Thermal and dynamic mechanical behavior of epoxy composites reinforced with post-consumed yerba mate

Yerba mate (YM) is widely consumed in Latin American countries, and its residues can be used as bio-resources such as reinforced in epoxy composites. The present work aims to produce epoxy resin composites and evaluate the influence of post-consumed YM as reinforcement. The concentrations of YM used were 5, 10, and 20% (wt/wt). Chemical, thermal, morphological, and dynamic mechanical behaviors were explored. The YM incorporation did not influence chemically on the epoxy structure and a pull-out phenomenon was observed as YM content increased. The YM at lower concentrations (5 and 10%) led to higher values of activation energies calculated from model-free isoconversional methods. On the other hand, the composite e/YM 20 wt% improved all dynamic-mechanical properties. YM proved to be a suitable and cheap reinforcement for epoxy resin.     

Multivariate process trajectories for molecular description of MF thermal curing and correlation with hydrolytic stability

During curing of thermosetting resins the technologically relevant properties of binders and coatings develop. However, curing is difficult to monitor due to the multitude of chemical and physical processes taking place. Precise prediction of specific technological properties based on molecular properties is very difficult. In this study, the potential of principal component analysis (PCA) and principal component regression (PCR) in the analysis of Fourier transform infrared (FTIR) spectra is demonstrated using the example of melamine-formaldehyde (MF) resin curing in solid state. FTIR/PCA-based reaction trajectories are used to visualize the influence of temperature on isothermal cure. An FTIR/PCR model for predicting the hydrolysis resistance of cured MF resin from their spectral fingerprints is presented which illustrates the advantages of FTIR/PCR compared to the combination differential scanning calorimetry/isoconversional kinetic analysis. The presented methodology is transferable to the curing reactions of any thermosetting resin and can be applied to model other technologically relevant final properties as well.     

Improving through-thickness conductivity of carbon fiber reinforced polymer using carbon nanotube/polyethylenimine at the interlaminar region

The through-thickness conductivity of carbon fiber reinforced polymer (CFRP) composite was increased by incorporating multiwalled carbon nanotubes in the interlaminar region. Carbon nanotubes (CNTs) were dispersed in a polyethylenimine (PEI) binder, which was then coated onto the carbon fiber fabric. Standard vacuum-assisted resin infusion process was applied to fabricate the composite laminates. This modification technique aims to enhance the electrical conductivity in through-thickness direction for the purpose of nondestructive testing, damage detection, and electromagnetic interference shielding. CNT concentrations ranging from 0 to 0.75 wt% were used and compared to pristine CFRP samples (reference). The through-thickness conductivity of the CFRP exhibited an improvement of up to 781% by adopting this technique. However, the dispersion of CNT in PEI led to a viscosity increase and poor wetting properties which resulted in the formation of voids/defects, poor adhesion (as shown in scanning electron micrographs) and the deterioration of the mechanical properties as manifested by interlaminar shear strength and dynamic mechanical analysis measurements.     

Self Assembly–Assisted Additive Manufacturing: Direct Ink Write 3D Printing of Epoxy–Amine Thermosets

The use of self‐assembling, pre‐polymer materials in 3D printing is rare, due to difficulties of facilitating printing with low molecular weight species and preserving their reactivity and/or functions on the macroscale. Akin to 3D printing of small molecules, examples of extrusion‐based printing of pre‐polymer thermosets are uncommon, arising from their limited rheological tuneability and slow reactions kinetics. The direct ink write (DIW) 3D printing of a two‐part resin, Epon 828 and Jeffamine D230, using a self‐assembly approach is reported. Through the addition of self‐assembling, ureidopyrimidinone‐modified Jeffamine D230 and nanoclay filler, suitable viscoelastic properties are obtained, enabling 3D printing of the epoxy–amine pre‐polymer resin. A significant increase in viscosity is observed, with an infinite shear rate viscosity of approximately two orders of magnitude higher than control resins, in addition to, an increase in yield strength and thixotropic behavior. Printing of simple geometries is demonstrated with parts showing excellent interlayer adhesion, unachievable using control resins.     

High biorenewable content acrylate photocurable resins for DLP 3D printing

Green chemistry and green engineering concepts have been combined to develop novel sustainable polymeric materials. Solvent free photocurable acrylate resins with biorenewable carbon content of 75%–82% suitable for application in DLP 3D printing technology were composed by commercially available bio-based materials, acrylated epoxidized soybean oil (AESO), isobornyl methacrylate (IBOMA), methacrylic ester (ME), tetrahydrofurfuryl acrylate (THFA), and tetrahydrofurfuryl methacrylate (THFMA). They demonstrated high printing accuracy and good adhesion between layers. The monitoring of photocross-linking kinetics of high biorenewable content acrylate photoresins by the real-time photorheometry and analysis of their rheological parameters was carried out. Synthesized polymers exhibited high yield of insoluble fraction and thermal decomposition temperature at the weight loss of 10% above 300°C. Polymers AESO/IBOMA and AESO/THFMA showed the highest values of tensile modulus and tensile strength. Biodegradability of the synthesized polymers AESO/ME, AESO/THFA, and AESO/THFMA was investigated by measuring oxygen consumption in a closed respirometer. Such AESO-based polymers can be a competitive solution to replace petroleum-derived polymeric materials in additive manufacturing and reduce the environmental impact.     

The activation of furfuryl alcohol polymerization by oxygen and its enhanced mechanical properties

The effect of oxygen and additional oxygen providers on furfuryl alcohol polymerization was investigated through chemical analyses and mechanical evaluation. NMR, UV–vis, Fourier transform infrared, and gas chromatography–mass spectrometry (GC–MS) results suggested that atmospheric oxygen and the further addition of an oxygen source functioned as an activator for the entire network polymerization. Interestingly, the construction of a conjugated structure on the furan linear chain, which is key to three-dimensional cross-linking, also appears to be accelerated in the presence of oxygen. Furthermore, the introduction of oxygen providers into the curing system successfully enhanced the mechanical properties of the cured furan resin.     

Biphenyl liquid crystal epoxy containing flexible chain: Synthesis and thermal properties

A biphenyl type liquid crystal epoxy (LCE) monomer 4,4′-di(2,3-epoxyhexyloxy)biphenyl (LCBP4) containing flexible chain was synthesized and the curing behavior was investigated using 4,4′-diaminodiphenylmethane (DDM) as the curing agent. The effect of curing condition on the formation of the liquid crystalline phase was examined. The cured samples show good mechanical properties and thermal stabilities. Moreover, the relationship between thermal conductivity and structure of liquid crystalline domain was also discussed. The samples show high thermal conductivity up to 0.28–0.31 W/(m*K), which is 1.5 times as high as that of conventional epoxy systems. In addition, thermal conductive filler, Al₂O₃, was introduced into LCBP4/DDM to obtain higher thermal conductive composites. When the content of Al₂O₃ was 80 wt%, the thermal conductivity of the composite reached to 1.86 W/(m*K), while that of diglycidyl ether of bisphenol A (Bis-A) epoxy resin/DDM/Al₂O₃ was 1.15 W/(m*K). Compared with Bis-A epoxy resin, the formation of liquid crystal domains in the cured LCE resin enhanced the thermal conductivity synergistically with the presence of Al₂O₃. Furthermore, the introduction of Al₂O₃ also slightly increased the thermal stabilities of the cured LCE.     

High‐performance thermosets with tailored properties derived from methacrylated eugenol and epoxy‐based vinyl ester

A renewable chemical, eugenol, is methacrylated to produce methacrylated eugenol (ME) employing the Steglich esterification reaction without any solvent. The resulting ME is used as a low‐viscosity co‐monomer to replace styrene in a commercial epoxy‐based vinyl ester resin (VE). The volatility and viscosity of ME and styrene are compared. The effect of ME loading and temperature on the viscosity of the VE–ME resin is investigated. Moreover, the thermomechanical properties, curing extent and thermal stability of the fully cured VE–ME thermosets are systematically examined. The results indicate that ME is a monomer with low volatility and low viscosity, and therefore the incorporation of ME monomer in VE resins allows significant reduction of viscosity. Moreover, the viscosity of the VE–ME resin can be tailored by adjusting the ME loadings and processing temperature to meet commercial liquid molding technology requirements. The glass transition temperatures of VE–ME thermosets range from 139 to 199 °C. In addition, more than 95% of the monomer is incorporated and fixed in the crosslinked network structure of VE–ME thermosets. Overall, the developed ME monomer exhibits promising potential for replacing styrene as an effective low‐viscosity co‐monomer. The VE–ME resins show great advantages for use in polymer matrices for high‐performance fiber‐reinforced composites. This work is of great significance to the vinyl ester industry by providing detailed experimental support. © 2018 Society of Chemical Industry     

Innovative conductive polymer composite coating for aircrafts lightning strike protection

A new kind of high electrical conductive epoxy coating with low filler rate was investigated for lightning strike protection (LSP) of carbon fiber reinforced polymer (CFRP). The coating without CFRP substrate was firstly studied. The influence of silver submicronic wires (AgSWs) with a high aspect ratio on the electrical behavior is observed; that is, the electrical resistivity evolution, the current density value, and the electrical conduction mechanisms as function of temperature. The preponderant electrical conduction mechanism is an Ohmic behavior. The higher level of conductivity obtained is 5.5 × 10⁵ S m⁻¹ for 9% vol of AgSWs. Lightning strike tests were carried out on an epoxy/AgSWs coating filled with 8% in volume (74 gsm) and deposited on a CFRP. The ultrasonic testing after lightning strike on coated CFRP shows no structural delamination and demonstrates the interest of this new route for an efficient LSP. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48700.     

Kinetics of the cyclization and isomerization reactions in polyacrylonitrile based carbon fiber precursors during thermal-oxidative stabilization

The kinetics of reactions in polyacrylonitrile (PAN) based carbon fiber (CF) production should be of significance to the guidance of process control, fiber structure formation. PAN precursor fibers were isothermally stabilized at 210, 225, 240, 255, and 270 °C, respectively, for 10 to 100 min in an air oven to study the kinetics of the cyclization and isomerization reactions. The structural evolution of PAN precursor fibers during thermal-oxidative stabilization was characterized by Fourier transform infrared (FTIR) spectroscopy and solid state ¹³C nuclear magnetic resonance (¹³C NMR). The results indicate that the FTIR absorbance of  CN (the resultant of the cyclization) in PAN shows a trend of first increasing and then decreasing. And then the NMR peak assigned to the carbon atoms linking imino groups ( NH ) proves the isomerization of  CN into  NH in pyridone structure. Based upon the FTIR absorbance method, the entire process of the cyclization and isomerization reactions is considered as a consecutive first-order reaction. A kinetic model for the consecutive reaction has been established via the evaluation of the reaction rate constants of two single reactions. According to the model, the simulated kinetic curves of the characteristic groups ( CN,  CN , and  NH ) conform to the FTIR absorbance trends of these groups based on experimental data. This study is expected to furnish in-depth information on the crucial reaction kinetics during stabilization of PAN precursors, which is of advantage to the process optimization of the CF production. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48819.