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Questions and Answers

May 22, 2001
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Questions and answers in the adhesives and sealants industry.

Question: What special requirements exist for bonding aluminum materials with structural adhesives? I have a lot of experience with several structural adhesives on steel, but I am concerned about potential problems that might occur when bonding aluminum.

Answer: Aluminum is clearly a different substrate than steel or other iron-containing metals with respect to obtaining a sound structural bond with adhesives. In addition to proper selection of a structural adhesive, the nature of the aluminum surface is critical to long-term bond integrity. Elemental aluminum is a terrible surface to try to bond with an adhesive. Luckily, aluminum doesn’t often exist in a pure form on the surface. There is almost always some form of oxide or conversion coating on the surface. These are often decent surfaces to use an adhesive on, but you do need to be aware of the type of surface you are dealing with.

One of the most common surface treatments found on commercial aluminum is phosphoric acid anodization. Chromic acid anodization is another commonly employed commercial process that results in a slightly different surface morphology on aluminum. Typically, aluminum anodized with phosphoric acid will result in more durable adhesive bonds than if the surface is treated with chromic acid, primarily due to the different rates of water absorption of the two resultant oxide levels.

Many other factors also come into play that should be considered. Aluminum generally presents a more challenging bonding surface than ferrous materials, and you should test both short- and long-term bond integrities carefully under required environmental conditions.

-BOB SMITH

Question: I recently encountered an unusual situation when installing a large ceramic kitchen. After installing the tile, I applied a siliconized acrylic latex caulk around the back splash. Later, when I was cleaning up the grout with muriatic acid, I noticed that the caulk I had applied was foaming and bubbling like it was reacting with the acid. I wiped most of it up but left some on a small area for quite a while. Later I saw the caulk was partially eaten away. What was going on, and is there something I can do to prevent this from happening again?

Answer: I’m surprised you haven’t encountered this before. Muriatic acid is essentially hydrochloric acid, a strong mineral acid. Muriatic acid is used to clean up excess dried grout because it reacts readily with the carbonate in the portland cement-based grout. However, many water-based caulk formulations also contain a carbonate filler in the form of calcium carbonate. When using muriatic acid to clean up grout, you can avoid this adverse reaction with the caulk by carefully avoiding getting the acid on the caulk, applying the caulk at the end of the job, or using a caulk that is filled with talc rather than calcium carbonate. In any event, you can apply additional sealant afterward if this situation is unavoidable.

-BOB SMITH

Question: I require an adhesive for sticking paper to tins.

Answer: I can only assume you mean applying labels to metal containers. There are any number of adhesives and processes available to accomplish this. Basically, there are two types of labels that are commonly in use. One makes use of a pressure sensitive adhesive that is pre-applied to the label stock (usually in rolls), and the label is then applied to the individual container. The other type of label is typically referred to as a “patch label,” and as the name implies, is supplied pre-cut and stacked. It can be applied either by the pre-application of a pressure sensitive adhesive, or more commonly by using a water-based adhesive to apply the label to the container when required. This latter process usually involves highly automated application equipment and typically is employed for very high-volume applications. For lower-volume applications, the more labor-intensive method of a pre-applied pressure sensitive label is often the most economical approach, despite the significantly higher cost of the label itself.

-BOB SMITH AND ROGER LOHMAN

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