NIPPON GOMU KYOKAISHI

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Several “synthetic rubber grade” magnesium oxides produced and commercially available in Japan were compared with “CR Grade” magnesium oxides produced abroad and imported into Japan.
Japanese magnesium oxides were classified by the manufactures as “light” or “light calcined” magnesium oxides, but, their higher bulk density suggested that most of them do not belong to the “light calcined” category of magnesium oxides.
Except for one brand, iodine absorption tests indicated that Japanese “synthetic rubber grade” magnesium oxides tested belong to the low activity grades. In Neoprene GNA compounds, the one exception, classified as high activity grade, gave good scorch resistance and a high state of cure based on modulus, while all other Japanese magnesium oxides tested gave less processing safety and bin storage stability according to their Mooney scorch time values.
The combination of 8phr of the low activity magnesium oxide and 5phr of Zine oxide gave less scorch resistance than the combination of 4phr of high activity magnesium oxide and 5phr of Zine oxide.
The combination of 16phr of low activity magnesium oxide and only 1phr of Zine oxide gave the same scorch resistance as that of 4phr of high activity magnesium oxide and 5phr of Zine oxide.

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Except for one brand, all Japanese magnesium oxides tested proved to be of low activity, notwithstanding their classification as “light calcined”. (Report No. 1)
Tests on Neoprene type W compounds showed that:
1) In gum stock compounds accelerated with 2-mencapto-imidazoline, Japanese low activities magnesium oxides. gave somewhat less scorch resistance than the high activity magnesium oxides. However, in lightly loaded compounds with carbon black, hard clay or whiting, the differences in magnesium oxide activity was of little significance.
2) In lightly loaded carbon black, clay or whiting compounds accelerated with 2-mercapto-imidazoline and tetra-methyl-thiuram-disulfide, the scorch resistance and state of cure based on modulus given by Japanese magnesium oxides were poorer, but the differences were not as great as in the case of Neoprene GNA compounds.
3) In lightly loading carbon black, clay or whiting compounds accelerated with tetra-methyl-thiuram-monosulfide, di-o-tolylguanidine and sulfur, the scorch resistance and states of cure based on modulus given by Japanese magnesium oxides were much lower.

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