Effect of Temperature and Humidity on the Formation of Dibutylurea in Benlate Fungicide

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Abstract
Benomyl [methyl 1-(butylcarbamoyl)-2-benzimidazolecarbamate] is the active ingredient in DuPont Benlate fungicides. The formation of N,N‘-dibutylurea (DBU), a phytotoxic degradation product of benomyl, in Benlate formulations was evaluated by analyzing Benlate samples maintained under simulated storage conditions and assessing the effects of temperature and humidity on sample moisture content, benomyl degradation, and the rate of DBU formation. Benomyl degraded during storage by the elimination of n-butylisocyanate (BIC) to form methyl 2-benzimidazole carbamate (MBC; carbendazim). Liberated BIC could then proceed to react with water to form DBU (first-order rate constant of 8.4 × 10-4 s -1). The degradation of benomyl and subsequent formation of DBU were dependent on the temperature and highly dependent on the humidity of the storage environment. At the lower humidity storage conditions the rates of DBU formation were significantly higher in the dry flowable (DF) formulation than in the wettable powder (WP) formulation. The initial moisture content of Benlate DF samples was higher than those of Benlate WP samples, although the Benlate WP samples absorbed more moisture upon incubation. These results may yield insight on the appearance of high levels of DBU found in some boxes and bags of Benlate DF and Benlate WP formulations. Keywords: Benlate; benomyl; fungicide; N,N-dibutylurea; formulation; carbendazim