Rheokinetics in curing process of polyfurfuryl alcohol: effect of homologous acid catalysts

Curing reaction of polyfurfuryl alcohol (PFA) resin was investigated using rheological measurements in the presence of different acid catalysts. A homologous series of dicarboxylic acid catalysts with even number of carbons of 2, 4 and 6 were chosen, i.e., oxalic acid (OX, 2 carbon), succinic acid (SU, 4 carbon) and adipic acid (AD, 6 carbon). Acidity of these catalysts in terms of pK _a was in the range of 1.25–4.43. Firstly, non-isothermal curing kinetics were investigated, and then, isothermal studies were performed at three different temperatures using 3 and 5 wt% of each acid catalyst. Isothermal cure behavior was best described by an empirical Arrhenius model of viscosity. An isoconversional method was applied to compute the changes in the effective activation energy as a function of degree of conversion. The change in activation energy at 50–10 kJ mol^?1 was observed for PFA/3 wt% OX system due to the vitrification and diffusion-controlled processes. Gel time, t _gel, was determined as a function of curing temperature, acid catalyst type and its concentration. The gelation time was consistent for all the formulations with an extent of conversion of about 0.7. Significant differences were observed in the curing behavior of resins in the presence of different acid catalysts. Increasing of the curing temperature significantly accelerated the curing process and increased curing rate constant of the PFA resin.     

Effect of processing conditions on the microencapsulation of 1-methylimidazole curing agent using solid epoxy resins

Among various methods for preparation of the non-reactive or latent curing agents for epoxy resins, encapsulation offers a promising and cost-effective method. This system has been used in one-part thermosetting adhesives or prepregs for developing advanced composite materials. In this study, 1-methylimidazole (1-MI) was microencapsulated using solid epoxy resins via solvent evaporation method. The effect of various types of shell materials or solid epoxy resins, different core-to-shell ratios of 30/70, 50/50, and 70/30, and a variety of mixing rates (500, 1000, and 1500 rpm) on the preparation of microcapsules were evaluated. The 1-MI content and microencapsulation efficiency were calculated using thermal gravimetric analysis (TGA). The shape and surface morphology of the prepared microcapsules were evaluated using scanning electron microscopy (SEM) technique. Gel time of the microcapsules mixed with liquid epoxy resin was also investigated. Results showed that by decreasing the core-to-shell ratio at different stirring rates, the encapsulation efficiency increased. The microencapsulation efficiency of 34.8% was obtained at shell-to-core (1-MI) ratio of 70/30 at stirring rate of 500 rpm. The results also showed that at higher mixing rates, 1-MI content in microcapsules was controlled by high shearing force. It has been found that the gel time of the epoxy resin/microcapsule samples was proportional to the imidazole content and it was in agreement with the data obtained by TGA analysis, as well.     

Development of self-healing coatings based on urea-formaldehyde/polyurethane microcapsules containing epoxy resin

In this study, the synthesis of urea-formaldehyde/polyurethane (UF/PU) microcapsules containing epoxy resin for self-healing and anti-corrosion coatings with good stability has been reported. Spherical microcapsules were prepared with a diameter of about 50–720 μm and a shell thickness of 0.6–0.7 μm via in situ polymerization in an oil-in-water emulsion using 2,4-toluene diisocyanate-based pre-polymer along with the urea-formaldehyde. Scanning electron microscopy (SEM) and optical microscopy (OM) were employed to evaluate the shape and morphology of the microcapsules. Fourier transform infrared (FTIR) spectroscopy showed the absence of free isocyanate groups within the microcapsule shell confirming the completion of shell formation reactions. OM illustrated that the microcapsules were stable over a period of 30-days in toluene and xylene. Increasing microcapsule loading improved crack repairing and anti-corrosion performance of the coating layer. Low-carbon steel coupons coated with an epoxy resin containing 10 wt% microcapsules and scribed using a scalpel blade showed no visible sign of corrosion after up to 5 weeks of exposure in a standard salt spray test chamber.     

Curing of poly(furfuryl alcohol) resin catalyzed by a homologous series of dicarboxylic acid catalysts: Kinetics and pot life

Curing kinetics and pot life are two vital characteristics for the application of poly(furfuryl alcohol) (PFA) resin because of the complexities both in the resin composition and curing mechanisms involved. However, few reports have provided a complete picture of PFA curing behavior. In this research, the effect of the addition of catalysts on the pot life and curing behavior of a PFA resin were evaluated. A homologous series of dicarboxylic acids [i.e., oxalic acid (OX), succinic acid (SU), and adipic acid (SA)] were used as the catalysts. Rheometric and nonisothermal differential scanning calorimetry (DSC) measurements and headspace gas chromatography/mass spectrometry analysis were carried out at 0, 6, and 24 h after the addition of the catalyst. The relaxation exponent (n), gel stiffness (S), and gel strength (A_F) of the prepared compositions were calculated with the Winter and Chambon and Gabriele rheological models. Furthermore, the curing kinetics were evaluated by the fitting of nonisothermal, multiple‐heating‐rate models. The DSC measurements showed a higher curing degree for samples containing OX catalyst compared to their counterparts containing either SU or AD. The rheometric findings supported an increased stiffness, gel strength, and curing development of the resin in the presence of OX compared to samples containing SU or AD. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44009.     

Adhesion performance of an epoxy clear coat on aluminum alloy in the presence of vinyl and amino-silane primers

The influence of amino and vinyl-silane-based treatments on the performance of an epoxy coated AA 1050 aluminum has been studied. The surface energy of the silane treated samples was determined using static contact angle measurement. Chemical interactions between the silane layer and aluminum substrates were also examined using FTIR-ATR spectroscopy. Pull-off adhesion was used under wet, dry and recovery conditions. The contact angle measurements showed a relative increase in the surface free energy of the silane treated specimens. For specimens treated with silane solutions below the IEP of aluminum, adhesion strengths were higher than those for pHs higher than IEP. This may suggest that an acid–base interaction occurs between the negatively charged silanol species and the positively charged AA surface. FTIR revealed two modes of interaction, one between silane and Al³⁺ (Si–O–Al), and the other indicative of a siloxane network (Si–O–Si). Thickness measurement showed that the silane layer is 100–200 nm thick indicating that the silane formed at the surface is not a monomolecular layer. Direct pull-off adhesion measurements of silane treated samples showed a significant improvement in initial bond strength compared with degreased aluminum substrates. However, those silane solutions having pHs higher than 8.4 showed inferior adhesion in comparison to those having lower pHs. The adhesion improvement was maintained, to some extent, after exposure to salt spray cabinet. In spite of good dry, wet and recovery adhesion, silane treated surfaces are not comparable with chromated AA aluminum in terms corrosion resistance.     

Adhesive strength of powder coated aluminium substrates

Adhesive strengths of differently pretreated powder coated aluminium substrates were evaluated using the pull-off and the tape tests in the dry and wet conditions. Various accelerated test chambers were employed to evaluate the corrosion behaviour of the samples. Scanning electron microscopy (SEM), with energy dispersive X-ray analysis (EDX) were also employed to the better understanding of the nature and morphology of the differently pretreated aluminium substrates prior to and after the accelerated tests.

Accelerated tests and wet adhesion measurements showed that the degreased samples demonstrated a better performance than Polyacrylic acid (PAA) treated samples. The chromate/phosphate conversion coating (CPCC) treated samples gave the best performance and the Polyacrylic acid/ hexafluorozirconic acid treated samples (PZr) gave marginally second best performance in anti-corrosive behaviour.

Based on the results obtained from the different experiments, it is argued that these studies may provide sufficient evidence to show that hydrolysis may destroy interface bonds between PAA and aluminium substrate. Moreover, SEM/EDX analysis, suggest that, the thin PAA polymeric layer was replaced by a crystalline, thick hydrated oxide layer, when samples were exposed to humid conditions.     

Curing of polyfurfuryl alcohol resin catalyzed by a homologous series of dicarboxylic acid catalysts. II. Swelling behavior and thermal properties

Polyfurfuryl alcohol (PFA), as a bio‐based resin made from lignocellulosic materials, was crosslinked using a homologous series of dicarboxylic acid catalysts consisting of oxalic, succinic, and adipic acids, which are different in their dissociation constants. Swelling behavior, thermal stability, and non‐oxidative char residue of the resulting networks were characterized as a function of the acids strength and their concentration. Swelling of the networks were investigated at room temperature in eight solvents differing in molecular size and solubility parameter. Using acetonitrile as a solvent, the swelling mechanism was explored by applying kinetic models to the swelling data. Dynamic swelling studies during 16–30 days supported non‐Fickian and anomalous diffusion mechanism in highly crosslinked samples supporting PFA chain rigidity and high crosslinking density of the networks. Polymer–solvent interaction parameter, molecular weight between crosslinks and crosslinking densities were also determined. According to the results, the extent of PFA crosslinking and non‐Fickian behavior of the swelling solvent diffusion through the networks are strongly dependent on the concentration and dissociation characteristics of the catalysts used. Thermal stability studies showed no significant differences between the compositions, up to 900 °C. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45770.     

Microencapsulation of 1-methylimidazole using solid epoxy resin: study on microcapsule residence time and properties of the system

In this study for the first time 1-methylimidazole was microencapsulated successfully by solid epoxy resin using solvent evaporation method. Fourier transform infrared spectroscopy, scanning electron microscopy, thermal gravimetric and differential scanning calorimetry were used for characterization of microcapsule and epoxy resin/microcapsule systems. The results revealed that although the solid epoxy shell was in contact with imidazole curing agent for a long time, it still remained as a thermoplastic. The chain length of the solid epoxy resin was grown in anionic polymerization process which led to an increase in the melting temperature from 64 to 78 °C. On the other hand, all epoxy functional groups of the shell did not participate in the curing reaction of resin with core curing agent of the microcapsule. The results showed that by increasing the residence time of the microcapsules, the number of epoxy groups of liquid epoxy decreased slightly but the rate of complex viscosity increase or the rate of curing reaction was increased and the cured epoxy system exhibited a single-phase morphology. On the other hand, in the presence of microcapsules the curing reaction of epoxy resin was successfully carried out and the curing temperature and the onset of viscosity increase or gel time at 120 °C were not more or less affected by sufficiently long contact time of epoxy and microcapsule.     

Evaluation of the weathering performance of basecoat/clearcoat automotive paint systems by electrochemical properties measurements

The aim of the present study was to investigate the possibility of evaluating the weathering performance of a basecoat/clearcoat automotive paint system through the determination of its electrochemical properties. To this end, the electrochemical properties of a basecoat/clearcoat automotive paint in a 3.5% solution of NaCl in deionized water were measured at different weathering exposure times. A constant phase element (CPE) was used for describing the electrochemical behavior of the coatings under test. The values of the CPE parameters, i.e. Y₀ (the CPE constant) and n (the CPE power) were subsequently correlated to the extent of photo-oxidation (as measured by appearance, surface roughness, FTIR, surface tension and adhesion measurements) of clearcoat at the surface, in the bulk and at the interface between the basecoat and the clearcoat. The result showed that the electrochemical parameters Y₀ and n provide ready means for comparing the weathering performances of basecoat/clearcoat automotive paint systems. Increases in the value of Y₀ together with decreases in the value of n with increasing weathering exposure times suggest increased possibilities for the onset of cracking in the clearcoat itself in addition to its propagation towards the basecoat. Additionally, sudden variations in the values of Y₀ and n are indicative of increases for the clearcoat to peel off.