Overview of Types, Processing, and Applications of Liquid Crystal Polymer LCP Gold Plated Copper Films

Compared with other ordinary organic polymers, LCP has a unique one-dimensional or two-dimensional remote molecular orientation, which is compatible with the characteristics of both polymers and liquid crystals, making it have excellent properties such as high heat resistance, high modulus, low melt viscosity, extremely small thermal expansion coefficient, low dielectric loss, and high strength. Its development is extremely rapid.

Scientists continuously improve the synthesis and process parameters to promote the continuous development of LCP polymer engineering and polymer chemistry, while also optimizing its performance and cost. DuPont, Celanese, PolyGram, Pruitt, Waters, and Gigabyte have over 30 products in LCP resin, fiber, film, and other fields.

These products are widely used in fields such as 5G communication, plugins, switches, relays, fiber optic cable structural components, composite materials, robotic arms, pump/valve components, functional components, etc., continuously promoting the development of LCP technology and related industry technologies.

At present, significant progress has been made in the performance research and application development of LCP materials, but there is still a lack of literature that systematically discusses LCP. This article provides an overview of the classification, fields, research status at home and abroad of LCP materials, and prospects for future development trends.

Classification of Liquid Crystal Polymers

Most liquid crystal compounds are composed of rod-shaped molecules, and their molecular structures have two characteristics:

According to the formation conditions of LCP, it can be divided into lyotropic LCP and thermally induced LCP. Dissolved LCP appears in a liquid crystal state in a certain concentration of solvent, while thermally induced LCP appears in a liquid crystal state at a certain temperature. According to its chemical structure, LCP can be divided into seven types: main chain type, side chain type, shell type, composite main and side chain type, mesh type, bowl type, and star type. According to the spatial arrangement of liquid crystal molecules, they can be divided into nematic phase, discotic columnar phase, smectic phase, and cholesteric phase.

The three classification methods intersect with each other, and the main chain LCP can include lyotropic LCP or thermally induced LCP, while the thermally induced LCP can also include main chain LCP or side chain LCP.

1. Lysogenic LCP

Liquid crystal polymers contain semi-rigid chain polymers, which produce a liquid crystal phase in a suitable solvent and within a certain concentration range. There are two common types of lyotropic LCP, one is the biological lyotropic LCP, such as peptides, cellulose, DNA, etc; Another type is synthetic lyotropic LCP, such as polyarylamide LCP and polyarylheterocyclic LCP.

Under the rigidity of molecular chains and strong intermolecular attraction, LCP fibers with one-dimensional orientation of the main chain exhibit excellent properties such as high strength, high modulus, high heat resistance, radiation resistance, and aging resistance. They are widely used in the high-performance fiber industry.

Liquid crystal polymer solutions have low viscosity and low energy consumption for film formation and spinning. In 1972, Dupont Corporation in the United States commercialized the "king of fibers" aramid (fully aromatic polyamide, Kevlar), which has excellent properties such as high strength, high modulus, and high temperature resistance. It has been widely used in bulletproof vests, tires, aircraft structures, and other fields. On this basis, He et al. further developed p-phenylbenzothiazole (PBZT) and poly (p-phenylbenzothiazole) fibers with higher temperature resistance, as shown in the structure diagram.

Polyaromatic heterocyclic dissolution induced LCP model

2 Heat induced LCP

Thermally induced LCP lags behind solution induced LCP and belongs to special engineering plastics. It has excellent properties such as higher mechanical strength, lower melt viscosity, lower thermal expansion coefficient, and low dielectric loss. It can not only be made into high-strength and high modulus fibers, but also be used for injection molding/extrusion processing of precision castings. During the melting process, thermally induced LCP is prone to molecular chain orientation, resulting in the formation of some microfiber structures that give the material a morphology and properties similar to fiber-reinforced materials. Therefore, it is also known as "self reinforcing plastics" and has made rapid progress in industry. Its representative product is fully aromatic polyester.

There are roughly three representative structures of typical thermally induced LCP, as shown in the figure. Among them, the melting points of Type I LCP, Type II LCP, and Type III LCP are 285~355 ℃, 180~260 ℃, and 64~215 ℃, respectively.

The main synthetic monomers of type I LCP are para hydroxybenzoic acid, terephthalic acid, and 4,4 '- hydroquinone, which have good heat resistance but poor processability. The main commercial products include Solvay Advanced Polymers' Xydar series and Sumitomo's Ekonol series.

Class II LCP monomers are 6-hydroxy-2-naphthoic acid and p-hydroxybenzoic acid. The "side step" effect generated by the naphthalene ring reduces the rigidity of the molecular chain segment, and the heat resistance and processability are between Class I and Class III. The main commercialized products are the Vectra series from Polyplastics.

Class III LCP monomers are copolymers of ethylene terephthalate and p-hydroxybenzoic acid. Due to the presence of aliphatic structures in the main chain and an increase in flexible segments, significant decomposition and hydrolysis phenomena occur at high temperatures. They have poor temperature and moisture resistance, but good processability. The main commercial product is the Rodrun series from Unitika company.

• Processing technology of liquid crystal polymer

As an anisotropic polymer material, LCP has processing advantages such as good flowability and low molding pressure, and is compatible with traditional molding processes such as injection molding, extrusion, and wire drawing. The products prepared have excellent properties such as high tensile strength and good toughness.

1. Extrusion molding process

Extrusion molding plays an important role in plastic processing and is one of the main methods for polymer processing and molding. In 1845, Richard Brooman and Hen ⁃ ry Bewley from England successfully developed the world's first plunger extruder, and in 1876, William Kiel and John Prior from the United States successfully developed the first single screw extruder. After a century and a half of development, over 60% of plastic products are produced through extrusion molding.

LCP extrusion molding has also attracted widespread attention. Tchoudakov et al.'s research results show that increasing processing temperature or shear rate will increase the electrical resistivity of LCP/carbon black; Filipea et al. found that the elastic modulus and comprehensive viscosity of LCP/PP decrease continuously with the advancement of the screw. Zhang et al.'s research shows that nano clay fillers can significantly improve the compatibility of LCP nylon 6 composite materials.

2 Injection molding process

Injection molding has the advantages of precise device size, high feasibility of producing complex structural products, high degree of automation, and short cycle. It is an important plastic product processing method and has wide applications in fields such as automobiles, electronic appliances, and healthcare.

The injection molding process includes complex steps such as mold closing, injection, pressure holding, cooling, mold opening, and ejection. Products are prone to defects such as shrinkage, silver lines, warping, bubbles, and melt cracks. However, with the continuous efforts of technical personnel, new equipment and technologies such as micro hole injection molding, nano injection molding, electromagnetic injection molding, foam injection molding, gas assisted injection molding, and vibration injection molding have emerged one after another. The micro hole foam injection equipment shown in the figure can effectively improve the lightweighting and functionalization of materials.

Chen et al. studied the effect of glass fiber and LCP on the number of cycles of polypropylene reinforced composites through injection molding. The results showed that after multiple cycles of injection molding, the tensile strength of glass fiber reinforced composites decreased by 50% after three cycles of injection molding, while the reinforcement performance of LCP did not show significant changes. Li et al. found that less than 10% LCP can increase the tensile strength and impact strength of PA by 17.7% and 45.5%, respectively.

3 Fiber Forming Process

Liquid crystal polymer fibers are divided into two categories: lyotropic LCP fibers and thermally induced LCP fibers. Soluble LCP fibers have excellent properties such as chemical corrosion resistance, weather aging resistance, and radiation resistance, and are widely used in military, aerospace, civilian, and other fields, such as DuPont's industrialized aromatic polyamide (Kevlar) in 1972 and Toyobo's industrialized poly (p-phenylene benzobisimidazole) fiber (Zylon) in 1998.

At the same time, thermally induced LCP fibers have also developed rapidly, showing excellent performance in UV resistance, dye compatibility, mechanical strength, wear resistance, and other aspects. They have been applied in aerospace, heavy ships, special ropes, and other fields, such as Kuraray's VectranHT fiber launched in 2008, and research on high thermal shell LCP fibers. In addition, in order to optimize the drawing process of LCP or further composite different materials, the blending fiber process has emerged, as shown in the figure.

Kim et al. studied the effect of LCP on the properties of poly (2,6-naphthalenediol ester) fibers, and the results showed that introducing a small amount of LCP can greatly improve the thermal stability of the fibers. At the same time, parameters such as tensile ratio, textile speed, and heating method also have a significant impact on the mechanical properties of the fibers. Choi et al. studied the effect of LCP on the properties of polytrimethylene terephthalate fibers, and the results showed that LCP has good compatibility with polytrimethylene terephthalate. As the LCP content increases, the tensile strength/modulus of the fibers continuously increases.

• Application areas and research status of LCP

Application and current status of LCP in the field of communication

In recent years, with the rapid development of electronic industries such as mobile data communication, industrial automation, and aerospace, the data traffic carried by the Internet of Things has been increasing, which has put forward further requirements for related electronic equipment and basic materials. As a printed circuit board (PCB) that carries information transmission, the challenges it faces are gradually upgrading from 4G MHz to 5G GHz, and then to higher frequencies in the future, constantly developing towards high-frequency, high-speed, and digital directions.

Research has shown that in order to ensure high-speed information transmission and low latency, in addition to requiring low copper foil transmission loss, PCB substrate materials are also required to have low dielectric transmission loss (TLD). The relationship between dielectric transmission loss and frequency, board dielectric constant (Dk), and board dielectric loss (Df) is shown in the equation. It can be seen that the decrease in Dk and Df of the board can effectively reduce the transmission loss of the medium and ensure high-speed signal transmission.