Effects of acid hydrolysis on microstructure of cured urea‐formaldehyde resins using atomic force microscopy

Abstract

This study investigated the effect of acid hydrolysis on the microstructure of cured urea-formaldehyde (UF) resins using atomic force microscopy (AFM) to better understand its hydrolytic degradation process which has been known to be responsible for the formaldehyde emission of wood-based composite panels. The AFM was scanned on both outer surface and facture surfaces of the thin films of cured UF resins that had been exposed to the etching of dilute hydrochloric acid to simulate their hydrolysis process. The AFM images showed two distinctive parts, which were classified as the hard and soft phases in cured UF resins. For the first time, this study reports the presence of thin filament-like crystalline structures on the fracture surface of cured UF resin. The soft phase of cured UF resins by ammonium chloride was much more easily hydrolyzed than those cured by ammonium sulfate, indicating that hardener types had a great impact on the hydrolytic degradation behavior of cured UF resins. The surface roughness measurement results also supported this result. The results of this study suggested that the soft phase was much more susceptible to the hydrolysis of cured UF resin than the hard phase.

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.…”

“…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.…”

Comparison of thermal curing behavior of liquid and solid urea–formaldehyde resins with different formaldehyde/urea mole ratios

“…Our group has been characterizing various features of UF resins, such as hydrolytic stability, morphology, crystallinity, and microstructure [10][11][12][13][14][15][16][17]. Moreover, this work is an extension of our works that have been focused on understanding the impact of F/U mole ratio to properties of liquid [15] or/and cured UF resins [11].…”

“…Thus, the information provided here adds new knowledge on the influence of F/U mole ratio on thermal curing kinetics of UF resins. We have described morphology and microstructure of cured UF resins in a solid film form [10,12]. Although liquid UF resins have been studied extensively, there is still an information gap in thermal curing behavior of partially cured solid UF resin.…”

Micro-Morphological Features of Cured Urea-Formaldehyde Adhesives Detected by Transmission Electron Microscopy

“…Further, as a binder, UF resin adhesives penetrated not only in cell lumens but also into tracheid walls [5]; thus interaction between wood and UF adhesives occurs continuously. Many studies have investigated the hydrolysis of UF resin adhesives to understand the mechanism of formaldehyde release, not only from pure cured UF resin adhesives [2,[6][7][8][9][10][11][12] but also from UF resin adhesive-bonded wood panel products [13][14][15][16]. The tendency toward hydrolytic degradation of a cured UF resin adhesives depends on its chemical structure and the degree of crosslinking and could be accelerated by high temperatures and strong acidic conditions [15].…”