JPS5944795B2 - Manufacturing method of polyvinylidene fluoride electrical functional device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an electrically functional element having large piezoelectricity and pyroelectricity made of polyvinylidene fluoride resin (hereinafter abbreviated as PVDF).

Recently, the development of polymer electrical functional elements having piezoelectricity and pyroelectricity has been particularly remarkable.

First, it was discovered that uniaxially stretched films of natural polymers such as wood and bone, as well as synthetic polypeptides, exhibited piezoelectric properties, and later certain synthetic polymer films were subjected to special treatments to exhibit large piezoelectric and pyroelectric properties. It was found that the In other words, by applying polarization treatment (electretization) by applying a high DC voltage at high temperatures to a polymer film obtained by means such as melt extrusion or stretching, so-called piezoelectric or pyroelectric properties can be obtained. This is the discovery of electrically conductive electret. Electrets such as PVDF, polyvinyl fluoride, and polyvinyl chloride are well known as such polymer electrofunctional devices, but among them, the piezoelectricity of PVDF electret, which is rich in β-type crystals, is extremely large and surpasses other types. do not have. PVDF is a highly polar crystalline polymer, and its crystal forms include at least two types: α type and β type.

When the resin is cooled and solidified from a molten state, the crystals become α-type, but even if α-type PVDF is polarized, its piezoelectricity is small. β-type PVDF exhibits great piezoelectricity due to polarization treatment
There are various methods for obtaining this, but the method of stretching an α-type film at a relatively low temperature is preferred because it is industrially easiest. In this case, the ratio of crystal transition from α type to β type increases as the stretching temperature decreases and as the stretching ratio increases,
For example, if the film is stretched by 130% or more in at least one direction at 130°C or lower, a film with many β-type crystals will be obtained, and as the temperature further decreases, the β-type crystals will increase, and the temperature at the stretching point will be 50°C.
It is known that an almost perfect β-type film can be obtained at temperatures below °C. It has been conventionally believed that there is a close relationship between the ratio of crystal transition and piezoelectricity, and generally speaking, the larger the amount of transition to the β type, the greater the piezoelectricity. Therefore, it is preferred to stretch at a low temperature and at a high magnification. On the other hand, the performance of piezoelectric electrets also depends on the polarization conditions, and generally speaking, the higher the polarization temperature and the greater the applied electric field strength, the greater piezoelectricity can be obtained.

However, high polarization temperatures and large electric field strengths can cause dielectric breakdown of the film, and even if attempts are made to further increase the temperature and electric field strength in order to obtain greater piezoelectricity,
The upper limit of piezoelectricity is determined by this wall of dielectric withstand voltage.

In order to improve the dielectric strength voltage of the film, even if the purity of the polymer is increased or the thickness unevenness of the film is eliminated as much as possible, there is an inherent limit to the dielectric strength voltage, and it is difficult to expect a significant improvement in piezoelectricity. Rather, it is extremely desirable, especially from an industrial point of view, to study the stretching conditions in more detail and to obtain a film that exhibits high piezoelectricity at a lower polarization temperature and electric field strength. Based on the above-mentioned viewpoint, the present inventors have developed a PV that exhibits high piezoelectricity at a lower polarization temperature and electric field strength.
In order to obtain a DF film, we conducted a more detailed study on the stretching of PVDF, and as a result, we discovered a new fact that was previously unknown, leading to the present invention. That is, when the present inventors uniaxially stretched an α-type PVDF film obtained by melt extrusion to form a β-type film, they found that there was a close relationship between the contraction rate in the width direction of the film and the piezoelectricity. This will be explained using a diagram. As shown in Fig. 1, an α-type PVDF film a is uniaxially stretched to form a β-type film b.
When the film is stretched, a constriction is created in the center of the film and the width W of the film is reduced. changes to WVC. Such a constriction phenomenon caused by uniaxial stretching may be observed in all materials, not just plastics, but it is truly surprising that PVDFVC has a close relationship with piezoelectricity.
The inventors of the present invention focused on the relationship between the shrinkage rate in the width direction of the film due to uniaxial stretching and the piezoelectric constant, and as a result of various studies, we found that even at the same stretching ratio, the larger the shrinkage rate in the width direction, the greater the shrinkage rate in the width direction. It has been found that a piezoelectric constant can be obtained, and that a width shrinkage ratio of 25q1) or more is required to obtain a particularly large piezoelectric constant.

It is clear that the shrinkage rate in the width direction of the film due to uniaxial stretching increases as the stretching ratio increases, but it also depends on the length of the stretching zone with respect to the unit film width. Thus, the longer the stretching zone, the greater the shrinkage rate.

Therefore, it is difficult to uniformly determine the stretching conditions to obtain a width shrinkage of 25% or more.If the force/stretching ratio is limited to the natural stretching ratio, the length of the stretching zone before stretching It has been empirically confirmed that it is sufficient to set the value to 1/4.5 or more of the film width. Of course, if the stretching ratio is higher than the natural stretching ratio, the necessary width shrinkage can be obtained in a shorter stretching zone.

The stretching zone mentioned here refers to the part where stress is applied during stretching and is actually stretched.For example, in the case of a tensile tester, it is the part sandwiched between the chucks of the film, or in the case of a continuous stretching machine. In some cases, it refers to the part sandwiched between the front and rear rolls of film. When a heating zone is installed at a specific location within the area where stretching stress acts, this heating zone substantially becomes the stretching zone.

In addition, the stretching ratio means the value obtained by dividing the length of the film after stretching by the length of the film before stretching, and the natural stretching ratio means that the stress-strain curve of the film is the same as the yield point, as shown in Figure 3. It means the stretching ratio at the point where stress is reached. Yes, the stretching temperature and stretching ratio suitable for use in the present invention are 25°C to 130°C.
and 2.3 to 7 times.

Stretching at a temperature of 25° C. or lower causes breakage of the film, which is particularly disadvantageous from an industrial perspective, and stretching at a temperature of 130° C. or higher results in insufficient crystal transition, making it difficult to obtain a film rich in β-type crystals.

Regarding the magnification, if the magnification is less than 2.3 times, the stretching will not be completed completely and an unstretched portion will remain, and if the magnification is more than 7 times, the film will easily break, which is not practical. Further, as a stretching method for increasing width shrinkage, there is a two-stage stretching method.

The two-stage stretching method is a method in which a film that has been stretched at a relatively low temperature is further stretched at a relatively high temperature.The stretching at a low temperature effectively causes crystal transition, and the subsequent stretching at a high temperature causes width shrinkage. be enhanced. The piezoelectric constant of the PVDF film obtained in this way after polarization treatment is extremely large.
In particular, it exhibits a sufficient piezoelectric constant even when polarized with a small electric field strength, so it can prevent dielectric breakdown during polarization processing.
Its practical value is extremely high.

Hereinafter, the present invention will be explained in detail with reference to Examples. Example 1 An α-type PVDF film having a thickness of 65 μm and a width of 45111 mm was stretched at a temperature of 90° C. using a tensile tester to prepare a β-type film.

At this time, in order to obtain samples with different shrinkage rates in the width direction, stretching was carried out by changing the length of the stretching zone and the stretching ratio. Aluminum was vacuum-deposited on both sides of the obtained β-type film to serve as electrodes, and a
A DC electric field of 00 KV/al& was applied, and after 30 minutes, the electric field was maintained and cooled to room temperature. Figure 4 shows the piezoelectric constants D3, of the PVDF electrets with various width shrinkage ratios obtained in this way. It is plotted and shows that the larger the width shrinkage rate, the larger D3l is.

Figure 5 shows the relationship between the length of the stretching zone and the width shrinkage rate at the natural stretching ratio for a film with a width of 45 m. It can be seen that it is sufficient to set the width to ±(22.5 m for a film with a width of 45 wr1n) of the film width.

FIG. 6 shows the relationship between the stretching ratio and D3l for samples with stretching zones of 25 ma and 50 wmb, and shows that even at the same stretching ratio, the longer stretching zone 1 has a larger D3.

Fig. 7 shows the results of Fig. 6 in terms of the relationship between the degree of orientation of the stretched film and D3, and shows that even with the same degree of orientation, the longer the stretching zone, the larger the D3. .

The degree of orientation was expressed as the optical birefringence Δn. Example 2 An α-type PVDF film with a thickness of 65 μm and a width of 30 cm was stretched at 90° C. using a uniaxial continuous stretching machine to create two types of β-type films, A and B.

2A is the length of the stretching zone 10c1
B was set to 50 cm, and both stretching ratios were 4.
It was stretched at 1x. The width shrinkage rate of the obtained β-type film is A
was 20%, and B was 45%. When these two types of films were polarized in the same manner as in Example 1 and D3 was measured, A was 5.0×10 −7 Cgsesu. ．． B was 7.2×10 −7 CgseBu.

Example 3 Among the PVDF electrets obtained in Example 1, the length of the stretching zone was 50 m, the stretching ratio was 3.64, and the width shrinkage ratio was 4.
After aging 7% at 50℃ for 16 hours, D
3L showed 6.88×10-7 Cgsesu, and this sample was cooled from 50℃ at a rate of 1℃/min.
When the pyroelectric constant at 0℃ was measured, it was 4.7nc/℃・d
The pyroelectricity was also found to be sufficiently large.

[Brief explanation of the drawing]

FIGS. 1 and 2 show shrinkage in the width direction when an α-type PVDF film is uniaxially stretched.

Claims (1)

[Claims] 1 A uniaxially stretched polyvinylidene fluoride resin film obtained by applying tension to a polyvinylidene fluoride resin film and stretching it 2.3 to 7 times in the uniaxial direction at 25°C to 130°C, shrinking it by 25% or more in the width direction during stretching. A method for manufacturing a polyvinylidene fluoride electrical functional device, which comprises polarizing a vinylidene chloride resin film.