BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a dual cure polyorganosiloxane composition that is storage stable. The composition contains: (a) a polysiloxane having at least one vinyl functionality and at least one alkoxy functionality; (b) a polysiloxane with at least one mercapto functionality; (c) a silane crosslinker with at least two alkoxy groups per molecule; (d) a photoinitiator; and (e) a combination of zirconium tetraalkoxylate and alkylacetoacetate.

INTRODUCTION

Light and moisture dual cure compositions are useful, for example, in coating, encapsulation, potting and adhesive applications where exposure of all portions of a coating to light is difficult yet rapid curing of the coating is desirable. The light curing character of the composition allows curing by exposure to light such as ultraviolet (UV) light. The moisture cure mechanism serves to cure portions of the composition blocked from exposure to light (“shadow areas”). Dual cure mechanisms are valuable in coatings to enable moisture curing to complete curing when light cannot access all areas of a coating.

The light cure mechanism is typically triggered by irradiation with UV and/or visible light. One light cure mechanism utilizes a “thiol-ene” reactive system comprising thiol-containing reactants and reactants having carbon-carbon unsaturated bonds. Upon exposure to UV and/or visible light the thiol functionality reacts with and adds across a carbon-carbon unsaturated bond. Typically, the thiol-ene reactive system further comprises photoinitiator and a catalyst.

A challenge with thiol-ene and titanite catalyzed moisture cure reactive systems is that they become less effective over time, particularly when stored for months at temperature of 23° C. or higher, or when stored for weeks at temperatures of 50° C. or higher. That means the room temperature storage, and especially higher temperature storage capability of thiol-ene based dual cure reactive systems is relatively short and the reactive system loses its UV cure performance characteristics. This phenomenon is more common when the system includes photoinitiators such as ethyl (2 4 6-trimethylbenzoyl) phenylphosphinate, 2-hydroxy-2-methyl-1-phenyl-propan-1-one, and 1-hydroxy-cyclohexyl-phenyl-ketone. Therefore, such thiol-ene based dual cure systems require storage at refrigerated temperatures (typically 4° C. or lower) to maintain UV cure reactive properties. Hence, it is desirable to identify a thiol-ene based dual cure reactive system that retains UV cure performance even after aging at temperatures of 23° C. or higher.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a thiol-ene based dual cure reactive system that retains UV cure performance even after aging at temperatures of 23° C. or higher.

The present invention is a result of discovering that the moisture cure catalyst used in current systems contributes to the diminished shelf life and the unexpected discovery that a combination of zirconium tetraalkoxylate and alkylacetoacetate both serves as a moisture cure catalyst suitable for thiol-ene based dual cure reactive systems and does not diminish the reactive composition's dual cure behavior after aging the reactive composition for an extended period of time.

In a first aspect, the present invention is a composition comprising the following components: (a) a polysiloxane having at least one alkenyl functional group and at least one alkoxy functional group; (b) a mercapto-functional polysiloxane with at least one mercapto functionality; (c) a silane crosslinker with at least two alkoxy groups per molecule; (d) a photoinitiator; (e) a combination of zirconium tetraalkoxylate and alkylacetoacetate at a mole ratio in a range of 1:1 to 1:4.

Compositions of the present invention are useful as dual cure coatings that can be cured by exposure to light and by exposure to moisture.

