JPS5937596B2 - 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, speakers, and the like.

Among many polymeric materials, vinylidene fluoride resin is known to have the best piezoelectric properties (especially piezoelectric constant), but when polarized using the usual method,
It does not exhibit piezoelectric properties good enough to be used as a piezoelectric element in an 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 a 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. P containing many β-type crystals
VDF can be obtained by stretching a film of PVDF formed by any method at low temperature, or by forming a film from a solvent solution of PVDF by a solvent evaporation method.

In the method of this 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 high electric field at room temperature (for example, about 15°C to about 25°C). After undergoing pre-polarization treatment, approximately 80
After undergoing polarization treatment at a temperature of ~130° C. with an electric field lower than the above electric field, it is cooled to a temperature below the transfer point while this electric field remains applied.

The film obtained by this polarization treatment is approximately It has been experimentally confirmed that it has a piezoelectric constant of 20% or more. The actual operation begins by first providing a counter electrode by depositing a metal such as aluminum on both sides of the PVDF film and applying a high electric field between the electrodes at about room temperature.

In this pre-polarization process, the upper limit of the applied voltage is
It is determined by the limit at which no discharge occurs between the electrodes, and although this varies depending on the thickness of the PVDF film used and other conditions, it is generally about 10,001 cv/Cm. The PVDF film is then heated to a temperature below its softening point,
For example, it is heated to about 100° C., and before reaching this temperature, the electric field applied between the electrodes is lowered to a predetermined value. The PVDF film is then cooled to room temperature while this electric field is applied. The reason why the electric field applied to the PVDF film is lowered before it is heated to the final polarization temperature is that at temperatures of about 80 to 130°C, the firing voltage between the electrodes becomes low, and the initial high electric field This is because discharge will begin if left as is. The relationship between electric field and temperature in this process is shown in the drawing. According to the experimental results, the pre-polarization time is about 10-30 minutes, and the final polarization time is about 30 minutes.
Good results were obtained in ~120 minutes. The PVDF film obtained by such polarization treatment has a size of 24.1×1
It has a piezoelectric constant of 0.03 esu (Cgs), which is the highest value of 21.4
×103e8u (Cgs), and it is clear that it has piezoelectric properties sufficient as a piezoelectric element for an electro-acoustic transducer such as a microphone or a speaker.

[Brief explanation of the drawing]

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

Claims (1)

[Claims] 1 Performing preliminary polarization treatment by applying a high electric field at room temperature to a film of vinylidene fluoride resin containing β-type crystals, and then applying an electric field lower than the above electric field at a temperature higher than room temperature. 1. A method for polarizing vinylidene fluoride resin, which comprises performing a final polarization treatment using an electric field, and then cooling the film to room temperature while applying this electric field.