Why does heat make paper curl up?

Paper is an essential part of daily life. We use it for a variety of materials and utilized for various purposes, such as writing, printing, wrapping things, decorations, currency, and even for industrial and construction processes. While it is undoubtedly beneficial for us, don’t expose it to heat and fire, or else, it will start to curl up and burn, rendering it useless.

But, have you ever wondered why heat or fire makes paper curl up before eventually getting burnt?

Well, it boils down to the paper’s composition. It is made up of cellulose fibers derived from various sources, such as rags, wood, grasses, cotton, and other vegetation. Then, it is incorporated with additives like chalk or china clay. What’s surprising is that the standard paper we usually use for various daily purposes actually consists of 5% water, even when it is dry. When it burns, it loses all its water components and finally turns the paper into ashes.

Unlike metal, paper isn’t actually a good conductor of heat. That means that heat does not reach the whole lamina immediately. When a particular region of the paper burns, only that specific part is heated, dehydrating the area of its water constituents.

Now, since the cellulose fiber is an organic polymer, or macromolecules consist of many repeating monomer units, its tendency is to stretch when exposed to heat. Meanwhile, the additives, chalk, and clay, just absorbs heat sans any changes in state.

As the paper burns, only the side of the paper that is heated loses water. The other side does not experience any change as the flame does not heat it to a similar extent. Then, as hot air is inclined to go upward for being less dense, only the heated side of the paper swings upwards. The other side stays the same, as the additives, chalk and clay, mop ups the heat released on the other. Thus, one side moves, the other side stays, leading to the paper’s curling motion upon exposure to heat or flame.

Different types of paper will curl differently. For instance, thicker papers will curl more when heated compared to thinner sheets. That is because the thickness creates more separation from the opposite sides that are heated. On the other hand, the thinner the paper is, the lesser the curling motion will be noticed in the one direction.

Moreover, the material used in the paper will also play an important role. The water percentage of different types of paper can range from 2 to 10 percent. The more water it has, the harder it burns. The typical paper we use at home or school typically has around 5 percent water. Meanwhile, the newspaper is composed of more water at nearly 10%.

If you’d like to see how paper curls up in different conditions, you may try getting a thick sheet of paper, such as a folder or postcard. Soak one of its sides in water, and you will notice that only the dry part of the item curls. To see the hot air’s role in the paper’s curling motion, get an ordinary sheet of paper. Hold the paper horizontally and burn one of its corners. You will observe that it will swing upwards. Holding the sheet upside down while burning one of the edges again will still give you an upward curling motion.

More Readings:

Paper (Wikipedia)

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