Why does Elmer's glue change color as it dries?

Some days, I watch paint dry. Other days, it's laundry. Today, I watched Elmer's glue dry. Yes, I know. Everyone is envious of my exciting life.

A funny question hit me the other day, "Why does Elmer's glue change color as it dries?" When you squirt glue out of the bottle, it starts out a milky opaque white. As it dries, it changes color. The final color is clear. I had an inkling about why this might happen, but I thought I should double check some facts before I settled on that explanation.

Wet glue (right), partly dried glue (left), and
closeup of partly dried glue

What is Elmer's glue made out of?

The first thing I wanted to know is "what is the stuff made out of"?  Everyone knows that it's made from trimmings from the hoofs of heifers when they go to the salon. Right? Naturally, I went to the Elmer's glue website to verify this.

"Elmer's Glues are chemical based. They are made or formulated from chemicals which are synthesized (created by Man). These chemicals were originally obtained or manufactured from petroleum, natural gas and other raw materials found in Nature. The exact formula and specific ingredients used in making Elmer's products are considered proprietary information, therefore, we cannot share those with you...

Elmer's does not use animals or animal parts to make glue. Our products are made from synthetic materials and are not derived from processing horses, cows or any other animals."

http://elmers.com/about/faqs/general/

So I booked a flight to Columbus, Ohio where Elmer lives. On the way to the airport, I picked up a cool spy camera, some camouflage pants, wire cutters (to cut through the barbed wire), and a couple of Snicker's Bars in case I needed to bribe a security guard. Or if I got hungry. I go all out for these blog posts, I tell you.

I suppose I could have just looked up "Elmer's glue" in Wikipedia. Then I could maybe have followed through to the link on polyvinyl acetate. But that thought just didn't occur to me until I was crouching down at the fence, dodging the searchlight from the guard's tower, and was trying to cut through the fence with nail clippers. My psychiatrist doesn't want me to have a wire cutters.

Here's what Wikipedia would have told me: Elmer's glue is an emulsion of polyvinyl acetate in water.

I don't know about you, but I was really excited when I heard this from the security guard as we chowed down on Snicker's bars. Luckily, he has a fondness for Snicker's bars, or I would be writing this from a maximum security prison. 

The clues are falling into place nicely. Now, all I need to know is the index of refraction of PVA. A little more googling, and I came up with this wonderful tidbit. The refractive index of polyvinyl acetate is 1.46 to 1.47. That's exactly what I wanted to know.

Explanation of wet glue 

Elmer's glue is primarily made of particles of polyvinyl acetate suspended (technically, emulsified) in water. It looks milky because those particles have a different index of refraction than the water they are in. (Water has an index of refraction of about 1.3.)  

Fresnel's law says that when light encounters a change in the index of refraction - as when light hits a polyvinyl acetate molecule - some light reflects. As a result, it changes direction. The same thing applies to light inside a polyvinyl acetate particle. Some light reflects. If there are enough of these encounters, none of the incident light will make it through to the underlying substrate, which is to say, the stuff will be transparent.

The actual photomicrograph shown below is an actual photomicrograph of particles of polyvinyl acetate in water. (This is wet Elmer's glue, by the way.) Imagine that you are a cute little photon (a tiny tiny particle of light) trying to make your way through this glop of wet glue. Anywhere you go, you run into a particle and you bounce. Chances are, you will bounce back in the direction that you came.

Particles of polyvinyl acetate suspended in water

(actual photomicrograph)

Now imagine that you are a person watching a zillion of these photons. What will you see?

First point... Not many photons are going to make it all the way through this glop of wet glue. That is to say, if you look at the wet glue, you won't see anything on the other side. In other words, wet glue is opaque, or at least mostly kinda.

Second point... Water is mostly kinda clear. That is to say, as photons bounce around between particles of polyvinyl acetat, they are likely to keep bouncing until they exit, rather than get absorbed. Note that this is also true of polyvinyl acetate. Note also that this holds for all wavelengths of light. In other words, wet glue is white, rather than gray or blue.

So what happens when the ink is dry?

The key thing that makes glue opaque and white is the combination of two components that have different indices of refraction. When glue dries, one of those components go away. Dried Elmer's Glue is mostly just a monolithic mass of polyvinyl acetate, so there is no opportunity  for light to bounce around. Light mostly just kinda goes right through the dried glue. And hence it's clear.

So, now you have an answer when you kindergartner asks why glue changes color. Naturally, when I explained this to my kids - years ago - they responded "Oh... of course, Dad. It's because of the Kubelka-Munk parameters."