[kat.rawlings]

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

[Stepan]

Aluminum Oxide used in grinding (abrasive) tools is so called alpha form of Al2O3 and is insoluble in ANY acids including vinegar. For analysis, I would burn the plastic, collect the ash and test it Al2O3 using XRD 

[kat.rawlings]

Aluminum Oxide used in grinding (abrasive) tools is so called alpha form of Al2O3 and is insoluble in ANY acids including vinegar. For analysis, I would burn the plastic, collect the ash and test it Al2O3 using XRD 

Thanks for your help. However, we need to test plastic parts that we then want to use. Is there any non-destructive testing that we could carry out?

[Fleaker]

Alumina (corundum) is too hard relative to any plastic--that is why it's getting stuck. Switch to something else.

Vinegar will not work! If your plastic is not a polyamide or a polyester that can saponify and otherwise hydrolyse, perhaps a concentrated solution of sodium hydroxide can solve your alumina woes?

I use hot, concentrated lye to remove silica, alumina, and aluminasilicates from incoming materials (i.e. picture platinum crucibles and fusion boats that have glass stuck to them).

Steam the parts, and then base digest to form aluminate Al(OH)₄^-

I'm pretty sure Stepan is right, alpha alumina is acid insoluble (gamma alumina will dissolve in HF/H₂SO₄)


To answer your original question: the cheapest way to test to see if it is on the surface would be an optical microscope and looking for the abrasive. Otherwise, get a portable XRF gun and see if it has aluminum in it: http://www.niton.com/?sflang=en

[Stepan]

I agree, Stereo Microscope with magnification 10-60 would be the least expensive nondestructive technique. It would be also the most sensitive as you can probably detect a few Carborundum particles per you sample which is way less than 0.01%. The only problem is: the method will be qualitative.

[kat.rawlings]

Thank you for your reply. 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 concentrated sodium hydroxide as was suggested. Thank you for your help.

[Enthalpy]

If I remember properly, alumina is fluorescent. You might use a UV lamp (got for 3$ a pocket lamp from China that includes UV and laser) to detect alumina powder by sight, maybe with a magnifier, provided the plastic doesn't fluoresce.

[Stepan]

If I remember properly, alumina is fluorescent. You might use a UV lamp (got for 3$ a pocket lamp from China that includes UV and laser) to detect alumina powder by sight, maybe with a magnifier, provided the plastic doesn't fluoresce.

Not sure about this one. And even it it is fluorescent, how to quantify it?

[Stepan]

I do not know about your process, but you can try other abrasive materials, which would not interfere with your applications. Let say sodium chloride or baking soda. The last one is used in dentistry as "soda blasting" to polish teeth.

[kat.rawlings]

If I remember properly, alumina is fluorescent. You might use a UV lamp (got for 3$ a pocket lamp from China that includes UV and laser) to detect alumina powder by sight, maybe with a magnifier, provided the plastic doesn't fluoresce.

Not sure about this one. And even it it is fluorescent, how to quantify it?

Somebody else has already mentioned using a UV lamp on a different forum but you're right Stepan, I'm guessing it would be very difficult to quantify it. I am definately thinking it would be better to avoid using the aluminium oxide, even though it gave brilliant results, and to try a different media that wouldn't cause this whole contamination problem. Any suggestions anyone?

Kat

[kat.rawlings]

I do not know about your process, but you can try other abrasive materials, which would not interfere with your applications. Let say sodium chloride or baking soda. The last one is used in dentistry as "soda blasting" to polish teeth.

Thanks, soda blasting has been mentioned several times, I will have to try it

Kat

[Enthalpy]

If your object in convex, try a rotating brush?

If sanding, you might try a powder that evaporates afterwards (CO₂ ice, used on aeroplanes, but cold). Or melts (water ice)?

[kat.rawlings]

If your object in convex, try a rotating brush?

If sanding, you might try a powder that evaporates afterwards (CO₂ ice, used on aeroplanes, but cold). Or melts (water ice)?

Due to there being lots of small grooves and ribs in the part I am guessing it would be difficult to use a rotating brush.

We have looked into dry ice blasting (CO2 ice) and the set-up costs and running costs look far too high. We would also need to modify our air supply due to the large amounts of air that are needed. Also, after contacting various dry ice blasting companies it appears as though it might not even work for our requirements. We will be sending some samples off to be dry ice blasted sometime soon to see what results it does give on polypropylene. If dry ice blasting does work then it may be more economical to send work off to one of the companies for them to blast for us so this is still a possibility.

I will have a look into ice blasting with water now.

Thanks for the advice.

[kat.rawlings]

If your object in convex, try a rotating brush?

If sanding, you might try a powder that evaporates afterwards (CO₂ ice, used on aeroplanes, but cold). Or melts (water ice)?

Thanks again for the ideas. I just thought I would update you on where I am up to at the moment. Feel free to correct me if I’m wrong about any of this.

Water ice blasting doesn’t look like it’s going to work either. It looks similar to dry ice blasting but instead of using dry ice as the blast media, water is used, which would suggest the running costs will be lower. It requires 75.7 litres of water an hour, which could get expensive. Specialist equipment would need to be purchased and the air supply would need to be modified, meaning set-up costs will be high. Also, I couldn’t find a UK based company who supplies water ice blast equipment/services. Even if we could get this equipment, I’m not sure if it would suit our requirements because there is no mention of blasting plastics. I’m thinking I might have to give up on this idea.

[Stepan]

I have seen people were using a fluidized bed for similar application. The fluidization can be achieved either due to air or liquid movement or by ultrasound.