Bonded films cohesion

In these expressions k is the rate constant, p and E are the entropy term and the energy of activation respectively for breaking the chemical bonds Z is the collision rate. It will be noted that the first and third of the additional factors for interfacial reaction, i.e. the surface pressure II and the film cohesion have been included in the accessibility function tj). This is defined as the relative accessibility of the potentially reactive groups in the film it is unity at high areas when II 0 and when the cohesion also becomes negligible. 

The effects of bonding temperature on (he apparent peel strength per unit width are shown in Figure 2. In the bonding experiments at 120 °C, adhesion of the EO-1 films caimot be obtained with which the T-peel test was not performed. Interfacial failmes were observed for the bonded films of EO-2 and EO-3 samples at 120 C. At the lowest peel rate tested, 2 mm/min, the apparent peel sfrength are about 0.11 + 0.02 and 0.25 + 0.02 N/mm for EO-2 and EO-3 samples, respectively. As the bonding temperature increased to 130 °C, interfacial failure was observed for EO-1 sample and cohesive failure for all the other two samples. The fractal analysis will be carried out only on the fractured surfaces resulted in interfacial failures imder these conditions. 

Adhesives. Poly(vinyl alcohol) is used as a component in a wide variety of general-purpose adhesives to bond ceUulosic materials, such as paper and paperboard, wood textiles, some metal foils, and porous ceramic surfaces, to each other. It is also an effective binder for pigments and other finely divided powders. Both fully and partially hydrolyzed grades are used. Sensitivity to water increases with decreasing degree of hydrolysis and the addition of plasticizer. Poly(vinyl alcohol) in many appHcations is employed as an additive to other polymer systems to improve the cohesive strength, film flexibiUty, moisture resistance, and other properties. It is incorporated into a wide variety of adhesives through its use as a protective coUoid in emulsion p olymerization. 

Fillers. Fillers are not commonly added to CR adhesives. Calcium carbonate or clay can be primarily added to reduce cost in high-solids CR mastics. Maximum bond strength is obtained using fillers with low particle size (lower than 5 [jim) and intermediate oil absorption (30 g/100 g filler). In general, fillers reduce the specific adhesion and cohesion strength of adhesive films. Although polychloroprene is inherently flame retardant, aluminium trihydrate, zinc borate, antimony trioxide or... 

Sol-gel film.s deposited on a grit-blasted aluminum surface give performance close to PAA bonds (Fig. 22) with generally cohesive failures observed in wedge tests. Given that one application of this treatment is repair, the performance is... 

Oleic acid (Figure 4.25) gives a much more expanded film than the corresponding saturated acid, stearic acid - i.e. ir is greater for any value of A. Because of the double bond there is less cohesion between the hydrocarbon chains than for stearic acid and a greater... 

These properties have a profound effect on the processing properties of the uncured adhesive and on the end properties of the fully cured product. The properties determined by physical chemistry affect both the cohesive strength of the adhesive film as well as the degree of adhesion to the substrate. They also affect the permanence and durability of the adhesive bond once it is placed into service. 

In hot mixing or elevated-temperature curing of an epoxy system, vapor pressure could also be of concern relative to the quality of the adhesive bond. If the components in an epoxy system become too hot, boiling can occur, resulting in gas bubbles. If gas bubbles become trapped in the cured adhesive film, they can lead to reduction of cohesive strength and stress risers. For many adhesive applications, particularly those in the electrical and electronic industries (due to possible ionization of air voids), complete removal of any gas bubbles from the epoxy is essential. 

Specialty product for bonding polyester sewing threads. Exhibits very high adhesion to polyester imparting excellent cohesion. Dries to a slightly tacky film which improves package build and stability. 

In inorganic systems, ionic bonds across interfaces must also be considered as electrostatic adhesion. In a number of instances, oxide or other films are deliberately created on metal surfaces to promote adhesion with other systems, such as polymers. The bond between the metal oxide and the metal itself is primarily electrostatic and is usually stronger than the cohesive strength of the metal oxide. 

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