Possible encapsulation material?

[the pu man]

Advice on finding the right material

Hello All, I have been reading this thread with interest. I have been involved within the thermoset resin systems business now for some 17 years and have begun development projects with a number of major solar players and it appears you guys may need some help! Why everyone has chosen to use silicone is somewhat strange. Silicone or at least the 2pk silicone systems for electronic and electrical gives one major benefit over standard thermosetting systems in that it has a high working temperature range IE it can run from - 60 to 200 deg c. We currently produce a range of Polyurethanes, a range we have produced for over 20 years which are clear, exhibit excellent UV stability (certainly higher than that or comparable to silicone -

[russ]

Welcome to the pu man!

A number of people will be very interested in your knowledge on the sealing topic!
Again - welcome,
Russ

[Jason]

I agree with Russ.

It's nice to have someone in a different industry with a different POV.

Should be interesting.

Thanks for registering and I look forward to your posts!

[Need4Speed]

Why everyone has chosen to use silicone is somewhat strange.

What about curing agents releasing acids, or is that only in epoxies?

[the pu man]

possible encapulsation material

Hi Need for Speed,

Curing agents releasing acids should not occur when using a PU. The reaction is between the OH of the Polyol and the NCO 2 in the Isocyanate. You can drive the reaction through catalyst addition in the Polyol but this is general as extremely low % of the mix and done during the manufacture rather than you adding before mix.

Essentially the PU reaction is exothermic which means it requires nothing but the two parts at the correct ratio - this is the important bit, you go off ratio you get overdose on one of the parts which leads to poor cure and sticky mess. This is different from Silicone which uses a catyalyst to drive the reaction and often the more or less you use the quicker or slower the reaction. Epoxy is similar to Polyurethane however it tends to be more forgiving, although they are quite agressive chemicals which usually are denoted as corrosive which I think may be where you are seeing problems with Acid.

Again reffering to the above the one thing to watch on PU is a potential secondary reaction which is Water IE H20. In combination with the OH and NCO2 it produces CO2 so gas bubbles. You are probably sitting on this reaction as you read this!!

Looking at what is trying to be done water in locality is a big no no anyway.

The points to consider in this application are
1. Adhesion to glass or polycarbonate sheet (seems most people use glass but there is a grade of Polycarbonate - makrolon from Bayer which is a really tough clear UV stable sheet with fantastic impact resistance. If you have a dvd then thats the same stuff). We currently have clients making bullet proof and hurricane resistant glass which uses the clear polyurethane as the interlayer (very similar to this application in some respects).
2. Adhesion to solar cell
3. impervious to water
4. capable of thermal cycling without loss of adhesion or cracking
5. UV stable
6. Easy to mix and pour
7. none reactive during and after cure - this is why you can only consider the exothermic cure products and why if you try a one part this is going to lead to issues.

In honesty there are so many types of Epoxies and they can differ widely in ingredients so it may be that the issue you refer may well be specific to a certain type of Epoxy due to the constiuent ingredients.

[Need4Speed]

Again reffering to the above the one thing to watch on PU is a potential secondary reaction which is Water IE H20. In combination with the OH and NCO2 it produces CO2 so gas bubbles. You are probably sitting on this reaction as you read this!!

Looking at what is trying to be done water in locality is a big no no anyway.

Are you saying liquid water should be avoided during the casting process of the PU, or should ambient air humidity be low?

[the pu man]

Both.

The reactive ingredients in 2 part Polyurethane are hydroscopic. if you look at each individual consituent the Polyol will uptake moisture but you dont know its there until you mix it and then you get Carbon dixoide gas formation. In the Isocyanate it will react to form a very hard UREA crystal, again gassing as it forms these crystals. This is why in most 2 part polyurethanes once you open the container you need to use it all up.

During the cure the hydroscopic nature gradually reduces as the polymerisation occurs.

If you are in a high humidity environment, the open time is long and the cure thinkness is thin film these can lead to atmospheric contamination, which appears as bubbling.

If you were going to use it for solar cell you would find that you can process much quicker and use a little heat after you have poured it to help speed up the cure and stop any potential contamination from atmospheric moisture.

Bear in mind this is worst case. It is not a case that as soon as you open to atmosphere you have a problem.

Any liquid water is an absolute no no however there should be absolutely no reason why you would want to start adding water to the products nor any reason why there would be water present on your solar cells, unless of course you want to make a foam.

[Need4Speed]

What shore hardness should I be looking for in a PU product? I know this is not your product but maybe you can provide some advice:

I can get Smooth-On's Clear Flex 50 (PU rubber) or Crystal Clear 200 (PU plastic) from a local supplier. I'm leaning towards Clear Flex 50 because it has lower shore hardness (50A vs 80D). Also the viscosity is lower (250 cps vs 600 cps). Clear Flex 50's shrinkage is specified @ 0.0015 in./in.

Any reason I should not be using these products?

[the pu man]

The crystal clear 200 is a no no. It is way too rigid. The clear flex 50 is more appropriate.

Hardness is measured on a given scale. It begins on the OO scale which is for gels and foams and then moves to the A when it moves above a gel like state. There are other scales in between but generally our industry uses 00 A and then D.

00 - foams and gels
A - gel to semi rigid
D - semi rigid to rigid

For example and 10 A material will be skin like in hardness, soft and squidgy.
60 - 65 A roughly a car tyre
90 - 95 A standard tap washer
90 D - Glass, bone

The scales actually overlap so somethinG which is say 90 A will also appear on the D as approximately 35 - 40 D.

Silicones register in the A scale and are never rigid so any alternative needs to replicate this hardness.

Soft material is very importatnt as it allows expansion and contraction during temperature cycling. Imagine that you have several differing materials / substrates each one reacting very differently to temperature. If you have too many mismatches then you will risk losing adhesion and or breaking the cells or even the glass. If you get the delamination of the resin system you will end up with water penetration.

Going to a sfot material, even though this will have a much higher thermal expansion and contraction charateristic, will actualy negate the issue. They tend to place low stess during cycling becuase they are soft, they also have good elongation which allows the material to expand and contract, to little elongation IE rigid and it cracks - pretty standard technology in the electronics and electrical industry when dealing with PCB boards and delicate components - go to rigid and it the encapsulant will literaly rip the components from the board during cycling.

The 50 would be my choice as it is low shore rubber like with good elongation. However I dont know what the adhesion to glass would be like. It mentions in the MSDS use of plasticisers within the Polyol. These can cause issue if they are used to soften. Some do not actually react in the system and can migrate over time, they can also act as a release agent. I would give it a go on a small area. Get your glass and run a test area to see if it sticks, obviously make sure the glass is clean and you use a cleaner which is not going to put any greasy film onto it.

In terms of shrinkage you are pouring in thin film. Shrinakge is linear and is based on mass and temperature - remember these systems are exothermic reaction IE the bigger the mass mixed, the higher the temperature the faster and more extreme the reaction between the two chemicals. When you pour in thin film the shrinkage would not be an issue, its also another reason to use soft material as rigid materials will always shrink more. Bear in mind however that this system has a pot life of 25 minutes from the start of mixing. Make sure you are prepared. Also bear in mind that if you go and mix this system agressively you will put bubbles into it. Another tip is to make sure you decup into a new mixing vessel after the first mix. The biggest failure through hand mixing products is unmixed material splashed around the sides of the vessel being incorporated into the mix when you pour. If you mix in one vessel decup and then remix in a new vessel you wont run the risk of unmixed incoporation. Another trick is after you have poured the material get a hairdryer and blow the surface of the liquid. The heat from the hairdryer will expand the bubbles and pop them.

One word of warning - I can not clearly see the exact type of Isocyanate being used here for the part A. Some can be particularly toxic. We dont tend to use these unless we have to but I do know a lot of companies still sell these types. There is also mercury in this product which is pretty much close to being banned across the world so be aware that this product may change in the next couple of years.

Unfortunately we are a big manufacturer so we dont do small lots of material. I will see if one of our distributors is prepared to stock and market our system as we do have a number of distrbutors who market to specific industries.

Let me know how the Smooth on fairs and if you need any more help.

[the pu man]

Sorry forgot to mention that you need to check out the recommend working temperature range of the system to ensure it is compatible.

[dlmcbm]

Has anyone tried the Sashco "through the roof" yet? If not can anyone tell if its good to use or not. I can buy this local and $30 a gallon sounds great to me.

http://www.sashcosealants.com/home_improvement/Through_The_Roof.aspx

Thanks

[the pu man]

the sashco product is a sealant for use in thin film cure applications. It will probably not cure very well if you try to get any build height on it. It may work for sticking cells to glass but for back encapsulation I think you will struggle.

[RifRaf]

has anyone tried any of these products on some panels yet? any pictures or test results? all sounds good but would really like to see some panels made this way

[the pu man]

Hi Rif Raf we are making up some panels over the next two weeks. We have two products being tried out. We also have a UV weathering chamber so we are going to put panels in at highest UV setting with water program every hour. It gets real hot as well at 65 deg c. Will post when we have some results. We will be shooting vids etc for the build.

[Aqua]

pu man
Am in the UK and have two 200w panels to make is there any way I can help test the products you are trying.