Further tests were conducted using different forms of both propylene and ethylene glycol. You can see the different tests on my experimental propellants 2 page. In the end, it seems food flavor extract works the best of all the locally available forms. I am now using spearmint extract, it is clear, and has no color changing effects on the propellant. It also has, I believe, about the highest percent of pure propylene glycol of all the extracts. It also has the added benefit of imparting a nice spearmint scent to the lab.
I mix a standard batch of propellant using 65% (from the bag) KNO3, and 35% powdered sugar. The dry ingredients are heated to melting in the standard manner, just before casting I add 2% extract by weight of the dry ingredients to the melted propellant, and quickly stir it in. I stir for about 15 to 20 seconds, turn off the heat and pour the propellant into the casting forms. I'm not going to say it turns it into water, but I have never been able to grab my melting pot and pour a grain, in the past I would have to scoop it up and push it out into the mold. Then I would have to tamp the propellant into the mold to remove air voids. I cast some grains with 5% Mg, replacing 5% of the sugar. The propellant was so thick it would stand up on its own when melted, the extract thinned it to almost pour able, about the same as regular propellant without the Mg or glycol.
Here's the extract I use. It's made here in Iowa so I doubt if it's available in a wide area.
So what's the big deal. Well, it makes casting much easier. And probably safer because it's easier to handle. But the real advantage is grain density. After starting to use the propylene glycol, my grain density has risen about 10%. From 1.574 grams/cc to 1.763 grams / cc. Or .06357 lb/ci of an ideal .0686 lb/ci. Grain density is important for a couple of reasons. First of all, you can pack more propellant in the same place, and that's always good. Second, it means there are fewer voids in the propellant. And that means a more consistent burn rate and fewer unexpected cato's from voids causing large burn surface areas and the resulting over pressurizing of the motor.
Give it a try. Find some flavor extract that only contains propylene glycol and the flavor extract. The propylene glycol should be the first ingredient listed. Stay away from any that include water in the ingredients list. The 2% added does not seem to change the propellant burn rate significantly, but I will perform more tests in the future. At this time, I now cast all my grains using the extract, and they are the best formed grains I have ever made.
Update 21 April, 2005:
I finally ordered some pure propylene glycol. I have been using the food flavoring, assuming the propylene glycol in the flavoring was the key to thinning the melted propellant. I ordered a quart from The Chemistry Store for about $10, that should last a long time. I've been getting the dual outboard rocket ready for its first flight, and it was time to get some grains cast for it. So I used this chance to try the pure propylene glycol.
Here's my quart of PG and the casting stands after the grains were cast.
I made a 550 gram batch, using standard 65% KNO3 and 35% powdered sugar. Once it was melted I added 2% of the batch total weight in PG (11 grams). As expected, once stirred in the propellant thinned nicely.
Now I must mention, I've had some people tell me they tried the PG in their propellant and they claim it didn't help. I can tell you it doesn't help at all with xylitol based propellant, so I'd guess it won't work with sorbitol either. That's a big point, a lot of people think I'm using sorbitol, I'm not, it's KNSU; as in KNO3 and sucrose (powdered table sugar).
The best way I have to quantify it is this: After the propellant is melted, and before the PG is added, I can turn my melting pot upside down and nothing comes out. To fill a mold without PG I need to literally spoon it into the mold with two spoons, one to scoop it up and the other to scrape it off the first spoon and into the mold. With the PG added, I can pour the first 60% of the melted propellant into the mold by itself, the rest I use a large spoon to scrape down the sides of the melting pot to get it to flow into the mold.