JP-7856988-B2 - Method for producing low molecular weight polytetrafluoroethylene

Inventors

• 田中 孝之
• 佐藤 数行
• 塚本 充郎
• 大島 明博
• 長澤 尚胤
• 清藤 一
• 山▲崎▼ 翔太

Assignees

• ダイキン工業株式会社
• 国立大学法人大阪大学
• 国立研究開発法人量子科学技術研究開発機構

Dates

Publication Date

20260512

Application Date

20230516

Priority Date

20220516

Claims (12)

1. The process includes (1) irradiating high molecular weight polytetrafluoroethylene having a standard specific gravity of 2.130 or higher and 2.230 or lower with radiation such that the ratio of the maximum dose rate to the minimum dose rate (maximum dose rate / minimum dose rate) is 1.55 or lower, to obtain low molecular weight polytetrafluoroethylene having a melt viscosity at 380°C of 1.0 × 10² Pa·s or higher and 7.0 × 10⁵ Pa·s or lower. Multiple irradiation containers filled with the aforementioned high molecular weight polytetrafluoroethylene are arranged in a row, and the irradiation is performed. A method for producing low molecular weight polytetrafluoroethylene.
2. The manufacturing method according to claim 1, wherein the minimum absorbed dose of the radiation is 200 kGy or more.
3. The manufacturing method according to claim 1 or 2, wherein the irradiation is carried out substantially in the absence of oxygen.
4. The manufacturing method according to claim 1 or 2, wherein the radiation is an electron beam, gamma ray, or X-ray.
5. The manufacturing method according to claim 1 or 2, wherein the irradiation is performed with a distance of 10 m or less from the radiation source to the furthest part of the high molecular weight polytetrafluoroethylene.
6. The manufacturing method according to claim 1 or 2, wherein the irradiation is performed with a distance of 5 cm or more from the radiation source to the nearest portion of the high molecular weight polytetrafluoroethylene.
7. The manufacturing method according to claim 1 or 2, wherein the high molecular weight polytetrafluoroethylene is placed at a position facing the effective region of the radiation source, and the irradiation is performed, and the effective region is a region where the distance from the center of the radiation source is 95% or less of the distance from the center to the end of the radiation source.
8. The manufacturing method according to claim 1 or 2, wherein the irradiation is performed on the high molecular weight polytetrafluoroethylene filled in an irradiation container made of at least one material selected from the group consisting of metals, glass, ceramics, and organic materials.
9. The manufacturing method according to claim 8, wherein the irradiation container is cylindrical and prismatic.
10. The manufacturing method according to claim 8, wherein the irradiation container has at least one surface selected from the group consisting of a plate-shaped surface, a mesh-like surface, and a surface having a slit-shaped opening.
11. The manufacturing method according to claim 1 or 2, wherein both the high molecular weight polytetrafluoroethylene and the low molecular weight polytetrafluoroethylene are in powder form.
12. The manufacturing method according to claim 1 or 2, further comprising step (2) of heating the high molecular weight polytetrafluoroethylene to above its primary melting point before step (1), wherein the molded article has a specific gravity of 1.0 g/ cm³ or more.

Description

This disclosure relates to a method for producing low molecular weight polytetrafluoroethylene. Low molecular weight polytetrafluoroethylenes (also known as "polytetrafluoroethylene wax" or "polytetrafluoroethylene micropowder") with molecular weights ranging from several thousand to several hundred thousand have excellent chemical stability, extremely low surface energy, and are resistant to fibrillation. Therefore, they are used as additives to improve lubricity and the texture of coating surfaces in the manufacture of plastics, inks, cosmetics, paints, greases, etc. (see, for example, Patent Document 1). Methods for producing low molecular weight polytetrafluoroethylene include polymerization, radiolysis, and thermal decomposition. Patent document 2 describes a method for producing low molecular weight polytetrafluoroethylene by radiolysis. Japanese Patent Application Publication No. 10-147617International Publication No. 2020/013336 This figure shows an example of a radiation irradiation method.This is a cross-sectional view showing an example of an irradiation vessel filled with high molecular weight PTFE.This is a cross-sectional view showing another example of an irradiation vessel filled with high molecular weight PTFE.This figure shows an example of the arrangement of the irradiation container.This figure shows the positional relationship between the radiation source and the irradiated object, as well as the sampling locations, in Example 1 and Comparative Example 1.This figure shows the positional relationship between the radiation source and the irradiated object, as well as the sampling locations, in Examples 2, 4-7 and Comparative Examples 2-6.This diagram shows the positional relationship of the main drum in Example 3.This figure shows the sampling locations in Example 3.This figure shows the positional relationship between the radiation source and the irradiated object, as well as the sampling locations, in Example 8.This figure shows the state in Example 8 where five 18-liter cans are stacked. Conventionally, when high molecular weight PTFE is irradiated with radiation, there is a problem in that the absorbed radiation dose differs greatly depending on the position within the irradiation chamber, resulting in large variations in the molecular weight of the resulting low molecular weight PTFE. The inventors have discovered that by irradiating the material such that the ratio of the maximum dose rate to the minimum dose rate of radiation falls within a specific range, the variation in the molecular weight of the resulting low molecular weight PTFE can be suppressed, and have thus completed the manufacturing method disclosed herein. The following provides a detailed explanation of this disclosure. In this specification, there are instances where "～" is used to indicate a numerical range from a lower limit to an upper limit. In these instances, the numerical range refers to a range of values that are greater than or equal to the lower limit and less than or equal to the upper limit, including the lower limit itself and the upper limit itself. This disclosure relates to a method for producing low molecular weight PTFE, comprising step (1) irradiating high molecular weight PTFE with radiation such that the ratio of the maximum dose rate to the minimum dose rate (maximum dose rate/minimum dose rate) is 1.55 or less, thereby obtaining low molecular weight PTFE having a melt viscosity at 380°C of 1.0 × 10² Pa·s or more and 7.0 × 10⁵ Pa·s or less. In step (1), high molecular weight PTFE is irradiated with radiation such that the ratio of the maximum dose rate to the minimum dose rate (hereinafter also referred to as the dose rate ratio) is 1.55 or less. By keeping the dose rate ratio within the above range, low molecular weight PTFE with small molecular weight variation is obtained. The above dose rate ratio is preferably 1.50 or less, more preferably 1.45 or less, even more preferably 1.40 or less, even more preferably 1.35 or less, and particularly preferably 1.30 or less. The above dose rate ratio may also be 1.00 or higher, or 1.10 or higher. The above dose rate ratio is determined as the ratio of the maximum and minimum absorbed dose rates obtained at multiple locations within the irradiation chamber containing the sample. The above absorbed dose rates are determined at least at the location where the lowest dose value is expected (e.g., the center of the irradiation chamber or the inner surface of the irradiation chamber furthest from the radiation source) and at the location where the highest dose is expected (e.g., the inner surface of the irradiation chamber closest to the radiation source). The absorbed dose rate can be measured by two methods: direct measurement using a chemical dosimeter and computational simulation using the Monte Carlo simulation code "PHITS" (Journal of Nuclear Science and Technology, 2018, Vol. 55, No. 6, pp. 684-690). Alanine dosimeters and PMMA dosimeters can be used