| PATENT ASSIGNEE'S COUNTRY | Japan |
| UPDATE | 01.00 |
| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 04.01.00 |
| PATENT TITLE |
Method of manufacturing modified polytetrafluoroethylkene fine powder |
| PATENT ABSTRACT |
Polymerization of tetrafluoroethylene is carried out in an aqueous medium to manufacture a type of modified polytetrafluoroethylene fine powder, containing perfluorobutyl ethylene and hexafluoropropylene, with excellent extrusion property under a high reduction ration and with high thermal stability. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | 17.09.96 |
| PATENT FOREIGN APPLICATION PRIORITY DATA | This data is not available for free |
| PATENT REFERENCES CITED | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. A method of manufacturing modified polytetrafluoroethylene powder by a polymerization reaction comprising copolymerizing tetrafluoroethylene with perfluorobutyl ethylene and hexafluoropropylene in an aqueous medium in the presence of a fluorine-containing dispersing agent at a temperature in the range of 10.degree.-90.degree. C. and under an average pressure in the range of 6-30 kg/cm.sup.2 G, said copolymerizing being carried out by feeding said tetrafluoroethylene to said aqueous medium, feeding said perfluorobutyl ethylene to said aqueous medium in the early stage of the polymerization reaction, feeding said hexafluoropropylene to said aqueous medium after at least 75% of said tetrafluoroethylene to be copolymerized has been consumed in said polymerization reaction, and after completing said polymerization reaction, coagulating the resultant primary grains of said modified polytetrafluoroethylene and drying the resultant coagulum to obtain said modified polytetrafluoroethylene powder, said feeding of said perfluorobutyl ethylene and said hexafluoropropylene being carried out so that said polytetrafluoroethylene powder contains 0.01-0.07 wt % of said perfluorobutyl ethylene and 0.01-0.05 wt % of said hexafluoropropylene, the total amount of said perfluorobutyl ethylene and said hexafluoropropylene present in said polytetrafluoroethylene powder being 0.03-0.08 wt %, based on the weight of tetrafluoroethylene in said powder, the average grain size of said primary grains being in the range of 0.1-0.5 .mu.m, the spheroidicity of said primary grains being 1.5 or lower, and the thermal degradation index of said powder being 20 or lower. 2. The method of manufacturing modified polytetrafluoroethylene fine powder described in claim 1, wherein the amount of perfluorobutyl ethylene fed to said aqueous medium is in the range of 0.01-0.1 wt % with respect to the amount of copolymerized tetrafluoroethylene, and the amount of hexafluoropropylene fed to said aqueous medium is in the range of 0.05-0.75 wt % with respect to the amount of copolymerized tetrafluoroethylene. 3. The method of claim 1 wherein said feeding of hexafluoropropylene to said aqueous medium is begun after the perfluorobutyl ethylene fed to said aqueous medium is consumed in said polymerization reaction. 4. The method of claim 1 comprising stopping said polymerization reaction prior to copolymerizing said hexafluoropropylene with said tetrafluoroethylene, said stopping including the step of removing any of said perfluorobutyl ethylene and said tetrafluoroethylene that has not been consumed in said polymerization reaction and thereafter carrying out said copolymerizing of said hexafluoropropylene with said tetrafluoroethylene. 5. The method of claim 1 wherein said feeding of said perfluorobutyl ethylene and said hexafluoropropylene to said aqueous medium is effective whereby said polytetrafluoroethylene powder is paste extrudable at a reduction ratio of 2500:1 to form a continuous smooth beading which is free of surface waviness. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
FIELD OF THE INVENTION This invention pertains to a method of manufacturing polytetrafluoroethylene (PTFE) fine powder, which has a low extrusion pressure in the paste extrusion molding operation, allows extrusion at a relatively high reduction ratio (RR), and has excellent thermal stability. BACKGROUND OF THE INVENTION By adding a small amount of comonomer into PTFE homopolymer, it is possible to modify the PTFE polymer, which does not allow melt molding but allows formation of fibrils, so as to favor paste extrusion. The polymer prepared in this way is called modified PTFE so as to be distinguished from copolymer of tetrafluoroethylene (TFE) which allows melt molding. For the paste extrusion molding, in order to improve the productivity, the tendency is to increase the RR. RR is represented by the ratio S2:S1, where S1 represents the cross-sectional area at the outlet of the die of the extruder and S2 represents the cross-sectional area of the cylinder that supplies the extrusion powder. Consequently, there is a demand for development of polymers which allow extrusion at higher RR, and which have molding with good appearance and high strength. Usually, as RR is increased, the extrusion pressure rises, and the extruded molding develops a waviness, rough surface, cracks, or breakage. Consequently, it becomes impossible to obtain normal moldings. The reason is believed to be as follows: When PTFE is extruded, the primary grains of PTFE receive shear, leading to the formation of fibrils. At the same time, orientation takes place in the extrusion direction of the primary grains, and friction takes place among grains. The fibril formation and friction appear as the extrusion pressure. Consequently, the extrusion pressure depends on the degree of fibril formation and the friction force. Under a high RR, these are promoted, leading to an increase in the extrusion pressure and poor quality of the molding. In order to improve the extrusion property under high RR, various techniques have been proposed. In the polymerization method of PTFE disclosed in Japanese Kokoku Patent No. Sho 37 [1962]-4643, before 70% of the prescribed amount of TFE is consumed, a modifier is added into the polymerization system. Examples of the modifiers include hexafluoropropylene (HFP) and other perfluoroalkyltrifluoroethylenes, and methanol and other chain transfer agents. By the addition of a modifier into PTFE, the crystallinity of the polymer can be decreased, and the formation of fibrils can be suppressed. Consequently, the addition of modifier into PTFE can suppress excessive rise of the extrusion pressure, and to alleviate the problem of poor moldings. The purpose of the method proposed in U.S. Pat. No. 4,792,594 is identical to that in the aforementioned approach disclosed in Japanese Kokoku Pat. No. Sho 37 [1962]-4643. The modifier used in this method is perfluorobutyl ethylene (PFBE). In the method disclosed in Japanese Kokoku Pat. No. Sho 56 [1981]-26242, at the time point when the reaction has been carried out for 70-85%, chlorotrifluoroethylene (CTFE) is added. Due to CTFE, a shell modification is produced for the portion near the surface of the primary grains. Consequently, a lower extrusion pressure is displayed, and the extrusion property at high RR is excellent for the obtained fine powder. By using the methods disclosed in Japanese Kokoku Pat. No. Sho 37 [1962]-4643 and U.S. Pat. No. 4,792,594, the paste extrusion property of the PTFE fine powder can be improved. However, in order to improve the productivity further, there is a demand for the development of PTFE fine powder that allows molding at even higher RR. For the fine powder prepared according to the method disclosed in Japanese Kokoku Pat. No. Sho 56 [1981]-26242, the extrusion property at high RR is better than that of the fine powders prepared using the aforementioned two methods. However, the fine powder prepared in this method has a high thermal degradation index (TDI), that is, a poor heat resistance. The TDI is an index derived from the difference in density of moldings prepared with different baking times. Larger values of the thermal degradation index correspond to larger differences in density. This indicates that the polymer molecular chains are severed by heat, leading to a decrease in the molecular weight. The fine powder for high-RR extrusion can be used to form electrical cable coating and fine tubes, which can be used in automobiles, airplanes, precision machines, etc., which require higher quality. In particular, cases of use of the fine powder in manufacturing peripheral parts for engines of automobiles are increasing, and the requirement for heat resistance is very strict for the electrical cables and tubes used in these cases. Consequently, in these fields, there is a demand for the development of fine powder with high heat resistance. For example, the CTFE modified fine powder disclosed in Japanese Kokoku Pat. No. Sho 56 [1981]-26242 exhibits a TDI in the range of 30-50. On the other hand, the modified fine powders disclosed in Japanese Kokoku Pat. No. Sho 37 [1962]-4643 and U.S. Pat. No. 4,792,594 exhibit a TDI in the range of 0-20. The purpose of this invention is to solve the aforementioned problems of the conventional methods by providing a method of manufacturing PTFE fine powder with high extrusion property at high RR and with excellent thermal stability. SUMMARY OF THE INVENTION This invention provides a method of manufacturing modified polytetrafluoroethylene powder by a polymerization reaction comprising copolymerizing tetrafluorethylene with perfluorobutyl ethylene and hexafluoropropylene in an aqueous medium in the presence of a fluorine-containing dispersing agent at a temperature in the range of 10.degree.-90.degree. C. and under an average pressure in the range of 6-30 kg/cm.sup.2 G, said copolymerizing being carried out by feeding said tetrafluoroethylene to said aqueous medium, feeding said perfluorobutyl ethylene to said aqueous medium in the early stage of the polymerization reaction, feeding said hexafluoropropylene to said aqueous medium after at least 75% of said tetrafluoroethylene to be copolymerized has been consumed in said polymerization reaction, and after completing said polymerization reaction, coagulating the resultant primary grains of said modified polytetrafluoroethylene and drying the resultant coagulum to obtain said modified polytetrafluoroethylene powder, said feeding of said perfluorobutyl ethylene and said hexafluoropropylene being carried out so that said polytetrafluoroethylene powder contains 0.01-0.07 wt % of said perfluorobutyl ethylene and 0.01-0.05 wt % of said hexafluoropropylene, the total amount of said perfluorobutyl ethylene and said hexafluoropropylene present in said polytetrafluoroethylene powder being 0.03-0.08 wt %, based on the weight of tetrafluoroethylene in said powder, the average grain size of said primary grains being in the range of 0.1-0.5 .mu.m, the spheroidicity of said primary grains being 1.5 or lower, and the thermal degradation index of said powder being 20 or lower. DETAILED DESCRIPTION OF THE INVENTION According to this invention, it is possible to manufacture a type of fine powder with improved paste extrusion property at high RR, while the thermal stability of PTFE can be maintained. Consequently, by using the fine powder of this invention, it is possible to manufacture the electrical cables used for sensors located in the periphery of the engines of automobiles and airplanes, and to manufacture the tubes used for manufacturing control cables, at a high productivity. Also, the molding manufactured from the fine powder prepared using the method of this invention has high dimensional stability and high mechanical strength. This invention provides a process for manufacturing of modified polytetrafluoroethylene fine powder with an average grain size of the primary grains in the range of 0.1-0.51.mu.m, in which the spheroidicity of the primary grains is 1.5 or lower, and the thermal degradation index is 20 or lower. According to this invention, polymerization of tetrafluoroethylene is carried out in an aqueous medium in the presence of a fluorine-containing dispersing agent at a temperature in the range of 10-90.degree. C. and under an average pressure in the range of 6-30 kg/cm.sup.2 G. In the initial stage of the reaction, perfluorobutyl ethylene is loaded into the reaction system, and the polymerization is started together with tetrafluoroethylene. After at least 75% of the amount of TFE that should take part in the reaction is consumed, hexafluoropropylene is loaded into the reaction system, and the polymerization is then carried out until all of the amount of TFE that should take part in the reaction is consumed. The primary grains of the modified PTFE obtained in the polymerization are coagulated and dried to obtain the PTFE fine powder. By adjusting the amount of PFBE and the amount of HFP loaded, it is possible to manufacture modified PTFE fine powder with the following features: With respect to the reacted TFE, the amount of PFBE contained is in the range of 0.01-0.07 wt %, the amount of HFP contained is in the range of 0.01-0.05 wt %, and the combined amount of PFBE and HFP contained is in the range of 0.03-0.08 wt %. That is, the primary grains of the modified PTFE obtained using method of this invention are made of a core of PFBE-modified PTFE and a shell of HFP-modified PTFE. The method of manufacturing this invention is basically the method disclosed in U.S. Pat. No. 2,559,752 by Berry except for the comonomer feeds to the polymerization and for other aspects described hereinafter. More specifically, it is a method of polymerizing TFE and a modifier in an aqueous medium made of water, a reaction initiator and a dispersing agent. Examples of the reaction initiators that can be used include succinoyl peroxide (DSP) and other water-soluble organic peroxides or ammonium persulfate (APS), potassium persulfate and other persulfates. Examples of the dispersing agents that can be used include ammonium perfluorooctanoate, ammonium perfluorononanoate, and other fluoro dispersing agents. The reaction initiators may be used either alone or as a mixture of several types. The primary grain size can be adjusted by changing the concentration of the dispersing agent. The polymerization is performed under a pressure in the range of 6-30 kg/cm2G by means of autogeneous gas pressurization. Usually, the pressure is maintained constant during the reaction. If the pressure is too low, the reaction rate becomes too slow. On the other hand, when the pressure is too high, the reaction rate becomes too fast, and it becomes difficult to control the temperature. The polymerization temperature is in the range of 10.degree.-90.degree. C. If the temperature is too low, the reaction does not progress. On the other hand, if the temperature is too high, the primary grain size becomes too large. This is undesirable. PFBE, one of the modifiers used in this invention, may be added all in a single round into the reaction system, added intermittently, or added continuously in the formation of the core of the primary grains of modified PTFE. Anyway, it is important to load it at the initial stage of the reaction. In particular, the method in which PFBE is added all in a single round before the start of the reaction is preferred, as this method can simplify the operation. As PFBE affects the reaction rate, the primary grain size, and the reactivity of HFP, the amount of PFBE added with respect to the weight of TFE that should take part in the reaction should be in the range of 0.01-0.1 wt %. In this case, the content of PFBE with respect to the weight of TFE should be in the range of 0.01-0.07 wt %, or preferably in the range of 0.02-0.07 wt %. If the amount of PFBE added is too large, the reaction rate becomes too low, and the reaction may stop progressing in some cases. Also, the primary grain size becomes too small. In addition, in polymerization of the shell, as the residual PFBE not consumed in the polymerization of the core lowers the reactivity of HFP, it becomes difficult for the polymerization of the shell containing an appropriate amount of HFP. Thus, preferably, the PFBE is either consumed or not present when the copolymerization of TFE with HFP is being carried out. HFP is introduced into the reaction system at the time when the TFE needed for forming the core is consumed. That is, HFP is introduced at the time point when at least 75% of TFE that should take part in the reaction is consumed. In this case, several approaches may be adopted. In one approach, the feeding of TFE and stirring are ceased at the time point when polymerization of the core is ended, the residual monomer in the system is released (removed), the pressure in the system is decreased to below the vapor pressure of HFP, and HFP is fed into the system. This insures that PFBE is not present when the copolymerization of TFE and HFP is carried out. In another method, HFP is fed into the system without ceasing the feeding of TFE and stirring. As the content of HFP can significantly affect the extrusion property, the amount added with respect to the weight of TFE should be in the range of 0.05-0.75 wt %. In this case, the content of HFP with respect to the weight of reacted TFE is in the range of 0.01-0.05 wt %. When the reaction proceeds to the point when the polymer concentration becomes 20-50 wt %, the feeding of TFE is stopped, stirring is stopped, and the residual monomer is released to outside the system, and the reaction is ended. Then, the aqueous dispersion of the polymer (referred to as a dispersion hereinafter) is removed from the autoclave, followed by coagulation and drying. The coagulation is carried out by stirring forcibly using a stirrer after water is added to the dispersion until the polymer concentration becomes 10-25 wt %, or when the pH is adjusted to neutral or alkaline in some cases. Drying of the coagulated powder is carried out by using hot air or another means. The preferable drying temperature is in the range of 100 .degree.-200.degree. C. As the drying temperature is increased, the extrusion pressure rises. Consequently, it is preferred that drying be carried out at a low temperature, such as a temperature in the range of 100.degree.-140.degree. C. The modified PTFE fine powder prepared using the aforementioned method of manufacturing this invention has excellent extrusion property at high RR and high thermal stability, the purpose of this invention. In addition, while the high mechanical strength characteristic of the pure PTFE is maintained, the transparency is better than that of the pure PTFE, and the shrinking rate is smaller. Consequently, the dimensional stability is better, the extrusion property at high RR is excellent, and the high thermal stability characteristic of the pure PTFE can be maintained. If the content of HFP is too small, the transparency and dimensional stability are poor. Also, when the content of HFP is too small, the formation of fibrils takes place excessively, and extrusion at high RR becomes difficult. On the other hand, when the content of HFP is too large, the formation of fibrils becomes insufficient, and high-quality molding cannot be formed. Consequently, with respect to the weight of the reacted TFE, the amount of PFBE contained should be in the range of 0.01-0.07 wt %, or preferably in the range of 0.02-0.07 wt %, and the amount of HFP contained should be in the range of 0.01-0.05 wt %. The total amount of PFBE and HFP combined should be in the range of 0.03-0.08 wt %. As far as the ratio by weight of the core to the shell is concerned, if the proportion of the shell in the overall grain is too large, the paste extrusion pressure becomes higher, and extrusion at high RR becomes inappropriate. On the other hand, when the proportion of the shell is too small, the effect in suppressing the formation of fibrils becomes less significant, and the paste extrusion pressure becomes higher. Consequently, the ratio by weight of the shell to the core should be in the range of 75:25 to 95:5. Also, the thermal degradation index (TDI) should be 20 or smaller, preferably 10 or smaller. For the fine powder meeting these conditions, a high thermal stability is displayed. The primary grain size and its shape are factors that affect the extrusion pressure. It is believed that, the frictional surface among grains generated by orientation of the primary grains during the aforementioned extrusion operation varies as a function of the primary grain size. That is, as the grain size becomes larger, the friction surface becomes smaller, and the extrusion pressure decreases. However, if the grain size is too large, problems arise when the grains are blended with pigment, and lumps are formed as the grains are blended with the assisting agent. Consequently, the grain size should be in the range of 0.1-0.5 .mu.m, or preferably in the range of 0.15-0.3 .mu.m. As the shape of the primary grains approaches being spherical, the friction force when the primary grains are oriented is promoted. Consequently, the spheroidicity of the primary grains,as defined below, should be 1.5 or smaller. When the modified PTFE fine powder prepared using the method of manufacturing this invention meeting the aforementioned conditions is subject to paste extrusion under RR of 2500, the bead-shaped extrusion moldings (referred to as beads hereinafter) have an appearance of No. 6 or better. The conditions of the paste extrusion molding and the bead No. will be defined later. In the following, this invention will be explained in more detail with reference to application examples and comparative examples. |
| PATENT EXAMPLES | This data is not available for free |
| PATENT PHOTOCOPY | Available on request |
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