Curing of Cyanoacrylates

The fast cure of cyanoacrylates can in many applications offer benefits to the production engineer, as process times will be fast and clamp times are usually short. The thinner the bond line the faster the cure. Note however that the joint must not be disturbed during the cure cycle as the cyanoacrylate will not properly re-configure itself if the molecular chains are broken at the start of the polymerisation. 

Cyanoacrylates are best suited to the smaller bond areas in rubber bonding applications, as larger areas often require more time to position the parts, and the fast cure of cyanoacrylates make this difficult. Typical applications are illustrated in Section 10.11. 

Since the anionic cure of cyanoacrylates proceeds as a result of basic catalysis, acids should naturally be considered as stabilizers. Acids of either the Lewis or protonic types have been used successfully. 

The sensitivity of the cure of cyanoacrylates to various substrates, notably acidic surfaces which inhibit or slow the anionic cure, has also been a recurrent problem. This had been overcome by the use of various basic surface acti-... 

Water vapor from the atmosphere is the curing agent for some silicone and isocyanate adhesives. Adsorbed surface water causes the cure of cyanoacrylates. Such adhesives are often packed in polyolefin containers, which are naturally permeable to water. Hence, the passage of water through the container wall has the possibility to shorten shelf life. 

Solvent cleaning with solvents such as isopropyl alcohol is generally a very acceptable cleaning method although it should be noted that sometimes these solvents can stress crack or craze some amorphous thermoplastics [7]. They can also remove all traces of moisture from the surface and this can slow down or even inhibit the cure of cyanoacrylates. 

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). 

Cyanoacrylate adhesives (Super-Glues) are materials which rapidly polymerize at room temperature. The standard monomer for a cyanoacrylate adhesive is ethyl 2-cyanoacrylate [7085-85-0], which readily undergoes anionic polymerization. Very rapid cure of these materials has made them widely used in the electronics industry for speaker magnet mounting, as weU as for wire tacking and other apphcations requiring rapid assembly. Anionic polymerization of a cyanoacrylate adhesive is normally initiated by water. Therefore, atmospheric humidity or the surface moisture content must be at a certain level for polymerization to take place. These adhesives are not cross-linked as are the surface-activated acryhcs. Rather, the cyanoacrylate material is a thermoplastic, and thus, the adhesives typically have poor temperature resistance. 

Cyanoacrylate adhesives cure by anionic polymerization. This reaction is catalyzed by weak bases (such as water), so the adhesives are generally stabilized by the inclusion of a weak acid in the formulation. While adhesion of cyanoacrylates to bare metals and many polymers is excellent, bonding to polyolefins requires a surface modifying primer. Solutions of chlorinated polyolefin oligomers, fran-sition metal complexes, and organic bases such as tertiary amines can greatly enhance cyanoacrylate adhesion to these surfaces [72]. The solvent is a critical component of these primers, as solvent swelling of the surface facilitates inter-... 

Presence of Water on Surfaces to be Bonded. Substances in the Earth s atmosphere adsorb water from it, the water in many instances being bound tenaciously to the surface. This provides the basis for curing of the cyanoacrylate adhesives, which takes place on contact with surfaces covered with such a water layer. 

The cured 2-cyanoacrylic ester polymers are relatively nontoxic. Oral doses of 6400 mg /kg failed to kill laboratory rats. Mild skin irritation was observed widi guinea pigs, but diere was no evidence of sensitization or absorption through die skin (15). 

Both, the liquid and cured 2-cyanoacrylic esters support combustion. These adhesives should not be used near sparks, heat, or open flame, or in areas of acute fire hazard. Highly exothermic polymerization can occur from direct addition of catalytic substances such as water, alcohols, and bases such as amines, ammonia, or caustics, or from contamination with any of the available surface activator solutions. 

At this point, an important difference compared to moisture curing, one-component polyurethane adhesives described in Section 4.2.2 has to be mentioned. While for cyanoacrylates already small traces of moisture are sufficient for a quick polymerization, polyurethanes require a considerably higher moislure amount for complete curing, because in such bonds, water in chemically bound form becomes a component of the adhesive layer. In the case of cyanoacrylates, water is merely the starter of the curing reaction. 

The initiator that drives the polymerization or curing reaction of cyanoacrylate adhesives can be any of the chemicals that generate free radicals. Electromagnetic... 

In general, standard industrial cyanoacrylates do not operate effectively above 180°F (see Fig. 4). However, the new allyl types of cyanoacrylates can operate as high as 480°F before the bond loses sufficient strength to be operationally effective (see Table 2). Allyl cyanoacrylates for metal-bonding applications have proven effective in wave solder and under-hood (automotive) applications. In Fig. 4, bonded assemblies are cured at room temperature for 24 h. The assemblies are heated for 2 h and tested hot. 

The use of one-component organic systems for impregnating structures is attended by considerable difficulties for example, the cost of cyanoacrylate monomers is high they can be used only for the impregnation of absolutely dry structures they are quickly polymerized and hence cannot penetrate deeply into materials. The use of radiation for acrylate polymerization is hazardous, expensive, and technicsdly complicated. When polyisocyanates cure by interaction with water, carbon dioxide is liberated and comes out to the siuface where it forms continuous pores ... 

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... 

A common method of wire-tacking today is to use cyanoacrylate adhesive and primer. In this process, a drop of cyanoacrylate is placed on top of the wire to be tacked, followed by a drop of primer. The resulting wire-tack is usually serviceable in 5 - 15 seconds although several hours can be required for full cure. The usage of the wire tack, although limited by a lack of moisture, temperature and impact resistance, has been finding increasing application in the electrical industry. 

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