Microstructure and Quantitative Micromechanical Analysis of Wood Cell–Emulsion Polymer Isocyanate and Urea–Formaldehyde Interphases

Abstract

Emulsion polymer isocyanate (EPI) and urea-formaldehyde (UF) were selected as typical resin systems to investigate the microstructure of wood-adhesive interphases by fluorescence microscopy (FM) and confocal laser scanning microscopy (CLSM). Further, a quantitative micromechanical analysis of the interphases was conducted using nanoindentation. The FM results showed that the UF resin could penetrate the wood to a greater extent than the EPI resin, and that the average penetration depth for these two resin systems was higher in the case of latewood. CLSM allowed visualization of the resin distribution with contrasting colors, showing that the EPI resin could not penetrate the cell wall, whereas UF resin could enter the cell walls. The micromechanical properties of the cell walls were almost unaffected by EPI penetration but were significantly affected by UF penetration, especially in the first cell wall from the glueline. This further confirmed that only cell walls with resin penetration can improve the mechanical properties of the interphase regions.

Beyond What Meets the Eye: Imaging and Imagining Wood Mechanical–Structural Properties

“…CLSM has become an incredibly versatile technique ( Figure ). It has found a variety of uses in wood research, [ 95,96,109 ] including to visualize 2D and 3D ultrastructure [ 97,98,106,107,109–112 ] (Figure 7a,c), measure cell dimensions, [ 103,105,108 ] and determine the cellulose microfibril angle; [ 113,114 ] study cell wall polymer composition [ 107,109,112,115 ] (Figure 7a–c); inspect (the effects of) liquid penetration, including water [ 116,117 ] and polymers [ 118,119 ] (Figure 7d); and assess ageing and (bio)degradation (Figure 7b). [ 120–122 ] Most of these ultrastructural observations can inform or aid discussion of structural properties of natural and/or modified wood.…”

“…[ 129 ] CLSM is well poised to study both the aspects and has been frequently used for this purpose, for a wide range of polymer adhesives, coatings, treatments and grafting. [ 118,119,130 ] CLSM is often used in a complementary way to other microscopy techniques in this area of research. CLSM enables measurement of penetration depth, the identification of fluid transport pathways, the localization of polymer or cell wall modification (e.g., delignification), and even the quantitative visualization of which polymers can penetrate into the cell wall microporosity (Figure 7d).…”

“…CLSM enables measurement of penetration depth, the identification of fluid transport pathways, the localization of polymer or cell wall modification (e.g., delignification), and even the quantitative visualization of which polymers can penetrate into the cell wall microporosity (Figure 7d). The latter can be complemented with micromechanical studies, such as nanoindentation tests, in which hardness and elastic modulus of cell walls, as a distance from the bond line, are measured [ 119 ] Nanomechanical measurements using AFM have also been used in combination with optical imaging to map the structure of wood (see Section 7).…”

Examination of wood adhesive bonds via MicroCT: The influence of pre-gluing surface machining treatments for southern pine, spotted gum, and Darwin stringybark timbers

“…Diverse technologies and methods have been adopted to examine glue lines and adhesive penetration in wood. These methods include light microscopy (LM), scanning electron microscopy (SEM), backscatter electron imaging, wavelength dispersive spectroscopy, fluorescence microscopy (FM), X-ray fluorescence microscopy, transmission electron microscopy (TEM), UV-microscopy, electron energy loss microscopy, chemical state X-ray microscopy, confocal laser scanning microscopy (CLSM), scanning thermal microscopy, nanoindentation, small-angle neutron scattering, and micro X-ray computed tomography (MicroCT) (Modzel et al 2011;Qin et al 2017;Jakes et al 2018). All of these technologies and approaches have advantages and disadvantages in terms of the nature and level of information generated, scale, ease of use, sample preparation requirements, resolution, cost, data set size, and complexity.…”

“…All of these technologies and approaches have advantages and disadvantages in terms of the nature and level of information generated, scale, ease of use, sample preparation requirements, resolution, cost, data set size, and complexity. However, various researchers have demonstrated the particular usefulness of MicroCT in studying wood adhesive bonds in 3D (Evans et al 2010;Modzel et al 2011;Hass et al 2012;Kamke et al 2014;Paris 2014;Bastani et al 2016;McKinley et al 2016;Qin et al 2017;Jakes et al 2018).…”