pic2 shows a 2"x2"x2" solid silicone cube that cured enough in two hours to be removed from its plastic box form.



For years I have been looking for an inexpensive way to create a flexible skin covering for robots and electronic circuits. I have tried several kinds of casting urethane rubber and silicone rubber. They all have their difficulties and either set up to fast or too slow. They are too thin or they are too thick. They are also very expensive in small quantities. Added to that is the problem that they have a very limited shelf life and usually must be used within six months. Sugru is great, but it is not affordable for making larger structures.



I and many others have tried using the inexpensive silicone caulk that is readily available from hardware stores. It is used to seal roofing and glass windows. It works fine but has the problem that it can only be used by putting it on thinly and waiting a long time for it to cure. It is also hard to work. It must be smoothed immediately while it is very sticky. Otherwise, the surface cures quickly and then forms a gummy film while the inside remains soft and wet. It has a smoothing time of seconds rather than minutes. If you put it on too thick the inside will remain soft and can take several days to finally cure. People have tried all kinds of additives in an attempt to make it cure in a more useful manner. I have found those additives to be unusable for my purposes.



So I wanted to add a catalyst that would help the silicone to cure from the inside out rather than just from the outside in.



As I understand it, 100% silicone caulk works by the moisture in the air initiating the polymerization of the silicone. So it cures from the outside in and as it does, it allows the water vapor to slowly seep inside and eventually cure the unexposed silicone. While it cures, it gives off Acetic acid (vinegar is diluted acetic acid) which is the strong smell you will notice if you use it.



I experimented with quite a few additives to try and introduce some moisture into the uncured silicone. Several of them worked to some degree, but the hands down favorite was also the least expensive.



It turns out that corn starch is highly absorbent and when sitting around in an open box it will absorb moisture from the air. It is an extremely fine powder that diffuses evenly in mixtures. By adding the right amount of corn starch, the sticky silicone is somewhat stiffened and very quickly starts to set up from the inside out. While it still sets up faster on the surface than in the middle, the whole thing will set up in five minutes to 2 hours no matter what the thickness. The actual curing time depends on the temperature, the humidity, the amount of corn starch added, and the speed at which it was mixed.



So that's it. Oogoo is corn starch and clear silicone caulk mixed together and then molded by hand or by forms to create just about anything you can imagine that needs to be adhesive initially and solid yet flexible when cured.





