Curing Kinetics Research Articles

RELAXATION PROCESSES IN SYNTHESIS AND MODIFICATION OF POLYMERS IN ELECTROMAGNETIC FIELDS. PART D: INCREASING RESEARCH OF CHEMICAL PROCESSES UNDER ELECTROMAGNETIC FIELDS – THE MAIN ROUTE TOWARDS FUTURE ADDITIVE TECHNOLOGIES

The paper studies of kinetics of epoxy resin curing by dielectric analysis method. Kinetic parameters largely depend on the measurement mode. During curing, time dependence tgδ passes two maximum loss, the first of which occurs only in the case of measurement at low frequencies. The position of both maxima is largely determined by frequency at which the measurements are performed. The latter circumstance is very important, due to the fact that the time of occurrence of the maximum is usually associated with gelation of the system, the first maximum determines the transition from viscous state to viscoelastic one, and corresponds to the onset of gelation process. In connection with these methods the possibility of using them in additive technologies is discussed.

Cure kinetics characterization of soy‐based epoxy resins for infusion moulding process

AbstractThis study presents the kinetics of the reaction of diglycidyl ether of bisphenol A (DGEBA) based epoxy resin in the presence of epoxidized soybean oil (ESO) cured with triethylene tetramine (TETA). An isothermal differential scanning calorimetric (DSC) study is carried out to propose a model to analyze the cure kinetics of bio‐based resin. The proposed model is also compared with the Kamal model. The results clearly highlight that the proposed model attains a reasonable percentage of improvement in predicting cure data. Adding ESO to the conventional system decreases the reaction enthalpy and increases the activation energy of the system. The outcome of the study confirms the feasibility of the proposed material system for an infusion process. Further, tensile property improvement is obtained through using an infusion process for the proposed material system.

Cure kinetics of bio-based epoxy resin developed from epoxidized cardanol–formaldehyde and diglycidyl ether of bisphenol–A networks

A bio-based epoxy resin was synthesized from cardanol which is natural renewable resource. Subsequently, diglycidyl ether of bisphenol A (DGEBA) was toughened with three different mass proportions of bio-based epoxidized cardanol–formaldehyde resin (ECF) (i.e., 20, 40, 60 mass%). In this study, the curing kinetics of amine-cured modified resin was studied by non-isothermal differential scanning calorimetry analysis. The apparent activation energy obtained by Kissinger and Flynn–Wall–Ozawa methods was 48 and 52 kJ mol−1, respectively. The two-parameter Sestak–Berggren autocatalytic model was used to obtain the reaction orders m and n. The curves obtained by the Malek method show good agreement with the experimental data for bio-based epoxy systems.

Green Epoxy Resin System Based on Lignin and Tung Oil and Its Application in Epoxy Asphalt

In this work, lignin-based epoxy was synthesized by reacting epichlorohydrin (ECH) and partially depolymerized lignin (PDL) and then used for modification of asphalt. The Diels–Alder adduct of methyl esters of eleostearic acid and maleic anhydride (ME-MA) was synthesized and used as a biobased curing agent. The structure of PDL-epoxy was characterized by FTIR and 31P NMR. Nonisothermal curing kinetics and thermal properties of the cured epoxy resins were studied by differential scanning analysis, dynamic mechanical analysis, and thermogravimetric analysis, respectively. Curing behaviors of PDL-epoxy and a commercial epoxy DER332 were compared. Effects of epoxy content on rheological properties of the modified asphalt binder were studied using a parallel plate rheometer. Results show that the elastic behavior of asphalt binder at elevated temperatures was improved with increase in epoxy content.

Morphogenesis of Photo-Polymerized Dimethacrylate Networks, Kinetics of Curing and Viscoelastic Parameters

The paper refers to the process of dimethacrylate networks morphogenesis. These stiff and highly cross-linked networks have been extensively used as a polymeric matrix of dental composites for decades. In the study, common co-monomer mixtures used in dental resin formulations were employed. This includes rigid aromatic base monomers, bisphenol A glycerolate dimethacrylate (Bis-GMA) and its ethoxylated alternative (Bis-EMA). Flexible aliphatic monomer, triethylene glycol dimethacrylate (TEGDMA), was used as the viscosity reducer. Kinetics of the polymerization process was studied regarding the structural differences and varying molar ratio of the co-monomers. Kinetic data provided the base for understanding the supra-molecular structure evolution. Consequently, an attempt to quantify the relationship between the resulting network morphology and complex viscoelastic moduli was made. Curing kinetics was studied using differential photo-calorimetry (DPC). Complex modulus was measured using dynamic-mechanical analysis (DMA). Thermal degradation kinetic data (TGA) were used in order to confirm the estimated morphology of cured networks. Reactivity of the monomer is derived from its molecular structure. The potential for non-covalent physical interactions along with monomer backbone rigidity significantly decrease polymerization rate and resulting double bond conversion. The diffusion-controlled kinetics dominates over the chemically controlled kinetics throughout most of the polymerization process. Dilution by the low viscous and flexible monomer shifts the diffusion-controlled kinetics to the later stages of the polymerization. However, the flexibility of the monomer backbone promotes the origination of structural heterogeneities, characterized by micro-gel domains formation. This is associated particularly with the anomalous pendant double bond reactivity and ineffective cross-linking.

Effect of nature of fibrous substrate of composite membranes on their structure, conductive properties, and selectivity

The effect of fibers selected for investigation on the kinetics of synthesis and curing of the polymer matrix has been revealed; structural features of Polikon K materials formed on polyacrylonitrile, polyester, and novolac phenol–formaldehyde fibers have been studied; and the performance characteristics of the materials have been determined.

Modeling and Simulation of Cure Kinetics and Flow in Cavity-Molded Composites

Modeling and Simulation of Cure Kinetics and Flow in Cavity-Molded Composites

Rubber Nanodomains Reinforced Epoxy Resin

ABSTRACTIt has been reported that the addition of liquid rubbers, like poly(dimethylsiloxane) (PDMS), to epoxy resins alter the final morphology, increase the toughness and influence the curing kinetics. Due to immiscibility, there is phase separation of the elastomeric phase during curing giving rise to microdomains embedded in the epoxic matrix. The resultant heterogeneous morphology obtained after the reaction controls to an important extent the properties of the epoxy composite. Here we report a method to obtain well-dispersed rubber nanodomains of silyl-diglycidyl ether terminated polydimethyl siloxane (PDMS-DGE) in diglycidyl ether of bisphenol-A (DGEBA) epoxy by using a prepolymerization step. Light scattering and optical microscopy showed that initial mixing of pre-polymerized rubber produced phase separation with micron-scale droplet formation. However, as the curing reaction proceeded, the rubber domains decreased below optical resolution, light scattering intensity reached a maximum and then decreased. Finally, rubber nanodomains of about 100 nm size were formed at the end of curing reaction, as revealed by transmission electron microscopy (TEM). The pre-polymerization step induced a two-fold increase in gel time, tgel, due to lesser active groups available for reaction. Strikingly, tensile modulus and toughness increased, suggesting rubber-epoxy interaction. The final nanocomposite also exhibited higher thermal stability and char formation.

Influence of diaminobenzoyl‐functionalized multiwalled carbon nanotubes on the nonisothermal curing kinetics, dynamic mechanical properties, and thermal conductivity of epoxy–anhydride composites

ABSTRACTTo obtain advanced materials with a high thermal dissipation, the addition of multiwalled carbon nanotubes containing diverse functionality groups, that is, as‐received multiwalled carbon nanotubes (AS‐MWCNTs) and diaminobenzoyl multiwalled carbon nanotubes (DA‐MWCNTs), to epoxy–anhydride composites was accomplished. According to nonisothermal differential scanning calorimetry analysis, the reactive functional groups present on the surfaces of the AS‐MWCNTs and DA‐MWCNTs accelerated the nucleophilic addition reaction of epoxy composites. Because of the difference in the reactivities of these functional groups toward epoxy groups, the distinction of fractional conversion and the reaction rate of the curing process were remarkably evident at the early stage. A suitable kinetic model was effectively elucidated with the Málek approach. The curing kinetics could best be described by a two‐parameter autocatalytic model as a truncated Šesták–Berggren model. The DA‐MWCNTs achieved effective load transfer and active heat conductive pathways; this resulted in good dynamic mechanical and thermal properties. As a result, the diglycidyl ether of bisphenol A/DA‐MWCNTs constituted an effective system with enhanced heat dissipation of materials for electronic applications. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43567.

Isothermal curing kinetics of a flame retardant epoxy resin containing DOPO investigated by DSC and rheology

The isothermal curing kinetics of a flame-retardant epoxy resin containing DOPO (1.5wt% of phosphorus) with hardener DICY was investigated using differential scanning calorimetry and oscillatory shear rheometry. The glass transition temperature Tg increased with conversion α measured by DSC. When the curing temperature is lower than Tg∞ of the completely cured epoxy, the curing kinetics at high α became diffusion-control due to that cured epoxy restricted the diffusion of the reactants to decelerate curing. The modified Kamal model including the diffusion factor described curing conversion rate curves quite well. The curing degree θ was established with relative storage modulus, which was measured by the cyclic frequency sweep, and revealed that the transition from liquid to solid during curing appeared later with a slower rate than that for the conversion change, because the network began to form after formation of branched polymer chains. At the modulus transition time, the epoxy samples cured at different temperatures had the same conversion of 0.409, indicating that above this conversion of reactants the effective network was formed to bear the stress.

Using rheometry to study the curing kinetics of glycidyl azide polymer spherical propellant by non-isothermal method

The curing kinetics of GAP spherical propellant was studied by rheological non-isothermal measurement. The kinetic parameters were determined by rheometry based on conversion degree α described by storage modulus. The non-isothermal method was first used in rheometry as conversion α determined by a novel method, considering that storage modulus variation was a complex process which increased with polymerization and decreased as the temperature increased. The variation of apparent activation energy was obtained by the isoconversional method of Kissinger–Akahira–Sunose. It was found that Ea is little lower in the range of 0.05 < α < 0.50 than it is in the range of 0.50 < α < 0.70. During the complete curing process, apparent activation energy can be considered a constant. A new kinetic equation had been established and showed great agreement with experimental data. Altogether, the rheological non-isothermal method can be used to investigate the mechanism and kinetics of thermosetting reactions, and the obtained knowledge of the curing kinetic will form a contribution to the study of GAP spherical propellant.

Preparation, curing, and properties of boron-containing bisphenol-S formaldehyde resin/o-cresol formaldehyde epoxy resin/nano-SiO2 composites

Boron-containing bisphenol-S formaldehyde resin (BBPSFR) with different amounts of nano-SiO2 by in situ formation was used to cure o-cresol formaldehyde epoxy resin (o-CFER). The curing kinetics, dynamic mechanical properties, and thermal stability of BBPSFR/o-CFER/nano-SiO2 composites (BCS) were investigated by differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermogravimetry (TG), and thermogravimetrymass spectrometry (TGMS). Morphology of nano-SiO2-containing BBPSFR and glass fiber laminates of the BCS were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The mechanical properties and electrical properties were also determined. The results showed that nano-SiO2 accelerated the curing process and decreased the curing temperature; the non-isothermal curing kinetics of the BCS can be described by the two-parameter (m, n) Sestak-Berggren kinetic model, and the average value of m was 0.32 and n was 1.00. The thermal stability was enhanced by the addition of nano-SiO2, especially at higher temperatures, and the residual weight increased with increasing nano-SiO2 content. Incorporation of 6 wt% of nano-SiO2 increased the impact strength from 105 to 149 kJ/m2 and storage modulus at ambient from 6.85 to 12.7 GPa, and the TEM photograph of which showed that nano-SiO2 particles (about 50 nm) were dispersed in the matrix more uniformly. The volume resistance, R v, and dielectric constant, e, slightly increased when the nano-SiO2 content was 3 wt%.

Non-isothermal curing kinetics, chemorheological behaviour, and IR spectral study of two trifunctional phenylethynyl-terminated imide oligomers compared with the corresponding bifunctional structure

Two trifunctional phenylethynyl-terminated imide oligomers, m-TPEPA and p-TPEPA, were systematically compared with the corresponding bifunctional phenylethynyl-terminated imide oligomer, BPEPA. The non-isothermal curing kinetics, Master Plots method, and rheological behaviour of the three high performance oligomers were systematically studied by dynamic DSC, small amplitude oscillatory shear rheometry, and infrared spectroscopy. The results show that the activation energy ( Ea) depended on the extent of conversion evaluated with three different methods, and the lower Ea values of the trifunctional oligomers, which may lead to different curing reactions, compared with the sustained growth of Ea values of BPEPA. The chemorheological properties of the oligomers were measured and fit numerically with the dual Arrhenius model and gel model. Moreover, the degree of cure (agel ) at the gel time (tgel ) was calculated by the value of glass transform temperature at different curing temperature combined with rheological and isothermal DSC results. According to structural changes during the curing reaction characterised using FTIR spectra, it was inferred that the structures of the cured trifunctional imide resins have cis-configurations, which may change the part of the configuration from cis to trimerisation with the lower activation energies found in the later part of the conversion, while the difunctional imide structure is trans.

Printing continuously graded interpenetrating polymer networks of acrylate/epoxy by manipulating cationic network formation during stereolithography

Ultra-violet (UV) laser assisted stereolithography is used to print graded interpenetrating polymer networks (IPNs) by controlling network formation. Unlike the traditional process where structural change in IPNs is achieved by varying the feeding ratio of monomers or polymer precursors, in this demonstration property is changed by controlled termination of network formation. A photo-initiated process is used to construct IPNs by a combination of radical and cationic network formation in an acrylate/epoxy system. The extent of the cationic network formation is used to control the final properties of the system. Rapid-Scan Fourier Transformation Infrared Spectroscopy (RS-FTIR) is used to track the curing kinetics of the two networks and identify key parameters to control the final properties. Atomic force microscopy (AFM) and differential scanning calorimetry (DSC) confirm the formation of homogenous IPNs, whereas nano-indentation indicates that properties vary with the extent of cationic network formation. The curing characteristics are used to design and demonstrate printing of graded IPNs that show two orders of magnitude variation in mechanical properties in the millimeter scale.

CURING KINETICS OF CONCRETE WITH UKD-1-NANOCARBON ADDITIVE IN VERSION OF NON WARM-UP TECHNOLOGY

The paper presents results of investigations on influence of domestic complex chemical additive containing structured carbon nanomaterial and characterized by combined (curing acceleration and plasticizing) effect on concrete curing kinetics in non warm-up version of monolithic concreting technology. Standardized and original methods have revealed experimentally regularities in increase of strength for heavy cement concrete under the influence of specific accelerating UKD-1 additive component and the additive itself according to its dosage (within the range of 0.5–2.0 % of cement mass). Growth rate dependences for the concrete with UKD-1 additive on basic technological factors: cement quality, water-cement ratio value, concrete composition and concrete mixture consistency, curing environment temperature and heat-insulating characteristics of shuttering blocks have been evaluated in the paper. Such approach has made it possible to substantiate rational modes and parameters of conditions for concrete curing with the UKD-1 additive in order to ensure strength (under compression) in the 50–100 % range of the required value at the project age of 28 days without energy consumption for acceleration of its curing.

Reinforcement of norbornene-based nanocomposites with norbornene functionalized multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWNTs) were functionalized with benzoic acid groups via an “on water” technique and further derivatized with norbornene groups. The norbornene groups grafted onto the MWNT surface were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and field-emission scanning electron microscopy (FE-SEM) to confirm covalent functionalization. Observations of suspensions and transmission electron microscopy (TEM) and SEM images of nanocomposites suggested improved dispersibility of norbornene-functionalized MWNTs (nMWNTs) in dicyclopentadiene/5-norbornene-2-carboxylic acid (DCPD/NCA) monomers relative to pristine MWNTs (pMWNTs). Cure kinetics and mechanical properties of MWNT/poly(DCPD/NCA) nanocomposites with various functionalized nanotube loadings (0.1–0.6wt%) were investigated by means of differential scanning calorimetry (DSC), a universal testing machine (UTM), and dynamic mechanical analysis (DMA). It was found from DSC that the functionalized nanotubes have an acceleration effect on the cure process, which is likely related to enhanced heat transfer from the carbon nanotubes. The swelling ratio increased with increasing nanotube loading, revealing that the formation of crosslinks was hindered by the addition of nanotubes. The incorporation of the functionalized nanotubes resulted in improved Young’s modulus and yield strength compared with neat poly(DCPD/NCA), far beyond the enhancement with pMWNTs. Although a decrease in the elongation at break was observed as the nanotube loading increases, the tensile toughness remained almost unchanged up to 0.4wt% nMWNT loading. Furthermore, the addition of the nMWNTs also shifted the glass transition toward higher temperatures. These enhancements are presumably attributed to better nanotube dispersion and stronger nanotube-matrix interfacial affinity.

Isothermal curing kinetics and mechanism of DGEBA epoxy resin with phthalide-containing aromatic diamine

The curing reaction of diglycidyl ether of bisphenol A (DGEBA) with a novel aromatic diamine containing phthalide structure (BAPP) was studied by differential scanning calorimetery (DSC). The optimal formulation of DGEBA/BAPP system was obtained in terms of the curing behavior and glass transition temperature determined by dynamic DSC. The nth-order reaction model and autocatalytic model were used to analyze the curing kinetics and mechanism based on the isothermal DSC technique. The results indicated that the Kamal autocatalytic model had a great agreement with the experimental data from the start of curing reaction to the initiation of diffusion control stage. Diffusion-controlled effect was also taken into account and then the extended Kamal model could describe precisely the curing reaction of BAPP/DGEBA in the entire conversion range.

Kinetics of multifunctional thiol-epoxy click reactions studied by differential scanning calorimetry: Effects of catalysis and functionality

Isothermal reaction kinetics of thiol-epoxy (multifunctional thiol-difunctional epoxy) reactions with and without external catalyst 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) were studied using differential scanning calorimetry (DSC). Autocatalytic behavior is observed both without DBU and with DBU, which is attributed to autocatalysis effects of hydroxyl products of the reaction. The isothermal curing kinetics are well described by the Kamal autocatalytic model, which is expressed as dα/dt=(k1 + k2αm)(1 − α)n (α: fractional conversion; dα/dt: apparent rate of reaction). Parameters of the reaction, including reaction orders m and n, apparent rate parameters k1 and k2, and activation energies are determined by fitting the experimental data to the Kamal model. For the thiol-epoxy reaction without DBU, the overall reaction order (m + n) is approximately 3 (with m ≈ 2 and n ≈ 1) but decreases to 2 (with m ≈ 1 and n ≈ 1) upon addition of DBU. Based on the most commonly described mechanism for nucleophilic thiol-epoxy reactions in the literature, new reaction schemes are proposed by incorporating the hydroxyl autocatalysis effect as well as the effect of external catalyst DBU on apparent reaction order. The proposed reaction schemes allow for excellent fits to reaction kinetics for thiol-epoxy reactions with and without DBU. In contrast to the strong effect of external DBU catalyst on reaction kinetics, changing the thiol monomer functionality leads to relatively small changes in apparent rate parameters and no change in the order of reaction or reaction mechanism.

Cure kinetics of epoxy resin using 1,2,4,5-benzenetetracarboxylic acid/2-ethyl-4-methylimidazole salt as a latent hardener

1,2,4,5-Benzenetetracarboxylic acid/2-ethyl-4-methylimidazole (PMA/2E4MI) salt was prepared as a latent hardener for epoxy resin, and Fourier transform infrared was used to characterise its structure. The thermal latent curing property of PMA/2E4MI salt in diglycidyl ether of bisphenol A epoxy resin was investigated by storage stability, gelation time and differential scanning calorimetry measurements. The non-isothermal cure processes of diglycidyl ether of bisphenol A epoxy resin using PMA/2E4MI as a latent hardener in comparison with a common hardener, 2E4MI alone, were analysed adopting the Avrami and Arrhenius methods. The methods revealed a transition temperature (Ttr) at which the cure reaction mechanism showed a significant change. The obtained apparent activation energy (Ea) values of diglycidyl ether of bisphenol A cured with PMA/2E4MI were obviously larger than those of diglycidyl ether of bisphenol A cured with 2E4MI, especially in the early reaction stage. The results suggested that PMA/2E4MI salt possesses thermal latent curing properties for epoxy resins.

Studying the effect of hyperbranched polymer modification on the kinetics of curing reactions and physical/mechanical properties of UV-curable coatings

Abstract A polyester-amide based hyperbranched polymer with hydroxyl end-groups is modified by the aid of acrylic functionalities. Two levels of modification, i.e. 40% and 65%, are considered. The unmodified hyperbranched polymer and the modified ones are added to UV-curable monomeric and oligomeric systems. Rheology measurements are conducted to investigate the effects of hyperbranched polymer on the rheological behavior. In order to assess the curing kinetics and physical/mechanical characteristics of the samples, photo-DSC and DMA experiments are exploited, respectively. The effect of viscosity and number of UV-curable functionalities of the hyperbranched polymer on the curing level and kinetics of the curing reactions are discussed. Dynamic mechanical analysis shows that the addition of hyperbranched polymer in the formulation increases a 25% in storage modulus of the glassy region and cross-linking density of the UV-curable coating. Furthermore, it is also seen that the existence of the hyperbranched polymer increases the viscosity of Trimethylolpropane triacrylate (TMPTA)-based system while it is decreasing the viscosity of oligomeric-based of the epoxy di-acrylate system. It is obtained that the hyperbranched polymer makes the curing level in the monomeric and the oligomeric system, improve up to 15% and 10% correspondingly. It is also observed that T g s of the coatings containing the hyperbranched polymer increases up to 5.9 °C.