Due to the condensation reaction of hydroxylated melamine molecules is possible to take place in the whole pH range, especially in acidic water medium or at high temperature, losing its water solubility. In this work, freeze drying perfectly removes water at pretty low temperature, avoiding condensation reaction of hydroxylated melamine. Therefore, a hydroxylated melamine in solid form (MOH-S) was prepared. It can be not only dissolved in water after storing for 180 days but also well dispersed in waterborne polyurethane emulsion. The addition of hydroxylated melamine significantly improves the tensile strength of polyurethane films by 170%. After soaking the films in water for 24 h, the tensile strength of WPU/30%MOH-S film lost only 6%, but 34% loss for the neat. Fourier transform infrared spectroscopy peak fitting demonstrates that the H-bonding degree of N─H in WPU/30%MOH-S film is 20.72 that is much higher than 3.33 in neat WPU. This indicates that H-bonding cross-linked structure is formed between polyurethane chains and hydroxylated melamine molecules. This work provides an effective strategy for preparing stable hydroxylated melamine and enhancing the strength of polyurethane film.

中文翻译：

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羟基化三聚氰胺H键交联提高聚氨酯薄膜强度

由于羟基化三聚氰胺分子的缩合反应有可能在整个pH范围内发生，特别是在酸性水介质或高温下，失去其水溶性。在这项工作中，冷冻干燥在相当低的温度下完美地去除了水分，避免了羟基化三聚氰胺的缩合反应。因此，制备了固体形式的羟基化三聚氰胺（MOH-S）。贮存180天后不仅能溶于水，而且能很好地分散在水性聚氨酯乳液中。羟基化三聚氰胺的加入使聚氨酯薄膜的拉伸强度显着提高了170%。将薄膜在水中浸泡 24 h 后，WPU/30%MOH-S 薄膜的拉伸强度仅损失 6%，而纯薄膜的拉伸强度损失 34%。傅里叶变换红外光谱峰拟合表明，WPU/30%MOH-S薄膜中N─H的H键合度为20.72，远高于纯WPU中的3.33。这表明聚氨酯链与羟基化三聚氰胺分子之间形成了氢键交联结构。该工作为制备稳定的羟基化三聚氰胺和提高聚氨酯薄膜的强度提供了有效的策略。