Brigitte Defoort

Electron-beam initiated polymerization of acrylate compositions 1 : FTIR monitoring of incremental irradiation

Abstract The electron-beam induced polymerization of some representative formulations including acrylate functional oligomers and diluents has been investigated by means of FTIR spectroscopy applied to films that were cured under a nitrogen flow. In order to gain a deeper insight into the reactivity of the polymerizable systems, the conversion–dose relationship was examined with emphasis on the following points : depth cure profile of the films, and the additivity of effects of incremental radiation doses on monomer conversion. It was shown to be possible to reproduce the actual polymerization profile from discontinuous measurements. This remarkable result is tentatively explained by the geometry of the samples causing limited thermal effects and by the minor influence of possible inhibition and post-polymerization that could influence each of the incremental transformations compared to a single large dose treatment. This method provides a fine tool for revealing differences in kinetic behavior between polymerizable mixtures of various compositions.

Thermal effects on the network structure of diglycidylether of bisphenol-A polymerized by electron-beam in the presence of an iodonium salt

Abstract The cationic polymerization of diglycidylether of bisphenol A (DGEBA) initiated in the presence of a diaryliodonium salt (DAIS) by electron beam irradiation has been studied by FTIR spectroscopy and by dynamic mechanical thermal analysis (DMA). The obtained results show the gradual increase of the temperature for the network thermo-mechanical transition ( T α , associated with the glass transition temperature T g ) over a broad range of conversion ( π ) and reveal a peculiar behavior at high conversion. In this domain ( π >0.90), the materials T g is shown to decrease when conversion approaches unity. Moreover, the post-irradiation thermal treatment of the materials, that generally yields effective post-polymerization, appears to induce a decrease of T g , with an amplitude correlated with the amount of DAIS in the formulation. Owing to the particular nature of the propagating centers in cationic polymerization, the thermal relaxation of ionic clusters trapped in the glassy matrix can be reasonably invoked as a possible cause for this behavior.

Electron‐beam initiated polymerization of acrylate compositions, 2. Simulation of thermal effects in thin films

The thermal effects taking place during the electron beam-induced polymerization of acrylate type formulations were numerically simulated on the basis of the general heat equation applied to a one-dimensional system, The nature, the dimensions and the environment of the polymerizing medium were defined for representing the actual conditions of kinetic experiments performed with a 175 kV laboratory accelerator and FTIR monitoring. The modeled system was constituted of a polymerizable composition coated onto a NaCl plate, initially at 20°C in gaseous nitrogen at the same constant temperature, with or without a PET film covering the reactive layer, Polymerization profiles describing the progress of the reaction as a function of dose were modeled on a phenomenological basis from actual data obtained by discontinuous FTIR monitoring of typical epoxy acrylate or polyurethane acrylate compositions. The influence of the reactive layer thickness (10/100 μm), dose rate (10-110 kGy.s -1 ), maximum polymerization heat (200-400 J.g -1 ) on the temperature-time variations was examined for continuous irradiation. In spite of the relatively small thickness of the reactive layer, significant temperature rise is simulated when heat production is large and fast compared to energy dissipation at the reactive layer boundaries. The obtained data substantiate the fact that upon fractionated EB-treatment with small dose increments (down to 0.6 kGy per pass) at low dose rate (down to 10 kGy.s -1 ) the heat release can be considered weak and without noticeable influence on the conversion data processed for a detailed kinetic analysis. For example, a maximal temperature rise of 6°C was calculated for a fractionated irradiation of 2 kGy increments at 19 kGy.s -1 applied to a polymerizable formulation releasing a maximum enthalpy of 300 J.g -1 .

Electron-Beam Initiated Polymerization of Acrylate Compositions, 6. Influence of Processing Parameters on the Curing Kinetics of an Epoxy Acrylate Blend

The electron beam (EB) induced polymerization of a typical epoxy acrylate (EP-AC) formulation designed for high performance fibre-reinforced composites has been investigated in order to quantify the influence of various processing parameters on polymerization kinetics. Crosslinking polymerization was first conducted on thin EP-AC films coated on a NaCl plate and irradiated in nitrogen with incremental EB-dose delivered by a 175 kV laboratory processor. In such conditions and with the particular geometry of the samples, thermal effects are small, inhibition by air is avoided and post-polymerization is reduced. The actual conversion vs. dose profiles can be reconstructed from discontinuous transmission FTIR measurements following each incremental dose application. Two limiting kinetic regimes were shown to be dependent on the square root of the dose rate (Ḋ) in the initial stage, and proportional to Ḋ in the final stage, as an expectable consequence of the change in chain termination mechanism. A second series of polymerizations was conducted with the reactive layer covered by a PET film that isolates the polymerizable medium from ambient air. The influence of the dose rate and of the temperature was examined under these conditions which allow some post-polymerization to take place. The activation energy determined from the Arrhenius plot drawn from the data recorded during the first regime was about 4 kJ mol–1. The polymerization that proceeds in the third kinetic regime was not significantly dependent on temperature, as expected for a process requiring segmental mobility that takes place in the glassy state. This kinetic study provides a useful set of information for developing a model based on the change of termination mechanism in a medium that is gradually solidifying upon simultaneous network densification and monomer consumption.

Electron-beam initiated polymerization of acrylate compositions 4: effects of pulsed irradiation parameters on curing kinetics

Abstract The electron beam induced curing of a typical epoxy acrylate formulation designed for high performance fiber reinforced composites has been investigated in order to quantify the influence of the beam parameters on polymerization kinetics. Experimental results illustrating the effects of dose rate on curing kinetics observed for a pulsed irradiation of the epoxy acrylate resin are detailed and compared to those of a continuous beam. Variations of the beam frequency in the case of pulsed irradiation do not lead to the same dose rate dependence of the polymerization rate as do current variations in continuous irradiation, or changes in the distance between gun and sample during pulsed irradiation. Simulations of the free radical concentration profiles using a reasonable selection of values for the rate constants provide the basis of tentative explanations and contribute to having a good control over the industrial process.