Laws of nature
Two of the most basic physical properties that define a paperboard are grammage and thickness. These properties tell us how much fibres and coating that is used for one square meter of board, and what thickness that results in. Based on grammage and thickness the density, or the bulk which is the density inverted, can be calculated and is often used to indicate whether a paperboard has a high or a low thickness at a given grammage. E.g. low density = high bulk = high thickness.
Most of the differences in the appearance and performance properties are a result of the types and amounts of pulps used. Within each type of paperboard there are a range of properties depending on the exact fibre composition, furnish, coating application and manufacturing technique. Within certain limits set by nature, each boardmaker can achieve a number of combinations. The main traits of these combinations will be apparent by evaluating thickness, grammage and density.
With a given amount of fibres and amount of coating the grammage is set. Deriving from this the boardmaker tries to optimise stiffness while trying to maintain strength in the sheet. Within the same grammage, optimization of strength by increased amount of chemical pulp will lead to increase in density and loss of thickness. When optimizing stiffness the thickness, or bulk plays an important role together with the strength properties of outer plies in a multi-ply construction. Coating and calendering will affect thickness negatively but promote print- and print finishing result positively. A high amount of coating also limits the amount of fibres within a given grammage, thus decreasing strength or stiffness depending of which types of fibre you spare for the weight compensation.
Strength properties are often described and discussed in relation to density and stiffness performance often discussed in relation to thickness as described in the following chapter.
Grammage (ISO 536)
The grammage specifies the weight of the paperboard per unit area in g/m2.
Test method and equipment
Grammage is scanned constantly on-line on the paperboard machine and linked automatically to the process control of moisture and pulp flow. Laboratory tests are routinely done to check the calibration of the on-line equipment. These tests are done by weighing sheets of specified size in a controlled atmosphere.
Grammage (plastic-coated products)
The coat weight of the plastic is the weight per unit area and it is normally expressed in g/m2.
Test method and equipment
During production the grammage is measured continuously on-line with an IR device. For calibration and other investigations a gravimetric method is used. The principle is as follows:
1. Cut out a sample with specified area (normally by using a punch, 0.5 dm2 or 1 dm2).
2. Carefully rinse all fibres from the plastic film by soaking in diluted NaOH solution.
3. Dry and weigh on a balance (with at least 0.001 g accuracy).
4. Calculate the result in g/m2.
Grammage is controlled through the flow of pulp to the headboxes (fibre distribution units) on the paperboard machine. Since grammage includes the amount of fibre and moisture in the paperboard, the two together play an important role in the consistency and uniformity of the paperboard characteristics.
By consistency we mean low variation against time of manufacturing, and by uniformity, low variation across the paperboard web.
Thickness (ISO 534)
Thickness is the distance between two parallel measuring devices over a specified area on either side of the sheet and is expressed in micrometres.
Test method and equipment
As with grammage, the thickness is recorded on-line on the paperboard machine. Laboratory tests are done to determine the thickness of the sample tested and to check calibration of the on-line equipment.
Density is the expression used to describe the compactness of the paperboard. Density is calculated as the ratio of grammage and thickness in kg/m3. An increasing thickness at constant grammage results in a lowering of density with density being reciprocally proportional to thickness.
Density (ISO 534)
Bulk is the inverse of density and is expressed in cm3/g.
Test methods and equipment
This property is calculated from measurements of grammage and thickness.
Click to enlarge.
The main influence on density is the type of fibre used. Mechanical fibres give the potential for lower density than chemical fibre. Multi-ply forming also enables density to be reduced during the forming process. Fibre treatment and mixing of the fibres in the different paperboard layers is used to optimise density.
Surface smoothness conflicts with density (stiffness), as increased calendering to improve smoothness reduces thickness, giving higher density and hence a lower stiffness.
Within the same type of paperboard, at constant density, the thickness increases with increasing grammage.
Mechanically processed fibres normally give higher thickness compared to chemically processed fibres for a given grammage. Multi-ply forming benefits high thickness compared to single-ply forming. Increased calendering reduces the thickness but gives a smoother surface.
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 are also constraints governed by natural laws which limit the number of combinations. This section explains some of the most important factors and their interaction, and will clarify some of the basic relationships, in order to assist in better decision making in the challenging world of paperboard selection.
The appearance and performance properties of paperboard can be described in terms such as:
• whiteness, smoothness, and gloss
• chemical character and purity
• elasticity, strength, and density.
Click to enlarge.
Fibre and paperboard properties
Most of these properties are directly or indirectly dependent on the type and characteristics of the basic raw material, i.e. the fibres.
Two main types of fibres are used for all types of paperboard, those produced by mechanically or chemically processing the wood. Due to these very different treatments the properties of the resulting fibres (or pulps as they are known in the industry) are also very different. Some of the basic fibre and paperboard properties are summarised in the illustration below.
All paperboards require a certain combination of whiteness and strength to meet appearance and performance demands.
The most common types of pulps give the combinations shown in the illustration on the previous page.
By mixing mechanical and chemical fibre in a multiply technique, the paperboard maker can optimize the raw material usage and tailor the end use demands to the paperboard properties. A complication is that some of the demands are entirely contradictory. Such an example is to obtain both maximum stiffness and strength with a given fibre composition.
The strength, flexibility, and consolidation behaviour of chemically processed fibres results in well formed, dense, and strong products. Mechanically processed fibres have in these respects the opposite characteristics resulting in open, bulky and weak, but stiff products. As both stiffness and strength are important the boardmaker has to achieve a compromise.
This is done by treating and mixing the fibres and using the multi-ply technique. The illustrations show some physical relationships for single ply sheets.
The multi-ply technique is used to optimise the stiffness and to achieve the desired appearance and surface properties with a minimum use of fibres. With Folding Box Board this is done by putting the high bulk mechanical pulp in the middle layers and the dense, strong, and smooth chemical pulp in the surface layers. By adding a layer of pigment coating a further enhancement of the appearance is achieved. Even if 100 % chemical pulp is used in all plies, the plies are treated differently to make use of the multiply principle.
For a given fibre composition and ply construction the stiffness is strongly dependent on the thickness of the paperboard. In theory stiffness is proportional to the cube of thickness. The strength properties are usually proportional to the weight (grammage).
Click to enlarge images.
Paperboard grades and properties
The dominating types of pulp used for paperboard are made from primary wood fibres separated via mechanical and chemical processes, and pulp made from recycled fibres. Due to the quite different characteristics of these pulps, they are utilised in four different types of paperboard:
• bleached chemical pulps – Solid Bleached Board (SBB)
• unbleached chemical pulps - Solid Unbleached Board (SUB)
• mainly mechanical pulps – Folding Box Board (FBB)
• mainly recycled pulps – White Lined Chipboard (WLC).
Most of the differences in the appearance and performance properties are a result of the types and amounts of pulps used. Within each type of paperboard there are a range of properties depending on the exact fibre composition and manufacturing technique. Within certain limits set by nature, each boardmaker can achieve a number of combinations.
In each case, it is important to match the required appearance and performance against the paperboard’s specific characteristics. Based on a large amount of test data for various grades combined with experienced judgement, some typical fibre-dependant differences can be identified. The indicated levels can be moved upwards or downwards, but within limits depending on the type and amount of fibres, ply composition and manufacturing conditions.
Stiffness – strength – appearance
Stiffness and strength are two basic paperboard properties which have a major influence on the mechanical paperboard performance of paperboard. They have a crucial effect on the boards protective properties and also influence carton shape and appearance.
The laws of nature make it impossible to maximize strength and stiffness simultaneously. Every application is a compromise to find the best balance.
Types of fibres
The physical properties of paperboard are determined largely by the types and amounts of fibres used. While these are the key factors influencing most properties, bending stiffness is the property that is most dependent on fibre composition as well as ply construction, specifically, on whether the paperboard consists of one or several plies.
The two main types of fibres used for paperboard are mechanically or chemically processed. Due to the very different treatment of wood in the mechanical and chemical processes, the resulting properties of the two types of fibre differ considerably.
When a paper product is recycled, the repulped fibres still ultimately originate from the same two sources, i.e. mechanical and chemical fibres. During recycling the fibres become contaminated and worn (lose strength). Depending on the application, the maximum number of cycles possible is theoretically in the range of 7– 8 but in many cases is only 2– 4 depending on the original pulping method, etc. Therefore primary fibres are always needed to maintain the quality of recycled products. Typical values for the different types of fibres are given in the following illustration.
Click to enlarge.