Dov Katz

A thermo-mechanical study of some epoxy adhesives subjected to combined dynamic and static stresses

Abstract The object of the present study was to investigate the effect of superimposed dynamic and static stresses on mechanical and thermal properties of some epoxy adhesives. It was found that combinations of shear creep and torsional oscillations, applied simultaneously to adhesive joints at temperatures within the glassy range of the adhesive, led to strengthening of the joints in shear and to an increase in the glass transition temperature of the adhesive. Similar loading stresses applied at temperatures close to the Tg of the adhesive, led to opposite effects on the above mentioned properties of the joints. The width of the glassy-rubbery transition of the adhesives increased, in the whole range of temperatures used in this study and for all epoxy compositions, as a result of subjecting the joints to superimposed dynamic and static stresses. The broadening of the glass transition was interpreted as a result of the stiffening of polymer network during the combined stressing experiments. A linear rela...

The Effect of Crosslink Density and Complex Fields of Stresses on the Network Chain Mobility of Model Epoxy Resins

The viscoelastic properties of model epoxy resins were investigated using a tensile stress relaxation technique. Simultaneously with the applied tensile strain, the specimens were subjected to torsional oscillations in a specially designed instrument. The crosslinking density of the resins was controlled by the length of the aliphatic chain of the diamine hardener. Smooth master curves were obtained by Computed Aided Superposition (CAS) using a software written especially for this work. The shape of the master curves changed significantly as a function of the frequency of torsional oscillations. Based on previously published data, as well as on pertinent information from literature, it was concluded that the variations in the shape of the master curves reflect corresponding changes in the spectrum of relaxation times of the investigated resins. It is suggested that the observed phenomena are caused by physical processes, such as short-range orientation and physical aging, which lead to densification of the network and a corresponding increase in its stiffness.

The Effect of Superimposed Dynamic and Static Stresses on the Stress Relaxation Rates of Model Epoxy Resins

Abstract Model epoxy resins with controlled crosslinking densities were prepared from a commercially available DGEBA (Epon Resin 825® from Shell Chemical) and α,ω-aliphatic diamines having different lengths of the aliphatic chain. The mixtures were always prepared in stoichiometric ratio. Dumbbell specimens, either with a cylindrical (with thin walls) or a rectangular cross section, have been subjected to combined static tensile strains (applied vertically) and dynamic torsional strains (applied horizontally). The experiments were performed at temperatures encompassing the glassy and the rubbery range of each investigated resin. The apparatus, by which the combined dynamic and static stresses were applied, was equipped with load cells for measurement of stresses developed in the specimens, both in tension and in shear. In this paper, results concerning the effect of the above combinations of stresses on stress relaxation rates in tension are shown. The main conclusion of this study is that combinations of...

Thermo-Mechanical Behavior of Highly Crosslinked Epoxy Resins in Complex Stress Fields

Single Lap Joints (SLJ) were subjected at different temperatures to combined static and dynamic stresses. The static stresses were applied vertically (shear creep), while the dynamic ones were applied horizontally as torsional oscillations with different frequencies. It was found that the thermo-mechanical history influenced strongly both the shear strength of the SLJ and also the T9 of the adhesives.

Correlation Between the Total Morphology of an Epoxy System and Some of its Properties

Abstract Due to the ever growing use of epoxy resins for structural applications, more information is required about their structure-properties relationship. Much work was done on this subject by us and by others, but many questions still remain unanswered. Some problems which require more detailed studies are the influence of factors like compositional changes in the resin (ratio of prepolymer-curing agent), postcuring and cooling of the polymerized resin from temperatures above Tg to room temperature, on physical properties of the product.