Loss of adhesion

The most common problem in the paint layers, which can have a wide variety of causes, is loss of adhesion. Upon drying of the medium, the paint layers develop shrinkage cracks. In itself, this is not a particularly worrisome phenomenon, but, if through any cause the adhesion between paint layers and ground or between ground and support is lost, the paint begins to flake. First the flakes cud up, and finally become completely detached and lost. 

For wear resistance and low friction, coatings of PTFE or M0S2 generally have been satisfactory. Use of low thermal expansion filler in PTFE helps minimise cracking and loss of adhesion from metal substrates with their lower coefficients of expansion. 

Poor preparation of the substrate can result in loss of adhesion, pitting, roughness, lower corrosion resistance, smears, and stains. Because electroplating takes place at the exact molecular surface of a work, it is important that the substrate surface be absolutely clean and receptive to the plating. In the effort to get the substrate into this condition, several separate steps may be required, and it is in these cleaning steps that most of the problems associated with plating arise. 

ActivatedL yer Loss. Loss of the catalytic layer is the third method of deactivation. Attrition, erosion, or loss of adhesion and exfoHation of the active catalytic layer aU. result in loss of catalyst performance. The monolithic honeycomb catalyst is designed to be resistant to aU. of these mechanisms. There is some erosion of the inlet edge of the cells at the entrance to the monolithic honeycomb, but this loss is minor. The peUetted catalyst is more susceptible to attrition losses because the pellets in the catalytic bed mb against each other. Improvements in the design of the peUetted converter, the surface hardness of the peUets, and the depth of the active layer of the peUets also minimise loss of catalyst performance from attrition in that converter. 

Fig. 5-8 Total adhesion loss of a 500-/xm-thick coating of EP (liquid lacquer), 0.2 M NaCI, galvanostatic = -1.5 /tA nrr, 5 years at 25"C. Left coating with a pin pore loss of adhesion due to cathodic disbonding. Right pore-free coating loss of adhesion due to electro-osmotic transport of H O. In both cases the loose coating was removed at the end of the experiment.

Cathodic protection of reinforcing steel with impressed current is a relatively new protection method. It was used experimentally at the end of the 1950s [21,22] for renovating steel-reinforced concrete structures damaged by corrosion, but not pursued further because of a lack of suitable anode materials so that driving voltages of 15 to 200 V had to be applied. Also, from previous experience [23-26], loss of adhesion between the steel and concrete due to cathodic alkalinity [see Eqs. (2-17) and (2-19)] was feared, which discouraged further technical development. 

This chapter first reviews the general structures and properties of silicone polymers. It goes on to describe the crosslinking chemistry and the properties of the crosslinked networks. The promotion of both adhesive and cohesive strength is then discussed. The build up of adhesion and the loss of adhesive strength are explained in the light of the fundamental theories of adhesion. The final section of the chapter illustrates the use of silicones in various adhesion applications and leads to the design of specific adhesive and sealant products. 

Since the locus of failure can clearly distinguish between adhesive and cohesive failures, the following discussion separates loss of adherence into loss of adhesion and loss of cohesion. In the loss of cohesion it is the polysiloxane network that degrades, which can be dealt with independently of the substrate. The loss of adhesion, however, is dependent on the cure chemistry of the silicone, the chemical and physical properties of the substrates, and the specific mechanisms of adhesion involved. 

Loss of adhesion occurs at the silicone substrate interface and two main mechanisms can be outlined the formation of a weak boundary layer (WBL) and the breaking of adhesive bonds. 

The homopolymers, which are formed from alkyl cyanoacrylate monomers, are inherently brittle. For applications which require a toughened adhesive, rubbers or elastomers can be added to improve toughness, without a substantial loss of adhesion. The rubbers and elastomers which have been used for toughening, include ethylene/acrylate copolymers, acrylonitrile/butadiene/styrene (ABS) copolymers, and methacrylate/butadiene/styrene (MBS) copolymers. In general, the toughening agents are incorporated into the adhesive at 5-20 wt.% of the monomer. 

The lapshear tensile shear strength of formulation A without any rubber, declines to ca. 50% of its original strength after 24 h at 12I C. In contrast, the lapshear ten.sile shear strength of formulation C exhibits no loss of adhesion under the same conditions. 

B. Degradation of the fiber-matrix interface resulting in loss of adhesion and interfacial bond strength. 

Surface preparation of concrete consists mainly of removing laitence, form oils and air pockets. Laitence is the fine cement powder that floats to the surface of concrete when it is placed. Coatings applied over such a powdery, weak layer will lose adhesion. Form oils are used for the easy stripping of forms or shuttering. Their presence will also cause loss of adhesion of subsequent coatings. Forms should be coated with non-migratory hard coatings and the use of oils or waxes prohibited. 

Air pockets or bubbles are left on the surface of all concrete. Good vibration and placing techniques will reduce their number but not eliminate them. Many air pockets have a small opening on the surface in relation to their size. Paints will not penetrate into such holes, with the result that air or solvent is trapped and subsequent expansion will cause the coating to blister. In addition, some air pockets are covered with a thin layer of cement that also has no strength and will cause loss of adhesion. 

Blistering the formation of dome-shaped projections or blisters in paints or varnish films by local loss of adhesion and lifting of the film from the underlying surface. Such blisters may contain liquid, vapour, gas or crystals. 

The flow behaviour of rubber on a mill is dependent on the material, nip width, roll speed and temperature, and certain combinations can give flow instabilities, the worst case from the mixing point of view being bagging , i.e., loss of adhesion of the rubber compound to the mill rolls. A decrease in nip width, an increase in speed or temperature, can overcome this problem. 

The tensile adhesion values show no correlation with the extent of corrosion the bisphenol A epoxy cured with a polyamide amine showed blistering, which represents a complete loss of adhesion. The polyester showed cohesive failure at less than 1000 hours of exposure, so a true adhesion value could not be determined. The other epoxies and the vinyl ester all had values in the 150-200 psi range, with no apparent relationship to the amount of corrosion. 

Loss of Adhesion When Wet. Many coatings, particularly those applied to a roughened surface, show excellent tensile adhesion to steel but lose this adhesion after exposure to pure water at room or elevated temperatures. A thin film of water at the interface is apparently responsible for the loss of adhesion. If the coating is allowed to dry without destructively testing the adhesion, the dried coating often exhibits the original tensile adhesion. The phenomenon is... 

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