Natural rubber latex, whether or not pre-vulcanised

40.01 - Natural rubber, balata, gutta-percha, guayule, chicle and similar natural gums, in primary forms or in plates, sheets or stnp. 4001.10 - Natural rubber latex, whether or not pre-vulcanised 400 1.21 400 1.22 4001.29 4001.30 - Natural rubber in other forms : - - Smoked sheets - - Technically specified natural rubber (TSNR) - - Other - Balata, gutta-percha, guayule, chicle and similar natural gums This heading includes : (A) Natural rubber latex (whether or not pre-vulcanised). Natural rubber latex is the liquid secreted rincipally by rubber trees and, in the species Hevea brasiliensis. This liqui consists of an aqueous solution mineral substances (proteins, fatty acids and their derivatives, salts, sugars containing in suspension 30 % to 40 % of rubber (i.e., polytsoprene of weight). 'f This part includes : (1) Stabilised or concentrated natural rubber latex. Rubber latex tmds to coagulate spontaneously a few hours after ta ping; it must therefore be stabilised in order to ensure preservation and to prevent putre action or coagulation. This is usually done by adding ammonia to latex in the proportion of 5 to 7 grams per litre of latex, which roduces a product known as " l l l ammonia" or FA type. A second stabilising met od which produces " low ammonia " or LA type is to add a very small quanti (1 to 2 grams per litre of latex) of a low concentration mixture of ammonia an substances such as tetramethylthiuramdisulphide and zinc oxide. P R There are also freeze-resistant natural rubber latexes stabilised by the addition, in particular, of minute quantities of sodium salicylate or formaldehyde and intended for use in cold countries. Natural rubber latexes are concentrated (mainly for transport purposes) by various methods (e.g., by centrifuging, evaporating, creaming). The rubber content of commercial latexes is usual1 between 60 % and 62 %; higher concentrations are also found and in some cases the so ~dcontent may exceed 70 %. f (2) Thermosensitive (heat-sensitive) natural rubber latexes. These are obtained by adding heat-sensitive agents. When heated these es of latexes gel faster than non-thermosensitive latex. They are generally use for the manufacture of dipped or moulded articles or for the production of foam or sponge rubber. (3) Electro~ositive latexes. These latexes are also known as "reversed electric charge latexes , because they are obtained by reversing the charge of the particles of a normal concentrated latex. Ths is usually achieved by adding cationic surface-active agents. The use of such latexes counters the tendency of most textile fibres to resist rubber impregnation (because like ordinary latex, the electrostatic charge of the fibres in an alkaline environment is negative). r (4) Pre-vulcanised natural rubber latex. This is obtained b the reaction of vulcanising agents on latex under heat-treatment at a temperature general y below 100 "C. The rubber particles contained in the latex are vulcanised by adding an excess of precipitated or colloidal sulphur, zinc oxide and accelerators (e.g., dithiocarbamate). The degree of vulcanisation of the finished product can be altered at will, by varyin the temperature, the heating time or the proportions of the in edients used. Normally, on y the outer walls of the rubber particles are vulcanised. In or er to avoid over-vulcanisation of the latex, excess ingredients are removed, by centrifugation, after heating. f r The appearance of pre-vulcanised latex is the same as that of normal latex. Their combined sulphur content is usually of the order of 1 %. The use of pre-vulcanised latex allows a number of operations (e.g., grinding, compoun@g) to be by-passed. It is used in the manufacture of dipped and cast articles (pharmaceutical and surgcal goods) and, increasingly, in the textile industry and as an adhesive. It is also used in the manufacture of certain grades of paper and of composition leather, and gives a rubber having excellent electrical insulation properties (because of its low content of proteins and soluble substances). Natural rubber latex is shipped either in internally coated drums (containing about 200 1) or in bulk. (B) Natural rubber in other forms. For the purpose of this traditionally shipped from the plantation factones natural rubber certain special improve the quality of the finished treated must not, however, must not contain by Note 5 (A). Coa lation of natural rubber latex takes place in tanks of various shapes, which may be fitterwith movable partitions. In order to se sate the rubber globules from the aqueous serum, the latex is coa lated b slight acidi cation with, for example, 1 % acetic acid or 0.5 % formic acid. At e end o the coagulation process the coagulum is removed either as slabs or as a continuous strip. C B R Subse uent treatment differs according to whether smoked sheets or ale or brown crepes, re-agg omerated granules or free-flowmg powders are being producecf ? (1) Rubber sheets and crepes. For the manufacture of sheets, the rubber strip is fed into a rolling mill in which the final set of embossed rolls leave the surface with characteristic markings (to facilitate drying by increasing the evaporation area). As the rubber strip about 3 to 4 rnm thick) emerges from the mill it is sliced into sheets. These are then p aced in either a drying shed or a smoke house. The purpose of smoking is to dry the rubber and to impregnate it with creosotic substances which serve as anti-oxidants and antiseptics. I For the manufacture of pale cre e, the rubber coa lurn is fed into a battery of cre ing machines. The frst machines ave grooved rol ers whereas the last machines gave smooth rollers, rotating at different speeds. This operation takes place under a constant stream of water so that the rubber is thoroughly washed. It is then b e d , at room temperature or in hot air, in a ventilated drylng shed. Two or more plies of crepe may be superimposed to form slabs of sole crepe. R P Sheets are also made by the following rocess : after the latex has been coagulated in c lindrical tanks, the coagulum is slicex by sawing, into a long strip which a cut hto s eets and dried (generally without smoking). x d Some types of rubber (in particular crepe other than pale crepe are not manufactured direct1 by the coagulation of latex, but are produce by the subsequent re-agg omeration and washing in " cre ing machines ' of the coa la obtained during tappmg or factory processing. The resu ting sheets, of varying thicYnesses, am dried in the same way as pale crepe. r S Natural rubber as described above is usually marketed according to its a pearance in the forms and grades corresponding to international standards set up by t ie concerned international organisations. The most common types are smoked sheets and cuttin s thereof, pale crepes and cuttings thereof, brown crepes and ribbed and air drie sheets. (2) Technically specified natural rubber (TSNR). This is dry raw natural rubber which has been processed, tested and graded into five general grades (5L,5, 10, 20 and 50) according to the specifications in the following table : Table : Grade of TSNR and maximum allowable limits for each parameter GRADE PARAMETER Dirt retained on 325 mesh (rnax. % wt.) Ash content (max. % wt.) 0.60 0.60 0.75 1.00 Nitrogen content (rnax. % wt.) 0.70 0.70 0.70 0.70 0.70 Volatile matter (rnax. % wt.) Wallace rapid plasticity - min. initial value (Po) Plasticity retention index, PRI (min. %) 1.00 1.00 1.00 1.00 1.00 Colour limit (Lovibond scale, max.) 6.00 - - - - 1.50 TSNR must be accompanied by a test certificate, issued b the competent authorities of the producin country, specifying the grade, speci?f cations and test results of the rubber. Eertain producing countries may have ades with more stringent s ecifications than those stated in the table above. T NR is packed in bales of 1/3 kg and wrapped in polyeth lene. Normall 30 or 36 such bales are either palletised and covered by polyethy ene liners on Xe inside or shrink-wra ped with polyethylene. Each bale or pallet has specific markings to show the gra e, weight, producer code, etc. & g r B (3) Re-agglomerated rubber granules. r The techniques used for processing granulated rubber are desi ed to give cleaner products with constant properties and a better appearance than rub er sheets or crepes. The manufacturing process comprises granulation of the coagulum, particularly thorough cleaning, drylng and compression into bales. A wide range of machines may be used for granulation, such as rotary blade choppers, cross hammer mills, pelleting machines and creping machines. The urely mechanical action of these may be reinforced by the adhtion of very smal quantities (0.2 to 0.7 %) of castor oil, zinc stearate or other crumbling agents, incorporated in the latex before coa ation. These crumbling agents have no effect on the subsequent use or properties of e rubber. f The granules are dried in semi-continuous trolley-type dryers, conveyor belt tunnel dryers or extruder-dryers. The dried anules are finally compressed, under high pressure, into parallel ipedal bales wei$ng fi-om 32 to 36 kg. Re-agglomerated rubber granules are usua y sold with guaranteed technical specifications. "R (4) Free-flowing powders of natural rubber. These are prepared as indicated in paragraph (3) above but without compression. In order to prevent the anules from re-agglomerating under the action of their own weight, they are blende during preparation with powdered inert substances such as talc or other anti-tack agents. f Rubber powders can also be obtained by injecting into the drying chambers, with the latex, an inert substance, such as siliceous earth, expressly to prevent agglomeration of the particles. (5) Special types of natural rubber. Various special types of natural rubber can be obtained in the forms described in (1) to (4) above. The principal types are : (a) CV (constant viscosity) rubber and LV (low viscosity) rubber. CV rubber is obtained by adding a very small quantity (0.15 %) of hydroxylamine before coagulation and LV rubber by adding, also before coagulation, a small quantity of mineral oil. The h droxylamine prevents the spontaneous increase in the viscosity of the natura rubber during storage. Use of these rubbers enables manufacturers to forecast mastication periods. (b) Peptised rubber. This is obtained by adding to the latex, before coagulation, a proximately 0,5 % of a peptising agent, which reduces the viscosity of the rub er during the drying operation. This rubber accordingly requires a shorter period of mastication. E (c) Superior processing rubber. This is obtained either b coagulatin a mixture of ordinary and pre-vulcanised latex or by mixing nawa latex coagu urn with pre-vulcanised latex coagulum; its use makes extrusion and calendering easier. T f (d) Purified rubber. This is obtained without addition of foreign substances, by a variation of the normal process of rubber production, for example, by centrifuging the latex. It is used in the re aration of chlorinated rubber and in the mufacture of certain wlcanised goo& glecttric cables, etc.) whose pro erties would suffer from the presence of the impurities normally contained in rub er. E (e) Skim rubber This is obtained by coagulating the by-product of latex skim. (f) Anticrystallising rubber. This is obtained by adding thiobenzoic acid to latex before coagulation; it thus becomes fieeze-reststant. (C) Balata. Balata m, or balata, is extracted from the latex of certain plants of the Sapotaceae family, especi ly from the bullet-tree (Manilkara bidentata) found mainly in Braz~l. f' Balata is a reddish roduct, most1 shipped in blocks weighing up to 50 kg, but sometimes also in sheets from to 6 mm thic . E It is mainly used for the manufacture of conveyor or transmission belts or beltin . It is also used, mixed with gutta-percha, in the manufacture of submarine cables and of go f balls. f (D) Gutta-percha. Gutta-percha is extracted from the latex of various trees of the Sapotaceae family (e.g., of the genus Palaquium and the genus Payena). It is yellow or yellow-reddish. Accordin to its origin, it is shipped either in cakes weighing 0.5 to 3 kg, or in blocks of 25 to 2 kg. t In addition to its uses, when mixed with balata, in the manufacture of submarine cables, golf balls and belting, it is also used for making sealing rings for pumps or valves, flax spinning rollers, linings for tanks, bottles for hydrofluoric acid, adhestves, etc. (E) Guayule gum. This is extracted from the latex of the P~rtheniumargentatum, a shrub originating in Mexico. Guayule rubber is generally shipped in cakes or sheets. (I? Chicle ) gum. This is extracted from the latex contained in the bark of certain trees of the Sapotaceae farnily grown in the tropical regions of America. This p is reddish, and is generally shipped in cakes of various sizes or in blocks weighmg about 10 kg. It is mainly used for the manufacture of chewing-gum. It is also used for the manufacture of certain surgical tapes and of dental goods. (G) Similar natural gums, for example,jelutong. In order to be classified in this heading, these gums must be rubber-like in character. (H) Intermixtures of any of the foregoing products. This heading excludes (a) Intermixtures of any product of this heading with any product of heading 40.02 (heading 40.02). s, compounded, (b) Natural rubber, balata, gutta-percha, guayule, chicle and similar natural before or after coagulation, with substances forbidden by Note 5 ( ) to this Chapter (heading 40.05 or 40.06). h