Ask Dr. Dave -- October 2002

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I keep coming across the term "cohesives." Is this different from adhesives?

Cohesives (sometimes called cold adhesives) are adhesives that stick only to themselves. They are applied to both substrates being assembled and then pressed together to create a bond. The term "contact adhesive" is frequently applied to these systems when sold into retail markets. In industry, they are used widely as packaging adhesives. Cohesives are particularly useful for packaging temperature-sensitive products like chocolate and ice cream. Historically, the two most common materials used were natural rubber latex and solvent solutions of polychloroprene. Because of environmental and safety concerns over solvents, latex versions of polychloroprene have been developed.

We are a manufacturer of control systems for furnaces. We need to cover an adjustment screw to discourage the end user from touching it and also make it "tamper-detectable." We know there are tamper-proofing adhesives available, but how can we see if a screw has been interfered with?

You are right; several companies market adhesives that can encapsulate an adjustment screw to stop loosening or to prevent tampering. This is often done using an epoxy adhesive, a hot melt or a UV-cured acrylic. The problem with this technique is that someone can certainly remove the encapsulant, adjust the screw and then replace the encapsulant.

A good way to detect whether a screw has been interfered with is to cover it with a unique type of encapsulant that will show whether it has been removed and is not readily available to an end user. The simplest way to do this is to add a readily detectable ingredient to the encapsulant such as an unusual color or a fluorescing material so you can detect your material under a black light.

The ultimate encapsulant I have found for this application is one that is formulated to be strong enough to hold the screw in place but instantly shatters if someone tries to remove it. This type of material is not easy to replace but should be easy to formulate by your adhesive supplier. Tell him or her that instead of trying to toughen the adhesive to make it very brittle instead!

I am interested in using a UV adhesive for a medical product. However, the adhesive will be exposed to high temperatures for short periods of time (100 deg C for two seconds). If the material burns, does it have the potential to outgas harmful vapors? Are there other non-obvious concerns I should be aware of?

You really should not have any problems with exposing a properly formulated UV adhesive to a relatively low temperature like this, particularly with the short exposure. UV adhesives are crosslinked systems that will routinely perform continuously at temperatures above 100 deg C. The adhesives will certainly not burn at this temperature, but you should check with your adhesive supplier to see if the adhesive is approved for use in medical devices and does not contain any unusual ingredients that might be very volatile at that temperature.

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