Chemistry of Wood Pulp Paper

Paper is a common material used in the binding of books, primarily used for the textblock with text and / or images printed on it. The process of making paper has a long history that dates back thousands of years and spans many countries and continents. Wood is used as the raw material for making the paper in some manufacturing processes. When wood is used it tends to be pulped by mechanical or chemical means, or sometimes a combination of both of these. Using wood can result in many different qualities of paper depending on the wood used and the production methods applied. The papers made from wood pulp all present conservators with challenges when it comes to preserving them. There are many methods used by conservators to tackle the issues and depending on the paper in question and the surrounding context these can often be used to improve the lifespan of the paper.

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The earliest records regarding the manufacture of paper dates back to 105 AD in China where Cai Lun a court official is credited with inventing the process using textile rags (Holik, 2006). There is however archaeological evidence of paper fragments found in China that predate this (Basbanes, 2013). Despite efforts made by the Chinese government to keep the paper making process a secret it did eventually begin to spread, firstly to Korea and Japan and the along the silk road towards the Middle East and North Africa which it reached by the 9th century (Basbanes, 2013). During the 11th made it way into Europe where linen and hemp rags were the most common materials used. Throughout all of this time period changes and innovations were made to the equipment used for the paper making and different materials were used depending on the location and what was readily available and suitable for use (Holik, 2006). Some of these materials were rags some small shrubs but the main component contained in each of them that is used to make the paper is cellulose. In the 19th century wood comes into used as a new material for paper making in Europe.

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There are 3 major components that make up wood, cellulose, hemicellulose and lignin. Cellulose is the desired product for making paper as they form long chains that give the paper its strength and flexibility. Hemicellulose is similar to cellulose, but it forms in random structures which do not form strong and long chains meaning they are weak and not a desired product for the paper. Lignin acts to hold cellulose and hemicellulose fibres tightly together in the wood but are unnecessary for paper and have poor aging qualities making them undesirable (Batterham and Milne, 2014). Having lignin present in the paper can disrupt the hydrogen bonds that hold the fibres of the paper together (Biermann, 1996). Lignin makes up about 30% of a piece of wood (Basbanes, 2013).  Having a new material in wood new ways of producing the pulp containing the fibre used to make the paper were required, these new methods being mechanical and chemical pulping (Bajpai, 2012). Previously to this when old linen and cotton rags were being used the process had included letting the fibres begin to rot before beating them in a slurry with slacked lime. This lime added alkaline compounds into the fibres which remained after the paper had been made (Holik, 2006).

One of the most commonly used materials for the binding of books is paper. The majority of paper made for binding books from the 19th century onwards has been wood pulp paper. Before this paper was made from rags usually from cotton or linen. Paper is primarily used to create a books textblock, the paper is folded and gathered into sections before being sewn together. Generally, this paper will have text or pictures printed on it. The more modern wood pulp paper does not tend to age as well as earlier papers made from cotton and linen. When folded and bound into a book in this fashion there is a strain put on the fold in the page as it will be opened and closed during use. Paper that is stronger and has more flexibility will endure this stress better than a weak and brittle paper will. Different methods of manufacturing the wood pulp paper will result in paper with a variety of different properties. Paper in books is generally more at risk of damage along the edges of the leaves where the paper can be easily torn or folded. A lot of wood pulp paper will rapidly deteriorate and become more acidic over time as the materials making up the paper degrade. This often causes discolouration to the paper that can obscure the text or image on the leaf. Other modern books are made by adhering individual leaves directly into a cover, this is known as ‘perfect binding’. These are often cheaper books and therefore will often have lower quality paper used for their production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . . . . . . . . . . . . . .. . Mechanical pulping of the fibres is done by grinding them up and relying on pure mechanical force to break the bonds between fibres. This also tends to mean the fibres themselves breaking up into smaller fragments as they go through the treatment resulting in shorter fibres and a weaker paper as a result (Bajpai, 2012). The mechanical pulping method does not remove any of the lignin or hemicellulose so there also end up in the finished paper (Basbanes, 2013). The fragmented nature of the fibres while making a weaker paper also gives a flatter surface more suitable for printing on. Mechanical wood pulp paper does not tend to have good aging properties due to the amount of lignin that ends up in the paper (Bajpai, 2012). Mechanical pulping will result in a high yield, meaning that a lot of the material from the wood gets used in the paper and there is little wastage. The yield from mechanical pulping tends to sit around 92-96% (Biermann, 1996). Mechanical pulp makes up 20% of all new pulp material used (Bajpai, 2012). 

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Chemical pulping of wood separates out the fibres by using a chemical process rather than a mechanical one. The chemicals do no cut the fibres like in mechanical pulping so they are left intact and are longer and stronger than the mechanical pulp counterpart (Biermann, 1996). Theoretically chemical pulping is designed to remove all of the materials in the wood other than cellulose. Practically this never actually occurs as there will always be some amount of other material mixed in after the treatment (Bajpai, 2012). Chemical pulping works to break apart the fibres in the wood by removing the lignin which acts like a glue holding the fibres to one another (Basbanes, 2013). The chemical pulping can be done with different chemical processes, sulphate, sulphite and soda processes are the most commonly used ones (Basbanes, 2013). Because the lignin gets removed from the pulp the paper that is made from it will have better ageing properties. Because this process removes material from the pulp it dos not use as much of the original mass of the wood as mechanical pulping does. With Chemical pulping the yield is only 40-50% of the original mass (Biermann, 1996). Currently chemical pulping is the most common form of producing pulp for making paper (Bajpai, 2012). Wood pulp is not always made using only chemical or only mechanical pulping, often the two are used in combination (Biermann, 1996).

The end result of any paper being produced is going to depend on what exactly the material being used to create it is. The fibres from different kinds of wood are going to have different structures. This will affect the physical makeup of the paper and will determine how strong and how flexible the paper is. If the wood used for the pulp is a softwood it will likely have longer and stronger fibres which will give the paper strength. On the other hand hard woods produce smaller fibres which will produce a flatter surface more appropriate for printing on (Holik, 2006).

References

Bajpai, P., 2012. Biotechnology for Pulp and Paper Processing. https://doi.org/10.1007/978-1-4614-1409-4

Basbanes, N.A., 2013. On Paper: The Everything of Its Two-Thousand-Year History. Knopf Doubleday Publishing Group.

Batterham, I., Milne, C., 2014. 20th Century Paper Quality in the National Archives of Australia. Q. J. Br. Assoc. Pap. Hist. 29.

Biermann, C.J., 1996. Handbook of Pulping and Papermaking. Elsevier.

Dwan, A., 1987. Paper Complexity and the Interpretation of Conservation Research 18.

Gurnagul, N., Howard, R.C., Zou, X., Uesaka, T., Page, D.H., 1993. The-mechanical-permanence-of-paper-a-literature-review.pdf. J. Pulp Pap. Sci. 19, 8.

Holik, H. (Ed.), 2006. Handbook of paper and board. Wiley-VCH, Weinheim.

Lee, S.Y., Baty, J., Minter, W., n.d. Study of the Aging Behavior of Rosin-Alum Sized Papers by Analysis of Mechanical Strength, Optical Properties, and Chemical Composition Following Accelerated Aging 2.

Zervos, S., Alexopoulou, I., 2015. Paper conservation methods: a literature review. Cellulose 22, 2859–2897. https://doi.org/10.1007/s10570-015-0699-7