Fluororesin In Different Industries--Semi-conducting industry
Since the Japanese Kawai discovered in 1969 that the polymer material polyvinylidene fluoride (PVDF) has a very strong piezoelectric effect, the research passion for PVDF film has not faded for decades. PVDF is a semi-crystalline high polymer which forms a chain structure from a repeating unit (CH2CF2)n. Since the fluorine atom connected to the carbon atom is small in volume and large in electronegativity, and further, -CH2- is a strong dipole, so CF2 is easily oriented along the electric field force, and the molecular chain is easily arranged in a regular manner, so that it has a high crystallinity. (about 50% to 80%). PVDF has at least four crystal structures, α, β, γ and δ. The α-type molecular chain has a helical structure, the dipole is antiparallel, and is a non-polar crystal. The β-type molecular chain has a planar sawtooth structure, and the dipoles are arranged in parallel in the same direction. It is a polar crystal; the γ and δ types are also polar crystals, but the polarity is smaller than the β type. Under normal conditions, the PVDF film is not polarized, but after rolling and stretching, the α-type crystal in the original film becomes a β-type crystal structure. When the PVDF film after stretching and polarization is subjected to external force or deformation in a certain direction, the polarization plane of the material will generate a certain electric charge, that is, the piezoelectric effect, which converts the pressure signal into an electrical signal, thereby becoming a very Useful pressure sensor. Compared with the commonly used piezoelectric ceramics and piezoelectric crystals, PVDF piezoelectric materials have the following advantages: (1) the density is small, only 1.78 g/cm3, which is one quarter of the piezoelectric ceramic material PZT, high elasticity and suppleness. Good properties, can be processed into a specific shape, easy to fit the surface to be tested, high mechanical strength, impact resistance;
(2) high output voltage, under the same conditions, its output voltage is 10 times higher than ceramic materials;
(3) The electric strength is high, it can withstand a strong electric field of up to 75 V/um. At this time, most of the piezoelectric ceramics have been depolarized;
(4) the acoustic impedance is low, only one tenth of the piezoelectric ceramic PZT. , close to water, human tissue and viscose;
(5) wide frequency response, from 10-3Hz to 109Hz can convert electromechanical effects, and vibration mode is simple;
(6) resistant to chemical and oily corrosion, can be processed into large Area and complex shape of the film;
(7) cheap, therefore, PVDF piezoelectric film can be made into audio transducers, electromechanical transducers and devices, ultrasonic and underwater transducers and infrared and optical devices.
Although PTFE has a strong polar group (-F), due to the symmetry of the arrangement, the polarities of the groups on both sides of the main chain cancel each other out to form a non-polar molecule; at the same time, due to the helical structure of the main chain, the fluorine atom The size is such that it forms a tight cylindrical outer shell on the surface of the carbon atom chain, effectively preventing the action of foreign objects. This makes PTFE have outstanding chemical inertness, excellent storage capacity and low cohesive energy density. It is the most excellent resin for the fluoropolymer family and one of the first fluoropolymer materials used in the field of electrets.
FEP is another non-polar material. Its special structure allows FEP to maintain the outstanding properties of PTFE (dielectric properties, chemical inertness and low hygroscopicity, these three properties are the basic elements of the formation of high-quality index space charge electret), and the mechanical modification has been realized. It is a kind of non-polar polymer electret material with excellent comprehensive properties such as flexibility, dimensional stability and easy processing. In the industrial electret field, especially in the production of electret acoustic sensors, FEP has always been the leader, widely used in the production of various types of electro-acoustic and acoustic sensors, including for ordinary telephones and voice-activated telephones, walkie-talkies, fax machines , tape recorders, karaoke machines, hearing aids, eavesdroppers, voice-activated toys, voice-activated switches and sound sensors for voice-activated computers, etc., cover more than 80% of all types of sound sensors.
Compared with FEP, PFA can work continuously at 260 °C, while FEP has a maximum operating temperature of only 205 °C; PFA's folding resistance (106) is about 100 times that of FEP or PTFE, and PFA also exhibits low gas permeability. And negligible hygroscopicity; the volume resistivity of PFA is slightly higher than FEP, and the surface resistivity is one order of magnitude higher than FEP. With these features, PFA is also used in electret acoustic sensors.
In addition, modified fluororesins are also used in the semiconductor industry. For example, in 1988, DuPont's ethylene epoxidized fats replaced ethylene fluorinated polymers under the trade name Teflon AF. A completely amorphous fluoropolymer with excellent electrical, optical and mechanical properties, mainly used as a circuit-off layer and computer chip for a new generation of high-speed computers; an insulating layer for microelectronic circuits, a medium for hybrid channel connection/sandwich integrated circuits Material; sealing layer for integrated line inserts, special circuit board; coating or film for releasing material.