Carbon Fiber Reinforced Plastics (CFRPs) is a combination of carbon fiber and resin with high strength and stiffness, and the weight of the composite is less than half the weight of similar rigid and ultra-high tensile steel plates. One of the most important factors affecting the properties of composite materials is adhesion at fiber-matrix interfaces. Sizing refers to coating the surface of a fiber with a thin polymer and in a way that has become common, most commercial fibers are now coated with sizing materials.
The physical and chemical changes of the carbon fiber surface according to the size of the materials were analyzed by TGA, SEM, XPS and FT-IR. As a result, EP and PA sized materials were proven to be the most suitable for the T-RTM process.
Introduction …
The fiber-matrix bond is strongly associated with the load transfer efficiency within the composite, thus influencing the overall mechanical properties of the material15. The weakest part of CFRP is the interphase between the fiber and the resin due to the inert surface of carbon fiber. Then, depending on the fiber/size/matrix compatibility, it increases the adhesion between the fibers and the matrix15.
These chemical reactions affect fiber-matrix bonding, which in turn potentially affects the overall mechanical properties of the composite. Using the T-RTM process, coated carbon fiber reinforced A-PA6 composites were fabricated, and the analysis of the mechanical properties of composite materials and the chemical reaction analysis of sizing/A-PA6 investigated the effects of sizing on the fiber matrix bonding.
Literature Review
Reactive Processing of Thermoplastic PA6
With studies, the results showed 3.7 and 2.9 wt. % catalyst to activator ratio, 150℃ processing temperature, 25s polymerization time, less than 5 minutes production cycle and 98% monomer conversion.
Improvement Method for Fiber-Matrix Adhesion …
The T-RTM process was performed with nanomaterial-dispersed resin and plasma-treated carbon fiber to create an A-PA6 nanomaterial/plasma-treated carbon fiber reinforced woven composite. In particular, the exfoliated A-PA6/plasma treated graphite nanoplatelet reinforced carbon fiber composite had a higher improvement in in-plane shear strength. Different improvement methods for fiber matrix adhesion: (a) growth of CNTs on SiC5 woven fabric, (b) carbon fibers grown with ZnO nanorods coated with CNT6 modified silane coupling agent.
Sizing Process
The HMSA-1 sizing agent created acted as a coupling agent by forming a covalent bone with the vinyl ester resin and functional groups on the carbon fiber surface, and the interlaminar shear strength (ILSS) was improved by 20.7%. Allred et al.8 improved the thermooxidative ability and mechanical performance of fibers through bonding on the carbon fiber surface (Fig. The bonding material coated on the commercial carbon fiber was removed and post-treated using the coupling agent and polyimide resin.
After subjecting it to a high temperature of 343℃, the results of mechanical tests showed that the post-treated sample had the least amount of microcracks, weight loss and the strongest thermally stable interface. Composite mechanical properties with different fiber treatment and thermal aging conditions: (a) short beam shear strength and (b) in-plane shear strength8. In addition, surface energy changes due to bonding materials coated with carbon fiber affect fiber-matrix adhesion.
Dai et al.9, 46, 47 demonstrated the effect on changes in surface energy and IFSS by size removal of commercial carbon fibers (Figure 5). Commercial carbon fibers coated with sizing material were debonded and then debonded by acetone extraction. The result showed that the size-reduced carbon fibers had a higher IFSS than the commercial carbon fibers and had the same tendency with the increase in the ratio of the dispersive components of the surface energy.
Experimental …
Materials …
Sample Preparation …
- Desizing Process
- Sizing Process
- T-RTM Process …
First, we moistened the woven carbon fiber for one minute in an adhesive solution designed for each adhesive material, then removed it and heated it in an oven. The weight of the carbon fiber before and after the gluing process was measured at least five times to calculate the glue content. The concentration of adhesive materials, temperature and time of heat treatment were optimized for each adhesive material to achieve a suitable adhesive content of 1 to 1.5% by weight, and the result is shown in Table 1.
Coated carbon fiber fabrics were placed inside the mold, then catalyst (3.7 wt%) and activator (2.9 wt%) with ε-caprolactam were melted in the individual tanks A and B maintaining nitrogen environment. After the temperature of the mold reached a suitable processing temperature of 145℃, the matrix solutions in individual tanks A and B were immediately mixed and transferred into the mold with vacuum (75 kPa).
Characterization and Mechanical Testing
IFSS and short beam strength of composites were measured using a universal testing machine (UTM, 5982, Instron). In the fiber bundle pull-out test, one end of the 24k carbon fiber bundle (T-700) was embedded into the A-PA6 matrix and the embedded length was 3~5mm. An experiment was performed at a crosshead speed of 0.5 mm/min, and the sample was measured at least 10 times per sample.
For the short beam test, the specimen data and test method were obtained from ASTM D2344. The test was performed at least 10 times for each sizing material at a cross head speed of 1 mm/min. The short beam tension when the crosshead movement was 4 mm was defined as “short beam strength” in this paper.
The chemical reactions between the sizing materials and the A-PA6 matrix were investigated by analyzing the specific peaks using FT-IR spectroscopy (NicoletTM iS50, Thermo Fisher Scientific Inc., USA). For ATR-FTIR measurements, the NicoletTM iS50 was equipped with an ATR Smart MIRAcleTM module (PIKE Technologies, Inc., USA). A germanium crystal (18 mm diameter) was used and the A-PA6 polymer size materials/composites were analyzed in transmission mode at cm-1 scan interval at room temperature.
To calculate the void content of the composite, CTAn and CTVol, 2D/3D processing and analysis programs from Bruker, were used with the constant threshold value determined by optical equation62.
Results and Discussion …
- TGA of Coated Carbon Fiber
- SEM Morphological Analysis of Coated Carbon Fiber …
- XPS Analysis of Coated Carbon Fiber ….…
- Surface Energy Analysis of Coated Carbon Fiber ….….…
- Fiber-Bundle Pull-out Test …
- Short Beam Shear Test …
- Reinforcement Mechanism
- Anionic Ring Opening Polymerization of ε-caprolactam
- Chemical Reactions of Sizing Materials/A-PA6
- FTIR Spectroscopy of Sizing Materials/A-PA6 ….….…
- X-ray Micro CT Analysis
- Fracture Surface Analysis …
The XPS analysis was performed to investigate the chemical composition and functional groups of the coated carbon fiber surface. First, the chemical composition of the carbon fiber surface changed with the bonding materials and it is shown in Table 2. The C 1s peak spectrum of the coated carbon fiber surface was deconvolved into several Gaussian-Lorentzian (8:2) peaks6, 64 -67 by the CasaXPS program, and the results was shown in table 3 and fig.
The total surface energy and the polar/dispersive component ratio of fibers are related to the matrix impregnability in the fabrics and therefore influence the properties of the composite69-73. In addition, the A-PA6 matrix has a high polar component ratio due to hydrogen bonding of the amide group. It was confirmed that most of the coated CF/A-PA6 composites had higher IFSS than the reduced CF/A-PA6 composite.
To measure the short beam strength of the composite, a short beam shear test was performed according to ASTM 2344 standard. As a result, these shear stresses will cause an interlaminar shear fracture in the central region of the specimen thickness. Therefore, “short beam stress” was defined as short beam stress when the crosshead displacement is 4 mm, which is the thickness of the specimen.
This has the same trend with the results of XPS and surface energy, indicating that activated carbon percentage and surface energy of fiber have a significant effect on fiber-matrix adhesion of the composite materials. The isocyanate group, for AS79 (Fig. 15b), is formed during the A-PA6 polymerization because the process temperature is higher than the deblocking temperature of the activator. PA adhesive material is polymer composed of the repeated amide groups as A-PA6 matrix, which forms a strong hydrogen bond between them (Fig. 15f).
The corresponding peaks disappeared in PU/A-PA6, indicating the possibility of the reaction occurring. In the case of the EP-coated sample, it was confirmed that the EP-coated carbon fibers and the A-PA6 resin were closely bonded, and a large part of the resin was well attached to the surface of the debonded fibers.
Summary …
This led to void formation in the fiber-matrix interphase and weakened interfacial strength, and it was proven by the fracture surface analysis.
Conclusions and Recommendation for Future Work
Conclusions
Recommendations for Future Work
Combining the above studies, it can be expected that the improvement of fiber-matrix adhesion through the sizing and the simplification of the T-RTM process can be achieved simultaneously if the fiber surface is coated successfully and without modification by CPLS and activator (Fig .21). ). Gnadinger, F., Middendorf, P., Fox, B., Interfacial shear strength studies of experimental carbon fibers, new thermoset polyurethane and epoxy matrices and tailor-made adhesives, Compos Sci Technol. Thiagarajan, A., Palaniradja, K., Velmurugan, K., Effect of interfacial bonding on impact properties of chopped glass fiber polymer nanocomposites, Compos Interface.
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