Ask Dr. Dave

Ask Dr. Dave

In his December 2008 column, Dr. Dave discusses sealants appropriate for use on freezers and the use of threadlockers for assembling screws into plastics.

Question: I noticed that the sealant in my -86°C freezer is coming off.  I would like to replace it, but can you tell me if so-called “normal” window and door silicone sealants are appropriate for that temperature range?

Answer: Exposure to low temperature affects most elastomeric materials, resulting in decreased compression and possible leakage. Extreme cold also affects seals by making them less flexible and brittle. However, you will find that most silicones will still give some flexibility at the low temperature of your freezer. Although silicone manufacturers do offer some special low-temperature grades, I would be surprised if a window and door sealant grade didn’t work for you. You should certainly make sure that the silicone is fully cured at room temperature before exposing it to low temperature.

Question: Can I use a threadlocking adhesive for assembling screws into plastics?

Answer: You have to be very careful when using anaerobic sealants on plastics. Though anaerobics usually don’t present problems for thermoset plastics (e.g., fiberglass or epoxy), they can cause crazing and cracking when the liquid is in contact with several thermoplastics for an extended period of time. The secret to preventing this is to convert the liquid to a solid very quickly by using an activator. In extreme cases, or when it is not possible or convenient to use an activator, you can use a cyanoacrylate-based threadlocker.


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