Re: Attn: Ken Johnsen, why does the rubber cement need to be applied to both?

Ben,

You are quite correct in your observation that rubber cement works by a 
mechanism of cohesion [also called autohesion, or 'self-sticking-to-self] but 
this is true both for the paper gluing example and the rubber gluing example, 
provided there is cement on both pieces of paper.

Cohesion occurs when the long polymer chains of the adhesive material are able 
to penetrate and mix with the polymer chains of the adherend [the substrate]. 
This process is also called 'interdigitation' in reference to the simile of 
fingers of opposite hands interlaced, as if in prayer. It is much harder to 
seperate hands with interlaced fingers than when the hands simply lay one on 
the other.

When rubber cement is used to bond rubber to itself, the solvent in the cement 
swells the substrates somewhat and facilitates the interdigitation process. 
After the solvent evaporates, it is hard to distinguish just where the joint 
lies. The bonding force is not a chemical bond  --  no bonds are made or 
broken; the strength of the bond is purely a physical phenomenon involving van 
der Walls and London forces between two intimately mixed and chemically similar 
non-polar hydrocarbon rubber molecules.

Paper is a polar carbohydrate, however, and wouldn't be expected to have any 
physical attraction to rubber. But, as you note, paper is porous and allows an 
adhesive to penetrate into its internal structure. This happens with rubber 
cement [just as with Elmer's Glue] when the solvent evaporates, leaving only 
dry rubber behind physically wrapped around the fibers in the surface of the 
paper.

When this dry rubber coating contacts another piece of plain paper, nothing 
happens, because the solvent that provides mobility to the adhesive is absent. 
But if another rubber coated piece of paper is encountered cohesion 
[autohesion, interdigitation] occurs rapidly and an adhesive bond is formed.

Elmer's Glue is made from polyvinylacetate [the same as found in common latex 
wall paint] and is polar, which favors interaction and attraction to polar 
cellulose, and is much more rigid than rubber and has a higher tensile strength 
which is all manifested in stronger paper-to-paper bonds than rubber cement.

Thanks for your perceptive comments, Ben, and keep reading The MAD Scientist!

Ken Johnsen