JPS5937597B2 - Polarization method of vinylidene fluoride resin - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for polarizing vinylidene fluoride resin.

Certain polymeric materials exhibit piezoelectric effects when polarized under appropriate conditions, and are therefore expected to be used as piezoelectric elements in microphones and other devices.

Among many polymeric materials, vinylidene fluoride resin is known to have the best piezoelectric properties (especially piezoelectric constant), but when polarized in the usual way,
It does not exhibit piezoelectric properties good enough to be used as a piezoelectric element in the electro-acoustic transducer as described above. This invention utilizes vinylidene fluoride resin (hereinafter referred to as PVDF) under specific conditions.
This is based on the new knowledge that a piezoelectric element with an extremely high piezoelectric constant can be obtained by polarizing the PVDF is crystalline, and there are α-type and β-type crystals. α-type crystals have a large dipole in the direction perpendicular to the molecular axis, but because the moments are antiparallel in the crystal, they have a structure in which the dipole moments cancel each other out. However, the β-type crystal has a large dipole moment in the direction perpendicular to the molecular axis and has a planar zigzag molecular structure. PVDF containing a large amount of β-type crystals is said to have favorable properties as a piezoelectric element. PVDF containing many β-type crystals is PVDF formed by any method.
or by stretching a film of P at low temperature.
It can be obtained by forming a film from a solvent solution of VDF by a solvent evaporation method.

In the method of the present invention, a PVDF film containing a large amount of β-type crystals obtained by the above-mentioned or other suitable method is first subjected to a preparative treatment using a high electric field at a relatively low temperature of about 40 to 80°C. After undergoing a polarization treatment, and then a final polarization treatment using an electric field lower than the above electric field at a temperature higher than the above temperature, it is cooled to room temperature with this electric field still applied.

The film obtained by this polarization treatment is about 20% higher than the film obtained by heating the film to a temperature of 80 to 130°C while applying a nearly constant electric field and then cooling it. It was experimentally confirmed that it has a piezoelectric constant of % or more. The actual operation is to first provide a pair of electrodes by vapor-depositing metal such as aluminum on both sides of the PVDF film, and while applying a predetermined high electric field of approximately 800 to 1800 KV/C77L between the electrodes, Begin by heating the film to a temperature of about 40-80°C.

Because the temperature of the film is low, no discharge occurs even when such a high electric field is applied. Then, this film
A temperature between the above temperature and the softening point, for example about 100°C
At this stage, the electric field applied between the electrodes is increased to a predetermined value (for example, about 800 KV/
c! n). The PVl)F film is then cooled to room temperature while this electric field is applied. PV
Before the DF film is heated to the temperature in the second stage,
The reason for lowering the electric field applied is that at this temperature, the discharge starting voltage between the electrodes becomes low, and if the initial high electric field remains, discharge will begin. The relationship between electric field and temperature during this process is shown in the drawing. According to the experimental results, good results were obtained when the film was kept at a low temperature for about 10 minutes during the preliminary polarization process and about 60 minutes for the final polarization. PVD obtained by such polarization treatment
The F film has a piezoelectric constant of 25.8 x 103 esu (Cgs), and this value is higher than that of PVDF by conventional polarization treatment.
This is significantly higher than the maximum value of the film, 21.4×10 3 esu (Cgs), and it is clear that the film has piezoelectric properties sufficient for use as a piezoelectric element in an electro-acoustic transducer such as a microphone or a speaker.

[Brief explanation of drawings]

The drawing is a graph showing the relationship between voltage and temperature in the method of this invention.

Claims (1)

[Claims] 1. A film of vinylidene fluoride resin containing β-type crystals is pre-polarized by applying a high electric field at a first temperature within the range of 40°C to 80°C, and then A final polarization process is performed by applying an electric field lower than the electric field at a second temperature higher than the temperature of the film, and then the film is cooled to room temperature while the electric field is applied. Polarization method of vinylidene fluoride resin.