The addition of limited amount of urea to resin to the resin solution just before use, either directly or in a hardener is a well-known and often recommended way of reducing the smell of formaldehyde. But it is not easy to determine quantitatively what type of product this contributes, whether resin or insoluble methylene urea compound; it depends on the molar ratio, the pH on addition, the rate at which the pH falls during curing, and the temperature.
It will be noticed that the term “water tolerant” is used. This expression, for reasons that are given later, is often more accurate and therefore is preferable to “water-solubility.” Here, the term is used to apply to the behaviour of the uncured resin, and should not be confused with “water-resistance,” a term that is applied to the cured adhesive.
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