What is a paper analysis

New insights into the chemical structure of paper


1 Raman image of the cross-section of a multi-layer coated special paper New insights into the chemical structure of paper Applications of Raman microscopy in paper analysis With a few exceptions, paper is a very complex product. In addition to the pulp, it contains many different components, such as fillers, pigments, sizing agents, wet strength agents and other chemical additives that are important for the use and processing properties of paper. The surface of the paper also often consists of coatings that contain several components and can be multilayered. This complicated composition of papers is always a great challenge for paper analysis, especially when ingredients are present in low concentrations and their microscopic distribution in the paper cross-section is to be examined. The application of Raman spectroscopy or Raman microscopy offers new possibilities for solving these questions. Enrico Pigorsch, Heidenau Paper Technology Foundation, Matthias Finger, Heidenau Paper Technology Foundation, Steffen Thiele, Dresden University of Technology, Bioanalytical Chemistry, E. Brunner, Dresden University of Technology, Bioanalytical Chemistry, Raman Spectroscopy Infrared (IR) and near-infrared (NIR) -Spectroscopy has long been used for paper analysis. Both methods have in common with Raman spectroscopy that they can be used to observe molecular vibrations. However, while IR spectroscopy is based on the absorption of (infrared) radiation, the Raman spectrum is created by a scattering phenomenon. 1 The sample is irradiated with monochromatic light (laser). The radiation that is backscattered by the sample (the molecules) is measured. During the scattering process, the molecules can absorb energy from the excitation radiation and are excited to vibrate. As a result, the scattered radiation then has a lower energy (greater wavelength). These energy shifts (Raman shifts) are recorded as the Raman spectrum. The Raman bands, like the IR bands, can therefore be assigned to specific molecular vibrations. IR and Raman spectroscopy differ in terms of the observability of molecular vibrations and therefore complement each other and are complementary. In the IR spectrum one mainly observes vibrations in which the dipole moment of the molecule changes, i.e. usually when two different atoms vibrate against each other (C-H, C = O, C-N, etc.). In contrast, vibrations are Raman-active when the polarizability of the molecule changes. This means that vibrations can also be observed or give intense bands in which the same atoms vibrate against each other (C-C, C = C, etc.). 582 weekly paper for paper production 9/2015

2 A major advantage of Raman spectroscopy compared to IR spectroscopy for paper analysis is that the intense IR bands of the OH and CO vibrations of the cellulose and the adsorbed water are practically non-existent in the Raman spectra, as these Molecular vibrations have little or no Raman activity. As a result, the characteristic bands of the other paper constituents can be observed and analyzed much better. Furthermore, the Raman bands are relatively narrow, so that there is less band overlap. Fig. 1 shows the comparison of the Raman spectrum with the IR spectrum of cellulose (eucalyptus pulp). Similar bands or band positions can be seen in both spectra. In the Raman spectrum, however, the intense and broad bands of the OH stretching vibrations at 3335 cm -1, the OH deformation vibrations at 1639 cm -1 and the C-O vibrations around 1040 cm -1 are missing. Raman spectroscopic measurement technology Thanks to the technological development of the measurement technology on which it is based, Raman spectroscopy has developed from a purely scientifically usable analysis method to a method that can be routinely used in the laboratory for about 15 years. The main technical advances were the development of diode lasers of different wavelengths, of highly sensitive CCD detectors and of confocal Raman microscopes. 2 With confocal Raman microscopy in particular, the essential advantages of Raman measurement technology, such as the high spatial resolution of up to 1 µm and the high chemical specificity, can be used extensively. Many additives only occur in very low concentrations in the paper or in the coating and are therefore difficult or impossible to detect, especially if only integrating measurements can be carried out over a larger measuring spot. Due to the high spatial resolution of Raman microscopy, the detection can be significantly improved, since the measurement is carried out precisely on the substance deposits, such as, for. B. on a fiber or in cavities of the fiber network. Interfering factors in Raman measurements can be fluorescence and absorption effects. The occurrence of these effects depends on the laser wavelength used and the laser power as well as on interfering substances in the paper sample. Fluorescence bands superimpose the actual Raman spectrum and absorption phenomena lead to heating of the sample and to burns, especially with colored or dark papers. These disturbances can be minimized by choosing a suitable laser wavelength and by adapting the laser power. A laser wavelength of 785 nm offers a good compromise between sufficient Raman signal strength and low fluorescence tendency. More signal intensity can be achieved with a shorter and therefore more energetic wavelength of 532 nm. However, this increases the likelihood of fluorescence occurring. Both laser wavelengths were used for the Raman measurements on papers shown below. The measurements were carried out on a Raman microscope HoloLab Series 5000 (Kaiser Optical Systems) and on a Raman microscope alpha 300M + (WITec GmbH). (Fig. 2) The laser power on the samples was between 10 and 30 mw. Fig. 1: IR and Raman spectrum of cellulose (eucalyptus pulp) Fig. 2: Raman microscope alpha 300 M + (WITec GmbH) 9 / 2015 weekly paper for paper production 583

3 Raman imaging A Raman microscope offers the possibility of generating high spatial resolution chemical Raman images, which make the presence and distribution of ingredients on the paper surface or in cross-section visible. The surfaces to be measured are scanned point by point (mapping). The color coding of the Raman images can take place as a function of the intensities of characteristic bands of the different substances (univariate evaluation) or by the multivariate evaluation of spectral differences in all Raman spectra of the data set. 3 The visualized chemical information from the Raman images is an important and meaningful addition to the previous standard method for analyzing the paper structure in the z-direction, scanning electron microscopy (SEM). SEM allows the fiber structure and the layer structure of paper to be monitored with high spatial resolution represent from below 1 µm. The disadvantage, however, is that practically no chemical information is obtained and identification, in particular of organic substances, is not possible. In contrast, the Raman images, which have a spatial resolution similar to that of SEM images, can provide additional chemical information on the visually recognizable structures of the SEM images. Raman imaging, in combination with scanning electron microscopy, enables new insights into the chemical structure of paper and the distribution of ingredients in a way that was not possible with the previously used analytical methods. The possible applications of this new measurement technology will expand even further due to the progressive metrological development. The first device systems with a direct combination of SEM and Raman spectroscopy have been on the market since 2014. 4 The PTS has already presented the possibilities and applications of Raman microscopy for the chemical analysis of paper in several publications. 5, 6, 7, 8 The following shows some application examples of Raman spectroscopic investigations on paper systems relevant in practice. Analysis of the layer structure of paper and cardboard The structure of multi-layer coatings on paper and cardboard can be visualized with high spatial resolution and chemically analyzed using Raman imaging measurements. In Fig. 4, the SEM image of the cross section of a photo inkjet paper is shown on the left. A multi-layer structure can be seen. But only the Raman measurement is able to precisely determine the chemical composition of the layers. The corresponding Raman image is shown on the right. The intensities of characteristic Raman bands of the chemical compounds contained were used for the color coding of the layers. All layers of the paper could be dissolved. The Raman spectra of the three layers and the base paper are shown in Fig. 5. Fig. 3: Raman spectrum of a paper coating Fig. 4: SEM and Raman image of the cross section of a photo inkjet paper Analysis of coating compositions The analysis of the chemical composition of coatings on paper and cardboard can be much more detailed and can be carried out more clearly than with the previously standard ATR-IR measurements (ATR Attenuated Total Reflectance). Fig. 3 shows the Raman spectrum of a paper coating. The characteristic Raman bands of the white pigment kaolin can be seen at 3696 and 3622 cm -1 and of the binder polyvinyl acetate at 2935, 2874 and 1731 cm -1. Due to the high spatial resolution of the Raman measurement, blue pigment particles added to the coating color, such as the violet color pigment PV23, can also be detected. It can be recognized by the characteristic band triplet at 1434, 1392 and 1348 cm -1. The intense and sharp Raman bands of titanium dioxide at 640, 517 and 397 cm -1 can be assigned to the modification anatase. Fig. 5: Raman spectra of the coating layers of photo inkjet paper 584 weekly paper for paper manufacture 9/2015

4 What is remarkable about this result is that the Raman measurement also made it possible to make the optically almost indistinguishable lower layer of the color receiving layer (red) visible. The visualization of components in the paper that cannot be visually recognized in the SEM image is one of the main advantages of Raman microscopy. Distribution analysis of paper components in the z-direction The distribution of ingredients and additives along the paper cross-section (z-direction) has a significant influence on the usage and processing properties as well as the functionality of paper. Knowledge of the z-distribution of paper components is therefore important. With its high spatial resolution and high chemical specificity, Raman microscopy enables the detection of the smallest differences in concentration of substances and thus the visualization and distribution analysis of paper components along the paper cross-section. Possible applications include Investigations into line binder distribution, starch distribution, resin migration (decorative papers), penetration depth of printing inks and distribution of impregnating agents. The following example in Fig. 6 shows the distribution analysis of starch in a two-ply abrasive base paper. 5 The two layers contain mass starch and are glued together with spray starch. Furthermore, surface thickness was applied on both sides. In order to obtain a representative picture of the thickness distribution throughout the paper, the Raman measurements were made on four sections of the cross-section at a distance of 500 µm. The distance between the measuring points is 2 µm. The color coding of the Raman image was carried out on the basis of the intensity of the characteristic Raman band with a thickness of 855 cm −1. In Fig. 7 the corresponding spectral range is shown with the further strength band at 938 cm -1. The spectra are normalized to the cellulose band at 900 cm -1. In the Raman image, the areas with starch are shown in yellow-red, with red areas indicating higher starch concentrations. A distribution curve for the thickness was calculated from the intensities of the Raman band at 855 cm -1 in all four measured paper cross-sectional areas. (Fig. 8) It clearly shows the increased starch concentrations on the paper surfaces and at the glue point in the middle. Investigations on historical documents and works of art on paper Raman spectroscopy is also used in a variety of ways for the chemical analysis and forensic investigation of paper documents and works of art. The majority of the known works, however, are investigations of printing inks and paints on paper. 9, 10 Only relatively few studies have so far dealt with the analysis of the paper carrier itself. 11,12 A closer look at the chemical composition reveals that paper does not only consist of the deliberately added ingredients. In addition, there are substances in paper that are carried along with the main components that arise from other compounds during paper manufacture or that get into the paper during use or aging. It is known that even old papers (before 1800) contain small amounts of calcium carbonate, which are produced by hard water or by the use of milk of lime (Ca (OH) 2) in rag processing. 13 Fig. 6: Raman image of the starch distribution (yellow-red areas) in an abrasive base paper Fig. 7: Raman spectra of areas of different starch concentrations along the cross-section of an abrasive base paper Fig. 8: Distribution curve for starch in the abrasive base paper 9/2015 weekly paper for paper production 585

5 If it is possible to detect and identify these substances, which only occur in traces, additional important, meaningful and ultimately conclusive information on the type and time of manufacture, the actual equality of papers and the use of the papers, etc. be won. Fig. 9 shows a clear example of the efficiency of the imaging Raman spectroscopy and the informative value of the analysis results. 8 In a similar way, statements about other smallest substance particles or traces of chemical compounds in the paper can be made from the Raman spectroscopic measurements. Fig. 9 shows the Raman image from the surface of an old paper from 1938. The fiber structure (gray) can be represented from the chemical information in the Raman spectra. In addition, particles of three different forms of calcium sulfate CaSO4 are found, the plaster form CaSO4. 2H2O (yellow, Raman band at 1008 cm -1), the partially dehydrated form bassanite CaSO4. 0.5H2O (blue, 1015 cm -1) and the anhydrite CaSO4 (red, 1017 cm -1). It is assumed that CaSO4 was not introduced into the paper as the actual filler, but is initially formed as gypsum from the sulphate of the sizing aid aluminum sulphate and the calcium ions in the process water. When the paper web is dried in the paper machine, dehydration takes place in stages at temperatures above 120 C. Summary The application examples presented demonstrate the great potential and new possibilities that Raman microscopy offers for paper analysis. In combination with scanning electron microscopy, new insights into the chemical structure of paper and its correlation with the usage and processing properties of paper and paper products can be obtained. This opens up new analysis options for product development, process optimization and quality control for the paper industry. Acknowledgments The presented results were inter alia. achieved in the context of the research projects IK-MF Raman-Chemical-Imaging and IK-MF paper age determination, which were funded in the program "Innovationskompetenz Ost (INNO-KOM-Ost)" with funds from the Federal Ministry for Economic Affairs and Energy (BMWi). We would like to take this opportunity to thank you for this. References 1 R. L. McCreery, Raman Spectroscopy for Chemical Analysis, Wiley Interscience, New York Th. Dieing, O. Hollricher and J. Toporski (Eds.), Confocal Raman Microscopy. Springer Series in Optical Sciences 158, Springer Verlag, Berlin Heidelberg 2010 Fig. 9: Raman image of a paper surface with particles of different shapes of CaSO4 3R. SalzerandH.W. Siesler (Eds.), InfraredandRamanSpectroscopicImaging, Wiley-VCHVerlag, Weinheim www.witec.de/products/rise-raman-imaging-scanning-electron-microscopy () 5E. Pigorsch, M. Finger, S. Thiele and E. Brunner, Analysis of Starch Distribution in the Paper Cross Section by Raman Microscopy, Appl. Spectrosc. 67 (1) (2013) E. Pigorsch, M. Finger, St. Thiele, E. Brunner, New Possibilities for Line Analysis in the Z Direction Using Raman Microscopic Measurements, 26th PTS Streicherei Symposium, Munich M. Finger, E .Pigorsch, G. Gärtner, St. Thiele and E. Brunner, Analysis of Packaging Paper and Board along the Cross-Section by Raman Microscopy, 8th CTP / PTS International Symposium on Packaging Design and Recycling, March 2014, Grenoble 8 E. Pigorsch , M.Finger, St. Thiele and E. Brunner, Application of Raman Microscopy to Analysis of Paper in Documents and Works of Art, 8th International Conference on the Application of Raman Spectroscopy in Art and Archeology, Wroclaw, 1-5 September JM Chamlers, H.G.M. Edwards and M.D. Hargraeves (Eds.), Infrared and Raman Spectroscopy in Forensic Science, John Wiley & Sons, Ltd, M. Manso and M.L. Carvalho, Application of spectroscopic techniques for the study of paper documents: A survey, Spectochim. Acta B 64 (2009) V. Librando, Z. Minniti and S. Larusso, Ancient and Modern Paper Characterization by FTIR and Micro-Raman Spectroscopy, Conserv. Sci. Cult. Herit. 11 (2011) A. Balakhnina et al., Raman Microscopy of Old Paper Samples with Foxing, Appl. Spectrosc. 68 (4) (2014) V.W. Clapp, The story of permanent / durable book-paper, Restaurator 1 (1972) Wochenblatt für Papierfabrikation 9/2015