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Old August 7th 10, 04:46 PM posted to rec.boats.building
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Default do all (most?) epoxies bond to each other?


I am Tosk wrote:
I believe silica is just a thickening agent to help prevent
sagging on vertical surfaces.


Silica is more than a thickener, it's a thixotropic agent.

This means that the resin+silica fluid has a lower viscosity when you
stress it (ie when painting or spraying or mixing). When you leave it
alone, the viscosity will increase a lot (quickly), making it a kind of
gel which will indeed prevent sagging.

With a thickener like wood flour, the mix will still flow when set on a
non-horizontal surface (albeit more slowly). You'd have to add
unworkable (with a brush) amounts to slow down the flowing to the point
where it stays put until the epoxy cures.

The silica is kind of a miracle thickener which "knows" what it should
be doing, but, as mentioned in the thread, the end result is hard to sand.

I think it's quite usual to mix in both silica and another thickener
like micro-balloons or wood flour, to adjust the viscosity properties
and the end density and hardness to what you are doing.

jf

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Old August 8th 10, 04:38 PM posted to rec.boats.building
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Default do all (most?) epoxies bond to each other?

In article , [email protected]
sevres.org says...

I am Tosk wrote:
I believe silica is just a thickening agent to help prevent
sagging on vertical surfaces.


Silica is more than a thickener, it's a thixotropic agent.

This means that the resin+silica fluid has a lower viscosity when you
stress it (ie when painting or spraying or mixing). When you leave it
alone, the viscosity will increase a lot (quickly), making it a kind of
gel which will indeed prevent sagging.

With a thickener like wood flour, the mix will still flow when set on a
non-horizontal surface (albeit more slowly). You'd have to add
unworkable (with a brush) amounts to slow down the flowing to the point
where it stays put until the epoxy cures.

The silica is kind of a miracle thickener which "knows" what it should
be doing, but, as mentioned in the thread, the end result is hard to sand.

I think it's quite usual to mix in both silica and another thickener
like micro-balloons or wood flour, to adjust the viscosity properties
and the end density and hardness to what you are doing.

jf


I used combinations of silica (yes, hard to sand) and maple or pine
flour for texture and color... Maple cures about the color of chocolate,
and pine about the color of peanut butter...

--
Rowdy Mouse Racing - We race for cheese!


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