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How to reduce the viscosity of Silicone caulk without weakening adhesion property?

I am looking to screen some patterns with silicone on different surfaces like hardboard, MDF, carboard etc. For that I diluted Silicone caulk with Xylene and added some normal oil paint. I was able to get positive result except that the dilution weakened the adhesion property of the silicone which ultimately failed the whole experiment.

The screened layer was about 1mm thick. The cured layer could easily be removed by gentle rubbing with finger or thumb.

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Downunder35m4 months ago

Welcome to the club ;)
It might help to understand how silicone chaulk actually works:
The standard silicone is "activated" with the presence of moisture, which causes cross-linking and the release of acetic acid - the vinegar smell.
This linking it what gives silicone the rubber like texture and strenght.

Now, you might have already noticed that different grades and colors result a different strength, for example white silicone is quite soft, while natural and black silicone tends to go quite "hard".
This is mainly due to the fillers and pigments used.
Too much of these fillers and the silicone becomes weak and is unable to form long enough linking chains - what you got...
In any case (from my experience) adding water or water based paint always results in a great lack of adhesion, good for molds but useless if the stuff actually needs to stick.
Sometimes you can get away by putting a thin film of undiluted silicone chaulk onto the the surface right before adding the mix, this way you get adhesion to the object plus a surface the mix can bind to.

In terms of making it thinner without loosing the good properties I had mixed results.
If the final goal is have a cured silicone that is much softer you might be out of luck with normal and cheap ingredients.
But if you need it thin for screening purposes while still having a good and hard result when cured you can try this:
Use a small container for mixing, like a joghurt tub.
Squeeze some silicone into it then add !!a little amount!! of M.E.K. and mix well.
MEK is Methyl Ethyl Ketone and can be found as priming fluid for PVC pipe gluing in your hardware store.
The trick is to be fast and good with the mixing.
At first it won't mix well and you will be tempted to add more MEK, only do so if the entire mix is evenly and still "hard".
Once fully mixed it is much easier to add more MEK if required.
Here is what actually happens and how you can make use of it:
The MEK drastically reduces the silicones ability to stick together.
Almost like adding acetone to grease, at first it lumps but then becomes more like oil....
You need to be aware that is causes slight shrinking and also requires a longer curing time, especially in thick layers which I don't recommend.
The good thing is that it will still stick to almost anything with the exception of certain plastics - MEK dissolves it so at least the surface will be ruined in the process.
To get layers more than a few mm thick it is best to repeat the process and add more layers before the old one fully cures.
It should still be smelly and quite soft but not sticky on the surface.
I diluted silicone this way to the point where I could use it in a spray can...

abuhafss (author)  Downunder35m4 months ago

Thanks for your input, Downunder35m.

Yes, I needed to thin the silicone for screening without loosing the adhesion property.

For coloring what paint should I use?

Longer curing time is good but would it require hot air for curing?

Yes, the screening should be done maybe thrice.

For another application ------- Can the silicon be diluted to viscosity easy flow-able like honey, with this method? The requirement is 5 to 10mm thick layer as a sealant in a HDPE container.

For coloring I would go with pigments or even try simple food coloring.
No hot air required, just enough airflow to allow for the MEK to evaporate.
If not too thick the curing time is about 3 days for a 5mm layer till it no longer smells.
And yes, you get it quite thin this way but to really get a smooth surface a vibrator or similar is a good idea unless you have tools where the stuff refuses to adhere to.
The vibrations cause the silicone to become more viscous and small bumps or dints go flat.

abuhafss (author)  Downunder35m4 months ago

As mentioned in my last post, now I am talking about two applications:

The first one, 1mm layer screened thrice thru a silk-screen film. I hope that doesn't require vibration and the curing time would be 30-60 minutes, am I right? How the screen should be cleaned/washed after use; with MEK or Xylene?

The second one, 5mm layer poured into a HDPE container. If the silicone is thinned to honey-like viscosity, it shouldn't create bumps but dints are possible. Do you think 5mm layer will really be cured in 3 days? I am concerned about the top surface (being cured earlier) prevent the rest material from getting moisture from air. Can corn-starch sort out this issue?

Lastly, would you please elaborate "The trick is to be fast and good with the mixing". For preparing a batch of say 1Kg, should I use a mixer?

abuhafss (author)  abuhafss4 months ago

It worked! Thanks Downunder35m.

Next, I am working on spraying a thin layer. I tried but the layer have a dull or smoked appearance. Could it be, somehow made glossy appearance?

Only way I know to make it glossy is with some oil in it but never tried it as I could not find a suitable oil.

abuhafss (author)  Downunder35m4 months ago

Could you please specify the criteria for suitability of oil?

I have done very little experimenting in the shiny field...
The only time I did I used a tiny amount of oil based paint as I wanted some color as well.
Did get the color but the rest was just a mess.
Problem is that oil won't freely mix with silicone :(
In general you tend to get a smoother and shinier surface if you don't have one ;)
Or to be precise: If you use a form, like the bottom of an icecreme container then the mating surface will be very shiny.
Basically comes down to how smooth you can get the surface....

steveastrouk4 months ago

Get the two part mould making resin, its much simpler.

rickharris4 months ago

Suggest you try to find this 2 part food grade silicone molding silicone.

It looks in the video about as viscous as screen printing ink as it is.