Adhesive, selection temperature resistance

As with PSA, the phenolics are added primarily for increased cohesive strength and temperature resistance ([216], pp. 284-306). More phenolic is used in adhesives with higher strength requirements, e.g. for metal-metal bonding. Resins based on /j-/-butyl phenolics are most commonly selected ([216], pp. 284-306). They are usually present in the adhesive at 35-50 parts per 100 rubber (phr), with typical optima at 40-45 phr ([216], pp. 284-306). Significant deviation from this optimum may have drastic effects. 

Chemical vapor deposition (CVD) is an atomistic surface modification process where a thin solid coating is deposited on an underlying heated substrate via a chemical reaction from the vapor or gas phase. The occurrence of this chemical reaction is an essential characteristic of the CVD method. The chemical reaction is generally activated thermally by resistance heat, RF, plasma and laser. Furthermore, the effects of the process variables such as temperature, pressure, flow rates, and input concentrations on these reactions must be understood. With proper selection of process parameters, the coating structure/properties such as hardness, toughness, elastic modulus, adhesion, thermal shock resistance and corrosion, wear and oxidation resistance can be controlled or tailored for a variety of applications. The optimum experimental parameters and the level to which... 

Once the fluoropolymer sheet, film, etc., has been etched, it is an adherable surface. The choice of adhesive depends on the required service conditions and, to some extent, the substrate. Some of the considerations in the selection of adhesive include chemical resistance, flexibility, and temperature resistance. 

There is not a best adhesive for universal environments. For example, an adhesive providing maximum resistance to acids, in all probability, will provide poor resistance to bases. It is difficult to select an adhesive that will not degrade in two widely differing chemical environments. In general, the adhesives that are most resistant to high temperatures usually exhibit the best resistance to chemicals and solvents. Table 2.9 provides the relative compatibilities of s)mthetic adhesives in selected environments. 

The first group can be described as absolute requirements , i.e. the requirements that have to be fulfilled without any compromises. Typical examples of these are temperature resistance or resistance to chemical or other types of environmental attack. Also certain mechanical properties may become absolute requirements if, for example, a minimum value is set for the property in the specification. Any adhesive failing to meet any of the absolute requirements leads to that particular adhesive being dropped from of the selection process. If none of the adhesives satisfies all the absolute requirements, it is obvious that either the requirements that have been set are unrealistic and have to be reconsidered or adhesive bonding is not the correct method for the case studied. 

The mechanism of reaction is an important determinant in the adhesive-selection process. For example, if the processing requirements dictate that only a room-temperamre-curing adhesive system can be used, then temperature resistance may be sacrificed. There are very few room-temperature-curing adhesives that exhibit good elevated-temperature resistance because of the lack of extensive cross-linking and the weaker nature of the polymers formed with room-temperaturecuring formulations. 

Because of the sensitivity of these adhesives to temperatures in the range of 25-65 °C (depending on the adhesive formulation), the fire resistance of an adhesive-repaired joint would depend primarily on joint design and on the additional measures taken to protect the bond-line. Also, adhesives nsed in these applications can display significantly different viscoelastic responses over the temperature ranges attained normally in service. Thus, in some applications, temperature-induced creep is a risk factor that needs to be considered cautiously when selecting the adhesive for that particular application. 

Below the MFFT the film is chalky and non-continuous. At temperatures very much higher than the Tg, the film will be soft with poor adhesion, and abrasion resistance. Most emulsion resins intended for ambient temperature applications have a in the range of 0°C-30°C. Although it is possible to include additives specifically to reduce the MFFT, these could possibly leach out of the film, and it is considered a much more desirable practice to control the Tg (and hence the MFFT) of the polymer by selection of the monomer type and composition. For this reason most emulsion polymers are copolymers. In addition to T, other factors dictate the choice of monomer, such as the performance characteristics conferred by a particular monomer. A copolymer is designed to give a balance of properties in the final film, including such parameters as adhesion, flexibility, abrasion resistance, water resistance and ease of film forming (coalescence). 

Table 11.8 presents a typical triethylenetetramine cured epoxy adhesive formulated with selected fillers. In this formulation the use of aluminum powder and alumina increases substantially the resistance of the adhesive to boiling water.7 This is also true when DETA is used as the curing agent.8 A typical room temperature cured aliphatic amine cured epoxy adhesive for general-purpose use is shown in Table 11.9. This shows the difference that is achieved in shear strength by curing at elevated temperatures versus room temperature.

Adhesion Occurs due to metal-metal contact dynamic process scratches, grooves, severe tearing and deep grooving, seizure may occur Avoid overheating of lubricant use additives with lubricant to form film Use grease at higher temperatures select materials with better resistance as well as hard coatings... 

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