TYPES OF NATURAL RUBBER.
Natural Rubber (NR) is produced from the milky white liquid
called latex harvested from rubber trees by the process
of tapping. Latex is a dispersion of rubber in water. The
latex that flows out from the rubber tree on tapping is
channeled into an attached container mainly polythene cups.
Around 80% of the crop from rubber plantation is in the
form of latex.
The Latex which gets dried up on the tapping panel (tree
lace) and the collection cups (shell scrap) also form part
of the crop and are called field coagulum. It is collected
by the tapper in a basket just before tapping. About 20
% of the total crop from rubber plantation is in the form
of field coagulum.
Latex and field coagulum are highly susceptible to degradation
due to bacterial contamination on keeping. Hence they are
processed into different marketable forms of natural rubber.
Marketable forms of Natural Rubber .
The most important forms of Natural Rubber marketed are
Sheet Rubber (both RSS and ADS), Technically Specified Block
Rubber (TSR), Crepe Rubber, Preserved Latex Concentrates,
and Specialty forms of rubber.
Among these, the first three are in dry form and almost
90% of the total Natural Rubber produced in the world is
at present marketed in these types. In the international
market, these three broad types are available in over 40
grades mainly consisting of Ribbed Smoked Sheets (RSS) and
Technically Specified Rubber (TSR) in block form.
The fourth type - Preserved Latex Concentrates - is produced
in all important rubber-growing countries and about 10%
of world production of Natural Rubber is processed and marketed
in this type. Besides these 4 types, there are a number
of Speciality Natural Rubbers which include Natural Rubber
in liquid form as well as in chemically modified forms and
in blends and composites.
The
details of each type of Natural Rubber with the grades involved
and the grade description are given below:
1.
Sheet Rubber
Ribbed
Smoked Sheets(RSS) are marketed based on visual assessment
of quality. To establish acceptable grades for commercial
purposes, the International Rubber Quality and Packing Conference
has stipulated the grade requirements and the details are
given in the Green Book published by the Rubber Manufactures
Association (RMA) inc., Washington D C, United
States of America. More...
2.
Technically Specified Rubber (TSR).
Natural
rubber in dry marketable forms were traded in earlier days
based on visual assessment of quality only. This was a limitation
for RSS and Crepe rubber with the end users. Hence efforts
were made to grade natural rubber based on certain technical
properties which could provide guarantees to the consumers
of a certain minimum quality. The International Standards
Organization (ISO) developed the first draft standards in
1964 in its meeting held in India
for the grading and marketing of Block Rubber or Technically
Specified Rubber (TSR). More...
3.
Crepe Rubbers
When
coagulated latex or any form of field coagulum like tree
lace, shell scrap is passed several times through creping
mills with heavy rolls, a crinkly lace-like rubber will
be obtained. This when air dried is called crepe rubber.
There are different types of crepe rubber called Pre-coagulated
crepe, Sole crepe (Pre-coagulated thin crepe), Pale latex
crepe (PLC), Estate brown crepe, Remilled crepe, Smoked
blanket crepe, Flat bark crepe etc.
4.
Latex Concentrates
Fresh
Latex collected from rubber plantation contains only 30
to 40 % dry rubber content, balance being mainly water.
Fresh latex, because of the high water content and presence
of non rubber solids, it is susceptible to bacterial attack
and hence a perishable material.
It can not be used directly for manufacturing any rubber
products. Therefore a proper system for preservation
and concentration was developed. Now different technologies
are adopted for producing stable latex concentrates with
suitable latex preservatives. Different types of latex
concentrates with a dry rubber content of up to 60 % are
produced either by centrifuging or creaming.
The
following are the important types of latex concentrates.
i)
Creamed
Concentrates
– Ammonium alginate or tamarind seed powder solution
is added to ammonia preserved field latex. The rubber
containing particles form clusters at the top separating
the serum fraction below. Creamed concentrates are
ideal for thread rubber production, form production etc.
It has relatively high viscosity and low ZST. Creamed
concentrates are available in HA or LA types.
ii)
Centrifuged
Latex Concentrates
- The process of production of centrifuged latex concentrate
involves the separation of preserved latex into two fractions,
using a high speed centrifuge, one containing the concentrated
latex and the other containing serum. Both HA and LA types
are available. Centrifuged latex concentrates accounts for
over 90% of world latex concentrates production.
| ISI specifications
for Centrifuged Concentrated Latex |
| |
Type
H A
|
Type
L A
|
| i)
Dry rubber content, percent by mass, Min
|
60.00 |
60.00 |
|
ii) Non-rubber
solids, percent by mass, Max |
2.00 |
2.00 |
|
iii) Coagulum
content, percent by mass of latex, Max
|
0.05 |
0.05 |
|
iv) Sludge content,
percent by mass, Max |
0.10 |
0.10 |
|
v) Alkalinity
as ammonia, percent by mass of latex
|
0.6 (Min) |
0.3 (Max) |
|
vi) KOH number,
Max |
1.0 |
1.0 |
|
vii) Mechanical stability, s, Min |
475 |
475 |
|
viii) Volatile fatty acid number, Max |
0.15 |
0.15
|
|
ix) Copper content, ppm
of total solids, Max |
8 |
8 |
|
x) Manganese
content, ppm of total solids,
Max |
8 |
8 |
iii)
Skim
rubber
– The serum separated during centrifuging of
latex contains 3 to 8 % of dry rubber. This is the
skim rubber. It is a low quality product which contains
many of the non rubbers.
5
Specialty forms of natural rubber
With
the development of synthetic rubbers, a complex balance
of technical properties and cost emerged as the decisive
factor in the selection of usage of rubber. To meet
this challenge, different special forms of natural rubber
have been evolved to make it suitable for certain specific
applications. Some of them are oil extended natural
rubber (OENR), Thermoplastic natural rubber (TPNR), De proteinised
natural rubber (DPNR), Superior
processing rubbers, Graft co-polymer rubbers etc.
