3. Baseboard physical properties Performance properties 111 Laws of nature 112 Flatness and stability 116 Strength and toughness 123 Defi ned physical properties 125 Tensile strength 125 Compression strength 126 Complex physical properties 128 Tearing resistance 128 Impact burst strength 128 Delamination, interlaminar strength 129 Stiffness 134 Box compression strength 138 Performance properties 110 Reference Manual | IGGESUND PAPERBOARD Performance properties Performance properties The performance properties are related to the physical Measurable properties characteristics of the paperboard. These properties relate The measurable properties are those which are com- to how the paperboard will withstand the surrounding monly found when describing the technical data for a environment. The following performance properties are paperboard grade. The methods described are those described in this chapter: most commonly used. They are also used by Iggesund • fl atness and dimensional stability Paperboard. • strength and toughness For further information about specific testing equip- • stiffness ment see the manufacturers’ product catalogues • box compression strength. (e.g. Lorentzen & Wettre). The measuring methods for the properties listed are described in the following pages: • tensile strength • tearing resistance • interlayer strength, plybond • bending stiffness • bending resistance • bending moment • density • dimensional properties - Flatness • grammage • grammage (plastic-coated products) • thickness • compression strength • moisture content • stiffness • curl and twist. IGGESUND PAPERBOARD | Reference Manual 111 Laws of nature Laws of nature Two of the most basic physical properties that defi ne a pa- Test method and equipment perboard are grammage and thickness. These properties During production the grammage is measured con- tell us how much fibres and coating that is used for one tinuously on-line with an IR device. For calibration and square meter of board, and what thickness that results other investigations a gravimetric method is used. The in. Based on grammage and thickness the density, or the principle is as follows: bulk which is the density inverted, can be calculated and 1. Cut out a sample with specified area (normally by is often used to indicate whether a paperboard has a high using a punch, 0.5 dm2 or 1 dm2). or a low thickness at a given grammage. E.g. low density = 2. Carefully rinse all fi bres from the plastic fi lm by high bulk = high thickness. soaking in diluted NaOH solution. Most of the differences in the appearance and perform- 3. Dry and weigh on a balance (with at least 0.001 g ance properties are a result of the types and amounts of accuracy). pulps used. Within each type of paperboard there are a 4. Calculate the result in g/m2. range of properties depending on the exact fibre com- position, furnish, coating application and manufacturing Key characteristics technique. Within certain limits set by nature, each board- Grammage is controlled through the flow of pulp to the maker can achieve a number of combinations. The main headboxes (fibre distribution units) on the paperboard traits of these combinations will be apparent by evaluating machine. Since grammage includes the amount of fibre thickness, grammage and density. and moisture in the paperboard, the two together play With a given amount of fi bres and amount of coating an important role in the consistency and uniformity of the the grammage is set. Deriving from this the boardmaker paperboard characteristics. tries to optimise stiffness while trying to maintain strength By consistency we mean low variation against time of in the sheet. Within the same grammage, optimization of manufacturing, and by uniformity, low variation across the strength by increased amount of chemical pulp will lead to paperboard web. increase in density and loss of thickness. When optimiz- ing stiffness the thickness, or bulk plays an important role Measurable properties together with the strength properties of outer plies in a Thickness (ISO 534) multi-ply construction. Coating and calendering will affect Thickness is the distance between two parallel meas- thickness negatively but promote print- and print fi nishing uring devices over a specifi ed area on either side of result positively. A high amount of coating also limits the the sheet and is expressed in micrometres. amount of fi bres within a given grammage, thus decreas- ing strength or stiffness depending of which types of fi bre Test method and equipment you spare for the weight compensation. As with grammage, the thickness is recorded on-line Strength properties are often described and discussed on the paperboard machine. Laboratory tests are in relation to density and stiffness performance often done to determine the thickness of the sample tested discussed in relation to thickness as described in the fol- and to check calibration of the on-line equipment. lowing chapter. Density Measurable properties Density is the expression used to describe the compact- Grammage (ISO 536) ness of the paperboard. Density is calculated as the ratio The grammage specifi es the weight of the paper- of grammage and thickness in kg/m3. An increasing thick- board per unit area in g/m2. ness at constant grammage results in a lowering of density with density being reciprocally proportional to thickness. Test method and equipment Grammage is scanned constantly on-line on the Measurable properties paperboard machine and linked automatically to the Density (ISO 534) process control of moisture and pulp fl ow. Laboratory grammage (g/m2) Density (kg/m3) = x 1000 tests are routinely done to check the calibration of the thickness (μm) on-line equipment. These tests are done by weighing sheets of specifi ed size in a controlled atmosphere. Bulk is the inverse of density and is expressed in cm3/g. Grammage (plastic-coated products) Test methods and equipment The coat weight of the plastic is the weight per unit This property is calculated from measurements of area and it is normally expressed in g/m2. grammage and thickness. 112 Reference Manual | IGGESUND PAPERBOARD Laws of nature Key characteristics The appearance and performance properties of paper- The main influence on density is the type of fibre used. board can be described in terms such as: Mechanical fi bres give the potential for lower density than • whiteness, smoothness, and gloss chemical fi bre. Multi-ply forming also enables density to be • chemical character and purity reduced during the forming process. Fibre treatment and • elasticity, strength, and density. mixing of the fibres in the different paperboard layers is used to optimise density. Fibre and paperboard properties Surface smoothness confl icts with density (stiffness), Most of these properties are directly or indirectly depend- as increased calendering to improve smoothness reduces ent on the type and characteristics of the basic raw mate- thickness, giving higher density and hence a lower stiffness. rial, i.e. the fi bres. Within the same type of paperboard, at constant den- sity, the thickness increases with increasing grammage. Mechanically processed fi bres normally give higher thick- ness compared to chemically processed fi bres for a given grammage. Multi-ply forming benefi ts high thickness compared to single-ply forming. Increased calendering PULP PROPERTIES reduces the thickness but gives a smoother surface. MECHANICAL FIBRE CHEMICAL FIBRE Basic relationships Short Long Rigid Flexible Graphics and packaging applications place demands on paperboard that are dependent on combinations of appearance and performance properties. The type and amount of fibre as well as the manufacturing technique permit a large number of possible options. However, there PAPERBOARD PROPERTIES are also constraints governed by natural laws which limit 100% MECHANICAL FIBRE 100% CHEMICAL FIBRE the number of combinations. This section explains some of the most important factors and their interaction, and will High bulk Dense Stiff Strong clarify some of the basic relationships, in order to assist in better decision making in the challenging world of paper- board selection. 100 Weak white Strong white 90 Bleached and unbleached mechanical pulps 80 Chemical pulps, bleached 70 60 50 40 WHITENESS, % 30 20 10 Chemical pulps, unbleached Weak brown/grey Recycled fibres Strong brown 0 10 20 30 40 50 60 70 80 90 100 110 120 STRENGTH, kNm/kg IGGESUND PAPERBOARD | Reference Manual 113 Laws of nature Two main types of fibres are used for all types of paper- in these respects the opposite characteristics resulting in board, those produced by mechanically or chemically open, bulky and weak, but stiff products. As both stiffness processing the wood. Due to these very different treat- and strength are important the boardmaker has to achieve ments the properties of the resulting fibres (or pulps as a compromise. they are known in the industry) are also very different. This is done by treating and mixing the fi bres and us- Some of the basic fibre and paperboard properties are ing the multi-ply technique. The illustrations show some summarised in the illustration below. physical relationships for single ply sheets. All paperboards require a certain combination of white- The multi-ply technique is used to optimise the stiff- ness and strength to meet appearance and performance ness and to achieve the desired appearance and surface demands. properties with a minimum use of fibres. With Folding The most common types of pulps give the combinations Box Board this is done by putting the high bulk mechani- shown in the illustration on the previous page. cal pulp in the middle layers and the dense, strong, and By mixing mechanical and chemical fibre in a multiply smooth chemical pulp in the surface layers. By adding a technique, the paperboard maker can optimize the raw layer of pigment coating a further enhancement of the material usage and tailor the end use demands to the appearance is achieved. Even if 100 % chemical pulp is paperboard properties. A complication is that some of the used in all plies, the plies are treated differently to make demands are entirely contradictory.