Proper modification of pure PTFE can improve its comprehensive performance and expand its application in various fields. At present, the modification of PTFE mainly uses the composite principle, which is combined with other materials to make up its own defects. The methods of modification include: surface modification, filling modification, blending modification and so on.
The surface modification
PTFE’s very low surface activity and non adhesion limit the bonding with other materials, especially the bond between PTFE thin film and other skeleton materials. Therefore, it is necessary to modify the surface of PTFE material to improve its surface activity. PTFE commonly used surface modification technology.
Surface activation technique
Radiation grafting method. By irradiation of high energy radiation, the surface of the film can be grafted with other materials under certain conditions, and a layer of PTFE film which is tightly combined with the framework material is formed.
Plasma activation method. With an inert gas such as argon, helium plasma treatment of PTFE material, the surface etching, and carbon fluorine bonds and carbon carbon bonds fracturing, generating a large number of free radicals, but also can be introduced into the active groups, greatly increase the surface free energy of PTFE, improve the wettability and adhesion, which can be formed the adhesive layer on the surface of the surface activation. In general, the stick relay can be 10 times larger than before treatment.
Potassium acetate activation method. Immersing the PTFE in the molten potassium acetate, and the active layer could be formed after a certain amount of time to be treated at a proper temperature.
Mixed liquor activation treatment. PTFE in a certain proportion of sodium hydroxide, two propylene based melamine mixture to heat treatment for a certain period of time can improve the surface activity.
Corona discharge activation. The PTFE can be formed into adhesive bonded active layer by the corona treatment of the proper time and a certain atmosphere.
Tesla transformer discharge activation treatment. The transformer is used to discharge the PTFE, and the surface of the transformer is activated.
The surface corrosion of PTFE mainly includes
The ammonia solution of sodium metal can be treated with a certain concentration of sodium ammonia solution for a certain period of time, so that the contact angle of PTFE can be reduced from 108 to 52 degrees, which can be used as a composite of epoxy resin and other materials.
Sodium naphthalene solution in tetrahydrofuran. PTFE with a certain concentration of sodium, naphthalene, tetrahydrofuran solution processing can achieve the goal of modification.
Alkali metal amalgam .The use of alkali metal amalgam corrosion of PTFE, which makes the surface carbonized and easy to be wetted and bonded.
Osmic acid corrosion. The use of osmium acid corrosion can increase the hydrophilicity of PTFE, which contributes to the composite molding.
The corrosion of iron pentacarbonyl. Using iron pentacarbonyl solution impregnates PTFE, which can improve its the surface corrosion and activity.
Surface deposition modification
Impregnating the PTFE in the colloidal solution of some metal hydroxide, which makes the colloidal particles deposited on the surface of PTFE, increasing the wetting angle, and improving the surface activity, which is easy to be combined with other materials.
The above surface modification methods are mainly applied to PTFE thin films. Usually, when the PTFE film is treated properly, it can be combined with other materials, and can be widely used in the design and manufacture of chemical anti-corrosion lining, sealing products and lubricating device. The basic design ideas of these methods, such as the introduction of polar groups, increase the interface bonding force; or to eliminate the weak interface layer, forming a strengthened surface layer; or to adjust the surface roughness, to give anchor effect. In short, various methods have their advantages and disadvantages, reasonable application or to find a new surface activation treatment is the main way to expand the application of PTFE thin film.
Filling modification of PTFE
Filling modification is adding filler to the PTFE, so as to improve and overcome the defects of pure PTFE, while maintaining its original advantages on the basis of the composite effect, improve the comprehensive performance. The filler can make the PTFE wear resistance increased about 1000 times; creep resistance increased 1.5 ~ 4.5 times at room temperature, 1.5 times at high temperature; flexural modulus increased 2 ~ 3 times; the hardness increased by 10% ~ 30%; the highest thermal conductivity increased 2 times; linear expansion coefficient reduced by about 1/2.
The filling properties of PTFE are closely related with the filler’s properties, content and technology, the basic principle for the general choices of filler include.The filler can withstand the sintering temperature of PTFE. The filler can improve PTFE’s abrasion resistance, mechanical strength, or increase the thermal conductivity coefficient, reduce the linear expansion etc. When in use, the filler will not effect other contact metals or fluid. The current commonly used fillers can be divided into 3 categories: inorganic materials, organic materials and metal materials.
Inorganic filler materials. The commonly used inorganic filler are mainly glass fiber, graphite, MoS2, carbon fiber etc.
Filled PTFE glass fiber is generally no alkali glass fiber, the filling amount is generally 15% ~ 25%, the ratio of length to diameter of 5 ~ 10. The hardness of the PTFE composited by the glass fiber can be generally increased by 10%, wear resistance can be increased by more than 500 times, creep resistance and resistance to cold flow properties have a greater degree of improvement and improvement.
Adding MoS2 helps to increase the rigidity and hardness of PTFE products, reduce the wear of the early start. In general, there’s little dosage of MoS2 in filling PTFE, and it often combines with other fillers.
The graphite can be used alone, and can be used in conjunction with the glass fiber or carbon black. Graphite filled PTFE has excellent chemical resistance, compression creep and better thermal conductivity, but its wear resistance worse than glass fiber filled PTFE difference. Only a small amount of carbon fiber filled PTFE can achieve carbon and graphite filling efficiency, but also has a very superior tensile properties. The wear resistance of the products in air and water can be greatly improved, and it has good creep resistance at room temperature and high temperature, but the biggest limitation to the application of this kind of material is the high cost.
Metal filling material. In order to improve the mechanical properties, thermal conductivity and dimensional stability of PTFE, it’s common to fill PTFE with iron, copper, lead, molybdenum, tungsten, silver and other metals, especially with copper and its alloys.
The copper filled PTFE can improve the creep resistance, compressive strength, hardness and dimensional stability of products, but the corrosion resistance and dielectric properties decrease. The suitable dosage of copper powder is 60%, at the moment, the limit value of PV can reach 29000, higher than that of other general materials.
Blending modification of PTFE
The blending modification of PTFE are familiar with the basic principle of filling modification generally, often refers to blend with other organic polymers, in order to improve its processing performance and use performance. In the blending modification, the PTFE is usually used only as a filling agent, and the processing method of the main material is usually used as the processing method of the blending material. The use of blending modification with PTFE has the following advantages:(1)The existing material and the existing process can be used to develop new varieties, and the investment is small, and the effect is fast; (2) Many varieties can be developed and the range is wide, such as the thermoplastic plastics, rubber, thermosetting resins, coatings, sealant, etc.;(3) The method of injection, molding curing, coating solidification and other methods of the main body material can be processed more efficiently and less energy consumption than the PTFE molding and sintering process;(4) Other polymers can improve the processing performance and use performance, such as heat resistance, chemical resistance, in particular, friction, wear resistance can be greatly improved.
The basic principle of blending is the principle of similar compatibility, solubility parameter and surface tension. PTFE blending modification engineering plastics mainly have polycarbonate (PC), poly formaldehyde (POM), nylon 66 (PA6), etc.. Modified engineering plastics not only maintain its original characteristics, especially the processing performance, but also improve its wear resistance, and improve the critical value of PV.
The rubber of PTFE modification is mainly silicone rubber, fluorine rubber. The wear resistance of the modified rubber is greatly improved. In some synthetic rubber and its coating, the blending of PTFE will greatly improve the chemical stability, oil resistance and solvent resistance of the rubber.
PTFE modified thermosetting resins are less. Often using a small amount of low molecular weight PTFE modifies the resin, and its wear performance is greatly improved; in addition, adding PTFE in curing type sealant, without reducing the sealing characteristics, can improve the use of temperature and chemical stability, while increasing lubrication.
In addition to the above modification method, introducing a small amount of non fluorine group into the PTFE chain, and the block is grafted to destroy the symmetry, so the modified PTFE is obtained by the method of thermoplastic processing. The performance of this product is similar to that of PTFE, but the performance of this product is greatly improved, so it is being paid more and more attention.
In addition, PTFE dispersion, paste and PTFE powder and expanded PTFE (EPTFE) are widely used in various industries because of its good processing performance.
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