RUBBER

Latex preservation and concentration

Preserved field latex

Field latex is preserved using suitable preservative for long-term storage.  The processing of preserved field latex consists essentially of adding the preservative (usually ammonia, minimum 1%) to the sieved latex, bulking, settling, blending and packing.  Field latex can also be preserved with LATZ  (Low ammonia – TMTD – zinc oxide) system.

Latex concentrate

Two important methods of processing latex into preserved latex concentrate are commercially practised.

Mix a creaming agent such as ammonium alginate or tamarind seed powder with properly preserved field latex and allow the latex to separate into two layers, an upper layer of concentrated latex and a lower layer of serum containing very little rubber. The lower layer of serum is removed, leaving the latex concentrate having about 50-55% DRC (dry rubber content), which is often tested, packed and marketed.

Separate the preserved field latex into two fractions, one containing the concentrated latex of more than 60% dry rubber and the other containing 4-8% dry rubber (skim latex).

Skim latex is generally coagulated with sulphuric acid, made into crepe, dried and marketed as skim rubber, which is a low-grade rubber.

Ribbed smoked sheet (RSS)

Latex is coagulated in suitable containers into thin slabs of coagulum and rolled through a set of smooth rollers followed by a grooved set and dried to obtain sheet rubber.  Depending upon the drying method, sheet rubbers are classified into two:

• Ribbed smoked sheets
• Air dried sheets (Pale amber unsmoked sheets).

The latex collected is brought to the processing centre before pre-coagulation sets in.  In cases where the latex is found to be prone to pre-coagulation, an anticoagulant is used.
Latex brought to the centre is strained through 40 and 60 mesh stainless steel sieves.  The volume of latex is measured with a standard vessel and a calibrated rod.  The dry rubber content (DRC) can be estimated with a metrolac, which is a special type of hydrometer calibrated to directly read the DRC.

Latex is diluted in bulking tanks to a standard consistency of 1/2 kg of dry rubber for every 4 litre of the diluted latex (12.5% DRC).

The diluted latex is allowed to stand in the bulking tank for a fixed time (usually 15 to 20 minutes) for the heavy dirt particles to sediment.

The diluted latex is drawn out from the bulking tank without disturbing the sediment layer of impurities into the coagulation pans or tanks.  Four litres of latex is usually transferred to each pan.

Coagulation

Formic acid or acetic acid is generally used for coagulation.  Only diluted acid should be used for coagulation and should be thoroughly mixed with latex.

Catalyst AC (100 ml of a 5 per cent solution) and sulphuric acid are also used by growers. 300 ml of a 0.5% solution of the sulphuric acid is required for same day sheeting and 250 ml for next day sheeting.

Coagulum from latex often shows a tendency for surface darkening.  To prevent this, a small quantity of sodium bisulphite (1.2 g per kg DRC), dissolved in water may be added to the diluted latex before coagulation.

After coagulation, the coagulum is removed from the pan or tank and thoroughly washed in running water.

They are rolled either in a sheeting battery or smooth rollers to a thickness of 3 mm and finally passed through the grooved roller.  While sheeting, the coagulum is continuously washed.  The sheets are again washed in running water in a tank.

Mould growth on sheet rubber can be prevented by treating freshly machined sheet in a dilute solution of para-nitrophenol (PNP).  The concentration of para-nitrophenol is 0.05 to 0.1% in water (100 litres of the solution for treating 100 sheets).  The wet sheets are allowed to drip on reapers arranged in a well-ventilated dripping shed.

Smoking and smoke house

The sheets after two or three hours of dripping in shade are placed in the smoke house where the temperature is maintained between 40-60°C.  In the smoke house, sheets are dried gradually whereby blisters are avoided.  In addition, the creosotic substances present in the smoke prevent mould growth on smoked sheets.

Smoke the sheets on the first day at a low temperature (40-45°C).  Sheets can be dried by placing them on the first day on the reapers at the bottom region of the smoke house and at the higher regions on the subsequent days of smoking.

Four days of smoking is generally sufficient under normal conditions, but during the rainy season five to six days are required for satisfactory drying of sheets.

The smoke house is a chamber in which, the sheets can be placed on reapers, a furnace outside the chamber and a flue duct connecting the furnace to the chamber.  Smoke houses are of two types, those in which the furnace is inside the drying chamber and those in which the furnace is outside (the ground-floor type and the tunnel-type).

Other methods of drying rubber sheets

In solar-cum-smoke drying, hot air from the solar collectors is blown into the drying chamber in which the sheets are placed on reapers placed on trolleys.  The system also contains a furnace for burning firewood which acts as a back-up heat source for maintaining the inside temperature during night and also on cloudy days.

Storage

The sheets after grading are packed in 50 kg bales.  The grades are marked on the bales and marketed.

Source: www.rubberboard.org.in

Last updated: 24-05-2008