Effect of Heat Treatment under Different Atmospheres on the Bonding Properties and Mechanism of Ceramiziable Heat-Resistant Adhesive

In this study, a heat-resistant adhesive was prepared using molybdenum-phenolic (Mo-PF) resin as the matrix and TiB2 particle as the ceramizable filler for bonding Al2O3 ceramic substrates. Firstly, Fourier transform infrared (FTIR) was used to characterize the chemical structure of the Mo-PF. Subsequently, thermo gravimetric analysis (TGA) and shear strength testing were employed to investigate the effects of heat treatment in different atmospheres on the thermal stability and residual bonding properties of the adhesive. To further explore the bonding mechanism of the adhesive after heat treatment in different atmospheres, scanning electron microscopy (SEM), compressive strength testing, and X-ray diffraction (XRD) were utilized to analyze the microstructure, mechanical strength, and composition evolution of the adhesive at different temperatures. The bonding strength of Al2O3 joints showed a trend of initially decreasing and then increasing after different temperature heat treatment in air, with the shear strength reaching a maximum value of 25.68 MPa after treatment at 1200 °C. And the bonding strength of Al2O3 joints decreased slowly with the increase of temperature in nitrogen. In air, the ceramicization reaction at a high temperature enabled the mechanical strength of the adhesive to rise despite the continuous pyrolysis of the resin. However, the TiB2 filler in nitrogen did not react, and the properties of the adhesive showed a decreasing tendency with the pyrolysis of the resin.

The Hyperbranched Polyester Reinforced Unsaturated Polyester Resin

We report a method of reinforcing and toughening unsaturated polyester resin (UPR) with a kind of hyperbranched polyester (HBP-1). Polyethylene glycol with different molecular weight was used as the core molecule of the preparation reaction, and the reaction product of phthalic anhydride and glycerol was used as the branching unit. The esterification reaction of polycondensation occurred, and then the hydroxyl-terminated hyperbranched polyester was prepared. The reaction product of maleic anhydride and isooctanol was added to the prepared hydroxyl-terminated hyperbranched polyester for esterification reaction. Both ends of the hyperbranched polyester had unsaturated double bond to obtain the hyperbranched polyester (HBP-1). The effects of this treatment on the morphology, mechanical properties and thermal properties of the composites were studied in detail. The HBP-1 was investigated by Fourier Transform Infrared Spectroscopy (FT-IR). The HBP-1/UPR composites were investigated by Thermogravimetric Analysis (TGA), Dynamic Mechanical Analysis (DMA), mechanical properties analysis and Scanning Electron Microscope (SEM). The results showed that HBP-1 enhanced the thermostability and mechanical properties of UPR. However, DMA indicated that the addition of HBP-1 cannot effectively improve the thermodynamic properties of UPR due to the flexible chain in HBP-1 structure. The HBP-1 improves tensile strength, bending strength and impact strength compared to neat UPR.

Diphenolic Acid-Derived Hyperbranched Epoxy Thermosets with High Mechanical Strength and Toughness

Diglycidyl ether of bisphenol A (DGEBA) is a kind of widely used epoxy resin, but its thermosets normally show high brittleness and poor impact resistance due to the intrinsic rigid aromatic rings, which limit its application greatly. To avoid this drawback, we proposed a method to prepare a series of hyperbranched epoxies (HBEPs) with different molecular weights. After HBEPs were cured with methyl tetrahydrophthalic anhydride (MTHPA), characterizations were carried out to evaluate the properties of the cured HBEP samples. Testing results indicate that the hyperbranched thermosets can achieve excellent mechanical strength and toughness (tensile strength: 89.2 MPa, bending strength: 129.6 MPa, elongation at break: 6.1%, toughness: 4.5 MJ m-3, and impact strength: 6.7 kJ m-2), which are superior to those of the thermosets of commercial DGEBA (tensile strength: 81.2 MPa, bending strength: 108.2 MPa, elongation at break: 3.0%, toughness: 1.5 MJ m-3, and impact strength: 4.2 kJ m-2). In addition, HBEP with the highest molecular weight and degree of branching shows the best comprehensive mechanical properties. All hyperbranched thermosets exhibit high glass-transition temperatures (T g) and thermostability, which further illustrates the potential application value of HBEPs.

Thermal Properties and Fracture Toughness of Epoxy Nanocomposites Loaded with Hyperbranched-Polymers-Based Core/Shell Nanoparticles

Synthesized silicon oxide (silica) nanoparticles were functionalized with a hyperbranched polymer (HBP) achieving a core/shell nanoparticles (CSNPs) morphology. CSNPs were characterized by Fourier Transform Infrared (FTIR) spectroscopy, Transmission Electron Microscopy (TEM), and Thermogravimetric Analysis (TGA). A core diameter of about 250 nm with a 15 nm thick shell was revealed using TEM images. An aeronautical epoxy resin was loaded with the synthesized CSNPs at different percentages and thermal properties, such as thermal stability and dynamic mechanical properties, were investigated with the use of different techniques. Although the incorporation of 2.5 wt% of CSNPs induces a ~4 °C reduction of the hosting matrix glass transition temperature, a slight increase of the storage modulus of about ~10% was also measured. The Kissinger Method was employed in order to study the thermal stability of the nanocomposites; the degradation activation energies that resulted were higher for the sample loaded with low filler content with a maximum increase of both degradation step energies of about ~77% and ~20%, respectively. Finally, fracture toughness analysis revealed that both the critical stress intensity factor (K_IC) and critical strain energy release rate (G_IC) increased with the CSNPs content, reporting an increase of about 32% and 74%, respectively, for the higher filler loading.

Tannic Acid as a Bio-Based Modifier of Epoxy/Anhydride Thermosets

Toughening an epoxy resin by bio-based modifiers without trade-offs in its modulus, mechanical strength, and other properties is still a big challenge. This paper presents an approach to modify epoxy resin with tannic acid (TA) as a bio-based feedstock. Carboxylic acid-modified tannic acid (TA⁻COOH) was first prepared through a simple esterification between TA and methylhexahydrophthalic anhydride, and then used as a modifier for the epoxy/anhydride curing system. Owing to the chemical modification, TA⁻COOH could easily disperse in epoxy resin and showed adequate interface interaction between TA⁻COOH and epoxy matrix, in avoid of phase separation. The use of TA⁻COOH in different proportions as modifier of epoxy/anhydride thermosets was studied. The results showed that TA⁻COOH could significantly improve the toughness with a great increase in impact strength under a low loading amount. Moreover, the addition of TA⁻COOH also simultaneously improved the tensile strength, elongation at break and glass transition temperature. The toughening and reinforcing mechanism was studied by scanning electron microscopy (SEM), dynamic mechanical analysis (DMA) and thermal mechanical analysis (TMA), which should be owned to the synergistic effect of good interface interaction, aromatic structure, decreasing of cross linking density and increasing of free volume. This approach allows us to utilize the renewable tannic acid as an effective modifier for epoxy resin with good mechanical and thermal properties.