Cured adhesives physical properties

Binders and Resins. The choice of binder is the most important ingredient choice in the formulation process because the binder affects the performance properties of a paint more than any other single ingredient (3). The physical properties of binders required for paints include the abiHty to dry or cure under various ambient conditions, good adhesion to various substrates, abrasion resistance, washabiHty, flexibiHty, water resistance, and ultraviolet light resistance. The balance of these required properties is mosdy dependent on whether the paint is being developed for interior or exterior appHcations. 

A chemical property of silicones is the possibility of building reactivity on the polymer [1,32,33]. This allows the building of cured silicone networks of controlled molecular architectures with specific adhesion properties while maintaining the inherent physical properties of the PDMS chains. The combination of the unique bulk characteristics of the silicone networks, the surface properties of the PDMS segments, and the specificity and controllability of the reactive groups, produces unique materials useful as adhesives, protective encapsulants, coatings and sealants. 

Since the locus of failure can clearly distinguish between adhesive and cohesive failures, the following discussion separates loss of adherence into loss of adhesion and loss of cohesion. In the loss of cohesion it is the polysiloxane network that degrades, which can be dealt with independently of the substrate. The loss of adhesion, however, is dependent on the cure chemistry of the silicone, the chemical and physical properties of the substrates, and the specific mechanisms of adhesion involved. 

The physical properties of automotive enamels are in large part determined by the crosslink structure developed in the paint films during the baking process. Enamels which are not cured sufficiently (undercured) are generally sensitive to humidity and solvents. In addition, they may be prone to chipping and cold cracking. Faints which have been baked excessively (overcured) exhibit intercoat adhesion failure. That is, subsequent coats... 

The composition and functionality of the systems greatly vary according to the substrate material to be coated, as they demand different curing conditions, adapted physical properties and characteristics as well as functionality for better wetting and adhesion. Dyes, pigments... 

Proper formulation of epoxy adhesives requires knowledge of the chemical reactions that lead to polymerization as well as the chemical and physical properties of both the uncured mixture and the cured material. This chapter reviews the general principles of epoxy resin chemistry including synthesis of the epoxy monomer itself and its possible polymerization reactions. 

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. 

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 addition to good adhesion, an adhesive must have satisfactory cohesive strength and durability once it is cured. The physical and chemical properties of the cured product are determined by... 

DADS melts at 135°C and is employed stoichiometrically with DGEBA at 33.5 pph. Fortunately, it is relatively unreactive so it can be mixed with epoxy resin at elevated temperatures. It can also be used in epoxy solutions to provide an adhesive formulation for manufacturing supported or unsupported film with long shelf life. Because of the low reactivity of the system, DADS is generally employed at a concentration that is about 10 percent greater than stoichiometry, or an accelerator, such as BF3-MEA, is employed at about 0.5 to 2 pph. When DADS is mixed with liquid DGEBA resin, it provides a pot life of 3 h at 100°C and requires a rather extended high-temperature cure to achieve optimal physical properties. 

The tertiary amine salts are claimed to provide epoxy formulations with very good adhesion to metal. The cured resins also show a hydrophobic effect when in contact with water or at high humidities. The strength, toughness, and elongation (4.7 percent) of the cured epoxy resin are very good. However, heat distortion temperature is only in the range of 70 to 80°C, and chemical resistance is relatively poor for an epoxy. The physical properties fall off rapidly with any rise in temperature. 

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). 

Epoxy-polysulfide systems can be formulated either as a liquid DGEBA epoxy mixed with liquid poly sulfide polymer or as an epoxy-terminated polysulfide polymer either may be cured with a tertiary amine such as DMP-30. Table 11.19 describes the formulation and shows the physical properties of these epoxy-polysulfide adhesives compared to an unmodified epoxy adhesive. 

Radiation is used to crosslink (Fig. 14.2) or cure organic resins into durable coatings or adhesives having excellent physical properties with high chemical and temperature resistance. Radiation curing technology involves at least four considerations type of radiation... 

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