Properties of cured adhesives

All the other test requirements deal with the performance and reliability of the adhesive joint, starting with the cure schedule that very often conditions the quality of the attachment. The bond strength requirements have been established to ensure that the assembly can be subjected to the subsequent manufacturing steps 

The first observation is that gold-filled polyimide IP 685 is more stable than the silver-filled version IP 680 at any temperature. After 200 h at 250°C, the weight loss of IP 685 is negligible, while the residual weights are 98 and 96% at 300 and 350°C, respectively. During the same period, the weight loss of IP 680 is 6% at 250°C, 9% after 100 h at 300°C, and 14% in less than 1 h at 350°C. For the silver-filled epoxy adhesive IP 670, the residual weights are 91% after 90 h at 250°C and 89% after 38 h at 300°C. 

Because of the long duration of this test, the thermal stability is determined by thermogravimetric analysis (TGA) in accordance with ASTM D3850 using 10 mg 

500°C (75%) corresponds to the amount of silver loaded into the epoxy adhesive composition. 

A major requirement for devices attached with an organic material is an adhesive strength sufficient to last the fife of the circuits and to withstand the environmental. 

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This section reviews the chemistry behind waterborne epoxy adhesives and the formulation possibilities. The characteristics of epoxy dispersions and the performance properties of cured adhesive films are addressed. The advantages and disadvantages of these adhesive systems are discussed with the focus on determining whether waterborne epoxy systems can replace traditional epoxy adhesives. 

For example, the physical properties of cured adhesives can be evaluated by measuring stress-strain properties. This can be done with a testing machine as shown in Fig. 1. The machine records force applied to a specimen of defined shaped as a function of either separation of the jaws of the specimen holder or the elongation of the sample. 

Ozonization of lignin forms derivatives of muconic acid that have the unique chemical structure of conjugated double bonds with two carboxyl groups. These derivatives have great potential for chemical modification. The ozonized lignin of white birch was soluble in epoxy resin at 120°C, and the free carboxyl groups were found to react with epoxide. This paper discusses developmental work on the preparation of pre-reacted ozonized lignin/epoxy resins the dynamic mechanical properties of cured resins and preliminary results of the application of these resins as wood adhesives. 

Reacts by accelerating or retarding adhesive cure Properties of modified adhesive affect bond strength ... 

The general properties of cured and uncured epoxy resins are reviewed in Chap. 3. The chemical structures of the resin and curing agent will determine these physical properties. They will also determine, to a great extent, the surface chemistry and adhesion properties of the final product. 

TABLE 3.1 Important Properties of Epoxy Adhesives during Various Stages of Cure... 

The properties of epoxy adhesives in their uncured condition will determine primarily how easily the adhesive can be processed, applied, and cured. They will also determine, to some extent, the performance characteristics of the cured joint. The properties of the individual components as well as that for the mixed formulation are important. 

Cure rate of an actual adhesive film can also be determined by several useful analytical methods. With these methods, fundamental properties of the adhesive, such as dielectric loss, mechanical damping, or exotherm, are measured as a function of time and temperature as the adhesive cures. Several of these test methods are described in Chap. 20. 

Diluents will also affect the performance properties of the adhesive. Diluents generally lower the degree of crosslinking and degrade the physical properties of the cured epoxy. This reduction in crosslink density increases the resiliency of the adhesive, but it also reduces tensile strength as well as heat and chemical resistance. These effects are more pronounced at elevated temperatures than at room temperature. The degree of these effects will depend on whether the diluent has epoxy functionality (reactive diluents) or whether the diluent is incapable of reacting with the epoxy system (nonreactive diluents). 

Fillers and extenders are used in epoxy adhesive formulations to improve properties and to lower cost. Properties that can be selectively improved include both the processing properties of the adhesive as well as its performance properties in a cured joint. However, the use of fillers can also impair certain properties. Typically, the formulator has to balance the improvements against property decline. The advantages and disadvantages of filler addition in epoxy formulations are listed in Table 9.1. Common fillers used in epoxy formulations and the properties that they are used to modify are shown in Table 9.2. 

Probably the most effective accelerator for dicyandiamide systems is the substituted ureas because of their synergistic contribution to the performance properties of the adhesive and their exceptionally good latency. It has been shown that adding 10 pph of a substituted urea to 10 pph of dicyandiamide will produce an adhesive system for liquid DGEBA epoxy resins that can cure in only 90 min at 110°C. Yet this adhesive will exhibit a shelf life of 3 to 6 weeks at room temperature. Cures can be achieved at temperature even down to 85°C if longer cure times are acceptable.10... 

Solid shapes or powders are formulated from solid or liquid (when used as a B stage) epoxy resins and curing agent. Fillers, additives, and other modifiers are often used as they are with liquid or paste epoxy adhesive formulations. However, consideration must be given to the flow properties of the adhesive when heated as well as the application properties. 

Monomers are usually low-MW, monofunctional materials that chemically incorporate into the cured coating rather than volatilize into the atmosphere, as is common with solvent diluents. Monomer diluents are chosen on the basis of providing good solvency, effectively reducing the viscosity of the oligomer without excessively retarding the cure rate. Certain diluents will contribute to the physical properties of the adhesive. However, generally they provide soft, thermoplastic films because of their linear and uncrosslinked nature. 

Figure 15.22 shows the long-term effect of a heat cured one-part epoxy adhesive to various chemical environments. As can be seen, the temperature of the immersion medium is a significant factor in the aging properties of the adhesive. As the temperature increases, the adhesive generally adsorbs more fluid, and the degradation rate increases. 

The main raw materials used in epoxy adhesive formulations (resins and curing agents) can be synthesized in a variety of ways to create many different products. Epoxies react readily via several polymerization mechanisms. The extent of crosslinking is an important determinant of the final properties of the adhesive. Crosslinking can be controlled by the choice of resin and curing agent and by the curing conditions. 

Filler Adhesive component in a solid, finely dispersed form that specifically modifies the processing properties of the adhesive and the properties of the adhesive layer (e.g., metal particles in electrically conductive adhesives, chalkstone, carbon black to increase viscosity). Fillers are not reactive partners in adhesive curing. 

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