polymer resins

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Revision as of 12:00, 23 January 2018 by Root (talk | contribs) (→‎freezing)
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freezing

Many resins after the curing agent has been mixed may be stored in a freezer for extended periods of time. The temperature will affect the rate of cure. A normal home fridge at 0F/-18C will almost halt the curing process, but not completely. The resin may feel hard and frozen, but it still continues to cure. This can be an excellent way to cast very thick sections that would otherwise overheat due to run-away thermal acceleration of the curing process. Thick epoxy casting that might actually get hot enough to ignite in a room at room temperature can be stored in a freezer for several days to a week or more. Then the casting may be removed and allowed to warm up. At this stage, the casting will likely not be totally cured. It may have the texture of a heavy putty or hard ice cream. It may hold its shape somewhat or it may sag a bit. At this stage, enough of the resin should have cured that it is no longer in danger of thermal run-away. It may get a little warm as the rest of the resin finishes curing, but it should be able to dissipate the heat.

This is also a good way to store extra resin or to get different flow properties that would otherwise be impossible. Because the resin will cure more evenly the partially cured resin will be thicker, but not lumpy. It behaves more predictably than resins that have had thixotropic compounds added. It has properties you don't otherwise see in resins that cure normally at room temperature (I mean the room's actual ambient temperature. The resin obviously gets hot while curing. Even resin stored in a freezer should be a tiny bit warmer than the freezer because the chemical reaction curing the resin continues, albeit more very slowly)

I have not tested but this may also be a good way to cast a resin seal and limit shrinkage of the resin which might cause pulling away from the seal. I am thinking of epoxy on glass seals in particular. Be sure to use a low-outgassing resin if working with a vacuum. If shrinkage is a concern then investigate a thermoset resin named benzoxazine which apparently has near zero shrinkage. I have not researched its outgassing properties to see if it is suitable for vacuum seals.

glass fiber

adhesion with silanes

Silane treatment of glass to improve epoxy bond adhesion. Silanes are a broad group of chemicals when deposited on a glass will change the surface chemistry to be more or less favorable to certain types of bonds. In the biological sciences, silanization is often used to prevent live samples from sticking to the glass walls of containers or microscope slides. When silanization is used in polymer sciences it is most often used to improve the adhesion strength of glass and resin. This can be critical for vacuum seals. There are many silane agents. Coupling type: agent:

universal epoxy-silane
Dow Corning Z-6032 vinylbenzylaminoethylaminopropyltrimethoxysilane
epoxy-silane
γ-glycidoxypropyltrimethoxysilane
amino-silane
N-β-(N-vinylbenzylaminoethyl)-γ-aminopropyltrimethoxysilane
hydrochloride, cationic-silane 
vinyl-silane
acrylic-silane
mercapto-silane
Methanethiol

See also #freezing for a possible method to cure resin without stress from shrinkage. I believe that because many resins cure at a higher temperature they shrink when they cool back to ambient temperature. If this is the cause then partially curing in a freezer may prevent shrinkage stress. A thermoset resin named benzoxazine apparently has near zero shrinkage. I have not researched its outgassing properties to see if it is suitable for vacuum seals.

testing

A well known test method is defined by the IPC (Institute for Interconnecting and Packaging Electronic Circuits), IPC TM-650 2.6.16 Pressure Vessel Method for Glass Epoxy Laminate. This test method exposing epoxy-glass laminates to a pressure cooker.

heat transfer -- thermal conductivity

http://www.advanceddiamond.com/

http://www.aitechnology.com/

Links

Epoxy Technology -- See Tech Tips
Making Polymers Terrific site that gives a good polymer foundation.

references