A. Apicella

Environmental degradation of the electrical and thermal properties of organic insulating materials

The sorption behaviour of water molecules in epoxy-based thermosetting networks is discussed and related to the modifications of the polymer properties. The hypothesized sorption modes and the corresponding mechanisms of plasticization are discussed on the basis of experimental liquid-sorption tests, differential scanning calorimetry (DSC), and electrical analyses. Two modes of moisture sorption are assumed: a sorption leading to an increase in the free volume of the system, and adsorption of the water molecules due to hydrogen bonding to hydrophilic groups present in the network and to the surfaces of “holes” which make up the excess free volume of the glassy polymer. Electrical investigation improved the understanding of the hygrothermal ageing phenomena. Water conditioning of the composites modified the surface and bulk resistivities as a consequence of microstructural damage and plasticization.

Solvent induced crystallization in poly(aryl-ether-ether-ketone)

Sorptions of methylene chloride and methylene chloride/n-heptane liquid solutions in poly(aryl-ether-ether-ketone) (PEEK) are analysed and interpreted assuming multiple transport mechanisms. The presence of the highly interacting methylene chloride is responsible for anomalous sorption behaviours. PEEK films immersed in methylene chloride poor n-heptane solutions sorb by non-ideal Fickian diffusion, while limiting Case II and diffusion controlled swelling accompany equilibration in progressively richer methylene chloride solutions. The optical microscopy observation of cryogenically fractured samples conditioned in pure liquid methylene chloride at 5, 20 and 36 ° C for different times, revealed the presence of a sharp moving front. Differential scanning calorimetry (DSC) indicates that the glass transition temperature of the penetrated shells was always lower than the treatment temperature. The high levels of solvent swelling and plasticization were probably responsible for the crystallinity detected in DSC thermograms and wide angle X-ray scattering (WAXS) diffractograms of the solvent treated and initially amorphous samples. The differences of WAXS diffractograms of samples crystallized by immersion in methylene chloride at 5 and 36 ° C for various times and by thermal treatment are discussed. Increasing levels of crystallinity were induced in the samples equilibrated in progressively richer methylene chloride solutions.

Thermal, mechanical and rheological evolution during the processing of multilayer thermosetting composite structures

Abstract The rheological, mechanical and thermokinetic aspects of the cure of unsaturated polyester based laminated composites were analyzed by means of a numerical model. The model was developed using the heat transfer and heat generation characteristics of polymerizable systems coupled with the viscoelastic properties of the layer materials. The calculated evolution of the temperature profiles in the single layers were used to analyze the corresponding thermomechanical dimensional stability. First, the temperature profiles were calculated according to appropriate kinetic and heat transfer models and then the corresponding dimensional changes were evaluated as a function of time. The viscoelastic parameters used in the model were measured in stress relaxation compression tests.

Solvent mixtures sorption in amorphous peek

SummaryThe sorption kinetics and equilibrium isotherms of methylene chloride, n-heptane and of methylene chloride/n-heptane mixtures in glassy amorphous Poly(aryl-ether-ether-ketone) (PEEK) have been investigated. Ideal Fickian diffusion, anomalous non-ideal Fickian diffusion, “Case II” sorption mechanism and diffusion controlled swelling were observed depending on temperature, on solvent type and on external solvent activity. Gaschromatographic analysis performed on PEEK samples contacted with methylene chloride/n-heptane mixtures indicated that the presence of methylene chloride enhances n-heptane mobility and equilibrium sorbed amount.

Compositional influence on toughness of structural acrylic adhesives

The fracture behaviour and toughness of modified acrylic adhesives, based on mixtures of various acrylic monomers and rubbers polymerized by a radical mechanism have been investigated. The influence of the matrix and rubber composition on bulk morphology and mechanical properties is analysed by means of transmission and scanning electron microscopy, and dynamic-mechanical measurements. Both microtexture of the material and resin-rubber compatibility have been found to significantly influence the fracture behaviour in tension-impact tests.