Embedded single carbon fibre to sense the thermomechanical behavior of an epoxy during the cure process

A new approach that uses a single carbon fibre for sensing the thermomechanical behavior of an epoxy during the cure cycle is presented. By recording and analyzing the electrical resistance and temperature history of a carbon fibre embedded inside an epoxy specimen during the cure cycle, the interaction between the carbon fibre and the surrounding polymer can be revealed. Compared with reported TMA and DMA results, this embedded carbon fibre sensor approach successfully detects the glass transition zone covering the final transition temperature, the main transition temperature, and the starting transition temperature that respectively have similar values as: (i) T_g corresponding to the abrupt change in CTE, (ii) T_g by storage modulus (E′) onset, and (iii) the upper temperature limit for the linear relationship between E′ and the temperature. The future applications for this sensor method are also discussed.     

Crystallization study of Te-Bi-Se glasses

Crystallization studies are carried out under non- isothermal conditions with samples heated at several uniform rates. The dependence of the glass transition temperature (T_g), the crystalline temperature (T_c) and the peak temperature of crystallization (T_p) on the composition and heating rate (β) has been studied. For a memory/switching material, the thermal stability and ease of glass formation are of crucial importance. The glass transition temperature, T_g, increases slightly with the variation of Bi content. From the heating rate dependence of T_g, the activation energy for glass transition (E_t) has been evaluated. The results are discussed on the basis of Kissinger’s approach and are interpreted using the chemically ordered network model (CONM).     

Novel nanocomposites based on epoxy resin/epoxy-functionalized polydimethylsiloxane reinforced with POSS

The purpose of the present study is to develop novel nanocomposites based on diglycidylether of bisphenol A (DGEBA) combined with diglycidylether-terminated polydimethylsiloxane (DG-PDMS), reinforced with 10 wt.% (mono-/octa) epoxy POSS nanocages (MEP or OEP-POSS). DG-PDMS and POSS compounds were covalently incorporated into DGEBA resin via copolymerization of epoxy groups. The effect of both DG-PDMS and POSS nanoparticles on the curing reaction, glass transition temperature (T_g), thermal stability, hardness and morphology of DGEBA/DG-PDMS ± POSS nanocomposites were studied by DSC, FTIR, DMA, TGA, SEM/EDX, AFM and contact angle measurements. SEM/EDX and AFM results prove that OEP-POSS is well dispersed within DGEBA/DG-PDMS polymer matrix, while MEP-POSS forms large POSS aggregates. The thermo-mechanical properties of POSS based nanocomposites are also in good correlation with morphology features. MEP-POSS based nanocomposite with heterogeneous dispersion of POSS aggregates exhibits lower T_g value and thermal stability in comparison with OEP-POSS nanocomposite which exhibits a nanoscale dispersion of the POSS cages. The obtained T_g of OEP-POSS based nanocomposite increases with 31 °C in comparison with the unreinforced matrix. Moreover, this nanocomposite shows the highest storage modulus (E′) and hardness.     

Shape memory property of microcrystalline cellulose–poly(ε-caprolactone) polymer network with broad transition temperature

Preparation of shape memory polymers (SMPs) with broad transition temperature was an effective method to realize multishape memory effect. In this study, a novel SMP with a broad glass transition temperature (T _g) based on microcrystalline cellulose was prepared. The structure of the SMP was analyzed by Fourier transform infrared spectroscopy and proton nuclear magnetic resonance, which can prove the successful synthesis of the material. The thermal properties were investigated with differential scanning calorimetry and dynamic mechanical analysis (DMA). The dual- and multishape memory effects were also quantificationally analyzed by DMA. Further, the influence of programming temperature within T _g on dual-shape memory effect was investigated, and a 1D model was built to explain their relationship.     

Dynamic mechanical analysis of carbon/epoxy composites for structural pipeline repair

The viscoelastic behavior of a carbon fiber/epoxy matrix composite material system used for pipeline repair has been evaluated though dynamic mechanical analysis. The effects of the heating rate, frequency, and measurement method on the glass transition temperature (T_g) were studied. The increase in T_g with frequency was related to the activation energy of the glass transition relaxation. The activation energy can be used for prediction of long term performance. The measured tan delta peak T_g’s of room temperature cured and post-cured composite specimens ranged from 60 to 129 °C. Analysis of T_g data at various cure states was used to determine use temperature limits for the composite repair system.     

Performances at high temperature of RC bridge decks strengthened with EBR-FRP

Fiber reinforced polymer (FRP) can be successfully used to externally strengthen reinforced concrete (RC) bridges where fire is not a primary concern. Nevertheless, common maintenance activity on a bridge deck, such as the laying of bituminous paving, can easily lead the FRP to temperatures higher than the glass transition temperature, T_g. Exceeding T_g does not necessarily imply a drastic reduction in strength and stiffness of the reinforcement. Nevertheless, the softening of the resin implies a drastic reduction in its adhesion properties. Therefore, the efficiency of the strengthening system for existing structures, which mainly depends on the effectiveness of the bond between FRP and concrete, is significantly affected by temperature.The relationships suggested by Italian and American codes in order to evaluate the limit strain for FRP debonding at normal temperature are modified to take into account the effect of high temperature. Then, performances at high temperature of RC bridge decks strengthened with externally bonded FRP plates (EBR) are investigated by considering thermal fields in the structural members which are different from the normal ones. Both fire and the laying of bituminous paving on the decks are considered. In addition, the thicknesses of the slabs and the protective layer are varied to assess their influence on the thermal field in the slabs. The results are discussed with reference to both ultimate and serviceability limit states.     

Woven hybrid biocomposites: Dynamic mechanical and thermal properties

The dynamic mechanical and thermal analysis of oil palm empty fruit bunch (EFB)/woven jute fibre (J_w) reinforced epoxy hybrid composites were carried out. The storage modulus (E′) was found to decrease with temperature in all cases, and hybrid composites had showed better values of E′ at glass transition temperature (T_g) compared to EFB and epoxy. Loss modulus showed shifts in the T_g of the polymer matrix with the addition of fibre as reinforcing phase, which indicate that fibre plays an important role in case of T_g. The Tan δ peak height was minimum for jute composites and maximum for epoxy matrix. Complex modulus variations and phase behaviour of the hybrid composites was studied by Cole-Cole analysis. Thermal analysis result indicates an increase in thermal stability of EFB composite with the incorporation of woven jute fibres. Hybridization of EFB composite with J_w fibres enhanced the dynamic mechanical and thermal properties.     

Preparation of Microcellular Epoxy Foams through a Limited‐Foaming Process: A Contradiction with the Time–Temperature–Transformation Cure Diagram

3D cross‐linking networks are generated through chemical reactions between thermosetting epoxy resin and hardener during curing. The curing degree of epoxy material can be increased by increasing curing temperature and/or time. The epoxy material must then be fully cured through a postcuring process to optimize its material characteristics. Here, a limited‐foaming method is introduced for the preparation of microcellular epoxy foams (Lim‐foams) with improved cell morphology, high thermal expansion coefficient, and good compressive properties. Lim‐foams exhibit a lower glass transition temperature (T_g) and curing degree than epoxy foams fabricated through free‐foaming process (Fre‐foams). Surprisingly, however, the T_g of Lim‐foams is unaffected by postcuring temperature and time. This phenomenon, which is related to high gas pressure in the bubbles, contradicts that indicated by the time–temperature–transformation cure diagram. High bubble pressure promotes the movement of molecular chains under heating at low temperature and simultaneously suppresses the etherification cross‐linking reaction during post‐curing.     

Study on short ramie fiber/poly(lactic acid) composites compatibilized by maleic anhydride

Short ramie fiber reinforced poly(lactic acid) (PLA) composites without and with maleic anhydride (MA) were developed. The influence of PLA-g-MA as a compatibilizer on the properties of the composites was studied. The tensile, flexural and impact strength of the composites have improvements with the addition of PLA-g-MA. The morphology of fracture surface evaluated by SEM indicates that the composites with the addition of PLA-g-MA can get better adhesion between the fiber and the matrix. And the Vicat softening temperature and the degradation temperature of the composites are increased with the addition of PLA-g-MA. However, PLA-g-MA leads the glass transition temperature (T_g) decrease according to the DSC results.     

Influence of post-curing and temperature effects on bulk density,glass transition and stress-strain behaviour of imidazole-cured epoxy network

The effects of post-curing and temperature on the glass transition, bulk density and stress-strain behaviour in the glassy and rubbery state of 2-ethyl-4-methyl imidazole-cured epoxy network have been evaluated by differential scanning calorimetry (DSC), water displacement and tensile testing. The glass transition temperature,T _g, was found to increase with increasing post-cure temperature and the size of the base line shift in the glass transition region on the DSC thermogram can serve as an indicator of the extent of cure. At room temperature, the decrease in bulk density with increasing extent of cure may be attributed to the additional cross-linking, adding molecular constraints to the thermal constraints. Thus, a higher free volume atT _g can be expected to remain in the glassy state as the sample is slowly cooled through the glass transition temperature. In the investigation on the temperature dependence of the tensile mechanical properties, a fracture envelope was obtained. The tensile strength, Young's modulus and ultimate elongation in the glassy and rubbery state are discussed in detail.     

Correlation between glass transition temperature and melting temperature in metallic glasses

We report that the glass transition temperature (T_g) of a variety of metallic glasses (MGs) correlates with the eutectic or peritectic temperature of two main components corresponding stoichiometric proportion in their binary phase diagram. The correlation suggests that the T_g of MGs is mainly determined by their solvent of two base components, which have composition close to the eutectic and peritectic points in the binary phase diagram and the weakest link in amorphous structure. The results have implications for understanding the structure and glass transition in MGs and for predicting and designing metallic glasses with a desirable T_g.     

Mechanical characterization of steel/CFRP double strap joints at elevated temperatures

This paper examines the mechanical performance of steel/CFRP adhesively-bonded double strap joints at elevated temperatures around the glass transition temperature (T_g, 42 °C) of the adhesive. A series of joints with different bond lengths were tested to failure at temperatures between 20 °C and 60 °C. It was found that the joint failure mode changed from adherend failure to debonding failure as the temperature approached T_g. In addition, the ultimate load and joint stiffness decreased significantly at temperatures near to and greater than T_g, while the effective bond length increased with temperature. Based on the ultimate load prediction model developed by Hart-Smith for double lap joints and kinetic modelling of the mechanical degradation of the adhesive, a mechanism-based model is proposed to describe the change of effective bond length, stiffness and strength degradation for steel/CFRP double strap joints at elevated temperatures. The modelling results were validated by the corresponding experimental measurements.     

The influence of hydrothermal conditioning on the Mode-I,thermal and flexural properties of Carbon/Benzoxazine composites with a thermoplastic toughening interlayer

Carbon/Benzoxazine laminates with and without non-woven thermoplastic fibrous polyamide (PA) veils at the interlaminar regions were manufactured using Vacuum Assisted Resin Transfer Moulding (VARTM). The effect of the interlaminar thermoplastic veils on the Mode-I strain energy release rate (G_IC), flexural stiffness, glass transition temperature (T_g) and water absorption behaviour was determined using two commercially available Benzoxazine resins. Despite an increase in the maximum moisture content, the veils greatly enhanced G_IC by an increase in fibre bridging of PA fibres, with concurrent reductions in flexural stiffness. Water ingress resulted in large reductions in the T_g, although no significant change was observed due to the PA interlayers. Fibre bridging and fibre pull-out were the main mechanisms by which the veils assisted in resisting delamination. The presence of the water was observed to degrade mechanical properties due to a reduction in fibre/matrix interfacial strength, molecular degradation and plasticisation of the matrix.     

Effect of elevated service temperature on bond between FRP EBR systems and concrete

The method to increase the ultimate capacity of reinforced concrete elements, by means of Fiber Reinforced Polymers bonded on their substrate with thermosetting resins, represents a technique useable worldwide. The bonding materials used, generally epoxy resins, are sensitive to a temperature increase. In fact, the curing process of epoxies leads to low glass transition temperature (T_g) values, that may cause a relevant decay of the mechanical properties of the adhesive, even under service thermal conditions. The reduction can influence the bond performance, compromising the effectiveness of the reinforcing technique. In this work the interface behaviour FRP–concrete at elevated service temperatures is analyzed. The experimental results show a relevant influence of the temperature on bond performance, in terms of type of failure of the interface, effective bond length and bond strength.     

Service life assessment and moisture influence on bio-based thermosetting resins

In this study, three different types of bio-based resins are compared to a conventional oil-based epoxy in terms of moisture uptake, long-term properties and its influence of moisture and glass transition temperature, T _g. Moisture uptake is determined by means of gravimetric method, time temperature superposition (TTSP), and T _g data obtained in dynamic mechanical thermal analysis (DMTA). Moisture uptake show Fickian diffuison behavour for all resins, saturation level and diffusion coefficient however differ. The long-term properties is characterised by creep compliance master curves created by means of TTSP. The examined bio-based resins are compatible to the reference epoxy in term of stability up to 3–10 years. Comparison between master curves for virgin, wet, and dried material show that moisture present in the specimen increases creep rate, and that some of this increase remains after drying of samples. T _g measurements show that moisture inside the specimen decreases T _g; this is anticipated because of the plasticizing effect of water. The overall conclusions are that the bio-based resins of polyester, and epoxy type are comparable in performance with oil-based epoxy, LY556 and they can be used to develop high-performance composites.     

Effects of dispersion and interfacial modification on the macroscale properties of TiO2 polymer–matrix nanocomposites

This paper quantifies how the quality of dispersion and the quality of the interfacial interaction between TiO₂ nanoparticles and host polymer independently affect benchmark properties such as glass transition temperature (T_g), elastic modulus and loss modulus. By examining these composites with differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and scanning electron microscopy (SEM), we demonstrate changes in properties depending on the adhesive/wetting or repulsive/dewetting interactions the nanoparticles have with the bulk polymer. We further quantify the dispersion of TiO₂ nanoparticles in polymethylmethacrylate (PMMA) matrices by a digital–optical method and correlate those values to the degree of T_g depression compared to neat PMMA. Samples with the same weight percent of nanoparticles but better dispersion show larger shifts in T_g.     

Characterization of strengthened rapidly quenched Zr-based alloys

The nominal composition components of alloy Zr_66.4Nb_6.4Ni_8.7Cu_10.5Al₈ (Alloy A) were fabricated and characterized. The strengthening of in-situ alloys depends on the role of both the glassy matrix and the second phases. The glass transition and the crystallization kinetics were studied using DSC and X-ray diffraction as a function of element distribution. The amorphous and semi-crystalline structures were identified with the existence of nano crystals in the alloy nominal compositions. The Elastic compression modulus were found to increase with transition to crystallite phase. Where as, the microhardness decreases dramatically with the change from crystalline to amorphous phase. The compression fracture surface shows classic veins behavior. In mode of continuous heating and adiabatic annealing the glass transition, T _g , and the crystalline peak, T _p , temperatures display a strong dependency on heating rate. The activation energy for glass transition and crystallization were determined as E _g  = 226 KJ/mol based on Kissinger method, but during the isothermal process E _g  = 121 KJ/mol.     

Effect of surfactant treatment on thermal stability and mechanical properties of CNT/polybenzoxazine nanocomposites

The mechanical and thermo-mechanical properties of polybenzoxazine nanocomposites containing multi-walled carbon nanotubes (MWCNTs) functionalized with surfactant are studied. The results are specifically compared with the corresponding properties of epoxy-based nanocomposites. The CNTs bring about significant improvements in flexural strength, flexural modulus, storage modulus and glass transition temperature, T_g, of CNT/polybenzoxazine nanocomposites at the expense of impact fracture toughness. The surfactant treatment has a beneficial effect on the improvement of these properties, except the impact toughness, through enhanced CNT dispersion and interfacial interaction. The former four properties are in general higher for the CNT/polybenzoxazine nanocomposites than the epoxy counterparts, and vice versa for the impact toughness. The addition of CNTs has an ameliorating effect of lowering the coefficient of thermal expansion (CTE) of polybenzoxazine nanocomposites in both the regions below and above T_g, whereas the reverse is true for the epoxy nanocomposites. This observation has a particular implication of exploiting the CNT/polybenzoxazine nanocomposites in applications requiring low shrinkage and accurate dimensional control.     

Crystallization kinetics of selenium-tellerium glasses

Bulk glasses of the compositions Se₇₀Te₃₀ and Se₈₀Te₂₀ were prepared by the melt quenching technique. Differential thermal analyses were performed at different heating rates. The values of the glass transition temperature,T _g, the crystallization temperature,T _c, and the peak temperature of crystallization,T _p, were found to depend on the composition and the heating rate. The activation energy for the glass transition,E _i, as well as the activation energy for crystallization,E _c, were evaluated from the heating rate dependence ofT _g andT _p. The crystallization mechanism was examined through analysis of the data under non-isothermal conditions. The results indicated that surface crystallization is dominant for both compositions.     

Effects of aluminium nitride inclusions on thermal and electrical properties of epoxy and polypropylene: An experimental investigation

Thermal and dielectric properties of polymers reinforced with micro-sized aluminium nitride (AlN) particles have been studied. A set of epoxy–AlN composites, with filler content ranging from 0 to 25 vol% is prepared by hand lay-up technique. With similar filler loading, polypropylene -AlN composites are fabricated by compression molding technique. Density (ρ_c), effective thermal conductivity (k_eff), glass transition temperature (T_g), coefficient of thermal expansion (CTE) and dielectric constant (ε_c) of these composites are measured experimentally. The various experimental data were interpreted using appropriate theoretical models. Incorporation of AlN in both the resin increases the k_eff and T_g whereas CTE of composite decreases favourably. The dielectric constant of the composite also found to get modified with filler content. With improved thermal and modified dielectric characteristics, these AlN filled polymer composites can possibly be used for microelectronics applications.