# Lumen maintenance for PC or PMMA lens



## cpf user (Aug 23, 2014)

Anyone have any longevity data or experience with polycarbonate or acrylic primary lenses, especially on 3w power LEDs ?

I need to make sure it will keep 90% or more transmission for 16,000 hours. 

Note that I'm referring to the dome lens, not an added secondary lens.


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## JohnR66 (Aug 25, 2014)

More information of the light's intended use would be helpful. Acrylic is the better choice of the two as it will not yellow or haze from the light energy over that span of time. PC, as used in car headlamp covers, tend to yellow and haze from UV light and weathering, however, there may be better quality grades available and may not even be an issue at all with your LED application. PC has the highest impact resistance but acrylic has a harder surface and won't haze due to micro surface scratches as much as uncoated PC.


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## cpf user (Aug 25, 2014)

The usage is for inside of products that need monochromatic light; there is no exposure to sunlight. Currently we use glass primary lenses, but they were discontinued.

The 3w LED stars are encapsulated in polyurethane.


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## Harold_B (Aug 27, 2014)

I would contact a manufacturer of the material you are considering before designing around it. Perhaps you already know this but different plastics including different grades of PMMA or PC have different UL fire ratings. Why not silicone like most LEDs?


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## cpf user (Sep 5, 2014)

Silicone allows impurities in the encapsulant to degrade the LED to failure. Glass lenses prevented this. 

Anyone know of an LED manufacturing contact who can put glass lenses on?


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## SemiMan (Sep 5, 2014)

cpf user said:


> Silicone allows impurities in the encapsulant to degrade the LED to failure. Glass lenses prevented this.
> 
> Anyone know of an LED manufacturing contact who can put glass lenses on?




I think you have been provided grossly inaccurate information.

Virtually every PC high powered LED is encapsulated in silicone. Impurities in the grade of silicone used for LED encapsulation are virtually non-existent. This is optical/semiconductor grade silicone that is used.

Silicone is currently about the only viable encapsulant. It withstands the very high temperatures of LED die at the extremes, i.e. 150C, it is an excellent binder for the phosphor, it does not yellow under intense blue and even near UV light, and most importantly it does not place undue mechanical strain on the bond wires, die, etc. that occur in the fairly extensive thermal shock that occurs with high powered LEDs.

Secondary lenses are either uv stabilized polycarbonate or PMMA, PMMA is almost always preferred as high quality optical PMMA only hazes about 10% after 100,000 hours of exposure to fairly significant levels of sunlight and its transmission rate is high, even compared to glass.

Glass secondary lenses are used, mainly for COB arrays as few companies have the ability to precision mould glass to the level required for for small optics.

I am not aware of any urethane encapsulation directly on LEDs, though I know there are people who have overmoulded LED arrays in urethane conformal coating for weather resistance, but good luck getting any LED vendors to approve that. Silicone conformal coating has also been used this way. You are betting to put on a secondary optic and seal the overall assembly.

Semiman


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## cpf user (Sep 5, 2014)

It's the conformal coating that I mean by encapsulant. The polyurethane is put on top of the LED lens.


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## SemiMan (Sep 5, 2014)

cpf user said:


> It's the conformal coating that I mean by encapsulant. The polyurethane is put on top of the LED lens.



I have to check my notes, emails, etc. There was a company making a conformal coating specifically for going over the LEDs that they did lots of thermal testing and UV testing for long life (50K+ hour operation). I don't think it was officially blessed by any LED companies, but I know several transportation lighting companies used it. I believe it was silicone based.

This is what you are looking for?

Semiman


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## cpf user (Sep 5, 2014)

No it really needs to be pourable into a container where the LEDs are mounted, so the LEDs can go into salt water. About 1/2" thickness of material over the LED.


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## SemiMan (Sep 5, 2014)

cpf user said:


> No it really needs to be pourable into a container where the LEDs are mounted, so the LEDs can go into salt water. About 1/2" thickness of material over the LED.



How deep of water?

I would suggest one of several methods:

1) Use a plastic cover/lens assembly, either polycarb or pmma. Neither will yellow after 16K hours assuming polycarb is UV stabilized. Make a thick assembly and gasket it and mechanically affix it. You could also use flat glass in the same way.

2) Buy an off the shelf clear lensed box that is rated for under-water (and salt).

3) A two part encapsulation scheme. A) silicone first at a reasonable thickness over the LEDs. The silicon is mechanically the best suited. Then over-mould with a mechanically stiffer material.

Semiman


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## RoGuE_StreaK (Sep 6, 2014)

I know Harold B's said about the Dow Corning optical silicones in the past, from recently checking out the specs they looked like a good option, but I can't find any info whatsoever on pricing or availability. Also didn't find much info on possible mould materials and required temperatures, seems like a mix'n'pour at room temp into silicone moulds and a bake at 140C for a minute to cure?
One of them (MS-1001) had a Shore A of 87 / Shore D of 25, so not quite _hard_, but not exactly soft either?


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## Harold_B (Sep 6, 2014)

The moldable silicones are primarily for secondary optics like lenses, light pipes, etc. and are intended to be produced in low pressure injection molding machines or compression molds. They have castable silicones for producing LED domes directly onto the die but I'm not too familiar with those. In the States there's Ellsworth Adhesives and Kraydon that carry Dow Corning silicones. They can give recommendations and pricing.

The OP seems pretty intent on PC or PMMA. This is one of those applications where a picture or a sketch would be very helpful.


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## mercrazy (Sep 7, 2014)

good luck finding an adhesive to attach a cover lens. what i've tested gets absorbed into the LED's silicone lens or the silicone used to attach the glass lens. don't trust their published approved adhesives because they all don't work. you have to run it at least 100 hours to see the problem. a silicone would probably work but i need a stronger, impact resistant adhesive.


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## Harold_B (Sep 7, 2014)

mercrazy - unless you are trying to encapsulate an LED like the OP you would be better to start a new thread and then be specific about your issues. I'm still not entirely sure about what the OP is trying to do but it would seem like the application is potting an existing LED with a material that won't yellow or degrade over time and exposure to salt water. If you are trying to secure an optic to a silicone dome or layer by putting goop in the light path you will have a difficult time with that.


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## SemiMan (Sep 7, 2014)

mercrazy said:


> good luck finding an adhesive to attach a cover lens. what i've tested gets absorbed into the LED's silicone lens or the silicone used to attach the glass lens. don't trust their published approved adhesives because they all don't work. you have to run it at least 100 hours to see the problem. a silicone would probably work but i need a stronger, impact resistant adhesive.



There is a big difference between potting compounds, conformal coatings and adhesives. Adhesives will include significant solvents, binders, etc. that will attack a dome. Potting compounds and conformal coatings will outgas little so they don't form bubbles. They are not inert though.

The conformal coating that was developed for overcoating for automotive was silicone if I remember. It will last 10's of thousands of hours, 100 would not show anything.

Semiman


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## cpf user (Sep 11, 2014)

> silicone first at a reasonable thickness over the LEDs



This was my original way, but the particular silicone had a strong candy smell to it, and after a week or two, the LED would fail. So like was said above, it takes a while for the material to attack the dome. The domes in those cases were plastic.

Then I found glass; it solved the problem, but then they stopped making them. I'm now trying polyurethane encapsulant over the bare LED, which has a PC or PMMA dome. 

As for outgassing or impurities or solvents, I wonder if letting it cure longer before operating the LEDs would help?


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## SemiMan (Sep 13, 2014)

cpf user said:


> This was my original way, but the particular silicone had a strong candy smell to it, and after a week or two, the LED would fail. So like was said above, it takes a while for the material to attack the dome. The domes in those cases were plastic.
> 
> Then I found glass; it solved the problem, but then they stopped making them. I'm now trying polyurethane encapsulant over the bare LED, which has a PC or PMMA dome.
> 
> As for outgassing or impurities or solvents, I wonder if letting it cure longer before operating the LEDs would help?



Not sure what you mean by bare LED with PC or PMMA dome. Bare LEDs have pretty much exclusively silicone domes or epoxy domes.

If the Silicon had a strong candy smell to it, then it sounds like commercial sealant, not electronics/optical grade. People try to cut corners and use standard hardware store silicon sealant to protect electronics .. bad move. The bonding additives are mildly corrosive. Electronics/optical grade (the right ones) do not have these additives.

Semiman


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## cpf user (Sep 13, 2014)

All the chinese leds have plastic domes, which have silicone domes under them. One supplier was able to provide glass, but no others.

The encapsulant/potting that we are using now is more of a standard clear (slightly amber) polyurethane. Needs to cure very firm, with a slight flex. Any you'd recommend?


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## SemiMan (Sep 13, 2014)

cpf user said:


> All the chinese leds have plastic domes, which have silicone domes under them. One supplier was able to provide glass, but no others.
> 
> The encapsulant/potting that we are using now is more of a standard clear (slightly amber) polyurethane. Needs to cure very firm, with a slight flex. Any you'd recommend?




I would not say all of them, I would actually say it is a pretty rare LED that has anything over top of the silicone. Cree did that with the XR, but pretty much no one does anything other than silicone as it is not needed and adds cost and impacts optics. There are epoxy ones, but they are low power.

Do you need a hard(er) plastic potting compound? I am not sure of all the properties you are looking for to be able to properly suggest something.


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## cpf user (Sep 14, 2014)

It needs to be waterproof, for underwater marine operation. It has about 1/2" of potting/encapsulation, which also heat-sinks it a bit. Optical clarity is not important since it is monochromatic (red), and the the slightly-amber PU that we have been using lets the red get through.


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