UV stabilized plastic reccomendation 5

[fade2blue]

I have a thermoformed HDPE cover that is used outdoors as a machine guard. After a few months in service the part becomes brittle and fails. Can anyone reccomend a material that has similar mechanical poperties with enhanced UV stability? The color must be white or a carbon black would be substituted. Also the production volume is fairly low so a higher priced stocked material would be preffered to an X ton minimum order. Would a TiO2 additive help?

 

[Pud]

http://en.wikipedia.org/wiki/Royalex

H

http://www.tynevalleyplastics.co.uk

 

[Demon3]

There are plenty of options.

Firstly, the HDPE would work with carbon black and proper UV stabilizers. Other alternatives are PP and ASA (the latter much more expensive).

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[Demon3]

PVC is another great choice for outdoors and has the advantage that it's intrinsically flame retardant and cheap.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[patprimmer]

PVC would be my first choice or UV stabilised HDPE or UV stabilised PP

Regards
Pat
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[btrueblood]

I like Pud's suggestion best, but then I like Royalex too, makes great watercraft hulls. For the outdoor machine guard application of the OP, it would save weight for the same stiffness, and has a gentler failure (i.e, a ductile yield rather than a brittle snap like PVC can give).

But, I also think the OP owes three stars for the prior posters, all of whom gave quite good info., as usual.

 

[fade2blue]

Thanks for the replies. Does white PVC have better UV properties than a white PP or HDPE?

 

[patprimmer]

Yes, presuming the same pigment formulation.

Not all pigments of the same colour offer the same UV protection.

eg, Carbon black is impervious to UV light while Nigroseine has no effect at all. Both are black.

Also particle size and quality of dispersion of the pigment has a significant effect om results.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[HDS]

A weatherable sign grade of PC could work but it is not as easy to mold and the shrinkage may not match up with HDPE. However, it will make a nice strong guard. Look through the Spartech catalog for other options.

 

[patprimmer]

I normally do not recommend PC for machine guards because of the influence of oils on it re solvent stress crack.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[HDS]

Good point Pat. I have seen PC guards self destruct because of exposure to brake clean.

 

[fade2blue]

I have found a UV resistant HDPE called Marine Board. Would it have the same processing capacbilitites as a standard HDPE?

 

[Demon3]

Check the MFI to see if the viscosity is the same as the PE you are used to.

UV stable PE should process the same as all you need is 1 % carbon black and the right UV stabilizer, e.g. HAS (also known as HALS in the old literature).

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[patprimmer]

If they are both extruded sheet with about the same dimensions and form, I would expect they are both probably fractional MFI and would process the same, however that is not a guarantee and as Chris states you should check.

If the supplier gives you a blank look when you ask what is the MFI, you can get a real rough idea by placing small offcuts of both, side by side in an oven and test them as they start to melt. If they are both about as bendy or malleable as each other you are probably OK. It is a real rough test, but might be all you have available.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[HDS]

You can give it a try. Send the data sheet to your thermoformer.

MFI isn't as important with thermoforming like it is with injection molding. There is a hot strength test which is more relevant (but I forget it's name).

I would worry more about forming temperature and shrinkage. The orientation of PE sheet is important and can make a big difference in the sag of the sheet and the shrinkage. However, if you ordering small lots you won't get to specify this.

 

[patprimmer]

Attempts to describe viscosity of melted plastics getts a lot of different approaches and terminology depending of whos telling for what reason.

MFI is used to attempt to describe how easy a plastic will flow in the mould It is used by pretty much injection moulders. It is a poor method but is easy to do and comprehend.

Molecular Weight is used by polymer chemists.

Melt viscosity is used by physicists.

Melt strength is used by blow moulders and extruders in a descriptive manner, but they still use MFI for a quantitative measure.

You want high melt strength which is low MFI. Fractional MFI is less than or a fraction of 1.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[Demon3]

Great post Pat.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[automatic2]

Extruded millions of feet of vinyl siding, and I can tell you that PVC has poor UV characteristics unless it specificallay has an inhibitor blended in.

 

[Demon3]

Same for many plastics. Even ASA which is extruded onto PVC siding to improve weatherability needs UV stabilizers and titanium dioxide to withstand UV.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[fade2blue]

So TiO2 is used as a UV stabilizer? One material manufacturer said it was an antioxidant and didn't specifically serve as a UV blocker. My thermoformer thinks the Marine board will form the same as regular HDPE.