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

Diplomarbeit von Christian Rompf

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.

Table 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
4.	Summary	83
5.	Sources	84
6.	List of Figures	86

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, or chemically with the aid of chemical additives. The dewatering of the suspension is one of the most important factors, which affect filler distribution in the sheet. Whereas, the fibres in suspension are fixed at the top on the forming wire, the smaller fines that are not attached to the suspended fibres are transported through the mat. The fines are small enough to pass through the pores of the mat. Filler particles, which are fixed to the fibres by colloidal adhesion prior to the mat-forming process, are a good portion of the total pigments retained in the mat.

In the drainage process, each layer of fibres in the mat captures an amount of the free particles. The particles that are not captured in the top layers are partially captured by the lower layers of the mat. Therefore the retained particles are accumulated at a greater rate in the lower layers of the mat. The filler concentration is higher at the wire side of the sheet and steadily decreases toward the topside. There are other parameters which modify the distribution of fillers found in the finished sheet: For example, the concentration of fillers in sheets made on a Fourdrinier-wire has its maximum at or near the top side.

Filler distribution depends on machine speed, average filler content and filler retention. Increasing the machine speed enhances top side layers. Furthermore increasing filler content increases the amount of fillers in the top side layers. On the other hand, low filler retention reduces the filler content in the extreme top side layers, but has no effect on the distribution near the wire side.

Paper produced on a Fourdrinier-wire always has asymmetric distributions of fillers, since the drainage is 100% through the bottom surface. To improve the symmetry of the filler distribution one can use a hybrid former taking 20 – 30% of the drainage through the top surface. However, there will still be a lower filler contents on the surface than in the centre.

Measuring filler content and distribution:

Measurement methods to characterize the content of filler particles in paper have been available for a long time. As to the distribution of fillers in the structure of the paper, the most common characterization technique is based on ashing a sheet that has been separated into layers. Here, paper is split into layers using either adhesive tape or the Beloit sheet splitter. Subsequently the individual splits are ashed at 900°C. The ash content is converted to filler content, neglecting the contribution of fibres and other additives. The data can be used to draw graphics of filler content (or ash weight) in the thickness direction of the sample.

Another method to measure fillers in the z-direction is to use a combination of an X-ray analysis (Energy Dispersing X-Ray Analysis, EDXRA) and a Scanning Electron Microscopy (SEM). Large structures such as fibres or, when higher magnifications are used, smaller ones such as clay particles clinging to the fibres, can be imaged by SEM. Micrographs of the cross-sectional paper structure are acquired using a Scanning Electron Microscope. The acquired grey-scale image is segmented into three binary images showing fibres, fibres and fillers and only fillers. The cross section is divided into a certain number of layers. Within each layer, the cross sectional area fraction of fillers is assessed using an image analysis.