Curing of "odorless" silicone caulk

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SUMMARY

The curing process of "odorless" silicone caulk, specifically GE Silicone II, relies on moisture exposure, similar to traditional silicone caulk that emits a vinegar smell. The primary curing agent in odorless silicone is MPDMS (methoxypolydimethylsiloxane), which is formed through a condensation reaction involving PDMS (polydimethylsiloxane) and trimethylsiloxane. The presence of HMDS (hexamethyldisilazane) acts as an initiator for polymerization, potentially activated by humid air rather than direct water contact, which can hinder the curing process. Mixing additives like acrylic paint or glycerine with GE Silicone II results in weak curing outcomes, emphasizing the need for careful consideration of components when attempting to accelerate the curing process.

PREREQUISITES
  • Understanding of silicone chemistry, specifically polymerization processes.
  • Familiarity with silicone caulk types, including GE Silicone II.
  • Knowledge of chemical components such as MPDMS, PDMS, and HMDS.
  • Ability to interpret Material Safety Data Sheets (MSDS).
NEXT STEPS
  • Research the chemical properties and curing mechanisms of MPDMS in silicone caulks.
  • Explore the effects of humidity on the curing process of silicone caulk.
  • Investigate the compatibility of various additives with GE Silicone II for enhanced curing.
  • Learn about alternative silicone formulations and their respective curing agents.
USEFUL FOR

This discussion is beneficial for chemists, materials scientists, and DIY enthusiasts interested in understanding the curing mechanisms of silicone caulks, particularly those seeking to optimize the performance of GE Silicone II in various applications.

Stephen Tashi
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What causes the "odorless" type of silicone caulk to cure once it has been extruded from its tube, but allows it to remain uncured while it is in the tube.

My understanding is that the type of silicone caulk that smells like vinegar cures outside the tube because it is exposed to moisture - but what activates the cure of caulk that is "odorless" or "low oder" or "GE silicone II" ?
 
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Most likely it is still a moisture that is necessary for polymerization.
 
There are many water based things that can be mixed with non-odorless silicone caulk that make it cure quickly - such as acrylic paint, glycerine, soapy water. (Videos on the web show doing this to make flexible molds. I've tried these methods and they do work.) However, when those things are mixed with GE Silicone II caulk, it either doesn't cure or cures to a very weak material. This is why I'm curious about curing process for odorless silicone caulk.
 
I don't know the exact process, but when in doubt, check the MSDS:
http://www.homedepot.com/catalog/pdfImages/2d/2d9622c5-268d-4ab0-8dce-32dd186b8b72.pdf
The main component is MPDMS, which is formed by a condensation reaction of PDMS and the trimethylsiloxane. The HMDS is probably an initiator (it tends to methylate SiOH groups). But HMDS is quite reactive with water. It's possible that humid air is enough to start breaking down the HMDS to initiate polymerization, whereas mixing water directly simply destroys all the HMDS at the outset. But again, this is just a guess.

Edit: abbreviations are as follows.
HMDS = hexamethyldisilazane
PDMS = polydimethylsiloxane
MPDMS = methoxypolydimethylsiloxane
For completeness:
MSDS = material safety data sheet
SiOH = silanol functional group
 

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