Papermaking has been around for about 4,000 years. The Egyptians had a process of making papyrus, a plant based paper product. The Chinese in the 2nd century BC had a paper making process involving a fibrous matter. In the 8th century, Arabs had learned the art of paper making, which explains the rise of their intellectual endeavors and learning.
It is one of those basic products that make our civilization what it is. But how is it made? Although the process has been refined over the years, the process today is less due to innovation than to efficiency of existing processes. It uses wood and it consists of eight stages: debarking, chipping, pulping, bleaching, paper machine, blade coater, supercalender, and sheet converting.
The Composition of Wood
Wood is a renewable resource that consists mainly of cellulous, lignin, and a small portion of terpenes, resins, and fatty acids. The breakdown of wood is 45% cellulose and 30% hemi-cellulose. It also contains 23% lignin, which is a resinous material that binds the cellulose fibers together. It has less than 5% substances like terpenes, resins and fatty oils and acids.
Because the process starts with cutting trees, the bark from trees must be removed before the wood can be used for papermaking. A large, spinning and rotating drum that makes the logs move around is used to debark the trees. As the logs rub against each other, the bark comes off.
Before the logs are pulped, they are cut into small pieces or chips. A large rotating knife goes in circles and cuts the logs into chips.
The pulping process separates the cellulose and hemi-cellulose from the lignin and removes other tree oils and resins. This process is important because the remaining fibers are used to produce the product we know as paper. There are two main pulping processes, chemical pulping and mechanical pulping.
Chemical or Kraft Pulping
Chemical pulping produces very pure cellulose fibers. It is the most common form of chemical pulping in North America. In Kraft pulping the wood chips are cooked in sodium hydroxide and sodium sulfide to produce a strong dark brown pulp. It is a highly efficient process for removing lignin and resins in softwoods while still producing a high-quality pulp. Wood wastes are burned for fuel and more than 95 percent of the pulping chemicals are recovered for re-use.
However, there are some problems. The release of hydrogen sulfide and the mercaptan family of sulfides can cause the characteristic paper-town smell of rotten eggs, although the use of scrubbers in mills has reduced odor emissions over the past 20 years. A second problem is that cellulose fibers that are lost during the Kraft process can be discharged with wastewater, and can build up fiber beds, causing environmental problems around wastewater pipes. In this case too, technical improvements have largely eliminated the problem. Because of the dark color of the resulting pulp, however, Kraft fibers require considerable bleaching to make them usable.
The second pulping process is mechanical pulping. It is a process that forces debarked logs against a grinding stone or rotating metal disks to produce pulp. This is both good and bad. The percentage of usable pulp is higher, but the resulting paper quality is lower. As much as 95 percent of the wood resource is turned into pulp, in contrast to Kraft pulping which is only 45-50 percent.
But the paper quality is lower because mechanical pulping is an energy-intensive process that is not as efficient as Kraft because the lignin and tree resins remain in the pulp. The grinding process breaks the cellulose fibers when tearing them apart, which decreases pulp strength. As a result, the paper has a weak fiber network and the high lignin content, which causes it to darken when it is exposed to sunlight. Paper produced by this process is used mainly for newsprint, telephone books, and similar applications where the high quality paper is not really needed.
Steam and Chemical Treating
In a variation on the mechanical pulping process, the chips are steam-treated beforehand. This process also called thermo-mechanical pulping (TMP) reduces the energy consumption process by softening chips with steam prior to grinding. And then going a step further, in chemo-thermomechanical pulping (CTMP) the wood chips are soaked with sulfur-based chemicals prior to steaming. This allows the lignin and resin to be extracted from the wood, which results in a stronger pulp. Pulp produced by CTMP can be used in the production of coated papers.
Bleaching is a very important step. Bleaching is used to purify and clean the pulp. Bleaching removes lignin, which affects the purity of the fiber. Kraft mills use chlorine gas and chlorine dioxide to bleach pulp, while most mechanical pulp bleaching operations use peroxide. Chlorine gas removes most of the remaining lignin, while the chlorine dioxide whitens lignin that cannot be removed.
- Paper Machine
The paper machine involves four sections. It starts with a step called stock preparation. During stock preparation, wood fiber is mixed with water and minerals in a mixture that is more than 99% water. In modern paper machines, the water is drained from the top and the bottom. When the water drains, a web of paper is formed. The next step is the press section. Here the paper is squeezed between rollers and a felt (a large mat of nylon and polyester filaments). This removes more water from the paper. After the press section, 40-50% of the paper is still made up of water. In the third section, the dryer section, dryers remove even more water from the paper. After the dryer section, only 2-6% of the paper is made up of water. In high-quality machines, a size press coating is put onto the paper next. A size press coater applies a light coating of starch and minerals. This makes the paper even stronger and smoother. After the size press coating is put on, the paper is dried again in the after-dryer section. Finally, the paper is wound into a giant roll.
- Blade Coater:
For coated papers, a blade coater puts a thin coating onto the finished paper. The paper glides under a blade that deposits a coating onto the paper surface. The different coatings make the paper very good to use in printing.
This process makes the paper glossy and smooth. The supercalendering machine is made up of many large rollers. The rollers put the paper under a lot of pressure and heat it up. After supercalendering, the paper is cut into smaller rolls.
- Sheet Converting:
Sheet converting is the last stage of papermaking. Some of the paper rolls are cut into sheets, while others are packaged for shipping to customers.
The steps outlined above have been around for centuries. The production techniques have been modified, made more efficient, and cost-effective. However, wood is still the basis of papermaking, and that is an old product.