Natural Rubber

Natural Rubber (NB)

            Many plants produce a milky sap, also referred to latex which is colloidal caoutchoouc dispersion in an aqueous medium. The latex-producing plants of which there are many known belong to different botanical families and they are predominantly found in tropical climates.

Not all caoutchouc-producing plants are harvested for industrial purposes because the yield is either too small the caoutchouc content in the latex too low, or the caoutchouc contains too many resinous impurities.

Early plantation economies used Ficus elastica, de Castilloa, Funtumia and Manihot plants, but they were soon displaced by the Hevea brasiliensis, because the latter gives a much greater yield of a superior caoutchouc.

Modern plantations the Hevea brasiliensis is cultivated. It is a tree up to 20 meters high, with a deep taproot and the first harvests can only be expected from trees which are at least six years old.

In modern plantations the preferred method of propagation is by various vegetative propagation of individual is referred to “clone”. Plants are created by bringing together a wood bud scion and a rootstock to combine in a tree a strong and disease-resistant root system, a stem with a tapresistant bark, and a canopy that is strong and well developed.

Another, albeit limited method of improving yield is the stimulation of latex flow by means of various chemical, such as chloroethyl phosphoric acid or aminotrichloropicolinic acid. After application, these chemical penetrate the bark and produce ethylene within the plant.

latex_tappedProducers

The largest producer of NR in Thailand with 1.5 million ha and about 0.6 million tons or 15% production share. In addition, there are Malaysia , Indonesia, India and China, Sri Lanka and numerous other Asiatic, Africal, and Amercan countries located in latitudes of about 100 on either side of the equator.

            Tapping of NR latex. In the Hevea, these capillary vessels are longitudinally continuous cells arranged as sheaths concentric with the outer bark. The majority of these vessels is found near the cambium in a 2 to 3 mm thick zone, and the diameter of individual vessels is about 20 to 50 um. If the vessels are severed by a cut in the bark of the tree, latex flows along the cut very slowly and it coagulates after 2 to 5 hours, due to evaporation, thus plugging the vessels to prevent further flow.

The most  used system of tapping is the “half spiral” method, which allows tapping every other day. Using gouges and , more recently, specially designed tapping knives, about 0.5 mm to 1-2 mm of cambium is removed. Expert tappers are also capable of making full spiral cuts aroung the complete tree trunk. This very difficult tapping method is called S1/d4 that mean a full spiral cut every fourth day.

            Coagulation, Processing of  the Coagulate, Sheets and Crepe. Method for recovering the caoutchouc is acid coagulation. The coagulating agent is mostly formic or acetic acid. By collecting the latex in large tanks, agood measure of crossblending of latex from trees of different ages and form different locations is achieved.

ribbed_smokeNomally, the latex is diluted with water to a solids content of up to 12 to 18%. The more dilutes the latex, the greater amount of acid is required for coagulation. The iso-electric point is reached at a pH of 5.1 to 4.8 under which conditions the latex coagulates.

The coagulum has to be processed immediately because it changes properties in air under the influence of bacteria.

The coagulation method for the various NR grades differs but little. There are two standard ways of processing the coagulum. First to drying by exposure to hot wood smoke to produce a smoked sheet and air dry to produced pale crepe.

            The special grades that have to specific requirements.

Initial Concentration Rubber (ICR) is produced from diluted latex.

Superier Processing Rubber (SP/PA Grades). Containing up to 50% of a crosslinked phase are lebelled SP caoutchouc, e.g. SP20 , SP40 with a 20% and 40% crosslinded blend component.

Oil Extended Natural Rubber (OE-NR).Contains 5 to 40 phr of either, naphthenic or aromatic oil.

Deproteinated Natural Rubber (DP-NR), also referred to as low nitrogen natural rubber(LN-NR)

Heveaplus MG Grades. These are graft copolymer of NR bound with antioxidants polymethylmethacrylate.

Expoxidized Natural rubber (ENR) is a new class of NR and it is available with a degree of epoxidation of 10 to 50% to prevents the ring opening reaction.

Thermoplastic NR is NR and polypropylene which has been crosslinked with peroxide.

Depolymerized NR is a liquid NR of syrup-like consistency. It was specially developed for liquid rubber processing technology.

Powdered or Particulate NR is obtained by grinding NR, but it is of no particular importance.

Peptized NR has proved itselt in the rubber insustry, because it is particularly pure, it can be processed very economically and it gives good vulcanizate properties.

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