|MadSci Network: Chemistry|
Thanks for your question. Because papermaking is a complex operation I will give you an overview of how paper is made, and then I will discuss some of the specific processes where sodium hydroxide is used.
You can look at a virtual tour of the history of papermaking that will give some details about modern technology at http://www.afandpa.org/Content/NavigationMenu/Pulp_and_Paper/Paper_-_the_history_and_making/Paper_-_the_history_and_making.htm
The papermaking process is divided into two main parts. The first involves pulping, the process of removing the wood fibers from the wood. The second is papermaking, where the fibers are formed into a sheet of paper.
Pulping is when small woodchips are treated mechanically or chemically to separate the individual fibers from each other, the way they are in the tree.
Mechanical pulping uses elevated temperatures and an abrasive surface to pull apart the individual fibers. The lignin that holds the fibers together in the wood remains with the pulp. (Lignin is like a glue that holds the fibers together to give the tree strength. It is also responsible for paper turning yellow as it ages.)
Chemical pulping utilizes chemicals to dissolve the lignin and free the pulp fibers. Chemical pulping is the most common method of pulping. There are several types of chemical pulping, but I will not discuss them here. (If you want more information, look in the Handbook of Pulping and Papermaking by Christopher Biermann.)
After the fibers are released from the wood, they can be bleached to increase the brightness of the pulp. This can also be done in a number of ways that I will not elaborate on here. After bleaching, the fibers are sent to the paper machine, where they will be formed into the type of paper desired (newspaper, linerboard, tissue paper).
The fibers are mixed with large amounts of water before they are sent to the paper machine. (The initial dilution has every 1 g of fiber diluted with approximately 200 g of water, or about 99.5% water.) This mixture of fiber and water, known as slurry, also contains any of a number of additives used to alter the strength or optical properties of the paper. The slurry is sent to the paper machine where it is placed on a moving wire mesh screen where most of the water will drain away from the fibers.
After some of the water drains away from the fibers, it moves to the press section where water is removed as the newly formed sheet of paper is squeezed between rolls. After the press section, about 50% of the sheet is still water. The remaining water will be removed by heating the sheet to evaporate the water from the fibers in the dryer section. This is done mainly by heating large metal cylinders and running the paper through them until it is dry.
Paper is made continuously on large paper machines that are sometimes over 10 meters wide. The machine is very long, mostly because of the dryer section, often reaching lengths over 150 meters. At the end of the machine, the paper is collected on a big roll until it reaches the desired size (possibly around 20 tons). The paper is then cut into sizes that are easier for consumers to handle.
Now, I will answer the question about using sodium hydroxide in the process. Because paper is made in so many different ways, it is impossible to give you every type of situation where any chemical is used. I will give you some of the common uses. Most are associated with the pulping process that I described previously.
Sodium hydroxide is used to help regenerate the chemicals used to pulp the wood chips into fibers in the chemical pulping process, which allows the pulp mill to reuse many of the chemicals and reduce the cost of producing pulp. It is also used in one of the stages of some common bleaching processes to increase the efficiency of the particular stage of the bleaching.
Sodium hydroxide is also used is in paper recycling. The sodium hydroxide causes the fibers to swell when they are placed in a solution with the sodium hydroxide. This swelling helps to separate the ink that is attached to the fibers allowing the fibers to be reused.
The final application of sodium hydroxide that I will present is simply for pH control. It is used for this in many processes where the efficiency of the process is affected. In these applications, the goal is to optimize the chemicals being used to save money and make the process more environmentally friendly. For example, when sodium hypochlorite is used as a bleaching chemical, sodium hydroxide is used to help keep the pH as close to 8.0 as possible, because sodium hypochlorite is most effective at that pH.
Try the links in the MadSci Library for more information on Chemistry.