|MadSci Network: Other|
Thanks for the question, Kat. Waterproofing of paper extends far beyond the underwater writing paper you used. Cardboard milk and juice cartons need to repel water (or milk or juice) and even normal writing paper is able to control the amount of liquid absorption so the ink does not run and smudge. Sizing is papermaking jargon for the ability of the paper to hold out water. There are two main types of sizing: internal sizing, which is added to the pulp fiber when the paper is being made, and surface sizing that is added to the surface of the sheet of paper. Wood fibers are hydrophilic materials, meaning they like water. Therefore making waterproof paper requires adding something to the fibers to increase their hydrophobicity (water hating). These additives are known as sizing. The type of sizing used depends on the conditions used in the papermaking process (acid, neutral, or alkaline). In general, a hydrophobic chemical is bonded to the wood fibers and after the paper is formed, it repels the water that comes into contact with the paper. This type of sizing can range from waxes to modified tree resins to low-molecular weight carbon polymers. The additives used in surface sizing are very similar to those used in internal sizing but the application method is different. Here, an aqueous solution of the sizing is impregnated into the paper similarly to a coating operation. The paper is then dried to remove the water it picked up with the surface size. I would imagine that your underwater paper was made with a combination of internal and surface sizing to ensure the proper water holdout was attained. I hope this answered your question because sizing of paper is a fairly extensive topic.
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