Description:
Polyvinylidene Fluoride (PVDF) has been used in outdoor decorative films and paints, corrosion resistant equipment, and flame retardant and high temperature electronics insulations since 1964. The unique molecular structure of a carbon backbone with alternating hydrogen and fluorine atoms on the molecular chain leads to a broad range of useful properties both in coating and melt processing applications.

High Melt Strength (HMS) Polyvinylidene Fluoride Kynar® HMS grades have been developed at Arkema Inc. These Kynar® HMS grades exhibit high melt strength and sag resistance during extrusion, making them excellent candidates for processes such as extrusion blow molding, thermoforming and extrusion blown film. These enhanced properties were obtained by the introduction of long chain branching.

The growing interest in engineering resins for blow molding applications is driven by the desire to take advantage of the economics of this process in the fabrication of hollow objects such as bottles, totes, large containers and many other complex shapes that cannot be produced by conventional processes. However, many of the engineering resins were originally developed for injection molding and profile extrusion and have properties such as melt viscosity, melt elasticity, melting point and thermal stability that are tailored to these applications. They are not well suited for processes where high uniaxial and biaxial stretching is involved. While the use of modified screw designs to lower compression ratios and long transition zones help avoid stagnation flows, the key hurdle in blow molding remains the formation of the parison. This is primarily due to excessive sag, that is, the inability of the melt to withstand gravity and blowing forces before and during molding.

Thermoforming of plastics has long been practiced in industry for the manufacturing of large and complex objects. While previously limited to applications wherein dimensional stability and high tolerances were not required, today’s processes and advanced resin properties allow the fabrication of sophisticated objects with tight tolerances. The property requirements for given resins for thermoforming processes and applications have been clearly identified in industry. Among the key parameters are the rheological properties, which need to be well balanced to meet the process requirements from the sheet formation step to vacuum and stretch forming. Among the critical resin properties are the viscosity control and the melt strength. In particular, high melt strength is recognized as a key factor in this process, and has been shown to allow wider processing windows, higher sag resistance and better product fabrication.