The chemistry of erasers (rubbers)

[CaptainPanic]

I just made a small error, and I wanted to erase some of my writing on paper. I took the eraser I have, and removed the pencil writing. Then I realized that I have no idea how that works!

 

First of all, when I write, the friction between the (soft) graphite and the paper causes some graphite to stay behind in and around the fibres of the paper. That part I understand (although it's a bit crude). But how the hell can an eraser get that graphite off the paper again? Does it have such affinity for graphite? Is it just an interfacial phenomenon, or is there a reaction?

 

Wikipedia does not explain how it works. This website says that the graphite "sticks" to the rubber (not a very scientific explanation).

 

For something so common, I feel ridiculous that I don't know it.

[doG]

It's an abrasive process basically. The eraser scratches the graphite particles off of the fibers of the paper. Harder erasers will also remove some of the fibers from the surface of the paper. That's why erasers made to remove ink are much harder than pencil erasers because you generally have to remove the fibers stained with ink to remove ink marks on paper. At the other extreme are kneaded erasers that are designed to be gentle on the paper and absorbent to soft materials like graphite or charcoal.

[StringJunky]

I just made a small error, and I wanted to erase some of my writing on paper. I took the eraser I have, and removed the pencil writing. Then I realized that I have no idea how that works!

 

First of all, when I write, the friction between the (soft) graphite and the paper causes some graphite to stay behind in and around the fibres of the paper. That part I understand (although it's a bit crude). But how the hell can an eraser get that graphite off the paper again? Does it have such affinity for graphite? Is it just an interfacial phenomenon, or is there a reaction?

 

Wikipedia does not explain how it works. This website says that the graphite "sticks" to the rubber (not a very scientific explanation).

 

For something so common, I feel ridiculous that I don't know it.

 

One explanation I came across is the friction causes rubber bits to come off the harder erasers which forms small spindles and these spindles reach between the fibres and pick up the graphite fragments on the outside of them.

 

The kneadable erasers reach between the fibres and wrap its material around the fragments...they get blacker with time,

Edited April 17, 2012 by StringJunky

[Suxamethonium]

At the same time, you can erase pencil using other things- e.g. scrunched up masking tape, other pieces of paper, etc. Some work better than others.

 

.. I think the point I was making was I don't know if it is specifically a chemical process, it might just be mechanical (with the chemical properties of the polymer designed to make it more preferable for the purpose (i.e. flexible, non-abrasive towards paper, etc).

[CaptainPanic]

Thanks for the feedback so far.

 

Let's talk only about the erasers that are gentle on the paper: those which can only remove the graphite.

 

I guess the most important question I had was what makes the graphite stick to the rubber? Are we talking some good old vd Waals attraction? And is the difference in polarity of any significance, like when you're determining whether something will dissolve into a solvent or not? The rubber and the graphite are apolar, but the paper is polar (paper = cellulose = sugar polymer). Or are we dealing with some double bond interactions? Could it even be covalent (I guess not)?

[ewmon]

I know that "dried out" erasers don't erase, so softness matters, along with the chemical(s) that make them soft. I also know that rubber washers used in faucets will dry out by simply sitting on the shelf, so if you buy a package of them and use only a few, they will dry out after a few years and not be soft and pliable enough to seal against a leak.

[Suxamethonium]

This is probably a stupid question.. but how does an eraser dry out? (If you mean the cracking seen in erasers, particularly at the ends of pencils, I thought it was just UV/light degradation of the rubber polymer).

 

From http://www.madehow.com/Volume-5/Eraser.html

 

An eraser is mostly rubber (synthetic or latex), with a few extra excipients for things like colour/flexibility etc. The rubber is vulcanised for stability- but the article implies this is not related to the erasers ability to function. This would imply non-polar interactions with the graphite. However, the article also said that bread was used effectively before the use of rubber/latex was widespread. Starch and cellulose and other polymers in bread are all very polar... Hence I still feel that it is more the mechanical 'rubbing' action that is most responsible, perhaps the non-polar interactions of synthetic/natural rubbers give it the improvement seen when comparing bread and rubber as erasers?

[ewmon]

how does an eraser dry out?

I don't know, but rubber bands and automotive radiator hoses and vacuum hoses also dry out. The rubber becomes like hard plastic and inflexible. Have you seen any old rubber bands?

[hypervalent_iodine]

That's not a process of 'drying out' though. As Suxamethonium said, they become brittle when the polymer chains are degraded/broken up. This can happen due to overuse (i.e. too much mechanical strain), UV exposure or due to heat.

[ewmon]

That's not a process of 'drying out' though.

Okay, thank you.

 

With erasers, I was thinking of those ancient pencils we often find tucked away in a drawer, etc. The graphite writes perfectly fine, but when we try to use the eraser — wow, what happened? I assumed that what happens to rubber washers happens to erasers, and I was told that rubber washers used in faucets will "dry out" by simply sitting on the shelf. What about the excipients that give erasers their flexibility — would they eventually evaporate?

 

So, regardless of what hardens soft erasers, not only do they no longer erase, but they leave marks themselves. Flexibility seems to correlate with erasability. I also think everyone can agree that a certain amount of abrading of the eraser occurs with erasing (except maybe with those kneadable grey art erasers ... but are they made of rubber?).