low viscosity epoxy resin degassing 1

[acoustoelectronics]

Hallo everybody,
I have a problem, which has been discussed here before from some colleagues, but i can not find the thread. I am trying to deposit thin epoxy resin layers by the means of a spin coater (thickness of the layer 2-8 microns. The degassing of a low viscosity epoxy resin (500 mPas) under vacuum (1x10-3 bar) creates some problems, because even after 20 minutes there are still some air bubbles appear in the resin's volume (the resin's pot life is 30 minutes, so i can not afford to degas more than 20, because after that the resin will change its viscosity and this will cause an additional drawbacks during spin coating.)

Here I've read that one possibility is to reduce the resin's surface tension either by increasing the temperature or by additives such as BYK 051-054.

My question is what would happen with the resin's precuring time if i add BYK 051-054? Does this change the curing process and will this influence the overall stability of the full cured thin epoxy resin layer (will the resin be rigid or it will be viscoelastic after full cure)? And lastly what would happen if i add a different additive (acetone or heptane)? Thank you very much in advance.

K.E.

 

[Compositepro]

You are probably making a mistake by applying vacuum too long. All epoxies will boil at that level of vacuum. The materials coming out are dissolved air, moisture, and organic materials with a vapor pressure. You will never reach a point where all the bubbles disappear under vacuum. Also realize that the pressure you mention is very low and that the hydrostatic pressure one mm under the surface of the liquid will be 1000 times greater. So stirring can be helpful.

You need to apply vacuum for a few minutes to remove all air (which will be expanded by the volatile moisture and organics and flushed away). Then release the vacuum, and all the remaining bubbles will collapse and completely disappear. Acetone and heptane are simply volatile organics that help to remove air bubbles as I just described. At your level of vacuum you probably don't need to add any. But if you were to add acetone you still have to release vacuum to collapse the last acetone bubbles.

 

[acoustoelectronics]

Thank you very much for the quick response. Perhaps you are correct, but there are still some ambiguities, which confuse me. First of all, i've did such degassing two years ago in UK, using vacuum oven Fisher Scientific OV-11. With this equipment and epoxy resin with viscosity 650 mPas (150 mPas higher than the currently used), i've observed several effects, including boiling of the resin and visible huge air separation. In fact the air was forming kind of a big balloon (a lot of air bubbles connected to one another and disapper from the resin simultaneously). After 20 min deggasing by this technology the resin was almoust totally air free and the deposited layers were uniform.

Nowadays i am repeating the experiment with exactly the same steps, but using different equipment (older one) and epoxy resin with lower viscosity (i was expecting to have even better results). The result is totally different - the resin does not boil at all....by applying the vacuum you can see only slight air separation, then this big air balloon observed previously with OV-11. First we thought that there is some leaks, but after checking we found out that the set up is correct. I've started to read in internet about this problem and subsequently found this site, where was written about the possibility that the surface tension of the resin can play major role in the degassing process.

Can you please provide some comments about this curious mismatch between the current results and these from UK, although we are using the same technological steps. Thans once again.

Kind regards:
/K.E./

 

[Compositepro]

Different resins will have completely different degassing characteristics, and there is no reason there should be correlations between them. Elasticity of the surface layer plays a big role in bubble stability. This is not the same as surface tension. Volatile content is another major factor. Most resins can be purified by vacuum distillation, which means that excessive vacuum will simply remove resin. A bubble in you degasser means nothing if it is not in your part or film. I'm sure you are not curing the spin-cast resin film under vacuum. If you are, that would be a mistake.

 

[acoustoelectronics]

Thank you very much once again...no i am not curing the resin film under vacuum. The curing process is conducted under room temperature conditions in an open lab, not in a chamber or vacuum oven. The problem is that the bubbles appear in the film after deposition...this is what i am trying to avoid. I want to produce thin and uniform resin layers with lack of air in it. Do you have any constructive suggestions to overcome this problem, except the addition of acetone or heptane? I am thinking to try with ultrasound as well... Actually i have tried it, it improves the degassing, but causes sufficient changes in the viscosity of the resin (increased viscosity even in the time range of the pot life, where normally the viscosity should be constant).

Kind regards:
/K.E./

 

[Compositepro]

Bubbles cannot appear by magic. Either there are bubbles in the resin to begin with or in spin casting the resin trapped some air in the texture of the substrate you are coating. Some resins can generate volatile reaction by-products during cure or by reaction with moisture in the air. For example, urethanes react with moisture to generate CO2, and some epoxy curing agents are moisture sensitive.

 

[acoustoelectronics]

Thanks for the tip .