RRIM training manual on natural rubber processing

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 this
refresher course on NR processing and comment on some areas where greater
care needs to be exercised.
Although the SMR scheme was introduced 17 years age, it was only in the
last year that the volume of SMR exported exceeded that of the conventional
sheets. Over 40% of Malaysian rubber is still sold as sheets and crepes. It is
therefore appropriate that this course covers the processing not only of SMR
but 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, size
reduction into crumbs and drying. These various steps are shown
schematically in Figure 1. The different techniques involved in the
production of different grades of NR are considered in detail in this course.

FIGURE 1. SCHEMATIC REPRESENTATION OF NR PROCESSING
Those in charge of NR processing factories should also have a knowledge
of the various types of machines used in processing, the care and
maintenance of these machines, the packaging of rubber and quality and
inventory 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 important
subject 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 the
development of the Malaysian rubber industry. It marked the most significant
changes in the processing and presentation of NR since the beginning of the
rubber industry in the country. The production of NR to technical
specifications and the improved packaging and presentation helped to put NR
in a more equitable position in its competition with SBR, the general purpose
synthetic rubber.
The success of the SMR scheme was made possible by developments of
new processes for converting latex and field coagulum into dry rubber. The
most notable of these were processes for converting rubber into crumbs and
the drying of rubber at the higher temperature of 100°C. The new processes
require that the relevant factory personnel understand the characteristics of
the raw materials and the effect of the different processing techniques and
conditions on rubber properties.
It would be useful to highlight some of the differences between the
conventional and new processes.

(i) Preservation
SMR processing machinery are expensive and this requires that the
processing factories are centralised for economies of scale. Thus typically,
SMR processing factories have production capacities of 10 to 50 tonnes per
day compared to the one to two tonnes per day RSS factories. Latex has
therefore to be transported over a greater distance. In order to prevent
precoagulation, the tendency is therefore to add a relatively higher level of
preservative, which is usually ammonia. This in turn requires a higher level
of formic acid to effect complete coagulation. Higher ammoniation may also
cause slower drying. Two other preservative systems containing low levels of
ammonia, viz. ammonia-hydroxylamine and ammonia-boric acid, have been
developed. These have been used only with varying success and have not
been widely used.


(ii) Coagulation
In producing RSS or crepe rubber, the processing parameters are more
uniform. Formic acid is virtually the only acid used; the d.r.c. of coagulation
and the amount of acid used are standardised. In SMR production the
coagulation conditions tend to be more variable. Although formic acid is the
recommended acid, sulphuric acid is not always excluded. The d.r.c. of
coagulation is not uniform; the amount of acid used can differ considerably
from batch to batch. Different chemicals are used to produce the different
grades of SMR. For example, SMR CV made by the Heveacrumb process
uses 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. For
example, the level of ammoniation, the pH of coagulation and the maturation
time of the coagulation affect the viscosity, modulus and cure behaviour of

rubber. Varying the processing conditions even a little at random may affect
the consistency in properties. It is therefore of utmost importance that those
involved in SMR production understand the influence of chemicals and
processing conditions on the properties of rubber.


(iii) Conversion of coagulum into crumbs
Conventional rubber processing machinery consists of crepers and sheeting
batteries. In SMR production, a wide range of machines may be used,
differing in design, function and performance. For example, size reduction of
the coagulum may be carried out in a crumbier, granulator, creperhammermill, extruder, shredder or prebreaker. Though the merits and defects
of these machines have been known through experience over a number of
years, there is still no general consensus on the best set of machines for SMR
production. The great diversity of machinery also means that the SMR
factory must keep a larger number and variety of spares and use a core
complicated maintenance schedule.


(iv) Drying
Unlike the conventional grades, the conditions of drying of SMR vary
greatly. Though meet dryers used for SMR production use the same basic
principle, 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 or
underdrying unless the conditions are strictly monitored. It is also necessary
to ensure that the dryers undergo cleaning and general maintenance at regular
intervals 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, rubber
processing has been converted into a science from an art. A proper
understanding of rubber processing requires an appreciation of the distinct
characteristics of the different raw materials and the influence of processing
techniques, processing conditions and the type of equipment used on the
grades of rubber produced and their properties. The main objective of this
course is to disseminate this knowledge to personnel in the factory
management.


The Growth of SMR
Initially the volume of SMR exported increased rapidly as the consumers
were enthusiastic about the technical quality and the improved presentation of
SMR. The growth was rapid in the SMR 10 and 20 grades but slower in the
latex grades such as SMR L and CV, possibly due to their relatively high
prices. However, the growth rate of SMR dropped sharply towards the end of
the 1970s (Table 1). It was then believed that this was due to the shortage of
field coagulum since all the available material was being converted to SMR
10 or 20.
TABLE 1. GROWTH OF SMR
Year

% increase over previous year

1970

62

1973

30

1976

20

1979

4

1980

-3

1981

12

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 this
increase, about 58,000 tonnes were due to SMR 10 and 20. In 1981, these
two grades constituted about 72% of total SMR exported and about 30% of
total NR production. It is difficult to attribute the real reasons for the sharp
increase and only time will tell whether 1981 marked a new trend in the
growth of SMR.


SMR GP
SMR GP was introduced about four years ago in order to stimulate the

growth of SMR and at the same time to convert more of the smallholder
rubber into SMR. It is prepared from a blend of 60% of latex-derived
material (latex and unsmoked sheets) and 40% of field coagulum. It is
viscosity stabilised at 60-70 Mooney units and sold to SMR 10 specifications.
The consumers have generally appreciated the quality of SMR GP and the
savings it will yield in the mixing costs but have been unwilling to pay the
extra premium needed for its production. Whether SMR GP would grow to
become the biggest volume SMR as envisaged can only be seen when the
present recession in the industrialised countries is over.


Prospects for the Future
Natural rubber is being sold today at price below that of SBR. The buffer
stock 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 machinery
continue to increase, following the trend of the 1970s. The cost of chemicals
has also generally increased though some chemicals have become cheaper in
1982 due to reduced demand and stable oil prices (Table 2). These together
with the rising wages have substantially increased the cost of rubber
production. 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. The
position is actually worse than it looks because the fuel prices are artificially
low, being heavily subsidised by the Government.
TABLE 2. COST OF CHEMICALS
1975 1980 1982
$
$
$
Ammonia - per kg

1.25 1.51 2.65

Formic acid - per kg

1.54 1.93 1.76

Hydroxylamine - per kg 4.40 4.60 3.75
Fuel - per litre

0.22

30 0.50

When business conditions improve in the industrialised countries and tyre
companies reopen their plants, it is probable that they would demand a more
uniform rubber with consistent properties and produced to tighter
specifications. These demands can be met only if the processing techniques
and conditions are standardised and strictly adhered to within each factory.
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 transported
over short distances within the estate or small-holding itself and rarely to
distant estates or group processing centres for processing. Low levels of

preservatives such as ammonia, sodium sulphite and less frequently
formaldehyde are adequate for the purpose of keeping the latex stable.
In the production of the new block SMR rubbers, however, the trend
towards greater centralised processing has resulted in the transport of latex
over 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 field
latex adequately to ensure trouble-free processing.
The use of ammonia for preserving latex, however, has disadvantages: the
ammonia-preserved latex requires more acid for coagulation; when used at
high levels, the ammonia can impart a dark brown colour on the rubber and it
may also extend the drying time.
This paper describes two practical composite preservative systems which
are economically competitive with ammonia but which also have certain
advantages over the ammonia system. These composite systems involve the
use of either hydroxylamine neutral sulphate or boric acid with ammonia.
The first system is recommended for the production of viscosity-stabilised
rubbers, 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 effective
preservative for latex and a strong alkali. Therefore, when hydroxylamine salt
is used in combination with ammonia, a more effective preservative system
could 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% wton-rubber level, which is also the level recommended for viscosity
stabilisation of the rubber.
TABLE 1. CRITICAL LEVEL OF HYDROXYLAMINE NS

Preservative system
Hydroxylamine NS % wt
d.r.c.

Preservation (h)

Ammonia % wt on
latex

1a

2b

3c Mean

0

0

3.5 1.5 2.5

2.5

0

0.05

5.5 5.5 4.5

5.2

0.05

0.05

5.5 6.5 5.5

5.8

0.10

0.05

8.5 11.5 10.5

6.8

0.15

0.05

16.5 20.5 19.5 18.8

Experiment was carried out with:
(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-ammonia
system 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
d.r.c.

Ammonia % wt on
latex

0

0

3.5 1.5 2.5

0

0.05

5.5 5.5 4.5

5.2

0.15

0.02

9.5 11.5 7.5

9.9

0

0.10

14.5 17.5 18.5 16.8

0.15

0.05

16.5 20.5 19.5 18.8

0

0.15

33.5 28.5 27.5 29.8

0.15

0.08

31.5 28.5 35.5 31.8

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

1a

2b

3c Mean
2.5


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.


Economics
The composite system reduces the level of ammonia usage by half and,
consequently, requires 60% less acid during coagulation. For long periods of
preservation (> 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 to
crumbs which often take longer to dry. By reducing the level of
ammoniation, the composite system produces crumbs which dry normally in
four to five days.
Tabel 3. Effectiveness of the hydroxylamine-ammonia system in field trials

Preservative system

Duration of preservation (h)

Hydroxylamine NS Ammonia %
% wt on d.r.c.
wt on latex

Estate latex

Smallholder
latex
5
(Upto 4
p.m.)
5 - 11 (4 10 p.m.)

A

0.15

0.03

11
(Upto 10 p.m.)

B

0.15

0.05

11 - 19 (10 - 6 a.m.)

C

0.15

0.07

19 - 30
11 - 20
(noon to early evening on (10 p.m. - 7
the following day)
a.m.)

Table 4. Economic advantages of the hydroxylamine-ammonia system
Price per
kg

Chemical

Hydroxylamine
sulphate (HNS)a

neutral

Ammonia
system
Levelb

HNS/Ammonia
system

CostcCostcb
Level
¢/kg
¢/kg

$4.60

0.15%

0.69

0.15%

0.69

Antonia

$2.31

0.14%

1.08

0.07%

0.54

Formic 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).


Recommendations
Hydroxylamine-ammonia composite system is specially recommended for
use in the production of viscosity-stabilised (CV and LV) rubber. It should
not be used in the production of SMR L and other types of rubber.
Hydroxylamine NS with ammonia has been found very effective for
preserving estate and smallholder latex. The effect of hydroxylamine NS on
latex preservation is most marked at the level of 0.15% wt-on-rubber. For a
given period of preservation, the level of ammonia required in the
hydroxylamine NS-ammonia system is half that required in the conventional
ammonia 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 SYSTEMS
Preservative systems
Hydroxylamine NS with
ammonia
% wt on
rubber

Duration of preservation (h)

% wt on
latex

Estate latex

Smallholder
latex
5
(Upto 4
p.m.
5-11
(4-10 p.m.)

A

0.15

0.03

11
(Upto 10 p.m.)

B

0.15

0.05

11-19
(10-6 a.m.)

C

0.15

0.07

19-30 (noon to early evening on the
11-20
following day)
(10-7 p.m.)

The preservative should be added to the latex in the field collection
stations.
The hydroxylamine NS and ammonia should be contained in one stock
solution. This is more convenient to use and reduces the chance of error. The
stock 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 the
average field d.r.c. of the latex source or the bulk d.r.c., if latex from more
than one source is bulked. When used at this level, there is no need for further
addition of hydroxylamine in the factory. The storage hardening properties of
the resulting rubber are identical to those of CV rubber, prepared in the
normal 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 of
field latex. It is, however, not as economically efficient as ammonia for long
term preservation. Its principal advantage is that unlike ammonia, it does not
discolour the rubber even when used at high levels.

Current experiments suggest that a composite system of boric acid and
ammonia, 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 are
particularly 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 of
latex which was particularly prone to precoagulation was carried out. The
preservation 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
Duration of preservation
(h)

Preservative system
Boric acid % w/w on
latex

Ammonia % w/w on
latex

Estate
latex

Smallholder
latex

0.2

0.03

16 (12)

11 - 15

0.3

0.02

18

14 - 18


0.3

0.05

-

18 - 19

0.3

0.07

32 - 60

29 - 40

0.4

0.03

-

20 - 26

0.4

0.05

-

24 - 30

0.5

0.03

34 - 44

20 - 27

0.5

0.05

39 - 43

20 - 43

-

0.15

44 - 56

43 - 50

Figure in parenthesis refers to a large commercial trial.