Substrate cohesive strength

Polyurethane adhesives are known for excellent adhesion, flexibihty, toughness, high cohesive strength, and fast cure rates. Polyurethane adhesives rely on the curing of multifunctional isocyanate-terrninated prepolymers with moisture or on the reaction with the substrate, eg, wood and ceUulosic fibers. Two-component adhesives consist of an isocyanate prepolymer, which is cured with low equivalent weight diols, polyols, diamines, or polyamines. Such systems can be used neat or as solution. The two components are kept separately before apphcation. Two-component polyurethane systems are also used as hot-melt adhesives. 

These types of polar monomer provide sites for hydrogen bonding which increase the cohesive strength of the PSA because of strong inter-chain interaction, and they can also allow for hydrogen bonding or other polar interactions with some substrates. 

Cohesive strength is the internal strength of an adhesive or the ability of the adhesive to resist splitting. Unlike tack and adhesion strength, cohesive strength is not influenced by the substrate. 

The electrowinning process is connected with higher power consumption but, on the other hand, the electrolytically produced zinc has higher purity. Therefore, further investigations are in progress. The main factors that must be considered in electrowinning process are (1) the electrochemical properties of the cathode materials, (2) the effect of ionic impurities in the electrolyte, and (3) the cohesion strength between the deposited metal and its substrate. 

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. 

There are several possible solutions to the expansion mismatch problem. One is to use a resilient adhesive that deforms with the substrate during temperature change. The penalty in this case is possible creep of the adhesives, and highly deformable adhesives usually have low cohesive strength. Another approach is to adjust the thermal expansion coefficient of the adhesive to a value that is nearer to that of the substrate. This is generally accomplished by selection of a different adhesive or by formulating the adhesive with specific fillers to tailor the thermal expansion. A third possible solution is to coat one or both substrates with a primer. This substance can provide either resiliency at the interface or an intermediate thermal expansion coefficient that will help reduce the overall stress in the joint. 

It is usually not possible to employ a sufficiently large filler loading to accomplish the degree of thermal expansion modification required to match the substrate. High loading volumes increase viscosity to the point where the adhesive cannot be applied or cannot wet the substrate. For some applications and with some fillers, loading volumes up to 200 pph may be employed, but optimum cohesive strength values are usually obtained with lesser amounts. 

The main use of adhesives in labelling applications is in the form of pressure-sensitives, i.e., sticky labels attached either directly or indirectly (behind a potential barrier layer) to a foodstuff. Pressure-sensitive adhesives are a distinct category of adhesives that in dry form are permanently tacky at room temperature. These adhesives will adhere to a variety of substrates when applied with pressure they do not require activation by water, heat or solvents and they have sufficient cohesive strength to be handled with the fingers or by mechanical means in labelling stations. 

The ability to construct and optimize self-assembled structures requires versatile substrates such as dendrons whose steric bulk, polarity and shape can be easily modified. Unfortunately relatively few systems are capable of producing large self-assembled structures with sufficient cohesive strength to withstand significant environmental changes. 

Adhesive characteristics of thin SPI-100 films could not be measured directly because adhesive forces are generally larger than cohesive forces. To obtain some information on adhesion values, thin high molecular weight SPI-100 films on substrates were overcoated with about. 1 mm of a commercially available polyimide (Product A) to provide greater cohesive strength than can be obtained with SPI-100 alone. The combined layers were then pulled in an Instron tester giving the results shown in Table II which also includes the values for commercial products A and B. 

Pull test results for all three substrates showed that the films enriched with tin have higher adhesive and cohesive strengths than those of higher carbon content. The metallic films acted as good water vapor permeability barriers in accord with what is expected for metal coatings. 

Considering only the adhesion of material B on substrate A and the cohesion of material B, the ideal adhesive strength amax(B/A) and cohesive strength Oma (B), are given by [70] ... 

To obtain maximum adhesion, the adhesive bond strength between the adhesive and adherend should be greater than the cohesive bond strength of the adhesive, as indicated in Fig. 6.2. (Of course, the overall strength is also limited by the cohesive strength of the substrates.)... 

Hot melts have to be applied hot (above meiting temperature) to wet the surface of the substrate. Then on cooling, the moiten polymer returns to its soiid form, providing good cohesive strength to the bond. They set very quickiy after they are applied, do not chemically react with the substrate, and do not generate solvent emissions. The primary disadvantage of hot melt adhesives is poor performance at elevated temperatures. 

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