Synthesis,morphology and physical properties of multi-walled carbon nanotube/biphenyl liquid crystalline epoxy composites

We have developed multi-walled carbon nanotube/liquid crystalline epoxy composites and studied the effects of incorporation carbon nanotubes (CNTs) on the morphology, thermal and mechanical properties of the composites. The CNTs are functionalized by liquid crystalline (LC) 4,4′-bis(2,3-epoxypropoxy) biphenyl (BP) epoxy resin for the ease of dispersion and the formation of long range ordered structure. The epoxy functionalized CNT (ef-CNT) were dispersed in the LC BP epoxy resin that can be thermal cured with an equivalent of 4,4′-diamino-diphenylsulfone to form composite. The curing process was monitored by polarized optical microscopy. The results indicate the LC resin was aligned along the CNTs to form fiber with dendritic structure initially then further on to obtain micro-sized spherical crystalline along with fibrous crystalline. With homogeneous dispersion and strong interaction between nanotubes and matrix, the composite containing 2.00 wt.% ef-CNT exhibits excellent thermal and mechanical properties. When the amount of ef-CNT exceeds 2.00 wt.%, vitrification stage of curing is fast reached, which lowers the degree of conversion. As compared with the neat resin, the composite containing 2.00 wt.% ef-CNT increases the glass transition temperature by 70.0 °C, the decomposition temperature by 13.8 °C, the storage modulus by 40.9%, and the microhardness by 63.3%.     

Effect of curing conditions on the dimensional and thermal stability of calcium phosphate cement for elevated temperature applications

Calcium phosphate cements (CPCs) are attractive materials for elevated temperature applications, like moulds to process thermoplastics up to 300 °C. The CPC resulting from the reaction of wollastonite with phosphoric acid cured at room temperature however contains hydrated phases like brushite, and is thus not stable when exposed to temperatures above 200 °C. A non-contact method based on digital image correlation demonstrated that isothermal curing at 60 °C reduces the thermal shrinkage up to 300 °C by 25%. This curing method results in the direct formation of the more stable monetite in a shorter curing time. The correlated results of TGA, pH of the filtration water, and DSC analysis on partially cured material indicate this. XRD diffractograms and SEM images in combination with EDX show the evolution of the transformation of wollastonite into monetite, and the structure and morphology of the formed material.     

Hydration kinetics modeling of the effect of curing temperature and pressure on the heat evolution of oil well cement

The heat evolution of Class G and Class H oil well cements cured under different temperatures (25 °C to 60 °C) and pressures (2 MPa to 45 MPa) was examined by isothermal calorimetry. Curing pressure was found to have a similar effect on cement hydration kinetics as curing temperature. Under isothermal and isobaric conditions, the dependency of cement hydration kinetics on curing temperature and pressure can be modeled by a scale factor which is related to the activation energy and the activation volume of the cement. The estimated apparent activation energy of the different cements at 2 MPa varies from 38.7 kJ/mol to 41.4 kJ/mol for the temperature range of 25 °C to 40 °C, which decreases slightly with increasing curing temperature and pressure. The estimated apparent activation volume of the cements at 25 °C varies from − 23.1 cm³/mol to − 25.9 cm³/mol for the pressure range studied here, which also decreases slightly in magnitude with increasing curing temperature.     

Permeability and elastic modulus of cement paste as a function of curing temperature

The permeability and elastic modulus of mature cement paste cured at temperatures between 8 °C and 60 °C were measured using a previously described beam bending method. The permeability increases by two orders of magnitude over this range, with most of the increase occurring when the curing temperature increases from 40 °C to 60 °C. The elastic modulus varies much less, decreasing by about 20% as the curing temperature increases from 20 °C to 60 °C. All specimens had very low permeability, k < 0.1 nm², despite having relatively high porosity, ? ~ 40%. Concomitant investigations of the microstructure using small angle neutron scattering and thermoporometry indicate that the porosity is characterized by nanometric pores, and that the characteristic size of pores controlling transport increases with curing temperature. The variation of the microstructure with curing temperature is attributed to changes in the pore structure of the calcium–silicate–hydrate reaction product. Both the empirical Carmen–Kozeny, and modified Carmen–Kozeny permeability models suggest that the tortuosity is very high regardless of curing temperature, ξ ~ 1000.     

The physical–chemical behaviour of amino cross-linkers and the effect of their chemistry on selected epoxy can coatings’ hydrolysis to melamine and to formaldehyde into aqueous food simulants

In this study, the suitability of four chemically different melamino cross-linkers for use in formulating epoxy coatings was investigated on the basis of the composition and of the tendency of cured coatings to hydrolyse to melamine and to formaldehyde when they were retorted in aqueous food simulants. The four cross-linkers were characterised for their composition identity, flow behaviour, thermal stability and for the presence of residual species. The different cross-linkers were used individually to cross-link selected epoxy coatings. The effects of the cross-linker chemistry, the curing conditions and the kinetics of the hydrolysis and subsequent migration processes, leading to melamine and formaldehyde were investigated following thermal treatments that were designed to represent the conditions of food sterilisation. The results show that each cross-linker type is different in its rheological characteristics, its solids content, its thermal behaviour and its physical properties. The chemistry of each cross-linker plays a major role in the manner in which the epoxy coatings undergo hydrolysis to release melamine and formaldehyde. The greatest migration of melamine (from an unpigmented epoxy anhydride coating, cured with the hexamethoxymethyl melamine cross-linker) into the 10% (v/v) aqueous ethanol food stimulant, after retorting at 131 °C, for 1 h was 525 μg/6 dm². The greatest migration of formaldehyde into the simulant was also from this coating at 11 μg/6 dm², when retorted at 131 °C for 1 h. The curing conditions affected the extent of the cross-linker hydrolysis. The influence of varying the curing time and the curing temperature was used to control the hydrolysis of the cross-linked, epoxy-based coatings. A decrease in the extent of cross-linker hydrolysis by 50–80% was achieved in all cases as the temperature of the curing was increased, in stages, from 160 °C to 200 °C.     

Compressive strength and microstructure of alkali-activated mortars with high ceramic waste content

The present work investigated alkali-activated mortars with high ceramic waste contents. Tile ceramic waste (TCW) was used as both a recycled aggregate (TCWA) and a precursor (TCWP) to obtain a binding matrix by the alkali-activation process. Mortars with natural siliceous (quartz) and calcareous (limestone) aggregates, and with other ceramic waste materials (red clay brick RCB and ceramic sanitaryware CSW waste), were also prepared for comparison purposes. Given the lower density and higher water absorption values of the ceramic aggregates, compared to the natural ones, it was necessary to adapt the preparation process of the recycled mortars by presaturating the aggregate with water before mixing with the TCWP alkali-activated paste. Aggregate type considerably determined the mechanical behaviour of the samples cured at 65 °C for 3 days. The mortars prepared with the siliceous aggregate presented poor mechanical properties, even when cured at 65 °C. The behaviour of the limestone aggregate mortars depended heavily on the applied curing temperature and, although they presented the best mechanical properties of all those cured at room temperature, their compressive strength reached a maximum when cured at 65 °C, and then decreased. The mechanical properties of the mortars prepared with TCWA progressively increased with curing time (53 MPa at 65 °C for 28 days). An optimum 50 wt% proportion was observed for the limestone/TCWA mortars (≈43 MPa, 3 days at 65 °C), whereas the mechanical properties of that prepared with siliceous particles (10 MPa) progressively increased with the TCWA content, up to 100 wt% substitution (23 MPa). Limestone particles interacted with the binding matrix, and played an interesting beneficial role at the 20 °C curing temperature, with a slight reduction when cured long term (28 days) at 65 °C. The results demonstrated a potential added value for these ceramic waste materials.     

Synthesis and properties of 1,3,4-oxadiazole-containing high-performance bismaleimide resins

Two novel bismaleimide (BMI) monomers containing 1,3,4-oxadiazole, i.e., 5-tert-butyl-1,3-di[5-(4-maleimidophenyl)-1,3,4-oxadiazole-2-yl]benzene (Buoxd) and 4,4′-[5-(4-maleimidophenyl)-1,3,4-oxadiazole-2-yl]diphenyldimethylsilane (Sioxd), were designed, synthesized and copolymerized with 4,4′-bismaleimidodiphenylmethane (BMDM) to yield a new series of high-performance bismaleimide resins. Both monomers obtained are readily soluble in common organic solvents, such as dichloromethane and chloroform, enabling an easy solution processing. The thermal properties of the two monomers were carefully studied by the differential scanning calorimetry (DSC), optical microscopy and thermogravimetric analysis (TGA, simultaneous DSC). The BMI resins based on a mixture of Buoxd (or Sioxd) and BMDM in a weight ratio of 10% were prepared. DSC investigations showed that the thermal curing of the BMI resins could be accomplished at a lower temperature than the thermal curing temperatures of Buoxd and Sioxd, and the thermal processing window, i.e., the temperature range between the melting transition and thermal curing process, was over 26 °C. The thermal properties and thermal mechanical properties of the resulting BMI resins were investigated by DSC, thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). No glass transition temperature was found in the range of 50–350 °C, and very good thermal stability (T_d > 490 °C in nitrogen) and high thermo-oxidative stability (T_d > 460 °C in air) were revealed. Composites composed of the above BMI resins and glass cloth were also prepared, which showed high bending modulus (>1.6 GPa) at a very high temperature (e.g., 400 °C).     

Effects of oxidation cross-linking and sintering additives (TiN,B) on the formation and heat-resistant performance of polymer-derived SiC(Ti,B) films

Effects of oxidation cross-linking and sintering additives (TiN, B) on the microstructure formation and heat-resistant performance of freestanding SiC(Ti, B) films synthesized from Ti, B-containing polycarbosilane (TiB-PCS) precursor were investigated. TiB-PCS green films were first cross-linked for 1 h, 2 h, 3 h and 4 h, respectively, and then pre-sintered at 950 °C. Finally, they were sintered at 1800 °C to complete the conversion from organic films to inorganic SiC(Ti, B) films. The results reveal that curing time has a great impact on the uniformity and density of SiC(Ti, B) films. TiB-PCS films cured for 3 h yield the best quality SiC(Ti, B) films, which are composed of β-SiC crystals, C clusters, α-SiC nano-crystals, a small amount of TiB₂ and B₄C. TiB₂ and B₄C are both steady phases which can inhibit abnormal growth of β-SiC, effectively reduce sintering temperature and help consume excess C from decomposition of amorphous SiOxCy. After high temperature annealing at 1500 °C, 1600 °C and 1700 °C in argon, SiC(Ti, B) films still keep excellent mechanical properties, which makes them attractive candidate materials for microelectromechanical systems (MEMS) used at ultra-high temperatures (exceeding 1500 °C).     

Influence of temperature on the hydration products of low pH cements

The chemical evolution of two hydrated “low pH” binders prepared from binary (60% Portland cement + 40% silica fume) or ternary (37.5% Portland cement + 32.5% silica fume + 30% fly-ash) mixtures was characterized over one year at 20 °C, 50 °C, and 80 °C. The main hydrates were Al-substituted C–S–H. Raising the temperature from 20 to 80 °C caused a lengthening and cross-linking of their silicate chains. Ettringite that formed in pastes stored at 20 °C was destabilized. Only traces of calcium sulfate (gypsum and/or anhydrite) reprecipitated after one year in some materials cured at 50 °C and 80 °C. The sulfates released were therefore partially adsorbed on the C–A–S–H and dissolved in the pore solution. The pore solution pH dropped by about 2 units as the temperature increased. Conversely, the soluble alkali fractions did not change significantly. Only the ternary binder resulted in a pore solution pH below 11 at the three temperatures studied.     

Bio-based thermosetting bismaleimide resins using eugenol,bieugenol and eugenol novolac

5,5′-Bieugenol (BEG) and eugenol novolac (EGN) were synthesized by the oxidative coupling reaction of eugenol (EG) and the addition–condensation reaction of EG with formaldehyde, respectively. The EG, BEG and EGN were prepolymerized with 4,4′-bismaleimidediphenylmethane (BMI) at 180 °C and then compression-molded at finally 250 °C for 6 h to produce cured EG/BMI (EB), BEG/BMI (BB) and EGN/BMI (NB) resins with eugenol/maleimide unit ratios of 1/1, 1/2 and 1/3. The FT-IR analysis of EBs and ¹³C NMR analysis of the model reaction product of EG/N-phenylmaleimide (PMI) 1/3 at 200 °C for 12 h suggested that the ene reaction and subsequent Diels-Alder/ene reactions mainly occurred for EBs. The FT-IR analyses of BBs and NBs supported the occurrence of ene reaction and subsequent thermal addition copolymerization in a similar manner to the well-known curing reaction of 2,2′-diallylbisphenol A and BMI. The glass transition temperature (T_g) and 5% weight loss temperature (T₅) of the cured resin increased with increasing BMI content, and EB 1/3 showed the highest T_g 377 °C and T₅ 475 °C. The flexural strengths and moduli of EBs and NBs were higher than those of BBs, and EB 1/2 showed the most balanced flexural strength and modulus (84.5 MPa and 2.75 GPa). The FE-SEM analysis revealed that there is no phase separation for all the cured resins.     

A morphological study of poly(butylene succinate)/poly(butylene adipate) blends with different blend ratios and crystallization processes

Morphologies of PBS/PBA blends varying in blend ratio at different crystallization temperatures of PBS component were studied using optical and atomic force microscopies. It was found that interspherulitic phase segregation of PBA takes place at high temperature, e.g. 100 °C, for all blend compositions due to the high diffusion length. On the contrary, no interspherulitic phase segregation of PBA at 75 °C occurs at all, consistent with a shorter diffusion length. It was further found that the PBA melt acted as a diluter, affecting the morphology of PBS, which in turn influenced the phase separation behavior of PBA remarkably. At 100 °C, with increasing PBA concentration, the resultant open structure of PBS caused by the diluting effect of the PBA melt makes it necessary to retain some portion of the PBA melt between the PBS lamellae. This leads to the concurrence of interlamellar and interspherulitic phase segregations. An even higher PBA content results in the occurrences of all three phase separation options, i.e. interlamellar, interfibrillar and interspherulitic phase segregations. At 75 °C, in blends with PBA as a minority phase, mainly interlamellar segregation of PBA occurs. For a 50/50 blend, interlamellar and interfibrillar phase segregations take place simultaneously. For a PBA in-rich blend, PBS forms only a spherulitic framework, filled in with the PBA lamellar crystals, indicating that interfibrillar mode is the main phase separation process.     

Effect of annealing temperature on the morphology and optical properties of ZnO tetrapods grown by a thermal evaporation technique

The effect of the thermal annealing temperature was investigated on ZnO tetrapods grown by a thermal evaporation method. The ZnO tetrapods were synthesized by thermal evaporation of Zn powder in air. The annealing was done in an oxygen gas environment at temperatures ranging from 400 to 1000 °C for 1 h. As the annealing temperature increased from 400 °C to 800 °C, the morphology of the tetrapod remained unchanged; however, the size of the tetrapods increased. With a further increase in the annealing temperature from 800 °C to 1000 °C, the ZnO tetrapod changed drastically to nanoneedles. As-grown and annealed samples had an identical crystal structure, which was a wurtzite structure. A strong and sharp ultraviolet emission at 380 nm was observed for the 600 °C –annealed sample indicating the high crystalline quality. The ultraviolet emission intensity decreased abruptly for the samples annealed at 800 °C and 1000 °C, which exhibits the degradation in crystallinity.     

Preparation of stable cross-linked enzyme aggregates (CLEAs) of NADH-dependent nitrate reductase and its use for silver nanoparticle synthesis from silver nitrate

The NADH-dependent nitrate reductase from Fusarium oxysporum cell extract was directly immobilized as cross linked enzyme aggregates (CLEAs) and investigated for the synthesis of silver nanoparticles by a reduction of silver nitrate. The effects of precipitant type and cross-linking on activity recovery of enzyme in CLEAs were studied. After aggregation of enzyme with ammonium sulfate followed by cross-linking formed aggregates for 4 h with 8 mM glutaraldehyde, 93% activity recovery was achieved in CLEAs with enhanced thermal stability at 50 °C and 40 °C. Scanning electron microscopy analysis showed that immobilized NADH-dependent nitrate reductase was of spherical structure. CLEAs showed 90% catalytic yield even after 4 cycles of repeated use in silver nanoparticle synthesis at pH 7.2 and temperature 35 °C.     

Preparation and characterization of novel film adhesives based on cyanate ester resin for bonding advanced radome

In this paper, the novel film adhesives based on phenolphthalein poly(ether sulfone) (PES-C) and epoxy (EP) modified cyanate ester resin (CE) were prepared for bonding an advanced radome. The film adhesives are convenient for applying to manufacture, possessing good adhesion strength, thermal durability and excellent dielectric property. The curing behaviors were confirmed by differential scanning calorimetry (DSC), showing that the main reaction pathways are not varied with adding PES-C but the reaction rates are evidently accelerated, and the film adhesives can be well cured at lower temperature of 177 °C. The adhesion strength was evaluated in lap shear strength and peel strength, indicating that the better adhesion strength is obtained with increasing in PES-C. The maximum value of lap shear strength is 33 MPa at room temperature. The thermal durability was determined by thermal aging tests of lap shear specimens, showing that the decrease in strength gets faster with adding PES-C, and the usability of film adhesives over 2000 h at 200 °C. The dielectric properties were measured by dielectric resonator methods, finding that the introduction of PES-C brings a positive effect on dielectric properties. The lowest value of determined dielectric loss is 0.0075 at 10 GHz.     

Synthesis of phenolic resol resins using cornstalk-derived bio-oil produced by direct liquefaction in hot-compressed phenol–water

For the synthesis of biomass-based resol resins, cornstalk powders were liquefied in a hot-compressed phenol–water (1:4, wt./wt.) medium at 300–350 °C. It was observed that essentially no phenol was reacted with the cornstalk degradation intermediates during the liquefaction process. The cornstalk-derived bio-oils contained oligomers of phenol and substituted phenols, originated primarily from the lignin component of the cornstalk feedstock. Using the cornstalk-derived bio-oils, resol resins were readily synthesized under the catalysis of sodium hydroxide. The biomass-derived resol resins were brown viscous liquids, possessing broad molecular weight distributions. In comparison with those of a conventional phenol resol resin, the properties of the bio-based resins were characterized by GPC, FTIR, DSC and TGA. The as-synthesized bio-oil resol resin exhibited typical properties of a thermosetting phenol–formaldehyde resin, e.g., exothermic curing temperatures at about 150–160 °C, and an acceptable residual carbon yield of ca 56% at 700 °C for the cured material.     

Effect of relative humidity on the reaction products of alkali activated fly ash

The alkali activation of fly ash (AAFA) is a chemical process in which the ash is mixed with an alkaline activator and cured at a mild temperature to generate compact solids. Both the curing conditions (temperature, time, relative humidity, etc.) and the nature and concentration of the alkali activator play a key role in the development of AAFA micro- and nanostructure, and consequently the properties of these materials. In the present study, fly ash was activated with a 15% water-glass (Na₂SiO₃) + 85% 10-M NaOH solution at 85 °C for 12 h, 7 and 30 days. Two curing methods were used, in which the variable was the relative humidity (RH).     

Thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites

The aim of this work is to investigate the thermal conversion of carbon fibres/polysiloxane composites to carbon fibres/ceramic composites. The conversion mechanism of four different resins to the ceramic phase in the presence of carbon fibres is investigated. The experiments were conducted in three temperature ranges, corresponding to composite manufacturing stages, namely up to 160 °C, 1000 °C and finally 1700 °C.The study reveals that the thermal conversion mechanism of pure resins in the presence of carbon fibres is similar to that without fibres up to 1000 °C. Above 1000 °C thermal decomposition occurs in both solid (composite matrix) and gas phases, and the presence of carbon fibres in resin matrix produces higher mass losses and higher porosity of the resulting composite samples in comparison to ceramic residue obtained from pure resin samples. XRD analysis shows that at temperature of 1700 °C composite matrices contain nanosized silicon carbide. SEM and EDS analyses indicate that due to the secondary decomposition of gaseous compounds released during pyrolysis a silicon carbide protective layer is created on the fibre surface and fibre–matrix interface. Moreover, nanosized silicon carbide filaments crystallize in composite pores.Owing to the presence of the protective silicon carbide layer created from the gas phase on the fibre–matrix interface, highly porous C/SiC composites show significantly high oxidation resistance.     

Curing green fibres infusible by electron beam irradiation for the preparation of SiBNC ceramic fibres

Curing green fibres infusible is an essential procedure for the preparation of SiBNC ceramic fibres. Previously, green fibres had been fabricated by one-pot synthesis of polyborosilazane (PBSZ) and melt-spinning. In this paper, we attempted to use the method of electron beam irradiation to crosslink green fibres. The variation of molecular structures from green fibres to cured fibres and the properties of sintered SiBNC fibres were investigated. Via electron beam irradiation, the free radicals are formed at the C atoms and Si atoms on the -N-SiH(CH₃)- main chain units and terminal -Si(CH₃)₃ groups. The radicals react with each other to produce cross-linking, coupling and grafting among PBSZ chains, which all contribute to improvement of the cross-linking density of green fibres. The cured fibres performed a high ceramic yield of 80.4 wt%. After pyrolysis at 1500 °C, SiBNC ceramic fibres were acquired, which exhibited a good flexibility with 12 µm in diameter and 1.22 GPa in tensile strength. The obtained fibres could remain amorphous up to 1700 °C and showed no mass loss at this temperature.     

Low temperature tensile lap-shear testing of adhesively bonded polyethylene pipe

This work studies the lap-shear strength performance of polyethylene pipeline bonded with acrylic adhesive in the temperature range -10 to +20 °C. Single lap shear test samples were firstly prepared at 20 °C under various clamping pressures and curing times to determine suitable conditions under which to prepare and test further samples at temperatures of -10, -5, 0, +5 and +20 °C. It was found that a decrease in curing/testing temperature to zero degrees resulted in a steady reduction in the lap-shear strength performance of the bonded joints from a mean value of 2.72 MPa at +20 °C to 1.15 MPa at 0 °C. Below zero degrees the strength of the bonded substrates was significantly reduced; no samples bonded at -5 °C had sufficient strength to test and only one sample bonded -10 °C was tested, which had very low strength of 0.105 MPa.     

Bio-based thermosetting resins composed of aliphatic polyol-derived polymaleimides and allyleugenol

Bio-based bismaleimide (2MPD), trismaleimide (3MGC) and tetramaleimide (4MDG) were synthesized by reactions of 4-isocyanatophenylmaleimide with 1,3-propanediol, glycerol and α,α′-diglycerol, respectively. Although 2MPD did not melt until the temperature where thermal decomposition starts, 3MGC and 4MDG exhibited broad melting temperatures with onset points at 165 °C and 124 °C, respectively. 3MGC and 4MDG were homogeneously prepolymerized at 170 °C with 2,4-diallyl-6-methoxyphenol (rAEG) which was prepared by the Claisen rearrangement of allyl-etherified eugenol (AEG). The prepolymers were compression-molded at 250 °C to produce cured rAEG/3MGC (A3Mxy) and rAEG/4MDG (A4Mxy) with the allyl/maleimide ratio of x/y = 1/1, 1/2 or 1/3. The FT-IR analysis revealed that the ene reaction of allyl and maleimide groups and subsequent addition copolymerization occurred for the cured resins. The thermal and mechanical properties of the cured resins were compared with those of the cured rAEG/4,4′-bismaleimidodiphenylmethane (BMI) (ABMxy) with the same allyl/maleimide ratio. A3M13 and A4M13 showed no inflection point of thermal expansion due to glass transition until 300 °C, which is a little lower than the thermo-degradation temperature. Flexural strengths and flexural strains at break for A3Ms and A4Ms increased with the polymaleimide contents, and those of A3M13 and A4M13 were much higher than those of ABM13.