Natural Rubber Latex PDF

Natural Rubber Latex 1 Natural rubber has been known to mankind for many centuries. It is obtained form latex which is found in over thousand species of plants and trees. of all these species, The plant Hevea Brasiliensis the sole source of commercial rubber today, due to...

Natural Rubber Latex 1 Natural rubber has been known to mankind for many centuries. It is obtained form latex which is found in over thousand species of plants and trees. of all these species, The plant Hevea Brasiliensis the sole source of commercial rubber today, due to its high productivity and economics 2  Rubber tree (Hevea Brasiliensis) 3 Natural Rubber Latex Obtain by tapping the trunk of rubber tree Milky fluid with relative density between 0.97 – 0.99 It is a colloidal system of rubber particles 4 Rubber Particle Each rubber particle is surrounded by a protective layer of phospholipids and proteins phospholipid Alpha - lecithin protein Alpha - globulin Latex particles are predominantly spherical with diameter 0.05 – 5 μm 5 6 Rubber Particle Particles carry negative charges Repulsion Repulsive forces Colloidal Stability 7 8 Composition of Fresh Latex Depends on, Clone Age of tree Chemical nature of the soil Tapping frequency 9 Natural Rubber Latex Typical Composition of Fresh Latex 10 Fresh NR latex can only be kept fluid for a few hours. After tapping in the morning, one observes a thickening of the latex in the afternoon, and in the evening the whole mass is coagulated. This coagulation may begin sooner or later, depending on circumstances. Spontaneous coagulation of field latex 11 Spontaneous coagulation of field latex Two theories have been proposed to explain the phenomenon of spontaneous coagulation. 12 13 Spontaneous coagulation of field latex Fluid of Biological origin latex Ideal medium for growth of bacteria 14  Bacteria decays sugar Produce simple organic acids (e.g. -lactic acid) H+ 15 H+ in acids can neutralize negative charge on protein membrane Rubber particles can now close together Rubber particles colloid with one another Break the protein membrane Rubber molecule combine with one another and entangle Causing latex to coagulate 16 PRODUCED 17 18 carboxylic acid with a long aliphatic chain Second mechanism attributes the effect to the liberation of fatty-acid anions through the hydrolysis of various lipid substances present in the latex. Lipids are molecules that contain hydrocarbons and make up the building blocks of the structure and function of living cells. 19 Such anions are then thought to be adsorbed at the surfaces of the rubber particles, possibly partially displacing adsorbed proteins. They then interact with divalent metal ions (Mg++, Ca++) These ions are either present in the latex initially or else are gradually released from complexes by the action of enzymes 20 21 Observation relating to spontaneous coagulations 22  pH of NR latex is 6-7 The pH of the latex does not usually fall below 6.3 -6.0 during spontaneous Coagulation. (whereas the pH of fresh natural rubber latex has to be reduced to below 5 for coagulation to be affected by acidification) Furthermore, spontaneous coagulation is not prevented by maintaining the pH 7 by the addition of, say, dilute alkali. 23 The process of spontaneous coagulation can be accelerated by the addition of small amounts of carboxylate soaps. larger additions cause retardation of the process. If Ca++ & Mg++ ions are absent in the latex then unpreserved NR latex can be kept in fluid condition until putrefaction starts. The anaerobic decomposition of organic matter by bacteria and fungi 24 The proportion of ether-soluble substances in the rubber coagulum which is obtained by precipitation with alcohol decreases with time after tapping. Clearly, these observations accord with the second of the theories outlined above, rather than with the first. 25 Preservation of latex In fresh latex spontaneous coagulation occurs within 8-24 hrs Prevention of spontaneous coagulation called as preservation Any material which could use to preserve latex against spontaneous coagulation & putrefaction is called as preservative 26 Requirements of a preservatives primary requirement of a preservative is that it should effectively preserve the latex against spontaneous coagulation and putrefaction. 27  Basic Requirements It should destroy latex micro-organisms, or at least suppress their activity and growth in the latex. It should enhance the colloid stability of the latex increase the charge density of the particle surfaces increase the activity of surface bound materials increase the degree of hydration of particle surfaces It should de-activate trace metal-ions, especially those of multivalent metals 28 Ancillary requirements It should be harmless both to people and to rubber. It should not discolour the latex or films of rubber obtained It should not impart an offensive odour to the latex. It should not interfere with established latex processes It should be cheap and convenient to handle. 29  Anticoagulant used for preserving the field latex Ammonia Sodium sulphite also function as bactericides in latex Formalin (38% solution) Washing soda (powder) The choice of chemical preservation of latex largely depends on how the latex is to be processed in the next stage 30 Process in the next stage Choice of anticoagulant (on weight of latex) RSS (Ribbed smoked sheet) 0.01% Ammonia production Crepe production 0.05% Sodium sulphite Centrifuging of latex 0.75% Ammonia Creaming of latex 1.25% Ammonia RSS from latex (likely to putrefy) 0.05% Sodium sulphite + 0.02% formalin 31  Ammonia as a Preservative 19 th century, ammonia has come to be regarded as the standard preservative for natural rubber latex 0.2% ammonia - Short term preservation 0.7% ammonia - long term preservation 32 Ammonia Act as a bactericide Imparts alkalinity to the aqueous phase (increase in the initial pH of the latex has) increase the charge density of particle surfaces Colloidal stability 33 Ammonia Ammonia can de-activate some multivalent metal ions Mg2+ + NH3 + HPO42- MgNH4PO4 Some metal ions may be precipitated as hydroxides. 34 Ammonia and the ancillary requirements Ammonia is not especially harmful to people. (unless it is inhaled in excessive quantities, or unless a concentrated solution comes into contact with the skin) It appears to have no effect upon natural rubber. It imparts no colour to the latex 35 Disadvantages of Ammonia Concentrated aqueous solutions is not very convenient to handle. (special precautions have to be taken ) Impart a strong odour on latex NH3 interferes with some established latex processes 36 Volatile Fatty acid Content (VFA) The VFA is an indication of the degree of preservation of the latex VFA number is Latex is well preserved 0.01 – 0.03 If VFA number is higher Latex is not well preserved Will not be used for the concentration process 37