Optical methods for determination of paper surface
topography
Hladnik, Chinga, Suhadolnik: Modern methods for determination of
paper surface topography1/22
Book: Analytical methods for structure characterization of
printing material (Editor: Diana Gregor-Svetec)
Modern methods for determination of paper surface topography
A. Hladnik, G. Chinga-Carrasco, A. Suhadolnik
1 Introduction
The quantification of surface topography is of major importance
for several industry sectors. For the paper industry, the surface
topography is essentially important for printing paper grades. The
topography affects several paper and print properties like gloss,
missing dots, ink distribution, ink transfer, mottling and picking.
It is thus important to have a detailed and standardised assessment
of surface topography for understanding its influence on the paper
and print characteristic details.
The quality of a given surface structure is commonly
characterised by the quantification of its roughness. Although
there are several roughness parameters (see Lipshitz et al. 1990;
Peltonen et al., 2004; Chinga et al., 2007b), the root-mean-square
is the most used roughness descriptor. The surface roughness may be
divided into several scales, each scale affecting a given paper or
print property.
A proper description of a given surface structure requires
reliable image acquisition devices. This is most important as the
quality of a given surface representation will determine or limit
the extraction of valuable numerical data, affecting a given
surface property. During the last two decades several methods have
been proposed for assessing the surface structure of paper
material. This includes stylus profilometry (see e.g. Wagberg and
Johansson, 1993; Enomae and LePoutre, 1995), laser profilometry
(Chinga 2004), photometric stereo method (Hansson and Johansson,
1999), CLSM (Béland XXXX; Dickson, 2005), SEM (Enomae et al. XXXX;
Reme and Kure, 2004), AFM (Niemi et al., 2002). This chapter will
focus on i) laser profilometry, ii) photometric stereo methods,
iii) SEM and iv) AFM. The mentioned characterization methods are
complementary and cover a wide range of roughness scales, from the
micron-level (laser profilometry, photometric stereo method) to the
nano-level (SEM and AFM).
2 Laser profilometry
In optical profilometry a sensing head scanns across the surface
of a given specimen to create profiles and 3D surface
representations of topography structure. Contrary to mechanical
stylus profilometry, laser profilometry applies no physical contact
between the sensor and the substrate surface, thus avoiding surface
damaging and resulting in a faster scanning. Laser profilometry is
also characterized by high vertical and lateral resolution.
2.1 Optical triangulation
Laser profilometry is often based on a principle referred to as
optical triangulation (Fig. 1). It comprises three basic elements:
a light source, imaging optics, and a photodetector (sensor). The
light source and imaging optics are used to generate a focused beam
of light that is projected onto a target surface. Although any
light source can be used for basic optical triangulation, the most
frequently applied are diode lasers, due to their high brightness,
narrow-band wavelength, and phase coherence []. An imaging lens
captures the scattered light and focuses it onto a photodetector,
which generates a signal that is proportional to the position of
the spot in its image plane. The incident light beam from the laser
source has to be well collimated to produce a uniformly small spot
size over the entire measuring range thus resulting in good spatial
resolution of the image. As the distance to the specimen surface
changes, the imaged spot shifts due to parallax. To generate a
three-dimensional image of the surface, the sensor is scanned in
two dimensions, thus generating a set of distance data that
represents the local surface topography.
The photodetector may be either a single-element
position-sensing device (PSD) or a charge-coupled device (CCD). CCD
arrays can be used to accurately determine the shape and intensity
distribution of the light spot on the detector, whereas PSDs only
determine the position of the spot’s centroid and total intensity.
On the other hand, CCD arrays have slower signal response and
require more signal processing circuitry than PSDs.
In addition to the above mentioned components, a dedicated
software is required to coordinate probe motion, data acquisition,
analysis, post-processing and reporting. Data may also be exported
for further analysis and processing with appropriate scientific
software packages.
Fig. 1. Optical triangulation principle. Reproduced from….
2.2 Alternative measurement principles
Along with a high-precision computer controlled X-Y movable
stage to which the specimen is attached, commercially available
instruments can be equipped with one of various point sensors
depending on the specific application. Laser profilometers by
Solarius [] utilize autofocus or confocal point sensors (see Table
1 for specifications), among others. In autofocus measurement (Fig.
2), condensed light is focused from a laser diode onto the specimen
surface. The reflected light is directed to a focus detector, which
measures deviations from the ideal focus to within a few
nanometers. The deviation in focus generates an error, which is
used to re-focus the objective. The position of the objective
represents an absolute measurement of the height.
In confocal principle, a point light source and detector pinhole
are used by the sensor to discriminate depth. The point light
source emitts the laser beam that is focused on a specimen through
an objective moving rapidly up and down. The maximum light
intensity occurs when the specimen lies within the focal plane of
the objective. As the objective moves closer to or farther from the
specimen, however, the reflected light reaching the pinhole is
defocused and does not pass through it. As a result, the quantity
of light received by a detector behind the pinhole decreases
rapidly. A detection signal is only generated when the maximum of
light goes through the pinhole. A precise height measurement of the
illuminated point is achieved by continuously scanning along the
z-axis.
Other sensor types can be applied as well. The chromatic white
light sensor [] is also based on the confocal technique. The
function of the pin-hole diaphragm is assumed by an objective lens
with high chromatic aberation, while a spectrophotometer is used to
measure the heights from colour differences. Due to its compact
design, this type of sensor is especially suited for measuring in
inaccessible places. The holographic sensor can be applied when
form and geometry with large differences in height are of
interest.
Fig. 2. Autofocus (left) and confocal (right) measurement
principle. Reproduced from…
Table 1. Sensor measurement specifications.
3 Photometric stereo method
Stereoscopy implies the use of stereo or binocular images to
derive height information. Optical and electron microscope images
can be used []. Thus, the range of resolution is quite wide, from
perhaps several tens of nanometers up to centimeters. This
technique requires significant computational resources, however
these are now commonly available. Modern analysis algorithms have
significantly improved both the speed and accuracy of this
technique, making it a much more viable option then it was in past
decades.
Suhadolnik !
4 Scanning electron microscopy
Resolution of the conventional optical – light – microscope is
limited by the visible light wavelength (λ = 400-700 nm) to
approximately 0.2 (m. In addition, out of focus light from points
outside the focal plane reduces image clarity. An important
progress in the investigation of material surfaces came about with
the development of an electron microscope – starting with the
transmission electron microscope (TEM) and continuing with the
scanning electron microscope (SEM). Principles of SEM were
developed in the early 1950s at the University of Cambridge, U.K.,
but the technique did not become commercially available until 1965.
Since then, numerous improvements have been made on the instrument
in terms of lens design, electron sources, detectors, and
electronic signal processing. Today, SEM is one the most widely
used analytical techniques providing means to study both the
morphology and composition of various materials. Its main
advantages [] are the high lateral resolution (1-10 nm), large
depth of focus (100 (m at 1000 x magnification), wide range of
magnifications (20-300000 x) and numerous types of
electron-specimen interaction that can be used for further material
examination or processing. The technique is being implemented in a
vast range of applications, such as semiconductor research and
manufacturing, metalurgy, biology, geology and also for paper and
print investigations.
A detailed discussion on various SEM components, functions and
capabilities has been provided by numerous authors – see, e.g. [, ]
– so only fundamentals of SEM and its modes of operation are
presented here. More emphasis will be given to the implementation
of this technique for paper- and print-related surface
analysis.
4.1 Instrument design
SEM basically contains two main components (Fig. 3) [5]: a
vacuum chamber and an electron-optical column. An electron gun is
placed on top of the column and its cathode filament can be made of
various materials: in conventional SEM, it is produced of tungsten
or LaB6 – usually with an additional electrode (Wehnelt) placed
between the cathode and anode – while in the modern field-emmission
gun SEM (FEGSEM) it is made of an extremely thin tungsten
monocrystal needle or a ZrO2 monocrystal attached to a tungsten
wire. Upon heating the cathode, a thermoionic (or field-emission in
FEGSEM) emission of electrons takes place. Primary electrons are
accelerated using anode voltage typically ranging from 500 to 30000
V.
A system of magnetic lenses – sometimes combined with
electrostatic ones – and apertures directs and focuses electron
beam onto the specimen surface. The condenser lenses and spray
apertures are responsible for changing the beam divergence angle
and therefore the probe current, thus affecting the probe diameter.
Purpose of the objective lens is to focus the electron beam into an
extremely fine spot on the surface of the sample: in modern
conventional SEM using tungsten cathode its diameter is as low as
10 nm. Scanning (rastering) of the focused electron beam across the
specimen surface is accomplished by special coils located in the
bottom part of the electron-optical column. The magnification and
scan velocity are varied by changing scan coil excitation.
Rastering together with the signals generated in the sample by the
incident electron beam (see below) are monitored simultaneously.
The signals are collected by special detectors, amplified,
displayed and stored in a computer memory (or on a photographic
film) for further image processing.
Vacuum in the SEM specimen chamber is 10-3 – 10-4 Pa. Some
modifications of this instrument operate at a much lower vacuum –
low vacuum SEM (LVSEM) – or at a controlled low pressure of certain
gases – environmental SEM (ESEM) and are especially suitable for
investigation of nonconductive samples containing water, oils,
organic solvents, etc.
Fig. 3. SEM schematic cross section. Reproduced from…
4.2 Electron beam-specimen interaction
When primary electrons hit the solid sample in a vacuum,
numerous signals are generated as a result of electrostatic
interactions with the nuclei and electrons of the target atoms
(Fig. 4). These interactions, i.e. scattering, can be either
elastic or inelastic. Elastic scattering is a result of primary
electron interactions with the specimen atomic nuclei. After
multiple scattering events and changes of direction, a portion –
about 30% – of the incident electrons orient themselves towards the
surface eventually leaving the sample. These electrons are called
back-scattered electrons (BSEs); all electrons having energy E >
50 eV belong to this category. The number of BSEs increases with an
increase in target's atomic number, Z.
In an inelastic scattering, electrons are losing their kinetic
energy due to their contact with the core and valence electrons of
the sample. This results in the emission of secondary electrons,
Auger electrons, characteristic X-rays, continuum X-rays
(Bremsstrahlung), etc. The excited valence electrons, called
secondary electrons (SEs), have low energy (E < 50 eV) and are
quickly absorbed by the sample, so only those generated close to
the surface – typical depth 50 nm – can be detected.
Conventional SEMs are normally equipped with BSE and SE
detectors, which provide topographical information about the
specimen surface, while generation of characteristic X-rays and
Auger electrons can, for example, be used in qualitative and
quantitative chemical analysis of sample composition (EDX, WDX,
AES).
Fig. 4. Signals generated upon interaction of primary electron
beam with a specimen in SEM. Reproduced from…
4.3 Preparation of paper samples
Samples to be analyzed by the conventional SEM need to be
electrically conductive, stable under vacuum and insensitive to the
local heat generated during electron beam-specimen interaction.
Pretreatment of nonconductive materials involves coating the
specimen surface with a thin metal or carbon layer of 5-40 nm
thickness. For the preparation of paper samples the following
procedure has been recommended []:
1. Cut a paper sample the size of the microscope stub with a
razor blade while taking care not to touch the surface with either
fingers or the instrument to avoid deforming and compressing the
paper.
2. Stick the paper sample onto the stub with double-sided tape,
preferably a conductive carbon tape. For thick or rough samples a
conductive pathway of silver paste or colloidal silver paint in an
ethanol base should be made between the paper surface and the metal
stub.
3. Leave the sample overnight in a desiccator to allow the
silver paint solvent to evaporate and to remove any possibly
present residual moisture.
4. Use a sputter coater or an evaporation coater to coat the
sample with a thin film (not exceeding 30 nm in thickness) of a
heavy metal, e.g. gold, palladium or a mixture of both. The film
thickness should be such that the surface details are not obscured;
for smooth samples, such as coated papers, 10 nm should be
appropriate. Papers with a rougher surface need thicker metal
layers to bridge small gaps and create a continuous conducting
layer.
By preparing cross-sections of the paper sheets, information
about paper structure in z-direction can be obtained using SEM
(Williams and Drummond, 1995?; Allem, 1998; Chinga and Helle,
2002). For such purposes, the paper sample should first be embedded
under vacuum in a polymer followed by cutting and polishing the
resulting stub. To improve the z-direction contrast, the sample
should be coated with a thin film of carbon to prevent
charging.
5 Atomic force microscopy
Together with scanning tunneling microscopy (STM), atomic force
microscopy (AFM) belongs to a family of scanning probe microscopy
techniques, which offer the highest resolution available for
studying surfaces of various materials. AFM was invented in 1986 by
Binning, Quate and Gerber [] and only three years later the first
commercial instrument was produced by Digital Instruments (USA).
Resolution of the surface features is typically on the nanometers
scale laterally and on the angstrom scale vertically, although
atomic resolution can be achieved under certain conditions [].
Sample preparation is quick and relatively simple, requiring no
stains, contrast agents, or conductive coatings (unlike e.g. in
SEM, see above). Other advantages of the technique include
nondestructiveness, potential to use a large variety of materials –
nonconductive, magnetic, elastic, etc. – and environmental
conditions – ambient air, various gases, humidity levels,
temperatures – and ability to study short- as well as long-range
molecular and atomic forces.
5.1 Operating principle
The probe used in AFM is an extremely sharp tip – typically less
than 5 μm tall and less than 10 nm in diameter at the apex [] –
integrated at the end of a 100-500 (m long cantilever. The
cantilever bends or deflects due to the forces between the tip and
the substrate surface while either the tip is scanned over the
sample, or the sample is scanned under the tip. Changes in the
angle of the cantilever caused by changes in sample topography
result in different voltage levels out of the detector. These
voltages are sent to a computer for processing and display of the
topographic image.
Most commercial instruments use optical techniques to detect the
position of the cantilever (Fig. 5). Here a light beam from a laser
diode bounces off the back of the cantilever and onto a
position-sensitive photo-detector (PSPD). Bending of the cantilever
causes a change in the laser beam position on the detector.
Fig. 5. Basic operational principle of AFM with an optical
detection system. Reproduced from ...
5.2 Cantilever, tip, scanner and detector (Tadeja,
Samardžija!)
Cantilever with the tip…
AFM scanner is made of piezoelectric ceramics… As mentioned
above, the sample can be mounted directly onto the scanner and
rastered underneath the cantilever tip, or the cantilever can be
mounted to a scanner tube and rastered over a sample fixed below
it. The former case is advantageous in imaging larger samples and
increases the speed of imaging.
Apart from the optical detection system shown in Fig. 5, …
5.3 Operational modes
Once the AFM has detected the cantilever deflection, it can
generate the topographic data with the Z feedback turned on or off.
With Z feedback off (constant-height mode), the spatial variation
of the cantilever deflection is used to generate the topographic
data set. With Z feedback on (constant-force mode), the image is
based on the Z motion of the scanner as it moves in the Z direction
to maintain a constant cantilever deflection.
In constant-force mode, which is generally preferred for most
applications, the speed of scanning is limited by the response time
of the feedback loop, but the total force exerted on the sample by
the tip is well controlled.
Constant-height mode is often used for taking atomic-scale
images of atomically flat surfaces, where the cantilever
deflections, and thus variations in applied force, are small. This
AFM mode is also utilized for recording real-time images of
changing surfaces, where high scan speed is necessary.
Contact mode
Noncontact mode
Intermittent contact (tappingTM) mode
6 Paper and print applications
The techniques described in this chapter are complementary, thus
enabling assessment of several surface characteristics and scale of
roughnesses.
During the last years laser profilometry (LP) has been used
extensively for assessing the surface topography of several paper
grades, including newsprints, SC and LWC paper. The method is fast,
fully automated, non-contact, non-destructive and capable of
assessing large areas (see Chinga-Carrasco et al., 2008). However,
LP has some limitations with respect to the detection and
description of steep gradients. Such limitation causes noise on the
digital images, especially along coarse surface fibres. Recently
Chinga et al. (2007), proposed to cover the surface with a layer of
gold before the image acquisition. The gold layer seems to reduce
internal reflections and reduces the amount of error height
values.
Suonstausta (2002) applied LP for assessing the effect of
coating and calendering on the surface structure of paper. The
study demonstrated one of the major advantages of profilometer
devices, i.e. a topographical map may be decomposed into several
scale of roughness. This gives the opportunity of assessing the
effect of a given scale of roughness on a specific respons.
Wavelength analysis was thus used for exploring the development of
the surface structure depending on a given coating composition and
calendering configuration (Suontausta 2002). Using LP, Holmstad et
al. (2004) studied the effect of temperature gradient calendering
on the surface structure of pilot calendered paper. According to
the authors, the calendering conditions used in the study had a
major effect on the uppermost surface structure, being the
calendering temperature the variable having the major impact on the
reduction of the surface roughness and thus the increasing of the
paper gloss.
LP has been applied for studying the effect of the surface
structure of SC and LWC paper on gloss (Suontausta 2002; Chinga
2004; Holmstad et al., 2004; Chinga-Carrasco et al., 2008). Gloss
is one of the most important paper and print quality parameters of
printing paper. Gloss may be affected by the surface roughness and
the mineral pigment particles, used as fillers or in a coating
layer. For the same roughness, clay yields usually higher gloss
compared to ground calcium carbonate (GCC) (see e.g. Stanislawska
and LePoutre, 1996). In addition, it has been demonstrated that the
amount of fillers in the surface layers explains most of the gloss
development of commercial SC papers (Chinga et al., 2007a).
Compared to SC paper, LWC paper has commonly higher gloss levels
due to the smoothing effect of the coating layer. In addition, when
SC paper is in contact with water a roughening phenomenon occurs
due to the swelling of the fibre material. The roughening is less
in coated paper (see e.g. Chinga et al., 2004). Contrary to water
application that causes roughening, a layer of printing ink usually
smoothens a given surface structure (Fig. XX).
Fig. XX. Upper row) Laser profilometry surface representations
of an unprinted (left) and printed (right) LWC sample from the same
local area. The calibration bars are given in micrometers. Lower
row) The corresponding 3D surface plot. The 3D plots are generated
with the Interactive 3D surface plot v.2.3 ImageJ plugin, by Kai
Uwe Barthel, Internationale Medieninformatik, Berlin, Germany.
A surface structure can be decomposed into gradients. A gradient
is a small facet having a given angle relative to the paper plane
(Fig. XX). For comparison purposes, such analysis has been
performed on the images from Fig. XX. Note that the gradient image
of the printed sample has a lower amount of high angles compared to
the gradient image of the unprinted sample. A lower amount of high
angles indicates that the surface has been smoothed due to the
printing ink (see also Chinga et al., 2004). Consequently, the
printed surface (Figs. XX and XX, right) will thus have a higher
gloss than the unprinted surface. This exemplifies one the benefits
of using a non-destructive surface assessment device in combination
with the appropriate image analysis for quantifying a given
response upon a finishing variable.
Fig. XX Gradient analysis of LWC samples. Upper row) Fig. XX
converted into gradients of the unprinted (left) and printed
(right) samples.Each gradient corresponds to an angle relative to
the paper plane. The calibration bar is in degrees. Lower row) the
corresponding histograms. Images generated by G. Chinga Carrasco,
PFI, Norway.
Based on a LP concept, Sung and Keller (2008) reported a new
method defined as a twin laser profilometer (TLP). The method
consists on two laser sensors, placed to each side of a sample
holder. This configuration makes it possible to acquire surface
profiles from each side of a paper sample. The profiles are thus
combined to generate a thickness map with a resolution of 1 m.
According to the authors, the TLP method yields the intrinsic
thickness of a given sample, thus eliminating the overestimation of
thickness that is characteristic of standard caliper methods (Sung
and Keller, 2008).
A photometric stereo method is a relatively simple, yet powerful
technique for assessing surface structures (Hansson and Johansson,
1999). The method is fast and is capable of assessing large areas.
However the maximum resolution of 5 m may be considered low for
some purposes such as the assessment of coated paper sub-micron
roughness and how this is affected by mineral pigment particles. On
the other side, an optical stereo method may be suitable for
assessing missing dots in rotogravure printing as the surface
cavities inducing missing dots seem in the range of 80 m in
diameter, thus reducing resolution requirements.
A clear example of the applicability of a photometric stereo
method is presented in Fig. XX. Surface depressions below -1 mm can
be quantified and related to the occurrence of missing dots. The
method seems to give reasonable results, detecting the surface
depressions causing a poor contact between the paper surface and
the gravure printing cylinder. A similar approach has been applied
in flexographic printing, where surface depressions may induce the
occurrence of unprinted areas (see Hansson and Johansson, 1999;
Barros and Johansson, 2006). In addition, Barros and Johansson,
2008 described the applicability of this instrumentation in
combination with bandpass filtering for finding the relationship
between surface topography and the reflectance of flexography
printed samples. The authors concluded that small differences in
surface roughness (wavelengths 0.8-1.6 mm) influenced ink
distribution and consequently print mottle in full-tone flexography
printing.
Fig. XX Photometric stereo method about missing dots.
Fig. XX presents a clear example of the capabilities of the
scanning electron microscope (SEM) for exploring the surface
structure of printing paper. SEM is a versatile device for
acquiring structural information. Images can be acquired at several
magnifications and with a resolution unattainable by other
techniques. Images can be acquired in secondary electron (SE) (Fig.
XX) or backscatter electron mode (BSE) (Fig. XX). Due to its
extensive capabilities, SEM has been a most used device for
assessing the structure of paper and prints (see e.g. Helle and
Johnsen, 1994?; Enomae et al., 1995?; Forseth and Helle, 1997;
Allem, 1998; Reme and Kure, 2003?; Chinga and Helle 2003; Zou et
al. XXXX; Eriksen and Gregersen, XXXX).
Fig. XX SEM-SE and SEM-BEI images from the same area.
The SEM is a powerful tool for assessing different
characteristics of the surface structure of paper. In SE-mode the
SEM gives a clear 3D impression of the topography of a given
surface. This capability has been used extensively for exploring
e.g. the surface development due to calendering (Holmstad et al.,
2004), the consolidation of coating layers (Enomae and LePoutre
XXXX) and the smoothening effect of printing inks (Chinga et al.,
2004), to name a few. Common for the mentioned studies is that the
SEM was used for exemplifying a given phenomenon, however no
quantification was performed. The SEM can also be used for
reconstructing surface structures (Fig. XX). The method is based on
stereo imaging and parallax (see e.g. Hein, 2001). Such surface
reconstruction makes it possible to perform a quantitative
assessment of the surface topography (see Helle and Johnsen, 1994;
Gregersen et al., 1995; Reme and Kure, 2004). Another approach has
been presented by Enomae et al. (1995??). The authors used a SEM
having two SE-detectors. Images were acquired from each side of the
vacuum chamber and a topographical height map was
reconstructed.
Fig. XX SEM stereo imaging of a newsprint sample. Note the
coarse surface fibre on the left side of the image. Use red/green
stereo glasses for better visualization. The SEM stereo image has
been acquired by G. Chinga Carrasco, PFI, Norway.
Surface coverage by a layer of fillers is an important
characteristic of printing paper. Coverage may determine some paper
and print quality properties. SEM in backscatter mode yields
contrast depending on the average atomic number of a given local
area. Mineral fillers such as clay and CaCO3 carbonate may appear
lighter that the matrix of darker fibres (Fig. XX). SEM-BEI mode
images with appropriate image segmentation and analysis procedures
make this technique suitable for quantification of coverage and
related characteristics. Such capability has been used for
quantifying the coverage of the coating layer on coated papers (see
e.g. Kaartovara 1989; Dickson et al., 2002; Forsström et al.,
2003;) and the coverage of fillers on SC paper surfaces (Chinga et
al., 2007). Coverage may be given in percentage and may be defined
as the ratio of areas covered with a coating layer to the whole
imaged area. Kaartovara (1989) quantified several statistics of
uncovered areas, such as percentage, average size and number of
uncoated areas. The author found a reduction of uncoated areas as
the coat weight was increased from approximately 8 to 20 g/m2.
However, care must be taken when using the SEM-BEI mode for
quantification of coverage, as the amount of uncovered areas will
depend on the accelerating voltage used during image acquisition in
the SEM–BEI mode. It is recommended to use low accelerating voltage
in order to assess only the uppermost layers of the paper surface.
The effect of accelerating voltage on the quantification of
coverage is depicted in Fig. XX.
Fig. XX Coverage and accelerating voltage. A) Secondary electron
image showing surface variations in a LWC sample. B)-F) Images
showing the same area, taken in BEI-mode with raising accelerating
voltage: 5, 10, 15, 20 and 25 kV resp. Note the gradually emerging
fibres when raising the accelerating voltage. Bar: 200 m. The SEM
images have been acquired by G. Chinga Carrasco, PFI, Norway.
In addition to surface assessment, the SEM is a powerful tool
for quantification of cross-sectional details of paper structure
(Allem, 1998; Chinga and Helle, 2002; Holmstad et al., 2004; Zou et
al., XXXX). Such quantification may be used for describing the
porosity (Allem 1998; Zhou XXXX?), fibre and pore cross-sectional
dimensions (Chinga et al., 2007), filler distribution (Holmstad et
al., 2004), fines distribution (Eriksen et al., 2006) and the
structural details of mineral pigment layers on paper (Chinga and
Helle, 2002). However, SEM has the limitation of yielding 2D images
of 3D structures. As an attempt to circumvent this limitation,
serial sectioning and serial grinding has been applied for making
3D reconstructions of paper structure (Aronsson et al., 2002?;
Chinga et al., 2004). This methods yield detailed information about
a given paper structure, though the method is time-consuming and
difficult to use as a standard analytical method for paper
structure assessment.
Atomic force microscope (AFM) is a most suitable scientific
device for quantifying the micro (1-100 m), sub-micron (0.1 – 1 m)
and nano-structure (1-100 nm) of paper and print surfaces.
According to Niemi et al. (2002) AFM have tree major advantages
compared to other microscopy techniques, i.e. i) no or little
preparation, ii) high resolution and three-dimensional surface
information and iii) the microscope can be used in environments
inaccessible with other techniques.
Fig. XX AFM image of a LWC paper sample and the corresponding 3D
surface representation. The calibration bar is given in
micrometers. The 3D plots are generated with the Interactive 3D
surface plot v.2.3 by Kai Uwe Barthel, Internationale
Medieninformatik, Berlin, Germany. The AFM image has been acquired
by B. Wang. Dept. of Physics, NTNU. Norway.
In addition to extracting 3D topographic information (see
Peltonen et al., 2004; Chinga-Carrasco et al., 2008), AFM can be
used in phase mode to distinguish different components in a coating
structure, such as pigment particles and latex (see Niemi et al.,
2002). The AFM is thus a suitable method for assessing the
structure of calendered coated paper. Rougher surfaces have to be
analysed with care, as the maximum movement in the z-direction
(height) is only a few micrometers. Local areas of fibre surfaces
may also be assessed thus giving a detailed description of pulp
fibres, including the different layers of the fibre walls with
their characteristic arrangements of microfibrils (see e.g. Niemi
et al., 2002).
Using AFM the structure of coating structures has been studied
in detail (Ström et al., 2003; Larsson et al., 2007; Järnström et
al., 2007; Chinga-Carrasco et al., 2008). Ström et al. (2003)
applied an AFM analysis for assessing the structure of XXXX. The
study proved the suitability of AFM for assessing the structure of
coated paper and prints. The topography was related to the print
gloss. A similar approach was applied by Järström et al. (2007) for
relating the surface topography of model calcium carbonate –based
coating layers to the corresponding gloss levels. The authors
described a set of roughness descriptors that can be applied for
exploring a surface structure in detail. The applicability of a
surface skewness parameter for exploring the topography development
at different scales was discussed. A coating layer structure is
also affected by the coating formulation. Calcium carbonate and
clay pigment particles have different shapes, i.e. ground calcium
carbonates particles are blocky, while clays are platey. They
affect the surface development and pore structure in coating layers
in different ways. Larson et al. (2007) performed a study to reveal
the effect of several blends on the surface and bulk structure of
coating layers. The behaviour of the coatings upon calendering was
also assessed. The results showed that increasing the amount of
clay caused a smoother surface, a more compact coating bulk
structure and consequently higher gloss levels. Most recently,
Chinga-Carrasco et al. (2008) showed the relative relationship
between several scales of roughness and gloss. Roughness below a
wavelength of approximately 1 m did not affect the gloss of LWC
paper significantly. The complementary capabilities of LP, AFM and
X-ray microtomography for assessing surface structures was also
demonstrated, thus giving valuable insight into the structure of
coating layers.
6 Conclusions
7 References
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