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Process requirements of twin-screw extruder
1 Twin-screw extruder plasticizing zone
In the plasticizing zone, the input resin is heated to form a molten state. Conditions such as heating temperature and time in this region are not particularly limited as long as the fluorine-containing polymer having a specific unstable terminal group is in a molten state. In the second half of the plasticizing zone, a devolatilization hole is preferably provided to remove at least a part of volatile components under normal pressure or reduced pressure. Then, the partial pressure of the volatile components in the stabilization region described later can be reduced, and the stabilization treatment can be effectively promoted.
2 Twin-screw extruder stabilization zone
In the stabilization treatment zone, melt-kneading is performed and specific unstable terminal groups are converted into a CF2 H. The stabilization treatment zone is preferably formed using kneading discs without propelling force. Here, melt kneading is performed in the presence of water and oxygen. It can be directly introduced into pure water, ion-exchanged water, etc. or water vapor; it can also be introduced along with the fluoropolymer to be treated.
Water is added in an amount equal to the number of water molecules of unstable terminal groups, but in practice it is preferably supplied in excess, for example at least 10 times the amount. The pressure in the stabilization area is generally controlled at 0.2-10MPa, and the set temperature is controlled at 280-430°C. The time to maintain pressurization is usually less than 10 minutes.
According to the principle of removing unstable end groups in the extruder, the melt kneading in the stabilization treatment zone is carried out in the presence of oxygen in addition to the presence of water. Air is usually used in terms of economy. In the presence of oxygen, carbon generated from the melt-kneaded product is also combusted, and coloration of the fluoropolymer can be suppressed. Oxygen is an inert gas to the unstable terminal carboxyl group and fluoroacyl group, which can improve the devolatilization efficiency of volatile components during the melting and mixing process.
Stabilizing the treated area generally increases the internal pressure. By increasing the pressure, the partial pressure of the removed high-temperature volatile components can be reduced, and the infiltration of water and oxygen introduced as needed can be promoted in the melt-kneaded product. As a result, the removal efficiency of high-temperature volatile components can be improved.
The temperature in this area is generally controlled at 330~410°C. If the temperature is too low, melting will be insufficient, and the surface renewal of the molten kneaded product will be insufficiently mixed with water or oxygen, making it difficult to promote degassing. If the temperature is too high, the quality of the fluoropolymer obtained will deteriorate.
The time of processing in the stabilization treatment area is not particularly limited, and depends on the temperature and the structure of the pressure extruder, but it is usually less than 10 minutes. If it is greater than or equal to 10 minutes, more shearing force will be applied, and the melted kneaded product will deteriorating.
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