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Analysis of unstable factors of twin-screw extruder granulator

by:TENGDA     2022-10-05

The following conditions are preferred for operation in the stabilization treatment zone of the kneader of the twin-screw extruder granulator: (1) In the presence of an oxygen-containing gas. Oxygen can convert vinyl end groups to 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.

Removal of vinyl end groups, which are generated from the bond between the carboxylic acid end group and the labile main chain under heat or shear force, can be treated as described above. Therefore, it can be seen from the above formula that if there is no oxygen present, the reaction will be stopped at the vinyl end group, and will not be converted into acid fluoride, which makes the treatment complicated. However, when the vinyl end group is heated, it can depolymerize to produce carbon, which makes the melt knead darker. The reaction equation is shown in the reaction equation.

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, 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.

Although the stabilization speed of the stable end group of the twin-screw extruder is increased, 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 element or alkaline earth metal element is 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.

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