Gel formation in natural rubber latex: 2. Effect of magnesium ion

The effect of Mg²⁺ 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 Mg²⁺ content after treatment with (NH₄)₂SO₄. The gel content of rubber from commercial HA-latex decreased significantly after (NH ₄)₂SO₄ treatment comparable to that of FL-latex. Long-storage commercial HA-latex containing 50% gel fraction showed no decrease in Mg²⁺ content after (NH₄)₂SO 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 Mg²⁺ content of rubber in long-storage commercial HA-latex, 0.005% (w/w rubber), decreased after treatment with (NH₄)₂SO₄, 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 (NH₄)₂SO₄ concentration. The gel content of FL- and HA-lattices increased with an increasing storage period in the presence and absence of (NH₄)₂SO₄. 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 Mg²⁺ ions. Whereas, the gel phase in long-storage commercial HA-latex is presumed to be a hard gel predominantly formed by covalent bonding.