Microwave processing of SiC nanoparticles infused polymer composites: Comparison of thermal and mechanical properties

The goal of this study is to compare thermal and mechanical properties of an epoxy resin system reinforced with SiC nanoparticles using both conventional thermal curing and microwave irradiation techniques. The microwave curing technique has shown potential benefits in processing polymeric nanocomposites by reducing the curing time without compromising the thermo‐mechanical performances of the materials. It was observed from this investigation that, the curing time was drastically reduced to ～30 min for microwave curing instead of 12 h room temperature curing with additional 6 h post curing at 75°C. Ductile behavior was more pronounced for microwave curing technique while thermal curing showed brittle like behavior as revealed from flexural test. The maximum strain to failure was increased by 25–40% for microwave‐cured nanocomposites over the room temperature cured nanocomposites for the same loading of nanofillers. The glass transition temperature (T_g) also increased by ～14°C while curing under microwave irradiation. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41708.     

Thermal and dynamic mechanical properties of microwave and heat‐cured poly(methyl methacrylate) used as dental base material

In this study, the particle size distribution, molecular weight, thermal analysis (TGA) differential scanning calorimetry (DSC) and thermogravimetric analysis, and dynamic mechanical analysis (DMA) of poly(methyl methacrylate) used as dental base material were investigated. The commercial raw material used were prepared for microwave curing, and they were cured by microwave and conventional heat methods. The average particle size of the powder studied (103.1 μm) were much larger than that of the commercial powders (50–78 μm) for conventional curing. The particle size dietribution were almost symmetrical and narrow. The viscosity‐average molecular weight were larger for microwave curing and increased with curing time. The glass transition temperature T_g measured (about 110°C) by DSC increased with curing period in microwave oven. The values of T_g were close to each other for both curing techniques. The degradation temperature range observed by TGA were 200–377°C. The movements of molecular chains in their conformations were studied by DMA in the form of changes in different mechanical properties with temperature. It was shown that crosslinking increased with increase of curing time. The changes were more noticeable in microwave curing compared to conventional heat curing. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2971–2978, 1999     

Effects of SiC whiskers on the mechanical properties and microstructure of SiC ceramics by reactive sintering

As-received and pre-coated SiC whiskers (SiC_w)/SiC ceramics were prepared by phenolic resin molding and reaction sintering at 1650 °C. The influence of SiC_w on the mechanical behaviors and morphology of the toughened reaction-bonded silicon carbide (RBSC) ceramics was evaluated. The fracture toughness of the composites reinforced with pre-coated SiC_w reached a peak value of 5.6 MPa m^1/2 at 15 wt% whiskers, which is higher than that of the RBSC with as-received SiC_w (fracture toughness of 3.4 MPa m^1/2). The surface of the whiskers was pre-coated with phenolic resin, which could form a SiC coating in situ after carbonization and reactive infiltration sintering. The coating not only protected the SiC whiskers from degradation but also provided moderate interfacial bonding, which is beneficial for whisker pull-out, whisker bridging and crack deflection.     

Comparison of the curing kinetics of a DGEBA/acid anhydride epoxy resin system using differential scanning calorimetry and a microwave‐heated calorimeter

The cure of an epoxy resin system, based upon a diglycidyl ether of bisphenol‐A (DGEBA) with HY917 (an acid anhydride hardener) and DY073 (an amine–phenol complex that acted as an accelerator), was investigated using a conventional differential scanning calorimeter and a microwave‐heated power‐compensated calorimeter. Dynamic cure of the epoxy resin using four different heating rates and isothermal cure using four different temperatures were carried out and the degree of cure and reaction rates were compared. The cure kinetics were analyzed using several kinetics models. The results showed different activation energies for conventional and microwave curing and suggested different reaction mechanisms were responsible for curing using the two heating methods. Resins cured using conventional heating showed higher glass transition temperatures than did those cured using microwave heating. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2054–2063, 2007     

Thermal and mechanical properties of microwave‐ and heat‐cured poly(methyl methacrylate) used as dental base material

The thermal and mechanical properties of dental base materials cured by microwave and conventional heat methods were studied. The commercial dental base poly(methyl metacrylate) (PMMA) powder and liquid were mixed in a 3/1 ratio. They were polymerized by a peroxy catalyst at 65°C, then cured with a boiling water temperature and microwave radiation for periods of 5, 10, 15, 20, 25, 30, and 35 min for heat curing and 1, 2, 3, 5, and 7 min for microwave radiation. The microwave radiation outputs used were 500 and 700 W. The products of 5‐min heat curing and 1‐, 2‐, and 7‐min microwave curing were soluble in chloroform. All the others were partially soluble. The viscosity‐average molecular weights of the soluble samples were about 1 × 10⁶. The thermal properties of the polymer samples were studied by differential scanning calorimetry (DSC). For the samples that were not cured completely, broad exothermic peaks at around 125°C were obtained in the DSC thermograms. The glass‐transition temperatures for completely cured samples were 110–120°C. The mechanical properties of the samples were determined from tensile and three‐point bending tests. The elastic modulus was highest for samples obtained by the conventional method with a 30‐min curing period. However, the bending modulus was highest for 7‐min cured samples in a 700‐W microwave. The mechanical strengths of the 700‐W output were higher than those at 500 W. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 251–256, 2003     

Spark plasma sintering of TiC–SiCw ceramics

Silicon carbide whiskers (SiC_w) in TiC had impressive impacts on the properties and made it possible for special applications which generally would not be conceivable with TiC alone. In the present work, SiC_w reinforced TiC based composites were prepared by spark plasma sintering (SPS) technique, at the temperature of 1900 °C under the pressure of 40 MPa for sintering time of 7 min. To test out the effects of different amount of SiC whisker (0, 10, 20 and 30 vol%) on the characteristics of TiC, the sintered samples were investigated about sinterability and physical-mechanical properties. Microstructure observations and density measurements confirmed that the composites were dense with uniformly distributed reinforcement, and the specimen doped with higher than 10 vol% SiC_w could attain higher relative density (>100%) than pure TiC and TiC–10 vol% SiC_w. Also, the highest values for hardness (29.04 GPa) and thermal conductivity (39.2 W/mK) were achieved in specimen containing 30 vol% SiC_w, whereas the optimum bending strength (644 MPa) was recorded in material containing 20 vol% SiC_w. It seems that one of the reasons which contributes to this trend of properties variation is the generation of near-stoichiometric TiC_x phase and new Ti₃SiC₂ compound.     

Comparative study on the properties of cross-linked cellulose nanocrystals/chitosan film composites with conventional heating and microwave curing

Cross-linking of chitosan film composites was carried out by using conventional heating and microwave curing methods in this study. Non-cross-linked and glutaraldehyde (GA) cross-linked neat chitosan and cellulose nanocrystals (CNC)/chitosan film composites were cured by either conventional oven heating or microwave irradiation. Tensile strength and Young's modulus of chitosan composites were enhanced significantly by the addition of CNC and GA especially for the microwave-cured samples. The changes in chemical interaction of the chitosan film composites was determined by Fourier transform infrared (FTIR) spectroscopy. The microwave-cured GA-cross-linked chitosan film composites were more thermally stable than non-cross-linked and conventionally heated GA-cross-linked chitosan film composites due to the formation of a more stable structure between GA and chitosan. Nevertheless, the reduced antimicrobial efficacy of film composites against Escherichia coli, Bacillus subtilis, and Saccharomyces cerevisiae was observed in cross-linked film composites compared with non-cross-linked composites.     

Microwave‐irradiated ring‐opening polymerization of D,L‐lactide under atmosphere

Poly(D ,L ‐lactide) (PDLLA) was synthesized by microwave‐irradiated ring‐opening polymerization catalyzed by stannous octoate (Sn(Oct)₂) under atmosphere. The effects of heating medium, monomer purity, catalyst concentration, microwave irradiation time, and vacuum level were discussed. Under the appropriate conditions such as carborundum (SiC) as heating‐medium, 0.15% catalyst, lactide with purity above 99.9%, 450 W microwave power, 30 min irradiation time, and atmosphere, PDLLA with a viscosity–average molecular weight (M_η) over 2.0 × 10⁵ and a yield over 85% was obtained. The dismission of vacuum to ring‐opening polymerization of D ,L ‐lactide (DLLA) under microwave irradiation simplified the process greatly. The temperature under microwave irradiation and conventional heating was compared. The largely enhanced ring‐opening polymerization rate of DLLA under microwave irradiation was the coeffect of thermal effects and microwave effects. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2244–2247, 2006     

Preparation and mechanical properties of SiCw-reinforced WC-10Ni3Al cemented carbide by microwave sintering

SiC_w-reinforced WC-10Ni₃Al cemented carbide was prepared by microwave sintering method, and the effects of the sintering temperature and SiC_w content on the microstructure and mechanical properties of WC-10Ni₃Al cemented carbide were investigated; the promotion effect and strengthening mechanism of SiC_w were then analysed. The experimental results showed that the relative density, hardness, flexural strength and fracture toughness of WC-10Ni₃Al cemented carbide increased and then decreased with increasing SiC_w addition and sintering temperature. When the sintering temperature was 1500 °C and the content of SiC_w was 0.3 wt%, the sample reached the highest mechanical properties and had a relative density of 96.5%, hardness of 1570 HV, flexural strength of 1275 MPa and fracture toughness of 13.1 MPa mm^1/2, which were 4.0%, 23.1%, 12.5% and 8.1% higher than those of the sample without SiC_w, respectively. During microwave sintering of WC-Ni₃Al, the addition of an appropriate SiC_w content can increase the microwave absorption of the sample, and produce many micro-high-temperature regions within the sample, which can accelerate the generation of the Ni₃Al liquid phase. This promotes liquid phase flow to fill pores and rearrange the WC grains, thereby improving density and mechanical properties of the sample. The strengthening mechanisms of SiC_w on microwave sintered WC-Ni₃Al consist of promoting densification enhancement, fine-grained strengthening, and solid solution strengthening of Ni₃Al by Si atoms.     

Comparison of the curing kinetics of the RTM6 epoxy resin system using differential scanning calorimetry and a microwave‐heated calorimeter

The cure of a commercial epoxy resin system, RTM6, was investigated using a conventional differential scanning calorimeter and a microwave‐heated calorimeter. Two curing methods, dynamic and isothermal, were carried out and the degree of cure and the reaction rates were compared. Several kinetics models ranging from a simple nth order model to more complicated models comprising nth order and autocatalytic kinetics models were used to describe the curing processes. The results showed that the resin cured isothermally showed similar cure times and final degree of cure using both conventional and microwave heating methods, suggesting similar curing mechanisms using both heating methods. The dynamic curing data were, however, different using two heating methods, possibly suggesting different curing mechanisms. Near‐infrared spectroscopy showed that in the dynamic curing of RTM6 using microwave heating, the epoxy‐amine reaction proceeded more rapidly than did the epoxy‐hydroxyl reaction. This was not the case during conventional curing of this resin. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3658–3668, 2006     

Preparation and mechanical performance of SiCw/geopolymer composites through direct ink writing

Traditional geopolymer structures benefit from the durability, irradiation resistance, and environmental friendliness, but their brittleness limits their applications where repeated, impactive strains are imparted. This situation may be alleviated if complex structures with tunable geometry can be fabricated, for example, through 3D printing based on direct ink writing (DIW). In this research, SiC whisker/geopolymer (SiC_w/GP) composites were fabricated by the DIW for the first time. The rheological behaviors of the SiC_w/GP inks and the fracture behaviors of the printed samples were explored. The modified printing inks exhibited non-Newtonian fluid behavior (shear-thinning). Subsequently, a series of lightweight architected structures were fabricated, and they revealed how reinforcement and architecture of the printed structures could influence both the strength and toughness of the SiC_w/GP composites.     

Synthesis of polyurethane acrylates by hydrogenated castor oil and dimer‐based polyester diol and study on pressure‐sensitive adhesive

Synthesis of polyurethane acrylate (PUA) and preparation of the UV‐cured pressure‐sensitive adhesives (PSA) are reported. Molecular weight (M_w) (by gel permeation chromatography) and viscosity (η*) of PUA were measured. Characterization of PUA and PSA before and after UV‐curing was made by FTIR. Increase of the hydroxyls from hydrogenated castor oil/hydroxyls from dimer‐based polyester diol (OH_HCO/OH_Diol) ratio decreased the M_w and η* value of PUA. Dynamic viscoelastic properties (by dynamic rheological spectrometer) and performance of the UV‐cured PSA were also studied. Increase of the OH_HCO/OH_Diol ratio increased the storage modulus (G′), the loss modulus (G″), and complex viscosity (Eta*) of the UV‐cured PSA, which, in turn, enhanced holding power and shear adhesion failure temperature (SAFT) and yet decreased peeling strength. Substitution of OB for DBTDL depressed the M_w and η* value of PUA, while the G″ and Eta* values of the UV‐cured PSA were elevated, which, in turn, increased the holding power and SAFT and yet depressed the peeling strength. Elevation of the tackifying resin content depressed the G′, G″, and Eta* values of the cured PSA and yet increased glass transition temperatures (T_g) of PSA, measured by differential scanning calorimetry. Peeling strength of PSA elevated as increasing the tackifying resin, while the holding power and SAFT fell. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1814–1821, 2005     

Processing and mechanical properties of B4C-SiCw ceramics densified by spark plasma sintering

Homogenous distribution of whiskers in the ceramic matrix is difficult to be achieved. To solve this problem, B₄C-SiC_w powder mixtures were freeze dried from a slurry dispersed by cellulose nanofibrils (CellNF) in this work. Dense B₄C ceramics reinforced with various amounts of SiC_w up to 12 wt% were consolidated by spark plasma sintering (SPS) at 1800 °C for 10 min under 50 MPa. During this process, CellNF was converted into carbon nanostructures. As iron impurities exist in the starting B₄C and SiC_w powders, both thermodynamic calculations and microstructure observations suggest the dissolution and precipitation of SiC_w in the liquids composed of Fe-Si-B-C occurred during sintering. Although not all the SiC_w grains were kept in the final ceramics, B₄C-9 wt% SiC_w ceramics sintered at 1800 °C still exhibit excellent Vickers hardness (35.5 ± 0.8 GPa), flexural strength (560 ± 9 MPa) and fracture toughness (5.1 ± 0.2 MPa·m^1/2), possibly contributed by the high-density stacking faults and twins in their SiC grains, no matter in whisker or particulate forms.     

The preparation and properties of SiCw/B4C composites infiltrated with molten silicon

Silicon carbide whisker (SiC_w) toughened B₄C composites have been prepared by pressureless infiltration of B₄C–SiC_w–C preforms with molten silicon under vacuum at 1500 °C. The effect of SiC_w addition on bulk density, hardness, bending strength, fracture toughness and microstructure of SiC_w/B₄C composites is discussed. It is revealed that the addition of SiC_w improves the fracture toughness of B₄C ceramic, but reduces its bending strength at the same time. The maximum fracture toughness for SiC_w/B₄C composite with 24 wt% SiC_w addition is 4.88 MPa m^1/2, which is about 9% higher than that of the one without SiC_w, but at the same time, the bending strength reduces to the minimum value 243 MPa, reduced by 25%. XRD analysis shows that the phase composition of reaction bonded SiC_w/B₄C composites is B₄C, SiC, Si, and B₁₂ (C, Si, B)₃, with no residual C. And the main toughening mechanism of SiC_w is whisker pulling up.     

Dual‐curing thermosetting monomer containing both propargyl ether and phthalonitrile groups

Novel self‐curing monomer containing two thermosetting groups, namely propargyl ether and phthalonitrile (PN) in a molecular structure, is synthesized and investigated. The study of catalyzed and uncatalyzed curing is performed and high heat release during curing is observed. This disadvantage can be adjusted by catalysis of propargyl ether polymerization with Ni (II), Co (II), and Cu (II) salts. The cured monomer possesses high thermal properties featured to phthalonitrile matrices (Heat deflection temperature, HDT = 428 °C, T_5% = 499 °C) and moderate mechanical properties (E = 4.9 ± 0.65 GPa, G_IC = 106 ± 26 J/m²), it can be applied as a high temperature matrix for carbon fiber reinforced plastics (CFRP). Low melt viscosity (223 mPa s at 120 °C) of the monomer provides a possibility to consider its application for composite material formation by vacuum infusion or resin transfer molding (RTM) techniques, which are exceptionally rarely applied for matrices with HDT > 300 °C but allow to obtain composites of complex shape with minimal joining parts. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 133, 44786.     

Influence of accelerated ageing on methacrylate‐based denture resins heterogeneity as viewed by ESR‐spin‐probe method

The electron‐spin‐resonance (ESR) spin‐probe method, was used to study the heterogeneity of denture resins based on poly(methyl methacrylate). Results for three resins processed by microwave energy, conventional curing and cold curing (depending on the curing procedure and exposed to ageing in various environmental conditions) were compared. All three cured resins were stored over the same time (1200 h) in distilled water at ambient temperature and in artificial saliva at 348 K. The temperature‐dependent ESR spectra of a spin probe dispersed in the denture resins are analyzed in terms of line‐shapes and line‐widths. The appearance of two spectral components was taken as an indication of resin heterogeneity. The results reveal that the cold‐cured resin has a lower local density in comparison with microwave and conventionally cured resin. The amount of residual monomer also contributes to the local motion of polymer segments. The change of denture resins exposed to ageing is influenced both by the structure of the original resin and the ageing conditions. Restricted motion of a spin probe incorporated into the acrylic resins exposed to accelerated ageing suggests additional crosslinking of polymer chains. The differences are observed for all the investigated resins, but the highest change is observed with the cold‐cured resin. The ESR results are accompanied by T_g and T_m measurements. Copyright © 2005 Society of Chemical Industry     

Epoxy/imidazolium-based ionic liquid systems: The effect of the hardener on the curing behavior,thermal stability,and microwave absorbing properties

A new transparent microwave absorbing coating was developed by compounding 1-butyl-3-methyl imidazolium tetrafluoroborate (bmim.BF₄) ionic liquid (IL) with diglycidyl ether of bisphenol A-type epoxy resin. The systems were crosslinked with the IL alone or combined with conventional hardeners, as anhydride or aromatic amine. The curing behavior was investigated by thermal and spectroscopic analysis performed at high temperatures. Neat bmim.BF₄ was able to cure epoxy resin, giving rise to networks with outstanding thermal stability compared with the systems cured with anhydride or aromatic amine. bmim.BF₄ accelerated the curing process in the presence of aromatic amine but retarded this event when anhydride was used as an external curing agent. The glass-transition temperature evaluated by dynamic mechanical analysis decreased when the amount of IL increased, which can be attributed to side reactions during the curing process, as well as the plasticizing effect of IL. The epoxy networks cured with bmim.BF₄ alone or in combination with anhydride or aromatic amine were transparent and presented considerable microwave absorbing properties in the X-band frequency range (8–12 GHz), being the best performance observed for the systems cured with bmim.BF4/anhydride curing system, with reflection loss value around −16 dB at 11.3 GHz. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48326.     

Polymerization of diisopropyl fumarate under microwave irradiation

Diisopropyl fumarate (DiPF), a representative monomer of dialkyl fumarates, was polymerized by microwave irradiation at three different powers (140, 210, and 280 W), using a domestic microwave oven. The nature and concentration of initiators [2,2′‐azobisisobutyronitrile (AIBN) and benzoyl peroxide (BP)], power and energy of microwave irradiation on the conversion, weight average molecular weight (M_w), and polydispersity index (M_w/M_n) were analyzed. The results indicate that the microwave conditions have a significant nonthermal effect in enhancing the polymerization rate of DiPF. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3785–3791, 2007     

Processing and properties of polycrystalline cubic boron nitride reinforced by SiC whiskers

Dense polycrystalline cBN (PcBN)–SiCw composites were fabricated by a two-step method: First, SiO₂ was coated on the surface of cubic boron nitride (cBN) particles by the sol-gel method. Then, silicon carbide whisker (SiC_w)- coated cBN powder was prepared by carbon thermal reaction between SiO₂ and carbon powders at 1500°C for 2 hour. Then, cBN–SiC_w complex powders were sintered by high-pressure and high-temperature sintering technology using Al, B, and C as sintering additives. The phase compositions and microstructures of cBN–SiC_w composites were investigated by X-ray diffraction and scanning electron microscopy, respectively. It was found that the SiC_w and Al₃BC₃ had been fabricated by in situ reaction, which cannot only promote densification but also improve mechanical properties. The relative density of PcBN composites increased from 96.3% to 99.4% with increasing SiC_w contents from 5 to 20 wt%. Meanwhile, the Vickers hardness, fracture toughness and flexural strength of as-obtained composites exhibited a similar trend as that of relative density. The composite contained 20 wt% of SiC_w exhibited the highest Vickers hardness and fracture toughness of 42.7 ± 1.9 GPa and 6.52 ± 0.21 MPa•m^1/2, respectively. At the same time, the flexural strength reached 406 ± 21 MPa.     

Low-temperature sintered (Ti,Zr, Nb,Ta, Mo)C-based composites toughened with damage-free SiCw

The high sintering temperature would have a great tendency to damage the morphology and thus properties of the silicon carbide whisker (SiC_w) in high entropy carbide-silicon carbide whisker (HEC-SiC_w) composites, which, in turn, would impact the effectiveness of the operative toughening mechanisms. The objective of this study was to achieve full contributions to the toughening effects of SiC_w by preparing (Ti, Zr, Nb, Ta, Mo)C-SiC_w composites at low temperature (1600 ℃) using cobalt as additives. Results showed that the fracture toughness of the (Ti, Zr, Nb, Ta, Mo)C bulk reinforced with 20 vol% SiC_w and 5 vol% Co was 7.2 MPa?m^1/2, which was much higher than that of the (Ti, Zr, Nb, Ta, Mo)C bulk only sintered with 5 vol% Co (3.4 MPa?m^1/2). Meanwhile, it was also higher than that of the reported HEC-20 vol% SiC_w composite sintered at 2000 ℃ (4.3 MPa?m^1/2). For the fracture toughness of HEC-SiC_w composites, it was significantly increased by the introduction of damage-free SiC_w.