NR is present to the extent of about 94% in the non aqueous component of latex and the other main items are carbohydrates, proteins, sterols, lipids, carotenoids, etc. The non-rubber components present in latex have pronounced

effect on the reactions of NR E.g.: in the rate of vulcanization.

NR is structurally almost completely Cis 1,4 Polyisoprene the monomer joined in head to tail fashion. The polymer is also found to have chain branching resulting from the presence of small numbers of non-hydrocanbon groups distributed along the chains. Thus carbonyl, epoxy, lactone and amine groups have been reported to be present in the hydrocarbon chain. The phenomena known as ‘Storage hardening’ or NR studies have revealed the presence of a sequence of 3 trans units/chain in hevea rubber hydro-carbon. The likely structure of NR is given below.

Where X is H or COR, n 10000.

The presence of a double bond in BR makes the molecule reactive; addition to the double bond and substitution of the 7 active hydrogen atoms (one hydrogen atom viz., Vinylic hydrogen is not reactive) are possible. The presence of a - CH3 group in the monomer increases the electron availability at the double at the double bond thereby increasing its reactivity towards electrophilic reagents. The 3 alkyl groups help to stabilise the resulting positive ion or free radical derived from the olefin by hydrogen abstraction from the L - carbon atoms. Thus NR undergoes addition reactions with Chlorine, Bromine, Iodine Chloride, Hydrogen halides, Hypochlorus Acid, etc. Epoxidation of NR using reagents like Hydrogen peroxide and acetic acid has evinced considerable interest recently since epoxidised NR has been found to possess properties like oil resistance, lower air permeability than NR, etc. Presence of double bond also makes the NR susceptible to chain degradation. Thus during the processing of NR in a consumer’s factory, it is broken down in a mixing mill to lower molecular weight (say MW 4,00,000 to 10,00,000). Under more degradation, say when MW falls to less than 1,50,000 NR gets changed to Liquid Rubber.

Presence of double bond in NR also necessitates giving protection to degradation from the attack of oxygen, ozone, etc., by way of incorporation of compounding ingredients classified as Antidegradants.

Having described in brief the salient points which contribute to the use of NR in rubber products manufacture, let us try to trace out the development of the synthetic analogue of NR namely the synthetic Cis 1, 4 Polyisprene. Prior to 1954, all attempts to produce a replica or NR had ended up in failures because the emulsion polymerisation technique being followed then for the production of synthetic rubbers was not having precise control on the geometry of the polymer molecule produced. Karl Zeigler in Switzerland and Giulio Natta in Italy discovered during 1953/54 a novel means for producing stereo regular polymers using special catalysts and the dream of polymer chemists to synthesize NR came true when in December 1954 Goodrich Company applied the Zeigler / Natta catalysis for the discovery was that NR which was uniquely placed before had to complete with its synthetic Cis Polyisoprene (IR) - one having a Cis content of only 94% produced using Lithium and Lithium Alkyls catalysts and a high Cis variety with about 98% Cis 1, 4 made using Titanium Tetrachloride and Aluminium trialkyls as catalysts. In the case of Ti-IR, the remaining 2% is 3, 4 structure and in Li-IR, Cis 1, 4 is 90-92% trans 1, 4 is 2 - 3% and 3, 4 structure about 6 - 7%.

Let us now try to have a comparative assessment of the different properties of NR and IR. It is proposed to examine the raw rubber properties the processing characteristics and technological properties of vulcanisates of NR and IR. Conventional forms of NR like sheet and crepe are graded by means of visual standards assessing the colour, presence or absence of extraneous foreign matter (dirt), bubbles, blisters, etc., and within each form there are different grades (35 grades in total). Visual grading as it is subjective often leads to malpractices and trade disputes and the criteria for grading often do not have any relation to the technological properties of NR. In the case of IR, it is specified by technical parameters which have some bearing on the vulcanizate properties. In the latter half of 1960s, NR also began to be made available in technically specified form akin to IR. Compact bales of NR wrapped in dispersible thin polyethylene film facilitates mechanical handling and protection from external contamination.

Colour of the raw rubber is a very important factor for the selection for the manufacture of certain items. A polymer with light colour is always preferred in the production of light coloured rubber articles. A comparison of this quality of NR and IR reveals that the conventional grades of NR like smoked sheet and Estate Brown Crepe are dark coloured and hence not the first choice whereas, IR is ideal. However, Pale Latex Crepe (PLC) and light coloured crumb rubber from latex find easy acceptance. Again purity of the polymer is the key factor guiding selection of raw rubber for the manufacture of pharmaceutical rubber goods. Here also IR is easily accepted.

GRADING AND PACKING OF RUBBER