Polytetrafluoroethylene (PTFE) has excellent properties such as good heat resistance, insulation, self-lubrication, non-flammability, and non-stickiness due to the strong fluorine-carbon bonds contained in its composition. At the same time, due to its high-temperature resistance and stable chemical properties, it can resist "aqua regia" corrosion, thus gaining the reputation of "the king of plastics." It is widely used in defense, mechanical industry, and medical materials, especially in the field of tribology. Therefore, 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 wear, and poor creep resistance, it is subject to certain restrictions in actual application and production. Therefore, researchers have been committed to finding an excellent method to improve its mechanical properties without changing the advantages of PTFE itself, thereby expanding its application field. The modification of PTFE is mainly to combine with other materials to compensate for the defects of PTFE itself, mainly including surface modification, blending modification and filling modification. Among them, blending modification and filling modification are mainly used in the preparation of composite materials, while surface chemical modification is mainly aimed at bonding problems.
PVDF and PTFE
1. Surface modification
PTFE has a very low surface activity and outstanding non-stickiness, which reduces the degree of adhesion with other materials. Surface modification can not only improve its surface inertness and compatibility with fillers but also improve the surface activity of the matrix material. The current main methods for chemical modification of PTFE surface are plasma treatment, radiation treatment, and chemical solution treatment. These methods are to remove surface fluoride ions and graft highly active functional groups on the surface to achieve the purpose of improving the activity of the matrix material.
Plasma modification bombards the surface of the sample with high-energy plasma, transfers energy to the molecules on the surface of the sample, causes thermal etching, crosslinking, degradation, and oxidation reactions on the sample, and causes the C-F bond and C-C bond on the surface of the sample to break, generate a large number of free radicals or introduce certain polar groups, thereby optimizing the performance of the sample surface. The modification of the material surface by low-temperature plasma treatment can be divided into plasma surface etching, plasma bonding, plasma vapor deposition, plasma liquid deposition, and plasma surface grafting.
High-energy radiation can trigger graft polymerization and give the polymer some unique properties, such as improving its hydrophilicity, biocompatibility, conductivity, etc. The radiation-treated PTFE surface 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 is easy to bond, making the PTFE surface rough and increasing the bonding area. Commonly used radiation sources in radiation grafting include gamma rays such as cobalt-60, cesium-137, and strontium-90, as well as various types of accelerators such as X-ray tubes, linear accelerators, and cyclotrons.
PTFE can be treated with chemicals to improve its surface activity. These chemicals include sodium-naphthalene tetrahydrofuran solution, ammonia solution of metallic sodium, alkali metal amalgam, pentacarbonyl iron solution, etc. The sodium-naphthalene treatment solution is obtained by dissolving or complexing equal amounts of sodium and naphthalene in active ethers such as tetrahydrofuran and ethylene glycol dimethyl ether. Sodium transfers the outermost electrons to the empty orbit of naphthalene to form anion free radicals, which then form ion pairs with sodium and release a large amount of resonance energy; then the naphthalene anions are transferred to PTFE, destroying the C-F bond and removing some fluorine atoms on the surface, thus forming a carbonized layer and some polar groups on the PT-FE surface. There are active groups such as hydroxyl, carbonyl, and carboxyl on the surface of the treated PTFE, which improves the bonding properties of the PTFE surface
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2. Blending modification
The basic principle of blending is the principle of like dissolves like, so the solubility value and surface tension of the blended materials must be similar. Blending PTFE with other engineering plastics can achieve the purpose of complementary advantages while integrating the strengths of each component, thereby expanding the application field to a certain extent. In blending modification, PTFE can be used as both a matrix material and a filler to reinforce other polymers. Here, we mainly introduce polyphenylene ester (POB), polyphenylene sulfide (PPS), and polyetheretherketone (PEEK).
POB has excellent compressive creep resistance and high hardness. Blending with PTFE can make up for the shortcomings of PTFE and improve the mechanical and tribological properties of PTFE.
Unlike POB, PPS has excellent wear resistance, solvent resistance, heat resistance, and easy manufacturing. It is widely used in aerospace and other fields. It can also be used as a matrix for superhydrophobic coatings. PTFE has the advantages of potential density, high thermal stability, high chemical inertness, low surface energy, and good self-lubricating ability. Blending PPS with PTFE is an ideal choice for improving the tribological properties of hydrophobic coatings.
PEEK and PTFE are both common matrix materials in solid lubricating composites. Cai Zhenjie et al. prepared PTFE-modified PEEK composites and studied the mechanical properties and wear mechanisms. When the mass fraction of PTFE micro powder is 5%, the friction coefficient is reduced from 0.35 to about 0.3, and the volume wear is minimized. This composite material can be used not only in the mechanical field but also in the medical field.
Blending modification is simpler and pollution-free than surface chemical modification. Still, it is generally only modified with polymers, which limits the addition of inorganic fillers such as metals, ceramics, and fibers, resulting in limited performance in improving the strength, hardness, and thermal conductivity of composite materials. In addition, the high inertness of PTFE makes it less compatible with other polymers. The surface needs to be treated before modification, or some specific components need to be added during the modification process to improve compatibility.
3. Filling modification
Filling modified PTFE is a simple and effective method. Adding fillers and additives can significantly improve the mechanical properties of PTFE, especially creep and wear rate. Commonly used fillers include glass fiber, carbon fiber, graphite, molybdenum disulfide, bronze, steel, etc.
Graphite is a good solid lubricant. Filling graphite in PTFE can not only greatly reduce the wear of PTFE composites but also improve the thermal conductivity and poor compression creep of PTFE.
Molybdenum disulfide (MoS2) has a lower friction coefficient than graphite and is stable in nature, so it is widely used. However, the price of MoS2 is very high. The performance of tungsten disulfide (WS2) is similar to that of MoS2, but the dry friction performance of WS2 is superior. MoS2 and WS2 can both improve the friction stability and wear resistance of composite materials while improving mechanical properties. Compared with pure PTFE, the friction stability of WS2 filling can be improved by about 33.3%. If the composite filling is used, the wear resistance can be improved by 2.3% compared with the single filling.
Carbon fiber (CF) has a high specific strength, high modulus, low density, excellent wear resistance, and creep properties. Carbon fiber is essential for reducing creep, increasing hardness, increasing flexibility, and compression modulus.
Polytetrafluoroethylene mixed with carbon fiber compounds has high thermal conductivity and low thermal expansion coefficient. Carbon fiber is inert to strong alkali and hydrofluoric acid (glass fiber can tolerate these two acids). These parts are very suitable for manufacturing automotive parts such as shock ab sorbers.
GF has always been favored in the production of industrial friction materials due to its high strength, high modulus and relatively low price, and is more widely used than CF in the field of polymer filling and modification.
Potassium titanate whisker (PTW) has much better mechanical properties than commonly used GF, CF, etc., due to its unique, highly ordered crystal structure. The addition of PTW can greatly improve the strength and wear resistance of the composite material while also improving the stiffness and toughness of the composite material, both strengthening and toughening, changing the previous phenomenon of improving one property while sacrificing another when modifying GF and CF. It also has stable chemical properties, good thermal insulation, and wear resistance. However, the filling and modification effect of PTW is better than GF and CF, and the compatibility between PTW and the matrix material needs to be further improved.
PTFE filled with bronze, this compound has excellent thermal conductivity and electrical conductivity, making it very suitable for applications with extreme loads and temperatures.
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