8 CHARACTERIZATION OF PAPER PROPERTIES
8.2 Surface topography
The surface topography of paper is an important char-acteristic influencing, as mentioned earlier, the wetting, adhesion and friction properties, as well as the light scattering of paper, and thereby its visual appearance in terms of gloss. Consequently, it is one of the key control parameters in the evaluation and design of paper prod-ucts. The surface topography of paper can be controlled by various treatments including coating and surface siz-ing treatments and calendersiz-ing in hard and soft nips. The process used depends on the value of the end product, as well as on the specific end-use requirements for strength, appearance and printability. The surface topography is consequently a key measure in the development of, e.g.
new coating colours and coating procedures and during press trials for new paper machine settings.
Surface topography is technically the variation of the height z-direction along the lateral x- and y-co-ordinate space of the paper surface. A map of z(x, y) gives a three-dimensional image of the surface.
This information can be used in itself to evaluate the
Intensity
Non-sized sheet
Sized sheet
Intensity
Binding energy (eV)
Figure 7.41. Typical XPS (ESCA) spectrum obtained on a coated paper grade, where peaks due to emission of core electrons can be clearly seen. The positions of these peaks are used to identify the elements present and to provide information about their oxidation states and chemical surroundings. The intensities of the peaks are furthermore used for quantification
topography. However, it is common and more practical to use various statistical measures. The subject of exactly which such parameters, as well as that of different models for light interaction with paper, is vast and it suffices here to say that many parameters are based on the calculation of some deviation of the surface structure from a best-fitted surface of well-known geometry, such as a plane or sphere, to the measured topography. It is important to be aware of the differences between the different measures and to consult suitable monographs on the subject before deciding on which parameter to use as the measure of topography. Depending on what the purpose of the measurement is, the different measures may be more or less suitable as they might emphasize or diminish certain effects.
Another powerful tool when analysing surface topog-raphy is filtering, which can be used to divide the topography according to the length scale of the sur-face features. By employing, for instance, wavelength filtering, much like the filtering of sound or any other frequency/wavelength-dependent property, it is possible to distinguish between roughness on different scales.
The interesting scale for paper is often set by the size of the building stones at the paper surface, i.e. the fibres, pigments or pigment aggregates. The division is, how-ever, somewhat arbitrary and the cut-off values between different ranges will be strongly dependent on the type of surface and the practical question of interest. In the case of the visual appearance, the interesting scale is set by the wavelengths of white light. In such cases, topography studies can gain much in terms of practical relevance by being combined with test panel studies in order to distinguish, for instance, within which wave-length band structural variations are most disturbing to the end-user.
With decreasing wavelength, or increasing spatial frequency, the different ranges are usually referred to as form (or shape), waviness, roughness and noise. As the very long wavelength variations rarely are of great importance, at least not in relation to surface chemistry, such variations can be removed by fitting an appropriate surface (e.g. a plane or sphere) to the form of the sample.
Applying a low- and a high-pass filter will then divide the topography into the remaining three waviness, form and noise regimes. The noise is usually as much a measure of the sensitivity of the instrument as it is an intrinsic characteristic of the substrate and can therefore often be omitted. What remains then are the waviness and roughness features, which can be used in the pursuit of structure-property relationships, e.g. correlating with experimental spreading, gloss variation and printability measurements.
When studying topography images, especially two-dimensional (2D) measurements over large distances, it is important to bear in mind the frequently very large aspect ratio between the scales on the z-direction, on the one hand, and xy-plane, on the other.
8.2.1 Experimental methods
There are a number of experimental techniques available for measuring the surface topography of paper and other materials. Several methods are commonly employed by paper manufacturers for fast assessment of the average surface roughness. Such techniques include the use of air-leak-type instruments as well as indirect measurements of light scattering.
Direct imaging of surface topographical features can be obtained by using either contact methods, where a sharp tip is used to track the height variations in the plane of the surface, or techniques relying on the interaction of a light probe with the surface. Since the paper surface is often quite soft, it is important when using contact techniques to ensure that the measured response is not convoluted with characteristics of the measuring device. The measurement speed, resolution (in both lateral and normal directions) and measurement range are among the characteristics which determine the most suitable technique for a particular user and problem area. Figure 7.43 shows the resolution of a few chosen contact and non-contact methods. Note already at this stage that one should in presentations of topography-related results always present the range used in the experiment, since statistical roughness measures are
Wavelength
Figure 7.43. The resolution of different experimental tech-niques used for surface topography studies. (Redrawn form Stout, K. J. and Blunt, L., Surface Coating TechnoL, 71, 69-81 (1995). Reprinted with permission from Elsevier Science)
Amplitude
usually dependent on the measurement area. We divide the discussion of the techniques into indirect, contact and non-contact or optical methods, according to their modes of interaction with the surface. The different methods have both different technical limitations and different resolutions in normal as well as in the lateral directions.
Atomic force microscopy (AFM) provides the highest lateral resolution images on paper but has a limited xy-range, whereas the stylus, autofocus and interferometry techniques have a much lower lateral resolution but larger working window.
Indirect methods
Common methods used by the paper industry for obtain-ing indirect surface roughness measures are those based on measurements of the amount of air that leaks out under the rim of a pressurized cup placed upside down on the paper surface. Two commercial instru-ments based on this principle are the Bendtsen and the Parker-Print-Surf (PPS) test. Such techniques provide an average value which is rather related to the surface roughness but do not provide a map of the surface and information about local properties. A serious problem with these methods is that they are not suitable for highly porous materials in which a significant fraction of air can be transported through the paper. However, they are simple and rapid to apply and are therefore frequently used in industry. Their primary usefulness is to follow the product reproducibility. The results obtained with the PPS method has, however, been scaled to physical roughness values and the technique is frequently used in the evaluation of paper for different printing tech-niques. As mentioned above in Section 7.4, the Bristow wheel wetting technique can also be used for evaluat-ing surface roughness, or rather surface volume. Other indirect measures include light scattering methods but these require the use of an optical model for calculat-ing roughness properties, and the input data are seldom known for constructing such models.
Contact methods
In contact methods, a sharp object is contacted with the sample and the variation in height position of this stylus is recorded as it is scans the x- and/or ^-directions of the surface. Contact methods are available both for 2D and three-dimensional (3D) measurements. From a 3D z(x, y) map it is possible to calculate all relevant topography measures. However, since the method relies on the contact between the surface and a sharp object
which is scanned along the surface while in contact, considerable damage may be done to soft materials such as paper. The size of the stylus will also limit the xy resolution. Only features larger than the radius of the stylus tip can be measured. The maximum amplitude is of course also limited by the size and shape of the tip that is used for imaging the paper surface.
The principle of atomic force microscopy, (AFM) is very similar to the stylus contact method. A sharp nano-sized tip is used here to image small areas of a surface. Although a stylus is used, the probe and the surface do not necessarily need to be in physical contact when using the AFM technique. The reason for this is that the technique is highly sensitive, thus allowing measurement of more long-range physical interactions, such as electrostatic fields, on the colloidal scale. There exist various methods by which a constant force can be maintained between the tip and the sample without coming into physical contact with the imaging tip.
This makes AFM ideal for imaging both hard and soft samples. The disadvantage of the technique is its small maximum measurement area, which is typically 50 x 50 |iim for paper. When determining roughness parameters from AFM data, it is recommended to use input from many repeated measurements in order to get reasonable statistics. The resolution is, on the other hand, very good and lateral structures of the order of a few nanometers can be detected without difficulty. The resolution in the vertical direction is even better, i.e.
fractions of nanometers. Figure 7.44 shows examples of topography images of a glossy and a matt paper imaged by using AFM. When determining roughness parameters from AFM data, it is recommended to use input from many repeated measurements in order to get reasonable statistics from the small scan size used with this technique. The AFM technique can be applied in both air and liquid ambient media, which is unique and very useful when studying, e.g. swelling of fibres in water. Furthermore, as was discussed previously, AFM can be used for much more than simply determining the topography. This includes mapping the viscoelastic response of surfaces and thus gaining information about the chemical heterogeneity of a surface, and measuring normal and lateral force interactions between surfaces.
Non-contact methods
One of two basic principles is employed in most com-mercial optical non-contact methods. The first is the laser-focusing technique, which works according to the same basic principle as a CD-player, and can best be described as an optical equivalent of the stylus
method. The local height z(x, y) is, by this method, determined from the measured position of a surface-reflected laser light probe at some distance away from the substrate as this surface is being moved in the xy-plane. A 3D map of the surface can then be con-structed from this set of data. This method also has the advantage of being a non-contact method and hav-ing quite a large lateral measurement range, as can be seen from Figure 7.43. However, the technique is quite slow when compared to the non-contact inter-ferometric technique, which is the second principle on which many commercial non-contact profilometers are based.
The basis of the interferometric technique is that when white light is used to illuminate both the sample and an internal reference surface, the reflected light from these two sources will produce a standing-wave pattern due to interference of the electromagnetic waves. The fringe pattern of reinforced and extinguished light is analysed after being transferred to a computer via a charge-coupled device (CCD)-camera. By scanning the focus of the interferometer microscope in the z-direction, fringe patterns corresponding to different heights on the surface are obtained and can be analysed in order to determine the variation in height at different positions on the surface. The lateral resolution obtained with this technique is of the order of 1 um, while the surface height information can be resolved down to about 1 nm. The area investigated is dependent on the objective used, as with all microscopes, but it can easily be increased by using a "stitching" methodology in which several images are assembled into a larger combined image. The technique has similar limitations as the stylus and laser-focusing techniques, but is much faster and it also allows easy filtering of the roughness in different wavelength bands, as is nicely illustrated in Figure 7.45. From experience gained at the
Wavelength band/|im
Figure 7.45. Roughness data of glossy, silk and matt paper grades as a function of the wavelength band. The coating colour recipes of the silk and matt grades are identical; however, the silk grade has been calendered in a soft nip. It can be clearly seen that the effect of calendering is largest for the larger roughness features and is not visible at low wavelengths, thus indicating that pigments and primary aggregate structures are unaffected by soft-calendering. The roughness for the glossy paper grade calendered in a hard nip is much smaller at all wavelengths when compared with the other grades. The papers featured in the plots are the same as those used in the spreading experiments shown in Figure 7.36
authors' laboratory, this technique, together with AFM, provides a very powerful combination in the study of the surface roughness of paper and similar materials which have roughness features that span several orders of magnitude. Common to all techniques used to study surface topography are problems related to very rapidly varying surface features. As mentioned above, contact techniques may run into problems because the stylus tip is relatively large and can be convoluted with the surface features. Such problems also provide difficulties for the optical non-contact techniques and should not be neglected.
Figure 7.44. AFM images of (a) a glossy and (b) a matt paper with a scan size of 4 x 4 um: x- and y-axes, 1.000 um/division;
z-axis, 1000 nm/division. Roughness data for the same papers measured by the white-light interferometric technique are provided in Figure 7.45. The same paper grades are also used in the spreading experiments shown in Figure 7.36
Roughness (rms values)/|im