Defined physical properties
Well-defined physical properties are:
• tensile strength
• compression strength.
Paperboard has an elastic behaviour up to a given limit – the elastic limit. This means that the force applied to the paperboard is proportional to the deformation caused by the applied force. If the force is removed the paperboard regains its original dimensions. This is summed up by Hooke’s law:
Elastic and plastic behaviour of paperboard.
Click to enlarge.
Paperboard deformed beyond the elastic limit has a plastic behaviour. This means that the applied force is no longer proportional to the deformation. When the force is removed the paperboard does not regain its original dimensions – it has become elongated. The value of the elastic limit is typically 0.2–0.5 % relative elongation.
After exceeding the elastic limit a permanent change of dimensions occurs. In practice this is a common reason for misregister during web printing.
The modulus of elasticity, E also known as Young’s modulus, is proportional to the initial slope of the curve.
Tensile strength and elongation
Tensile strength is the force per unit width which is required to rupture a strip of paperboard. The value is usually determined in a tensile testing machine where the paperboard test strip is gradually pulled to failure. The maximum force and elongation are recorded.
The maximum elongation, i.e. at break, emax is also called strain to failure.
Tensile strength (ISO 1924-2)
Tensile strength is evaluated by measuring the force required to break a 15 mm wide paperboard strip. The force is divided by the sample width and the result is expressed in kN/m.
Tensile strength = F/w
F = maximum force
w = initial sample width
Test methods and equipment
All Iggesund Paperboard’s products are tested using a tensile tester. Besides tensile strength, the instrument also measures the elongation at break, the tensile energy absorption, and the tensile stiffness (elastic modulus).
Because paperboard is a porous material consisting of a large number of fibres in a complex structure, its physical behaviour is different under tension than under compression after passing the elastic limit. Under tension the fibre network is gradually strained, aligning more and more fibres to carry the increasing load until the test strip and the fibres completely break (are separated from each other).
Under compression, the fibre network, which can be seen as consisting of a large number of supporting columns, collapses in the microscale by compression failure in the fibres themselves and by fibre buckling and bending.
After compression failure the fibre strength is basically unchanged, which means that the tensile strength is maintained with the material holding together. The different mechanism of compression also means that the compression strength is always 2–3 times lower than the tensile strength. This is unique for paper and paperboard and is the fundamental explanation as to why paperboard has the ability to be creased and folded.
Maximum tensile strength at failure.
Click to enlarge images.
Compression strength is defined as the maximum force per unit width a strip of paperboard can withstand under compression (also known as edge or short span compression strength).
Maximum compression, or strain at failure, is given by emax in %.
Compression strength. Click to enlarge.
Paperboard is a strong material in relation to its weight and thickness. Most paperboard grades for graphical and packaging applications have a thickness below 1 mm. To prevent the thin paperboard strip from bending (elastic buckling) the free length must be small in relation to the thickness. Otherwise it is not possible to record the maximum force the material can withstand. The only way to do this is to reduce the free length of the test strip to 0.7 mm. This allows us to determine the intrinsic compression strength.