Effect of Thermo-Treatment and Alkyl Ketene Dimer (AKD)
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Article Wetting Behavior of Alder (Alnus cordata (Loisel) Duby) Wood Surface: Effect of Thermo-Treatment and Alkyl Ketene Dimer (AKD) Teresa Lovaglio 1, Wolfgang Gindl-Altmutter 2, Tillmann Meints 3, Nicola Moretti 1 and Luigi Todaro 1,* 1 School of Agricultural, Forestry, Food and Environmental Science, University of Basilicata, V. le Ateneo Lucano 10, 85100 Potenza, Italy 2 Department of Materials Science and Process Engineering, University of Natural Resources and Life Science Vienna, Konrad Lorenzstrasse 24, 3430 Tulln, Austria 3 Kompetenzzentrum Holz GmbH, Altenbergerstrasse 69, 4040 Linz, Austria * Correspondence: [email protected]; Tel.: +39-0971-205340 Received: 26 July 2019; Accepted: 3 September 2019; Published: 5 September 2019 Abstract: The main purpose of this study was to investigate the hydrophobic effect and chemical changes induced by thermo-treatment and alkyl ketene dimer (AKD) on the surface properties of Alder (Alnus cordata (Loisel) Duby) wood before and after an artificial weathering test. Thermal treatment was conducted at a temperature of 200 ◦C for 4 h in a thermo-vacuum cylinder. Then, the paper sizing agent, AKD at different concentrations of a solution of 0.1%, 0.5% and 10% was used as a potential hydrophobizing reagent for untreated and thermally treated alder wood surfaces. The contact angle measurement, ATR-FTIR analysis and colour variation were carried out for the samples. The preliminary results revealed that the contact angle values of the wood materials increased with thermal modification. However, the influence of the thermal treatment on hydrophobicity was small when compared to the substantial effect of the AKD application in this respect, and also after the artificial weathering test. The FTIR analysis supported the hypothesis that AKD could make bonds chemically stable even when using a small concentration of AKD. The findings acquired in this work provide important information for future research and the utilization of the AKD on lesser-used wood species. Keywords: underused wood species; thermo-vacuum treatment; surface degradation; hydrophobic effect 1. Introduction It is known fact that wood undergoes degradation processes when exposed to exterior conditions. In particular, the surface is affected by exposure to light and water. The hygroscopic nature of wood produces some undesirable properties [1]. Due to water absorption and desorption, it is easily subjected to the biological attack of fungi and insects as well as dimensional instability. All of these adverse factors influence the overall properties of wood in outdoor use. To enhance the performance of wood under critical environmental conditions, different approaches may be applied. There are several chemical treatments to preserve wood surfaces, but these could cause toxic emissions during the production, service life and after use [2]. An important alternative to preserve wood is thermal modification. As noted by Hill [3], the heating temperature used during the treatment provoked changes in the wood. Thermo-treatment removes water from wood and causes changes in the chemical composition of wood and ultrastructure through autocatalytic reactions and cross-linking of the Forests 2019, 10, 770; doi:10.3390/f10090770 www.mdpi.com/journal/forests Forests 2019, 10, 770 2 of 16 cell-wall constituents [4,5]. The wood became less hygroscopic, and an improvement in dimensional stability could be observed [6]. However, thermally modified wood shows a general decrease in mechanical properties. The colour change is not permanent in time when the wood is exposed to exterior situations. Decay resistance does show some improvement, however is not recommended to use thermally treated wood in ground contact situations [3,6]. Hemicellulose degradation starts with the hydrolytic cleavage of acetyl followed by carboxyl groups from the side chains, which in the presence of temperature and water, leads to the formation of acetic and formic acids. Afterwards, organic acids catalyze the further hydrolysis of the cell wall polysaccharides. The corresponding sugars can be further dehydrated to aldehydes, with furfural and hydroxymethylfurfural being typical products originating from pentoses and hexoses [7,8]. The degradation of amorphous cellulose also occurs which implies the inaccessibility of hydroxyl groups to water molecules. This is one of the factors that contribute to a decrease of equilibrium moisture content. However, the lignin degrades and its content increases and new methylene bridges arise which are responsible for crosslinking [9–11]. At the same time, heat treatments modify the wettability of wood, which is directly related to the chemical changes of the wood structure, especially to the degradation of the hemicelluloses. The effectiveness of water-repellency treatment can be defined as the ability of a treatment to prevent or control the rate of liquid water uptake [12]. Several direct and indirect analytical approaches have been adopted to wood samples, including the measurement of wettability through the water contact angle (WCA) [12,13]. The relevance of the determination of the wettability of wood and its significance in wood science was critically discussed by Petriˇcand Oven [13]. Numerous investigations were carried out on the wettability changes of wood during heat treatment. General statement thermo-treatment has induced a reduced wettability [3]. However, Huang et al. [14] stated that induced weathering increased the wettability of heat-treated wood. Weathering is a complex process that modifies and degrades the overall molecular structure of wood and wood-based products. The ultraviolet (UV) radiation is one of the main responsible of the photodegradation, causing the fastest and the strongest degradation of the wood components. The quantum energies of UV light break many of the chemical bonds presented in the wood constituents [15]. Thus, a certain degree of surface degradation of the treated wood used in outdoor conditions would still be expected. Therefore, surface protection of thermo-treated wood products would be necessary to enhance the overall effective service life of the final product [15]. While thermal modification of wood is an important alternative for improving technological performance, numerous studies have proposed making wood materials water-repellent by using various derivatives of anhydride, epoxide, alkyl chloride, aldehyde and chlorosilane [16], such as octadecyltrichlorosilane, which according to Hui et al. [17], have exhibited hydrophobic properties. Other chemical treatments of esterification, such as acetylation of cellulose [18] or natural fibers [19], can also be used. An interesting treatment regarding the use of fluorine, which is a chemical element with the highest electronegativity, has led to hydrophobic characteristics in carbon fiber [20]. For wood, fluorination converts the bond between the carbon and hydroxyl groups into C–F bonds, thus making it hydrophobic [21]. Alkyl ketene dimer (AKD) can be also used to make wood surfaces hydrophobic [22] and is a widely used hydrophobization chemical in the papermaking industry [23]. Compared with other chemical compounds, it exhibits low toxicity, requires no special equipment, and the solvents involved are recyclable. Each AKD molecule is composed of two alkyl chains and one hetero four-member ring that reacts to hydroxyl groups to bind β-ketoester linkages [23,24]. There were three different hypotheses of the mechanisms of the reaction of AKD with cellulose materials. The first hypothesis states that the substance could react directly with cellulose hydroxyl (principally with C6 hydroxyl) as it is less sterically hindered when compared to the hydroxyl group Forests 2019, 10, 770 3 of 16 on C2 or C3 [25–28]. The second one indicates that the mechanism of the reaction of AKD and water molecules is faster than cellulose even if it produces an unstable β-keto acid [29,30]. The third hypothesis takes into account whether the reaction could occur between AKD molecules forming an oligomer [31]. Other authors [32,33] have argued that the reaction between the hydroxyl groups in cellulose and AKD is essential to repel water. However, although many studies were carried out related to the use of AKD on the wood surface, the investigation of the chemical reaction after an accelerating weathering test remains poorly investigated. During the last decades, the demand for sustainable material removing the use of toxic chemicals obtaining materials with great potential and durability has been increasing. For this reason, it has raised the necessity to modify the wood products. High-value wood resources might be obtained with different processes, including impregnation, chemical modification or thermal treatments, among others. The potential economic gain is more attractive when considering lesser-used or minor wood species of relatively low market value. The effects of thermal treatments on properties of lesser-used hardwood species, such as Alder wood, have been understudied [15]. Italian Alder (Alnus cordata (Loisel) Duby) can grow very rapidly (up to 30 m tall and reaching 70–80 cm in diameter) in the soil at increased humidity along watercourses [34–36]. Italian alder grows along the watershed, even at high altitudes mainly in Southern Italy. These species have been introduced in Sardinia and the Southern Alps. Recent plantations were also established in other European countries including France, Spain, Portugal, England, and the Netherlands. It has also been recently introduced