Let KY CHEMICAL briefly introduce the structure and technology of the double screw extruder.
The plasticizing zone is to heat the injected resin to form a molten state. The conditions such as the heating temperature and time in this region are not particularly limited as long as the fluoropolymer having a specific unstable terminal group is in a molten state.
In the latter half of the plasticizing zone of the double screw extruder, it is preferable to provide devolatilization holes to remove at least a part of volatile components under normal pressure or reduced pressure. Then, the partial pressure of volatile components in the stabilization region described later can be reduced, and the stabilization treatment can be effectively promoted.
The stabilization treatment area is for melt mixing and converting specific unstable end groups into -CF2H. The stabilization treatment area is preferably formed using a kneading disc without propulsive force.
Here, the melt mixing is carried out in the presence of water and oxygen. It can be directly introduced into pure water, ion-exchanged water or water vapor; it can also be introduced along with the fluoropolymer to be treated.
The amount of water added is the same as the number of unstable terminal groups, but it is actually preferable to supply an excess amount, for example, at least 10 times the amount. The pressure in the stabilization zone is generally controlled at 0.2~10MPa, and the set temperature is controlled at 280~430℃. The time to maintain pressurization is usually less than 10 minutes.
According to the principle of removing unstable end groups in a twin screw extruder machine, it is recommended that the melt mixing in the stabilization zone be carried out in the presence of water, but also in the presence of oxygen. In terms of economy, air is usually used.
In the presence of oxygen, the carbon generated by the molten kneaded product is also burned, which can inhibit the coloring of the fluoropolymer. Oxygen is an inert gas to the unstable terminal groups carboxyl and fluorinated acyl groups. During the process of melt mixing, it can improve the devolatilization efficiency of volatile components.
The devolatilization zone removes high-temperature volatile components generated by the decomposition of specific unstable terminal groups. The pressure varies according to the melting state of the molten mixture and the screw operating conditions of the mixer. Generally, the pressure is reduced to the extent that the molten kneaded product does not intrude into the exhaust nozzle. It can be 0.01 to 0.1 MPa, and the temperature is controlled at 250. ℃ around.
In the presence of a certain amount of water and oxygen, FEP extruding through a double screw extruder can effectively remove the unstable terminal groups of FEP. Simplifies the processing process of FEP unstable end groups and saves production costs.
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