Composite bag material is defined in the national standard GB/T 3961-1993 as a solid product composed of two or more independent physical phases, including bonding materials (matrix) and granular, fiber or sheet materials, which is called composite material. In short, composite material is a kind of multiphase material composed of two or more components with different properties and different shapes through composite means. 1. Composition of composite materials (1) Matrix (matrix phase): a single material constituting the continuous phase of composite materials, such as the resin in glass fiber reinforced plastics (GRP) is the matrix. (2) Reinforcing material (reinforcing phase): The material in the composite that does not constitute a continuous phase. It is dispersed and contained by the matrix, such as glass fiber in fiberglass. (3) Interface of composite material (interface phase): an interface between reinforcement phase and matrix phase is called composite interface. Each phase of the composite can be physically separated at the interface. However, through the analysis and research on the microstructure level, it is found that the reinforcement phase and matrix phase near the composite interface have complex structures different from both the matrix phase and the reinforcement component itself due to the complex physical and chemical changes during the composite. It is also found that this structure and morphology will have an impact on the macro performance of the composite, Therefore, the micro area near the interface where the structure and properties change can also be used as a phase of the composite, called the interface phase. 2. Compared with traditional materials, composite materials have the following characteristics: ① designability. The mechanical, mechanical, thermal, acoustic, optical, electrical, anti-corrosion, anti-aging and other physical and chemical properties of the composite materials can meet the requirements of the use of the parts and environmental conditions. Through the selection and matching of component materials, interface control and other material design methods, the expected purpose can be achieved to a large extent to meet the use performance. ② The identity of materials and structures. The component forming of traditional materials is through the reprocessing of materials. During the processing, the materials do not change in composition and chemistry, while composite components and materials are formed at the same time. It is formed from the component materials of composite materials when they are compounded into materials. Generally, composite materials are no longer processed into composite components. Because of this feature of composite materials, its structural integrity is good, which can greatly reduce the number of parts and connections, thus shortening the processing cycle, reducing costs and improving the reliability of components. ③ Give play to the advantages of compound effect. Although composite materials are formed by the composite process of each component material, it is not a simple mixture of several materials, but a new performance is formed according to the composite effect, which is the only composite material. ④ Dependence of material properties on composite process. During the forming process of composite structure, there are physical and chemical changes of component materials. The process is very complex. The performance of components depends heavily on the process method, process parameters, and process. At the same time, it is difficult to accurately control the process parameters during the forming process. Compared with traditional materials, composite materials have the following advantages in performance: ① The specific strength and specific modulus are large. The composite material is “lightweight and high-strength”. For example, the specific strength of carbon fiber reinforced epoxy resin composite is 5 times higher than that of steel, 4 times higher than that of aluminum alloy, 3.5 times higher than that of titanium alloy, and the specific modulus is 4 times higher than that of steel, aluminum, and titanium. ② Good fatigue resistance. ③ Good damping performance. The composite interface has a greater ability to absorb vibration energy, resulting in higher vibration damping of materials. ④ High damage safety. The failure of composite materials does not occur suddenly like traditional materials, but goes through a series of processes such as damage, cracking, interface debonding, fiber fracture, etc., thus delaying the sudden occurrence of failure. Flexible packaging material composite process and equipment. 3. Composite materials are classified according to different standards and requirements. Composite materials usually have the following classification methods. (1) It can be divided into structural composite materials and functional composite materials according to different service properties. (2) It can be divided into resin matrix composites, metal matrix composites and inorganic non-metallic matrix composites according to the type of matrix materials. (3) It can be divided into continuous fiber reinforced composites, fiber fabrics, braid reinforced composites, sheet reinforced composites, short fibers or whiskers reinforced composites, and particle reinforced composites according to the morphology of dispersed phases. (4) It can be divided into carbon fiber composite, glass fiber composite, organic fiber composite and ceramic fiber composite according to the category of reinforcement fibers. 4. Composite material performance Composite principle Composite materials are composed of two or more materials with different chemical components and different properties. What rules should they follow to composite, and how to make the overall performance of the composite better than that of the component materials, that is, retain the desired characteristics (such as high strength, high rigidity, light weight), and suppress the unexpected characteristics (low ductility). The composite principle is to discuss this problem, The following is a brief introduction to relevant issues. (1) Interaction between matrix and reinforcement In composites, the interaction between matrix and reinforcement is shown by the properties and strength of the interface formed. Generally speaking, the interface is not only the geometric interface of two materials, but also the interface layer with a certain thickness. In this area, rapid changes occur, and there are various complex physical, chemical and mechanical effects. In actual composite materials, the combination between matrix and reinforcement can be divided into the following categories. ① Mechanical combination. There is no chemical reaction between the matrix and the reinforcement, which is purely mechanical connection. This combination is achieved by friction between the rough fiber surface and the matrix, and can only bear the load in the direction parallel to the fiber. ② Wetting and diffusion. In the process of composite, the liquid cluster spreads and wets on the surface of the reinforcement material, and then mutual atoms or molecules diffuse and penetrate to form an interface. ③ Reaction binding. The chemical reaction takes place between the matrix and the reinforcement, and the interface shape ④ is combined. Several combinations of the above combinations are the most common. (2) Compatibility between matrix and reinforcement refers to the degree of coordination and cooperation between components of composite materials in the process of manufacturing and use. It is related to whether each component material can effectively play its role, and whether the overall structure and performance of composite materials are durable and stable for a long time. ① Physical compatibility. Physical compatibility requires that the mechanical properties and other physical properties of each phase component materials can be coordinated and matched when the composite materials are subjected to stress and environmental temperature changes. Among them, mechanical compatibility mainly means that the matrix should have enough toughness and strength to evenly transfer the external load to the reinforcement without obvious discontinuity; Thermal compatibility is another content of physical compatibility. Composite materials require that the matrix and reinforcement materials have equivalent thermal expansion coefficients and a reasonable combination of expansion coefficients (sometimes thermal conductivity also needs to be considered), so as not to damage the mechanical properties of composite materials due to harmful additional stresses generated in the composite materials after high temperature or during cyclic heating. ② Chemical compatibility. Chemical compatibility is relatively complex, including thermodynamic compatibility and chemical reaction (reaction between matrix and reinforcement) compatibility.
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