Short Communication—A Novel Sample Preparation Method That Enables Ultrathin Sectioning of Urea-Formaldehyde Resin for Imaging by Transmission Electron Microscopy

Abstract

Urea-formaldehyde (UF) resin is widely used as an adhesive for the manufacture of a range of wood and fiber based products. Although the microstructure of this resin has been examined at high resolution by field-emission scanning electron microscopy and atomic force microscopy, transmission electron microscopy (TEM) has thus far not been used, perhaps because of difficulties in ultrathin sectioning this resin in cured (polymerized) state. In the technical note presented here, a novel sample preparation method is described which enabled us to examine the microstructural morphology of UF resin by transmission electron microscopy in ultrathin sections, revealing the presence of spherical particles within the resin. Our initial attempt to ultrathin section the resin directly was not successful as it was too brittle to trim blocks for sectioning. Then, we developed a sample preparation technique that involved impregnation of Pinus radiata wood tissues with the UF resin, and then embedding of resin impregnated wood tissues with Spurr's low viscosity embedding medium, which has been widely employed in plant and wood ultrastructure work. The TEM images illustrated and the information on the microstructural morphology of the UF resin presented are based on this novel sample preparation approach.

MRT Letter: High resolution SEM imaging of nano‐architecture of cured urea‐formaldehyde resin using plasma coating of osmium

“…However, concerted efforts are being made to minimize formaldehyde emission through changes in formulations aimed to reduce the proportion of formaldehyde in the UF mix, without compromising on desirable properties of this resin as a wood adhesive (Pizzi et al, ; Gu et al, ; Park and Jeong, ). Developing more environmentally friendly technologies for engineering products based on lower formaldehyde content in the UF resin mix will require a thorough understanding of the physical and chemical characteristics of UF resin in liquid and cured states, and among the high resolution imaging tools being used field‐emission scanning electron microscopes (FE‐SEM) (Park and Jeong, ) and transmission electron microscopes (Singh et al, ) are providing valuable information on microstructural features of UF resins prepared varying formulations and the factors related to resin curing.…”

Urea-formaldehyde resin penetration into Pinus radiata tracheid walls assessed by TEM-EDXS

“…UF resin of 1.0 formaldehyde/urea (F/U) mole ratio (nonvolatile solids content 60.9 wt%, viscosity 195 mPa s, gel time 194 s) was prepared according to Park and Jeong (2011), and P. radiata veneers were glued according to Singh et al (2013), which were specifically developed to facilitate the ultrathin sectioning of resin-penetrated wood tissues.…”

Hydroxymethyl furfural-modified urea–formaldehyde resin: synthesis and properties

“…UF resins cannot form a non-cracked and uniform coating due to high brittleness. Singh et al (2013) have also mentioned the brittleness of UF resin and their problems for making a smooth and uniform surface to investigate the morphology of UF resin by SEM and TEM. In the current work, the UF resin did not generate such a film on a wood plate too.…”

“…However, because of their very low costs, non-flammability, high curing rate, and the light color, UF resins are still used in vast amounts and as the most important adhesive by far in the production of wood based panels (Zorba et al 2008;Abdullah and Park 2010;Basta et al 2011;Patel et al 2013). UF resins show some disadvantages, for example, low water-susceptibility (Christjanson et al 2002(Christjanson et al , 2006, low heat-stability (Dim 2011), non-uniform film formation (Singh et al 2013) and, most importantly, emission of carcinogenic free formaldehyde (Birkeland et al 2010). For solving the latter problem, reducing the formaldehyde (F)/urea (U) molar ratio in the synthesis is one the most favorable approaches (Tohmura et al 2000;Ferra 2010).…”