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How do erasers work?

These days, erasers may seem like a relic from history but anyone taking multiple-choice exams understands the importance of a good eraser. So how do they work?

These days we are accustomed to pressing the delete key and a word on the screen disappears, ready to be replaced by a few keystrokes. But that doesn't mean pencils and erasers have disappeared. On multiple-choice exams, little circles are still coloured in with pencils and if a mistake is made, an eraser is needed. Let's glimpse into their history...

When Europeans first came to South America, they found that the Natives were making interesting use of the sap of a tree. They formed the wet latex into balls and allowed them to dry. This is how the first bouncing balls were born! Explorers took some samples of the stuff back to Europe where Joseph Priestley, the brilliant English chemist who, among other things, discovered soda water and oxygen, found a use for the material. It would neatly rub pencil marks off of paper! He gave it the name "rubber", a name that is still used in England to describe, what we refer to, as the "eraser".

How does it work?

By the physical action of abrasion. Pencils deposit a thin layer of graphite on paper, a layer that can be removed with the gentle rubbing of an eraser. The trick, of course, is to remove the graphite without damaging the paper. So we do not want a substance that is too abrasive. Chemists have worked on this problem and have improved on the original rubber. For one, the rubber is vulcanized, meaning it has been treated with sulfur. This increases the resiliency and reduces the rigidity. A fine pumice powder made of about 75% silicon dioxide and 25% aluminum oxide is added to the rubber. This also serves as an abrasive. Basically, this is the stuff that is used on sandpaper but is ground extremely fine for inclusion in erasers. Why does it not work on ink as well? Ink is absorbed into the paper, unlike graphite. So rubbing the surface isn’t enough. Deeper penetration is needed, which of course, will damage the paper.

Why is the traditional eraser pink? Today, erasers are usually made from synthetic rubber, which is a polymer. When it is made, a chemical called an accelerator is added to help join the monomers, or small molecules, into a polymer. The original accelerator had a pink color; the erasers sold well and the tradition has been maintained. So much so that today, a pink dye is actually used.

Just like everything else, erasers have changed with the times. Modern science has led to improvements. Polyvinyl chloride, or PVC, may be familiar to people as the “vinyl” of car roofs or old-fashioned long-playing records, but this synthetic polymer can also be formulated into erasers. To make it soft and pliable, a plasticizer such as dioctyl phthalate is added, and to improve abrasion qualities some calcium carbonate, or chalk, is included. Vinyl erasers tend to be more gentle on paper than the rubber variety. They also have a few other advantages. For example, white vinyl erasers can clean ivory very well. You also don't have to worry about allergies with vinyl, as you would with rubber and latex. Of course, there is concern about the environmental consequences of phthalates which have been accused of having endocrine-disruptive effects. There’s no free lunch. But let’s not rub that in.


@JoeSchwarcz

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