Characterization, film adhesion testing

In a typical probe test, the probe is brought in contact with the surface of the adhesive at a velocity Va and subsequently removed at a constant velocity Vdeb-The stress applied to the adhesive film is at first compressive, then becomes tensile until failure is initiated, typically by the formation of cavities. The plateau stress is then representative of the high-strain deformation of the cavity walls in the tensile direction. This geometry is ideally suited to characterize the adhesive properties of a fibrillating PSA and in particular of two specific properties, namely its resistance to the formation of cavities under a nearly hydrostatic stress, and its ability to form stable fibrils upon debonding [26]. 

In order to characterize the adhesive properties of the blends we used mainly the instrumented probe test developed in our own laboratory [35]. While it is in principle identical to the type of probe tester presented by Zosel [36] and currently commercialized by Stable Microsystems under the name Texture Analyzer, our design has introduced some important differences, shown schematically in Fig. 22.7. We have added a 45° mirror and a video camera to be able to visualize the contact area through a transparent substrate, and a very stiff tripod to be able to adjust the parallelism between the adhesive film and the probe in order to maximize the contact area. Additionally, compared with the commercial instrument, the compliance of our probe tester is much lower, avoiding some of the interpretation problems associated with testing a stiff layer with a compliant... 

The principal aim of the present work was to study the modification of polymer surfaces by plasma treatment as well as the interaction of a metal film with these treated surfaces by surface analytical techniques. As shown above, the results indeed show increased interaction after surface treatment under certain conditions. However, ultimately, improved adhesion should also be observed. For this purpose preliminary expenments with aluminum films on plasma treated polypropylene were carried out. These films were not evaporated under in-situ conditions, but in a separate electron beam evaporator under identical conditions for the different polymer surfaces. A simple Scotch tape test was performed in order to characterize the adhesion qualitatively. In agreement with the surface analytical results, the as-received polypropylene surfaces show poor adhesion as the aluminum film and can be peeled off completely. A five second nitrogen plasma treatment however leads to a film which adheres well and cannot be lifted off by the same tape. Hnally, a 120 s ocatment in a nitrogen plaana leads to an oveitreatmcntof the surface characterized by a low adherion again. 

A simple form of the scratch test is used for assessing paint adhesion (see Paint constitution and adhesion). The scratch test, as described in this article, is perhaps best regarded as a useful comparative method of characterizing thin film adhesion. The test variables used should be carefully characterized and caution exercised in comparing results obtained in different laboratories. 

Abrasion test (characterization) Testing film adhesion and abrasion resistance by rubbing, impacting, or sliding in contact with another surface or surfaces. Examples Tumble test, tabor test, eraser test. 

For analytical purposes and an initial characterization, quick tests (duration minutes to few hours) are sufficient. However, the estimation of the usefulness as an industrial material needs long-term testing (months to years) in different environments (air, water, solvents, etc). The numerous other tests employed in engineering practice to determine mechanical (and other) properties, as well as the special methods for testing rubbers, films, fibers, foams, coatings, and adhesives, will not be dealt with here. 

Transmission Electron Microscopy (TEM) has been used to characterize aluminum thin films thermally evaporated (vacuum around 10 4 Torr) on Polyethyleneterephtalate (Mylar) and to correlate the crystallographic structure of the system Al/Mylar and the adhesion of the aluminum films. The adhesion of these films has been measured by a Peel test technique. For the polymer, an amorphous layer (t=12 nm) followed by a crystalline film have been observed on a Corona treated film and the opposite configuration has been found on a bi-axially stretched film. Some spherical precipitation ana interdiffusion zones have also be observed in the Mylar for the films which have the lower coefficient of adhesion (100 g/inch). The main conclusion is the augmentation of the adhesion of the aluminum film as the size of the grains decreases and/or as the microroughness of the Al/Mylar interface increases. 

Aluminum was deposited onto different PET films and some of the properties (adhesion and structure) of these thin films were characterized with a Peel test and by TEM. For the deposition on a non treated polymer we find that as soon as the film is... 

Aligned multiwall CNT arrays were synthesized as a basis for a microstructured catalyst, which was then tested in the Fischer-Tropsch reaction in a microchannel reactor [269]. Fabrication of such a structured catalyst first involved MOCVD of a thin but dense A1203 film on a FeCrAlY foam to enhance the adhesion between the catalyst and the metal substrate. Then, multiwall CNTs were deposited uniformly on the substrate by controlled catalytic decomposition of ethene. Coating the outer surfaces of the nanotube bundles with an active catalyst layer results in a unique hierarchical structure with small interstitial spaces between the carbon bundles. The microstructured catalyst was characterized by the excellent thermal conductivity inherent to CNTs, and heat could be efficiently removed from the catalytically active sites during the exothermic Fischer-Tropsch synthesis. 

In material characterization, shear strength is the maximum shear stress a material is capable of sustaining. In testing, the shear stress is caused by a shear or torsion load and is based on the original specimen dimensions. Shear strength is rarely a factor in molded and extruded plastic products because of their relatively thick wall sections. However, it can be important in film and sheet products and in adhesive joints. 

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