Gas Permeability of vinyl tube; and why it sucks.

A project log for Modular Continuous Track System

A inexpensive modular tank track system that can be used for robotic and radio controlled vehicles.

jupdykejupdyke 09/06/2016 at 21:110 Comments

It has been a little while since I really posted anything other than a brief update. I really have not had anything interesting to publish or share. I moved my setup to my basement, this required first cleaning up the basement, building a workbench, moving everything, running new outlet wires, adding lights and getting a dehumidifier. It was a lot of work, but worth it to have a dedicated space.

I have not been running the resin molding system too much lately. This is mostly because I got parts injection molded and have several thousand black Tank Chain treads. :) Since the vast majority of my orders are black track, I no longer mold those using my resin system. I only make the custom colors now.

This means that the resin system has been sitting for quite a while. One issue with the resin I am using is the shelf life once opened. The Part A resin will harden without being mixed with Part B. And I have noticed that Part B will turn almost black. This completely ruins it for making custom colors because once it is black it is nearly impossible to get any other color.

I have been storing my resin in my custom dispenser using Corny Kegs and they work GREAT! I knew the resin would harden if left open and didn't want it to harden inside the kegs. So I added some R134a to the kegs as a dry gas blanket. This worked great, and even after a month in the keg the resin was still liquid. But I noticed the Part A in the clear vinyl tubes coming from my keg was solid, and the Part B was black. This was never an issue when I was making a lot of parts becuase I was running the resin through the tube every day or two.

Lucky for me I had enough spare tube to cut it off and replace it. This was annoying but only took 1-2 hours to get everything back into working order. But it made me think, why was the resin hardening in the tubes?

Well I am not 100% sure yet, but I have a good guess. After talking with my buddy who brews beer, he mentioned that the vinyl tube has some gas permeability. Meaning that gas leaks through the tube. Normally this is not much of a problem. The amount that leaks out is a small percentage. But if you are storing CO2 kegs under pressure with 20 feet of vinyl tubing, the percentage might be enough to care about.

For my application it is the opposite problem. The air can be getting through the vinyl tube and causing the resin to harden. It doesnt happen fast, but after 2 weeks I now have a clogged up dispenser.

Here is a good link which gives some approximate numbers for how much gas can leak from various types of tubes.,E,C&htmlfile=SelectingTubing.htm&Title=Search

If you look at the bottom you can see that vinyl tube has a value of 360 for C02, 40 for O2, and 80 for N2. The units are a little confusing at first {cc-mmsec-cm2-cm Hg } x 10-10, but basically you need to know the pressure the tube is under and the surface area. I don't need to calculate the value, but I can use these values to select a better type of tubing.

I am not 100% sure that this is the problem, but it definitely makes sense. Being exposed to humid air definitely causes the resin to harden. This is why having the R134A dry gas blanket in the kegs is so important. Another idea I had was UV light exposure could be hardening the resin. Simple solution to both is to order some opaque tube which is not air permeable.

I will report back my results when I get them.