Carbon fiber is a fiber material composed of graphite with a carbon content of more than 90%, which is prepared from organic fiber precursors such as polyacrylonitrile fiber, viscose fiber, and asphalt fiber through reaction processes such as pre-oxidation, carbonization and graphitization, sizing, and heat treatment. In the carbonization process, carbon fiber is pre-stretched along the axial direction, so graphite crystallites have a preferential orientation to endow carbon fiber with the characteristics of high strength and high modulus. In addition, carbon fiber also has excellent characteristics such as low density, ultra-high temperature resistance, corrosion resistance, atomic radiation resistance, thermal conductivity, and electrical conductivity, making carbon fiber an important reinforcing fiber in composite materials. Carbon fiber composite materials are formed by the combination of resin, metal or ceramics as the matrix and carbon fiber as the reinforcement, and through the optimal design of the matrix type, carbon fiber content and distribution, the components with excellent comprehensive performance can be prepared for aircraft, rockets, satellites and other aerospace vehicles.
With the wide application of carbon fiber and its composite materials in aerospace, automotive, civil construction, sports equipment and other fields, its demand is also growing steadily. China from the mid-70s of the 20th century began to research and develop carbon fiber localization technology, after nearly 50 years of development, China's carbon fiber engineering technology and industrialization construction has made a decisive breakthrough, domestic carbon fiber and its composite materials have been widely used in China's construction industry, sporting goods, shipbuilding industry, aerospace industry and advanced fields.
Standardization of carbon fibers and composite materials: an example in the field of building materials
The basic physical and chemical properties of carbon fiber and matrix are the basis for the research and development, product preparation and application of carbon fiber composite materials, and a variety of additives will be used in the preparation process of carbon fiber, matrix and composite materials in order to obtain specific excellent properties. The composition, structure and properties of carbon fiber composites are the decisive factors in the structure-performance relationship in the research and development process of composite materials. Therefore, the composition analysis, structural characterization and performance analysis of carbon fiber composite materials are indispensable key links in the product preparation process, and the quality control and healthy and sustainable development of carbon fiber composite products depend on the relevant standards in the product application field.
The status quo of carbon fiber standardization
In the current standards in China, standard test methods for carbon fiber carbon content, density, metal element content, diameter, root number, cross-section and other standard test methods have been established for the analysis and structural characterization of carbon fiber. According to the performance analysis of carbon fiber, the determination methods of thermal stability, volume resistivity, tensile properties of multifilament and tensile strength of nodules were established. For polyacrylonitrile carbon fiber, relevant standards for the determination of precursor structure and morphology have been formulated. The above standards generally cover the analysis of carbon fiber composition and some of the thermal, electrical and mechanical properties.
However, the microstructure that affects the properties of carbon fibers, such as the degree of graphitization, internal defects, aggregate structure, etc., as well as the thermal conductivity and thermal expansion coefficient of carbon fibers, and other performance test standard methods have not yet been established.
Standardization status of carbon fiber composite materials for building materials
Carbon fiber reinforced concrete.
At present, the relevant international standards for carbon fiber reinforced concrete include the "ASTM C1116 Standard Specification for Fiber Reinforced Concrete" and "ASTM C1609 Standard Test Method for Flexural Properties of Fiber Reinforced Concrete" issued by the American Society for Testing and Materials, and the existing "GB T 16309-1996 Terminology of Fiber Reinforced Cement and Its Products" in China, and the industry standard "CJJ T 280-2018 Technical Standard for Fiber Reinforced Composite Reinforced Concrete Bridges" and "JG T 348— 2011 Fiber Reinforced Concrete Decorative Wall Panels. The carbon fiber content, mechanical properties, permeability and electrical conductivity of carbon fiber reinforced concrete composites are the key factors affecting their performance as functional materials, and there are no relevant standards and test methods at present.
Carbon fiber reinforced resin.
The resin content, uniformity, volatile content and mechanical properties of carbon fiber reinforced plastics are the key indicators affecting the performance of composite materials. For the composition analysis of carbon fiber reinforced plastics, standard test methods for resin content, pore content and fiber volume content have been established in China.
As an intermediate material of composite materials, carbon fiber prepreg is a prepreg sheet product made by impregnating carbon fiber in a resin matrix, and the matrix used mainly includes polyester resin, epoxy resin, thermoplastic resin, etc. For prepreg resins with carbon fiber as reinforcing materials and thermosetting resin as matrix, a standard test method for performance determination was established, including the determination of resin content, gel time, resin fluidity, volatile content, tensile strength, mass per unit area, etc. For epoxy resin carbon fiber prepreg, China has established product specifications.
In the molding process of carbon fiber reinforced resin composites, there is an interface layer between the carbon fiber and the resin matrix. When the reinforcing phase and the matrix phase are compounded in two phases, physical, chemical and mechanical effects will occur in the interface layer, and the interfacial bonding of carbon fiber and resin is particularly important to the performance of composite materials. The interface between carbon fiber and resin is a key factor in determining the performance of carbon fiber composites, and it is also a key link in product quality control. There is currently no standard method for evaluating the interface properties of carbon fiber and resin.
For the performance test of carbon fiber reinforced plastics, China has established the "GB T 1446-2005 General Principles of Performance Test Methods for Fiber Reinforced Plastics" and "GB T 1450".2-2005 Test Method for Stamping Shear Strength of Fiber Reinforced Plastics", "GB T 1451-2005 Test Method for Impact Toughness of Fiber Reinforced Plastics Charpy Beam", "GB T 28889-2012 Test Method for In-plane Shear Properties of Composite Materials" and other standards. Standard test methods have also been established for the combustion properties of fiber-reinforced plastics.
The application of carbon fiber composite materials in the field of building materials should strictly follow the requirements of various standards, including the use standards and construction standards of carbon fiber composite materials. At present, China has established the "GB T 21490-2008 Carbon Fiber Sheet for Structural Reinforcement and Repair" and "CECS 146-2003 (2007) Carbon Fiber Sheet Reinforcement Concrete Structure Technical Specification", which standardizes the reinforcement application technology in the field of carbon fiber sheet building materials, and plays an important role in improving the reinforcement technology and developing the fiber sheet reinforcement industry.
To sum up, for the application of carbon fiber and its composite materials in the field of building materials, China has made significant progress in standardization. However, for carbon fiber composites with different compositions, different specific functions and different application fields, there is still a lack of product specifications, and the key characteristic components and their contents, interface structures, standard methods for the determination of relevant functional indicators and construction technical regulations that affect the performance are not perfect. The establishment of such specifications and standard methods is of great significance for testing and evaluating the quality of carbon fiber composite materials and related products in the field of building materials, and promoting the universal application of carbon fiber and its composite materials in the field of building materials.
Application prospect of carbon fiber and composite materials: an example in the field of building materials
As China's economy has shifted from a stage of rapid growth to a stage of high-quality development, it is necessary to accelerate the construction of a manufacturing power, improve the basic capacity of the industry and the level of the industrial chain, and promote China's industry to move towards the middle and high end of the global value chain. As a kind of technical specification, standards play an important role in maintaining the healthy development of the market economy and are also an important manifestation of national competitiveness.
As a high-performance, multi-functional advanced composite material, carbon fiber composite material can be used as structural building materials and functional building materials. With the breakthrough of production and process technology, the cost of carbon fiber composite materials has been reduced, and there will be a huge demand for carbon fiber composite materials in the construction field. Although carbon fiber composite materials have the characteristics of heavy bearing, light weight and high strength, and have good mechanical and physical properties, they can meet the needs of building structures and conform to the development trend of ecological civilization construction. However, there are still many deficiencies in the development of carbon fiber composite materials in the field of building materials, especially the product specifications, test and evaluation methods and standards of carbon fiber composite materials for building materials are not perfect, so that this high-performance material has not been widely used.
The expansion and development of the application of carbon fiber composite materials in the field of building materials has put forward an urgent demand for the standardization of carbon fiber composite materials in the field of building materials. The first is the development of carbon fiber composite material testing standards. Through the optimization and verification of existing group standards and industry standards, the method is optimized, and the development of relevant national standards is carried out. Secondly, the development of detection methods for carbon fiber composites should be accelerated. In view of the lack of detection methods in the process of use and construction, the development of detection methods such as thermal conductivity of carbon fiber, thermal expansion coefficient, mechanical properties, damp and heat resistance of carbon fiber composite rods, conductivity, shielding performance and corrosion resistance of carbon fiber functional composites was carried out, so as to provide key technical support for the specific application of carbon fiber composites for building materials.
With the evaluation and promotion of green building materials product labeling included in the ecological civilization construction, urban planning and construction systems, the declaration, evaluation and production and research and development of green building materials need to evaluate the environmental safety of the whole life cycle of building materials. Studying the environmental impact of carbon fiber composites throughout their life cycle is conducive to promoting their recycling. Therefore, it is also necessary to strengthen the research on the safety and environmental impact assessment methods of carbon fiber composite materials in the whole life cycle. This kind of work can provide technical support and basic data for the detection of the environmental impact of carbon fiber composites in the whole life cycle, which is of great significance for improving the quality and safety of carbon fiber composites, and promoting the wide application of carbon fiber composites in the field of building materials.
Further reading. The world's three major special fibers丨carbon fiber, aramid fiber and ultra-high molecular weight polyethylene.
1 carbon fiber. The preparation of carbon fibers is a highly complex process that includes high-temperature conversion and multiple processes. Through high-temperature treatment, organic fibers are converted into inorganic fibers with a carbon content of more than 90%, giving them excellent high temperature resistance and strength characteristics. Carbon fiber is widely used in aerospace, sports equipment, new energy vehicles and other fields because of its characteristics of being as thin as a hair, light as a feather, and as strong as steel. It not only changes the lightweight design of products, but also raises the performance standards of various industries.
2. Aramid fibers.
With its high strength, high modulus, high temperature resistance, corrosion resistance and other characteristics, aramid fiber has become the preferred material in the field of protection. Its molecular structure gives it excellent tensile and compression resistance. Aramid fibers are widely used in protective equipment such as bulletproof vests, chemical protective clothing, and fireproof clothing. In the military, police, fire and other fields, the excellent performance of aramid fiber ensures personal safety.
3. Ultra-high molecular weight polyethylene fiber.
Ultra-high molecular weight polyethylene fibers have an extremely high molecular weight and density, giving them excellent abrasion, chemical and UV resistance. It is stronger than steel and lighter than carbon fiber. UHMWPE fibers show strong potential in medicine, aerospace, sports equipment and other fields. In orthopedic implant materials, spacecraft structures, sports equipment manufacturing, etc., the unique properties of UHMWPE fibers are widely used.