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.     

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.     

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.     

Photolytic and free‐radical polymerization of monomethyl maleate esters of epoxidized plant oil triglycerides

Epoxidized soybean oil (ESO) was reacted with monomethyl maleate with AMC‐2 catalyst. Monomethyl maleate was found to react with 65% of the available epoxy groups to give the monomethyl maleic esters of ESO (MESO). ¹H‐NMR, ¹³C‐NMR, and IR spectra of the new derivative confirmed the proposed structure. The NMR spectra revealed that the average number of monomethyl maleate groups per triglyceride molecule was 2.6. MESO was photopolymerized with ultraviolet light and was free radically homopolymerized and copolymerized with styrene (STY), vinyl acetate (VA), and methylmethacrylate (MMA). MESO was also reacted with maleic anhydride at the newly formed hydroxyl groups to give maleinized MESO, (MESOMA), which now contained 4.9 maleate unsaturations per triglyceride. Dynamic mechanical analysis revealed the dynamic modulus for styrene copolymers of MESO and maleinized MESO as 105 and 140 MPa, respectively. Both of these plant oil derived monomers are good candidates for a practical and economical liquid molding resin. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 626–633, 2007     

Rheology of chemically modified triglycerides

The rheology of chemically modified plant oils was determined to aid in their processing for polymers and other applications. Epoxidized and acrylated triglycerides were derived from various plant oils and model triglycerides. The viscosity of the unmodified oils decreased as the level of oil unsaturation increased. However, the viscosity of the epoxidized oils increased slightly as the level of epoxidation increased. Furthermore, the viscosity of the acrylated oils increased exponentially as the level of acrylation increased because of increased polarity. In addition, the viscosity of the acrylated oils increased as the average distance of the acrylate groups from the fatty acid chain ends decreased. Chemically modified and unmodified oils did not exhibit any shear‐thinning behavior or any memory of shear history. The temperature dependence of the viscosity followed the Arrhenius law, and the activation energy decreased linearly with the level of acrylation. The addition of comonomers, such as styrene, reduced the viscosity of the acrylated oils exponentially as the comonomer concentration increased. However, the glass‐transition temperature and modulus of the triglyceride‐based polymers decreased as the styrene content decreased. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 774–783, 2005     

Limitation of the Coalescence of Evolutive Droplets by the Use of Copolymers in a Thermoplastic/Thermoset Blend

Summary: Polystyrene (PS)/epoxy‐amine (DGEBA‐MDEA) is a thermoplastic/thermoset precursor blend which is miscible at high temperature (177 °C), and which phase separates under the polymerization of the epoxy‐amine system. Previous studies have shown that the morphology of this blend polymerized under shear is coarse and irregular because the dispersed epoxy‐amine domains coalesce before they gel. Several styrene‐methyl methacrylate and a styrene‐butadiene‐styrene block copolymers have been added to the PS/DGEBA‐MDEA 60/40 blend in order to limit the coalescence and thus obtain a finer morphology. Two of the copolymers used were reactive either with the epoxy or with the amine. It was shown that the addition of 15 wt.‐% of non reactive copolymer had a positive but limited effect on the size of the final epoxy‐amine particles. The copolymer remained at the interface at the early stages of the polymerization. However, it was pulled out by the shear forces around the gel point of the epoxy domains. Most of the non reactive copolymer was present in the shape of micelles at the end of the process. On the other hand, the reactive copolymers were able to establish covalent bonds with the epoxy‐amine drops and hence were not extracted at all. Consequently they allowed the decrease the size of the particles by a factor of 15. Despite this, the observation of the morphology at different stages of the polymerization has revealed that the copolymer moved at the interface of the epoxy domains during the collision of two droplets. The movements of fluids into the epoxy domains pushed the copolymer out of the inter‐droplet zone so that it could not prevent the drainage of the liquid film between the droplets and consequently their coalescence.

TEM showing that the layer of copolymer (in dark grey) has moved along the interface of epoxy‐amine drops during their successful collision in a polystyrene‐rich matrix.     

Thermal and physical properties of allyl PPO and its composite

The thermal behavior of allyl PPO and its cured resin were investigated. In the allyl PPO curing process, the specific temperatures were T_gel = 173.6°C, T_cure = 225.4°C, and T_treat = 237.7°C, and the activation energy (E_a) was 122 kJ/mol. The average number of PPO molecular units between two crosslinking points was about 20. In the degradation process of cured allyl PPO resin, the temperature at which mass loss equaled 1% was much higher than 300°C. The E_a for degradation was calculated as 125 kJ/mol, with a degradation fraction (α) in the range of 0.15–0.65, or 117 kJ/mol with an α of 0.10–0.90. The most probable mechanism function of decomposition of the cured allyl PPO resin was f(α) = 2(1 ? α)^3/2 or g(α) = (1 ? α)^?1/2 ? 1. The thermocompressed laminate of the allyl PPO blending with an additive resin (made from BDM and DP) exhibited the desired properties. ©2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4111–4115, 2006     

A new thermoset for separation of polystyrene and naphthalene in preparative chromatography

Many research groups in recent years have demonstrated the importance of obtaining new materials and reducing environmental impact. In this context, the chemical modification of cellulose and its derivatives has received much attention. This study synthesized cellulose acetate gel (CAMDIH) obtained through the modification of cellulose acetate (CA) with a degree of substitution of 2.5, by crosslinking reactions using 4,4′‐diphenylmethane diisocyanate in homogeneous medium. The formation of crosslinks were observed by the presence of Fourier transform infrared spectroscopy absorption bands at 3046 and 864 cm^?1, which correspond to the absorption of aromatic groups associated with the incorporation of 4,4′‐diphenylmethane diisocyanate in the CA structure. The potential applications of the gel as a stationary phase were tested using column chromatography in the fractionation and separation of standard solutions of polystyrene and naphthalene. The findings showed the effectiveness of the gel as a stationary state in the separation of mixture compounds. Furthermore, the study found that CAMDHI is an innovative material when considering its simple synthesis and the low costs involved in the process. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46063.