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 resin.

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.

The ingredients

So, the material is:
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 ball milling. 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 be improved...
First, grind KNO3 in a fine powder and weight it according to the recipe (see below). 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 slurry.

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:
  1. Resin warms up by itself and may take fire(!!!!)
  2. Even if it doesn't take fire by itself, you could accidentally set it on fire by placing it near a heat source!
Pay attention to these warnings, especially if you are preparing relatively thick slabs of resin.

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 %) KNO3 resin Fe2O3 S
Mix 1 70% 30% - -
Mix 2 70% 28.5% 1.5% -
Mix 3 70% 28% 1% 0.5%

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):

Mix 1. Mix 2. Mix 3.
kno3_polyester_propellant kno3_polyester_ironoxide_propellant kno3_polyester_ironoxide_sulfur_propellant

Burning tests

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 propellants. In particular: Some low quality videos can be found here for the three mixtures.
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:


To improve the experiments and propellant one should at least:
  1. Find out the chemical composistion of the polyester resin employed and use stoichiometric quantities for the mixtures;
  2. Grind the KNO3 in a ball mill and cook it to reduce humidity (see Richard Nakka's suggestions);
  3. Evacuate the propellants to remove voids (see Richard Nakka's suggestions);
  4. 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 <-