THE latex, brought to the factory in tanks or buckets as it is collected from the trees, contains as a rule from 30 to 40 per cent, of pure caoutchouc. The latter is obtained in a state of considerable purity by the successive processes of coagulation, washing and drying. The method of preparing biscuit rubber by hand, as practised during the earliest beginnings of the industry, is now of little more than historical interest. It may serve however to illustrate in their simplest form the nature of the chief processes concerned.
In this method the latex is first strained through a brass or copper wire sieve of small gauge, and then generally diluted with one or more times its own bulk of water. The diluted latex is next agitated with a dilute solution of acetic acid, containing about one volume of pure acid for every 1000 volumes of pure latex. The acidified latex is then poured into round shallow pans to set. Coagulation begins almost immediately, but the pans are allowed to stand for several hours until a firm cake of rubber of the consistency of cream cheese has formed upon the surface. The remaining liquid is by this time perfectly clear and free from rubber. The cake of rubber is then removed, washed in clean water, and rolled out thin on a board with a common rolling pin or a bottle, in order to express as much water as possible The resulting biscuits are then spread out to dry on racks in an airy and darkened room. In favourable weather the drying may be complete in from 15 to 20 days, if the biscuits do not exceed one-tenth of an inch in thickness. The resulting product consists of thin round sheets of rubber. If all goes well, the biscuits are semi-transparent and of a uniform honey colour.
The first step towards more elaborate processes of manufacture was the substitution of a simple rolling machine for the bottle or rolling pin; but with the increase of crops from large estates, hand labour has largely been ousted by the introduction of heavy machinery.
The site for the factory should combine a central position on the estate with accessibility to the main road or other means of transit leading to the port of shipment A plentiful supply of good clean water is essential for washing. It is generally best for the factory to lie at the lowest possible level, since the incoming latex is heavier for transit than the outgoing rubber. Water power should be made use of if available, but this is comparatively rare on rubber estates. The source of power usually employed consists of steam or suction gas or liquid fuel engines, of which the latter are probably the more popular. A supply of steam may however be required for heating purposes.
The amount of power required depends upon the size of the estate. On a plantation of 500 acres provision will have to be made for turning out 1000 pounds of dry rubber daily at certain seasons of the year. The treatment of the latex cannot be delayed, and machinery must be available for dealing with the maximum crop at any season. For this output about 45 horse power will be required for the washing and crêiping machines, in addition to about 8 horse power for vacuum driers, if these are adopted.
Several firms of engineers now specialize in the building of rubber factories and in the provision of suitable machinery. The details of structure and equipment best adapted to the needs of any particular estate can best be settled in consultation with a reliable firm. One of the most important details to be considered relates to the lighting of the factory. Plenty of light is desirable for working the washing, rolling and crêping machines, but in the subsequent stages of the process of manufacture an excess of light is to be avoided, owing to the injurious effect of light upon the rubber. Rooms intended for the drying and storage of the rubber require to be specially well screened from light, either by curtains or by painting the glass of the windows.
It is usual to begin with a factory of moderate size, which is capable of expansion as the crops of rubber increase; and it should be borne in mind when drawing up the original plans, that provision may ultimately have to be made for dealing with a crop of more than two pounds of rubber daily from every acre of the planted area. Another essential point is the necessity for the utmost cleanliness at every stage of manufacture. Any carelessness in this respect is liable to lead to the production of discoloured or even tacky rubber, with a corresponding diminution in the value of the product It is worth while to copy from modern hospital buildings the method of rounding off all corners where walls and floors meet. Internal surfaces, wherever possible, should be of smooth cement. Floors should be sloped so as to drain into cemented channels for greater ease in washing. A plentiful supply of clean water is essential.