Furnish and papermaking effects

In the following, we discuss some general factors affecting bending stiffness.

Each pulp has its own potential for bending stiffness but paper machine conditions and converting treatments cause the final level to be lower than the full potential. The most important pulp property is bulk. The specific elastic modulus is less important. The effect of the pulp can be characterised by the following rules of thumb:

• Flexible fibres give dense paper and bending stiffness is low.
• Stiff fibres give bulky paper and bending stiffness is high.
• The stiffer the inter-fibre bonds, the better the bending stiffness of paper.

Chemical pulps usually have a better elastic modulus than mechanical pulps. However, mechanical pulp fibres are stiffer than chemical pulp fibres, thus giving the paper higher thickness (higher bulk). Because thickness is more important to bending stiffness than is elastic modulus, mechanical pulps have higher potential bending stiffness than chemical pulps. However, the situation changes if paper is calendered to a constant thickness (constant density). Then, the stronger inter-fibre bonding and higher elastic modulus given by chemical pulp produces a stiffer paper than mechanical pulp. Of the mechanical pulps, TMP has a greater potential for bending stiffness than PGW, because of its stiffer fibres.

Beating is a good example of the counteractive effects of bonding ability and fibre flexibility. Beating increases both of these and therefore the elastic modulus of paper increases strongly. However, since flexible fibres also decrease paper thickness, bending stiffness may not change at all.

It would be quite easy to choose a pulp that maximises bending stiffness, if that were the only requirement. In practice, bending stiffness has to be considered against other criteria, such as constant smoothness or constant gloss. In a study of TMP-based newsprint ¹ it was found that very good smoothness/stiffness combinations could be obtained by separate processing of fibre fractions. However, to make full use of this potential, lighter calendering than is possible with state-of-the art equipment would be required.

The forming section of a paper machine affects the z-directional variations of paper structure, particularly the fines and filler distributions. These distributions can also affect bending stiffness. For example, in one case where a Fourdrinier was replaced by a gap former, bending stiffness increased by about 10%, although the bulk remained constant ². The increase was caused by changes in either the layered fibre orientation or fines distribution.

The anisotropy of bending stiffness can be affected by altering fibre orientation, wet strains and drying stresses in a rather straightforward manner. In this case, the bending stiffness ratio changes in proportion to the elastic modulus ratio. For example, increased fibre orientation anisotropy increases bending stiffness in MD but decreases it in CD. Higher wet strain or drying tension increase MD stiffness but have little effect on the CD value. Increased drying shrinkage, on the other hand, decreases CD stiffness, particularly at the edges of the web. The importance of stiffness anisotropy depends on the end use of the paper or board. In printed matter, MD stiffness is usually critical, but if CD stiffness decreases too much, runnability problems may occur. In folding boxboard, smaller CD bending stiffness is the critical factor, and therefore a low degree of anisotropy is favoured.

In wet pressing, the bonding degree of paper increases but thickness decreases and therefore the bending stiffness decreases. The density distribution of paper in the z-direction can also be altered by wet pressing to improve bending stiffness ³. The main effect of calendering is a decrease in thickness when stiffness is considered. Therefore, bending stiffness decreases approximately in proportion to thickness squared (Eq. 1). Under certain conditions also the specific elastic modulus changes. Nevertheless, this happens only in extremely heavy machine calendering — where the specific elastic modulus decreases — or if the moisture content of paper exceeds 10%. In the latter case, the specific elastic modulus can increase by tens of per cent ⁴, which reduces the harmful effect of decreasing thickness. The use of heavy calendering is limited to special paper grades like glassine, because of other harmful effects such as a decrease in opacity.

Coating and surface sizing increase bending stiffness because bonded material is added on the surfaces. If, in addition, the elastic modulus on the surface layers is high, the effect is especially big. The elastic modulus of coatings varies in a wide range ^5-6, E = 3–25 GPa, so the bending stiffness of paper can be altered by modifying the coating colour. Although the range is large, the practical values are near the lower end. Although coating stiffens paper, the bending stiffness index of coated paper grades is generally lower than that of the corresponding uncoated grade. This comes from the high density of coatings, ρ = 1,300–2,000 kg/m³.

The additional thickness brought by the coating layer does not depend very much on the coating colour and it cannot be used to control stiffness. However, the elastic modulus of coatings is sensitive to the pigment and binder. The modulus is high for pigment particles that have a large shape factor and small void volume. Kaolin with its platelike particles usually has a higher modulus than calcium carbonate ^6-7, but the moduli of the two pigments vary a lot and can even overlap (see Table 1).

Table 1. Examples of elastic modulus of coating formulations. The binder was a mixture of SB latex and CMC and similar in all test point ⁵.

In binders, the highest elastic modulus is obtained with starch and latexes with a high glass transition temperature. However, these hard binders also have negative effects, like the poor brightness and print mottle caused by starch. Moreover, hard binders cause fold breaks in the coating layer ^8-9. Increasing the binder content decreases the elastic modulus if a soft latex is used, but increases it if a hard binder is used. Multilayer coatings can be designed to optimise multiple properties. For example, precoating would give stiffness and top coating would be selected for best interaction with ink.

Surface sizing does not increase the thickness of paper like coating, but it does increase bending stiffness because the size increases the elastic modulus of the fibre network. The effect of starch on elastic modulus is shown in Figure 1. In a study ¹⁰, the effect of surface sizing on the elastic modulus and bending stiffness was found to be linked with drying shrinkage. Although starch would increase the bonding of the fibre network and thus also the elastic modulus, starch also increases the shrinkage potential, and shrinkage in turn decreases the elastic modulus. A further effect is that shrinkage also makes the fibres crimp, which increases thickness. The net effect is an increase in bending stiffness even with a decrease in the elastic modulus. However, in the study starch was added as a 1% solution, which exaggerates the effect of water compared to normal surface sizing, where the consistency is at least 10 times greater.

The effect of surface sizing is strongest if the size penetrates only to the surface layers, in which case the starch content of the surface layers can be tens of per cent ¹¹. The penetration depends on the sizing equipment. A size press forces the size deep into the paper while, e.g., short dwell time application (SDTA) applies it only on the surfaces. SDTA can give more than 50% higher bending stiffness for the same amount of size ¹².

Figure 1. Net elastic modulus effect of starch plotted as a function of starch content in the cross and machine directions. Starch was applied as a 1% solution and drying shrinkage prevented. The highest starch contents correspond to the content in the surface layers of paper where starch is efficiently concentrated in the surface ¹³.