3.1. XRD Results
An X-ray diffractogram of pine wood samples, including the crystal lattice assignments, is shown in
Figure 1. The diffraction pattern is similar to that of cellulose, which was characterised by two main peaks and a broad amorphous background band.
Figure 1.
X-ray diffraction (XRD) diffractograms of uncoated wood samples. (A) control; (B) sample exposed to natural weathering; (C) sample artificially weathered.
Figure 1.
X-ray diffraction (XRD) diffractograms of uncoated wood samples. (A) control; (B) sample exposed to natural weathering; (C) sample artificially weathered.
The lower angle peak was the result of the merging of the diffraction peaks at 2θ = 15° and 16.5° into a broader one, as also reported in the literature [
23], where it is assigned to the [001] crystalline plane. The peak observed at 2θ = 22.4° was assigned to the [002] crystalline plane and was used for the calculation of the crystallinity index CI
XRD.
The effect of natural and artificial weathering on the X-ray diffractograms of uncoated pine wood can be observed in
Figure 1. The peak at 2θ = 22.4° became sharper and its height increased significantly while the height of the other peak at 15.5° remained substantially unchanged. As expected, the effect of weathering was less evident on coated samples, as shown in
Figure 2, because of the protective action of the coating.
Figure 2.
XRD diffractograms of coated wood samples. (A) control; (B) sample exposed to natural weathering; (C) sample artificially weathered.
Figure 2.
XRD diffractograms of coated wood samples. (A) control; (B) sample exposed to natural weathering; (C) sample artificially weathered.
The changes in diffractograms were analysed in order to determine the crystallinity index (CI) and the thickness of crystallites, whose values are reported in
Table 1. The values of CI
XRD agree with those reported by Andersson
et al. [
30] who found a crystallinity of wood varying from 24% to 31% for Scots pine wood not exposed to weather. In
Table 1, ∆CI
XRD and ∆
thickness represent the percentage of variation in properties compared to the control specimen of the same group (coated and uncoated).
The increase of CI can be attributed to the degradation caused by weathering, which reduces the amorphous fractions of wood and, consequently, enriches the relative crystalline content, taking into account that less than one third of the wood polysaccharides are crystalline and the remaining wood constituent are hemicelluloses, pectic substances, or amorphous and para-crystalline regions of the cellulose fibrils. If those amorphous polysaccharides are degraded more than the crystalline cellulose, the overall crystallinity content is expected to increase. The apparent increase in the crystalline fraction of cellulose observed in the studied wood sample well agrees with that reported by Fackler
et al. [
20] and Salaita
et al. [
31] on wood subjected to fungal decay and weathering, respectively.
XRD measurements on samples naturally weathered for two years were also performed. The results were not significantly different from those obtained on one year weathered samples. This suggests that degradation of the amorphous phases could become a steady process when some of the species produced by degradation and which are insoluble in water may form a surface layer which protects the underlying wood from further degradation.
Table 1.
Crystallinity index CIXRD and crystallite thickness as obtained by XRD measurements on weathered wood.
Table 1.
Crystallinity index CIXRD and crystallite thickness as obtained by XRD measurements on weathered wood.
Sample | CIXRD (%) | ∆CIXRD (%) | Crystallite thickness (nm) | ∆thickness (%) |
---|
COATED | | | | |
control | 22.1 | – | 3.10 | – |
naturally aged | 27.4 | 24% | 3.50 | 13% |
artificially aged | 28.1 | 27% | 3.61 | 16% |
UNCOATED | | | | |
control | 33.1 | – | 3.11 | – |
naturally aged | 46.3 | 40% | 3.80 | 22% |
artificially aged | 63.1 | 90% | 4.09 | 31% |
The crystallite thicknesses, determined by applying the Scherrer formula (
Table 1) for pine wood was 3.1 nm, in agreement with the literature [
30]. Weathering resulted in an increase of the crystallite thickness from 3.11 to 3.84 nm for uncoated wood and from 3.10 to 3.61 nm for coated wood. This phenomenon has been also observed in the literature for heat-treated wood [
19]. In this case, it can be assumed that a reduction of molecular weight of cellulose due to thermo-oxidative processes is associated with an increase of CI. Howell
et al. [
19] hypothesised that the apparent changes in CI and crystallite size may be due to a re-crystallization of the semicrystalline wood component after the removal of the amorphous fraction (lignin and hemicellulose). However, this process is difficult to observe with FT-IR because of the difficulty to separate the intensity bands related to hemicelluloses and semicrystalline cellulose.
3.2. FT-IR Results
Since FTIR spectra of wood samples feature overlapping bands, all spectra were analysed after a deconvolution.
Figure 3 and
Figure 4 show FT-IR spectra in the fingerprint region between 1800 and 800 cm
−1 for coated and uncoated pine wood samples, respectively, at different weathering conditions.
The assignments of characteristic IR bands to various components of wood are summarised in
Table 2. The band at 1735 cm
−1 was characteristic of an unconjugated carbonyl group typical of xylan and hemicelluloses. Lignin bands were found at 1595 cm
−1 and 1512 cm
−1 for C=C stretching of the aromatic ring, at 1463 cm
−1 for CH
3 bending and at 1269 cm
−1 for CO stretching in lignin (guaiacyl) and hemicellulose. Finally, typical bands assigned to cellulose were located at 1425 cm
−1 and 1375 cm
−1 for CH
2 and CH bending mode, respectively, at 1163 cm
−1 and 897 cm
−1 and at 1336 cm
−1 for hydroxyl bending, and 1317 cm
−1 for CH
2 wagging, which distinguished between amorphous and crystallised I cellulose [
20,
29].
Figure 3.
Deconvoluted Fourier transform infrared (FT-IR) spectra of coated wood samples. (A) control; (B) sample exposed to natural weathering; (C) sample artificially weathered.
Figure 3.
Deconvoluted Fourier transform infrared (FT-IR) spectra of coated wood samples. (A) control; (B) sample exposed to natural weathering; (C) sample artificially weathered.
Table 2.
Assignments of characteristic absorption IR bands of wood samples in fingerprint region.
Table 2.
Assignments of characteristic absorption IR bands of wood samples in fingerprint region.
Wavenumber (cm−1) | Functional group | Assignment |
---|
1735 | C=O stretching in unconjugated ketones aldehydes and carboxyl | Xylan and hemicellulose |
1595 | C=C stretching of the aromatic ring | Lignin |
1512 | C=C stretching of the aromatic ring | Lignin |
1463 | Asymmetric bending in CH3 | Lignin |
1425 | CH2 bending | Cellulose (crystallised I and amorphous) |
1375 | CH bending | Cellulose |
1336 | OH in plane bending | Cellulose (amorphous) |
1317 | CH2 wagging | Cellulose (crystallised I) |
1269 | CO stretching | Lignin and hemicellulose |
1163 | COC asym. bridge oxygen stretching | Cellulose |
897 | asym. Out of phase ring stretching | Cellulose |
The changes in IR spectra due to weathering of uncoated samples are shown in
Figure 4. The band at 1425 cm
−1 slightly shifted to higher wavelength in aged samples (curves B and C in
Figure 4 and
Figure 5). According to Colom
et al. [
12], a shift of the band at 1425 cm
−1 (assigned to amorphous and crystallised cellulose) to 1430 cm
−1 (characteristic of crystallised cellulose), indicated that the amorphous area of the cellulosic component was more affected by the degradation process, or that partially degraded cellulose was capable of forming new and larger crystals, as also observed by XRD.
The band at 1163 cm
−1 (
Figure 4) slightly shifted to higher wavelengths and became narrower. Usually in crystallised cellulose, this band was located at 1163 cm
−1 while in amorphous cellulose at 1156 cm
−1. A qualitative decrease of amorphous cellulose content can be thus assumed in uncoated samples exposed to weathering [
12,
20].
Figure 4.
Deconvoluted FT-IR spectra of uncoated wood samples. (A) control; (B) sample exposed to natural weathering; (C) sample artificially weathered.
Figure 4.
Deconvoluted FT-IR spectra of uncoated wood samples. (A) control; (B) sample exposed to natural weathering; (C) sample artificially weathered.
Figure 5.
Deconvoluted FT-IR spectra of uncoated wood samples in the region 1300–1450 cm−1. (A) control; (B) sample exposed to natural weathering; (C) sample artificially weathered.
Figure 5.
Deconvoluted FT-IR spectra of uncoated wood samples in the region 1300–1450 cm−1. (A) control; (B) sample exposed to natural weathering; (C) sample artificially weathered.
After weathering the C=O stretching band at 1740–1720 cm
−1 can increase, due to an oxidative degradation of lignin caused mainly by UV light [
1]. Furthermore, the C=O stretching band at 1740–1720 cm
−1 can decrease, due to degradation of acetyl groups [
20,
29]. Moreover, a shoulder around 1730 cm
−1 can be due to oxidised cellulose and lignin [
1,
20,
29]. No considerable changes in the absorbance at 1735 cm
−1 were found for coated samples after ageing (
Figure 3), suggesting no photo-oxidation of the wood surface or consistent deacetylation of hemicellulose. On the other hand, a significant decrease in the absorbance value at 1735 cm
−1 (from 0.51 to 0.15) for uncoated wood after artificial ageing (curve C in
Figure 4) is suggestive of a consistent degradation of acetyl groups promoted only by artificial treatment onto unprotected wood surfaces. A detailed analysis of carbonyl groups region for wood plastic composites subject to degradation was described by Fabiyi
et al. [
32,
33]. Four different carbonyl groups were found in the region 1800–1680 cm
−1, mainly as overlapped bands, assigned to γ-lactone (1800–1765 cm
−1), ester (1745–1730 cm
−1), hydrogen-bonded carboxylic acids (1725–1715 cm
−1) and conjugated ketones (1700–1685 cm
−1). They found an increasing concentration of both esters at 1735 cm
−1 and carboxylic acid at 1715 cm
−1 upon xenon-arc and UVA weathering, while with longer exposure time, these concentrations began to decrease [
32]. In our study, for coated samples, the region between 1740 cm
−1 and 1710 cm
−1 (esters and carboxylic acids) remained almost similar after ageing. The uncoated samples after natural ageing showed a very slight decrease of the ester signal and a small signal at 1714 cm
−1 due to carboxylic acid. After artificial ageing, only a shoulder at 1714 cm
−1 was observed in the uncoated samples confirming that, after a stronger ageing, both ester and carboxylic acid underwent remarkable degradation.
A quantitative analysis was carried out, focused on the changes of the intensity of the absorptions at 1735, 1512, 1463, 1425, 1375, 1336, 1317, 1269, 1163, and 897 cm
−1. All the intensities of IR bands were normalised to the intensity of the 1024 cm
−1 band in the deconvoluted spectra. Even if the chosen band, due to both lignin and carbohydrate C–O stretching, did not remain completely constant in all spectra, it represents one of the less variable bands during the ageing. All data are summarised in
Table 3.
Table 3.
FT-IR absorbances average values of wood samples. All values are normalised to 1024 cm−1.
Table 3.
FT-IR absorbances average values of wood samples. All values are normalised to 1024 cm−1.
| Sample | Band frequency (cm−1) |
---|
| 1735 | 1512 | 1463 | 1425 | 1375 | 1336 | 1317 | 1269 | 1163 | 897 |
---|
Absorbance (a.u.) | COATED | | | | | | | | | | |
control | 0.52 | 1.06 | 0.59 | 0.44 | 0.34 | 0.11 | 0.14 | 0.46 | 0.60 | 0.21 |
naturally aged | 0.43 | 0.99 | 0.59 | 0.46 | 0.38 | 0.15 | 0.19 | 0.41 | 0.64 | 0.24 |
artificially aged | 0.61 | 1.06 | 0.66 | 0.47 | 0.44 | 0.16 | 0.20 | 0.38 | 0.65 | 0.29 |
UNCOATED | | | | | | | | | | |
control | 0.51 | 1.27 | 0.72 | 0.62 | 0.45 | 0.28 | 0.32 | 0.43 | 0.67 | 0.31 |
naturally aged | 0.40 | 0.80 | 0.50 | 0.45 | 0.37 | 0.25 | 0.29 | 0.27 | 0.70 | 0.27 |
artificially aged | 0.15 | 0.61 | 0.39 | 0.39 | 0.38 | 0.22 | 0.30 | 0.24 | 0.80 | 0.30 |
Characteristic lignin bands at 1512, 1465 and 1269 cm−1 show a similar trend. In fact, all these three bands did not change significantly after weathering of coated wood while they showed a significant decrease in absorbance values for uncoated wood samples. An absorbance reduction of 0.47; 0.22 and 0.16 in naturally weathered samples and a reduction of 0.66; 0.33; 0.19 in artificially weathered samples has been found in uncoated samples due to a severe degradation of lignin, whereas lignin degradation seems mostly preserved by a protective layer.
Even if absorbance values considered individually give useful information on wood degradation during the ageing process, additional information has been obtained from height ratios calculated between them. The height ratios I
1375/I
1512 and I
1317/I
1336 are reported in
Table 4.
Table 4.
Absorbance ratios of wood samples with the indication of percentage variation in properties between aged and control samples.
Table 4.
Absorbance ratios of wood samples with the indication of percentage variation in properties between aged and control samples.
Sample | CIFTIR I1375/I1512 | ∆CIFTIR (%) | I1317/I1336 | ∆ (%) |
---|
COATED | | | | |
control | 0.32 | – | 1.27 | – |
naturally aged | 0.38 | 19 | 1.27 | 0 |
artificially aged | 0.42 | 31 | 1.25 | −1 |
UNCOATED | | | | |
control | 0.35 | – | 1.14 | – |
naturally aged | 0.47 | 34 | 1.16 | 2 |
artificially aged | 0.63 | 80 | 1.36 | 19 |
The absorbance ratio I
1375/I
1512 can be used as the crystallinity index (CI
FTIR) of wood considering that the peak at 1375 cm
−1 was characteristic of cellulose and that at 1512 cm
−1 of lignin [
20]. This ratio increased for the coated wood sample after weathering, thereby indicating the degradation of lignin when wood was protected by a coating layer. In accordance to XRD results discussed above, a higher CI was found for uncoated wood, especially when artificial ageing was used, where a significant degradation of lignin was observed.
The absorbance ratio I
1317/I
1336 provided additional information concerning the difference in the degradation process of amorphous and crystalline cellulose. Since the 1317 cm
−1 and 1336 cm
−1 bands were related to the contents in crystallised I and amorphous cellulose, respectively, as reported in
Table 1, an increase in the ratio indicated an increase in crystallinity [
12]. The ratio remained almost constant in all samples except for the uncoated artificially aged sample where a decrease was found. This indicated that the degradation of the amorphous cellulose component became significant only if wood was exposed to the artificial weathering conditions.
Therefore, the FTIR results confirmed the reduction of the lignin and hemicellulose component of wood samples due to weathering as also found by XRD analysis. Moreover, the FTIR technique was able to distinguish between the behaviour of lignin and hemicellulose, suggesting that hemicellulose underwent to a severe degradation only in the artificial ageing of uncoated wood samples.
Assuming CI
FTIR as the I
1375/I
1512 absorbance ratio, the results obtained with the two techniques did not provide the same values of CI in absolute terms. However, as reported in
Figure 6, the results obtained by the two techniques presented the same growth trend with the weathering. Therefore, it was possible to correlate the CI obtained by XRD (CI
XRD) with that one obtained by FT-IR spectroscopy (CI
FTIR).
A correlation between CIFTIR (I1375/I1512) and CIXRD values gave a high correlation coefficient (0.94). The CI obtained from FT-IR (CIFTIR) showed to correlate well with the corresponding CI obtained by X-ray diffraction (CIXRD).
It should be highlighted that the paint was applied on a rough surface and the ageing was responsible for the changing of both the coating thickness and colour. Therefore, it cannot be excluded that the complete removal of the coating may be associated with the removal of a very small amount of wood, in the order of tens of microns, at the interface between coating and wood. This may lead to an underestimation of the values of the crystallinity index. Despite this, the significant differences found in the CI values among control and weathered samples, obtained by XRD and FTIR, demonstrate how both the techniques are sensitive to small changes due to weathering. This is further proof of the reliability of the proposed spectroscopic approach to monitor the degradation of wood.
Figure 6.
Crystallinity index (CI) of wood cellulose determined by XRD and FT-IR.
Figure 6.
Crystallinity index (CI) of wood cellulose determined by XRD and FT-IR.