Letter 27086

How can I tell if brightener has been used?  

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We are working with a part that has an olive drab finish and we bond rubber to it. We currently get the parts from a plating house, vapor degrease them and then apply an adhesive. Every once in a while we will run into a situation where we will get a mass amount of bond failures (60%). The adhesive supplier always asks about brighteners, because the use of them will interfere with bonding. So my question is, what exactly is a brightener and how can I tell if one batch has brightener and another does not?

Paul R. Rudolph
Chemist for automotive supplier - Churubusco, Indiana, U.S.A.

You are lucky to have an insightful adhesive supplier, Mr. Rudolph. While it is never possible to know for sure if a theory of the cause of failure is correct without actually testing it, my guess is you are right on the money.

The olive drab finish you are looking at is presumably zinc plating followed by an olive drab chromate conversion coating. All zinc plating requires brighteners (addition agents) in order to deposit in a smooth, useful, bright finish. However, there are at least three general classes of zinc plating baths (cyanide-based, acid-based, and alkaline non-cyanide based); each requires different kinds of brighteners. On top of that, the brighteners are supplied as proprietaries by many different suppliers and may further differ for that reason. Finally, the plating shop can very easily add too much brightener and precipitate an emergency.

Usually an excess of brightener will reveal itself earlier in the process because it will be difficult to get the olive drab chromate to come out right if the zinc was over brightened. You still have a problem on your hands of determining whether the plating shop has multiple plating lines, with one zinc tank or chromate tank being different than others, or tracking down whether the problem was an accidental over-addition of brightener. But I think you are well on you way to solving the problem. Basically though, it will not be up to you to examine the parts for the presencer of brighteners, all parts have them; it will be up to the plating shop to institute process control and QA which assures they do not cause this adhesion problem.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Is one of the plating methods better than another for bonding?

Paul R. Rudolph
- Churubusco, IN, USA

Well, I understand that the alkaline non-cyanide zincs are difficult to chromate because of a greater tendancy towards having excess brighteners contaminating the surface. But this is book knowledge; so now it is hopefully time to turn you over to a reader who does more than one of these processes and can give you a real-world assessment of whether changing the finish would actually improve the bonding.

Ted Mooney, P.E.
finishing.com Inc. - Brick, NJ

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