Why can an eraser erase pencil marks?

Pencils and erasers are both convenient stationery. We can write freely and erase easily. Why can erasers erase words written with pencils? What are pencils and erasers made of? What is the difference between pencils and colored pencils? Let's understand the principles of pencils and erasers.

The lead of a pencil is made of a mixture of "graphite" and "clay", sintered at a high temperature of more than 1000℃, and then treated with oil. When we write or draw on paper with a pencil, the lead will rub against the paper fibers, and the graphite particles in it will be scraped off and arranged on the surface of the paper (see the figure below). In this way, the pencil can write or draw on paper. Although the surface of the paper looks flat to the naked eye, in fact, the paper is uneven because the fibers of the raw material "pulp" are mixed together when making paper. Therefore, the pencil lead can be scraped off by these fibers, and the graphite will remain on the paper.

When you write on paper with a pencil, the graphite particles in the pencil core are arranged on the surface of the paper (left). When you rub an eraser on the paper, the stickiness of the eraser causes the graphite particles to adhere to the eraser, causing the text to disappear (middle). The right picture is an enlarged view of the eraser structure, where plasticizers and fillers are sandwiched between vinyl chloride molecules.

In addition, pencils are marked with hardness marks such as "H" and "B", with a total of 17 types. These differences in hardness are produced by changing the ratio of graphite and clay. The higher the proportion of clay, the harder the lead will be, and the amount of graphite left on the surface of the paper will be reduced accordingly, so the text will appear very light. Conversely, the softer the lead, the higher the proportion of graphite, and the thicker the handwriting will appear.

Part 1

Attach graphite to the eraser to erase the writing

Erasers can erase graphite particles on the surface of paper. They are usually a very flexible plastic, made of a mixture of "polyvinyl chloride" and "plasticizers" that can enhance the flexibility of plastics. Plastics made using this process have a higher viscosity than paper. Therefore, compared to paper, graphite particles arranged on the surface of paper are more easily adsorbed on the eraser. After rubbing the eraser on the paper a few times, the force used during the friction and the viscosity of the eraser cause the graphite particles originally arranged on the surface of the paper to be transferred to the eraser. In this way, the eraser can erase the text written by the pencil. The used part of the eraser will fall off in the form of eraser crumbs, revealing a brand new side. The erasers commonly used now have "filling materials" such as calcium carbonate sandwiched between the molecules of polyvinyl chloride. Because of the presence of this filling material, the molecules of polyvinyl chloride are not easy to stick together, and the external force applied to the eraser can destroy its shape. Therefore, after rubbing the eraser on the paper a few times, the eraser crumbs will fall off the eraser.

Part 2

Use "Frosting" to erase color

"Frosted eraser" is similar to eraser. But the difference is that it is made of a sharp particle - "quartz sand" mixed with "synthetic rubber". The main purpose of frosted eraser is to erase colored ink. The method it uses is not to absorb the ink particles that penetrate into the paper onto the eraser, but to use the quartz sand particles to scrape off part of the paper, thereby erasing the ink color.

Part 3

Eraser crumbs are produced differently

There are different types of erasers. Some erasers easily produce eraser crumbs after erasing text, and the crumbs are relatively fine; others produce a large mass of crumbs that tend to gather together. According to Ushiwakama, director of the quality assurance department of Tombow Pencil Co., Ltd., erasers that easily produce eraser crumbs and can easily erase text contain small bubbles inside, and the crumbs easily fall off the eraser. On the contrary, if eraser crumbs tend to gather together and it takes a lot of effort to erase text, it should be that some kind of "polymer" (polymer), that is, thin and strong fibers, has been added to the eraser, which makes it difficult to separate the eraser crumbs from the eraser.

If you leave your eraser in a pencil case or desk drawer for a while, it is very likely that the eraser will stick to the plastic ruler. This is because the plasticizer in the eraser migrates to the surrounding plastic products over time.

In rubber, plasticizers exist between the molecules of polyvinyl chloride. However, under the influence of high temperature and other conditions, plasticizers become very mobile and may migrate to other plastic products that come into contact with the rubber. To prevent this from happening, rubbers are generally stored in places where they are not easily exposed to plastics, such as wrapping them in paper.

Part 4

Colored pencils cannot be erased because they seep into the paper

Colored pencils are very similar to pencils. But unlike pencils, the writing written by colored pencils is difficult to erase with an eraser. Why is that?

The lead of a colored pencil is made by mixing and solidifying a coloring ingredient, "pigment" (colored powder), and a fat-like ingredient, "wax." The pigment is dispersed in the wax and becomes one with it.

The top half of the image shows the difference between writing on paper with a pencil (left) and with a colored pencil (right). The bottom half of the image shows a magnified version of each. When writing with a colored pencil, the color spreads until it fills in between the fibers of the paper.

The pigment and wax particles of colored pencils penetrate into the "fiber gaps" of paper (pictured above). It is difficult to reattach the particles that have penetrated into the fiber gaps to the surface of the eraser, so the colored pencil writing cannot be erased cleanly with an eraser. If you insist on erasing it, the heat generated by the friction between the eraser and the paper will make the wax flow easily, causing its color to spread around, so you need to pay more attention to this.

Source: Science World