Gel formation in natural rubber latex: 1. Effect of (NH₄) ₂HPO₄ and TMTD/ZnO additives

The rubber from commercially obtained high-ammonia latex (commercial HA-latex) increased in gel content significantly after long preservation of the latex with 1.0% w/v tetramethylthiuram disulfide (TMTD) and zinc oxide (ZnO). Deproteinization of the HA-latex did not decrease the gel content. The gel fraction of deproteinized commercial HA-latex (DPHA-latex) was not solubilized by toluene containing 1.0% ethanol, showing that the gel fraction is composed of chemically crosslinked rubber. The addition of (NH₄) ₂HPO₄, which is usually added to fresh latex (FL-latex) to remove excess amounts of Mg²⁺ ions by centrifugation, decreased the gel formation in FL-latex preserved with 0.6 % v/v NH₄OH due to the removal of Mg²⁺ ions. The excess amounts of (NH₄) ₂HPO₄ accelerated the gel formation in preserved FL-latex and commercial HA-latex. The addition of 0.1% w/v TMTD/ZnO to preserved FL-latex treated with 5% w/v (NH₄)₂HPO₄ caused an increase of gel content during storage. The gel formation in the commercial HA-latex during long time storage was presumed to be caused by excess amounts of Mg²⁺ ions, TMTD/ZnO and (NH₄)₂HPO ₄. Here, TMTD/ZnO accelerated the gel formation at the initial stage of storage, while (NH₄)₂HPO₄ affected in long storage.