Dynamic Differential Scanning Calorimetry Measurements Research Articles

Adhesives Based on Poly(glycidyl methacrylate‐co‐butyl acrylate) with Controlled Structure: Curing Behavior and Adhesion Properties on Metal Substrates

AbstractThe adhesion properties of poly(glycidyl methacrylate (GMA)‐co‐butyl acrylate (BA)) statistical copolymers, synthesized by atom transfer radical polymerization (ATRP), are investigated employing three different curing agents or hardeners, such as diethanolamine (DEA), dicyandiamide (DICY), and 2‐cyanoacetamide (2‐CA) on copper, iron, brass, aluminum, and titanium metal surfaces. This work describes the treatment of the different surfaces, establishes the optimal curing conditions from differential scanning calorimetry (DSC) analysis of these novel adhesive systems, and evaluates the results of the single‐lap shear test for metal joints. Thus, by dynamic DSC measurements of the mixtures, a low curing temperature of 90 °C is defined when DEA is used as a curative; while systems based on DICY and 2‐CA require temperatures of 150 °C and 160 °C, respectively. In addition, the curing process of this controlled acrylic copolymer with DICY exhibits a singular behavior, possibly due to the curing reaction mechanism, where multiple epoxy‐amine ring‐opening polyaddition reactions take place between DICY's active hydrogens and epoxy groups of poly(GMA‐co‐BA). This latter curing system shows the highest adhesion features with lap‐shear strength at room temperature of 15.5 MPa, using copper as metallic substrate; however, the best results are obtained using 2‐CA as curing agent with aluminum and iron.

Aspects of halloysite nanotubes integration in epoxy nanocomposites: Curing kinetics and deformation mechanism

AbstractThe curing kinetics of diglycidyl ether of bisphenol A epoxy resin using amine‐based hardener in the presence of halloysite nanotubes (HNTs) are investigated by performing isothermal and dynamic differential scanning calorimetry measurements, followed by a detailed analysis of the experimental data via the kinetic free method and three common model‐based analysis. Kamal and Sourour model gave the best fit for the experimental data and thus, was further modified by Williams–Landel–Ferry diffusion control equations to calculate the diffusion constants near the glass transition temperature (Tg). The optimized model is used to find the curing kinetics parameters of epoxy/HNTs nanocomposites, and those finding are further validated and explained by performing rheometric measurements. Noncatalytic and autocatalytic curing reactions and diffusion constants are all functions of the amount of HNTs. HNTs shift the curing reaction slightly toward higher temperatures and promote etherification reactions, leading to an increase in the curing enthalpy, as extra bonds will be formed between HNTs and epoxy resin during the polymerization. The combination of acceleration and inhibition mechanisms dictates the complex kinetics of the reactions at high concentrations of HNTs, while the produced local topology of the polymeric network depends highly on HNTs weight ratio. Furthermore, to investigate the mechanical performance of the prepared nanocomposites under flexural loads, three points bending tests are performed in both modes, dynamic and static, followed by a fractography analysis of the fractured surfaces. The mechanical properties have improved by the addition of pristine HNTs without compromising the Tg of the polymer. This study provides a set of observational relationships between various chemical interactions during polymerization and the final topology and performance of the polymer, which in turn aims to bridge the gap in the literature between the curing kinetics of epoxy/HNTs nanocomposites and the performance of these nanocomposites under various thermal and mechanical stresses.

Thermal history method for identification of autocatalytic decomposition reactions of energetic materials

The thermal stability during the autocatalytic decomposition of a material depends on the intrinsic properties of the substance such as its kinetic triplet, in addition to its thermal history. In this reported study, simulations were conducted on select substances using the first-order reaction model (F1) and the general autocatalytic reaction model (Cn). Through changing the initial conversion rate, the thermal history's influence on the dynamic differential scanning calorimetry (DSC) curves was investigated based on the n-order reaction and autocatalytic reaction. A thermal history method for determining the autocatalytic decompositions of four energetic materials were established using the results of Roduit et al. and a simulated combustion. The isothermal DSC measurements and Swiss method were also employed to validate the reliability of the proposed thermal history method. Results of the simulations and experiments indicated that the influence on the dynamic DSC curves was significant based on data in the autocatalytic reactions. Thermal stability was decreased with a lower initial temperature and peak temperature. The influence was more significant on the energetic materials with greater autocatalytic potential. The results of this study indicated that the proposed thermal history method can be successfully used to quickly and effectively identify the autocatalysis characteristics of energetic materials.

Effects of size and shape of dispersed poly(butylene terephthalate) on isothermal crystallization kinetics and morphology of poly(lactic acid) blends

The isothermal crystallization kinetics and morphology of the poly(lactic acid) (PLA) blends containing three different sizes of both spherical and fibrous poly(butylene terephthalate) (PBT) domains have been comparatively investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The dynamic DSC measurement reveals that PBT domains significantly increase the degree of crystallinity of the PLA. Furthermore, the Avrami model is employed to evaluate the crystallization kinetics under isothermal conditions and it is found that PBT acts as nucleating agent, leading to a high overall crystallization rate constant k and shortened crystallization half time t1/2. Furthermore, the crystallization rate of PLA is promoted with the incorporation of PBT with a large specific surface area. The average Avrami index n of all samples lies within the range of 3.3 − 4.0, suggesting that morphologies of PBT do not affect the nucleation mechanism; however, the depression of equilibrium melting temperature in the blends ascribes the reductions of perfectness and size of the PLA crystallites. Besides, the nucleation of PLA crystallites around PBT fibers is probably faster than those around PBT spheres because the PBT chains oriented at the fiber surface as a result of flow‐induced crystallization during melt stretching may serve as the primary nuclei for PLA chains to drastically crystallize at the fiber surface. POLYM. ENG. SCI., 56:258–268, 2016. © 2015 Society of Plastics Engineers

Thermal and bio-disintegration properties of poly(lactic acid)/natural rubber/organoclay nanocomposites

The crystallisation behaviour of a poly(lactic acid) (PLA)/natural rubber (NR) blend compatibilised through the addition of organoclays located at the interface between both polymers was evaluated by differential scanning calorimetry (DSC), rheological and wide angle X-ray diffraction (WAXD) measurements. NR acted as a nucleating agent for the PLA matrix, leading to a decrease of PLA cold crystallisation temperature during dynamic DSC measurements and an increase of the crystallisation rate followed by rheological measurements during isothermal crystallisation. Nevertheless, this effect was suppressed when adding 3wt.% of organoclays due to the particular morphology of the nanocomposites. Moreover, the organoclays increased the thermal degradation temperature of the PLA/NR blend and delayed its disintegration in compost, without affecting the disintegration percentage after one month of experiment.

Kinetics and Mechanism of Formaldehyde-Free Phenol-Glucose Novolac Resin Cured with an Epoxy

Abstract Formaldehyde used in manufacture and curing of phenol formaldehyde (PF) resins is a big challenge for the PF industry with respect to sustainability and human health. Glucose has grand potential to replace formaldehyde in synthesis of bio-based phenolic novolac resins curable with hexamethylenetetramine (HMTA, derived from formaldehyde), an obstacle to achieve formaldehyde-free curing. This study reports the curing of phenol-glucose novolac resin with a bis-phenol-A type epoxy without using HMTA. The curing process was investigated by using dynamic differential scanning calorimetry measurements at different heating rates. Apparent activation energy of the curing reaction is 109.6 kJ/mol. Sestak–Berggren equation model was determined to be the reaction model according to Málek methods. The model-predicted reaction rates showed a good agreement with the experimental results. The results showed that the curing process was complicated; the kinetics expression was shown to be first-order autocatalytic reaction.

Synthesis and characterization of a cured epoxy resin with a benzoxazine monomer containing allyl groups

AbstractVinyl‐terminated benzoxazine (VB‐a), which can be polymerized through ring‐opening polymerization, was synthesized through the Mannich condensation of bisphenol A, formaldehyde, and allylamine. This VB‐a monomer was then blended with epoxy resin and then concurrently thermally cured to form an epoxy/VB‐a copolymer network. To understand the curing kinetics of this epoxy/VB‐a copolymer, dynamic differential scanning calorimetry measurements were performed by the Kissinger and Flynn–Wall–Ozawa methods. Fourier transform infrared (FTIR) analyses revealed the presence of thermal curing reactions and hydrogen‐bonding interactions of the epoxy/VB‐a copolymers. Meanwhile, a significant enhancement of the ring‐opening and allyl polymerizations of the epoxy was observed. For these interpenetrating polymer networks, dynamic mechanical analysis and thermogravimetric analysis results indicate that the thermal properties increased with increasing VB‐a content in the epoxy/VB‐a copolymers. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Cure characterization of the ring‐opening metathesis polymerization of linseed oil‐based thermosetting resins

AbstractBACKGROUND: Bio‐based polymers from vegetable oils are excellent alternatives to petroleum‐based resins for both environmental and economic reasons. A detailed understanding of the cure behavior of bio‐based polymers is essential to optimize cure schedules and the final properties of the polymers. In this work, the cure of newly developed linseed oil‐based thermosetting resins, synthesized using Grubbs' first‐generation catalyst and a bis‐norbornadiene cross‐linking agent by ring‐opening metathesis polymerization, is characterized using differential scanning calorimetry (DSC) and parallel plate oscillatory rheometry.RESULTS: Experimental results reveal that the rate of cure increases and the gel time decreases with increasing cross‐linker loading; however, the activation energy of the cure does not vary systematically with cross‐linker loading. Phenomenological reaction models are used to describe the dynamic DSC measurements and to determine the kinetic parameters which facilitate cure predictions under isothermal conditions.CONCLUSION: This work demonstrates that the cure kinetics of a linseed oil‐based thermosetting resin can be controlled by varying the cross‐linker loading. Furthermore, the kinetic parameters and cure rates at any cross‐linker loading for this system can be described by a simple autocatalytic reaction model which facilitates development of cure schedules. Copyright © 2009 Society of Chemical Industry

Polymers based on N,N‐diglycidylaniline. I. Investigations of the curing kinetics by dynamic differential scanning calorimetry measurements

AbstractN,N‐Diglycidylaniline was reacted with aniline (yielding polymer EP‐1) and the newly synthesized chromophore 4‐(phenylazo)aniline (yielding polymer EP‐2). The curing kinetics of these two epoxy resin systems was studied in dynamic experiments by means of differential scanning calorimetry. Kinetic parameters such as the activation energy and frequency factor were estimated with the Ozawa method [E(O) and A(O), respectively], the Kissinger method [E(K) and A(K), respectively], and the modified Avrami method [E(A) and A(A), respectively]. The activation energy and frequency factor of EP‐1 were much lower than those of EP‐2 estimated with the Ozawa, Kissinger, and Avrami methods. The activation energy and frequency factor for EP‐1 determined with the Ozawa method [E(O) = 55.8 kJ/mol, A(O) = 10 × 103 1/s] and the Avrami method [E(A) = 56.4 kJ/mol, A(A) = 9.2 × 103 1/s] were higher than those determined with the Kissinger method [E(K) = 51.0 kJ/mol, A(K) = 2 × 103 1/s]. In the case of EP‐2, the kinetic parameters calculated with the Ozawa model [E(O) = 140.4 kJ/mol, A(O) = 12.3 × 1013 1/s] and the Kissinger model [E(K) = 139.9 kJ/mol, A(K) = 10.9 × 1013 1/s] were higher than those calculated with the Avrami model [E(A) = 130.4 kJ/mol, A(A) = 7.9 × 1012 1/s]. The obtained polymers were characterized with Fourier transform infrared, 1H‐NMR, differential scanning calorimetry, and ultraviolet–visible spectroscopy. The polymers exhibited low glass‐transition temperatures in the range of 57–79°C and good solubility in common organic solvents. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Kinetic Analysis via Microwave Dielectric Measurements

In the chemical industry, a key step for the assessment of thermal risk and runaway is the identification of autocatalytic reactions. In this field, many estimation methods based on dynamic differential scanning calorimetry (DSC) measurements have been developed. As a rule, a screening of the most hazardous chemicals is conducted. In the first step, the measured heat release curve from dynamic DSC gives the apparent activation energy level. In the second step, the characteristic behaviour of a possible autocatalytic decomposition is pointed out with an isothermal DSC measurement. Unfortunately, when an endothermic process precedes the exothermic decomposition, identification is rather difficult or even impossible. A new method based on microwave dielectric measurements of chemicals has been developed. This method consists of a preliminary recording of the initial reactants and final product dielectric properties. Then the normalized extent of a chemical process is determined from the behaviour of its complex permittivity. As an illustration, the thermal decomposition of a well known curing agent, 2,2’-azobis-isobutyronitrile is studied.

Autocatalytic decomposition reactions, hazards and detection

In the chemical industry, estimation methods based on dynamic Differential Scanning Calorimetry (DSC) measurements have been developed for a preliminary screening of the risk associated to a chemical operation. An important point for the assessment of thermal risk is the identification of autocatalytic reactions. These types of reactions require our special attention and should be clearly distinguished from nth order reactions. Until today, the most reliable tool for the identification of autocatalytic decompositions was an isothermal DSC measurement. A new screening method based on dynamic DSC measurements for the identification of autocatalytic decompositions has been developed. The method consists of fitting a first order kinetic model to the measured heat release rate curve (from dynamic DSC measurement) and determining the apparent activation energy. If the apparent activation energy is higher than 220 kJ/mol, the decomposition is autocatalytic. By taking into account the cases with the border value of the apparent activation energy (180–220 kJ/mol), as well as the exception to this method, it should be possible to apply our new screening tool to about 80% of the cases.

Effect of cellulose crystallinity on the progress of thermal oxidative degradation of paper

Dynamic differential scanning calorimetry (DSC) using paper samples of different compositions has evidenced varying degrees of endothermic activity prior to the exothermic, oxidative decomposition of cellulose under static air. Whereas cotton cellulose papers displayed a significant endothermic activity, wood-pulp papers showed a much less pronounced effect or no activity at all. DSC measurements using microcrystalline cellulose submitted to different extents of milling indicated that the observed endotherm is related to the degree of crystallinity of cellulose. Crystallinity decrease is accompanied by a decrease in the area of the endothermic peak. Thermal disruption of cellulose crystalline domains is therefore believed to be the reason for the appearance of an endotherm in the thermograms of paper. The higher degree of crystallinity of cotton cellulose in comparison to wood-pulp cellulose accounts for the more pronounced endothermic activity observed for cotton papers. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 61–66, 2000

On structural relaxation of Zr 52Ni 26Al 22 metallic glass

The relation between the dynamic differential scanning calorimetry measurements and the thermal history of the Zr 52Ni 26Al 22 metallic glassy ribbon was studied using the distribution of non-linear relaxation times model (DNLR). The double principal relaxation times spectrum, i.e. two structural relaxation modes had to be assumed.

Estimation of the time to maximum rate using dynamic DSC experiments

Abstract The probability of thermal risk may be described by the time to maximum rate under adiabatic conditions (TMR ad ). In this paper a screening method based on dynamic differential scanning calorimetry (DSC) measurements was studied in order to show that, when using the TMR ad criterion, no process would be assessed as safe when it could, in fact, be critical. The method of investigation was based on dynamic simulation. In this, the DSC measurement equipment and five different reaction types (simple nth order, consecutive, branched and autocatalytic reactions) were described in mathematical terms and simulated using MATLAB. The reliability of the estimation was checked by comparing the simulated TMR ad with the estimated TMR ad based on dynamic DSC measurements and isothermal assessment procedures. The TMR ad values calculated from simulation of dynamic DSC runs were always found to lie in the range of 10–70% of those obtained by dynamic simulation of the adiabatic case. Owing to some uncertainties with regard to autocatalytic reactions, a method was developed to identify them. The application of the total method to experimental measurements showed a good correspondence to the results obtained by simulation. The estimation method is therefore a good tool for preliminary screening and may be applied at the early stages in process design to save both time and money and without loss of safety.

Thermal Degradation and Thermooxidation of O-Acetyl-(4-O-methyl-D-glucurono)-D-xylan and Related Derivatives

Four polysaccharide samples related to (4-O-methyl-D-glucurono)-D-xylan were analyzed by thermogravimetry (TG) and differential scanning calorimetry (DSC) in nitrogen and oxygen environments. The calculated difference in the measured TG-residues run in different environments and obtained at temperatures close to 300°C by a dynamic experiment indicate the action of oxygen on the formation of gaseous products. The rats constants and activation energies of the degradation process in both environments indicate the increase in stability due to the protection of hydroxyls by acetylation. In an inert environment xylans have one broad multicomponent exotherm, as determined by dynamic DSC measurement. In an oxygen environment two or three independent exotherms are observed. The pretreatment of xylan with trifluoroacetic acid (solvent used for acetylation) and acetylation lowers the exothermic effect of both the thermolysis and thermooxidation. The recycling of the acetylated xylan to Na + -salt form leads to a greater exothermic effect of the degradation in both environments.

Heat Curing Behavior of Light-cured Composite Resins Investigated by Dynamic Differential Scanning Calorimetry

The heat curing behavior of light-cured restorative composite resins, light-cured crown and bridge veneering resins, pure dimethacrylate monomers, Bis-GMA monomers containing various initiators, and monomer mixtures were investigated by slow heating at a constant rate with differential scanning calorimetry (DSC) without any light irradiation--the so-called dynamic DSC measurement. Some of the light-cured restorative composite resins and the light-cured crown and bridge veneering resins showed a sharp exothermic peak due to heat curing. Pure TEDMA also showed a sharp exothermic peak, whereas the Bis-GMA monomers containing catalysts for light curing showed no exothermic peak. It seems that heat curing behavior of light-cured composite resins depends not on the decomposition of camphorquinone, but on that of the monomer itself.

A nonisothermal differential scanning calorimetry study of the curing kinetics of an unsaturated polyester system

AbstractNonisothermal differential scanning calorimetry (DSC) measurements were used to estimate the kinetic parameters for the curing reaction of a commercial unsaturated polyester resin. The reaction rate expression was derived from a mechanistic kinetic model based on the concept of free radical polymerization, accounting also for the diffusion controlled reaction. The total heat of reaction was evaluated in experiments run at very low scan speeds and using high amount of initiator. The kinetic parameters were found to show no dependency on the heating rate nor on the initiator concentration. Dynamic DSC measurements provide reliable kinetic data over a broad range of temperatures that is mostly significant for process simulations.

The curing reaction of an epoxide resin with 2‐ethyl‐4‐methylimidazole, a calorimetric study of the kinetics of formation of epoxide‐imidazole adducts

AbstractThe kinetics of formation of the 1:1 and 2:1 adducts of (in terms of equivalents of epoxide) 2,3‐epoxypropyloxybenzene (2) and 2,2‐bis[p‐(2,3‐epoxypropyloxy)phenyl]‐propane (1) with 2‐ethyl‐4‐methylimidazole (3) have been investigated by differential scanning calorimetry (DSC). A comparison of isothermal and dynamic data for the 1:1 system 2/3 showed that the dynamic rate cannot simply be equated to the isothermal rate, but the true dynamic rate equation, dα/dt = (∂α/∂tT)+(∂α/∂Tt) (dT/dt), gave good agreement with the isothermal data. The dynamic DSC measurements indicate that all of the reactions approximate to first order kinetics and have similar values for activation energy (E = 81–93 kJ/mol) and pre‐exponential factor (log A ≈ 9–10). The results support the suggestion that adduct formation is likely to be the first stage of the resin curing reaction catalyzed by 3.