Formulators Forum—The Effects of Processing Conditions on FEVE Topcoat Powder Systems

Fluoroethylene vinyl ether (FEVE) resins were developed to withstand exterior weathering environments for decades. FEVE resins are synthesized from two monomers— fluoroethylene and vinyl ether. Undergoing radical polymerization, they produce alternating, amorphous A-B- type polymers. These high-performance polymers create a strong backbone resin with multiple properties. FEVE resins contain functional hydroxyl groups, allowing the resin to crosslink with standard aliphatic isocyanates. In addition, the incorporation of vinyl ether groups into the polymer provides solvent solubility, making it an incredibly easy material for paint formulation.

The fluoroethylene groups provide remarkable strength to FEVE resins against ultraviolet (UV) light. Traditionally, total fluorine content is used as an indicator of overall UV performance. But with FEVE resins, the fluoroethylene units alternate with the vinyl ether groups, creating steric hindrance and chemical protection for the neighboring vinyl ether units. FEVE resins’ incredible performance against the sun’s UV radiation makes them an excellent material for long-term exterior weathering. Paint coatings formulated with FEVE resins endure harsh exterior weathering with minimal loss of appearance and performance. FEVE coatings have been used to protect bridges and other large structures for decades.

In contrast to preparing liquid coatings for application, powder coatings do not require attention to the raw materials’ order of addition. Instead, the conditions within each step required for powder processing play a critical role in the final coating’s features. After formulation design, raw materials are weighed out and lightly dispersed in a pre-mix. The pre-mixed material is fed into an extruder where heat and shear forces are applied to evenly disperse and distribute the raw materials. After the material is discharged from the extruder, a chill roll cools and presses the homogenized material into flakes. The flakes are then milled and sieved to a specific particle size and distribution for electrostatic spray application and heated oven cure. Each step’s processing parameters are dependent on raw material properties and formulation chemistry.

Screw design heavily influences the quality of mixing for the compounded powder. If a powder resin has a high glass transition (Tg) value, the extruder’s screw design requires higher mixing elements for the resin to reach the glass transition and the successful dispersion of material aggregates. Some of the considerations for effective mixing range from designing screw elements for high dispersion mixing, distributive mixing, and fill zones. The element layouts along the screw shaft also affect material flow and shearing as it undergoes dispersive and distributive mixing. When a screw design complements the FEVE resin properties, the powder system has a wide range of setting parameters that yield various surface effects. 100% FEVE resin coatings typically have high gloss, excellent pigment dispersion, and last over 10,000 hours of accelerated weathering.

However, when powder systems undergo ineffective melting and mixing or the extruder’s processing conditions are set well out of controlled material flow during extrusion, the final coating often yields poor surface appearances such as voids or pinholes. SEM-EDX microscopy also shows large material aggregates scattered throughout the film. The poor dispersion of material may also contribute to failures in flexibility, adhesion, and impact resistance. In addition, these powder systems typically struggle to survive long- term accelerated weathering and accelerated corrosive environments required for FEVE coatings. When extrusion parameters are selected to have high energy output, heat- sensitive materials are affected as temperatures in the barrel increase from high screw rotation speed combined with high mixing forces. Excessive shearing yields coatings with lower gloss, variation in color, and physical performance failures. When screw rotation is set too low, the extruder endures high work energy input and low energy output. These parameters create irregular torque and barrel jamming. Powder systems with identical formulations result in significant variations in surface appearance and performance when extruder screw designs or conditions are manipulated.

A powder formulation could produce significant variations in surface appearance and performance when processing conditions are changed. As a result, the powder manufacturer needs to have the processing conditions complement the raw materials’ properties and formulation chemistry to achieve the desired characteristics of the final coating. In addition, setting and process parameters for extrusion are dependent on each other, meaning adjustments in compounding requires extensive investigations into selecting optimal processing conditions.

Connie Przeslawski is a product development chemist at AGC Chemicals Americas, Inc.