If you saw Griswold's post on the new wahoo gun that he is working on, you may have caught his mention of "syntactic". This is a material that we have been playing with for a couple of years now, and is the core material that we used for most of our carbon fiber guns.
Regular foams, like the two part polyurethane foams, consist of air bubbles trapped in rigid polyurethane. They have an obvious limitation in that the air bubbles can break, and saturate with water. This can happen even in the foam floatation of boats, so you can imagine that the problem could be even worse at the increased water pressure when diving. As the foam absorbs more water, it loses buoyancy, which is usually the thing it was supposed to provide in the first place. Heavier density foams are more resistant, but not bulletproof.
Enter syntactic foam. Syntactic foam gets its buoyancy from tiny hollow glass spheres (sometimes called micro balloons) mixed in an epoxy resin. Glass has very high compressive strength, and the uniform spherical shape of the micro balloons makes syntactic foam crush proof to amazing depths. This is the stuff that they use to provide buoyancy on deep sea submersibles, and floatation buoys for deep oil drilling applications. Depending on the types of resin, and size / thickness of the glass spheres, it can hold its shape at thousands of feet deep. The advantages in gun making are clear - The spheres don't absorb water, and the epoxy will never rot.
We experimented with quite a few different versions of the stuff. The Polywood product from Industrial Polymers http://www.industrialpolymers.…id_casting_urethanes.html is a mix and pour material that can be used to fill tubes, hollow handles, etc. It has a specific gravity of about 0.53, which means that it is only slightly lighter than teak. But it cuts and machines well, so it's a nice option for filling an odd shapped space.
We made some syntactic of our own for a while, using 16 lb density polyurethane foam and ceramic microspheres. It works, and it's inexpensive, but it is definately not as stiff as the real deal.
Later, we used some pre-fabricated blocks from CRG at http://www.crgrp.com/syntactics.shtml. This is the stuff that we used to shape the core for most of our carbon fiber guns, and for the tube on my current hybrid gun. When cut into a long slender shape, it is not quite as stiff as teak. But it is considerably lighter with a specific gravity of around 0.45. That is important when using the carbon fiber, because the CF is quite a bit heavier than wood. The syntactic cuts very nicely with wood working tools, though the glass beads are abrasive and will dull blades quickly. Because it is an epoxy resin, it bonds very well with the epoxy saturated carbon fiber cloth that we laid up around it. It holds screws and other fasteners surprisingly well, about like soft pine. For areas that see higher stresses, like around the trigger mechanism or reel mount, we typically use a piece of delrin embeded in the syntactic for extra strength.
Syntactic is not as strong as wood when it comes to bending, so it could never replace wood on its own. But that was not an issue for our carbon fiber guns. We just needed something to define the shape. Once cured, the carbon fiber does all the work. The syntactic is definately not a perfect material. But for certain applications, it sure is a nice tool to have in the box.
At one point, Chad and I were very careful and considered our use of the syntactic materials to be our little secret. But building guns is not something that we do to feed our kids, so the time seemed right to share.
