HC7 is a low-cost, moderate-viscosity, PAPI-type isocyanate mixture that produces unparalleled cure rates for R45 HTPB, at astoundingly low proportions of curative. One part R45 with 0.1 parts HC7, with no plasticizer, will begin to thicken at room temperature in about 30-60 minutes; cure is essentially complete after just two to four hours. Plasticizer will slow the cure slightly and produce a more rubbery propellant, but cure is still essentially complete overnight. Up to 0.5 part of plasticizer may be added per one part R45. Other curatives (DDI, etc.) may be mixed with HC7 to provide longer potlife and slower cure. If used alone, HC7 is best used for propellants that will be processed at room temperature.
Please note that propellant additives may affect cure rates and
times. Individual experimentation may be required to establish the
conditions needed for a particular propellant.
Mr. Fiberglass Brand "Hydroxy-Cure" as a Curing Agent For R45 HTPB
A test showed that 4.01 grams of this HC-7 and 27.1 grams of R45 thickened in about 30 minutes, was firm but tacky in 90 minutes, and was completely cured overnight.
Four test batches of propellant were prepared, of varying composition. All were prepared at room temperature and were vacuum-processed.
|
Ingredient |
I |
II |
III |
IV |
|
Ammonium perchlorate 400 um |
16.24 |
16.89 |
26.00 |
16.30 |
|
Ammonium perchlorate 200 um |
48.72 |
50.67 |
52.80 |
48.91 |
|
Ammonium perchlorate 90 um |
9.74 |
10.13 |
|
9.78 |
|
Aluminum |
7.00 |
3.00 |
2.00 |
2.00 |
|
Magnesium, 2 micron |
2.00 |
3.00 |
|
|
|
Copper chromite |
0.30 |
|
|
|
|
Iron oxide (red) |
|
0.30 |
|
|
|
Carbon black |
|
|
0.20 |
|
|
Copper oxide (black) |
|
|
|
1.00 |
|
R45 |
11.06 |
11.37 |
17.00 |
13.00 |
|
Hydroxy-cure |
1.64 |
1.33 |
1.70 |
1.30 |
|
Tepanol |
0.30 |
0.30 |
0.30 |
0.50 |
|
2-ethylhexyl acrylate |
3.00 |
|
|
|
|
Dioctyl adipate |
|
3.00 |
|
7.20 |
|
|
|
|
|
|
|
Solids content |
84.00 |
84.00 |
81.00 |
78.00 |
|
Curative (per 1 part R45) |
0.148 |
0.117 |
0.100 |
0.100 |
Batch I (largest proportion of curative) began to firm slightly about 30-45 minutes after packing. It was tackless about 4 hours after packing and cut very easily into separate grains with a knife. Unfortunately, the next day, Batch I was found to be very hard. Thin portions would break very easily when bent. However, the propellant was quite strong and would probably be adequate at modest chamber pressure.
For Batch II the proportion of curative was reduced. Batch II cured slightly more slowly than Batch I. After five hours, it was cut into grains. Again, after full cure, the propellant was quite hard though slightly flexible. Thin sections could be bent somewhat before cracking.
For Batch III the curative was further reduced. This batch began to set slightly by the end of packing. It was gummy-stiff in about an hour, and almost tackless in about three hours. It appears that the lack of plasticizer (a diluent) caused an increase in the net concentration of each of the two components and a consequent increase in cure rate. This batch when cured was quite hard (again, because of the lack of plasticizer) but was a bit flexible; a cored grain would give slightly when squeezed.
For Batch IV the proportion of curative was kept the same as for III, but plasticizer was added. This batch remained soft enough to mold for about 90 minutes or more. Four hours after adding the curative, the batch was still very tacky but no material adhered to the finger. The next day, the batch was cured but was very slightly tacky, possibly because of the large proportion of plasticizer used. It was quite flexible and a piece about ¼" thick could be bent almost double without cracking or tearing. It was also quite strong but easily cut with a razor knife.
Both Batch III and IV had a slight odor of R45 after cure, indicating an excess of R45 in the mixture.
Conclusions and Recommendations:
Approximately 10 parts of Hydroxy-cure to 100 parts R45 appears to be a proper ratio for strong but reasonably flexible propellant. The mixture will tolerate over 50 parts of plasticizer per 100 parts R45, but slightly lower levels of plasticizer might be preferable. Room temperature processing is probably necessary, especially if plasticizer is omitted. Pot life for a mixture with R45, Hydroxy-Cure, and plasticizer is on the order of 60-90 minutes. For a longer pot life, Hydroxy-Cure might be used along with a slower curative such as DDI.