A method for improving the color difference of the appearance of composite material products and composite material products thereof

Abstract

The present invention discloses a method for improving the color difference of the appearance of a composite product and the composite product thereof, and belongs to the field of composite material preparation. In view of the problem that the cost of improving the color difference of the appearance of existing composite products is high and the effect is poor, the present invention provides a method for improving the color difference of the appearance of a composite product, which includes preparing a fiber premix, placing the fiber premix in a molding mold for compression molding, and obtaining a composite product; wherein in step S2, a fiber felt premix is ​​also laid in a molding cavity in the molding mold, and the size of the fiber felt premix matches the size of the composite product. The present invention lays the fiber felt premix in the molding cavity, so that the fiber premix is ​​more evenly distributed during the molding process, the mixing of the fiber and the resin is more consistent, and the uneven color caused by excessive local resin or fiber aggregation is avoided. The resin content on the surface of the composite product finally prepared is effectively controlled, and the product surface has no defects such as appearance color difference, glue accumulation, and fiber agglomeration, and the cost is low.

Description

Technical Field

[0001] The invention belongs to the technical field of composite material preparation, and more specifically, relates to a method for improving the color difference of the appearance of a composite material product and the composite material product. Background Art

[0002] The chopped fiber premix integrally molded composite material is a method in which continuous fibers (carbon fiber, glass fiber, alumina fiber, quartz fiber, etc.) are cut into chopped fibers of a certain length by using equipment, and then the fibers are impregnated with resin to form a premix, which is then molded into a product. When the chopped fibers are impregnated with resin, a large amount of resin and fibers will gather into a fiber-resin mass. Before molding the product, the fiber mass needs to be torn loose by hand or by equipment. After airing, it is molded into a chopped fiber premix integrally molded composite material product; this type of composite material premix has good wettability, high production efficiency, high repeatability, and low production cost. It is suitable for mass production of products. In addition, the fiber length of the premix can be designed, and the product performance can be improved by changing the fiber length. However, when tearing the fiber-resin mass, manual tearing will consume a large amount of labor, and equipment tearing will form fiber bundles, resulting in incomplete tearing. The fiber bundle absorbs the resin and the fiber to form fiber strips, and the fiber strip resin is much higher than the premix in the ideal state. Therefore, after the product is molded, the fiber strips exposed on the mold surface of the product will have serious local glue accumulation due to the high resin content, which will cause color defects in the product appearance, such as Figure 1 shown.

[0003] Corresponding improvements have also been made to address the above problems. For example, Chinese patent application number CN201210458493.4, published on April 17, 2013, discloses a method for reducing surface defects of fiber-reinforced thermoplastic composite products, a fiber-reinforced thermoplastic composite material obtained by the method, and a product made of the composite material. The method comprises the following steps: (a) providing a mold having a female mold and a male mold that can be molded together; (b) laying one or more layers of fiber woven cloth and one or more layers of elastic fiber products on the mold in a stacked manner, or laying a stack containing one or more layers of fiber woven cloth and one or more layers of elastic fiber products on the mold, wherein the elastic fiber product is one or more of fiber felt and three-dimensional fiber woven fabric; (c) closing the female mold and the male mold, and heating the mold; (d) injecting a thermoplastic resin composition into the mold; and (e) after the thermoplastic resin composition is solidified and the mold is cooled, demolding to obtain a product. The shortcoming of this patent is that the quality of fiber weaving and laying of elastic fiber products cannot be controlled, and they are easily separated from the mold, thus affecting the final molding effect.

[0004] Another example is Chinese patent application number CN202310837108.5, published on September 19, 2023. The patent discloses a molding method for a composite antenna window with high surface finish and small color difference, including the following steps: 1) mold design: a closed compression mold is used, and a first upper mold insert, a second upper mold insert and a lower mold insert are provided inside the closed compression mold, and a feeding cavity is formed after cooperating with each other; 2) making prepreg; 3) mold preparation; 4) unloading; 5) laying: after ensuring that the sheet is wrinkle-free and deformed, the non-porous whole paving layer is laid on the lower mold insert; 6) the first upper mold insert is pressed in the center on the laid whole paving layer, and then the frame sheet is laid on the whole paving layer, and then the second upper mold insert is lowered to be inserted into the first upper mold insert, and the second upper mold insert is controlled to fall vertically and fit with the frame sheet; 7) mold closing; 8) curing. The shortcomings of this patent are: the process is cumbersome, the preparation cycle is long and the cost is high. Summary of the invention

[0005] 1. Problems to be solved

[0006] In view of the problem that the cost of improving the color difference of the appearance of existing composite products is high and the effect is poor, the present invention provides a method for improving the color difference of the appearance of composite products and composite products thereof. The present invention lays a fiber felt premix in the molding cavity, so that the fiber premix is ​​more evenly distributed during the molding process, and the mixing of the fiber and the resin is more consistent, avoiding the uneven color caused by excessive local resin or fiber aggregation. The resin content on the surface of the composite product finally prepared is effectively controlled, and the product surface has no defects such as appearance color difference, glue accumulation, and fiber agglomeration, and the cost is low; at the same time, the product made by the present application has excellent ablation resistance when used as an ablation insulation product.

[0007] 2. Technical solution

[0008] To solve the above problems, the present invention adopts the following technical solutions.

[0009] A method for improving the color difference of the appearance of a composite material product comprises the following steps:

[0010] S1: preparing fiber premix;

[0011] S2: Laying a fiber felt premix in a molding cavity in a molding mold, wherein the size of the fiber felt premix matches the size of the composite product;

[0012] S3: placing the fiber premix in a molding mold for compression molding to obtain a composite material product.

[0013] By adopting the above technical scheme, fiber felt premix is ​​laid in the molding cavity. Since the fiber felt premix itself has uniform fiber distribution, the fiber premix can be more evenly distributed during the molding process, and the fiber and resin are mixed more consistently, avoiding uneven color caused by excessive local resin or fiber aggregation. The resin content on the surface of the composite product finally prepared is effectively controlled, and the product surface has no defects such as color difference, glue accumulation, and fiber agglomeration. The size of the fiber felt premix matches the size of the composite product, so that all visible surfaces of the composite product use the same fiber felt premix, with good fiber dispersion and reduced material waste. At the same time, the product made by this invention has excellent ablation resistance when used as an ablative insulation product.

[0014] The whole method is simple to operate and does not require the introduction of complex equipment or methods. It only requires the preparation of fiber felt premixes. The process is simple and easy to use, and it can effectively improve the color difference of the appearance of the final composite product while being low in cost.

[0015] Furthermore, the fiber mat premix is ​​prepared as follows:

[0016] Determine the size of the fiber felt premix according to the inner and outer surface sizes of the composite product;

[0017] Determine the required amount of fiber and resin based on the size of the fiber mat premix;

[0018] The fibers are spread on a glass plate covered with a polytetrafluoroethylene film, and the resin is sprayed onto the fibers to form a fiber mat premix.

[0019] By adopting the above technical scheme, the size of the fiber felt premix is ​​determined by the composite product to ensure that the size of the fiber felt premix and the product are perfectly matched, thereby avoiding product defects caused by size mismatch; and the precise matching of the fiber amount and the resin amount can minimize the waste of materials; the entire preparation method is relatively simple and does not require complicated equipment and process steps. By mixing the fiber and resin directly on the glass plate, the preparation process is greatly simplified, thereby improving production efficiency.

[0020] Furthermore, after the resin is sprayed onto the fibers, the fibers are compacted so that the resin evenly impregnates the fibers.

[0021] By adopting the above technical scheme, the resin can be further evenly penetrated into the fiber through the compaction operation, and the surface of the finally prepared fiber felt premix can be smoother through the compaction operation, making it easier to fit with the molding mold, reducing the generation of bubbles and defects, and further improving the surface quality.

[0022] Furthermore, after the fibers are laid on the glass plate, the fibers are shaken loose, specifically, the fibers are shaken loose using tweezers; a polytetrafluoroethylene film is laid on the glass plate.

[0023] By adopting the above technical solution, the fluffing operation avoids the agglomeration of fibers, so that in the subsequent resin infiltration and compaction process, the resin can penetrate more fully and evenly between the fibers, avoiding local uneven resin content; at the same time, the fluffing operation can effectively remove impurities and dust in the fibers, improving the purity of the fibers; and the use of tweezers can ensure a good fluffing effect, which is easy to use and control and avoids direct contact to contaminate the fibers.

[0024] Furthermore, the step S3 also includes at least one operation of relieving the pressure of the molding die to perform molding venting during the model forming process.

[0025] By adopting the above technical scheme, during the molding process, only the pressure of the molding mold is released to perform the molding exhaust operation. When the fiber premix is ​​molded, after the external pressure is released, the pressure in the molding mold drops rapidly, and pores are formed between the molding mold and the fiber premix, which is conducive to the discharge of gas and completes the exhaust, further reducing the gas content in the fiber premix and the bubbles generated during the molding process, thereby ensuring that the final composite material product has fewer internal pores, improving the product performance, and extending the service life of the product.

[0026] Furthermore, in the step S3, during the compression molding process, when the resin in the fiber premix is ​​in a molten state, the molding mold is subjected to only a pressure relief operation, and then pressure is applied to the molding mold again to complete the molding of the premix.

[0027] By adopting the above technical scheme, when the resin in the fiber premix is ​​in a molten state, the resin in the molding mold is reacting violently. At this time, the temperature of the resin system is greater than 100°C, and the violently reacting resin will carry out a large amount of water vapor and other small gas molecules. By removing the external pressure from the molding mold, the internal pressure of the molding mold drops rapidly, and pores are formed between the molding mold and the fiber premix. The small gas molecules volatilized by the violently reacting resin can be fully released from the pores. The small gas molecules in the premix can be carried out during the release process, thereby ensuring that the final composite material product has fewer internal pores, thereby improving product quality.

[0028] Furthermore, the pressure-releasing operation specifically includes: removing the pressure applied from the outside to the forming mold, and keeping the upper male mold and the lower female mold in the forming mold in contact and closed; and at the same time, the pressure relief time is maintained at 30s to 60s.

[0029] With the above technical solution, since the pressure is released when the resin reacts most violently, the release time should not be too long. If it is too long, it will directly lead to the extension of the entire molding process, thereby reducing production efficiency and increasing production costs. In addition, if the exhaust time is too long, the resin will not harden completely in the subsequent process, thus affecting the final quality. The pressure release time should not be too short. If it is too short, the gas may not be fully discharged, affecting the final quality.

[0030] Furthermore, after the pressure on the molding die is released, pressure is applied to the molding die again, and the applied pressure is the maximum pressure required for molding the composite material product; and the pressure is applied for a time when the resin is in a molten state.

[0031] By adopting the above technical solution, maximum pressure is applied when the resin is in a molten state, so that the resin can be completely filled into every corner of the molding mold, thereby eliminating defects such as bubbles and gaps, and improving the density and uniformity of the product; and accelerating the flow and filling process of the resin, shortening the production cycle, and improving production efficiency; at the same time, the resin is evenly distributed in the molding mold, reducing the wear and damage of the molding mold due to uneven force, thereby extending the service life of the molding mold.

[0032] A composite material product is prepared by using any of the above methods for improving the color difference of the appearance of a composite material product, and has excellent ablation resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a schematic diagram of a composite material product in the prior art;

[0034] Figure 2 A schematic diagram of a composite material product prepared using the present application;

[0035] Figure 3 It is a schematic diagram of the surface of the composite material product prepared by the present application after high temperature washing;

[0036] Figure 4 It is a schematic diagram of the surface of a composite product directly molded using fiber premix after high temperature washing;

[0037] Figure 5 Schematic diagram of the fiber mat premix in this application. DETAILED DESCRIPTION

[0038] The present invention is further described below in conjunction with specific embodiments and drawings.

[0039] A method for improving the color difference of the appearance of a composite material product comprises the following steps:

[0040] S1: preparing fiber premix;

[0041] S2: Laying a fiber felt premix in a molding cavity in a molding mold, wherein the size of the fiber felt premix matches the size of the composite product;

[0042] S3: placing the fiber premix in a molding mold for compression molding to obtain a composite material product.

[0043] In this embodiment, specifically, by laying the fiber felt premix in the molding cavity in the molding mold, because the molding cavity is the position where the fiber premix is ​​molded into the shape of the final product, the fiber felt premix is ​​arranged at this place, because the fiber felt premix itself has uniform fiber distribution, the fiber premix can be more evenly distributed during the molding process, the fiber and resin are mixed more consistently, and the uneven color caused by excessive local resin or fiber aggregation is avoided; the resin content on the surface of the composite material product finally prepared is effectively controlled, and the product surface has no defects such as appearance color difference, glue accumulation, fiber agglomeration, etc., such as Figure 2 As shown. And the size of the fiber felt premix matches the size of the composite product, so that all visible surfaces of the composite product use the same fiber felt premix, the fiber dispersion is good, and the waste of materials is reduced.

[0044] It is worth mentioning that in the prior art, the improvement of the color difference in the appearance of the final composite product is mostly done by improving the components of the composite material. However, improving the components will lead to increased costs and cumbersome subsequent preparation processes, and the improvement effect is not obvious; or optimizing the molding process. The adjustment of the molding process may involve changes in equipment, molds, etc., which requires a high level of technology and cost investment. If the surface treatment process is not properly operated, it may damage the surface of the product and affect the quality.

[0045] Therefore, the present application utilizes the laying of fiber felt premix in the molding cavity to significantly improve the appearance color difference quality of the overall molded composite material of the fiber premix, with good designability and high molding efficiency; the entire method is simple to operate and does not require the introduction of complex equipment or methods. It only requires the preparation of the fiber felt premix. The process is simple and easy to use, which effectively improves the appearance color difference of the final composite product while being low in cost.

[0046] In one specific embodiment, the fiber mat premix is ​​prepared as follows:

[0047] Determine the size of the fiber felt premix according to the inner and outer surface sizes of the composite product; specifically, in this step, draw the unfolded shapes of the inner and outer surfaces of the composite product through a two-dimensional drawing of the composite product, that is, the outer surface refers to the outer contour of the composite product, and the inner surface refers to the internal shape of the composite product; determine the size of the fiber felt premix according to the unfolded plane shapes of the inner and outer surfaces of the composite product;

[0048] Determine the required fiber and resin amounts according to the size of the fiber mat premix and the prepreg technical requirements (glue content and fiber content); specifically, if 100g of fiber mat premix with 50% glue content is required, 50g of fiber and m1g of resin glue are required. If the solid content of the effective ingredient in the resin glue is G, G*m1*X=50g, X is the correction factor, half of which is 1.05-1.10, and needs to fluctuate according to the type of resin and the resin curing process;

[0049] The fibers are spread on a glass plate covered with a polytetrafluoroethylene film, and the resin is sprayed onto the fibers to form a fiber mat premix.

[0050] Specifically, the size of the fiber mat premix is ​​determined by the composite product to ensure that the size of the fiber mat premix and the product are perfectly matched, thereby avoiding product defects caused by size mismatch; and the precise matching of the fiber amount and the resin amount can minimize material waste; the entire preparation method is relatively simple and does not require complex equipment and process steps. By directly mixing the fiber and the resin on the glass plate, the preparation process is greatly simplified, thereby improving production efficiency, and the prepared fiber mat premix is Figure 5 As shown, the fibers are fully impregnated with the resin.

[0051] In a specific embodiment, after spraying the resin onto the fibers, the fibers are compacted to allow the resin to evenly impregnate the fibers.

[0052] Specifically, when a certain amount of resin is evenly sandblasted onto the fiber, a nylon roller is held to fully compact the fiber sprayed with the glue so that the resin fully infiltrates the fiber. During the rolling process, the distribution of fibers and resins in the fiber felt premix needs to be observed, and the uneven distribution of fibers and resins is evenly distributed on the entire fiber felt premix. After the rolling is completed, the fiber felt premix is ​​visually inspected, and no fiber bundles or resin accumulation is required on the fiber felt premix. The compaction operation ensures that the resin penetrates into every tiny part of the fiber more evenly, avoids the formation of cavities or accumulation of resin between fibers, and compresses the gaps between the fibers and the resin through compaction, making the material more compact, which makes the final prepared fiber felt premix have higher strength and better wear resistance. More importantly, the compaction operation makes the surface of the prepared fiber felt premix smooth, making it easier to fit the molding mold, reducing the generation of bubbles and defects, and further improving the processing quality.

[0053] In a specific embodiment, after the fibers are laid on the glass plate, the fibers are further fluffed, specifically, the fibers are fluffed using tweezers; a polytetrafluoroethylene film is laid on the glass plate.

[0054] In this embodiment, the preparation of the fiber felt premix includes fluffing the fibers before they are impregnated with resin, so that the fibers become more fluffy and dispersed, reducing entanglement and agglomeration between the fibers, providing a good foundation for subsequent impregnation and compaction operations, allowing the resin to penetrate more evenly between the fibers, and improving the bonding strength between the fibers and the resin; at the same time, the fluffing operation can also effectively remove impurities and dust in the fibers, ensuring the purity of the fibers.

[0055] More specifically, the fibers are loosened with tweezers. The fine tip of the tweezers can reach into the fine part of the fiber, ensuring that each fiber can be fully loosened and the loosening effect is guaranteed. Compared with the method of loosening directly by hand, it can avoid direct contact with the fiber and cause contamination to the fiber, ensuring the purity of the fiber and then improving the fiber quality. The fibers are evenly laid on the glass plate with polytetrafluoroethylene film to ensure that the resin can penetrate more evenly between the fibers during the impregnation process; and the uniform fiber distribution can make the prepared fiber mat premix have more consistent physical and chemical properties, improving the quality of the fiber mat premix.

[0056] In a specific embodiment, the step S3 also includes at least one operation of only relieving the pressure of the molding die to perform molding venting during the molding process.

[0057] Specifically, in this embodiment, the method of molding exhaust is to only relieve the pressure on the molding die, that is, to keep the molding die in the same state during the molding process, and only relieve the external pressure on the molding die. When the fiber premix is ​​molded, after the external pressure is relieved, the pressure inside the molding die drops rapidly, and pores are formed between the molding die and the fiber premix, which is conducive to the discharge of gas, completes the exhaust, and further reduces the gas content in the fiber premix and the bubbles generated during the molding process, thereby ensuring that the final composite material product has fewer internal pores, improving product performance, and extending the service life of the product. This molding exhaust method is easy to operate, does not require additional parts to be added to the molding die to achieve exhaust operations, saves costs and does not cause other interference, and ensures the quality of composite material products.

[0058] In a specific embodiment, during the compression molding process in step S3, when the resin in the fiber premix is ​​in a molten state, the molding mold is subjected to only a pressure release operation, and then pressure is applied to the molding mold again to complete the molding of the premix.

[0059] It is worth mentioning here that since the fiber premix is ​​formed by impregnating the fiber with resin, the compression molding process is to gradually apply pressure to the product while the molding mold and the fiber premix are heated. The resin is reacting violently in the molten state. At this time, the temperature of the resin system is greater than 100°C. The violently reacting resin will carry out a large amount of water vapor and other small gas molecules; when the resin is in a molten state, only the pressure of the molding mold is released, and the internal pressure of the molding mold drops rapidly, forming pores between the molding mold and the fiber premix. The small gas molecules volatilized by the violently reacting resin can be fully released from the pores, and the small gas molecules in the premix can be carried out during the release process, thereby ensuring that the final composite material product has fewer internal pores and the product quality is improved.

[0060] The traditional molding process is to continuously pressurize and gradually apply pressing pressure to the product. When the temperature of the heating system rises to the resin gel temperature, the resin has completely softened and is close to a fluid state due to the existence of pressure. At this time, the internal reinforcement system (referring to the fiber) of the product has become dense, and a resin-rich layer will form on the surface of the product, and the resin layer is also reacting violently. Because of the pressure, the molding system is close to a sealed state, and the air, water vapor and other small gas molecules carried out by the violent reaction and volatilization of the resin in the mold cannot be discharged. Therefore, the small gas molecules that are not discharged will affect the product quality.

[0061] In a specific embodiment, the pressure-releasing operation specifically includes: removing the pressure applied from the outside to the forming mold, and keeping the upper male mold and the lower female mold in the forming mold in contact and closed; and at the same time, the pressure relief time is maintained at 30s to 60s.

[0062] Specifically, in this embodiment, only the pressure relief operation is to remove the external pressure on the molding die, so that the molding die is in a state of no force and the molding die maintains the upper male mold and the lower female mold in contact and closed state, that is, the upper male mold is not raised, so as to avoid the problem of product scrapping caused by the collapse of the mixed material in the molding die. Because the resin molten state will last for 10 to 20 minutes, the time for pressure relief should not be too long or too short. Too long will directly lead to the extension of the time of the entire molding process, thereby reducing production efficiency and increasing production costs. In addition, too long exhaust time will cause the resin to harden incompletely in the subsequent process, thus affecting the final quality; too short pressure relief should not be too short, because too short may cause the gas to be unable to be fully discharged, affecting the final quality.

[0063] In a specific embodiment, after the pressure on the molding die is released, pressure is applied to the molding die again, and the applied pressure is the maximum pressure required for molding the composite product; and the pressure is applied for a time when the resin is in a molten state.

[0064] Applying maximum pressure when the resin is in a molten state allows the resin to completely fill every corner of the molding mold, thereby eliminating defects such as bubbles and gaps, and improving the density and uniformity of the product; and accelerating the flow and filling process of the resin, shortening the production cycle, and improving production efficiency; at the same time, the resin is evenly distributed in the molding mold, reducing the wear and damage of the molding mold due to uneven force, thereby extending the service life of the molding mold.

[0065] In a specific embodiment, a composite material product is prepared by using the above-mentioned method for improving the color difference of the appearance of a composite material product. The composite material product prepared by the method has excellent ablation resistance when used as an ablation heat insulation product. Figure 3 The product made by this application method has a relatively smooth, uniform and dense surface after high-temperature scouring and ablation; Figure 4 It is a product that is directly molded using fiber premix. After being washed at high temperature, its surface has localized shedding and falling off after ablation. The reason is that the resin content on the surface is uneven. In areas with less resin content, the resin decomposes after ablation, and the fibers have nowhere to attach, so they are washed off by the airflow.

[0066] The examples described in the present invention are merely descriptions of the preferred implementation modes of the present invention, and are not intended to limit the concept and scope of the present invention. Without departing from the design concept of the present invention, various modifications and improvements made to the technical solutions of the present invention by engineers and technicians in this field should all fall within the protection scope of the present invention.

Claims

1. A method for improving the color difference of the appearance of a composite product, characterized in that: The steps include: S1: preparing fiber premix; S2: Laying a fiber felt premix in a molding cavity in a molding mold, wherein the size of the fiber felt premix matches the size of the composite product; S3: placing the fiber premix in a molding mold for compression molding to obtain a composite material product.

2. The method for improving the color difference of the appearance of a composite product according to claim 1, characterized in that: The fiber mat premix is ​​prepared as follows: Determine the size of the fiber felt premix according to the inner and outer surface sizes of the composite product; Determine the required amount of fiber and resin based on the size of the fiber mat premix; The fibers are spread on a glass plate covered with a polytetrafluoroethylene film, and the resin is sprayed onto the fibers to form a fiber mat premix.

3. The method for improving the color difference of the appearance of a composite product according to claim 2, characterized in that: After the resin is sprayed onto the fibers, the fibers are compacted to allow the resin to evenly impregnate the fibers.

4. A method for improving the color difference of the appearance of a composite product according to claim 2 or 3, characterized in that: After the fibers are laid on the glass plate, the fibers are further fluffed, specifically, the fibers are fluffed using tweezers; a polytetrafluoroethylene film is laid on the glass plate.

5. The method for improving the color difference of the appearance of a composite product according to claim 1, characterized in that: The step S3 also includes at least one operation of relieving the pressure of the molding die to perform molding venting during the molding process.

6. The method for improving the color difference of the appearance of a composite product according to claim 5, characterized in that: In the step S3, during the compression molding process, when the resin in the fiber premix is ​​in a molten state, the molding die is subjected to a pressure-releasing operation, and then pressure is applied to the molding die again to complete the molding of the premix.

7. A method for improving the color difference of the appearance of a composite product according to claim 5 or 6, characterized in that: The pressure-releasing operation specifically includes: removing the pressure applied by the outside to the forming mold, and keeping the upper male mold and the lower female mold in the forming mold in contact and closed; and at the same time, the pressure relief time is maintained at 30s to 60s.

8. The method for improving the color difference of the appearance of a composite product according to claim 7, characterized in that: After the pressure on the molding die is released, pressure is applied to the molding die again. The applied pressure is the maximum pressure required for molding the composite material product. The pressure is applied for a period of time when the resin is in a molten state.

9. A composite material product, characterized in that: The composite material product is prepared by the method for improving the color difference of the appearance of the composite material product as described in any one of claims 1 to 8.

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