The unique convertibility of paperboard

Abstract

AbstractAlthough the tensile strength or bending stiffness of paperboard is high, in general the material possesses good fold, creasing or scoring properties. Both crease and fold resistance are surprisingly low, as is the rate of crack propagation along the deformation lines. The background to this characteristic is explained.It is shown experimentally that the folding resistance before and after creasing, MB and M' respectively, are strongly correlated with the (in plane) compression strength of the material, approximately according to MB ˜τBC · τ2 and M'/MB ˜ ϵBc, where t is the board thickness, τBC and ϵBc are the compression breaking stress and strain respectively.The strength in compression is several times lower than in tension, both in terms of stress and strain. Consequently, the manner in which the material yields, both in folding and in creasing fracture, originates from the compression regions, i.e. the concave parts of the deformation zone. The tension level at this time may only be 30–50% of its maximum. The breaking limit in tension is reached far later than in compression, and can effectively be ignored.Combined shear stresses (out of plane) of various magnitudes, applied during folding, do not change the relationships found. The material is insensitive to shear stress at least up to folding angles of 90°.Crack propagation along the cut, folded or creased lines is directly related to the in‐plane compression strength of the material, approximately according to l˜Lc−1(τBC/T87‐)2, where /is the crack length, τer is the tension breaking stress and τc‐1 is the brittleness characterized by the critical tensile span of the material, Lc.The rate of crack propagation is greatly reduced in comparison to materials with more equal compression and tension strength and can be as little as 75 or 90%, due to the actual strength ratio. Additionally, paperboard has a relatively high critical span length, Lc, of the same order as that of various synthetic fibre composites, which reduces crack propagation even further.It is necessary to keep the compression strength as low as possible in the development of paperboard; if compression strength needs to be raised, e.g. for end‐usability, tensile strength also needs to be increased, or brittleness reduced, at least in the outer layers of the material. If these adjustments are not made, the properties that make for good convertibility are degraded, according to the results presented.