A Cast-in-place Process For Fabricating Bicontinuous Fiber-Reinforced Epoxy Composites

The application scope of epoxy composites is expanding,making it one of the most popular categories in the field of materials science and engineering.Generally,epoxy composites are made from epoxy resin and other fillers,including particles and fibers—several methods for fabricating the composites,such as traditional mixing,hand lay-up,and vacuum infusion.In the molding or direct-mixing method,the epoxy liquid fluidity is necessary,especially if the mold has a complex shape.The fluidity can be enhanced by decreasing the percentage of filler in the liquid polymers.However,high loading of filling within the epoxy is often not desirable,resulting in increasing voids formation and filler aggregation in the resulting composite.In the case of a filler-epoxy composite,the higher the filler/epoxy ratio,the lower the composite's mechanical performance.The mechanical interlocking between the fillers and the epoxy matrix is essential to adhering if the fibers form spring-shaped or a three-dimensional network.Besides,the filler should have a rough surface to permit a small degree of mechanical entanglement to occur to improve mechanical properties for composites.Furthermore,the aspect ratio of the fiber strongly affects the mechanical performance of the composites.The bonding between the fibers and the epoxy matrix increases with the increase in the aspect ratio of the fibers,resulting from the increase in the contact area between the fiber and the resin.This factor helps to improve mechanical performance.Besides,the pulled-out is more in short fiber composites compared with long fiber composites.To obtain composites with high mechanical performance,we designed a cast-in-place process as a new method for modeling a bicontinuous composite with a high mechanical performance by perfusing epoxy resin into a three-dimensional network structure of short fibers.The cast-in-place process with foam can overcome the aggregation of short fillers in the epoxy matrix,ensuring most of the stress being transferred throughout the 3D skeleton structure.The cast-in-place process was much better than the direct-mixing method for preparing composites with high mechanical performance.The effects of different filler shapes and sizes and composites manufacturing processes on mechanical properties were systematically discussed.(1)As a comparison consideration,Helical carbon nanotubes(HCNTs)and multi walled carbon nanotubes(MWCNTs)were blended respectively with the epoxy matrix using the direct-mixing method for preparing epoxy-based composites.The mechanical performances of the composites were investigated.The effects of loading and fillers modification within the epoxy composites on the mechanical properties were studied.The flexural strength and strain of HCNTs/epoxy composites are 20.0 and 47.7% higher than that of the MWCNTs/epoxy composites at the filler content of 0.37vol%.The tensile strength of the composite also improved.For composites of MWCNTs/epoxy,the samples displayed the tensile strength improvement by 46.1% at the filler loading 0.37vol% compared with pure epoxy.Simultaneously,the HCNTs/epoxy composites showed improvement by 51.2% at the same filler loading(0.37vol%)increments as pure epoxy.HCNTs were found to be more efficient than MWCNTs as reinforcing fillers for the epoxy matrix,having excellent flexibility.However,the flexural strength and the flexural strain were enhanced by 72.0 and325.0%,respectively,compared to pure epoxy after adding a small amount of the KH560 modified HCNTs.Simultaneously,the tensile strength and young's modulus of KHCNTs/epoxy composites were 51.3 and 270.9% higher than those of pure epoxy.The results showed that the presence of silane molecules improved the dispersion of HCNTs in epoxy and the interfacial interaction.SEM images of the fractured composite cross-sections revealed that the HCNTs were not pulled from the epoxy matrix but were fractured with an epoxy matrix compared to the straight MWCNTs subjected to pulling and cracking upon exposure to stresses.This can be attributed to the unique shape of HCNTs.HCNTs have a larger specific surface area than MWCNTs,which helps develop the composites' mechanical performance because of the strong bonding(chemical and mechanical bonding)between the surface of the HCNTs and the epoxy matrix.Straight MWCNTs have a smooth surface as well as have low dispersion and weak bonding with epoxy resin;thus the epoxy composite prepared to have low interface bonding.Better cross-linking is important for efficient transfer of stress from the matrix of epoxy to the carbon nanotubes;this helps decrease stress concentrations besides increases overall mechanical performance.(2)A cast-in-place process was designed and applied for directly perfusing epoxy resin into the three-dimensional nanofibers foam(3D NFFs)towards a bicontinuous composite that displayed high mechanical performance.Firstly,the 3D NFFs skeletons with densities from 0.028 to 0.218 g/cm3 were obtained by regulating the nanofibers growth conditions.Secondly,the epoxy composites were prepared with different loadings of the surfacemodified nanofibers from 5.2 to 36.7% by volume.In the formation of 3D NFFs,the nanofibers were well entangled,which caused the infusing epoxy to be immersed uniformly into the porous 3D NFFs,resulting in a bicontinuous composite.Mechanical properties such as compressive and flexural behavior have been studied.The cast-in-place process composites showed remarkably improved compressive and flexural properties compared to the direct-mixing composites.Besides,it was found that the compressive and flexural properties increase with the increase of fiber contents;the remarkable enhancement in mechanical performance started at the loading of 10.8–14.3vol%.Compared to the pure epoxy,the compressive strength of the cast-in-placed 3D NFFs/epoxy composite was improved by 436.8% at the loading of 10.8vol%,and the flexural strength was increased by133.5% at the loading of 14.3vol%.In contrast,the compressive and flexural strength of the dispersed NFPs/epoxy composite prepared through the traditional mixing method was increased by 173.6% at the loading of 10.8vol% and 63.1% at the loading of 14.3vol%,respectively.As to the cast-in-place composites,the strength increased due to the nanofibers foam's flexibility,high strength,and sufficient entanglement,resulting in highly effective stress transfer.The mechanical properties of the direct-mixing composite resulted from poor bonding between the nanofibers and the epoxy.The aggregation of the fibers resulted in the low-stress transmission from the matrix to the fibers.(3)Short basalt fibers defects in an epoxy composite when using the conventional mixing process can be solved by using the cast-in-placed process.In the cast-in-placed process,the epoxy resin is poured into 3D basalt fibers foam(3D BFFs).In this study,the assembly basalt fibers foam has been made of short fibers to reinforce the epoxy composite using the cast-in-place process.A good dispersion of basalt fibers inside the epoxy matrix using the cast-in-placed process gave high mechanical performance compared to the directmixing method.The 3D bicontinuous BFFs/epoxy composites with suitable fibers dispersion within a matrix are essential in controlling the transfer of stress from the epoxy to the basalt fibers.The direct-mixing method used for dispersing basalt fibers in epoxy composite was usually unevenly distributed,reducing the benefit of using the short basalt fibers to enhance the epoxy composites' mechanical performance to the cast-in-place composite.To guarantee the advantage of 3D bicontinuous BFFs/epoxy composites and the possibility of our study,we compared the performance of the composites at the same loading(0,13,21,and 31vol%)of basalt fibers powder and 3D basalt fibers foam.However,for the direct-mixing of basalt fiber/epoxy composites,the composites displayed a little enhance in flexural properties.The remarkable improvement was 72.0% at the basalt fiber content of 21vol%.At the 3D basalt fibers foam loading of 21vol%,the flexural strength increased by 173.5% compared with the pure epoxy.Besides,the maximum tensile strength of direct-mixing composite enhancement of 9.1% was obtained with basalt fibers of 21vol%.The fusion of epoxy liquid into 3D basalt fibers foam was observed a significant improvement in the tensile strength.The tensile strength of 3D basalt fibers foam/epoxy composite was increased by 28.7% relative to pure epoxy at the basalt fiber loading of 21vol%.SEM microscopy showed that the cast-in-place process using 3D fibers foam provides a sufficient entanglement of the basalt fibers.The homogeneous distribution and crosslinking of fibers play a significant role in improving the mechanical properties of the composite.The study also reported the formation of carbon nanofiber on the basalt fibers surface to form(BFFs-CNF),where the CVD process obtained the BFFs-CNFs.The growth loading of CNFs on basalt fibers was adjusted by the letting amount of acetylene flow rate for the in a situ CVD process.The growing CNFs on the basalt fibers surface help increase the basalt fibers surface area and enhance the interaction between the basalt fiber and the epoxy.Remarkably,the flexural strength and the flexural strain were improved by 109.0 and 28.2%,respectively,compared to the pure epoxy after growing CNFs on the basalt fibers surface.The improvement in mechanical properties is attributed to the strong adhesion of the fiber with the epoxy matrix.Conversely,grafting CNFs onto the basalt fibers surface reduces epoxy composites' mechanical properties compared to the original BFFs/epoxy.The insufficient reinforcement on mechanical properties could be ascribed to the deterioration in basalt fiber tensile strength upon exposure to high temperature during the CVD process.(4)The epoxy resin was cast with silane coupling agents and woven basalt fibers(W-BFs)using the cast-in-place process.Silane coupling agents(KH550,KH570,and KH590)were respectively blended with epoxy resin at room temperature,and then the woven basalt fibers were impregnated with the epoxy solution.It was found that the addition of KH590,KH570,or KH550 could remarkably increase the mechanical properties of the composites compared to untreated woven basalt fibers/epoxy composite.Compared with the untreated W-BFs/EP composite(173.2 MPa),when the content of KH590,KH570 and KH550 is 9wt%,the tensile strength reaches a maximum 307.6,258.1,and 311.0 MPa,which demonstrated increases by 77.6,49.0 and 79.6%,respectively.While the flexural strength of the WBFs/epoxy composite modified with 9wt% KH590,KH570 and KH550 improved by 65.3,81.5,and 74.5%,respectively,compared with untreated W-BFs/EP composite(271.5 MPa).Herein,the enhancement of the mechanical performance of composites is assigned to the transfer of the effective stress from the epoxy to the basalt fibers,which arises from the increase of the epoxy cross-linking chain as well as the enhanced interfacial adhesion between the epoxy and woven basalt fibers.The basalt fiber is a silicate-based composite with the same composition as the silane coupling agent.Silane coupling agent could react with the hydroxyl groups of silicon at the surface of basalt fiber,forming a new bonding of-Si-O-Si-,resulting in a more uniform network structure of silicon-oxide resulting in increased mechanical performance.However,the flexural strength of the epoxy resin modified with 6wt% of KH560,KH590,KH570,or KH550 was strengthened by 69.4,57.6,83.4,and 84.7%,respectively.Whereas the flexural strain improved by 75.0,292.8,281.8,and 342.8% with the same silane coupling agent concentration(6wt%),respectively.When silane coupling agents were mixed with epoxy resin,it could interact with resin through the epoxide groups and silanol groups and thus entered the cross-linked network,which acted as cross-linking sites in some extent,causing better cross-linking and thus higher flexural strength.