i

EFFECTS OF COATING FORMULATIONS ON THERMAL

PROPERTIES OF COATING LAYERS

by

CHONG LIANG

A thesis submitted in conformity with the requirements for the

Degree of Master of Applied Science

Department of Chemical Engineering and Applied Chemistry

University of Toronto

Copyright by Chong Liang (2009)

ii

EFFECTS OF COATING FORMULATIONS ON THERMAL

PROPERTIES OF COATING LAYERS

Chong Liang

Degree of Master of Applied Science

Department of Chemical Engineering and Applied Chemistry

University of Toronto

2009

Abstract

The effects of coating formulation on thermal characteristics of coating layers

were systematically studied for xerographic toner fusion on coated papers. Model

coatings were formulated using three types of ground calcium carbonate and one kaolin

pigments, each mixed with 6, 10, 18, and 25 pph of a styrene butadiene latex binder.

Porosity was found to be a key parameter for coating thermal conductivity adjustment,

and was determined by the latex concentration. The particle size distribution and

morphology of pigments also affect the overall thermal characteristics of coating layers.

Print qualities on model coated papers were evaluated by print gloss measurement, toner

adhesion test, and pair-wise visual ranking, and it was proved that print gloss is reduced

by increasing the bulk thermal conductivity of coating layers. The coating layer consisted

of Covercarb HP pigment and 10 pph of latex was found to have the best performance in

the three print quality evaluation tests.

iii

Acknowledgements

I would like to sincerely thank Professor Ning Yan, my supervisor, for her

in-depth knowledge on the subject, timely guidance, and great encouragement; the

success of this research would not be possible without her. I deeply appreciate this

wonderful opportunity that she has given me.

I would also like to show my appreciation to Professor Ramin Farnood, for

allowing me to use his lab facilities and giving me stimulating discussions on the research

from various student seminars. My appreciation is also with the members of my

examination committee, namely, Professor Farnood and Professor Mohini Sain, for their

interests in my work and the valuable feedback.

The technical assistance and advice from Sabina Di Risio, Syed Sabitha, and

Carlos Quijano-Solis from Professor Yans research group, and Pooya Azadi, Yalda Azimi,

and Kieron Moore from Professor Farnoods group are gratefully acknowledged. The

friendship, help, and caring of these people, as well as Stacy Filenkova, Sherlyn Chen,

Alice Yip, Anna Ho, and many other people from labs ES2008 and WB419 and at the

Pulp and Paper Centre had made my masters study enjoyable.

The financial support from the Natural Sciences and Engineering Research

Council of Canada (NSERC), FP Innovations (Paprican Division), and the member

companies of the Surface Science III Research Consortium at the Pulp and Paper Centre

iv

of University of Toronto is gratefully acknowledged. Special thanks go to Paprican and

Dr. Xuejun Zou, Dr. David Vidal, Ms. Sylvie St-Amour, and Ms. Sylvie Sauriol for the

permission and training on the use of their AutoPore IV mercury intrusion porosimeter

and the plentiful feedback on my project. Special thanks also go to the Xerox Research

Centre of Canada and Ms. Gail Song for the permission and training on the use of their

Nanoflash apparatus. I am also thankful to Omya AG, Imerys, and BASF Canada for

providing the sample materials needed for this study.

Finally, I would like to thank my friends and colleagues in the Department of

Chemical Engineering and Applied Chemistry, University of Toronto for keeping me

motivated, and making the two years of my master study unforgettable.

Thank God for all His blessings, faithfulness, and love on me.

v

Table of Contents

Abstract ............................................................................................................................................ ii

Acknowledgements ......................................................................................................................... iii

Table of Contents ............................................................................................................................. v

List of Figures ................................................................................................................................. ix

List of Tables ................................................................................................................................. xii

Chapter 1 Introduction..1

1.1 Background and Industrial Significance ................................................................................ 1

1.2 Objectives ............................................................................................................................. 3

1.3 Research Approach ................................................................................................................ 3

Chapter 2 Literature Review5

2.1 Introduction ............................................................................................................................ 5

2.2 Coating Structure and Components ....................................................................................... 5

2.2.1 Overview of Coating Structure ...................................................................................... 5

2.2.2 Pigment .......................................................................................................................... 6

2.2.3 Binder ........................................................................................................................... 10

2.2.4 Chemical Additives ...................................................................................................... 13

2.2.5 Air Pores ...................................................................................................................... 13

2.3 Studies on Toner Fusion and Heat Transfer ......................................................................... 14

2.4 Studies on Thermal Conductivity of Composite Materials .................................................. 17

2.4.1 Experimental Techniques ............................................................................................. 17

2.4.1.1 Steady State Methods ........................................................................................... 17

2.4.1.2 Transient Methods ................................................................................................ 19

2.4.1.3 Microthermal Analysis ......................................................................................... 23

2.4.2 Analytical Models ........................................................................................................ 26

2.4.3 Conclusion ................................................................................................................... 29

vi

Chapter 3 Experimental Procedures..30

3.1 Introduction .......................................................................................................................... 30

3.2 Experimental Design ............................................................................................................ 30

3.3 Materials .............................................................................................................................. 31

3.3.1 Pigments ....................................................................................................................... 31

3.3.2 Binder ........................................................................................................................... 31

3.3.3 Coating Substrates ........................................................................................................ 32

3.4 Coating Colour Preparation and Coating Application ......................................................... 33

3.4.1 Preparation of Model Coating Colours ........................................................................ 33

3.4.2 Coating Application Techniques .................................................................................. 34

3.5 Bulk Thermal Property Measuring Techniques.................................................................... 35

3.5.1 Specific Heat Capacity Measurement .......................................................................... 35

3.5.2 Thermal Diffusivity Measurement ............................................................................... 36

3.5.3 Porosity and Apparent Density Measurements ............................................................ 37

3.5.4 Calculation of Bulk Thermal Conductivity .................................................................. 38

3.6 Surface Thermal Conductivity Measuring Techniques ........................................................ 38

3.6.1 Image Acquisition by Microthermal Analysis ............................................................. 38

3.6.2 Image Processing and Analysis .................................................................................... 39

3.7 Print Quality Evaluation ...................................................................................................... 41

3.7.1 Print Gloss Measurement ............................................................................................. 41

3.7.2 Toner Adhesion Test .................................................................................................... 42

3.7.3 Pair-wise Visual Ranking ............................................................................................. 43

Chapter 4 Results and Discussions.44

4.1 Introduction .......................................................................................................................... 44

4.2 Results and Discussions on Bulk Thermal Conductivity ..................................................... 44

4.2.1 Summary of Experimental Data ................................................................................... 44

4.2.2 Effects of Latex Concentration .................................................................................... 45

4.2.2.1 Effect of Latex Concentration on Porosity and Pore Size .................................... 45

4.2.2.2 Effect of Latex Concentration on Density ............................................................ 48

4.2.2.3 Effect of Latex Concentration on Specific Heat Capacity .................................... 49

4.2.2.4 Effect of Latex Concentration on Thermal Diffusivity ......................................... 51

vii

4.2.2.5 Effect of Latex Concentration on Thermal Conductivity ..................................... 52

4.2.2.6 Modelling Thermal Conductivity by Geometric Mean Model ............................. 54

4.2.3 Effects of Pigment Particle Size ................................................................................... 55

4.2.3.1 Effect of Pigment Particle Size on Porosity and Pore Size ................................... 55

4.2.3.2 Effect of Pigment Particle Size on Density........................................................... 56

4.2.3.3 Effect of Pigment Particle Size on Specific Heat Capacity .................................. 56

4.2.3.4 Effect of Pigment Particle Size on Thermal Diffusivity ....................................... 57

4.2.3.5 Effect of Pigment Particle Size on Thermal Conductivity .................................... 57

4.2.4 Effects of Pigment Particle Size Distribution .............................................................. 58

4.2.4.1 Effect of Pigment Particle Size Distribution on Porosity and Pore Size .............. 58

4.2.4.2 Effect of Pigment Particle Size Distribution on Density ...................................... 60

4.2.4.3 Effect of Pigment Particle Size Distribution on Specific Heat Capacity .............. 60

4.2.4.4 Effect of Pigment Particle Size Distribution on Thermal Diffusivity ................... 61

4.2.4.5 Effect of Pigment Particle Size Distribution on Thermal Conductivity ............... 61

4.2.5 Effects of Pigment Morphology ................................................................................... 61

4.2.5.1 Effect of Pigment Morphology on Porosity and Pore Size ................................... 62

4.2.5.2 Effect of Pigment Morphology on Density ........................................................... 62

4.2.5.3 Effect of Pigment Morphology on Specific Heat Capacity .................................. 63

4.2.5.4 Effect of Pigment Morphology on Thermal Diffusivity ....................................... 63

4.2.5.5 Effect of Pigment Morphology on Thermal Conductivity .................................... 63

4.2.6 Effects of Temperature ................................................................................................. 64

4.2.6.1 Effect of Temperature on Specific Heat Capacity ................................................ 64

4.2.6.2 Effect of Temperature on Thermal Diffusivity ..................................................... 65

4.2.6.3 Effect of Temperature on Thermal Conductivity .................................................. 66

4.3 Discussions on Surface Thermal Conductivity .................................................................... 67

4.3.1 Analysis on Topography Images .................................................................................. 67

4.3.1.1 RMS Roughness ................................................................................................... 70

4.3.1.2 Distribution of Pigments on Coating Layer Surface ............................................. 72

4.3.2 Analysis on Thermal Conductivity Images .................................................................. 73

4.3.2.1 Comparison of Surface and Bulk Thermal Conductivity ..................................... 74

4.3.2.2 Comparison of Surface Structure and Surface Thermal Conductivity ................. 75

viii

4.4 Discussions on Print Quality of Model Coated Papers ........................................................ 76

4.4.1 Print Gloss Results and Discussion .............................................................................. 76

4.4.2 Toner Adhesion Test Results and Discussion .............................................................. 79

4.4.3 Visual Ranking Test Results and Discussion ............................................................... 82

4.4.4 Summary of the Print Quality Test Results .................................................................. 85

Chapter 5 Conclusions and Recommendations86

5.1 Conclusions .......................................................................................................................... 86

5.2 Limitations ........................................................................................................................... 89

5.3 Future Recommendations .................................................................................................... 90

References.91

Appendix A: Experimental Results of Porosity Measurements .............................................. 102

Appendix B: Experimental Results of Heat Capacity Measurements ...................................... 107

Appendix C: Analytical Models of Effective Thermal Conductivity Calculations

for Multi-phase Materials .............................................................................. 110

ix

List of Figures

Figure 1. 1 Experimental Approach of This Thesis ......................................................................... 4

Figure 2. 1 Classification of Binders ............................................................................................. 11

Figure 2. 2 Schematic Diagram of Wollaston Wire Probe and TA Operation ............................. 24

Figure 3. 1 Pigment Particle Size Distributions ............................................................................. 32

Figure 3. 2 Q1000 Differential Scanning Calorimeter ................................................................... 35

Figure 3. 3 LFA 447 NanoFlash Xenon Flash Apparatus ............................................................ 36

Figure 3. 4 AutoPore IV 9500 Mercury Intrusion Porosimeter ..................................................... 37

Figure 3. 5 RhopointTM Novo-GlossTM Statistical Glossmeter ...................................................... 41

Figure 3. 6 IGT Printability Tester ................................................................................................. 42

Figure 3. 7 Pair-wise Ranking Method .......................................................................................... 43

Figure 4. 1 Effect of Latex Concentration on Porosity .................................................................. 46

Figure 4. 2 Effect of Latex Concentration on Pore Size (on Volume Basis) .................................. 47

Figure 4. 3 Effect of Latex Concentration on Apparent Density ................................................... 48

Figure 4. 4 Effect of Latex Concentration on Specific Heat Capacity at 25C .............................. 49

Figure 4. 5 Effect of Latex Concentration on Thermal Diffusivity at 25C ................................... 51

Figure 4. 6 Effect of Latex Concentration on Thermal Conductivity at 25C ............................... 52

Figure 4. 7 Thermal Conductivity Data (Dots) Fitted by Geometric Mean Model (Curves) ........ 54

Figure 4. 8 Effect of Pigment Particle Size Broadness on Porosity ............................................... 59

Figure 4. 9 Effect of Pigment Particle Size Broadness on Pore Size (on Volume Basis) .............. 59

Figure 4. 10 Effect of Pigment Particle Size Broadness on Apparent Density .............................. 60

Figure 4. 11 Effect of Temperature on Specific Heat Capacity ..................................................... 64

Figure 4. 12 Effect of Temperature on Thermal Diffusivity of HC60 Coating Layers .................. 65

Figure 4. 13 Effect of Temperature on Thermal Conductivity of HC60 Coating Layers .............. 66

Figure 4. 14 Topography and Thermal Conductivity Images of HC90-10, HC90-18,

HC90-25, HC60-10 Coating Layers ............................................................................... 68

Figure 4. 15 Topography and Thermal Conductivity Images of CCHP-10 and

CPDG-10 Coating Layers, Fused Toner Layer, and Xerox Coated Paper ...................... 69

Figure 4. 16 Average RMS Roughness of Coating Layers ............................................................ 70

Figure 4. 17 Effect of Porosity on RMS Roughness of Coating Layers ........................................ 71

x

Figure 4. 18 Relative (Percent) Standard Deviation of Grey Level of Black and

White Topography of Coating Layers ............................................................................. 72

Figure 4. 19 Comparison between Bulk Thermal Conductivity and Surface Thermal

Conductivity of Coating Layers ...................................................................................... 74

Figure 4. 20 Comparison between RMS Roughness of Coating Layers and Surface

Thermal Conductivity ..................................................................................................... 75

Figure 4. 21 Comparison between RMS Roughness Uniformity and Surface

Thermal Conductivity Uniformity .................................................................................. 75

Figure 4. 22 Effect of Bulk Thermal Conductivity of Coating Layers on Print Gloss

of Coated Paper ............................................................................................................... 76

Figure 4. 23 Effect of RMS Roughness of Coating Layers on Print Gloss of Coated Paper ......... 77

Figure 4. 24 Effect of Bulk Thermal Conductivity of Coating Layers on the RMS

Roughness of Model Coated Paper ................................................................................. 78

Figure 4. 25 Effect of Surface Thermal Conductivity (Normalized Thermal Power

Dissipation) of Coating Layers on Print Gloss of Coated Paper ..................................... 78

Figure 4. 26 Effect of Bulk Thermal Conductivity of Coating Layers on Toner

Adhesion on Coated Paper .............................................................................................. 80

Figure 4. 27 Effect of Surface Thermal Conductivity (Normalized Thermal Power

Dissipation) of Coating Layers on Toner Adhesion on Coated Paper ............................. 81

Figure 4. 28 Effect of RMS Roughness of Coating Layers on Toner Adhesion on

Coated Paper ................................................................................................................... 81

Figure 4. 29 Effect of Bulk Thermal Conductivity of Coating Layers on Ranking of

Coated Paper ................................................................................................................... 82

Figure 4. 30 Effect of Porosity of Coating Layers on Ranking of Coated Paper ........................... 83

Figure 4. 31 Effect of RMS Roughness of Coating Layers on Ranking of Coated Paper ............. 83

Figure 4. 32 Effect of Surface Thermal Conductivity (Normalized Thermal Power

Dissipation) of Coating Layers on Ranking of Coated Paper ......................................... 84

Figure A. 1 Mercury Intrusion Plots of HC90 Coating Layers .................................................... 102

Figure A. 2 Mercury Intrusion Plots of HC60 Coating Layers .................................................... 103

Figure A. 3 Mercury Intrusion Plots of CCHP Coating Layers ................................................... 104

Figure A. 4 Mercury Intrusion Plots of CPDG Coating Layers ................................................... 105

xi

Figure B. 1 Specific Heat Capacity Plots of HC 90 Coating Layers ........................................... 107

Figure B. 2 Specific Heat Capacity Plots of HC 60 Coating Layers ........................................... 107

Figure B. 3 Specific Heat Capacity Plots of CCHP Coating Layers ........................................... 108

Figure B. 4 Specific Heat Capacity Plots of CPDG Coating Layers ........................................... 108

xii

List of Tables

Table 2. 1 Chemical Composition and Physical Properties of Ground Calcium Carbonate ............ 8

Table 2. 2 Chemical Composition and Physical Properties of Precipitated Calcium Carbonate ..... 9

Table 2. 3 Chemical Composition and Physical Properties of Kaolin Clay................................... 10

Table 2. 4 Relative Binding Power of Various Binders ................................................................. 11

Table 2. 5 Characteristics and Performance of Three Major Paper Coating Latexes .................... 12

Table 3. 1 Experimental Design of Model Coating Colours .......................................................... 30

Table 3. 2 Characteristics of Pigments ........................................................................................... 31

Table 3. 3 Coating Colour Formulas .............................................................................................. 33

Table 4. 1 Thermal Properties of Model Coating Layers Measured at 25C ................................. 44

Table 4. 2 Thermal Properties of HC60 Coating Layers Measured at 40, 60, and 80C ............ 45

Table 4. 3 Literature Values of Density of Coating Components .................................................. 48

Table 4. 4 Literature Values of Specific Heat Capacity of Coating Components .......................... 50

Table 4. 5 Cp pigment and Cp latex Values used in the Parallel Model .................................................. 50

Table 4. 6 Literature Values of Thermal Diffusivity of Coating Components ............................... 52

Table 4. 7 Literature Values of Thermal Conductivity of Coating Components ............................ 53

Table 4. 8 Average RMS Roughness of Coating Layers ................................................................ 70

Table 4. 9 Grey Level of Topography of Coating Layers .............................................................. 72

Table 4. 10 Data Extracted from the Thermal Conductivity Images ............................................. 73

Table 4. 11 Average Print Gloss of Model Coated Papers ............................................................. 76

Table 4. 12 Toner Remain Percentage after IGT Pick Test on Coated Paper ................................. 79

Table 4. 13 Ranking of Toner Adhesion on Coated Papers ............................................................ 80

Table 4. 14 Ranking of Print Quality of Coated Papers ................................................................. 82

Table 4. 15 Summary of the Print Quality Test Results ................................................................. 85

Table 5. 1 Effect of Individual Coating Formulation Parameter on Porosity and

Thermal Properties of Coating Layers ......................................................................... 87

Table A. 1 Mercury Intrusion Data of All Coating Layers........................................................... 106

Table B. 1 Specific Heat Capacity Data of All Coating Layers ................................................... 109

1

Chapter 1 Introduction

1.1 Background and Industrial Significance

In recent years, digital printing has the largest growth in the printing industry.

This is mainly due to the advanced technologies developed for a new generation of

large-scale xerography digital presses which can print on a wide variety of paper

substrates at a high speed (e.g. 100 pages per minute).

Xerography is an electrostatic dry-ink printing technology invented by Chester

Carlson in the 1950s1. For modern xerography, there are six steps in this cyclic process2:

1) Charging a photoconductive belt;

2) Generating a latent image on the photoconductive belt by image-wise light exposure;

3) Developing the latent image by brushing charged pigment powders (toner) onto the

image area;

4) Transferring the toner from the photoconductive belt to paper;

5) Fusing the toner on the paper in a fuser consisting of a heating roll and a pressure roll;

6) Discharging the photoconductive belt and cleaning the residual toner.

Toner fusion, the fifth step in the xerography process, consumes the largest

amount of energy and is the key step in determining print quality. When toner and paper

go through the fuser, the toner powders absorb the thermal energy provided by the

heating roll, and then melt, coalesce, and spread (wet) on the paper surface. Since this

process only takes several milliseconds in the high-speed xerography printing presses, the

heat transfer at the fuser nip is of great importance.

2

In many applications, paper is used as the printing substrate. To enhance print

quality, paper is coated with inorganic pigments and latex binders to obtain better

smoothness, whiteness, and gloss. However, studies have shown that the coating layer

affects the effective heat transfer area for toner, and its thermal properties have an inverse

effect on the toner temperature at the toner-coating interface3. In addition to the possible

heat sink effect caused by the coated paper substrate4, these problematic phenomena

result in insufficient heat supply for toner fusion, subjecting the industry to costly

printing operations and potential reduced print quality on coated paper.

Heat transfer during toner fusion has been a subject of study for a number of years.

Analytical techniques or empirical correlations (e.g. Maxwell and Carman-Kozeny

correlations) have been used to study heat transfer through paper based on the effects of

fuser roll configurations or paper properties. Experimental work on thermal properties of

paper only emerged in the past few years, still showing the shortage on experimental data.

Furthermore, the number of studies on the thermal effects of coating layers is very limited.

Using a systematic experimental approach, this thesis provides a better understanding of

realistic coating thermal properties. This knowledge will enable paper coating suppliers

to engineer optimum coating with better raw material selection, or papermakers to

develop advanced coated papers to better suit the printing applications with the

high-speed xerography digital presses. The experimental techniques presented in this

thesis also give some insights to papermakers for developing cost-effective methods to

test product properties.

3

1.2 Objectives

To study the effects of coating compositions on the thermal characteristics of coating

layers using a systematic experimental approach.

To relate thermal properties of coating layers to the toner fusion performance on

coated papers.

1.3 Research Approach

The experimental approach taken in this study is illustrated in Figure 1.1. The

steps involved are as follows:

1) Formulating model coating colours.

2) Producing standalone coating layers by applying the model coating colours on

plastic films or in plastic casts.

3) Measuring the bulk thermal properties of model coating layers by:

i. Measuring porosity and apparent density using mercury intrusion porosimetry

ii. Measuring thermal diffusivity using a light flash apparatus

iii. Measuring specific heat capacity using differential scanning calorimetry

iv. Calculating bulk thermal conductivity by multiplying apparent density, thermal

diffusivity, and specific heat capacity

4) Measuring the surface thermal conductivity of model coating layers by:

i. Acquiring topography and surface thermal conductivity images using

microthermal analysis

ii. Processing and analyzing the images to obtain quantitative information

5) Applying the model coating colours on paper substrates followed by printing 100%

4

solid black using a Xerox printer

6) Evaluating the influence of coating thermal conductivity on the print quality by print

gloss measurement, toner adhesion test, and pair-wise visual ranking

Figure 1. 1 Experimental Approach of This Thesis

Formulation of Model Coating Colours

Formation of Model Coating Layers

Measurements of Bulk Thermal Properties of Coating Layers

Porosity and Apparent Density Measurements

Thermal Diffusivity Measurement

Specific Heat Capacity Measurement

Bulk Thermal Conductivity Calculation

Measurement of Surface Thermal Conductivity of Coating Layers

Topography and Thermal Conductivity Image Acquisition

Image Processing and Analyses

Preparation of Xerography-printed

Coated Papers

Print Quality Evaluations

Pair-wise Visual Ranking Toner Adhesion Test Print Gloss Measurement

5

Chapter 2 Literature Review

2.1 Introduction

In order to engineer optimum coating formulations or to develop advanced coated

papers to improve the thermal effectiveness in toner fusion, it is essential to have a

comprehensive understanding on the basics of coating structures, heat transfer in toner

fusion, as well as the measuring techniques of thermal properties of materials. This

chapter is a review of the literature related to these topics: Section 2.2 introduces the

main components of paper coating, including pigments, binders, chemical additives, and

pores; Section 2.3 reviews the studies conducted over the past decades on heat transfer in

the applications of toner fusion and papermaking; and, Section 2.4 discusses the

numerous techniques used to measure thermal conductivities of different materials, and

the reason for choosing those outlined in Section 1.3 for this thesis.

2.2 Coating Structure and Components

2.2.1 Overview of Coating Structure

Paper coatings fill the cavities and cover the highest sitting fibers on the surface

of base papers. Therefore, coated papers have several advantages compared to uncoated

ones, including increased surface smoothness, surface strength, glossiness, brightness,

whiteness, and decreased ink absorption and dusting. In general, the printing properties of

paper are improved by the coating.

Coating structure was first defined by Lepoutre5 as the spatial arrangement of

pigment particles and binder. Two forms of coating structures were distinguished: the wet

6

and the dry structures. The wet structure comprises of a suspension of dispersed colloidal

pigment and binder particles along with chemical additives in water as described by

Lehtinen6. Colloidal interactions among particles and rheology of the suspension (also

called coating colour) are often studied for coating application processes and the

formation of the coating layer on base paper. The coating layer, that is, the dry structure,

forms after the water from the coating suspension evaporates and the pigment and binder

particles consolidate. Air pores are formed during the consolidation, and this structural

component has a very strong influence on the optical, mechanical, and fluid absorption

properties of the coated paper. The advantages of coated papers mentioned earlier are

mainly due to the properties of the dry coating layer of the end product.

In summary, a coating layer is a dry porous structure with polydispersed spatial

arrangements of pigment, binder, air pores, and chemical additives. These four

components are reviewed in the following sections in more details.

2.2.2 Pigment

Pigment is the most abundant component in the coating, ranging from 80% 95%

by weight, or 70% by volume6. The pigment particle size, particle size distribution, and

morphology (including shape and aspect ratio) all have influence on the packing

arrangement within a coating structure and have significant impact on the printed

properties of coated papers7,8.

7

Depending on the chemical compositions, pigments can be classified as organic

and inorganic. Inorganic pigments are typically used in the paper industry, due to their

abundance and low cost. The following briefly introduces the most common types of

minerals used as inorganic pigments: ground or precipitated calcium carbonate and kaolin

clay. Other types of pigments, including talc, titanium dioxide, and synthetic plastic

pigments, are also used in the paper industry, but will not be discussed in this review.

Ground Calcium Carbonate (GCC)

Ground calcium carbonate (GCC) is one of the most commonly used pigments in

the paper industry, since it has the most favourable price/performance ratio. GCC

pigments have lower binder demand than clay, therefore, GCC coating is in general

cheaper than clay coating. Also, GCC pigments give good brightness and print gloss,

resulting in coated papers generally more acceptable to consumers. In addition, GCC

coating colours have favourable rheological properties that can provide good runnability

for coaters and energy saving for the coating process. GCC pigments are made from

natural calcite mines by grinding and screening. Their chemical composition and physical

properties can be seen in Table 2.1.

8

Table 2. 1 Chemical Composition and Physical Properties of Ground Calcium Carbonate

Chemical Composition CaCO3, MgCO3 (2-3%)

Density (kg/m3) 2.7

Particle Size (m) 0.7 2

Particle Shape Cubic, prismatic

Particle Aspect Ratio 1

Refractive Index 1.56 1.65

ISO-brightness 87 97

Form of delivery Powder slurry (65 78 % solid content)

Solubility Dissolves under acidic conditions

Dispersion Anionic / cationic

Surface Area (m2/g) 2 20

Reference: Various locations in the publication of Lehtinen6

Precipitated Calcium Carbonate (PCC)

Precipitated calcium carbonate (PCC) is manufactured from limestone through a

series of chemical reactions and purification processes. The refinery results in highly pure

pigments, and the particles can be designed to have various sizes and morphologies based

on the needs of customers. In general, PCC coated papers have high brightness and

opacity, and adjustable ink setting properties which are beneficial for printability.

However, PCC remains a minor player in the global coating pigment market due to its

higher cost compared to conventional coating pigments. The chemical composition and

physical properties of PCC pigments can be seen in Table 2.2.

9

Table 2. 2 Chemical Composition and Physical Properties of Precipitated Calcium Carbonate

Chemical Composition CaCO3

Density (kg/m3) 2.7

Particle Size (m) 0.1 1.0

Particle Shape Aragonite, prismatic

Particle Aspect Ratio 2 4

Refractive Index 1.59

ISO-brightness 96 99

Form of delivery Powder slurry

(71 75 % solid content)

Solubility Dissolves under acidic conditions

Dispersion Anionic / cationic

Surface Area (m2/g) 3 13

Reference: Various locations in the publication of Lehtinen6

Kaolin Clay

The use of kaolin clay as paper filler or coating pigment has lasted for many years.

From the original mineral form, kaolin pigments are manufactured through a serious of

extraction and refining processes. Kaolin is an intrinsically valuable coating pigment

because of its platy particle shape, good colour (white or near white), and the relative

ease to be processed to a fine particle size. Compared with GCC, the kaolin pigments

generally give higher print gloss on the coated paper. With modern technologies, kaolin

pigments can be engineered to obtain very high aspect ratios, or aggregated by either

thermal or chemical means9 in order to increase the light-scattering coefficient and create

intra-particle pores. This type of advanced engineered kaolin can improve the optical

properties of coated papers drastically. The chemical composition and physical properties

of conventional kaolin pigments can be seen in Table 2.3.

10

Table 2. 3 Chemical Composition and Physical Properties of Kaolin Clay

Chemical Composition Al2O3 2SiO2 2H2O

Density (kg/m3) 2.58

Particle Size (m) 0.3 5

Particle Shape Platy (hexagonal)

Particle Aspect Ratio 8 20

Refractive Index 1.56

ISO-brightness 80 90

Form of delivery Powder slurry

(62 71 % solid content)

Solubility Insoluble

Dispersion Alkaline conditions

Surface Area (m2/g) 18 25

Reference: Various locations in the publication of Lehtinen6

2.2.3 Binder

Binders are the second most abundant component by weight in coating

formulations. The functions of binders are fourfold: binding pigment particles to base

papers, binding pigment particles to each other, partially filling voids between pigment

particles, and, affecting the viscosity and water retention of a coating colour.

The classification of binders is well summarized in Figure 2.1, and their relative

binding strengths are listed in Table 2.46.

11

Figure 2. 1 Classification of Binders

Table 2. 4 Relative Binding Power of Various Binders6

Binder Amount Needed to Achieve Same Coating Strength (in Parts)

Polyvinyl Alcohol 1

Carboxy Methyl Cellulose 1

Styrene Butadiene Latex 2 2.5

Acrylic Latex 2 2.5

Polyvinyl Acetate Latex 2 3

Starch 2.5 3

Soy Protein 2.5 3

Latex

Latexes are manufactured by emulsion polymerization of simple monomers.

Properties that can be adjusted in latexes include the chemical composition, particle size,

molecular weight, degree of crosslinking, pH, surface tension, glass transition

temperature, and minimum film formation temperature6,10. Latexes are widely used as

binders in paper coating. Bergh11 showed that styrene butadiene latex accounted for 70%

Binder

Water Soluble Binder Insoluble Binder

Natural Polymer Synthetic Polymer

Starch

Protein

Cellulose derivatives

Polyvinyl alcohol (PVA, PVOH)

Latex

Styrene butadiene (SB) latex

Styrene acrylate (SA) latex

Polyvinyl acetate (PVAc) latex

12

of the binder consumption in the paper industry in 1997, while styrene acrylic latex stood

at the second place with 16%. Table 2.5 summarizes the performance of the three major

latexes in paper coatings.

Table 2. 5 Characteristics and Performance of Three Major Paper Coating Latexes

Latex Type Molecular Structure Paper Coating Performance

Styrene Butadiene Crosslinked copolymer

High binding strength; High-solids coating blade runnability; High coating gloss; High ink gloss

Styrene Acrylic Copolymer

High-solids coating blade runnability; High coating gloss; High ink gloss; Light stable coatings

Polyvinyl Acetate Linear homopolymer

Blister resisting coatings; Better fibre coverage; More porous coatings; Paperboard coatings

Lehtinen6, Bergh11, and Heiser12 gave comprehensive reviews on the effects of

latex properties on paper coating structures, interactions among coating components,

rheology and runnability of coating formulations, coating gloss, ink gloss, and printability

of coated papers. Eklund et al.13,14 further found that increasing the binder concentration

in the coating formulation results in a less porous coating structure, and coatings

containing SB latex show higher hydrophobicity than coatings containing SA latex,

decreasing the water sorption of coated papers.

13

2.2.4 Chemical Additives

In addition to pigments and binders, trace amounts of chemicals are added in

coating colours to fulfill various functions. These chemical additives include dispersants,

pH control agents, foam control agents, water retention and rheology modifiers, colorants,

optical brightening agents, lubricants, insolubilizers, and preservatives. Due to the small

amount required, chemical additives have very little direct contributions to the formation

of a coating layer (the dry coating structure)6.

2.2.5 Air Pores

As discussed earlier, due to the spatial arrangement of pigment and binder

particles, voids (air pores) exist in a coating layer. The porous structure of a coating layer

has a strong influence on its optical (opacity, reflectance), mechanical (strength,

compressibility), and fluid absorption (ink and water absorption) properties5.

In the past, researchers have attempted to model pore structures theoretically15,16;

however, this is not an easy task due to the complex poly-dispersed arrangement of

particles in the coating layer. Some characteristics of the pore structure, such as pore size,

pore size distribution, and total pore volume, are more often obtained experimentally by

mercury intrusion porosimetry (MIP)17, air permeability, and oil absorption. However, the

real pore geometry and the interconnectivity of the pores still cannot be assessed.

14

2.3 Studies on Toner Fusion and Heat Transfer

For high-speed xerographic printing, high thermal energy must be supplied in a

short period of time to fuse the toner onto the paper. This process is mostly affected by

the heat flux to the toner and the stress applied during fusion. More than 50% of the total

electricity consumption of a copier is caused by the fuser rolls18. Toner fusion thus has

been a focus in many research works for the paper and printing industries.

In 1992, Mitsuya et al.19 studied the optimization of temperature and pressure of a

heat roll for the improvement of toner fusion. A heat flux profile was calculated

numerically on a four-layer model consisting of paper, toner, a fluoride surface, and an

aluminum core of the heat roll. The resulting heat flux to the toner decreased with

increasing heat roll thickness due to the lower heat transfer from its aluminum core. The

effect of pressure on the toner melting property was also examined. Mitsuya et al.19 have

established a basic one-dimensional model for heat flux calculations in toner fusion.

However, the model is oversimplified and the case of toner fusion on coated papers has

not yet been analyzed.

Sanders and Rutland20 studied the fusing fixation on a set of 25 model papers

printed in three different high-speed copiers using a crease test. It was found that the

macroscopic contact area between fused toner and paper fibres is controlled by the fusing

conditions (temperature and pressure) and by the toner melt rheology, while the surface

energies of paper and toner determine the degree of microscopic wetting.

15

Bandyopadhyay et al.21 studied the transient thermal response of coated papers

passing between fusing rolls. They found that paper and coating parameters affect the

fusing process in xerography. In their study, a sheet of coated paper was considered as a

two-dimensional porous medium subjected to a thermal pulse moving along the surface

boundaries. Using finite volume analysis, the temperature and moisture content of the

coated paper were modeled. Thermo-physical properties of the base sheet and coating

materials were also studied. It was found that the moisture content of a base sheet

declines dramatically at the beginning of fusing, but it gradually reaches its steady-state

value while the temperature field of the base sheet experiences a maximum at 500

milliseconds after the initiation of fusing. Coating parameters were found to affect toner

fusion; however, this finding was not explained in detail.

Maijala et al.22 in 2004 studied the heat transfer in the thermal fixing of

electrostatically deposited dry coating powders on paper surface. In this study, an

one-dimensional model comprising a heat source layer, a coating powder layer, a heat

sink layer, and two aluminum foil layers (between coating powder and heat source/heat

sink) was used to determine the thermal conductivity and heat transfer coefficient of the

coating layer. In addition, transient temperature responses at various depths in the coating

layer were simulated. Very rapid heat transfer in the fixation nip was found and the

authors concluded that sufficient fixation of the coating powder layer can be achieved

within a few milliseconds.

16

Verhnes et al. 18 further gave a comprehensive review on toner fusion/fixing in

electrophotography printing. Relationships between physically properties and

temperature of the three major components in a fusing nip, namely toner, paper, and fuser

roll, were discussed. In addition, the authors demonstrated a method to model the

temperature field in the fixing nip by applying heat equations to a one-dimensional

system including 4 layers: cylinder, air, toner, and paper.

Recently, Azadi3 used a discrete element method to simulate unsteady-state heat

transfer at the fuser-toner and toner-coating layer interfaces during the xerographic fusing

process. The model coating layers consisted of randomly arranged spherical pigment and

latex particles with realistic size distributions. Effects of coating characteristics, toner size,

multiple toner layers, toner melting energy and toner thermal conductivity on the

unsteady state heat transfer in the fusing process were investigated. Results showed that

the temperature variation highly depended on the toner size, toner melting energy and the

fuser roll temperature. Moreover, thermal conductivity of coating materials played a

significant role in determining toner temperatures. However, these conclusions have not

been verified with experimental results.

A critical review of the literature tells that there has been a comprehensive study

on toner fusion in xerography printing, detailing the effects of various factors including

toner properties, paper properties, fuser roll configurations, air gap formed between

components, and fusing process conditions. However, the contributions of the coating

layer itself have not been recognized until recently. The thermal and surface properties of

17

paper can be altered significantly with the addition of a porous coating layer. On the other

hand, due to the complexity of the paper structure, more researchers prefer to study toner

fusion and heat transfer using numerical modelling or computer simulations.

Assumptions have to be made in order to simplify the calculations, or, complicated

modeling techniques requiring hours of computation are used. Therefore, it is necessary

to investigate the effects of coating layers on toner fusion with the support of

experimental data.

2.4 Studies on Thermal Conductivity of Composite Materials

As mentioned earlier, a coating layer is a thin dry structure of polydispersed

spatial arrangements of pigment, binder, air pores, and chemical additives. Therefore, one

can gain some insights on the characterization techniques for thermal properties of

coating layers by reviewing literature on thermal properties of porous, composite, or thin

film materials. Experimental methods (Section 2.4.1) for the measurement of thermal

conductivity can be classified in two categories: steady state methods and transient

methods. There are also a number of analytical models for calculating the theoretical bulk

thermal conductivity of composites (Section 2.4.2).

2.4.1 Experimental Techniques

2.4.1.1 Steady State Methods

As described in the review from Tzeng et al.23, the steady state method was

developed for measuring apparent (or overall) thermal conductivity of thin composite

materials at the steady-state heat flux condition. In principle, a specimen is sandwiched

18

between a heat source and a heat sink both with thermal sensing elements. When the heat

flow through the entire system reaches steady state, the known amount of heat per unit

time and the temperature difference across the specimen are measured and from which

the thermal conductivity of the specimen is calculated. To ensure consistent heat flow,

good surface contact between the specimen and the heat source and heat sink must be

maintained. Therefore, surface characteristics of specimens such as compressibility and

conformability are very important to the accuracy and repeatability of the test results.

This testing principle was used in a number of thermal conductivity studies for

clay-based materials or polymer composites. Al-Malah et al.24 tested the thermal

conductivities of bentonite- and calcium carbonate-based insulating materials using a

Hilton B480 device25, which has a hot plate whose vertical position can be adjusted to fit

samples with various thicknesses, a water chiller, and a spring device to detect the

pressure load on the sample. Dondi et al.26 also used a hot plate experimental setup to test

the thermal conductivities of 29 types of clay bricks. In the work of Keith et al.27, the

through-plane thermal conductivities of several carbon-filled liquid crystal polymer

composites were tested using the ASTM F433 guarded heat flow meter method28. This is

a comparative method in which thermal conductivities of test specimens are calculated or

interpolated in reference to the calibration standards and the contact resistance incurred

between standard materials and the heating/cooling elements.

For paper-related samples, the problem of contact resistance between surfaces of

specimens and heating/cooling elements is more significant. Sanders and Forsyth29 have

19

modified the steady-state thermal conductivity cell by installing a copper column with

optically flat surface above and underneath the specimen, in order to apply pressure to the

specimen to reduce its surface contact resistance. However, this modification has altered

the apparent density of the specimen and created additional thermal interfaces within the

system, therefore, the test results might be representative in real applications.

2.4.1.2 Transient Methods

Transient techniques measure the transient (time-dependent) response of a sample

to thermal signals. The heating element usually also serves as the thermal sensor.

Compared to steady state methods, the advantages of transient thermal conductivity

measurements are threefold:

l Measurements start before heat flow reaches steady state, thus requiring shorter

experimental time;

l Depending on the configuration of the heating elements, smaller or no contact area

between sample and heating elements is needed, thus requiring smaller sample size

and having smaller or no contact resistance problem;

l Smaller temperature differentials across specimens can be detected, thus more

suitable for thin materials or materials with small thermal conductivities.

Hot Disk/Wire Method

The transient plate source (hot disk) method and hot wire method are most

common in thermal conductivity measurements for insulating bricks and powder or

fibrous materials.

20

Bouguerra et al.30 used the transient plane source technique to measure thermal

conductivities of quartz, calcite, and kaolinite by placing a nickel foil between two layers

of test materials of equal thickness. The nickel foil acted as a plane heat source and

temperature sensor; its resistivity was temperature dependent, thus the thermal

conductivities of the test materials were determined based on the voltage drop of the

system. Using the same technique, Al-Ajlan31 measured the thermal conductivities of

insulation materials including polystyrene, polyurethane board, lightweight concrete,

glass fibre, and rock wool.

Ogacho et al.32 employed a transient hot wire method in the investigation of the

effective thermal conductivity of a set of kaolinite-based refractory with various

concentrations of vegetable-based binder. In Ogacho et al.s work, a nichrome wire (hot

wire) with thermocouples was sandwiched between two semi-cylindrical specimens. With

known electrical power per unit length of the hot wire dissipated to the heating system,

the thermal conductivities of test materials were determined using a correlation between

the hot wire surface temperature and time. The hot wire method was also used in the

work of Franco33 for measuring thermal conductivities of non-metallic building materials.

A modification of the transient hot wire method is the heat-pulse method, which

uses a hypodermic needle probe inserted in test specimens. When an electrical current is

passed through the heater on the probe for a short period of time, the temperature history

of the heaters surface will take on a characteristic form based on the thermal properties

of test materials. This method was used in Ochsner et al.s study on the thermal properties

21

of soils in situ34.

Light/Laser Flash Method

The light flash or laser flash method is used to measure thermal diffusivities and

heat capacities of thin solid materials. Thermal conductivities can be found based on the

relationship K = Cp D , where K, Cp, D, and represent the bulk thermal conductivity,

specific heat capacity, thermal diffusivity, and apparent density of a sample35,37,41,42,82.

Since this is a non-contact measurement, the problem of contact resistance between

testing samples and temperature sensors or heating elements is eliminated.

The flash method was first developed by Parker et al.35 in 1960. In this method,

the energy of a high-intensity short-duration light pulse was absorbed by the front surface

of a thermally insulated specimen a few millimeters thick coated with camphor black.

The resulting temperature history of the rear surface of the specimen was measured by a

thermocouple and recorded with an oscilloscope and camera, and was used to determine

the thermal diffusivity of the specimen. The heat capacity of the specimen was found by

the maximum temperature indicated by the thermocouple. Parker et al. used this

technique to measure the thermal properties of metals including copper, silver, iron,

nickel, aluminum, tin, zinc, and some alloys at 22C and 135C. Cowan36 later modified

the calculation procedures of this method to account for tests at very high temperature

(2500 K or higher).

22

Zhao and Schabel37 adopted this method to measure thermal properties of paper

using a Xenon lamp as the light source and an infrared detector as the thermal sensor.

With a build-in furnace, the thermal diffusivity of graphite-coated paper samples was

measured from 20C to 190C.The experimental results were found highly repeatable.

Based on the same principle, Miloevi et al.38 simultaneously measured the

thermal diffusivity and estimated the thermal contact resistance of thin solid films (nickel

film on aluminum substrate) and coatings (PTFE coating on steel substrate) using a

two-dimensional laser flash method.

Other Techniques

Other thermal conductivity/diffusivity testing techniques that have been used on

paper samples include:

l Kerekes method39,40, in which temperature measurements were made on a large

paper web passing over a heated calender roll using an infrared thermometer;

l AC Joule heating method41, in which a paper specimen attached with a thin gold

layer was inserted between two glass slides, and the temperature variations at the

rear surface of the specimen were detected by the gold layer sensor acting as a

resistance thermometer; and

l Thermoacoustic42,43 method, in which the thermal diffusivity of a sample is found by

measuring characteristics of the thermal waves (temperature history, amplitude of

thermal excitation, and angular frequency) resulted from a periodic heating on the

sample surface.

23

2.4.1.3 Microthermal Analysis

Microthermal analysis (TA) is an image analysis technology combining scanning

probe microscopy (SPM) with thermal analysis. It is a transient thermal property

measuring technique, but different from the ones mentioned earlier, it is considered as

qualitative or semi-quantitative.

TA is powerful in terms of its ability to acquire simultaneous topography and

thermal conductivity image of a surface with sub-micron spatial resolutions. A Wollaston

wire probe, developed originally by Dinwiddie et al.44 in the 1990s and further by

Hammiche et al.45,46,47,48, is used as the scanning probe and heating element. The

Wollaston wire consists of a 5-m diameter platinum core surrounded by silver coating,

with one end mounted to piezoelectric elements and serves as a cantilever, and the other

end forming a V-shaped tip with a diameter of 1 m49. A schematic diagram of the

assembly is shown in Figure 2.2.

As required by heat conduction, TA operates in the contact mode. When the

Wollaston wire probe scans a surface, changes in the deflection of the cantilever caused

by the surface topography are optically monitored by a photodetector via a reflected laser

beam, and are converted into an electrical signal generating a feedback to control the

vertical position of the cantilever and thus the tip. Displaying the tips vertical position

against its lateral position results in a topographic image for the surface. Simultaneously,

an electrical current passes through the wire, and the heat generated at the tip due to the

large electrical resistance of platinum is transferred to the sample surface50. As the probe

24

encounters an area of the sample with high thermal conductivity, more heat is absorbed

into the sample, so the electrical power required to maintain a constant tip temperature is

increased. When the thermal conductivity is low, the power dissipation is reduced. In this

way the thermal conductivity of a sample surface can be mapped. In order to eliminate

any interference from the environment, a reference probe measuring heat dissipation into

the air is used in conjunction with the regular probe.

Figure 2. 2 Schematic Diagram of Wollaston Wire Probe and TA Operation

TA has been employed in a number of leading-edge studies on surface or

sub-surface thermal characteristics of composites. Hammiche et al. successfully imaged

copper particles embedded under a polymer film45, and individual components in a

Piezo Scanner

Cantilever Mount

Wollaston Wire Tip

Sample

Laser Beam

Laser

Mirror

Photodetector

Electronic Control

Unit

Feedback Signal

Mirror

25

number of immiscible polymer blends46,48 , based on the large contrast between the

intrinsic thermal conductivities of the two materials in the same composite. They also

found that by increasing the probe temperature, distributions of components at deeper

levels inside the composites can be observed. Woodward et al.51 imaged the cross section

of a multilayer packaging material, revealing the supporting resin, the different polymer

layers, and the interstitial metal layer by the different power dissipation intensities shown

on the thermal conductivity map. Keating52 demonstrated the applicability of TA in the

study of adhesion temperature of the topical coatings of Tyvek HDPE sheets, fusion of

the heat-processed polyethylene fibres in a bundle, the surface crystallinity of PET pellets,

and phase changes of toughen Nylon 66.

TA can be well used to test materials with low thermal conductivities. Tsukruk et

al.53 found that the thermal tip and sample surface may become a poor conductive

counterpart for heat dissipation for platinum, silicon, and gold, however, for polymers,

glass, and silicon nitride, heat dissipation at the contact point of the tip is directly

proportional to the surface thermal conductivities of test materials. Fischer54 also

concluded that TA shows highest sensitivity for small thermal conductivity, and it is

useful in a thermal conductivity range between 0.05 and 20 W/mK.

It is possible to obtain quantitative information on thermal conductivity from the

power dissipation shown on the thermal conductivity map, given that the junction

temperature between the thermal tip and sample surface and the effective diameter of the

tip are known55. However, such information is very difficult to find. Fischer54 developed

26

a comparative method to determine thermal conductivity of samples by correlating the

power dissipation obtained from thermal maps of numerous standard materials with their

known thermal conductivities. Harding et al. also attempted to quantify the thermal

conductivity data via a statistical approach56. Although their regression model was able to

reduce variation in the thermal image due to the topography effect, it has also reduced the

apparent ability to distinguish between different components in the composites on the

thermal images.

More literatures regarding the operating principles and applications of TA can be

found elsewhere57.

2.4.2 Analytical Models

The modeling of the effective thermal conductivity of heterogeneous multi-phase

composites has been a subject of study for decades. Originated from Maxwells work,

researchers have put tremendous efforts in the development of analytical models that can

capture the contributions of both the thermal conductivity and the structure of each

constituent of the composite to the overall thermal conductivity. A list of these models in

mathematical expressions can be found in Appendix C.

Brailsford and Major58 in 1964 suggested that the maxima and the minima of the

thermal conductivity can be obtained by the parallel model and the series model,

respectively. They also developed mathematical expressions for two-phase structures

with one continuous and one dispersed phase or with two randomly mixed phases. The

27

predicted temperature-dependent thermal conductivities of water saturated and air

saturated glass balls and rubber plates were found to be in close agreement with

experimental data. However, empirical modifications to their mathematical expressions

were needed in order to better predict the thermal conductivity of porous sandstones

which were not saturated with either air or water.

Chaudhary and Bhandari also59 proved that the models developed by Braislford

and Major were not accurate by comparing experimental data to the calculated thermal

conductivities of moist porous rocks. Based on that, they proposed a model to find an

empirical geometric mean of the maximum and minimum bounds obtained from the

parallel and the series models.

Behrens60 took into considerations of thermal waves travelling through the

inter-component spacing within two-phase composites with isotropic constituents and

derived analytical solutions to find the average thermal conductivities. He further

explained in detail the modifications in his calculations for an orthorhombic composite, a

lamellar composite, a filamentary composite, and a composite with cubic symmetry.

Progelhof et al.61 later summarized the developments of theoretical thermal

conductivity calculation models from Behrens, Bruggeman, Cheng and Vachon, Jefferson

and Witzell, Lewis and Nielsen, Maxwell, and Russell. Among these, the Lewis and

Nielsen semi-theoretical model was found to be the most accurate since it accounts for

many geometric shapes, orientations, and packing of the dispersed phase in a composite

28

and the direction of the heat flow through the structure. Bauer62 also developed

semi-empirical models that can be applied on porous materials with a wide range of

porosity, pore types, and pore sizes.

Based on the various effective models used in food engineering and heat transfer,

Carson et al.63 recently developed a systematic approach to model the thermal

conductivity of unfrozen, porous food. Extended from Carsons work, Wang et al.64 used

the parallel model, the series model, two Maxwell-Eucken models, the effective medium

approximation (EMT), and combinations of those five, to calculate the effective thermal

conductivity of materials with various microscopic structures, ranging from homogenous

multilayer structures to randomly dispersed multi-phase structures. Wang et al.65 further

developed a new fundamental model to predict thermal conductivity of materials with

co-continuous inclusions.

Since all of the analytical models discussed above have limitations, the major one

being their limited ability to account for the microscopic distortion of the temperature

field in the neighborhood of an individual inclusion, some researchers turned to

numerical models for predicting the effective thermal conductivity of composites. Few

major numerical methods include the finite element method66,67, the discrete element

method3, and the lattice Boltzmann model68.

29

2.4.3 Conclusion

It is known that contact resistance is the major obstacle for thermal conductivity

or heat transfer measurements for paper-related materials. Based on the literature review

on various experimental techniques, it is believed that the effective bulk thermal

conductivities (K) of coating layers can be obtained by first measuring the thermal

diffusivity (D) and specific heat capacity (Cp) using a non-contacting technique such as

light/laser flash method, followed by calculation with the equation K = Cp D . In this

way, the contact resistance problem can be minimized and the three major thermal

properties mentioned above can also be studied at the same time. The surface thermal

conductivities of coating layers can be studied using microthermal analysis. In addition,

since the focus of this thesis is a systematic experimental approach for studying thermal

properties, analytical models may be used only as a validation tool for experimental

results.

30

Chapter 3 Experimental Procedures

3.1 Introduction

This chapter explains the details of the experimental approach shown in Figure

1.1. The attributes of model coating components, coating colour formulation, coating

application techniques, thermal property measurement and analysis techniques for model

coating layers, and print quality evaluation tests on model coated papers are discussed.

3.2 Experimental Design

Since the main objective of this study was to determine the effect of coating

formulation on the thermal properties of coating layers, only the attributes of the model

coating colours were varied (see Table 3.1). The coating technique, coating substrate, coat

weight, and conditions for coating layer formation were kept constant for the same type

of test or measurement. In addition, no calendering was involved in this study.

Table 3. 1 Experimental Design of Model Coating Colours

Factor Measuring Parameter Number of Variations

Varied Pigment

Particle size 2

Particle size distribution 2

Morphology 2

Binder Concentration 4

Fixed

Binder Type (chemical composition and

glass transition temperature)

1 Solid content Total solid weight percentage

pH value pH value

Additives Type (chemical composition)

Concentration

31

3.3 Materials

3.3.1 Pigments

Three types of ground calcium carbonate (GCC) and one type of kaolin clay

pigments were used in this study. They were received from commercial suppliers in

slurry forms. Table 3.2 summarizes the relevant characteristics of the pigment particles

while Figure 3.1 illustrates the pigment particle size distributions (PSD) determined by

the SediGraphTM sedimentation technique69.

Table 3. 2 Characteristics of Pigments

Note: a. Median is defined as D50 (50 percentile).

b. Distribution is defined as either:

i. Percentage of particles having diameter smaller than 2 m;

ii. Broadness of PSD width, defined as (D70 D30)/D50. 30, 50, 70 are the percentages of

particles smaller than the diameter D.

3.3.2 Binder

A latex binder composed of styrene-butadiene copolymer (SB) was used in this

study. The glass transition temperature (Tg) of this latex was -23C. It was received from

a commercial supplier as a dispersion in water.

Code Name Pigment Type

Particle Size

Morphology Mediana

Distributionb

% < 2 m Broadness

HC90 Hydrocarb 90 GCC 0.61 m 95.1 1.01 Spherical

HC60 Hydrocarb 60 GCC 1.38 m 63.8 1.16 Spherical

CCHP Covercarb HP GCC 0.64 m 97.7 0.73 Spherical

CPDG Capim DG Kaolin 0.71 m 88.5 1.06 Platy

32

Figure 3. 1 Pigment Particle Size Distributions

3.3.3 Coating Substrates

Three types of coating substrates were used in this study for different purposes

(refer to Section 3.4.2 for details):

CADO transparent wrap (100% polypropylene, film thickness: 28 0.6 m): used

in bench-top machine rod coating

Fisherbrand standard polystyrene Petri dishes: used in cast coating

Xerox Digital Color Elite Silk Cover paper (80 lb, 8 in x 11.5 in, commercially

coated on both sides): used in bench-top hand-held rod coating

33

3.4 Coating Colour Preparation and Coating Application

3.4.1 Preparation of Model Coating Colours

Sixteen coating colours were formulated according to Table 3.3 by applying a

lab-scale batch technique6. Only one type of pigment was mixed with the latex binder in

each coating colour formula. The amount of each component was calculated as dry

quantities from mass balances, using the mass of dry pigment as a basis (e.g. 1 part per

hundred or 1 pph equals 1 g of dry component per 100 g of dry pigment). Since the raw

materials were received in a slurry form, various amounts of distilled water were added to

all coating colours to adjust their overall solid content to 60% by weight. No other

chemicals (such as pH buffer or dispersants) were used.

Table 3. 3 Coating Colour Formulas

Sample Code Pigment

Parts in Weight (pph) Binder

Parts in Weight (pph)

Solid Content

HC90-6 Hydrocarb

90 (GCC)

100 SB Latex

6

All adjusted to 60% w/w with distilled

water

HC90-10 10

HC90-18 18

HC90-25 25

HC60-6 Hydrocarb

60 (GCC)

100 SB Latex

6

HC60-10 10

HC60-18 18

HC60-25 25

CCHP-6 Covercarb

HP (GCC)

100 SB Latex

6

CCHP-10 10

CCHP-18 18

CCHP-25 25

CPDG-6

Capim DG (Kaolin) 100

SB Latex

6

CPDG-10 10

CPDG-18 18

CPDG-25 25

34

3.4.2 Coating Application Techniques

Three coating application techniques were used in this study for different

purposes as described below.

1) Bench-top machine rod coating

A standard procedure using an ENDUPAP-Universal Coating Machine

(developed by Centre Technique de lIndustrie des Papiers, Cartons et Celluloses de

Grenoble) was followed to coat the CADO polypropylene film. A #52 metering rod was

used and the coater was run at the speed of 1 m/min. After the coating was dried at 80

100C for several minutes, the polypropylene film was peeled off from the dry coating

layers. Since the thickness of the resulted coating layers was not desirable for thermal

diffusivity measurement (Section 3.5.3), these coating layers were not used in any

measurement or test thereafter.

2) Cast coating

Standard polystyrene Petri dishes were used as the cast for coating. Prior to the

addition of coating colour, a thin layer of silicone release oil was applied to the inner wall

of each Petri dish. Based on the desirable dry coating layer thickness, a consistent

calculated amount of coating colour was dozed into each Petri dish. All Petri dishes were

kept levelled and uncovered, and the coating colours were dried under ambient condition

over night. Coating layers with a thickness of 1.0 0.08 mm (coat weight: 1800 151

g/m2) were obtained, and used in the measurements of the bulk thermal properties

(Section 3.5).

35

3) Bench-top hand-held rod coating

For the evaluation of print quality resulted by the model coating (Section 3.6),

selected model coating colours were applied on the Xerox paper described in Section

3.3.3 using a hand-held #12 metering rod. The coated paper samples were dried under

ambient condition for 30 minutes. The coat weight was 20 0.7 g/m2.

3.5 Bulk Thermal Property Measuring Techniques

3.5.1 Specific Heat Capacity Measurement

The specific heat capacity of model coating layers was measured using a Q1000

differential scanning calorimeter (DSC) developed by TA Instruments, USA70.

As sample preparation, approximately 6 mg of coating layer was sealed in a

standard aluminum sample pan, whose mass was also measured. One empty aluminum

pan was used as a reference. The DSC was operated under the ramp mode for the

temperature range of 10 110C with a steady heating rate of 10C/min. By analyzing

the power dissipation required to raise the specimen to the set temperature, the heat

capacity of the specimen was determined. The specific heat capacity was then calculated

by dividing the heat capacity by the weight of the specimen.

Figure 3. 2 Q1000 Differential Scanning Calorimeter70

36

3.5.2 Thermal Diffusivity Measurement

The thermal diffusivity of model coating layers was measured using an LFA 447

NanoFlash Xenon flash apparatus developed by Netzsch Instrument Inc, Germany71.

Prior to the measurement, the coating layer samples were cut into discs with 0.5

inch (1.27 cm) diameter using a hole-punch. The thickness of the discs was measured

using an electronic micrometer calliper. The specimens were then coated with 3 5

sprays of liquid graphite on both top and bottom sides. The liquid graphite was dried at

room conditions for 3 5 minutes to form a uniform layer, so that the coating layers were

able to absorb a consistent amount of light energy emitted by the LFA instrument.

When a coating layer specimen was loaded into the furnace of the instrument, the

bottom side was heated by a short light pulse with pre-set power, and the temperature rise

on the top side of the specimen was measured by an infrared detector. By analyzing the

resulting temperature change with time, the thermal diffusivity of the specimen was

determined. For each specimen, the light pulse process was repeated 5 times. To study the

temperature effect on thermal diffusivity, for the HC60 coating layers, the thermal

diffusivity measurement process was conducted under 25C, 40C, 60C, and 80C.

Figure 3. 3 LFA 447 NanoFlash Xenon Flash Apparatus71

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3.5.3 Porosity and Apparent Density Measurements

Mercury intrusion porosimetry characterizes a samples porosity by measuring the

volume of liquid mercury intruded in the sample under increasing pressures. In this study,

the porosity of model coating layers was measured using an AutoPore IV 9500 mercury

intrusion porosimeter developed by Micromeritics Instrument Corporation, USA72.

As sample preparation, a known mass of a model coating layer was sealed in a

penetrometer (sample holder). The penetrometer was inserted into the low pressure port

of the porosimeter (exerting 0 25 psia of pressure) for air evacuation and initial mercury

intrusion, followed by the high pressure port (exerting 25 31000 psia of pressure) for

further mercury intrusion. The sample holder was standardized so that the exact volume

of liquid mercury intruded to the sample after every pressure increment was known.

Since the intrusion pressure is inversely proportional to the sample pore size, from the

cumulative intruded mercury volume vs. pressure data recorded by the instrument, the

cumulative pore volume distributions according to pore size, total pore volume, total pore

surface area, median pore diameter, and the skeletal and apparent densities of the coating

layer sample were determined.

Figure 3. 4 AutoPore IV 9500 Mercury Intrusion Porosimeter72 (On the Right: Sealed

Penetrometer, Enlarged)

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3.5.4 Calculation of Bulk Thermal Conductivity

After the specific heat capacity (Cp), thermal diffusivity (D), and apparent density

() of a coating layer sample were measured, the bulk thermal conductivity (K) of the

sample was calculated using the equation K = Cp D .

3.6 Surface Thermal Conductivity Measuring Techniques

3.6.1 Image Acquisition by Microthermal Analysis

Microthermal analysis (TA) was performed on the model coated papers to obtain

surface topography and relative thermal conductivity images of the coating layers. The

instrument consists of a TA 2990 analyzer incorporated with an ExplorerTM SPM

modular stage microscope. A V-shaped 5 m diameter platinum/rhodium thermal resistor

probe with a temperature coefficient of 0.00165/K, nominal resistance of 2.1 Ohms, and a

spring constant of 5 20 N/m was used as an active sensor for sample scanning and

measuring. Another thermal resistor probe with the same specifications was used as a

reference probe measuring changes of thermal properties in ambient air. The following

lists the operating conditions of the system used in this study:

Sample scanning area = 50 m x 50 m

Speed = 0.5 Hz

Resolution = 0.33 m/pixel 0.17 m/pixel

Sample temperature = 26 C

Probe temperature = 76 C

More details regarding the instrument and its operating principles can be found in Section

2.4.1.3.

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3.6.2 Image Processing and Analysis

The topography and thermal conductivity images of the coating layers were

processed using SPMLab 6.02, a software package provided in the TA system, and

Image J, a public domain image analysis program developed by Rasband and National

Institute of Health, USA73. The following are the processing procedures in sequence.

1) Elimination of Tilting Effect on Topography

Due to physical limitations on the mounting of specimens on the sample port in

the TA analyzer, the height values for topography sensed by the thermal resistor probe

could be biased by the tilt of specimens. This tilting effect was corrected by applying a

Levelling function using SPMLab 6.02, fitting a surface to the initially observed

topography and then subtracting the height values pixel by pixel of the fitted surface from

those of the initial image. In this study, a 1st order plane was used as the fitted surface.

2) Roughness Measurement on Topography

After levelling, the Area Analysis function in SPMLab 6.02 was used to obtain the

micro roughness of the coating layers. The micro roughness was calculated as the root

mean square (RMS) deviation from the mean surface on each topography image.

3) Grey Level Measurement

Both the topography and thermal conductivity images obtained from TA were

converted to 8-bit black and white (B&W) images using Image J. The mean grey level

and its standard deviation of each B&W image were obtained by the Histogram function.

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4) Deduction of Quantitative Information from Thermal Conductivity Images

Using the Histogram function, the maximum, minimum, and mean grey level of

each B&W thermal image were obtained. The range in grey level for each image was

calculated using the equation Grey level range = Max Min. A ratio of the mean grey

level to the range was calculated using Ratio = (Mean Min)/Grey level range. From

each original thermal conductivity image, the maximum and minimum electrical powers

shown on the scale were obtained. The range in power for each image was calculated

using Power range = Max Min. The relative mean power was then calculated using

Relative mean power = Power range x Ratio. The mean power for each thermal image

was then calculated using Mean power = Relative mean power + Min power.

Assumptions were made that the operating conditions for TA were kept constant

throughout the entire imaging process, so that the power requirements for imaging all

coating layers could be normalized on the same basis. Careful calibration of the

instrument and handling of samples were carried out in order to meet the requirement.

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3.7 Print Quality Evaluation

For the print quality evaluation, 6 selected model coating colours (HC90-10,

HC90-18, HC90-25, HC60-10, CCHP-10, and CPDG-10) were applied on Xerox Digital

Color Elite Silk Cover paper (Section 3.4.2). The dried coated paper samples were then

printed with a 100% solid black area of 8 inch x 10 inch by a Xerox Workcentre 7345

multifunction copier under the high resolution mode (with pre-set printing speed and

amount of toner used). In addition, a same solid black area was printed on original Xerox

paper (specifications listed in Section 3.3.3) and used as control.

3.7.1 Print Gloss Measurement

Print gloss of the coated paper samples was measured using a RhopointTM

Novo-GlossTM Statistical Glossmeter, developed by Rhopoint Instrumentation Ltd,

England. The instrument was calibrated against a standard black glass tile with gloss

units (GU) of 89.9, 93.8, and 99.0 for the measurement angel of 20, 60, and 75,

respectively. The samples used in the visual ranking test (see Section 3.7.3) were used in

this measurement to avoid inconsistency in print quality evaluation. The specular gloss of

the 100% black print on the samples was measured at 75 according to the TAPPI Test

Methods T 480.

Figure 3. 5 RhopointTM Novo-GlossTM Statistical Glossmeter

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3.7.2 Toner Adhesion Test

Toner adhesion on the coated paper samples was measured using an IGT AIC2

Printability Tester (Fi