Despite impressive properties, Polyvinylidene fluoride (PVDF) polymer has remained a niche product. For over half a century, PVDF has found application in outdoor decorative films and paints, corrosion-resistant equipment and flame-retardant electronics insulation. PVDF has long term UV stability, excellent resistance to harsh chemicals like acids, chlorine and bromine, and high thermal stability for increased flame and smoke resistance. However, it has limited use in melt processing. Alternation of hydrogen and fluorine atoms on the molecular chain provides additional properties like abrasion resistance, radiation stability and a lower melting point than most fluoropolymers, which allows for lower processing temperatures similar to those of polyolefins. Other benefits are lower density than most fluoropolymers and good tensile strength at up to 150 degree C.

With the aim of improving processing efficiency, Arkema Inc. has done extensive development work to alter homopolymer PVDF, thus broadening its use. Arkema is manufacturing HMS resins at a pilot facility and is sampling the materials with key customers.
New Kynar High Melt Strength (HMS) PVDF grades are chain-branched polymer resins that exhibit high melt strength and sag resistance during extrusion, making them forerunners for extrusion blow molding, thermoforming, and blown film extrusion. Kynar HMS grades exhibit balanced rheological properties, allowing them to be used in many more processes and applications than conventional PVDF resins. Combining good melt strength and draw-down ratio with sag resistance and strain hardening, they are specifically recommended for extrusion blow molding, thermoforming and blown film extrusion. They can also be used in rigid foam extrusion and potentially in fiber spinning. Mechanical and physical properties remain comparable to those of conventional Kynar resins. The ability to modify polymer architecture to balance melt viscosity with elasticity allows these PVDF grades to be cost-competitive even as the material�s performance attributes are maintained. Conventionally, commodity polymers such as polyolefins, PET and ABS have ruled these processes, interest in ETPs is growing.

In thermoforming, the key parameters are well balanced rheological properties to meet the requirements for sheet formation, vacuum stretching and forming. Few critical properties are viscosity-control and melt strength. High melt strength allows wider processing windows and higher sag resistance. Sag resistance is the most important property in thermoforming, is temperature dependent and its measurement involves evaluating a form of creep resistance in the melt under no load. Improved melt strength and sag resistance makes Kynar HMS a strong candidate for vacuum forming of thin sheet into liners, open trays and heat-sealed parts for chemical storage applications.

In blown film extrusion, balanced rheological properties allow high blow-up ratios, excellent bubble stability and enhanced optical properties at low thicknesses. The process requires relatively low viscosity for ease of processing and elimination of surface defects like sharkskin, and also require sufficient melt strength and sag resistance to support the bubble weight and withstand controlled biaxial stretching to allow production of thin films. In blown film, Kynar HMS expands the processing window and maintains PVDF properties such as tensile strength, impact resistance, and chemical resistance. It offers excellent balance between melt strength and elongation, high sag resistance at low viscosity, and high die swell. These enhanced properties were obtained by the introduction of long-chain branching.