Indentation test for adhesion measurement of polyimide films

An indentation technique was used to determine the adhesion of polyimide films on a ceramic substrate. Experimental results were obtained by indenting 13 μm thick polyimide films with a conical indenter at different indentation loads. Among the process variables investigated were the amount of adhesion promoter added to the polyimide and the exposure to temperature and humidity. The technique provides a measure of the bond strength, based on the analysis of indentation debonding of thin films. For well adhered films, no debonding could be induced, indicating the usefulness of the test only for the poorly bonded films.     

Analysis of test methods for UV durability predictions of polymer coatings

The purpose of this paper is to review procedures which are used for the evaluation of the durability of polymer coatings. In particular, methods of environmental acceleration and techniques of assessment of coating degradation have been examined, with an emphasis upon those which may produce reliable fast answer durability predictions. The advantages and disadvantages of the various exposure regimes currently used (such as Florida exposure, EMMAQUA or artificial light sources) have been discussed in terms of correlation with actual durability data and of degree of acceleration. A general rule of thumb is that the correlation of accelerated methods with natural exposure is inversely proportional to the degree of acceleration used. The common physical methods used to assess the extent of coating durability have been presented, with the general drawbacks to these techniques being highlighted. Finally, the benefits and drawbacks of a number of chemical techniques (in particular electron spin resonance (ESR), Fourier transform infrared spectroscopy (FTIR), hydroperoxide determination and chemiluminescence), which in principal could provide durability information in a fraction of the time of the physical techniques mentioned earlier, have been discussed. Of these, both ESR and FTIR spectroscopy show particular potential because of the short exposure times necessary to obtain significant results under UVA exposure.     

Mechanics of the indentation test and its use to assess the adhesion of polymeric coatings

The indentation test provides a simple means by which the adhesion of coatings can be qualitatively assessed. On the way to establishing a quantitative measurement of the adhesion strength of coatings and films, it is important that the mechanics of this test are clearly understood. To investigate the influence of factors such as the coating thickness, the indenter radius, and friction during the test, numerical simulations of the indentation of a typical polymeric coating, polymethylmethacrylate (PMMA), bonded to a rigid substrate were conducted by using the finite element method. The stress generated during the indentation test were obtained by employing an accurate constitutive model of the elastic-viscoplastic behaviour of the polymeric coating under consideration. The results of this analysis illustrate the effects of the factors mentioned above on the deformation of the coating during indentation, its confinement, and interfacial shear, and the normal, shear, and hoop stress distributions occurring during indentation. These results provide insight into the possible failure mechanisms operative during the indentation of thin coatings and the important effects of the coating thickness during such tests.     

Dimethyl secondary amine chain extenders: a conceptual approach to in situ generation of advanced epoxy resins for rapid cure, low VOC coatings

Solid epoxy resin oligomers have traditionally been utilized for coatings that combine fast dry-to-touch speed and good flexibility, properties greatly in demand in many applications such as marine and industrial maintenance coatings. Unfortunately, solid epoxy resins require formulation with large quantities of solvent that make the attainment of modern VOC regulations difficult or impossible. Coatings formulated from low molecular weight liquid epoxy resin (LER) on the other hand, can more easily meet VOC challenges, but dry slowly and tend to be brittle. This article explores the concept of using fast reacting, difunctional amine chain extenders to generate epoxy oligomers in situ as a means of meeting these opposing property demands. Methylamine-terminated poly(N-methylazetidine) (p-NMAz) is prepared in a two-step process involving the Michael addition of methylamine to acrylonitrile followed by hydrogenation in a methylamine-containing atmosphere to yield an oligomer stream with an M _n of about 250. Hydrogenation of isophthalonitrile in a methylamine-containing atmosphere yields N,N′-dimethyl-meta-xylylenediamine (DMMXDA). Competitive pseudo-first-order kinetic measurements conducted in isopropyl alcohol indicate these amines react approximately 3–3.6 times faster with phenyl glycidyl ether (PGE) than the primary amine meta-xylylenediamine (MXDA). These chain extenders can be formulated with traditional multifunctional amine crosslinkers to yield coatings with lower VOC, faster dry speed, and better flexibility compared with corresponding coatings formulated without the chain extender. Consistent with their chemical structures, p-NMAz proved capable of yielding coatings with the best impact resistance and mandrel bend properties, while DMMXDA yielded coatings with better water and corrosion resistance properties.