Comparative Study of Brightness/Whiteness Using Various Analytical Methods on Coated Papers Containing Colorants

Fluorescent Whitening Agents (FWA), dyes and pigmented inks are used extensively in the paper industry to enhance the appearance and optical performance of coated and uncoated papers. There are several different systems in use for the description and specification of color and optical properties of paper, using colorimeters, spectrophotometers and brightness meters. CIE and TAPPI are the most commonly used color systems in the paper industry. In this study, three different colors (blue, black and red), and FWA were added into coating formulations from three to five addition levels. Coated LWC papers were measured for their optical properties with two different spectrophotometers and a brightimeter. The CIE whiteness formulation (1996) was analyzed on slightly colored papers. It was found that in some cases, calculated CIE whiteness values increased with increasing amount of colors in the coating layer, although the papers appeared darker or redder to the observers. Small deviations in measured CIE tristimulus functions X, Y, and Z often caused significant changes in calculated CIE, and Ganz whiteness values. Hunter whiteness values also followed the same pattern as CIE and Ganz whiteness, although it is less sensitive to small changes in the tristimulus values. The results show the deficiencies in the current measurement techniques for assessment of brightness and whiteness of wood containing papers and coatings containing optical brightening agents. INTRODUCTION: Other than physical properties of a paper, its appearance is very important, especially to the printer and to the final reader. Today, many free sheet and groundwood containing offset, gravure and ink jet grades contain FWAs (1). Appearance is also important in several other paper grades such as tissue papers. For this reason, there have been many efforts to produce paper at high whiteness levels. The bleaching of pulp, addition of fillers into the paper, coating the paper, and addition of FWAs serve to increase the whiteness of papers. However, their contribution to whiteness and/or brightness of the paper and paperboard is limited due to strength and porosity issues. High whiteness on papers can be achieved only with the proper usage of FWAs (2). Whiteness and brightness are manifestations of optical properties and they cannot be measured directly. Evaluation of whiteness in terms of its visual wavelengths can be made by visual appearance or by instrumental measurements. Only the physical property, the spectral reflectance of a sample can be measured. There are several different systems in use for the description and specification of color and optical properties of paper, using colorimeters, spectrophotometers and brightimeters. CIE whiteness (3) and TAPPI brightness (4) are the most commonly used measurements in the paper industry. However, CIE whiteness, and TAPPI brightness are different concepts and TAPPI brightness may lead to misleading assessments of optical properties of papers that contain dyes and/or FWA’s. Likewise, CIE whiteness may give a different impression of relative whitenesses of two samples relative to what a human observer would conclude. Secondly, measured spectral reflectance is not a standard fixed quantity. It is influenced by a variety of factors. Geometry of the measurement device, aperture, light source, filters and measurement set up all influence the data that are acquired from an instrument. These factors may differ from one instrument to another. Thus, comparing data from at least two different instruments could be an appropriate approach to enable mills and their customers to have more accurate and reliable data for optical analyses of paper that contains dye or FWA. Besides the configuration of the measuring device, the base paper may also complicate the usefulness of the measurement techniques and interpretation of the optical data. This is especially true for LWC (Light Weight Coated Paper) papers that contain large amounts of mechanical pulp. The high amounts of lignin present in this grade cause the product to age rapidly, which results in the yellowing of the paper with time. LWC papers may also exhibit differing properties and different shades of colors from one manufacturer to another, depending on the wood species, proportions of the wood species, and the ratio of mechanical pulp to chemical pulp used in the production of the base paper. Generally speaking, papers containing high amounts of dark, mechanical wood fibers, influence the optical properties of the applied coating more than non-wood containing papers. All these factors make the development of a standard optical measurement method for this grade very difficult. As a result, there has not been a SWOP (5) standard adopted for this grade (6). For this reason, a LWC grade paper was selected for this study. The work herein was performed to better understand the difficulties and complications faced when trying to perform brightness and whiteness measurements on these papers in an attempt to meet the commercial specifications of the printer. TECHNICAL CONSIDERATIONS: When light strikes an object it may be transmitted, scattered, reflected or absorbed and all these may occur separately or in combination. The object appears white if it totally reflects the light, and scatters diffusively at all wavelengths of the visible spectrum. The object appears colored if some wavelengths of the light are reflected while the others are absorbed. It is black if the object absorbs all the wavelengths of the light in the visible spectrum (1,7). By another definition white is the achromatic object color of greatest lightness, characteristically perceived to belong to objects that reflect diffusely nearly all the incident energy throughout the visible spectrum (2,8). In general, all objects absorb illuminating light energy to some degree. The color white, as with any color, can be interpreted in a three-dimensional color space (9). For example, it can be described by hue, saturation and lightness (7). The color white is distinguished by its high lightness, its very low (ideally zero) saturation, and it is felt to be more attractive with a bluish cast rather than yellowish cast (10,11). Depending on the hues of whites, the perception may differ. For example, an object with a bluish cast will be perceived whiter than an object that has a yellow cast, where saturation and lightness are the same for both objects (7,11). 1. Whiteness vs. TAPPI Brightness: Whiteness and brightness are sometimes used interchangeably when comparing the relative whiteness of different papers or when defining how white a specific paper is. However, while these two terms are related, their scientific definitions differ (2,12). Papers having the same degree of brightness, in fact, may differ greatly in visual appearance (1,2). TAPPI brightness is based on the filter chosen to measure the reflectance of the pulp in the region most sensitive to the effects of bleaching (1). Whiteness of paper is measured by the reflectance of a paper’s surface for all wavelengths of the visible spectrum. Brightness of paper is measured by comparing the amount of light, of a prescribed single wavelength (457 nm) in the blue region of the spectrum, reflected by a pad of that paper to the amount that is reflected by an arbitrary standard having 100 reflectance at this same wavelength. The method is defined by TAPPI method T452 (4). The selected standard is magnesium carbonate. This method states that this procedure is applicable to all naturally colored pulps, papers and paperboard. The TAPPI brightness measurement can be deceptive when the papers that contain dyes or FWAs are optically evaluated because the TAPPI brightness measurement technique ignores more than two-thirds of the visible light spectrum. One example of this is the papers that contain FWAs. Modern papers containing FWAs, tinting dyes and inks cannot be properly evaluated by a simple assessment in the blue area (10). Thus, brightness is not a complete description of the visual appearance of the paper (12). For these reasons, the TAPPI brightness measurement method should only be used for comparing undyed and FWA free sheets, where the brightness measured depends only on the degree of blue light absorbed by the pulp or coating (7). Assessment by the TAPPI method generally functions very well only if the reflectance properties of the papers to be compared are similar (10). 2. Whiteness Evaluation: Whiteness can be evaluated both visually and instrumentally (7). However, neither color nor white can be measured directly. Only, the physical property, the spectral reflectance of a sample can be measured. Instrumental characterization of whiteness is made in two steps. First, reflectance spectra are measured. Then, whiteness assessments are developed through some type of graphical or numerical manipulation of the data (2,8). Measured spectral reflectance is not a standard fixed quantity. It is influenced by the characteristics of the measurement device. Full geometry of the illuminating chamber is incorporated in the measuring results. Another two factors that influence the reflectance spectra are the size of the aperture, and whether gloss is excluded or included in measurements (10). For this reason, not all the instruments used today give identical results. Different light sources and different filters give different and subjective assessments (13). Therefore, sometimes comparing data from two different instruments could be an appropriate approach. Accurate data are obtained if the incident light is strictly controlled. For this reason, special instrument design and calibration, and illuminant adaptation is required when measuring paper that contains FWAs (1). Both the selection and the quantity of illuminating light is very important when instrumental evaluation of whiteness on FWA containing papers is to be made, since FWAs are only excited by UV energy. Illuminating light should match as closely as possible to the energy distribution of the daylight standard (D65) both in the visible and ultraviolet spectrum at all times. An appropriate whiteness measurement for measuring dyed or optically whitened papers should include: a) a reproducible ultraviolet rich light source like Xenon arc lamp. [This kind of a light source is closely matched with daylight (D65 standard) both in UV and in visible regions of the spectrum.] b) software or absorbing filters to enable calibrating the UV portion of the illuminant to a constant value, c) a reverse optical system that prevents absorption of the UV and enables light from the source to reflect on the sheet before it is focused on the detector, d) a detector that measures the whole visible spectrum (400-700 nm) with at least 20 nm accuracy, e) proper software to calculate various whiteness values from measured tristimulus values (7). The most effective method to measure optical properties of a FWA containing paper is with a xenon lamp because the radiance of xenon is fairly similar to the energy spectrum of the reference illuminant (D65). Another advantage of xenon light source is that it has a relatively uniform, high-energy emission (1,14). EXPERIMENTAL PROCEDURES A coating suitable for a LWC rotogravure printed-paper grade was prepared according to the formulation outlined in Table 1. The coatings consisted mostly of delaminated clay and SBR latex, and contained smaller amounts of calcined clay, TiO2, plastic pigment, calcium stearate lubricant and ammonium zirconium carbonate crosslinker. For the coatings containing FWA, the TiO2 was replaced with the same volume of plastic pigment, since TiO2 adversely influences FWA’s UV absorbance and so its whitening performance (2,15). Three different dyes [blue (Ciba-Pergasol Blue PTD), black (CibaPergasol Black LVC) and red (CibaPergasol Red 2B)] were added into the coating at three addition levels. Table 1. Coating formulation used in the experiments (See Appendix A for supplier information). Coating Ingredient Parts