Crystallinity and domain size of cured urea–formaldehyde resin adhesives with different formaldehyde/urea mole ratios

As a part of understanding hydrolytic degradation of cured UF resins responsible for formaldehyde emission, this study investigated the crystallinity and domain size of cured urea–formaldehyde (UF) resins with different formaldehyde/urea (F/U) mole ratios using wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) techniques. Both techniques were employed to calculate the crystallinity and domain size of cured UF resins as a function of F/U mole ratio. As the F/U mole ratio decreases, the crystallinity of cured UF resins increases from 26% to 48%. The SAXS provided useful information on the spatial arrangement of the crystalline domains and of their average distance. As the F/U mole ratio decreased from 1.6 to 1.0, the average distance between domains in cured UF resins decreased from 39 nm to 34 nm while the crystallite size increased from 1.3 nm to 5 nm. These results suggested that the crystallites were more closely packed with each other in cured UF resins when the F/U mole ratio decreased and the ordered domains were more randomly dispersed in high F/U mole ratio than low F/U mole ratio of UF resins. The greater amount of these ordered crystallites in cured UF resins of low F/U mole ratio was found to be correlated to the improved hydrolytic stability of the cured resins.

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► Both WAXD and SAXS showed crystalline characteristic of UF resins.
► The crystallinity ranged from 26% to 48%.
► The crystallite size increased from 1.3 to 5 nm as the F/U mole ratio decreased.
► The average distance between domains decreased from 39 to 34 nm.
► The crystallites were more closely packed in cured UF resins of lower F/U mole ratio.