Pulps produced from Acacia species are competitive in the world market of hardwood pulps now dominated by Eucalyptus. However, pulps produced from Acacia species, namely from Acacia mangium, are emerging in the hardwood pulps market and an increasing number of commercial plantations is developing in Asia (Griffin et al., 2014).

A. melanoxylon was also the focus of research for the wood technological quality, e.g. wood volume (Rucha et al., 2011), heartwood variation (Knapic et al., 2006), basic density (Santos et al., 2012a; Igartúa & Monteoliva, 2009), wood macroscopic properties and fiber characteristics (Santos et al. 2013; Tavares et al., 2011) and mechanical properties (Machado et al., 2014). A. melanoxylon is also interesting for pulping as evaluated by different authors regarding pulp yields and properties (Santos et al., 2012b; Anjos et al., 2011; Lourenço et al., 2008; Santos et al. 2007; Santos et al., 2006; Santos et al., 2005).

One important pulp quality parameter is the ISO brightness, which is correlated with the pulp kappa number, i.e. pulp brightness can sometimes increase with the decrease in kappa number.

Brightness of pulp is the numerical value of the reflectance factor of a sample with respect to blue light of specific spectral and geometric characteristics. It is one of the most important characteristics in pulp characterization given their relation to the bleaching process, the kinds of fibre used in the pulp produced and also correlated with same important chemical characteristics of the pulp, namely kappa number (Santos et al., 2012b; Santos et al., 2008a; Santos et al., 2008b). Given this correlation existing with the chemical pulp composition and brightness is also possible use Near-infrared (NIR) spectroscopy with PLS calibration to predict the brightness.

In addition unbleached Kraft pulps, brightness information is required for planning of bleaching sequences and consumption of bleaching chemicals which are related with the initial brightness of the pulp.

Given Near-infrared (NIR) spectroscopy it is one of the most powerful non-destructive techniques, has been applied to the determination of various properties in numerous materials, such as food (Wang et al., 2014; Fu et al., 2012), ethanol (Xu et al., 2013), and wood lignin (Zhang et al., 2011; Yao et al., 2010; Alves et al., 2006; Hodge & Woodbridge, 2004; Poke et al., 2004; Schwanninger & Hinterstoisser, 2001) and cellulose (Tyson et al., 2010; Hein et al., 2009; Poke et al., 2006), wood properties and basic density (Santos et al., 2012a; Alves et al., 2012; Sykes et al., 2005; Kelley et al., 2004; Hauksson et al., 2001), pulp yield, kappa number and mechanical and optical properties of pulps (Santos et al., 2014; Downes et al., 2009; Schimleck et al., 2006; Fardim et al., 2005; Schimleck et al., 2005; Fardim et al., 2002).

NIR spectroscopic procedures are well suited to an automated on-line application for continuous control of various properties of the material, and the same collected spectral information may be used to estimate different parameters, if the specific models have been developed and validated. This paper focuses on the development of a NIR-PLSR model for the prediction of the ISO brightness of unbleached Kraft pulps produced under identical pulping conditions, as it would be the case in pulp mill operation, using A. melanoxylon pulps as case study.

Moreover the brightness analysis in pulp is already made with a not painful methodology this research could improve the work in a laboratory or in factory because many other parameters are available to be analysed with NIR infrared spectroscopy and all analyses could be performed in the same equipment.

The ISO brightness of the unbleached pulps of A. melanoxylon obtained in this study ranged from 18.9 to 47.9 %, with an average of 28.0 % and a standard deviation of 6.1% (Table 1). This variability represents a good data scattering and covered a wide range of levels of pulp brightness that improve the accuracy of the final model proposed. The two sets showed similar statistics.

The coupled spectral information from two wavenumber ranges from 9404 to 7498 cm-1 and 4605 to 4243 cm-1 was used for calibration, as found by automated optimization. The near-infrared data from the 48 samples of the calibration set were regressed against their experimentally determined ISO brightness, and the results obtained with the various pre-processing of the raw spectral data are summarized in Table 2. The obtained rank ranged from two (MinMax, VecNor, 1stDer+VecNor) to six (no spectral pre-processing), while the coefficients of determination (r2) ranged from 88.1 % to 99.4 %, and RMSECV from 0.4 to 1.5 %. When using the test set validation, the coefficients of determination ranged from 90.6 % to 99.5 %.

The best model was selected by using the 1stDer (first derivative) of the spectral data and the corresponding plot of NIR-PLSR predicted versus the laboratorial determined ISO brightness is shown in Figure 1. Both the cross-validation and the validation showed high correlation between predicted and determined values, with a RMSEP of 0.5 %, a rank of five and no outliers.

In this study, the ISO brightness of hardwood unbleached kraft pulps is predicted with a very high accuracy and precision (0.5 %) and with RPD = 14.7. These results compare very favourably with the few data available, as shown in Table 3 that summarizes the models published in the bibliography regarding estimation of pulp brightness using NIR spectroscopy. Fardim et al., (2002), using unbleached and unrefined kraft pulp, reported a root mean square error of prediction of 1.3 % for ISO brightness, with 3 PLS components and 1stDer as pre-processing.

The second model published by Fardim et al., (2005), was based on one sample of Eucalyptus grandis wood that was pulped and refined with different conditions, presented a RMSECV = 1.2 %, with a Rank = 6.

Nilsson et al. (2005) proposed two models for prediction of ISO brightness in thermomechanical pulps of Norway spruce bleached with hydrogen peroxide (45 samples) and sodium dithionite (45 samples). In both cases it was possible to predict the ISO brightness of the bleached thermomechanical pulps from near-infrared spectra on pulp with lower errors of prediction (RMSEP).

The ratios of performance to deviation (RPD) may be used to evaluate if the prediction models fulfill the requirements of the AACC Method 39-00 for process control that require a RPD ≥ 8 (AACC Method 39-00:15). The RPD was introduced by Williams and Norris (2004) as the ratio between the standard deviation of the reference data of the validation set and the standard error of prediction of a cross-validation or of the test set validation.

In the present case, the RPD for the validation of the NIR PLS-R model was 14.7 (Table 2), thereby allowing the conclusion that it is applicable for process control.

Therefore pulp NIR spectra may be used in prediction of ISO brightness. It is noteworthy that once NIR spectra are obtained, they may provide additional information e.g. kappa number of the pulp.

The results demonstrated the potential of using NIR spectra for prediction of ISO brightness of unbleached hardwood pulps in a variation range of 18.9 to 47.9 %, with an accuracy that fulfils the requirements to be used in industrial control processes.