Enhancing Thermally Modified Wood Stability against Discoloration

Abstract

Thermal modification of wood has gained its niche in the production of materials that are mainly used for outdoor applications, where the stability of aesthetic appearances is very important. In the present research, spectral sensitivity to discoloration of thermally modified (TM) aspen wood was assessed and, based on these results, the possibility to delay discoloration due to weathering by non-film forming coating containing transparent iron oxides in the formulation was studied. The effect of including organic light stabilizers (UVA and HALS) in coatings as well as pretreatment with lignin stabilizer (HALS) was evaluated. Artificial and outdoor weathering was used for testing the efficiency of different coating formulations on TM wood discoloration. For color measurements and discoloration assessment, the CIELAB color model was used. Significant differences between the spectral sensitivity of unmodified and TM wood was observed by implying that different strategies could be effective for their photostabilization. From the studied concepts, the inclusion of the transparent red iron oxide into the base formulation of the non-film forming coating was found to be the most effective approach for enhancing TM wood photostability against discoloration due to weathering.

1. Introduction

Thermal treatment has been found to be a feasible modification method for increasing wood hydrophobicity, biological durability, and dimensional stability [1,2,3]. In addition, such modification is considered to be more environmentally friendly compared to chemical modification or preservative treatment because no chemicals are used. Furthermore, thermal modification does not entail any obstacles for easy disposal of wood after its service life. Apart from the enhanced functionality, during thermal modification, the appearance of wood is essentially altered by forming a lighter or darker brown color, which for some species is observed to be more homogenous compared to the color of unmodified wood [4,5]. Furthermore, the characteristic brown color imparted to wood by thermal modification is often regarded as an extra or even one of the main benefits as it is highly favored by customers [6,7,8].

Among the variety of possible TM wood applications, the dominating areas are decking, claddings, façades, garden accessories, and similar outdoor applications [2]. Given this aspect of TM wood application, the stability of the aesthetic appearance is of great importance for its successful competition with other materials in the area [2]. The surface of wood used outdoors is degraded by weathering, which is a set of complex phenomena mostly including complementary, synergetic, and antagonistic effects of solar irradiation, water, fungi, and atmosphere [9,10]. Although wood functionality usually is not deteriorated due to weathering, which is known to exclusively be a superficial phenomenon, it exerts disastrous effect on the aesthetic value of wood [10,11]. Some researches claim improved weathering resistance of wood due to thermal treatment [12,13]. Nevertheless, discoloration of TM wood during outdoor exposure has been observed to be quite a rapid process [13,14,15].

Among different wood protection strategies against weathering, the most widespread approach is the use of coatings due to economical, application simplicity, as well as custom practice considerations [10]. The use of transparent coatings is the most common practice for wood protection from discoloration with simultaneous retention of its natural color and texture [16]. However, due to the high photosensitivity of wood, photoprotective additives such as UV absorbers (UVA), which convert the absorbed UV radiation into heat, and hindered amine light stabilizers (HALS), which inhibit autooxidation by scavenging free radicals, are routinely included in coating formulations to improve their performance. In addition, it has been found that the inclusion of such additives into formulation does not reduce adhesion strength between the coating and wood, which is an important characteristic for providing good performance of the coating [17]. The combination of organic UVA and HALS has proved to exert a synergistic effect at inhibiting both coating and wood photodegradation [18,19,20]. Light stabilizers of both types are often included in a single commercial additive for wood coatings. A variety of UVA chemical compositions and product forms differing in efficiency, spectral coverage, and resistance to weathering has been developed to improve the performance of coatings [21,22]. Promising results have also been reached when a concept about the wood photostabilization by pre-treatment with HALS inhibiting lignin photooxidation was implemented [23]. However, organic UVA may be decomposed by solar irradiation, resulting in the loss of their efficiency [24]. Recently a number of studies have focused on different aspects of incorporation of inorganic nanoparticles into transparent coating formulations to enhance performance of wood coatings as they have inherent high photostability and, what is of high importance, they are non-toxic [15,23]. The most widely used UV-absorbing inorganic additives are zinc oxide (ZnO) and titanium dioxide (TiO₂) due to their high absorption of radiation mainly in the UV range, thus providing good retention of the natural wood color after application of coating [15,20,21]. Another easily available inorganic UVA are iron oxides. However, considering transparent coatings for pale woods, iron oxides are recognized as inappropriate because of their typical strong absorption not only in UV but also in wide spectral region of visible light, resulting in inevitable alteration of the wood original appearance [23,25].

Although wood is substantially transformed during thermal modification, in a number of studies, it has been observed that the coating formulations intended for the use on unmodified wood suits as well for application on TM wood as a substrate [26,27,28]. Nonetheless, the increased hydrophobic character of heat-treated wood and the changes of the polarity of surface energy of wood could cause problems with the adhesion of waterborne coatings [29].

Concerning TM wood protection against photodegradation, different approaches have been studied. Introduction of visible light non-absorbing inorganic and organic UVA as well as HALS into coating formulations has been found to be ineffective for TM wood protection from discoloration [30,31,32]. A promising result was achieved when TM wood was treated with TiO₂ sol [33]. However, due to the complexity of the treatment process, the proposed approach does not seem to be an appropriate solution for commercial purposes.

The focus of the present research was the investigation of the possibility to improve TM wood resistance to color change. TM wood spectral sensitivity to photodiscoloration was evaluated since such data are important for the designing of effective protective coatings but hardly any information is reported in the literature. Such knowledge provides the possibility to tailor coating formulation matching TM wood peculiarities. Basing on these results, the inclusion of transparent iron oxides into the non-film forming coating formulation was tested. The use of iron oxides could be a simple, effective, and economical way to reduce TM wood photodiscoloration during outdoor exposure without substantially altering its characteristic appearance. In addition, the effect of wood pretreatment with the lignin stabilizer and the inclusion of a light stabilizer containing organic UVA and HALS into coating formulation were evaluated. European aspen (Populus tremula L.) was used for the experiment as it is a fast-growing species with inadequate commercial value and thermal modification could be considered a potential way to increase its market share.

2. Materials and Methods

2.1. Wood Material

Aspen (Populus tremula L.) wood was used for the study. Defect-free boards measuring 1000 mm × 100 mm × 25 mm were subjected to thermal modification in a WTT (Wood Treatment Technology, Brande, Denmark) experimental wood modification device, in a water vapor medium under elevated pressure (0.6 MPa) at 170 °C, for 1 h at the peak temperature. Before preparing the specimens, the boards of unmodified and TM wood were conditioned for a month in an atmosphere of 20 °C and 65% relative humidity. The specimens measuring 150 mm × 70 mm × 15 mm for exposure to solar radiation and to artificial UV weathering and the specimens measuring 300 mm × 90 mm × 20 mm for outdoor exposure on weathering racks were cut from the boards. For all experiments, six replicate specimens from different boards were prepared for each treatment. The surfaces of all specimens were furnished by planing.

2.2. Color and Reflectance Spectra Measurement

A portable spectrophotometer CM-2500d Konica Minolta, Tokyo, Japan (standard illuminant D65, d/8° measuring geometry, 10° standard observer, measuring area Ø 8 mm) was used for measurements of wood reflectance spectra and color measurements. Reflectance spectra were recorded for the wavelengths ranging from 360 to 740 nm, using a scanning interval of 10 nm. Color was expressed according to the CIELAB color model (Commission Internationale de l’Eclairage 1976) as the color parameters L*, a*, b*. Measurements were performed at five surface points and average values were calculated for representation of each specimen color and reflectance spectra. The color change (∆E_ab) was calculated from the differences between the initial and resulting values of color parameters ∆L*, ∆a*, ∆b* according to the equation:

$$∆E_{ab}=\sqrt{{\left(∆L\right)}^2+{\left(∆a\right)}^2+{\left(∆b\right)}^2}$$

(1)

For evaluation of color change, the measurements were always performed at the same spots on the tested surface of the specimen.

2.3. Coating Formulations and Finishing of Specimens

The base coating formulation was solvent-borne alkyd prepared in a laboratory by using long oil and medium oil alkyd resins provided by a local paint factory. The only additive was cobalt siccative and the solid content of the base formulation was 20%. Pigmented coatings were prepared by adding to the base formulation equal amounts of yellow, red, or a mixture (1:1) of both iron oxides with pigment concentration in the dry coating film being 16%. Sicoflush^® L Red 2817 and Sicoflush^® L Yellow 1916 (from the BASF, Ludwigshafen, Germany) pigment pastes for solvent-based coatings containing transparent pigments were used. The coatings were brush applied on the specimens in two coats for the artificial weathering experiment and two and three coats for outdoor weathering. In preliminary experiments with the base coating formulation, it was assessed that the spread rate of 160 g/m² is the upper limit for uniform coverage without formation of a visibly detectable film. The spread rate was assessed by weighing specimens before and after finishing. The average spread rate was 116 g/m² for two coats and 136 g/m² for three coats. For evaluating the effect of light stabilizer containing organic UVA and HALS on prevention TM wood from discoloration due to UV irradiation, 2% (based on the dry weight of the alkyd resins) of a commercial light stabilizer Tinuvin 5060 (from the BASF, Ludwigshafen, Germany) was added into the pigmented formulation (containing the mixture of both iron oxides) and into the base non-pigmented formulation. The specimens were then conditioned at RH 65% and 20 °C for two weeks to assure proper curing of the coating. For the experiment in which usefulness of TM wood pre-treatment with the lignin stabilizer (HALS) was evaluated, before exposure to UV irradiation, unmodified and TM wood specimens were pre-treated with lignin photostabilizing product “Lignostab 1198 L” of BASF production by spraying it on the specimen surface.

2.4. Weathering Experiments

Two outdoor experiments were carried out during the present study. They both were located in Riga (56°58′ N 24°11′ E). To assess spectral sensitivity of TM wood, the effect of incident light spectral composition on unmodified and TM wood discoloration was studied. The specimens of unmodified and TM wood specimens were exposed to full solar radiation as well as to solar radiation transmitted through glass filters of different absorption spectra. The experiment was performed by exposure of specimens to solar irradiation outdoors during summer between May and August, when the incident solar radiation has the highest intensity. Specimens were exposed outdoors only on sunny hours, when the intensity of the total UV radiation (290 to 390 nm) was above 10 W/m². During exposure, the average UV radiation intensity was 23 W/m² and the average solar radiation intensity was 590 W/m². UV light meter (Lutron UV-340, Taipei City, Taiwan) and solar power meter (Amprobe SOLAR-100, Glottertal, Germany) were applied for the measurements of radiation intensity. The total exposure time was 100 h. The specimens were kept in the dark while they were not exposed outdoors.

During the second outdoor experiment, the specimens with pigmented coatings were exposed on weathering racks inclined at 45° and facing south. The experiment was carried out from January to November; the total exposure time was 220 days.

Artificial weathering was performed by exposing the specimens to UV irradiation in an accelerating weathering chamber QUV (Q-Lab, Westlake, OH, USA). UVA-340 type fluorescent lamps, which simulate the UV portion (295–360 nm) of the solar radiation, were used with irradiation intensity of 0.89 W/m² at 340 nm.

3. Results and Discussion

3.1. TM Wood Color and Its Photosensitivity

The color of pale wood species indicates a poor amount of chromophores in their chemical composition as only a small portion of the incident visible light is absorbed while the majority is reflected. On the contrary, one of the consequences of wood subjection to high temperature treatment is the changed composition of its chromophores, imparting wood the characteristic brown color (Figure 1), which is a result of intense visible light absorption.

It has been found that the main chemical transformations processes in wood during thermal treatment imparting wood the typical brown color are condensation and oxidation reactions, leading to an extensive formation of conjugated structures [34,35]. It is suggested that the formation of quinoid compounds due to degradation and oxidation of lignin and aromatic extractives could be involved in wood color changes upon thermal treatment [35]. In addition, most of the chromophoric substances are of high molecular mass and/or degree of cross-linking [7]. However, precise composition and formation processes of the chromophores of TM wood is still unknown. From the reflectance spectra (Figure 2), it can be seen that due to thermal modification, wood reflectance decreases in the whole range of the visible light with more pronounced changes in the wavelength range above 500 nm, indicating substantially increased absorption in this region. It implies that colored quinoid with the typical absorption at the shorter wavelength region of visible light are not the dominant cause of wood color changes due to thermal treatment.

Although the absorption of radiation of definite wavelengths is mandatory for wood photo-discoloration, the absorption of light does not necessarily lead to chemical transformations. Moreover, only the chemical changes affecting chromophores result in discoloration [36]. Therefore, information about spectral sensitivity of the material is important to develop coating formulations providing effective protection, especially when transparent coatings are preferable for the maintenance of wood’s natural appearance. For the identification of thermally modified wood, spectral sensitivity and evaluating the changes caused by the thermal treatment, both unmodified and thermally modified wood specimens were exposed to direct solar radiation and to solar radiation filtered through glass filters transmitting definite wavelength ranges of the solar spectrum. The results of the experiment (Figure 3) clearly demonstrate differences in spectral sensitivity regarding discoloration of the two tested wood types.

Substantially greater discoloration of the unmodified wood subjected to full solar spectrum compared to TM wood implies that in general the wood transformations during thermal treatment impart to wood a higher color stability. In addition, much greater improvement of resistance to discoloration due to the wood thermal treatment can be observed when specimens covered by filters transmitting solely UV radiation (295–400 nm) are compared. It is easy to see that TM wood exposure to the UV portion of the solar radiation, which caused even greater discoloration of unmodified wood than radiation of full solar spectrum, resulted in much smaller color changes compared with those entailed by the full solar radiation. Reduced discoloration caused by UV irradiation due to wood thermal modification has been observed in a number of researches [37,38,39,40,41]. The phenomena of enhanced resistance to photodiscoloration of TM wood is associated mainly with chemical transformations of lignin, which is well recognized to be the key wood component responsible for its discoloration [37,38,42,43]. However, in the literature, contradicting results have also been reported showing greater discoloration for TM wood than for unmodified wood caused by UV irradiation during artificial weathering [44,45,46]. Such discrepancies in results could be caused by differences in wood species, thermal treatment processes, as well as design of experiments used in the studies.

However, the exposure of TM wood to the visible light portion of the solar radiation (360–900 nm) resulted in substantial discoloration, while only minor color changes were detected for unmodified wood. In addition, essentially greater discoloration was observed for TM wood specimens covered with filters transmitting radiation in the range of 515-900 nm compared with the specimens covered with UV light transmitting filters. Considerable changes in color were detected even for the TM wood specimens exposed to the radiation of wavelengths longer than 600 nm. Due to the complicated chemical structure of wood, precise mechanisms of wood photochemical transformations have not been entirely established [10,43]. However, the main reactions of photochemical transformations are similar for common organic materials and start with the formation of free radicals, which is initiated by the absorption of photons of adequate energy for cleavage off a radical [47]. The results of wood discoloration implies that TM wood contains chromophores, the transformation of which can be caused by less energetic photons of visible light in addition to UV radiation. These results suggest that shielding from considerable portion of the visible light could be an effective protection strategy to prevent TM wood discoloration and contribute to retention of its natural appearance, especially when the envisaged application includes exposure to solar radiation. Apart from differences in spectral sensitivity, there are cardinal differences in the discoloration pattern of unmodified and TM wood. For unmodified wood, photodiscoloration shows up as a darkening and is pertained to transformations of lignin with formation of chromophores such as quinones and quinine-like structures [23,36,38]. In contrary, TM wood color becomes lighter, implying a reduction in chromophoric structures that absorb visible light. Obviously, radically different processes are involved in transformation of chromophores for unmodified and TM wood.

3.2. Effect of Lignin Stabilizer

It has been found that, similar to unmodified wood, lignin is the most light-sensitive wood component also in TM wood [39,45,46] Therefore, an experiment was carried out by employing an approach that has proved its suitability in case of unmodified wood [23]. The approach includes wood pretreatment prior to coating application with a lignin stabilizer, which is a special HALS designed for trapping the radicals forming at the wood surface. The results proved that pretreatment with the lignin stabilizer can reduce rate and amount of unmodified wood discoloration caused by exposure to UV irradiation (Figure 4).

An adverse effect of pretreatment with lignin stabilizer was detected for TM wood when more than twice as large discoloration was observed for the pretreated specimens compared to those without pretreatment. From these results, it is clearly evident that the application of lignin stabilizer did not improve TM wood resistance to UV-caused discoloration. The adverse effect of the lignin stabilizer on discoloration of unmodified and TM wood could be related to the differences of the two wood types regarding chemical structures of the components involved in the processes of free radicals formation and wood discoloration. It has been proved by measurements of EPR (electron paramagnetic resonance) spectroscopy that during thermal treatment, stable free radicals are formed in wood, which may also take part in photodiscoloration processes of TM wood [38,48]. Miklečić et al. [32] also found that pre-treatment of TM wood with lignin stabilizer of HALS type did not prove efficient and even caused increase of discoloration. However, Kocaefe and Saha [49] have reported better TM wood color protection effective use of coatings modified by adding a HALS type lignin stabilizer and bark extracts compared to that of organic UVA and HALS for softwood (jack pine), while similar discoloration was observed for the tested TM hardwoods (aspen and birch) for both additive combinations. These results imply that there is no consensus on the effect of HALS type lignin stabilizer on the discoloration of TM wood.

3.3. Effect of Incorporation of Pigments into Coating on TM Wood Color

As mentioned above, when maintenance of the natural appearance of wood is desirable, iron oxides are usually avoided in transparent wood coatings because of the color they impart to the coating due to the characteristic absorption in visible light range. However, this property could be beneficial in the case of TM wood for which considerable discoloration was caused by exposure to visible light (Figure 3), implying the necessity to screen wood from this part of solar radiation. In addition, due to the inherent brown color of TM wood, application of colored iron oxides should not lead to excessive transformations of the TM wood original color, which itself absorbs visible light significantly. For evaluation of the effect of transparent iron oxide pigments on TM wood appearance, reflectance spectra of wood coated with unpigmented and pigmented formulations were compared (Figure 5).

The reflectance spectra demonstrate that the application of all the tested coating formulations reduced the amount of the reflected radiation of the TM wood surface in the whole range of visible light with greater reduction recorded for the longer wavelengths. However, the differences between the effect of unpigmented formulation and those containing transparent iron oxides were statistically insignificant. In addition, the pigmented coatings imparted quite similar appearance regardless of the applied pigments (red, yellow or a mixture). These results indicate that the use of iron oxide pigments in coating formulations does not cause extra changes in TM wood color, apart from those imparted by the coating base matrix.

3.4. Effect of Incorporation of a Light Stabilizer Additive into Pigmented Coating Formulation

Iron oxide pigments absorb certain radiation in both UV and visible light range, depending on the oxide type. The effect of addition of organic UVA into pigmented coating formulation was tested by exposure specimens to UV irradiation in an artificial weathering chamber for 1500 h with regular evaluation of surface discoloration. The results showed that the addition of organic UVA to the coating formulation provided only slight delay of discoloration in the very beginning (Figure 6).

For the specimens finished with both types of coatings with the UVA additive, discoloration during the first 100 h (in the chart the unit of exposure time is a square root of an hour) was quite similar and slightly smaller than color changes detected for specimens coated with the pigmented formulation without UVA additive, implying a positive effect of the additive in delaying discoloration of TM wood. On the other hand, the further exposure resulted in almost equal discoloration of specimens with pigmented coatings, regardless of extra UVA addition or not, while the specimens treated with the unpigmented formulation that contained UVA discolored substantially more. It indicates that UVA additive in the transparent formulation containing iron oxide pigments exerts certain positive effect against TM wood discoloration only for the initial period but is ineffective for long-term protection. Therefore, there is no ground for use of UVA in such coatings when prevention of TM wood discoloration is considered. This finding is consistent with a research reporting that addition of organic UV absorbers to dark pigmented systems does not improve their performance in protection of unmodified wood from discoloration [23]. However, the long-term efficiency differs depending on the UVA chemistry [22]. The UVA included in the light stabilizer used in the experiments was of the BTZ (2-(2-hydroxyphenyl)-benzotriazile) class, which has been found to provide less long-lasting protection than those of HPT (2-hydroxy-phenyl-s-triazine) [22]. For more well-grounded inference, studies focusing on other UVA types should be performed. In addition, the potential importance of the UVA additive in elimination of coating microfilm degradation should be investigated as unimpaired coating film may be crucial for proper wood protection in outdoor environments, where, apart from solar radiation, other factors contribute to weathering with moisture exhibiting the most serious effect.

3.5. Effect of Red and Yellow Iron Oxide Pigmenst in Coating Formulation on TM Wood Discoloration during Natural Weathering

In the artificial weathering chamber, only the potential effect of UV radiation was simulated without considering the contribution of visible light which, as it has previously been identified (Figure 3), has a strong implication or even dominance in the total TM wood discoloration under exposure to solar radiation. Yellow and red iron oxides have similar UV absorption capacity but significantly differ in visible light absorption with the absorbance significantly red-shifted for the red iron oxide [50]. Mixture of both iron oxides as the pigment additive was used in the coating formulation for the artificial weathering test with UV irradiation. Taking into account the differences in visible light absorption spectra of yellow and red iron oxides, formulations containing only one iron oxide or mixture of both were used for the outdoor test to evaluate the protective potency depending on the kind of iron oxide. Two sets of specimens differing in number of coats were prepared for exposure to outdoor weathering. The results of specimen discoloration caused by 220 days outdoor exposure are presented in Figure 7. The results of discoloration clearly show that the red iron oxide has a higher capacity for prevention of TM wood discoloration.

As expected, less color changes were detected for all the tested formulations when three coats were applied. However for both sets of specimens, a similar trend was observed regarding the protective force of each formulation. The results of outdoor weathering clearly show that the coating formulation with red iron oxide prevented TM wood from discoloration more effectively than the other two formulations. This effect was especially pronounced in the case of three coats when almost twice less discoloration was detected for specimens with coating containing red iron oxide than for those with yellow iron oxide. Higher efficiency of the red iron oxide in comparison with the yellow one considering protection from discoloration was observed also in a case of unmodified pine sapwood [25]. The authors propose that the observed phenomena is due to the masking effect of the red iron. However, when TM wood is considered, a more possible explanation for red iron oxide’s better performance in reducing discoloration is the red-shifted absorbance, resulting in less solar radiation of wavelengths that cause TM wood discoloration reaching wood.

The addition of nanoparticles as well as pigment concentration may affect other coating properties [51]. Therefore, further research is needed to optimize coating formulations by assessing the most eligible pigment concentrations, taking into account all aspects of coating performance. In addition, systematic investigation into the compatibility and interaction of iron oxides with different kinds of coating systems regarding binders, solvents, and other additives should be carried out.

4. Conclusions

The study has shown that TM wood considerably differs from unmodified wood regarding wood spectral sensitivity. For unmodified wood, UV radiation is the dominant cause of photodiscoloration. In contrary, the wide wavelength range of visible light was found to cause significant discoloration of TM wood. This finding suggests that a cardinally different strategy should be applied for providing effective protection of TM wood from the main strategy used for unmodified wood, which mostly focuses on protecting wood surface from the UV radiation. In addition, the results showed that pre-treatment of wood with a lignin stabilizer designed for unmodified wood is not effective for the protection of TM wood from discoloration.

Inclusion of iron oxides in transparent coating formulation does not radically alter the appearance of the finished TM wood. The obtained results suggest that including of iron oxides and preferentially the red one into transparent coating formulations can provide effective TM wood protection from photodiscoloration. On the other hand, addition of the light stabilizer containing organic UV absorber and HALS to the pigmented coating formulation does not improve the color retention of TM wood.