Furnish and papermaking effects in compression
Compressive strength is affected by much the same factors as elastic modulus or tensile strength, that is, the properties of the fibres and the fibre network. Naturally, the effects may vary depending on the paper property in focus. The generally significant properties are the length, flexibility and bonding ability of fibres. These depend, in turn, on the tree species, pulping method and beating level. The important network properties are fibre orientation, network density and inter-fibre bonding. Fibre orientation is affected by, e.g., the jet-to-wire speed difference (see: Laminar shear on wire), while density and inter-fibre bonding are affected by wet pressing and beating, and of course again by the fibre properties. Chemicals may also affect bond strength. Finally, drying shrinkage affects all strength properties.
The compressive strength of the fibres themselves have an effect on the compressive strength of paper and board. In addition, fibre properties such as dimensions and flexibility affect the network structure. All these effects depend on fibre morphology, length, thickness and microfibril angle. Experiments show that fibre thickness has the biggest effect. If fibre thickness increases from 3 μm to 6 μm, the compression index decreases by over 20% (Figure 1). Fibre length and the microfibril angle have a smaller effect. For example, if the fibre length decreases from 2.1 mm to 1.25 mm, the tensile strength index drops by 22% but the compression index by only 1 8%. A change in the microfibril angle from 5° to 25° decreases the compression index by a little less than 10%.
Figure 1. Effect of fibre thickness (double wall) and microfibril angle on compression index for kraft softwood pulps. The numbers indicate microfibril angles. The dashed lines are fits to the data for the 5° and 25° microfibril angles 1.
It has also been argued that the compressive strength of a fibre depends almost as much on the lignin as on the cellulose 2. This argument is supported by, for example, the observation that in kraft pulping the compressive strength of a paper or board is less dependent on the pulping yield than is the elastic modulus, although both have a maximum at the same yield (Figure 2).
Figure 2. Compression index and tensile strength index vs. yield in kraft pulping 2.
Damage in pulp fibres affects the compressive strength. In a low-density network, fibre curl and wrinkles reduce the buckling load discussed in the preceding section. The final fibre failure usually starts from microscopic damage in the microfibrils. The microfibrils are not homogeneous. Loose areas occur naturally and reduce the compression strength of the fibre 2. Sachs noticed that shear failure takes place at the S1–S2 boundary in the fibres, and that this part is weak because the fibril angles of the layers are so different, creating shear forces 3. Further damage can be caused in pulping and papermaking. For example, in high-consistency beating, the fibres collapse in their axial direction, and dislocations and misalignments appear in the microfibrils. However, it has been claimed that the dislocations can heal during paper drying 2, which would reduce their negative effects.
When using multiple furnishes, the compression index of a board can be approximated to be a linear combination of the compression indices of the components. This is valid for homogeneous blends and layered structures 4. This is naturally very useful in the evaluation of furnish components. No similar relationship holds for tensile strength, presumably because tensile strength is more dependent on bond strength than is compressive strength.
Table 1. Qualitative effect of various factors on tensile and compressive strength.
| Factor | Compressive strength | Tensile strength |
| beating | + | + |
| fibre orientation | + in MD | ++ in MD |
| – in CD | – – in CD | |
| felted sheet | + or ± 0 | – |
| wet pressing | + | + |
| drying shrinkage | – – | – |
| humidity | – – | – |
| sizing | ++ | ++ |
The qualitative effects of some process variables on tensile and compressive strength are compared in Table 1. For example, beating and wet pressing improve compressive strength9 but the simultaneous increase in density is harmful for paperboard grades other than linerboard because bending stiffness decreases. In beating, the dependence of compressive strength on density can be linearised for reasonable changes in the density range so that
(1)
where ρ is the board density and ρ0 is a threshold density, which in chemical pulps is usually ρ0 = 300–500 kg/m3 2,5. From the equation it can be seen that also the compression index increases with density. The elastic modulus behaves in the same way in beating and even the threshold density is about the same. Thus the changes in compressive strength that beating induces seem to arise primarily through changes in elastic modulus. Notice that compressive strength differs from tensile strength in this respect, since in beating tensile strength increases faster than the elastic modulus.
The effect of wet pressing on compressive strength and elastic modulus can also be quantified by Eq. (1) – though with different parameter values. At very high densities the changes level off. The effect of wet pressing depends also on the pressing method. According to Figure 3, press drying or extended nip pressing are more favourable than conventional wet pressing.
Figure 3. Schematic dependence of general strength properties on paper density that is changed by various unit processes. The vertical marks for pressing and calendering indicate successive nips 6.
The positive effect of sizing on compressive strength arises from stiffer fibres and stronger bonds. Internal sizing is more effective than surface sizing. The former can increase compressive strength by more than a third 7. However, especially the effect of starch disappears at high moisture content because starch absorbs moisture. This reduces the strength of inter-fibre bonding. In general, moisture affects compressive strength more than tensile strength 8. This is bad, because board packages are often stored in high humidity (storage in cold, for example). A special case of compressive failure is accelerated creep under cyclic humidity.
Table 2. Increase in mechanical properties of a board when free drying is changed to restrained drying. Results from handsheets 9.
| MD | CD | |
| Elastic modulus | 48% | 79% |
| Tensile strength | 11% | 20% |
| Compressive strength | 36% | 34% |
Table 2 shows that drying shrinkage or the prevention of it affects tensile strength less than compressive strength and the largest effect is seen in elastic modulus. The difference between compressive and tensile strength arises here from the compressive deformations of fibres induced by drying shrinkage. When paper is strained in tension, these fibre wall deformations straighten out before the fibres and bonds start to break. Therefore, drying shrinkage increases breaking strain but does not affect tensile strength as much. On the other hand, in compression, the drying compressions introduce crimps into fibres, making them more vulnerable to bending and thus the sheet more sensitive to compressive failure. The prevention of CD drying shrinkage on a board machine would give smaller improvements because the starting point is already somewhere between free and restrained drying.
