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Preparation and properties of PTFE anti-friction coating

Polytetrafluoroethylene (PTFE) is considered the best solid lubricant, its friction coefficient is only 0.05 within 320 ℃ temperature, even smaller than the graphite friction factor 0.07. PTFE because its good lubricity and chemical stability, is widely used as industrial lubricants. Perfluoro surfactant is a type of fluorine-containing surfactant. Compared with the traditional surfactants, fluorosurfactants have high surface activity, high heat stability, high chemical stability, low surface tension, low use concentration and other excellent features. Most importantly, the fluorine-containing surfactant can be used as a dispersant, and the fluorine-containing surfactant can be used as a dispersant in the dispersion polymerization of various fluororesins.

Bonded solid lubricating film is organic and inorganic binder as the base material, the small friction coefficient solid as a solid lubricant, together with solvents and surfactants made of composite lubricating paint, by dip or spray to form coating.In this paper, epoxy resin as the base material, by adding polytetrafluoroethylene and perfluorinated surfactants (FC-400) made of self-lubricating friction-reducing coating. According to orthogonal experimental design to get different formulations of paint, and then coated on the polished iron which has been processed, at room temperature curing, get friction coating. After a series of tests, the best formula was screened out. The physical and chemical properties of the coatings were measured by a SpectrumOne infrared spectrometer, an MM-200 friction and wear tester, a Y-4Q X-ray diffractometer, a P / N2500052 SERIAL contact angle gage and a differential thermal analyzer.

Experimental method
The substrate used in the experiment is A4 steel sheet with size 20mm*20mm. The substrate was polished by model 200 #, 600 #, 800 #, 1500 # water-resistant sandpaper to remove the substrate surface oxide and impurities, with 15% hydrochloric acid and 15% sulfuric acid mixed solution pickling 2 min, with acetone and ethanol mixed solution in the beaker ultrasonic cleaning 10 min to remove the sample surface of organic matter, and then activated with 3% hydrochloric acid. According to orthogonal experiment design, select the amount of epoxy resin and curing agent are 3 g, the other reagents were tested according to a 4 factor 4 level orthogonal experiment table(see Table 1).

Table 1 4 factors 4 level orthogonal experiment table
Table 1
According to the different formulations the orthogonal table, the weighed polytetrafluoroethylene powder and the graphite powder were mixed and shaken for 3 min in an ultrasonic oscillator, so that the polytetrafluoroethylene powder and the graphite powder are mixed uniformly. Take 4 mL of acetone, dissolve the dispersant FC-400, mix well, pay attention to slow stirring, to avoid excessive air bubbles. The mixed PTFE and graphite powder dissolved in acetone to dissolve the dispersant, the rapid mixing evenly. Pour the mixed mixture into a beaker with epoxy and EP curing agent, and mix well. The surface treatment of iron into the coating in the infiltration of 5 ~ 10 s, with the centrifuge to shake the sample, select the appropriate film thickness, that is the sample.

Results and discussion
Tribological properties
The friction coefficient of the coating was measured with a MM-200 friction and wear tester manufactured in Japan. The test was carried out at a load of 10 N at a speed of 180 r / min, the results of orthogonal experimental analysis are shown in Table 2.
Table 2 Friction factor analysis orthogonal table
Table 2
In table 2, Kij is the i-th factor of the j-level test results and the value of the sum. For example, KA is the sum of the values of the first level of factor A for each test result, that is KA =0.144 +0.055 +0.122 +0.054 =0.375. ki is the ki value divided by the number of occurrences of that level, for example, KA means that the KA value is divided by the factor A factor of the first occurrence of the number of times, that is kA =KA/4 =0.375 /4 =0.094. Ri is the maximum difference between the ki values in the i-th factor, for example, the RA value is the difference between the maximum value of the kA value minus the minimum value, that is RA =kA -kA=0.12 -0.072 =0.048.

As can be seen from Table 2, the influence of each factor on the friction coefficient is small. We can see from RC
Table 3 is the contact angle of each factor and the impact of various factors on the contact angle, we can see from RB
Fig.1 X-ray diffraction spectrum of epoxy resin-PTFE composite coating

Infrared spectroscopy
The bond length and bond angle of the molecule can be determined by infrared spectroscopy, and thus infer the three-dimensional configuration of molecules. By infrared spectroscopy, which can determine the presence of organic functional groups in the sample, can determine the chemical structure of the coating.
Fig.2
Fig.2 Infrared spectrum of epoxy resin-PTFE composite coating

Fig.2 is infrared spectrum of epoxy resin-PTFE composite coating. The strongest band of PTFE appeared in the frequency range of 1 250 to 1 110 cm-1, the characteristic frequencies of 1 157 .2, 1 180 .3 and 1 245.9 cm-1 are in the strongest spectral band of PTFE, the characteristic absorption peak is mainly caused by the stretching vibration of C-F bond, which indicates the presence of PTFE in the composite coating. And an absorption peak at the frequency of 3 448.5 cm-1, the absorption peak is in the frequency range of 3000 ~ 4000 cm-1, which is caused by the absorption of O-H bond stretching vibration, indicating that the composite coating contains – O-H bond material, inferred that the epoxy resin curing agent in the curing chain by the chain generated. The characteristic frequencies of 1 245 cm-1 and 879.5 cm-1 are between 870 and 1 280 cm-1 of cyclic ether bond, from this inference that it contain ether bond. And the absorption peak appeared at the frequency of 829.3 cm-1, and these absorption peaks appeared in the frequency range of epoxy compound, which indicated the existence of epoxy compound in the coating.

Differential thermal analysis
Using the differential thermal analysis, the melting point, decomposition temperature and so on of the samples can be obtained, and the thermal stability of the samples in a certain temperature range can also be seen.
Figure 3 is the differential thermal analysis of the epoxy resin -PTFE composite coating at temperature below 600℃. As can be seen from Figure 3, there is an endothermic peak at 381.34℃, which may be caused by the decomposition of epoxy resin. There is also an endothermic peak at 541.60℃ which is due to the decomposition of PTFE. Indicating epoxy resin PTFE composite coating at 300℃ is a stable state, can work properly.
Figure 3
Figure 3 Differential thermal analysis of epoxy resin – PTFE composite coating

Scanning electron microscopy and energy spectrum analysis
The surface of the sample and the surface composition of the sample were analyzed by scanning electron microscopy. The results are shown in Table 4 and Fig.4.
Table 4 Relative proportions of coating surface composition
Table 4
Figure 4
Figure 4 Epoxy resin – PTFE composite
It can be seen from Figure 4 that the surface of the epoxy resin- PTFE coating is relatively uniformly smooth and level. You can also see a small amount of white particles, it is caused by small amount of PTFE gathered. As can be seen from the relative proportions of the surface compositions of Table 4, the surface of the composite coating mainly contains F and C2 elements, and the mass ratio of F to C is 1: 7.15. The F element is one of the light elements, and the F element can be analyzed by electron microscopy. It is shown that the content of F in the composite coating is larger, that is, the content of PTFE is more.

Conclusion
Using the orthogonal experiment and the range analysis, the formula of epoxy resin- PTFE composite coating with the smallest friction factor was obtained, that is, epoxy resin and solid agent 3 g, PTFE 0.9 g, graphite 0.2 g, acetone 7 mL, dispersant FC-400 0.012 g, the minimum friction factor 0.037, the average thickness of about 300 micron. X-ray analysis showed that PTFE in coating is crystalline structure, epoxy resin is amorphous structure, the emergence of carbon peak shows the presence of graphite. FTIR and DTA showed that PTFE, graphite etc. are present as monomers in the coating, and in the stable below 300 ℃. Electron microscopy and energy dispersive X-ray spectroscopy (EDS) show that the surface of the composite coating is uniformly smooth and level, with a small amount of PTFE gathered, the mass ratio of F to C is 1: 7.15, indicating that the content of PTFE in the composite coating is higher, which plays a decisive role in reducing the friction. From the wetting properties of the coating and the medium water, due to the presence of PTFE in the coating, the contact angle is generally larger than without PTFE, indicating that the coating is more suitable for dry friction in the environment.

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Research progress of PTFE modification

Polytetrafluoroethylene (PTFE) resin was invented by Dr. Champlain Kate (Plankett) in 1938 and formally put into industrial production by Du Pont in 1950. PTFE as high crystallization of tetrafluoroethylene monomer polymer is a kind of thermoplastics of white waxy feeling. In PTFE, fluorine atoms to replace the hydrogen atoms in the polyethylene, due to the radius of the fluorine atoms (0.064 nm) is greater than the radius of the hydrogen atom (0.028 nm), makes carbon – carbon chain of polyethylene flat, at full stretch and conformation is reverse to the PTFE spiral conformation. The helical conformation is surrounded in the PTFE are susceptible to chemical attack skeleton of carbon chain, formed a close completely “fluorine generation” protective layer, makes the main chain of the PTFE is not affected by the outside world any reagent, make PTFE has unmatched by other materials in solvent resistance, chemical stability and low of cohesive energy density; At the same time, carbon – fluorine keys is extremely strong, the key can reach 460.2 kJ/mol, than carbon – hydrogen bonding (410 kJ/mol) and carbon – carbon bonds (372 kJ/mol) is high, the PTFE has good thermal stability and chemical inertness. In addition of fluorine atoms electro negativity is great, and tetrafluoroethylene monomer with perfect symmetry and make the PTFE intermolecular attraction and low surface energy, so that the PTFE has very low surface friction coefficient and good ductility when low temperature; At the same time also make PTFE creep resistant ability is bad, it’s easy to have a cold flow phenomenon. No branch of PTFE symmetrical main chain structure also makes it highly crystalline, so it is difficult to machining.

Pure PTFE is suitably modified, can improve the comprehensive performance, and expand its applications in various fields. at present, the modification of PTFE is mainly based on the principle of composite; it combined with other materials, to make up for the defects of PTFE. Modified methods mainly include: surface modification, filling modification and blending modification, etc.

The surface modification of PTFE
The extremely low surface activity and tackiness of PTFE limits its ability to combine with other materials, especially the adhesive of the PTFE film skeleton with other materials. At present, the solution of the PTFE adhesive, mainly through the surface of the PTFE activation to improve its surface tension, then choose the suitable adhesive for bonding. PTFE surface activation generally based on several methods such as the displacement, crosslinking, grafting, oxidation and recrystallization. Technology of surface modification of PTFE commonly used are: the method of reducing agent (sodium – naphthalene solution permutation method), high temperature melting method, plasma treatment, laser radiation chemical processing method, silicate modification method, force method, etc.

Reducing agent method (sodium – naphthalene solution permutation method)
Sodium – naphthalene solution permutation method in all the modification methods are now known to effect is better, is widely used. Its principle is: the Na to the outermost electron transfer to naphthalene empty orbit, form anion free radicals; With Na + ion pair formation, can release a lot of resonance, to generate the dark green metal mixture of organic compounds. These compounds of high reactivity, contact with PTFE, sodium can destroy C – F key, ripped the partial fluorine atoms on the surface of the PTFE, left on the surface of carbide layer and the – CH, CO and C = C, – COOH polar groups; The depth of carbonation layer is about 0.05 ~ 1 micron, the surface tension of the PTFE by 18.5 x 10-3 N/m increase to 50.0 x 10-3 N/m, the surface has high polarity and high surface energy.

With this method the activation of PTFE with epoxy – polyamide adhesive bonding, the shear strength can be more than 10.7 MPa. According to the data reported, it cannot reach many industrial requirements on bonding strength when PTFE shear strength of bonding under 10.7 MPa. Xu Baoguo by sodium naphthalene solution activation of PTFE surface, and form a complete set of J – 2021 adhesive is developed, the shear strength reach 13.7 MPa above, better meet the requirement of some of the applications of industrial sector.

This method also has some obvious disadvantages, such as: glued surfaces dark or black, under the environment of high temperature resistance is reduced, surface cementing performance under long-term exposure to the light will greatly decrease; Makes the method of application is limited by a lot. Also have reported, after adding carbon black PTFE and adhesive and hydroquinone, light resistance can be greatly improved.

Besides sodium -naphthalene tetrahydrofuran corrosion liquid sodium, sodium -biphenyl dioxane, sodium -naphthalene ethylene glycol dimethyl ether and other processing liquid also has a good effect.

High temperature melting method
The basic principle of this method is: under the high temperature, crystal morphology change, make the surface of the PTFE embedded some high surface energy, easy adhesive material such as SiO2, Al powder, etc.; After cooling will form a layer on the surface of PTFE modified layer can be sticky substances are embedded. Because of the easy sticky material has entered the PTFE surface molecules, destroy it equivalent to intermolecular destruction; so, the bonding strength is high. This method has the advantage of weatherability, wet and heat resistance than other methods, suitable for outdoor use for a long time; Shortcoming in high temperature sintered PTFE emit a kind of toxic substances, and PTFE membrane shape is not easy to maintain.

Plasma treatment
Plasma processing is exposed to the gas plasma polymerized to material, the use of plasma bombardment material surface, causing many changes the structure of the polymer material and the surface modification of polymer materials. The active substances in plasma can various interactions with polymer material surface; plasma processing mechanism of polymer surface is different. Has been reported with CF4 plasma gases, C2F6, CF3H, CF3Cl, CF3Br, NH3, N2, NO, O2 and H2O, CO2, SO2, H2 / N2 and CF4 / O2, O2 / He, air, He, Ar, Kr, Ne, etc. Badey J P disposes PTFE by microwave plasma downstream processing, makes it surface modification. PTFE is disposed With O2 / N2 / O2, surface without modification; Using NH3 plasma treatment, the PTFE surface polarity composition increase, the hydrophilic also increase.

Several parameters which easy to adjust such as operating gas pressure, electric field frequency, power and action time in plasma treatment process , can obtain the ideal control and produce good results. Such as under the condition of high electric/gas density ratio of PTFE surface modification, surface modification of PTFE can be compared with the usual process increased significantly.

PTFE and the adhesion about the aluminum metal were studied by Zhang E C and others. They first use argon plasma (frequency is 40 kHz, power is 35 W, argon gas pressure is 80 Pa) for preprocessing of PTFE and exposed to the atmosphere about 10 min to produce oxide and peroxide; Then conducted on the glycerin alcohol acrylate (GMA) grafted copolymerization, then thermal evaporation of aluminum, results make the adhesion force that PTFE with GMA grafted polymer and Al is 22 times as big as between PTFE and Al, it is also only three times after Ar plasma pretreatment.

Laser radiation method
Laser surface processing with high power density of laser beam, in the form of non-contact heating surface, with the help of the material surface itself conduction cooled, live up to its surface modification process. It has many advantages in material processing, the other surface treatment technology is difficult to match: (1) The energy transfer is convenient, can choose to be processed work piece surface is the local reinforcement; (2) The energy concentration, the processing time is short, small heat affected zone, laser processing, deformation is small; (3) Processing complicated shape of work piece surface, and easy to realize automatic production; (4) Modification effect is more significant: high speed, high efficiency, low cost; (5) Usually can handle only some sheet metal, not suitable for dealing with thicker plate; (6) Due to the laser damage to the human eye, affect the safety of the staff, so be committed to the development of safety facilities.

Steps of this method are as follows: put PTFE membrane in monomers of fumaric acid, methyl acrylate, styrene polymerization, such as with Co60 radiation, the monomer grafting polymerization on the PTFE membrane surface, thus in PTFE membrane surface to form a layer of adhesive of graft polymer easily. PTFE in three dimensional directions after grafting grew up, and keep shape, but lost the original luster and lubrication; Surface roughness increases with increasing the amount of grafted, but surface does not change color, and surface resistance in a wet environment change.

Silicate activation method
Porous PTFE with SiX4 after processing, then through hydrolysis, can achieve the purpose of make PTFE surface activation; this is “silicate modification method”.
As early as 1959, Herr has used SiCl4 processing and hydrolyzed to silicate surface treatment methods of PTFE and the formation mechanism of surface oxide layer are discussed in this paper. Thereafter Rossbach use SiF4 to activate PTFE surface, using ESCA (X-ray photoelectron spectroscopic analysis) to verify the modification result. On the basis of this, the modification technology by Mohammed step further. They think: traditional modification method can change the chemical structure of PTFE, thereby the influence of different degree of the inherent structure of PTFE; and this modification will not change the chemical structure of PTFE, and can achieve the goal that make its surface activation.

Force chemical treatment method
“Force chemical treatment” is glued on coated with adhesive friction material surface, by chemical action, the adhesive molecules combine with material surface chemical bond, thus greatly improve the bonding strength of joint. This is a new method used to glue stick plastic.

The principle is the polymer surface under the action of external force, strength degradation and form free radical chains, and adhesive molecules form covalent bond, produce strong interface bonding. This has been confirmed by the ESR (electron spin resonance spectroscopy) and ATRIR (internal reflection FTIR).Table 1 lists the joints Bonding strength contrast of PTFE after chemical treatment and processing of sand paper burnish, the bonding strength of contrast.
Table 1 Adhesive joint shear strength contrast of PTFE
Table 1
Filling modification of PTFE
Suitable for filling PTFE packing must meet the following requirements: (1) under the condition of 380 ~ 400 ℃ sintering stability;(2) particle size is 150 microns or less;(3)not moisture absorption;(4) under the condition of sintering, itself not cluster;5. Do not react with PTFE. According to these requirements, the commonly used fillers are: glass fiber, carbon fiber, graphite, quartz sand, molybdenum disulfide, amorphous carbon, calcium fluoride, bronze powder, ceramic, polyimide, polystyrene, poly hydroxy acid ester, etc.

The application of filled PTFE products components according to fill products environment to decide, with single filler filling, also there are several kinds of filler composite filling. Such as: 20% SiO2 filled PTFE used as the oxygen compressor piston ring,10% polyimide,15% glass fiber, 5% graphite composite filled PTFE used as hydrogen booster piston ring, etc. The single packing of PTFE can improve some performance, but adverse influence on the performance of some other. Such as glass fiber filled PTFE resistance to wear life can increase 1 000 ~ 2 000 times, use the PV value of about 10 times; But the friction coefficient increases, load variables to reduce about 10%.Again, such as the addition of graphite improved PTFE dimension stability, drug resistance, resistant to compression creep and thermal conductivity; But abrasion resistance is poor. Therefore, usually adopt several packing composites filling to filled PTFE products with excellent properties. As applied to the injection molding machine of PTFE bearing consists of PTFE filled with glass fiber, graphite, carbon fiber, copper powder and Fe2O3, its mechanical properties, electrical conductivity, cold flow property and wear resistance are improved; Applied to the guide rail with boring machine is made of glass fiber, graphite, bronze powder, MoS2 filled PTFE composite materials; Used to import equipment accessories and high strength wear resistant filling PTFE seals, is made from 15% 10% glass fiber, carbon fiber filled PTFE, its friction coefficient is 0.17, the abrasion value is 0.000 1 mm, grinding crack width is 3.7 mm, reach imported equipment and more random accessories required.

The blending modification of PTFE
Liquid crystal polymer modification of PTFE
Liquid crystal polymer (LCP) is a unique member of the family of polymers; it refers to the liquid when some parts of the macromolecular chain can still each other are arranged orderly in the polymer. Polyphenylene ester is also called the poly hydroxy benzoic acid benzene ester, is a kind of liquid crystal polymer and the type of chain linear molecules, it has high crystallinity (greater than 90%), make it difficult to melt flow, generally uses the moulding. In 1970 the United States emery company first successful develop the product. Polyphenylene esters in larger temperature range has high rigidity, high compression creep resistance, high thermal stability, even if heated to 538 ℃ is not melting, at the same time both heat conduction and insulation as well as excellent solvent resistance and radiation resistance. Because of the special molding process of PTFE, so used for liquid crystal polymer modified PTFE must have very high heat resistance, and the molten state of viscosity is very big, can satisfy the PTFE sintering conditions. Meet the above conditions of liquid crystal polymer with all together p-hydroxy benzoic acid benzene ester is the most suitable. Began in the 1960 s, American Carborundum started manufacture of PTFE/polyphenylene ester material .In 1973, Japan Daikin company produced and sold the materials, the commodity was called “Polyflon 7060”.After 1979, Japan’s Sumitomo chemical industrial company from emery bought and modified polyphenyl ester production technology, to produce the blending materials and had a large number of listed, its brand has EkonolS200, EkonolS230, EkonolS300 and EkonolS330, etc. Since the 1980 s, the material market demand increased rapidly.

According to reports, to use type liquid crystal polymer as modifier of PTFE, the wear resistance of PTFE can be improved more than 100 times, significantly improved the wear resistance of PTFE, and maintained the characteristic of low friction coefficient.

Liquid crystal polymer modified PTFE abrasion mechanism is: the liquid crystal polymer melt after high temperature has excellent liquidity, formation heat transfer; the result of the heat transfer inside the liquid crystal polymer in PTFE matrix to move form the gap along the PTFE micro fiber. In alloy composition ratio and sintering temperature is appropriate, these micro fiber end are interconnected to form a dense and uniform in the PTFE matrix three-dimensional network; These networks tightly envelope PTFE matrix and reinforcement effect, greatly restricted and prevented the PTFE belt wear, the wear and tear of PTFE turned into tiny particles plough wear, thus improved the wear form of PTFE and course, raised the wear properties of PTFE.

Other plastic modification of PTFE
Thierry mix PTFE with copolymer (FEP) of tetrafluoroethylene and hexafluoropropylene after electronic radiation, through high heat sintering in nitrogen self-lubricating composite materials. Research shows that: when the filling amount of FEP were 35% and 50% respectively, the corresponding friction coefficient of the composite material is 0.09 and 0.11, are lower than the pure PTFE friction coefficient; When sliding speed reach 0.01 m/s and 0.05 m/s, the wear rate of the composite is 8.6 x 10-6 mm/(N, m) and 7.7 x 10-6 mm/(N, m), were far lower than the wear rate of pure PTFE [each 0.43 x 10-6 mm/(N, m) and 1.2 x 10-6 mm/(N, m)];Compared with the hard particles filled PTFE composites, no etching on the surface of side effects. When the blend contains 35% FEP, can significantly improve the tensile strength of the PTFE, explain these two kinds of polymer compatibility is good. Japan develop a PTFE/PFA (soluble PTFE) material, made ball valve sealing material, it not only improves durability but also products easy to hot welding. Abroad with PTFE and polyurethane block copolymer (FPV) making artificial heart blood and blood circulation of elastic material, this material is not only meet the durability, flexibility, strength, and molding requirements, but also in contact with the blood will not make bad blood, also won’t destroy blood components, the most important thing is not to form blood clots.

Polyphenylene Sulfide Modified PTFE Blend Materials(PTFE/PPS) has excellent creep resistance and dimensional stability. The blending material, PPS mass fraction is 20% ~ 40% commonly, in order to improve the mechanical properties of blend material, usually also filled with other inorganic materials, such as glass fiber, carbon fiber, etc.

PTFE emulsion used for metal spraying, the base material has poor adhesion and membrane is soft. In order to improve these properties, usually with other high polymer materials, such as phenolic resin, polypropylene resin, epoxy resin, polyimide, PPS, etc.) And its blending, can make the modified PTFE coating film hardness, abrasion resistance, and make the film at low temperature. for example, the coating of PTFE and PPS blending has strong adhesion to the metal, heat resistance and corrosion resistance, they can withstand oil, acid, can be used as the heat protective coating of organic matter, inorganic salt chemical industry equipment, can also be used on the cooker, pastry mould, rubber mould and film coating.

Nanomaterials are new materials with excellent performance which develop in recent years, it has good plasticity and toughness, its hardness and strength is 4 ~ 5 times higher than ordinary liquid crystal materials. It is essential to the wear and tear of PTFE in macromolecular slip or fracture under the action of external force, so that the material is pulling on the surface of the crystalline region and the slice to accidentally, caused by adhesive wear. After filling A12O3 in the PTFE, the hardness, compression strength, elastic modulus and creep performance can been improved to varying degrees, and this is because in its contact with the metal surface, packing plays a role of bearing .PTFE macromolecular implicated by filling at the same time, unlike in the filled PTFE as easily, and obviously improve the wear resistance.

It is important to note that although the nano Al2O3 can improve the wear resistance of PTFE, it can lead to severe plastic deformation; and not the content of nano filler is higher, the wear resistance of filled PTFE composite material is better. He Chunxia used to measure the filled PTFE wear mass loss and the relationship between load data; When the mass fraction of nano Al2O3 is 10%, material performance is better. So only in adding the right amount of nano Al2O3 in substrate, can make the hardness and strength of the PTFE increased, can not only improve the wear resistance of PTFE, but also will not lead to severe plastic deformation.

Expectation
Modified PTFE material is a promising new type of PTFE, should vigorously develop, and make its commercialization, variety series.

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pdfResearch progress of PTFE modification.pdf

Hydrophilic Modification of Teflon Hollow Fiber Membrane by Ar Plasma

Teflon (PTFE) because of its excellent chemical stability of high temperature resistance, known as the “plastic king”, it is often used as a membrane separation material. But because the material molecular structure is highly symmetrical, causing its surface hydrophobicity to be strong, this affects Teflon in the bonding and liquid filtration and other applications. This question had been troubled Teflon coated fabric suppliers for many years. In order to make up for these shortcomings, the major Teflon coated suppliers in the world spent a huge investment on the PTFE membrane modified.

At present, the major suppliers commonly used chemical method, high temperature method and plasma method and other methods to deal with PTFE membrane surface. But the surface of the Teflon film treated by the chemical method is obviously darkened and the ontology of the material is affected. PTFE will evaporate toxic substances.in high temperature melting process. And the plasma method has the effect of changing the surface properties of the material in a short time without affecting the inherent properties of the substrate, and the process is dry processing, saving energy protection environment.

In this paper, Ar is used as the treatment medium to treat the Teflon membrane, the grafted AA monomer further enhances its hydrophilicity to address the timeliness of plasma treatment. The effects of different plasma treatments and grafting processes on the hydrophilicity of Teflon films were investigated, and the contact angle was used to characterize the hydrophilicity of FTIR. The solution and the corresponding results in this paper, there have a huge reference value for the suppliers of Teflon coated fabric, Teflon adhesive tape and Teflon conveyor belt.

Experiment
Major reagents and equipment
Teflon hollow fiber membrane, Lab homemade; Argon (purity ≥99.99%), Acrylic acid, anhydrous ethanol (AR).
DSA25S contact angle measuring instrument, Germany Kruss company; BTF-1200C-S-SL plasma processing system; IR Affinity-1 Fourier transform infrared spectrometer.

Sample preparation
Ar plasma pretreatment

The Teflon membrane was ultrasonically cleaned with absolute ethanol for 30 min, after drying, put it in the reaction chamber of the quartz stent, Vacuum to 2 ~ 6Pa, through argon, draining the residual air in the reaction chamber, adjust the needle to control the gas flow in 10~60cm3·min-1, adjust the pressure to 20 ~ 100Pa, After stabilization, start the RF power source, the Teflon film was pretreated by plasma, processing power of 100 ~ 500W, processing time of 30 ~ 300s.

Surface graft polymerization
After the plasma pretreatment of the sample with air contact oxidation for a period of time, put in the concentration of 5% to 45% of the AA solution, heated to 3070, the processing time is 214h, After the reaction, remove it and wash it in 40 distilled water for 12 hours.
Performance test and structural characterization
Contact angle
The contact angle of the PTFE film surface was measured using a contact angle meter, and the measurement was carried out at a room temperature and humidity of 30% RH, using micro-injectors to control the droplets in about 2μL, the average of the five points measured on the same sample surface is taken as the final contact angle.

ATR-FTIR
The change of membrane surface structure before and after modification was analyzed by attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), ATR-FTIR analysis of the scanning frequency is 30 times, scanning range is 600 ~ 4500cm-1, the minimum resolution is 2cm-1.

Results and discussion
Effect of Plasma Treatment Conditions on Surface Wettability of Teflon Membrane
By testing the plasma processing power time and the gas flow rate of PTFE membrane surface contact angle, explore the best conditions for plasma treatment.

Effect of Plasma Treatment Power on Surface Wettability of PTFE Membrane

The effect of the contact angle of the film surface on the treated power is shown in Table 1, Membrane surface wettability with power changes shown in Figure 1.

Table1 Effect of power in Ar plasma on PTFE membrane contact angle
Table1
Fig.1
Fig.1 Teflon membrane wettability change with different plasma powers

As can be seen from Table 1, The contact angle of the surface of the Teflon membrane is 125°. When the power increases from 0W to 300W, the contact angle decreases from 125°to 65°. When the power increases from 300W to 500W, the contact angle becomes 58°.As the power increases, the contact angle of the membrane surface initially decreases rapidly and then slowly and stabilizes. Figure 1 shows that the wettability of the membrane with the power increase gradually improved, and ultimately no longer significantly changed.

The reason is that the level of power represents the amount of energy that can be accepted on the surface of the membrane, and the power increases so that the energy obtained by the quantitative argon molecules also increases, the chemical bond of the membrane surface molecules is increased by the probability of being opened, and the number of hydrophilic groups formed by further combining the free radicals in the plasma increases, thereby reducing the contact angle of the film surface. However, when the power is too large, the plasma generated free radicals between the probability of binding will increase, can not effectively combine the free surface of the membrane surface.
Considering, the best processing power is 300W.
Effect of Plasma Treatment Time on Surface Wettability of Teflon Membrane
By changing the treatment time, to study its effect on the film surface wettability, the results shown in Figure 2
Fig. 2
Fig. 2 Teflon membrane contact angle change with different plasma times
It can be seen from Fig. 2 that the contact angle of the membrane surface is different when the processing time is different, and the contact angle increases gradually with time, and then begins to rise and stabilize after a minimum value. The contact angle decreases rapidly in a short time and reaches a minimum of 52°at 120 s, and the contact angle increases after more than 120 s.
The reason may be related to the mechanism of plasma reaction. As shown in Figure 3. When the time is short, the active particles in the Ar plasma bombard the membrane surface, resulting in the membrane surface of the C-F bond fracture, generating peroxides, These free radicals formed by the destruction of the membrane matrix recombine with other free radicals in the plasma, forming a polar group on the membrane surface, thereby significantly improving the wettability of the film surface. When the time reaches a certain limit, the free radicals generated on the membrane surface also reach a critical value, and then extended the time will increase the probability of the new generation of free radicals between the cross-linking reaction until the membrane surface polarity The rate of increase to achieve dynamic equilibrium, contact angle is no longer significantly increased.
Fig. 3
Fig. 3 The principle of plasma treatment on the hydrophilic modification of PTFE membrane

According to the above, both to ensure that the hydrophilic modification effect can improve the experimental efficiency of the best processing time is 120s.

Effect of Plasma Flow Rate on Surface Wettability of PTFE Membrane
Table 2 Effect of different plasma gas flows on PTFE membrane contact angle
Table 2
The effect of the gas flow on the film surface wettability was investigated by changing the gas flow rate. The results are shown in Table 2.

It can be seen from Table 2 that when the gas flow rate is 30cm3·min-1, the contact angle of the membrane surface is at least 54°, which is 61° lower than that of the original film. When the flow rate is increased to 50cm3·min- The contact angle becomes 65°.
As the gas flow increases, the contact angle of the membrane surface decreases first. This is because as the gas flow increases, the concentration of argon ions in the plasma generator increases rapidly, but also accelerates the rate of regeneration of the ionized gas, Resulting in enhanced deflagration on the membrane surface, thus generating more free radicals, a large amount of hydrophilic groups are polymerized on the surface of the film so that the contact angle after treatment decreases first. But when the gas flow exceeds 40cm3·min-1, the contact angle of the membrane surface increases. This is due to the increase in gas flow, resulting in reduced vacuum in the plasma generator, shortening the activity of high-energy particles free path, and thus can not fully stimulate the gas molecules in the reactor.

Effect of Grafting Conditions on Surface Wettability of Teflon Membrane
Effect of Acrylic Acid Concentration on Surface Wettability of Teflon Membrane
The concentration of AA monomer has a great effect on the wettability of the membrane surface, as shown in Fig 4.
Fig. 4
Fig. 4 PTFE membrane contact angle change with different AA volume concentrations

It can be seen from Fig. 4 that the wettability of the membrane surface increases with the increase of AA concentration, and the contact angle decreases to 52° when the AA concentration reaches 20%. This is because when the AA concentration is low, the solution viscosity in the reactor is low, the monomer is easy to approach the free radicals on the membrane surface, which can lead to the graft polymerization. When the AA concentration continues to increase, the contact angle is slightly increased. This is because when the AA concentration is too high, the AA monomer self-aggregation or homopolymerization rate is accelerated, so that the monomer can not react with the free radicals on the membrane surface.
Fig. 5
Fig. 5 Infrared spectrogram of Teflon membrane with different AA volume concentrations

The ATR-FTIR test was performed on the membrane surface. The results are shown in Fig.5. It can be seen from Fig. 5 that the plasma treated membranes have peaks that characterize AA grafts, and the positions and shapes of the peaks are basically the same. When the AA concentration of 20%, the measured bands appear rich in bands near 3300 cm-1, should be -OH stretching vibration peak, the band at 2800-3000 cm-1 is the stretching vibration of saturated C-H, near 1730cm-1 appear the C = O stretching vibration peak, 1149cm-1 and 1210cm-1 are C-F stretching vibration peaks. These characteristic peaks demonstrate that AA is successfully grafted onto the membrane surface.

Effect of Grafting Reaction Time and Temperature on Surface Wettability of PTFE Membrane

The effect of graft reaction time and temperature on the contact angle of the membrane surface is shown in Fig.6.
Fig. 6
Fig. 6 Effect of different grafting times and temperatures on PTFE membrane wettability

As can be seen from Figure 6, As the temperature increases, the contact angle of the membrane surface gradually decreases, and when the temperature is 50 ℃, the minimum contact angle is 58°. This is because the temperature rise makes the free radicals on the membrane surface and the AA monomer in the solution activated, the contact angle in the early reaction with the temperature rise and decreased. When the temperature is more than 50 ℃, the contact angle be increased. May be caused by high temperature self-polymerization of AA monomer, so that the solution becomes viscous and hinder the grafting rate.

Conclusion
The results of this paper show that, Teflon hollow fiber membrane by Ar plasma treatment and then grafted AA monomer, the number of hydrophilic groups on the membrane surface increased significantly, surface energy and surface activity increased, the hydrophilicity is greatly improved and the modification effect is long, Thus broadening the scope of application of Teflon hollow fiber membrane for membrane separation systems in the field of waste water treatment.

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Friction and wear properties of different nano materials mixed with graphite filled PTFE composite material

Polytetrafluoroethylene (PTFE) has excellent heat resistance, corrosion resistance and good electrical insulating properties, it is a kind of important matrix composite materials for sliding friction parts. But due to low hardness, poor wear resistance of pure PTFE, the modification of PT FE in recent decades to a lot of researches, found in adding graphite, PTFE Mo S2, copper powder, glass fiber, carbon fiber, etc., can significantly improve the strength, hardness and wear resistance, etc. Many studies have shown that mixed filling materials than single filler material to improve the friction and wear properties of PT FE is better. Graphite is a good solid lubrication materials, filling in the PTFE graphite can further improve their tribological properties, graphite filled PT FE has become more widely in engineering application. Nanometer materials is the excellent performance of new materials developed in recent years, due to the nanometer material has good plasticity and toughness, its strength and hardness is higher than ordinary coarse grain materials 4 ~ 5 times. Current research on modification of polymer with nano materials much attention, in recent years the domestic existing unit to carry out this work. The author of SiO2, TiO2, Al2 O3 three different nano materials mixed with graphite filled PTFE composite material has carried on the friction and wear performance test, comparing their tribological performance, they wear mechanisms are also discussed.

Test method
The PTFE average particle size is 50 microns, graphite’s particle size is 1. 5 ~ 30 microns, the performance of three kinds nanometer materials of SiO2, TiO2, Al2 O3 are shown in table 1. The three kinds of nanometer materials respectively by 10% and 5% mass fraction of graphite join the PTFE, according to the following process to make different nanometer materials filled PTFE composite material specimen:
PTFE+ nano material – graphite→ mechanical stirring blending→compression molding→blank products→sawing→polish→specimen
Table 1 The performance of the nano materials
Table 1
M M – 200 type friction and wear testing machine is adopted to improve the friction and wear test, the rotating speed of 200 r/min, the conditions of dry friction sliding, couple of 45 # steel ring, the surface roughness Ra to 0.08 ~0.12 microns. Wear mass loss of samples are measured under different loading (with one over ten thousand of the photoelectric analytical balance), and the coefficient of friction. Use JMS – 6300 scanning electron microscope to observe and analyze (SEM) of nanometer mixed with graphite filler composites wear surface morphology, and adopt Brinell hardness tester to measure hardness, load is 62. 5 N, loading time is 60 s.

Test results and discussion
Wear performance
Figure 1 shows the three kinds of nanometer materials and their mixed with graphite filler PT FE and the grinding quality of the pure PTFE under different load loss. Can be seen from figure 1, filled PTFE composites wear mass loss is much smaller than the pure PTFE, nanometer materials and their mixed with graphite filler PT FE wear resistance than pure graphite composite mixed filling PTFE composite material is much better than that of nanometer materials and their mixed with graphite filler PTFE composite material with high wear resistance, which can effectively improve the wear resistance of PTFE. Also suggests that  single graphite filler of PTFE to improve the wear resistance of the result is bad, are not equal to its single nanometer material of PTFE mixed with graphite filled PTFE composite material, may be the graphite as nanometer materials can have the effect of effective support load, and graphite played a good lubricant and can form transfer film on the friction process to reduce wear materials, to make the nanometer materials and their mixed with graphite filler of PT FE composite material has good abrasion resistance. Can be seen from the figure 1 (b), three kinds of material mixed with graphite filler to PT FE abrasion resistance is different, the influence of the nanometer SiO2 nano TiO2 – graphite – graphite, the two materials on wear resistance of PTFE increased more, especially nano SiO2 – very effective to improve the wear resistance of PTFE and graphite materials and Al2 O3 – graphite material with nano SiO2 – graphite, TiO2 – graphite material, compared to the improve the wear resistance of PTFE is relatively less, and also can be seen from the figure 1 (a), a single nanometer SiO2 is very effective to improve the wear resistance of PTFE, but trials found that SiO2 / PTFE composite materials prone to cracking phenomenon, the reason is worth further exploration in the future.
Figure 1
Figure 1 The relationship between wear mass loss and the load of nanometer materials and its mixed with graphite filler material filling PTFE.

Figure 1 also shows that PTFE and three kinds of nano and its mixed with graphite filled PTFE wear mass loss increase with the increase of load, PTFE wear mass loss increased faster, and the wear mass loss of filled PTFE composites increase more gently, and three kinds of nanometer materials – graphite filling is far smaller than the pure PTFE composite materials, nano – graphite filled PTFE material can improve the wear resistance. That the essence of wear about PTFE under the action of external force, macromolecular chain will slip or rupture, so that the material is pulled out of the land of crystalline region and the slice on the surface of the transferred to accidentally, caused by adhesive wear. Due to the hardness of PTFE and the shear strength is lower than metal, with the steel mill, PTFE wear mainly in itself. After PTFE filled with nano – graphite composite material, its hardness, compressive strength, elastic modulus and creep performance are improved, when its contact with the metal surface, packing plays a role of bearing load, PTFE macromolecular implicated by padding at the same time, it is not as easy as in the filled PTFE relief, and obviously improve the wear resistance.

Figure 2 is the rigidity of PTFE and nano graphite filled composite materials, we can find that, nano materials and nano – graphite filled PTFE material can improve the hardness, the hardness of PTFE composites increase will make its wear resistance also increase. Nano material filling and its corresponding nano – hardness of mixed graphite filled PTFE composites were similar, three kinds of nano – graphite filler material, nano – SiO2 – graphite materials to improve the hardness of PT FE is more, so its corresponding wear ability is good, and nano – PT FE Al2 O3 – graphite composite materials is small, therefore, the hardness of the PTFE composite material has smaller, wear ability is poor, which further shows that nano – graphite is mixed filling materials have played a role in bearing load.
Figure 2
Figure 2 PTFE, nanometer material and its hardness with graphite filled PTFE composite material

Figure 3 shows three different nanometer materials and graphite filled PTFE samples under two load worn surface morphology of the SEM. It can be seen that different nano – almost the wear surface of graphite filled PTFE has different degree of furrows and plastic deformation, and furrows and plastic deformation degree and the filling material and the size of the load, the load is small (100 N), furrows and plastic deformation are relatively mild, and can see the wear and sliding in the same direction, their gall scratches relatively clear [figure 3 (a), 3 (c), 3 (e)];When load increases, furrows and plastic deformation are aggravating, especially nano Al2 O3 – graphite filled PTFE, when load increases, the sample the furrows of the SEM photos visible on the surface of the larger, and appeared surface flaking off, show the characteristics of fatigue wear. And nano SiO2 – graphite mixed filling materials, when the load increases (250 N), the surface is still visible, which is similar to those of low load and grinding crack, explain the material in the loading load range is not sensitive to loading, from the previous figure 1 also shows that load increase the added value of this kind of composite material the erosion rate is very small, showing good abrasion resistance. Also can be seen from the picture, in three groups of filling material, nano SiO2 – graphite mixed filling materials in load under the action of wear and tear, and the plough cut effect of plastic deformation is small, so the nano SiO2 – mixed graphite filled PTFE composites showed better resistance to wear, mixing of nano SiO2 – graphite filler material is one of the effective material of PTFE improve wear resistance.
Figure 3
Figure 3 SEM photographs of worn surfaces of PTFE filled with different nano – materials and graphite

Friction performance
Figure 4 shows the PTFE and different nanometer materials – graphite filler PT FE composite friction coefficient along with the change of load. It can be seen that different nano – graphite material for PTFE composite material with different friction coefficient, the influence of different nano – graphite filled PTFE composite material of the friction coefficient with the increase of load are different. Nano SiO2 – graphite filler PTFE composite materials and PTFE are basic same, friction coefficient and load increase slightly to reduce the friction coefficient, and nano TiO2 – graphite filled PTFE composites slightly bigger than a pure PTFE friction coefficient, and slightly increased along with the increase of load, nano Al2 O3 – graphite filled PTFE composite material is smaller than the pure PTFE, its relationship with load is more complicated. All in all, three kinds of nanometer materials – mixed graphite filled PTFE composite, the friction coefficient of nanometer Al2 O3 – graphite filling PTFE composite material is smaller than the others, nano TiO2 – graphite filled PTFE composite material of the friction coefficient is bigger, the corresponding relationship between the abrasion performance above, nano – SiO2 – wear resistance of graphite filled PTFE composites is much better than nano Al2 O3 – graphite filled PTFE composite material, it shows that SiO2 – graphite filler mixed filling materials for PTFE is a kind of good filling material, its wear resistance of PT FE increased significantly, and the coefficient of friction and pure PT FE are basic same. Although nano Al2 O3 – graphite mixed filling materials can reduce the friction coefficient of PT FE, to improve the wear resistance of PT FE is worse than SiO2 and TiO2 – graphite – graphite filled PTFE composite material performance.
Figure 4
Figure 4 The relationship between load and friction coefficient of different nanometer materials filling PTFE

Conclusions
(1) Nano- graphite mix packing and can improve the hardness and wear resistance of PTFE, they improve the wear resistance of the reason is because the filler material bearing load and graphite lubricant and the three kinds of nano – graphite filled PTFE composite material, mixed with SiO2 – graphite filling effect is best, its wear resistance of PTFE increased more.
(2) Different nano – graphite filled PTFE composite material is different, the friction coefficient of nano SiO2 – graphite friction coefficient of PTFE composites filled with PTFE is basically the same, the friction coefficient of nano TiO2 – graphite filled PTFE than pure PTFE increases slightly, and nano Al2 O3 – graphite filled PTFE composite material is smaller than the pure PTFE.

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Analysis of thermal expansion properties of PTFE matrix composites

PTTE (polytetrafluoroethylene) and its composites as excellent self-lubrication material both in industry and aerospace fields are widely used, the thermal expansion characteristics and linear expansion coefficient as a very important physical parameters on the products design and use effect is large. Application of PTFE material temperature range is very wide, but there are changes at room temperature, while accompanying changes in structure and size. Because PTFE material bigger than linear expansion coefficient of metal materials, and large temperature changes, it is generally only larger temperature ranges of average linear expansion coefficient. Considering the phase change process of PTFE and composites of precision mechanical structure clearance and the use of a larger impact performance, the author analyzes the pure PTFE and aramid fiber reinforced PTFE matrix composites in thermal expansion between the – 100 ℃~ + 250 ℃.

Room temperature phase transition of PTFE
PTFE material with special spiral chain structure, along the main chain of C – C plane trans position, reverse for about 17 ° Angle. Under normal pressure, there are two reversible transitions in PTFE crystals at about 19 ℃ and 30 ℃. Under 19 ℃, the repeat unit contains 13 CF2, repeat distance of 1. 68 nm, the repeat distance chain be reversed in 180 ℃, unit cell belongs to triclinic crystal system. As shown in figure 1 in 19 ℃ PTFE material part through a crystal phase diagram type transformation, in 19 ℃ above spiral slightly spread, repeat unit contains 15 CF2, repeat distance of 1. 95 nm, is within the repeat distance chain turn 180 ℃, molecular accumulation into nearly hexagonal cylinder, unit cell of hexagonal system. As shown in figure 1, PTFE material in normal temperature zone in the phase diagram to mutually Ⅱ under 19 ℃, mutually Ⅳ between 19 ~ 30 ℃, PTFE has other phase transformation under high pressure.
Figure 1
Figure 1 The partial phase diagram of PTFE material

PTFE crystals in triclinic crystal system into a crystal structure transition of hexagonal system, volume increased about 1. 2%. At 30 ℃, PTFE crystal crystallization relaxation, the helix of the chain becomes irregularly wound, when 30 ℃ volume change about 10% of the 19 ℃.Due to the crystal transformation and crystallization temperature point in common temperature range, the relaxation of almost all application condition will override this temperature range. In the process of crystal transformation and crystallization relaxation, PTFE volume changes obviously, corresponding to the application performance of PTFE and its composites have an impact.

The preparation of PTFE matrix composites
PTFE matrix composites using powder blending, cold pressing molding and sintering temperature control of preparation process. PTFE powder produced by light chemical industry research institute, the particle size of 20 ~ 40 microns, density of 2. 2 g/cm3.Enhancement for commercially available with aramid fiber, fiber diameter of 10 ~ 12 mu m, length is about 100 microns, the density of 1.45 g/cm3. Strengthening agent quality percentage adopts 15% and 15% respectively. Reinforcing fiber with PTFE matrix after mechanical mixture, under pressure from 60 M P a pressure maintaining 10 min molding, then in sintering furnace control of sintering temperature, sintering temperature of 380 ℃, 300 min sintering time, l/cooling rate of 20 ℃ / M in, along with the furnace cooling. The cooled sintered specimen is machined to produce the required test piece.

Measurement and Analysis of Linear Expansion
Linear expansion coefficient of characterization of objects caused by temperature rise 1 ℃ which affects the ratio of growth and its original length. Set an initial length of an object as L0, L is the length of the increment when the object temperature T, the cable expansion coefficient alpha αL is:
fig 0

With German D IL 402 c type thermal expansion instrument measuring the linear expansion coefficient of PTFE matrix composites, measure the direction parallel to the direction of specimen molding pressure. Sample 5 mm in diameter, length of about 25 mm. Experimental analysis temperature range: – 100 ~ + 250 ℃, low temperature zone by liquid nitrogen for evaporative cooling way, after – 100 ℃ keep 3 min began to heat up. Adopted in 0 ~ + 50 ℃ temperature zone in the heating rate of 2 ℃ / m, other zones are used in the heating rate of 5 ℃ / m.

PTFE material and adding 15% and 25% respectively of aramid fiber thermal expansion of PTFE composites curves as shown in figure 2 ~ 4, respectively. Abscissa is temperature, ordinate is amount for sample linear expansion. According to the definition, the linear expansion coefficient curve 2 ~ 4 by differential and linear change corresponding linear expansion coefficient curve, as shown in figure 5 ~ 7, respectively.

From figure 2 ~ 4, aramid fiber reinforced PTFE matrix composites show almost consistent with pure PTFE material thermal expansion characteristics, below 0 ℃ and 50 ℃ above sample length increase are almost linearly with the temperature, the turning point between 0 ~ 50 ℃.After adding aramid fiber materials of the thermal expansion amount is reduced, and the reduction increases with the increase of additive content. Three kinds of material in different temperature range of the average linear expansion coefficient values as shown in table 1, in – 100 ~ 0 ℃ interval linear expansion coefficient of value only half of the 50 ~ 250 ℃ range value, within the range of the two interior expansion coefficient with the increase of the filler content reduced slightly, and within the range of 0 ~ 50 ℃ insider expansion coefficient increases slightly with increasing filler content.

Table 1 The average linear expansion coefficient of PTFE composite materials ×10 – 5K – 1
Table 1
As can be seen from figure 5 ~ 7, linear expansion of aramid fiber reinforced PTFE matrix composites curve profile consistent with that of pure PTFE material. Under 0 ℃, three kinds of linear expansion coefficient of material all showed increasing trend with the increase of temperature, linear expansion coefficient of the material is pure PTFE under 10 x 11-5 K – 1, after adding 15% and 25% of aramid fiber linear expansion coefficient are reduced to 10 x 10-5 and 9 x 10 K – 1-5 K – 1 the following, and expansion curve flattens. In 50 ~ 250 ℃ range, linear expansion coefficient of the same with the temperature increasing, the pure PTFE for linear expansion coefficient (7. 4 ~ 27. 5) x 10 – K – 1, 5 and add respectively 15% and 25% after aramid fiber linear expansion coefficient (6. 3 ~ 26. 2) x 10-5 K – 1 and 6. 3 ~ 23. (1) x 10-5 K – 1.In 0 ~ 50 ℃ range, due to containing PTFE material phase transition point, linear expansion curve changes appear several twists and turns, but the 3 kinds of materials are at 25. 5 ℃ at its highest point, near a crystallization relaxation near 30 ℃.Crystal structure transition temperature point of the three kinds of materials have some deviation, pure PTFE material appeared in the vicinity of 17 ℃ crystal structure transformation, and transformation point after adding 15% and 25% of aramid fiber are reduced to around 13 ℃.And after adding filler PTFE material general tendency of lower linear expansion coefficient, high coefficient of linear expansion of PTFE material is from low to high in turn, PTFE + 15% aramid fiber composites, PTFE + 25% of aramid fiber composite materials. In the case of phase change can achieve maximum linear expansion coefficient average linear expansion coefficient of 3 ~ 5 times.
fig
Generally, adding graphite in PTFE and inorganic fillers such as glass fiber filler does not participate in the crystallization of the PTFE molecular chains, crystal transition temperature of PTFE is not having an impact. Aramid fiber made of PTFE crystal transition temperature the temperature offset, peak temperature relaxation of crystallization temperature and expansion is essentially the same, description of aramid fiber in a certain degree of participation in the crystallization of the PTFE molecule chain, copolymer copolymer results makes grain size small, Crystal changes are more prone to swelling, and reflects the crystalline transition region line expansion coefficient becomes larger.
fig 67
Conclusion
The linear expansion coefficient of PTFE matrix composites with the temperature change is bigger, linear expansion coefficient is far less than that of low temperature zone high value. With aramid fiber as reinforcing agent, the decrease of average linear expansion coefficient, the filler content is higher, the smaller the average linear expansion coefficient. After adding filler PTFE composites compared with pure PTFE material crystal structure transition temperature drift in the direction of low temperature, crystallization temperature relaxation has not big change, but in the crystal type of filler content in the transformation process, the more the linear expansion coefficient, the greater the maximum linear expansion coefficient is 3 ~ 5 times the average linear expansion coefficient.

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PTFE PDMS/PVDF composite membrane preparation and pervaporation properties

Organic wastewater containing halogen is difficult to degrade organics, the common characteristics of these pollutants is toxic, complex components, high chemical oxygen demand, the general microbial almost no degradation effect to its, if these substances without governance to environmental emissions, would seriously pollute the environment and endanger human health. Meanwhile, halogenated organic compounds are important chemical raw material; its recovery has important economic value. Process of pervaporation (PV) is a new type of membrane separation process, it is especially suitable for the separation of the small amounts of organic pollutants in wastewater, and it has obvious technical and economic advantages. Pervaporation process play a key role is the core element of film, from the point of view of literature retrieval condition, materials used in silicone rubber polydimethylsiloxane (PDMS). Chandak prepared silica zeolite modified PDMS composite pervaporation membrane , and used for separation of chlorinated organic aqueous solution; Salehi, etc were used in pervaporation PDMS – PES crosslinked composite membrane, investigated its removal from aqueous solution, toluene and chloroform pervaporation performance; Bennett preparation containing different organic functional groups such as side chain of the modified PDMS silicon rubber membrane, with chloroform solution system testing the pervaporation performance; Tadahiro prepared PDMS-PMMA composite film by dimethyl siloxane crosslinking  methyl methacrylate (mma) , studied the pervaporation separation of VOC pervaporation properties of aqueous solution. And with strong hydrophobicity, high crystallinity and stability good polytetrafluoroethylene (PTFE) modified PDMS is relatively rare, this paper was prepared with poly (vinylidene fluoride) (PVDF) ultrafiltration membrane as the basement membrane, PTFE superfine powder filling PDMS membrane for pervaporation of active cortex (PTFE – PDMS/PVDF) composite membrane, taking chloroform solution as separation system composite membrane pervaporation performance was studied.

Experimental part
The preparation of PTFE-PDMS/PVDF composite membranes
According to take the right amount of silicon rubber PDMS, dissolved in hexane, in proportion to join the right amount of PTFE powder, TEOS crosslinking agent, mechanical stirring 2 h after ultrasonic dispersion of PTFE, join DBTL catalyst, fully stir until sticky solution, dumping on PVDF ultrafiltration membrane, extending into membrane, place 2 h at room temperature, vacuum drying oven crosslinking on 60 ℃ in the 4 h, quick PTFE – PDMS/PVDF composite films. In the process of preparation of PDMS: TEOS: DBTL, the mass ratio is 10:1:0. 5.By PTFE: PDMS  the mass ratio prepare  the five (0:20, 1.5:20, 6:20, 10:20, 15:20)  different composite membrane.SEM analyze and adopt  the Japanese JMS – 5600 LV scanning electron microscope, contact Angle measurement by Shanghai Calvin JC2000D1 type of contact Angle meter.
The Pervaporation experiments of PTFE-PDMS/PVDF composite membrane
Pervaporation separation experiment device see references. Sample analysis use UV spectrophotometer. Membrane separation factor alpha, permeate flux J and separation index PSI is the main parameter that characterization of pervaporation membrane separation performance, respectively defined as:
pic 1
Mass transfer model
Pervaporation series of low concentration organic solution mass transfer process is generally used resistance model, the component i and flux Ji are proportional to across the membrane mass transfer driving force:
pic 2
The results and discussion

SEM analysis

From figure 1 (a) can be seen, no modified silicone rubber membrane surface showed very good flatness, and no found holes and cracks. But after joining PTFE composite membrane surface is uneven structure, and a large number of PTFE particles distribution in PDMS surface, the edge between PDMS and no phase separation phenomenon, the rough surface morphology significantly increased the membrane surface and the surface hydrophobicity enhanced composite membrane (water contact Angle change as shown in figure 2), to strengthen the pervaporation performance, as shown in figure 1 (b – c).From figure 1 (c), ultrafiltration membrane surface is about 25 microns thickness of PTFE filled with PDMS active cortex.
Figure 1
Figure 1 SEM. a-PDMS membrane surface (magnification:×5000) ;b-PTFE-PDMS/PVDF membrane surface (magnification:×200) ;c-PTFE-PDMS/PVDF membrane section (magnification:×500)
Figure 2
Figure 2 The impact of PTFE content on contact angle of composite membrane

The impact of PTFE content on the composite membrane pervaporation performance
The figure 3 and figure 4 shows that filling amount of PTFE film 15:20, and to explain hydrophobic nano PTFE and PDMS compatibility are good, and the addition of PTFE significantly improves the pervaporation properties of PDMS composite membrane. Water flux showed a trend of increase gradually and far greater than chloroform; Chloroform flux, the separation factor and separate index showed a trend of decrease after the first increase. Shingjiang Jessie thought the aggregation particle filling PDMS membrane help water flux to improve. By section 3.1, PTFE particles are the state of aggregation. So its content is increased, the water flux increased, chloroform adsorption quantity increases, the flux increases; and, because of the large water impetus the chloroform water flux is greater than chloroform. when the content of PTFE more than10:20, and get a lot of PTFE will add destroy the PDMS membrane structure of continuous phase, lead to water at this time spread more likely , the water flux increases with chloroform flux decline and separation factor decrease.
Figure 3
Figure 3 The impact of PTFE content on the composite membrane flux, water flux and the flux of chloroform (feed liquid temperature 60 ℃ and permeate side pressure 2.2 mmHg, slurry concentration 660 mg L – 1, the material liquid flow 400 mL min – 1)

Figure 4
Figure 4 The impact of PTFE content on the composite membrane separation factor and the separation index (feed liquid temperature 60 ℃, permeate side pressure 2.2 mmHg, slurry concentration 660 mg L – 1, the material liquid flow 400 mL min – 1)

The impact of material liquid flow rate on pervaporation performance

The mass ratio is 10:20 PTFE PDMS/PVDF composite membrane pervaporation performance results under different flow rate as shown in figure 5, and 6.With the increase of feed liquid velocity, total flux, chloroform, water fluxes and separation factor in increased rapidly, while less than 200 mL min – 1 when the flow rate is greater than 200 mL min – 1 change is steady, the experimental results and Park using PDMS composite membrane pervaporation VOC aqueous solution, the conclusion is very similar. Due to the effect of dissolve of the composite membrane of chloroform strong, easily happened under the condition of low velocity phenomenon of concentration polarization, but with the increase of flow velocity and boundary layer thinning, concentration polarization effect weakened and is beneficial to chloroform and mass transfer process, so when the flow rate is less than 200 mL min – 1 flux and separation factor are increased with the increase of flow velocity; And when the flow rate is greater than 200 mL min – 1 flux and separation factor change steady, shows that the boundary layer and mass transfer resistance is small, the mass transfer resistance is controlled by membrane resistance.
Figure 5
Figure 5 The impact of material liquid on the composite membrane material total flow flux, the water flux and flux chloroform (150 mg L – 1 material liquid concentration, feed liquid temperature 50 ℃, permeate side pressure 2.2 mmHg)

Figure 6
Figure 6 The impact of material liquid flow on the composite membrane separation factor (150 mg · L 1 material liquid concentration, feed liquid temperature 50 ℃, permeate side pressure 2.2 mmHg)

The impact of material liquid concentration on the pervaporation performance
PTFE material liquid concentration on the mass ratio 10:20, and get – PDMS/PVDF composite membrane pervaporation performance impact is shown in figure 7, 8.With the increase of the concentration of slurry, the total flux and chloroform flux linear increase slowly, and the water flux changes smaller; Separation factor with the increase of the concentration of material liquid drop sharply. The material liquid flow 400 mL min – 1 eliminate the influence of concentration polarization (see section 3.3), the type, the water mass transfer driving force changes with the increase of concentration of chloroform, so the water flux change is very small and chloroform flux linear increase. When chloroform concentrations greater than 200 PPM, film inside a large amount of dissolved chloroform by membrane of the groups in law of organic affinity effect and make the transfer resistance increases, chloroform molecules of mass transfer rate is reduced, the separation factor is on the decline.
Figure 7
Figure 7 The impact of feed concentration on the composite membrane flux , water flux and the flux of chloroform  (feed temperature 50 ℃, permeate side pressure 2.2 mmHg, material liquid flow rate, 400 ml min – 1)

Figure 8
Figure 8 The impact of feed concentration of composite membrane separation factor (feed temperature 50 ℃, permeate side pressure 2.2 mmHg, material liquid flow rate, 400 ml min – 1)

Process analysis of mass transfer
From figure 9, 10, with the increase of feed liquid velocity, boundary layer thickness gradually thinning, Kb increase gradually, Rb gradually decreased; When the flow rate is greater than 200 mL min – 1, Kt gradually close to Km, Rt gradually close to the Rm, illustrate the Rb, chloroform and mass transfer process is controlled by the Rm. When the flow is 10 mL min – 1, Rb is Rm 29 times, chloroform and mass transfer process is controlled by Rb, thus, phenomenon of concentration polarization in chloroform plays a considerable role in the process of mass transfer, so the pervaporation chloroform solution should be carried out under high flow velocity, in order to overcome the influence of the concentration polarization. From figure 10, flow under the condition of 400 mL min – 1 Kt basic equal to Km, so in figure 11, with the increase of the concentration of chloroform in the material liquid, the chloroform Kt increases linearly, and water Kt has linear decreasing trend, namely chloroform Km linear increase, and the water Km linear decrease. This suggests that a greater influence on the concentration of chloroform in the material liquid Km, this is mainly due to the strong hydrophilic organic composite film features generated by the results.

Figure 9
Figure 9 The impact of material liquid velocity of chloroform and total mass transfer coefficient, boundary layer and mass transfer coefficient and the membrane mass transfer coefficient (150 mg · L 1 material liquid concentration, feed liquid temperature 50 ℃, permeate side pressure 2.2 mmHg)

Figure 10
Figure 10 The impact of material liquid velocity on mass transfer resistance of chloroform (150 mg · L 1 material liquid concentration, feed liquid temperature 50 ℃, permeate side pressure 2.2 mmHg)

Figure 11
Figure 11 The impact of concentration of materials on total mass transfer coefficient of chloroform and water(feed temperature 50 ℃, permeate side pressure 2.2 mmHg, material liquid flow rate, 400 ml min – 1)

Conclusion

PTFE PDMS/PVDF composite membrane was prepared, and in chloroform solution system study of pervaporation properties of the composite membrane. Increased, PTFE filler in PDMS membrane showed a trend of increasing total flux and water flux, flux and chloroform, the separation factor and separate index showed a trend of decrease after the first increase, when PTFE: PDMS is 10:20 for mass ratio, PTFE PDMS/PVDF composite membrane pervaporation performance was the best; With the increase of feed liquid velocity, total flux, chloroform, water fluxes and separation factor in increased rapidly, while less than 200 mL min – 1 when the flow rate is greater than 200 mL min – 1 change to level off; With the increase of the concentration of slurry, the total permeation flux and chloroform flux increase slowly, and the water flux changes smaller; Separation factor dramatically reduced with the increase of concentration of feed solution.

Chloroform and mass transfer process was analyzed, when the flow is 10 mL min – 1, the boundary layer resistance Rb is 29 times the Rm of the membrane resistance and mass transfer process is controlled by boundary layer resistance; And when the flow rate is greater than 200 mL min – 1, chloroform and mass transfer process is controlled by the membrane mass transfer resistance, thus, phenomenon of concentration polarization in chloroform plays a considerable role in the process of mass transfer, so the pervaporation chloroform solution should be carried out under high flow velocity, in order to overcome the influence of the concentration polarization.

Symbol explanation:
A The effective area of membrane,m2
C Molarity,mol L−1
G Quality,g
JPermeate flux,g m−2 h−1
KMass transfer coefficient,m s−1
R Mass transfer resistance,s m−1 t Operating time,h
XRaw material liquid component mole fraction
Y Penetrating fluid component mole fraction Superscript
f Raw material liquid
p penetrating fluid Subscript
b boundary layer
i component
m membrane
t total
C chloroform
W water

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pdfPTFE PDMS PVDF composite membrane preparation and pervaporation properties.pdf

Effect of Adhesive Teflon Emulsion on Properties of Gas Diffused Electrode Prepared by Ethanol Pretreatment

With manganese dioxide as catalyst, activated carbon as catalyst support, teflon as the binder of gas diffusion electrode is a three-phase reaction groove electrode interface. Mainly used in alkaline metal air batteries, for the reduction of oxygen to provide a reaction site. With the increased demand for high-power power battery, the nature of the gas diffusion electrode was also hit by the unprecedented challenges. Both require the use of gas diffusion electrode low cost, but also requires the electrode has a higher output power. Since the use of manganese dioxide as the oxygen reduction catalyst, the use of Pt, Au and other precious metals as a catalyst when the electrode cost.

In determining the catalyst, the main factor determining the performance of the gas diffusion electrode is the microstructure of the electrode. The ideal gas diffusion electrode structure on the one hand must have good gas permeability to meet the high power of the electrode when the gas reactant supply; On the other hand have a good porous system, and can form enough three-phase interface, improve the gas diffusion electrode chemical energy into electrical energy capacity. By increasing the effective porosity of gas diffusion electrode structure to increase electrochemical reaction sites become one of the method of improving electrode performance. Adding a certain amount of pore-forming agent in the electrode can increase the permeability of the electrode to the reaction gas and reduce the concentration polarization. The formation of the porous system of the gas diffusion electrode is related to the structure of the catalyst support used, Carbon nanotubes as a catalyst carrier can make the gas diffusion electrode to form a unique network structure, greatly reducing the transmission of reactive gas barriers, while increasing the electrode electrochemical reaction activation point. In addition, fiber, activated carbon, nickel fiber mixture, etc. through the sintering to form a unique highly efficient porous structure, and the thickness does not exceed 2/3 of the conventional electrode. This electrode not only has good electronic conductivity, but also has a high efficiency of electrochemical performance.

The structure of the gas diffusion electrode is divided into a gas diffusion layer and a reactive layer (catalyst layer). The gas diffusion layer mainly exercises the reaction gas transmission function, and the reactive layer is mainly the place where the electrochemical reaction is provided. teflon emulsion as adhesive of gas diffusion electrode, On the one hand with the function of adhesive electrode material, on the other hand with the microstructure of the electrode itself is closely linked. In this paper, the pretreatment of teflon is added to the production process of gas diffusion electrode, the influence of the properties of the diffusion electrode on the diffusion electrode and the microstructure characteristic parameters of the gas diffusion electrode were discussed. The effect of the teflon powder on the performance of the gas diffusion electrode was discussed.

The experiment
The manufacture of gas diffusion electrode
Production process of gas diffusion layer. The acetylene black, graphite, activated carbon by 6: 3: 4 mass ratio with 20000 r·min-1 mixer evenly mixed, a certain amount of 30% (w, mass fraction) of teflon emulsion is mixed with the corresponding amount of ethanol (anhydrous ethanol, analytically pure), and than the two mixtures were uniformly blended and applied to the side of the foamed nickel, bake in an oven at 150 ℃ for 30 min.

Production process of catalytic layer. First, acetylene black, graphite, activated carbon, manganese dioxide by 2: 7: 9: 54 mass ratio with the same speed mixer evenly mixed, then a certain amount of 30% teflon latex mixed with appropriate amount of ethanol, and than the two mixtures were uniformly blended and applied to the side of the foamed nickel, bake in an oven at 150 ℃ for 30 min.

The baked electrode is naturally cooled to room temperature, A 0.1 mm thick teflon film was attached to the gas diffusion layer side to prevent the electrolyte leakage. Finally, the electrode is rolled and the thickness is about 0.5 mm.

Gas diffusion electrode performance testing
Gas diffusion electrode effective area is 70 cm2, with zinc electrode assembled into zinc air battery, 7 mol·L-1 KOH solution as the electrolyte, at room temperature and atmospheric pressure in the air discharge performance test. Zinc-air batteries in the first 30 minutes before the start of work, zinc electrode can be considered no change. It is generally believed that the overpotential of the zinc air battery is mainly produced by the cathode, In the experiment with calomel reference electrode measurements that, when the current density in 200, mA at 2 cm – produced by zinc electrode overpotential was only about 0.02 V, compared with the overpotential of gas diffusion electrode is a very small amount, it can be ignored. When increasing battery working current density of overpotential can approximate thought is caused by a gas diffusion electrode. Compared with zinc air battery voltage value under same working current density can determine gas diffusion electrode discharge performance. Each kind of experiment to repeat more than three times at least, to ensure the reliability of the experimental results.

Through automatic microporous physical and chemical adsorption instrument (American Microm – eritics ASAP2020M + C) to a single point of gas diffusion electrode BET specific surface area, Langmuir specific surface area, pore distribution, and total pore volume and area of the test. By scanning electron microscope (SEM, American FEI, Sirion200) observe apparent morphology of electrode.

The pretreatment of ethanol
Teflon film teflon membrane immersed in ethanol solution, and then at 150 ℃ conditions, the ethanol completely volatile, observed throughout the process of teflon membrane morphology changes.

The results and discussion
Discharge performance of gas diffusion electrode
Only the diffusion layer side teflon emulsion is pretreated
From the battery discharge results (as shown in figure 1), only the gas diffusion layer of teflon adhesive emulsion after pretreatment, other things being equal, compared with the adhesive without pretreatment of electrode, the current density is 110 cm, mA – 2, gas diffusion electrode polarization overpotential reduced 23 mV. This indicates that the gas diffusion layer adhesive after ethanol pretreatment can improve the performance of the electrode, but the magnitude is very small.
Fig 1

Only the catalyst layer side teflon emulsion was pretreated
Fig.2 shows the polarization curves of the catalytic layer teflon before and after ethanol pretreatment. As can be seen from Fig. 2, only the catalytic layer of adhesive teflon emulsion after ethanol pretreatment it can also significantly improve the performance of gas diffusion electrode. At relatively low current density (less than 50 mA · cm-2), the adhesive of the catalyst layer was not significantly different from the pretreated one. When the working current density exceeds 100 mA · cm-2, the polarization overpotential of the pretreated gas diffusion electrode of teflon emulsion is obviously lower than that without pretreatment. When the working current density is about 140 mA · cm-2, compared with the conventional electrode, the catalytic layer of teflon emulsion after pretreatment of gas diffusion electrode polarization overpotential was reduced 47 mV. This shows that the relatively high current density at work, the catalytic layer adhesive after ethanol pretreatment can significantly improve the performance of gas diffusion electrode.
Fig 2

Changes of teflon Membrane Before and After Pretreatment with Ethanol
Fig.3 is a photograph of a teflon film in ethanol. As shown in fig.3, the white teflon film is translucent in ethanol and can be completely immersed in ethanol, hydrophobic teflon membranes are hydrophilic in nature. After 150 ° C baking to completely evaporate ethanol, teflon film change into the original white, and was curved (see Figure 4). After treated with ethanol, teflon membrane has a certain degree of contraction.
Fig 3

Fig 4

Discussion
Teflon film in ethanol will be water swelling, and the ethanol is completely evaporated, the teflon film will shrink and resume, but the chemical properties do not change. teflon membrane and teflon emulsion is the same kind of material of two different forms, with the same chemical properties. teflon emulsion as adhesive will swell when mixed with ethanol, and after the powdery electrode material is uniformly blended, the teflon emulsion and the electrode material form micelles. The micelles and micelles are bonded to each other to form a catalytic layer and a gas diffusion layer of the gas diffusion electrode, and then at the condition of 150 ℃ drying, ethanol and water evaporation, teflon shrinkage, to a certain extent, the gas diffusion electrode played a role in making holes. Microscopic observation on electrode proved, adhesive after pretreatment, far more than the pore structure of the gas diffusion electrode without pretreatment of electrode. Comparison of gas diffusion electrode SEM photos (see figure 5 and figure 6) can be concluded that the adhesive without pretreatment of gas diffusion electrode apparent morphology show flat shape, pore and fissure is not obvious. After pretreatment, the surface of the gas diffusion electrode catalytic layer is almost covered with fissures and pores, and particles with different sizes of particles appear.
Fig 5

Fig 6
Figure 7 is BET test results, the straight line is based on p/(Q (p0 – p)) as the ordinate and  (p/p0) as abscissa.  The BET equation can calculate the gas diffusion electrode single saturated adsorption amount, Vm and Vm characterization of specific surface area:
Fig h
Type p as adsorbate in partial pressure; P0 as the adsorbent of saturated vapor pressure; Q as actual amount adsorbed sample; The Vm as single sample saturated adsorption capacity; C as adsorption capacity related to the sample of constant, this type of value is 122.846875.

Fig 7
The calculated results show that the specific surface area of the electrode catalytic layer is 41.6962 m2 · g-1 after pretreatment; The diffusion layer was pretreated followed by 37.6578 m2 g-1; Untreated electrodes, specific surface area is 36.6563 m2 g – 1.. Figure 8 shows the Langmuir test results, with p/Q as the ordinate, p as the abscissa. By the Langmuir isotherm adsorption model equation calculates the Vm (m2 g – 1).
type 2
Fig 8

Value type b is adsorption coefficient, in this value is 3.453 x 10-4 Pa-1. After the pretreatment of the catalytic layer, the specific surface area of the electrode is the largest, which is 63.8552 m2 · g-1; after preprocessing the electrode diffusion layer, specific surface area is 57.4402 m2 g – 1; without pretreatment of electrode, specific surface area is 56.0801 m2 g – 1. This shows that the catalytic layer adhesive after ethanol pretreatment can significantly increase the specific surface of electrode, gas diffusion layer of specific surface area increased after pretreatment is not very obvious. The different diameter and contribution to the electrode surface as shown in figure 9, whether electrode after ethanol pretreatment, the aperture between 1-2 nm porous provided almost no change, specific surface area can be speculated that the number of microporous aperture is mainly related to the production of electrode materials used. The catalytic layer adhesive after alcohol pretreatment, the micropores with pore size between 2.5 and 10.0 nm increase significantly, and the specific surface area of micropores in pore size increases. The gas diffusion layer has no obvious change after pretreatment, performance compared with conventional electrodes in the aperture range of electrode surface have no obvious change. This shows that the binder after ethanol pretreatment, can increase the catalytic pore size between 2.5-10.0 nm between the number of micropores.
Fig 9

The main function of the gas diffusion layer is to adsorb and transfer the reaction gas. At the same time, the electrolyte is transferred from the catalytic layer to the diffusion layer, and a new electrochemical reaction site can be formed in the diffusion layer. Increasing the specific surface area of gas diffusion layer , on the one hand, reduce the reaction of gas transmission, on the other hand also can form more new reaction activation points, improving electrode performance. But the gas diffusion layer is thin, the thickness is about 1/10 the thickness of electrode, the electrode overall contribution to the surface is not big. So the gas diffusion layer adhesive after pretreatment, the specific surface area increases is not very obvious, showing a slight improvement in electrode discharge performance (see Fig.1).

The main function of the gas diffusion electrode catalytic layer is to provide a reaction site for electrochemical reactions (also known as reaction activation points). Generally, the number of effective reaction sites is used to define the ability of the gas diffusion electrode to convert chemical energy into electrical energy. The more effective reaction sites, the greater the total gas diffusion electrode current contributes. So in the same condition of discharge voltage, effective place more gas diffusion electrode output current. The purpose of the catalytic layer of porous gas diffusion electrode is for more effective reaction activation point. teflon adhesive emulsion after pretreatment, catalyst layers obviously increase the specific surface area and porosity, effective reaction sites will also be increased accordingly. So under the condition of high current density, polarization overpotential is lower than conventional gas diffusion electrode in the same conditions (see Fig. 2).

The best content of ethanol
As ethanol is first mixed with teflon emulsio, there should be an optimum ratio between the amount of teflon emulsion and the amount of ethanol added. Figure 10 is ethanol electrode discharge performance of teflon emulsion with different ratio. It can be seen from figure 10, teflon emulsion with ethanol ratio of 4:1, the quality of the work in the same current density of gas diffusion electrode polarization potential minimum. The reason is the amount of ethanol added is too low, teflon expansion is not sufficient, the gas diffusion electrode pore formation did not achieve the best condition; Ethanol added too much, may lead to the initial bonding between the electrode powder material is not borne by the teflon, but by the ethanol, after high temperature drying, the original particles temporarily bonded by ethanol will relax, or even fall off, to the opposite effect.
Fig 10

Conclusion
Teflon in ethanol can be water swelling, until the ethanol and water completely evaporate teflon will shrink. The teflon emulsion is pretreated when the gas diffusion electrode is made to increase the electrode pore structure and specific surface area. The catalytic layer after pretreatment, specific surface area and pore structure increase more obviously, the catalytic layer increased pore structure can increase the gas diffusion electrode electrochemical reaction effectively. When under the condition of high current density, electrode polarization potential is relatively small; the gas diffusion electrode performance improvement is relatively significant. The increase of the specific surface area and pore structure of the electrode after the pretreatment of the gas diffusion layer is not obvious, and the improvement of the electrode performance is also limited. When the ratio of the mass of the teflon emulsion to the ethanol is 4: 1, the electrode performance is the best.

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pdfEffect of Adhesive Teflon Emulsion on Properties of Gas Diffused Electrode Prepared by Ethanol Pretreatment.pdf

Study on Modified Polytetrafluoroethylene Composites Filled with Inorganic Nanoparticles

Polytetrafluoroethylene (PTFE) has excellent antifriction and self lubrication, but dimension stability is poor, the poor wear resistance, usually adopts the micron grade graphite, molybdenum disulfide, copper powder, glass fiber and other inorganic particles are filled and modify PTFE. Micron structured particle filling modification of PTFE, however, there are still some limitations, such as the contradiction between products softness, toughness and wear resistance, the improvement of the performance of the products processing is also limited.

Nanometer material is the excellent performance new material and developed quickly in recent years, it has the good physical and chemical properties which many micron-grade materials do not have, such as high particle strength, high specific surface area and high surface energy, etc., composite with plastic, will produce very strong interface function and good modification effect. This paper studies modification effect of inorganic nanometer particle on PTFE, focuses on the nanometer SiO2 and Al2O3 effect on the improvement of PTFE wear-resistance, results obtained excellent comprehensive properties of modified PTFE composites, the material is very suitable for the preparation of automobile engine crankshaft oil seal and other seals.

The experiment part
Raw materials
PTFE: JF – 4TM suspended powder, particle size of 10 ~ 50 microns. Nanometer SiO2, Al2O3:30 ~ 40 nm particle size, specific surface area is equal or greater than 20 g/m2.

Modified PTFE material production process:
1

Main instrument
Universal testing machine: UH – I, Friction and wear testing machine: the MM – 200, Shaw hardness tester model: LX – D.

The performance test
The tensile strength and elongation at break: according to HG/T 2902-1997 to test; Abrasion and friction coefficient, according to GB/T 3960-1983 to test; Shaw hardness: according to GB/T 2411-2411 to test.

Results and discussion
The selection of dispersion mode of Inorganic Nanoparticles
The most important problem of nano-materials modified polymer is the nano-materials evenly dispersed in the polymer. For ordinary polymer, dispersion method includes solution blending method, melt blending method, melt intercalation method, etc. But PTFE almost has no solvent, the melt viscosity in the molten state is also high, the above dispersion method can not be used. Therefore, in this paper, the use of mechanical mixing, ultrasonic mixing and other decentralized way. The experimental results are shown in table 1. Table 1 shows that scattered in various ways, mechanical mixing and airflow crushing combination is the best way to disperse. In mechanical and ultrasonic mix combination way of dispersion, the modification effect is changed with the change of ultrasonic intensity, when the intensity of ultrasonic is 300 mA, the modification effect is better.
Table 1
Table 1 inorganic nanoparticles dispersion effect on the properties of modified PTFE material

Dispersing the inorganic nanopowders dispersed by mechanical mixing and airflow comminution of the modified PTFE composites, the state of the blended structure was measured by atomic force microscopy (AFM), the results are shown in figure 1. Figure 1 shows that inorganic nanoparticles is basically in the form of a single nanoparticles dispersion in the modified PTFE material, It is described that the dispersion effect of the dispersion method using the mechanical mixing and the airflow pulverization combination is good; and the dispersion method has the advantages of simple operation and stability and very conducive to industrial production.
Figure 1
Figure 1 Atomic Force Microscopic Morphology of Nano – particle Modified PTFE Composites

Effect of airflow crushing process on properties of PTFE modified by Inorganic Nanoparticles
The specific conditions were experimented on the dispersion process of mechanical mixing and airflow comminution, the results are shown in Table 2 and figure 2. Table 2 results show that when the air is pulverized, the choice of feed pressure 0.50 MPa, air crushing pressure of 0.75 MPa process conditions, which is advantageous for improving the performance of the modified PTFE material. If the feed pressure, airflow crushing pressure is too low, the mixing is not sufficient; If the feed pressure, air flow crushing pressure is too high, then PTFE will have a certain degradation, will result in modified products, tensile strength and hardness decreased too fast. Figure 2 shows that the wear amount of the modified PTFE material decreases with the increase of the number of times of air flow crushing, this is because with the increase of the number of airflow pulverization, nanoparticles with PTFE mixed more evenly, the effect of the nanoparticles modified is better, reflected in the wear of PTFE composite materials to further reduce.

Table 2 Effect of Airflow Crushing Pressure on Properties of Modified PTFE Material
tab 2

Figure 2
Figure 2 Effect of the number of times of airflow crushing on wear performance of PTFE composites

Effect of Inorganic Nanoparticles on Mechanical Properties of Modified PTFE Materials
Figure 3
Figure 3 Effect of Nanometer Particle Type and Dosage on Mechanical Properties of PTFE Composites

The effect of the kind and amount of inorganic nanoparticles on the properties of modified PTFE is shown in Figure 3. As can be seen from a and b of Fig. 3, the addition of nano-SiO2 reduces the mechanical properties of PTFE, The addition of nano-Al2O3 increases the tensile strength and elongation at break of PTFE in a certain range, but with the further increase in the amount of PTFE material, the mechanical properties of PTFE materials decreased, and the decrease is smaller than that of the nano-SiO2. Therefore, filling the modified PTFE with 0 ~ 3% nano-Al2O3 is beneficial to improve the mechanical properties of PTFE composites. It can be seen from Figure 3c, Adding nano-SiO2 and nano-Al2O3 into PTFE, have significantly improved the wear resistance of PTFE materials, especially a small amount (3%) of nano-SiO2 modification, the PTFE material on the wear resistance has greatly improved, further increase the amount, the improvement in abrasion resistance is not significant. When the amount of nano-Al2O3 is more than 7%, the same effect can be achieved by nano-SiO2 modification.

Single nano-particles filling modification of PTFE material has certain limitation, it can only improve the performance of PTFE material in one aspect. In order to get excellent properties of modified PTFE material, nanometer Al2O3 and nano – SiO2 composite filling modification of PTFE experiment, the experimental results are shown in table 3. Experimental results showed that nano Al2O3 tensile strength, elongation at break of PTFE material, nano – SiO2 significantly improve the abrasion resistance of PTFE material, two aspects to improve effectively together, add 2% SiO2 and 3% nano Al2O3, the modified PTFE composite having excellent tensile properties of tensile strength of 27.4 MPa, elongation at break of 306.7%, Shao D hardness of 60.0, abrasion of 0.001 g and coefficient of friction of 0.20.

Table 3 Effect of Nano – particle Composite Filling on Properties of PTFE
Table 3

Application test
After the nano-particles modified PTFE composite materials made of automotive oil seal, the bench durability tests have been carried out. Test condition: test oil 15w / 40; Test oil temperature (120 + 3) ℃; Shaft beating 0.38 mm; Sitting holes eccentric 0.38 mm; Shaft speed 3400 r/min; Test period 120 ℃ x 20 h x 3 400 r/min + x 4 h * 0 r/min at room temperature. The test results show that There was no leaking oil in any form in 240 hours, far more than 150 hours of time required by CES10599; After the test by the demolition, the oil seal on the axis almost no wear. In the ordinary PTFE oil seal test, found that the shaft has obvious wear; From test after two main oil seal lip piece of wear, wear resistance of PTFE is better than the ordinary contrast samples. Therefore, with the experimental nano-particles modified PTFE composite made of oil seal, can better meet the needs of the automotive industry.

Conclusions
A. The inorganic nanoparticles can be uniformly dispersed in PTFE by a combination of mechanical mixing and airflow comminution.
B. The tensile strength and elongation at break of modified PTFE were improved by the addition of 0 ~ 3% nano -Al2O3; When the dosage is increased, the mechanical properties of the modified PTFE are decreased, but the rate of decrease is slower.
C. 3% of the amount of nano-SiO2 significantly improved the wear resistance of modified PTFE materials.
D. Nano-Al2O3 and nano-SiO2 composite modified PTFE, obtained a comprehensive performance of PTFE wear-resistant materials; the tensile strength of 27.4 MPa, the elongation at break 306.7%, shao (D) 60.0 hardness, abrasion quantity 0.001 g, 0.20 friction coefficient, the modified material is very suitable for the preparation of automobile engine shaft oil seal.

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pdfStudy on Modified Polytetrafluoroethylene Composites Filled with Inorganic Nanoparticles.pdf

Structure and performance of PTFE fiber

PTFE is linear polymer fluoride, it is a kind of highly symmetric nonpolar polymer compound, its structure is linear macromolecule structure, spiral and molecular chain conformation. According to the apparent color different, PTFE fiber can be divided into the brown and white fiber two kinds. Brown PTFE fiber carrier usually in spinning, the fabric is very soft, and low friction factor, widely used in machinery industry, oil free, dynamic and static anti-wear areas. White PTFE fiber is generally by membrane crack cutting method, the fiber filter material can improve filtering section, so as to improve the accuracy of filtering.

The performance of PTFE fiber

Chemical resistance
The key of “C – F” in PTFE molecules has a very high bonding energy, molecular chain appear conformation which is spiral, nonpolar and crystal structure, This makes the PTFE has very excellent chemical resistance. In addition to the strong element fluoride and fluoride medium (such as trifluoride chloride), molten alkali metals and 300 ℃ of sodium hydroxide, corrosion of PTFE almost without any chemical reagent, all of the strong acid, strong alkali, strong oxidizing agents, salt for PTFE are no effect, even in aqua regia boil, its quality and performance are also don’t have any change. For organic compounds, in addition to aromatic hydrocarbons and halide amine on the slight swelling, other organic solvent has no effect on PTFE, so is known as “plastics king”.

Thermal properties

PTFE has very excellent high and low temperature resistance, can be used in – 190 ~ 260 ℃ temperature for long time. It can not only bear the instant high temperature of up to 290 ℃, but is not brittle below -260 ℃, still can keep a deflection. So in some poor conditions, PTFE products are often used.

Mechanical properties

Because PTFE is nonpolar molecular chain, the mutual attraction between macromolecules is very small; Combined with the molecular chain is not branched chain of high rigidity, entanglement is very small, leading to poor mechanical properties of PTFE. Under long-term load, PTFE will have large creep, prone to cold flow phenomenon. But its fatigue resistance is excellent, general won’t appear permanent fatigue damage.

In addition to the above excellent performance, PTFE also has a good flame retardant performance, excellent lubricity, water repellency and electrical insulating properties, resist ultraviolet ageing resistance, etc.

pdfThe research progress of PTFE fiber preparation technology.pdf

The preparation and modification of Teflon film

Modification of Teflon film

For Teflon film which has been formed, the following methods can be used to change or increasing the some properties of thin films, in order to achieve the experiment or industrial demand.

(1) The PTFE impregnated in some metal hydroxide colloid solution, such as iron hydroxide and hydroxide tin hydroxide colloid, gel ion deposition in PTFE membrane surface, can increase the wetting Angle.

(2) Iron hydroxide colloid adsorbed to the PTFE membrane, and polymerization of acrylic acid (AA) hydrophilic monomers, hydrophilic strong PTFE microporous membrane can be obtained.

(3) With the inert gas such as argon, helium plasma processing PTFE membrane, cohesive force than before treatment increased 10 times. PTFE membrane after plasma treatment, and then AA chemical treatment, AA of strong hydrophilic is grafted to the surface of PTFE, and makes the PTFE film is of good surface infiltration characteristics.

This paper reviews laser pulse deposition, pyrolytic amorphous PTFE precipitation method and impregnated method of coated Teflon film. In addition to these three methods, method of spin coating is commonly used with Teflon film, which obtain the quickest method, it is not the limitation on the sedimentary materials without a substantial, the thickness of thin film gets below the dip coating method. Through the above analysis, in the laser pulse deposition method, using different membrane characteristics of target material to produce slightly different. Powder compacting of PTFE as a target material, the thin film in high resistivity, and high temperature of unpolarized light has high transparency; polished block of PTFE as a target material, the thin film is lower for visible light transparency, in the form of cohesion is very poor. Pyrolysis of PTFE amorphous precipitation, the thin film has many holes, but after condensation formed by light transparent film with amorphous structure, and no hole under high magnification. Method of dip coating can change the deposition of thin film microstructure, operation is easy, high cost performance is higher. These methods have their advantages and disadvantages, in practice, should according to different conditions and demand to choose different coating methods. For example, in the double liquid zoom lens experiments, choose the tantalum pentoxide zoom lens as a double liquid dielectric layer, dielectric layer outside should be coated with a layer of hydrophobic membrane. According to the above advantages and disadvantages of various coating method, considering the base is a special cylindrical, this study thinks that dip coating technique is more suitable for double liquid zoom lens.

pdfThe preparation and modification of Teflon film.pdf