JPH0342224A - Method for extrusion molding of polyimide and polyimide pellet used therein - Google Patents

Abstract

PURPOSE:To stably mold a product reduced in thickness fluctuation and excellent in mechanical strength and electric insulating properties by melting polyimide having a specific repeating structural unit under heating to process the same into a pellet form and heat-treating the pellets to set a degree of crystallization to specific % or more before supplying the same to an extrusion molding machine to perform molding. CONSTITUTION:Polyimide having a repeating structural unit represented by formula [I] (wherein x is a single bond or a hexafluoroisopropylidene group) is heat-treated to form polyimide pellets usually having a degree of crystallization of 5% or more. In this case, the heat treatment temp. is pref. 250 - 370 deg.C. When said temp. is below 250 deg.C, crystallization is very slow and, even when the temp. exceeds 370 deg.C, crystallization is slow and said temp. is not pref. because the resin begins to melt. When the degree of crystallization is below 5%, it is not pref. because the extrusion amount at the time of extrusion varies and extrusion becomes impossible according to circumstances and foaming or die refuse is generated. The upper limit of a degree of crystallization is not especially prescribed but a pref. range of a degree of crystallization is 5 - 50%. Further, stirring is pref. performed during heat treatment in order to prevent the partial fusion of the resin.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a thermoplastic polyimide extrusion molding method and polyimide pellets used in this method. More specifically, in extrusion molding of polyimide, it is a method that can stably produce products with a stable extrusion rate and without bubbles or die scum, in which polyimide with a specific structure is pre-crystallized to a crystallinity within a specific range. The present invention relates to a method of supplying converted polyimide to an extruder and molding it, and polyimide pellets applied to this method.

[Conventional technology]

Aromatic polyimide has the highest heat resistance among organic polymers, as well as excellent mechanical properties, chemical resistance, and electrical insulation properties, and is used in the electrical and electronic industry, machinery industry, nuclear power, automobile industry, etc. It is widely used in the field of

In recent years, thermoplastic polyimides and methods for molding them have been reported.

Among these types of polyimides, polyimides having repeating structural units represented by the general formula (I) (wherein, X represents a single bond or a hexafluoroisopropylidene group) were developed as polyimides with good extrusion moldability. (For example, Japanese Patent Application Laid-Open No. 62-20
5124, JP-A-62-241923).

These polyimides have thermoplastic properties and can be used to produce products such as fibers, films, sheets, electric wires, rods, plates, or tube materials by the well-known extrusion method. However, among these polyimides and their molding methods, no fully satisfactory molding method suitable for these polyimides has yet been proposed for application to extrusion molding.

[Problem to be solved by the invention]

Polyimide having a repeating structural unit represented by the general formula CI) has good applicability to extrusion molding compared to other polyimides that are difficult to mold, and is usually processed into pellets and then put into an extrusion molding machine. supplied and molded.

However, even with such polyimide, if the manufactured polyimide powder is processed into pellets and fed as is to an extrusion molding machine, extrusion molding cannot be performed smoothly, and even if a molded product is obtained, Air bubbles and goo dregs occur in the product, making it difficult to obtain a product of stable quality.

This phenomenon occurs because the resin supplied to the extrusion molding machine undergoes rapid plasticization due to the temperature and pressure received by the supply and/or compression parts of the screw, and depending on the rotation speed of the screw, the resin may In some cases, the material gets wrapped around the material and cannot be pushed out at all. In addition, even when no blockage occurs, it has been found that gases such as air, which are transported together with rapidly plasticized resin, are not removed at the compression part of the screw, which should be removed, and are transported to the die, generating bubbles. .

When such a phenomenon occurs, for example, in fiber processing, there are problems such as denier fluctuations due to fluctuations in extrusion amount, yarn breakage due to the generation of air bubbles, etc.
In sheet processing, there are problems such as thickness fluctuations due to changes in the extrusion rate, appearance defects and mechanical strength decreases due to the generation of air bubbles or die scum, etc. Furthermore, in rod and pipe processing, problems occur due to changes in the extrusion rate. There are problems such as variations in the diameter, a decrease in strength due to the generation of internal air bubbles, and poor appearance due to die dregs.Furthermore, in wire processing, there is a problem that electrical insulation is likely to deteriorate due to the generation of air bubbles or die dregs. Occur.

As a solution to these problems, it is possible to change the temperature conditions of the extruder, such as lowering the extruder cylinder temperature to a temperature at which plasticization of the resin does not proceed rapidly, that is, 250°C or less, but with this method, The viscosity of the molten resin becomes abnormally high, making it impossible to extrude, or the resin deteriorates due to a large amount of shear heat generation, which is undesirable. In addition, it may be possible to change the screw shape of the extruder, or the extrusion rate may fluctuate (so-called surging phenomenon), and in some cases, extrusion molding may become impossible, making it difficult to solve the problem.

As described above, even with the above-mentioned polyimide, there are problems to be solved in the extrusion molding method, and unless these problems are solved, it will be difficult to obtain molded products with the required performance.

It is therefore an object of the present invention to provide a significantly improved method for extruding polyimides having excellent mechanical properties, chemical resistance and electrical insulation properties.

Another object is to provide an improved extrusion method that can be applied to polyimides and provides molded articles of stable quality in order to expand the applicability of polyimides of specific structures.

[Means for solving problems]

The present inventors have made extensive studies to achieve this objective. As a result, the polyimide having the repeating structural unit represented by the general formula (I) produced by polymerization is usually in the form of a semi-crystalline powder, but when applied to extrusion molding, pellets are preferable as a supply form. Surprisingly, even though the polyimide is crystalline, the crystallinity of the polyimide decreases significantly during pellet production, resulting in an amorphous state. It has been found that the various problems mentioned above occur when the water is supplied to the water. Furthermore, it is possible to make semi-crystalline polyimide by heating amorphous polyimide processed into pellets under specific temperature conditions, and when this is fed to an extrusion molding machine and molded, various types of The inventors have discovered that the problem can be solved and have achieved the present invention.

That is, in the method of the present invention, when a polyimide having a repeating structural unit represented by the general formula CI) (wherein X represents a single bond or a hexafluoroisopropylidene group) is supplied to a melt extruder, polymerization is performed. Processing the obtained polyimide into pellets by heat-melting and heat-treating the resulting polyimide pellets, which have a crystallinity of 596 or more, and supplying this polyimide to an extrusion molding machine to mold it. This is a polyimide extrusion molding method characterized by the following.

The present invention aims to improve the above-mentioned problems in extrusion molding of polyimide, and can produce polyimide having repeating structural units represented by general formula [I] in a practically satisfactory state, that is, in terms of extrusion amount. The purpose of the present invention is to provide a method for stably molding a product that prevents fluctuations in thickness, bubbles, die residue, etc., has little wall thickness fluctuation, and has excellent mechanical strength and electrical insulation.

The polyimide in the present invention has the general formula (1) (wherein,
is a polyimide having a repeating structural unit represented by a single bond or a hexafluoroisopropylidene group).

These polyimides are composed of pyromellitic dianhydride and 4,
4′-bis(3-aminophenoxy)biphenyl or 2,2-bis(3-aminophenoxyphenyl)-1,
The polyamic acid represented by the following formula [■] (wherein, X has the same meaning as above) is heated through a dehydration condensation reaction with 1,1,3,3,3-hexafluoropropane. It can be obtained by imidization either physically or chemically.

The polyimide to be supplied to the extrusion molding machine in the present invention is processed into pellets by heating and melting the polyimide powder produced by the above method or the polyimide powder that has been heat-treated in advance, and then heat-treating the pellets. The resulting crystallized polyimide pellets. That is, the manufactured polyimide powder is processed and molded into pellets such as tablets and cylinders, and heat treated, or the manufactured polyimide powder is heat-treated and the powdered product is processed and molded into pellets such as tablets and cylinders. The polyimide pellets were then further heat treated to produce crystallized polyimide pellets that can be applied to molding machines. A method in which polyimide powder is heat-treated before being processed into pellets, that is, pellets produced by the latter method, can produce molded products with stable quality without the inclusion of air bubbles in the molded products.

The polyimide pellet in the present invention is prepared by a commonly used melt extrusion method. The polyimide powder obtained by polymerization is preferably heat-treated at 250 to 370°C for 1 minute to 50 hours, and then supplied to an extruder. It is preferable to dry the polyimide at 120 to 350° C. for 3 to 24 hours before supplying it to an extruder, so that the water content of the polyimide is 200 ppm or less.

The extrusion temperature is in the range of 300 to 450°C, preferably 350 to 430°C. The diameter of the strand extruded in a molten state is not particularly limited, but is generally about 1 to 5 mm. The molten strand may be cut into pellets after being water-cooled or air-cooled, or the molten strand may be cut without cooling, so-called hot cutting, and then water-cooled or air-cooled.

The extruder used may be of a commonly used type, but a vent type or one equipped with a vacuum hopper is preferably used.

The polyimide pellets thus obtained are either semi-crystalline or amorphous before pellet processing. In the present invention, it is necessary to heat-treat this amorphous polyimide pellet to form a polyimide pellet with a crystallinity of 5% or more.

The conditions for the heat treatment are as follows.

The heat treatment temperature is preferably 250 to 370°C. If the heat treatment temperature is less than 250° C., crystallization will be very slow and it will be impractical, so it is not preferable. Further, even if the temperature exceeds 370°C, crystallization is slow and the resin starts to melt, which is not preferable because welding or decomposition of the resins tends to occur. Particularly preferably, the temperature is 290 to 330°C.

The heat treatment time varies depending on the heat treatment temperature and degree of crystallinity, so it is not particularly limited, and is usually 1 minute to 5 minutes.
Heat treatment is performed for 0 hours. If the time is less than 1 minute, sufficient crystallization will not occur, resulting in fluctuations in the amount of extrusion, inability to extrude, and generation of bubbles, which is not preferable. Further, if the time exceeds 50 hours, thermal decomposition may occur, causing discoloration and foaming during extrusion, which is not preferable. A particularly practically preferable heat treatment time is about 30 minutes to IO hours.

The polyimide having a repeating structural unit represented by the general formula (1) applied to the present invention is heat-treated under the above conditions to form polyimide pellets whose crystallinity is usually 5% or more, or In order to obtain polyimide pellets with a polyimide content of 5% or more, the heat treatment temperature may be further increased, the heat treatment time may be further extended, or the heat treatment may be repeated two or more times. Sometimes you can get it. If the crystallinity is less than 5%, the extrusion rate during extrusion may fluctuate or extrusion may become impossible, resulting in foaming, die residue, etc., which is not preferable. Although the upper limit of the degree of crystallinity is not particularly specified, the degree of crystallinity of the pellets obtained by heat treatment is at most 50%, and the preferable range of the degree of crystallinity is 5 to 50%, more preferably 10 to 50%.

The heat treatment is preferably carried out in air or an inert gas such as nitrogen, carbon dioxide, helium, neon or argon, and particularly preferably in an inert gas to prevent oxidative deterioration of the polyimide. Further, it is preferable to stir the resin during the heat treatment to prevent partial fusion of the resin.

The degree of crystallinity of the polyimide for molding in the present invention is determined by the density gradient tube method. That is, the density of the amorphous material obtained by rapidly cooling the polyimide melt and the crystal density of the crystallized material are obtained from X-ray diffraction and structural analysis, and then the density of the polyimide (sample) is measured. and the formula (I
II) It can be obtained by (1).

Incidentally, the polyimide of the general formula (I) may contain additives such as inorganic fillers such as carbon and glass, and pigments as long as crystallization is not significantly inhibited.

In the method of the present invention, the crystalline polyimide pellets with a crystallinity of 5% or more prepared as described above are supplied to a well-known melt extruder, heated and melted, molded by a die, and cooled. It is solidified and formed into fibers, films, sheets, covered wires, plates, rods, pipes, etc.

At this time, the moisture content of the polyimide was reduced to 200 pI before supplying the produced polyimide pellets to the melt extruder.
)m or less. In order to reduce the moisture content of polyimide to 2001) pm or less, there are no particular limitations on the method. Hold for 24 hours. Furthermore, it is effective to replace the atmosphere with air, nitrogen, etc., and furthermore, the treatment may be performed under reduced pressure.

The molding temperature in the melt extruder is in the range of 300-450°C, preferably 350-430'C. If the molding temperature is less than 300°C, the resin will not melt and extrusion will be difficult. In addition, when the temperature exceeds 450℃, the decomposition of the resin progresses,
This is undesirable because decomposed foaming, die lines, decomposed sludge, etc. are generated, impairing the appearance and performance of the product.

stomach.

Further, the melt extruder used may be of a commonly used general type, or a vent type or one equipped with a vacuum hopper is preferably used.

〔Example〕

Hereinafter, the present invention will be explained in more detail with reference to Examples.

The method for measuring the characteristic values of polyimide described in the Examples is shown below.

(1) Glass transition temperature, melting point Glass transition temperature (Tg) and melting point (Tm) were measured by DSC method. The sample was about 10 mg and measured at a heating rate of 4°C/min, and Tm was defined as the peak temperature of the melting curve.

(2) Measured using a melt viscosity enhancement type flow tester, 200 seconds ~
The apparent viscosity at an apparent printing speed of 1 and 400° C. was calculated.

(3) Crystallinity Density (Xobs) at 23°C was measured by toluene-carbon tetrachloride density gradient tube method, and calculated from the amorphous density (Xam) and crystal density (Xcr) using the following formula.

Here, Crystal density of synthesis example 1 1.459 g/aIr Amorphous density 1.327 glcd Synthesis example compound word 2 density 1.438 g/at Amorphous density 1.292 g/al [Synthesis example 1] Stirrer A reaction vessel equipped with a reflux condenser and a nitrogen inlet tube was charged with 368.4 g (1 mol) of 4.4'-bis(3-aminophenoxy)biphenyl and 2344 g of N,N-dimethylacetamide, and nitrogen was added. Under an atmosphere, 218.1 g (1 mol) of pyromellitic dianhydride was added in portions while being careful not to increase the solution temperature, and about 218.1 g (1 mol) of pyromellitic dianhydride was added at room temperature.
Stirred for 0 hours. To the thus obtained polyamic acid solution, 30.3 g (0.3 mol) of triethylamine and 30.6 g (0.3 mol) of acetic anhydride were added over about 30 minutes, followed by stirring for about 30 minutes. Add 2 to this solution
000 g of methanol was charged, and the polyimide powder was filtered out at 30°C. After washing the obtained polyimide powder with methanol and acetone, it was heated for 300 minutes under a nitrogen atmosphere.
It was dried at ℃ for 8 hours to obtain 517 g of polyimide powder.

The obtained polyimide has a Tg of 248℃1Tm or 386℃
1 It is a crystalline resin with a crystallinity of 35% and a melt viscosity of 45%.
There was a 00 voice.

The obtained polyimide powder was dried at 180°C for 24 hours, and
5Io+! Melted at 410°C using a vented extruder,
Extruded through a 2m diameter nozzle and allowed to cool naturally to approximately 1.8
Got a strand of cityφ. This was cut into approximately 3 mm pieces in the length direction to obtain pellets. This polyimide resin pellet has a cold crystallization peak (Tcc) of 296°C in DSC.
Further, the resin had a density of 1.327 g/c&, and was an amorphous resin with a crystallinity of 0%.

This pellet is called polyimide A.

[Synthesis Example 2] 4,4°-bis(3-aminophenoxyphenyl)-1,l, 1.3.3.3-hexafluoro was added to a reaction vessel equipped with a stirrer, a reflux condenser, and a nitrogen introduction tube. 259.2 g (0.5 mol) of propane and 1713 g of m-cresol were charged, and 109.1 g (0.5 mol) of pyromellitic dianhydride was added in a nitrogen atmosphere, taking care not to increase the solution temperature. The mixture was added in portions and stirred at room temperature for about 2 hours.

Subsequently, the temperature was increased under nitrogen atmosphere. It turned into an orange transparent solution at around 60°C, and when the temperature was further raised and stirring was continued at 150°C, pale yellow polyimide powder began to gradually precipitate in about 1 hour. After stirring under heating for 5 hours, the mixture was filtered to obtain polyimide powder. After washing this powder with methanol and acetone, it was dried under reduced pressure at 180° C. for 24 hours, and further dried at 300° C. for 8 hours in a nitrogen atmosphere to obtain 319 g of polyimide powder.

The obtained polyimide has a Tg of 271″C and a Tm of 390
It is a crystalline resin with a degree of crystallinity of 28% at ℃1, and a melt viscosity of 6
It was 500 voices.

The obtained polyimide powder was dried at 180°C for 24 hours, and
It was melted at 410"C using a 5mm vented extruder, extruded through a nozzle with a diameter of 2mm, and left to cool naturally to obtain a strand of about 1.8mm. This was cut into lengths of about 3ms in the longitudinal direction to obtain pellets. This polyimide resin pellet is DS
A cold crystallization peak (Tcc) was observed at 296°C in C, and it was an amorphous resin with a density of 1.292 g/cnl' and a crystallinity of 096. This pellet will be referred to as polyimide B.

[Synthesis Example 3] Same as Synthesis 1 except that 400.5 g (1 mol) of 4.4′-bis(3-aminophenoxyphenyl)-sulfite was used instead of 4.4-bis(3-aminophenoxyphenyl)biphenyl. Polyimitopelesoto was obtained in the same manner. The glass transition temperature of this pellet by DSC method is 230°C
No crystalline melting point was observed.

This pellet is called polyimide C.

[Examples 1 to 5] 2 kg of polyimide A was placed in a stainless steel container and placed in a closed hot air dryer. The inside of the dryer was sufficiently replaced with nitrogen, and the nitrogen was flowed at a rate of 310 m/min.
The mixture was heat-treated at ℃ for 5 hours and then returned to room temperature. The crystallinity of the pellets after heat treatment was calculated by the density method and was 31%.

The pellets were dried at 180°C for 24 hours, fed to a 25° extruder equipped with a vacuum hopper, heated and melted at 410°C, extruded through a 150° wide slit die (gap 0.5 mm), and released naturally in the air. Cool, about 0.5 thick! A sheet of IIm was obtained. At this time, the screw rotation speed was varied in the range of 5 to +50 rpm as shown in Table 1, but no problems such as fluctuation in the extrusion amount or generation of bubbles were observed.

Details of the results are shown in Table 1.

[Examples 6 to 10] Polyimide A was heated in air by changing the temperature and time. In these examples, the polyimide A was taken out of the dryer immediately after the heat treatment, and cooled by blowing room temperature air without stirring. Five types of pellets with different degrees of crystallinity were obtained. A sheet was obtained in the same manner as in Example 2 using these pellets. As a result, no problems such as fluctuation in extrusion amount or generation of bubbles were observed. Details of the results are shown in Table 1.

[Comparative Examples 1 to 5] Polyimide A was not heat-treated and sheet-molded in the same manner as in Example 1 at the screw rotation speed shown in Table 1. As a result, fluctuations in extrusion rate, inability to extrude, and generation of bubbles were observed. Details of the results are shown in Table 1.

[Comparative Example 6] Polyimide 8 was heat treated under the conditions shown in Table 1 to obtain pellets with a crystallinity of 3.5%. This was formed into a sheet in the same manner as in Example 2. As a result, fluctuations in the extrusion rate,
Generation of bubbles was observed. Details of the results are shown in Table 1.

[Example If] The same method as in Example 2 except that pellets that had been heat-treated in the same manner as in Example 2 were used, a nozzle with a diameter of 0.3 mm was attached to the extruder during melt extrusion, and the take-up speed was 2 m/min. The raw yarn was obtained. As a result, the average extrusion amount was 7.5 g.
/min, the fluctuation width was 3%, and the fluctuation in the extrusion amount was small. Also, there was no problem with thread breakage.

[Comparative Example 7] An attempt was made to obtain yarn in the same manner as in Example 11 except that polyimide A was used, but yarn breakage occurred frequently and yarn could not be obtained.

[Example 12] Using the same pellets as in Example 2, a round bar with a diameter of 20 mm was produced using a sizing die that was fitted with a nozzle of 20 B in diameter during melt extrusion and kept at about 50°C. As a result, the variation in diameter was about 5%, there were no bubbles inside, and a good molded product was obtained.

[Comparative Example 8] An attempt was made to obtain a round bar in the same manner as in Example 12, except that polyimide A was used. However, the extrusion rate fluctuated significantly, and significant air bubbles were formed inside the bar, so it could not be called a round bar.

(Example +3) When polyimide B was heat-treated at 260° C. for 28 hours and at 310° C. for 5 hours in a nitrogen atmosphere, a crystalline pellet with a crystallinity of 23% was obtained.

The pellets were dried at 180°C for 24 hours, fed to a 25mm extruder, heated and melted at 400°C, extruded through a 150mm wide slit die (gap 0.5mm), allowed to cool naturally in the air, and then heated to about 0.0000°C. A sheet of 5au was obtained. At this time, the screw rotation speed was 10 rpm, or the extrusion amount fluctuation was 3.
%, and no problems such as bubbles were observed.

[Examples 14 to 16] Polyimide B was heat treated according to Example 1 under the conditions (temperature, time) shown in Table 1 to obtain three types of pellets with different degrees of crystallinity. These pellets were dried at 180"C for 24 hours, fed to a 25mm extruder, heated and melted at 400°C, extruded through a 150-wide slit die (gap 0.5-), allowed to cool naturally in the air, A sheet of about 0.5 mm was obtained.

At this time, the screw rotation speed was set at IOrpm, and no problems such as fluctuations in the extrusion amount or bubbles were observed. Details of the results are shown in Table 1.

[Comparative Examples 9 to 10] Polyimide B was used as it was (Comparative Example 10), or polyimide B was heated to
A pellet with a crystallinity of 2.596 was obtained by heat treatment for a minute (Comparative Example 9). These pellets were extruded into sheets in the same manner as in Example 14. However, fluctuations in the extrusion amount and generation of bubbles were observed. Details of the results are shown in Table 1.

[Example 17] Raw yarn was produced in the same manner as in Example 2, except that the pellet used in Example 13 was used, a nozzle with a diameter of 0.3 mm was attached to the extruder during melt extrusion, and the take-up speed was 2 m/min. I got it.

As a result, the average extrusion J was 16.8 g/min, and the fluctuation range was 6%.
and the fluctuation of the extrusion amount was small. Also, there was no problem with thread breakage.

(Comparative Example II) An attempt was made to obtain a raw yarn in the same manner as in Example 17 except that polyimide B was used, or yarn breakage occurred frequently and no yarn could be obtained.

[Example 18] Using the pellet used in Example 13, a round bar with a diameter of 20 mm was manufactured using a sizing die that was fitted with a nozzle with a diameter of 20n+I11 and maintained at about 50° C. during melt extrusion. As a result, the variation in diameter was about 8%, there were no bubbles inside, and a good molded product was obtained.

[Comparative Example 12] An attempt was made to obtain a round bar in the same manner as in Example 18, except that polyimide B was used. However, the extrusion rate fluctuated significantly, and significant air bubbles were formed inside the bar, so it could not be called a round bar.

[Comparative Example 13] Polyimide C was heat-treated at 250° C. for 48 hours in a nitrogen atmosphere, and no crystallinity was observed by DSC and X-ray methods. Heat-treated polyimide C at 180℃ for 24 hours
After drying for a while, sheet molding was performed in the same manner as in Example 2. As a result, the average extrusion rate was f17.5 g/nin, the variation range was 53%, and the extrusion amount fluctuated greatly, and bubbles and die residue were observed in the obtained sheet.

[Effects of the Invention] The extrusion molding method for cross-polyimide in the present invention not only solves fundamental problems in extrusion molding, such as fluctuations in the extrusion amount during extrusion molding and the inability to extrude, but also solves As the gas is removed as the resin melts, it is possible to solve secondary problems such as gas contamination into the product and die scum caused by it, and is extremely effective in extrusion molding of polyimide resin.

Claims (1)

[Claims] 1. A repeating structural unit represented by the general formula [I] ▲There are mathematical formulas, chemical formulas, tables, etc.▼ [I] (In the formula, X represents a single bond or a hexafluoroisopropylidene group) 1. A polyimide extrusion molding method, which comprises processing polyimide having a polyimide into pellets, heat-treating the pellets, and supplying the polyimide having a crystallinity of 5% or more to a melt extruder. 2. The method according to claim 1, wherein the heat treatment is carried out at a temperature of 250 to 370°C. 3. The method according to claim 1, wherein the crystallinity is 5 to 50%. 4. General formula obtained by polymerization [I] ▲There are mathematical formulas, chemical formulas, tables, etc.▼ [I] (In the formula, X represents a single bond or a hexafluoroisopropylidene group) Repeating structural unit represented by A polyimide pellet for extrusion molding in which a polyimide having the following formula is processed into pellets, which are then heat-treated to have a crystallinity of 5% or more. 5.Claim 4, wherein the heat treatment is performed at a temperature of 250 to 370°C.
Polyimide pellets as described. 6. The polyimide pellet according to claim 4, which has a crystallinity of 5 to 50%.