In papermaking, as in gunpowder, the Chinese were technologically ahead of the west. Paper was first made in China in the 6th century AD. By 748 AD, they were publishing newspapers. In the west, only parchment (made from sheepskin) and papyrus (made from the plant of that name) were available, and both were expensive. In 751, Arabs captured some Chinese papermakers and set up a paper mill at Samarkand. By 793, there was a mill in Baghdad. Gradually, the technology was passed to Christendom, with paper mills built in Spain by 1150 and France by 1198.
Parchment and vellum are produced by splitting sheep or goatskin. This produces a very fine and durable writing surface, but a very expensive one. Papyrus is made from the papyrus reed. The dried reed is separated to produce long fibers and laid side by side. Then another layer of fibers is laid on top of and perpendicular to the first. Finally, the two layers are beaten until they form a single sheet of papyrus. This is also very labor intensive and so papyrus is an expensive commodity.
True paper, like papyrus, is made of plant fibers. These fibers are separated, either mechanically by pounding the plant material (pulverizing!), or chemically, by soaking the plant material in a chemical bath which dissolves the lignin which holds the cellulose fibers together. In practice, a combination of mechanical and chemical pulping is used. Once the fibers are separated, the pulp is lifted on a screen, or deckle, which catches the plant fibers with the fibers running all different directions. Whereas with papyrus, the fibers had to be laid parallel to one another in each layer, the paper fibers run every which way. Consequently, paper-making is much more easily mechanized than papyrus-making.
In the orient, rice is the plant of choice for paper making. Such paper is made directly from the plant. In the west, cotton and linen were the traditional fibers of choice. While is is possible to make paper directly from these fibers, more commonly pulp was produced from rags. So the fibers were spun into yarn, woven into cloth, used until the cloth was beyond repair, and then pulped to make paper. We still refer to the "rag content" even today.
The invention of the printing press in the mid fifteenth century created an ever-increasing demand for low-cost paper. The problem was that the fibers used for making paper, cotton and linen, were of the more expensive variety. Attempts to make paper from wood were frustrated by poor quality. Today, only newsprint is made from the mechanical pulping of wood in which the entire log is ground to a pulp. Paper made from this pulp is weak and tan in color. It yellows quickly and becomes brittle with age.
In the nineteenth century, it a chemical pulping process was discovered which allowed wood to be pulped for strong, white, relatively long-lasting paper. The goal of chemical pulping is to remove all the parts of the wood which are not cellulose. Chief among these is lignin, a carbohydrate which cements adjacent cells together in the wood. This material can be solublized by cooking the pulp either in strong acid or strong base.
Like fat, lignin is a polymer of long chain organic acids and alcohols. Recall that lye can be used to hydrolyse insoluble fat into fatty acids and glycerol, which are both soluble in water. In the same way, lye can be used to hydrolyse insoluble lignin into the acids and alcohols of which it is composed. These acids and alcohols are soluble in water and so can be separated from the insoluble cellulose we need for making paper.
Originally, wood pulp was cooked in lye (NaOH) alone, but this produced a rather weak paper. The addition of sodium sulfide to the pulp produced a much stronger paper. This process is called the kraft process from the German word for strong. About 80% of this kraft pulp is wood and the remaining 20% is lye and sodium sulfide. The pulp is cooked, or digested, at 170 C for 3 hours until most of the lignin is solublized. The liquid is drained off and the pulp is washed to remove the chemicals.
The resulting pulp is dark brown in color. If it is made directly into paper, the resulting paper is strong but brown. This is the kind of paper out of which grocery sacks are made. Corrugated cardboard is also made from this kind of paper. If white paper is desired, the pulp must be bleached, a topic which we will visit later in the semester.
One problem with the kraft process (as opposed to mechanical pulping) is the liquid that remains from the pulping. This black liquor contains soda ash, sodium sulfide, and lignin. In the past, this would have been discharged directly into a lake or river, causing significant water pollution. Not only does this damage the environment, but it also wastes the potentially useful components of the liquid waste. Economical production of kraft paper relies on the recycling of these components.
Let us begin with this nasty, black liquid very similar to that left over from your pulping process. This liquid contains alkali's like lye and soda ash. First, sodium sulfate is added to the black brew to provide sulfur. Sodium sulfate is manufactured from sulfuric acid in a process we will see when we discuss acids. The black brew is sprayed into a furnace where it is burned. The water is quickly vaporized. The organic components from the lignin burn to charcoal and provide a reducing environment in the furnace. Two other reactions occur in the furnace simultaneously:
Na2SO4(s) + 2 C(s) -----> Na2S(s) + 2 CO2(g)
2 NaOH(s) + CO2(g) -----> Na2CO3(s) + H2O(g)
That is, the organic components of the brew burn, the water is vaporized, leftover lye is converted to soda ash, and the sodium sulfate is reduced to sodium sulfide. The only solid products are sodium sulfide and soda ash, a mixture referred to as "black ash."
But the kraft process requires sodium sulfide and lye, not soda ash. You should already have an idea for the conversion process:
In a separate kiln, limestone is converted to lime:
CaCO3 -----> CaO(s) + CO2(g)
The lime is slaked:
CaO(s) + H2O(l) -----> Ca(OH)2(aq)
The slaked lime is added to the black ash, where it reacts with the soda
ash, leaving the sodium sulfide untouched:
Ca(OH)2(aq) + Na2CO3(aq) -----> 2 NaOH(aq) + CaCO3(s)
The limestone produced in this reaction is filtered off and sent back to the lime kiln, and the solution, containing lye and sodium sulfide is ready to be used again for making paper. By this process, pollution is drastically cut while dependence on purchased lye is reduced.
One remaining problem for this process is the possibility that sodium sulfide in the furnace may produce sulfur emissions. As we well know from metal smelting, sulfur compounds are quite nasty smelling. You are already familiar with sulfur dioxide. You may also be familiar with mercaptoethanol--the stench added to natural gas and propane. The problem of air quality around kraft paper mills is a continuing one and efforts to reduce emissions are costly to the mills.
Once the pulp is produced, it is diluted in water to produce a very thin suspension of cellulose. For handmade papers, the deckle (essentially a screen) is dipped into the pulp and lifted out, retaining a thin layer of pulp on the surface. Once the water has drained, the wet paper is transferred, or couched to a piece of cloth and then dried. In large-scale paper making, the deckle becomes a continuous belt rising out of the pulp and delivering the paper into a press, where the water is pressed out and a smooth surface is acheived.
The paper quiz will consist of three questions on the following topics:
You will use an alkaline pulping process for making paper from dried grass or straw. Each person making paper will need about 1 cubic foot of grass clippings or straw. You may cook your pulp as a tribe, but each person must make his own paper from the pulp. Douglas W. Jones has written an extensive set of instructions which you may follow. You may substitute a solution of 1.5 tbsp lye/gallon of water for the wood ash solution.
Your paper must be at least 8 inches square and be fine enough and smooth enough that you can write legibly upon it. It will most certainly be dark green or brown, though, a situation which we will correct when we discuss bleach.