional case, such as the special preparation of “slab” rubber.
Care in Mixing.—It is essential that the mixture of dilute acid and latex should be thoroughly intimate. This can only be attained by careful manipulation, especially in the case of sheet preparation. Where crepe rubber is to be made it may be permissible to employ a solution stronger than 1 per cent., but it is not advised. The acid should be poured into the latex while stirring, and the agitation should continue for such a period as to ensure thorough mixing in all parts.
It will be appreciated that in the preparation of sheet rubber this period may not be unduly prolonged, otherwise the latex will have begun to coagulate before skimming and the placing of the partitions in their respective slots can be effected. Furthermore, while in the preliminary treatment for crepe rubber, the formation of enclosed bubbles and surface froth is immaterial. For sheet preparation it is essential that the stirring shall be done so carefully as to try to avoid internal bubbles and to reduce surface froth to a minimum. For crepe-making a perforated board, with handle attached at right angles to the face of the board, may be used; but in shallow sheet-coagulating tanks, broad paddles (which may or may not be perforated) give good results as long as there is a sufficient number used to cover the area of the tank in reasonable time. Obviously also, where the area of any tank or compartment is of any appreciable size, the dilute acid solution should be poured in from various points so as to obtain a good even distribution. In some cases the acid is distributed from a sprinkling can, but this is a refinement which experience shows to be unnecessary. In actual practice, working on a tank measuring 12 ft. by 4 ft., no difficulty is found if coolies pour in acid solution from four points. The degree of success depends entirely upon experience and efficient supervision. This remark applies equally to the use of various devices, such as rakes with broad teeth, used as stirring implements. There is room for display of ingenuity in this direction, and it is found often that, while they are used successfully on one estate, they may be condemned on another.
Two Views of Dilution and Mixing Tanks.
Below, on the right, coagulating tanks. At the far end strainers. Each dilution tank is of equal capacity to the corresponding coagulating tank.