Science for Handpapermakers
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Vol. II : BEATING
In this first of the Science for Handpapermakers series, Why Paper is Paper, the scientific explanations were presented which can account for the remarkable manner in which a wet mat of fibers, when dried, spontaneously converts into the coherent material we call paper. As all papermakers quickly discover, however, this spontaneous behaviour guarantees only a coherent sheet. The quality of the sheet, smooth or rough, opaque or transparent, stiff or limp, or whether it achieves any of the other infinite esthetic possibilities which it can easily compass, depends on the craft of the papermaker.
The ultimate goal of the craft person - the perfect sheet of paper, or the perfect stuff for forming or sculpting, or anything else - can only be achieved by discovering satisfactory answers to four question. Where will I source my fibers? How will i extract and purify these fibers? How will I beat these fibers? How will I wet form these fibers to give them the potential that drying will consummate?
For this second presentation, I have chosen to explore how science perceives the third question - how will I beat these fibers? The reason for making this selection is not because it is necessarily the most important question to answer, but because it is the most mysterious. Even the uninitiated would have little difficulty appreciating the significance of the first two questions, and also of the last one. However, ....beating? It sounds and looks like a very abusive procedure, and it is far from obvious why it is needed. To the uninitiated it would certainly be mysterious. The truth is, however, that it is also mysterious to the rest of us, whether skilled crafts people, or scientists!
Scientific ignorance therefore limits the scope of this presentation. The scientific appreciation of the commercial beating of wood fibers is very far from complete. Non-woody fibers have simply been ignored. This presentation is best described, then, as a personal scientific perspective developed over many years of active engagement with the mysteries of beating. My primary goal is quite selfish. I am hoping that this presentation will help provide a language bridge that will give me and other interested scientists access to the extensive knowledge of beating which many paper artists and hand papermakers have necessarily acquired in order to reliably achieve their craft objectives. Being able to pool our knowledge will, I believe produce insights that will otherwise remain transparent to both our communities. And so to the task.
What Comes to the Beater:
What happens in a beater depends not only upon the characteristics of the beater being used, but also the character of the fibers being introduced. In spite of the fact that all papermaking fibers are cellulosic, they can be incredibly different. For instance, they come in many different sizes as show in Figures 1 and 2.
Although they all are basically hollow cylinders, they can have thick walls or thin walls or anywhere in-between, as illustrated in Figure 3. These walls are built up in a complicated way by cellulose fibers that spiral around the axis of the cylinder. These spirals can be steep or flat, or anywhere in-between, as pictured in Figure 4. But things get even worse!
The fibers that come to the beater are originally parts of plants. These plants grew their fibers for their own exclusive use, whether as stems for structural support, as transport devices for distributing nutrients, or as a useful means to implement reproductive strategies. To make them effective plants frequently use a rich admixture of other materials and structures, creating blends that look something like what is shown in Figure 5. Here we see fibers embedded in a complex three-dimensional structure consisting of various materials, principally lignin with some admixture of pentosans and trace amounts of many other things. The percentage of the non-cellulosic component is dependent on the plant species as indicated in Figure 6, and can be seen to vary widely.
What makes things worse is that before the fiber can be delivered to the beater it needs to be stripped from its non-cellulosic associates. To disengage the fibers from the plant structure requires chemical or biological degradation of the non-cellulose portion of the structure till it can be washed free. This can be done slowly or quickly in a wide variety of different ways. The way that is chosen and the time allowed to the process will strongly influence how the fibers will emerge. Any means used to degrade the interfiber material will, if generally to a much lesser degree, also attack the fiber. So we see that not only are fibers grown with very different properties by different plants, but that for a given plant the manner in which the fibers are extracted and purified will also strongly influence their character.
We can now return to the beater with appropriate humility. What will happen to the fibers when subjected to exactly the same beating strategy will depend not only on what fibers are selected (e.g. cotton, flax, abaca, etc.) but the detailed history of those fibers including the idiosyncrasies of growth and age which distinguish different harvest areas, and the specific manner in which the plant harvest has been reduced to yield the purified papermaking fibers.
The mystery of beating is greatly enhanced by the mystery of the fiber and explains why the expected results of beating the next batch of a favorite fiber can turn out, unexpectantly, to be different from the last batch. Different histories produce different quality fibers.
Currently, the consequences of beating specific fibers is far better understood by the crafts person than by the scientist. However, science has revealed some useful insights into what beating tends to do to fibers. It is these tendencies that we will now explore.
The beating action, depending on its nature and severity, can cause fiber fracture, producing a reduction in the average geometric length of the fiber. This action has historically been referred to as fiber cutting (Figure 7). It can be deliberate, as in the treatment of flax, hemp, and cotton, in order to reduce fiber length to a manageable level for wet forming. It can also be an inadvertent by-product in the pursuit of other beating goals.
The cell wall structure (the wall of the fiber cylinder) is built up from cellulose molecules which are extremely hydrophillic, a scientific word meaning "water loving". Beating is conducted in the presence of water and when the beating action in any way disrupts the fiber surface, water is quick to enter and attach itself to newly available cellulose surfaces. This invasion causes a swelling of the cell wall and therefore of the whole fiber as illustrated in Figure 8. The magnitude of this swelling effect is very dependent upon the fiber structure. Wood fibers will readily increase their diameter by up to as much as 50% in response to the swelling tendency induced by beating. Fiber swelling is unquestionably a principal cause of many of the characteristics acquired by paper when made from a beaten wood pulp.
Although fibers in the plant are typically straight, the process of purification tends to reveal or create surface defects, which, once the plant's supporting matrix disappears, become sites of surface weakness which enhance kinking and curling tendencies. Thus when seen floating around in water or examined on paper surfaces fibers will appear kinked and curled rather than straight.
It is a property of cylindrical shapes that they become stiffer as their diameter increases. Any tendency for fibers to swell will therefore increase the stiffness of the wet fiber. This stiffening will induce fiber straightening, just as a collapsed and rolled up party tube will straighten out when blown into. The tendency for fibers to straighten out (Figure 8) and thus to assume less curled configurations in a sheet of paper is a direct and inevitable consequence of the fiber swelling induced by beating. Whether fibers are curly or straight will effect the network structure created when a sheet is formed, significantly altering the physical properties of the derivative paper. Although fiber curl can be induced by certain special and specific mechanical treatments, under conditions of conventional beating, decurling is an important, if uncontrolled, side effect induced by fiber swelling.
Fiber swelling increases the wet stiffness of the fiber, but when the fiber is squeezed during pressing, and the water in the lumen, the central opening in the fiber, is expelled, the swollen cell wall is more easily compressed. The fiber becomes more conformable in respect to other fibers in the mat. Also the fiber wall tends to collapse more readily upon itself, eliminating the lumen. In it's swollen state, the increase in diameter translates into a larger perimeter, which upon collapse, produces a major increase in the contact area with other fibers. This is illustrated in figure 9. This increase in contact area together with the increased compressibility of the fiber allows the surface tension forces active during drying to create larger regions where hydrogen bonding takes hold. The net result is an increase in the degree of bonding yielding a stronger, denser, less porous paper.
The water which penetrates the cell wall during fiber swelling acts to cause the separation and rearrangement of internal surfaces. This rearrangement frequently eliminates constrictions (see figure 10) which pre-existed in the cell wall of the untreated fiber. Such constrictions, when present, induce load concentrations when the fiber is stressed, causing premature failure. By eliminating constrictions, fiber swelling produces a fiber which when bonded into the dry network of paper allows stress to flow more uniformly, thus increasing both the strength of the fiber and of the paper which contains the fibers.
It is this combination of increased fiber diameter and the loosening of the cell wall structure caused by fiber swelling which translate into the most conspicuous changes in the character of paper as the source fibers are beaten.
Fibrillation and Fines:
The surface of an unbeaten fiber is relatively smooth. The effect of beating is to cause bits of the fiber surface to come loose, increasingly giving the surface the appearance of having grown hairs. These hairy surface outcroppings are called fibrils, and the manner of their occurrence is described as fibrillation. If the beating action is sufficiently severe or prolonged, surface fibrils can be totally dislodged from the fiber, becoming very small particles which are free to wander around and ultimately seek a new home when retained and dried into a sheet of paper. These separated fibrils are called fines. It is the fibrillated surfaces of the fibers and the independent fines which are largely responsible for the drainage characteristics of the stock on the papermaker's mold or on the wire of a papermachine. The more fibrillation and the more fines that beating produces, the slower will be the stock on the wire or mold, meaning that water will more slowly pass through the forming mat. In the commercial world such a pulp is said to have a lower freeness.
Fibrils and fines interact with the fiber network during drying in such a way as to promote interfiber bonding. The fibrils of two adjacent fibers will be the first parts to come in contact during pressing, giving the fibers an advanced guard, so to speak, for co-mingling. Fines will tend to follow water into the crevasses between fibers, and gradually be drawn into bonding sites as the water evaporates during drying. The presence of fibrils and fines means that fiber surfaces can be bonded together even though their main surfaces may not have been pulled into the intimate contact necessary to cause direct hydrogen bonding. This is illustrated in the bottom section of Figure 11.
The beating actions that have been described can be summarized in the following manner:
|Cutting||Reduces average fiber length|
|Fiber Swelling||Increases bonding potential
Strengthens individual fibers
|Decurling||Increases effective length
Alters sheet structure
|Reduces pulp freeness
Increases bonding potential
What interests scientists, and I should think also crafts people, is how much of each of these things does a given beater do to a given batch of fibers, and how much of each do we really need in order to produce the paper we really want?
For the scientist to begin to be able to answer those kinds of questions requires a system of measurement which quantifies these effects. At Pulmac we have been working for many years to perfect a system which attempts to do just this. But that is a story to be told at a later date.
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