JPH0220416B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing polyvinylidene fluoride resin molded products, and more specifically, polyvinylidene fluoride resin (hereinafter referred to as "PVDF") has a crystal structure of β type It mainly relates to a method for manufacturing a structure in which the polar axis of the crystal is oriented in a specific direction.

As is well known, PVDF has α-type, β-type and γ-type.
In the case of β-type crystals, the dipoles in the molecular chains within the crystal are oriented in a certain direction, and there is a polar axis. Therefore, β
It is known that PVDF films with a type crystal structure exhibit large piezoelectric constants, pyroelectric constants, and dielectric constants. Such a β-type crystal structure can be easily obtained, for example, by low-temperature axial stretching.
However, the polar axes of this cold-axially stretched film are usually randomly oriented within the cross section of the film. Furthermore, the film stretched below the crystal melting point has a fibrillar structure and has the disadvantage of being extremely easily torn.

It is expected that selectively orienting the polar axis of such β-type crystals in a specific direction within the film will provide even better electrical properties. However, it is not easy to obtain such a film, and it is only known that it can be produced by the following method. One method is JP-A-55
- PVDF quenched as disclosed in No. 17957
This is a method of biaxially stretching a film under specific conditions. Another method is corona poling during stretching (Proceedings of the Society of Polymer Science, Vol. 30, No. 3, 2J30, No.
677 (1981)). Another method is JP-A-55
This method is disclosed in No. 166981, and involves cold rolling with rolls after uniaxial stretching. All of these methods are cold-stretched, and the molded products obtained by these methods have the magnification shown in Figure 1 as an example.
As is clear from the 10,000x scanning electron micrograph, since it has a microfibrillar structure, microvoids tend to occur between the microfibrils within the molded product.

The present invention differs from these prior art techniques in that it provides a method for molding PVDF that does not exhibit a microfibrillar structure, has a crystal structure mainly consisting of a β-type structure, and has a polar axis with a selective orientation. purpose.

In order to prevent PVDF from exhibiting a microfibrillar structure, it is sufficient to stretch it from an amorphous state. However, unlike polyethylene terephthalate and nylon, PVDF, like polyethylene and polyoxymethylene, has a glass transition point much lower than room temperature, and its crystallization rate is extremely fast, so even if it is rapidly cooled, the crystallization temperature will still be low. An amorphous state cannot be obtained at temperatures below. That is, once cooled, the molded product cannot assume an amorphous state, and cold stretching from the amorphous state is impossible.

Also, US Patent No. 3580829 or Special Publication No. 51
According to Publication No. 3498, a cross-linked PVDF tube is expanded to about 2 to 3 times its diameter at a temperature above its melting point, and after cooling and solidifying while maintaining its internal pressure, other objects are placed inside the tube. It has been proposed to obtain an elastic memory material such as a heat-shrinkable tube by utilizing the fact that when a tube is inserted and heated above its melting point, the tube returns to approximately its size before being stretched. Although the molded product obtained by this method does not have microfibrils, its crystal structure exhibits an α-type structure in many cases, including when PVDF is composed of vinylidene fluoride homopomer. It is.
Only copolymers consisting of vinylidene fluoride and certain comonomers, such as vinyl fluoride, exhibit a β-type structure. Moreover, even in these cases, the preferred orientation of the polar axis is not shown.

Therefore, the present inventor attempted to stretch from a non-crystallized state, that is, from a molten state at a high temperature.
At such stretching temperatures, the thermal movement of molecular chains is extremely active, and even if measures such as rapid cooling immediately after stretching are adopted, it is of course impossible to obtain a molded product with the polar axis selectively oriented, and PVDF In many cases, including vinylidene fluoride homopolymers, β
It turns out that type structure is not possible.

However, when such cross-linked PVDF is stretched in a molten state in two directions to a stretching ratio close to breaking,
It has been found that when cooled while maintaining the stretched state, the crystal structure not only mainly has a β-type structure but also exhibits the selective orientation of the polar axis, which is the object of the present invention.

Therefore, the method according to the present invention uses crosslinked polyvinylidene fluoride resin (PVDF) as described below.
is stretched in a molten state in at least two directions to a stretching ratio close to breakage, and then cooled while maintaining the stretched state.

PVDF in this invention is not limited to vinylidene fluoride homopolymer, and contains vinylidene fluoride in an amount of about 50 mol% or more, preferably about 70 mol%.
The above, more preferably about 90 mol% or more, and one or more comonomers that can be copolymerized with this, such as vinyl fluoride, ethylene trifluoride, ethylene chloride trifluoride, ethylene tetrafluoride, propylene hexafluoride, etc. or a composition containing at least about 50% by weight of at least one of these polymers. Examples of components that can constitute such a composition include, of course, acid esters such as polymethyl methacrylate, polymethyl acrylate, and polyvinyl acetate, which are known as polymers that are compatible with polyvinylidene fluoride homopolymer. , polycarbonate, polyethylene terephthalate, and other polymers that can be mixed but cannot be said to be compatible; polyester plasticizers; terephthalic acid esters; potassium chloride, sodium chloride, titanium oxide, carbon black, aluminum oxide powder, barium titanate, flavans. Examples include TRON and the like.

The degree of crosslinking of the crosslinked PVDF used in this invention is such that the gel fraction is about 20 to 75%, preferably about 30 to 65%,
More preferably it is about 35-60%. If it is too small than the above range, it will flow when stretched at a temperature above the crystal melting point, making it difficult for the molecular chains to be highly oriented; This is because the molecular chains are difficult to be highly oriented due to excessive crosslinking.

Here, the gel fraction means that when PVDF is dissolved in dimethylacetamide, which is a solvent that dissolves well,
It is a value expressed in % of the ratio of the gel content remaining when uncrosslinked substances are extracted from the solution to the amount of resin before extraction. At this time, use enough solvent so that the concentration of the dimethylacetamide solution is 1% or less,
The extraction temperature was 100°C and the extraction time was 24 hours.

Examples of the crosslinking agent that can be used to crosslink the crosslinked PVDF used in this invention include cyanurates such as triallyl cyanurate, triallyl trimellitate, tripropargyl cyanurate, dipropargyl allyl cyanurate, and diallyl propargyl cyanurate; Known crosslinking agents such as isocyanurates such as triallyl isocyanurate, tripropargyl isocyanurate, dipropargylallyl isocyanurate, and diallylpropargyl isocyanurate are used.

In this invention, the cross-linked PVDF used is
It can be produced by crosslinking PVDF as described above using a crosslinking agent in a conventional manner. The obtained cross-linked PVDF is stretched in at least two directions at a suitable elongation range at a temperature above its crystal melting temperature and below its decomposition start temperature, and then cooled at a temperature below its crystal melting point while maintaining its elongation. Ru.

In Figure 2, the solid line is crosslinked PVDF and the dotted line is uncrosslinked PVDF.
This is an example of the apparent stress-strain curve of PVDF. Here, the temperatures shown for the solid lines and dotted lines are ambient temperatures. When the ambient temperature is 170°C, the polymer obtained by suspension polymerization is below the crystal melting point of PVDF, so even cross-linked PVDF can be uncross-linked.
PVDF also undergoes netting stretching and exhibits a microfibrillar structure. On the other hand, the ambient temperature
If the temperature is above the crystalline melting point of PVDF, no netting stretching will occur. Furthermore, in the case of crosslinked PVDF, as shown in FIG. 2, when the elongation is small, the stress increases only slightly as the elongation increases. On the other hand, stretching in a stretching region where the increase in stress that changes with increase in elongation can be considered to be approximately constant is herein referred to as "flow stretching". Such a flow-stretching region is shown, for example, as C ₂ -D ₂ in FIG. Although not shown, such flow stretching is also observed in the case of uncrosslinked PVDF. Even if such flow-stretched PVDF is cooled, not only does the preferential orientation of the polar axis not occur,
As mentioned above, in most cases, only the α structure can be obtained. However, when cross-linked PVDF is further stretched above its crystal melting point, unlike the previous flow stretching, even a slight increase in elongation causes a large increase in stress, and this trend continues until breaking point B is reached. By stretching in at least two directions beyond such a flow-stretching region, and cooling and solidifying while maintaining this state, selective orientation of the polar axis can be achieved, and the crystal structure can be changed from that in the flow-stretching region. Even if the material can only have an α-type structure upon stretching, it will now exhibit a β-type structure.
Moreover, the closer this elongation is to the breaking point B, the higher the proportion of the β-type structure becomes, and the preferential orientation of the polar axis becomes more clear.

Furthermore, to explain FIG. 2 in more detail,
When stretched beyond the netting stretching region, for example C ₁ - D ₁ or the fluid stretching region, for example C ₂ - D ₂ , the increase in apparent stress (ΔS) per unit elongation (ΔL) is The rate of change ΔS/ΔL increases as the stretching ratio increases, but the rate of change per unit elongation, ie, d ² S/dL ² reaches its maximum value at a certain stretching ratio. This point is designated as A, and in FIG. 2, curves at different stretching temperatures are indicated as A ₁ , A ₂ , and A ₃ . In addition, in Fig. 2, the bottom line cannot display _A4 , but it is recognized at 950% elongation. That is, A indicates the point on the apparent stress-strain curve where the apparent stress increases most rapidly, which appears at an elongation close to the breaking point, that is, the point where the apparent stress increases the most.

If the elongation at point A is L _A , the breaking point is B, and the elongation is L _B , then the sum of the elongations in at least two directions is slightly smaller than L _A , that is, L _A −2/5
By elongating the elongation within a range that is greater than the elongation of (L _B − L _A ) and smaller than L _B , the crystal structure can be changed to only the α-type structure by the subsequent cooling. Even those that do not have a β-type structure come to exhibit a preferential orientation of the polar axis. In particular, by setting the elongation to be larger than L _A −1/5 (L _B −L _A ) and smaller than L _B , especially larger than L _A and smaller than L _B , the crystal structure will noticeably exhibit a β-type structure. In addition, the selective orientation of the polar axis can be made more clearly.

In this invention, the stretching in at least two directions may be simultaneous biaxial stretching such as inflation or sequential biaxial stretching, or may be further performed by post-stretching. Furthermore, it is desirable that the stretching directions be perpendicular to each other, but the stretching directions are not particularly limited thereto.

For example, in the case of stretching in two directions, it is not preferable for one elongation to be extremely small and the other to be extremely large; it is desirable that the elongations be approximately the same. That is, the ratio of the elongation in the maximum elongation direction to the elongation in the minimum elongation direction is preferably about 5 or less, and more preferably about 3 or less. By determining the elongation ratio in this way, the selective orientation of the polar axis can be carried out more clearly.

Such stretching is carried out at a temperature higher than the crystal melting point of crosslinked PVDF and lower than the decomposition initiation temperature. Industrially, in order to melt faster, the stretching is preferably carried out at a temperature range of about 5°C or more higher than the crystal melting point, but not about 100°C or more higher than the crystal melting point, more preferably about 15°C higher than the crystal melting point. The temperature is higher than
In addition, it is preferable to carry out the reaction at a temperature not higher than about 80° C. above the crystal melting point.

Furthermore, the sum of elongations in at least two directions here refers to the sum of elongations in two orthogonal directions in the case of inflation, for example, and refers to the sum of each elongation when post-stretching is performed after biaxial stretching. shall be taken as a thing.

In this invention, after stretching as described above, the material is cooled at a temperature lower than the crystal melting point in order to crystallize while maintaining the degree of elongation. This cooling is preferably carried out at a temperature below which the maximum crystallization rate is obtained, more preferably at a temperature below about 40° C., whereby a high mechanical strength can be obtained.

As a more specific example of the manufacturing method according to the present invention, PVDF containing a crosslinking agent is extruded into a tube shape, crosslinked with γ-rays or β-type radiation, and then one end is pinched and the other end is extruded. There is a method in which a tube-like film is obtained by blowing an inert gas into the material and heating it in a high-temperature furnace to a temperature above the crystal melting point to cause inflation, and then cooling the tube-like material while applying internal pressure. In this method, the tube is expanded by blowing gas unless prevented by the crosslinking, and the crosslinking allows the tube to expand to a size where no further expansion is possible. At this time, by adjusting the gas blowing pressure appropriately, the tube can be expanded without breaking and so that the sum of the elongations in two directions falls within the above-mentioned range. From this state, by cooling and crystallizing as described above, it is possible to obtain a film whose crystal structure is mainly a β-type structure and whose polar axis is selectively oriented. Note that such a film can be made to have an extremely uniform thickness both in the lengthwise direction and in the circumferential direction, as long as there is no large thickness unevenness in the original tube before stretching. In addition, the film produced by inflation in such a molten state has a higher stretching ratio than cold stretching, and can be easily made into a thin film.Since there is no microfibrillar structure, the surface is smooth, and the inside of the film is smooth. can exhibit a structure without microvoids.

When employing the inflation method described above, the radiation dose depends on the amount of crosslinking agent, the type of polymer, the degree of polymerization, etc., but the amount that should be adopted will not be explained unless specifically explained. Those skilled in the art will be able to select one as appropriate.

The molded product obtained by the method according to the present invention mainly has a β-type crystal structure, and the β-type structure was quantified by the method described below. That is, the absorbance D ₅₁₀ at 510 cm ^-1 , which is infrared absorption based on β-type crystals, and α
The absorbance D ₅₃₀ at 530 cm ^-1 , which is infrared absorption based on the type crystal, was determined by drawing a baseline as shown in FIG. 6, and the proportion in the β-type crystal system was determined using the following formula.

D ₅₁₀ / D ₅₁₀ + D ₅₃₀ × 100 (%) In this invention, “mainly has β-type structure”
means that the value calculated using the above formula is approximately 50% or more,
Preferably it is 65% or more, more preferably 75% or more.

In addition, the molded product obtained by the method according to the present invention exhibits a selective orientation of the polar axis, but the "selective orientation of the polar axis" here refers not only to the ideal double orientation state but also to the wide-angle This refers to the case where the diffraction images of the (200) and (110) planes of a β-type crystal form a 6-point image or 6 arc-shaped images in an X-ray diffraction photograph. This wide-angle X-ray diffraction photograph is obtained by setting a cross-section of a film cut perpendicularly to the direction of the highest stretching ratio among two or more stretching directions, with the film width direction vertically vertically. Photographs are taken by injecting CuKα X-rays. When a typical wide-angle X-ray photograph obtained by the manufacturing method according to the present invention is shown in FIG. 3, it can be seen that there are six arc images.

The more the directions of all the polar axes of the β-type crystal are oriented in limited directions, the more the six-point image is observed as six spots in the sharpness. However, as the orientation of the polar axis becomes insufficient, the 6-point image becomes blurred, and the 6-point image spots become 6 arc-shaped images. However, as described above, as long as there are six arc-shaped images, it can be said that the selective orientation of the polar axis in this invention can be achieved. Furthermore, if the orientation is disturbed and the dipoles are oriented randomly within the cross section of the film, they will be observed in a ring shape on the same circumference as shown in FIG.

The present invention will be explained below with reference to Examples.

Example 1 Vinylidene fluoride homopolymer 100 with a degree of polymerization of 1350
2 parts by weight of triallyl isocyanurate as a crosslinking agent was added to the parts by weight, thoroughly mixed and blended using a Henschel mixer, and extruded into a tube shape with an outer diameter of about 15 mm and a wall thickness of about 0.4 mm using an extruder. turned into γ of 4 Mrad at room temperature in this tube.
It was cross-linked by radiation irradiation. The gel fraction of this tube was 50%.

The tube thus obtained was inflated in a heating furnace maintained at 210° C. by pinching one end and blowing air through the other end to melt it. after that,
The inflated tube was taken out and air cooled while the internal pressure was still applied. The circumferential direction of the film thus obtained is approximately 8 times larger, and the longitudinal direction is approximately 8 times larger.
Stretched 4.5 times, the apparent stress strain curve is approximately L _A
It corresponded to the elongation of . Further, the thickness of the film was approximately 11 μm, and a uniform film with very little thickness unevenness was obtained.

This film's {D ₅₁₀ / (D ₅₁₀ + D ₅₃₀ )} x 100
The rate was 91%, and its wide-angle X-ray photograph is shown in Figure 3. Furthermore, the dielectric constant of this film at 25℃ and 30Hz is 14.0, and the tensile strength is 21Kg/mm ²
It was hot. The tensile strength was determined by making five samples each with a sample length of 100 mm parallel to the circumferential direction during film molding and a width of 10 mm in the perpendicular direction. This is an average value measured using a mold.

Furthermore, the surface of the film was observed using an electron micrograph, but as shown in FIG. 5, no fibrils were observed.

Comparative Example 1 A 9μ thick sheet made of vinylidene fluoride homopolymer with a degree of polymerization of 1350 was stretched 4.0 times in the extrusion direction at 100°C, and then 6.0 times in the direction perpendicular to the extrusion direction at 150°C. I got the film.
An electron micrograph of the surface of the film is shown in FIG. 1, and the presence of fibrils is clearly recognized.
Further, the dielectric constant of this film at 25° C. and 30 Hz was 15.0.

Example 2 Vinylidene fluoride homopolymer 100 with a degree of polymerization of 1000
1 part by weight of triallylisocyanurate as a crosslinking agent was added to each part by weight, and the tube was extruded and crystallized in the same manner as in Example 1 to form a tube with an outer diameter of about 15 mm and a wall thickness of about 0.5 mm. in
Crosslinking was performed by irradiation with 3 Mrad of gamma rays. The gel fraction of this tube was 30%.

The film obtained by inflating this tube under the same conditions as in Example 1 had a circumferential direction of approximately
It was stretched 12 times and about 7 times in the length direction. This elongation is approximately L _A in the apparent stress-strain curve.
And this film's {D ₅₁₀ /D ₅₁₀ +D ₅₃₀ )}×
100 was 80%. Furthermore, the wide-angle X-ray photograph showed a 6-point arc image, almost the same as in Figure 3. Observation of the surface of the film using electron micrographs showed almost the same pattern as shown in FIG. 1 in Example 1, and no fibrillar structure was observed.

Furthermore, the tensile strength of this film was 23 Kg/mm ² and the dielectric constant at 25° C. and 35 Hz was 13.5.

Example 3 Vinylidene fluoride homopolymer 100 with a degree of polymerization of 1000
4 parts by weight of triallyl isocyanurate as a crosslinking agent was added to the weight part, and the same procedure as in Example 1 was performed to extrude and crystallize the tube into a tube with an outer diameter of about 15 mmφ and an inner thickness of about 0.4 mm. in
Crosslinking was performed by 8 Mrad γ-ray irradiation. The gel fraction of this tube was 75%.

This tube was inflated under substantially the same conditions as in Example 1. The obtained film was stretched approximately three times in the circumferential direction and approximately twice in the longitudinal direction. The apparent stress-strain curve for this elongation is approximately L _A ,
This film's {D ₅₁₀ / (D ₅₁₀ + D ₅₃₀ )} x 100 is
It was 95%. Further, no fibrillar structure could be observed in the electron micrograph.

Furthermore, the tensile strength of this film was 25 Kg/mm ² and the dielectric constant at 25° C. and 35 Hz was 13.5.

Comparative Example 2 Vinylidene fluoride homopolymer 100 with a degree of polymerization of 1100
4 parts by weight of triallylisocyanurate as a crosslinking agent was added to the parts by weight, and extruded and crystallized into a tube with an outer diameter of about 14 mmφ and a wall thickness of about 0.5 mm in the same manner as in Example 1. at room temperature
Crosslinking was performed by irradiating with γ-rays at 16 Mrad. The gel fraction of this tube was 85%.

This tube was inflated under the same conditions as in Example 1. The obtained film had been stretched approximately twice as much in the circumferential direction and approximately 1.6 times as long in the longitudinal direction.
This film's {D ₅₁₀ / (D ₅₁₀ + D ₅₃₀ )} x 100 is
The crystal structure was 96%, and the crystal structure was mainly a β-type structure. Further, no fibrillar structure could be observed in the electron micrograph. However, almost no selective orientation of the polar axis was observed, and the wide-angle X-ray photograph was almost the same as that in FIG. Moreover, such a film was brown in color and was found to have poor thermal stability.

As is clear from the above-mentioned Examples, the film obtained by the method according to the present invention mainly has a β-type structure with the polar axis selectively oriented, has no microfibrils, and has the above-mentioned tensile strength. A film with a tensile strength of 12 Kg/mm ² or more, preferably 15 Kg/mm ² or more, and 20 Kg/mm ² or more if manufactured under more preferable conditions, can be obtained by the test method, and has good transparency and surface smoothness. It's an excellent film.

The polyvinylidene fluoride resin molded product obtained by the method according to the present invention has the above-mentioned characteristics as well as the characteristics of conventional PVDF, so it can be used, for example, as a film for capacitors, piezoelectric, pyroelectric, etc. It is particularly preferably used as electrical materials such as films, as outdoor or indoor building materials, and as materials that come into contact with chemicals.

[Brief explanation of drawings]

Figure 1 is an electron micrograph of a crystal surface obtained in a comparative example showing the presence of microfibrils, Figure 2 is a graph showing an example of an apparent stress-strain curve, Figures 3 and 4. 5 is a wide-angle X-ray photograph of the crystal surface, FIG. 5 is an electron micrograph of the crystal surface obtained in Example 1 showing the absence of microfibrils, and FIG. 6 is a graph showing the infrared absorbance ratio.

Claims (1)

[Scope of Claims] 1. Stretching a crosslinked polyvinylidene fluoride resin having a gel fraction of 20 to 75% in at least two directions at a temperature higher than the melting temperature of its crystals but lower than the decomposition starting temperature of the resin. In the apparent stress-strain curve, the sum of the elongations is L _{A , which is the elongation at the point of maximum increase in apparent stress, and L B ,} which is the elongation at the breaking point.
Then, L B is larger than L _A −2/5 (L _B − L _A ), and L _B
1. A method for producing a polyvinylidene fluoride resin molded product, which comprises elongating the polyvinylidene fluoride resin molded product to a smaller range, and then cooling it at a temperature lower than the crystal melting point while maintaining such elongation. 2. The crosslinked polyvinylidene fluoride resin having a gel fraction of 20 to 75% is obtained by extrusion molding a composition consisting of a polyvinylidene fluoride resin and a crosslinking agent and by radiation crosslinking. A method for producing a polyvinylidene fluoride resin molded product as described in 2. 3. A method for producing a polyvinylidene fluoride resin molded product according to claim 1 or 2, wherein the stretching is performed by inflation. 4. The polyvinylidene fluoride resin according to any one of claims 1 to 3, wherein the sum of elongations is greater than L _A -1/5 (L _B - L _A ). Method of manufacturing molded products. 5. The method for producing a polyvinylidene fluoride resin molded product according to claim 4, wherein the sum of elongations is greater than _LA . 6. The method for producing a polyvinylidene fluoride resin molded product according to any one of claims 1 to 5, wherein the gel fraction is 30 to 65%. 7. The method for producing a polyvinylidene fluoride resin molded product according to claim 6, wherein the gel fraction is 35 to 60%. 8. Claims 1 to 8, characterized in that the step of stretching in at least two directions involves stretching in two directions, and the ratio of the elongation in the maximum elongation direction to the elongation in the minimum elongation direction is 5 or less. The method for producing a polyvinylidene fluoride resin molded product according to any one of Item 7. 9. The method for producing a polyvinylidene fluoride resin molded product according to claim 8, wherein the ratio of the elongation in the maximum elongation direction to the elongation in the minimum elongation direction is 3 or less.