RRIM training manual on natural rubber processing

PRESERVATIVE SYSTEMS FOR FIELD LATEXNg Chiew Sum Rubber Research Institute of Malaysia In the preparation of conventional RSS, field latex is normally transportedover short distances wit

RRIM TRAINING MANUAL ON

NATURAL RUBBER PROCESSING

Rubber Research Institute of Malaysia

eBook created (04/01/‘16): QuocSan

SOME ASPECTS OF NATURAL RUBBER PROCESSING

A Subramaniam

Rubber Research Institute of Malaysia

In this introductory lecture I shall touch on the scope and content of thisrefresher course on NR processing and comment on some areas where greatercare needs to be exercised

Although the SMR scheme was introduced 17 years age, it was only in thelast year that the volume of SMR exported exceeded that of the conventionalsheets Over 40% of Malaysian rubber is still sold as sheets and crepes It istherefore appropriate that this course covers the processing not only of SMRbut also of conventional grades as well as latex concentrate

Stapes In NR Processing

In the processing of latex into dry rubber, the basic steps involved are:(i) preservation of latex,

(ii) coagulation,

(iii) conversion of coagulum into sheets, crepes or crumbs,

and (iv) drying

For field coagulum, processing involves cleaning and blending, sizereduction into crumbs and drying These various steps are shownschematically in Figure 1 The different techniques involved in theproduction of different grades of NR are considered in detail in this course

FIGURE 1 SCHEMATIC REPRESENTATION OF NR PROCESSINGThose in charge of NR processing factories should also have a knowledge

of the various types of machines used in processing, the care andmaintenance of these machines, the packaging of rubber and quality andinventory control All these topics are covered in the lectures that follow.Finally, there is the problem of effluent discharge from rubber factories,whether these be washings from the centrifuge bowl, serum from coagulation

of field latex or skim or the discharge from remiller factories This importantsubject is discussed in the final part of this course

SMR and New Processes

The introduction of the SMR scheme in 1965 was a milestone in thedevelopment of the Malaysian rubber industry It marked the most significantchanges in the processing and presentation of NR since the beginning of therubber industry in the country The production of NR to technicalspecifications and the improved packaging and presentation helped to put NR

in a more equitable position in its competition with SBR, the general purposesynthetic rubber

The success of the SMR scheme was made possible by developments ofnew processes for converting latex and field coagulum into dry rubber Themost notable of these were processes for converting rubber into crumbs andthe drying of rubber at the higher temperature of 100°C The new processesrequire that the relevant factory personnel understand the characteristics ofthe raw materials and the effect of the different processing techniques andconditions on rubber properties

It would be useful to highlight some of the differences between theconventional and new processes

(i) Preservation

SMR processing machinery are expensive and this requires that theprocessing factories are centralised for economies of scale Thus typically,SMR processing factories have production capacities of 10 to 50 tonnes perday compared to the one to two tonnes per day RSS factories Latex hastherefore to be transported over a greater distance In order to preventprecoagulation, the tendency is therefore to add a relatively higher level ofpreservative, which is usually ammonia This in turn requires a higher level

of formic acid to effect complete coagulation Higher ammoniation may alsocause slower drying Two other preservative systems containing low levels ofammonia, viz ammonia-hydroxylamine and ammonia-boric acid, have beendeveloped These have been used only with varying success and have notbeen widely used

(ii) Coagulation

In producing RSS or crepe rubber, the processing parameters are moreuniform Formic acid is virtually the only acid used; the d.r.c of coagulationand the amount of acid used are standardised In SMR production thecoagulation conditions tend to be more variable Although formic acid is therecommended acid, sulphuric acid is not always excluded The d.r.c ofcoagulation is not uniform; the amount of acid used can differ considerablyfrom batch to batch Different chemicals are used to produce the differentgrades of SMR For example, SMR CV made by the Heveacrumb processuses hydroxylamine neutral sulphate, castor oil and emulsifier; SMR 20 may

be dipped in phosphoric acid to improve its PRI

The coagulation and processing conditions affect the rubber properties Forexample, the level of ammoniation, the pH of coagulation and the maturationtime of the coagulation affect the viscosity, modulus and cure behaviour ofrubber Varying the processing conditions even a little at random may affectthe consistency in properties It is therefore of utmost importance that thoseinvolved in SMR production understand the influence of chemicals andprocessing conditions on the properties of rubber

(iii) Conversion of coagulum into crumbs

Conventional rubber processing machinery consists of crepers and sheetingbatteries In SMR production, a wide range of machines may be used,differing in design, function and performance For example, size reduction ofthe coagulum may be carried out in a crumbier, granulator, creper-hammermill, extruder, shredder or prebreaker Though the merits and defects

of these machines have been known through experience over a number ofyears, there is still no general consensus on the best set of machines for SMRproduction The great diversity of machinery also means that the SMRfactory must keep a larger number and variety of spares and use a corecomplicated maintenance schedule

(iv) Drying

Unlike the conventional grades, the conditions of drying of SMR varygreatly Though meet dryers used for SMR production use the same basicprinciple, i.e through-air circulation drying at 100°C, they differ in design,mechanical construction and efficiency While the recommended temperature

of drying is 100°C, higher temperatures are used in many factories to speed

up the drying process This may cause problems of overdrying orunderdrying unless the conditions are strictly monitored It is also necessary

to ensure that the dryers undergo cleaning and general maintenance at regularintervals in order to prevent contamination of the rubber by soot and rust

Science of NR Processing

It can be said that with the introduction of the new process rubbers, rubberprocessing has been converted into a science from an art A properunderstanding of rubber processing requires an appreciation of the distinctcharacteristics of the different raw materials and the influence of processingtechniques, processing conditions and the type of equipment used on thegrades of rubber produced and their properties The main objective of thiscourse is to disseminate this knowledge to personnel in the factorymanagement

The Growth of SMR

Initially the volume of SMR exported increased rapidly as the consumerswere enthusiastic about the technical quality and the improved presentation ofSMR The growth was rapid in the SMR 10 and 20 grades but slower in thelatex grades such as SMR L and CV, possibly due to their relatively highprices However, the growth rate of SMR dropped sharply towards the end ofthe 1970s (Table 1) It was then believed that this was due to the shortage offield coagulum since all the available material was being converted to SMR

10 or 20

TABLE 1 GROWTH OF SMRYear % increase over previous year

However, 1981 has turned out to be a surprise with a substantial increase

of about 68,000 tonnes in the export of SMR over the previous year Of thisincrease, about 58,000 tonnes were due to SMR 10 and 20 In 1981, thesetwo grades constituted about 72% of total SMR exported and about 30% oftotal NR production It is difficult to attribute the real reasons for the sharpincrease and only time will tell whether 1981 marked a new trend in thegrowth of SMR

SMR GP

SMR GP was introduced about four years ago in order to stimulate thegrowth of SMR and at the same time to convert more of the smallholderrubber into SMR It is prepared from a blend of 60% of latex-derivedmaterial (latex and unsmoked sheets) and 40% of field coagulum It isviscosity stabilised at 60-70 Mooney units and sold to SMR 10 specifications.The consumers have generally appreciated the quality of SMR GP and thesavings it will yield in the mixing costs but have been unwilling to pay theextra premium needed for its production Whether SMR GP would grow tobecome the biggest volume SMR as envisaged can only be seen when thepresent recession in the industrialised countries is over

Prospects for the Future

Natural rubber is being sold today at price below that of SBR The bufferstock operations which concentrate only on certain grades have not been able

to boost NR prices

On the other hand, the costs of factory buildings and processing machinerycontinue to increase, following the trend of the 1970s The cost of chemicalshas also generally increased though some chemicals have become cheaper in

1982 due to reduced demand and stable oil prices (Table 2) These togetherwith the rising wages have substantially increased the cost of rubberproduction It is said that, at the present prices of NR, only those estates with

a large proportion of high yielding clones continue to make a profit Theposition is actually worse than it looks because the fuel prices are artificiallylow, being heavily subsidised by the Government

TABLE 2 COST OF CHEMICALS

At the same time, it is necessary to critically review the processing methods

so as to reduce the costs of production, e.g by introducing more automation

in rubber processing

PRESERVATIVE SYSTEMS FOR FIELD LATEX

Ng Chiew Sum

Rubber Research Institute of Malaysia

In the preparation of conventional RSS, field latex is normally transportedover short distances within the estate or small-holding itself and rarely todistant estates or group processing centres for processing Low levels ofpreservatives such as ammonia, sodium sulphite and less frequentlyformaldehyde are adequate for the purpose of keeping the latex stable

In the production of the new block SMR rubbers, however, the trendtowards greater centralised processing has resulted in the transport of latexover longer distances In these circumstances, ammonia has established itself

as the most effective and widely used preservative of field latex High levels

of ammonia (0.05 - 0.15% wt on latex) are often required to preserve fieldlatex adequately to ensure trouble-free processing

The use of ammonia for preserving latex, however, has disadvantages: theammonia-preserved latex requires more acid for coagulation; when used athigh levels, the ammonia can impart a dark brown colour on the rubber and itmay also extend the drying time

This paper describes two practical composite preservative systems whichare economically competitive with ammonia but which also have certainadvantages over the ammonia system These composite systems involve theuse of either hydroxylamine neutral sulphate or boric acid with ammonia.The first system is recommended for the production of viscosity-stabilisedrubbers, SMR CV and LV; the second for light coloured rubbers, SMR L

HYDROXYLAMINE – AMMONIA COMPOSITE SYSTEM

Although hydroxylamine is a bactericide, it does not preserve field latex.This is because of its acidity Ammonia, on the other hand, is an effectivepreservative for latex and a strong alkali Therefore, when hydroxylamine salt

is used in combination with ammonia, a more effective preservative systemcould be expected (The cheaper hydroxylamine neutral sulphate (NS) is

preferred) Table 1 shows that this, in fact, is the case The most dramatic

increase in preservation time is when hydroxylamine NS is at the 0.15% on-rubber level, which is also the level recommended for viscositystabilisation of the rubber

wt-TABLE 1 CRITICAL LEVEL OF HYDROXYLAMINE NS

Preservative system Preservation (h)Hydroxylamine NS % wt

(a) late tapping latex;

(b) latex from trees rested for one tapping cycle;

(c) latex from normal tapping

As hydroxylamine and ammonia both inhibit the proliferation of bacteria

in latex, the composite system out-performs the conventional ammonia

system Data presented in Table 2 shows that for a given period of

preservation, the level of ammonia required in the hydroxylamine-ammoniasystem is half that required in the conventional ammonia system

TABLE 2 COMPARATIVE EFFECTIVENESS OF THE

HYDROXYLAMINE/AMMONIA AND THE CONVENTIONAL

AMMONIA SYSTEMS

Preservative system Preservation time (h)Hydroxylamine NS % wt

0 0.15 33.5 28.5 27.5 29.80.15 0.08 31.5 28.5 35.5 31.8

Footnotes: a, b, c are same as in Table 1.

Field Trials

The effectiveness of the composite system was demonstrated in field trials

As expected, the preservative is less effective with small-holder latex than

with estate latex (Table 3) Nevertheless, it is much more superior to the

conventional ammonia system

The composite system reduces the level of ammonia usage by half and,consequently, requires 60% less acid during coagulation For long periods ofpreservation (> 20 h) the estimated cost saving can exceed 4 ¢/kg (rubber)

(Table 4).

Latex preserved for long periods with high levels of ammonia gives rise tocrumbs which often take longer to dry By reducing the level ofammoniation, the composite system produces crumbs which dry normally infour to five days

Tabel 3 Effectiveness of the hydroxylamine-ammonia system in field trials

Preservative system Duration of preservation (h)

(Upto 10 p.m.)

5(Upto 4p.m.)

B 0.15 0.05 11 - 19 (10 - 6 a.m.) 5 11 (4

-10 p.m.)

19 - 30(noon to early evening onthe following day)

11 - 20(10 p.m - 7a.m.)Table 4 Economic advantages of the hydroxylamine-ammonia system

Chemical Price per

kg

Ammoniasystem

HNS/Ammoniasystem

sulphate (HNS)a $4.60 0.15% 0.69 0.15% 0.69Antonia $2.31 0.14% 1.08 0.07% 0.54Formic acid $1.79 3.00% 5.37 0.63% 1.13

Total 7.14 2.36

a For the preparation of SMR CV, the same amount of hydroxylamine will

be required

b The levels of hydroxylamine NS and formic acid are based on the weight

of dry rubber and that of ammonia on the weight of latex of 30% d.r.c

c Based on ¢/kg (rubber)

Hydroxylamine-ammonia composite system is specially recommended foruse in the production of viscosity-stabilised (CV and LV) rubber It shouldnot be used in the production of SMR L and other types of rubber

Hydroxylamine NS with ammonia has been found very effective forpreserving estate and smallholder latex The effect of hydroxylamine NS onlatex preservation is most marked at the level of 0.15% wt-on-rubber For agiven period of preservation, the level of ammonia required in thehydroxylamine NS-ammonia system is half that required in the conventionalammonia system

The three recommended preservative systems (A, B and C) are shown in

Table 5 In most situations, two preservative systems should suffice; one for

short-term, and the other for long-term preservation

TABLE 5 RECOMMENDED HYDROXYLAMINE-AMMONIA

PRESERVATIVE SYSTEMSPreservative systems

Smallholderlatex

(Upto 10 p.m.)

5(Upto 4p.m

(10-6 a.m.)

5-11(4-10 p.m.)

C 0.15 0.07 19-30 (noon to early evening on the

following day)

11-20(10-7 p.m.)The preservative should be added to the latex in the field collectionstations

The hydroxylamine NS and ammonia should be contained in one stocksolution This is more convenient to use and reduces the chance of error Thestock solution should be prepared fresh on the day before use It can,however, be kept for at least three months without losing its effectiveness

The level of hydroxylamine in the latex should be 0.15% wt on theaverage field d.r.c of the latex source or the bulk d.r.c., if latex from morethan one source is bulked When used at this level, there is no need for furtheraddition of hydroxylamine in the factory The storage hardening properties ofthe resulting rubber are identical to those of CV rubber, prepared in thenormal way.

Details of stock solution preparation and dosage are given in the Appendix.

BORIC ACID-AMMONIA COMPOSITE SYSTEM

Boric acid is presently used in a limited way for short term preservation offield latex It is, however, not as economically efficient as ammonia for longterm preservation Its principal advantage is that unlike ammonia, it does notdiscolour the rubber even when used at high levels

Current experiments suggest that a composite system of boric acid andammonia, while being very effective for the preservation of field latex, does

not impair the light colour qualities of the resultant rubber Figure 1 shows

the results of small scale trials At low levels of boric acid, i.e below 0.2%

wt on latex, the effect of increasing ammonia content is not marked.Combinations employing 0.4 to 0.5% boric acid with 0.07% wt ammonia areparticularly effective and are equivalent to the conventional ammonia system

at 0.15% wt on latex

Field Trials

Figure 1 Effectiveness of the Boric acid – Ammonia System

The effectiveness of the composite system was demonstrated in field trials

using gallon quantities of latex which were representative of the bulk Table 6

summarises the field data A large commercial trial involving 8,000 litres oflatex which was particularly prone to precoagulation was carried out Thepreservation time of 12 hours for the boric acid (0.2% wt) - ammonia (0.03%

wt) system confirms the data presented in Table 6.

Table 6 Effectiveness of the boric acid-ammonia system in field trials

Preservative system Duration of preservation

(h)Boric acid % w/w on

latex

Ammonia % w/w on

latex

Estatelatex

Smallholderlatex

Raw Rubber Properties

Block rubbers were prepared from latices preserved with the boric ammonia system and the conventional ammonia system of equivalent

acid-effectiveness Data presented in Table 7 shows that the colour of the rubber

derived from the composite system is considerably lighter and meets theSMR 5L specifications

Table 7 Raw rubber properties

Preservative system (% wt on latex) Dirt % wt P0 PRI MOD

value ColourAmmonia 0.2% 0.004 60 83 6.0 9

system (Table 8) The other main advantage of the composite system is that it

enables the production of light coloured rubber which is not possible withlatex preserved for long periods at high ammonia levels

Table 8 Economic advantages of the boric acid-ammonia system

Chemical Price/kg

Ammonia system BA/Ammonia systemLevela Cost (¢/kg)b Levela Cost (¢/kg)bBoric acid (BA) $1.70 - - 0.50% 2.83

The boric acid-ammonia composite system is recommended for use in theproduction of light-coloured (SMR L) rubbers, especially when the latexmust be kept fluid for a long time

The three recommended preservative systems are given in the following

Table 9.

Table 9 Recommended boric acid-ammonia preservative systems

Preservative system Duration of preservation

(h)Boric acid % w/w on

latex

Ammonia % w/w on

latex

Estatelatex

Smallholderlatex

solution (kg or lb)

System B Stocksolution (kg or lb)

System C stocksolution (kg or lb)

2) The volume (litre or gallon) of stock solution to be added to 100 litres or

100 gallons of latex is given along the two ‘dosage’.

3) The three preservative systems (A, B and C) are described in the text

Composite systems consisting of two chemicals have proved to be moreeconomically efficient and technically better than the conventional ammoniasystem for the preservation of field latex The hydroxylamine neutralsulphate-ammonia composite system is more economical to use than theconventional ammonia system

It is now recommended for the preservation of latex intended for theproduction of SMR CV and SMR LV rubbers The boric acid-ammoniasystem has the advantage of being able to produce a light colour rubber toSMR L standards in situations where the high level of preservative in theconventional ammonia system would impair the colour of rubber

HYDROXYLAMINE-AMMONIA SYSTEM

Preparation of Stock Solution

The stock solution can be prepared by dissolving the calculation amount ofhydroxylamine NS (Formula II) in water While keeping the receptacle cool,the required amount of ammonia gas (Formula I) is bubbled into thehydroxylamine solution To allow for evaporation losses, slightly more thanthe calculated quantity of ammonia should be used

It is important to decide on the level of ammonia that is to be used in thecomposite system before preparing the stock solution

If the intended level of ammonia preservation exceeds 0 05% wt on latex,the stock solution should contain 5% wt of ammonia; otherwise, a 3% wtstock solution would be suitable These levels ensure that the preservativewill not excessively dilute the latex

The strength of the hydroxylamine NS in the stock solution depends on theaverage d.r.c of the particular source or bulk of field latex, and on the level

of ammonia preservation

Ammonia requirement The weight of ammonia gas required in the stock

solution is given by:

Sa = strength of ammonia in stock solution (%)

Hydroxylamine requirement The weight of hydroxylamine required in the

stock solution is given by:

d.r.c = average of field latex source or bulk (%)

W = weight of water (kg or lb)

Sa = strength of ammonia in stock solution (% wt)

La = desired level of ammonia preservation (wt on latex)

For a given set of conditions, Sa, La and d.r.c are known Formulae I and

II can be reduced to very single forms where Wa and Wh are dependent only

on W

Dosage When the stock solution prepared has been prepared as described

in the preceding section, it can be used as if it contained only ammonia Theamount of stock solution required is given by:

Formula III

V =La × VI / Sa

where

V = volume of stock solution (lit or gal)

La = desired level of ammonia preservation (% wt on latex)

VI = volume of latex (lit or gal)

Sa = strength of ammonia in stock solution (% wt)

When the dosage of stock solution is determined, the level ofhydroxylamine in the latex will be sufficient to stabilise the viscosity of therubber – i.e at 0.15% wt on rubber

The level of ammonia (La) used in Formulae II an III must be the same;otherwise, the preserved latex will contain too little or too muchhydroxylamine

The required quantities of ammonia and hydroxylamine NS to make up

100 lit (10 gal) stock solution add the dosage of stock solution for every 100

lit (100 gal) of latex are indicated in Table 11.

TABLE 11 PREPARATION AND DOSAGE OF STOCK SOLUTION

kg orlb

kg orlb

kg orlb

kg orlb

kg orlb

kg orlb

kg orlb

kg orlb

kg orlb

1) The respective weights (kg or lb) of ammonia gas and hydroxylamine

NS required to prepare a 100 lit or 10 gal stock solution are given undercolumns ‘NH3’ and ‘HNS’

2) The volume (litre or gallon) of stock solution to be added to 100 lit (100gal) of latex is given under column ‘dosage’

3) The three preservative systems (A, B and C) are described in text

RIBBED SMOKED SHEET (RSS)

P S Rama Rao

Rubber Research Institute of Malaysia

General Considerations

The quality of the final produce would depend upon the various amenitiesavailable in the factory and the subsequent treatment given

The most important single factor in the manufacture of good quality RSS is

to ensure an acceptable degree of cleanliness at all stages of preparation Aplentiful supply of clean water is required for the dilution of the latex and forwashing the rubber during the sheeting process

Latex should be collected as soon as possible without any undue delayeither from the field or elsewhere It should be transported to the factory soonafter collection with least delay This is to avoid any premature coagulation

of latex The use of anticoagulants can be resorted to if the latex is to betransported from outlying divisions/areas

Sodium Sulphite is the most commonly used anti-coagulant The dosagemay vary depending upon the individual requirements However, as a guide,

it should not exceed 0.05% on dry rubber basis

Factory Layout

The layout of the coagulation, smoking and packing facilities suitable forprocessing 1,000 kg per day of RSS is illustrated in the Appendix 1 Thecoagulation and packing areas may be extended under certain circumstancesdepending upon the requirements

Coagulation and Milling Facilities

The coagulating tanks are paired and brought up to the chute as shown inthe drawing

The deeper 18” coagulating tank is preferred as it can hold more rubber perdollar of capital investment and helps reduce handling in the factory to aminimum Seven tanks are sufficient for a 1,000 kg/day crop Whencalculating factory capacity, allow for the peak crop which will be about ⅓more than the average crop

Tanks with 90-partition plates are preferred to those with 75-partitionplates as the former produce thinner coagulum and hence thinner sheetswhich dry rapidly A higher DRC of coagulation can be used in such tanks,thus reducing handling costs The standard aluminium tanks cost about

$4,100 each and are available at Diethelms & Co., Petaling Jaya Thestandard wooden with inner aluminium lining tanks cost about half the price.Continuous sheet coagulation is preferred to separate sheet coagulation Acontinuous coagulum offers several advantages, namely, less handling, higherthroughput per hour, less folded edges, etc

Separate sheet coagulating tanks can be converted to continuous sheettanks by cutting or folding over part of one end of each partition plate Thecut or folded section should leave a clearance of ¾” to 1” between thepartition plate and tank side depending on the number of partition plates Thecut or folded section should commence at about 3” from the top When theplates are inserted in the coagulating tank, the section should enable the ends

of adjacent coagula to be joined A schematic diagram of the modified plate

as in Appendix 2 No difficulty in conversion is encountered with thealuminium tanks For wooden tanks lined with aluminium it is necessary toavoid ‘catches’ (2” thick) on top of both sides of the tank to hold the partitionplates steady and ensure even spacing Guides at the base of the tank areessential for continuous sheeting The coagulation shall preferably be carriedout at 12.5% d.r.c and at 4.5 - 4.7 pH

To facilitate the cutting of continuous sheets, a rotating aluminium/woodencutting drum is necessary

iv) + Lee Woh Engineering Works,

55 Jalan Kilang, Malacca 350v) + Tal Sing Engineering Works,

74 Jalan Kilang, Malacca 350vi) + Tan Hock Seng Engineering Factory, 9, Jalan

Ismail, Muar, Johore 350

* The Huttenbach is specially designed for continuous sheeting althoughthis company can offer several modified machines at differing costs Theabsence of overall guard makes operation of this type of battery easier thanthe very close roll types when feeding continuous coagulum

+ No brand name available

Bush bearings tend to wear during normal operation and produce excessiveclearance between the marker rolls Ball or roller bearings are moresatisfactory and should always be used on the marker rolls

Two men are required to operate each of the above-mentioned sheetingbatteries Any other type which requires more than two men to operate is noteconomical

Plans of the RRIM 1100, 2200, 3300 and 4400 type smokehouse, furnace,truck, etc are obtainable from the Rubber Research Institute of Malaysia onrequest The rated capacity of the RRIM 2200 type smokehouse is 1,000 kgrubber per day This smokehouse can be extended to accommodate smallincreases in crop (i.e up to 1,200 kg/ day)

The location of the smokehouse, relative to the factory, can be changeddepending on the site Site prone to flooding should be avoided