I’m sure everyone has seen it from time to time. You spill water on a sheet of paper, and it curls, deforms and warps. Why? Why can it go back to stay flat which is the original shape?

Paper curling

Addressing curl problems require the researcher to look at the process and very details at the structure of paper to find out why curl is occurring. To find out the reasons, there are few basics facts about paper that we need to know. Don`t be surprise if one of the reason is because paper is alive!

It is alive in the sense that it reacts to changes around the surrounding environment. If the relative humidity increases, paper absorbs water, and if the humidity decreases, it loses water. Paper fibers are shaped like little drinking straws, and as they absorb water, they get fatter but not longer. Have you ever wonder perhaps this is the same reason why libraries are always cold?

If you do, then you are right! As has been reported by Chloe Lei (2014), a librarian at Hong Kong Baptist University, there are cases where ten thousands of books have been destroyed in mold outbreaks.  This is due to the conditions of warmth and humidity that encourage the growth of mold and proliferation of pests such as insects.

Paper is not homogeneous. Since paper is thin, we tend to think that a piece of paper would have the same level of flat surface. In fact, paper has top and bottom. If the moisture content of the paper changed, the top and the bottom of the sheet often react differently. That differences result in paper curl (Charles Green & Jim Atkins, 2011).

Paper cross section
Source: The Problem Of Paper Curl

Apart from this, paper is also directional. Due to the manufacturing process, more of the fibers are aligned in the web flow direction, or machine-direction (MD) that sometimes referred as ‘grain-long’. In easier words, paper have grain just like wood. The grain in paper comes from how the fibers of the paper are arranged.

Since more of the fibres are aligned in the MD, when the fibres swell due to increased humidity, the sheet usually curls with the axis of curl in the MD. Paper has built-in stresses due to the manufacturing process.

If paper is moistened on one side only, the fires on that side swell, and that side of the paper gets longer, so the paper curls away from the wet side. As the wetter side of the paper dries out, the paper slowly loses its curl and then is flat again. But then a curious thing happens.

It does not stop there. The paper then begins to curl toward what was the wetter side. The moisture has relieved some of the stresses in the one side of the paper and that side gets shorter. So curl forms (or moves) toward the side from which moisture moves last or toward the side from which moisture migrates last.

There are various types of curl that can be observed in paper. Based on the shape and the principle axis of the curl, we can define it as CD (cross-machine direction), MD (machine direction) and diagonal.

If the curl axis is along the MD direction of paper, we call it CD curl. Similarly, MD curl has axis along CD direction, and diagonal curl along diagonal direction. Dependent on the side of the curvature (concave side), each form is further classified as top side or bottom side curl.

To understand why the paper does this, we have to take a look at what paper is made of. Now what are paper made of? Most papers are made out of trees and the raw material for paper is cellulose. Cellulose is a substance found in plant cells which allows the plants to remain hard and strong.

Cellulose
Source: Shutterstock

Cellulose consists of three elements: carbon, oxygen and hydrogen (C6H10O5). Cellulose is made by hydrogen bonding in water without special adhesives or chemicals. Basically, paper made of cellulose looks smooth with the naked eye, but if you look closely under a microscope, there is a lot of empty space between the connected molecules.

Cellulose molecules
Source: Advancing the characterization of cellulose materials with state-of-the-art equipment

When paper gets wet, water molecules enter the void and break the bond. That’s why papers with disorganized molecular structures no longer have a smooth appearance.In addition, paper contains ingredients such as grass and gelatin, that becomes less bulky when they are soaked in water.

Because of this paper changes its shape when it gets wet. The nature of cellulose in paper is also related to the paper changing its shape. Cellulose fibers in paper have different diameters and lengths, so when the paper soaks in water, some of the cellulosic fibers quickly dry up and shrink quickly, while others dry slowly, reducing the fiber’s size slowly.

Because of this time difference, when paper dries up, it becomes deformed. So it’s because of this small fibres of different cellulose that your toilet paper gets so soggy that it can’t be used anymore.

Wet paper

 

Thankfully you can improve the condition of your wet paper if you know some tricks. One of these tricks is freezing the paper. When paper freezes, it increases in volume. So when wet paper is put in the refrigerator, it spreads out allowing it to become smoother in the process.

Drying the wet paper with an iron also improves its smoothness. This is because the difference in the drying speed of the stamping of the cellulose fibers in the paper is reduced due to the temperature and pressure of the iron, reducing deformation.

References:

  1. https://indianapublicmedia.org/amomentofscience/chemical-cellulose-paper/https://www.bbc.co.uk/bitesize/topics/znyycdm/articles/z2d2gdm
  2. https://www.wikihow.com/Dry-Wet-Paper
  3. http://www.cartonhub.com/en/paper-curl-issues/
  4. https://www.academia.edu/11217381/The_problem_of_Paper_Curl?auto=download
  5. https://www.strathmoreartist.com/faq-full/how-can-i-determine-the-grain-direction-of-paper.html
  6. https://www.scientificamerican.com/article/folded-or-flat-paper-towel-which-one-absorbs-more-water/

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