Cyanoacrylate poor adhesion

Durability is one of the most important aspects of the performance of a structural adhesive. The durability of an adhesive joint is the sum total of its responses to environmental effects such as heat, moisture, other chemicals, radiation, and mechanical stresses. Cyanoacrylate-based adhesives have a reputation for poor durability, especially when bonding metals... 

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. 

Cyanoacrylates are one-part, highly polar thermoplastic polymers. The resin monomers cure in seconds when in contact with a weak base such as the moisture that is present on most surfaces. Many cyanoacrylate-adhesive formulations are commercially available, but not widely used in electronics assembly because of their poor resistance to solvents and moisture at elevated temperatures (>70 °C). Cyanoacrylates have relatively low impact and peel strengths and may be brittle unless toughened by the addition of elastomeric resins. 

Ordinarily solvent cementing or thermal welding is used with PMMA. These methods provide stronger joints than adhesive bonding. Adhesives used are cyanoacrylates, second-generation acrylics, and epoxies, each of which provides good adhesion but poor resistance to thermal aging. "... 

Disadvantages of cyanoacrylate adhesives include poor thermal and moisture resistance on metals and glass, brittleness, sensitivity to surface preparation and poor cure through... 

There are several reasons why cyanoacrylates are attractive as adhesives. They are easy to apply, one-part, 100% reactive, storage-stable adhesives. They cure rapidly at room temperature when spread in thin films between substrate surfaces, and they form strong bonds between a variety of substrates. However, cyanoacrylates do have several serious shortcomings including poor heat resistance, poor moisture resistance, poor peel and impact resistance, and limited ability to fill gaps and to bond porous substrates. The poor durability and impact resistance have been particular limitations in metal-to-metal bonding. 

The moisture resistance of cyanoacrylate adhesives, like the heat resistance, is poor, and the problem is particularly apparent on metals. A cyanoacrylate bond responds in several ways to a moist environment. Polyalkyl cyanoacrylates are very susceptible to alkaline hydrolysis, which degrades the molecular weight rapidly (see Section II.F.). Corrosion products generated at the interface may catalyze this hydrolysis. Loss of adhesion due to displacement of the adhesive from the adherend by moisture is a common occurrence with many adhesives. Evidence that this mechanism... 

Vitreous materials and ceramics respond best to cleaning with water-based cleaners followed by water rinsing and drying. The alkalinity of these substances makes them poor candidates for cyanoacrylate bonding because the durability is so poor. Also, thin adhesives tend to wick into the porous surface of the ceramic before curing, further weakening the bond. 

Durability. Cyanoacrylates suffer from poor heat and moisture durability. This failing is pronounced on metal adherends, but minimal on most plastic or rubber adherends. Poor heat resistance is due to several causes the thermoplastic nature of the polycyanoacrylate, the tendency to retropoly-merize, and the loss of adhesion experienced on heat aging of cyanoacrylate bonds. The poor moisture resistance is due in part to the hydrolytic degradation of the polymer and in part to the loss of adhesion caused by exposure to moisture. 

High RH can also be detrimental as the cyanoacrylate can sometimes cure so fast that the adhesive cures before it has properly adhered to the surface and the resulting bond is poor. 

The cyanoacrylate adhesives are more rigid and less resistant to moisture than acrylate acid diester adhesives. They are available only as low-viscosity liquids that cure in seconds at room temperature without the need of a primer. The cyanoacrylate adhesives bond well to a variety of substrates, as shown in Table 7.25, but have relatively poor thermal resistance. Modifications of the original cyanoacrylate resins have been introduced to provide faster cures, higher strengths with some plastics, and greater thermal resistance. 

Cyanoacrylates are probably the most versatile adhesives for plastics with excellent adhesion to most plastics. Limitations include gapfilling only to 0.5 mm, temperature resistance to -100 "C and poor impact resistance. 

Anyone who has used cyanoacrylates on a continuous basis in an area with poor ventilation is familiar with the pungent, irritating, acrylic-type odor. The relatively high vapor pressure of the cyanoacrylate monomers which contributes to the odor can also cause evaporation from uncured fillets the vapors precipitate as a white haze on surfaces adjacent to the bondline. On bright, decorative, or transparent parts this may be unacceptable, often requiring the use of an alternative adhesive. Good ventilation helps, but not universally. This has led to the development and marketing of alkoxyalkyl cyanoacrylate adhesives, which structurally are merely ethyl cyanoacrylates with methoxy or ethoxy... 

Other examples are less obvious and may go unrecognized. For example, cyanoacrylate adhesive is a solvent for some polymer surfaces, and may actually dissolve and mix with some of the surface before it hardens. As a result, the bonds exhibit excellent durability toward moisture and other environmental stresses, seemingly in contradiction to the adhesive s poor reputation for service under these conditions. The interpenetration provides durability which is absent when the same adhesive is used on an inorganic substrate. 

EPDM can be difficult to bond (Table 4.4) and many cyanoacrylates will not show particularly good adhesion to EPDM. However, some speciality grades of cyanoacrylate will show good adhesion to EPDM without the use of primer. The silicones, UV acrylics and two-part acrylics all show relatively poor performance on EPDM. 

One of the benefits of adhesives is that they will fill gaps between poorly fitting parts or parts with large manufacturing tolerances. The cure of epoxies is not affected by the gap between the parts but cyanoacrylates are very much driven by this gap and are limited to gaps of less than 0.2 mm. The gap therefore has an important bearing on the family of adhesives that will be best suited for the application. 

Tlie distinction between structural and nonstructural bonds is not always clear. For example, is a hot melt adhesive used in retaining a fabric s plies structural or nonstructural One may argue that such an adhesive can be placed in either classification. However, the superglues (cyanoacrylates) are classified as structural adhesives even though they have poor resistance to moisture and heat. 

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