PTFE Structure, Properties, Application And Modification

Polytetrafluoroethylene (PTFE), also known as [teflon" and [Plastic King", is widely used in various industries due to its high temperature resistance, corrosion resistance, solvent resistance, high insulation, biological inertness, etc. PTFE is a fluorine atom instead of Polyethylene hydrogen atom. PTFE is a kind of polyethylene hydrogen atom replaced by fluorine atom, only by the C and F two elements through the covalent combination of the polymer does not contain branched chain.

I. Structure and Properties of PTFE

1. Structural characteristics

Polytetrafluoroethylene is a homopolymer of tetrafluoroethylene, which can be produced by suspension, dispersion, emulsion and other polymerization methods. Its structure is

Polytetrafluoroethylene (PTFE) is a super strong material and is the only fluoroplastic that can be used as an engineering plastic.

2. Main Properties

The relative molecular weight of PTFE is very large, thus the size of relative molecular weight has no obvious effect on the strength, but the crystallinity has obvious effect on the rigidity, toughness, elongation and strength of PTFE products.

The density of PTFE is about 2.2g/cm3, the surface is smooth and waxy, and the contact angle to water is 114°~115°.PT-FE is usually milky white and opaque, but the quenched products have a certain degree of transparency, almost non-absorbent, with a low permeability to water vapor and nitrogen, and decreases with the increase of density.

PTFE tensile strength, elongation, elasticity, hardness, permeability, dielectric strength and molding pressure, sintering temperature and time, cooling rate and other processing conditions, because the processing conditions affect the product porosity and crystallinity. High molding pressure, sintering in the mold and cooling under pressure, can reduce the voids in the product, thus improving its mechanical strength.PTFE has a low modulus of elasticity, easy to creep. Creep is the reason why PTFE can be used in gaskets, raw material tapes, elastic tapes, etc. to act as a seal.

The hardness of PTFE is low, but can be improved by adding fillers.

PTFE's friction factor is the smallest of all solid materials, and does not change with temperature. Its static friction factor is less than the kinetic friction factor, therefore, PTFE bearings start smoothly, resistance is small, can be used as a low-speed high-load bearings, low-speed rotation without noise.

The thermal conductivity of PTFE is low, and can be improved by adding metal filler.

PTFE melting point of 327 ℃, heat distortion temperature of 50 ~ 60 ℃ (ISO R75 A method) or 130 ~ 140 ℃ (B method), the use of temperature -200 ~ 260 ℃, non-combustible. PTFE thermal stability is the highest thermoplastic, in 204 ~ 327 ℃ when the degradation of less, so do not need to heat stabilizer.

Polytetrafluoroethylene relative molecular mass is large, the low hundreds of thousands, high up to more than 10 million, generally millions (polymerization degree in 104 orders of magnitude, and polyethylene only in 103). General crystallinity of 90 to 95%, melting temperature of 327 to 342 ℃. Polytetrafluoroethylene molecule CF2 units arranged in a sawtooth shape, due to the fluorine atom radius is slightly larger than the hydrogen, so adjacent CF2 units can not be completely according to the trans cross orientation, but rather the formation of a helical twisted chain, fluorine atoms cover almost the entire surface of the polymer chain. This molecular structure explains the various properties of PTFE.

Second, the application of PTFE

PTFE's chemical resistance is the best, so it is most used in anti-corrosion materials, with a wide range of applications; PTFE's electrical properties are excellent, so it is used as an insulating material in the electronic and electrical industry; PTFE's friction factor is small, wear resistance is good, so it is used in the production of wear-resistant materials in the mechanical industry, sliding parts and seals, and so on.

PTFE is commonly used in bridges and buildings as load-bearing support seat. In addition, according to the PTFE film treatment with selective permeability, can be used as a separation material, selective transmission of gas or liquid. Its porous membrane can be used for gas-liquid separation, gas-air separation and liquid-liquid separation, can also be used for filtration of corrosive liquids. In addition to this, PTFE also has a wide range of applications in the medical, electronics, construction and other industries, such as PTFE membranes can be used as human organs, including artificial blood vessels, heart valves and so on.

1. PTFE applications in the field of 5G

FR4 copper-clad laminates commonly used in the communication industry use epoxy resin as the substrate material, but its loss is large and not suitable for high-frequency communication.

5G field of high-frequency copper cladding board requirements are low dielectric constant and low dielectric loss factor, and the 5G field has its own characteristics (microwave and millimeter wave applications) on the copper cladding board higher requirements.

Polytetrafluoroethylene resin is currently the lowest dielectric constant polymer material, its dielectric properties and dielectric loss to meet the requirements of the 5G field of communication base stations. Therefore, PTFE is gradually used in 5G, aerospace, military and other high-frequency communications, and the copper-clad laminates made of it are called high-frequency copper-clad laminates.

In addition, PTFE is often used to make semi-flexible coaxial cables, RF coaxial cables, radar antenna plates, etc. in 5G field.

2. Application of PTFE in hydrogen energy industry

In the field of hydrogen energy, PTFE is mainly used in the sealing of alkaline electrolyzer, as well as in the enhancement of proton exchange membrane in PEM fuel cells and electrolyzed water.

In alkaline electrolyzer, the sealing gasket as the main component, both sealing and insulating two major roles. Leakage is one of the most important factors affecting the life and safety of alkaline electrolyzers. The compression resilience and creep relaxation of the sealing gasket is an important index to measure the performance of the sealing gasket. Domestic alkaline electrolyzer sealing materials have been upgraded through many iterations of asbestos rubber sheet - [cloth gasket in one" diaphragm gasket - polytetrafluoroethylene (PTFE) type filled gaskets and so on. At this stage, the commonly used sealing gaskets for electrolysis tanks in China are mainly PTFE filled gaskets, which are modified by glass fiber, alumina, graphite and other reinforcing fillers, and then molded and sintered to form sealing gaskets.

In fuel cell and PEM electrolysis water, the proton exchange membrane to the development of thin, but thin homogeneous perfluorinated sulfonic acid membrane life can not meet the requirements of fuel cells and PEM electrolysis water. The fuel cell proton membranes on the market today are often composite proton exchange membranes, using ePTFE as a composite material with perfluorinated sulfonic acid membrane. Expanded polytetrafluoroethylene membrane (ePTFE) is a porous three-dimensional mesh microstructure with micrometer or submicrometer scale.

At present, the global ePTFE market by the United States Gore, Nitto Denko, Donaldson and a few other manufacturers, the domestic Pan Asia Microtransmission breakthroughs ePTFE production technology by virtue of continuous technological groping, successfully breaking the overseas monopoly. But the overall domestic ePTFE film focus on the low-end market, not much in the high-end market.

Third, PTFE modification technology

Polytetrafluoroethylene (PTFE) due to the composition of the strong fluorocarbon bond contained in the composition of good heat resistance, insulation, self-lubricating properties, as well as non-combustible, non-stick and other excellent properties, while its high temperature resistance and chemical stability and resistance to corrosion of [aqua regia" ability to obtain the [King of Plastics [It is widely used in the fields of national defense, mechanical industry and medical materials, especially in the field of tribology, so in the field of engineering plastics, PTFE has become one of the materials favored by researchers.

However, due to the shortcomings of PTFE such as low hardness, easy to wear and poor creep resistance, PTFE has been limited in practical applications, so researchers have been committed to finding an excellent way to improve the mechanical properties of PTFE without altering its own advantages, so as to expand its applications. The modification of PTFE is mainly combined with other materials to compensate for the shortcomings of PTFE, mainly including surface modification, blending modification, filling modification, and so on. The modification of PTFE is mainly to combine with other materials to compensate for the defects of PTFE itself, which mainly includes surface modification, blending modification, filling modification, of which blending modification and filling modification are mainly used in the preparation of composites, while surface chemical modification is mainly for the problem of adhesion.

1. Surface modification

PTFE due to very low surface activity and outstanding non-stick, reduce the degree of adhesion with other materials, and surface modification can not only improve its surface inertia and compatibility with the filler, but also improve the surface activity of the matrix material. PTFE surface chemical modification is currently the main method of plasma treatment, radiation treatment method and chemical solution treatment method is mainly. These methods are to remove surface fluorine ions, the more active functional groups grafted on the surface, to improve the activity of the substrate material.

Plasma modification through the high energy state of the plasma bombardment of the surface of the specimen, the energy transfer to the molecules of the surface layer of the specimen, so that the specimen undergoes thermal corrosion, crosslinking, degradation and oxidation reactions, and the surface of the specimen C-F bond and C-C bond fracture, the generation of a large number of free radicals or the introduction of certain polar groups, so as to optimize the performance of the surface of the specimen. The modification of the material surface by low-temperature plasma treatment can be categorized into plasma surface etching, plasma bonding, plasma vapor deposition, plasma liquid deposition and plasma surface grafting.

High-energy radiation can initiate graft polymerization reactions and confer some unique properties to the polymer, such as improving its hydrophilicity, biocompatibility, electrical conductivity, etc. Radiation-treated PTFE surfaces can be directly grafted with hydrophilic monomers such as acrylic acid, acrylamide, styrene and styrene/maleic anhydride to form a layer of grafted polymer that can be easily bonded, resulting in a roughened PTFE surface and a larger bonding area. Radiation grafting in the commonly used radiation sources are cobalt-60, cesium-137 and strontium-90 and other γ-rays, in addition to various types of gas pedals, such as X-ray tubes, linear gas pedals and cyclotron and so on.

PTFE can be treated with chemicals to improve its surface activity, these chemicals include sodium-naphthalene tetrahydrofuran solution, ammonia solution of sodium metal, alkali metal amalgam, iron pentacarbonyl solution, etc. Sodium-naphthalene treatment solutions are obtained by dissolving or complexing equal amounts of sodium and naphthalene in reactive ethers such as tetrahydrofuran and ethylene glycol dimethyl ether. Sodium transfers the outermost electrons to the naphthalene's empty orbitals to form anionic radicals, which then form ionic pairs with sodium and release a large amount of resonance energy; the naphthalene-based anions are then transferred to PTFE, destroying the C-F bond and causing it to shed some of the fluorine atoms on the surface, which results in the formation of a carbonized layer and a number of polar groups on the surface of PT-FE. Reactive groups such as hydroxyl, carbonyl and carboxyl groups exist on the treated PTFE surface, which improves the adhesive properties of the PTFE surface.

2. Blending modification

The basic principle of blending is the principle of similarity and solubility, so the solubility value and surface tension of the materials to be blended must be similar. Blending and modifying PTFE with other engineering plastics can achieve the purpose of complementing each other's strengths while integrating the strengths of each component, thus expanding the application fields to a certain extent. In the blend modification, PTFE can be used as both a base material and a filler to reinforce other polymers. Here we mainly introduce polyphenylene ester (POB), Polyphenylene Sulfide ( PPS ) and polyether ether ketone ( PEEK ).

POB has excellent pressure creep resistance, high hardness, and PTFE blending can make up for the shortcomings of PTFE, improve the mechanical properties of PTFE and tribological properties.

Unlike POB, PPS has excellent abrasion resistance, solvent resistance, heat resistance and ease of manufacture, etc. It is widely used in aerospace and other fields, and can also be used as a substrate for superhydrophobic coatings. PTFE, on the other hand, has the advantages of potential biocompactness, high thermal stability, high chemical inertness, low surface energy, and good self-lubricating ability, etc. The blending of PPS with PTFE is an ideal choice to improve the tribological properties of hydrophobic coatings.

Both PEEK and PTFE are common matrix materials in solid lubricated composites. Zhenjie Cai et al. prepared PTFE-modified PEEK composites and investigated the mechanical properties and wear mechanisms. When the mass fraction of PTFE micropowder was 5%, the friction coefficient was reduced from 0.35 to about 0.3, and the volume wear was minimized. The composites can be applied not only in the mechanical field but also in the medical field.

Blending modification is simpler and non-polluting than surface chemical modification, but generally only with polymer blending modification, restricting the addition of metals, ceramics, fibers and other inorganic fillers, resulting in the improvement of composite materials in terms of strength, hardness and thermal conductivity is limited, in addition to the high inertia of PTFE so that it is less compatible with other polymers, the need to treat the surface prior to the modification or in the process of modification to add some specific Before modification, it is necessary to treat the surface or add some specific ingredients in the modification process to improve the compatibility.

3. Filling modification

Filling modification of PTFE is a simple and effective way to add fillers and additives can significantly improve the mechanical properties of PTFE, especially creep and wear rate. Commonly used fillers are glass fiber, carbon fiber, graphite, molybdenum disulfide, bronze, steel and so on.

Graphite is a good solid lubricant, and filling PTFE with graphite not only significantly reduces the wear of PTFE composites, but also improves the thermal conductivity of PTFE as well as poor compression creep and other characteristics.

Molybdenum disulfide (MoS2) has a lower coefficient of friction than graphite and is stable and therefore widely used. However, the price of MoS2 is very high, tungsten disulfide (WS2 ) and MoS2 performance is not much different, but the dry friction performance of WS2 is more superior. MoS2 and WS2 in improving the mechanical properties at the same time can improve the friction stability of the composite material and abrasion resistance, and compared with the pure PTFE, the friction stability of the filler WS2 can be improved by 33.3% or so, and if composite filler, abrasion resistance can be increased by 2.3% compared with a single filler. In case of composite filling, the wear resistance can be increased by 2.3% compared with single filling.

Carbon fiber (CF) has high specific strength, high modulus, low density, excellent wear resistance and creep properties. Carbon fibers are essential for reducing creep, improving stiffness, and increasing flexibility and compression modulus. PTFE blended with carbon fiber compounds has high thermal conductivity and a low coefficient of thermal expansion. Carbon fiber is inert to strong bases and hydrofluoric acid (glass fiber is resistant to both). These parts are ideally suited for the manufacture of automotive parts such as shock absorbers.

GF is favored in the production of industrial friction materials because of its high strength, high modulus, and relatively low price, and is more widely used than CF in the field of filler modification of polymers.

The mechanical properties of potassium titanate whisker (PTW) are much better than those of commonly used GF and CF due to its unique highly ordered crystal structure. Adding PTW can greatly improve the strength and wear-resistant properties of composites, and at the same time improve the stiffness and toughness of composites, both enhancement and toughening, changing the phenomenon of improving one property but sacrificing another when modifying GF and CF in the past, as well as the chemical stability, heat insulation and wear-resistant properties are better. Although the effect of PTW filler modification is better than GF, CF, but the compatibility between PTW and the base material needs to be further improved.

Filled with bronze PTFE, this compound has excellent thermal and electrical conductivity, making it ideal for applications with extreme loads and temperatures.

Summary of PTFE

PTFE has excellent comprehensive performance, is the most widely used species of fluoroplastics, playing an increasingly important role. With the progress of science and technology, pure PTFE can not meet the market demand, so the modification of PTFE has become an inevitable trend, which is mainly based on surface modification, blending modification and filling modification. At present, China has become the main producer of PTFE, and at the same time has basically mastered the modified PTFE molding and processing methods, but compared with foreign countries, technology and product quality, there is still a big gap, so in the modified PTFE research, processing and application of different working conditions under the tribological behavior and mechanism need to be more in-depth study.