China's high-strength and high-mode PI fiber preparation technology is leading the world, and follow-up research focuses on three aspects

As a leader in polymer materials, polyimide (PI) enjoys the reputation
of "problem solver" and "the most promising polymer material of the 21st century".
Following films, engineering plastics, resins, etc.
, PI fiber was also introduced to the market in the late 1980s and widely used, but the mechanical properties are not high, and they use their high temperature resistance and flame retardant properties to prepare high temperature filter bags, flame retardant fire suits, etc
.
Because of the strong designability of its structure, the wide range of monomer sources, the relatively large regulatory space of performance, and the high rigidity structure, it also indicates that it may achieve high strength and high mode
.
Therefore, a lot of work has been done by domestic and foreign scientists in terms of structural regulation and mechanical property improvement, but there has been no breakthrough in the industrialization of high-strength and high-mode PI fibers and products to the market
.
Until the past five years, Jiangsu Xiannuo New Material Technology Co.
, Ltd.
products have gradually been accepted by users, on February 1, 2021, it led the drafting of the national standard GB/T 39027-2020 "high-strength high-mold PI filament" officially implemented, marking the high-strength high-model PI fiber is different from the heat-resistant PI fiber, has an independent product standard
.

High-strength high-mode PI fiber is a new type of high-performance organic fiber, with outstanding mechanical properties, high and low temperature resistance, aging resistance and low moisture absorption, low dielectric power, high insulation and other performance characteristics, is the most comprehensive performance, the greatest development potential of high-performance organic fiber, can play an important role in aerospace, national defense and military industry, transportation, power electronics and other fields, is one of
the key materials for typical military and civilian use.
However, due to the short industrialization and commercialization time, it is still in the initial stage of development, and its development still revolves around fiber performance improvement, large-scale preparation and cost reduction, application technology development, market development and so on
.
The development status and new progress of application in recent years deserve attention
.

Research progress

High-strength and high-mode PI fiber not only has the characteristics of high-strength and high-mold high-performance organic fibers, but also inherits the excellent performance
of PI materials such as high and low temperature, radiation resistance, low dielectric and flame retardant.
Its long-term use temperature can reach 300 °C, 5% thermal decomposition temperature is above 530 °C, and it will not be brittle
in liquid helium at -269 °C.
Its dielectric constant is about 3.
4~3.
6, and has wide-frequency transmission performance; the limit oxygen index is 38%~50%, which is a flame-retardant self-extinguishing material
.

After decades of development, domestic and foreign scholars have done a lot of work
in preparation methods, structural design and optimization.
In terms of preparation, a one-step method and a two-step method are
formed.
The one-step method is to prepare PI fibers by spraying, solidification, washing, drawing and other processes using soluble PI solution as spinning liquid
.
The spinning process of this method is easy to control, and through structural and process optimization, the tensile strength and modulus of the prepared high-strength high-mold fibers reach 3.
0GPa and 130GPa
, respectively.
However, most PI solubility is poor, and obtaining soluble PI requires special monomer and structural design, which makes large-scale preparation difficult
.
Therefore, the one-step method is currently only in the laboratory study
.
The two-step method is to use the precursor of PI - polyamide acid solution as the spinning fluid, in the spinning process need to add a step of high temperature cyclization, increasing the complexity of the spinning process, thereby affecting the performance and stability of the fiber, but the monomer source is wide, is conducive to the structural design and cost control of the fiber, so most domestic researchers are using this method, the mechanical properties are also continuously improved
。 The research team of Beijing University of Chemical Technology uses molecular simulation technology to analyze and predict the performance of PI in multiple dimensions such as atoms, molecular chains and even polymer condensed matter, to guide the molecular structure design of high-strength and high-mode PI fibers, and through a large number of experimental work such as copolymerization and process optimization, the fiber strength and modulus prepared by the laboratory are much higher than those of PI fibers
prepared by one-step method.

Industrialization process

Although many scientific research institutions at home and abroad have carried out research on high-strength and high-mold PI fibers, there is relatively little industrialization information, and there are no typical successful application cases
.
Research in Russia and the United States was relatively early, but did not achieve engineering and industrialization
.
According to reports, Japan's IST company has carried out research on high-strength high-mode PI fibers, and the tensile strength of laboratory-prepared fibers has reached 3.
0GPa, and is currently being engineered
.

China in the preparation technology of high-strength high-mode PI fiber is in a leading state, Beijing University of Chemical Technology and Jiangsu Xiannuo New Material Technology Co.
, Ltd.
cooperation for ten years, focusing on the research and preparation of high-strength high-mode PI fiber, the development of integrated continuous preparation technology with independent intellectual property rights, the sequence of breakthroughs in engineering and industrialization of key technologies, independent design and construction of the first ten tons and 100 tons of high-strength high-mode PI fiber integrated continuous preparation production line, the development of high-strength and high-mode two series of products
。 Among them, the tensile strength of high-strength products can reach more than 4.
0GPa, the modulus is about 120GPa, and the tensile strength and modulus of high-modulus products can reach 3.
5GPa and 160GPa respectively, and according to user requirements and structural design, the product is customized
.
In 2016 and 2022, through the appraisal of scientific and technological achievements, the overall technology reached the international leading level
.

Application of new advances

As a new type of high-performance organic fiber, high-strength and high-mold PI fiber has no typical application cases and application research data and experience to refer to and support in the
early stage.
In recent years, with the cultivation and excavation of the market, as well as the advancement of application research and application verification, the application scenarios of high-strength and high-mode PI fibers have become more clear, and the performance of fibers has also been recognized by users, and is gradually playing a role
in aviation, aerospace, bulletproof, electronics and high-end manufacturing.

In the field of aviation structural materials, high-strength and high-mode PI fibers can be used to make secondary structural materials for large aircraft, such as cabin doors, windows, wings, fairing surfaces, etc.
, due to the advantages of light weight, high strength and high mold, impact resistance, wear resistance, etc.
, and can also make in-flight ceilings, bulkheads, etc
.
The use of high-strength high-mode PI fiber mixed with carbon fiber composite material for aircraft engine containment can significantly improve the weight and containment of
fan receivers.
In the aerospace field, the use of high-strength high-mode PI fiber excellent creep resistance, radiation resistance and thermal stability, its woven rope can be used for space tether, truss mesh expandable antenna, etc.
, woven can be used in solar wings, antennas, solar cell arrays and other flexible or semi-rigid lightweight structures, to meet the service capacity requirements
of wide temperature range and long radiation environment.
For some applications that withstand extreme high and low temperatures, such as space, rocket engines, etc.
, high-strength and high-mode PI fibers can also be used as a reinforcement layer for temperature-resistant optical cables, forming structural support for cable cores to ensure uniform and reliable
when the optical cables are stressed.
High-strength high-mode PI fibers have a strong strain rate effect, that is, with the increase of strain rate, its fracture strength is significantly improved
.
At the same time, its high strength, high toughness and moderate modulus can meet the requirements of high ballistic efficacy, and has the advantages of high and low temperature resistance and environmental aging, so it has a good application prospect
in the fields of bulletproof helmets, body armor, bulletproof armor and so on.
High-strength high-mode PI fiber has the characteristics of low dielectric power, compared with quartz glass fiber, has better mechanical properties and lower density, can prepare structural transmission composites, used in ships, aircraft radar radome, in order to achieve effective weight reduction while significantly improving the comprehensive performance
of radome.
The composite material prepared by high-strength high-mode PI fiber and epoxy resin can reduce weight by more than 20% compared with glass fiber composites, and has excellent broadband transmission performance, with an average transmittance rate of more than
95%.
Using the characteristics of high strength and high mold PI fiber reinforced composite material with high impact toughness, the composite cylinder made of composite material can reduce the performance loss caused by the crack propagation of the composite material, make up for the defect of insufficient toughness of carbon fiber reinforced composite materials, and is expected to obtain important applications
in oxygen storage cylinders, aerospace pressure vessels, fuel cell hydrogen storage cylinders and other equipment.

Future outlook

In recent years, China's high-strength and high-mode PI fiber production technology, production scale and product performance have made great progress, the product has gradually been recognized by various application fields, performance advantages gradually appeared, but there is still a certain gap
from large-scale application.
Therefore, follow-up research should be carried out from the following three aspects:

First, through the molecular structure design and spinning process optimization to further improve fiber performance to meet the needs of different application areas;

The second is to strengthen the development of downstream products and their performance research in different application scenarios, solid basic data, and further expand the application field of high-strength and high-mode PI fibers;

The third is to expand the scale of production, reduce costs, promote industrialization development, and make high-strength and high-mold PI fibers play a greater role
.