posted on 2024-03-05, 16:41authored byTing Lin, Yuchen Wu, Elizabeth Santos, Xiaoyun Chen, Frederic Gubbels, Nick Shephard, Carol Mohler, Dongchan Ahn, Tzu-Chi Kuo, Zhan Chen
Silicone
elastomers are widely used in many industrial applications,
including coatings, adhesives, and sealants. Room-temperature vulcanized
(RTV) silicone, a major subcategory of silicone elastomers, undergoes
molecular structural transformations during condensation curing, which
affect their mechanical, thermal, and chemical properties. The role
of reactive hydroxyl (−OH) groups in the curing reaction of RTV silicone is crucial but not well
understood, particularly when multiple sources of hydroxyl groups
are present in a formulated product. This work aims to elucidate the
interfacial molecular structural changes and origins of interfacial
reactive hydroxyl groups in RTV silicone during curing, focusing on
the methoxy groups at interfaces and their relationship to adhesion.
Sum frequency generation (SFG) vibrational spectroscopy is an in situ
nondestructive technique used in this study to investigate the interfacial
molecular structure of select RTV formulations at the buried interface
at different levels of cure. The primary sources of hydroxyl groups
required for interfacial reactions in the initial curing stage are
found to be those on the substrate surface rather than those from
the ingress of ambient moisture. The silylation treatment of silica
substrates eliminates interfacial hydroxyl groups, which greatly impact
the silicone interfacial behavior and properties (e.g., adhesion).
This study establishes the correlation between interfacial molecular
structural changes in RTV silicones and their effect on adhesion strength.
It also highlights the power of SFG spectroscopy as a unique tool
for studying chemical and structural changes at RTV silicone/substrate
interface in situ and in real time during curing. This work provides
valuable insights into the interfacial chemistry of RTV silicone and
its implications for material performance and application development,
aiding in the development of improved silicone adhesives.