Photodegradation stability of huminated European pine (Pinus sylvestris L.) microveneers

2.1 Wood microveneers

European pine (Pinus sylvestris L.) sapwood boards were supplied by a local sawmill in Kronoberg, Sweden, with an average density of 490 kg m⁻³. Wood blocks measuring 100 × 15 × 50 mm³ (L × R × T), free of knots and visible defects were cut from the boards and submerged in distilled water for one week. The microveneer strips measuring 100 mm × 15 mm × 100 µm (L × R × T) were sectionized using a sliding microtome (Reichert-Jung, Nussloch, Germany) with a disposable blade, as described previously (Hosseinpourpia and Mai 2016a,b,c).

2.2 Humination of microveneers

Humin, produced at pilot plant scale, was kindly provided by Avantium Renewable Polymers B.V. (Amsterdam, the Netherlands) through the dehydration process of fructose and glucose, containing 80 % humins and 20 % methyl levulinate. Citric acid (CA, 98 %) and succinic acid (SA, 99 %) were purchased from Sigma Aldrich (St. Louis, MI, USA) and used as catalysts. The modification solution was prepared by homogeneous mixing humins with various concentrations e.g., 1.5 %, 3 %, and 4.5 % (v/v), of catalysts (CA or SA) in a vessel submerged in an oil bath at 60 °C temperature for 120 min, then cooled down to the ambient temperature and directly used for modification of microveneers. Eight groups of twenty-eight (28) oven-dried microveneers (103 °C/24 h) were vacuum-impregnated (4 h, 370 mbar) with the modification solutions, according to Table 1. After impregnation, the excess solution was blotted off the micro-veneers with tissue paper. The curing process was carried out by step-wise oven temperature increment from 60 °C to 100 °C with a 20 °C/h ramp and then immediately to 140 °C for 10 h following. Also, a group of 28 samples were impregnated with water, cured at 140 °C, and served as a control. The weight changes of the samples due to the humination were calculated according to Hosseinpourpia and Mai (2016a), while the leaching (EN 84 2020) test was performed to remove the unreacted resins and unbound polymers. Additionally, to assess the efficiency of chemical modification on microveneers, the WPG ratio was calculated. All samples were stored at 20 ± 2 °C and 65 ± 5 RH for 14 days prior to further analysis.

2.3 Accelerated weathering test

Microveneers were placed in a QUV Weathering Tester–Model QUV Spray (Q-Panel, Lab Products, Cleveland, USA), by clamping their two ends in a PET frame and exposing the uncovered radial area to weathering condition, according to the standard EN 927-6 (2006). The weathering process in the QUV machine was carried out by exposing each group to fluorescent ultraviolet (UV) lamps and water spray for 0, 48, 96, and 144 h following cycles consisting of 2.5 h UV exposure at 60 °C and 0.5 h cold-water spray and a sub-cycle of UV-A light (highest absorption at 340 nm and 0.89 W m⁻² nm⁻¹). The oven-dried (103 °C/24 h) weight of the samples before and after the accelerated weathering test was used for mass change assessments.

2.4 FT-IR spectroscopy

The chemical structure of wood microveneers due to humination was analyzed with FTIR spectroscopy using Nicolet-Nexus, Waltham (MA, USA). The analyses were conducted in attenuated reflection (ATR) mode, with 64 scans at 4 cm⁻¹ resolutions ranging from 4000 to 755 cm⁻¹ and at room temperature. To ensure reproducible and constant force, the specimens were pressed onto the ATR-crystal with an integrated applicator. A background spectrum with an empty specimen compartment was recorded before observations and automatically removed from the spectra in the subsequent experiments. The spectra were baseline corrected, and the vector was normalized using OriginLab 2021b (Northampton, Massachusetts, United States). Two analyses were performed per sample, and average spectra were reported.

2.5 Micro-tensile strength

The finite span testing with a free clamping length of 50 mm and a testing rate of 2 mm min⁻¹ was performed using a universal mechanical testing machine (10 kN MTS Exceed E43, MTS Systems Corporation, MN, USA). Seven samples were tested per group and after each exposure period, and the strength loss was assessed by using the strength values of unweathered (0 h) control samples.

2.6 Statistical analysis

Statistical differences between mechanical properties of huminated and control microveneers with different weathering cycles, were performed by a one-way analysis of variance (ANOVA) of properties of microveneers using IBM SPSS statistical software, version 26 (IBM Corporation, New York, USA). The differences between significant means were analyzed with Duncan’s multiple-range test at a 0.05 significance level.

3.1 Weight changes by humination

The weight of Scots pine microveneers were considerably increased after modification with humins-based formulas. Figure 1a shows the WPG of huminated microveneers as a function of acid catalysts and their respective concentrations. The huminated samples at 1.5 % and 3 % concentrations of SA showed significantly higher WPG in comparison with the other modified samples (ANOVA, α = 0.05). The average WPG value of 189 % was obtained by samples modified with HSA 3 %. This might be explained by the self-polymerization reaction through esterification between humins and succinic acid, which may result in the formation of polymeric matrix with higher molecular weight in the wood cells. No statistically significant difference was observed in the WPG values of microveneers modified with sole humins and humins-CA formulas. The WPG values were considerably decreased after two weeks of leaching, while the values followed a similar trend as unleached samples. The highest leaching rate was obtained from the samples modified with the HCA 1.5 % and HSA 3 % formulas, while the ones modified samples showed more stable deposition of humins macromolecules. This could be related to the removal of unreacted humins and unbound polymer from wood cells as evident in the color of the leachate. Moreover, the limitation of humins to penetrate into the wood cell walls due to the high molecular weight and more allocation in the cell lumens could be an additional reason for the relatively high leaching rate (Liu et al. 2022; Sangregorio et al. 2020). In general, the WPG of huminated microveneers with the average values of above 100 % was considerably higher than the other resin modification systems such as 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU), phenol-formaldehyde resin and glutaraldehyde that were reported previously (Emmerich and Militz 2020; Hosseinpourpia et al. 2016a; Passauer et al. 2021; Xiao et al. 2012). The effectiveness of the modification can be evaluated by analyzing the WPG ratio (WPGr) due to the leaching test in wood microveneers, as depicted in Figure 1b. No obvious trends were observed between the WPGr values of huminated samples as a function of CA and SA catalysts, while the microveneers modified with HCA 1.5 % and HSA 3 % formulas showed statistically significantly lower fixation levels after the leaching than the other treated samples. The samples modified with sole humins illustrated the highest retention value after leaching. The microveneer samples modified with sole humins illustrated the highest retention value after leaching, although it is statistically insignificant to other formulas containing acidic catalysts, e.g., HCA 3 %, HCA 4.5 % and HSA 1.5 %. This might be either due to the formation of smaller molecule weight products during the curing process and penetration into the wood cells, or the creation of water-stable matrices in the lumens.

Figure 1:

Weight percentage gain (WPG) of huminated microveneers in the presence of citric acid (CA) and succinic acid (SA) catalysts before and after leaching (a) and calculated the weight percentage gain ratio (WPG_r) (b). Different superscript letters indicate a significant difference between the samples with different treatment solutions at p < 0.05 level.

3.2 FTIR spectroscopy

The changes in the chemical structure of European pine microveneers after modifications were analyzed by FTIR spectroscopy (Figure 2). While Figure 2a and b illustrated the chemical changes of microveneers modified with humins and humins combined with CA and SA catalysts, for a proper interpretation of the chemical interaction of humins-based formulas with the wood polymers, the FTIR spectra of microveneers modified with sole catalysts were also presented in Figure 2c and d. Obvious changes across all regions of spectra were observed after the modification of microveneers. The stretching vibration at 3350 cm⁻¹, which is assigned to the hydroxyl group, was decreased due to the humination modification. However, the humination of wood microveneers in the presence of CA at the higher concentration levels of acid, e.g., 3 % and 4.5 %, led to a slight increase in the absorption band of hydroxyl groups as compared with the sole humins-treated wood. This trend was similar to the sole CA-treated microveneers. This might be explained by the cleavage of glycosidic linkage in the structure of wood polysaccharides, and hydroxyl groups and carboxylic groups of CA, which resulted in the formation of new hydroxyl groups (Lee et al. 2020; Nypelö et al. 2021). The peak at 2910 cm⁻¹ is associated with C–H stretching of aromatic methoxyl, methylene and methyl groups, with almost similar intensity observed in all samples modified with CA-containing formulas. The huminated microveneers in the presence of SA catalyst showed slightly different changes in their chemical structure by having two distinct peaks at 2930 cm⁻¹ and 2858 cm⁻¹, which are assigned to the CH2 signals of esterified wood samples (Chang and Chang 2001; Rosu et al. 2010). As can be seen in Figure 2d, the intensity of peaks at 2930 cm⁻¹ and 2858 cm⁻¹ increased by increasing the concentration of SA. The peaks between 1722 cm⁻¹ and 1730 cm⁻¹ in huminated samples could be attributed to C=O stretching in the ester bonds (Coates 2000), caused by the esterification reaction between SA and CA catalysts and hydroxyl groups of wood polymers, as indicated in Figure 2c and d (Del Menezzi et al. 2018; Jin et al. 2023; Kurkowiak et al. 2022). The absorbance of these peaks was considerably increased by humination in the presence of acidic catalysts. A slight vibration at 1730 cm⁻¹ in unmodified microveneers corresponds to unconjugated C=O stretching of polysaccharides and acetyl groups in the wood (Esteves et al. 2013; Lupoi et al. 2015). Similar to unmodified reference samples, the CA and SA modified microveneers showed a wide absorption between 1610 cm⁻¹ to 1640 cm⁻¹, which might be related to the C=C bond of aromatic ring vibration of the phenyl propane skeleton (Hemmilä et al. 2020). This absorption was changed to a distinct peak with a slight shift to 1664 cm⁻¹ in acid modified samples, which could be overlapped with the vibration of furan rings of humins in huminated samples in the presence of catalysts (Sangregorio et al. 2018, 2020). The vibrations at 1515 cm⁻¹ and 1430 cm⁻¹ in unmodified and acid-modified samples were associated with the aromatic ring vibration of the phenyl propane skeleton (Hemmilä et al. 2020). However, the huminated samples showed absorbances at 1515 cm⁻¹, which could be related to the C=C in the furan ring (Patil and Lund 2011), and the disappearance of the peak at 1430 cm⁻¹. The intensity of the various absorptions was obviously changed as a function of modification formulas (Figure 2e). A slight increase at 1515 cm⁻¹ vibration in the huminated samples in the presence of CA catalyst (Figure 2e) might be related to the side reactions with the wood polysaccharides, e.g., hemicelluloses, resulting in the formation of new furanic acids (Huang et al. 2011). The huminated wood microveneers illustrated new peaks in the region between 755 and 805 cm⁻¹ with almost similar absorbance intensities in the samples modified with sole humins and humins-CA formulas and lower intensities in humins-SA modified samples (Figure 2e). These peaks could be respectively related to the C–H plane deformation and wagging in furan rings of humins (Sangregorio et al. 2020, 2018). The FTIR analyses confirmed that the chemical structure of wood microveneers was highly modified by humination modification.

Figure 2:

Fourier transform infrared spectroscopy (FTIR) spectra of modified microveneers, (a) humins-citric acid (HCA), (b) humins-succinic acid (HSA), (c) reference-succinic acid (SA), (d) reference-citric acid (CA), and (e) intensity of major absorptions as calculated by integral of the areas under the peaks.

3.3 Photodegradation behavior of huminated microveneers

The effect of photodegradation on the performance of unmodified and modified microveneers was studied by means of their mass loss (ML) and micro-tensile strength changes after exposure to the accelerated weathering test in a QUV machine. The MLs of the samples were apparently increased by increasing the exposure durations from 48 h to 144 h (Figure 3a). Overall, during artificial accelerated weathering, the unmodified reference samples illustrated the highest MLs, which were significantly higher than the modified ones (α = 0.05). Although no obvious trend was observed among different modifications, the huminated microveneers in the presence of the SA catalyst showed slightly lower average ML values than the CA catalyst at identical concentrations. Except for the HSA 4.5 % modified microveneers after 96 h of exposure, no statistically significant differences were observed between different levels of catalyst concentrations and exposure time. The mass loss ratio (MLr) of the samples showed that the huminated microveneers at 3 % and 4.5 % of SA catalyst with respective 10.7 % and 10.5 % MLr were the most photostable samples after 144 h exposure to artificial accelerated weathering (Figure 3b). As reported previously, the MLs of wood microveneers due to the photodegradation of wood polymers mainly occurs due to the fragmentation of wood polymers, initially photo-sensitive ones like lignin and then holocelluloses, and finally leaching the degraded polymers from wood surfaces (Gascón-Garrido et al. 2016; Xie et al. 2005). The low ML of huminated microveneers with or without acidic catalysts might be related to the distribution and bonding of humins within the wood cells. However, it is not evident if this occurred through merely the formation of a corset-like photo-stable resin matrix around wood polymers, e.g., lignin, for protecting against photodegradation and/or jointly by the substitution of wood hydroxyl groups with active functional groups in humins and presented catalysts.

Figure 3:

Mass loss (ML) of reference and modified microveneers after 48 h, 96 h, and 144 h weathering (a) and calculated mass loss ratio (ML_r) of the samples after 144 h of weathering (b). Different superscript letters indicate a significant difference between the reference and with different treatment solutions samples at p < 0.05 level.

The finite-span micro-tensile strength evaluation of wood microveneers was previously reported to be an accurate measure of the depolymerization level of wood polymers (Derbyshire and Miller 1981; Evans and Schmalzl 1989; Klüppel and Mai 2012). Figure 4 shows substantial changes in the tensile strength of microveneers as a function of humination and artificial accelerated weathering. In unweathered samples, humination considerably reduced the tensile strength of wood microveneers (Figure 4a–c). A major impact was observed in the samples modified in the presence of CA, as the microveneers showed larger tensile strength losses, i.e., veneers modified with HCA 4.5 % illustrated 42.8 % lower tensile strength than the reference sample (Figure 4a). The modification with HSA, however, followed a different pattern by providing about 50 % tensile strength loss in the microveneers modified with 4.5 % SA and larger strength losses at lower SA levels as compared with unmodified reference samples. The tensile strength of microveneers decreased drastically after the artificial accelerated weathering in comparison with the unmodified reference samples. Although, unmodified wood microveneers showed steeper tensile strength reduction in comparison with the modified samples during the first 48 h exposure time, a gradual reduction was observed with increasing the exposure time. In contrast, modified microveneers demonstrated almost constant and gradual strength losses until 96 h of exposure and then drastic reduction after 144 h of accelerated weathering, where the highest tensile strength loss of almost 70 % was observed in the huminated microveneers in the presence of 4.5 % SA (Figure 4b). The differences in the tensile strength of unmodified and modified microveneers might be explained by the modification and weathering effect on the polymeric structure of wood. The tensile strength loss of unweathered huminated wood might be related to the rigidity of the auto-crosslinked humins network (Tosi et al. 2018) in the wood matrix, even though this rigidity was unable to provide considerable interfiber strength to the wood structure after artificial accelerated weathering. In the HCA-modified microveneers before weathering, the polysaccharides degradation of wood, e.g., hemicellulose and amorphous part of cellulose, by CA could be considered as an additional reason for the strong strength reductions (Ando and Umemura 2021; Gauss et al. 2021; Lee et al. 2020; Mihulja et al. 2021). As reported by Klüppel and Mai (2012), the finite span tensile strength of wood microveneers is sensitive to the degradation of polysaccharides. This degradation effect, however, seemed to be offset by the formation of the photo-stable cross-linked wood-humins-CA network at 3 % and 4.5 % CA concentration level. These results are in accordance with Xie et al. (2005), who reported a lower percentage of micro-tensile strength loss in DMDHEU-treated wood after 144 h of artificial accelerated weathering. Slight tensile strength loss of unweathered HSA-modified microveneers might be related to the formation of a crosslinked network with partial degradation of lignin caused by SA (Kim et al. 2021). As previously claimed, lignin limits the accessibility of water to the polysaccharides and mainly contributes to increasing the wet tensile strength of wood veneers (Klüppel and Mai 2012), and thus lignin degradation by SA in unweathered microveneers showed a marginal effect on the tensile strength of wood. However, due to the fact that the finite-span tensile strength is affected by the network, the role of lignin became apparent, and leaching of degraded lignin fractions from the wood structure due to weathering resulted in obvious tensile strength loss in huminated microveneers at higher concentration of SA after 144 h accelerated weathering test. Overall, the results indicated the effect of humins on protecting wood polymers against photodegradation. The role of catalysts in the humination of wood became more apparent by evaluating the tensile strength after 144 h of exposure, where the samples modified with humins in the presence of 3 % CA and also 1.5 % SA illustrated statistically significant lower tensile strength loss than the other formulas.

Figure 4:

Tensile strength (MPa) (a, c) during QUV weathering in unweathered, 48 h, 96 h, 144 h and tensile strength loss (%) (b, d) of the reference and modified microveneers with humins and different concentrations of citric acid (CA) and succinic acid (SA) after 144 h exposure. Different superscript letters indicate a significant difference between the reference and with different treatment solutions samples at p < 0.05 level.