The present invention relates to a method for the manufacture of fluting or test liner, as well as a product manufactured by the method.

Fluting is a packaging board which constitutes a fluted layer for corrugated board. Test liner is a kraft-faced liner made of recycled paper, which is used e.g. as a top liner for corrugated board. These packaging boards are traditionally manufactured by surface sizing, without calendering. One problem in using recycled fibers is the loss of strength in comparison with the use of virgin fibers.

It is an objective of the present invention to provide an improved method for the manufacture of fluting or test liner, which enables a substantial gain in strength properties with considerable savings in raw material as less grammage is required for reaching a given strength level.

In order to accomplish this objective, a method of the invention is characterized in that the method comprises treating a fibrous web in at least one process step with a metal belt calender, comprising a metal belt adapted to circle around at least one guide element, outside said belt being provided at least one counter-element establishing a contact surface with the belt, such that the belt and the counter- element establish therebetween a web treatment zone of adjustable length, the web to be treated being passed therethrough, that the method comprises setting a web dwell time in the treatment zone at about 5-200 ms, that the metal belt and/or the counter-element has its temperature set at about 20-400 ⁰ C, and that a contact pressure applied to the web within the treatment zone is regulated over the range of about 0.01 MPa to about 200 MPa.

The term contact pressure refers to a sum of pressure effects applied to a web in the treatment zone between a belt and a counter-element, resulting from belt tension and/or a compression load exerted by possible press elements inside the belt. Setting the contact pressure for a given pressure value or range of pressure values is effected by selecting a suitable belt material, allowing the use of a desired tension, and, if necessary, suitable press elements capable of increasing pressure with respect to a pressure achieved by a belt alone. It should be noted that,

depending on the assembly constituted by belt and counter-elements as well as by possible press elements, it will be possible to cover either a portion of the contact pressure regulating range, in which case the transition to another pressure value or range of pressure values is effected by replacing elements of the assembly as necessary, or, with an appropriate assembly, to cover the entire contact pressure regulating range of about 0.01 MPa to about 70 MPa or even about 0.01 MPa to about 200 MPa. For example, the compression achieved by belt tension alone is not much with respect to what is achieved by press elements, whereby, in solutions implemented without press elements, the regulating range lies closer to the lower limit, e.g. within the range of about 0.01 MPa to about 5 MPa. With the use of press elements, the regulating range can be e.g. from about 5 MPa to about 70 MPa, preferably from about 7 MPa to about 50 MPa or e.g. from about 70 MPa to about 200 MPa.

The inventive method, which makes use of metal belt calendering for enhancing the strength of fluting or test liner, also enables the manufacturing of these board grades even without surface sizing, which has been traditionally used for increased strength.

Typical values for flutings are as follows:

CMT test (Concora Medium Test)

CMT test expresses the strength of fluting. In the test a sample is corrugated between heated corrugating rolls, whereafter the sample is secured and held together with a tape for keeping the flutes properly spaced from each other. This is to imitate single-sided corrugated board. Breaking load is applied perpendicularly to

the plane of paper and the breaking force (N) is measured. CMT value is the maximum force the sample can withstand before it breaks.

SCT test (Short Span Compression Tesfl

SCT is a compression strength property measured both in machine direction (MD) and in cross-direction (CD). A specimen of 15 mm in width is secured to two clamps 0,7 mm apart from each other. The clamps are forced towards each other, the length being reduced and the stress in the specimen increased until failure occurs in the specimen. A measurement is effected in either direction and the measuring result is expressed in units kN/m.

The invention will now be described in more detail with reference to the accompanying drawing, in which:

Hg. 1 shows in a schematic side view one metal belt calender suitable for implementing a method of the invention, and

Hg. 2 shows test results for the effects of an additional load on strength at various thermo roll temperatures.

Hg. 1 shows one metal belt calender suitable for implementing a method of the invention, comprising a metal-fabricated calendering belt 2 circling around guide rolls 3, at least some of said guide rolls being displaceable for adjusting the belt 2 for a desired tension. The calendering belt 2 travels around a roll 5 disposed outside the belt loop for establishing a calendering zone between the belt 2 and the roll 5. A material web W to be calendered proceeds through the calendering zone, being exposed to a desired pressure impulse and thermal effect as a function of time. In fig. 1, a dash-dot line 9 is used to represent the shape of a pressure impulse when the calendering belt loop 2 has disposed thereinside a nip roll 4, which functions as a press element and presses the belt against the roll 5, thus establishing a higher pressure treatment area within the calendering zone. On the other hand, a dash-dot line 8 is used to represent the shape of a pressure impulse when the contact pressure acting within the calendering zone is only established by a tension of the belt 2, the roll 4 being disengaged from a pressing contact with the belt 2 (or with

no roll 4 at all fitted inside the belt loop 2). The roll 5, as well as the roll 4, may or may not be a deflection-compensated roll and it is selected from the group consisting of: a flexible surface roll, such a polymer-coated roll, a rubber-coated roll or an elastomer surface roll, a shoe roll, a thermo roll, a metal roll, a fibrous roll and a composite roll. The roll 4 can be replaced as a press element with some other profilable or fixed-profile press element, which moreover may consist of several segments successive in cross-machine direction. The press element 4 designed as a roll may likewise consist of several segments successive in cross-machine direction. The press element 4 may have its surface designed as continuous or discontinuous. Furthermore, the press element 4 can be designed displaceable for modifying the treatment zone length and/or belt tension. The metal belt may also be coated. In the embodiment shown in fig. 1, the nip roll comprises a shoe roll. Reference numeral 6 represents heating elements, such as for example an induction heater, an infrared radiator, a gas burner or a capacitive heater.

Preliminary test runs were conducted according to the invention in a metal belt calender intended for trial operation by using surface-sized fluting with a grammage of 110 g/m ² , which was treated in the metal belt calender at various thermo roll temperatures and at various loads applied by an additional stress roll placed inside the belt loop. The tests were conducted by using three rolls of unequal strengths, the respective strength values thereof being 1,75 SCT (CD) (P2), 1,805 SCT (CD) (P3) and 1,83 SCT (CD) (P4). The test runs revealed that metal belt calendering provided a distinct increase in strength with respect to the initial strength of a base web. For example, with a dwell time of 100 ms and a belt-tension induced pressure of 0.2 MPa, the strength of a base web increased from 1,83 SCT (CD) to 1,975 SCT (CD) at a thermo roll temperature of 17O ⁰ C and at an additional load of 50 kN/m (fig. 2). This represents a strength gain of about 8%, whereby the grammage can be reduced at the same rate to still maintain a substantially original strength value. Even a small additional load of 10 kN/m enabled achieving the strength of 1,895 SCT (CD), i.e. a gain of 3,5%.

When the dwell time was reduced to 65 ms, while the thermo roll temperature was 17O ⁰ C and the additional load was 70 kN/m, the strength of a base web rose from 1,83 SCT (CD) to 1,935 SCT (CD), i.e. about 5,7%. When applying a dwell time of 65 ms and an additional load of 50 kN/m with a thermo roll temperature of 125 ⁰ C,

the strength value of 1,93 SCT (CD) was achieved on a base web strength of 1,805 SCT (CD), i.e. a gain of over 10% in this instance.

The belt temperature had been set at 7O ⁰ C in the tests.

The foregoing examples demonstrate that the inventive method is capable of providing a considerable gain in strength, and hence saving in material, with respect to traditional surface-sized fluting, the same applying also to test liner. The inventive method is also applicable to the manufacturing of fluting or test liner without surface sizing.