Thermotropic liquid crystal polymers are also called thermotropic polymers. During the heating process, the polymer can be transformed from solid (crystalline or glassy) to liquid crystal at a certain temperature. Liquid crystal polymer (LCP) is a polymer substance composed of rigid molecular chains, which can have both the fluidity of liquid and the anisotropic state of crystal physical properties (this state is called liquid crystal state) under certain physical conditions. Liquid crystal polymers are divided into three categories: lyotropic liquid crystal polymers (LLCP), thermotropic liquid crystal polymers (TLCP) and pressure-tropic liquid crystal polymers.

Introduction

Thermotropic liquid crystal polymers were first developed after the American Eastman Kodak Company discovered that PET modified p-hydroxybenzoic acid (PHB/PET) showed thermotropic liquid crystals in 1976. It was not until the mid-to-late 1980s that it entered the practical stage. The American Dartco Company first put the "Xydar" liquid crystal polymer on the market, and then several companies in the United States and Japan also developed liquid crystal polymers. Liquid crystal polymers are increasingly valued by various countries for their excellent comprehensive properties in heat, electricity, mechanics and chemistry. Their products are introduced into various high-tech applications and are known as super engineering plastics.

Performance of Thermotropic Liquid Crystal Polymers

Compared with other organic polymer materials, liquid crystal polymers have a unique molecular structure and thermal behavior. Their molecules are composed of rigid rod-shaped macromolecular chains. After being melted by heat or dissolved by solvents, they form a liquid crystal state with both solid and liquid properties. This special phase structure of liquid crystal polymers leads to the following characteristics: self-reinforcement effect; small linear expansion coefficient; excellent heat resistance; self-flame retardant; low melt viscosity and good fluidity; small molding shrinkage; good chemical resistance, etc.

Due to different chemical structures and modification methods, the performance of liquid crystal polymer products varies greatly, but they still have many common excellent characteristics.

1. High strength, high modulus and other excellent mechanical properties

Due to the self-reinforcement characteristics of liquid crystal polymers, they can reach or even exceed the mechanical strength and elastic modulus level of ordinary engineering plastics reinforced with tens of percent glass fiber without reinforcement, and the latter is even higher after being reinforced with glass fiber or carbon fiber, reaching an abnormally high level. Liquid crystal polymers also have excellent friction and wear properties, and creep properties are negligible.

2. Outstanding heat resistance

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

3. Excellent flame retardancy

Without adding flame retardants, thermotropic liquid crystal polymer materials are self-extinguishing to flames, and can reach UL-94V-0 level flame retardancy. They do not drip in flames and do not produce toxic smoke. Xydar has a smoke density of 3 to 5 measured by the smoke method NBS-D4. These are rare in plastics, and they are one of the best plastics for safety.

4. Good weathering and radiation resistance, transparent to microwaves

The weathering resistance of liquid crystal polymers is better than that of most plastics. Xydar maintains excellent performance after 4000 hours of accelerated weathering. Vectra maintains 90-100% of its performance indicators after 2000 hours of weathering irradiation, and maintains more than 50% of its tensile strength and elongation after 180 days of high temperature (200буC) aging. Liquid crystal polymers are not significantly reduced in performance after 6700 hours of carbon arc accelerated ultraviolet irradiation or 10 megarads of Co60 nuclear radiation. They are transparent to microwave radiation and not easy to heat up.

6. Excellent electrical properties

Liquid crystal polymer has high electrical performance indicators, and its dielectric strength is much higher than that of general engineering plastics when its thickness is small.

7. Excellent molding performance

Liquid crystal polymer has low melt viscosity and good fluidity, so the molding pressure is low and the cycle is short. It can be processed into thin-walled, slender and complex-shaped products; no release agent or post-treatment is required when processing liquid crystal polymer, and because the molecules of liquid crystal polymer material form a solid oriented layer on the surface in contact with the metal mold, the surface of the processed workpiece is very flat and smooth.

Disadvantages

Since the orientation of thermotropic liquid crystal polymer materials is strong in the flow direction but weak in the vertical direction, the surface of the workpiece shows strong anisotropy;

1. At the confluence of two strands of materials in the mold cavity, the strength is reduced because the formation of crystals is in the direction of the welding line, so the design of the mold is to take this point into full consideration;

2. Thin molded products are brittle;

3. Since the thermotropic liquid crystal polymer material itself is opaque, the possibility of coloring it is limited;

4. The price is relatively expensive, so using it will increase the cost.

Application

Thermotropic liquid crystal polymer is a high-performance special engineering plastic that came out in the early 1980s. Due to its excellent comprehensive properties, it has been rapidly and widely used in the fields of chemical industry, electronic communications, military machinery, aerospace, automobile manufacturing, etc.

Development Trends

Development trends of thermotropic liquid crystal polymers:

First, expand the scale of production and develop cheap monomers to produce thermotropic liquid crystal polymers to reduce the production cost and sales price of resins;

Second, through copolymerization modification, such as introducing bending structures and asymmetric structures in the macromolecular chain, develop thermotropic liquid crystal polymer resins with better comprehensive performance;

Third, in order to further improve the performance of thermotropic liquid crystal polymers, the use of reinforcement and filling modifications can not only suppress the anisotropy of thermotropic liquid crystal polymers, improve the strength at high temperatures, but also give them some special functions, expand the application field, and reduce costs and improve market competitiveness;

Fourth, thermotropic liquid crystal polymers are blended and modified with thermoplastics, especially with high-performance, difficult-to-process special engineering plastics, which can improve the molding and processing performance of difficult-to-process engineering plastics, improve mechanical properties, reduce the linear expansion coefficient of thermoplastics, and thus improve their dimensional stability. At the same time, it can also improve the wear resistance of thermotropic liquid crystal polymers and overcome the anisotropy of thermotropic liquid crystal polymers.

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