Complex physical properties
Complex (less well-defined) physical properties are:
• box compression strength
• tearing resistance
• impact or burst strength
• delamination or interlaminar strength
• surface strength.
Stiffness and box compression strength are described in separate sections.
The strength properties are increased by increasing grammage. Moreover, the potential for high tensile strength is governed by the type of fibre and the production method, e.g. chemically processed, long fibres from species such as pine and spruce give the best results. Methods of fibre treatment, e.g. refining, are also important. The ratio of strength in MD/CD is dependent on the forming process at the wet end of the paperboard machine.
Impact, burst strength
Several more or less complicated methods exist to evaluate the physical resistance of paperboard to impact or penetration loads. The most commonly used method is the burst test, which is a modified paper test method.
Burst or puncture test methods are complicated because many physical parameters are involved. Parameters such as tensile strength and elongation, bending stiffness, and tear are involved depending on the geometric conditions for the test used. This makes it very difficult to draw more specific conclusions. However, in this case a higher value does mean a strong and tough material that is tolerant of various types of stresses and strains.
Tearing resistance is a property based on a method which attempts to simulate the tearing of paper (tearing perpendicular to the plane of the sheet).
After an initial cut, tearing is performed according to well-defined testing conditions. Despite this the physical meaning of the value (the tearing energy) is less obvious.
In general, high tearing resistance depends on the general strength level (tensile strength and elongation) and the amount of long, well-bonded fibres (more and longer fibres give a better result).
Tearing resistance (ISO 1974)
Tearing resistance is the force required to tear the paperboard from an initial cut.
Test method and equipment
A tear tester of the Elmendorf type is used for this test. The test is made in both the machine direction (MD)
and cross direction (CD) and is expressed in mN.
Principle of burst strength.
Tearing resistance tester.
Click to enlarge images.
The method is fairly good for paper but less useful for paperboard. Due to the way the test is performed, values for thick paper and paperboard are influenced by stiffness due to bending during tearing. Another factor is that the tearing of, for instance, multi-ply paperboard sheets sometimes changes the mode of failure from tearing intodelamination, or a combination of the two. Due to these circumstances tearing resistance has a limited practical value considering its complexity and built-in errors. However, the value does indicate if the paperboard is brittle or tough.
Methods have been identified that, under controlled conditions, are able to measure in-plane propagation resis- tance. This is a scientific value which means there are also methods to quantify tear and delamination behaviour according to material physics.
Good tearing resistance is needed in almost every packaging or graphical application, e.g. tearing strips for opening of a package, hanging displays for a blister package, book covers, brochures, etc.
Delamination, interlaminar strength
Delamination strength or interlaminar strength is usually defined by a number of methods designed to measure the force or energy required to separate or delaminate the interior structure of paperboard – i.e. the bonding within or between the plies, not the interface between the fibres and coating or within the coating itself.
For many graphical and packaging applications a certain level of interlaminar strength should be maintained. This should be high enough to make edges, corners and flaps resist handling damage but low enough to allow for good delamination during creasing and folding. The fact that paperboard is a relatively thin but strong material with a complex porous fibre structure makes it extremely difficult to develop accurate and reliable test methods.
Methods have been developed to measure delamination by using pulling (z-strength), peeling or combinations of the two. The complex loading conditions, very often using tape between the paperboard and the testing unit, create a number of restrictions and potential errors in these measuring methods.
Due to their complexity the established methods do not explain interlaminar strength. A method based on z-directional toughness has been developed by Innventia (formerly STFI-Packforsk). This method eliminates previous difficulties and measures a well-defined physical quantity.
Principles of different interlaminar strength evaluations. Click to enlarge.
Click to enlarge.
Chemically processed fibres from wood containing long fibres, like pine and spruce, together with optimised fibre treatment (refining), give the best potential for high interlayer strength.
In packaging applications a frequent failure, which is due to low strength, is the failure of the tearing strip where only the top layer tears off.
Interlayer strength, plybond (TAPPI 569)
As a multi-ply paperboard is built from several layers of fibres, it is important that these layers are well bonded together. interlayer strength is the expression used to quantify this property and may be measured using a variety of techniques.
Test method and equipment
The method used for all Iggesund Paperboard products is plybond using a Scott Bond type tester. In this test the energy needed to delaminate a sample by applying a perpendicular force to the paperboard surface is quantified. The test result is expressed in J/m² and the principle of the method is shown in this illustration.
The typical differences in mechanical behaviour can be explained by observing the loading and elongation behaviour (stress-strain behaviour). Recording the force and elongation during tensile testing allows us to obtain the curves shown in the illustration.
Products containing 100 % chemical pulp are approximately three times stronger than those with 100 % mechanical pulp, and the elongation value is some 50 % higher. The main reason is that chemically processed pulp gives long, well-bonded fibres resulting in strong and dense products. The initial slope of the two curves corresponds to the differences in the modulus of elasticity
Paperboard consists of a fibrous network, and due to the manufacturing process more fibres are aligned parallel to the running direction of the paperboard machine. Therefore the physical properties of paperboard are directionally dependent. This means that parallel to the running direction, (the machine direction), the products are typically stronger than in the cross machine direction. Usually these directions are abbreviated MD (machine direction) and CD (cross direction). This means that paperboard is stiffer and stronger in the MD and consequently weaker in the CD. On the other hand, the elongation is less in the MD and greater in the CD. These directional-dependent differences have a large influence in many user applications, not only for physical protection but also for printing requirements such as register control, curl and flatness, creasing, and folding.
The differences in the two directions should be as low as possible and are usually referred to as the MD/CD ratio for stiffness. An established way of averaging the differences in the various directions of the paperboard is to calculate the geometric mean value
In this way it is possible to facilitate comparison of the levels of different materials regardless of the MD/CD ratio. The typical differences in the tensile elongation curves are shown below.
Specific material properties
Paperboard is a fibrous porous network consisting of cellulose fibre material and air. Consequently the loadbearing elements, the fibres, only partly fill the volume or the cross section of a paperboard strip.
The apparent stress, s, when a load, F, per unit width (b, width of test piece) is acting on the total cross-sectional area A, is given by:
where ρF and ρ are the fibre and sheet density respectively. Thus the specific stress acting on a sheet is equal to the specific stress acting on the fibres.
In this context the asterisk (*) denotes a normalisation with regard to density.
Paper and paperboard may in many cases be treated as a homogeneous engineering material in spite of their fibrous, porous structure. However, since paperboard in its end use is in most cases judged by its properties per unit width, the properties: tensile failure stress (st), compression failure stress (sc), and elastic modulus (E), which are expressed in terms of force per unit cross-sectionalarea, are less suitable quantities for the characterisation of different products. Furthermore, these properties are sensitive to changes in sheet thickness (produced by e.g. calendering) even though the changes in thickness may not be accompanied by any real change in extensional properties. This difficulty may be overcome by expressing the sheet properties by the expressions: (st × t), (sc × t), and (E × t), i.e. by multiplying the tensile failure stress, compression failure stress, and elastic modulus, respectively, by the thickness (t) to give properties having the dimensions of force per unit width.
Elasticity, strength and elongation.
Differences in tensile elongation.
Click to enlarge.
Since these expressions are dependent on the basis weight of the sheet, the measurement may finally be normalised by dividing by the basis weight (w). Expressions for the specific tensile failure stress, specific compression failure stress, and specific elastic modulus are then obtained, which are equivalent to dividing the tensile failure stress, compression failure stress and elastic modulus by the density. These specific material properties are thus identical with the strength indices commonly used in the paperboard industry, as shown in the table below.
Basic features that have an impact on strength and
• type of pulp
• moisture content
• amount of pulp
• bulk or density.
There are numerous ways of measuring strength and toughness. The elastic modulus is calculated using stress and strain data from tensile testing. The strain to failure, or elongation, is recorded when the sample fails during the tensile strength measurement.
Hooke’s law may be modified to apply to specific properties:
In a material that contains air it is useful to use specific properties, which means that the strength is related to the mass of the material.
The force per unit width, F, acting on the test
sample of paperboard is borne by the
cross-sectional area occupied by the fibres.
Click to enlarge.
| Specific tensile failure stress
|Specific compression failure stress
||Tensile stiffness index
Law of mixtures
Defined properties such as modulus of elasticity, tensile and compression strength follow the laws of mixtures. This means that the final strength of the product is determined by the amount and strength of the ingoing components.
Typical values for different types of fibres
The following illustrations give some typical ranges of properties for major types of fibres used for paperboard. The illustrations indicate the levels and ranges typically found and also indicate the strong effect of density on the physical properties.
Tensile stiffness vs. sheet density for boards
made from different raw materials. Density is
varied by varying the wet-pressing.
Tensile index vs. sheet density for boards from
different fibre sources. The density is varied
by varying the wet-pressing.
The compression index vs. density for boards
made from different raw materials. The density
is varied by varying the wet-pressing.
Tensile strain vs. sheet density for boards made from different fibre sources. The results are obtained for boards dried under restraint. The density is varied by varying the wet-pressing.
Click to enlarge images.