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Research progress on lyotropic aramid liquid crystals

Research progress of lyotropic aramid liquid crystal Research Progress of Lyotropic Aramid Liquid Crystal Abstract: This article gives a brief introduction to the varieties, synthe…

Research progress of lyotropic aramid liquid crystal

Research Progress of Lyotropic Aramid Liquid Crystal

Abstract: This article gives a brief introduction to the varieties, synthesis and processing methods of aramid fiber, and makes a simple comparison of different varieties of aramid fiber and the different synthesis and processing methods of aramid fiber. Made a brief analysis of the external development situation and put forward some views on the development prospects of aramid fiber.

Keywords: aramid, liquid crystal, synthesis, processing, fiber, development

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Progress in Study on Lyotropic Liquid Crystal Aramid

Abstract: This paper introduces several kind of aramids,and their synthesis and processing methods.This paper compares these methods,and also analyzes the development of aramids in our country and overseas.Then we put forward our views on progress in aramid.

Keyword: Aramid, Liquid Cristal, Synthesis, Manufacture, Fiber, Progress

I. Introduction

Aramid is a new high-tech synthetic material and is the general name for aromatic polyamide. Compared with ordinary polyamide materials such as nylon 6 and nylon 66, the relatively soft carbon chains on the molecular chain are replaced by rigid benzene ring structures. The structural characteristics of aromatic polyacyl materials show lyotropic liquid crystallinity and are an important main chain polymer liquid crystal. Therefore, aramid fiber has excellent properties such as ultra-high strength, high modulus and high temperature resistance. Aramid fiber has been widely used in national defense industry, aerospace, electromechanical, construction, automobiles, sporting goods and other aspects of the national economy. Such as aramid body armor, helmets, spacecraft, aircraft and other base materials, etc. It is estimated that aramid products are used in body armor, helmets, etc., accounting for about 7 to 8%; aerospace materials and sports materials account for about 40%; tire frame materials, conveyor belt materials, etc. account for about 20%, and High-strength ropes and other aspects account for about 13%.

2. Naming of aramid fiber

Since the late 1960s, a variety of aramid fibers have been available and industrially produced. There are many types of aramid fibers, and there are many ways to classify them.

The first naming method is divided into para-aramid and meta-aramid based on structure. The monomers of para-aramid are terephthalic acid and para-phenylenediamine. The functional groups on the monomers are para-position. The chain segments obtained by polymerization are relatively regular, have good high temperature resistance, strength and high modulus. Para-aramid is mainly represented by DuPont’s Kevlar series products. The monomers of meta-aramid are isophthalic acid and meta-phenylenediamine. The functional group on the monomer is meta. The chain segments obtained by polymerization are in a zigzag shape and are resistant to high temperatures, but their strength and modulus are relatively low. Meta-aramid is mainly represented by DuPont’s Nomex series products.

The second naming method is divided according to the structure. For example, the para position is the 14th position on the benzene ring, and the meta position is the 13th position on the benzene ring. For example, the polymerized monomer of aramid 14 is para-aminobenzoic acid, and aramid 1414 is the aforementioned. The para-aramid fiber mentioned above, aramid fiber 1313 is the meta-aramid fiber mentioned above.

The third naming method is based on the type of polymerized monomer. As mentioned earlier, aramid 14 is also called aramid type I, and aramid 1414 and aramid 1313 are also called aramid type II. When adding third unit monomers such as 4,4′-diaminodiphenyl ether, 5(6)-amino- Aramid fibers obtained from 2-(4-aminophenyl)benzimidazole and the like can be called aramid type III. When the third unit monomer has a heterocyclic structure, it is often called heterocyclic aramid.

3. Preparation process of aramid fiber

1. Synthesis method of aramid resin

1.1. Low temperature solution polymerization method

It is obtained by reacting aromatic diamines and aromatic diacyl chlorides in polar solvents. The polycondensation reaction is carried out at low temperature (usually 0-5℃), and alkali or alkaline earth metal salts are added as co-solvents to improve the stability of the resin in the solvent. Solubility and stability, after concentration adjustment, this solution can be directly used for wet spinning, beating and film making. It has the advantages of less solvent consumption and high production efficiency. Currently, medium-temperature solution polycondensation is widely used in production.

In industrial research, there are only a few commercialized monomers that can be selected. Technicians’ research on low-temperature solution polymerization mainly focuses on the selection of reaction solvents and co-solvents. The solvents used in the initial reaction are mainly amides, such as hexamethylphosphoramide, diethylformamide, N-alkylpyrrolidone, etc. [1]. As the degree of polymerization increases, the solubility of the polymer in the solvent decreases and it will precipitate from the solvent, which is detrimental to product quality and subsequent processing, so it is necessary to add a cosolvent. The co-solvent can be alkali or alkaline earth metal salts such as calcium chloride, lithium chloride, etc., and the maximum addition amount can reach 5% of the solvent amount [2]. You can also choose quaternary ammonium chloride, such as methyltri-n-butyl chloride. Ammonium chloride【3】. Because the price of quaternary ammonium chloride is high, calcium chloride and lithium chloride are often used in industrial production. In comparison, the solubilizing effect of calcium chloride is slightly worse than that of lithium chloride, and the dosage is larger, but the price is cheaper. .

Because of some shortcomings of amide compounds themselves: for example, hexamethylphosphoramide is carcinogenic.It was not until 2004 that this technology was broken through. Yantai Spandex Co., Ltd. took the lead in realizing the industrial production of meta-aramid in China. my country’s research on para-aramid fiber began in 1972. In 2007, the 100 t/a para-aramid polymer preparation pilot research project of Shanghai Aimaida Company and Donghua University passed the appraisal organized by the China Textile Industry Association. In 2008, Yizheng The 100-ton para-aramid fiber pilot projects of Chemical Fiber Group Corporation and Yantai Spandex Co., Ltd. have also passed expert appraisals. At present, several thousand-ton para-aramid fiber production lines have been completed in China and will be put into production in 2011. In 1977, the Shanghai Synthetic Resin Research Institute also conducted research on polyparaben (aramid 14), and completed pilot testing in 1981. However, the rapid development of aramid 1313 and aramid 1414 made people more interested in aramid 14. weaken.

The prospects of aramid products are so promising that Chinese scientific researchers and engineering technicians have devoted themselves to the research of aramid products. In the past two years, domestic aramid technology has improved significantly, and many manufacturers have successively made breakthroughs and launched aramid projects.

Domestic aramid product manufacturers

At present, aramid 1313 in type II aramid is mainly used in insulating paper and laminates for electrical equipment, high-temperature flue gas filtration, automobiles, transportation, and special protective clothing and protective equipment. As people’s safety awareness increases, the demand for various protective equipment is increasing. Meta-aramid is a permanently flame-retardant fiber that does not burn, drip, or stick to the skin after burning. Especially when encountering a sudden strong temperature of 900℃-1500℃, the cloth surface will quickly carbonize and thicken, forming a unique thermal insulation barrier with outstanding fire protection effect. It is especially suitable as fire-fighting clothing, high-temperature work clothing, etc. and insulating materials. Aramid 1414 has the characteristics of high strength, high elastic modulus and heat resistance. It has important applications in military, aerospace, industry, IT and other industries, especially in body armor and bulletproof armor in the military industry.

With the demand for high-functional materials, the demand for aramid fiber will increase to a considerable extent, and the research on aramid fiber products will be further deepened. Especially after the localization of spandex aramid in Yantai, Shandong, it broke the monopoly of aramid production by large foreign companies. In recent years, with the improvement of domestic research levels and technical capabilities, many companies have the ability to produce aramid. It is possible for aristocrats in the aramid material industry to join ordinary people.

In terms of industrialization, because the number of commercialized aramid monomers is limited. There are currently four commonly used monomers for aramid II type. Aramid III type mostly adds heterocyclic diamine or 3,4′-diaminodiphenyl ether as the third monomer. Basically, the raw materials have been fixed, and spinning Most of the equipment is complete sets produced by specialized manufacturers. Therefore, research focuses on the selection of reaction solvents and co-solvents, as well as the optimization of reaction conditions and processing parameters, in order to obtain products with better quality and suitable cost.

In terms of theoretical research, the selection of comonomers becomes more important, how to obtain aramid fibers with better solubility and easier processing. In particular, all current aramid products produce lyotropic liquid crystals, and the processing methods are relatively monotonous. Can thermotropic aramid liquid crystals be developed? Aramid can be melt-spun, extruded, injection molded, etc., which makes the processing methods more diverse.

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Author: clsrich