Influence of Hydrolytic Degradation on the Morphology of Cured Urea-Formaldehyde Resins of Different Formaldehyde/Urea Mole Ratios

Abstract

In an effort to understand the hydrolytic degradation process of cured urea-formaldehyde (UF) resins responsible for the formaldehyde emission of wood-based composite panels, this study analyzed the influence of acid hydrolysis on the morphology of cured UF resins with different formaldehyde/urea (F/U) mole ratios such as 1.6, 1.4, 1.2 and 1.0. Field emission-scanning electron microscopy (FE-SEM) was employed to observe both exterior and fracture surfaces on thin films of cured UF resins before and after the etching with hydrochloric acid as a simulation of the hydrolytic degradation process. FE-SEM images showed that the exterior surface of cured UF resin with the F/U mole ratio of 1.0 had spherical structures after the acid hydrolysis while the other cured UF resins were not the case. However, the fracture surface observation showed that all the samples possessed spherical structures in the cured state of UF resins although their occurrence and size decreased as the F/U mole ratio increased. For the first time, we found the spherical structures in cured UF resins of higher F/U mole ratio of 1.4. After the acid hydrolysis, the spherical structures became a much predominant at the fracture surface. These results indicated that the spherical structures in cured UF resinswere much more resistant to the hydrolytic degradation by the acid than amorphous region.

Microstructure of Cured Urea-Formaldehyde Resins Modified by Rubber Latex Emulsion after Hydrolytic Degradation

“…Latex is an amorphous elastomer which is not prone to hydrolysis. The crystalline part of UF resin is also resistant to hydrolytic degradation as reported in the previous publications (Park and Jeong 2011a;2011b;2011c;Park and Causin 2013;Singh et al 2014). that they were partially covered by the latex when they were blended (Fig.…”

“…6c and 6d also revealed cavities and fragments that were distributed throughout the samples. The fragments could be part of distinct crystalline particles because the shape was irregular and similar to those observed in the previous work (Park and Jeong 2011a;Singh et al 2014). The length of fragments varied from the longest of 157 nm to the shortest of 9 nm, with the average of 57 ± 38 nm.…”

“…Moreover, John and Dunker (1986) and Motter (1990) stated that the UF colloidal dispersion has been shown to be partly crystalline. Further, evidence was provided that crystallinity was inherent within cured UF resins using SEM (Park and Jeong 2011a), small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXD) (Park and Causin 2013). Recently, Singh et al (2013) reported evidence of heterogeneity of cured UF resin covering up to nanoscale; coarsely granular region of UF resins that contain both globular/nodular particles and crystalline structures, as crystal aggregations and small developing crystals.…”

“…Some studies investigated the hydrolysis of UF resins to understand the mechanism of bond strength loss and formaldehyde released using neat cured UF resin (Dutkiewicz 1983;1984;Tohmura et al 2000;Ringena et al 2006;Abdullah and Park 2009;Park and Jeong 2011a;2011b), and UF resin-bonded wood panel products (Irleand Bolton 1988;Ebewele et al1991;Park et al 2006;Roffael and Hüster 2012). Furthermore, some authors have attempted to examine the morphology of cured UF resins.…”

“…Furthermore, some authors have attempted to examine the morphology of cured UF resins. Park and Jeong (2011a;2011c) investigated effects of acid hydrolysis on the microstructure and micromorphology of cured UF resins using atomic force microscopy (AFM) and scanning electron microscopy (SEM), respectively. Their results showed two distinctive phases, hard and soft phases in AFM images.…”