40 m. Rubber production in Hevea is entirely from the bark. Tree stumps without leaves have been taped and have continued to yield latex for long periods,tapping of a felled tree continues for 4-6 weeks
particularly good quality, and the yield of the trees is said to be high. Bark texture, thickness, latex-vessel rows, latex-vessel size etc., play significant role on the yield of rubber.
Tapping
When the rubber tree has reached a circumference of 45-55 cm at about 0.9 m from the ground, it is large enough to be tapped. The Rubber Research Insitute of Malaya (RRIM) has made the following recommendations for the initial tapping of young rubber trees.
1. Alternate day tapping of clonal `seedling’ through a half spiral cut usually results in a physiological disorder called brown bast.
2. Tapping towards a junction of bark of different ages leads to severe drop in yield.
3. Bark of the second renewal is often thin and yield from it is disappointing
Rubber is obtained from the tree by a system of tapping that consists of paring off a small amount of bark- just sufficient to open up the ends of the latex vessels. A flexible template is
used to mark the limits of the tapping panel on the tree and the desired slope of the tapping is cut. The latex vessels in the bark of Hevea do not ascend vertically but have a slight slope to the right. To intercept maximum number of latex vessels, the tapping cut is sloped upwards to the left at an angle of about 300 above the horizontal.
The tapping cut is opened along the mark made in laying out the panel, and a
vertical groove is made in the bark at the lower end of the tapping cut. This vertical groove is to direct the latex to the spout that is driven into the bark about 4 inches below the lower end of the tapping cut. Some five tapping cuts are required to bring the cut to the correct depth and during this period the yield of latex is low. For tapping the tree tapping equipment like knife, spout, latex cup, cup hanger, and ammonia solution are necessary.
Collection of Latex
The latex is collected in a large receptacle and a small quantity of ammonia is added to avoid pre- coagulation of the latex.
Straining
The latex is assembled and passed through an aluminum plate 0.01 cm perforation spaced 0.31 cm from center to center, to remove large particles such as bark shavings, twigs, leaves etc.
Rubber content of latex
The rubber content of the latex is estimated by measuring the specific gravity of latex and estimates the rubber content on the basis of the relative specific gravity of the major constituents. For measuring the specific gravity of the latex, Metrolac hydrometer or latex meter are used.
Dilution and Second straining
The rubber is then diluted with water to a concentration of 12-15 percent. After a short wait for the precipitation of heavy particles of sand, the latex is strained through a fine screen which will catch the small particles of dirt and trash that pass through the coarse screen.
Concentration
Fresh latex has a rubber content of 30-35 percent that must be increased to 60-70 percent. This concentration of latex is accomplished by centrifuging in a modified cream separator, by creaming with alginates or other suitable creaming agents, or by evaporation in the presence of protective colloids.
Coagulation
The latex is then coagulated by treatment with acetic and formic acids. If ammonia has been added to the latex, it is necessary to add sufficient acid to neutralize the ammonia as well as to coagulate the latex. The latex coagulates into a thick sheet.
Sheeting
In coagulation, the rubber must be passed through power rolls to reduce the thickness sufficiently to assure adequate drying. The sheet is passed through a marking roll after it has reached its final thickness, and sheeting is done in mills with smooth rolls.
Smoking
Sheeted rubber is dried in a smokehouse where temperature is maintained below 500C, Sheet rolled to a thickness of 2.5 mm should dry under these conditions within four days.
Grading
Before packing for shipment, the rubber is inspected carefully for defects or blemishes and graded accordingly.
Shipment
Plantation latex is normally shipped in drums, tanks, or tankers. The containers in which the latex is shipped must be thoroughly sterilized.
Vulcanization
Good year (1839) discovered the process of vulcanization. Rubber heated with various quantities of sulphur (only about 2 percent for soft rubber, as much as 40 percent for hard rubber) retained its firmness at normal temperatures and did not become tacky. Alexander Parkes (1846) found that thin films of rubber could be changed from the plastic to elastic state by the use of sulphur monochloride, and this change could be accomplished without heat. This is called cold vulcanization of rubber.
Mechanism
The rubber hydrocarbon, polyprene, combines with sulphur without the evolution of sulphureted hydrogen.
The vulcanization process is thus an addition reaction. The vulcanization process results in the formation of a continuous series of addition products of sulphur and polyprene. The upper limit of this series is represented by the compound C100H160S20 and the lower limit by the compound C100H160S.
According to L.A. Wood (1957), United States National Bureau of Standards, vulcanization is a chemical reaction in which the physical properties of a rubber are changed in the direction of decreased plastic flow, less surface thickness and increased tensile strength by reacting it with sulphur or other suitable agents.
Spread of Rubber Industry
Following Goodyear’s invention, factories for manufacture of articles made of rubber began to spring up in the principal industrial countries. Solid rubber tyres were first used on road vehicles in 1867. In 1888 Dunlop invented the pneumatic bicycle tyre, from which the motorcar tyre was developed.
Uses of Rubber
Many kinds of articles can be fabricated from rubber: strong structural materials, soft, yielding comfortable materials, resilient, elastic materials, conductors and non-conductors of electricity, shock absorbers, mountings for motors and other machinery, transmission belts, gaskets, hoses for transportation gases and liquids, transparent material, translucent materials, articles, or clothing to keep out rain or to control figure, sports goods, cements, paints, plantics, pharmaceuticals, and above all, tyres , the chief outlet for rubber.