Tailoring Crosslink Density and Index in 2K Waterborne PVDF Coatings

By Agnes Beaugendre, R.W. Skilton, Kurt Wood

Poly(vinylidene fluoride) (PVDF)-based topcoats¹ have been used on monumental buildings around the world for many years, to meet the needs of architects and engineers for the highest levels of decorative property durability and substrate protec­tion. In these coatings, the polymer binder is typi­cally a blend of 70-80 wt% PVDF resin with 20-30 wt% of a miscible acrylic resin. In most of these systems, a high-temperature bake (230-250° C) is required for alloying of the polymer components. This requirement has historically limited the use of this technology mainly to substrates that can with­stand these baking conditions (e.g., metals).

In re­cent years, however, the baking limitation has been lifted, as new water-based hybrid resin technology has become commercially available.² This technol­ogy combines PVDF copolymers and acrylic resins in pre-alloyed form,³ in proportions similar to the baked systems. The new water-based technology allows for the application of durable PVDF-based coatings under field-applied and low-temperature bake OEM conditions, with dramatically lower levels of emitted volatile organic compounds (VOC).

Hydroxy-functional PVDF (polyvinylidene fluoride)-acrylic waterborne hybrid dispersions, crosslinked with water-dispersible polyisocyanates, develop film properties through both polymer entanglements and crosslinking reactions. Since the majority component PVDF does not contain crosslinkable groups, the crosslink density which can be generated is limited. Increasing the index (isocyanate:hydroxyl equivalent ratio) above 1.0 can increase the crosslink density, through reac­tion of the isocyanate with ambient water to yield urea linkages.

However, there are significant draw­backs to this approach (e.g., shorter formulation potlife and slow hardness development). In this article, we describe new experiments involving blends of hydroxy-functional PVDF-acrylic hybrids with various oligomeric and polymeric polyols, which allow for independent adjustment of the crosslink density and the index to meet differing application requirements.