METHOD FOR SURFACE TREATING CORRUGATED BASE PAPERS IN ORDER TO INCREASE STRENGTH The invention relates to a method for surface treating corrugated base papers with a surface treating agent in order to increase strength.

It is known that during the last years waste materials have been extensively recycled on many areas. Obviously, it has several advantages; namely, on one hand, it facilitates decreasing the further pollution of the overcharged environment, on the other hand, the secondary use of base materials and/or energy is economically advantageous.

This trend has emerged also in the paper industry; the use of waste paper instead of primary fibres (semi- cellulose, cellulose) is ever increasing. Several manufacturers use only waste paper in their production processes. However, certain properties of the paper products manufactured from waste papers, which are collected, processed (chemically and mechanically) and used several times, gradually deteriorate. This is especially true for the strength properties. In case of various products, e. g. corrugated base papers, the appropriate strength is of premium importance.

It is well known in the art that corrugated base papers are often treated in order to increase their strength.

It is also known that in the paper dryer section of the manufacturing process a sizing press is applied in modern machines used for producing corrugated base paper. The sizing presses can be excellently used for surface treating papers.

The paper strength is attempted to be increased by surface treatment, and, for the time being, starch is widespreadly used for the mentioned surface treatment. For this purpose oxidized or enzymatically decomposed starches of plant origin are used which can be applied on the paper surface with the sizing press in the dryer section. The use of starch increases the number hydrogen bridges between fibres which in turn increase the strength of corrugated base papers.

Though the starch used for surface treatment increases strength, it cannot impart enough rigidity (CMT, SCT, etc.) to corrugated base papers. Therefore, a surface treatment is needed which further increases the paper strength.

However, Hungarian patent No. 215 679 describes a composition for corrugated boards which can be applied on the paper surface with a sizing press and contains 50 to 99 parts per weight lignosulfonate (black liquor) from pulp mills and 1 to 50 parts per weight additives (based on dry weight). The mentioned additive contains a mixture of sodium chloride and carbamide or sodium chloride or carbamide. According to the patent mentioned above, the composition is especially useful for increasing the CMT value (the resistance (measured in N) of the corrugated base paper sheet against the maximal compression force acting perpendicularly to the plane of the sample, Hungarian standard MSZ 5664-77) and the rupture strength.

For brevity, this composition will be called LIQUWELL in the Description and Examples.

LIQUWELL is a remarkable improvement as compared with starch, however, the strength properties of paper should be preferably further improved in various uses.

Surprisingly, we have found that in a surface treating material, containing the appropriate amount and

concentration of starch and LIQUWELL, the two components interact and the strength and rigidity of the paper unexpectedly increase. Thus, the CMT value and SCT value (test of short span compressive strength, Hungarian standard MSZ ISO-150-9895) can be considerably increased and the rupture strength can be also improved-According to our observations, the starch strengthens the fibre-to-fibre connections and LIQUWELL imparts rigidity to the upper layers of the paper. This synergism could not be predicted by experts.

An other unexpected effect is that the solids content of the mixture applied with the sizing press in the dryer section of the paper machine can be considerably increased.

The increase of the solids content leads to significant savings in drying energy.

Thus, the invention relates to a method for surface treating corrugated base papers where a surface treating agent containing starch and a composition containing 50 to 99 parts per weight lignosulfonate from pulp mills and 1 to 50 parts per weight additives-the surface treating agent contains at least 5 parts weight of the mentioned composition-is applied on the product surface with a sizing press in the dryer section of the paper machine.

The embodiment of the invention is illustrated with the following examples, however, the invention is not limited to them.

The paper machine specifications are as follows in all examples: width: 2.6 m; velocity: 760 m/min.

Examples 1 and 2<BR> Produced paper: corrugated base paper of 105 g/m2 mass E.g. Surface treating Applied Solids Applied Evaporated Drying No. agent materi-al, content in water, water, energy, g/m2 working g/m2 t/hour GJ/t tank, % 1 Enzymatic starch 7 10 63 13.8 5.85 2 LIQWELL/starch, 7 20 28 7.45 4.62 20-80 % As it can be seen from the above examples, the same amount of surface treating agent<BR> according to the invention (Example 2) leads to double amount of applied solids and half<BR> amount of applied water to be removed when compared with (comparative) Example 1, where<BR> only starch is applied as surface treating agent. Therefore, in the embodiment according to<BR> the invention the amount of evaporated water is significantly less and, in turn, the method<BR> requires significantly less drying energy than those using only starch.

Examples 3 to 6 Produced paper: corrugated base paper of 105 g/m2 mass E. g. Surface treating Strength Applied Material Applied No. agent CMT30, material, density water, N 9/M2 in 9/m2 working tank, % tank, % 4 Starch/Liquwell, 200 20 32 4 Starch/Liquwell, 200 8 20 32 80-20 % 5 Starch 145 4.1 6 64.2 6 Starch/Liquwell 215 6. 25 15 35. 4 55-45 %

Examples 3 to 6 lead to the same conclusions as Examples 1 and 2, and it can be also observed that the CMT30 values have been highly improved in the embodiment according to the invention as compared with the surface treating agent containing only starch.