Method for producing composition containing low molecular weight polytetrafluoroethylene

1. A method for producing a composition containing low molecular weight polytetrafluoroethylene, comprising:

(0) carrying out classification treatment on a composition containing high molecular weight polytetrafluoroethylene to remove coarse particles;

(I) irradiating the composition containing high molecular weight polytetrafluoroethylene in an amount of 90 mass parts or more, based on 100 mass parts of the composition, with ionizing radiation to obtain a composition containing low molecular weight polytetrafluoroethylene and a low molecular weight fluorine-containing compound having a functional group or its derivative, wherein the low molecular weight polytetrafluoroethylene has a melt viscosity at 380° C. in the range of 1.0×10 2 to 7.0×10 5 Pa·s, and the low molecular weight fluorine-containing compound having a functional group or its derivative is an acid having a low molecular weight fluorine-containing carbon chain; and

(II) carrying out grinding treatment on the composition obtained in the step (I);

after the grinding treatment, carrying out fluorination treatment on the composition; and

after the fluorination treatment, carrying out classification treatment on the composition to remove coarse particles.

2. The method for producing a composition containing low molecular weight polytetrafluoroethylene according to claim 1 , wherein the fluorination treatment is carried out by bringing the composition containing low molecular weight polytetrafluoroethylene obtained in the step (I) and at least a fluorinating agent into contact with each other.

3. The method for producing a composition containing low molecular weight polytetrafluoroethylene according to claim 2 , wherein the fluorinating agent is a nucleophilic fluorinating agent or an electrophilic fluorinating agent.

4. The method for producing a composition containing low molecular weight polytetrafluoroethylene according to claim 2 , wherein the fluorinating agent is fluorine gas.

5. The method for producing a composition containing low molecular weight polytetrafluoroethylene according to claim 1 , wherein the composition containing high molecular weight polytetrafluoroethylene is powder.

6. The method for producing a composition containing low molecular weight polytetrafluoroethylene according to claim 1 , wherein the low molecular weight fluorine-containing compound having a functional group is an acid having a fluorine-containing carbon chain having 6 to 14 carbon atoms or a derivative thereof.

7. The method for producing a composition containing low molecular weight polytetrafluoroethylene according to claim 6 , wherein a content of the acid having a fluorine-containing carbon chain having 6 to 14 carbon atoms or a derivative thereof in the composition containing low molecular weight polytetrafluoroethylene obtained in the step (II) is 50 mass ppb or less based on the low molecular weight polytetrafluoroethylene.

8. The method for producing a composition containing low molecular weight polytetrafluoroethylene according to claim 6 , wherein the acid having a fluorine-containing carbon chain having 6 to 14 carbon atoms or a derivative thereof contains perfluorooctanoic acid or a salt thereof; and

a content of the perfluorooctanoic acid and a salt thereof in the composition containing low molecular weight polytetrafluoroethylene obtained in the step (II) is less than 25 mass ppb based on the low molecular weight polytetrafluoroethylene.