Södra Research Shows Progress with tailor-Made Softwood Pulps for Premium Tissue

Through a more refined selection of wood, in conjunction with modified cooking processes, softwood pulp can be tailored to optimise the properties that are most desirable for tissue making.

Anne Kristin Holen and Björn Dillner, Södra Cell Research and Development, Tofte, Norway

Bleached chemical softwood pulp is a main component in most high quality tissue products. The principal function of this pulp is to give the reinforcement strength necessary both for production on a high speed machine as well as strength of the tissue product itself. This strength must be achieved without sacrificing other important quality attributes like softness, bulk or water absorption capacity. A mixed pulp slurry consists of billions of fibres. Each fibre has individual characteristics, made by nature in different shapes and dimensions. This variation could be a problem if the target is to make the same products from all fibres.

Utilising these differences, however, gives the pulp producer a possibility to make pulps with fibres that are suitable for a specific end use. The most obvious example of this is the use of hardwood and softwood pulps. The differences in properties between them are well known and the pulps have a clearly different range of applications.

There are also variations within softwoods, depending on the species, the place of growth and the age of the tree. Through knowledge of the raw material, these differences can be utilised by sorting the wood and producing pulps with different profiles.

Bonding depends on fibre flexibility.

The strength of the fibre network is of fundamental importance for the strength of the paper web. In products like tissue it is the bonding strength between the fibres, rather than the strength of the individual fibre, that is most important. The bonds in the fibre web are hydrogen bonds between the surfaces of the fibres. Every point on the surface of a bleached chemical fibre can make a bond if it is in contact with another fibre. Because of this, it is the fibre collapsibility and flexibility that is determining the potential of the fibre to make bonds, since the fibres have to be flexible to intertwine.

To imagine this, think of the fibres as straws of spaghetti. Before swelling (cooking in boiling water), the straws are inflexible, and even if they were soaked in cold water they would not bond well to each other. However, after cooking the straws are flexible and will stick together if you do not stir them. A simplified illustration of the differences information of a fibre web from stiff and flexible fibres is shown in Figure 1. Note the much higher contact area with the flexible fibres.

FIBRE FLEXIBILITY IS TO A LARGE DEGREE DEPENDENT ON THE FIBRE WALL THICKNESS, since slender fibres have greater possibility to intertwine. However, the fibre flexibility might also be improved by changing the chemical composition of the fibre wall making the structure more able to swell in water. This is illustrated in Figure 2. In practice, the tensile strength is the most commonly used property to evaluate the combined effect of fibre flexibility and bonding capacity. To get a pulp with a high unbeaten tensile strength is often the optimal choice for the tissue producer. Probably this is a good approach, even though the pulp normally will be slightly beaten before production of the tissue, since these pulps have the highest fibre flexibility. However, other characteristics like the bonding area or the stiffness of the fibres could give increased knowledge about their behaviour on the paper machine.

ONE FACTOR THAT HAS AN INFLUENCE ON THE TENSILE STRENGTH IS THE SHAPE OF THE FIBRE. Kinks and curls along the fibre decrease the tensile strength but do not have any impact on the flexibility. Since the tissue paper is curled in the creping process, the influence of fibre curl on the properties of the final tissue is not clear.

Every softwood fibre could give a satisfactorily high tensile strength for all kinds of tissue products if it is exposed to enough beating. However, there are trade offs, as extended beating costs energy, and other quality parameters like softness and dewatering capacity will suffer. The ideal fibre for tissue thus is a flexible one, with good potential for making fibre-fibre bonds with a minimum of mechanical treatment.

Another fibre property of importance is the fibre charge. Charge on the fibre surface is important for the retention of wet strength agents and affects the chemistry on the paper machine. There are significant differences in charge between different softwood pulps, depending mainly on the production process. They are, however, small compared to the differences between softwood and hardwood or chemical pulps and CTMP.

Wood origin and age play an important role.

It is well known that different wood species have fibres with different dimensions, but there are also large variations between fibres from the same species depending on the origin and the age of the wood. Figure 3 illustrates the development of tensile strength by beating in a PFI-mill for laboratory pulps from six Norwegian softwood selections. These vary by species, age and origin. Clearly beating increases tensile strength. It can be seen that spruce pulps in general have faster development of tensile strength than pine pulps, but there are also significant differences between old and young wood from the same species. The fibre dimensions of these pulps can explain these differences. Scandinavian spruce has, in general, longer and more slender fibres than pine, and young wood has shorter and thinner fibres than older fibres. From Figure 4 it can be concluded that the pulps with thick-walled fibres have the lowest unbeaten tensile index.

AT SÖDRA, THIS KNOWLEDGE IS BEING UTILIZED IN THE PRODUCTION OF INDUSTRIAL MARKET PULPS. At our Tofte mill the wood is sorted to produce pulps especially designed for different products. Wood from young spruce with slender fibres is used for tissue pulp, while the more coarse spruce fibres have good properties as reinforcement pulp in printing papers.

A third quality, which is a mix of spruce and pine, is used in products that need bulk and stiffness. Coarseness, fibre length and unbeaten tensile index for these qualities are shown in Figure 5 (see page 60).

THE TISSUE PULP FROM TOFTE HAS BEEN WELL ACCEPTED AMONG THE TISSUE PRODUCERS AND THE MARKET for this kind of pulp is increasing. To meet the demand, our Varo mill has developed a similar pulp for tissue through systematic development of raw material supply and the production process. The result from these changes has been an increase in the unbeaten tensile of more than 10% as illustrated in Figure 6.

Modified cooking shows promise for more improvement.

Research at Södra has shown that modification of the cooking process can also improve the suitability of these pulps for tissue grades. While these methods are not yet being used on an industrial scale, they will most probably be introduced at the mills in the near future. The most promising research is focused on retention of hemicelluloses to improve fibre flexibility.

Fibre wall thickness is the most important property that influences the fibre flexibility, but the chemical composition of the fibre wall also plays a role. The fibre wall of a bleached chemical fibre consists of two groups of complex carbohydrates: cellulose and hemicellulose. The cellulose is based on partly-crystalline microfibrils and is the reinforcement material giving the fibre strength, while the hemicellulose, which embeds the microfibrils is the softener. Hemicellulose is amorphous and swells when the fibre is immersed in water. A higher level of cellulose in the fibre wall improves the fibre strength, but not the flexibility or bonding properties. To improve these important properties, it is the hemicellulose content which plays a key role. A higher level of hemicellulose in the pulp should make the pulp more flexible and lead to better bonding.

One method for increasing the carbohydrate yield is to use additives in the cooking process. Addition of anthraquinone (AQ) during digesting accelerates the delignification and increases the yield of carbohydrates. The use of polysulphide (PS) protects the emicelluloses and increases the hemicellulose yield relatively to cellulose. The addition of PS and AQ together has a synergistic effect on the carbohydrate yield from the cook. The influence of addition of PS and AQ on the tensile strength of laboratory pine pulps is shown in Figure 7, with the modified pulps showing better tensile strength than the reference pulps.

IN SUMMARY, IT CAN BE STATED THAT OUR UNDERSTANDING OF THE INFLUENCE OF FIBRE CHARACTERISTICS ON TISSUE QUALITY IS INCREASING ALL THE TIME. Through this increased knowledge about fibre properties and critical quality attributes for different paper grades, the pulp producer is able to produce a pulp with properties more specifically designed for the final product. In a pulp suitable for tissue, the fibres should be flexible and give a high bonding strength. This target can be reached through choice of the right raw material and further modification of the pulping process.

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