Plastic weathering

[dalbert]

I am working on a 2-piece weatherproof enclosure for electronics that control a streetlight. The enclosure sits on top of the streetlight and so the top piece (cap) will be exposed to weather (UV, rain, wind, temperature -40C to +70C) and impact (hail) for extended periods (10-20 years). The top piece is snap-fit onto the base so it must also have a little flexibility. So I need something UL-746C f1 outdoor rated and UL-94 HB fire rated that meets the above criteria...and of course cost is always an issue.

I am considering PC stabilized with HALS and colored with TiO2 (light color to minimize the thermal rise that would be hard on the electronics inside) or Carbon Black. Other options might be a PC-ABS blend for lower cost and better cold-weather characteristics. Are these the best choices? Should I be considering PP or PA?

Any experience with materials, thicknesses, and stabilizers for long outdoor life would be much appreciated. Thanks!

 

[TomDOT]

HALS are a good choice for a stabilizer. Be aware that TiO2 spawns free radicals when exposed to UV, and the anatase form is particularly effective at this. Refer to ASTM D 476 when specifying grades of TiO2.

 

[patprimmer]

If I used PC, I would take rather than give advice to the supplier as it is a strange beast and does not always react as expected to additives.

PP is very poor in all qualities your after except price and snap fit. It is crumby to UV, FR and cold impact. The addive packages you need and the interaction between various packages is problematic to say the least

A polyester/urethane block copolymer like Hytrel will be expensive, but might hit all your goals without a huge additive package

Also for nylons, the additives to fix some problems will exacerbate others.

Cold impact FR, UV combo is the tricky bit

Regards
Pat
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[Pud]

PVC? (I thinks it's called Vinyl in the USA?)
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[Pud]

...or a FR PBT?

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[Compositepro]

I believe most plastic electrical enclosures are PVC.

 

[IRstuff]

Kynar, or PVDF, is used for lots of exterior coatings, particularly roofing. 20-yr old Kynar coated aluminum roofs look as good as when they were installed. Supposedly, it can be molded into structural components.

TTFN
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[dalbert]

Thanks to everyone for their replies and suggestions!

PC remains a top contender, but I've looked into some of the materials discussed and am researching them further; any thoughts on relative costs would be appreciated. I've listed the main concerns and relevant links below:

PVC: an interesting choice: low cost with many UL 746C formulations. My main concerns is low-temperature impact resistance which falls off drastically below 10C. Maybe a fiber reinforced PVC?

http://www.matbase.com/material/polymers/commodity/hard-pvc/properties

(see here:

http://www.madisongroup.com/services/failure/casepipe/pipeanalysis.html).

PUR: Better at low temperature, but high cost and concerns with hydrolytic degradation (especially for polyester polyurethanes).

http://www.matbase.com/material/polymers/thermosets/pur/properties

FR PBT: An interesting option...I need to look into this more. Low temperature is an issue with PBT, but the FR seems to help.

http://www.matbase.com/material/polymers/engineering/pbt/properties

http://www.matbase.com/material/polymers/reinforced/pbt-gf30/properties

 

[patprimmer]

PVC is very good re flame retardancy and is quite good, but not great to UV. It is also getting a bit rubbery at 70 deg C. The tougher grades are less resistant to temperature. It produces hydrochloric acid vapours when it burns

Most PU is fairly poor to UV and its very rubbery. It produces cyanide gas when it burns if that matters.

PBT will have cold impact issues, but is very easily made FR and UV resitant. Toughened grades are available that can also still have good UV and FR with reasonable additive packages.

The polyester block elastomers like Hytrel are basically PBT with some PU incorperated to improve impact.



Regards
Pat
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[IRstuff]

One option for weathering is to pick a base material and coat it with Kynar. Roofs and sidings are done that way.

TTFN
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[TomDOT]

Lumiflon is another fluoropolymer coating with weathering resistance similar to Kynar (they claim better than.) Kynar has been around longer.

 

[Mad88]

Hey dalbert,

from my experience I would use a blend of ABS/ASA.
They use ASA for all kind of outdoor applications.

The problem will be that you get the blend only in sheet stock, so the only way to form it will be thermoforming.

Let me know if you would like to have more information's about the material.

 

[patprimmer]

Why would you bother blending it instead of just using ASA.

You can also get PC/ASA blends, but once again, why. What advantage would it bring to PC

Regards
Pat
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[Pud]

Pat, as it's sheet (thermoforming?), I assume it will be a co-extrusion - ABS clad with ASA. Maybe...
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[patprimmer]

Has the OP indicated it is a part that can be thermoformed. I don't think he says either way, but does indicate possible materials that are not normally available as sheet.

Regards
Pat
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[Pud]

Sorry Pat, not the OP - Mad88's post.
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[dalbert]

Thanks to all for providing so many responses so quickly.

To answer a few questions (and ask a few more):

- The target process is injection molding

- The bottom piece has has brass electrical contacts molded/pressed in that will be soldered to the internal electronics; the soldering process will make the pins very hot so I need a polymer with a high melting point...I am considering PA6-GF30 or PC-GF30.

- Also, the electronics include a wireless transceiver so Carbon Black is out as a UV stabilizer/pigment because it absorbs RF energy.

 

[patprimmer]

Well that throws a different light on it. The polymer generally considered most suitable for close contact with soldering applications is GF nylon 6.6. GF PET or PBT is used if better dimensional control is required.

Any other surprises?

UV stabilised nylons 6 and 6.6 are extensively used in street light housings here where we have extended exposure to the highest UV in the world and they must also pass the Pelican Test.

6.6 is normally used if the housing gets very hot short term. 6 is better if it gets fairly hot long term, but both types are used a lot.

Regards
Pat
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[nicklam89]

- Also, the electronics include a wireless transceiver so Carbon Black is out as a UV stabilizer/pigment because it absorbs RF energy.

dalbert,

I found this comment interesting and I wonder if you have any further comments or information on materials that are suitable for housing electronics for RF transmission?

Thanks

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