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The principle of stability of twin screw extruder

by:TENGDA     2022-10-06

The stabilization speed of the stabilized end group of the twin-screw extruder pelletizer increases, but the coloring of the polymer cannot be completely excluded, and the polymer itself deteriorates, showing a trend of decreasing melt viscosity. For example, in melt-kneading, the above-mentioned alkali metal elements or alkaline earth metal elements are less than or equal to 2ppm (10-g/g) of the melt-kneaded product. This requires the synthesis and post-processing of FEP resin to strictly control the metal content. Processing of residual metals at elevated temperatures may degrade and decompose fluoropolymers. This decomposition may result in discoloration, deterioration, and clogging of the nozzle. These conditions can cause molecular fragments to form on the surface of the nozzle exit, negatively affecting the process when forming the wire. It can also cause cracks or cracks in the coating, so-called cracking. If cracking occurs, it is necessary to stop the coating process, rework, and have to wait until the system reaches equilibrium again. This makes it difficult to achieve long working hours, resulting in a deterioration in productivity.

The following conditions are preferred for operation in the stabilization treatment zone of the twin-screw extruder kneader: (1) In the presence of an oxygen-containing gas. Oxygen can convert vinyl end groups into acid fluoride end groups, and can also oxidize carbon atoms generated by depolymerization of vinyl end groups to CO. It is preferable to use air directly from an economical point of view. (2) In the presence of water. Water stabilizes the carboxylic acid end groups—CF2H and converts the acid fluoride to the carboxylic acid.

The reaction to remove the carboxylic acid end group is shown in the reaction formula. The reaction is carried out under heating, usually at 200-450~C. In order to promote the reaction, an effective method is to add a reaction accelerator such as a compound containing an alkali metal, an alkaline earth metal, or an ammonium salt; or an amine, an alcohol, an amine or a salt thereof, and the like. Specific examples include hydroxides such as potassium hydroxide and sodium hydroxide, carbonates such as potassium carbonate and calcium carbonate, sulfates such as potassium sulfate, nitrates such as potassium nitrate, ammonium salts such as ammonium hydroxide, ammonia, alcohols such as methanol and ethanol class, amines or their salts. When an alkali metal or alkaline earth metal compound is used, it becomes a CF2H terminal; when an ammonium salt or ammonia or amine is used, the terminal group is an amide (-CONHz), and when the temperature is high, a part becomes a CF2H terminal; when an alcohol is used, the terminal group is an alkyl ester.

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