Science for Handpapermakers |
[ Back to Papermaking ] |
Vol. I : WHY PAPER IS PAPER
Introduction:
The artistic activity and the scientific activity are deemed by many to occupy opposite ends of the creativity spectrum. This gap creates communication problems which frequently seem unbridgeable. However, artists and craftspeople who "make" paper have discovered that paper is far more than the commodity item which conveys the printed word to our eyes, wipes our bottoms, and fills our landfills. This presentation seeks to add a scientific dimension to this sensitivity to paper; to illuminate aspects of paper science which contribute to the spirit and essence of paper, that strange chameleon-like material with it's notorious for doing it's own thing in spite of the best efforts of either artist or technologist to control it's form. In this perplexing task, the artist has as a primary focus, the question HOW?. How do I make this stuff come out right? On the other hand, the scientist brings to this same task a focus on the question WHY?. This presentation offers, therefore, a bridge from aspects of how to aspects of why.
Background:
Looked at from the perspective of no prior knowledge it is immediately evident that the connection between a water slurry of vegetable fibers, and a sheet if paper, is far from the intuitively obvious. Although paper is said to have been invented in China by Tsai Lun in 104 AD, it is far more likely that it developed after someone recognized the significance of a fortuitous event such as a dry mat of fibers deposited on a mud bank. In this sense paper was a serendipidous discovery not an invention. This underlines the above fact that the connection between a slurry of fibers and a sheet of paper is far from obvious and also explains why papermaking could have been maintained as a secret art, as it was, for hundreds of years.
But HOW, this discovery that indeed paper could be made from a slurry of fibers, came almost two thousand years before the question WHY was answered. To explain why a slurry of fibers can be converted to a sheet of paper required the development of a number of scientific ideas. The puzzle was only completed some 60 years ago by a Canadian scientist, Boyd Campbell.
The Strange Behaviour of Water
Fig 1 shows a list of common elements, headed by hydrogen and oxygen. When hydrogen unites with the elements in this list it produces a number of well known gases; viz. ammonia, the odor associated with strong liquid cleaners; hydrogen chloride, the gas which produces hydrochloric acid; hydrogen sulfide, the odor associated with rotten eggs; and methane, the greenhouse gas produced by the decomposition of animal and vegetable matter. When oxygen unites with these same elements it too produces a number of equally well known gases; viz. nitrous oxides, the pollutant that escapes from the tail pipes of automobiles; chlorine dioxide, a widely used bleaching agent for wood pulp; sulfur dioxide, the gas emitted by coal burning power plants that contributes to the acidification of rain; and carbon dioxide, the primary greenhouse gas. All these are predictably gases because they all consist of small, light weight molecules.
When hydrogen unites with oxygen in produces H2O, a molecule which is lighter than all but one of these gases. But it doesn't fit the mold. It isn't a gas. And it is to this strange and anomalous behaviour that we owe our existence. Because if water was a gas, life as we know it could not have evolved on planet earth.
Fig 2 shows water molecules floating about. These molecules consist of an oxygen core with two hydrogen dumbbells. It turns out that the oxygen atom is a bit of a playboy when it comes to hydrogen. It tends to switch allegiances so that it is never quite clear which hydrogen atoms in the immediate vicinity are connected to which oxygen cores. The dotted lines in Fig 2 illustrate this confusion. This vicarious attraction of oxygen and hydrogen creates sufficient coalescing forces to cause water, which should be a gas, to become the liquid which makes us possible. Also, this behaviour is part of science which explains paper.
Affinity of Liquids with solids
Solids and liquids have an interesting relationship which can be made manifest by placing a small drop of any liquid on a flat surface of any solid. What one observes is a range of behaviour shown in Fig 3. Sometimes the drop sits up as a sphere, as when mercury spills on a floor and rolls away as little balls, or as a drop of water sits up on a Teflon coated frying pan or freshly waxed car. Alternatively, the drop smears out or spreads as water does on most surfaces. This behaviour is scientifically characterized by measuring the contact angle (Fig 4). If the angle is high, such as for the mercury drop, it means that the atoms in the liquid are attracted much more to each other than to the atoms of the solid surface. As the contact angle declines it means that the liquid atoms are finding the atoms of the solid ever more congenial, until it is clear that the liquid atoms have a greater affinity for the solid surface atoms than they do for their own atoms. When water is the liquid the behaviour of different surfaces will vary from hydrophobic, the attitude of Teflon, to hydrophillic, the attitude of glass.
Surface Tension:
Figure 5 shows the two sets of glass microscope slides which have had handles glued to them. The surfaces of one set have been covered with a layer of paraffin wax, the other set is untreated. A drop of water is squeezed between each set of surfaces. The consequences are radically different. The paraffin coated surfaces fall away from each other, while the glass surfaces adhere strongly.
What is happening is illustrated in Fig 6. The film of water between the paraffin surfaces has no affinity for these hydrophobic surfaces. The water seeks to avoid contact and so the free surface at the edges forms an outward spherical bulge. With the glass surfaces, which are hydrophillic, the water seeks contact and so its outer free surface forms an inward spherical contraction.
Surface tension is the name given to the forces which necessarily exist in these surface skins which separate the liquid from the air. The existence of surface tension at the air-water interface requires that an equilibrium pressure difference must exist across any curved surface interface. The magnitude of this pressure difference is inversely proportional to the radius of curvature. This means that as the radius of curvature becomes smaller, the pressure differential across the curved surface becomes larger.
For the paraffin plates the radius of curvature of the air-water interface is positive. The surface tension at this interface therefore induces an increased internal pressure which acts to push the paraffin surfaces apart. The harder the paraffin surfaces are squeezed together, the smaller will be the radius of curvature and the larger will become the internal pressure acting to push apart the plates.
The free surface of the water contained between the glass surfaces exhibits a negative radius of curvature (Fig 7). This means that a negative pressure will be creating internally which acts to pull the glass plates closer together. Trying to pull the glass plates apart is resisted by this internal negative pressure and so the glass surfaces are effectively glued together. This consequence of water's surface tension is a further scientific clue needed to explain the WHY of paper.
Cellulose:
A hundred and fifty years ago rags provided the primary source of papermaking fiber. A contemporary pronouncement suggested that:
The common link between reeds and ropes, cotton and linen rags, and wood, was not deduced until chemists elucidated the structure of a basic material they called cellulose. Fig 8 shows what cellulose looks like. It is a large molecule built up by linking together a large number of identical units one after the other, like forging a long chain of many links. The basic link is a sugar
called glucose. It consists of a ring of carbon atoms bristling with what are called hydroxyl (OH) groups. These should be hauntingly reminiscent of the water molecule (HOH) discussed earlier.
Cellulose is a polymer built up by linking together in a linear fashion the glucose monomers. However, every second glucose monomer is attached upside down. This is very interesting, because another material was elucidated about the same time to have a very similar molecular structure (Fig 9). This material also is built up in a linear fashion by adding glucose monomers. The only difference from the cellulose is that all the glucose monomers are linked together with the same orientation. This material is starch. Cows digest cellulose back to glucose, and humans digest starch for the same reason. The only difference between starch and cellulose is simply reversing every second glucose monomer. Yet that makes it indigestible to humans, and the fundamental common link in all materials which have ever been found suitable to make natural paper.
Boyd Campbell
Now all the pieces of the puzzle are available and we can see how Boyd Campbell put them together in order to explain why paper is paper. Boyd Campbell's idea is illustrated in Fig 10. He recognized that cellulose was extremely hydrophillic and therefore in a wet sheet of paper, after pressing, the fibers would be interconnected by a whole series of water droplets. The free surfaces of these water droplets would each have a negative radius of curvature and would therefore create internal negative pressure tending to hold the fibers together. Campbell reasoned that during drying, all the interconnecting water droplets would start to shrink, and as they did so the free surface radii of curvature would get smaller and smaller, exerting larger and larger tensions, pulling the fibers close and closer together (Fig 11).
At the molecular level Boyd Campbell visualized the picture illustrated in Fig 12. Here two cellulose surfaces of adjacent fibers are separated by a film of water molecules. As the film recedes during drying, surface tension forces bring the cellulose surfaces closer together. Finally, the peculiar propensity for oxygen molecules to be attracted to hydrogen molecules, the thing that made water a liquid and not a gas, begins to create linkages between the hydroxyl groups of the adjacent cellulose surfaces, holding them permanently together. This connection between adjacent hydroxyl groups as oxygen links with two hydrogen atoms is called a hydrogen bond. It is the hydrogen bond that makes water liquid so that human life is possible, and holds cellulose fibers together after dying so that humans can make paper.
Boyd Campbell's Classic Experiment:
To satisfy himself that this picture of why fibers became paper was actually true, Boyd Campbell made a wet sheet of paper and then froze it so that the fibers were interconnected by rigid ice. He then placed the frozen sheet in a special environment where the ice would disappear without melting. That is, the ice would go from the solid state to the gaseous state by the process known as sublimation. Boyd Campbell reasoned that under these conditions the interconnecting ice would disappear without pulling the fibers close together. If they weren't pulled together the cellulose molecules in adjacent fibers would be too far apart for hydrogen bonds to form and so the sheet after drying by subliming away the ice, would have no connections between fibers.
He ran the experiment and found, no doubt to his great satisfaction, that indeed as he predicted, he produced a sheet of essentially unbonded fibers which could literally be blown to the winds.
An Easier Experiment:
Another way of showing this same interconnection is to make a wet sheet and then displace the water in the fiber with a non polar solvent. When this has been done the wet sheet consists of fibers interspersed with a liquid which has no affinity for cellulose. The liquid free surfaces, as shown in Fig 6, bulge outward producing a positive radius of curvature. This in turn creates a positive internal pressure which tends to push the fibers further apart. As this liquid evaporates during drying, the surface tension forces act in the opposite way and do not pull the fibers together. So, as in the case with the freeze dried sheet, the fibers are not pulled close enough together for hydrogen bonding to occur. Such a sheet will be equally weak and can be readily pulled apart, like thistledown.
That's Why Paper is Paper:
Thus ends the story of how science has come to perceive paper. It is the consequence of a unique aspect of water, the liquid that should be a gas; of cellulose, the upside down, hydroxyl bristling glucose polymer that feeds the ruminants of the world; of the hydrophillic relationship which brings surface tension as the essential compacting agent operative during drying; and of the unique attractions that hydrogen inspires in oxygen which produces the final hydrogen bond which holds the dry fibers together in that great and astonishing miracle - PAPER.