Premature adhesive failure

Grip layer. A layer of coarse aggregate may be bonded to the steel to form a rough layer onto which fresh concrete is subsequently poured. This technique has been the subject of some research involving slab panels, tensile reinforcement being provided in the form of a thin soffit steel plate. However, premature adhesive failures have been reported and an improved bonding technique is considered to be necessary(9). 

The exact cause of premature adhesive failure is very difficult to determine. If the adhesive does not wet the surface of the substrate completely, the bond strength is certain to be less than maximal. Internal stresses occur in adhesive joints because of a natural tendency of the adhesive to shrink during setting, and because of differences in physical properties of adhesive and substrate. The coefficient of thermal expansion of adhesive and adherend should be as close as possible to minimize the stresses that may develop during thermal cycling or after cooling from an elevated temperature cure. Fillers are often used to modify the thermal expansion characteristics of adhesives and limit internal stresses. Another way to accommodate these stresses is to use relatively elastic adhesives. 

Loading to the design ultimate, or a multiple thereof, will give confidence that the connections in the complete structure are capable of performing satisfactorily. Unless a linear behaviour up to collapse of the connection can be assumed, such tests give little indication of the ultimate margin of safety. However, they confirm that premature adhesive failure will not occur. 

Extracellular matrices (ECM) are the primary structural materials found in connective tissue in vertebrates that serve to maintain tissue shape (skin), aid in locomotion (bone), transmit and absorb mechanical loads (tendon and ligament), prevent premature mechanical failure (tendon, ligament, skin, and blood vessel wall), partition cells and tissues into functional units (fascia), act as scaffolds that define tissue and organ architecture (organ parenchyma), act as storage devices for elastic energy (tendon and blood vessel wall), and as the substrate for cell adhesion, growth, and differentiation of a variety of cell types. 

The adhesives will bond almost all materials (though a primer may be needed with some), except polyolefin plastics (eg Polythene) and other low surface-energy types such as fluoropolymers (eg Teflon) and silicone-based rubbers. Alkaline glass may cause premature bond failure and all glasses should be silane primed if at all possible, as this considerably improves the joint s humidity resistance. May stress crack stressed mouldings or susceptible plastics - polycarbonate, for example. 

Early implanted microelectronic devices for neurological prostheses, including the cardiac pacemaker, suffered premature encapsulation failures, probably because the function of the encapsulant had received insufficient thought. Subsequent research, conducted at the Medical Research Council, showed that the important property required of an encapsulant was not impermeability, as had been assumed, but adhesion. It was shown that bonds could be made between mbber and alumina or between mbber and titanium which could withstand boiling at pHs between 4 and 10 for at least 200 days. This was thought to correspond to a life of about 50 years, at 37C, in a patient. 3 refs. 

Insoluble Sulfur. In natural mbber compounds, insoluble sulfur is used for adhesion to brass-coated wire, a necessary component in steel-belted radial tires. The adhesion of mbber to the brass-plated steel cord during vulcanization improves with high sulfur levels ( 3.5%). Ordinary rhombic sulfur blooms at this dose level. Crystals of sulfur on the surface to be bonded destroy building tack and lead to premature failure of the tire. Rubber mixtures containing insoluble sulfur must be kept cool (<100°C) or the amorphous polymeric form converts to rhombic crystals. 

Poor adhesion of membrane to metal is the leading cause of failure in solid-state potentiometric sensors [116], For glass membranes, the mismatch of thermal coefficients of expansion between thin glass membrane and metal (mostly Pt) has been attributed to premature failure due to hairline crack formations in the glass layer [60], For polymer-based membranes, water vapor penetration was reported to compromise the membrane-metal interface, therefore affecting the sensor s performance. 

One form of film break-up has been mentioned previously, in which the surface of a relatively thick film becomes consolidated over a soft unconsolidated layer. This can lead to shear in the soft layer, especially with high non-conformal loading, and the consolidated layer will break away. Loss of adhesion and film break-up can also occur, especially with a bonded film, if the surface pre-treatment has been badly performed, so that the coating simply fails to adhere and breaks away. These cases may be considered as premature failures caused by poor film preparation. 

---