aluminum soda can coating 2

[karankanodia]

i have been trying to find out what coating is used inside the soda cans which prevent the liquid from reacting with the aluminum (soda cans are made of aluminum).

Also, once that is identified i need to find a chemical that could be used to remove the coating. The goal is to obtain the aluminum without the coating.

Note: i do not have enough chemistry background so it will be extremely helpful to get as detailed info. as possible.

 

[bchoate]

I am answering your question but you may not understand some of the chemical aspects. Interior can coatings designed to prevent migration of metal salts into the contained product are called "comestible polymeric coatings". The coatings ars polymers typically used in coil coating. The exact nature of the coatings isn't available since most are proprietary to manufacturers who continuously look for better coatings.

Most are epichorohydrin - bisphenol A epoxy;s cross-linked with urea-formaldhyde or vinyl resins or other materials. These polymers are used because of the good adhesion to the can metal and because of the very low extractables from the polymers.

Removing the polymer isn't an easy task. Recyclers handle this by incineration which is an energy intensive process. There are some patents looking at biodegradation of the polymer. The process is not simple and requires hours to remove the polymer. Solvents that could dissolve the polymer are things like tetrahydrofuran (THF) or ortho-dichlorobenene. A solvent based process will have a lot of environmental concerns. The need for the aluminum in the cans will have to economically support the cost of the process. There is a large recycling industry because the cost of the aluminum is less than that of aluminum from ore. It takes about 30, 12 oz. beverage cans to provide 1 lb of aluminum.
Bill Choate

 

[karankanodia]

Thanks for all the info on that Bill. I was also wondering if someone help me figure out the material used to coat the outside of the can which is used for advertising

 

[unclesyd]

What quality do you need to clean?

There are several methods that will take you can to bare metal with no loss of material.
Pyrolysis would be the cheapest but maybe limited by the presence of Chlorine.
The next option would be Fluidized Bed as Chlorine is not near the problem as with Pyrolysis.

I have cleaned quite few coated aluminum cans over the years in a Pyrolysis Oven for different Science Fair Projects.

 

[karankanodia]

I am trying to retain as much reflectivity as possible. so i am striving for maximum cleaning.

 

[bchoate]

karankanodia
Your last post expressed a need to retain as much reflectivity as possible. By that I am assuming that you want as high a gloss or shine on the metal as possible. Aluminum self passivates by the formation of an oxide coating. This gives aluminum it's satin gray appearance. Most of the surface treatments used to remove the exterior graphics and coatings will either result in loss of aluminum or accelerate oxide formation. The can surface may not have the gloss that a new beverage can has.

Two piece beverage can exterior coatings and graphics are a complex system of very thin layers. A typical system may start with a clear or white urethane acrylate base coat. Graphics are developed by printing up to six colors over the base coat. The can is protected by a final over print clear varnish to maintain the surface of the can and the graphics. These components may be UV curable acrylic materials. There are thermally curable systems also.

Removal of the exterior coatings leaves the surface susceptible to oxide formation. Systems used to remove the coatings may activate the aluminum for oxidation. Maintaining a high surface gloss probably requires a protective varnish or polish.

B. Choate

 

[karankanodia]

what form of aluminum is obtained after the coatings (both inside and outside) are removed?

 

[unclesyd]

It is the same alloy and as stated above it will have an oxide surface which is grayish. It want have much reflectivity. To restore some reflectivity you will have to mechanically polish it with special Al polishing materials.

It maybe possible to retin some of the shine if you could use a chemical solvent stripping system. Unless there are thousands to do the cost would be prohibitive.

If you can reveal the end use there might be some alternatives.

 

[karankanodia]

Well i am trying to figure out a way to recycle the aluminum of soda cans into solar concentrators. And for that i need to retain the highest amount of reflectivity as cheaply as possible.

 

[dgallup]

Is the inside reflective enough as it is? It would not be too hard to cut the top & bottom out, split them length wise and form them into a reflector.