EVA plastic would be another candidate for widely available plastics
that melt in approximately that temperature range. You can get it as
bulk pellets intended for hot-melt glue applications. For example:
https://www.hotmelt.com/products/infinity-coolpack-low-temp-packaging-bulk-hot-melt
Viscosity might be a little higher than you'd like at only 100C, though.
On Thu, Jul 9, 2020 at 6:13 PM Ben Brockert <wikkit@xxxxxxxxx> wrote:
Bill,
I googled around a bit but didn't find many plastics that are higher
than Mojave in summer temperature but lower than 100 C. PEG 8000 is
about as close as I got, https://amzn.to/2ZbzHNc
Melts at 160-165 deg F, sg 1.111. Have not put it into pypropep to see
what it does.
Independent of the "thermoplastic APCP" use case, it seems like an
interesting addition to try in KNSB motors. KNSB is melted and cast,
but is fairly brittle and stiff and can't generally be case bonded.
PEG is specifically used as an additive to other plastics to make them
more flexible. It is fairly water soluble, but then so is KN and SB so
it doesn't change that aspect of the motors.
Agreed that hands-off melting and casting seems like a good idea, but
from Troy it sounds like that is fairly well sorted out already.
On Wed, Jul 8, 2020 at 1:01 PM William Claybaugh <wclaybaugh2@xxxxxxxxx>
wrote:
Ben:
The RRI (circa 1968) use to melt tar in an electrically heated pot and then
mix in Potassium Perchlorate so there is an existence proof that a “low”
melting point binder can be mixed with oxidizer w/o ignition.
My sources say AP ignites a little above 400 degrees F so a binder that
forms a low viscosity fluid at around the BP of water will in theory work
fine if there is no possibility of hot spots.
If I were going in that direction I would want to automate the mixing
rather than stand over an open pot of hot binder, AP, and Al, stirring. I
would also expect that the casting process would require starting with a
heated motor / mandrel and then cooling it, presumably from bottom to top,
assuming a vertical pour. Again, I would not be willing to be standing
next to this setup during pour.
I’m thinking of putting it on the to-do list but I’m cautious.
Bill
On Wed, Jul 8, 2020 at 12:36 PM Ben Brockert <wikkit@xxxxxxxxx> wrote:
In the "sugar" world where dry mixing and melting is common, they did
some basic sensitivity tests of putting the propellant on the skillet
or whatever and seeing how much excess heating it takes to set it off.
It turns out that it's usually quite a lot, though it's a proof test
that should be redone when things like catalysts are added.
I don't know how AP mixes compare for autoginition or decomposition
temperatures. But you can run the process with an inert oxidizer
stand-in, to look for hot spots and see how the viscosity works out at
a given dry proportion to melted.
Supposedly the big "hybrid" of doom has (or had) some oxidizer mixed
into the fuel grain to increase performance, so there's probably
someone out there who would really like to talk about casting a ton of
thermoplastic nylon with oxidizer.
On Wed, Jul 8, 2020 at 8:13 AM William Claybaugh <wclaybaugh2@xxxxxxxxx>
wrote:
There are today plenty of “rubbery” thermoplastics available. If we
accept somewhat lower performance then a professional propellant, it
seems—in principle—that making and casting solid propellant using a
thermoplastic binder should be relatively simple, as previous RRI
experience w/ asphalt suggests.
The problem for me has been figuring out a relatively safe process:
Dry mixing the ingredients and then melting seems—again, to me—too
sporty.
Dry mixing the Al and binder seems safe enough; melting that and
breaking into pellets for later processing also seems safe up to the
point that AP is added.
Any thoughts as to how to cast a thermoplastic grain w/o having to mix
dry fuel and oxidizer before melting?
Bill