Wanted to share some testing I've done in order to gain a deeper understanding of how to work with silicone and repair my > 1 year old silicone doll.
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Silicone adherence tests were performed on the back of the doll (pictured on the left here
viewtopic.php?t=178099) that is made of platinum silicone with a shore hardness between OO-10 and OO-20. I don't have a shore hardness tester so am just making a guess based on some mixtures I've created. There are lots of shore hardness scales like "shore OO" and "shore A" and even though they don't line up, here are 2 charts I've found that help.
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I created 6 adherence test mixtures, from 10% psycho paint with 90% ecoflex OO-20, up to 100% psycho paint and no ecoflex. The reason for creating these mixtures is the psycho paint has a shore hardness of A-30 (between a rubber band and pencil eraser) so it is fairly hard depending on where it is being used on the doll. So the more ecoflex that is added, the softer the resulting mixture will become.
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I added a half drop of a product called "thi-vex" that thickens each mixture so they will not run off the back of the doll, along with a little flesh pigment. The thi-vex doesn't make the resulting silicone harder, it just makes the mixture thicker so instead of being a flowable liquid, it becomes a non-flowable paste.
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From left to right, here are the cured mixtures and the progression as they were removed. At only 10% psycho paint it was extremely tough to remove the cured silicone and took several minutes of picking and scraping. As you can see, some damage occurred when scraping so you can imagine the adherence was very strong. The 20% psycho paint acted about the same as the 10% mixture, but once the concentration was increased to 30%, major damage started to occur while attempting to remove it. Past 30% it was no longer possible to remove any part of the mixture, all of it had to be cut away.
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The doll I tested on also has a harder mixture of silicone used for its head, arms, and lower legs; but I did not perform this detailed testing on the harder silicone. However, based on my airbrush testing
viewtopic.php?t=175971 I found the adherence to harder silicone was much less strong. I was able to remove 100% psycho paint from the hard > 1 year old silicone but it was extremely difficult and I had prepared half of the painted area with petroleum jelly to act as a release agent. I was able to get the paint to peel from this area and it was strong enough to also peel itself off the raw area that had no jelly. I did the same petrolum jelly prep on the back of the doll for the tests pictured above but unexpectedly the samples bonded much more than expected. Going into this I thought I would be able to peel off even the 100% psycho paint but that obviously was not the case and at least in this doll's case, the softer the platinum silicone the better the adherence
Now the complication with using only psycho paint and ecoflex is the shore hardness of the resulting mixture. When I first started looking to repair silicone (
viewtopic.php?p=2431091#p2431091) I purchased some sil-poxy and later a no-name silicone doll adhesive and finally some psycho paint + pigment, but all of these result in repairs that are much harder than the doll's silicone. Psycho paint is about A-30, sil-poxy is much harder, maybe A-40, while the no-name adhesive is maybe A-25. I liked the no-name adhesive the best and found repairs using it will last several months, but since it's still much harder than the underlying silicone it's not a perfect solution.
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I estimate the softer parts of this particular doll to have a shore hardness = OO-10 and the only way to achieve that softness is to introduce another product called "slacker" into the mix. This makes the silicone soft but also makes it sticky, and the more slacker the softer and stickier it becomes. Smooth-On recommends adding no more than 50% slacker and I found the resulting silicone is like the inside of a cooked marshmellow: extremely sticky and so soft you can barely feel it. It also will not return to its original shape. At 35% slacker and 65% ecoflex the cured mixture is still extremely sticky but it can be stuck and unstuck from itself and returns to its original shape easily. I feel like this ratio is close to the softness of a soft TPE like those used in fleshlights (so it's softer than a typical TPE doll).
To make a particular hardness of silicone you simply need to place your softer and harder mixtures on the same scale, e.g. psycho paint is about OO-72 on the OO scale and the ecoflex is OO-20. If we mix 50% psycho paint and 50% ecoflex, the resulting mixture will cure to a hardness of about OO-46 ((20 + 72) / 2). Using ratios allow you to create a mixture of any hardness between the two you are working with, and I found it helpful to create samples of several different ratios to have as a reference.
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After doing all these tests I think the following approach would be good for performing large repairs. First coat the raw, torn surfaces with an extremely thin layer of 30% psycho paint + 30% slacker + 40% ecoflex OO-20 to create a surface of fresh silicone that can be easily bonded to. This is the softest mixture I found that holds permanently and is strong, but it may be harder than a soft silicone used in a doll, so make it as thin as possible. Then mix ecoflex with slacker to create the softness that matches your doll's silicone, e.g. 30% slacker and 70% ecoflex OO-20, and use it to fill in the larger missing areas for the repair. After it cures the resulting soft silicone will be sticky and you can coat it with a thin layer of ecoflex to remove the stickiness.
In the end I decided to create a vaginal insert for my doll rather than performing another repair and will make a new topic about this project. Good luck out there with your own repairs.
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