A rapid and quantitative method for assessing the whiteness of whitened lignin based on an in-depth analysis of reported methods

Highlights

• A model for characterizing the brightness of whitened lignin was proposed in light of the Munsell color system.

• The lignin and dark-colored objects can be characterized quickly and quantitatively with the proposed method.

• An in-depth analysis on the defects of previously reported methods was presented.

Abstract

The modification of lignin to a lighter color has been a concern in lignin valorization. In the previous reports on lignin color reduction, the methods of assessing the whiteness of lignin varied from each other, due to the absence of a universal method for characterizing the lightness/darkness of dark samples. In this paper, the reported methods for assessing the whiteness of lignin were analyzed, including “comparison of absorbance curve”, “absorbance at specific wavelength”, “ISO or TAPPI brightness” and “photograph observation”. Our study revealed that various defects existed in these methods. In light of the Munsell color system, we herein proposed a quantitative method for evaluating the whiteness of lignin, based on the finding that the sum of the reflectance of samples with the same Munsell value (whiteness) are substantially equal. A model for converting the reflectance of lignin to Munsell value was established, and the effectiveness of the model was verified and discussed. The standard deviation of the model ranges from 0.23-1.10. The applicability of the method to liquid was also discussed in the paper. Moreover, because the model was derived from the Munsell color system, it can also be applied to characterize other medium to dark-colored objects besides lignin.

Introduction

Lignin is the second most abundant natural renewable polymer [1], most of which is produced as a byproduct of the pulping and paper industry. Lignin valorization has gained increasing attention in the past decades. To date, it has been successfully modified and used as dye dispersants [2,3], fibers [[4], [5], [6]], drug additives [[7], [8], [9]], polymer blends [10,11], etc. However, most of the lignin is still burned as a heat resource in the paper mill, because of its dark color and many other reasons. Several chromophores that account for the black color of lignin, such as quinone methides, quinones, conjugated CC structures and free radicals, have been reported [[12], [13], [14]]. To expand the application range of lignin, many methods for lignin whitening have been reported in recent years. However, because there is not a universal method for characterizing the lightness/darkness of dark objectives such as lignin, the previously reported method for evaluating the effectiveness of lignin color reduction varied from each other.

Wang and Zhang used the “absorbance curve” to demonstrate the whiteness difference [15,16], where the upper curve represents more absorption and hence a lower level of whiteness; and the lower curve represents less absorption and thus a higher level of whiteness. This method is intuitive when the whiteness difference of the compared samples is obvious. However, if the two curves crossed in the visible light range, then it is difficult to differentiate the samples by their relative positions. Another drawback of the method is “the concentration problem”. To compare the absorbance curves, the tested samples must be prepared with the same concentration. Some researchers prepared the solution with the same mass concentration [16], while other researchers holds that, only when the absorbance at 204 nm (the characteristic peak of lignin) reaches the same value, can the tested lignin solution reach the same concentration [15,17]. However, there has been no conclusion about which procedure is more desirable so far.

In some studies, the whiteness difference was shown by demonstrating pre- and post-treated lignin photographs [18,19]. This method depends on subjective evaluation, and the results cannot be presented quantitatively. Moreover, as our research reveals, the appearance of lignin powder is influenced greatly by the grinding conditions which should be defined explicitly when demonstrating the photographs of solid samples.

ISO and TAPPI issued a series of standards for evaluating the brightness of paper and boards [20,21] that were also introduced into the process of assessing the whiteness of lignin [2,22]. However, as explained by ISO 2470-1:2016, this method is designed for “white and near white paper, pulp and board” [20]. While the color of lignin, even the whitened lignin, is far from white or near white, which suggests the evaluation of lignin whiteness is out of the scope of this method. Although the measured brightness of the whitened lignin increased as reported, the obtained brightness values were not convincing. Lin and Zhang [2,23] improved this method by dropping the lignin solution on white cloth and paper, and then performing measurement on the stained cloth or paper. The improved process is reasonable, although the testing was complex and time-consuming.

The Munsell color system is a universal method for color characterization that is widely used in industries such as packaging, painting, and fabricating, etc. This system was designed by Albert H. Munsell in 1905 and has since evolved, and it is now defined by the international standard-ASTM D1535 [24]. In the system, three parameters are specified, the hue, chroma and value, where the value represents the lightness or darkness. The Munsell value (Mv) is divided into 11 levels (0−10), where higher levels indicate the tested objects are whiter. The Munsell color system is usually presented as the Munsell book of color, which contains color patches with varied hue, chroma and value. One of the advantages of the Munsell color system is that the Mv is evenly divided, and it agrees with the human visual perception [24]. Theoretically, for each lignin sample, a similar color patch can be located in the Munsell color system, and the whiteness of the lignin is therefore represented with the Mv of the matched patch. This method is accurate and quantitative; however, it is obviously a laborious task to locate a color patch through visual matching. Besides, the color patches in the Munsell book are vulnerable when exposed to the chemical environment.

In this study, it is surprisingly found that the sum of the reflectances of samples with the same Mv were substantially equal (especially the ones with lower brightness), regardless of their hues. Based on this finding, a mathematical model converting the reflectance to Mv was established, which makes it possible to obtain the Mv of lignin from a spectrometer instead of visual matching. In this paper, the effectiveness of the model was verified, and the values obtained with the proposed method were consistent with the result from the matching process, which indicates the proposed method can be used for lignin whiteness characterization. Moreover, compared with the Munsell color system, the Mv obtained with this method is preferable in practice, because the Mv levels in the Munsell color system is discrete and limited to 11, while the Mv of most of the samples is not precisely the given value as Munsell system defined. This method can also be applied to solution samples by using its transmittance, which is also discussed in the paper.