Ask Dr. Dave / Columns

Ask Dr. Dave

April 1, 2005
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April 2005

QUESTION: I have some terra cotta sculptures with clean breaks that I want to repair. I spoke to a museum and they recommended that I use an acrylic adhesive. Unfortunately, the term acrylic adhesive seems to be a very broad one. Could you please help me to choose an adhesive?

ANSWER: Acrylic adhesives would probably do a very good job repairing these objects. The most suitable would be the tough acrylic type that uses an adhesive/surface activator system, but these adhesives are not readily available these days outside of the industrial and marine industries. As an alternative, I would suggest that you try a good-quality, two-component epoxy adhesive from a hardware store or home center. Most epoxy adhesives you will find there will be called "fast" or "5-minute" and are designed for consumers seeking instant repairs of broken objects. In your case, I would recommend a slower-curing version - look for terms like "one-hour working time " or "extra working time." This will give you lots of time to mix the adhesive, apply it to the pieces and position them accurately. It is also possible to color the epoxy to get the best-looking cosmetic repair. If you have any sculptures with missing parts or voids, the epoxy putties that I covered in my April 2004 column will do a great repair job and can be sanded after curing to give perfect-looking restorations.

QUESTION: What is the best product for the exterior caulking of a home for long-lasting repair?

ANSWER: You will see a range of products being sold these days including silicones, acrylics and so called "siliconized" acrylics. I have absolutely no doubt that RTV silicone is the best product from a technical point of view, and I know that manufacturers of these types of sealants often give 50-year or lifetime guarantees. Silicones have great resistance to weathering and tremendous flexibility, which means they can withstand large joint movements caused by the expansion and contraction of your house with changes in temperature. However, you need to ask yourself a few questions before using the silicones. Do you want to pay a relatively high price for a silicone or would you be happy with a product with a less-expensive sealant that might only last 10-20 years? Can you live with the color of silicones (normally clear or white) or would you need to paint over the sealant? (Most silicones do not accept paint readily.) Water-based acrylic caulks have become very popular over the last few years because, while not quite as good as silicone, their performance still exceeds the time most people live in a house, they are readily paintable and the wet sealants clean easily with water.

Q&A Exchange is written by Dr. Dave Dunn of F.L.D.Enterprises, a technical consultancy and full-service industrial market research firm specializing in the adhesives, sealants, specialty rubbers, and plastics fields. Dr. Dave is a former vice president and director of Loctite Corp., and has spent many years troubleshooting adhesive and sealant problems. Questions for publication should be directed to him at 242 Trails End, Aurora OH 44202; (330) 562-2930; FAX (865) 251-9687; e-mail DrDave242@att.net ; or visit http://www.fldenterprises.com .

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