Rheological behavior of an epoxy/amine system near the gel point

Network Formation and Physical Properties of Epoxy Resins for Future Practical Applications

Epoxy resins are used in various fields in a wide range of applications such as coatings, adhesives, modeling compounds, impregnation materials, high-performance composites, insulating materials, and encapsulating and packaging materials for electronic devices. To achieve the desired properties, it is necessary to obtain a better understanding of how the network formation and physical state change involved in the curing reaction affect the resultant network architecture and physical properties. However, this is not necessarily easy because of their infusibility at higher temperatures and insolubility in organic solvents. In this paper, we summarize the knowledge related to these issues which has been gathered using various experimental techniques in conjunction with molecular dynamics simulations. This should provide useful ideas for researchers who aim to design and construct various thermosetting polymer systems including currently popular materials such as vitrimers over epoxy resins.

Determining the gel point of an epoxy resin by various theological methods

The gel point of TGDDM-based epoxy resin MY721 (Ciba Geigy) mixed with DDS (Sigma) was determined using multiwave, dynamic-time and steady-time tests. The gel times from each test agreed within experimental variations and the effects of temperature, sample size and shear rate are discussed. New methods of determining the gel point of thermosetting resins are discussed using multiwave tests and normal stress measurements during steady shear tests.

The effect of impurities on gel times for TGDDM epoxy resins cured with DDS

The impurities present in the commercial (Ciba-Geigy) N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane (TGDDM) resins MY720 and MY72I and a purified TGDDM resin were studied using high-performance liquid chromatography and infrared spectroscopy. The cure kinetics for the three resins cured with 27% 4,4'-diaminodiphenylsulfone (DDS) were described using a previously developed kinetic model and near-infrared spectroscopy with the catalysis of epoxide reactions by impurity hydroxyl groups being seen to have a large effect on the initial rate of epoxide reaction. The gel times were determined using the crossover point between storage and loss moduli (i.e. tan (= 1) as measured using dynamic rheometry. An impurity effect on the-degree of epoxide reaction necessary for gelation to occur was identified and an empirical relationship was developed to quantify this effect. An approach was proposed for the prediction of gel times for TGDDM resins of any purity cured with DDS combining the use of the kinetic model and empirical gel time function.

Monitoring the Cure State of Thermosetting Resins by Ultrasound

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.

Rheological and chemical analysis of reverse gelation in a covalently crosslinked Diels-Alder polymer network

A network polymer, incorporating dynamic and reversible crosslinks, was synthesized using the Diels-Alder reaction. Fourier transform infrared (FTIR) spectroscopy was used to characterize the reaction rate and thermodynamic equilibrium over a broad temperature range. Equilibrium conversion of the furan and maleimide varied from 74% at 85°C to 24% at 155°C, demonstrating significant depolymerization via the retro-Diels-Alder reaction. The gel point temperature, as determined by rheometry using the Winter-Chambon criterion, was 92°C, corresponding to a gel-point conversion of 71%, consistent with the Flory-Stockmayer equation. The scaling exponents for the complex moduli, viscosity, and plateau modulus, in the vicinity of the gel-point, were determined and compared with experimental and theoretical literature values. Further, the material exhibited a low frequency relaxation owing to dynamic rearrangement of crosslinks by the Diels-Alder and retro-Diels-Alder reactions.

Rheology of asphalts modified with glycidylmethacrylate functionalized polymers

Asphalt is known to be a colloidal suspension in which asphaltenes are covered by a stabilizing phase of polar resins and form complex micelles that are dispersed in the oily maltenic phase. In order to enhance its mechanical properties (e.g., in road paving), asphalts are often loaded with polymeric materials, thereby obtaining blends that can have different physical or chemical structures, depending on the composition of the added polymer. Asphalts modified by the addition of reactive ethylene terpolymers were prepared and their dielectric and rheological properties were measured both before and after a cure at high temperature. Even if it is not possible to determine the exact nature of the chemical interactions between asphalt and polymer, master curves obtained from dynamic data clearly show that during the cure the material tends to the behavior of a cross-linked network.