Many areas of the world do not possess indigenous supplies of wood fiber. In these areas-North Africa, Central Asia and the Middle East-demand for paper will grow, and it is important to find supplies of papermaking fibers other than wood. Plenty of non-wood plant fiber is available in these areas. For example, China is the biggest producer of straw pulp in the world. Over 60% of the 5 x 106 tons of paper per year produced in their 1000 mills (Table 2) comes from straw, mostly wheat (Haifeng 1988).
Regardless of fiber source, the effect of morphology will be the same. This paper will necessarily focus on wood fibers, but effects of wood fiber morphology can be applied to fiber from other sources.
The subject of fiber-to-fiber bonding has always been recognized for its importance in the area of sheet structure and its effect on paper properties. There are two types of fiber bonding associated to a fibrous network structure: (1) intra-fiber bonding, mainly for fiber stiffness, and (2) inter-fiber bonding, which is mainly for sheet strength. Probably the most influential aspect of how a sheet bonds together is the effect of fiber morphology We know that the properties of any given paper product made of fiber from wood A will often vary markedly from an identical product made of fiber from wood B, even though the conversion variables are identical. These differences in properties can be attributed to the morphological difference between the two wood species. Understanding the influence that pulp fiber morphology has on paper properties has great potential value to the pulp and paper industry Especially so, in projecting their effect on products or, evaluating new crop sources for their fiber supply. Instead of costly and time-consuming pulping evaluations, it may be possible to use selected fiber measurements to estimate the papermaking potential from unutilized tree components, to give direction to tree improvement work by clarifying the fiber properties most selected for genetic control and, to possibly predict the most efficient blend of pulps to achieve a desired set of product properties.
In the case of unbeaten hardwood pulps, 78% of the variation in stretch properties can be accounted for by fiber length and 70% by fibril angle. However, after beating, the effect of fiber length is less and fibril angle becomes the dominant single variable. Although the dependence on fibril angle is lessened, it accounts for 45% of the variation in stretch of sheets made from beaten hardwood fiber. Multiple regression showed that 92% of the variation in stretch of sheets made from unbeaten pulps could be accounted for by fiber length and fibril angle. The same two fiber characteristics accounted for 75% of the variation in unbeaten pulps.
The effect of fiber length was discussed previously in respect to stretch properties. Bursting strength of sheets made from unbeaten hardwood pulps also show a dependence upon fiber length (Fig. 4).
Visually, it is easy to grasp the significance of cell wall thickness on paper properties, in particular, to those in which bonding is important (Fig. 5). As mentioned, with softwood pulps, fiber length per se does not influence sheet strength properties. In our studies on fiber morphology and its effect on sheet properties, we have found by multiple regression analysis that the majority of the variation in bursting and tensile strength could be accounted for by fiber length and cell wall thickness. It was from these studies that we developed a new pulp index referred to as the L/T ratio, i.e., the ratio of fiber length, to cell wall thickness. The data in Fig. 6 demonstrates that the L/T ratio accounts for 92% of the variation in tensile strength of unbeaten softwood pulps and 84% of the variation after beating. Using L/T as a measure of fiber flexibility is an excellent indicator for the potential use of a softwood pulp for paper products requiring high strength.
The bursting strength of unbeaten hardwood pulps is shown to be influenced by fiber length. The dependence of unbeaten hardwood pulp on fiber length may be influenced by the heterogeneity of these pulps. Ifs been shown that the presence of a high percentage of parenchyma cells can reduce sheet strength by inhibiting fiber-to-fiber bond formation (Horn 1978) (Table 3). Therefore, the parenchyma cell content of any new fiber source should be a consideration in assessing its potential use in papermaking.
However, after beating, the L/T ratio becomes the dominant factor for both bursting strength and tensile strength of sheets made from hardwood pulps (Fig. 7, 8). This most probably reflects the greater degree of fiber collapse which results from beating. The fibers become more flexible and conformable, which in turn provides for more area to be developed for bonding along the fibers length. Also, beating reduces the identity of parenchyma cells and, in a sense, adds bonding material to the furnish. Therefore, bursting and tensile strength, being dependent upon the formation of fiber-to-fiber bonds, is greatly influenced by fiber length and cell wall thickness.
Parenchyma cells. Parenchyma cells do affect the strength of pulp sheets (Horn 1978). The variation in bursting and tensile strength due to parenchyma cell (fines) content is shown in Table 3. There is only a slight improvement in strength with the removal of parenchyma cells from a white oak kraft pulp, This can, however, be attributed to the poor bonding of thick-walled short fibers characteristic to oak. If, on the other hand, those same parenchyma cells removed from the oak are added to a low parenchyma content red alder furnish, the result is the lowering of the bursting and tensile strengths. This reduction in strength occurs even though the cell wall thickness of red alder is considerably less than that of the oak.
Vessel elements. The percentage by weight in hardwood pulps is extremely low and, not surprisingly, their effect on sheet strength is minimal (Horn 1978). Table 4 shows the effect of vessel elements on tensile strength. The presence or absence of vessel elements at the percentages found in the original pulp furnish has little influence on the ultimate tensile strength of hardwood pulps.
There are instances where the effect of a particular morphological factor may differ between those of hardwood and softwood fibers. Such is the case with fiber length. With softwood fibers, fiber length per se does not show a relationship to any sheet property. On the other hand, fiber length can be an important factor in such hardwood sheet properties as bursting and tensile strength, tearing strength, and stretch.
Sheet properties dependent upon fiber-to-fiber bond formation for strength development are dependent upon a combination of fiber length and cell wall thickness. These morphological characteristics combined, reported as a ratio of length to thickness (L/T), can be used to assess the potential of fibers for developing strength properties such as bursting and tensile strength.
Without question, sheet properties are dependent upon the morphological characteristic of the pulp fiber. Processing variables, such as beating, will affect sheet strength. However, everything else being equal, sheet properties are most influenced by the morphology of the fibers.
|Number of||Annual production|
(thousands of metric tons)
|Location||Paper and |
|Pulp mills||Paper board||Pulp||Per capita|
|Species||Finesz (%)||Burst |
|Tensile strength (psi)|
400 ml CSF
400 ml CSF
|Fig. 1. Influence of fibril angle on the stretch of pulp sheets made of unbeaten, unbleached softwood kraft pulp fibers.|
|Fig. 2. Influence of fiber length on the tearing resistance of pulp sheets made of unbeaten, unbleached hardwood kraft pulp fibers.||Fig. 3. Influence of fiber length on tearing resistance of pulp sheets made of unbleached hardwood kraft pulp fibers (beaten to 400 ml CSF).|
|Fig. 4. Relationship of bursting strength to fiber length of pulp sheets made of unbleached, unbeaten hardwood kraft pulp fibers.|
Fig. 5. Cross-sections of western larch fibers (Larix occidentalis) and of Port Orford cedar fibers (Chamaecyparis lowsoniana).
|Fig. 6. Relationship of tensile strength to L/T ratio of: (B) pulp sheets made of unbleached, unbeaten softwood kraft pulp fibers and (B1) after beating to 500 ml CSF.|
|Fig. 7. Relationship of bursting strength to L/T ratio of pulp sheets made of unbleached hardwood kraft pulp fibers beaten to 400 ml CSF.|
|Fig. 8. Relationship of tensile strength to L/T ratio of pulp sheets made from unbleached hardwood kraft pulp fibers beaten to 400 ml CSF.|