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Homemade silicones and casting/repair compounds


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#1 Orpheus

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Posted 23 February 2011 - 12:28 AM

a1.jpg
"This is Jane. She's from Ireland and she's lovely." [I know this, because it says so at the top of the Sugru website.]

In 2003, Jane, a former sculptor, was a graduate student in Product Design at the Royal College of Art in London. She bemoaned the fact that you had to buy new products when minor parts broke, or upgrade to a new device to get minor added surface improvements/features. This led to a five-year collaboration with two retired Corning engineers and a silicone chemist, resulting in a product called "Sugru" (from the Irish (Gaeilge) word "sugradh" ("play")] -- a nontoxic, self-adhesive, self-curing, moldable silicone elastomer with many useful properties for the home tinkerer. She marketed batches intermittently by 2009, and by 2010, it was in regular ongoing production, much beloved by "makers" everywhere.

As you know by now, this is the sort of story I love, so it is with heavy heart that I report that I haven't touched the stuff, and never intend to (unless someone wants to send me some for testing) -- and despite being "beloved" by Makers, it isn't much used by them either. (It isn't uncommon for products in he Maker movement to be far more honored in theory than in practice, I was shocked to learn that after 7 years, only 100K-150K Arduinos have yet been sold in the world -- and most are likely dead or gathering dust)

What turns me off Sugru? Well, the cost, for one: a "Smart Hacks Super Pack" (three sealed 5g sachets in each of four colors (Black, Green, Orange and Blue) for UK 11 = US$16.50 = 13.20, give or take plus shipping from the UK -- or about $10/oz when all is said and done. Worst yet, each sachet only has a reliable unopened shelf life of 6 mos, and once you open it, all bets are off. Now, I'll acknowledge that many projects don't need more than 5g of silicone putty, but it's still rather paltry, making Sugru far too expensive for many projects.

I'm also disappointed by its temperature range, -65 to 180 C (= -85 to 365F) and fixed hardness (70 Shore A). While these are quite adequate for the kinds of application Jane intended around the house, Sugru is basically a nontoxic RTV silicone, a material that is widely available in auto, hardware and cooking shops, with a wide variety of properties, for as little as $5 for a 10oz tube (vs ~$90 for 10oz of a single color of Sugru)

There are drawbacks to using various RTV formulations, but for the $20 you'd invest in Sugru's recommended starter pack, plus a few common household materials, you can have a wide assortment of moldable or sculptable RTV silicones with higher temperature ranges (550F=290C or higher), more/less rigidity, higher/lower density, foam or solid -- even food-safe silicones for molds or cookware. Oddly, though many tinkers use silicones in their projects, customized home silicones haven't been well explored with few exceptions.

(BTW, Sugru's 350F rating may seem fine for baking, but I'd want a higher safety margin because oven temperatures fluctuate around the setpoint and the IR from the heating elements will create surface temperatures higher than the air temperature that is measured by the thermostat. In fact, It's a good idea to put a tray under any silicone bakeware to protect it from direct IR, regardless of its rated temperature. Also, though Sugru is 'nontoxic', it isn't food grade. You can use it for casserole handles, but not food contact)

I guess the best place to start is with my very first custom silicone as a boy. They say you never forget your first...

Edited by Orpheus, 03 March 2011 - 04:49 PM.
changed title to broaden scope


#2 D.Rabbit

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Posted 24 February 2011 - 01:25 AM

I took some time to explore the site.
The videos in the blog tab are fun, and one might even find a job there.

The applications appear to be endless.
Nice to see they came out with white, so you can now blend your colors to match a little closer.

The price and the shelf life are a bit of a draw back but like all new to the market items, once they catch on, the price should come down.
Good xyzt to you, = a web greeting that includes all time zones and planets.
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#3 Orpheus

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Posted 25 February 2011 - 02:17 AM

Actually, it occurs to me that accounts of my early experiments won't make much sense without some basic background, because it's all about the chemistry, even if we only need to know "kitchen chemistry".

Types of Common Household Silicones to Tinker With
1) Acetoxy: this is the one-part self-curing RTV goo most commonly used in automotive/gasket [e.g. the regular permatex series) home repair applications. These are really "water cured" silicones that react to humidity in the air, releasing a *strong* vapor of vinegar (acetic acid) and initially forming a flexible hardened "skin". Even in fully cured form, some household acetoxy silicones remain very soft and flexible for ten or more years after cure because they cure by forming straight chains and "harden" because these chains are tangled together. Others are fairly rigid because their molecule are branched, and form a 3D matrix chains). Their "intermittent" heat tolerance may be listed as up to 550F (290C) or more, but consider their intended use: if it's used for engine seals that will last for years of daily use, most tinker projects would count as "intermittent", and if you don't mind regular resealing, they may be usable at higher temps. I've successfully used "600F" silicones to bind/seal glass wool for limited sessions at near 1000F (550C), and silicone impregnated with commercial basalt fiber might survive brief exposures to higher temperatures.

Water-cured silicones may cure just millimeters deep, or somewhat deeper over time or perhaps eventually full thickness.  Even silicones with excellent liquid resistance are often surprisingly permeable to gas --often hundreds of times as oxygen-permeable as butyl rubber-- and water molecules are about half the size of oxygen molecules. We can't mix water directly into thee uncured silicone because it'll form a liquid-tight skin immediately on contact, and normal curing depends on humidity slowly seeping in (at least as far as it can)

2) Oxime: Commonly called 'odorless' or 'neutral', these are also one-part water-cured silicones that are often sold as aquarium sealant/adhesive, silicone bathtub/exterior caulk [get 100% silicone, not silicone-containing acrylic caulk, which frankly sucks) or building adhesives. They may have a mild initial odor of vinegar or ammonia left over from initial manufacture, but it soon fades to a scent like silicone lubricant. Oxime-cure silicones sold as sealants (like 100% silicone caulk or gas) may be gas tight enough that humidity can't reach more than 1-3 mm deep after 24 hours (or ever). Oximes are theoreticakly capable of similar heat resistance to acetoxy silicones, but common household oximes, like silicone caulk, may only be rated to ~300F. 10-oz caulk-gun tubes (~$5) are often the least expensive form of silicone available in local stores.

3) Tin Cure: This is a broad class of (usually) base/catalyst silicones--say, a few drops of liquid catalyst per ounce of liquid silicone. These are commonly used in making molds, which can last many years. They may be sprayable, pourable or brushable (you can thin them slightly with hardware store xylene) but often need a 'mother' (firm backing) of plaster or plastic behind the silicone, for structural strength. Many Sn-cure silicones are classed as nontoxic, but are not recommended for food contact, due to the trace tin. Some cause irritation in some people after prolonged skin contact (eyeglass pads, prosthetics) but they are fine for ordinary routine handling. Medical grade tin-cure silicones exist, but obviously, you should thoroughly understand the specific documentation before attempting these applications. These silicones are quite forgiving and most will cure well under almost any conditions (underwater, moderate heat/cold, oils, salts etc.), but some formulations require heat/IR or UV/visible light to sustain the curing process.

4) Platinum cure: (sometimes Rhodium) are generally binary [you mix equal amounts of two components] and heat-cured at say 210-260F (100-125C) (e.g. a toaster oven or an electric skillet full of clean sand), but a few work at room temperature. Most medica/food grade silicones are Pt-cured, because Pt is safer than Sn (tin). Avoid pt-cure silicones listed as solid/powder or injectable/extrudable but not pourable.

Pt-cure silicones tend to be pickier than tin-cure, and some have a reputation for punishing everything from a bad hair day to distant moral lapses, but I haven't had any particular issues with craft/hobby/kitchen brands -- except once when the platinum mildly catalyzed a reaction I was working on, and freaked me out.

5) Peroxide cure: "Do. Not. Want." This is the oldest cure chemistry. It requires an organic peroxide (not hydrogen peroxide), has a short working time and the uncured silicone must be refrigerated. The end product is much like a Pt-cure, but the properties can be adjusted by changing the concentration of the catalyst, curing temperature, etc. These are usually cured at an elevated temperature, and often need a post-cure bake to drive off organic byproducts. A factory can control conditions precisely to use these as a cheaper and more versatile alternative to Pt-cure in many applications (e.g. silicone tubing, food service, medicine) or even produce superior properties like higher temperature resistance. For thin layers (adhesive tapes, paper coatings) the silicone base is often exposed to peroxide in a separate step after coating, making the process more like a catalyst or binary system than a one-part cure.

#4 Orpheus

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Posted 02 March 2011 - 01:17 AM

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 "just because"
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 671F @ 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)

Ultrablack1b.jpg
[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.

#5 Orpheus

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Posted 24 March 2011 - 11:57 PM

I had high hopes of doing actual tests with measurements/observations/photos, but it looks like this will be a cool spring, and the local indoor humidity may take a while to rise to "normal" levels, so I'll just describe past results.

You can make a good, cheap molding/sculpting silicone by mixing 100% silicone caulk ($3/10oz for acetoxy) with cornstarch ($1/lb). It works best with the cheaper acetoxy type [strong vinegar smell], but the oxime type works too -- if its says "odorless" or some such, you're dealing with oxime caulk.

The cornstarch doesn't participate in the reaction, it's just "hygroscopic", meaning it absorbs moisture from the air when it's sitting in the box, and is easy to [visibly] mix uniform with the caulk. It probably absorbed plenty of moisture on its way from the factory to you, so just use it straight from the box: stir it into the silicone caulk in a plastic cup for use in molding; for sculpting; stir and wait until the mixture has a nonsticky clay-like consistency.

The cornstarch:caulk ratio isn't critical: 1:5 will work, and so will 2:1. However, less cornstarch means slower curing (and greater workable time) while more cornstarch esp. above 1:1 starts to affect the properties of the silicone. For sculptural craft work, this won't matter much, but I personally like the translucency of clear caulk with minimal cornstarch (vs the equally popular white caulks of the same brands)

I sometimes use less than 1:5 cornstarch with 1-5 drops/oz of drug store glycerin. Glycerin by itself will work, but it's clear, so it's hard to tell when it is completely mixed (and easy to introduce undesired bubbles) Glycerin also *does* participate in the reaction somewhat, and will "terminate the polymer chains", so you should use it sparingly, because it does weaken the silicone a bit, and probably affects temperature tolerance, etc.

Some hobbyists are now calling this mix Oo-goo, after Jane's Sugru (though the homemade versions have been used at least 3 decades longer) but the two are very different to work with. Sugru is a putty that cures relatively firm and sticks well to most surfaces, including glass, metal and glazed ceramics. Oogoo starts off sticky but (depending on the exact mix) usually isn't still sticky by the time you reach the putty like stage (as we'll discuss later, you can improve its adhesion with additives like silanes). In fact, oogoo can be so smooth and slick that it often needs no release agent when made into a mold or casting, and when one is needed, simple soap often suffices.

Even when it is applied in the sticky stage, some formulations of oogoo may peel off smooth surfaces if you apply shear forces, or "shrink off" if you use a solvent as part of the mix (as we will do in later formulations). The joke is "nothing sticks to Oogoo except Oogoo", and while the situation isn't quite that dire, it's nice to remember that uncured oogoo *will* stick to cured oogoo, in case you have to make changes or repairs later.

I've read reports that acrylic art paints will work to cure as well as color the silicone/cornstarch mixture. I like using linseed oil-based paints or powdered pigments. You can get some amazingly vibrant colors! If the colors of Polymer Modeling Clay (Sculpey, Fimo, etc.) make you happy (as they do me) you'll love the colors of oogoo.

Actually, it occurs to me that many of you will be using an opaque white (or other color) caulk, rather than the clear type I prefer. You may want to add a little colorant to assess how well the cornstarch is mixed. White on white is harder to see than white on clear. If you want to end up with white, try starting with beige or light gray caulk.

#6 Orpheus

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Posted 02 April 2011 - 04:12 PM

Oogoo colors well with phosphorescent (glow in the dark) or fluorescent (glow in UV) pigments. It also encapsulates zinc-sulfide phosphorescents, which otherwise have a permanent sulfur stink, that can rub onto your fingers

If you start with a clear silicone caulk, and use clear glycerin to cure it, you can gen get some cool transparent glowing castings.

It can also be thinned with a number of solvents to make a paint which is not only more durable than most glow in he dark paints, but can be brighter, since the base is transparent. The silicone seals the phosphorescent powder, creating a more durable surface than the commercial phosphorescent paints I've seen, and also seal the strongly glowing but stinky zinc sulfide phosphorescents.

Xylene and mineral spirits (sold at paint/hardware shops) will work, but they aren't very volatile and give off fumes for days. Naphtha evaporates more quickly, and creates a paint which is stronger and sticks better to very smooth surfaces like metal, glass and ceramics. However Naphtha (also sold as "white gas", "camp fuel" and "lighter fluid") is very flammable, so let it dry outside or in a *well* ventilated area, like a garage with a fan.

Be aware that VM&P Naphtha isn't really naphtha, but a slightly less voltile mix of petroleum distillates (mostly pentane) It should work, but I haven't tried it. It's also called "petroleum ether", but isn't really an ether at all (which is a good thing, because common ethers aren't just flammable but downright explosive)

Phosphorescents and Fluorescents can be expensive, but the "chalk powder" sold for carpenter's chalk lines and ground marking comes in fluorescent or phosphorescent versions which are quite cheap in the larger (1-lb or more) containers. As with so many products, the big containers cost little more than the "convenient" small refill sizes.

#7 pol098

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Posted 01 June 2012 - 09:23 PM

Just a factual update, not trying to make any points: I've just looked at the Sugru website, and prices are as of June 2012 much lower than reported before, about US$18 + postage for eight 5g packs, the same for one 100g pack.

#8 Orpheus

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Posted 02 June 2012 - 12:32 AM

Excellent news! Thank  you!

#9 homebrewer

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Posted 15 May 2014 - 01:35 PM

Hi Guys,

Greetings from England.

I have read your postings about silicons very carefully and the reference to 3D-printing
which is the center of  my interest.

In another weblink I have read that some people use "acetoxy silicone polymer" in their
3D-printing appication, something I would like to replicate.

My application calls for a "acetoxy silicone polymer" material which has a good elasticity,
is abbrasive resistant and can be 3D-printed.

Perhaps you can guide me, your help is much apppreciated.

#10 Orpheus

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Posted 30 June 2014 - 11:05 PM

You might want to look into paste extrusion printers. They don't get much play, but I think the field is wide open for a breakthrough.

Here's one very simple example:

http://player.vimeo.com/video/98488940




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