Analysis Of Wet Winding Process For Resin-Based Composite Materials

 

Due to the limitations of raw materials and process control methods, in the early days, especially in the fields of aerospace, winding molding was mostly carried out in the form of prepreg tape dry method. This molding method has a good production environment and can obtain composite products with excellent performance. However, the required equipment is complex, the investment is large, and the product cost is high.

 

Wet winding molding is to directly wind the reinforcing material on the core mold after passing through the resin bath for curing molding. The lubricating effect of the resin can reduce the wear of the reinforcing yarn during transmission, which is beneficial to the strength of the fiber. More importantly, since the prepreg tape production step is omitted, the equipment required for wet winding molding is simple, the process is less and the production cycle is shorter, which can effectively improve production efficiency and reduce manufacturing costs.

 

Data show that replacing dry winding with wet winding can reduce costs by about 40%. Therefore, in the winding molding of products, there is a trend to replace dry winding with wet winding.

 

The main factors limiting the widespread application of wet winding molding are mainly

 

There are fewer types of resin matrices suitable for wet winding molding;

 

The wet winding process control is difficult to control, etc.

 

 

Reinforcement Materials For Wet Winding

 

The reinforcement materials for winding molding are mainly glass fiber, aramid fiber and carbon fiber. Glass fiber is the earliest reinforcement material, and the original one was ordinary E glass fiber.

 

In the 1960s, high-strength glass fiber came out and was immediately widely used. Aramid fiber came out in the 1970s. Its tensile strength is equivalent to that of high-strength glass fiber, but its density is only 1.45, and its specific strength is the highest level at that time. It became the main reinforcement material for composite solid rocket engine shells in the 1970s and 1980s. Carbon fiber appeared relatively early, but its application in many structural materials was limited due to its low elongation at break in the early stage. In addition, the density of carbon fiber is relatively large, which affects the competitiveness of this fiber.

 

Since the resin matrix does not allow the addition of inactive solvents in wet winding and has a high viscosity, in order to ensure that the reinforcement material is fully impregnated during the winding process, wet winding requires the reinforcement material to have better wettability than dry winding.

 

Among the above three main reinforcement materials, glass fiber has the best wettability to resin, so it is more suitable for wet winding process. Aramid fiber is an organic fiber with a typical “skin-core” structure in cross section. The skin is a relatively complete fibril structure of rigid polymer macromolecules straightened and arranged, surrounding a relatively loose microcrystalline core structure.

 

This structure determines that the longitudinal strength of the fiber is high, the transverse strength is low and the fiber splitting is easy to occur. The wet winding process allows the resin-impregnated fiber to be directly wound on the core mold, which simplifies the winding process after pre-impregnation and reduces the chance of surface damage during fiber winding.

 

The aramid fiber-wound solid rocket engine shells in the United States and the former Soviet Union are mostly formed by wet winding. Carbon fiber has poor wettability with resin, but manufacturers improve the wettability of carbon fiber by adding coating materials before leaving the factory, so that it can be used for wet winding.

 

In addition, carbon fiber is a brittle material, and the wet process reduces the wear and tear of the fiber during winding under the lubrication of the resin, which makes carbon fiber more suitable for wet winding process. With the development of carbon fiber manufacturing technology, carbon fiber is expected to completely replace aramid fiber in the field of structural materials.

 

With the progress of technological research, organic fibers with higher performance have emerged, such as ultra-high molecular weight polyethylene fibers and PBo fibers. This type of fiber is characterized by ultra-high strength, but contains fewer or no polar groups in the molecular chain structure, has strong surface inertness, and has poor wetting and bonding with the resin matrix.

 

Wang Baiya et al. conducted a preliminary study on the wet winding process of PBo fibers. The results show that the bonding between PBo and the epoxy matrix is ​​poor, making it difficult to exert the high-strength characteristics of PBo fibers. The surface corona treatment of PBo fibers does not significantly improve their wettability. More research is needed on the application of this type of fiber in wet winding.

 

The main form of reinforcement material for wet winding molding is untwisted roving. Because the fiber bands formed after the untwisted rovings are ply-ply, there are overlaps, crosses and gaps between the fibers, and the tension of the rovings involved in the ply-ply is uneven, so the mechanical properties are not fully utilized. The lateral shear and compression performance of composite materials obtained by winding with woven cloth is much better than that of fiber winding, especially for components working under multiple loads, cloth tape winding can obtain higher structural efficiency.

 

However, research on wet winding of cloth tape is still relatively rare. According to foreign reports, Owen and Griffith used unsaturated polyester as the resin matrix and successfully made a thin-walled cylinder by wet winding of glass cloth tape for testing static performance and fatigue performance.

 

Research On Resin Matrix For Wet Winding

 

For the matrix resin for wet winding molding, in addition to providing excellent thermomechanical properties, it is also required to have a low initial viscosity at the working temperature and maintain a low viscosity state for a long time (i.e., the applicable period) at this temperature. Commonly used matrix resins are mainly unsaturated polyester and epoxy resin.

 

The outstanding advantages of unsaturated polyester are low viscosity and low price, but its gelation speed is fast and the comprehensive performance of the cured product is not high. It is mainly used in the field of civil low-pressure structural materials with low requirements. At present, the main research direction of unsaturated polyester for wet winding is to improve performance, prolong and controllable gel time. Zhou Wenying and others developed an unsaturated polyester system for wet winding of environmentally friendly water treatment containers.

 

The NOL ring test and container fatigue and explosion test show that the formula system has low viscosity and excellent winding process performance, good toughness, good interface bonding effect with glass fiber, and fiber strength conversion rate of more than 80%. The mechanical properties of the water treatment container wound by it are not lower than the original ring resin system, but the cost is half of the original system.

 

Due to its good adhesion, chemical corrosion resistance and low shrinkage, epoxy resin has been widely used in the field of advanced composite materials. However, compared with unsaturated polyester, epoxy resin has the disadvantages of high price and high viscosity. When used as a resin matrix for wet winding, the current research is mainly to improve its wet processability without excessively reducing the thermomechanical properties of the resin, that is, by selecting an appropriate diluent to reduce the viscosity of the system; by selecting an appropriate curing agent and accelerator to reduce the curing temperature of the system and extend the applicability of the system.

 

Zhang Chunhua et al. used epoxy resin TDE-85 as the main resin and a self-made modified aromatic amine as the curing agent to prepare a low-viscosity resin system. The viscosity at 25°C was only 0.41Pa.s, and the viscosity was lower than 0.6Pa.s after 8h, indicating that the system has a long service life. The tensile strength of the cast body after curing with the curing system of 90°C/2h+120°C/1h+160°C/3h reached 97MPa, the tensile modulus reached 3.8GPa, and the elongation at break reached 4.3%. The wettability of the resin matrix and T-700 carbon fiber was studied by wet winding a Φ150mm small container. The results showed that the fiber strength conversion rate was as high as 86.5%, and the pressure vessel characteristic coefficient (PV/WC) value was as high as 39.8km.

 

Wang Bin et al. conducted a series of studies on epoxy resin formulations for wet winding. For aramid fibers, a series of wet formulations were prepared by compounding different diluents and curing agents. HR18A was selected for a preliminary study of wet winding processability. The results showed that the shear strength of aramid NOL ring was about 51-55MPa, the strength conversion rate was above 87%, and the PV/Wc value of a Φ150mm small container reached 36.3km.

 

Aiming at the application characteristics of all-composite high-pressure gas cylinders for natural gas vehicles, a medium-temperature curing epoxy formula HE2 was prepared by adding active diluents and homemade accelerators with epoxy resin E-51 as the main resin and tough DDM as the curing agent. The initial viscosity of the resin system was 0.72 Pa.s, and the viscosity only increased to 2.32 Pa.s after 10h, indicating that the system has a long applicability period.

Due to the limitations of raw materials and process control methods, in the early days, especially in the fields of aerospace, winding molding was mostly carried out in the form of prepreg tape dry method. This molding method has a good production environment and can obtain composite products with excellent performance. However, the required equipment is complex, the investment is large, and the product cost is high. Wet winding molding is to directly wind the reinforcing material on the core mold after passing through the resin bath for curing molding. The lubricating effect of the resin can reduce the wear of the reinforcing yarn during transmission, which is beneficial to the strength of the fiber. More importantly, since the prepreg tape production step is omitted, the equipment required for wet winding molding is simple, the process is less and the production cycle is shorter, which can effectively improve production efficiency and reduce manufacturing costs. Data show that replacing dry winding with wet winding can reduce costs by about 40%. Therefore, in the winding molding of products, there is a trend to replace dry winding with wet winding. The main factors limiting the widespread application of wet winding molding are mainly There are fewer types of resin matrices suitable for wet winding molding; The wet winding process control is difficult to control, etc. Reinforcement Materials For Wet Winding The reinforcement materials for winding molding are mainly glass fiber, aramid fiber and carbon fiber. Glass fiber is the earliest reinforcement material, and the original one was ordinary E glass fiber. In the 1960s, high-strength glass fiber came out and was immediately widely used. Aramid fiber came out in the 1970s. Its tensile strength is equivalent to that of high-strength glass fiber, but its density is only 1.45, and its specific strength is the highest level at that time. It became the main reinforcement material for composite solid rocket engine shells in the 1970s and 1980s. Carbon fiber appeared relatively early, but its application in many structural materials was limited due to its low elongation at break in the early stage. In addition, the density of carbon fiber is relatively large, which affects the competitiveness of this fiber. Since the resin matrix does not allow the addition of inactive solvents in wet winding and has a high viscosity, in order to ensure that the reinforcement material is fully impregnated during the winding process, wet winding requires the reinforcement material to have better wettability than dry winding. Among the above three main reinforcement materials, glass fiber has the best wettability to resin, so it is more suitable for wet winding process. Aramid fiber is an organic fiber with a typical "skin-core" structure in cross section. The skin is a relatively complete fibril structure of rigid polymer macromolecules straightened and arranged, surrounding a relatively loose microcrystalline core structure. This structure determines that the longitudinal strength of the fiber is high, the transverse strength is low and the fiber splitting is easy to occur. The wet winding process allows the resin-impregnated fiber to be directly wound on the core mold, which simplifies the winding process after pre-impregnation and reduces the chance of surface damage during fiber winding. The aramid fiber-wound solid rocket engine shells in the United States and the former Soviet Union are mostly formed by wet winding. Carbon fiber has poor wettability with resin, but manufacturers improve the wettability of carbon fiber by adding coating materials before leaving the factory, so that it can be used for wet winding. In addition, carbon fiber is a brittle material, and the wet process reduces the wear and tear of the fiber during winding under the lubrication of the resin, which makes carbon fiber more suitable for wet winding process. With the development of carbon fiber manufacturing technology, carbon fiber is expected to completely replace aramid fiber in the field of structural materials. With the progress of technological research, organic fibers with higher performance have emerged, such as ultra-high molecular weight polyethylene fibers and PBo fibers. This type of fiber is characterized by ultra-high strength, but contains fewer or no polar groups in the molecular chain structure, has strong surface inertness, and has poor wetting and bonding with the resin matrix. Wang Baiya et al. conducted a preliminary study on the wet winding process of PBo fibers. The results show that the bonding between PBo and the epoxy matrix is poor, making it difficult to exert the high-strength characteristics of PBo fibers. The surface corona treatment of PBo fibers does not significantly improve their wettability. More research is needed on the application of this type of fiber in wet winding. The main form of reinforcement material for wet winding molding is untwisted roving. Because the fiber bands formed after the untwisted rovings are ply-ply, there are overlaps, crosses and gaps between the fibers, and the tension of the rovings involved in the ply-ply is uneven, so the mechanical properties are not fully utilized. The lateral shear and compression performance of composite materials obtained by winding with woven cloth is much better than that of fiber winding, especially for components working under multiple loads, cloth tape winding can obtain higher structural efficiency. However, research on wet winding of cloth tape is still relatively rare. According to foreign reports, Owen and Griffith used unsaturated polyester as the resin matrix and successfully made a thin-walled cylinder by wet winding of glass cloth tape for testing static performance and fatigue performance. Research On Resin Matrix For Wet Winding For the matrix resin for wet winding molding, in addition to providing excellent thermomechanical properties, it is also required to have a low initial viscosity at the working temperature and maintain a low viscosity state for a long time (i.e., the applicable period) at this temperature. Commonly used matrix resins are mainly unsaturated polyester and epoxy resin. The outstanding advantages of unsaturated polyester are low viscosity and low price, but its gelation speed is fast and the comprehensive performance of the cured product is not high. It is mainly used in the field of civil low-pressure structural materials with low requirements. At present, the main research direction of unsaturated polyester for wet winding is to improve performance, prolong and controllable gel time. Zhou Wenying and others developed an unsaturated polyester system for wet winding of environmentally friendly water treatment containers. The NOL ring test and container fatigue and explosion test show that the formula system has low viscosity and excellent winding process performance, good toughness, good interface bonding effect with glass fiber, and fiber strength conversion rate of more than 80%. The mechanical properties of the water treatment container wound by it are not lower than the original ring resin system, but the cost is half of the original system. Due to its good adhesion, chemical corrosion resistance and low shrinkage, epoxy resin has been widely used in the field of advanced composite materials. However, compared with unsaturated polyester, epoxy resin has the disadvantages of high price and high viscosity. When used as a resin matrix for wet winding, the current research is mainly to improve its wet processability without excessively reducing the thermomechanical properties of the resin, that is, by selecting an appropriate diluent to reduce the viscosity of the system; by selecting an appropriate curing agent and accelerator to reduce the curing temperature of the system and extend the applicability of the system. Zhang Chunhua et al. used epoxy resin TDE-85 as the main resin and a self-made modified aromatic amine as the curing agent to prepare a low-viscosity resin system. The viscosity at 25°C was only 0.41Pa.s, and the viscosity was lower than 0.6Pa.s after 8h, indicating that the system has a long service life. The tensile strength of the cast body after curing with the curing system of 90°C/2h+120°C/1h+160°C/3h reached 97MPa, the tensile modulus reached 3.8GPa, and the elongation at break reached 4.3%. The wettability of the resin matrix and T-700 carbon fiber was studied by wet winding a Φ150mm small container. The results showed that the fiber strength conversion rate was as high as 86.5%, and the pressure vessel characteristic coefficient (PV/WC) value was as high as 39.8km. Wang Bin et al. conducted a series of studies on epoxy resin formulations for wet winding. For aramid fibers, a series of wet formulations were prepared by compounding different diluents and curing agents. HR18A was selected for a preliminary study of wet winding processability. The results showed that the shear strength of aramid NOL ring was about 51-55MPa, the strength conversion rate was above 87%, and the PV/Wc value of a Φ150mm small container reached 36.3km. Aiming at the application characteristics of all-composite high-pressure gas cylinders for natural gas vehicles, a medium-temperature curing epoxy formula HE2 was prepared by adding active diluents and homemade accelerators with epoxy resin E-51 as the main resin and tough DDM as the curing agent. The initial viscosity of the resin system was 0.72 Pa.s, and the viscosity only increased to 2.32 Pa.s after 10h, indicating that the system has a long applicability period.

 

 

 

In order to meet the requirements of high toughness and high heat resistance of the resin matrix for the solid rocket engine casing, Chen et al. used T-51TDE-85 as the main resin, and added high-temperature resistant component modified bismaleimide copolymer and toughening agent modified DDM curing agent to the system to prepare high-temperature resistant wet winding resin formula and high-toughness wet winding epoxy formula respectively.

 

A small container of Φ150mm was wet-wound with high-toughness formula as the matrix and T-700 carbon fiber as the reinforcement material. The test results show that the strength conversion rate of carbon fiber is as high as 89.4%, and the PV/Wc value is as high as 40.1 km, indicating that the matrix has good compatibility with T-700 carbon fiber.

 

Research On Wet Winding Molding Process

 

Reinforcement materials and resin matrix for wet winding are necessary material conditions for the smooth progress of wet winding molding, but the performance of the product and the full play of material properties are largely determined by the execution of the winding process. Compared with dry molding, the biggest difficulty of wet winding molding lies in the precise control of the glue content and the uniform distribution of the glue liquid.

 

There are many factors that affect the glue content in the wet winding process, such as winding tension, resin viscosity, and glue scraping system. These factors affect each other, thus determining the difficulty of controlling the wet winding process. At present, there are few reports on the systematic research of the wet winding process, and the application of wet winding molding is mainly in the fields of civil building materials and pipelines with low requirements. Ren Penggang et al. applied the fiber wet winding technology to the molding of composite hollow insulators and conducted a preliminary study on the wet molding process. The results show that fiber wet winding is better than the vacuum impregnation cloth tape winding molding method, and can obtain materials with good density and waterproof diffusion.

 

Liu Bingyu et al. preliminarily explored the wet winding molding process in the molding of carbon fiber composite tape shells. The higher winding tension is conducive to the full impregnation of the yarn tape and the squeezing out of the excess glue to make the material more dense, but the excessive tension causes the yarn tape to wear during the transmission process. The glue content tends to gradually decrease. The study believes that this is because after the yarn is wound on the core mold, the glue on the yarn migrates outward under the action of tension.

 

The winding tension is a very important process parameter in wet winding. A larger winding tension is conducive to the yarn being fully impregnated with glue and squeezing out excess glue to make the material more compact. However, excessive tension will cause the yarn to wear during the transmission process, which is not conducive to the development of fiber strength. It will also cause excessive glue extrusion and the product glue content is too low.

 

Chen Shile et al. studied the effect of tension on the strength of T-700 and T-800 carbon fiber wet winding and found that the high-modulus quality T-800 is more brittle and has poor wear resistance, and is more susceptible to wear during the winding process than T-700.

 

Fiber wear during fiber winding is an important factor affecting fiber strength conversion. Therefore, reducing the winding tension can improve the fiber strength to a certain extent. However, when the tension is reduced to a certain extent, the interlaminar shear strength will be reduced, which is not conducive to the overall development of fiber strength. The tension range of wet-wound carbon fiber is generally 3-8% of the fiber breaking strength. For T-800, the upper limit should be less than 4%, that is, the tension of T-800 carbon fiber should be selected between 40 and 48N per group, and T-700 can be higher than 50N per group.

 

Conclusion and Prospect

 

Currently, all commonly used reinforcing materials can be used for wet winding, while the newly emerging super-strong organic fibers, such as ultra-high molecular weight polyethylene fibers and PBO fibers, need further research for wet winding due to their large surface inertia. The limited variety of applicable resin matrices is one of the main limiting factors for the further application of wet winding. At present, some excellent epoxy and non-epoxy wet winding formulas have been developed.

 

Future research directions should focus on solving the contradiction between low-temperature curability and room-temperature applicability in the formula, and prolonging the applicability of the system while reducing the curing temperature of the system. The wet winding process is one of the important factors affecting the performance of the product. In view of this, future research should focus on the effects of glue viscosity, winding tension and glue extrusion conditions on glue content, so as to make the resin viscosity adjustable in the winding process and accurately quantify the relationship between winding tension, glue content and product performance.

 

Winding molding has become one of the most important means in the manufacturing process of composite materials. Compared with dry prepreg molding, wet winding molding does not require pre-preparation and refrigeration of prepreg tapes, the required equipment is simple, and the manufacturing process is simplified, so it can effectively reduce the manufacturing cost of composite materials.

 

In view of the increasingly fierce market competition and increasing cost pressure, wet winding molding will surely develop greatly, especially in the field of advanced composite materials such as aerospace, where dry winding molding currently dominates, wet winding molding may replace dry molding.

 

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