Wet adhesion

The abiHty to tailor both head and tail groups of the constituent molecules makes SAMs exceUent systems for a more fundamental understanding of phenomena affected by competing intermolecular, molecular—substrate and molecule—solvent interactions, such as ordering and growth, wetting, adhesion, lubrication, and corrosion. Because SAMs are weU-defined and accessible, they are good model systems for studies of physical chemistry and statistical physics in two dimensions, and the crossover to three dimensions. 

Adhesives for paper tubes, paperboard, cormgated paperboard, and laminated fiber board are made from dispersions of clays suspended with fully hydrolyzed poly(vinyl alcohol). Addition of boric acid improves wet tack and reduces penetration into porous surfaces (312,313). The tackified grades have higher solution viscosity than unmodified PVA and must be maintained at pH 4.6—4.9 for optimum wet adhesion. 

Primers for Metal. If reasonably high performance is required ia the end product and unless cost is of paramount importance, a minimum of two coats, usually a primer and a top coat, should be appHed to metal. For highest performance, primer vehicles should provide good wet adhesion, be saponification resistant, and have low viscosity to permit penetration of the vehicle iato microsurface irregularities ia the substrate. Color, color retention, exterior durabiHty, and other such properties are generally not important ia primers. Resia systems such as those including bisphenol A epoxy resias which provide superior wet adhesion can thus be used ia spite of their poor exterior durabiHty. 

Plasticizers soften the film and increase the adhesion and the setting speed. The most common are phthalates, adipates and benzoates. The amount added can be in a broad range of 10-50%. They affect the swelling and softening of the PVAc emulsion particles, ensure film formation at room temperature, and the tack of the still wet adhesive. They also provide improved moisture resistance of the bond. Disadvantages are the lower resistance of the bond line against heat, possible migration of the plasticizers and enhanced cold flow. 

When corrosion develops on painted steel the question is often raised as to whether corrosion was a result of paint failure or the paint failure was caused by corrosion. Several studies have shown that adhesion forces are reduced greatly after water soaking or even at very high humidity -and it has been argued that film detachment by water usually precedes underfilm corrosion . Against this view others have claimed that those paints known to have reduced wet adhesion, e.g. those based on alkyd resins, are not uniquely, or even especially, subject to underfilm corrosion Several factors should be considered in this discussion ... 

Westheimer, Frank, 16 727, 741 Westinghouse AP600 reactor, 17 595 Westinghouse Bettis Laboratory, 17 573 Westinghouse Model 412 pressurized water reactor, 17 574-577 West Nile encephalitis, 14 338 West Nile virus (WNV), 3 135, 137 antiviral therapy, 3 165-168 infection process, 3 164-165 Weston cell, 15 750 Wet adhesion... 

Wet adhesives, that is, polymers that become sticky upon hydration. 

Wet adhesion phenomena represent a potentially fruitful area of research since so little is known. Some of the important questions are (1) How does one measure quantitatively the magnitude of the adhesion when the coating is wet (2) What is the governing principle that determines whether or not water collects at an organic coating/metal interface (3) What is the thickness of the water layer at the interface and what determines the thickness A recent paper (1.) correlates the wet adhesion properties of a phosphated surface with the crystalline nature of the zinc phosphate at the metal surface. 

The interfacial chemistry of corrosion-induced failure has also been studied for coatings with relatively poor resistance to water (poor wet adhesion) for these materials, the corrosion-induced failure typically involved little chemical change, but appeared to involve the same displacement mechanism observed for hvunidity induced adhesion loss (19). XPS spectra showing this mode of failure are reproduced in Figure 1 for this coating, essentially identical spectra were obtained from all surfaces analyzed, independent of test conditions. 

Too much enphasis has be given to adhesion under dry conditions. However, corrosion is only possible if enough water is present in the ooating/metal interface to provide the electrolyte for the corrosion elements to operate. This condition is hardly imaginable without a previous significant reduction or even the loss of adhesion. Therefore "wet adhesion" is considered to be of crucial inportance to corrosion protection by organic coatings (9). 

On the other hand it is advantageous to choose a primer esdiibiting optimal wet adhesion and simultaneously optimal barrier properties. 

In two-layer coating sterns the best choice is to endow both layers with the barrier effect and choose a binder having good wet adhesion. Other ooimbinations are less effective or even not reasonable (Figure 3). 

POOR WET ADHESION, BUT NON-BARRIER BASE COAT ENABLES ACTION OF ACTIVE PIGMENT AND TOP COAT RETARDS LOSS OF ADHESION... 

In all these cases we actually have not a "twofold-protection" but interdependent as well as ocmplementary mechanisms. The intact coating stem protect by its barrier action and, possible, 1 good wet adhesion, vhereas the electrochanical mechanism must be restricted to coating defects. 

It was also found that the lignin-resol resin adhesives satisfied the JIS requirements for non-volatile content, pH value, viscosity, gel time, and dry and wet adhesion strength. Furthermore, the low temperature curability typically found in amino resin adhesives could also be achieved. Thus, it can be concluded that an effective utilization of lignin is possible with simultaneous improvement of the properties of resol resin adhesives. 

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