<table><caption>Table 4-5 Precoat formulations – 100 parts substitution</caption><thead><tr><th></th><th>Cond. 17</th><th>Cond. 18</th><th>Cond. 19</th><th>Cond. 20</th><th>Cond. 21</th></tr></thead><tbody><tr><td>BPSD GCC (0.659µm)</td><td>100</td><td></td><td></td><td></td><td></td></tr><tr><td>BPSD GCC (0.3598µm)</td><td></td><td>100</td><td></td><td></td><td></td></tr><tr><td>PCC (0.499µm)</td><td></td><td></td><td>100</td><td></td><td></td></tr><tr><td>Brazilian clay (0.548µm)</td><td></td><td></td><td></td><td>100</td><td></td></tr><tr><td>NPSD GCC (0.6593µm)</td><td></td><td></td><td></td><td></td><td>100</td></tr><tr><td>Latex SBR</td><td>10</td><td>10</td><td>10</td><td>10</td><td>10</td></tr><tr><td>Optical Brightener Hexa.</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td><td>0.5</td></tr><tr><td>Calcium Stearate</td><td>0.4</td><td>0.4</td><td>0.4</td><td>0.4</td><td>0.4</td></tr><tr><td>Carboxy Methyl Cellulose(CMC)</td><td>0.7</td><td>0.7</td><td>0.7</td><td>0.7</td><td>0.7</td></tr><tr><td>Solids</td><td>66%</td><td>66%</td><td>64%</td><td>64%</td><td>64%</td></tr></tbody></table>

## 4.3 Top Coated Samples

The 20 precoated rolls in the pilot coater facility have been top coated with a CLC 7000 (Cylindrical Laboratory Coater or CLC for short) because the CLC does not require as much paper as a pilot coater. We needed to do use the CLC as the number of coatings and/or different settings is high and the cost of performing all the conditions on pilot equipment would have been prohibitive.

<table><caption>Table 4-6 Precoat Operation Parameters - Coater</caption><tr><td>Coating Unit</td><td>Jet Applicator</td></tr><tr><td>Speed</td><td>1400 m/min</td></tr><tr><td>First Side to Coat</td><td>Rough</td></tr><tr><td>Coat Weight Application</td><td>12 g/m²</td></tr><tr><td>Moisture</td><td>4.5% for the rough side</td></tr><tr><td>Blade</td><td>0.457 / 84 mm / 45°</td></tr><tr><td>Dwell Length</td><td>350 mm</td></tr><tr><td>Jet Opening</td><td>0.9 mm</td></tr><tr><td>Jet Angle</td><td>34°</td></tr><tr><td>Gap - Jet/Baking Roll</td><td>10 mm</td></tr><tr><td>Beam Angle</td><td>45° - Constant Tip angle</td></tr></table>
The formulations used for the top coat are listed in the table 4-7. The CLC conditions for the coating are as follows: speed of 800 m/min, pre-heating of 10 seconds at 80% intensity, drying time of 30 seconds at 100% intensity, and coating length of 5 meters (on a 760 mm width).

The coating blade is 0.457 mm thick with a 50° tip angle. Remembering that the precoat was applied on both sides of the base stock paper, the top coating was always applied on the same precoat side: the one corresponding to the smooth side of the base paper as seen in Figure 4-4.

<figure><img src="image_64.png" /><figcaption>Figure 4-4 Schematic representation of the cross section of the final paper samples produced for the study</figcaption></figure>

### 4.3.1 Control of the top coat weight for the CLC samples

When adding the top coat layer, each CLC coating run provides a 14 cm wide coated strip in a helicoidal form. Twelve samples are cut out of the 14 cm wide strip using a template 12.5 cm by 40 cm for a total area of 500 cm² per sample. In parallel, a sample with the same size (500 cm²) is cut out from the “uncoated paper” (it is actually the pilot precoated samples) portion from each CLC run. The “uncoated paper” sample serves as the reference to calculate precisely the coat weight applied with the CLC.

The target coat weight is 12 ± 0.5 g/m². All 500 cm² top coated samples are weighed and only the samples within ± 0.5 g/m² tolerance are kept for the study. A minimum of 15 CLC samples at the correct coat weight are needed for each precoat/topcoat combination.
<table><caption>Table 4-7 Topcoat Formulations used with the CLC</caption><thead><tr><th></th><th>solids</th><th>1</th><th>2</th><th>3</th></tr></thead><tbody><tr><td>BPSD GCC (0.659 µm)</td><td>76%</td><td>70</td><td>70</td><td>70</td></tr><tr><td>High glossing clay (0.469µm)</td><td>70%</td><td>30</td><td>30</td><td>30</td></tr><tr><td>Polyacrylate salt dispersant</td><td>40%</td><td>0.15</td><td>0.15</td><td>0.15</td></tr><tr><td>S/B/ACN</td><td>50</td><td>12</td><td></td><td></td></tr><tr><td>SA</td><td>50</td><td></td><td>12</td><td></td></tr><tr><td>S/A/ACN</td><td>50</td><td></td><td></td><td>12</td></tr><tr><td>Optical Brightener Hexasulpho-</td><td>100</td><td>0.7</td><td>0.7</td><td>0.7</td></tr><tr><td>Calcium Stearate</td><td>50</td><td>0.7</td><td>0.7</td><td>0.7</td></tr><tr><td>Ammonium zirconium carbonate</td><td>32</td><td>0.3</td><td>0.3</td><td>0.3</td></tr><tr><td>Carboxy Methyl Cellulose(CMC)</td><td>90</td><td>0.5</td><td>0.5</td><td>0.5</td></tr><tr><td>Solids</td><td></td><td>65%</td><td>65%</td><td>65%</td></tr></tbody></table>

## 4.4 Calendering the top coated samples

All CLC top coated samples have been calendered. The calender used is a laboratory supercalender with 2 nips formed by 2 cotton rolls and one steel roll. The calendering is performed to obtain a gloss target of 70 [21] for the control condition (Group 0) for each latex used in the study: i.e. S/B/ACN latex, S/A latex, and S/A/CAN latex. Once the calendering conditions were found for each latex, all other group samples containing the same latex were done using the same latex calendering conditions. The laboratory calendering conditions are listed in the table 4-8.

<table><caption>Table 4-8 Laboratory Calendering Conditions</caption><tr><td>Speed</td><td>14.7 m/min</td></tr><tr><td>Sample side on steel</td><td>Smooth</td></tr><tr><td>Number of passes</td><td>1</td></tr><tr><td>Steel Temperature</td><td>60°C</td></tr><tr><td>Nips Pressure - S/B/ACN latex</td><td>34.46 bar</td></tr><tr><td>Nips Pressure - SA latex</td><td>34.46 bar</td></tr><tr><td>Nips Pressure - S/A/ACN latex</td><td>20.68 bar</td></tr></table>
# Chapter 5 - Results and Analysis

## 5.1 Introduction

As a reminder, the purpose of present work is to understand the effects of precoat components/parameters on the structure, both of the precoat and of the final double-coated paper, and on the printability of double-coated paper grades.

In order to achieve that, from a standard precoat containing 100% GCC and coated with 3 different latexes, we have modify the double-coated papers by replacing 20% of the GCC by clay in the precoat (also called 20% clay substitution or Group 1), calendaring either the base paper or the precoat before precoat and/or top coat application (Group 2), substituting the standard precoat latex by an acrylic latex and the carboxy methyl cellulose (CMC) by an amphoteric thickener *i.e.* protein: (Group 3), and finally replacing 100% of the standard GCC by different GCC or precipitated calcium carbonates (PCC) or Brazilian clay (Group 4).

To facilitate the analysis of results, we have summarized the experimental design conditions in Table 5-1 which outlines the overall approach of our study: *i.e.* the 20 different precoated conditions. It should also be reminded that the same pigments and additives systems are applied as topcoat but with 3 different latexes within each of the 20 precoat conditions (*i.e.* for a grand total of 60 conditions).

The three latex chemistries examined are styrene/butadiene/acrylonitrile (S/B/ACN), a styrene/acrylic (S/A), and a styrene/acrylic/acrylonitrile (S/A/ACN) as these are the most commonly used in the industry for the considered coating applications.

The variations within the various coating structures brought about by above modifications are analysed by mercury intrusion as explained in Chapter 4.
<table><caption>**Table 5-1** Precoated formulations related with the group and the condition number</caption><thead><tr><th>Precoated formulation</th><th>Group</th><th>Samples</th></tr></thead><tbody><tr><td>100 parts GCC (1.4 µm)</td><td>Control</td><td>3 topcoat = 3 samples</td></tr><tr><td>20 parts High glossing clay (0.469 µm)</td><td rowspan="4">Group 1<br>20 parts clay<br>substitution</td><td rowspan="4">3 topcoat x 4 precoat<br>= 12 conditions</td></tr><tr><td>20 parts No.2 clay (2.13 µm)</td></tr><tr><td>20 parts delaminated clay (5.312 µm)</td></tr><tr><td>20 parts High aspect ratio clay (6.329 µm)</td></tr><tr><td>Base paper calendered 100°C - 50 kN/m</td><td rowspan="2">Group 2<br>Base paper &amp;</td><td rowspan="6">3 topcoat x<br>4 calendered base paper<br>and<br>4 calendered precoat<br>= 24 conditions</td></tr><tr><td>Base paper calendered 100°C - 200 kN/m</td></tr><tr><td>Precoat calendered 100°C - 50 kN/m</td><td rowspan="2">Precoat<br>calendering group</td></tr><tr><td>Precoat calendered 100°C - 200 kN/m</td></tr><tr><td>Base paper calendered 200°C - 50 kN/m</td><td rowspan="4">Precoat calendered 200°C - 200 kN/m</td><td rowspan="4"></td></tr><tr><td>Precoat calendered 200°C - 50 kN/m</td></tr><tr><td>Precoat calendered 200°C - 200 kN/m</td></tr><tr><td>Precoat calendered 200°C - 200 kN/m</td></tr><tr><td>Precoat 100 parts GCC (1.4 µm) + Acrylic<br>Latex</td><td>Group 3<br>SA + Protein<br>substitution group</td><td>3 topcoat x 2 precoat<br>6 = conditions</td></tr><tr><td>Precoat 100 parts GCC (1.4 µm) + Protein</td><td></td><td></td></tr><tr><td>100 parts BPSD GCC (0.659 µm)</td><td rowspan="5">Group 4<br>100 parts pigment<br>substitution</td><td rowspan="5">3 topcoat x 5 precoat<br>= 15 conditions</td></tr><tr><td>100 parts BPSD GCC (0.3598 µm)</td></tr><tr><td>100 parts PCC (0.499 µm)</td></tr><tr><td>100 parts Brazilian clay (0.548 µm)</td></tr><tr><td>100 parts NPSD GCC (0.6593 µm)</td></tr></tbody></table>

Similarly, the effect of the structure variations brought about by above modifications on printability can be explained by contact angle analysis and DELTACK curves. For the DELTACK curves we are focusing on the comparison of the actual curves, namely through average values for total condition, and maximum tack force and time. The link between surface structure modification and printability is also supported by the analysis of SEM pictures.

All results have been further validated through statistical analysis using the projection on latent structure method.
## 5.2 Effects of 20 parts clay substitution in the precoat layer (Group 1)

The Group 1 includes 5 different conditions. The first condition has a precoat formulation that includes 100 parts GCC (1.4 µm) and is considered as the reference comparison point. The 4 other conditions are 80 parts GCC (1.4 µm) as in the reference but now with 20 parts clay having a different particle size distribution, and aspect ratio, as listed in table 5-2. In essence, the 20% clay substitution is the variable between each formulation. The aspect ratio of the clay has not here been considered as a key variable (as it will be seen from our results).

<table><caption>Table 5-2 Conditions for the 20 parts clay substitution - Group 1</caption><thead><tr><td>Precoated formulation</td><td>Group</td><td>Formulation number</td></tr></thead><tbody><tr><td>100 parts GCC (1.4 µm)</td><td>Control</td><td>3 topcoat = 3 samples</td></tr><tr><td>20 parts High glossing clay (0.469 µm)</td><td rowspan="4">Group 1<br>20 parts clay<br>substitution</td><td rowspan="4">3 topcoat x 4 precoat<br>= 12 conditions</td></tr><tr><td>20 parts No.2 clay (2.13 µm)</td></tr><tr><td>20 parts delaminated clay (5.312 µm)</td></tr><tr><td>20 parts High aspect ratio clay (6.329 µm)</td></tr></tbody></table>

### 5.2.1 Analysis of the precoat and topcoat structures: 20 parts clay substitution (Group 1)

#### 5.2.1.1 Analysis of the precoat and topcoat structure through pore size distribution

Figure 5-1 presents the pore size distribution of the precoat for the reference paper (in blue, average particle size 1.4 µm) and the 4 samples with 20 parts clay substitution by clays of increasing average particle size from 0.469 µm to 6.329 µm. We can conclude that the pore diameter increases and the volume slightly increases with the increasing of the average particle diameter and the aspect ratio (high aspect ratio). As all precoats contain the same latex (S/B), the structure modification is due to the network of pigments, the way they arrange in the matrix. The smaller the particle size, the easier it is for the particle to fill-in existing pores.
Intrusion in Precoating layer
Precoat 20 parts clay substitution group

<figure><img src="image_65.png" /><figcaption>Figure 5-1 Mercury Intrusion in Precoated layer – Precoat 20 parts clay substitution Group</figcaption></figure>

Figure 5.2 presents the pore distribution for the precoat (standard or reference condition) and the pore distribution of the topcoat for the 3 latexes used in the study. It appears clearly that applying a topcoat on a precoat has a direct influence on the pore distribution of the precoat. The pore volume decreases for all precoat structure once the topcoat is applied. However, using different latexes does not change the resulting structure of the precoat. Indeed, by analysing the 3 curves corresponding to the 3 latexes on Figure 5-2, it is apparent that the precoat structure is exactly the same with the 3 different topcoat conditions.

**Important remark on the analysis of the mercury intrusion graphs.** It is here worth to note that the curves in Figure 5-2 originate from 3 different mercury intrusion tests but that the curves are quite coherent: i.e. the apex in the precoat in pore size distribution occurs at the same pore diameter for all 3 samples. The comment we are making here for Figure 5-2 is true for all curves we have analysed in our study. We consider that such an accurate location of the apexes and curves is a confirmation that our technique of data treatment is valid. It is also why we are able to perform a fine and detailed comparison of mercury intrusion curves and isolate the various effects of pigment,
latexes, and of the all conditions of our study. Subsequently, the “subtraction” of the precoat data becomes feasible to allow for the sole top coat analysis. The analysis of the topcoat distribution is done below for the 3 latexes, using the data “subtraction” method.

<figure><img src="image_66.png" /><figcaption>Figure 5-2 Mercury Intrusion in Coating structure for condition with Standard Precoat - 100 parts GCC (1.4µm) - uncalendered</figcaption></figure>

Figures 5-3, 5-4, and 5-5 present the topcoat pore distribution for the calendered samples for the 3 latexes used in the topcoat: i.e. S/B/ACN, S/A latex, and S/A/ACN, respectively. Each Figure present 5 different curves: one corresponding to the reference precoat (dark blue), and 4 curves corresponding to the 20 parts clay substitution: i.e. high glossing clay (orange), Nº 2 clay (pale blue), delaminated clay (grey), and high aspect ratio clay (brick red).

For all topcoat structures, the cut-off gap in pore size is 0.03 µm to 0.14 µm. The cut-off value and the pore interval is selected to correspond to the minimum in the pore size distribution between the precoat and the topcoat for the standard precoat (reference) using the S/B/ACN latex. It is easy to verify in Figure 5-3 that the reference mercury intrusion curve (in dark blue) intersects the x-axis (0 value in mL/g) at 0.14 µm. All the
other conditions within the group in Figure 5-3 cross the x-axis to/or very close to
0.14 µm.

The S/A latex (Figure 5-4) and S/A/ACN (Figure 5-5) present a very different effect on
the topcoat structure. For the S/A latex, all the curves below 0.1 µm are under the x-axis
(remembering it is an artefact due to the data treatment) indicating that, for this latex, the
topcoat has a lower pore volume than the topcoat with the S/B/ACN latex. The opposite
is true for the S/A/ACN latex where all curves are above the x-axis, indicating a higher
topcoat pore volume compared to the topcoat with the S/B/ACN latex. We conclude
that the latexes have an influence on the topcoat pore structure although we do not have,
at this time, an explanation to such an effect we surmise that it might be related to the
chemical structure and/or gel content of the latexes. The characteristics of the latexes are
listed in table 4.1.

The changes in pore volume and diameter observed with the precoat (see Figure 5-1) are
transposed on the topcoat: these vary as a function of the clay used for the 20 part clay
substitution. The only difference is the change on the coating structure is only in pore
volume as related to the clay used: for instance, the high glossing clay always presents
the lowest pore volume and the high aspect ration clay always presents the highest pore
volume. It is true for the 3 topcoat latexes (S/B/ACN, S/A and S/A/ACN) as it can be
verified in Figures 5-3, 5-4 and 5-5. The analysis will be further detailed using the aver-
age pigment size as an independent variable.

5.2.1.2 Influence of the precoat (20 parts substitution) on the top-coat/precoat combined structure

Figure 5-6 shows the total pore volume for the combined precoat/topcoat layers structure
as a function of the clay average particle size in the 20 part substitution. The precoat
only curve (without topcoat) is used as a reference for the initial structure before topcoat
application.
Intrusion in Topcoating layer
20 parts clay substitution group
Topcoat Latex SB/ACN

<figure><img src="image_67.png" /><figcaption>Figure 5-3 Mercury Intrusion in Topcoating Layer - 20 parts clay substitution group - S/B/ACN latex - uncalendered</figcaption></figure>

Intrusion in Topcoating layer
20 parts clay substitution group - Latex SA

<figure><img src="image_68.png" /><figcaption>Figure 5-4 Mercury Intrusion in Topcoating Layer - 20 parts clay substitution group - S/A latex - uncalendered</figcaption></figure>
<figure><img src="image_69.png" /><figcaption>Figure 5-5 Mercury Intrusion in Topcoating Layer - 20 parts clay substitution group - Latex S/A/ACN - uncalendered</figcaption></figure>

Figure 5-7 is identical but after calendering of the topcoat. For both uncalendered and calendered samples, the total pore volume of the precoat/ topcoat layers increases with the pigment size used as the 20 parts replacement in the precoat. We will come back to the physical explanation later when analyzing the topcoat structure variations as a function of the precoat 20 part substitution.

Both for the uncalendered (Figure 5-6) and the calendered samples (Figure 5-7), the latex S/A/ACN shows a higher pore volume than the other latexes (S/B/ACN and SA) notwithstanding the fact that the pressure we use to obtain a target gloss of 70 is about 20.7 bar for the S/A/ACN latex compared to 34.5 bar for the 2 other latexes. The difference in structure, already present in the uncalendered samples, is maintained in the calendered samples.

As far as gloss development is concerned, the acrylonitrile (ACN) is known to improve gloss development: it is probably the reason why the pressure needed to obtain a 70 gloss is lower for the S/A/ACN latex. However, it does not occur for the S/B/ACN latex. The gel content of the S/B/ACN latex being lower than the S/A/ACN latex might
be the reason why the calendering pressures to obtain a 70 gloss are the same for the S/A
and S/B/ACN latexes.

<figure><img src="image_70.png" /><figcaption>Figure 5-6 Total Pore Volume (TPV) Uncalendered Paper for Group 1 - Comparing the 3 latexes</figcaption></figure>

<figure><img src="image_71.png" /><figcaption>Figure 5-7 Total Pore Volume (TPV) Calendered Paper for Group 1 - Comparing the 3 latexes</figcaption></figure>
Figure 5-8 presents the average pore diameter of the precoat/topcoat layer structure to increase with the pigment size of the precoat 20 part substitution. It is apparent that the precoat structure is more open with larger particle size pigment than with smaller particle size pigment used in the 20 parts substitution. The effect is less apparent for the precoat/topcoat combined structure, whatever the latex used. Therefore, we should look at the topcoat structure only to analyse the influence of the precoat 20 part pigment substitution.

<figure><img src="image_72.png" /><figcaption>Figure 5-8 Average Pore Diameter (APD) Calendered Paper for Group 1 - Comparing the 3 latexes</figcaption></figure>

### 5.2.1.3 Influence of the precoat structure (20 parts substitution) on the topcoat structure

As said previously, the increase in total pore volume as a function of the pigment particle size in the precoat layer (Figures 5-6 and 5-7) needs further explanation. Due to the fact that both Figures show that the precoat total pore volume increases with the particle size, we may surmise that it is solely due to the increase in the precoat. However Figure 5-9 shows that the topcoat total pore volume increases with the pigment average particle
size in the precoat. It is true for the 3 different latexes used in the topcoat although the topcoat layers have all the same pigment system (70 parts GCC and 30 parts N°1 glossing clay). It can then be concluded that the precoat structure has a direct effect on the structure of the topcoat. We propose that three physical principles may explain such an effect:

1. forced penetration of the topcoat coating color in the precoat structure: an open structure with large pores (as defined by pore diameter) bringing about more penetration than a closed structure with small pores,

2. capillary penetration within the precoat structure: smaller pores favouring capillary penetration,

3. preferential topcoat pigment penetration within the precoat structure: smaller pigments (N°1 high glossing clay) penetrating more easily the precoat structure than larger size pigment (GCC).

<figure><img src="image_73.png" /><figcaption>Figure 5-9 Total Pore Volume (TPV) Topcoated layer for Group 1 as a function of the particle size of the precoat layer - Comparing the 3 latexes</figcaption></figure>
Forced penetration implies that larger pores lead to more penetration of the topcoat layer within the precoat layer: i.e. leaving less coating layer on top and subsequently a smaller pore volume. According to the fact that the topcoat pore volume increases (linearly, Figure 5-9) with the precoat average particle size, this hypothesis can be ruled out: the forced penetration is not the right explanation.

The effect of capillary penetration can be further analysed by looking at the average pore diameter of the topcoat layer. For instance, analyzing the average pore diameter in Figure 5-10 of the topcoat structure with the S/A/ACN latex, the pore size of the topcoat is independent of the pigment average particle size in the precoat. The increase in pore volume can then only be accounted for by a large number of pores of the same diameter. In other words, for a close structure in the precoat, there is less pores remaining than in the open structure. It means that more coating color penetrates the precoat structure when pores are smaller: it can only be explained by capillary penetration that is favoured by smaller capillaries, here smaller pore sizes (in the precoat structure). We concluded that capillary penetration should be considered as an acting force as can be seen in Figure 5-11.

However, when analyzing the topcoat structures related to the S/A and S/B/ACN latexes, we see that the average pore diameter increases slightly, but significantly, with the pigment average particle size in the precoat. It means that another phenomenon is taking place.

We propose “preferential topcoat pigment penetration” in the precoat as the solid phase of the topcoat coating color contains two pigments of different sizes (around 0.15 - 0.20 µm for the clay and around 0.66 µm for the GCC) as can be seen in Figure 5-12. Large pores in the precoat may absorb smaller pigments from the topcoat, whereas leaving a larger pore structure in the topcoat. For these two latexes, we surmise that there is a combination of the two phenomena: capillary penetration and preferential pigment penetration.
<figure><img src="image_74.png" /><figcaption>Average pore diameter (4V/A) (μm)
TOPCOATING LAYER
Precoat 20 parts clay substitution group</figcaption></figure>

**Figure 5-10** Average Pore Diameter (APD) Topcoated layer for Group 1 as a function of the particle size of the precoat layer - Comparing the 3 latexes

<figure><img src="image_75.png" /></figure>

**Figure 5-11** Capillary theory related to smaller pores in the precoat. Top-coat: GCC is in purple-blue, clay in light red. Precoat: GCC is in orange and Nº1 fine clay is in yellow.
<figure><img src="image_76.png" /><figcaption>**Figure 5-12** Preferential movement theory related to bigger pores in the pre-coat. Topcoat: GCC is in purple-blue, clay in light red. Precoat: GCC is in orange and high aspect ratio clay is in grey.</figcaption></figure>

**5.2.2 Printability variations related to 20 parts clay substitution in the pre-coat (Group 1)**

As presented in Section 3.8, the printability of the double coated samples is evaluated by the DELTACK method for each latex. A typical graph is shown in Figure 5-13 for latex S/B/ACN and the 4 samples of the 20 parts clay substitution group. For all 3 latexes, the shape of the curves is somehow similar.

The same comment is valid for the remaining groups. Consequently, and for clarity sake, all the remaining graphs are presented in Appendix 2. As explained in Section 3.8.2, the key information deduced from the DELTACK Method are the time to reach maximum tack and the maximum tack value as these two parameters can be related to the topcoat/precoat layer structure and the composition, mainly latex type, of the topcoat layer.
Deltac Average Curves for Latex SB/ACN
20 parts Clay Substitution Group

<figure><img src="image_77.png" /><figcaption>Figure 5-13 DELTACK – 20% clay substitution Group, S/B/ACN latex</figcaption></figure>

Figure 5-14 shows the maximum tack force (maximum force, in N) as a function of the time (in sec) to reach the maximum force. The S/A and S/A/ACN latexes are regrouped and there seems to be no relationship relating time and force. However, for the S/B/ACN, the maximum force increases linearly with time; in other words, when the ink tack force is reach sooner, it is also lower in force value. We surmised that it might be due to the structure of the topcoat. Figure 5-15 shows the time to reach the maximum force as a function of the pore volume of the topcoat/precoat structure. Again, the data are grouped as a function of the latexes. When analysing the S/B/CAN values, it can be seen that the low times (to reach maximum force) correspond to small pore volumes and that the high times correspond to high pore volume. Consequently, going back to Figure 5-14, we conclude that, for the S/B/ACN latex, the minimum tack force correspond to a low pore volume, and that the maximum tack forces correspond to large pore volume. From an ink transfer standpoint, large volume allow for more ink to penetrate the structure than small volume. In essence, a large pore volume in the topcoat/precoat structure will lead to a thinner ink film, after multiple transfers as it occurs in the DELTACK method and by consequence to a higher tack force. The phenomenon can be further modulated by solvent penetration within the topcoat structure, according to the average pore diameter of the topcoat/precoat structure.
Precoat 20 parts clay substitution (Group 1)
CALENDERED SAMPLES

<figure><img src="image_78.png" /><figcaption>Figure 5-14 Time to reach the maximum force function of the maximum force</figcaption></figure>

Precoat 20 parts clay substitution (Group 1)
CALENDERED SAMPLES

<figure><img src="image_79.png" /><figcaption>Figure 5-15 Total Pore Volume function Time to reach the maximum force</figcaption></figure>

Another conclusion from the analysis of Figures 5-14 and 5-15, is that the key information relating the structure of the topcoat/precoat layer will be deduced from the analysis of the maximum tack force as a function of both the pore volume, allowing for the capacity to absorb a given volume of ink, and the average pore diameter, to take into account the capillary phenomenon. Figures 5-17 and 5-18 show the maximum (tack) force
as a function of the pore volume and the average pore diameter of the topcoat/precoat structure, accordingly. In addition to the grouping related to the 3 latexes, we find that a apex (minimum for the volume, maximum for the average pore size) occurs for both, the S/B/ACN and S/A latexes. The fact that no apex occurs with the S/A/ACN latex might simply be due to the lack of data at low values in either pore volume or diameter. Con- sidering, for instance, the curves (in red) corresponding to the S/A latex, two different situations occur. On the one hand, the same maximum tack force originates from struc- ture having two largely different pore volumes (Figure 5-17) but the same average pore diameter (Figure 5-18), as seen from data points corresponding to the downward arrows. On the other hand, the same maximum tack force originates from structure having the same pore volume (Figure 5-17) but two largely different average pore diameters (Fig- ure 5-18), as seen from data points corresponding to the upward arrows.

Obviously to explain such behaviour of the topcoat/precoat structure, at least two phe- nomena must occur and have opposite effects. Let us consider the case where we obtain the same maximum tack force with a) the same pore volume (upwards arrows, Figure 5-17) and b) largely different pore diameters (upward arrows, Figure 5-18). To under- stand the phenomenon that we know to be related to the topcoat/precoat structure as we have illustrated in Figures 5-11 and 5-12, we need to have some information on the top- coat structure only. Such information is provided by the Figures 5-9 and 5-10 which show both the average pore size and the pore volume of the topcoat to increase linearly with the pigment size of the precoat structure (20 parts substitution). In Figure 5-17, the available volume for ink penetration provided by the topcoat/precoat structure is the same, and we could conclude that the ink transfer is the same and that the remaining ink layer on top of the topcoat structure is the same, providing the same maximum tack force. We propose here that the maximum tack force is due to a more complex phenom- ena as the average pore size of the two structures is widely different: i.e. the ink transfer should not the same (ref. work by [104, 105]). In essence, we propose and surmise that ink transfer is different because the topcoat structures are different. In effect, the arrow (case a) corresponding to the low pore size in precoat/topcoat structure corresponds to very fine pores and a very small volume in the topcoat structure (see Figures 5-9 and 5-10). In parallel, the arrow (case b) corresponding to the high pore size (in pre-
coat/topcoat structure) corresponds to a larger pores and larger volume in the topcoat structure. Practically, it means that ink layer remaining of the surface in case ‘‘a’’ will be thicker than in case ‘‘b’’ (smaller pores and smaller available volume). However, considering the actual sizes of these pores, it also means that the structure with very small pores will favour, due to capillary penetration, the preferential absorption of ink solvent. In essence we will end up with a thicker ink film but with less solvent, i.e. a tackier ink. In the case of the larger pores, there will be no or less penetration of ink solvent. It is therefore quite feasible that the two effects being opposite, we will end up with the same maximum tack force. The phenomenon is illustrated in Figure 5-16.

<figure><img src="image_80.png" /></figure>

Bottom = precoat layer / middle = topcoat layer / top = ink film (green)

<caption>**Figure 5-16** Ink film thickness related to an equal ink tack force and the pre-coat structure: close precoat structure (left) and open precoat structure (right)</caption>

Furthermore, we also know that some precoat formulations containing high glossing clay (0.469 µm) and No. 2 clay (2.13 µm) produce a larger amount of small sized pores in the final pore size distribution after topcoat application and calendering than the pre-coats containing delaminated clay (5.312 µm) and high aspect ratio clay (6.329 µm). It is also known that for coatings of equal pore density (number of pores per unit area), the ink sets faster due to the larger pores. Compared at equal pore volume which is a function of size and pore density, coatings with smaller pores set ink faster because of the larger number of sites available for ink penetration by capillarity [9].
When analyzing the situation where a) the available pore volume of the topcoat/precoat structure is widely different (downwards arrows, Figure 5-17) but b) the average pore diameter of the topcoat/precoat structure is the same (downwards arrows, Figure 5-18), then, a similar analysis can be performed. In essence, we will end up with two different ink layers because the topcoat structure is modulated by the precoat structure. Therefore, due to the preferential solvent penetration, the maximum tack force will still approximately be the same. One should note that we have taken two extreme cases and that this compensation effect of solvent penetration controlling the ink tack and ink transfer controlling the ink thickness (and therefore also the tack force) is valid for all intermediary cases.

Although we understand that the proposed mechanism could be modeled (ref. work by [106]), it is outside the scope of present work. It should also be noted that the mechanism that we are here proposing is complementary to previous works and models proposed by [76, 107, 108] as is explained below.

<figure><img src="image_81.png" /><figcaption>Figure 5-17 DELTACK maximum force versus total pore volume for 20 parts clay substitution group</figcaption></figure>
Precoat 20 parts clay substitution (Group 1)
CALENDERED SAMPLES

<figure><img src="image_82.png" /><figcaption>Figure 5-18 DELTACK maximum force versus average pore diameter for 20 parts clay substitution group</figcaption></figure>

The slow ink setting trend containing latex with low Tg and acrylonitrile is seen for all the precoat and topcoat combinations. Latexes S/B/ACN has an ink setting behaviour that follow the coating porosity while latex S/A and S/A/ACN have a slower ink setting than could be predicted from the sole topcoat/precoat porosity.

The slow ink setting trends for topcoats with the latex containing acrylonitrile could be explained by the slowing effect of acrylonitrile and the well-known rule in coating formulation and physics: latex is about 10% of the coat weight, 20% of the coating volume, and 50% of the coating surface. Indeed, Rousu *et al.* [101] showed that low latex solubility parameter (butadiene < styrene = acrylate < acrylonitrile) leads to a large effect of oil interaction. Furthermore, latexes with low glass transition temperature (Tg) and percentage of gel (gel-%) show an increased interactivity with ink oils [109].

It also means that the setting of the ink can also be adjusted to be faster or slower according to the choice of ink. It implies that the differences we have analysed could be greater or smaller depending on the ink used in the printing operation.

Finally, we used a lower calendering pressure for the S/A/ACN latex coatings to achieve the same gloss target than all other coatings. The difference in calendering probably
produces a slightly different connectivity for the coating pores compared to the other coatings. We have not further analysed such a probability.

Last but not least, the DELTACK method is performed without fountain solution. The acceptance of different liquids (solvent, fountain solution, ink oils, etc.) to the pores will further complicate how the pore-to-throat diameter ratio, and the available volume of the small voids will influence the ink setting (and therefore the maximum tack force from the DELTACK method). It is still unclear how the latex with the increased solvent absorption (lower solubility parameter) shows a faster ink setting, and the question remains if the latex swelling may partly close or not surface pores. Consequently, the latex swelling effect may further reduce the coating porosity and capillary absorption [110- [111]-112].

The last comment pertains to the solubility parameter of the solvent within the ink. As the difference between the solubility parameter of the solvent within the ink and the chemistry of the latex increases, the rate of solvent absorption decreases. The addition of acrylonitrile to a latex polymer decreases the solubility parameter compared to styrene butadiene latex. The result is that the difference between the solubility parameter of the solvent and the acrylonitrile latex that of the styrene butadiene latex, results in a decreased ink set rate for the acrylonitrile containing latex [113, 114].

### 5.2.3 Contact Angle results comparison from Group 1 (20 parts clay substitution)

The contact angle measurements as a function of the average particle size of the precoat layer for the group 1 (20 parts clay substitution) are represented in Figures 5-19 and 5-20 for the uncalendered and calendered samples, respectively. It can be seen that the contact angle for the S/B/ACN is always higher than for the S/A/ACN and the S/A latexes, the effect being more pronounced for the uncalendered samples. It is confirmed by looking at the contact angle mean shown in table 5-3.

We surmised that it might be due to either to the non polar component of the S/B/ACN being higher than for the other two latexes or to differences in the film forming proper-
ties. In addition, for the S/B/ACN latex, the contact angle decreases as a function of the average particle size of the precoat layer whereas it is independent of the average particle size for the other two latexes. In addition, the calendering has a greater effect on the contact angle for the pigment with a higher PSD as the particles with the highest average particle size have also the highest aspect ratio.

It is important to remember here, the S/A/ACN latex has been calendered with a lower pressure than the two other ones. In effect, the S/B/ACN seems to be more affected by the calendering than either the S/A latex or the S/A/ACN latex. In this particular case, the S/B/ACN latex shows an important reduction in contact angle between the coated paper and calendered paper. For the two other latexes, the contact angle increases upon calendering. The effect of calendaring is classical and related to the reduction in surface roughness.

From above analysis, we can conclude that also the contact angle provides some information on the effect of the precoat, it is not sufficient to deduce useful in-depth information on the effect of precoat layer on the final surface properties of the paper. The results obtained in the other groups confirmed this analysis and therefore, we will not repeat it.

<figure><img src="image_83.png" /><figcaption>**Figure 5-19** Contact angle for Coated Paper– 20% clay substitution Group</figcaption></figure>
<figure><img src="image_84.png" /><figcaption>Figure 5-20 Contact angle for calendered Paper– 20% clay substitution Group</figcaption></figure>

<table><caption>Table 5-3 Mean Contact Angle (°) values for Group 1 – Coated and Calendered Paper</caption><thead><tr><th></th><th>S/B/ACN latex</th><th>S/A Latex</th><th>S/A/ACN latex</th></tr></thead><tbody><tr><td>Coated Paper</td><td>94.66°</td><td>60.56°</td><td>58.81°</td></tr><tr><td>Calendered Paper</td><td>75.31°</td><td>64.29°</td><td>70.85°</td></tr></tbody></table>

### 5.2.4 Summary: 20 parts substitution in the precoat layer (Group 1)

Present work has clearly demonstrated that substituting 20 parts of clay in 100 parts GCC precoat formulation permits a modulation of the coating structure to obtain the desired structural and printing properties. Similarly, we have shown that the topcoat structure is affected by the precoat and that a possible is capillary penetration, a preferential movement, or a forced penetration into the precoat structure by the topcoat. The forced penetration, or a part of it, probably originates from the rewetting of the precoat layer upon topcoat application.

Another key conclusion is that differences in printing properties as described by the DELTACK analysis are mainly related to the type of latex used in the topcoat layer.
The butadiene base latex is always higher in maximum tack force and time to get the maximum tack force compared to the styrene acrylic. The topcoat containing styrene acrylic present similar behaviour when comparing DELTACK curve shapes. Finally, we found that it is possible to have the same ink tack with very different pore structures, either with an equal pore volume and a very different average pore size of the structure or, in the opposite way, a very different pore volume with the same average pore size. We have proposed a fundamental mechanistic approach to explain this particular phenomenon. The phenomenon is related to the changes in topcoat structure coming from the different precoat structures. Overall, the structural changes in the complex two layer structure provided by the precoat/topcoat are able to modify how the ink and/or the ink solvent penetrate preferentially into the two layer structure resulting in a thick or thin ink layer with different final compositions but providing the same tack force.

## 5.3 Effect of calendering the base stock or the precoat layer (Group 2)

The group has been created to verify if calendering the base paper or the precoated layer could improve the final properties of the double coated paper sheet. As seen in table 5-4, the group comprises 8 conditions: 4 conditions are used to check the effect of calendering of the base paper before precoat/topcoat application and 4 conditions serve to check the effect of calendering the precoated layer before topcoat application. Two temperatures and two pressures are used for calendering, resulting in 4 different conditions. The 4 different conditions of temperature and pressure applied on the base stock or the precoated layer are the same to be able to verify if calendering temperature and pressure would have cumulative or opposite effects on the final double coated paper structure and properties.

The precoat and topcoat layers are identical for all 8 conditions. The precoated layer is 100 parts of GCC (1.4 µm) with a S/B latex. The complete formulation is described in table 4-2. The topcoat layer is a mix of 70 part of BPSD GCC (0.659 µm) and High glossing clay (0.469 µm). Due to the primordial importance of the latex effect seen previously, we still use 3 different latexes (i.e. 3 different samples), namely S/B/ACN, S/A, and S/A/ACN. The formulations are shown in table 4-7.
<table><caption>Table 5-4 Conditions for the calendering group - Group 2</caption><thead><tr><th>Precoated formulation</th><th>Group</th><th>Formulation number</th></tr></thead><tbody><tr><td>Base paper calendered 100°C - 50 kN/m</td><td rowspan="2">Group 2</td><td></td></tr><tr><td>Base paper calendered 100°C - 200 kN/m</td><td></td></tr><tr><td>Precoat calendered 100°C - 50 kN/m</td><td rowspan="2">Base paper &amp; Precoat</td><td>3 topcoat x</td></tr><tr><td>Precoat calendered 100°C - 200 kN/m</td><td>4 calendered base paper</td></tr><tr><td>Base paper calendered 200°C - 50 kN/m</td><td rowspan="3">calendering group</td><td>And</td></tr><tr><td>Base paper calendered 200°C - 200 kN/m</td><td>4 calendered precoat</td></tr><tr><td>Precoat calendered 200°C - 50 kN/m</td><td>= 24 conditions</td></tr><tr><td>Precoat calendered 200°C - 200 kN/m</td><td></td><td></td></tr></tbody></table>

### 5.3.1 Analysis of the precoat and topcoat structures: calendering base paper and precoated layer (Group 2)

In order to analyse the effect of precalendering either the base paper or the precoat on the double coated papers structure, it is necessary to detail how it affects the precoat and topcoat structures separately, then the total structure of the complex double coating layer. Indeed, it was shown previously that the two layers interact through capillary penetration during manufacturing. Figure 5-21 is a good example of the base paper while a calendering has been done. The Figure shows the calendering treatment of the base paper for group 2 when 200°C and 200 kN/m were applied to the paper.

<figure><img src="image_85.png" /><figcaption>Figure 5-21 Uncalendered (left) and calendered (right) base stock use in this study</figcaption></figure>
### 5.3.1.1 Analysis of the precoat and topcoat structure through pore size distribution

The various structures have been analysed using mercury intrusion and the normalization technique that has been described previously (Separation of the precoat and the topcoat layers).

#### Analysis of the precoat layer structure.

Figure 5-22 shows the pore size distribution of the precoat layer only for the precalendering of the base paper (4 conditions: see Table 5-4) and for the precalendering of the precoat layer (4 conditions: see Table 5-4). For these samples no top coat layer has yet been applied.

<figure><img src="image_86.png" /><figcaption>Figure 5-22 Mercury Intrusion in Precoated layer – Base Paper and Precoat Calendered Group</figcaption></figure>

At the planning stages of present work, we made the hypothesis that precalendering either the base stock or the precoat layer would result in a lower mercury intrusion volume while increasing the calendering temperature and/or the pressure applied. Upon precalendering of either the base paper or the precoat, the pore size distribution of the sole
precoat layer (Figure 5-22) is always lower than the pore distribution of the reference paper. Such is to be expected from the physics of calendering as soft nip calendering somehow reduces the pore size and pore volume. However, no measurable difference can be seen in Figure 5-22 between the pore size distributions corresponding to the 8 calendered conditions as far as precalendering in a first operation either the base paper or the precoat layer. The effect of the calendering conditions, even if minor, becomes clear when analyzing the total pore volume; it is due to the fact that calculating the total pore volume integrates the pore volume at all sizes.

Table 5-5 shows, as expected, that calendering at 200 kN/m results in a lower total pore volume than calendering at 50 kN/m. However, calendering at 100°C generates a larger total pore volume than calendering at 200°C. We will come back to this apparently surprising result when analyzing the combined precoat/top coat layer structure.

<table><caption>**Table 5-5** Average volume structure (mL/g) for the precoat layer (Group 2)</caption><thead><tr><th colspan="4">Soft Nips Pressures and Temperatures</th></tr><tr><th></th><th>100°C</th><th>200°C</th><th></th></tr></thead><tbody><tr><td>Base paper calendered - 50 kN/m</td><td>0.1909</td><td>0.2000</td><td>Average 50 kN/m</td></tr><tr><td>Precoat calendered - 50 kN/m</td><td>0.1869</td><td>0.1957</td><td><strong>0.1934</strong></td></tr><tr><td>Base paper calendered - 200 kN/m</td><td>0.1661</td><td>0.1895</td><td>Average 200 kN/m</td></tr><tr><td>Precoat calendered - 200 kN/m</td><td>0.1772</td><td>0.1972</td><td><strong>0.1825</strong></td></tr></tbody></table>

### Analysis of the topcoat layer structure.

Figure 5-23 shows the pore size distribution for the top coat layer only for the S/B/ACN latex (used in the top coat layer mix) for the precalendered base paper (top graph) and the precalendered precoat layer (bottom graph). The reference (standard) top coat layer has been added to both graphs; it corresponds to the case where neither the base paper nor the precoat layer has been calendered. Similar graphs, not shown here, were obtained for the S/A and the S/A/ACN latexes used in the top coat.
<figure><img src="image_87.png" /><figcaption>Figure 5-23 Mercury Intrusion in Topcoated layer – Base Paper and Precoat Calendered Group Separated – S/B/ACN latex</figcaption></figure>

As for the precoat analysis, the top coat structure corresponding to the reference paper, with no calendering of either base paper or precoat, show a more open structure than the samples precalendered (base paper or precoat) before top coat application. When the base paper is precalendered, the pore size distribution curve is higher for the 200°C cal-
endering temperature than for the 100°C temperature. The effect on the pore distribution curves is not as clear when dealing with the precalendering of the precoat layer. However, as shown previously, we surmise that the total pore volume which integrates the incremental volumes along the pore sizes might render the calendering temperature and pressure effects more apparent. The total pore volume of the top coat layer applied to precalendered base paper and precalendered precoat is shown in Table 5-6 for the 3 latexes.

When precalendering the precoat which contains styrene butadiene (SB) latex, the SB latex tends to plasticize with temperature. The effect being more pronounced at 200°C than at 100°C, we propose that at 100°C the precalendered precoat surface will be more open than at 200°C. We propose that a filtration effect, similar to the one already proposed to explain the filler substitution effect in the precoat (Printability variations related to 20 parts clay substitution in the precoat (Group 1)), applies here. In essence, coating colour filtration and segregation through an open structure, as represented by the surface of the precoat calendered at 100°C, should create a denser top coat structure, i.e. a smaller total pore volume, than filtration through a close structure, as represented by the surface of the precoat calendered at 200°C. It also means that the top coat layer packing is improved at 100°C.

Similarly, on the basis of our proposed mechanism, precalendering the base paper has an effect on the packing of the precoat: i.e. the total pore volume of the precoat layer, when applied on the base paper precalendered at 100°C should be smaller than at 200°C. This is confirmed from Table 5-5 results showing the total pore volume of the precoat layers. As the top coat layer is now applied to a denser precoat (when base paper is precalendered at 100°C), less filtration effect occurs than when the base paper is precalendered at 200°C. As before, the packing of the top coat layer on dense precoat structures provides an open structure: i.e. the total pore volume of the top coat layer when precalendering the base paper at 100°C should be higher than the total pore volume of the top coat layer when the base paper is precalendered at 200°C. This is confirmed in Table 5-6 showing the total pore volume of the top coat layers. The effect of precalen-
dering pressure, either on the base paper or on the precoat layer, although less apparent, confirms present filtration and packing mechanism.

<table><caption>**Table 5-6** Average pore volume (mL/g) for the topcoat layer for each latex (Group 2)</caption><thead><tr><th colspan="4">Soft Nips Pressures and Temperatures</th></tr><tr><th></th><th colspan="3">Topcoat latex</th></tr><tr><th></th><th>S/B/ACN</th><th>S/A</th><th>S/A/ACN</th></tr></thead><tbody><tr><td>Base paper calendered 100°C - 50 KN/m</td><td>0.0396</td><td>0.0384</td><td>0.0442</td></tr><tr><td>Base paper calendered 100°C - 200 KN/m</td><td>0.0434</td><td>0.0329</td><td>0.0325</td></tr><tr><td>Top coat average value corresponding of base paper at 100°C</td><td><u>0.0415</u></td><td><u>0.03565</u></td><td><u>0.03835</u></td></tr><tr><td>Base paper calendered 200°C - 50 KN/m</td><td>0.0308</td><td>0.0232</td><td>0.0368</td></tr><tr><td>Base paper calendered 200°C - 200 KN/m</td><td>0.0383</td><td>0.0361</td><td>0.0382</td></tr><tr><td>Top coat average value corresponding of base paper at 200°C</td><td><u>0.0345</u></td><td><u>0.02965</u></td><td><u>0.0375</u></td></tr><tr><td>Precoat calendered 100°C - 50 KN/m</td><td>0.0222</td><td>0.0361</td><td>0.0309</td></tr><tr><td>Precoat calendered 100°C - 200 KN/m</td><td>0.0422</td><td>0.0291</td><td>0.0329</td></tr><tr><td>Top coat average value corresponding to precoat layer at 100°C</td><td><u>0.0322</u></td><td><u>0.0326</u></td><td><u>0.0319</u></td></tr><tr><td>Precoat calendered 200°C - 50 KN/m</td><td>0.0422</td><td>0.0361</td><td>0.0373</td></tr><tr><td>Precoat calendered 200°C - 200 KN/m</td><td>0.0349</td><td>0.0426</td><td>0.039</td></tr><tr><td>Top coat average value corresponding to precoat layer at 200°C</td><td><u>0.0385</u></td><td><u>0.03935</u></td><td><u>0.03815</u></td></tr></tbody></table>

*Analysis of the combined precoat and topcoat layer structure.*

Figure 5-24 shows the pore size distribution for the whole precoat/top coat structures for the S/B/ACN latex (used in the precoat). Again, a reference (standard) pore size distribution shown corresponds to no calendering of either base paper or precoat layer. The tendencies for pore size distributions as analysed separately for the precoat and the top coat layer are here combined. It is however important to look at such a combined pore size distribution as we have seen (Printability variations related to 20 parts clay substitu-
tion in the precoat (Group 1)) that both coating layers affects the final printing characteristics of the double-coated paper.

As before, besides the reference paper, Figure 5-24 shows that the differences in pore size distribution among the various conditions are subtle; just the extreme conditions, for instance precalendering at 100°C and 50 kN/m presents the lowest distribution, essentially for the top coat portion of the graph.

<figure><img src="image_88.png" /><figcaption>**Figure 5-24** Mercury Intrusion in Coating Structure – Base Paper and Pre-coat Calendered Group</figcaption></figure>

The Figure 5-26 presents the total pore volume of the combined precoat and top coat layer structure for the reference paper (in yellow), the precalendered base paper (in blue), and the precalendered precoat layer (in purple) for the various precalendering temperatures and pressures. It can be seen that the effect of the precalendering temperature is preponderant over the effect of the precalendering pressure. Moreover, the effect of the temperature on the total pore volume of the combined precoat/top coat structure implies that the packing effect due to the surface pore closing is preponderant: i.e. at 100°C the total pore volume is always lower than at 200°C.
100° C / 50 kN/m

<figure><img src="image_89.png" /></figure>

100° C / 200 kN/m

<figure><img src="image_90.png" /><figcaption>**Figure 5-25** Total pore volume (mL/g) for the standard reference (yellow), calendered base paper (blue) and precoat layer (purple) for the different pressures and 100°C used in calendering.</figcaption></figure>
<figure><img src="image_91.png" /><figcaption>Figure 5-26 Total pore volume (mL/g) for the standard reference (yellow), calendered base paper (blue) and precoat layer (purple) for the different pressures and 200°C used in calendering.</figcaption></figure>
### 5.3.2 Printability variations related to precalendering the base stock or the precoat layer (group 2)

As for the previous series of experiments (Group 1), the effect of precalendering either the base paper or the precoat layer on printability is analysed through the DELTACK maximum force. Figure 5-27 presents the average value for the 8 precalendering conditions (2 temperatures, 2 pressures, and 2 layers: base stock and precoat) for the 3 latexes included in the top coat layer. The reason for showing average curves is that the curve shapes are similar from one condition to the other: the main difference, as shown in Figure 5-27, is related to the latex type.

<figure><img src="image_92.png" /><figcaption>Figure 5-27 DELTACK mean for each latex (8 calendering conditions)</figcaption></figure>

As we have seen that the total pore volume is the main difference when considering the effect of precalendering, Figure 5-28 presents the maximum DELTACK force obtained for the 8 precalendering conditions for the three latexes as a function of the average total pore volume. Actually, the analysis is somehow similar to the one realized for Group 1 as far as total pore volume is concerned. Considering that the precalendering temperature is the main factor controlling the total pore volume, we have identified on Figure 5-28 the temperature for the data points representing the 8 conditions. Considering the effect of temperature on the structure of the double-coated layer, and separately on each coating layer, the points regroup themselves as a function of the temperature. This is
however not surprising when considering that the total pore volume may originate from
two different structures. In essence, there is a preferential filtration of ink components
that will provide the same maximum DELTACK force for different structures (corre-
sponding here to different total pore volumes).

We have seen previously that the structure of the precoat, and consequent effect on the
structure of the top coat, is different whether the temperature is 100°C or 200°C. We
have also explained the phenomena by either plasticization, when precalendering the
precoat, or closing the surface pore structure, when precalendering the base paper. It
somehow confirms the mechanistic model we have proposed to explain the effect of the
structural changes brought about by the 20% substitution of pigments in the precoat for
5.2.1.3.

<figure><img src="image_93.png" /><figcaption>Figure 5-28 Localisation of the dots for the precalendering, both base paper and precoat, at 100°C and 200°C for the S/B/ACN, S/A, and S/A/ACN latex</figcaption></figure>

5.3.3 Summary: base paper and precoated layer calendering group
(Group 2)

The group 2 has been created to verify the effect on a double-coated paper of precalen-
dering either the base paper or the precoat layer. In all case, precalendering produces
lower pore size distribution, and consequently a lower total pore volume, than the reference paper, with no precalendering. Although the analysis of the pore distribution curves were difficult and that no clear trend could be easily established to understand how precalendering specifically affected the structures, they nevertheless led us to the conclusion that the structure thus produced by precalendering are different. The analysis of the total pore volume is found to be a powerful tool to analyse the difference in precalendering.

First we found that precalendering at 100°C results in a higher precoat pore volume than precalendering at 200°C. As expected, calendering at 200 kN/m also produces smaller total pore volume of the precoat than precalendering at 50 kN/m. More surprising are the results obtained for the topcoat structure upon precalendering either the base paper or the precoat. Here, two apparently opposite effects occur:

- when precalendering the base paper, the top coat total pore volume is higher at 100°C than at 200°C, on the opposite

- when precalendering the precoat layer, the top coat total pore volume is higher at 200°C than at 100°C.

The mechanism we propose to explain such apparently contradictory results is related to the filtration and packing of the structure that occurs during the coating operation. It is related to the movement of the water and materials when a wet coating layer is applied over another surface. Indeed, the surface pore structure being more or less open will control filtration and packing. High temperature precalendering of the precoat layer has a direct impact on the top coat structure. Precalendering the base paper has the same effect on the precoat structure but the situation is then necessarily reversed when the top coat is applied in a kind of "domino" effect.

Finally, we explained that it is feasible to obtain the same printing characteristics (maximum DELTACK force) with the different structures created by precalendering. Here too, the effect of temperature precalendering is found to be the predominant factor.
As a final conclusion, we show here that it is possible to control the double-coated layer micro-structure and down to nano-structure (30 nm) through precalendering and thus control the printing characteristics of double-coated papers.

## 5.4 Effect on Styrene/Acrylic and Protein substitution in precoat layer (Group 3)

The group 3 was designed to verify if a change in the chemistry of the precoat layer could have an effect on the final printability or the structure parameters of the coating layers. Groups 3 contains only 2 condition levels related to the chemistry of the components added, besides the pigments, in the coating colors. For the first condition, S/A latex replaces the latex S/B that was used for all other conditions, all groups considered. Latex S/A used in Group 3 is the same latex that is used in the topcoat layer. For the second condition, a protein (Procote 4200-S), i.e. an amphoteric molecule, replaces the CMC which was used as a thickener in all other conditions, all groups considered. A summary of the different conditions included in this group is presented in table 5-7.

<table><caption>**Table 5-7** Conditions for the S/A precoat latex and amphoteric thickener - Group 3</caption><thead><tr><th>Precoated formulation</th><th>Group</th><th>Formulation number</th></tr></thead><tbody><tr><td>100 parts GCC (1.4 µm)</td><td>Control</td><td>3 topcoat = 3 samples</td></tr><tr><td>Precoat 100 parts GCC (1.4 µm) + Acrylic Latex</td><td>Group 3</td><td>3 topcoat x 2 precoat</td></tr><tr><td>Precoat 100 parts GCC (1.4 µm) + Protein</td><td>SA + Protein<br>substitution group</td><td>6 = conditions</td></tr></tbody></table>

### 5.4.1 Analysis of the precoat and topcoat structures through pore size distribution: Styrene/Acrylic and Protein substitution (Group 3)

Figure 5-29 shows the pore size distribution of the precoat layer as a function of the pore diameter for the S/A latex and the protein substitution. The curve of the standard precoat is also included as a reference, for comparison. The pore size distributions for the two
conditions present similar effects that were observed in the Group one for the effects of 20 parts clay substitution.

When compared to the standard curve precoat formulation, the pore volumes as a function of pore diameter curves are lower than that of the standard precoat (Figure 5-29). Furthermore, the two curves representing the latex S/A and protein substitution are similar, practically identical.

<figure><img src="image_94.png" /><figcaption>**Figure 5-29** Mercury Intrusion in Precoating layer with 100 parts GCC (1.4 µm) –SA Latex and Amphoteric Thickener (protein)</figcaption></figure>

Figure 5-30 and 5-31 present the total pore volume of the whole coating structure, i.e. precoat and topcoat layers, for the 3 different latexes used to make the topcoat layers. The fourth curve in Figure 5-30 represents the precoat with acrylic latex substitution (S/A replacing S/B, GCC, 1.4 µm) and no topcoat applied; for the remaining 3 curves, S/A is used in the precoat layer. The fourth curve in Figure 5-30 represents the precoat with protein substitution (protein replacing CMC, GCC, 1.4 µm) and no topcoat applied; for the remaining 3 curves, protein is used in the precoat layer.

On the one hand, the analysis of each Figure shows that there is no difference in the pre-coat structure after the application of a topcoat layer. It is true for the curves obtained with the S/A latex substitution and the curves obtained with the protein substitution. On
the other hand, the comparison of the resulting precoat curves after a topcoat application
is quite different when comparing the effect of the S/A latex substitution (Figure 5-30)
and the effect of protein substitution (Figure 5-31): namely the shape of the curves cor-
responding to the precoat portion of the curves are quite different in the distribution of
the volumes as a function of decreasing pore diameter.

For the protein substitution, the curve has a larger base becoming broad (Figure 5-31) than for the acrylic substitution. Furthermore, for the protein substitution, the maximum pore volume obtained is lower than the S/A latex substitution in the precoat. Indeed, the pore volume corresponding to pores sizes in the 300-500 nm range is greater for the “S/A-CMC” than for the “S/B-protein” substitution. We surmise that the effect is due to a better compaction of the structure brought about by the protein.

When analyzing the pore volume for the topcoat, it is clearly seen that the total pore vol-
ume (area under the curves at very low pore diameter, below 150 nm) may be ranked as
follows: S/A/ACN > S/B/ACN > S/A. This is true for both the S/A and protein substitu-
tion (Figure 5-30 and 5-31). However, for the protein case, the total pore volume is lar-
ger, for the 3 latexes than for the acrylic substitution case. This might be due to the fil-
tration effect of the precoat compaction surmised above.

The “compacted” precoat structure for protein substitution has smaller pores that prevent
penetration of the topcoat components thus leaving an more open pore structure of the
topcoat than for the acrylic substitution. It is in agreement with the mechanisms of
structure development of coating layers we proposed in Group 1 and 2.

Figures 5-32, 5-33, and 5-34 present the pore volume as a function of the pore diameter for the final calendered double-coated paper (precoat/topcoat) for the acrylic latex and the protein substitution in the precoat for the 3 latex used in the topcoat, i.e. S/B/ACN (Fig. 5-32), S/A (Fig. 5-33), and S/A/ACN (Fig. 5-34).
Intrusion in Coating layer
Precoat 100 parts GCC (1.4 µm) + Acrylic Latex substitution
+ Topcoat

<figure><img src="image_95.png" /><figcaption>Figure 5-30 Mercury Intrusion in Coating structure for condition with 100 parts GCC (1.4 µm) and Acrylic Latex - uncalendered</figcaption></figure>

Intrusion in Coating layer
Precoat 100 parts GCC (1.4 µm) + Protein substitution
+ Topcoat

<figure><img src="image_96.png" /><figcaption>Figure 5-31 Mercury Intrusion in Coating structure for condition with 100 parts GCC (1.4 µm) – Amphoteric Thickener (protein) – Uncalendered</figcaption></figure>
The structures and the differences in the structures of precoat curves shapes are pre-
served, although the total pore volume (area under the curves) has been reduced, as ex-
pected from the calendering process. It is true for the 3 latexes used in the topcoat (Fig-
ure 5-33, 5-32 and 5-34). The differences observed in the pore structure of the precoat
layer are not directly transferred to the topcoat structure. The total pore volume of the
topcoat layer for the S/A/ACN is greater than for the S/A and SB/ACN cases. It might
simply be due to the fact that the calendering pressure was lower as we decided early on
to calender the samples at the same gloss target (70).

The other minor differences, i.e. the total pore volume of the topcoat (TPVtop) ranking as
follows: "protein TPVtop > S/A TPVtop" for the S/B/ACN in the topcoat (Fig. 5-32),
"protein TPVtop < S/A TPVtop" for the S/A latex in the topcoat (Fig. 5-33), and "protein
TPVtop = S/A TPVtop" (curves are on top of one another) for the S/A/ACN latex in the
topcoat (Fig. 5-34) might be due to combination of calendering pressure and the glass
transition temperature of the various latexes, 10°C, 23°C, and 40°C, respectively.

<figure><img src="image_97.png" /><figcaption>Figure 5-32 Mercury Intrusion in Calendered Structure with 100 parts GCC (1.4 µm) -S/B/ACN Latex and Amphoteric Thickener (protein)</figcaption></figure>
<figure><img src="image_98.png" /><figcaption>Figure 5-33 Mercury Intrusion in Calendered Structure with 100 parts GCC (1.4 µm) –SA Latex and Amphoteric Thickener (protein)</figcaption></figure>

<figure><img src="image_99.png" /><figcaption>Figure 5-34 Mercury Intrusion in Calendered Structure with 100 parts GCC (1.4 µm) –S/A/ACN Latex and Amphoteric Thickener (protein)</figcaption></figure>
Above observations are confirmed by analyzing Figure 5-35 which show the total pore volume (TPV) for each precoat/topcoat samples of the S/A and protein substitutions in the precoat composition and each latex used in the topcoat coating colour, i.e. S/B/ACN, S/A, and S/A/ACN. The standard precoat i.e. 100 parts GCC (1.4 µm) is also added as reference.

The protein (Procote 4200-S) has a lower TPV for the precoat layer which is maintained or even slightly increased upon topcoated application (Figure 5-35). As the protein is a colloid amphoteric material, it will create a close chemical and physical association with the pigment. On the contrary, the TPV of the precoated layer made with the acrylic latex (S/A) shows smaller values for the 3 latexes used for the topcoat (Figure 5-35). The S/A latex has smaller particle size than the S/B latex used for all other precoat conditions. It is probably the reason why we can observe such an effect.

<figure><img src="image_100.png" /><figcaption>**Figure 5-35** Total Pore Volume (TPV) Coated Paper for Group 3 - Comparing the 3 latexes. Precoat: all 3 groups 100 parts GCC (1.4 µm and (A): SBR+CMC, (B): S/A+CMC, and (C) SBR+Protein.</figcaption></figure>
### 5.4.2 DELTACK results comparison from Group 3 (SA latex and protein substitution)

First we will interest ourselves in comparing the effect of the protein and latex S/A substitution with the 3 different latexes used in the topcoat in order to evaluate the effect of the topcoat latexes among themselves, second we will look at the effect of using the latex S/A and protein substitution when compared to the standard practice of using SBR/CMC combination in the reference (or standard) double coated paper.

First, Figures 5-36, 5-37, and 5-38 present the DELTACK curves for the S/A and protein substitution in the precoat layer, for the latexes used in the topcoat layer, S/B/ACN, S/A, and S/A/ACN, respectively. For the 3 Figures, the curves for the S/A and protein substitution are similar for each top coat composition. The difference seen in Figure 5-36 for the S/B/ACN cannot easily be explained from the various topcoat and topcoat structures as evaluated in Figure 5-32: there could be a combination of pore volume and pore diameter, acting both on the volume availability and capillary absorption. Indeed the times (in seconds) available for absorption are sufficient in the DELTACK experiments for capillary absorption to have some importance when compared to the times occurring upon coating application a commercial coater.

Nevertheless, it appears to be a difference when comparing the various topcoat latexes. Figure 5-39 shows the DELTACK mean curves for the 3 latexes used for the topcoat layers. The shapes of the various curves are similar or equivalent to the ones observed for the previous conditions (Group 1 and 2). Using S/BACN in the topcoat coasting colour provides a higher surface resistance than using S/A or S/A/ACN but the tow last latexes provide better printability; which is coherent with commercial practice.

Second, Figure 5-40 shows that the acrylic latex used in the precoat condition may possibly react differently than the S/B-CMC used for the reference precoat layer. The maximum DELTACK force for the reference paper is higher and it occurs later than for the two substitutions cases.
The S/B latex used in the precoat has a bigger particle size than the 3 topcoat latex. It has also a low gel content. These two factors are known to decrease the bond strength and the ink setting [115].

Nevertheless, the results for the DELTACK presented previously show clearly that the application of a wet coating layer over another one modified the structure. Therefore, it confirms that the latex used in a precoat does affect how the topcoat layer applied over it will react, especially if the latex has quite different properties than the one normally used.

It should however be emphasised that we have only looked at two substitution conditions in the precoat layer and that more work should be performed to study these effects. In the course of this work, we may conclude that the substitution of SBR by S/A and CMC by protein is not truly worthy unless specific printing characteristic are targeted.

Deltac Average Curves for Latex SB/ACN
Acrylic Latex and Protein Precoated Layer Group

<figure><img src="image_101.png" /><figcaption>**Figure 5-36** DELTACK – S/A Precoating layer and protein Group, latex, S/B/ACN</figcaption></figure>
<figure><img src="image_102.png" /><figcaption>Deltac Average Curves for Latex S/A
Acrylic Latex and Protein Precoated Layer Group</figcaption></figure>

Figure 5-37 DELTACK – S/A Precoating layer and protein Group, S/A latex

<figure><img src="image_103.png" /><figcaption>Deltac Average Curves for Latex SA/ACN
Acrylic Latex and Protein Precoated Layer Group</figcaption></figure>

Figure 5-38 DELTACK – S/A Precoating layer and protein Group, latex, S/A/ACN latex
Deltac Average Curves
Acrylic Latex and Protein Precoated Layer (Group 3)

<figure><img src="image_104.png" /><figcaption>Figure 5-39 DELTACK mean value for the 3 topcoat latex conditions with S/A latex and Protein substitution)</figcaption></figure>

Deltac Average Curves for Latex S/A
Acrylic Latex and Protein Precoated Layer Group

<figure><img src="image_105.png" /><figcaption>Figure 5-40 DELTACK measurements for the S/A latex topcoat conditions (S/A latex and Protein substitution)</figcaption></figure>
### 5.4.3 Summary: Latex and thickener substitution in precoat layer (Group 3)

Using either a protein or a S/A latex in the precoat to substitute for an SB+CMC combination does not show significant improvement especially when the S/A latex precoat substitution is concerned. However, the protein substitution case presents an interesting behaviour as we can clearly demonstrate the effect of such a substitution on the structure of the coating layer. The protein definitively results in a broader pore distribution in the precoat layer. Finally, it is difficult to conclude on a general trend as just one condition has been studied. It is an area of coating improvement that could be studied specifically to understand as such a product changes the coating structure. It would be of future research interest to study the effect of structure/chemistry of the latexes on printing characteristics as the effect on the DELTACK results seen here are not really different than what we have analysed for the previous conditions.

## 5.5 Effect on 100 parts substitution in the precoat layer (group 4)

The last 5 conditions of the study of the influence of the precoat layer on the double-coated layer structures are the complete replacement (100 parts) of the pigment used in the precoat layer. All the pigments selected for such a replacement are representative of the pigments used commercially and extensively in the coated paper industry. A summary of the different conditions are listed in table 5-8.

<table><caption>Table 5-8 Conditions for 100 parts substitution - Group 4</caption><thead><tr><th>Precoated formulation</th><th>Group</th><th>Formulation number</th></tr></thead><tbody><tr><td>100 parts GCC (1.4 µm)</td><td>Control</td><td>3 topcoat = 3 samples</td></tr><tr><td>100 parts BPSD GCC (0.659 µm)</td><td rowspan="5">Group 4<br>100 parts pigment<br>substitution</td><td rowspan="5">3 topcoat x 5 precoat<br>= 15 conditions</td></tr><tr><td>100 parts BPSD GCC (0.3598 µm)</td></tr><tr><td>100 parts PCC (0.499 µm)</td></tr><tr><td>100 parts Brazilian clay (0.548 µm)</td></tr><tr><td>100 parts NPSD GCC (0.6593 µm)</td></tr></tbody></table>

BPSD: broad particle size distribution

NPSD: narrow particle size distribution.
### 5.5.1 Analysis of the uncalendered precoat and topcoat structures, pore size distribution: 100 parts precoat pigment substitution (group 4).

Figure 5-41 presents the pore volume distribution as a function of the pore diameter for the 5 pigments used in the precoat layer, i.e. GCC NPSD (0.6593 µm), PCC (0.499 µm), Brazilian clay (0.548 µm), GCC BPSD (0.659 µm), and GCC BPSD (0.3598 µm). The curve representing the standard precoat (100 parts GCC, 1.4 µm) structure is included for comparison. The curves clearly illustrate the difference in the structures obtained between a pigment with narrow particle size distribution (NPSD) and a pigment with a broad particle size distribution (BPSD). The NPSD pigment reaches a higher value in pore volume with a narrow pore size distribution.

It is interesting to note that the PCC which has the narrowest particle size distribution also presents the highest volume value (at 180 nm). We will see later that the analysis is confirmed when considering the total pore volume. Figure 5-42 shows the total precoat-top coat structure after the application of the topcoat. We have only illustrated here the tendencies for the latex S/B/ACN in Figure 5-42 although similar curves with similar characteristics were obtained with S/A latex and S/A/ACN latex. The part of the curves representing the precoat structure after topcoat application is always lower in volume than for the precoat structure before application of the topcoat layer. It was expected as it is exactly what occurred for all other conditions upon topcoat application. There are at least two effects occurring. First, application of the topcoat rewets the precoat structure thus disturbing and partly rearranging the precoat structure. Second, depending on the initial precoat structure, some topcoat components penetrate the precoat structure thus reducing the precoat total volume.

Another effect might be due to the shape of the pigment as they directly affect the pore shape and the related pore volume (packing efficiency, Figure 2-4 to Figure 2-9). Although Figure 5-42 represents the structures obtained for the S/B/ACN latex, the last assertion is true for the S/A and S/A/ACN latexes. Therefore, present analysis confirms previous findings on the influence of the precoat structure. It is possible to conclude that the precoat structure _greatly impact_ the final structure of coated paper.
Figures 5-43, 5-44, and 5-45 present the pore volume for the 100 part substitution group as a function of the pore diameter for the 3 topcoat latexes used in this study, i.e. S/B/ACN, S/A, and S/A/ACN, respectively. It should be reminded here that the negative values for the mercury intrusion appearing at the beginning of each curve is a calculation artefact related to the data treatment that allows us to compare each graph one to another. The curves for each latex present similar characteristics, almost identical, to the ones found for group 1 (5.2.1.1) corresponding to 20 parts substitution.

It is important to remember that the pigments/binder/chemicals system used for the topcoat in group 1 (20 parts substitution) and group 4 (100 parts substitution) is the same. Therefore, we may conclude that, due to the predominant influence of the precoat, all topcoats used in this study results in a certain type of structure pattern.

In addition, it should be emphasized that the pattern structure of the topcoat remains the same even with very different precoat pigments. Last but not least, it will nevertheless result in various precoat – topcoat structure combinations.

<figure><img src="image_106.png" /><figcaption>Figure 5-41 Mercury Intrusion in precoat layer – 100 parts substitution Group. The reference curve GCC is in red.</figcaption></figure>
Intrusion in Precoating and Topcoating Layer
Precoat 100 parts pigment substitution group
Topcoat Latex SB/ACN

<figure><img src="image_107.png" /><figcaption>Figure 5-42 Mercury Intrusion in Coating Structure layer – 100 parts substitution Group. Uncalendered samples.</figcaption></figure>

Intrusion in Topcoating layer
Precoat 100 parts pigment substitution group
Topcoat Latex SB/ACN

<figure><img src="image_108.png" /><figcaption>Figure 5-43 Mercury Intrusion in Topcoat Structure – 100 parts substitution Group – S/B/ACN latex. Uncalendered samples.</figcaption></figure>
<figure><img src="image_109.png" /><figcaption>Intrusion in Topcoating layer
Precoat 100 parts pigment substitution group
Topcoat Latex SA</figcaption></figure>

Figure 5-44 Mercury Intrusion in Topcoat Structure – 100 parts substitution Group – SA latex. Uncalendered samples.

<figure><img src="image_110.png" /><figcaption>Intrusion in Topcoating layer
Precoat 100 parts pigment substitution group
Topcoat Latex SA/ACN</figcaption></figure>

Figure 5-45 Mercury Intrusion in Topcoat Structure – 100 parts substitution Group – S/A/ACN latex. Uncalendered samples.
### 5.5.2 Analysis of the calendered precoat and topcoat structures total pore volume, total pore area, and average pore diameter: 100 parts pre-coat pigment substitution (group 4).

As the pore distributions for the calendered and uncalendered samples present the same pattern, we surmised that it was possible to obtain additional information when comparing global values for the 100 part substitution group and for each latex used in the top-coat; i.e. total pore volume, total pore area, and average pore diameter as characterizing both the calendering and the substitution effects.

In order to compare the effect of the 3 latexes, the total pore volume (TPV) is presented in Figure 5-46, the total pore area (TPA) in Figure 5-47, and the average pore diameter (APD) in Figure 5-48 as a function of the particle size of each pigment used in the pre-coat. The standard precoat (GCC 1.4 µm) is added as a reference. The parameters TPV, TPA, and APD are for the precoat/topcoat layers and for the calendered coated paper. The total pore volumes shown in Figure 5-46 clearly indicate that the 3 pigments with a broad distribution (BPSD have the same volume. As seen for Figure 5-41, the volume increases for pigments with narrow particle size distribution. Then, it is possible to conclude that the total pore volume is somehow inversely proportional to the particle size distribution. Figure 5-46 also shows the latex type has no effect on the total pore volume of the structure.

The fact that the latex has no influence on the total structure is coherent with the analysis performed for all other groups. In Figure 5-47, the total pore area for the broad particle size distribution pigment (BPSD-GCC) is inversely proportional to the particle size. The other pigments do not necessarily follow the same rule as it is especially seen for the PCC pigment. However, we do not have any other particle size for comparison.

Finally, as could be expected from the packing of the pigments, Figure 5-48 shows that the average pore diameter is directly related to the particle size of pigments used in the precoat.
<figure><img src="image_111.png" /><figcaption>Figure 5-46 Total Pore Volume (TPV) of the calendered precoat/topcoat structure for Group 4 - Comparing the 3 latexes. The samples are ordered in decreasing particle size distribution.</figcaption></figure>

<figure><img src="image_112.png" /><figcaption>Figure 5-47 Total Pore Area (TPA) Coated Paper for Group 4 - Comparing the 3 latexes. The samples are ordered in decreasing particle size distribution.</figcaption></figure>
<figure><img src="image_113.png" /><figcaption>Figure 5-48 Average Pore Diameter (APD) Coated Paper for Group 4 - Comparing the 3 latexes. Samples are ordered as a function of the particle size.</figcaption></figure>

### 5.5.3 DELTACK results comparison from Group 4 (100 parts pigments substitution)

As it will be seen in further details later (5.6), the shapes of the DELTACK curves are somehow similar for all conditions in present group 4 (100 parts substitution). Accordingly, Figure 5-49 show the DELTACK curves force as a function of time (measuring cycle) for the mean of all 5 pigment substitutions; i.e. BPSD-GCC, PCC, Brazilian clay, and NPSD-GCC, with one curve for each latex used in the topcoat, S/B/ACN, S/A, and S/A/ACN, respectively. As previously found for the other groups, the latex is the major factor influencing the DELTACK measurement.

Figures 5-50 and 5-51 correspond to the maximum force obtained for the DELTACK test in function of the total pore volume (5-50) and the average pore diameter (5-51) for each conditions of the group 4 and for each topcoat latex.
The arrows presented in both Figures are for matched data pairs: for instance, the data points corresponding to the green arrows in Figure 5-50, for pore volume, correspond to the same data in Figure 5-51 but for pore diameter. Furthermore, the colours are dark green and orange for S/B/ACN latex, lavender and pink for the S/A latex, and blue and black for the S/A/ACN latex. It is interesting by the observation of the graphics from Figure 5-50 and 5-51. When comparing the upwards and downwards arrows in both Figures, we immediately see that the pairs can be directly related and are fully coherent to the theory developed for group 1 (Figure 5-17) when only 20 parts substitution was concerned.

As seen previously for Group 1 (20 parts substitution), Figures 5-50 and 5-51 present two different opposite configurations. Indeed, a given maximum DELTACK force can be obtained with two configurations of pore volume and average pore diameter, namely: a) the same (or almost identical) pore volume (upwards arrows, Figure 5-50) but largely different average pore diameters (upward arrows, Figure 5-51), and b) largely different pore volumes (downward arrows, Figure 5-50 and same (or almost identical) pore diameter (downward arrows, Figure 5-51).

Deltac Average Curves
100 parts Pigment Substitution Group

<figure><img src="image_114.png" /><figcaption>**Figure 5-49** DELTACK mean for each latex (5 conditions – 100 parts substitution)</figcaption></figure>
As we proposed earlier, the maximum tack force is due to a complex phenomenon of preferential filtration and ink transfer build-up. As the average pore size of the two structures (case a and case b) is widely different, the ink transfer cannot be the same (ref. work by [104, 105]).

We proposed, and it is confirmed here, that ink transfer is different because the topcoat structures are different and that the filtration mechanisms of ink components is necessarily different for such different combinations of precoat/topcoat structures.

Practically, the thickness of the ink layer remaining on the surface might be different and still produce similar tack forces as very small pores will favour, due to capillary penetration, the preferential absorption of ink solvent.

We will then end up with a thicker ink film but with less solvent, i.e. a tackier ink. In the case of the larger pores, there will be no or less preferential penetration of ink solvent. It is therefore quite feasible that the two effects being opposite, we will end up with the same maximum tack force. Furthermore, we concluded that ink layer thicknesses on the topcoat structure are modulated by the underlying precoat structure.

Although we had taken two extreme cases in Group 1, we can now generalize our proposed mechanism as the compensation effect of solvent penetration controlling the ink tack, and the effect of ink transfer controlling the ink thickness, and therefore also the tack force, is valid for all double-coated structure cases.

Finally, the analysis being valid for all 3 latexes used in the topcoat structure, we further conclude that the maximum DELTACK force is a combination of latex type, as the predominant factor, and overall structure of the precoat/topcoat layer for the second factor explaining the mechanisms of force build-up through preferential filtration occurring during the sequential transfer cycles.
Precoat 100 parts pigments substitution (Group 4)
CALENDERED SAMPLES

<figure><img src="image_115.png" /><figcaption>Figure 5-50 DELTACK maximum force versus total pore volume for 100 parts pigments substitution group- all conditions</figcaption></figure>

Precoat 100 parts pigments substitution (Group 4)
CALENDERED SAMPLES

<figure><img src="image_116.png" /><figcaption>Figure 5-51 DELTACK maximum force versus average pore diameter for 100 parts pigments substitution group - all conditions</figcaption></figure>
### 5.5.4 Summary: 100 parts substitution in the precoat layer (Group 4)

The analysis of the structures of the samples obtained when substituting 100 part pigments for different pigments in the precoat layer structure allows us to confirm that the precoat structure *greatly impact* the final structure of coated paper. We have seen here the key importance of the particle size distribution and of the average particle size, and how they impact the precoat/topcoat structure. More specifically, we found, and explained how, the total pore volume is somehow inversely proportional to the particle size distribution. We also found that the total pore area decreases with particle size but, in the case of the broad particle size distribution pigment, it is inversely proportional to the particle size. Finally, as expected from pigment packing and influence on the coated layer pores, we confirmed that the average pore diameter is directly related to the particle size of pigments used in the precoat.

Last but not least, the mechanism we proposed to explain similar maximum ink tack force, as measured with the DELTACK, with widely different pore structures, namely similar total pore volumes obtained with widely different average pore diameters, and largely different total pore volumes obtained with similar average pore diameters, has been validated and confirmed. As a reminder, the theory is based on the effect of preferential solvent penetration controlling the ink tack, and the effect of ink transfer controlling the ink thickness.

## 5.6 DELTACK and ink tack comparison of Groups and latex – Average Curves

Along the analysis of the various modifications of the precoat layer: 20 parts of clay substitution (Group 1), precalendering of base stock paper and precoat layer (Group 2), latex and thickener substitution in the precoat (Group 3), and 100 part substitution of pigment in the precoat (Group 4), we mentioned many times that the DELTACK curves used to analyse the printing characteristic of the double-coated paper were somehow similar in shape and that the effect of the latex was the predominant factor for tack force build-up notwithstanding the fact that the maximum values were mainly structure dependent. Accordingly, we intend to show here that it is indeed the case by the comparison.
son of average curves calculated for each group; i.e. modification of the precoat structure.

Table 5-9 shows the average maximum ink tack, in force (N) and time to reach the maximum force (in seconds) for the average of all 20 various modifications of the precoat for each of the 3 latexes used in the topcoat (20 conditions per latex). The Table also shows the average determination coefficient (R²) obtained when fitting the 20 DELTAC curves with a second order polynomial (excluding the data corresponding to the levelling out of the curve at high time cycle values). The average total pore volume and average total pore area are provided for information only.

Figure 5-52 provides the general curve shapes obtained for all 20 precoat conditions, for each topcoat latex used in the study. The differences obtained among the 3 latexes while printing are here clearly illustrated. The S/B/ACN latex takes a longer time to get to the maximum pick force (Table 5-9 and Figure 5-52), i.e. a longer time for ink setting. It takes 50% less for the S/A latex and 32% less for the S/A/ACN latex. As previously explained, the gel content is a key factor that may explain such differences. The acrylic content (S/A and S/A/ACN) reduces the ink setting time but also the coating strength. Figures 5-53, 5-54, 5-55, 5-56, 5-57, and 5-58 show the results for each different group and specific precoat conditions: the control or reference (5-53), the 20 parts clay substitution (5-54), the precalendering effect (5-55), the latex and thickener substitution (5-56), and the 100 parts pigment substitution for the carbonates (5-57) and for the Brazilian clay (5-58).

It is evident that the 3 latexes present a similar behaviour within each single group with only the S/A latex for a 20 parts clay substitution having a different curve shape (Figure 5-54).

A major conclusion from above considerations is that, as detailed in our proposed mechanisms, is that ink transfer is mainly controlled by the topcoat layer as the 3 different latexes are used in the topcoat, while the tack force is further controlled by the whole precoat/topcoat structures induced by the precoat structure.
Deltac Average Curves for the 3 latexes

<figure><img src="image_117.png" /><figcaption>Figure 5-52 General DELTACK Comparison curves for the 3 latexes</figcaption></figure>

<table><caption>Table 5-9 Maximum ink tack force and time for all conditions for the 3 latexes used in this study</caption><thead><tr><th rowspan="3">Condition</th><th colspan="2">Max Ink Tack</th><th rowspan="2">R<sup>2</sup></th><th rowspan="2">TPV</th><th rowspan="2">TPA</th></tr><tr><th>Time</th><th>Force</th></tr><tr><th>(second)</th><th>(Newton)</th><th></th><th>mL/g</th><th>4V/A<br>(µm)</th></tr></thead><tbody><tr><td>S/B/ACN</td><td>10.16</td><td>1.56</td><td>0.9679</td><td>0.1686</td><td>0.098</td></tr><tr><td>S/A</td><td>4.37</td><td>1.23</td><td>0.9474</td><td>0.1641</td><td>0.092</td></tr><tr><td>S/A/ACN</td><td>6.72</td><td>1.17</td><td>0.9163</td><td>0.1731</td><td>0.103</td></tr></tbody></table>
<figure><img src="image_118.png" /><figcaption>Figure 5-53 DELTACK Curve for the control precoating layer – 100 parts GCC (1.4 µm)</figcaption></figure>

<figure><img src="image_119.png" /><figcaption>Figure 5-54 DELTACK Average Curves for the 3 latexes for the 20 parts substitution Group</figcaption></figure>
<figure><img src="image_120.png" /><figcaption>Figure 5-55 DELTACK Average Curves for the 3 latexes for the base paper and precoating layer calendering Group</figcaption></figure>

<figure><img src="image_121.png" /><figcaption>Figure 5-56 DELTACK Average Curves for the 3 latexes for the Acrylic latex and Protein substitution Group</figcaption></figure>
Deltac Average Curves for the 3 latexes
100 parts Pigment Substitution Group - Carbonates

<figure><img src="image_122.png" /><figcaption>Figure 5-57 DELTACK Average Curves for the 3 latexes for the 100 parts substitution Groups - Carbonates</figcaption></figure>

Deltac Average Curves for the 3 latexes
100 parts Pigment Substitution Group - Brazilian Clay

<figure><img src="image_123.png" /><figcaption>Figure 5-58 DELTACK average curves for the 3 latex - Brazilian clay</figcaption></figure>
## 5.7 Projection on a Latent Structure (Partial least Square) Study

In the analysis of the effect of the precoat variables such as pigment type, precalendering of the precoat layer and/or base paper, substitution of pigment, either 20 parts or 100 parts, latex and thickener substitution, we have seen that parameters such as pigment type, size, etc. had a significant influence on the double-coated paper structure. We have demonstrated that the latex type in the topcoat was the predominant factor for tack build-up (DELTACK force). We have also seen that the structural parameters, as induced by above variables, would have a complementary effect that explained the maximum tack force. In order to verify that the main parameters we proposed to explain the various effects on the structure and the printing characteristics (DELTACK) are indeed justified and that there are no other second order effect, we performed a partial least square analysis of the set of data developed in the course of present study. It should already be emphasized that this additional step is a mathematical verification step as it is not a first time analysis. For instance, we already know that latex is the key component from a printability standpoint.

### 5.7.1 General presentation of the PLS method

Basically PLS is a sort of Principal Component Analysis (PCA). PCA is a powerful and common tool in the analysis of study phenomena. Multivariate projection methods represent a useful and versatile mathematical tool in the modeling, monitoring, and prediction of the often complex problems and data structures encountered within data-rich disciplines in Research, Development and Production (RDP) [116]. One type of analysis included in the PCA is the projection on a latent structure (PLS) also knows as Partial Least Square.

PLS was first introduced by Wold *et al* ([116-119]) and has continuously grown since. An alternative term for PLS, and a more correct according to Wold [118], is “projection to latent structures”. However, the term “partial least squares” is still dominant in many areas. It is widely applied in the field of chemometrics, in sensory evaluation, and more recently, in the analysis of functional brain imaging data.
Briefly, projection on latent structure regression (PLS-regression) or partial least squares regression (PLS-regression) is a statistical method making relationship between principal components regression. Because both, the X and Y data are projected to new spaces, the PLS family of methods are known as bilinear factor models [117]. PLS method identifies directly X and Y values which allows to establish predictive cause and effects models. These predictions can be between the X values, between the Y values and between X and Y values.

PLS-regression is an important step in PLS path modeling, a multivariate data analysis technique that employs latent variables. The technique is often referred to as a form of variance-based or component-based structural equation modeling. It is used to find the fundamental relations between two matrices (X, above “variables” and Y, above “parameters”), i.e. a latent variable approach to modeling the covariance structures in these two spaces.

A PLS model will try to find the multidimensional direction in the X space that explains the maximum multidimensional variance direction in the Y space as illustrated in Figure 5-59 where the dispersion of the dots on the graphic is well explained by the 2 red axes. PLS-regression is particularly suited when the matrix of predictors has more variables than observations, which is our case, and when there is multicollinearity among X values. By contrast, standard regression will fail in these cases [117].

<figure><img src="image_124.png" /><figcaption>Figure 5-59 2 principal axis on a fish picture</figcaption></figure>
Practically, the program finds the axis in the three dimensional (3D) space giving the best correlation between *n* random variables as can be seen in Figure 5-60. The method is also known as Karhunen-Loeve transformation or Hotelling transformation (from Harold Hotelling). The results may be graphically displayed in different ways. It works because the methods capture the dominant, latent properties of the system under study.

<figure><img src="image_125.png" /><figcaption>**Figure 5-60** Graphic showing 2 principal components, the best correlation with Y and the projection on a surface</figcaption></figure>

By contrast to PCA, PLS regression finds components from X that are also relevant for Y. Specifically, PLS regression searches for a set of components (called latent vectors) that perform a simultaneous decomposition of X and Y with the constraint that these components explain as much as possible of the covariance between X and Y. This step generalizes PCA. It is followed by a regression step where the decomposition of X is used to predict Y. The technique is often referred to as a form of variance-based or component-based structural equation modeling [117, 119].

A more complete explanation of the PLS method is presented in Appendix 3.
5.7.2 PLS Results

The structure of the PLS analysis represented in table 5-10 was selected upon considera-
tion of the importance we previously found in the variables. Indeed, the 2 component
analysis includes latex type, the 3 components analysis adds the particle and pigment
size variables, and the 4 component analysis includes all data up to the precalendering
variables. However, the order of selection has no importance anymore when the 4 com-
ponent analysis is concerned.

Before going into detailed analysis, it should be stressed that the determination coeffi-
cients R² we found are often low, with values between 0.5 and 0.7. Of course, removing
the non-significant values would have helped to improve or raise the R² value. Indeed,
in PLS studies, the variables located inside a given area, limited +/-0.2 value for both
axes around 0, are normally less important. An example is presented in Figure 5-61
which shows the projection on a plane of the correlation coefficients for each variable as
related to 2 components (one per axis). Such data are sometimes removed from the
analysis as they do not have any significant effect on the final results. In our study, the
values inside the "+/- 0.2 square area" were intentionally kept into the analysis to con-
sider all available data. This decision has the effect of reducing the R² value while keep-
ing the correlations significant.

<figure><img src="image_126.png" /><figcaption>Figure 5-61 PLS Regression – Projection graphic with the +/- 0.2 square low correlation area</figcaption></figure>
<table><caption>Table 5-10 Modeling structure related to the components used</caption><thead><tr><th></th><th>2 components</th><th>3 components</th><th>4 components</th></tr></thead><tbody><tr><td colspan="4"><strong>Variables Y</strong></td></tr><tr><td>Time (DELTACK)</td><td>√</td><td>√</td><td>√</td></tr><tr><td>Force (DELTACK)</td><td>√</td><td>√</td><td>√</td></tr><tr><td>Contact Angle</td><td>√</td><td>√</td><td>√</td></tr><tr><td>Total (pre-coat/topcoat) Pore Volume</td><td>√</td><td>√</td><td>√</td></tr><tr><td>Precoat Pore Volume</td><td>√</td><td>√</td><td>√</td></tr><tr><td colspan="4"><strong>Variables X</strong></td></tr><tr><td>Particle Size</td><td></td><td>√</td><td>√</td></tr><tr><td>Pigment Type</td><td></td><td>√</td><td>√</td></tr><tr><td>Latex type</td><td>√</td><td>√</td><td>√</td></tr><tr><td>Base Stock Temp. Cal.</td><td></td><td></td><td>√</td></tr><tr><td>Base Stock Pres. Cal.</td><td></td><td></td><td>√</td></tr><tr><td>Precoat Temp. Cal.</td><td></td><td></td><td>√</td></tr><tr><td>Precoat Pres. Cal.</td><td></td><td></td><td>√</td></tr></tbody></table>

### 5.7.3 PLS Study for 2 components - Effect of precoat pore volume on the final results

As our initial hypothesis is to demonstrate if and how the precoat would determine the structure and printing characteristics of a double-coated paper, it is logical to first analyse the effect and importance of precoat volume to explain the data set (results). Consequently, two projection graphics at 4 components are performed and presented in Figures 5-62 and 5-63, one with the precoat pore volume inserted into the PLS study and the second without the precoat pore volume values.

Comparison of the two Figures indicates that the precoat pore volume itself has no (or very little) measurable effect on the other variables. Only a slight variations in the pro-
jection graphs occur by adding the precoat pore volume in the PLS analysis. In fact, as we have seen previously in the mechanistic and physical analysis, the precoat pore volume works in combination with one and/or more other variables to achieve a desired printability level.

### 5.7.4 Effect of the latex

A key conclusion of the mechanistic study is the predominant importance of the latex type of the topcoat on the maximum tack force value. Although quite not expected, the latex in the topcoat was not part of the initial hypothesis of the thesis focus on the precoat impact. To confirm the point, we have performed the 2 component analysis with the latex type as a variable. According to previous results (0), we could have done the same for the precoat volume. Another reason to use the latex here is to present and understand the PLS analysis with only 2 components. Results would have somehow been similar with the precoat volume and a 2 components analysis.

Table 5-11 which presents the PLS results, determination coefficients and correlation factors, Figure 5-64 which shows the importance factor (or weight on a normalized scale) and Figure 5-65 which presents the projection graphic are used to explain the results of the 2 component analysis. Although the PLS analysis clearly shows, as expected from the previous DETLTACK analysis, that the latex has a part of the variations in the printability characteristics, it does not explains all data variations. The 3 latexes used in the trial are responsible for only 0.19 (19%) in Y of the total variation (table 5-11).

The projection graphic in Figure 5-65 confirms that the 3 latexes behave in total opposition. The same graphic also indicates that the contact angle (CA), the pore volume for the uncalendered paper (VP), the pore volume for the calendered paper (VPC), the maximum DELTACK force (Force) and time (Time) are not very relevant.

Globally, the latex has a key impact but the variations must be explained with some other variables; whereas the need to perform the analysis with more than 2 components.
<figure><img src="image_127.png" /><figcaption>Figure 5-62 Graphic projection for the Component 1 and 2 without precoated pore volume variable</figcaption></figure>

<figure><img src="image_128.png" /><figcaption>Figure 5-63 Graphic projection for the Component 1 and 2 with precoated pore volume variable</figcaption></figure>

<table><caption>Table 5-11 Correlation factor R² for 2 components – Results</caption><caption>Correlation factor X and Y for 2 components<br>Type: PLS Observations (N)=60. Variables (K)=8 (X=3. Y=5), Qualitative (1. expanded=3)</caption><thead><tr><th>A</th><th>1</th><th>2</th></tr></thead><tbody><tr><td>R2X - Cent.</td><td>0.5</td><td>0.5</td></tr><tr><td>R2X(cum)</td><td>0.5</td><td>1</td></tr><tr><td>Eigen values</td><td>1.5</td><td>1.5</td></tr><tr><td>R2Y - Cent.</td><td>0.157</td><td>0.0337</td></tr><tr><td>R2Y(cum)</td><td>0.157</td><td>0.19</td></tr><tr><td>Q2</td><td>0.12</td><td>0.00445</td></tr><tr><td>Limit</td><td>0.05</td><td>0.05</td></tr><tr><td>Q2(cum)</td><td>0.12</td><td>0.124</td></tr><tr><td>Significance</td><td>R1</td><td>R2</td></tr><tr><td>Iterations</td><td>9</td><td>2</td></tr></tbody></table>
<figure><img src="image_129.png" /><figcaption>Figure 5-64 PLS Result (2 Components) – Graphic for the 3 latex</figcaption></figure>

<figure><img src="image_130.png" /><figcaption>Figure 5-65 PLS Result (2 Components) – Correlation factor R² projection graphic</figcaption></figure>
### 5.7.5 PLS study for 3 components

The 3 components PLS enable the analysis of the strength of the variables targeted by the analysis (Table 5-10).

Table 5-12 which presents the PLS results, determination coefficients and correlation factors, Figure 5-67 which presents the projection graphic and Figure 5-66 which shows the importance factor and are used to explain the results of the 3 components analysis.

We conclude from the analysis that the type of pigment (PCC, GCC, and Clay), the latex (S/B/ACN, SA, S/A/ACN), and the pigment size are the most important variables to explain the results (Figure 5-67 and 5-66). Together, all these variables explain 48 % (table 5-12) of the results. However, if we consider only the 2 first components, 45% of the results are explained.

<table><caption>**Table 5-12 Correlation factor R² for 3 components – Results**</caption><caption>Correlation factor X and Y for 3 components<br>Type: PLS Observations (N)=60. Variables (K)=12 (X=7. Y=5), Qualitative (2. expanded=6)</caption><thead><tr><th>A</th><th>1</th><th>2</th><th>3</th></tr></thead><tbody><tr><td>R2X - Cent.</td><td>0.207</td><td>0.234</td><td>0.225</td></tr><tr><td>R2X(cum)</td><td>0.207</td><td>0.441</td><td>0.666</td></tr><tr><td>Eigen values</td><td>1.45</td><td>1.64</td><td>1.58</td></tr><tr><td>R2Y - Cent.</td><td>0.299</td><td>0.154</td><td>0.0287</td></tr><tr><td>R2Y(cum)</td><td>0.299</td><td>0.453</td><td>0.482</td></tr><tr><td>Q2</td><td>0.235</td><td>0.185</td><td>-0.0361</td></tr><tr><td>Limit</td><td>0.05</td><td>0.05</td><td>0.05</td></tr><tr><td>Q2(cum)</td><td>0.235</td><td>0.376</td><td>0.354</td></tr><tr><td>Significance</td><td>R1</td><td>R1</td><td>R2</td></tr><tr><td>Iterations</td><td>20</td><td>8</td><td>23</td></tr></tbody></table>
<figure><img src="image_131.png" /></figure>

Figure 5-66 PLS Result (3 Components) – Most important variables in order of their importance

<figure><img src="image_132.png" /></figure>

Figure 5-67 PLS Result (3 Components) – Correlation factor R² projection graphic

### 5.7.6 PLS study for 4 components

The PLS analysis with 4 component is the most common way to perform most of the PLS analyses, especially if the results vary widely. We used it initially (0) to evaluate the precoat structural importance; i.e. our initial hypothesis. Table 5-13 which presents the PLS results, determination coefficients and correlation factors, Figure 5-70 which presents the projection graphic and Figure 5-69 which shows the importance factor and
are used to explain the results of the 3 components analysis. It should be noted that Figure 5-70 is identical to Figure 5-68 and is presented here to facilitate the analysis. Results in Table 5-13 indicate that the 4 components analysis explains 55.5% of data variations. Therefore, the addition of the 4th component explains approximately 7% more values than the 3 component analysis and 10% more when compared with 2 components (A=2 in table 5-13). This means that the addition of the 4th component has a minor effect and/or works in combination with another parameter of the study. The addition of the 4th component includes all the calendering variables in the PLS study. However, the results related to the calendering are inside and/or very close to the “+/- 0.2 square” area: i.e. with a low significance factor. Indeed, we could only see the importance of the pre-calendering when analyzing the factor separately (analysis of Group 2) which seems not to be the case here as all data are included in the PLS study.

<table><caption>**Table 5-13 Correlation factor R² for 4 components – Results**</caption><caption>Correlation factor X and Y for 4 components<br>Type: PLS Observations (N)=60. Variables (K)=18 (X=12. Y=6), Qualitative (2. expanded=6)</caption><thead><tr><th>A</th><th>1</th><th>2</th><th>3</th><th>4</th></tr></thead><tbody><tr><td>R2X - Cent.</td><td>0.183</td><td>0.136</td><td>0.16</td><td>0.162</td></tr><tr><td>R2X(cum)</td><td>0.183</td><td>0.318</td><td>0.478</td><td>0.64</td></tr><tr><td>Eigen values</td><td>2.19</td><td>1.63</td><td>1.91</td><td>1.94</td></tr><tr><td>R2Y - Cent.</td><td>0.312</td><td>0.144</td><td>0.0571</td><td>0.0415</td></tr><tr><td>R2Y(cum)</td><td>0.312</td><td>0.456</td><td>0.513</td><td>0.555</td></tr><tr><td>Q2</td><td>0.236</td><td>0.106</td><td>-0.027</td><td>0.0147</td></tr><tr><td>Limit</td><td>0.05</td><td>0.05</td><td>0.05</td><td>0.05</td></tr><tr><td>Q2(cum)</td><td>0.236</td><td>0.317</td><td>0.299</td><td>0.309</td></tr><tr><td>Significance</td><td>R1</td><td>R1</td><td>NS</td><td>R2</td></tr><tr><td>Iterations</td><td>11</td><td>14</td><td>29</td><td>8</td></tr></tbody></table>

Figure 5-69 shows the variables included in the 4th component analysis by order of importance. Like previous similar graphs, the PCC, GCC type, and the latex S/B/ACN and SA and clay are the most important values. As seen in Table 5-13, they explain over 45% of all the correlations between the data. Not surprisingly, by adding a 4th compo-
nent, the precalendering conditions now reappear in the importance factors. It even has a bigger impact than latex S/A/ACN and the particle size. In the same train of thought, the calendering of the base paper has a larger impact than precoated layer calendering. In addition, it is important to realize that in a 4 dimension space due to the 4 components, one has to analyse various plane projections. Figure 5-70 which shows the projection of the values for the 1st and 2nd component confirms the analysis performed with Figure 5-69. An additional information relates to the importance of the time and force related to tack build-up (DELTACK).

<figure><img src="image_133.png" /><figcaption>Figure 5-69 PLS Result (4 Components) – Most important variables in order of their importance</figcaption></figure>

<figure><img src="image_134.png" /><figcaption>Figure 5-70 PLS Result (4 Components) – Graphic projection for the Component 1 and 2 with precoated variable</figcaption></figure>
It further confirms that the topcoat latex is an important factor when combined with the other analysed values. Figure 5-71 provides the projection for the 3rd and 4th components and also confirms the analysis of the ranking of the various variables.

Figures 5-72 to 5-76 present the correlation coefficient obtained with the Ink Tack Force (5-72), the maximum force (5-73), the pore volume for the uncalendered double-coated paper (5-74), the calendered double-coated paper (5-75), and the contact angle (5-76), respectively and the other dependant variables of the study.

The five graphics simply show how selected dependant variables Y vary for each dependant variables X. The graphics confirm the importance of the 3 first variables appearing in the overall ordering presented in Figure 5-69, namely PCC (type), GCC (type) and S/B/ACN latex.

The other variables have a minor effect for the majority of the dependent variables Y evaluated. Figure 5-76 shows the interesting additional evaluation for the contact angle as we know that the variables affected by the contact angle measurement are very different, depending on both chemical and structural parameters. Indeed, the latexes S/B/ACN and S/A continue to have a key impact on the contact angle value.

Finally, the addition of a 4th component in the PLS analysis clearly demonstrates that the improvement of the determination coefficient (and correlation coefficients) becomes marginal. Although one may conclude that the addition of the variables for the additional 4th component will have a rather low impact on the final results when analysed globally, it mainly means that there is no need to pursue the analysis at the 5th component level.

In conclusion, the PLS analysis has mainly confirmed and validated initial mechanistic and structural analysis we performed. It therefore provides strength and confidence in our proposed mechanisms of precoat structural importance on the double-coated paper properties.
<figure><img src="image_135.png" /><figcaption>Figure 5-71 PLS Result (4 Components) – Graphic projection for the Component 3 and 4 with precoated variable</figcaption></figure>

<figure><img src="image_136.png" /><figcaption>Figure 5-72 PLS Result (4 Components) – Ink Tack Force</figcaption></figure>
<figure><img src="image_137.png" /><figcaption>Figure 5-73 PLS Result (4 Components) – Time to get the maximum force with DELTACK</figcaption></figure>

<figure><img src="image_138.png" /><figcaption>Figure 5-74 PLS Result (4 Components) – Pore Volume for uncalendered paper</figcaption></figure>
<figure><img src="image_139.png" /><figcaption>Figure 5-75 PLS Result (4 Components) – Pore Volume for calendered paper</figcaption></figure>

<figure><img src="image_140.png" /><figcaption>Figure 5-76 PLS Result (4 Components) – Contact Angle</figcaption></figure>
Chapter 6 - Conclusion

The objective of the work was to analyse the effect of the precoat on the properties,
mainly structure and printing characteristics as evaluated by the maximum force corre-
sponding to ink build-up upon repeated printing cycles (so-called DELTACK analysis).
In order to fully understand the importance of the precoat, we modified the precoat con-
sidering potential commercial practices such as substituting 20 parts clay in a 100 parts
GCC precoat (Group 1), precalendering either the base paper or the precoated paper
(Group 2), substituting either a protein or a S/A latex in the precoat for an SB+CMC
combination (Group 3), and changing the various pigments used in the precoating layer
(100 parts substitution, Group 4). Our hypothesis was that such modifications would
fundamentally modify the structure and even the physico-chemical characteristics of the
precoat, thus inducing modification of the topcoat structure, and by consequence, the
whole double-coated paper properties. Furthermore, we used 3 commercial standard
latexes in the topcoat in order to analyse how the precoat effects would be modulated
with different topcoats. The experimental design was developed to permit a full under-
standing of the different precoat conditions with an additional input created by the top-
coat latexes. In essence, the different precoat formulations allowed us to study the
modification and/or modulations of the pore structure of the double-coated paper due to
changes in the precoat structures and physico-chemical properties.

The present work has permitted an excellent evaluation of different precoat conditions
with an additional look on the topcoat latex for double coated paper. The different
groups were created to show how to pore structure is modified and/or modulated by
changes in the precoat structure (pigments, chemicals, precalendering). The general con-
clusion related to each groups are:

Present work has clearly demonstrated that substituting 20 parts of clay in 100 parts GCC precoat formulation permits a modulation of the coating structure to obtain the desired structural and printing properties. Similarly, we have shown that the topcoat structure is affected by the precoat and that the most probable explanation is a combination of capillary penetration, a preferential penetration within the precoat structure of the
smallest part of the topcoat pigment particle, i.e. a preferential pigment rearrangement, and a forced penetration into the precoat structure by the topcoat coating colour. The forced penetration, or a part of it, probably originates from the rewetting of the precoat layer upon topcoat application.

Another key conclusion related to 20 parts substitution in the precoat (Group 1) is that differences in printing properties as described by the DELTACK analysis are mainly related to the type of latex used in the topcoat layer. The butadiene base latex always presents higher maximum tack force and time than the styrene acrylic. We surmised it was related to the gel content of the latex. The topcoats containing styrene acrylic present similar behaviour when comparing DELTACK curve shapes.

Finally, we demonstrated how the same ink tack could be obtained with very different pore structures, either with equal pore volume and very different average pore size, or on the contrary, with very different pore volume and the same average pore size. We have proposed a fundamental mechanistic approach to explain this particular phenomenon. The mechanism is related to the changes in topcoat structure originating from the different precoat structures. Overall, the structural changes in a complex two layer structure provided by the precoat/topcoat will modify how the ink and/or the ink solvent penetrate preferentially into a two layer structure thus resulting in a thick or thin ink layer with different final compositions which will provide the same tack force.

The second approach to modify the structure of the precoat layer was by precalendering either the base paper or the precoat layer (Group 2). In all cases, precalendering produces lower pore size distribution, and consequently a lower total pore volume, than the reference paper, with no precalendering. Although the analysis of the pore distribution curves indicated no clear trend to easily understand how precalendering specifically affected the structures, it nevertheless led us to the conclusion that the structures thus produced by precalendering are different.

First, we showed that precalendering at 100°C results in a higher precoat pore volume than precalendering at 200°C. As expected, calendering to 200 kN/m also produces smaller total pore volume of the precoat than precalendering at 50 kN/m. More surpris-
ing are the results obtained for the topcoat structure upon precalendering either the base
paper or the precoat. Here, we demonstrated that two apparently opposite effects occur.
First, when precalendering the base paper, the top coat total pore volume is higher at
100°C than at 200°C. On the opposite, when precalendering the precoat layer, the top
coat total pore volume is higher at 200°C than at 100°C.

The mechanism we propose to explain such apparently contradictory results is basically
the same as the one we proposed to explain the ink tack. It is related to the filtration and
packing of the structure that occurs during the coating operation. It is also related to the
movement of the water and materials when a wet coating layer is applied over another
surface. Indeed, the surface pore structure being more or less open will control filtration
and packing. High temperature precalendering of the precoat layer has a direct impact
on the topcoat structure. Precalendering the base paper has the same effect on the pre-
coat structure but the situation is then necessarily reversed when the topcoat is applied in
a kind of “domino” effect.

We further explained how it was feasible to obtain the same printing characteristics with
the different structures created by precalendering. Here too, the effect of temperature
precalendering was the predominant factor. The analysis here developed is a powerful
tool to analyse such differences in precalendering. Finally, we demonstrated here that it
was possible to control the double-coated layer micro-structure, even down to the nano-
structure (at 30 nm and below) through precalendering and thus control the printing
characteristics of double-coated papers.

After partial modification of the pigment content and modification of the base pa-
per/precoat structure through calendering, we analysed how chemical modification of
the precoat would affect the structures. Accordingly, we showed that, using either a
protein or a S/A latex in the precoat to substitute for an SB+CMC combination (Group
3) does not result in significant improvements especially when the S/A latex precoat
substitution is concerned. However, the protein substitution case presented an interest-
ing behaviour and we were able to clearly demonstrate the effect of such a substitution
on the structure of the coating layer. Indeed, the protein definitively resulted in a
broader pore distribution in the precoat layer. However, it was difficult to generalize as just one condition has been studied.

Finally, considering that the most dramatic commercial change would be to "move to completely another type of pigment", we looked how different pigment types would modify the precoat structure and therefore the whole double-coated paper structure and properties. The analysis of the structures of the samples obtained when substituting 100 part pigments (Group 4) for different pigments in the precoat layer structure allowed to confirm that the precoat structure *greatly impact* the final structure of coated paper. We confirmed here the key importance of the particle size distribution and of the average particle size, and how they impact the precoat/topcoat structures. More specifically, we showed and explained how the total pore volume is somehow inversely proportional to the particle size distribution. Not only does the total pore area decrease with particle size but, in the case of the broad particle size distribution pigment, it is inversely proportional to the particle size. Finally, as expected from pigment packing and influence on the coated layer pores, we confirmed that the average pore diameter is directly related to the particle size of pigments used in the precoat.

Last but not least, the mechanism we proposed to explain similar maximum ink tack force, as measured with the DELTACK, with widely different pore structures, namely similar total pore volumes obtained with widely different average pore diameters, and largely different total pore volumes obtained with similar average pore diameters, was fully validated and confirmed with the precoat layers containing very different pigment types. As a reminder, the theory is based on the effect of preferential solvent penetration controlling the ink tack, and the effect of ink transfer controlling the ink thickness.

Present work brings two important contributions to the understanding of the physics and structure of double coated paper, and more specifically on the effect of the precoat layer on the double coated paper structure and properties.

Our first main contribution is the development of a normalisation method to treat the mercury intrusion data in such a way that it is now feasible to extract the maximum amount of structural information on such data sets. The method allowed us to isolate the
coating layers one from the other. It proposes a new way to treat mercury intrusion data sets to analyse the pore structure with good accuracy down to 30 nanometers, and probably lower. The method will be a new powerful tool to understand the flow dynamics at the nano level, for instance, as we have seen, how wet layers behave when applied one on top of the other. The new method enabled to segregate each layer structure separately, providing the potential to fine tune the analysis of multi-layered paper coating. We have thus developed a powerful tool to analyse how various coating colour formulations will affect each layered structure separately.

Our second main contribution is the development and proposal of a mechanism to explain how the same printing characteristics (as indicated by the maximum DELTACK force) can be obtained with two completely different structures, namely structures with the same pore volume but very different average pore diameters, and structures with widely different pore volumes but (almost) identical average pore diameter.

The reasons for obtaining similar printing characteristics when the average pore size of the two structures is widely different are rather complex. The mechanism we proposed, considers that ink transfer, in agreement with previous studies [104, 105], is indeed different because the topcoat structures are different. It should be noted that we have not validated it through modelling of fluid penetration as it was outside of the scope of present study. However, we showed that the low pore size case (case 1) in precoat/topcoat structure corresponds to very fine pores and a very small volume in the topcoat structure. In parallel, referring to the high pore size case (case 2), the precoat/topcoat structure corresponds to a larger pores and larger volume in the topcoat structure. Practically, it means that ink layer remaining of the surface in case 1 will be thicker than in case 2. In addition, considering the actual sizes of these pores, it also means that the structure with very small pores will favour, due to capillary penetration, the preferential absorption of ink solvent. In essence we will end up with a thicker ink film but with less solvent, i.e. a tackier ink. In the case of the larger pores, there will be no or less penetration of ink solvent. It is therefore quite feasible that the two effects being opposite, we will end up with the same maximum tack force.
From a commercial standpoint, we have demonstrated that the precoat has a more prac-
tical importance than usually accepted by industry practices. Generally, the precoat is
more or less considered as a “layer filling-in surface defects and pores”. We have shown
here that the precoat is an integral part of the double coated paper structure and printing
characteristics. In essence, it could be used to fine tune the desired double coated paper
properties. We have also shown that the precalendering has an important effect that
goes beyond the usual commercial control of the precoated paper thickness.

For future work, the effect of thickeners on the pore structure and printing properties
may be of interest as it may be related to coating improvement, specifically if one needs
to understand how such a product modifies the coating structure. It would be of com-
mercial research interest to study the effect of structure/chemistry of the latexes on print-
ing characteristics as the effect on the DELTACK results seen for the thickener substitu-
tion were not really different than what was analysed for the structural modification (20
parts substitution and precalendering). Last but not least, the modelling of fluid penetra-
tion in double layered structures to further understand ink tack, ink build-up, and there
resulting printing characteristics would be both a fundamental and an industry asset.
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# Appendix

## Appendix 1 Mercury Intrusion Test Procedure by Micromeritics and calculating values description

"The sample cup has a capillary stem attached and this capillary serves both as the mercury reservoir during analysis and as an element of the mercury volume transducer. Prior to the beginning of each analysis, the sample cup and capillary are filled with mercury. After filling, the main source of mercury is removed leaving only the mercury in the sample cup and capillary stem the combination being referred to as the penetrometer. Pressure is applied to the mercury in the capillary either by a gas (air) or a liquid (oil). The pressure is transmitted from the far end of the capillary to the mercury surrounding the sample in the sample cup. The capillary stem is constructed of glass (an electrical insulator), is filled with mercury (an electrical conductor), and the outer surface of the capillary stem is plated with metal (an electrical conductor). The combination of two concentric electrical conductors separated by an insulator produces a co-axial capacitor. The value of the capacitance is a function of the areas of the conductors, the dielectric constant of the insulator, and other physical parameters. In the case of this particular capacitor, the only variable is the area of the interior conductor as mercury leaves the capillary and enters the sample voids and pores, or as it moves back into the capillary when pressure is reduced. This is mechanically analogous to a mercury thermometer in which case mercury moves in and out of a calibrated capillary from a large bulb at one end. A small volume of mercury entering or leaving a small capillary causes the length (and area) of the mercury column to change significantly, thus providing volume-measuring sensitivity and resolution. In the case of the thermometer, the change in volume is proportional to the change in temperature by the coefficient of volumetric expansion of mercury. The capacitance value of the stem is monitored by a capacitance detector that, similar to the pressure transducer electronics, produces an electrical signal that is proportional to capacitance. Capacitance measurements are transformed into volume measurements by knowledge of the diameter of the precision capillary and the equation governing coaxial capacitors [120]".
<figure><img src="image_141.png" /><figcaption>Figure 6-1 Sample bulb and stem – cross section showing the different parts</figcaption></figure>

<figure><img src="image_142.png" /><figcaption>Figure 6-2 Bulb mounted for low pressure testing and fill with mercury</figcaption></figure>

The test generates many values calculated by the equipment program. The values are calculated considering the weight of the sample. The values obtained by the Micromeritics program are [120, 121]:

1. Total intrusion volume in mL/g which is the maximum volume of mercury intrusion (penetration) into the pores for the sample at the indicated pressure.
2. Skeletal Density in g/cm³ which is the mass of the sample divided by the sum of the volumes of the solid material in the sample and the volume of pores within the sample.

3. Porosity in % which is the percentage of pores in the sample or total intrusion divided by the total intrusion + 1/skeletal density x 100

4. Total Pore area in m²/g which is the area of the pore walls based on the assumption of cylindrical geometry and summed over the complete pressure range of mercury intrusion.

5. Median Pore Diameter volume in µm which is the pore diameter at which equal quantities of pore volume occur at both large and small diameters.

6. Median Pore Diameter Area in µm² which is the pore diameter at which equal quantities of pore wall area occurs at larger and smaller diameters

7. Average Pore Diameter – 4V/A in µm which is based upon the assumption that all pores are right circular cylinders. When the pore volume (V= πd² l / 4) is divided by the pore area (A = πdl), the pore diameter (d) is 4 V/A.

To simplify the analysis, the original Autopore data file is imported into an Excel file, regrouping the results in a single table that allow the plotting of required intrusion curves.
**Appendix 2** DELTACK Curves

**DELTACK Curves for 20 parts clay substitution (Group 1)**

<figure><img src="image_143.png" /><figcaption>**Figure 6-3** DELTACK Average Curves for S/B/ACN latex, S/A latex and S/A/ACN, 100 parts GCC (1.4 µm)</figcaption></figure>

<figure><img src="image_144.png" /><figcaption>**Figure 6-4** DELTACK – 20% clay substitution Group, S/B/ACN latex</figcaption></figure>
<figure><img src="image_145.png" /><figcaption>Deltac Average Curves for Latex S/A
20 parts Clay Substitution Group</figcaption></figure>

Figure 6-5 DELTACK – 20% clay substitution Group, S/A latex

<figure><img src="image_146.png" /><figcaption>Deltac Average Curves for Latex SA/ACN
20 parts Clay Substitution Group</figcaption></figure>

Figure 6-6 DELTACK – 20% clay substitution Group, S/A/ACN latex
DELTACK Curves for Base Paper and Precoat Calendered (Group 2)

<figure><img src="image_147.png" /><figcaption>Figure 6-7 DELTACK – Base Paper and Precoated layer Calendering Group, S/B/ACN latex</figcaption></figure>

<figure><img src="image_148.png" /><figcaption>Figure 6-8 DELTACK – Base Paper and Precoated layer Calendering Group, S/A latex</figcaption></figure>
Deltac Average Curves for Latex SA/ACN
Base Paper and Precoated layer Calendering Group

<figure><img src="image_149.png" /><figcaption>Figure 6-9 DELTACK – Base Paper and Precoated layer Calendering Group, S/A/ACN latex</figcaption></figure>

DELTACK Curves for S/A precoat latex and Thickener substitution (Group 3)

Deltac Average Curves for Latex SB/ACN
Acrylic Latex and Protein Precoated Layer Group

<figure><img src="image_150.png" /><figcaption>Figure 6-10 DELTACK – S/A Precoating layer and protein Group, latex, S/B/ACN</figcaption></figure>
<figure><img src="image_151.png" /><figcaption>Deltac Average Curves for Latex S/A
Acrylic Latex and Protein Precoated Layer Group</figcaption></figure>

Figure 6-11 DELTACK – S/A Precoating layer and protein Group, S/A latex

<figure><img src="image_152.png" /><figcaption>Deltac Average Curves for Latex SA/ACN
Acrylic Latex and Protein Precoated Layer Group</figcaption></figure>

Figure 6-12 DELTACK – S/A Precoating layer and protein Group, latex, S/A/ACN latex
DELTACK Curves for 100 parts pigment substitution (Group 4)

<figure><img src="image_153.png" /><figcaption>Figure 6-13 DELTACK – 100% Pigment substitution Group, latex, S/B/ACN</figcaption></figure>

<figure><img src="image_154.png" /><figcaption>Figure 6-14 DELTACK – 100% Pigment substitution Group, S/A latex</figcaption></figure>
<figure><img src="image_155.png" /><figcaption>Deltac Average Curves for Latex SA/ACN
100 parts Pigment Substitution Group</figcaption></figure>

Figure 6-15 DELTACK – 100% Pigment substitution Group, S/A/ACN latex
# Appendix 3 Projection on Latent structure (PLS)

## Definition of projection on latent structure

Partial least squares regression (PLS-regression) is a statistical method that bears some relation to principal components regression; instead of finding hyper planes of maximum variance between the response and independent variables, it finds a linear model by projecting the predicted variables and the observable variables to a new space. Because both the X and Y data are projected to new spaces, the PLS family of methods are known as bilinear factor models.

It is used to find the fundamental relations between two matrices (X and Y), i.e. a latent variable approach to modeling the covariance structures in these two spaces. A PLS model will try to find the multidimensional direction in the X space that explains the maximum multidimensional variance direction in the Y space. PLS-regression is particularly suited when the matrix of predictors has more variables than observations, and when there is multicollinearity among X values. By contrast, standard regression will fail in these cases.

PLS-regression is an important step in PLS path modeling, a multivariate data analysis technique that employs latent variables. This technique is often referred to as a form of variance-based or component-based structural equation modeling [122].

## Theory of the analysis (PLS)

Projection on latent structure (PLS-regression) or partial least squares regression (PLS-regression) is a statistical method that bears some relation to principal components regression; instead of finding hyper planes of maximum variance between the response and independent variables, it finds a linear model by projecting the predicted variables and the observable variables to a new space (Figure 5-60). Because both the X and Y data are projected to new spaces, the PLS family of methods are known as bilinear factor models [117, 119, 122, 123]. The basic equation of the multiple regression model is given by the equation 6-1:
$$Y = b_0 + b_1 X_1 + b_2 X_2 + \dots + b_p X_p \qquad \text{Equation 6-1}$$

The PLS-regression is an extension of the multiple regression model. The standard model for a single block is shown in equation 6-2 and 6-3 [123].

$$[X]_N^K = [T]_N^A * [P]_A^K + [E]_N^K \qquad \text{Equation 6-2}$$

$$[Y]_N^M = [U]_N^A * [C]_A^M + [F]_N^M \qquad \text{Equation 6-3}$$

X is the matrix of N trials where K variables are analysed. Matrix T contains the location of each observation respecting the new system of A latent vectors. The matrix P describes the rotation of latent vectors found by the analysis and respecting the original system. Finally, the residual from the least square fit are in the matrix E. Exactly the same development is done for the variables Y and the matrix are identified U, C and F.

The PLS regression is the analysis of 2 blocks at the same time. A set of K predictor from variables X are compared with a set of M response from variables Y. The latent vector Y is obtained in the same way as X and is shown in equation 6-2 and 6-3 [123].

Equation 6-2 and 6-3 are in relation each other. The objective of the PLS is to use these equations to predict Y from X. Each block are not calculated independently, and the information are exchanged between the 2 blocks to reduce the deviation from the regression line and is described by equation 6-4

$$u_{n,a} = b_a t_{n,a} + h_{n,a} \qquad \text{Equation 6-4}$$

Where u is the reduce vector representing X and Y, b the correlation factor between both matrix and finally h, the matrix of the residual data or error.

All these concepts are shown in Figure 6-16 [123]127].
<figure><img src="image_156.png" /><figcaption>Figure 6-16 Correlation using equation 6-2 and 6-3 and showing correlating latent factor</figcaption></figure>

## Projection on latent Structure (partial Least Square) Analysis

In addition to all the analysis done with graphics and tables, a PLS study was performed to verify if the facts and conclusions found are significant and accurate. The PLS study was done with the program SIMCA-P version 11. In statistic analysis, X and Y variables are defined like dependent and independent, accordingly. The PLS analysis refer to input and output variables. The variables X are normally the variables that can be manipulated and the variable Y defined like the answers to the variables X.