A rapid method of predicting wood chip decay using FTIR spectroscopy

Pulp & Paper Canada T 116 106:5 (2005) ❘ ❘ ❘ 41 HE ADVERSE EFFECTS of wood decay on chip quality, pulp yield, and pulp quality have been well documented [8,2024,27]. Decay can enter the chip supply either by utilization of decayed standing wood [14] or through prolonged or improper storage [16]. The net effect of using decayed wood for pulping is cost increases due to decreases in pulp yield, reduced pulp quality, and an increase in the variability of pulp properties as decay content fluctuates [20]. Pulp produced from decayed wood has lower tear and tensile strengths than pulp produced from equivalent sound wood [8,23,24]. In kraft pulping, decayed wood causes an increase in alkali consumption, kappa number, shive content and screen rejects [8,14,22,24]. For an increase of 1% in caustic solubility, mills can expect a decrease of 0.5 to 1% in pulp yield and an increase of 0.1 to 0.3% in effective alkali consumption depending on species mix [20]. Accurate quantification of decay would enable mills to optimize their chip handling procedures to minimize the effects of decay, evaluate the quality of incoming chips, optimize storage times and adjust cooking conditions. An important limitation of current chip storage and utilization procedures is the inability to accurately and rapidly determine the extent of decay present in wood chips. One percent caustic solubility (PAPTAC standard G.6, G.7) and buffering capacity are the best industrially applicable methods currently used to estimate decay content [4,21,24,25]. However, both methods are so laborious [4,10,12,23] that less than 10% of U.S. pulps mills perform these analyses routinely [4] with similar testing rates also expected in Canada. Hand sorting of screened chips is also used to estimate rot, but is plagued by inaccuracy. Other researchers have estimated decay content by using a ball-milling technique [13], but this is affected by moisture content, wood species, and is limited in its industrial applicability due to its dependence on chip dimensions and chipping technique. Infrared spectroscopy has been used to detect incipient decay in Douglas fir and Southern yellow pine, and to determine acetyl, lignin, glucose and xylose content in wood [6,9,17,18,24]. More recently, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and multivariate statistical analyses, such as Partial Least Squares (PLS), have been used to estimate weight loss due to the fungal decay of wood [2,3]. PLS is a multivariate statistical method that regresses instrumental data (in this case FTIR spectral data) against known reference methods. Data are grouped into linear combinations called factors that represent the major components of variance in the spectral and concentration data sets. These factors are used to define a subspace that can model concentration data in a more accurate manner [15]. In the present study, PLS calibration models have been developed that can predict 1% caustic solubility and buffering capacity from the FTIR spectra of trembling aspen, white spruce, lodgepole pine, sub-alpine fir and western hemlock decayed by various fungi. This provides a rapid means of estimating decay extent in a sample of wood chips and thus assessing the value of the wood for pulping. This is of particular interest when chip supplies are limited and for mills that must handle wood from over-mature stands and dead standing wood.