|MadSci Network: Physics
Harpreet, It's been my experience that paper will sometimes shrink after it has been soaked in water and dried out but not when it is still in the water. A good example of this is a US 1$ bill that has been thru the wash-dry cycle. I don't have a good reference on this and am drawing on my experience with polymers in general. There are several paper sites on the web listed here: http://www.tech centre.bc.ca/forestry/pulp+paper.html The one for McGill gives names and email addresses. There are several possible reasons for shrinking: 1. Polymer molecules in general exist in crystalline domains where sections of the molecule fold upon themselves to make a compact section. According to some sources [Morrison and Boyd, Organic chemistry for one] cellulose tends to exist as linear strands that are attracted to each other by hydrogen bonding and twist to make fibres. Water could break these intermolecular attractions and substitute intramolecular attractions upon drying causing each polymer molecule to take up less space. The cellulose fibers were originally made by plants and then mechanically treated and are not necessarily in the lowest energy formations of the individual polymer molecules. 2.Paper is usually rolled and dried under pressure of rollers. The dried paper is essentially stretched leaving residual strain in the paper. Wetting the paper allows the paper to contract when it redries without the stretching force being present. This combined with molecular folding rearrangement could also cause wrinkling of the sheet giving apparently greater shrinkage. 3.Paper is made with "sizing" materials. These materials are glues, gelatins, clays, certain soluble polymers and their purpose is to stiffen the paper and fill in the surface "holes: etc. They can be added as coatings after the paper is rolled or they can be added to the mix. In the latter case they coat the individual fibers in the paper itself and contribute to the volume of paper. These can be washed out of the paper and be another cause of shrinkage. These are ideas. Check out the web sites and send some emails with questions. also find a book on paper manufacturing that explains why some of these thing are done.
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