Molten fluoropolymers

Products of the degradation of molten fluoropolymers are highly corrosive, often containing hydrofluoric acid. The parts of the machine that come in contact with molten fluoropol5miers must be constructed from corrosion-resistant metals that are significantly more expensive than lower grades of steel. Corrosion of process surfaces can result in the contamination of the finished product and deterioration of its physical properties. 

Rheology of the molten fluoropolymers is of critical importance in processing these polymers. Fluoropolymers, and generally thermoplastic materials, must be processed below the velocity at which melt fracture occurs, referred to as the critical shear rate. Melt fracture in molten plastics takes place when the velocity of the resin (in flow) exceeds the critical... 

Monoaxially and biaxially oriented films of fluoropolymer are made by melt extrusion of the resin into flat webs or tubes. The main function of orientation is to enhance the mechanical properties of the film such as tensile break strength and tear resistance. The decision to orient is usually made according to the requirements of the end use for mechanical properties. All process surfaces that contact molten fluoropolymers must be corrosion resistant because of the formation of corrosive compounds such as HF and HCl from the high-temperature degradation of these plastics. 

After the molten polymer has been stored in the pot and reached the required temperature, it is transferred into the mold cavity under positive pressure. The pressure and rate of transfer are the significant variables of the process. Critical shear rates at which the molten fluoropolymers exhibit unsteady flow are fairly low when compared to other pol5miers (Table 6.42). Critical shear rate increases with temperature as seen in Figs. 6.54-6.56 for three commercial grades of FEP, PFA, and ETFE. Careful gate design and transfer rate is necessary to keep the flow below the critical shear rates. 

Rheology of the molten fluoropolymers is of critical importance in processing these polymers. Fluoropolymers, and generally thermoplastic materials, must be processed below the velocity at which melt fracture occurs, referred to as the critical shear rate. Melt fracture in molten plastics takes place when the velocity of the resin in flow exceeds the critical velocity, the point where the melt strength of the polymer is surpassed by internal stresses. Critical velocity of most fluoropolymers is usually much lower than most thermoplastics. Parts molded in a process where critical velocity is exceeded exhibit typical symptoms of melt fracture that is, they may have a frosty or cloudy surface. In some cases, a part may have a smooth and shiny surface but is internally fractured. 

Special metal alloys are specified for the construction of contact surfaces due to the corrosive properties of the molten fluoropolymer. 

Fluorine compounds from fluorite (fluorspar, CaF2) are used in water treatment (to suppress dental caries) and to make fluoropolymers (such as Teflon), lubricants, and refrigerants. Molten cryolite (Na3AlF6) is essential as a solvent for Al203 in the electrolytic production of aluminum metal, while the isotopic enrichment of uranium for nuclear power reactors is usually achieved by diffusion or gas centrifugation of volatile UF6. 

Temperatures required to adequately produce a molten layer of the surfaces vary from 210°C-290°C. Welds may be particularly susceptible to stress cracking in strongly alkali solutions where discoloration develops as a result of dehydrofluorination. VF2/ HFP fluoropolymers exhibit improved resistance to a high pH environment, where PVDF homopolymers have been known to become brittle over time. 

Selection of the fluoropolymer depends on the end use requirements, part design, and process conditions. Viscosity of the molten pol5nner has a great influence on the transfer rate into the cavity. Temperature changes can alter polymer viscosity as long as it has sufficient thermal stability in the given temperature range. Cleanliness of the melt can affect the quality of the part and thermal stability of the molten resin. 

It is possible to obtain a good bond between fluoropolymers themselves, without the use of adhesives, by application of heat and pressure. Pressure can help drive the molten pol5mier into the pores of the substrate. Bond strength is dependent on the mechanical interlocking that is achieved by the adhe-... 

The irradiation of fluoropolymers at elevated temperatures has been explored for the development of materials with better mechanical properties [35]. This arises because of the radiation-induced crossUnking of chains and subsequent higher network density in the resultant polymer [36]. Here, the irradiation is accomplished at a temperature higher than the melting point of the polymer. In the molten state, the polymer behaves as an amorphous matrix and the mobility of molecular chains is considerably enhanced. This promotes the mutual recombination of radicals, i.e., crossHnking involving chain end radicals and chain alkyl radicals [37]. 

Temperature uniformity of the molten resin is a common requirement regardless of the type of resin or part design. Screw plastilication is more effective than the melt pot method due to the static nature of the latter and poor thermal conductivity of fluoropolymers. Melt pots require significantly longer heat up time for the polymer to reach transfer temperature. There will always be a temperature gradient between the melt near the wall and the cooler material at the center of the pot. 

It is noticeable that the above mechanism is based on studies of fatty acid amides while other slip agents are also used for various purposes related with reduction of coefficient of fnction. Fluoropolymer additives, for example, are used to improve film extmsion in which they act in a similar manner during process as amides act in the final film. Fluoropolymer additive is also not compatible with polymer matrix. During extrusion it migrates to the surface of metal and forms film which has pronounced effect on production parameters. It reduces melt fraction, viscosity, shear rate, and gate pressure. This makes production faster and eneigy use lower. Some of these additives are developed in such a manner that they migrate only in a molten state but are immobilized within material after material solidifies. This makes them essentially absent from the film surface which in some post process operation is an important requirement. ... 

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