Bond failure, adhesives determination

Some of these techniques using electrons and photons as probes of the surface chemistry have been described in this symposium by other authors. In this paper methods of surface analyses using beams of ions will be described. Emphasis is placed on ion scattering spectrometry (ISS) and secondary ion mass spectrometry (SIMS). Examples are shown for adhesive bonding applications including determination of locus of failure, contamination, cleaning and thermal and chemical pretreatments. 

The fracture stress of adhesive-bonded joints (adhesion strength) is a consequence of processes that occur in the course of their formation, but all attempts to formulate a fundamental relationship between formation and failure of adhesive-bonded joints have so far been unsuccessful. This is mainly due to the lack of methods for meastuing adhesion that would permit determination of the failure equilibrium work. Accordingly, the relationship between the experimentally determined value of the adhesion strength and the thermodynamic characteristics can be one of correlation only. 

In our view, the failure to determine correlation between the thermodynamic characteristics of adhesion and the strength of adhesive-bonded joints lies in the fact that what was being studied was the relationship of the initial compoimd properties (in the hquid state) with the strength of joints in which the adhesive was in the cured state. In addition to ignoring the essential differences of the cured adhesive from the initial liquid, there was no accoimt for irregularity of the process of formation of adhesive-bonded joints itself, which means that the most favorable adhesive structure (from the thermodynamic point of view) is not realized for kinetic reasons. The necessity for accounting for these factors has been established in previous sections. 

FE may serve as a probe of the locus of fracture in composite materials and in illuminating failure mechanisms. The roles of contact charging and chemical bonding in adhesion may be determined by FE studies. 

Tables 1.3-1.5 show the result of analyses of several bonds between a substrate and a polyvinyl fluoride him using an acrylic adhesive. All surfaces were analyzed by electron spectroscopy for chemical analysis (ESC A). ESC A yields chemical analysis of organic surfaces in atomic percentage, with the exclusion of hydrogen, which is undetectable by this technique. To determine the type of bond failure, ESCA results for the failed surfaces are compared with those of the adhesive and the polyvinyl fluoride him.

Before actual bonding, the subjective water-break test or the quantitative and objective contact-angle test may be carried out. After bonding, the effectiveness of surface preparation may be determined by measuring the bond strength and determining the mode of the failure of the adhesive joint. 

The following techniques are useful for analysis and charaterization for the sizing on the fibres SEM, IR and 6PC of extracts, IGC and DMA (T ) for the fibre-matrix bond micro bond, micro failure with SEM [2], confocal laser scanning microscopy [3] DMA, flexural strength, ILSS and impact of the uni-directional composites (fixed fibre length, orientation and volume fraction distributions). After this detailed analysis, the fibres are tested in their application, which is an injection moulded compound for thermoplasts, where the microstructure (fibre length, fibre orientation and fibre-fibre distance distributions) as well as the fibre- matrix adhesion determine the mechanical properties. 

The carboxylic polyacrylates are effective as laminating adhesives for both flexible and rigid surfaces. The particular carboxylic acrylate used for a given laminating application is determined by the characteristics of the surfaces to be adhered, whether adhesive or cohesive bond failure is desired, the bond strength required of the adhesive, and many other factors. These may include stiffness, friability, extensibility, clarity, cold flow, moisture resistance, solvent or chemical resistance, heat and light insensitivity, radiation resistance, compatibility with resins and other polymeric materials or pigments, and vulcanizability. The carboxylic polyacrylates may be varied to meet the adhesive requirements of a specific application by ... 

The standard ISO 15109 Adhesives - Determination of the time to rupture of bonded joints under static load is focused on time to rupture of bonded lap-shear specimens under a specific static load. No time-dependent shear strain is recorded according to this standard. Initial stress levels are chosen as a percentage of the ultimate tensile lap-shear strength in quasistatic experiments in accordance with ISO 4587. The results are expressed as a graph of stress versus time to failure including remarks on the type of failure pattern for each specimen. 

In cases where a previously made bond has failed, examine the bond surfaces to determine the mode of failure. It will soon be noticed whether the adhesive has peeled from one or other of the surfaces showing poor affinity or improper preparation. Paint or other material that has pulled away with the adhesive will also be apparent. This is another way of identifying unsuitable surfaces. 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

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