Cyanoacrylate relative humidity

Moisture-curing adhesives are formulated with resins that react with atmospheric moisture to form a curedpolymer. Most ofthese resins are one-part cyanoacrylates, silicones, or polyurethanes. The key advantage, of course, is their ability to cure without the use of heat or other external form of energy. However, in many cases, several days of ambient exposure are required to achieve full properties since the rate of curing is dependent on the relative humidity and the thickness of the adhesive. If there is too much moisture on the surface, curing of polyurethanes occurs so quickly that... 

Water in the atmosphere is pure and free, and in a temperate country like the United Kingdom, the amount present, as expressed by the relative Humidity, remains fairly constant for most of the time. Cyanoacrylates, silicones and isocyanates are among the materials that may be available as one-part, moistnre-cnred adhesives. (Cyanoacrylate adhesives. Silicone adhesion). 

The properties of cyanoacrylate adhesives have been discussed in detail in the previous sections of this chapter. The evaluation of these adhesives is complemented in this section by comparisons with other structural adhesives. A specific comparison has been made of cyanoacrylate adhesives with a two-part epoxy, EC 2216 B/A, and a two-part polyurethane, EC 3549 B/A, all sold by 3M. The substrates on which the adhesives were tested were aluminum, polystyrene, and chloroprene. The results for room temperature water immersion, 90°F/90% relative humidity, and 150°F oven aging are shown in Tables XVI and XVII. 

The optimum cure condition for cyanoacrylates is when the relative humidity (RH) is between 40% RH and 60% RH. Lower RH, i.e., 20% RH, will result in a slower cure, and high RH (80% RH) result in a faster cure. A dry environment (low RH) often occurs when the work area heating system is switched on in the autumn months, thus drying out the local air adjacent to the work station. 

Cyanoacrylate polymers (Figure 8.2, Table 8.3 also see Figure A3.41) are polymerized in situ from monomers (Schoenberg, 1990). The reaction is initiated in the film of moisture on the surface. It is thus dependent on the nature of the surface and the relative humidity of the air. Alkaline surfaces such as glass promote rtqtid adhesion, whereas acid surfaces such as wood require polymerization activators. 

Cyanoacrylates are very dependent on the presence of small amounts of moisture on the surface and if the relative humidity in the working area is less than 25% RH, the cyanoacrylate will be slow to cure. Surface acidity can also interfere with the curing of cyanoacrylates (see Section 1.2). 

When a drop of cyanoaerylate adhesive is put on the surface of a part, the aeid stabilizer molecules react with the water molecules present on the surface of the part from the relative humidity in the air. The reaetion of the water and acid causes the aeid stabilizer to be neutralized. The cyanoacrylate moleeules then reaet with eaeh other and form polymer ehains without eross-linking. ... 

---