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Thermal expansion molding process (TEMP) is a sophisticated molding technique for creating complex sandwich composites. The core key of TEMP is a high‐performance, thermally expandable resin film. In this study, the epoxy resin is used to create a specific crosslinking network that can provide the support force required for the stable molding of the vesicles during the expansion process of the foaming agent. The curing process of the resin was determined by analyzing the change in the exothermic amount of the epoxy resin during the curing process and the flow behavior of the resin. The pore size distribution of the epoxy foam was investigated, and the expansion force of the thermally expandable resin was tested. The TEMP molding process was compared with the vacuum bag press molding (VBPM) process. The TEMP molding technique increased tensile and compressive strength by about 25%, flexural strength by 34.5%, and shear strength by 54.5%. Finally, the section on microanalysis of the comparative materials and the short beam bending experiments of the sandwich structure provides a theoretical foundation for developing complicated sandwich structures. Highlights: Preparation of thermally expandable epoxy prepregs with stabilized foaming by means of pre‐curing.The effects of curing temperature, foaming agent content and expansion multiplicity on the thermal expansion force of epoxy prepreg adhesives were investigated.Thermally expandable epoxy prepreg molding composites with different expansion multiplicities and analysis of the mechanical properties of the composites.Preparation of foam core sandwich composites by thermal expansion in‐mold monolithic molding process and their verification by bending experiments. [ABSTRACT FROM AUTHOR]

Efficient enhancement of the toughness of epoxy resins has been a bottleneck for expanding their suitability for advanced applications. Here, polysulfone (PSF) was adopted to toughen and modify the epoxy. The influences of PSF on the mechanical and thermal properties of the epoxy resin were systematically studied by optical microscopy, Fourier transform infrared spectrometer (FT-IR), differential scanning calorimetry (DSC), thermogravimetric analyzer (TG), dynamic mechanical thermal analyzer (DMA), mechanical tests and scanning electron microscope (SEM). The dissolution experimental results showed that PSF presents a good compatibility with the epoxy resin and could be well dissolved under controlled conditions. The introduction of PSF was found to promote the curing reaction of the epoxy resin without participating in the curing reaction and changing the curing mechanism as revealed by the FT-IR and DSC studies. The mechanical properties of PSF/epoxy resin blends showed that the fracture toughness and impact strength were significantly improved, which could be attributed to the bicontinuous phase structure of PSF/epoxy blends. Representative phase structures resulted from the reaction induced phase separation process were clearly observed in the PSF/epoxy blends during the curing process of epoxy resin, which presented dispersed particles, bicontinuous and phase inverted structures with the increase of the PSF content. Our work further confirmed that the thermal stability of the PSF/epoxy blends was slightly increased compared to that of the pure epoxy resin, mainly due to the good heat resistance of the PSF component. [ABSTRACT FROM AUTHOR]

To satisfy the light weight requirements of vehicles owing to the aggravation of environmental pollution, carbon-fiber (CF)-reinforced epoxy composites have been chosen as a substitute for traditional metal counterparts. Since the current processing methods such as resin transfer molding (RTM) and compression molding (CM) have many limitations, an integrated and optimal molding method needs to be developed. Herein, we prepared high-performance composites by an optimized molding method, namely compression resin transfer molding (CRTM), which combines the traditional RTM and CM selectively and comprehensively. Differential scanning calorimetry (DSC) and rotational rheometry were performed to optimize the molding parameters of CRTM. In addition, metallurgical microscopy test and mechanical tests were performed to evaluate the applicability of CRTM. The experimental results showed that the composites prepared by CRTM displayed superior mechanical properties than those of the composites prepared by RTM and CM. The composite prepared by CRTM showed up to 42.9%, 41.2%, 77.3%, and 5.3% increases in tensile strength, bending strength, interlaminar shear strength, and volume fraction, respectively, of the composites prepared by RTM. Meanwhile, the porosity decreased by 45.2 %. [ABSTRACT FROM AUTHOR]

The winding tension of the dry-wound forming process is uniform, the process is easy to control and the structure and performance of the prepared vehicle-mounted hydrogen composite pressure vessels are stable. Since the vehicle-mounted hydrogen pressure vessels must be subjected to rigorous fire experiments, the flame-retardant epoxy prepreg yarn technology is currently monopolized by foreign countries. The development of flame-retardant prepreg yarn with good processing properties is of great significance to the promotion and application of hydrogen pressure vessels. Here, a flame-retardant additive hexaphenoxy cyclotriphosphazene (HPCTP) was synthesized and dissolved in epoxy resin of prepreg yarn named P/N-EP. Then, the flame-retardant prepreg yarn was made by the solution impregnation method. Curing behavior, thermal analysis combustion behavior and mechanical properties of P/N-EP and tensile properties of flame-retardant prepreg yarn were investigated. Experimental results showed that the LOI value reached 36.4% at 10 phr and reached the V-0 level of the UL-94 test. The results about thermal stability of the P-N/EP resin system showed that with the increase of P-N content, T5% (temperature at 5% weight loss) and CY600 (carbon residue rate) both increased. The microstructure of carbon residue after UL-94 test was studied by electron microscope and laser Raman spectrometer. The results showed that with the increase of P-N content, the carbon layer structure became denser and the ID/IG decreased significantly, to achieve the flame-retardant effect by the condensed phase mechanism. [ABSTRACT FROM AUTHOR]

In this work, a dicyclopentadiene monomer/Grubbs 2 catalyst was added to a mixture of epoxy oligomers and methyl nadic anhydride to obtain a poly‐dicyclopentadiene (DCPD) modified epoxy cross‐linked network via sequential curing. The hybrid resin was used as a matrix for preparing carbon fiber composites. The results indicated that the addition of DCPD monomers significantly reduced the viscosity of the epoxy resin. The toughness and heat resistance of the co‐cured resin was improved without significant loss of strength, which was attributed to the high compatibility between epoxy and the modifier polymer. Regarding the composites, the flexural strength of the hybrid resin‐derived composite first increased and then diminished slightly. Meanwhile, the mode‐I interlaminar fracture toughness continued to increase with the increase of PDCPD content compared to the non‐modified composites. Generally, the change trend of the above properties in the composites was consistent with the change trend of the strength and toughness exhibited in the bulk resin with increasing PDCPD contents, while the change extent was inconsistent due to factors such as fiber volume content and interfacial bonding. Highlights: The epoxy/PDCPD hybrid resin system was obtained via sequential curing.The PDCPD as modifier can reduce the viscosity of the epoxy curing system.The toughness and heat resistance of the PDCPD modified epoxy resin was improved.The loss in flexural strength of the hybrid resin system was insignificant.The GIC of the epoxy composites modified with PDCPD improved appreciably. [ABSTRACT FROM AUTHOR]

Large tow carbon fiber (LTCF) has excellent performance and low cost. However, there are still many difficulties in the application of LTCFs, among which the problem of interfacial penetration is mainly solved. Building interfaces with sizing structures of fiber composites is an effective approach to achieve high performance. In this article, a water-based epoxy sizing agent suitable for LTCF fiber thermoset composites was studied. A comparison of the five LTCFs in this study shows that the wettability and contact angle of the sized LTCFs are significantly improved. In addition, the interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) increased from 52.3 to 71.0 MPa (35%) and 36.4 to 54.6 MPa (50%), respectively. Compared with other imported LTCFs, domestic large-size LTCFs have a wide variety and a large number of surface functional groups, thereby improving their wettability and covalent bonding with the resin matrix. This work has reference significance for the practical application and industrial production of LTCFRP. [ABSTRACT FROM AUTHOR]

In this research, a resin film of the MAM/EP blend curing system containing the triblock copolymer polymethyl methacrylate- b -poly-n-butyl acrylate- b -polymethyl methacrylate (MAM) was prepared. Such resin film was inserted into the interlaminar of rapid-curing composites to simultaneously improve both mode I, mode II fracture toughness and impact resistance. Both the Mode I and Mode II interlaminar fracture toughness of the composites were carefully investigated as a function of MAM content of the MAM/EP resin film (0, 20 and 40 phr@50 μm). A maximum improvement up to 109% and 70.2% on Mode I and Mode II fracture toughness, respectively was obtained with the introduction of the 40 phr@50 μm compared to the laminates with the introduction of the 0 phr@50 μm. Analysis of the fracture surfaces of the laminates elucidated that the distinctive improvement of the interlaminar fracture toughness could be attributed to the characteristic interlaminar structures of MAM in epoxy resin by self-assembly. The interleaved laminates also displayed an increase of 26.7% for the compression after impact (CAI) properties, and showed excellent impact resistance. The interlaminar shear strength (ILSS) of the composite was thereafter explored, which indicated slight enhancements to the strengths. [ABSTRACT FROM AUTHOR]

In this study, a series of GO with different oxidation levels were prepared via a modified Hummers' method, in which the oxidant/graphite ratio and oxidation time were varied. The change of GO structure as a function of oxidation level was investigated by X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction measurements. The prepared GO were used for reinforcing epoxy resins, and the surfaces of GO were functionalized by the amine hardener during the dispersion process. Microscopic observations show that the oxidation level has a significant impact on the dispersion state of GO in the epoxy matrix. Both GO with the low and high oxidation levels tend to have low degree of dispersion, even aggregate. The influences of oxidation level on the performances of epoxy/GO composites, including curing behaviors, rheological, thermal and mechanical properties, were investigated in detail. Notably, the GO with optimum level of oxidation (denominated as GO-4) shows superior toughening effects on the epoxy matrix, compared to the reported epoxy/GO composites. The incorporation of merely 0.2 wt% GO-4 would yield 56 and 128% enhancement in the critical stress intensity factor ( K IC ) and the critical strain energy release rate ( G IC ), respectively. [ABSTRACT FROM AUTHOR]

In this work, the tackified PDCPD (PDCPD-T) specialized for vacuum assistant resin infusion process (VARI) was developed. The epoxy resin and methyl nadic anhydride (NMA) was subsequently added to the PDCPD-T to improve the interfacial bonding with the carbon fibres. The results showed that the introduction of NMA-cured epoxy undermined the mechanical properties of the PDCPD-T owing to the agglomeration of the tackifier particles. Nonetheless, the mechanical properties of the CFRCs based on the co-cured blends of the PDCPD-T and the NMA-cured epoxy were comprehensively improved compared to the composites based on the PDCPD-T. The improvement was that a part of NMA-cured epoxy in the co-cured blends would phase separate on the surface of the carbon fibre, which formed the "bridge" connecting the matrix and the carbon fibre. In addition, the degree of the tackifier agglomeration would also decrease due to the decrease in the surrounding epoxy concentration. [Display omitted] • The continuous carbon fiber reinforced polydicyclopentadiene (PDCPD) composites based on the vacuum-assisted resin infusion process are realized by tackifying the dicyclopentadiene (DCPD) monomer. • The further addition of anhydride-cured epoxy resin to the tackified DCPD monomer solves the problem of poor interfacial bonding between the PDCPD resin and the carbon fiber. • The carbon fiber composites based on the modified PDCPD matrix are proved to be far superior in interlaminar fracture toughness and impact resistance than the epoxy-based composites. [ABSTRACT FROM AUTHOR]

In this study, a phase inversion approach was employed to fabricate four different thicknesses (20 μm to 100 μm) of polyethersulfone (PES) porous films with visible holes (1 hole/cm2) on the surface, applied as interleaves to improve the interlaminar fracture toughness of carbon fiber/epoxy composite laminates prepared by vacuum assistant resin infusion process (VARI). The film could dissolve into the epoxy resin which as clearly evident by the optical microscopy under a controlled condition. It was noted that the thickness of the resin-rich layer and the distribution of PES microspheres in the interlaminar layer depended on the thickness of the PES porous film. Also, mode I and mode II fracture toughness, interlaminar shear strength (ILSS), flexural properties as well as tensile properties of the CF/EP composites had all been characterized and analyzed in detail and in order. The results demonstrated that mode I and mode II fracture energies at the optimal interleaved laminates for the toughened system were increased by 61.5% and 55.1% compared to the composites without interleaves, respectively. Besides, PES porous film interleaved laminates displayed an increase of 32% for the interlaminar shear strength and no significant changes in the flexural and tensile properties. It was observed that the films induced toughening via two mechanisms: i) cohesive failure in a thicker resin region and ii) crack deflection and microcrack caused by PES microspheres, illustrated in the microstructure analysis of the failure surfaces and the observations of the crack propagation path. [ABSTRACT FROM AUTHOR]