Abstract Thermosetting urea–formaldehyde (UF) resins as the most common adhesives for wood-based composites emit formaldehyde, which forces producers to lower formaldehyde/urea (F/U) molar ratio for the UF resins synthesis. However, low-molar-ratio (below 1.0) UF resins have low formaldehyde emission at the expense of poor adhesion, which is responsible for the formation of crystalline domains as a result of hydrogen bonds between linear molecules. For the first time, this study reports the conversion of crystalline UF resins to amorphous polymers by blocking the hydrogen bonds, using transition metal ion-modified bentonite (TMI-BNT) nanoclay through in situ intercalation. The modified UF resins with 5% TMI-BNT showed an almost amorphous structure, faster curing and higher cross-linking density compared with those of neat resins, and resulted in 56.4% increase in the adhesion strength and 48.3% reduction in the formaldehyde emission. Thus, blocking hydrogen bonds in low F/U molar ratio UF resins with TMI-BNT converted crystalline UF resins to almost amorphous ones, resulting in a significant improvement in their adhesion with a low crystallinity.

中文翻译：

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使用改性纳米粘土将结晶热固性脲醛树脂转化为无定形聚合物

摘要 热固性脲醛 (UF) 树脂作为木基复合材料最常见的粘合剂会释放甲醛，这迫使生产商降低用于 UF 树脂合成的甲醛/脲 (F/U) 摩尔比。然而，低摩尔比（低于 1.0）UF 树脂的甲醛释放量较低，但附着力较差，这是由于线性分子之间的氢键导致形成结晶域的原因。本研究首次报道了通过原位嵌入过渡金属离子改性膨润土 (TMI-BNT) 纳米粘土，通过阻断氢键将结晶超滤树脂转化为无定形聚合物。与纯树脂相比，含有 5% TMI-BNT 的改性 UF 树脂显示出几乎无定形的结构、更快的固化和更高的交联密度，并导致粘合强度增加 56.4%，甲醛释放量减少 48.3%。因此，用 TMI-BNT 阻断低 F/U 摩尔比 UF 树脂中的氢键将结晶 UF 树脂转化为几乎无定形的树脂，从而显着改善其低结晶度的粘合力。