Polyester resin propellant - first tests
Cremona, December 2018.
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These are a couple of notes about a polyester resin + KNO3 propellant.
The question is: what do we get if we mix on the fly KNO3 with some polyester resin, let it dry and burn it?
Basically, we try out the well performed
composite propellant experiments by Richard Nakka
on a much coarser scale.
Note that he was using epoxy, while we try out polyester
ALERT! This experiment is quite dangerous and unhealthy
(for a number of reasons, such as fire, chemicals, smoke etc).
I'm not responsible for any sort of damage you may incur into.
So, the material is:
- KNO3 (potassium nitrate);
- Polyester resin + catalyzer;
- (optional) Fe2O3 (red iron oxide) and sulfur;
The KNO3 was of low grade (small balls used as fertilizer), and was
grinded manually, by placing it on a hard surface and rolling a glass
bottle over it, for about 5 - 10 minutes.
Needless to say, the result is much poorer than what we could obtain by
Relatively large grains of KNO3 are well visible in the following picture
of one of the propellants prepared.
Othe microscope views are provided below.
Resin was of polyester type (cheaper than epoxy).
Two out of the three propellants include either red iron oxide (Fe2O3),
which is used as a dye, or both red iron oxide and sulfur (bought at a
local drug store).
Preparation of the propellant
Preparing the propellant for this test is very easy, although much can
First, grind KNO3 in a fine powder and weight it according to the recipe
I grinded it by rolling for 5 / 10 minutes a glass bottle over the KNO3 small pellet balls,
put on a flat and hard surface.
Prepare the proper amount of resin and mix (mix well!) with the catalyzer.
Then, introduce the powder in and stir until you get a macroscopically uniform
Finally, place the slurry in a mould or on a surface of some kind.
You may want to wait for a while for the resin to polimerize.
Timings strongly depend on temperature and amount of catalyzer introduced.
In winter, you should try to do this in a warm environment, but beware:
Pay attention to these warnings, especially if you are preparing
relatively thick slabs of resin.
- Resin warms up by itself and may take fire(!!!!)
- Even if it doesn't take fire by itself, you could accidentally
set it on fire by placing it near a heat source!
In my case, I used a heat gun to keep the temperature around the propellant at around 30 Celsius.
Three mixtures were prepared, with the composition (in weigth) tablutated:
| (Mass %)
| Mix 1
| Mix 2
| Mix 3
Notes and pictures of the propellant
A few notes about the propellants.
Hardening and mechanical properties
Mixtures 1 and 2 dried easily, resulting in a solid slab after about 10 minutes since the catalyzer was
mixed to the resin.
Similar mechanical properties were shown.
Mixture 3, including sulfur, took significantly longer to solidify.
After 1 day, mixture 3 was still soft to the touch.
This could be due to a less careful temperature control (the night was approaching and I decided to keep
the heat gun a bit further), or a slightly different amount of catalyzer.
But I feel the real cause is the sulfur.
After 2 days, mixture 3 had solidified.
Mechanical properties were quite different than mix 1 and 2; mixture 3 being quite plastic, and could
be easily deformed before breaking.
Bubbles in the propellant and KNO3 grains
Richard Nakka strongly suggests to cook the KNO3 to remove humidity, and evacuate the propellants
using a vacuum system.
Bubbles would generate, otherwise.
I did not evacuate nor cook the KNO3, and many bubbles are clearly visible in the pictures, below.
I'm not sure wheter they come from the stirring process or from humidity.
The pictures here were taken with a cheap USB camera (10 euros on ebay):
The propellant burns!
And considering that we just put random percentages without even considering the chemistry of polyester
resin, I would say this was a moderate success.
In absolute terms, however, burning tests revealed a rather poor performance w.r.t. KNO3 + sugar
Some low quality videos can be found here for the three mixtures.
- Samples were quite hard to ignite, probably due to poor mixing and too big size of KNO3 grains;
- Combustion is not uniform and steady, because of large KNO3 grains, bubbles and probably poor
- When totally melt by the heat, all the propellants burned much better;
- The amount of smoke generated is very low (compared with KNO3-sugar propellants);
- A thick leftover is found after the combustion, which probably means non-ideal
VIDEO - Mixture 1: KNO3 + polyester resin
VIDEO - Mixture 2: KNO3 + polyester resin + Fe2O3
VIDEO - Mixture 3: KNO3 + polyester resin + Fe2O3 + S
The burning velocities obtained from the tests are purely indicative, as the propellant slabs
have different sizes, experiments were not repeated, the camera sucks etc etc.
Anyways, just to give an idea, burning velocities at ambient pressure (and temperature of about 280 K):
|| Burn velocity
| Mix 1
|| 130 mm/s
| Mix 2
|| 214 mm/s
| Mix 3
|| 133 mm/s
Also consider that:
- The first part of the mixture-1 video was cut: this mixture also takes a bit of time to ignite,
like mixture-2 and mixture-3;
- Mixture-3 could take more time to ignite than mixture-2 due to (maybe) poorer mixing of
- The sulfur version (mixture-3) burns slower (probably also due to the large bubbles), but
when the slab is completely melt, a flame persists and gives much more satisfaction
(improved mixing in the liquid phase).
To improve the experiments and propellant one should at least:
- Find out the chemical composistion of the polyester resin employed and use stoichiometric
quantities for the mixtures;
- Grind the KNO3 in a ball mill and cook it to reduce humidity (see Richard Nakka's suggestions);
- Evacuate the propellants to remove voids (see Richard Nakka's suggestions);
- Produce uniform samples for the testing, possibly thin and long, and test them attached in
mid-air with no side wind.
Finally, thanks to Elena Capelli for
helping out in preparing and testing these mixtures!
-Stefano -> BACK TO THE HOMEPAGE <-