Testing for Aluminium Oxide 4

[KatRawlings]

Hiya everyone,

I was just wondering if anybody knows how I can test for the presence of aluminium oxide? We have been shot blasting plastic with an aluminium oxide abrasive, so bits of this abrasive will be left on the surface of the plastic. We believe that we can remove this aluminium oxide using acetic acid (white vinegar). However, it is essential that there is no aluminium oxide left on the surface of the plastic. We therefore need some way to test for this.

Thanks

Kat

 

[unclesyd]

I think your best bet would a microscopic examination.
You could locate some particles embedded in the plastic and have a lab verify that it is Al2O3.

 

[arunmrao]

xray diffraction technique should be able to identify. As a suggestion,why do you need to blast a soft plastic with a hard abrasive? This is bound to embed.

 

[KatRawlings]

Thanks for advice, so i could try microscopic evaluation or XRD. A chemical analysis company has suggested XRF.
@arunmrao: We have tried about 15 abrasives on about 7 different plastics and aluminium oxide has been giving the best results.

 

[TVP]

XRF can be portable (hand-held), so this may be an advantage over XRD. Either way, a microscopic evaluation, even at 10x magnification, will probably be very helpful in understanding what the surface looks like after abrasive blasting.

 

[MintJulep]

How serious and expensive are the consequences of remaining aluminum oxide?

If you can't tolerate the presence of aluminum oxide embedded in your surface then the best course of action is not to blast the surface with aluminum oxide.

You will NEVER be able to test all the aluminum oxide away.

Do you seriously think that it's realistic to test and confirm the absence of aluminum oxide on every square millimeter of every part?

You are starting down a path that will be excessively time consuming and expensive and also guaranteed to fail. You should rethink.

 

[redpicker]

I agree with MintJulep.

Acetic acid will not remove aluminum oxide. Al2O3 is inert to acetic acid, so you aren't doing any better than soap and water would do (actually, soap and water may do better). Strong caustic (pH > 10) will dissolve Al2O3, but it may damage the palstic and will certianly damage any human tissue it comes into contact with (it is very dangerous), so a caustic wash may not be the best solution.

The best solution would be use a abrasive that is compatible with the product and processes

rp

 

[IRstuff]

Perhaps you need to review what you are trying to achieve with the shot blasting. This is seems to be a rather crude approach to achieving whatever your goal is. I'd imagine that the part in question is not that expensive, and the cost of inspection would most likely blow the price through the roof.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[tomwalz]

I've never sandblasted plastic with aluminum oxide but I do have some experience sand blasting tungsten carbide with aluminum oxide.

Aluminum oxide splits and shatters in sandblasting. It does this even when sandblasting a soft medium because the particles impact one another. One of the shapes formed with regularity could be described as ‘spears’. These aluminum oxide ‘spears’ penetrate and become embedded in tungsten carbide.

I would suspect that the spears formed by the breakdown of aluminum oxide grit would become deeply embedded in the plastic.

You may be able to find the report on this on the Internet. I remember seeing it because there was a lot of commotion about whether the aluminum oxide on the surface of tungsten carbide would interfere with brazing or not. Once we establish that the aluminum oxide penetrants occupied a miniscule percentage of the available bonding area I pretty well lost interest in the whole thing.


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[IRstuff]

Hydrofluoric and phosphoric acids are the only two common acids able to attack aluminum oxide:

http://accuratus.com/alumox.html

Note that HF is EXTREMELY dangerous; use with caution.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[KatRawlings]

Thank you all for your replies. When I posted originally I was thinking there might be an easy, non-destructive method for detecting aluminium oxide. Our main focus was for a particular medical application where it is essential that there is no aluminium oxide contaminating the products. I was therefore thinking we could remove the aluminium oxide chemically and then test each product to check that all aluminium oxide had actually been removed. From the responses I have had on this forum and elsewhere, it appears as though this will be far more difficult and expensive for us to do this than we originally anticipated, and it doesn’t appear as though this would be feasible. If we shot blast these products with aluminium oxide it is highly likely that there will still be some aluminium oxide embedded in the surface of the plastic, which will not be acceptable. Also, some of the parts have a 1m diameter so it will not be easy to check the entire surface for contamination. For this particular application I am therefore thinking it would be better to use an alternative method to shot blasting with aluminium, so that there is no risk of contamination.

It is possible that there could be a non-medical application where surface contamination wouldn’t be as serious and the removal of aluminium oxide would be mainly for aesthetic purposes. In this case we would be able to try a few different chemicals (and different lengths of exposure etc) to remove the aluminium oxide. I can therefore try hydrofluoric and phosphoric acids as suggested. Thank you for your help.

 

[IRstuff]

What's the intent of the shot blasting? One who think that a plastic could be made with the desired surface qualities in the first place, avoiding any further processing such as this.

Again, I caution you about HF, since it's extremely toxic and hazardous, and not necessarily something you want to be handled by untrained personnel.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Ron]

Have you considered shot blasting with dry ice? Leaves no residue.

 

[unclesyd]

I echo the warning on the use of HF acid unless you have been properly informed about its properties.
It is insidious as one drop on the skin especially the finger nails is quickly absorbed with resulting pain that feels like being pounded with a hammer. The affected area will take long medical and healing process.
It has to use in a fume hood. The use of an acid proof fume hood is very desirable.
It can not be worked in metal tanks.


Here two suppliers of plastic bleat media. Checkout both to see how it used, especially the Maxiblast Application

http://www.maxiblast.com/applications/

http://www.optiblast.com/products.asp

You could look at soda blasting, wet or dry.

http://www.armex.com/?gclid=CNGt88jjvaYCFQrt7QodCzRbGg

You could also try corn cobs or walnut shells.

http://www.kramerindustriesonline.com/blasting-media.htm

Or water ice blasting.

http://www.iceblast.net/faq00.htm

 

[KatRawlings]

Thanks for all the suggestions and also thanks for the HF warnings.

@ Ron: Do you have any experience with ice blasting plastics. I have already been talking to some companies about dry ice blasting. The general feel I'm getting is that it probably isn't right for achieving a good surface finish on plastics. A couple of companies have said I can send samples for testing so I am currently waiting for some samples to be prepared so I can send them off. However, even if this works it looks as though it is going to cost far too much for us to start using this process. We would have to send samples off to one of these companies to blast for us, as we do not have the capabilies on-site.

@ Unclesid: I am going to buy some bicarbonate of soda at the weekend so I can test out soda blasting. With regards to ice blasting our on site air capabilities are not adequate so it would be very expensive for us. Thanks for the plastic blast media links and also the natural abrasives link, very helpful.

Kat

 

[unclesyd]

Regular baking soda will work on some projects, but I don't think it will work your project. Here is the link to Armex Abrasives where we bought all our soda blasting media.
The second link is to grit-o-cob blasting media

http://www.armex.com/

http://www.gritocob.com/product-list.aspx

If you are in the US and live near a Harbor Freight Store they carry the Armex material as we a soda blasting pots.

 

[KatRawlings]

Thanks, I am in the UK so I will try to find a UK company that sells soda blasting media. Will that work with my shot blasting gun that I have been using? Or will I need to purchase proper soda blasting equipment?

 

[IRstuff]

Are you sure you need to do this? Is there an alternative, i.e., making or molding the plastic to the desired state in the first place?

There are lots of very good polymer and plastics guys here that might come up with a simpler solution to your problem.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[unclesyd]

It should work and if doesn't all you will have to do is change the nozzle. Keep the air as dry as possible.

 

[KatRawlings]

@unclesyd: Thanks, I'll post when I have tested with soda blasting media to let you know how it goes.

@IRStuff: The plastic cannot be made this way in the first place. It is complex engineering and must be cut and machined etc. There might be alternatives to shot blasting but the results that shot blasting has given are billiant. We therefore need some way to improve surface finish to this standard without using aluminium oxide, or alternatively we need to find a way to remove the al oxide afterwards. Maybe shot blasting with plastic blast media will work as our results for this are quite good. I'm sure we'll get there in the end.