Ultrasound monitoring of the cure kinetics of an epoxy resin: Identification, frequency and temperature dependence (original) (raw)

Monitoring the Cure State of Thermosetting Resins by Ultrasound

Materials, 2013

The propagation of low intensity ultrasound in a curing resin, acting as a high frequency oscillatory excitation, has been recently proposed as an ultrasonic dynamic mechanical analysis (UDMA) for cure monitoring. The technique measures sound velocity and attenuation, which are very sensitive to changes in the viscoelastic characteristics of the curing resin, since the velocity is related to the resin storage modulus and density, while the attenuation is related to the energy dissipation and scattering in the curing resin. The paper reviews the results obtained by the authors' research group in the last decade by means of in-house made ultrasonic set-ups for both contact and air-coupled ultrasonic experiments. The basics of the ultrasonic wave propagation in polymers and examples of measurements of the time-evolution of ultrasonic longitudinal modulus and chemical conversion of different thermosetting resins are presented. The effect of temperature on the cure kinetics, the comparison with rheological, low frequency dynamic mechanical and calorimetric results, and the correlation between ultrasonic modulus and crosslinking density will be also discussed. The paper highlights the reliability of ultrasonic wave propagation for monitoring the physical changes taking place during curing and the potential for online monitoring during polymer and polymer matrix composite processing.

Curing and post-curing viscoelastic monitoring of an epoxy resin

An experimental setup has been developed for the monitoring of ultrasonic parameters during polymerization in the context of the monitoring of composite plate production. An analytical approach is proposed based on the modeling of the wave velocity fitted by a Weibull distribution and was investigated to validate this approach by the Debye series modeling (DSM).The monitoring of the cured epoxy is also performed after curing in order to study the thermal transformation compared with DSC measurements. As a result, an approximated frequency-temperature (f, T) model is proposed for attenuation and velocity frequency and temperature dispersions.

Review Monitoring the Cure State of Thermosetting Resins

2013

The propagation of low intensity ultrasound in a curing resin, acting as a high frequency oscillatory excitation, has been recently proposed as an ultrasonic dynamic mechanical analysis (UDMA) for cure monitoring. The technique measures sound velocity and attenuation, which are very sensitive to changes in the viscoelastic characteristics of the curing resin, since the velocity is related to the resin storage modulus and density, while the attenuation is related to the energy dissipation and scattering in the curing resin. The paper reviews the results obtained by the authors' research group in the last decade by means of in-house made ultrasonic setups for both contact and air-coupled ultrasonic experiments. The basics of the ultrasonic wave propagation in polymers and examples of measurements of the time-evolution of ultrasonic longitudinal modulus and chemical conversion of different thermosetting resins are presented. The effect of temperature on the cure kinetics, the comparison with rheological, low frequency dynamic mechanical and calorimetric results, and the correlation between ultrasonic modulus and crosslinking density will be also discussed. The paper highlights the reliability of ultrasonic wave propagation for monitoring the physical changes taking place during curing and the potential for online monitoring during polymer and polymer matrix composite processing.

High temperature polymerization monitoring of an epoxy resin using ultrasound

IOP Conference Series: Materials Science and Engineering

In this study, the real time ultrasonic monitoring is investigated to quantify changes in physical and mechanical properties during the manufacture of composite structures. In this context, an experimental transmission was developed with the aim of characterizing a high temperature polymerization reaction and post-curing properties using an ultrasonic method. First, the monitoring of ultrasonic parameters of a thermosetting resin is carried out in a device reproducing the experimental conditions for manufacturing a composite material with a process known as RTM, that is to say an isothermal polymerization at T = 160°C. During this curing, the resin is changing from its initial viscous liquid state to its final viscous solid state. Between those states, a glassy transition stage is observed, during which the physical properties are strongly changing, i.e. an increase of the ultrasonic velocity up to its steady value and a transient increase of the ultrasonic attenuation. Second, the ultrasonic inspection of the thermosetting resin is performed during a heating and cooling process to study the temperature sensitivity after curing. This type of characterization leads to identifying the ultrasonic properties dependence before, during and after the glassy transition temperature Tg. Eventually, this study is composed of two complementary parts: the first is useful for the curing optimization, while the second one is fruitful for the post-processing characterization in a temperature range including the glassy transition temperature Tg.

Air-Coupled Ultrasonic Cure Monitoring of Unsaturated Polyester Resins

Macromolecular Symposia, 2007

Air-coupled ultrasound in one-side transmission mode has been applied to monitor the curing process of an unsaturated polyester resin, commonly used as a matrix in glass reinforced composites. The evolution of the resin mechanical properties during cure has been measured from the variation of longitudinal velocity. The technique has demonstrated to be a very powerful tool for detecting the structural changes occurring at gelation and vitrification. The no-contact ultrasonic results have been compared with those obtained using conventional contact ultrasonic measurements, characterized by direct coupling between the transducer and the resin, and with the rheological measurements. The good agreement among the results of the different techniques demonstrates the reliability of air-coupled ultrasound in monitoring the changes of viscoelastic properties occurring during the cure of thermosetting polymers. A further advantage of the proposed one-side ultrasonic technique is the absence of physical contact between the transducers and the sample, which is relevant during composite manufacturing, where probe contact could adversely affect the part quality or access from both sides is not practicable. No-contact ultrasonic cure monitoring is suitable for both stationary and moving liquid or solid samples in several process conditions such as moulding, filament winding, etc., opening the way to new applications of ultrasound in the composite industry.

Ultrasonic Methodology to Characterize the State of Cure

2003

As a polymer (like a thermoset resin) cures from liquid to solid, the cross-link density increases and this change is accompanied by a significant production of heat and an increase in material stiffness. The ability to predict the value of material stiffness at discrete times during the process of curing would be helpful in process monitoring. This research examines an epoxy-amine system during cure, and makes an in situ characterization using embedded piezoelectric (PZT) chip sensors. A new concept of using ultrasonic-time cure-temperature measurement is proposed for predicting the modulus at any given time during cure and at any given level of curing temperature. The reliable results obtained with this method may enable the quantitative characterization of the properties of the more complex state of cure in the cement-based materials.

Air-Coupled Ultrasound: A Novel Technique for Monitoring the Curing of Thermosetting Matrices

IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, 2007

A custom-made, air-coupled ultrasonic device was applied to cure monitoring of thick samples (7-10 mm) of unsaturated polyester resin at room temperature. A key point was the optimization of the experimental setup in order to propagate compression waves during the overall curing reaction by suitable placement of the noncontact transducers, placed on the same side of the test material, in the so-called pitch-catch configuration. The progress of polymerization was monitored through the variation of the time of flight of the propagating longitudinal waves. The exothermic character of the polymerization was taken into account by correcting the measured value of time of flight with that one in air, obtained by sampling the air velocity during the experiment.

Accurate Cure Modeling for Isothermal Processing of Fast Curing Epoxy Resins

Polymers

In this work a holistic approach for the characterization and mathematical modeling of the reaction kinetics of a fast epoxy resin is shown. Major composite manufacturing processes like resin transfer molding involve isothermal curing at temperatures far below the ultimate glass transition temperature. Hence, premature vitrification occurs during curing and consequently has to be taken into account by the kinetic model. In order to show the benefit of using a complex kinetic model, the Kamal-Malkin kinetic model is compared to the Grindling kinetic model in terms of prediction quality for isothermal processing. From the selected models, only the Grindling kinetic is capable of taking into account vitrification. Non-isothermal, isothermal and combined differential scanning calorimetry (DSC) measurements are conducted and processed for subsequent use for model parametrization. In order to demonstrate which DSC measurements are vital for proper cure modeling, both models are fitted to varying sets of measurements. Special attention is given to the evaluation of isothermal DSC measurements which are subject to deviations arising from unrecorded cross-linking prior to the beginning of the measurement as well as from physical aging effects. It is found that isothermal measurements are vital for accurate modeling of isothermal cure and cannot be neglected. Accurate cure predictions are achieved using the Grindling kinetic model.

Phase transformations during the cure of unsaturated polyester resins

Materials Science and Engineering: A, 2004

The phase transformations occurring during the cure of an unsaturated polyester resin have been investigated by different techniques, including high frequency dynamic-mechanical analysis by ultrasonic wave propagation. The increase of longitudinal sound velocity can be attributed to the increase of longitudinal modulus L while irreversible viscous losses are responsible for the increase of sound attenuation. A correspondence between the changes in the velocity and attenuation and the phase transformations (gelation and vitrification) can be observed. The ultrasonic properties have been compared with the gel time values obtained from parallel-plate rheological measurements. Finally, a DSC analysis has been carried out to compare the evolution of degree of reaction with that of the ultrasonic modulus.