Chapter 2 - Latex Concentrated (LC) Processing

2.0 Latex Concentrate (LC) Processing

• Latex concentrate production in Malaysia is about 25% of total natural rubber production and the export account for about 12.4%.

• An equal proportion is being consumed by local latex based industries, particularly glove manufacturers.

• The latex concentrate accounts for about 62.4% of the NR consumption the country.

• The increased consumption of latex concentrate in the local market has given rise to interest in the technical know how of latex concentrate production and the quality control measures of latex production.

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Latex must be exported as concentrate:

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2.2 THE METHODS OF CONCENTRATIONS

• Field latex from the tree is the raw material of the rubber industry. • The key manufacture of consistently high quality latex concentrate is careful process control and the maintenance of very clean equipment.

• The tree is tapped in the early morning and the field latex is collected and transferred to a large tanks. • Ammonia is added as a preservative, to inhibit the bacterial growth which would otherwise bring about spontaneous coagulation. • The amount of ammonia added at this stage is depends on the time between latex collection and concentration.

• Longer storage time requires larger quantities of ammonia. • The level used is normally between 0.1-0.6%.

The ammonia level can be adjusted according to whether high or low ammonia latex is being produced. • Before concentration of latex is carried out, the latex is tested for ammonia content, rubber content and volatile fatty acid (VFA) number.

• Rubber content should be between 25-35% by weight.

• The VFA is an indication of the degree of preservation of the latex.

• It measures the quantity of volatile fatty acids present in the latex. A VFA number of 0.010.03 in field latex means that latex is well preserved.

• A higher value of VFA number indicates that the latex is not well preserved and it will not be used for concentration process.

Tree : 30% DRC 70% Water

Concentration process

Latex conc: 60% DRC (contains much more rubber)

• Standard NR latex concentrates are categorized according to their methods of concentration and preservation. • Latex concentrates are commercially produced in Malaysia by the following process.

Latex Centrifuge Machine

A series of centrifuges machine

Field latex

Flow chart: Production of Latex Concentrate

Collection point

Bulking tanks

Centrifuge

Skim

Deammoniation tower

Concentrate latex Addition of NH3 / Secondary preservative

Blending tank

Skim recovery unit Skim coagulum

Storage tank Skim rubber Latex conc. export

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• Creaming and centrifuging methods involve the partial removal of non-rubber constituents relative to the rubber content. • The particle size distribution of the concentrate also differs from the initial latex (field latex) because of the proportion of the smaller particles is eliminated at the same time.

Centrifuging is regarded as a type of accelerated creaming process in which separation is affected by means of centrifugal force rather than a gravitational one.

• This type of latex concentrate possesses a high solid content (72% or more) and a high rubber content (68% or more). • Evaporated latex is used for impregnation, casting, adhesive and for preparing rubberized road surfaces.

“ When a man says I cannot, he has made a suggestion to himself. He has weakened his power of accomplishing that which otherwise would have been accomplished .

2.3 CONCENTRATION BY CREAMING PROCESS

1. Field latex is ammoniated at 0.7% w/w (weight/weight) on latex. Check latex during reception.

2. For high magnesium latex add phosphate in order to precipitate Mg out as sludge.

3. Addition of creaming agent (ammonium alginate).

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Master-batch Process Portion 1 a)Take out 12% of the latex from the bulk. b)Add ammonium alginate at 0.10% w/w on total crop DRC. Portion 2 a)Take out 6% of the latex from the bulk. b)Treated with a 70% ammonium oleate at 0.05% w/w on total crop.

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4. Add portion 1 and 2 to the bulk latex.

Stirred for 2 hours for complete mixing.

5. Bulk latex is then clarified (using centrifuge) or by filtering to remove phosphate sludge.

6. Ammonia level is again adjusted to 0.7% w/w on latex.

7. Bulk latex is heated by live steam to 40°C to optimise the creaming process.

8. Bulk latex is allowed to stand in the main creaming tank for 20 hours.

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9. After 20 hours, 40% of the bulk latex is drained off (skim latex, it contains 2% DRC). Carry out quality control test on skim latex (VFA and TSC).

10.Second day crop ( latex received on the second day). It is treated in the same way and pumped into the same creaming tank (step 1 to 8 are repeated) 25

11.The two crops are allowed to cream together.

12. 40% of the second day’s volume is removed after 20 hours of creaming.

13. This process is continued for 8 to 10 days until the creaming tank is full with 60% of the total crop over the period.

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Latex separation after 168 hour 27

14.The cream latex now contains of about 50% DRC. 15.The bulk is left undisturbed for 3 weeks to complete creaming.

16.After this remove, 30% of latex from this bulk. This skim latex contains about 4% DRC. 17.The original cream latex contains about 66-68% DRC. 28

18.Since the main creaming tank has a large diameter, a diffused fraction will occur.

60%

Cream 67% DRC Diffused Fraction

10% 30%

Skim 4%DRC

Fraction between the skim and concentrate latex. This diffused is run off separately into a narrow funnel bottom secondary creaming tank and it is allowed to cream for 2 weeks 29

19.After 2 weeks, 40% of the skim latex is removed.

The cream latex is added to the main bulk in the storage tank. Stirred for 1-2 days to ensure homogeneity.

20.The main bulk of the cream latex (latex concentrate) is treated as step 19.

21.Alternative method to process diffused fraction: Skim latex recovery • • •

Add ammonium alginate Creaming Coagulation & process into a pale crepe This is about 2-3% of the original crop

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Properties of creamed latex concentrate:    

Higher DRC (compare to centrifuge latex) Smaller rubber particle size TSC – DRC difference is slightly smaller KOH and VFA is better MST is very good (high)

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Advantages

Disadvantages

Simple and easy to operate

Slow operating process

Labour costs are low

Sensitivity to variations in field latex

Low operating cost & energy consumption

Inevitability after creaming during storage and transportation

Environment friendly Proportion of rubber lost in the skim is small

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2.4 CONCENTRATION BY EVAPORATION PROCESS

1. Latex is fed into the top of heated vertical tube by a side inlet.

2. A central rotor with blades throws the latex against the walls of the tube.

3. Latex spirals downwards, water will evaporate.

4. Vapour is drawn out of the top of the tube.

5. The clearance between the blades and the tube wall is only about 1mm, so that only a thin turbulent layer of latex is in contact with the wall.

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6. The evaporation time is no more than about 20sec, to reduce the possibility of degradation.

7. The concentrate from the bottom of the tube is preserved with potassium hydroxide.

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Advantages  High rubber and non rubber content

Disadvantages  The stability of the final latex is not good as centrifuged latex  Not suitable for manufacturing rubber articles which require less rubber content

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2.5 CONCENTRATION BY CENTRIFUGATION PROCESS

Centrifuge machine

Field latex

Flow chart: Production of Latex Concentrate

Collection point

Bulking tanks

Centrifuge

Skim

Deammoniation tower

Concentrate latex Addition of NH3 / Secondary preservative

Blending tank

Skim recovery unit Skim coagulum

Storage tank Skim rubber Latex conc. export

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Image of tapping cut, spout and cup on rubber tree

Image of collection point for natural rubber latex

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• Fresh latex exudes from the tree and becomes contaminated with bacteria. These bacteria grow rapidly on latex and eventually putrefy.

• Within a few hours after tapping, the latex flocculate and eventually form coagulum.

• This spontaneous coagulation of latex is undesirable and must be prevented.

• Thus, preservatives are normally added to field latex to inhibit the growth of bacteria and keep it fluid during processing, shipment and storage at the factory.

• Ammonia is the most cost effective preservative.

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• Preservation is carried out as early as possible. • Early preservation, inhibit bacterial growth at an early stage, hence giving lattices with low VFA number. • The importance of early preservation is showed in table 1 when a given level of ammonia is far more effective when applied early than a later time.

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% NH3 added (hrs after

VFA no. after 10 days

VFA no. after 50 days

1.0% (1h)

0.02

0.03

0.05% (1h) + 0.95% (9h)

0.03

0.07

0.05% (5h) + 0.95% (9h)

0.06

0.09

1.0% (9h)

0.06

0.13

tapping)

Table 1: Influence of time and addition of NH3 on VFA

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• Ammoniated field latex arrived at the factory

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• Field latex on arrival at the factory is sieved through a 80-mesh sieve into large reception tanks.

• A sample of the bulked latex is extracted from each tank for spot DRC determination prior to ammoniation.

• If the DRC of the latex is unsatisfactorily low (below 25%), the latex is rejected for concentrate production.

• If the DRC is suitable, ammonia gas is immediately added to increase the ammonia content of the latex to the above 0.4% • The ammoniated latex is then transferred to the settling tanks where it is kept overnight to allow settlement of the sludge.

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Image of reception of field latex at factory 50

• Sludge removal helps to increase the efficiency of the centrifugation.

• In case, where the latex is known to contain a high magnesium content, diammonium hydrogen phosphate is added to it to form magnesium ammonium phosphate which is settled with the natural sludge.

• The sludge is then settled and a sample of the latex bulk is taken prior to centrifugation.

• The VFA number of the latex sample is tested to ensure that the latex is well preserved.

• Well preserved latex should have a VFA number not greater than 0.05 (well preserved: 0.010.03%) and lattices with higher VFA number are not allowed for concentrate production.

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Field latex will pump into the reception tank

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Image of reception tank 53

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• A series of centrifuged machine

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• The basic principle employed in the centrifugation process is to break up a rotating mass of latex into a number of thin conical shells using a high speed rotating bowl containing a series of conical discs. • The resulting effect is a separation of the latex whereby the rubber-rich concentrate flows inwards towards the axis, and the denser rubber depleted portion or skims flow outwards from the axis of rotation.

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STAGES OF LATEX CONCENTRATION PROCESS

During centrifugation the following stages take place: •Removal of water from field latex DRC of 28 – 34% •Latex is concentrated to 60% DRC •Removal of 50 – 60% of VFA through serum •Removal of about 50% ammonia used as preservative in the field latex •A by product of DRC 4-10% known as skim latex is produced.

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Image of disc stack for centrifuge machine 59

• There are several types of centrifuges used in industry, the most commonly used model is the Laval Centrifuge.

• The Alfa-Laval centrifuge is a disc stack solids retaining concentrator.

• This means that the separated solids accumulated at the inside surfaces of the bowl and distributor walls, forming layers that are removed manually at intervals.

• A magnetic braking device reduces the time required to stop the bowl for cleaning.

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• There are only 3 adjustments which can be made on a centrifuge to vary its performance.

There are:

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SitiNor

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PROCESS OPERATION

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• The discharged concentrate requires no further sedimentation. The heavy sludge in the feed, already separated at the inside of distributor.

• Centrifuge performance is gauged by the bowl efficiency, which is defined as follows: Bowl efficiency : C ( F – S ) x 100 F(C–S) Where

C = Concentrate DRC F = Field Latex DRC S = Skim DRC

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• The efficiency of the centrifugation and the throughput act in opposite ways.

• The choice between achieving high efficiency and high throughput depends on economic considerations.

• High efficiency centrifugation requires a low throughput and a corresponding increase in number of machines and running costs, whereas low efficiency centrifugation need only a low capital outlay and minimal operating costs.

• In practice, a compromise operation is aimed at reasonably high throughputs and efficiencies.

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Concentrated latex outlet

Skim latex outlet

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Image of centrifuge machine layout at factory

Concentrated latex outlet

Skim latex outlet

Image of concentrated latex and skim latex outlet at centrifuge machine

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• Storage tank

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• The final stage of the concentration process is bulking and preservation.

• Immediately after the centrifuge, the cream passes via cream chute to the ammoniation tank.

• At the ammoniation tank, adjustments are made for the addition of low ammonia/high ammonia and lauric acid.

• These are necessary to give the required preservative level and also the mechanical stability in the final concentrate.

• While filling the latex into final storage tank, more than required percentage of ammonia is added initially and the amount of ammonia is reduced as the tank is filled.

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Type

Preservation details

HA

0.7% ammonia

MA

0.4 – 0.5% ammonia

LA-SPP

0.2% ammonia + 0.2% sodium pentachlorophenate

LA-BA

0.2% ammonia + 0.2% boric acid + 0.05% lauric acid

LA-ZDC

0.2% ammonia + 0.1% ZDC + 0.05% lauric acid

LA-TZ

0.2% ammonia + 0.013% TMTD + 0.013% ZnO + 0.05% lauric acid

PRESERVATION SYSTEMS FOR CENTRIFUGED CONCENTRATE 72

• The type of preservation used in general depends on the requirement of the consumer to which the latex is consigned.

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De-ammoniation tower

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• Skim rubber

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2.6

THE LATEX CONCENTRATE FACTORY

•The layout of the factory is designed as such to optimise the use of the gravity for the transfer of latex from one part of the factory to another. •For example, from the feed tank to machines, from machines to receiving or storage tanks and from storage tanks to drums or transportation tanks.

Tips to maintain the quality of latex concentate • Keeping a high standard of cleanliness of all collection vessels,

collection tanks, storage tanks and other utensils both in field and factory.

• Ammoniate latex to required level without delay. The normal level 0.3 – 0.7%.

• Latex should be stable free from pre coagulation.

• Field latex should have a VFA number between 0.01 – 0.03%.

• Should have sufficient capacity of storage (bulking) tanks to facilitate overnight desludging.

•Latices with higher levels of VFA number should not be used for concentrate production.

• DRC of field latex should not be less than 25%.

• Disinfection of tanks and pipelines with disinfectants such as formaldehyde, izal should be made compulsory soon after washing.

• The separator running time should be fixed so that the DRC of the concentrate is maintained at a reasonable level at the end of the run.

• Latex concentrate is ammoniated to required level.

• Latex concentrate is subjected to minimum handling and transfer after centrifugation

2.7

THE MODIFIED LATEX CONCENTRATES

• There are several ways of modifying a standard concentrate, each of which gives rise to a specialty latex. • Each of these specialty latices has certain characteristics, properties which are suitable for specific applications.

Double Centrifuged Latex • Double centrifuged latex is prepared by centrifugation diluting the concentrate to field latex DRC with ammonia, water and recentrifugation.

• The product has lower non-rubber content than a single centrifuged as reflected latex by the lower (TSC-DRC) value:TSC, %

DRC, %

TSC – DRC, %

Field latex

32.5

30

2.5

Centrifuged to 60% DRC

61.5

60

1.5

30% and centrifuged to 60% DRC

60.8

60

0.8

Prevulcanised Latex • Prevulcanised latex is prepared by heating natural rubber latex with vulcanising ingredients. This latex must be suitably treated with stabilisers such as potassium hydroxide/potassium laurate or potassium hydroxide/potassium caseinate. •Suitable formulation for the preparation of prevulcanised latex is as follows:Ingredients Dry weight 60% NR latex

100

50% Sulphur

1.0

50% ZDC

1.0

50% ZnO

0.2

20% KOH

0.4

10% potassium caeinate

0.2

• Heating is carried out at 70º C for 1½ - 2 hr. After prevulcanisation, the latex is clarified by passing through a centrifugal separator.

• This classification stage removes unreacted vulcanising ingredients, thus minimising post vulcanisation on storage.

• Consequently, change in viscosity and MST remains minimal.

• Pre-vulcanisation maybe carried out in the ammoniated field latex or ammoniated latex concentrate.

• Pre-vulcanised latex gives a film a better clarity and is used in the manufacturing of dipped and cast goods.

Freeze-thaw Stabilised Latex • When normal latex is frozen, the latex may become unstable.

• Thus, on thawing it may become more viscous and its mechanical stability decreased.

• In more severe and prolonged conditions of freezing the latex may gel.

• Freeze-thaw stabilised latex is prepared by adding 0.2% w/w on latex of sodium salicylate and 0.04 – 0.08% of lauric acid to normal latex concentrate.

• These chemicals impart stability to the latex, which on freezing and thawing, retains its fluid state.

• Freeze-thaw stabilised latex also has minimal viscosity and MST changes after each freezing and thawing cycle.

• It has apparent advantages over normal latex concentrate when used in temperate countries.

Methyl-methacrylate Grafted Latex • When methyl methacrylate is polymerised in natural rubber latex using a free radical catalyst, a substantial proportion of the polymethylmethacrylate (PMMA) formed becomes chemically grafted to the polystyrene chain.

• Two such commercially available graft polymers, Heveaplus MG30 and MG49, contains respectively 30% and 49% by weight of PMMA. • In latex form, both these latices are used in the manufacturing of adhesively for bonding dissimilar surfaces (e.g. PCV-NR) and as reinforcing agent in certain latex applications.

• These latices also impart better resistance to swelling by solvent and better tear strength in latex films.

Deprotenised Natural Rubber Latex • Deproteinised natural rubber latex (DPNR) is usually produced by the treatment of field latex with specific proteolytic enzymes in the presence of specific surfactant stabilizers followed by concentration by centrifugation.

Low viscosity (LCv) Latex • This concentrate is prepared from fresh centrifuged latex concentrate by treating it with 0.15% w/w on DRC of hydroxylamine neutral sulphate. • The addition of this chemical will stabilise the viscosity of the rubber in the latex.