How to Processing of Latex Concentrate

Natural latex remains of special importance in the rubber industry because it is still the main raw material for those rubber manufacturing operations using latex as distinct from a dry rubber.  Some of these operations are dipping processes, by which gloves, balloons, and similar thin walled articles are made.  Of equal or more importance are processes in the textile industry such as coatings, anti-fray treatments, dimension stabilization of woven and non-woven fabrics and proofing.

The use of natural latex for carpet backing now probably represents the largest single market for natural latex.  Foam rubber made from natural latex has a number of desirable features which are not matched by synthetic latex.  The many other industrial applications include adhesives, latex paints, latex plaster materials, and elastic thread.

Method that has been developed to concentrate latex, are: centrifuging, creaming, evaporation, and electro-decantation.  The latter method has never been put into commercial operation; creaming and evaporation are only used on a limited scale and it is not likely that these processes will be able to compete economically with centrifuging.  However, in the industrial cease it always means latex concentrate, i.e. Hevea latex in a concentrated (and purified) form.  This method therefore will be dealt with in this module.

Image of Natural Rubber Tapping

RECEPTION AT FACTORY AND FLOW-LINE

When the latex arrives at the factory the method of operation is as follows:

1. Draw a sample from the lorry tank and do a quick test for approximate DRC (latex meter) and ammonia content.  If the test shows the DRC to be unsatisfactory (because of rain dilution or for other reasons) the latex should not be centrifuged but coagulated, using standard methods
2. If the ammonia content is far below standard, provision should be made to blend the lot with others showing higher ammonia content.  However, this should be avoided by careful control of ammonization at the field receiving station.
3. Sieve the latex into the reception tank.
4. Transport the bulked latex from the reception tank to the dilution tank in the factory, either by gravity flow or by air pressure.  The purpose of dilution is to bring the latex to a standard DRC, in accordance with the separating efficiency of the centrifuge and the requirements for the concentrate to be produced (minimum 60 or 61 percent DRC).  Thus, it can be found to be of advantage to dilute the latex to a DRC in the range of 27-29 percent.  It stands to reason that for dilution only clear, pure water should be used.
5. After through mixing the latex is run straight into a battery of centrifuges for separation into a concentrate fraction and a skim fraction.  Both fractions are collected in tanks, preferably situated on a lower storey in the centrifuge building.  If a high ammonia concentrate is to be produced, it is further ammoniated by adding the appropriate amount of ammonia gas to bring it up to the required standard.  For low ammonia concentrate, other preservatives are added at this stage.
6. After through mixing the concentrate is ready for transportation: however, a period of storage up to two weeks before shipment is preferred.
7. The skim latex flows either through a pipeline or through an open gutter to a site outside the centrifuge building for further processing.

AMMONIZATION

In producing high ammonia concentrate the ammonia content after centrifuging is raised to 0.7 percent w/w, which allows a margin of safety above the minimum specification of 1.6 percent w/w in the water phase.  The permissible maximum in low ammonia concentrate is 1.0 percent w/w on the water phase or, approximately 0.37 percent w/w, although most low ammonia systems do not require more than 0.20 percent w/w.  Even then small additions to the freshly centrifuged concentrate may be needed to make up the difference between 0.20 percent w/w and the actual content.

Ammonia should be added to the concentrate as early as possible.  In each collecting tank a separate ammonia spreader should be installed with slow moving, mechanical paddles.  The tanks may be of any convenient size depending on the capacity of the plant, but preferably cylindrical.

PRESERVATIVE SYSTEMS

The “Standard Specification for Concentrated, Ammonia Preserved, Creamed and Centrifuged Natural Rubber Latex”, issued by the American Society for Testing and Materials (ASTM) D1076-69, distinguishes four types of natural latex concentrate as follows:

1. Type I:     Centrifuged natural latex preserved with ammonia only or by formaldehyde followed by ammonia
2. Type II:     Creamed natural latex preserved with ammonia only or by formaldehyde followed by ammonia
3. Type III:     Centrifuged natural latex preserved with low ammonia with other necessary preservations
4. Type IV:     Creamed natural latex preserved with low ammonia with other necessary preservations

The description of Type III indicates that with low ammonia concentrate other preservative are added after centrifuging; this is necessary to prevent coagulation and maintain or build up the required stability.

Preservatives may function as bactericides, enzyme inhibitors, sequestering agents or stabilizer.  They are often used in combination.  The following systems have been proven successful and put into commercial practice:

1. Santobrite (sodium pentachlorophenate – SPP) 0.2%, ammonia (NH3) 0.2%
2. Santobrite 0.1%, EDTA (ethylene diamine tetra acetic acid) 0.1%, ammonia 0.1%
3. ZDC (zinc diethyl dithiocarbamate) 0.2%, ammonia 0.2%
4. Boric acid 0.20-0.25%, lauric acid 0.03-0.06%, ammonia 0.2%

Preservatives are added to the concentrate as soon as possible after centrifuging, for in a concentrated solution or as a slurry.  For ease of operation, boric acid and lauric acid are first dissolved in strong ammonia water.  Boric acid acts as a preservative this also a destabilizer; to counterbalance this effect, lauric acid is added for soap reaction.  SPP concentrate is well known for its high mechanical stability.

After preservatives have been added the concentrate is properly blended and transferred into two storage tanks, preferably by gravity flow.  Two tanks are required so that one consignment can be mixed, added and shipped while the second is being prepared. Horizontal cylindrical tanks can be used up to 100000 liters capacity.  Beyond that size it is probably cheaper to build vertical cylindrical tanks with flat base. 

Experience has shown that a storage period of at least two weeks should be maintained the concentrate the opportunity to “ripen”, i.e. to build up its stability before this subjected to handling during transportation.  Practically all natural latex concentrate shipped overseas to the consuming countries; transportation to the port of exit is usually with cars or, if water transport is possible and available, by lighters.

QUALITY STANDARD
Unlike sheet and crepe rubber, latex concentrate is graded according to detailed technical specifications (as for block rubber).  Technical specifications are separately established by both the American Society for Testing Materials (ASTM) and the British Standards Institution (BSI).  National and international standards are substantially identical in their requirements.  Commercial latex is nowadays mainly prepared by centrifugation, with only two types made on a large scale:

1. HA (high-ammonia) latex, preserved with 0.7% ammonia
2. LA-TZ (low ammonia) latex, preserved with 0.2% ammonia + 0.025% TMTD/ ZnO + 0.04-0.05% lauric acid as ammonium laurate.

Posted by Teguh

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