Gasket Silicone materials
My first experience in curing silicones was with Permatex automotive gasket material, which is vaguely color coded -- but ONLY vaguely: a red Permatex may be Hi-Temp acetoxy, Hi-Temp Sensor-Safe (an oxime silicone that doesn't give off corrosive acetic acid), anerobic [which cures where sealed away from air, not exposed], etc. You might guess that (e.g.) Ultra-Black would be the best oxime equivalent to the basic Black (acetoxy), but that's not always true. Check the specs in the datasheets
, not the buzzwords: Hi-Temp Red doesn't have as high a temperature tolerance as Ultra-Copper, and the differences between different colors can be subtle or even unimportant.
Sodium SILICATE paste
If you're looking for a *really* heat tolerant hardening paste, consider Permatex Muffler and Tailpipe Sealant. It's a rigid sodium silicate paste, not a silicone, but it is rated for up to 1100C (2000F) continuous, while the most temperature resistant Permatex silicone gasket pastes are only rated for 315-345C (600-650F) continuous and 370C (700F) intermittent. I have personally tested/used these silicones at 50F above their rated temperatures, sometimes more, for brief exposures in well-ventilated usage, but I'm not going to advise anyone else to do so. You're on your own. I've never formally lab-tested the silicate paste, but I've used it to patch kilns, and it's the only household paste material I trust above 1000F (540C)
There are also a number of inexpensive so-called "refractories" or mineral-based mortars or powders, but they aren't used the same way as the silicone pastes, so I'm not going to discuss them. Many need to be fired to an appropriate high temperature/time promptly after application. They can be unreliable and even a little dangerous if improperly fired (e.g. trapped moisture can explode sending sharp chips flying, and causing a project to fail), so they are often not suited to "firing during use" -- even you are building/fixing a kiln, you want to fire the mortar fully, according to directions, before using the kiln for your actual work.
My First Custom Silicone
My first attempt at customization was to add baking soda to the Permatex Black (an acetoxy silicone). My thinking was simple. Any grade-schooler [which I was, at the time] knows that baking soda and vinegar neutralize each other to give off CO2, and many know that this reaction releases water. Acetoxy silicones don't cure well to any substantial thickness because they rely on water from the air to catalyze them, I was hoping the water released by bicarb+acetic acid would cure it at any thickness I wanted. (It does.)
I also hoped that the neutralized acetic fumes would be less corrosive in some future use (e.g. reactive metals or sensitive sensors), but though I probably used them that way at some point, nothing comes to mind. Soon thereafter non-corrosive, "sensor safe" (oxime) equivalents came out for just a few dollars more.
Finally, I hoped that I would be able to create a silicone foam from trapped CO2. Though many of these gasket materials remain "soft" and flexible indefinitely after curing, they aren't compressible enough to provide a great deal of cushioning or a reusable/variable thickness seal. I've had mixed experiences with creating a foam, possibly due to changes in formula, differences in various pastes (like viscosity) and the fairly limited amount of gas that would be generated. I might like to give that another try this year. I have ideas
Unfortunately I don't have any of the acetoxy Permatex on hand, so I decided to try mixing bicarb with the flexible oxime Permatex "Ultra Black", just to see what happens.
Oxime and Bicarb #1
I wouldn't expect you to test your samples to the detail I have here. In actual use, you'd probably only care about a few properties. I'm being more complete to give you some idea of the things you can easily test.
Since uncured oxime silicones are non-corrosive, I knew they must be fairly pH neutral (not acid/alkaline) non-corrosive), and probably cure at close to pH 7. Therefore I wasn't too worried that adding a weak alkali like sodium bicarb would interfere with the polymerization (I'd worried about that in my early experiments, though it turned out not to be a concern with any of the common silicones I worked with, including acetoxys)
For my first run, I just filled the bottom of a plastic sample cup with bicarb, placed a blob of silicon on top [I didn't weigh it, but based on hindsight comparison, it was probably ~1.5g) and kneaded/stirred it in the bicarb until it was non-sticky throughout (I probably mixed in more bicarb than strictly necessary ) and formed a ball, approximately 18mm in diameter. A 1.6g blob of plain silicone was my control
That workroom is always 67°±1F @ 20%
Relative Humidity. The spec sheet listed 24hr for a full thickness cure up to 1/4" = 6.25mm) at 50%
RH, but due to the dry conditions, the control blob only developed a 2.5mm cured wall in 36 hours. I only lifted the blob enough to see that there was still uncured paste, but this left a thin gap that let me measure the wall thickness later. It was interesting to see how distinct and sharp the wall was -- the room humidity had apparently diffused slowly into the material, achieving a near 100% cure as it went, with essentially 0% cure just a few tens of micrometers deeper. You could make use of this: e.g. make a silicone tube by laying silicone like toothpaste, curing it slowly in a low humidity chamber (a sealed box with some desiccant in it) then cutting off the ends and blowing out the uncured core with dry compressed air (Canned air is always dry, and you should always have a drier on your compressor)
[Test sample ball, cut in half, showing interior and exterior surfaces alongside a reference blob of pure Permatex Ultra Black. the test sample is irregular due to several abrasion tests performed after exposure to high temperature and various solvents. Both test and reference samples are actually somewhat lighter than they appear in this picture, despite the name "Ultra-Black"]
The cured experimental sample had the initial appearance of a dark gray mouse ball, and a similar firmness [allowing for the fact that mouse balls actually have a steel core) The surface hardness was Shore A 65-70 (comparable to a car tire) as opposed to Shore A 30 for the control. Surface texture was reasonably smooth, except for adherent bicarb powder, but washing with water to remove the bicarb seemed to make the surface slightly rougher, and less durable, probably due to pits left behind by the dissolved bicarb.
The Test Sample was susceptible to abrasion with a thumbnail, but the control sample was only affected if I really gouged and tore at it. Though the test sample did not appear porous, it seemed to slightly absorb some solvents, like Methyl Ethyl Ketone and Acetone, though none of the common household solvents I tried worked to smooth and seal the surface. I suspect the bicarb and some slight diffusion for the slight absorption.
The ~1.7mm cured ball weighed 6g, for a density of about 2, after allowing for small inner voids found on cutting the ball. Aside from the voids (probably due to a poor final knead before balling) the test mass was uniform silicone with bicarb dust, with no sign of uneven curing.
The large mass (6g vs 1.5g) and density difference (~2 vs 1.26) indicates that the silicone incorporated a LOT more bicarb (density: 2.173) than it seemed to, before it stopped being sticky -- about two volumes of bicarb per volume of silicone. That's both bad and good. On one hand, the uncured ultra-black was obnoxiously sticky/messy to work with (perhaps others aren't as bad), but on the other hand, this suggests that the silicone could incorporate very substantial amounts of active ingredients. For example, you should be able to get a very decent magnet by mixing iron powder/filings, and exposing it to a strong magnetic field during curing [to orient and magnetize the individual iron particles]. You could also incorporate a fair amount of mineral wool for heat resistance and reinforcement *beyond* the rated limit of the silicone alone.
I've done prior work with making conducting silicones, but never found an easily made mix with consistently low resistance. However, my prior work indicates that silicone saturated with inexpensive conducting powders like nickel, iron or carbon shields EMI very nearly as well as silicone saturated with silver powder, despite having 100-500 times the bulk resistance. The defense industry uses bulk conductivity as a cheap index of EM shielding (RF testing in a certified isolation lab is time consuming and expensive) and strongly favors silver. This knowledge could become a nice edge for your product over a conventional manufacturer.
Ultra-Black is rated to 400F (204C) continuous and 500F (260C) intermittent, but several successive 1 minute exposures to an open gas flame or a soldering iron at 600F and 700F showed no smoke, flame or degradation, outside a very slight surface expansion and some increased susceptibility to abrasion. I suspect this is due to the bicarb decomposing at 122F (70 C), absorbing energy and releasing CO2. You might try other fillers with breakdown temperatures closer to the silicone's limits, for an even better protective effect.
These are just a few of the properties you can measure at home involving "inert" additives --i.e. additives that do not participate in the chemistry of the silicone itself. When I promised kitchen-level chemistry, I meant it! The fanciest thing I used was a $20-30 eBay digital scale that reads down to 10 milligrams (you don't want higher precision because slight air currents will keep the reading from ever settling -- but read the ad carefully and make sure it *measures* in 10mg increments vs. merely *displaying* .01g increments)
I think I'll try to remember to pick up some acetoxy Permatex at the auto parts store and take another shot at making a silicone foam using just a *little* bicarb (e.g. 3% by weight, vs the 300% used here). Though the acetic acid released by acetoxy silicones is pretty strong (undiluted by water) there really isn't *much* of it, so adding too much bicarb wouldn't really help. In fact it would probably hinder the formation of sufficiently large bubbles, by quite a few mechanisms. Perhaps even 1% bicarb might be enough.