MadSci Network: Physics

Re: Why does paper shrink when it is soaked in water?

Date: Sat Mar 25 22:43:12 2000
Posted By: James Griepenburg, , Chemical consultant, Chemmet Services
Area of science: Physics
ID: 953204178.Ph


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: 
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  The cellulose fibers were originally made 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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