Effect of Thermal Modification on the Nano-Mechanical Properties of the Wood Cell Wall and Waterborne Polyacrylic Coating

Abstract

Masson pine (Pinus massoniana Lamb.) samples were heat-treated at different treatment temperatures (150, 170, and 190 °C), and the nano-mechanical properties of the wood cell wall, which was coated with a waterborne polyacrylic (WPA) lacquer product, were compared. The elastic modulus (Er) and hardness (H) of wood cell wall and the coating were measured and characterized by nanoindentation, and the influencing factors of mechanical properties during thermal modification were investigated by chemical composition analysis, contact angle analysis, and colorimetric analysis. The results showed that with the increase in the heat treatment temperature, the contact angle of the water on the wood’s surface and the colorimetric difference increased, while the content of the cellulose and hemicelluloses decreased. After thermal modification of 190 °C, the Er and H of the wood cell wall increased by 13.9% and 17.6%, respectively, and the Er and H of the WPA coating applied to the wood decreased by 12.1% and 22.2%. The Er and H of the interface between the coating and wood were lower than those near the coating’s surface. The Er and H of the cell wall at the interface between the coating and wood were lower than those far away from the coating. This study was of great significance for understanding the binding mechanism between coating and wood cell walls and improving the finishing technology of the wood materials after thermal modification.

Multiscale Mechanical Performance of Wood: From Nano- to Macro-Scale across Structure Hierarchy and Size Effects

“…For instance, the following results are reported: H NI = 0.35–0.42 GPa for Pinus massoniana Lamb. In [ 107 ], H NI = 0.41–0.53 GPa for Masson pine , coinciding within the measurement accuracy for EW and LW in [ 108 ] and H NI = 0.34–0.54 GPa for Pinus taeda , not discerning EW and LW, in [ 109 ]. Similar results are reported for H NI in cell walls junction through the middle lamella for Norway spruce .…”

Relationship between Thermal Diffusivity and Mechanical Properties of Wood

“…Besides, due to the similarity of cell walls' microstructure, nanohardness H n variations within the same sample and even between different wood species are not large. For instance, the measurements of H n in various pine species give the following values: 440-490 MPa for Pinus sylvestris L. [37], 350-420 MPa for Pinus massoniana Lamb., as reported in [38], and 410-530 MPa [39] and 340-540 MPa for Pinus taeda [40].…”

“…The study of the effective distribution of Young's modulus and microhardness based upon indentations with size corresponding to that of several cells revealed that the mechanical properties differences between early and late wood annual ring layers (Figures 6-8) were several times larger than those measured in cell walls at the nanoscale, which are usually around 10-20% [37][38][39]. Besides, the former technique require much less surface preparation and does not require a precise indentation targeting cell walls whose width is in the order of several micrometers.…”

“…To apply the nanoindentation technique to study soft biological materials characterized by inhomogeneity at various scale levels, the properties of anisotropy, and time dependencies, the choice of the testing protocols, including maximal load P max , duration of holding at maximal load, etc., becomes very important [36]. In studying the nano-/micromechanical properties of wood cell wall, P max is usually chosen in the range of 0.1-1 mN [37][38][39][40][41], so that the lateral size of the indent is much less than the cell wall width (2-5 µm). Such measurements provide interesting information about the contribution of different layers to cell wall stiffness and hardness.…”

Локальные Механические Свойства И Кольцевая Структура Древесины, Исследованные Методом Сканирующего Наноиндентирования

“…Так, в [40] в клеточных стенках сосны обыкновенной (Pinus sylvestris L.) H c в молодой древесине (7-е годовое кольцо) составляла в EW 439 МРа, а в LW -466 МРа; в зрелой древесине (74-е годовое кольцо) в EW -469 МРа, а в LW -489 МРа. В [61] в сосне (Pinus massoniana Lamb.) твердость клеточной стенки варьировалась в интервале 0.35−0.42 GPa независимо от того, была ли ее внутренняя полость заполнена пластиком при ламинировании или нет.…”