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Modified Silicone Polymers and Cleanroom Sealants


Modified Silicone sealants,  or also know as Modified Silane ( MS ) polymers  have been the product of choice and gaining popularity in construction, automotive, manufacturing, and DIY in recent years.  The  modified silicone formulations offer the combination of best properties of  weatherability of silicone,  and strength  and paintability of polyurethane ( PU ) while limiting their individual drawbacks.

The structure of  a MS polymer consists of  Polyether backbone which terminated with hydrolyzable dimethoxysilyl groups on both ends.

Typical Modified Silicone ( Silane )  Structure


The curing mechanism of  MS polymer is believed to be taken place in 2 steps :  First, the methoxysilane undergoes hydrolysis, by reacting with water ( atmospheric moisture ) in the presence of catalyst to form silanol groups. Subsequently,  these silanol groups  react  with one another or with another methoxysilane to form siloxane  linkages  -- formation of  macro molecular weight  MS Polymer.



Benefits of Modified Silane polymers:

* Silicone-free formulation, no silicone staining due to migration of  low molecule weight siloxanes
* Isocyanate-free
* Neutral cure system, odourless
* Free from blisters which usually associate with PU sealants
* Excellent adhesion to broad range of substrates without the need of primer; bonding dissimilar  substrate at the same time
* User friendly with good extrudability even at low temperature
* Excellent physical properties -  withstand stringent weathering conditions
* Maintain long term elasticity and bonding properties without significant changes
* Compatible with many paints and coating -- sealants can be painted after application

#  Performance Comparison of MS Polymer, Polyurethane, and Silicone Adhesives & Sealants



Although MS polymer sealants are generally recognised as all-round sealants, one should also evaluate  the limitations of  typical MS polymer sealants:
* should not be used in areas that  subjected to permanent immersion in water
* should not be use at temperature greater than 90 deg C
* should not be use with polypropylene, polyethylene, etc.


In view of the environmental friendliness of MS Polymers, being free from silicone and isocyanate, certain specialty MS sealants are virtually  having  very low level of outgassing,  thereby  meeting  the  requirement of  chemical contamination control in clean room environment.

    Scientists have identified  sources of problematic organic contaminants,  and its impact to manufacturing of  critical electronic components, integrated circuits, hard disc drives, optical products, precision instruments to name a few. The molecular contaminants can be:
* Siloxanes from silicone sealants    * Plasticizers, anti oxidants, additives, flame retardants from adhesives and sealants, plastics, polymers, paints and coatings    * Amines, amide    * Decomposition products

As far as  sealant for  cleanroom applications  is concerned,  MS polymer sealants with minimal outgassing tendency  are particularly suited for clean room construction, panel sealing and bonding. This is due to requirements of high degree cleanliness, air- tight sealing of cleanroom joints and semi-conductor production equipment.



Terostat MS 930 -- A gun-grade, one component sealant based on silane modified polymer, which cures by reaction with moisture to a soft elastic product. The skin formation and curing times are dependent on humidity and temperature, and the curing time also depends on joint depth.  *  free of solvents, isocyanates, silicones and PVC, and is odourless. * good adhesion to many substrates and is compatible with suitable paint systems. * good UV resistance and can therefore be used for interior and exterior applications.

Terostat MS 930




You may also like:
1. RTV Silicone Sealant Clear
2. Polyurethanes : Coatings, Adhesives and Sealant  from Google Books
3. Electronic Contact & Parts Cleaner


Reference:
# Loctite Elastomeric Adhesive and Sealants

Synonyms:  modified silicone, modified silane, MS polymer, silyl-terminated polyether, silyl-terminated polypropylene oxide