Adhesion lifetime

The modulus of elasticity can also influence the adhesion lifetime. Some sealants may harden with age as a result of plasticizer loss or continued cross-linking. As a sealant hardens, the modulus increases and more stress is placed on the substrate—sealant adhesive bond. If modulus forces become too high, the bond may faH adhesively or the substrate may faH cohesively, such as in concrete or asphalt. In either case the result is a faHed joint that wHl leak. 

Adhesion Life. A second key factor in determining the durabHity of a sealant is the abHity of the sealant to adhere to the substrate through its lifetime. A sealant may have exceHent resistance to uv effects, but if it has poor adhesion performance and faHs adhesively, it is of Httie use. The same can be said of a sealant with superior adhesion characteristics but poor resistance to uv. Either situation results in a short performance life. 

A sealant s adhesion is commonly studied by 180 degree peel tests such as ASTM C794 or by tensHe/adhesion joints tests such as ASTM C719. The adhesion test protocol should simulate actual field conditions as closely as possible. Sealants often have good adhesion to dry substrates, but this adhesion may be quickly destroyed by water. Because most sealants are exposed to water over their lifetime, adhesion testing should include exposure to water for some length of time. ASTM C719 is one of the better tests to determine a sealant s adhesion durabHity as it exposes sealants to seven days of water immersion. 

Impact strength also increased if the adhesion between the polymer and fiber is increased [240, 249]. The most promising method of modification of fiber-filled compositions is by pre-treating the fibers or adding to the matrix of specific depressants or modifiers with the aim of creating a chemical bond at the interphase. This improves the composition service lifetime, strength and thermal stability [250],... 

In the case of tungsten or copper CMP where alumina slurries are used, the pH of the solution must be greater than 9 or lower than 2 to avoid adhesion of the slurries in the porous structure of the brush (back to Fig. 13). This phenomenon, called the loading effect, increases the final particle levels on the wafers and therefore drastically reduces the brush lifetime. This effect can be greatly attenuated by injection of 0.5 to 2% ammonia, for example. 

This expression was derived by Bell (1978), who used Kramers theory to show that bond lifetime ean be shortened by an applied force in processes such as cell adhesion. Although Eq. (3.2) is quite useful, it is in practice limited, most notably by the fact that it assumes that xp is constant. Typically, measurements of force dependency are made under conditions in which force changes with time, and it is likely that the position of the transition state will move as the shape of the potential surface is perturbed by an applied force (Evans and Ritchie 1997 Hummer and Szabo 2003). Theoretical and empirical treatments of various cases have been put forth in the hterature, but they are outside the scope of this chapter and will not be reviewed here. 

Wear is defined as the progressive loss of material from a body caused by contact and relative movement of a contacting solid, liquid, or gas. The importance of understanding and minimizing wear in technical designs is obvious, but still today there are no reliable methods to theoretically predict the lifetime of a new design. Several equations are used to describe wear rates. One example is Archard s well-known law of adhesive wear [512], which describes the material loss per time ... 

For photochemical aging, it is well known that photooxidation affects only a thin superficial layer directly exposed to solar radiation - a few dozens of micrometers in the case of epoxies (Bellenger and Verdu, 1983). Thus the aging mode cannot control the material s lifetime in most cases (composites, adhesives), except for applications such as, for example, varnishes of automotive bodies (Bauer et ah, 1992). 

Barrier metals, as opposed to alloys like AuGeNi, are employed in many thin film metallization systems to promote adhesion and prevent interdiffusion. Gold is an excellent conductor, however, it has very poor adhesion to both Si and GaAs. Gold also shortens the device lifetime when it diffuses into an active region of the device. For this reason it is used in multilayered structures such as Ta/Pt/Ta/Au (50), W/Au (50) and Cr/Au (51). SIMS, AES and RBS have all been used effectively in studying metal-metal interdiffusion, to extract diffusion coefficients, and to estimate device lifetimes. 

As was found for electrochemical-based sensors, problems can occur with peeling of the membrane element from its substrate. PVC is not the only membrane used for optrodes other polymers include polyurethane, which displayed improved adhesion [125]. Ambrose and Meyerhoff [126] incorporated ion-selective species into thin liquid films of decyl methacrylate on glass and then photopolymerised the methacrylate, with simultaneous crosslinking and covalent attachment of the polymer to its substrate to give substantially improved lifetimes. 

Stability of sodium ISFETs. The PVC and KP-13 matrix ISFETs have some drift characteristics of a few mV per hour. The lifetimes of the both ISFETs are about 1 week. It is considered that the drift and durability is caused by the poor membrane adhesion to the ISFET device (9). The Urushi matrix ISFETs exhibited a drift <0.1 mV per hour and durability > 1 month because of the strong adhesion of the Na+ sensing membrane to the ISFET device. 

Ion selective membranes are the active, chemically selective component of many potentiometric ion sensors (7). They have been most successfully used with solution contacts on both sides of the membrane, and have been found to perform less satisfactorily when a solid state contact is made to one face. One approach that has been used to improve the lifetime of solid state devices coated with membranes has been to improve the adhesion of the film on the solid substrate (2-5). However, our results with this approach for plasticized polyvinylchloride (PVC) based membranes suggested it is important to understand the basic phenomena occurring inside these membranes in terms of solvent uptake, ion transport and membrane stress (4,6). We have previously reported on the design of an optical instrument that allows the concentration profiles inside PVC based ion sensitive membranes to be determined (7). In that study it was shown that water uptake occurs in two steps. A more detailed study of water transport has been undertaken since water is believed to play an important role in such membranes, but its exact function is poorly understood, and the quantitative data available on water in PVC membranes is not in good agreement (8-10). One key problem is to develop an understanding of the role of water uptake in polymer swelling and internal stress, since these factors appear to be related to the rapid failure of membranes on solid substrates. 

During reverse osmosis and ultrafiltration membrane concentration, polarization and fouling are the phenomena responsible for limiting the permeate flux during a cyclic operation (i.e., permeation followed by cleaning). That is, membrane lifetimes and permeate (i.e., pure water) fluxes are primarily affected by the phenomena of concentration polarization (i.e., solute build up) and fouling (e.g., microbial adhesion, gel layer formation, and solute adhesion) at the membrane surface [11]. 

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