The hottest thermotropic liquid crystal polymer en

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Thermotropic liquid crystal polymer engineering plastics

liquid crystal polymer (LCP) is a kind of polymer composed of rigid molecular chains, which can appear both the fluidity of liquid and the anisotropy of physical properties of crystal (this state is called liquid crystalline state) under certain physical conditions. Liquid crystal polymers include lyotropic liquid crystal polymers (llcp), thermotropic liquid crystal polymers (TLCP) and piezotropic liquid crystal polymers

thermotropic liquid crystal polymers

thermotropic liquid crystal polymers have been developed in the laboratory with dozens of varieties, which has further increased the breadth and depth of the utilization of plastic products, but few have truly realized industrialization. Dartco of the United States first put the liquid crystal polymer of "xydar" on the market, and then several companies in the United States, Japan and other countries also successively developed liquid crystal polymers. Table 1 lists the trade names and chemical compositions of several major TLCPs that have been successfully commercialized. Table 1 some commercialized TLCP

the United States is the main producer of TLCP, accounting for about 80% of the world's total output, followed by Japan, Western Europe and Russia

performance characteristics

compared with other organic polymer materials, TLCP has a relatively unique molecular structure and thermal behavior. Its molecules are composed of rigid rod-shaped macromolecular chains, which form a liquid crystal state with both solid and liquid properties after being heated and melted or dissolved by solvents. This special phase structure of TLCP leads to the following characteristics: it has self enhancement effect; The coefficient of linear expansion is small; Excellent heat resistance; Self flame retardant; Low melt viscosity and good fluidity; Small molding shrinkage; Good chemical resistance, etc

tlcp products have great differences in performance due to different chemical structures and modification methods, but they still have many common excellent characteristics as follows

mechanical properties

because TLCP has self reinforcing characteristics, it can reach or even exceed the mechanical strength and elastic modulus of ordinary engineering plastics reinforced with glass fiber without reinforcement. TLCP also has excellent friction and wear resistance, and the creep property can be ignored

heat resistance

xydar's melting point is 421 ℃, and it begins to decompose at 560 ℃ in air and 567 ℃ in nitrogen. Its thermal deformation temperature is as high as 355 ℃, and Ekonol's thermal deformation temperature is 293 ℃. Xydar can be used continuously at -50 ℃ ~240 ℃, and still has excellent impact toughness and dimensional stability. Xydar is not affected by the melting of tin welding alloy. Ekonol can withstand 320 ℃ solder immersion for 5 minutes, and glass fiber reinforced Vectra can also withstand 260 ℃ ~280 ℃ solder immersion for 10 seconds

flame retardancy

without adding flame retardant, TLCP material has self extinguishing property to the flame, which can reach UL-94 V-0 flame retardancy, and does not drip in the flame and produce toxic smoke. The smoke density measured by xydar according to the smoke method nbs-d4 is 3~5. These are rare in plastics. They are one of the best plastics in terms of fire safety

dimensional stability

the linear expansion coefficient of TLCP in the flow direction is generally cm/cm. ℃, which is comparable to metal and one order of magnitude smaller than general plastics. Because TLCP has crystallinity in the molten state, unlike ordinary crystalline plastics, which are processed into products and have volume shrinkage when cooled, the dimensional accuracy of products is high

weather aging resistance and radiation resistance

the weather resistance of TLCP is better than that of most plastics. Xydar still maintains excellent performance after accelerating the weather aging for 4000 hours. After exposure to Vectra for 2000 hours, the performance index remains 90% - 100%. After aging at high temperature (200 ℃) for 180 days, the tensile strength and elongation remain above 50%. The performance of TLCP was not significantly degraded after 6700 hours of carbon arc accelerated ultraviolet irradiation or 10 megarads of Co60 nuclear radiation. Transparent to microwave radiation, not easy to heat

electrical properties

tlcp has higher electrical performance indicators, and the dielectric strength of small thickness is much higher than that of general engineering plastics

chemical corrosion resistance

tlcp is not corroded or affected by all industrial solvents, fuel oil, detergent with an imported amount of $66.79 million, drift agent, hot water, acid with a concentration of 90% and lye with a concentration of 50% in a wide temperature range, and stress cracking does not occur under the action of solvents. For example, xydar is immersed in 20% sulfuric acid at 50 ℃ for 11 days, its tensile strength remains 98%, and its performance remains unchanged after immersion in hot water at 82 ℃ for 4000 hours

molding processability

tlcp melt has low viscosity and good fluidity, so the molding pressure is low and the cycle is short - new product design concepts such as "green supply chain" in Europe and the United States, "low carbon transformation" and zero emission in Japan are rising, which can be processed into products with thin walls, slender and complex shapes; There is no need for mold release agent and post-treatment when processing TLCP, and because the molecules of TLCP material form a solid directional layer on the surface in contact with the metal mold, the surface of the processed workpiece is very flat and smooth

tclp material also has some shortcomings: because the orientation of TLCP material is strong in the flow direction and weak in the vertical direction, the surface of the workpiece strongly shows anisotropy; At the convergence of two strands of material in the mold cavity, because the formation of crystallization is oriented according to the welding line, its strength is reduced, so it should be considered when designing the mold; Thin shaped products have brittleness; Because the TLCP material itself is opaque, the possibility of coloring it is limited; The price is expensive and the cost is high

processing of thermotropic liquid crystal polymers

tlcp can be processed and formed by melt spinning, injection, extrusion, molding, coating and other processes. Although the processing methods are different, they all use the high orientation of molecular chains in the liquid crystal state for molding and then cooling to fix the orientation, so as to obtain high mechanical properties

liquid crystal polymer processing and molding generally do not need special equipment, and conventional polymer processing equipment can be used. However, due to the high processing temperature of liquid crystal polymer, the capacity of its heating system and the material of the equipment should be fully considered when selecting the equipment, and it must withstand a long time of high temperature baking. On the other hand, due to the rod-shaped orientation of liquid crystal molecules, increasing the aspect ratio of the die outlet is conducive to the molecular orientation and improve the mechanical properties of the material

temperature affects the viscosity of the polymer, thus affecting the flow uniformity. The processing process must ensure that the melt temperature is uniform and there is a suitable flow pattern. Too high melt temperature will lead to too intense molecular motion and loss of orientation order, which will be unfavorable; If the temperature is low, the molecular chain cannot be fully extended and the superiority of liquid crystal state will be lost. Generally, the mold temperature can be controlled at 100 ℃ ~150 ℃ lower than the melt temperature

liquid crystal polymer molding also requires a certain pressure, but the pressure and molding rate should not be too high, otherwise it will lead to uneven melt flow, product defects and increased internal stress. In injection molding, the pressure is related to the injection volume. Generally, the injection volume is 50% - 70% of the barrel volume

Application of thermotropic liquid crystal polymers

thermotropic liquid crystal polymers have excellent comprehensive properties and are widely used in chemical industry, electronic communication, military machinery, aerospace, automobile manufacturing and other fields. The main uses of TLCP are listed in Table 2. Table 2 main uses of TLCP

in addition, many unique properties of TLCP make it increasingly widely used in the plastic processing aids industry. By using its liquid crystalline property, it can be used as a nucleating agent for pet crystallization and improve the processability of PET engineering plastics; Using its high strength and high modulus characteristics, it can be made into fibers to replace glass fibers and mineral fillers, so as to reduce the wear of equipment and reduce the specific gravity of materials, or TLCP can be directly combined with other resins in situ to improve the strength and modulus and realize the high performance of general-purpose plastics; Using its high fluidity, it can be used as a flow modifier for plastics that are difficult to be processed and molded

the excellent performance of liquid crystal polymerization and automatic storage of experimental results has attracted more and more attention. Its application field has been expanding, new products have been developed, and there are more and more kinds of industrialized liquid crystal polymers, which will play a greater role in industry, science and technology and people's life in the future. (end)

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