The local distribution of fillers in the z-direction of paper

Rompf, Christian

The local distribution of fillers in the z-direction of paper

Lokale Füllstoffverteilung in z-Richtung bei Papier

von Christian Rompf (Autor:in)

Ingenieurwissenschaften - Allgemeines

Zusammenfassung

Inhaltsangabe:Introduction:
It is important to determine the distribution fillers in thickness direction of paper. However, the techniques that are available are limited in precision and accuracy. This thesis describes a new method for the determination of fillers in the z-direction of paper. The method is based on a splitting technique that offers a good reproducibility and provides good layer uniformity. The new method is used to estimate the effects of different factors of influences affecting the fillers distribution.
During sheet forming a large amount of water is removed. This dewatering process creates an irregular distribution of fillers across the thickness of the paper. Therefore, the description of the distribution of fillers in the thickness direction is critical to optimize paper properties. The techniques currently available for this kind of characterization have limitations in terms of repeatability, resolution or precision. Another limitation to currently available techniques is the lack of a good sampling area. Considering the large effect of fillers on many paper properties, the ability to measure and thus control the filler distribution in the z-direction helps to control sheet structure, reduce two-sidedness and improve paper properties such as internal bond. Inhaltsverzeichnis:Table of Contents:
Acknowledgements2
Abstract3
Index of contents4
1.Introduction6
1.1Paper structure7
1.1.1Three dimensional network8
1.1.2Formation11
1.2Fillers12
1.2.1Effects of filler loading on the properties of paper12
1.2.2The main fillers:14
1.3Retention and dewatering16
1.3.1First-pass retention and true retention17
1.3.2Agglomeration and flocculation20
1.3.3Retention fundamentals:23
1.3.4Filler distribution24
1.4Measuring filler content and distribution25
2.Materials and methods28
2.1.1Measuring filler distribution28
2.1.2The procedure of splitting33
2.1.3Acquisition of the images35
3.Results and Discussion37
3.1Settings of the scanner38
3.2Different backgrounds40
3.3Repeatability of the method42
3.4Influence of the position of filler addition43
3.5SC Paper with 42,8 % GCC53
3.6TMP - Paper 20% GCC57
3.7Fine paper with 20 % and 30 % of GCC63
3.8Different positions of adding GCC73
4.Summary83
5.Sources84
6.List of Figures86 Textprobe:Text Sample:
Chapter 1.3.4, Filler distribution:
The distribution of filler material in the paper depends on the forming process. Fillers can be entrapped mechanically into the web, […]

Leseprobe

Inhaltsverzeichnis

Christian Rompf

The local distribution of fillers in the z-direction of paper

Lokale Füllstoffverteilung in z-Richtung bei Papier

ISBN: 978-3-8366-4446-4

Herstellung: Diplomica® Verlag GmbH, Hamburg, 2010

Zugl. Fachhochschule München, München, Deutschland, Diplomarbeit, 2009

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http://www.diplomica.de, Hamburg 2010

Christian Rompf

Acknowledgements

Acknowledgements

My first gratitude goes to my supervisor Marco Lucisano for giving me the opportunity to

work on this interesting project at Innventia and helping me during all my experiences in

Stockholm. The insightful discussions and the possibility to come to Marco with my questions

whenever I had one, let me learn a lot during my time at Innventia. Thanks are due to Prof.

Kleemann, who supervised my thesis and gave me the opportunity to write it in Sweden.

I wish to thank Mikael Bouveng for the technical support, the daily conversations and the

great sense of humour. I would like to also thank the other interns who have been working at

Innventia while I was there for the nice times during lunch or "fika" and to Markus Rehberger

for the daily "Servus".

Thanks to all the nice people that I met in Sweden, who made my stay in Stockholm

memorable, especially Jeff.

I dedicate this thesis to my family, Cecilia, Marc-David, Irmgard and Helmut, without whose

support over all the years, all of this would not have been possible.

Christian Rompf

Abstract

Abstract

Fillers are used in nearly all paper productions. They are important to improve the production

economy and surface properties of the paper. Therefore, measuring the distribution of fillers

in the sheet is important. There are some available measurement techniques to measure the

filler distribution in thickness direction, but they are limited in precision and accuracy.

Traditional methods often need expensive equipment and are limited to small samples.

In this thesis we used and improved a new technique to produce images of different layers of

paper in the z-direction. The burn-out technique successfully works for the acquisition of very

good images in plane. Those images are a foundation for further research on the distribution

of fillers and their behaviour in the inside of a paper sheet. The applied method is based on a

modified version of the burn-out method, traditionally used to characterize coating layers.

Analysing the images, one could notices that, on the two external surfaces fibres sometimes

seemed to hide fillers, which become more evident in the inner layers. Furthermore, the filler

distribution in most trials changes in z-direction with a slightly lower amount of fillers in the

inner layers. The position of fillers addition to the paper making process had an effect on the

way the fillers agglomerate and as a result affects the strength of the paper. It was also

possible to see wiremarkings on the fillers with the same geometrical features of the wire in

filler distribution.

Nearly 500 images have been generated during the thesis forming an extensive basis to see

how fillers are distributed in the z-direction of papers.

Christian Rompf

Index of contents

Index of contents

ACKNOWLEDGEMENTS... 2

ABSTRACT ... 3

INDEX OF CONTENTS ... 4

1 INTRODUCTION... 6

1.1 Paper structure ... 7

1.1.1

Three dimensional network...8

1.1.2

Formation ...11

1.2 Fillers... 12

1.2.1

Effects of filler loading on the properties of paper...12

1.2.2

The main fillers :...14

1.3 Retention and dewatering... 16

1.3.1

First-pass retention and true retention ...17

1.3.2

Agglomeration and flocculation ...20

1.3.3

Retention fundamentals:...23

1.3.4

Filler distribution ...24

1.4 Measuring filler content and distribution ... 25

2 MATERIALS AND METHODS ... 28

2.1.1

Measuring filler distribution ...28

2.1.2

The procedure of splitting ...33

2.1.3

Acquisition of the images...35

3 RESULTS AND DISCUSSION ... 37

3.1 Settings of the scanner... 38

3.2 Different backgrounds... 40

3.3 Repeatability of the method... 42

3.4 Influence of the position of filler addition... 43

3.5 SC Paper with 42,8 % GCC... 53

3.6 TMP Paper 20% GCC ... 57

3.7 Fine paper with 20 % and 30 % of GCC... 63

3.8 Different positions of adding GCC ... 73

Christian Rompf

Index of contents

4 SUMMARY... 83

5 SOURCES ... 84

6 LIST OF FIGURES ... 86

Christian Rompf

Introduction

Introduction

It is important to determine the distribution fillers in thickness direction of paper. However,

the techniques that are available are limited in precision and accuracy. This thesis describes a

new method for the determination of fillers in the z-direction of paper. The method is based

on a splitting technique that offers a good reproducibility and provides good layer uniformity.

The new method is used to estimate the effects of different factors of influences affecting the

fillers distribution.

During sheet forming a large amount of water is removed. This dewatering process creates an

irregular distribution of fillers across the thickness of the paper. Therefore, the description of

the distribution of fillers in the thickness direction is critical to optimize paper properties. The

techniques currently available for this kind of characterization have limitations in terms of

repeatability, resolution or precision. Another limitation to currently available techniques is

the lack of a good sampling area. Considering the large effect of fillers on many paper

properties, the ability to measure and thus control the filler distribution in the z-direction helps

to control sheet structure, reduce two-sidedness and improve paper properties such as internal

bond.

Christian Rompf

Introduction

1.1

Paper structure

Paper is a material defined by its production process. The British Paper and Board Industry

Federation has given a definition in 1978:

"Paper is a sheet or continuous web of material formed by the deposition of vegetables,

mineral, animal or synthetic fibres or their mixture with or without the addition of other

substances, from suspension in a liquid, vapour or gas, in such a way that the fibres are

intermeshed and bonded together." [1]

Fig. 1: Paper Structure [2].

Paper consists of fibres, fillers like CaCO

, polymers and other components that are arranged

in the form of a sheet. Most of the physical properties of paper are influenced by the structure

of the paper. For example, paper strength, depends on the degree to which fibres are

connected to each other and on the strength of the connecting bonds.

The connectivity or the degree of inter-fibre bonding has an influence on the mechanical

properties of paper. As the degree of bonding increases, the strength of paper increases. For

example, bathroom tissue paper has for example, a relatively low degree of bonding, whereas

packaging-papers have a relatively high degree of bonding and therefore have better strength

properties.

[2]

Christian Rompf

Introduction

Furthermore there is an important difference between fibre sources originating from

hardwood and softwood. The softwood fibre is almost two to three times longer than

hardwood fibre.

Fig. 2: Fibre Length [2].

1.1.1

Three dimensional network

The typical thickness of paper is 0.1 mm. The fibres are much longer than the thickness of the

paper and therefore the network they form may be understood as a two-dimensional structure

of paper as the network those fibres form is more or less planar. The two-dimensional

structure explains many paper properties, but the three-dimensional porous structure is also

important. The thickness-directional open space, which is ignored in the two-dimensional

simplification, gives paper its structure and determines how fluids are transported through the

sheet.

Christian Rompf

Introduction

Furthermore, the drainage may proceed according to either of two mechanisms: filtration or

thickening. [2]

Fig. 3: Filtration and thickening [2].

Filtration takes place when the suspended fibres are mobile or free to move independently.

Thus, in filtration a sharply defined boundary develops between the concentrated mat

deposited on the wire and the dilute suspension approaching it. The fibres are laid down

individually from a dilute fibre suspension. This forms a strongly layered sheet structure with

rather poor strength in z-direction.

Thickening happens when the fibres in the suspension are immobilized to form a coherent

network. There is no sharp boundary dividing the mat from the undrained suspension, and the

thickening mat increases steadily from the top to the wire side of the mat. In this way, the

thickening fibre network behaves like the mat deposited in filtration. The structure of the

resulting sheet will be more three-dimensional since the fibres are more entangled in the z-

direction. This makes the paper stronger the in z-direction, but also provides some losses in

the plane of the sheet. [3, 4]

Papermaking fibres are one to two orders of magnitude longer than a typical paper sheet

thickness. Due to the fibre dimensions, most of the fibre must be aligned in the plane of the

paper sheet. The adjustment of fibres in the thickness-direction can be layered or felted.

Christian Rompf

Introduction

Fig. 4: Layered and felted structure [2].

A layered network forms if the fibres land on the wire one after another. Fibres, therefore,

form a regular sequence in the vertical direction. A felted sheet structure forms at high

consistency or under pulsating drainage. The pressure pulse moves fibres back and forth so

that they cannot land on the wire freely. At high consistency, three-dimensional fibre

aggregates or flocs can be formed, that are then pressed in the planar sheet. Gap formers, like

the one that is used on the pilot paper machine at Innventia, lead to a layered paper structure.

A felted sheet structure should give better strength in the z-direction than a layered structure

as fibres are stronger than bonds. A fully layered paper can be split without breaking the

fibres. [2]

Furthermore, the thickness of the fibre is at least one order of magnitude smaller than the

thickness of paper. Therefore it is important to look at the structure in z-direction.

The conceptual description of the three-dimensional structure is far less developed than the

simplified two-dimensional structure. One complicating factor is the low thickness of paper,

which may mislead someone

to argue that a paper sheet consists of just two surface layers and

a few bulk layers in between. [2]

Christian Rompf

Introduction

1.1.2

Formation

Besides fibres, paper consists of fibre fragments, minerals, fillers and chemical additives.

They are all settled stochastically in the web. Even though not completely random, the

structure of paper is for many purposes nonuniform and irregular. Paper formation is the

nonuniform distribution of particles within the paper. It may also be understood as the

variability of the grammage of the paper. The uneven structure of paper can easily be

observed looking at a paper sheet held against a light source.

The grammage variability partly depends on the coincidence of single fibre deposition and

partly on fibre interactions and flocculation in the actual web forming process. Flocculation,

for example, increases the variability of grammage and so causes poor formation. The

hydrodynamics of the fibre suspension can help to decrease flocculation by breaking the flocs.

The uneven grammage distribution has an effect on many properties of paper.

On a paper machine there is often a relationship between formation and retention of fine

particles. Improvements of the formation usually lead to a decrease in retention.

Christian Rompf

Introduction

1.2

Fillers

Nowadays it is impossible to think about papermaking without additives like fillers. They are

used to improve the properties of the final product and to provide an economical production

process. Most graphical papers contain fillers as they fill out spaces between fibres,

smoothing the surface and improving properties such as evenness of formation, printability

and opacity. In addition, most fillers are cheaper than fibres and therefore reduce

manufacturing costs. However, fillers also have an undesirable effect on paper properties

which is related to the loss of bonding between fibres. This can result in a lower tensile

strength and stiffness and lintning in printing. [2]

1.2.1

Effects of filler loading on the properties of paper

The main effects of using fillers are:

Improvement of texture:

If a web consists only of fibres, it will have an uneven surface. Fillers can be used to improve

the surface topography and reduce the unevenness of the surface. The small filler particles fit

well between the long fibres and can place themselves within the fibre-network. The result of

using fillers is a more attractive appearance of the paper.

Improvement of print quality:

A good surface smoothness is required for good printing results on paper or board. For

example, missing dots occur due to lack of contact between the paper and the printing plate

and distort the printed picture.

Moreover, the printing ink must quickly be immobilized to prevent smudge deposits on the

next sheet. Fillers have a much better wettability than fibres and accept the printing ink better.

A system of finer capillaries is created by the addition of fillers which is essential for a

uniform ink acceptance.

Opacity:

Fillers are added to increase the opacity of the paper.

Christian Rompf

Introduction

Brightness:

Fillers are often added to papers to improve their brightness or whiteness. Papers of higher

whiteness appear more attractive.

Special physical properties:

In filter papers fillers are used to adjust the pore size according to the specifications for the

filtration rate and the degree of retention of the suspended material which should not pass

through the filter paper. Another example is cigarette paper, where fillers are used to control

the porosity and burning rate. As a general rule, filler addition reduces stiffness of paper and

increases its flexibility.

Negative effects of fillers

A high amount of fillers can result in limpness. Due to the reduction of the fibre-to-fibre

bonds, fillers increase the softness of the paper and lower its physical strength. [5]

The main fillers used in the paper industry are presented in Figure 6 and their main properties

are listed below.

Christian Rompf

Introduction

1.2.2

The main fillers :

Fig. 5: Amount of fillers in the Paper Industry [6].

Clay (or kaolin):

- Hydrated SiO

and A

- It is a cheap filler.

- Mined from natural deposits.

- Used in magazine and book paper.

- 40 90 % of clay particles are less than 2µm.

Titanium dioxide:

- It is an expensive filler.

- High brightness.

- Used frequently in North America and for high-brightness applications in Europe.

Talc (hydrated magnesium silicate):

- Mainly used in Europe.

- It consists of hydrated magnesium silicate.

- It is an expensive filler.

- high brightness and low abrasion.

[6]

Christian Rompf

Introduction

Calcium carbonate (chalk or limestone):

Calcium carbonate is one of the most important fillers. It exists in two general forms; the

natural product made by grinding stone is called ground calcium carbonate (GCC) and the

synthetic product, precipitated calcium carbonate (PCC).

Fig. 6: GCC [3].

Fig. 7: PCC [3].

GCC is common in Europe and it is used mainly in alkaline papermaking. Ground calcium

carbonate fillers disrupt fibre-to-fibre bonds within the sheet of paper less than many other

types of fillers. The reason for that is their low specific surface area. The low surface and the

absence of internal porosity in GCC result in little or no alkaline size reversion over time.

Besides that, the absence of internal porosity of GCC also improves the sheet drainage

compared to the one of PCC with a higher internal porosity.

If high sheet strength and low

sheet porosity are required, GCC is often the pigment of choice.

[7]

Christian Rompf

Introduction

1.3

Retention and dewatering

Paper is a complex material consisting of biological, synthetic and inorganic materials. A

proper retention of the individual components is critical to the properties of the paper as well

as with regards to minimizing costs.

Retention describes the amount of a material in the final product relative to the amount at

some earlier stage of the production process. [7

]

Fig. 8: Schematic of the short circulation and wire section of a paper machine at steady-state conditions and

definition of retention. Qx and cx are the volumetric flow [m3/s] and concentration [kg/m3] at position x.

Christian Rompf

Introduction

1.3.1

First-pass retention and true retention

First-pass retention

Several definitions of first-pass retention (FPR) can be found in literature. A popular one is

provided by the Papermaking Additives Committee of TAPPI where the defined first-pass

retention is defined as the headbox consistency minus the consistency in the tray collecting

the white water, divided by the headbox consistency:

100

)

(

FPR

[kg/m

]

is the headbox consistency and C

[kg/m

]

is the tray consistency, as illustrated in

Figure 9.

The first-pass retention can be described as a snapshot of what is happening in the forming

section of the paper machine. The value one gets out of the equation is an instantaneous

measurement of total solids retained and is used as a daily check of furnish and chemistry in a

white water loop. As mentioned, the value is only a snapshot and is subjected to considerable

error because all the liquid that is drained from the furnish to make the paper sheet, is not

collected in the trays in the forming zone. The accuracy of the result is very dependent on

how representative the tray water is. Nevertheless, first-pass retention provides an estimate

trend and can be useful to the operator of the paper machine. The first-pass retention of the

fillers depends on:

The size of the particles that are to be retained.

The size of the pores in the web and the geometry of the forming fabric.

The basis weight of the paper that is being manufactured.

The nature of the dewatering process during forming.

Christian Rompf

Introduction

True retention

True or over-all retention is related to the efficiency of the total papermaking system. A

definition suggested by TAPPI is, that true retention (R

) is the ratio of the tonnage conveyed

to the press to the tonnage discharged by the headbox. The measurements should be carried

out, when the machine is in an equilibrium state.

and c

are the volumetric flow [m

/s] and concentration [kg/m

] at position x, as defined in

Figure 9.

Comprehensive retention

The comprehensive retention provides an in-depth analysis of retention for a specific

production case. It requires detailed knowledge of the fibre fractions and consistency of both

the headbox and tray water. Comprehensive retention is defined as:

100

)

(

)

(

)

(

is the fibre fractions and c

and concentration [kg/m

] at position x, as defined in Figure 9.

The measurement of the comprehensive retention provides a more precisely measure of

retention than the methods usually used in paper mills. However, it is often too time-

consuming to be used routinely. As a result, determining the first-pass retention is quicker

than determining the comprehensive retention and can provide useful comparative

information to the papermakers. [7]

It can also be said that the `retention' means the ability of the system to retain the substances

within the system limits.

Particles are considered to be in one of two states, free or bound. A free particle exists in

suspension and is not associated with the retention medium on the opposite. On the other

hand, a bound particle is associated with the fibre and is unable to move freely in suspension.

Christian Rompf

Introduction

At any particular instant of time, those particles which are bound are considered to be

retained. In this way, retention can be defined as the process by which free particles are

converted to bound particles.

Furthermore, the retention of a component is influenced by most of the variables on the paper

machine, for instance the stock composition, the paper chemistry and the construction of the

forming section.

The fibre mat that is formed within the first few meters on the wire significantly affects he

retention. Therefore it is important to have a suitable design and arrangement of the

dewatering elements. The shorter the fibres and the higher the fines and filler content the

more difficult is it to achieve a good retention on the wire. A low grammage of the paper also

lowers the retention, as the filtrating layer is much thinner.

There are two basic retention mechanisms: the mechanical one, which is filtration or

mechanical retention (retention during sheet forming of those particles which are bigger than

the holes in the wire or the pores in paper web) and the chemical one, which is agglomeration

or flocculation (interaction between the boundary surfaces of the particles to form flocs)

Mechanical retention

The sheet forming process during papermaking can be regarded as a filtration process.

A three-dimensional layered network is formed on the wire by the fibres that are retained on

the wire. The sheet forming begins with the wire capturing the large fibres followed by those

fibres capturing the smaller fibres as the pores sizes decrease. After this process, the shape of

the wire can be found on the paper, and is identified as wire marking. Wire markings are a

negative copy of the wire on the structure of the paper sheet.

Details

Seiten
Erscheinungsform: Originalausgabe
Jahr: 2009
ISBN (eBook): 9783836644464
Dateigröße: 15.6 MB
Sprache: Englisch
Institution / Hochschule: Hochschule für angewandte Wissenschaften München – Verfahrenstechnik, Papiertechnik
Erscheinungsdatum: 2014 (April)
Note: 1,0
Schlagworte: füllstoffverteilung burn-out-technik papiertechnik innventia mikroskopie Filler

Autor

Christian Rompf (Autor:in)