APPLICATION NOTE

Quality Management andProduction Control in Paper
Manufacturing

WITec GmbH, Lise-Meitner-Str. 6, 89081 Ulm, Germany
fon +49 (0) 731 140 700, fax +49 (0) 731 140 70 200
info@WITec.de, www.WITec.de

Confocal Raman Imaging and correlative Raman-AFM
for chemical and structural characterization

Quality Management and Production Control in Paper Manufacturing

Confocal Raman Imaging and correlative Raman- Atomic Force Microscopy for chemical and structural characterization

Introduction

Materials & Methods

Papermaking has an over 2000-year-longmanufacturing history and launched a revolution in communication and packaging.
Over the centuries, paper’s contribution
to civilization is difficult to overstate andthe refinement of its production processes
continues to benefit many industries.
Paper typically consists of plant fibers fromwood. Those fibers are mainly composedof cellulose and lignin. The properties ofpaper are determined primarily by thefiber composition. Processes and chemicaltreatments such as pulping, bleaching,sizing and coating further influence thecharacteristics of the produced paper
including suitability for printing.
A thorough understanding of the microand nanostructures of the paper in its successive stages of production as well as thefinal paper composition at a sub-micrometer level is essential for further improvement of its properties. With nano-analytical investigation methods such asconfocal Raman microscopy and atomic
force microscopy (AFM) this detailed
knowledge is readily accessible.
Confocal Raman imaging reveals valuable
information about the spatial distributionof the chemical compounds while AFM
delivers high-resolution images of physicalsurface properties (e.g. roughness). In
combination, the techniques provide acomprehensive examination of the qualityof the produced paper.
This application note presents results ofconfocal Raman and AFM measurementsfrom several stages of the paper manufacturing process followed by examples thatprovide insight into printing and writingon paper.

All experiments were performed with aconfocal Raman-AFM microscope of theWITec alpha300 series. The alpha300
series is optimized for high-resolution
imaging of samples and is able to combine
several analytical measuring techniques in
one instrument such as diffraction-limited
confocal microscopy, chemically sensitive
Raman spectroscopy and cantilever-based
scanning probe microscopy, including AFMand SNOM. Using these instruments it ispossible to analyze the same sample position with various measuring techniques
sequentially or even simultaneously (Raman and AFM). Automated measurement
procedures and large area scans are alsooffered.
The microscope can be equipped with aselection of excitation lasers for confocal
Raman microscopy. For the measurements
presented in this application note, either afrequency-doubled 532 nm NdYAG laser ora 785 nm diode laser (NIR) was used. The
excitation light was polarized horizontally(in the x-direction) with respect to theRaman image. All measurements wereperformed using an ultra-high-throughput
spectrometer (UHTS) and back-illuminated
CCD cameras.
The distribution of chemical species on thesample was determined in Raman spectral
imaging mode. In this mode a complete
Raman spectrum is recorded at every
image pixel, producing a 2D array of Raman spectra. By extracting individual features such as Raman peak intensity, peakwidth or peak position from the recorded
spectra, Raman images can be generated
and fitting algorithms can be employed tovisualize the distribution of compounds
and characteristics. The illustration colors
are coded in the same way as the corresponding spectra. The acquisition time fora Raman spectrum within a 2D spectral

array was only a few milliseconds or 10sof milliseconds, allowing the acquisitionof thousands of Raman spectra withinminutes. The number of pixels and the integration times per spectrum are listed in
the sections describing the corresponding
Raman image throughout this document.
By rotating the microscope turret, it ispossible to switch between confocal Raman microscopy and AFM without losing
the sample position. The samples wereimaged in AFM AC Mode using ArrowFM
cantilevers (Nanoworld, Germany) witha resonance frequency in the range of
70–90 kHz and damping of r = 50 percent,recording both topography and phase
images simultaneously.

alpha300 Raman AFM microscope

Results & Discussion
Paper Production
Depending on the scope of use, paper has
to meet multiple requirements, leading
to variation in production methods. For
the production of packaging paper withshort fiber lengths and a yellowish color,mechanical pulping is sufficient. In thisprocess, lignin is not removed from thepulp, which offers a high production yield.
High quality paper for writing and printing
requires chemical pulping. In this process
the chemical structure of lignin is destroyed and dissolved in the pulp and can be
washed off of the cellulose fibers. Furthermore, the fiber length is preserved for greater strength. The finished paper is thenproduced by feeding the pulp into a paper
machine where it is formed as a paper web
and the water is removed by pressing anddrying. For specialty paper (photographica)

1095b)

1118relative wavenumbers (cm-1)

Fig. 1: Confocal Raman imaging of raw cellulose
fiber bundles. Video image (a), orientation-dependent Raman spectra of cellulose (b) andcolor-coded Raman image of cellulose fibers (c).
Image area 50x50 µm2, 128x128 pixels, integrationtime: 80 ms/spectrum.

paper, technical paper, decorative paper,etc.) further treatments such as coating,laminating or impregnating are necessary
in order to achieve the required properties.
In the following, the spectral differencesof pure cellulose are first illustrated thenfollowed by the analysis of residues on
cellulose fibers.
Raman imaging of a pure cellulose paper
webFig. 1a shows the video image of a paper
web consisting of pure cellulose fibers. The
red square in this video image indicates the area scanned in Raman spectral
imaging mode using green excitation anda high numerical aperture air objective. An
area of 50x50 µm2 was scanned and a 2D
spectral array of 128x128 complete Raman
spectra was recorded with an integration
time of 80 ms/spectrum. From this 2D

array of spectra, two distinct spectra forcellulose were extracted and are shownin Fig. 1b. A significant difference between the two spectra can be seen in theintensity variation of the Raman bands
at wavenumbers 1095 cm-1 and 1118 cm-1.
These two bands are characteristic for C-O
and C-C stretching, respectively, and arelaser polarization-direction dependent.
The distribution of these two Raman
spectra over the examined surface area ispresented in Fig. 1c. The color of the spectrain Fig. 1b matches the color in the Raman
image 1c. Thus, pixels that are bright green
show a high spectral similarity to thegreen spectrum, whereas bright red pixels
show a high spectral similarity to the redspectrum. The laser polarization direction
is indicated with the white arrow in Fig.
1c. Fibers oriented parallel to the laser
polarization direction show an increased
C-O stretching band at 1095 cm-1 (red)laser polarization directionc)

a)celluloseb)d)z-direction

resinc)relative wavenumbers (cm-1)

compared to fibers oriented perpendicular
to the laser polarization direction (green).
Thus, confocal Raman microscopy easily
reveals the orientation of cellulose fibers
in the paper web even if this cannot bedetermined optically.
Raman and AFM imaging of residues on
cellulose fibers
As mentioned before, lignin is washed offof the cellulose fibers during the chemicalpulping. Furthermore, small quantities ofextractives such as resin acids, triglycerides, steryl esters, fatty acids, and sterols
can be separated from wood by using
solvents such as acetone or dichloromethane. During the papermaking process,these extractives are partly released into
the circulation water. Today, efforts are
made to decrease the water consumption
in paper mills. Hence, water is re-circulated
in the process, which leads to a build-up of
dissolved and colloidal substances. The enrichment of extractives in the circulatingwater in particular leads to problems suchas precipitates, paper strength decreases
and foaming. The following two examples
show studies of residues and their distri-Fig. 2: Confocal Raman study of resin
distribution around cellulose fibers. Video
image (a), Raman spectra of resin (green)and cellulose (red) (b), color-coded Raman
image of a surface scan, image area 45x17µm2, 100x100 pixels, integration time:
112 ms/spectrum (c), and a depth profile,image area 45x40 µm2, 100x100 pixels,
integration time: 112 ms/spectrum (d).

9 µmbution on cellulose fibers.
Fig. 2 shows an example of a study of resin
distribution in paper. For this experimenta sheet of paper was soaked in water toreduce the strong fluorescence of thesample. The alpha300 confocal Raman
microscope was equipped with an NIR excitation laser and a back-illuminated deep
depletion CCD camera. A water immersion
objective (NA =1.0) was used for thesemeasurements. The video image in Fig.
2a shows several cellulose fibers oriented
almost perpendicular to the polarization
direction of the laser. The red square denotes the in-plane (x-y) scanned area of 45x17µm2. The blue line indicates where thedepth (x-z) scan was performed. For thein-plane scan an array of 100x100 Raman
spectra was recorded with an integration
time of 112 ms/spectrum. Fig. 2b shows thecharacteristic Raman spectra extracted
from the spectral array. The distributionof cellulose (red) and resin (green) on theanalyzed sample area is shown in Fig. 2c.
Fig. 2d shows a depth (x-z) scan of thesample. In this case an array of 100x100
Raman spectra was recorded over a rangeof 45x40 µm2. The Raman images presen-ted in Figs. 2c and 2d clearly show thatthe resin surrounds the cellulose fibers
without penetrating into them.

A further survey was performed using thecombination of confocal Raman and AFM
to detect traces of wood extractives onpure cellulose fibers. An area of 20x40 µm2
was scanned in Raman spectral imaging
mode by acquiring a spectral array of80x160 complete Raman spectra. From
this spectral array three distinct spectra
were extracted as shown in Fig. 3a. The
green and blue spectra are the orientation-dependent Raman spectra of cellulose,
whereas the red spectrum represents hexane extractives. The distribution of thesethree compounds on the analyzed sample
area is shown in the color-coded Raman
image in Fig. 3b. The white rectangle in Fig.
3b marks the area that was imaged in AFM
Raman b)AC mode. Fig. 3c shows the topography
image of a fiber bundle consisting of small,oriented fibrills, characteristic for cellulose fibers. The simultaneously recorded
AFM phase image (Fig. 3d) reveals in thecentral part an additional contrast which
can be attributed to the hexane extractive.
High-resolution AFM AC images recorded
from this region are shown in Figs. 3e (topography image) and 3f (phase image). The
hexane extractives show a clear contrast inthe phase image, revealing a material withdifferent mechanical properties than thecellulose fibrills underneath.

AFM Topography c)

AFM Phased)a)relative wavenumbers (cm-1)

hexanecellulose I
cellulose IIAFM Topographye)

AFM Phase f)

Fig. 3: Confocal Raman AFM images of traces of wood extractives on cellulose fibers: Raman spectra of cellulose (green and blue spectrum) and hexane
extract (red spectrum) (a), color-coded Raman image of a cellulose fiber. Image area 20x40 µm2, 80x160 pixels, integration time: 100 ms/spectrum. (b),high-resolution AFM topography (c) and phase image (d) of the fiber and zoom-ins (e: topography; f: phase).

a)b)Table 1: Roughness Parameters

Fig. 4: (a) The distribution of calcium carbonate(blue) on cellulose paper (red and green color
show the different orientations of the cellulose
fibers), image area 150x150 µm2, 150x150 pixels,
integration time: 90 ms/spectrum. (b) The distribution of starch (red), calcium carbonate (blue)
and a polymeric material (green) on cellulose
paper (yellow), image area 50x50 µm2, 100x100
pixels, integration time: 512 ms/spectrum.

Paper Finishing
The pure cellulose paper introduced in
the previous section is a yellowish, roughpaper, which can hardly be used for writing
and printing. To provide a paper surfacemore suitable for e.g. high-resolution
halftone screening, the raw paper is coatedwith a thin layer of calcium carbonate andother additives. Confocal Raman imaging
and AFM can provide valuable information on their distribution and effect as thefollowing examples will illustrate.
Fig. 4a shows an investigation of a calcium
carbonate coating on paper. This image
was obtained with a laser excitation wavelength of 532 nm and a 40x air objective(NA = 0.6). An area of 150x150 µm2 wasscanned in Raman spectral imaging mode

and an array of 150x150 Raman spectra
was recorded with an integration time of
90 ms/spectrum. Red and green in Fig. 4adenote the different orientation of thecellulose fibers, whereas blue shows thedistribution of calcium carbonate on thecoated paper. In Fig. 4b additional additives
used in paper coating were investigated.
The confocal Raman image of 50x50 µm2
reveals that the coating of this paper
sample contains, along with calcium carbonate (blue color), starch grains (red ) anda polymer matrix (green ). The cellulose isshown in yellow.
AFM can additionally contribute to theanalysis of paper finishing with high-resolution images that describe the surface roughness of the paper (Fig. 5). The
topography of raw paper is shown in Fig.

5a, revealing the fibrillar fine structureof cellulose fibers with diameters in thenm-range. This surface is rather rough andcannot be used for writing or printing. InFigs. 5b and 5c the topography of laminated paper is shown after applying one andtwo laminations, respectively. The roughness parameters calculated from thesedifferently processed paper surfaces using
the WITec software suite for data analysis
and evaluation are summarized in table
1. From these results it is evident that atleast two layers of lamination should beapplied in order to decrease the surface
roughness. The combined Raman-AFM studies give insight into the chemical natureof the additives and laminates as well as
their effect on the surface roughness.

a)

b)

c)

Fig. 5: High-resolution AFM images of untreated paper (a) and laminated paper after applying one layer (b) and two layers (c).

Paper Coating
Depending on the final application of thepaper, its coating can consist of several layers with different chemical compositions.
Confocal Raman imaging can contribute to
the analysis of paper layers in a nondestructive way. Fig. 6 shows a depth scan (x-zscan) obtained from multi-colored paper.
An area of 60x100 µm2 was scanned in thex-z plane and an array of 100x250 Raman
spectra was recorded with an integration
time of 50 ms/spectrum. This experiment
was performed using an excitation laser
wavelength of 532 nm and an oil immersion objective (NA = 1.25). The distribution of
the various materials in the layers can beclearly determined and imaged as shownin Fig. 6 (information about the chemicalcomposition remains undisclosed due toconfidentiality). The image clearly reveals
the presence of 10 different layers along

a depth of 100 µm. Most of the coatings
used form a well-defined interface without
penetrating into one another.
Another example of modified paper
is shown in Fig. 7. Two adhesives wereapplied to the paper surface to produce a
sticky paper label. In this experiment an
area of 50x40 µm2 was scanned in the x-z
plane and an array of 100x80 Raman spectra was acquired with an integration timeof 100 ms/spectrum. The measurements
were performed with an excitation laser
wavelength of 532 nm and an air objective(NA = 0.9).
polymer I
polymer IIcellulose
calcium carbonate

z-direction

z-direction

relative wavenumbers (cm-1)

Fig. 6: Depth scan through a multilayer coatingof a paper sample. Different colors in the image
represent different coating materials. Image area
60x100 µm2, 100x250 pixels, integration time: 50
ms/spectrum.

Fig. 7a shows the unique Raman spectra of
the two polymers (red and green) and thecharacteristic Raman spectra of cellulose(blue) and calcium carbonate (orange).
The color-coded depth profile (Fig. 7b)shows that the two polymers are sharply
distinguishable (red and green) from one
another as well as from the paper beneath(blue and yellow).

a)Fig. 7: Confocal Raman depthscan through a paper sticker.
(a) Raman spectra calculated
from the spectral array. (b)color-coded Raman imageof depth profile showing thetwo polymeric layers (redand green) applied on paper(yellow and blue). Image area
50x40 µm2, 100x80 pixels,
integration time: 100 ms/spectrum.

b)

a)paste I
paste IIink I
ink II

relative wavenumbers (cm-1)

An array of 128x128 complete Raman spectra was acquired over 90 µm in the x-direction and 50 µm in the z-direction with an
integration time of 100 ms/spectrum. The
distribution along the depth profile of theinks is shown in Fig. 8d, using the samecolor-code as Fig. 8c. This depth profile
clearly shows that the red-marked ink wasapplied on top of the green-marked ink,
thus showing the order in which theseinks were applied to the paper.

c)SummaryConfocal Raman imaging and atomic force
microscopy can be applied in all steps ofpaper production and printing to control,
examine and improve the process. During
the pulping and pressing process confocal
Raman imaging can provide informationon the orientation of cellulose fibers andseveral chemical residues from circulatingwater or wood extractives on cellulose
fibers can be identified unambiguously. In
further production processes such as finishing and coating, confocal Raman imaging
and AFM can reveal valuable information
about the distribution of the applied substances and the surface structure. Coatinglayers can be visualized and investigatedwith depth scans and the qualities ofprinting can be described in a non-destructive manner, without laborious sample
preparation.
Confocal Raman imaging combined withhigh resolution AFM contribute to a moredetailed understanding of the paper manufacturing process.

d)z-direction

Writing and Printing on Paper
Confocal Raman imaging can also provide
information about the qualities of inks
applied on paper. Fig. 8a shows a video
image recorded at the intersection of two
different inks. The area marked with thered frame (50x50 µm2) was scanned in
Raman spectral imaging mode by recording an array of 128x128 complete Ramanspectra with an integration time of 100
ms/spectrum. From the array of spectra,
three different Raman spectra were calculated. At some surface areas only luminescence could be detected (not shown as aspectrum), whereas other areas showed
the unique Raman spectra presentedin Fig. 8b. These two spectra show only
very small differences of which some arehighlighted with arrows in the figure. In
the color-coded Raman image presented inFig. 8c, blue denotes the luminescent areaswhile the distribution of the two inks arerepresented in red and green. An area where both inks overlap is clearly visible. Theorder in which these inks were applied tothe paper can be examined with a depthscan (x-z scan), performed along the blackline in the video image (Fig. 8a).

10 µmb)Fig. 8: Confocal Raman imaging of inks on paper. Video image (a), evaluated Raman spectra (b),color-coded Raman image recorded from the paper surface, image area 50x50 µm2, 128x128 pixels,
integration time: 100 ms/spectrum (c), and color-coded depth profile, image area 90x50 µm2, 128x128
pixels, integration time: 100 ms/spectrum (d). The inks are shown in red and green in the images.