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
There is good market for preserved latex concentrate, as it is an important raw material with a wide range of applications. Two important methods of processing latex into preserved latex concentrate are commercially practised.

1. Concentration by Creaming

The processing of latex into creamed concentrate involves the mixing of a creaming agent such as ammonium alginate or tamarind seed powder with properly preserved field latex and allowing 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

The processing of latex into latex concentrate by centrifugation involves the separation of 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.

Preservation of Centrifuged Latex
Centrifuged latices are commercially available as high ammonia (HA – minimum 0.6% ammonia) and low ammonia (LA - 0.2 to 0.3% ammonia) types. The former is preserved solely with ammonia and the latter contains one or more preservatives in addition to ammonia. The most popular LA type latex is low ammonia TMTD – Zinc oxide (LA-TZ) which contains 0.2 to 0.3% ammonia, 0.013% TMTD, 0.013% zinc oxide and 0.05% lauric acid.

Specifications
Preserved latex concentrates shall be graded and marketed in conformity with the standards specified by the Bureau of Indian Standards (BIS) as given in IS: 5430-1981 (centrifuged latex), IS: 11001-1984 (double centrifuged latex) and IS 13101-1991 (creamed latex).

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 and Air Dried Sheets (Pale Amber Unsmoked Sheets).

A major quantity of natural rubber produced in this country (about 74.7%) is marketed in sheet form at present, as it is the oldest and the simplest method of processing latex into a marketable form.

For processing latex into sheet rubber, it is important that 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) is estimated with a metrolac, which is a special type of hydrometer calibrated to directly read the DRC. However, laboratory methods are employed for accurate determination.

Latex is diluted in bulking tanks to a standard consistency of 1/2 kg of dry rubber for every 4 litres 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 sedimented 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. The quantity of acid required for satisfactory coagulation depends on various factors like the amount and type of anticoagulant used, the duration of coagulation, the season, and the nature of the latex.
The acid requirement may slightly change under varying conditions and can be fixed up by experience. Only diluted acid should be used for coagulation and should be thoroughly mixed with latex.

Catalyst AC and sulphuric acid are also used by growers. Catalyst AC is a dry powder and comparatively a safe coagulant. Normally 100 ml of a 5 per cent solution of this chemical is enough for making a ˝ kg sheet.

Since sulphuric acid is highly corrosive, care should be taken in its handling and dilution. 300 ml of a 0.5% solution of the 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 paranitrophenol (PNP). The concentration of paranitrophenol is 0.05 to 0.1% in water. 100 litres of the solution will be sufficient for treating 100 sheets. The wet sheets are allowed to drip on reapers arranged in a well-ventilated dripping shed.

Smoking and Smoke Houses
The sheets after two or three hours of dripping in shade are placed in the smoke house where the temperature is maintained between 40° and 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.

It is preferable to smoke the sheets on the first day at a low temperature (40° to 45°C). For the subsequent days (i.e. the second to the fourth day) the sheets are to be dried at a higher temperature (not exceeding 60°C) and fairly low relative humidity. 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.

There are various types of smoke houses, all working on the same general principle. The essential features of a smoke house are 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.

Generally, the sheets are turned on the reapers every day for uniform smoking and drying and to avoid reaper marks on dry sheets. 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.

Drying of sheets in a smoke house has definite advantages. It is quicker than sun-drying and does not cause oxidation by ultraviolet radiation. Inside the smoke house, there is only limited supply of air and it is mostly filled with smoke and carbon dioxide. Hence chances for oxidation of rubber are very limited, provided the temperature is within limits. Also, the creosotic i.e. materials present in smoke, get deposited on the surface of the sheets thereby preventing mould growth on sheets. The requirements of a good smoke house are:

* Minimum drying time
* Maintenance of temperature in the range of 40-60^o C
* Maximum fuel efficiency
* Minimum heat loss
* Minimum drying cost
* Easy loading/unloading of sheets
* Minimum labour requirement
* Continuous operation
* Good ventilation
* Minimum number of defective sheets

A smoke house consists of a chamber into which the sheets are loaded either on trolleys carrying reapers or on reapers fitted on a wooden framework. Smoke is generated in the furnace, which is usually outside the chamber. Smoke and hot air from the furnace are directed into the chamber through a flue. Air inlets and ventilators are provided at the bottom and top of the chamber respectively. These can be opened or closed for controlling temperature. Temperature can also be regulated by adjusting the rate of burning of the firewood by opening or closing the air inlets in the furnace door. A damper is usually provided at the main flue outlet, as a safety measure to prevent fire entering the chamber in an event of opening the furnace door. The chamber may be of brickwork with a reinforced concrete frame. The smoke house shall be provided with adequate drainage to facilitate removal of serum dripping from the sheets. The roof and the ceiling may be of asbestos sheets and the gap between the roof and the ceiling at the top of the walls shall be closed from all the four sides of the smoke house, so as to avoid heat loss due to air currents over the ceiling and to prevent condensed moisture containing carbon from dripping on sheets. Smoke houses are of two types, those in which the furnace is inside the drying chamber and those in which the furnace is outside.

Furnace Inside the Chamber
Smoke house of the pit-fired-type and trolley box type which are used in Sri Lanka come under this. In the pit-fired-type, smoke is generated by burning firewood in a central pit, which is inside the smoke house. A thick gauge galvanized iron sheet, slightly larger in size than the mouth of the pit, is fixed 25 cm above so as to spread the smoke. The sheets are hung inside the chamber and a minimum space of 180 cm between the fire and the bottom layer of the sheet is given to avoid overheating of the sheet near the pit. In the trolley box furnace-type, smoke is generated in a fire trolley on wheels. The advantage of this type of smoke house over the pit-fired-type is that the smoke house can be kept clean since operations such as loading of firewood and removing the ash can be done outside the smoke house.

Furnace Outside the Chamber
There are two types of smoke houses with external furnaces, the ground-floor type and the tunnel-type. In the ground-floor type, the smoke from the furnace is directed to the centre of the smoking chamber. The sheets are hung on reapers fitted on a wooden framework. Fresh sheets, after dripping, are put on reapers near the ground and on the next day they are taken from the lower reapers and hung on the upper ones.

The usual drying time in this type of smoke house is four to five days. The disadvantages of this type of smoke house are that only batch wise operation is possible and more labour is required, since sheets are replaced on the reapers every day.>

The details of construction and blueprints of various types of smoke houses are available for sale from RRII.

Solar-cum-Smoke Drying
Drying of sheets in direct sunlight increases the chances of oxidation of sheets, especially when they are exposed to sunlight for longer periods. However, solar energy could be indirectly used for drying the sheets using flat plate solar collectors. In this system, 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. Reports show that with the back-up heat source, drying time could be brought down to five days. Compared to conventional smoke drying, the saving in firewood using the solar-cum-smoke drier is around 60 to 70 per cent and the quality of sheets is comparable to that of sheets prepared by conventional smoke drying.

Sun Drying
Growers without smoke house facility dry their sheets in the open sun. It has been reported that sun drying of sheet beyond 2-3 days causes slight degradation of sheet apart from accumulation of atmospheric dust on it. Therefore, it is advisable to limit open sun drying to two days initially followed by smoke drying.

Air Drying
The sheets dried in hot air are called air-dried sheets.

Air-dried Sheet
This is a light coloured sheet prepared in the same way as ribbed smoked sheets but dried in a shed or tunnel in hot air instead of smoke. As colour of the sheets is an important consideration, use of sodium bisulphite, which inhibits enzymic discolouration of the sheets is necessary in this process. Usually a calculated volume of a solution of sodium bisulphite is added in the bulking tank so as to get 1.20 g of it in a kg of dry rubber. Air-dried sheets may fetch a better price compared to smoked sheets as they can be used as substitutes for pale latex crepe.

Trolley for Sun-cum-smoke Drying of Natural Rubber Sheets
Smallholders dry rubber sheets partially in sunlight and partially in a smoke dryer/kitchen hearth. During this process, a lot of dirt, dust, and other contaminants accumulate on the surface of the sheet leading to downgrading of the dried sheet. Drying of rubber sheets put on wooden rods fixed on trolleys can eliminate surface contamination to a great extent. The trolley can be used to dry rubber sheets in open sunlight as well as in a smoke dryer, thereby substantially reducing the drying time, surface contamination and improving the grade of the sheet.

The trolley for sunlight-cum-smoke drying of natural rubber sheets consists of four sliding layers, one guard layer and a top layer which are fixed on a mild steel (M.S.) angle framework. The sliding layers can be drawn out in four directions. Slots are cut, 75mm apart, on the sliding and fixed frames to insert wooden rods of 20mm diameter on which the rubber sheets are hung. For drying in open sunlight the trolley is rolled out of the smoke chamber and the sliding layers are drawn out in four directions thereby avoiding the shade of the top layer of sheets from falling on the bottom ones. This provides direct sunlight to sheets on all the layers, better aeration through layers of sheet and avoids water dripping from one layer to the other. The cumulative effect of all the above is faster and uniform drying of the sheets. During night the sheets on the trolley can be dried in a smoke house, for which the sliding layers are pulled in and arranged one above the other. As the trolley is designed for maximum use of sunlight with minimum handling, the period of drying is reduced. Since the trolley is fitted with swivel caster wheels of 100 mm diameter it can be easily moved in and out of the smoke house and rolled into the open space for sun drying.
The main frame of the trolley is made of M.S angles of 50x50x6mm size. The sliding and guard layers are of 35x35x5mm M.S angles and the top layer has 40x40x6mm M.S angles. The guard layer is intended for collecting the sheet that may accidentally fall down, thereby preventing it from catching fire in the smoke house. The sliding layer when drawn out are supported on 10mm. iron rods thereby providing overall balance of the trolley carrying the sheets. Each layer can carry 30 sheets of 500 g, leading to a total capacity of 75 kg. of dried sheets per trolley.

Trials conducted at RRII indicated 50-60 per cent savings in fire wood by the use of this new trolley for sun-cum-smoke drying.

Ribbed Smoked Sheet Cleaning Machine
A major portion of smallholder’s sheet rubber is of very poor quality, due to incomplete drying, mould growth and a lot of surface contamination. These problems could be overcome at least partially if such sheets are properly cleaned even after drying. The manual method of cleaning is laborious and costly. Hence, a mechanical device for cleaning of low quality sheet rubber is developed.

The machine consists of a set of cast iron/mild steel rolls having a specific design and which run at an rpm of 90, a set of brush rolls with 300 rpm and a third set of rolls (brush) of 360 rpm, arranged in a row. The length of each roll is 70 cm. The brushes are made of nylon bristles of 2.5 cm in length and are fixed on a PVC/nylon roll which has a steel shaft at its core. The sheets are first fed to the cast iron rolls and as they come out they are pulled by the first set of brush rolls under a forced spray of water which is applied on the sheet from the bottom and top sides. This semi-cleaned sheet passes through the second set of brush rolls under the same conditions as in the previous case. For easy movement of the sheets, rolls are arranged in a slanting manner. There are two motors, each of 2HP. One motor drives all the bottom rolls whereas the other one drives all the upper rolls. Rolls are arranged on mild steel framework which is provided with stainless steel/aluminium feeding and receiving trays. Aluminium plates are also provided between each set of rolls to prevent the sheets from falling down. The top rolls have an upper and lower movement facility to accommodate sheets of different thickness. This machine can clean dry sheets after soaking in water which will upgrade the quality of sheets on drying. The sheet is stretched due to the speed difference and brushed well when it passes through the rolls. Cleaning of both the surfaces of the sheets takes only two to three seconds, leading to an output of 20 to 30 sheets per minute depending on the setting of the rolls and size of sheets.

Major Defects in Smoked Sheets

The completely dried sheets are removed to the packing shed where they are carefully inspected and graded according to the standards published by the Rubber Manufacturers Association (RMA) Inc. Washington in Green Book. This system at present provides for six grades of ribbed smoked sheets, viz, RSS IX, RSS 1, RSS 2, RSS 3, RSS 4 and RSS 5. The grading of sheet rubber is carried out by visual examination. Normally this is accomplished by holding rubber sheets against light when the most obvious defects become apparent.