The effect of Mg 2+ ions on the gel formation in fresh and commercial high ammonia natural rubber latices (FL-latex and commercial HA-latex) was analyzed from the gel content and 2+ content after treatment with (NH 4 ) 2 SO 4 . The gel content of rubber from commercial HA-latex decreased significantly after (NH 4 ) 2 SO 4 treatment comparable to that of FL-latex. Long-storage commercial HA-latex containing 50% gel fraction showed no decrease in 2+ content after (NH 4 ) 2 SO 4 treatment. This gel fraction was not solubilized in toluene by the treatment of a proteolytic enzyme in latex or ethanol/toluene mixed solvent extraction of rubber. The 2+ content of rubber in long-storage commercial HA-latex, 0.005% (w/w rubber), decreased after treatment with (NH 4 ) 2 SO 4 , while the same treatment showed little change on FL-latex, 0.035%. The toluene soluble fraction of these latices showed a decrease in the M n value with an increase in the (NH 4 ) 2 SO 4 concentration. The gel content of FL- and HA-lattices increased with an increasing storage period in the presence and absence of (NH 4 ) 2 SO 4 . The initial rate of increase in the gel content was slow in the case of FL-latex. These findings indicate that the gel fraction in HA-latex is partly formed by ionic crosslinks caused by 2+ ions. Whereas, the gel phase in long-storage commercial HA-latex is presumed to be a hard gel predominantly formed by covalent bonding.
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Tarachiwin et al. (2003) studied this question.