Abstract

European policymakers and research funding in Europe have recently been pushing for reducing the dependency of European countries on imports of raw materials (e.g., proteins, energy, wood, etc.). Part of the strategy is to increase the utilization of all raw material that currently exists. A large number of poplar (Populus spp.) clones have been introduced in Europe (and elsewhere) for paper fiber supply but are considered relatively low grade and are not used in higher-value applications. Wood modification techniques have been used to help valorize underutilized wood materials and increase their performance with respect to durability, mechanical characteristics, and new forms and functions desired by consumers and designers alike (Sandberg et al. 2017). One such modification technique is thermo-hydro-mechanical (THM) treatment. THM treatments use heat, water, and mechanical pressure to compress and densify the wood material. This treatment results in increased density, hardness, abrasion resistance, and some strength properties. During the THM process, the wood cell is softened and relaxed by heat and moisture and compressed, resulting in densification without fracturing the cell walls. The densified wood will maintain its shape if it is cooled under pressure. One of the largest challenges facing this modification technique is dimensional stability. If this treated wood is exposed to high levels of moisture or water, it can partially revert to its original dimensions. This behavior is known as set-recovery. Impregnating the wood microstructure with various curing resins is one technique that has been proposed to increase dimensional stability and reduce set-recovery. The most success has been found with aqueous, phenol-based resins that are impregnated via vacuum treatment into the wood structure. After impregnation, a drying step is applied and followed by THM treatment (Stamm and Seborg 1941; Stamm 1959; Hill 2006; Gabrielli and Kamke 2008, 2010). During the THM treatment, the phenol molecules impart a plasticizing effect on the wood that further reduces cracking in the cell walls (Franke et al. 2016). Once these resins reach a certain temperature, they polymerize throughout the microstructure, improving the dimensional stability. While this approach has found success in the past, there are few commercial products available using this technique. Both THM treatments and phenol resins have been used in the past as wood modification methods to improve either mechanical properties or dimensional stability. This study investigated coupling these techniques to capture the benefits from both. As a trial application, the potential as flooring was explored using a newly developed phenol-based industrial resin and THM process (Kantner et al. 2019). The objective of this study was to explore the potential use of this resin in densified wood flooring or other high-wear applications using poplar.

Materials and Methods

The general approach of this study was to assess scratch resistance, hardness, and adhesive bond shear strength to gain insight into the viability of using this THM and resin treatment for flooring products.

THM treatment

After drying, impregnated specimens were densified in a hydraulic, 30-ton-capacity “Perfect” LZT-UK-30-L model hot press (Langzauner GmbH, Lambrechten, Austria) equipped with a water-cooling system. To achieve a target thickness of 10 mm, steel hard stops were used (Fig. 1). Two panels were pressed simultaneously within the press. Aluminum foil was placed on the top and bottom to protect the platens from excess resin. Pressing parameters are presented in Table 1. The temperature of 170°C is chosen to target softening of hemicelluloses and lignin in the wood, while the increase in temperature to 200°C for 2 minutes targets the degradation of hygroscopic components of wood after compression, especially hemicellulose. This has been found to improve the stabilization of densified wood (Kutnar and Šernek 2007). This approach allows compression without fracturing the cell wall and improved stability.

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Table 1 Pressing parameters for densification.

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To assess the level of densification achieved, dimensions and weight of specimens were measured before and after THM treatment. Densification ratio was calculated using ovendry density of the samples compared with the density after THM treatment.

Results

Results of the THM treatment are presented in Table 2. The THM treatment resulted in poplar specimens with an increased density from 0.37 to 1.03 g/cm³ (density ratio of 2.78). If we consider the maximum density possible as the wood cell wall density (∼1.54 g/cm³), there is still room for further densification. However, at these levels, cracking and damage are much more likely to occur. While not at this theoretical level of density, this final density is quite high compared with other THM treatments (0.79 g/cm³) applied to poplar (Kutnar et al. 2008) but similar to those found in THM studies with other wood species, like beech (Fagus sylvatica) (1.12 g/cm³; Schwarzkopf et al. 2017). This large increase in density is beneficial to some mechanical properties (stiffness, hardness, etc.) but does bring some aspects that should be taken into consideration. For example, it is known that the elevated temperature and steam conditions of the compression environment influence the set-recovery of compression deformation and that wood with higher levels of densification show the highest potential for set-recovery (Kutnar et al. 2009, Kutnar and Kamke 2012). This should be considered when designing products, and a lower density ratio may be desirable for applications involving water. The dark red-brown color observed in Figure 1 comes from the cured phenol resin.

Table 2 Results of thermo-hydro-mechanical densification treatment. Data are means with standard deviations shown in parentheses.

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Results of mechanical testing are presented in Table 3. As expected, the equivalent Janka hardness value of the THM-treated specimens is very high compared to untreated poplar (∼1,735 to 2,400 N) or other, more dense hardwoods, like white oak (Quercus alba), which has a hardness value of 6,050 N (Forest Products Laboratory 2010, Meier 2020). This hardness level is also quite high compared to other densified poplar in which the highest density achieved using poplar was 0.79 g/cm³ (Kutnar et al. 2008).

Table 3 Mechanical test data of impregnated, thermo-hydro-mechanical–treated specimens. Data are means with standard deviations shown in parentheses.

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The mean scratch resistance was found to be 2.49 N, which is significantly increased from natural wood with no THM treatments or phenol impregnation. Keskin et al. (2010) measured the scratch resistance of several wood species including poplar and oak and found values of 0.49 and 1.04, respectively.

The highest mean shear strength was found in Group 1 at 7.68 MPa, in which densified wood was adhered to a beech plywood panel. The higher mean strength in Group 1 versus Group 2 was attributed to the combination of two high-density surfaces with little void space in Group 2, inhibiting the development of a strong bonding interphase. The values for both groups are similar to those found in a study conducted by Blanchet et. al (2003), who reported a strength value of 4.3 MPa for PVA glue lines. In another study by Wang et al. (2017), oak surface layers were bonded to poplar plywood using a polyurethane reactive adhesive. Shear strength of their samples was near 3.0 MPa. Fang et al. (2012) also studied the use of THM treatments (without impregnated resin) on sugar maple (Acer saccharum) veneers for use in engineered wood flooring. They observed shear strength values of 13.1 MPa when gluing two densified pieces together. While many test factors (e.g., species, THM parameters, etc.) vary, it is possible that the use of impregnated phenol resins decreases the bond strength. However, shear strength values were still higher than uncompressed sugar maple bond lines used in flooring applications.

Conclusions

The THM treatment and impregnated phenol resin used in this study increased the scratch resistance, density, and hardness of natural poplar wood and created satisfactory bonding conditions for flooring purposes. The high levels of density and hardness may not be needed in practice, and more efficient flooring products could be created with modifications to the THM treatment targeting lower densities. These modifications may also allow for better bonding characteristics. Additionally, a lower solids level in the impregnated resin could also help lead to more realistic densities while maintaining dimensional stability and performance. With these optimizations, a high-quality flooring material used alone as a massive piece or as an upper laminate in engineered wood flooring could be produced from a low-density species like poplar. By using thinner sections of material in a laminate configuration, resin costs could be reduced while still providing the same performance.