Bond properties, adhesives, sources

Sources of Contaminants or Species Which Could Affect Adhesive Bonding and Bond Properties... 

FIGURE 19.8. In summary, adhesive joint failure may be caused by a number of factors some not connected with the actual adhesive bond. A common source of problems is the presence of trapped air or other contaminants that serve as loci for failure at the interface (a). Perhaps less common, but of significance, is the presence of contaminants or flaws in the bulk of the adhesive (b) or in the substrate (c) that weaken the physical properties of that phase and lead to joint failure. 

To minimize possible sources of error and improve the application procedure that directly enhances the bond properties, several tools can be used to stabilize the joining process. The utilization of static mixing tubes, which are available for almost every two-part adhesive system... 

Certain specifications and standards provide excellent tutorials on adhesives and sealants. For example, MIL-HDBK-691 offers a complete handbook on adhesive bonding, and MIL-HDBK-725 provides a guide to the properties and uses of adhesives. ASTM C 962 provides an excellent source of information regarding sealant joint design and the types of sealants that are appropriate for various substrates. Although this specification is primarily for construction sealants, much of the information that it contains is generally useful for other sealant applications. 

This has been made possible by the development of suitable adhesive systems that are able to bond the partieles together. The synthetie adhesives offer a consistency of performance that is diffieult to aehieve with natural produets sueh as tannins, and at a cost that has enabled rapid growth to be aehieved. At the same time the adhesive systems have shown a toleranee to a range of wood properties that has enabled most wood residues sources to be used. The environmental effects of formaldehyde emissions from panels made using UF adhesives have been successfully addressed. Initially these required higher resin addition rates to offset a loss in physieal property levels, but further development has reclaimed much of this additional cost. 

The effects of a total dose of ionizing radiation from a cobalt-60 source may be measured by Method 1019.5 of MIL-STD-883F. Although this method is intended for radiation effects on semiconductor devices, the radiation sources, doses, and procedures can be used to evaluate the effects on exposed adhesives, provided that changes in bond strength or other physical or electrical properties are measured. 

Amorphous copolymers of ethylene and propylene, EPM, also possess rubber-elastic properties. But they cannot be vulcanized with sulfur because of the absence of carbon-carbon double bonds, and so a special technique using peroxides as free radical sources for transfer reactions has had to be developed. However, polymerizing in a diene component such as, for example, cyclopentadiene or ethylidene norbornene, leads to the formation of what are known as EPDM rubbers with double bonds in the side chains. These can, on the one hand, be vulcanized in the classic way with sulfur, but, on the other hand, still have good aging properties. Consequently, EPDM rubbers are mainly used in automobile construction, the cable and construction industries, as well as for technical purposes. However, the EPDM rubbers have only slight self-adhesion, so that producing tires from cut sections is made more difficult. It is for this reason that EPDM rubbers are not used for tires. 

About 50-90% of the original double bonds are lost during cyclization. Phenol is a particularly good proton source, since it also acts as an antioxidant. The added phenol is incorporated partly as phenyl ether end groups and partly as substituted phenol. Cyclic rubber has a glass-transition temperature of about 90°C. After pretreatment it resembles vulcanized rubber, balata, or gutta percha in its properties. It is used as a binder for printing inks, lacquers, adhesives, etc. 

Table 7.10 lists common recommended surface treatments for plastic adherends. These treatments are necessary when plastics are to be joined with adhesives. Specific surface treatments for certain plastics and their effect on surface property characteristics are discussed in Sec. 7.6. Details regarding the surface treatment process parameters may also be found in ASTM D-2093 and various texts on adhesive bonding of plastics. An excellent source of information regarding prebond surface treatments is the suppher of the plastic resin that is being joined. 

In the rubber industry the distribution of particle size is considered to be important as it affects the mechanical properties and performance. Aggregate size also varies with particle size. Aggregates can have any shape or morphology. The fundamental property of the filler used in a filled elastomer is the particle size. This affects the reinforcement of elastomer most strongly. One of the sources of reinforcement between the carbon black surface and the rubber matrix is the van der Waals force attraction. Also, rubber chains are grafted onto the carbon black surface by covalent bonds. The interaction is caused by a reaction between the functional group at the carbon black particle surface and free radicals on polymer chains. Hence, filler-rubber interface is made up of complex physical-chemical interaction. The adhesion at the rubber-filler interface also affects the reinforcement of rubber. When the polymer composites are filled with spherical filler (aspect ratio of the particle is equal to unity), the modulus of the composite depends on the modulus, density, size, shape, volume ratio, and number of the incorporated particles. 

---