JP4846496B2 - Cross-linked polytetrafluoroethylene resin and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a crosslinked polytetrafluoroethylene resin having a powdery amorphous crosslinked structure, and its manufacturing method. <P>SOLUTION: The amorphous crosslinked polytetrafluoroethylene resin is characterized in that the crystal melting heat amount in the state of no thermal history after polymerization is less than 55 J/g and a crystal melting temperature is lower than 327&deg;C. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

  The present invention does not require a special step, can be made of a resin powder having a small particle diameter, and in particular, a cross-linked polytetrafluoroethylene resin and a cross-linked polytetrafluoroethylene that do not require removal of a solvent. The present invention relates to a method for producing a resin.

  Polytetrafluoroethylene (hereinafter referred to as PTFE) is widely used in the industry by adjusting its molecular weight and particle size. This is an engineering plastic resin for which demand is expected to increase more and more today, where technological sophistication and high added value of products are required. Specifically, taking advantage of the excellent features of PTFE (chemical resistance, heat resistance, low friction, low dielectric constant, etc.) as PTFE molded products, chemical equipment piping materials, electrical insulation materials, bearings, sliding materials, low elution It is used as various products such as high performance filter materials. In particular, PTFE powder is used as an additive to paints, inks, lubricants and the like.

  Currently, the raw material powder for PTFE molded products is produced by polymerizing tetrafluoroethylene monomer (hereinafter referred to as TFE) by a chemical catalyst method. The obtained PTFE molecular chain is linear, has almost no branching, and has high crystallinity. Commercially available PTFE raw material powder produced by emulsion polymerization or suspension polymerization is affected by molecular weight, but the degree of crystallinity in the state of not undergoing thermal history after polymerization exceeds 90% by either polymerization method. When measured with a differential scanning calorimeter at a rate of 10 ° C / min, the crystal melting temperature is 340 ° C or higher and the heat of crystal melting exceeds 55 J / g. PTFE obtained by these polymerization methods is fired (sintered) once at a temperature equal to or higher than the crystal melting temperature, whereby the melting point is shifted to 327 ° C. and the crystallization heat amount is reduced to 30 J / g or less. The degree of crystallinity decreases from a value exceeding 90% to several tens of% by firing.

  In addition, the average particle size of the primary particles of the obtained raw material powder is less than 1 μm, but the average particle size of the powder as a product of secondary agglomeration is about several tens of molding powders produced by suspension polymerization. In the fine powder produced by emulsion polymerization by emulsion polymerization, it is several hundred μm.

  In addition, some low molecular weight PTFE powders are irradiated with radiation in the air (in the presence of oxygen) to high molecular weight PTFE raw material powder, and the molecular weight is reduced by molecular chain scission by radiolysis, resulting in low molecular weight PTFE powder. Some have obtained. Of course, these low molecular weight PTFEs have a linear molecular structure without a cross-linked structure.

  PTFE that does not have these cross-linked structures has the excellent characteristics of the above-mentioned PTFE (chemical resistance, heat resistance, low friction, low dielectric constant, etc.), but an oxidation reaction occurs due to radiation irradiation in the air. There was a problem that oxygen was unavoidable in the molecular chain.

  Therefore, the present inventors have proposed a PTFE resin having a cross-linked structure formed by irradiating ionizing radiation (hereinafter referred to as radiation) at a temperature equal to or higher than the crystalline melting point of PTFE resin and in the absence of oxygen (patent). Reference 1). The PTFE resin having such a crosslinked structure has succeeded in greatly changing the characteristics. Such proposed PTFE resins are generally subjected to a crosslinking treatment using a polymerized PTFE resin as a raw material.

  Furthermore, ultrafine powdered polytetrafluoroethylene (PTFE) having a particle size of 1 μm or less obtained by irradiating tetrafluoroethylene (TFE) in an acetone solvent is also proposed (patent document). 2). According to such a proposal, the molecular weight can be controlled by controlling the conditions, and it is possible to produce ultrafine powdered PTFE having various molecular weights from low molecular weight to high molecular weight and having a crosslinked structure. ing.

  At present, there is no method for producing PTFE having a crosslinked structure directly from raw material monomers, and it is not commercially available. The same applies to other perfluororesins (PFA, FEP) as well as PTFE.

Although there are few methods for producing a crosslinked fluororesin powder, there are methods for mechanically pulverizing a crosslinked fluororesin to produce a crosslinked fluororesin powder (Patent Document 3, Patent Document 4, etc.).
Japanese Patent No. 3374552 Japanese Unexamined Patent Publication No. 2000-26614 JP-A-9-278907 JP 2000-129019

  However, in the conventional cross-linked PTFE resin described above, the raw material resins are fused with each other, and it is necessary to provide a special process called a mechanical pulverization process in order to obtain a powder, which makes the manufacturing complicated. There was a problem. Moreover, the average particle size of the resin powder obtained by these methods is only about 20 μm, and at present, development of a resin powder having a smaller particle size is desired. In addition, in the ultrafine powdered PTFE obtained by crosslinking TFE by irradiating radiation in an acetone solvent (the above-mentioned Patent Document 2), it is necessary to finally remove acetone as a solvent, and the removal of acetone is insufficient. If so, problems such as inability to guarantee the purity, which is a feature of PTFE, arise.

  The object of the present invention is to eliminate such drawbacks, and in particular, no special process is required, and it is possible to use a resin powder having a fine particle diameter, and further, it is not particularly necessary to remove the solvent. An object of the present invention is to provide a method for producing a certain polytetrafluoroethylene and a crosslinked polytetrafluoroethylene resin.

  As a result of intensive studies to solve such problems of PTFE, the present inventors have obtained a method for polymerizing monomers in a solid phase and a specific heat of crystal melting and a crystal melting temperature obtained by such a method. The inventors have found that PTFE resin can achieve the above object, and have completed the present invention.

  That is, the present invention has a crystal melting calorific value of less than 55 J / g after polymerization without undergoing a thermal history as measured by differential thermal scanning system analysis (DSC) at a heating rate of 10 ° C./min, and The present invention provides an amorphous cross-linked polytetrafluoroethylene resin having a crystal melting temperature of less than 327 ° C.

  Crystal melting temperature and crystallization calorie were obtained by thermal analysis (Diamond DSC manufactured by PerkinElmer) at a rate of temperature increase of 10 ° C / min and sampled amount of 5-10 mg in a nitrogen atmosphere (1st run) The peak temperature of the crystal melting curve during the temperature rising process was defined as the crystal melting temperature, and the peak area of the crystal melting curve during the temperature rising process was defined as the heat of crystal melting.

  Further, the present invention provides the above-mentioned cross-linked polycrystal characterized by performing a radiation irradiation step in which tetrafluoroethylene monomer is brought into a solid phase at a temperature below the melting point, and the tetrafluoroethylene monomer in the solid phase is irradiated with ionizing radiation. A method for producing a tetrafluoroethylene resin is provided.

  As described above, the present invention relates to an amorphous crosslinked polytetrafluoroethylene resin characterized in that the heat of crystal fusion is less than 55 J / g and the crystal melting temperature is less than 327 ° C. In addition, the production method of the present invention has an important significance in finding a process for directly producing a low-crystalline crosslinked PTFE having a crosslinked structure, which is impossible with the chemical catalyst polymerization method, using TFE as a raw material. . The cross-linking process in the production method of the present invention does not require pulverization of the cross-linked PTFE resin, does not increase the temperature above the crystallization melting point of PTFE, does not require the presence of a solvent such as acetone, and provides a cross-linked structure. It can be given, and is not in the conventional concept.

The crosslinked polytetrafluoroethylene resin of the present invention does not have the above-mentioned problems, does not require a special process, can be made of a resin powder having a fine particle size , and further does not particularly require removal of a solvent. It is.

In addition, according to the production method of the present invention, the above-mentioned crosslinked polytetrafluoroethylene resin of the present invention can be directly produced from the raw material TFE, and the above-mentioned PTFE powder can be produced, while the temperature is controlled while the TFE It can be obtained only by irradiating only the raw material, does not require a special process, can be made of a resin powder with a small particle size , and further does not require removal of the solvent. In addition, once the obtained PTFE powder is taken out and separated from TFE which is a raw material monomer, the obtained PTFE is again irradiated in the absence of oxygen and at a temperature equal to or higher than the melting point of TFE, thereby crosslinking. It is also possible to speed up the progress.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail.
The crosslinked polytetrafluoroethylene resin of the present invention is obtained by imparting a crosslinked structure to polytetrafluoroethylene obtained by polymerizing a tetrafluoroethylene monomer.

  The crosslinked polytetrafluoroethylene resin of the present invention is amorphous, has a heat of crystal fusion of less than 55 J / g, preferably 40 to 0 J / g, in a state where it has not undergone a thermal history after polymerization, The melting temperature is less than 327 ° C, preferably 320 ° C or less.

  The amount of heat of crystal melting can be measured by calculating the crystal melting peak area in the temperature rising process (temperature rising rate 10 ° C./min) using a differential scanning calorimeter (DSC). The crystal melting temperature can be measured by measuring the temperature of the crystal melting peak in the temperature rising process (temperature rising rate 10 ° C./min) with a differential scanning calorimeter (DSC).

In the present invention, “amorphous” is a feature contrary to crystallinity, and means a state in which molecular chains are not regularly arranged but are randomly arranged.
The crosslinked polytetrafluoroethylene resin preferably has a network structure with polymer chains having Y-shaped branches. Details of the polymer chain having a Y-shaped branch are as follows.

  The cross-linked polytetrafluoroethylene resin of the present invention does not have to be cross-linked all of the polytetrafluoroethylene molecules, and up to 2.5% of the polytetrafluoroethylene constituting the resin has a cross-linked structure. good.

  In consideration of utilization as an additive, the crosslinked polytetrafluoroethylene resin has a mean particle size of 20 μm or less, more preferably 10 μm or less for the purpose of maintaining the dispersibility and the appearance and texture of the added product. Of these particles, it is preferable.

The crosslinked polytetrafluoroethylene resin of the present invention having the characteristics as described above can be obtained by producing it by the production method of the present invention described later.
The crosslinked PTFE resin of the present invention is expected to be used as a raw material for molded products as a novel perfluoro resin and as an additive. It is also expected to be used as a raw material for new molded products by mixing with other fluororesins and various polymer resins.

Below, the manufacturing method of this invention is demonstrated.
The production method of the present invention is characterized by performing a radiation irradiation step in which a tetrafluoroethylene monomer is brought into a solid phase at a temperature below the melting point, and ionizing radiation is irradiated to the tetrafluoroethylene monomer in the solid phase.
[Radiation irradiation process]
In order to prevent oxidation during irradiation, it is preferable to irradiate TFE in the absence of an acid cord, that is, in a vacuum or in an inert gas atmosphere (nitrogen, argon, helium, etc.). Further, the irradiation of radiation can be carried out without particular limitation using a technique used for polymerization by this type of radiation. Examples of radiation used include electron beams, X-rays, neutrons, and high-energy ions.

The temperature during irradiation with radiation in the radiation irradiation step is preferably −142.5 ° C. or less from the viewpoint of maintaining the TFE monomer in a solid state, and more preferably −142.5 to −160 ° C.
The dose of radiation in the radiation irradiation step is 10 to 1000 kGy, but it is preferably 20 kGy or more in order to fully develop the characteristics of the crosslinked structure.

In addition, the oxygen concentration when performing the above irradiation is preferably 10 ^-2 torr or less in order to eliminate the influence of oxygen in the reaction process, and the influence of oxygen is sufficiently eliminated by a relatively simple vacuum device. In order to achieve this, it is more preferably 10 ⁻² to 10 ⁻⁵ torr.

By performing the radiation irradiation step, the amorphous crosslinked PTFE resin of the present invention, which is the final target product, is obtained as a reaction product.
Next, steps that can be performed other than the radiation irradiation step will be described.
[Re-irradiation process]
In the present invention, as a step other than the radiation irradiation step, after separating and removing unreacted tetrafluoroethylene monomer after the radiation irradiation step, the separated polytetrafluoroethylene has a melting point equal to or higher than the melting point of the tetrafluoroethylene monomer. A re-irradiation step of irradiating with ionizing radiation again at a temperature of −78 to 30 ° C. (room temperature) and an oxygen concentration of 10 ⁻² torr or less, preferably 10 ⁻² to 10 ⁻⁵ torr It is preferred to do so.

By performing the re-irradiation step, the progress of crosslinking can be promoted, and a crosslinked PTFE resin having a further improved crosslinking rate can be obtained.
The “isolated polytetrafluoroethylene” includes both those crosslinked by the radiation irradiation step and those not crosslinked.

The dose of radiation in the re-irradiation step is 10 to 1000 kGy, but it is preferably 10 to 400 kGy in order to promote the progress of crosslinking and to further improve the crosslinking rate.
[Pre-treatment process / Post-treatment process]
In the present invention, a pretreatment step such as TFE solid phase can be performed before the radiation irradiation step. Moreover, after the said radiation irradiation process or the said re-irradiation process, post-processing processes, such as a refinement | purification process, can also be performed.

EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not restrict | limited to a following example.
[Example 1]
About 2 ml of TFE was introduced from a TFE cylinder connected to the vacuum line into a glass ampoule connected to the same vacuum line and cooled to -196 ° C with liquid nitrogen, and then sufficiently frozen and degassed to 10 ^-4 torr. And sealed the glass ampoule. The same operation was performed several times to obtain a glass ampoule in which a plurality of TFEs were enclosed. These glass ampoules were cooled to −196 ° C. with liquid nitrogen and TFE was kept in a solid state, and irradiated with gamma rays of cobalt 60 at 10, 20, 104, 182, 400, 700 and 1000 kGy, respectively. The irradiation dose rate of gamma rays was 2.6 kGy / h. After the irradiation was completed, the glass ampule was heated to just above the melting point of TFE to remove TFE, and the thermal characteristics of the crosslinked PTFE resin powder that had not undergone a thermal history after polymerization were examined. For analysis of thermal characteristics, Diamond DSC manufactured by PerkinElmer was used, and the temperature was increased in a nitrogen atmosphere at a rate of temperature increase of 10 ° C./min. The crystal melting temperature and the heat of crystallization were evaluated by the first measurement result (1st run) by DSC (thermal analyzer).

[Example 2]
A glass ampoule containing TFE was cooled to −158 ° C., and gamma rays from cobalt 60 were irradiated to 20, 104, 182, 400, and 700 kGy, respectively, to obtain a crosslinked PTFE resin in the same manner as in Example 1. The obtained crosslinked PTFE resin powder was subjected to thermal analysis in the same manner as in Example 1.

[Comparative Example 1]
Except that the glass ampule filled with TFE was cooled to -78 ° C with a mixed refrigerant of dry ice and methanol to bring the TFE into a liquid phase and irradiated with gamma rays of cobalt 60 at 20, 104 and 182 kGy, respectively. Thus, a crosslinked PTFE resin was obtained, and the obtained crosslinked PTFE resin powder was subjected to thermal analysis in the same manner as in Example 1.

[Comparative Example 2]
A cross-linked PTFE resin was obtained in the same manner as in Example 1 except that the glass ampule filled with TFE was put into a liquid phase state at 25 ° C. and irradiated with gamma rays of cobalt 60 at 20, 104 and 182 kGy, respectively. The PTFE resin powder was subjected to thermal analysis in the same manner as in Example 1.

  Tables 1 and 2 and FIGS. 4 and 5 show the results of measurement of the crystal melting temperature and the amount of heat of crystal melting of the PTFE powders obtained in Examples 1 and 2 and Comparative Examples 1 and 2, respectively, by differential scanning calorimetry.

[Example 3 (Example in which re-irradiation step is performed)]
PTFE obtained by irradiation with 20 and 182 kGy in Example 1 was separated from unreacted TFE, and unreacted TFE was removed. The PTFE obtained by separation was again put into a glass ampule, and after sufficiently deaerated to 10 ⁻⁴ torr, the glass ampule was sealed. The same operation was performed several times to obtain a glass ampoule in which a plurality of samples were enclosed. Here, the separation of TFE was performed at a temperature just above the melting point of TFE. The reason for separating TFE at a temperature just above the melting point of TFE is to suppress polymerization (post polymerization) after irradiation. Thereafter, gamma rays with cobalt 60 were irradiated at 20, 182, and 400 kGy under the temperature conditions of 25 ° C., −78 ° C., and −196 ° C., respectively (re-irradiation step) to obtain a crosslinked PTFE resin.

  Tables 3 and 4 and FIGS. 6 to 9 show the measurement results of the crystal melting temperature and the crystal melting heat quantity obtained by differential scanning calorimetry of the PTFE powder obtained in Example 3, respectively. For comparison, the results of Example 1 are also shown in each figure.

[Evaluation]
(1) The PTFE powder obtained by irradiating 700 kGy of gamma rays at -196 ° C. in Example 1 was observed with a scanning electron microscope. An observation photograph is shown in FIG. The measurement was performed with a JEOL JSM-6700F at an acceleration voltage of 10 kV. As a result of observation, the average particle size of the obtained PTFE powder was about 1 to 10 μm, and most of the average particle size was about 2 to 5 μm.
(2) ¹⁹ F HS MAS NMR analysis results of PTFE obtained by irradiation with 700 kGy in the solid state at −196 ° C. in Example 1 and commercially available PTFE powder are shown in FIG. In the commercially available PTFE, -121.9ppm attributed to -CF ₂ -C F ₂ -CF _2- (underlined fluorine atoms are observed, the same applies hereinafter) is the only peak (90.5, 15.7, -228.1) And 334.1 are spinning sidebands of the same peak).

On the other hand, PTFE obtained by irradiation at -196 ° C has a signal of -184.5ppm attributed to> C F -(-3.4 and -215.8ppm are the spinning sidebands of the same peak) or -C F ₂ -CF A signal of −109.2 ppm attributed to (C F ₂ −) — C F ₂ − was observed. Both signals indicate the presence of tertiary carbon. Tertiary carbon branching is a site never seen in commercial PTFE on a straight chain. This point is a cross-linking point (branch) in PTFE unique to the cross-linked PTFE resin of the present invention obtained by irradiating TFE in a solid phase, and is evidence that a cross-linked structure is formed.

The signal attributed to -CF ₂ -C F ₃ is -82.2 ppm (24.1 is the spinning sideband of the same peak), and the signal attributed to -C F ₂ -CF ₃ is -126.4 ppm (-20.4 and- 232.5 is the spinning sideband of the same peak), and the signal attributed to the amorphous part of CF ₂ -CF (C F ₃ ) -CF _2- is -66.8 ppm, and the signal attributed to the same crystal part is- It was observed at 70.3 ppm. All are signals belonging to the end group, indicating that PTFE obtained by irradiating TFH in a solid phase has many molecular chain ends. In addition, a signal of -90.7 ppm is assigned to C F ₂ = CF-CF _2- , and a signal of -157.7 ppm is assigned to Rf-C F = C F -Rf. In addition to the crosslinked structure, various double bonds are formed in PTFE obtained by irradiating TFE in the solid state.

  The number of crosslinking points was calculated as follows. All signals in the NMR spectrum were assigned and the peak area of each signal was determined. Further, each signal was normalized by dividing the peak area by the number of fluorine (F) atoms resulting from the signal in each structural formula. The ratio of each signal to the total number of normalized values was calculated as a percentage, and the abundance ratio of each signal (structural formula) was determined. Table 5 shows the abundance ratio of signals having a branched structure.

From Table 5, the abundance ratio of the crosslinking points not containing CF ₃ branching is about 1% (peak number 4), and the inclusion ratio of CF ₃ branching is about 2.5% (peak number 2, 3 and 4). Therefore, the total number of crosslinking points (branching points) in PTFE obtained by irradiation with 700 kGy in the solid state at -196 ° C. is about 2.5%.

A method for obtaining the number of crosslinking points from ¹⁹ F HS MAS NMR spectrum data is also described in “Radiochemical Yields for Cross-Links and Branches in Radiation-Modified Poly (tetrafluoroethylene)” Macromolecules 2002, 35, 9079-9082.
(3) The yield of the PTFE powder obtained in Examples 1 and 2 (the amount of PTFE powder produced relative to the charged monomer) is shown in FIG. The yield was determined by the following formula.
Yield = (weight of produced PTFE powder) / (weight of charged monomer) × 100
(4) From the results of Examples 1 and 2, when the radiation irradiation step in the production method of the present invention is performed, the crystal melting temperature is lowered and the crystallization heat quantity is also reduced as the radiation dose increases. That is, it is shown that a crosslinked structure is formed by irradiation in a solid phase, the crystallinity of PTFE is lowered, and low crystalline PTFE can be obtained.

  Even in the same solid state, -158 ° C crosslinks faster than -196 ° C, and the yield increases. A temperature close to the TFE melting point of -142.5 ° C increases the yield. It turns out to be advantageous.

  On the other hand, as in Comparative Examples 1 and 2, the irradiation in the liquid phase state (−78 ° C. and 25 ° C.) showed no decrease in the crystal melting temperature, and the crystallization heat amount showed a large value. There is no decline. That is, it is shown that amorphous crosslinked PTFE cannot be obtained even when irradiation is performed under these temperature conditions.

  From the above results, it can be seen that the cross-linking reaction proceeds even when the TFE monomer is separated from the polymerized PTFE and only PTFE is irradiated in the absence of oxygen. Further, in the irradiation (re-irradiation step) after separating the TFE monomer and the polymerized PTFE, the crosslinking reaction proceeds even if irradiation is performed at a temperature higher than the melting point of TFE. Furthermore, in the case of irradiating only the PTFE separated from the TFE monomer, compared with the temperature condition of Example 1 at −196 ° C. for the polymerization, the cross-linking is caused by irradiation at a temperature higher than −196 ° C. It turns out that it progresses efficiently.

FIG. 3 is an observation photograph of PTFE obtained by irradiating gamma rays at 700 kGy at −196 ° C. with a scanning electron microscope in Example 1. FIG. It is a structural analysis result by ¹⁹ F HS MAS NMR of PTFE obtained in Example 1 by irradiating 700 kGy with gamma rays at -196 ° C. This is the yield of PTFE obtained by irradiating gamma rays at −196 ° C. and −158 ° C. in Examples 1 and 2 with respect to the charged TFE amount. It is the crystal melting temperature of PTFE obtained by irradiating TFE with gamma rays in Examples 1 and 2 and Comparative Examples 1 and 2. This is the heat of crystal melting of PTFE obtained by irradiating TFE with gamma rays in Examples 1 and 2 and Comparative Examples 1 and 2. This is the crystal melting temperature of PTFE irradiated with 20 gGy of gamma rays at −196 ° C. in Example 3 and then irradiated again with PTFE obtained by separation from TFE. This is the amount of heat of crystal melting of PTFE that was irradiated with 20 kGy of gamma rays at −196 ° C. in Example 3 and then irradiated again with PTFE obtained by separation from TFE. This is the crystal melting temperature of PTFE that was irradiated with gamma ray 182 kGy at −196 ° C. in Example 3 and then irradiated again with PTFE obtained by separation from TFE. This is the amount of heat of crystal melting of PTFE irradiated with PTFE obtained by irradiating TFE with gamma ray 182 kGy at −196 ° C. in Example 3 and then separating from TFE.

Claims (7)

A method for producing a crosslinked polytetrafluoroethylene resin, comprising: performing a radiation irradiation step of irradiating ionizable radiation to the tetrafluoroethylene monomer in a solid phase state, wherein the tetrafluoroethylene monomer is in a solid phase state at a temperature below the melting point. . After the radiation irradiation step, unreacted tetrafluoroethylene monomer is separated and removed, and the resulting polytetrafluoroethylene is in a state where the temperature is not less than the melting point of the tetrafluoroethylene monomer and the oxygen concentration is not more than 10 ⁻² torr. The method for producing a crosslinked polytetrafluoroethylene resin according to claim 1, wherein a re-irradiation step of irradiating ionizing radiation is performed again. The method according to claim 1, wherein a temperature at the time of irradiation with radiation in the radiation irradiation step is −142.5 ° C. or lower. The method according to claim 1, wherein the radiation dose in the radiation irradiation step is 10 to 1000 kGy. The method according to claim 2, wherein the radiation dose in the re-irradiation step is 10 to 1000 kGy. 3. The method according to claim 1 or 2, wherein the resin is in the form of a powder having an average particle size of 10 μm or less. The crosslinked structure is
























The method according to claim 1 or 2, which is a network structure of polymer chains having at least one Y-shaped branch represented by the following formula.