MadSci Network: Engineering

Re: What are 'grow toys' made of? Why do they expand in water?

Date: Mon Mar 31 13:20:16 2008
Posted By: Joseph Weeks, Engineer
Area of science: Engineering
ID: 1204659981.Eg

Adam, that is a great question. I called Kevin at Oriental Trading (, on the online retailers of these types of toys and he was quite helpful. His toys are made of Aqua-Keep, PVAC, and EVA, which are all various types of polymers, or very long chain molecules.

PVAC is an abbreviation for polyvinyl acetate which is a polymer mainly used as a binder or glue. White household glue is probably mostly polyvinyl acetate. EVA is an abbreviation for ethylene vinyl acetate, a soft and flexible polymer that is used in a variety of applications including mouth guards. As a thermoplastic polymer, it is the reason that mixtures of these three materials can be molded into a specific shape. The PVAC and EVA holds everything together, just like a cell membrane holds everything together in biology.

Aqua-Keep, as described here: is comprised mainly of sodium polyacrylate. Sodium polyacrylate was invented by Robert Niles Bashaw, Bobby Leroy Atkins and Billy Gene Harper working at Dow Chemical. The following is an illustration of this molecule before and after being exposed to water:

The real magic of these toys is due to the properties of sodium polyacrylate, one of several materials referred to as a "super adsorbent polymer." When exposed to water, the sodium polyacrylate swells and adsorbs up to 1000 times its own weight in water. Hydrogen bonding between the water molecules and the polymer allows so much water to be adsorbed. As the image tries to illustrate, the water molecules surround the polymer molecule, causing it to swell and stretch out. Chemically, the water molecules are trying to dilute the polar groups on the polymer chain.

A good introduction to super adsorbent polymers can be found here: The major use for sodium polyacrylate is in baby diapers, where the water adsorbing properties keep the diapers from leaking. These polymers are also sometimes added to soil to adsorb excess water when it is available, and then to gradually release it as the soil dries out. One other interesting use for these super adsorbent polymers is for making artificial snow for movies. Here is a great link that shows the process for making fake snow:

Because these polymers work adsorbing water, if there are dissolved salts in the water, they do not adsorb as much liquid as if they were adsorbing pure water. So, in a diaper, you are going to have to use more of the super adsorbent polymer to hold urine than if you were just trying to hold water.

A great science fair project might be to illustrate the effects of dissolved salts on osmotic pressure by putting varying amounts of salt in water in seeing what effect it has on the eventual size of the toy. Thanks for your question.

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