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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.
1) Concentration by creaming
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.
2) Concentration by centrifugation
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.
Acid requirement for coagulation of 4 litres of diluted latex containing ½
kg rubber:
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Acetic acid
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Formic acid
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For the next day sheeting
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3 ml diluted to 300 ml with water
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1.5 ml diluted to 300 ml with water
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For the same day sheeting
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4 ml diluted to 400 ml with water
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2 ml diluted to 400 ml with water
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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
(1) Solar-cum-smoke drying,
(2) Sun drying and
(3) Air drying
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
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