Ask Dr. Dave: What does fixture time mean?
Dave Dunn's October 2018 column.
Question: I see references to fixture time on adhesives technical data sheets. What exactly does that mean?
Answer: ASTM Standard D 1144 defines fixture time as “the shortest time required by an adhesive to develop handling strength such that test specimens can be removed from fixtures, unclamped, or handled without stressing the bond, thereby affecting bond strength.” Unfortunately, the standard leaves the handling time as a subjective measure without any specific strength requirements.
Some manufacturers have used tensile shear overlap specimens and defined the fixture time as the time required to develop a certain amount of psi of bond strength, often measured by the assembled bond being able to lift a specified weight. However, different manufacturers use different weights and times. You will have to choose your adhesive and then actually determine the shortest time to actually handle your parts.
Question: Please advise on the best way to tool sealants in the home after application.
Answer: I may upset some manufacturers with this advice, but I have had some negative and positive experiences with this over the years in the home. Many manufacturers supply a tool to help in tooling or recommend using something like a plastic spoon. The most common sealants used in the home are latex caulks and silicones. Latex caulks are very easy to tool; in fact, any tool or a wet finger will suffice. You can always wipe off any excess with a damp cloth.
The real secret with silicones is to apply a very narrow bead to the joint. Applying too much means the joint will never look good. After the application of a narrow bead, a wet index finger or two will give an attractive and effective seal.
Dr. Dave is a former vice president and director of Loctite Corp. and has spent many years in troubleshooting adhesive and sealant problems in the adhesives, sealants, specialty rubbers and plastics fields. Questions for publication should be directed to him at 242 Trails End, Aurora OH 44202; phone (440) 477-5164; email DrDave242@windstream.net; or visit www.fldenterprises.com.
Any views or opinions expressed in this column are those of the author and do not represent those of ASI, its staff, Editorial Advisory Board or BNP Media.