Henri Jullien

Polyurethane curing by a pulsed microwave field

Abstract The complete study of the crosslinking of polyurethane resins by means of a pulsed microwave field has been made, in order to describe the energy transfer processes in polymer films and to optimize the transfer efficiency. Experimental data show the energy transfer can be strongly enhanced by using specific pulse repetition frequencies which correspond to dielectric relaxation of polar structural elements in polymers. Moreover, the hardness of cured films can be also correlated with this pulse frequency specificity.

The microwave reaction of phenyl glycidyl ether with aniline on inorganic supports: a model for the microwave crosslinking of epoxy resins

Abstract The microwave reaction between equivalent amounts of phenyl glycidyl ether and aniline is studied on solid inorganic supports in ‘dry media’, as a model for the crosslinking reaction of epoxy resins, to determine any chemical peculiarities due to the use of microwave energy. The resulting products are compared with standards given by the conventional heating reaction. The nature of the support is a major parameter. With or without microwaves, clays or silica gel (and especially clays with lamellar structures, such as montmorillonite K 10 or Tonsil BW3) provide epoxy conversion extents higher than aluminas, owing to their increased acidic properties. Besides the epoxy-amine addition products, etherification products of the phenyl glycidyl ether are formed by both hydroxyl-epoxy and epoxy-epoxy competitive reactions. When compared to conventional heating, microwaves do not qualitatively change the reaction products, but quantitatively modify their proportions, because both microwaves and the structure of the inorganic supports influence the relative kinetics of the different reaction paths.

VARIATIONS OF THE DIELECTRIC PROPERTIES OF EPOXY RESINS DURING MICROWAVE CURING

Abstract The curing of a particular epoxy resin (diglycidylether of bisphenol-A) by three amino hardeners (4,4′-diaminodiphenylmethane, 4,4′-diaminodiphenylsulphone and 4,4′-diaminodiphenylether) is followed by dielectric measurements performed under microwave heating. The formation on heating of a molecular association, is suggested by the analysis of the dielectric variations on the basis of an additive contribution of all the known constituents in the mixture. The reversibility of this molecular association is demonstrated. Two possible interpretations are proposed and discussed. Dielectric constant variations during curing provide evidence that microwave heating is a very efficient way of energy transfer and for monitoring chemical processes, such as the cross-linking of epoxy resins.

Dielectric and geometric dependence of electric field and power distribution in a waveguide heterogeneously filled with lossy dielectrics

In microwave processing of dielectric materials which completely fill a waveguide, the distribution of the electric field within the material needs to be known. This paper presents the theoretical conditions for the microwave cure of large pieces, the size of which is more than the wavelength. The mathematical description of heterogeneously multilayer-filled waveguides presents certain difficulties because of the involved transcendental equations. A computer treatment to determine the electric-field spatial distribution is developed. The influence of the dielectric constants and the thickness of the dielectric materials on the spatial distribution of the electric field and power flow in each layer is studied. In particular, the field strength is enhanced in the dielectric with the highest permittivity. A numerical resolution of the transcendental equations defining the cutoff frequencies of propagation modes allows one to enumerate the modes, which can successively appear in a dielectric-loaded waveguide as functions of dielectric and geometric parameters. The attenuation constant and the microwave power dissipated in each material are determined. A balance sheet of energy is established.

Behaviour of film-forming polymers in a microwave electric field

Abstract The application of a microwave electric field to the formation of polyurethane films supported by a glass substrate has been described previously. The present work concerns the investigation of the separated contributions of a polymer and its solvent during the film formation in microwaves. The measurement technique allows the kinetic formation to be observed by means of a specific infra-red pyrometer which indicates the temperature variation of the film surface when submitted to the electric field inside a waveguide associated to a microwave power generator at 2.45 GHz. Present data show that dielectric relaxation losses of structural elements of the polymers form the dominant heating process in microwave coating.

Polymerization and Surface Treatment in a Large Volume Microwave Plasma Generator

AbstractSeveral experiments are reported oe the curing and cross-linking of polymer films deposited on metals and non-metals with the aim of evaluating the large volume microwave plasma generator (LMP®) technology. The LMP® technology allows, in principle, the generation of a low-temperature plasma of any desired length and it is therefore of great interest in industrial processes involving the treatment of large surface areas. This paper attempts to evaluate some plasma-polymer coatings produced on various metallic and non-metallic surfaces. It is shown that polyurethane films deposited on metal surfaces are effectively and quickly cross-linked when exposed to an argon microwave plasma; in addition, plasma-polymer coatings were formed from styrene and methylmethacrylate vapours introduced into the LMP reactor. The deposition of such plasma-polymers produced, in most cases, resistant and strongly adhesive coatings. However, the plasma-polymer coating was not found to possess a regular structure. Instead i...