Outline. COST E32 Meeting, Budapest, May 4-5, Coating layer microstructure and location of binder - results from SEM analysis

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1 Coating layer microstructure and location of binder - results from SEM analysis Tiina Pöhler, Kari Juvonen, Asko Sneck May 4-5, 26 1 Outline Introduction Quantification of coating pore structure Comparison of SEM and mercury porosimetry SB latex analysis Summary May 4-5,

Outline Introduction Quantification of coating pore structure

2 SEM images of coated paper at different scales Coat weight variation Macropores Micropores, binder Surface Crosssection ~2 mm ~1 µm ~1 µm May 4-5, 26 3 Quantification of microstructure Scanning electron microscope (SEM) imaging OsO 4 treatment to label SB latex Back-scattered electron (BSE) imaging in SEM Surface and cross-section images Image analysis Minimum of 3 images per sample Segmentation of image into three phases: pigments, pores and latex May 4-5,

imaging OsO 4 treatment to label SB latex Back-scattered electron (BSE) imaging in SEM Surface and cross-section

3 Images of coating cross-sections HC6 Coarse carbonate SB latex Pore Pigment SetaCarb Fine carbonate SPS Platy English clay Image size 19x14 µm 2 May 4-5, 26 5 Images of coating surface HC6 Coarse carbonate SB latex Pore Pigment SetaCarb Fine carbonate SPS Platy English clay Image size 25x19 µm 2 May 4-5,

26 5 Images of coating surface HC6 Coarse carbonate SB latex Pore Pigment

4 Parameters for coating layer structure Porosity [%], pore dimensions [µm] Pore orientation [degrees] Latex area fraction [%] Z-directional distributions for porosity and latex Both coating surface and cross-section Measures only coating layer(s) Images for visual examination Two-dimensional estimate May 4-5, 26 7 Coatings with different pigments Parameter HC6 SetaCarb SPS Porosity in cross-section 19.5 ± ± ±.8 [%] Porosity on surface [%] 5.2 ± ± ±.2 Number of pores on surface [#/µm 2 ].83 ± ± ±.7 ± 95% confidence interval Pilot coated papers, jet application 15 m/min Coat weight target 12 g/m², 11 pph SB latex May 4-5,

different pigments Parameter HC6 SetaCarb SPS Porosity in cross-section 19.5 ± 1.9 13.2 ±.4 12.4 ±.8 [%] Porosity on surface [%] 5.2 ±.2 6.4 ±.3 4.2 ±.2 Number of pores on surface [#/µm 2 ].

5 % of total pore area Pore size distributions SEM Cross-section Pore diameter, µm Pore volume, ml/g % of total pore area Mercury porosimeter 5 SEM Surface Pore diameter, µm HC6 SetaCarb SPS Pore diameter, µm May 4-5, 26 9 Porosity of pigment mixtures 2 Porosity, % Parts of CaCO 3 in coating color Cross-section Surface Pilot coated papers, jet application 15 m/min CW target 1 g/m2, CC75 and SPS, 9 pph SB latex May 4-5,

diameter, µm May 4-5, 26 9 Porosity of pigment mixtures 2 Porosity, % 15 1 5 2 4 6 8 1 Parts of CaCO 3 in coating color

6 Comparison of SEM and mercury porosimetry results SEM, µm Surface pore size Cross-section pore size.4 R 2 = R 2 = Hg-porosimeter, µm SEM, % Surface porosity Cross-section porosity Hg-porosimeter, % Pore sizes of SEM surface pores and Hg-pores are similar. SEM gives lower porosity values than Hg-porosimeter, the porosities do not correlate. May 4-5, SEM and mercury porosimetry The basic principles of the two methods are different The absolute level of SEM pore size/porosity values depend on the imaging (and image analysis) parameters used The methods rank samples in the same order according to pore size SEM sees large pores (~4 µm in HC6) in the coating layer These remain unseen with mercury porosimeter because of the ink bottle phenomenon Mercury pore size distributions resemble more the SEM surface pore size distribution (than the SEM cross-sections) May 4-5,

SEM gives lower porosity values than Hg-porosimeter, the porosities do not correlate.

7 SB latex on paper surface (HC9 pigment) pph 6 pph 12 pph 18 pph Latex area fraction, % Surface R 2 = Parts of SB latex Image size 25x19 µm² Detection limit ~3 pph (surface) Detection limit ~5 pph (cross-section) May 4-5, SEM in SB latex analysis Image analysis of BSE images gives latex area fraction [%]. Osmium labeled latex Analyzed latex is attached to the surface of the topmost pigment particles. Z-directional distribution is obtained by dividing the cross-section image into layers of 1 µm. May 4-5,

$26 13 SEM in SB latex analysis Image analysis of BSE images gives latex area fraction [%].$

8 Z-directional latex distribution Latex area fraction, % Cross-section Distance from surface, µm SB 9 pph 13 pph Surface layer had less latex than bulk. Variation in latex content increased towards the base paper. May 4-5, Summary Coating layer microstructure is quantified with SEM and image analysis. new information compared to mercury porosimeter new method for SB latex quantification Cross-section and surface images give supplementary information. pore dimensions and porosity are main results z-directional porosity and latex distributions. topcoat and precoat can be analyzed separately May 4-5,

May 4-5, 26 15 Summary Coating layer microstructure is quantified with SEM and image analysis.

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