Presently, commercial NR latex concentrate is preserved with either a high ammonia system (HA latex) at 0.7% ammonia or LA-TZ a low ammonia system (LA latex) at 0.2% ammonia) and 0.025% of 1:1 mixture of tetramethylthiuram disulphide (TMTD) and zinc oxide.
The HA system poses serious environmental problem such as strong smell due to the presence of large amount of ammonia. On the other hand, TMTD has been implicated to produce carcinogenic nitrosamines in rubber products and is an allergen causing contact dermatitis (Type IV allergy). Furthermore, zinc is considered toxic especially in food-contact applications and is a source of heavy metal in waste sludge and effluent water.
A synthetic carboxylate soap (Struktol LB 219) has been shown to be a suitable replacement of TMTD, but zinc oxide is still needed. Variants of LA-TZ with ultra-low ammonia content (0.05-0.1%) in the presence potassium hydroxide (0.10%) have also been developed e.g. ULA-ZN and KLA-ZN. However, LA-LB219, ULA-ZN and KLA-ZN. Other alternative systems such as Dowicil 150 (water soluble, zinc oxide free), LA-PRBL (water-soluble, zinc oxide free and LA-BZ (BZ-dibenyldithiocarbamate) had been investigated in the past, but these were not readily acceptable for commercial production so far. Systems based on Dowicil 150, LA-TZ and LA-BZ were found to be unsuitable for block rubber production due to the adverse effects on CV (constant viscosity) properties, PRI (Plasticity Retention Index) and colour.
There is, therefore, an urgent need to develop a better preservative system to replace those currently used as they can eventually become unsuitable due to the constraints of future regulatory requirements in the latex industry. Among the major considerations in the development of the preservative system for natural rubber latex is the necessity of complying with more stringent standards of environmental control in view of increasing world concern over the problem of environmental pollution,...
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