SERIAL SECTIONING OF COATED PAPER 
AS A NOVEL METHOD TO ANALYZE 
BINDER PENETRATION 
PLASTNI RAZREZ VZORCEV PREMAZANEGA PAPIRJA 
KOT NOVA METODA ZA ANALIZO PENETRACIJE VEZIV 
K. HOF€R\ fì. ORTN€R2, G. S. NVRNHONGO2, H. UUINT€R3, G. GÜßlTZ2, Ш. BRUČR1 
li 
ABSTRACT 
To understand and optimize mechanical and printing properties like e.g. picking resistance, information regarding the distribution 
of the coating binder and the penetration of the binder into the base paper is important. Especially water-soluble binders tend 
to follow the water phase into the base paper via drainage. Since these binders are used for pre-coatings these penetration 
phenomena play an important role. This work focuses on monitoring binder penetration of water-soluble coating binders into 
the base paper using serial sectioning combined with fluorescence microscopy (SSFM). This method allows a three-dimensional 
data analysis in an appropriate region of interest. Coating colors varying in their Abo Akademi Gravimetric Water Retention Value 
(AA-GWR value) were prepared to investigate differences in binder penetration depth into the base paper. 
Key words: coated paper, binder penetration, serial sectioning, fluorescence microscopy 
IZVLEČEK 
Informacija o porazdelitvi veziva premaza in o penetraciji veziva v osnovni papir je pomembna za razumevanje mehanskih 
in tiskarskih lastnosti, kot je npr. površinska trdnost. Posebno vodotopna veziva sledijo vodni fazi v osnovni papir preko 
odvodnjavanja. Ker se ta veziva uporabljajo za predpremaze, igra penetracija pomembno vlogo. To delo je osredotočeno 
na spremljanje penetracije veziva vodotopne premazne mase v osnovni papir z metodo serijskega razreza v kombinaciji s 
fluorescenčno mikroskopijo (SSFM). Metoda omogoča tridimenzionalno analizo podatkov v izbranem območju. Premazne mase z 
različnimi AA-GWR vrednostmi (gravimetrična vrednost zadrževanja vode Àbo Akademi) so bile pripravljene, da bi raziskali kako 
globoko penetrira vezivo v osnovni papir. 
Ključne besede: premazan papir, penetracija veziva, serijski razrez, fluorescenčna mikroskopija 
1 INTRODUCTION 
Binder migration/penetration and the 
distribution of binder within a coating 
layer play an important role regarding 
optical, mechanical and printing properties 
[1, 2]. Whalen-Shaw [3] defines binder 
penetration as the movement of binder 
towards base paper driven by drainage 
and binder migration as the movement 
of binder towards coating layer surface by 
evaporation. In this work the focus is on 
binder penetration. 
Several methods to analyze binder 
penetration/migration and binder 
distribution are presented in the literature. 
These methods can be separated into 
two groups including two-dimensional 
measurements at the coating layer surface 
(binder migration) and three-dimensional 
measurements (binder penetration and 
binder distribution within the coating layer): 
^ Two-dimensional observation of 
the coating layer surface has been 
assessed via Raman Spectroscopy 
[4], Raman Microscopy [5], X-ray 
photoelectron spectroscopy (XPS) also 
known as electron spectroscopy for 
chemical analysis (ESCA) [1], Time-of-
flight-secondary ion mass spectrometry 
(ToF-SIMS) and field-emission-scanning 
electron microscopy (FE-SEM) [6], and 
Atomic Force Microscopy (AFM) [7]. 
^ Three-dimensional observation of 
the coating binder penetration and 
its distribution has been assessed via 
Confocal Raman Microscopy (CRS) 
[5, 8], argon ion beam milling in 
combination with FE-SEM [9], cross-
section analysis with AFM [10] and 
Confocal Laser Scanning Microscopy 
(CLSM) [11, 12, 13]. 
To observe three-dimensional features of 
coating layers, our method combines serial 
sectioning with fluorescence microscopy. 
Wiltsche et al [14] developed an analysis 
routine for coating layer properties and 
fiber properties via serial sectioning in 
combination with light microscopy. To detect 
specific components of a coating color like 
binders, a second generation prototype 
(Figure 1) was developed and equipped with 
a fluorescent light source additionally to 
the visual light source [15]. It performs fully 
automated serial sectioning and microscopy 
of coated paper samples. Sample sizes of 
6 mm in CD (cross direction), 2000 mm in 
MD (machine direction) and 2 mm in ZD 
(z-direction) at resolution down to 
(0.47 X 0.47 X 1) |jm3 can be analyzed. 
Looking at the three-dimensional 
observation methods, advantages and 
disadvantages of our method (SSFM) 
need to be pointed out. Compared to 
CRS-analysis [5, 8], the possibility to 
observe images of each single cut is one 
big advantage of the presented method. 
However, no chemical information can 
be determined by SSFM. The selectivity 
regarding the detection of different binders 
is defined by the staining process. The area 
of interest in z-direction is limited to 
10-30|jm with С RS [8] compared to 2mm 
with our method. Also AFM [10] and 
FE-SEM [9] show smaller region of interest 
but higher resolution. 
Starch can also be detected after the 
staining process with SSFM, compared to 
CRS. The similarity of CRS signal of cellulose 
and starch prohibits a distinction of those 
two components [8]. Compared to CLSM 
methods, SSFM is a destructive method and 
CLSM sample preparation is easier because 
of the fact that no embedding is necessary. 
However, SSFM is not limited in information 
in z-direction as CLSM regarding light 
penetration depth [12]. 
The aim of this work was the establishment 
of an analytical method to determine the 
binder penetration of coating colors into 

Table I : The three different coating colors, their AA-GWR value, coating composition and solid content 
Preglednica 1 : Tri različne premazne mešanice, njihove AA-GWR vrednosti, sestava premaza in vsebnost suhe snovi 
Coating color with AA-GWR value Calcium Carbonate Binder Target Solid Content 
[g/nv] [%] [%] [%] 
F'iinder A 95 I 00 lo 70 
Binder В 400 I 00 lo 60 
Binder С 970 I 00 lo 60 
Figure I : Second generation prototype (serial sectioning in combination with fluorescence microscopy, SSFM) 
Slika 1 : Prototip druge generacije (serijski razrez v kombinaciji s fluorescenčno mikroskopijo) 
the base paper. Watanabe and Lepoutre 
[16] divide the consolidation of the coating 
layer during application and drying into 
three phases separated by the FCC (first 
critical concentration) and the SCC (second 
critical concentration). The first phase 
before reaching the FCC is interesting 
regarding the penetration of binder. Due 
to that fact we prepared three different 
coating colors varying in their AA-GWR 
value (Abo Akademi Gravimetric Water 
Retention Value) and determined the 
penetration depth of the binder into the 
base paper. 
2 MATERIALS AND METHODS 
2.1 Coating Color Preparation 
and Coating Process 
Coating colors with calcium carbonate, 
binder, water (to set the target solid 
content) and sodium hydroxide (to set the 
pH of 9) were prepared (see Table 1 ). The 
aim was to create coating colors with three 
totally different AA-GWR values due to the 
fact that the ability of a coating color to 
retain the water during the coating process 
plays an important role with respect to 
binder penetration. In Table 1 the AA-
GWR values and the solid contents are 
shown. The coating process was performed 
with a manual rod coater. As base paper, 
laboratory hand sheets with a grammage 
of 80 g/m2 made out of short fiber sulfate 
pulp with a low Kappa number (free of 
fluorescence) were used. 
2.2 Sample Preparation 
Paper samples are cut into pieces of 8 mm 
X 25 mm and infiltrated with TechnoVit 
7100 (cold curing embedding resin based 
on hydroxyethylmethacrylate). This solution 
has been stained with Sudan Black (Roth) 
(0,3 mass %) and stirred for 45 minutes. 
Staining is necessary to minimize out of 
focus light effects. Samples are infiltrated in 
a vacuum exsiccator, and a vacuum pump is 
switched on and off for 4 hours in intervals 
of 15 minutes. Infiltrated paper samples are 
embedded in gelatin capsules. Hardening 
time was 12 hours. Samples were then 
polished. 
2.3 Microtomy, Image Creation 
and Analysis 
Embedded samples were precutwith a glass 
knife and then cut with a 4mm diamond 
knife (Diatome). The slice thickness was set 
to 4 |jm and 300 cuts were performed. 
100 automatically stitched and aligned 
images with a distance of 12 pm 
were created. The dimension of the images 
was 0,5 mm in ZD and 5,4 mm in CD. For 
fluorescence microscopy a GFP-filter was 
used. Images were transformed to binary 
images using ImageJ and penetration depth 
into the base paper was calculated (adapted 
method described by Wiltsche et al [14]). 
Furthermore the coating layer thickness 
was measured via image analysis with the 
method described in Wiltsche et al [14]. 
3 RESULTS AND DISCUSSION 
The output of SSFM are images of the 
coated paper cross-sections and specific 
components can be detected. Figure 2 
shows the visual light image (BF) and the 
UV light image (GFP) of the paper cross-
sections of the three different coating 
colors. 
The coating layer can be seen on the 
left side of the BF images in Figure 2. To 
visualize the binder penetration depth into 
the base paper the contour of the coating 
layer of the BF image was superimposed 
to the GFP image (white line). The green 
colored areas in the GFP images indicate the 
location of the binder after the coating and 
drying process. Figure 2 shows that Binder 
A has the lowest and Binder С the highest 
penetration depth into the base paper. Due 
to the fact that a lower water retention 
value leads to less binder penetrating 
into the base paper, the water retention 
value (see Table 1 ) correlates well with the 
analyzed microtome images. The coating 
color with Binder A has a higher solid 
content. As Watanabe and Lepoutre [16] 
pointed out, the FCC and thus the initial 
solid content has an influence on binder 
penetration.In case of Binders В and С there 
is a depletion of binder in the coating layer. 
In the literature there is a contradiction 
regarding the amount of water-soluble 
binder (starch) in the surface near layers 
of the dry coating layer. Dappen [17] 
investigated the distribution of starch 
within the coating layer during drying and 
in dry coatings. Results showed that there 
is starch transported to the surface layers 
of the coating layer via evaporation. Du et 
al [1 ] found out that there is a depletion of 
starch at the coating surface when applying 
a coating color with a mixture of starch and 
latex as binder. During coating consolidation 
the starch cannot be deposited at the 
Figure 2: Cross-sections ot coated papers (E'F—VIS light microscope imiage, GFF'-UV light microscope imiage) 
using binders with different AA-GWR 
Slika 2: Prečni prerez premazanega papirja (mikroskopska slika: BF-s standardno svetlobno mikroskopijo, 
GFP-s fluorescenčno mikroskopijo) in uporabo veziv z različnimi AA-GWR vrednostmi 
BINDER C 

Table 2: Determination of coating layer thickness 
Preglednica 2: Določitev debeline prernaznega sloja 
Coating color with Coating layer thickness 
[|jm] 
Binder A I 1,90 
Binder В 15,56 
Binder С 12,0 
Table 3: 3D data analysis of binder penetration depth 
Preglednica 3:3D podatki analize globine penetracije 
veziva 
Sample Penetration depth into the base paper 
[|jm] 
Binder A 16,9 
Binder В 2 1,0 
Binder С 31,1 
Figure 3: Distribution curves of binder penetration into the base paper 
Slika 3: Pcrazdelitvene krivulje penetracije veziva v ronconi papir 
coating surface, due to its water-solubility, 
but is enriched in the coating pores. Also 
Chattopadhyay et al [18] found out that 
there is no enrichment of starch at the 
top of the coating layer when applying 
a mixture of latex and starch in coating 
formulations. 
The coating layer with Binder С is not 
fluorescent anymore because the binder 
completely penetrated into the base paper. 
This is an extreme case of binder penetration 
and this kind of binder cannot be used in 
conventional coating application. 
To control the coat weight applied to the 
base paper, the coating layer thickness was 
determined. In Table 2 the results of the 
calculation of the coating layer thickness 
are shown. Coating layer thickness of 
coating color A and С are nearly the same 
and so they can be compared directly. 
Unfortunately, the coating layer thickness 
of the coating color В was a little bit higher. 
There were difficulties to control applied 
amount of coating with the manual rod 
coater. 
In Table 3 the results of the 3D data 
analysis of an image stack (100 images) are 
presented. Binder A shows a penetration 
value of 16,9 pm compared to Binder 
С with 31,1 |jm. The binder follows the 
water, which cannot be retained in the 
coating color, into the porous system of the 
base paper. The lower the water retention 
of the coating color, the deeper the binder 
penetrates into the base paper (see Binder 
С in Table 3). 
Figure 3 shows the distribution curves of 
binder penetration into the base paper. 
Binder A and В show a narrow distribution 
and, in comparison, Binder С has a broad 
distribution. 
4 CONCLUSION AND 
OUTLOOK 
In conclusion serial sectioning of fluorescent 
coating layers (SSFM) is an appropriate 
method to obtain information about 
the binder penetration into the base 
paper. Cross-section images and 3D 
data analysis of an image stack can be 
correlated with water retention values 
of the coating colors. An important step 
regarding the establishment of this new 
binder penetration analysis method is the 
development of the staining process of 
each binder. So far, only water-soluble 
binders were analyzed. In the future, 
staining of latex, which is the most used 
conventional binder in coating, will be 
an important issue. Influences of coating 
application and drying method will also 
be investigated. 
ACKNOWLEDGEMENTS 
The authors acknowledge the industrial 
partners Sappi Gratkorn, Heinzel Pulp Pols, 
Norske Skog Bruck and Mondi Frantschach, 
the Austrian Research Promotion Agency 
(FFG), COMET, В M VIT, BMWFJ, Country 
of Styria and Carinthia for the financial 
support of the K-project Flippr. 
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Institute of Paper, Pulp and Fiber Technology, 
Graz University of Technology, Inffeldgasse 23, 
8010 Graz, Austria 
2University of Natural Resources and Life 
Sciences Vienna, Institute of Environmental 
Biotechnology, Konrad-Lorenz Str. 20, 3430 
Tu Un, Austria 
3Sappi Paper Holding, BruckerStrasse 21, 8101 
Gratkorn, Austria 
DI Karin Hofer, BSc 
DI DI (FH) Andreas Ortner 
Dr. Gibson S. Nyanhongo 
DI Dr. Heribert Winter 
Univ.-Prof. DI Dr. Georg Gübitz 
Univ.-Prof. DI Dr. Wolfgang Bauer