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Was the Contamination From Silicone?



Was the Contamination From Silicone?
Black enamel was sprayed onto metal followed by a clear coating when a contamination problems surfaced. What was causing the contamination?
Mysteries of Science

Transcript


A large metal fabrication plant's main work involved metal deco, the printing, painting, and coating of metal parts. This is most commonly done through spray painting with automatic machines. 

Any contamination on the parts will cause many kinds of defects. One of the worst contaminates is silicone oil or grease. It causes the paint to bead up into a phenomenon called "fish eyes", forming small circular ridges of coating. 

An important job was being run with black enamels sprayed onto metal and dried followed by a clear coating to add protection and gloss. As soon as the clear coat was sprayed "fish eyes" were present. 

The line was shut down and a search for the source of the contamination was conducted. But to no avail.

By the following day the problem had vanished only to return two weeks and one day later in the same spray room while a different part was being run.

What was causing the contamination?  

Here's the rest of the story.

A hunt began to find the source of contamination and the evidence pointed towards silicone. Years earlier the President of the company had banned the use of silicones and cautioned workers to be watchful not to bring in any lotion, hairspray or anything else that might contain silicone. 

Several paints had shown the problem but it always happened in the same area. In fact the "fish eye" problem had hit all the spray booths in this one particular room in the factory at the same time. 

A big meeting was held to see if they could solve the problem. An engineer spoke up, "Now don't laugh, but I think it is the wind. Haven't you noticed that it always happens when the wind comes out of the east?"

Wind out of the east is rare in the Midwest, but it was easy to check the weather data with the factory fish eye records.

The group headed for the roof and walked over to the air intake vent for the spray room below.

Something must be getting pulled into the fresh air intake when the wind blew from the east. There was one big exhaust pipe not too far from the spray room air intake and it was directly east.

So if the wind theory was right, it meant that exhaust from that nearby stack was being sucked into the plant room to cause the contamination problem.

The nearby stack was connected to the adhesive coating dryer. No silicone oil was used, but silicone had been an ingredient in the adhesives up until the president had banned the use. 

As a test some adhesive was smeared onto a metal sheet and paint was sprayed from an aerosol can. Immediately they saw the problem "fisheyes."

Finally an operator confessed that he was secretly using his own hidden supply of silicone to doctor up the adhesive batches to get them to perform better.

They had a simple solution, raise the height of the spray booth air intake vent. So the stack height was raised by five feet and the next time the wind blew off the east no problems. 

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