In the production process of FEP, because the initiator used in the aqueous phase polymerization reaction system is a persulfate such as ammonium persulfate or potassium persulfate, an unstable terminal group-carboxylic acid terminal will be generated during the reaction process.
The carboxylic acid end groups are converted to vinyl end groups or acyl fluoride end groups under melting conditions, or under melting. These end groups are thermally unstable and can decompose to produce volatile species that can cause bubbles or voids in the final product and can corrode processing equipment.
In addition, bonds to repeat units in the backbone of the fluoropolymer create unstable bonds. For example, in FEP, which is a TFE-HFP-based copolymer, the bonds between HFPs are unstable, and are cut by application of mechanical force during melt-kneading, resulting in unstable vinyl end groups. Initially, the two-screw extruder mainly played the role of granulation.
During the granulation process, a strong shear force is applied through the two-screw extruder, which can cut off the HFP-HFP bond and generate unstable end groups. However, in this method, since a kneader such as a two-screw extruder that can apply a strong shear force in a short period of time is used, the unstable bond of the main chain is cut off to produce an unstable vinyl end group, and the reaction stops, the treatment of the resulting unstable end groups is not carried out in the two-screw extruder. Now, the two-screw extruder can not only pelletize but also remove the unstable end groups in the process.
1. Removal of carboxylic acid end groups
The reaction is carried out under heating conditions, usually at 200~450°C. In order to promote the reaction, it is effective to add a reaction accelerator such as a compound containing alkali metal, alkaline earth metal, ammonium salt; or amines, alcohols, amines or their salts. When alkali metal or alkaline earth metal compound is used, it becomes CF2H end; when ammonium salt or ammonia, amine, the end group is amide, and when the temperature is high, part of it becomes -CF2H end; when alcohol is used, the end group is alkyl ester.
When the addition amount of the reaction accelerator is too large, although the stabilization speed of the stable terminal group of the two-screw extruder is increased, the coloring of the polymer cannot be completely eliminated, and the polymer itself deteriorates, showing a tendency to decrease the melt viscosity.
Residual metal processing at high temperatures may cause deterioration and decomposition of fluoropolymers. This decomposition may cause discoloration, deterioration, and nozzle clogging. These conditions can lead to molecular fragmentation at the surface of the nozzle outlet when forming wires, which negatively affects the process.
It is also possible to cause cracks or fissures in the coating, so-called cracking, in which case stopping the coating process, reworking is indispensable, and time must be waited until the system reaches equilibrium again. This makes it difficult to realize long working hours, resulting in deterioration of productivity.
2. Removal of vinyl end groups
Formation of carboxylic acid end groups can be done as described above. Therefore, it can be seen from the above formula that if there is no oxygen, the reaction will be stopped at the vinyl end group, and it will not be converted into acyl fluoride, which makes the treatment complicated. However, when the vinyl end group is heated, it can be depolymerized to produce charcoal, which makes the molten kneader darken.
For this reason, the following conditions are preferred for operation in the stabilization treatment zone of the two-screw extruder kneader, namely:
(1) In the presence of oxygen-containing gas. Oxygen can convert vinyl end groups into acyl fluoride end groups, and can also oxidize carbon atoms produced by depolymerization of vinyl end groups into CO. From an economic point of view, it is preferable to use air directly.
(2) In the presence of water. Water stabilizes the carboxylic acid end groups - CF2 H and converts the acid fluoride to the carboxylic acid.
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