Fluoropolymers melt viscosity

In 1972 Du Pont introduced Teflon PFA, a copolymer of tetrafluoroethylene and perfluorofpropyl vinyl ether) (CF2 = CFOCF2CF2CF3). Similar materials are now also produeed by Asahi Glass, Daikin, Hoechst and Monteflos and are commonly referred to as PFA fluoropolymers. In 1994 Hoechst introduced Hostaflon PFA-N, claimed to have significantly lower melt viscosities than earlier grades of material. 

Fluoropolymers. These form one of our oldest and most spectacular families of engineering plastics. Polytetrafluoroethylene was developed by DuPont over two decades ago, and more recently by Allied Chemical, Hoechst, ICI, Pennwalt, and other manufacturers as well. It combines unusually low adhesion and friction, high temperature and flame resistance, excellent electrical properties, and extreme chemical inertness. Its high melting point and melt viscosity make thermoplastic processing extremely difficult, so that many... 

With developments in equipment and the fluoropolymers that have high melt viscosities, the maximum extrusion rate using screws is normally limited by melt fracture. However, these plastics have exceptional melt strength. This characteristic makes possible ram extruding using a die with a large opening and apply a draw down of the extrudate to the desired insulation thickness. 

With the exception of two fluoropolymers, PVF and PTFE, the rest of the resins described in this entry can be processed by standard melt-processing techniques, such as injection, transfer and blow molding, extrusion, and rotational molding. Process equipment for fluoropolymers must be made from corrosion resistant alloys because of the corrosive compound that may be produced when fluoropolymers are heated above their melting points. Higher melt viscosity of these resins may require more powder and higher pressure rating equipment. 

Fluoropolymer manufacturers and suppliers have developed time-temperature-shear-rate data for melt viscosity or melt flow rate (index) to provide an assessment of the thermal stability of these polymers. Figures 6.1 and 6.2 show the melt viscosity of a few commercial grades of polyvinylidene fluoride as a function of temperature at a fixed shear rate. The relationships between melt viscosity and shear rate, and shear stress versus shear rate, are presented in Figs. 

There are two approaches to characterizing fluoropolymers for injection molding. The more fundamental methodology centers around the measurement of physical properties such as melt viscosity and thermal diffusivity to generate data for mathematical modeling (simulation) of injection molding processes. 

A number of factors should be considered in the design of a screw for plastication of fluoropol5miers in addition to those considered for conventional injection molding of other thermoplastics. The most significant additional factors are the high processing temperature, corrosive nature, high melt viscosity, and low critical shear of fluoropolymers. 

Pressure Rating Depending on the fluoroplastic, a mold rating of 7-28 MPa at 370°C is sufficient for the highest melting point and melt viscosity fluoropolymer. 

Vinylidene Fluoride Hexafluoropropylene Copolymer - Thermoplastic copolymer of vinylidene fluoride and hexafluoropropylene. Has better thermal stability, antistick, dielectric, and antifriction properties, and chemical resistance, but lower mechanical strength at room temperature and creep resistance, compared to incompletely fluorinated fluoropolymers. Processing by conventional thermoplastic techniques is difficult due to its high melt viscosity. Uses include chemical apparatus, containers, films, and coatings. Also called TAM... 

Known in Germany since 1933, polytetrafluoro-ethylene (PTEE) is a semicrystalline resin (92-98% crystallinity), with T = 342°C and melt viscosity of 7 - 10 GPas. Others, more common fluoro-polymers are polychlorotrifluoroethylene (PCTFE), Hostaflon commercialized in 1934, fluorinated ethylene-propylene (FEP), Teflon -FEP introduced in 1972, and numerous copolymers with Tjjj = 260 to 304°C, processable at = 315 to 425°C, and having the degradation temperature, deg 440°C. The fluoropolymers are... 

This book is the second of two volumes about fluoropolymers. The division of the volumes is based on the processing techniques of commercial fluoropolymers. Volume One covers the homopolymers of tetrafluoroethylene (TFE) or polytetrafluoroethylene plastics, which are processed by nontraditional techniques. The extremely high melt viscosity of TFE homopolymers precludes its processing by conventional melt processing methods such as injection molding and melt extrusion. The copolymers of TFE and other fluorocarbon polymers, which are processed by melt-processing methods, have been covered in Volume Two. This book is devoted to exploring the various perflu-orinated and partially fluorinated copolymers of tetrafluorethylene and chlorotrifluoroethylene. Polymers of vinyl fluoride and vinylidene fluoride that are, for the most part, melt-processible have been discussed in the second volume. 

The brief description of the commercially important fluoropolymers indicates the techniques by which they can be fabricated. Generally, the processing method is dependent on the rheology of the fiuoropoly-mer in question. Table 3.6 summarizes the structure-rheology-fabrication technique characteristics of various copolymers. Melt viscosity values represent a wide range of shear rate for melt processible polymers in Table 3.6. Volume One focuses on fluoropolymers which are processed by non-melt methods.The present volume is devoted to the melt processible fluoropolymers. 

Fluoropolymer melts are substantially more viscous than the majority of other thermoplastic polymers. Viscosity of the polymer melt varies inversely with the melt temperature at a constant shear rate. Melt viscosity decreases with increase in shear rate. Larger orifices have to be used with FEP and PFA to increase melt flow because of their high viscosity. They have fairly high melt strength and can be drawn down to the desired size. ETFE has lower melt strength than PFA and FEP and can not be drawn to the same extent. High output rates can be maintained with ETFE due to its relatively lower viscosity and higher critical shear rate. 

A normal feature of extrusion of fluoropolymers is an increase in MFR during the process. A limited amount of resin degradation occurs as a result of exposure to the operating temperatures, reducing the melt viscosity, and increasing the MFR. Some increase in MFR may be desirable depending on the type of the process and the resin. Table 8.7 lists the increase in MFR for different fluoropolymers. ETFE can degrade... 

Fluoropolymers require fairly high processing temperatures due to their high melting point and/or high melt viscosity. The issues related to safety in the use of... 

Fluoropolymers can be fabricated into hollow objects by rotomolding. They are, however, hard to mold due to their relatively high melting point and melt viscosity. The most frequently rotomolded fluoropolymers are PFA, FEP, PVDF, ETFE, and ECTFE. Fluoropolymers constitute about 1% of all rotomolded parts. 

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