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 (NH 4 ) 2 HPO 4 , which is usually added to fresh latex (FL-latex) to remove excess amounts of Mg 2+ ions by centrifugation, decreased the gel formation in FL-latex preserved with 0.6 % v/v NH 4 OH due to the removal of Mg 2+ ions. The excess amounts of (NH 4 ) 2 HPO 4 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 (NH 4 ) 2 HPO 4 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 Mg 2+ ions, TMTD/ZnO and (NH 4 ) 2 HPO 4 . Here, TMTD/ZnO accelerated the gel formation at the initial stage of storage, while (NH 4 ) 2 HPO 4 affected in long storage.