Adhesion-promoting factors

Under optimum conditions, hydrolysis is rapid, and precedes slower condensation reactions. However, the silanol groups formed in hydrolysis are very reactive, and condensation can be a significant factor even in dilute aqueous solutions. Understanding the kinetics of the hydrolysis and condensation reactions for a specific silane in solution is important because its effectiveness as an adhesion promoter is influenced by the extent of condensation, which, in turn, affects the structure of the interphase in composites. 

Evenness of treatment depends on factors such as the designs of the chamber and of the electrode, the levels of energy and pressure, and the period of time of exposure once established at satisfactory values, these parameters can be controlled fairly easily. The surface chemistry is similar to that with simple flame treatment but the use of a gaseous reactant means that even treatment of quite complex shapes is practicable. Besides even and repeatable results the method offers the additional advantage that adhesion promoters are not necessary—with associated savings in costs and enhanced environmental implications. 

A similar mechanism acts in the case of reactive adhesion promoters such as polyurethanes. Here water is a factor, as well as surface functional groups of fillers. The adhesion promoter is exhausted within the bulk of organic material (coating, adhesive, etc.) and unable to perform the task. 

Steric ( ) factors. Generally we observed that the use of even modest amounts of fatty acids or their esters in OH-rich binders substantially decreases the corrosion resistance. A possible explanation would be sterical shielding of the adhesion-promoting groups by the long fatty acid tails. 

Trachte and DiBenedetto, 1971 Wambach et ai, 1968). Since PPO is much more ductile at 25 C than the epoxy resins mentioned, the effects of filler and adhesion promoter on PPO should tend to resemble the effects on an epoxy resin in a ductile state (e.g., at 130°C). Indeed this is the case. The point is that a filler tends to increase surface roughness and hence y in an otherwise brittle matrix, especially if the filler-matrix adhesion is poor, but tends to inhibit plastic deformation (by constraints or by simple volume replacement) in an otherwise ductile matrix. Such effects are not accounted for in Nielsen s simple treatment (Section 12.1.2.3) and conceivably may occur as competitive mechanisms (see Figure 12.20). A useful summary of such competitive factors is given in Table 12.3 for the glass-bead-epoxy systems (DiBenedetto and Wambach, 1972) the discussion should be relevant to other cases as well. 

The discussion of heat durability in this section should make clear that this is a complex phenomenon. Embrittlement, retropolymerization, thermoplasticity, and the loss of adhesion are all factors affecting the adhesive s performance on metal surfaces. Based on the published state of the art, optimum cyanoacrylate heat durability could be achieved using a combination of a heat-resistant adhesion promoter, a crosslinking agent, and a plasticizer. The heat durability promoters discussed in this section are summarized in Table XI. 

To engineer bioactivity in materials we now commonly incorporate biologically motivated, biomimetic adhesion-promoting sites. Most frequently we also add factors, such as growth factors, to promote or suppress growth. It is also now established that we can, at least in part, mimic the less specific interactions of, for example, heparin, in this case with polyelectrolytes, in biomaterials design. 

At a first look the decrease of the peak current by a factor of 4—5 and the delay of die activation peak by a factor of 20-40, from 3-4 s to 60-120 s, does not seem to be sufficient for an effective improvement. In the case of an additional polymer coating, a factor of 20 increase in activation time might extend the lifetime of a product by several years. For this reason, aminophosphonate monolayers may prove suitable as adhesion promoters. 

Automotive Sealants In filling underbody dead spaces, foamed plastisols compete with urethanes and other products. Such plastisols are highly filled, often with over 100 phr calcium carbonate, and plasticizer levels of 100-140 phr. Adhesion promoters for metal bonding (Chapter 12) are used with 2-4 phr of 5-7 p,m AZO, yielding expansion factors of 1.5-2. 

---