A corrugated cardboard (1) comprising a first flat pre-treated linerboard (2), a second flat pre-treated linerboard (3), a corrugated flute (4) being arranged between the first and the second flat pre-treated linerboard (2, 3),
- wherein the corrugated flute (4) comprises an A-surface (5a), a B-surface (5b), and an internal flute material (6) arranged between the A- and B-surface (5a, 5b),
- the corrugated flute (4) comprises A-peaks (7a) and A-valleys (8a) which are arranged opposite each other, B-peaks (7b) and B-valleys (8b) which are arranged opposite each other,
- the first flat pre-treated linerboard (2) comprises a first inner surface (9a) and an opposite arranged first outer surface (10a),
- the second flat pre-treated linerboard (3) comprises a second inner surface (9b) and an opposite arranged second outer surface (10b),
- the first inner surface (9a) is connected against the A-peaks (7a) with a first re-activated adhesive, and
- the second inner surface (9b) is connected against the B-peaks (7b) with a second re-activated adhesive,
- wherein the first inner surface (9a) is pre-treated with the first re-activated adhesive being a first heat re-activated polymeric coating (11a) coated against the inner surface (9a), the second inner surface (9b) is pre-treated with the second re-activated adhesive being a second heat re-activated polymeric coating (11b) coated against the inner surface (9b),
- wherein the polymeric coatings (11a, 11b) on the respective inner surfaces (9a, 9b) have a thickness of 1-10 μm, and the outer surfaces (10a, 10b) are coating free,
- the A-peaks (7a) comprise a first A-bending zone (12a) and the B-peaks (7b) comprise a first B-bending zone (12b), such zones comprising fibre dislocations which are surface cracks (13) in the A- and B-surfaces created when the corrugated flute material (6) was corrugated and shaped with the A- and B-peaks (7a, 7b),
- the polymeric coatings (11a, 11b) are adhered in a connection between the inner surfaces (9a, 9b) and around the surface cracks (13) and in the surface cracks (13) against the internal flute material (6) of the peaks (7a, 7b), fibre ends (15) extend out and away in divergent and mixed directions from internal surfaces (16) of the surface cracks (13) and are emerged with the polymer coatings (11a, 11b) forming fibre reinforced connections (17) between the peaks (7a, 7b) and the inner surfaces (9a, 9b), and the second outer surface (10b) is smoother than the first outer surface (10a).