Composite materials are all combined with reinforcing fibers and a plastic material. The role of resin in composite materials is crucial. The choice of resin determines a series of characteristic process parameters, some mechanical properties and functionality (thermal properties, flammability, Environmental resistance, etc.), resin properties are also a key factor in understanding the mechanical properties of composite materials. When the resin is selected, the window that determines the range of processes and properties of the composite is automatically determined. Thermosetting resin is a commonly used resin type for resin matrix composites because of its good manufacturability. Thermoset resins are almost exclusively liquid or semi-solid at room temperature, and conceptually they are more like the monomers that make up the thermoplastic resin than the thermoplastic resin in the final state. Before thermosetting resins are cured, they can be processed into various shapes, but once cured using curing agents, initiators or heat, they cannot be shaped again because chemical bonds are formed during curing, making Small molecules are transformed into three-dimensional cross-linked rigid polymers with higher molecular weights.
There are many kinds of thermosetting resins, commonly used are phenolic resins, epoxy resins, bis-horse resins, vinyl resins, phenolic resins, etc.
(1) Phenolic resin is an early thermosetting resin with good adhesion, good heat resistance and dielectric properties after curing, and its outstanding features are excellent flame retardant properties, low heat release rate, low smoke density, and combustion. The gas released is less toxic. The processability is good, and the composite material components can be manufactured by molding, winding, hand lay-up, spraying, and pultrusion processes. A large number of phenolic resin-based composite materials are used in the interior decoration materials of civil aircraft.
(2) Epoxy resin is an early resin matrix used in aircraft structures. It is characterized by a wide variety of materials. Different curing agents and accelerators can obtain a curing temperature range from room temperature to 180 ℃; it has higher mechanical properties; Good fiber matching type; heat and humidity resistance; excellent toughness; excellent manufacturability (good coverage, moderate resin viscosity, good fluidity, pressurized bandwidth, etc.); suitable for overall co-curing molding of large components; cheap. The good molding process and outstanding toughness of epoxy resin make it occupy an important position in the resin matrix of advanced composite materials.
(3)Vinyl resin is recognized as one of the excellent corrosion-resistant resins. It can withstand most acids, alkalis, salt solutions and strong solvent media. It is widely used in papermaking, chemical industry, electronics, petroleum, storage and transportation, environmental protection, ships, Automotive Lighting Industry. It has the characteristics of unsaturated polyester and epoxy resin, so that it has both the excellent mechanical properties of epoxy resin and the good process performance of unsaturated polyester. In addition to outstanding corrosion resistance, this type of resin also has good heat resistance. It includes standard type, high temperature type, flame retardant type, impact resistance type and other varieties. The application of vinyl resin in fiber reinforced plastic (FRP) is mainly based on hand lay-up, especially in anti-corrosion applications. With the development of SMC, its application in this regard is also quite noticeable.
(4)Modified bismaleimide resin (referred to as bismaleimide resin) is developed to meet the requirements of new fighter jets for composite resin matrix. These requirements include: large components and complex profiles at 130 ℃ Manufacture of components, etc. Compared with epoxy resin, Shuangma resin is mainly characterized by superior humidity and heat resistance and high operating temperature; the disadvantage is that the manufacturability is not as good as epoxy resin, and the curing temperature is high (curing above 185 ℃), and requires a temperature of 200 ℃. Or for a long time at a temperature above 200 ℃.
(5)Cyanide (qing diacoustic) ester resin has low dielectric constant (2.8~3.2) and extremely small dielectric loss tangent (0.002~0.008), high glass transition temperature (240~290℃) , Low shrinkage, low moisture absorption, excellent mechanical properties and bonding properties, etc., and it has similar processing technology to epoxy resin.
At present, cyanate resins are mainly used in three aspects: printed circuit boards for high-speed digital and high-frequency, high-performance wave-transmitting structural materials and high-performance structural composite materials for aerospace.
To put it simply, epoxy resin, the performance of epoxy resin is not only related to the synthesis conditions, but also mainly depends on the molecular structure. The glycidyl group in epoxy resin is a flexible segment, which can reduce the viscosity of the resin and improve the process performance, but at the same time reduce the heat resistance of the cured resin. The main approaches to improve the thermal and mechanical properties of cured epoxy resins are low molecular weight and multifunctionalization to increase crosslink density and introduce rigid structures. Of course, the introduction of a rigid structure leads to a decrease in solubility and an increase in viscosity, which leads to a decrease in epoxy resin process performance. How to improve the temperature resistance of epoxy resin system is a very important aspect. From the point of view of resin and curing agent, the more functional groups, the greater the crosslinking density. The higher the Tg. Specific operation: Use multifunctional epoxy resin or curing agent, use high-purity epoxy resin. The commonly used method is to add a certain proportion of o-methyl acetaldehyde epoxy resin into the curing system, which has good effect and low cost. The larger the average molecular weight, the narrower the molecular weight distribution, and the higher the Tg. Specific operation: Use a multifunctional epoxy resin or curing agent or other methods with a relatively uniform molecular weight distribution.
As a high-performance resin matrix used as a composite matrix, its various properties, such as processability, thermophysical properties and mechanical properties, must meet the needs of practical applications. Resin matrix manufacturability includes solubility in solvents, melt viscosity (fluidity) and viscosity changes, and gel time changes with temperature (process window). The composition of the resin formulation and the choice of reaction temperature determine the chemical reaction kinetics (cure rate), chemical rheological properties (viscosity-temperature versus time), and chemical reaction thermodynamics (exothermic). Different processes have different requirements for resin viscosity. Generally speaking, for the winding process, the resin viscosity is generally around 500cPs; for the pultrusion process, the resin viscosity is around 800~1200cPs; for the vacuum introduction process, the resin viscosity is generally around 300cPs, and the RTM process may be higher, but Generally, it will not exceed 800cPs; for the prepreg process, the viscosity is required to be relatively high, generally around 30000~50000cPs. Of course, these viscosity requirements are related to the properties of the process, equipment and materials themselves, and are not static. Generally speaking, as the temperature increases, the viscosity of the resin decreases in the lower temperature range; however, as the temperature increases, the curing reaction of the resin also proceeds, kinetically speaking, the temperature The reaction rate doubles for every 10℃ increase, and this approximation is still useful for estimating when the viscosity of a reactive resin system increases to a certain critical viscosity point. For example, it takes 50 minutes for a resin system with a viscosity of 200cPs at 100℃ to increase its viscosity to 1000cPs, then the time required for the same resin system to increase its initial viscosity from less than 200cPs to 1000cPs at 110℃ is about 25 minutes. The selection of process parameters should fully consider the viscosity and gel time. For example, in the vacuum introduction process, it is necessary to ensure that the viscosity at the operating temperature is within the viscosity range required by the process, and the pot life of the resin at this temperature must be long enough to ensure that the resin can be imported. To sum up, the selection of resin type in the injection process must consider the gel point, filling time and temperature of the material. Other processes have a similar situation.
In the molding process, the size and shape of the part (mold), the type of reinforcement, and the process parameters determine the heat transfer rate and mass transfer process of the process. Resin cures exothermic heat, which is generated by the formation of chemical bonds. The more chemical bonds formed per unit volume per unit time, the more energy is released. The heat transfer coefficients of resins and their polymers are generally quite low. The rate of heat removal during polymerization cannot match the rate of heat generation. These incremental amounts of heat cause chemical reactions to proceed at a faster rate, resulting in more This self-accelerating reaction will eventually lead to stress failure or degradation of the part. This is more prominent in the manufacture of large-thickness composite parts, and it is particularly important to optimize the curing process path. The problem of local “temperature overshoot” caused by the high exothermic rate of prepreg curing, and the state difference (such as temperature difference) between the global process window and the local process window are all due to how to control the curing process. The “temperature uniformity” in the part (especially in the thickness direction of the part), to achieve “temperature uniformity” depends on the arrangement (or application) of some “unit technologies” in the “manufacturing system”. For thin parts, since a large amount of heat will be dissipated into the environment, the temperature rises gently, and sometimes the part will not be fully cured. At this time, auxiliary heat needs to be applied to complete the cross-linking reaction, that is, continuous heating.
The composite material non-autoclave forming technology is relative to the traditional autoclave forming technology. Broadly speaking, any composite material forming method that does not use autoclave equipment can be called non-autoclave forming technology. . So far, the application of non-autoclave molding technology in the aerospace field mainly includes the following directions: non-autoclave prepreg technology, liquid molding technology, prepreg compression molding technology, microwave curing technology, electron beam curing technology, Balanced pressure fluid forming technology. Among these technologies, OoA (Outof Autoclave) prepreg technology is closer to the traditional autoclave forming process, and has a wide range of manual laying and automatic laying process foundations, so it is regarded as a non-woven fabric that is likely to be realized on a large scale. Autoclave forming technology. An important reason for using an autoclave for high-performance composite parts is to provide sufficient pressure to the prepreg, greater than the vapor pressure of any gas during curing, to inhibit the formation of pores, and this is OoA prepreg The primary difficulty that technology needs to break through. Whether the porosity of the part can be controlled under vacuum pressure and its performance can reach the performance of autoclave cured laminate is an important criterion for evaluating the quality of OoA prepreg and its molding process.
The development of OoA prepreg technology first originated from the development of resin. There are three main points in the development of resins for OoA prepregs: one is to control the porosity of the molded parts, such as using addition reaction-cured resins to reduce volatiles in the curing reaction; the second is to improve the performance of the cured resins To achieve the resin properties formed by the autoclave process, including thermal properties and mechanical properties; the third is to ensure that the prepreg has good manufacturability, such as ensuring that the resin can flow under a pressure gradient of an atmospheric pressure, ensuring that it has a long viscosity life and Sufficient room temperature outside time, etc. Raw material manufacturers conduct material research and development according to specific design requirements and process methods. The main directions should include: improving mechanical properties, increasing external time, reducing curing temperature, and improving moisture and heat resistance. Some of these performance improvements are conflicting. , such as high toughness and low temperature curing. You need to find a balance point and consider it comprehensively!
In addition to resin development, the manufacturing method of prepreg also promotes the application development of OoA prepreg. The study found the importance of prepreg vacuum channels for making zero-porosity laminates. Subsequent studies have shown that semi-impregnated prepregs can effectively improve gas permeability. OoA prepregs are semi-impregnated with resin, and dry fibers are used as channels for exhaust gas. The gases and volatiles involved in the curing of the part can be Exhaust through channels such that the porosity of the final part is <1%.
The vacuum bagging process belongs to the non-autoclave forming (OoA) process. In short, it is a molding process that seals the product between the mold and the vacuum bag, and pressurizes the product by vacuuming to make the product more compact and better mechanical properties. The main manufacturing process is
First, a release agent or release cloth is applied to the layup mold (or glass sheet). The prepreg is inspected according to the standard of the prepreg used, mainly including the surface density, resin content, volatile matter and other information of the prepreg. Cut the prepreg to size. When cutting, pay attention to the direction of the fibers. Generally, the direction deviation of the fibers is required to be less than 1°. Number each blanking unit and record the prepreg number. When laying up layers, the layers should be laid in strict accordance with the lay-up order required on the lay-up record sheet, and the PE film or release paper should be connected along the direction of the fibers, and the air bubbles should be chased along the direction of the fibers. The scraper spreads out the prepreg and scrapes it out as much as possible to remove the air between the layers. When laying up, it is sometimes necessary to splicing prepregs, which must be spliced along the fiber direction. In the splicing process, overlap and less overlap should be achieved, and the splicing seams of each layer should be staggered. Generally, the splicing gap of unidirectional prepreg is as follows. 1mm; the braided prepreg is only allowed to overlap, not splicing, and the overlap width is 10~15mm. Next, pay attention to vacuum pre-compaction, and the thickness of pre-pumping varies according to different requirements. The purpose is to discharge the air trapped in the layup and the volatiles in the prepreg to ensure the internal quality of the component. Then there is the laying of auxiliary materials and vacuum bagging. Bag sealing and curing: The final requirement is to not be able to leak air. Note: The place where there is often air leakage is the sealant joint.
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Post time: May-23-2022
