Cyanoacrylate, curing

Cyanoacrylate Cures in 30-60 s Incompatible materials Rigid/ inflexible Disbondment with excessive moisture... 

Two of the most difficult types of adhesives to dispense are Cyanoacrylate adhesives and Anaerobic adhesives. Cyanoacrylates cure by interaction with the surface moisture on a component part. Anaerobics remain liquid in the presence of oxygen but automatically cure once enclosed in a joint. 

Aluminum can be cleaned in the same fashion as steel. Alternatively, it may be prepared for bonding by chemical etching. For instance, cyanoacrylates cure rapidly to give strong, durable bonds on aluminum etched by Forest Products Laboratory (FPL) chromic acid etching solution. 

Contrary to popular understanding, superglue does not"air dry." In fact, cyanoacrylates cure (convert from liquid to solid) in the presence of weak nucleophiles such as water. Under normal circumstances, a thin layer of water is present on almost all surfaces. The curing process, therefore, involves the reaction shown here. 

Figure 10.3 When the adhesive is between two close fitting surfaces the cyanoacrylate cures rapidly due to the surface moisture on both surfaces...

Cyanoacrylates cure quickly and care should be taken during the selection process to ensure that sufficient time is made available to position parts prior to the adhesive curing. 

Blooming occurs when cyanoacrylate molecules escape from the main body of the adhesive and react with water vapour in the surrounding air. The molecules of cyanoacrylate cure and then fall to the adjacent surface as a white powder. Even with rubber toughened cyanoacrylates, (which are often black in colour) it is the monomer which escapes and the deposit will still be white (see Figure 10.18). 

Cyanoacrylates cure to thermoplastic polymers and typically give very limited high-temperature performance. The Tg of polyethyl cyanoacrylate is -100 °C, and bonds at temperatures close to this fail dramatically. Early attempts to improve heat resistance involved the development of cyanoacrylates with an allylic group in the... 

The lower esters of the alkyl cyanoacrylates cure to clear, hard resins which exhibit good adhesion and tensile shear strengths to a wide variety of substrate combinations. They are, however, brittle and exhibit low peel and impact strengths. Higher viscosity versions of the lower esters filled with 5-10% of poly(methylmethacrylate) resins are somewhat less brittle and can offer double the impact strength of low viscosity monomeric adhesives, as well as significantly better peel strengths. 

Table 2. Cured Bulk Properties of Common 2-Cyanoacrylic Esters...

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-... 

Acrylic adhesives cure by a free radical chain growth mechanism. In contrast, epoxy and urethane adhesives cure by a step growth mechanism. This has a major impact on the cure kinetics, as well as the composition of the adhesive during cure ([9], pp. 6-9). Cyanoacrylate adhesives (such as Super Glue ) also cure by chain growth, but the mechanism is ionic with initiation by surface moisture. 

Ethyl cyanoacrylate is the monomer which is most widely used in both consumer and industrial applications, because of its combination of fast cure speed and ease of manufacture. 

Cyanoacrylates. Fast curing cyanoacrylates are used for holding shims in place during assembly of components and other applications where fast cure is useful. 

Cyanoacetic acid, 2 138, 139 and esters, 2 7 244-245 Cyanoacrylate adhesives, 2 539-540 Cyanoacrylate vapors, 22 102 Cyanobacteria, in nitrogen fixation, 2 7 302 Cyanobacterial associations, in nitrogen fixation, 27 299-300 Cyanocobalamin, 7 238 25 803-804 Cyanoethene. See Acrylonitrile (AN) l-Cyanoethyl-2-ethyl-4-methylimidazole (2EMZ-CN) curing catalyst, 20 17 2V-Cyanoethylated toluenediamines, 25 197... 

The adhesion was measured by fastening a lead anchor of known area (2,84 cm ) to the coating with a cyanoacrylate adhesive (Loctite 414) and after curing, pulling it off normal to the surface with a Dillon tensile tester. The force to remove the coating was divided by the area of attachment to convert it to a normalized tensile adhesion value. 

Ihe polymers of the 2-cyanoacrylic esters, more commonly known as the alkyl 2-cyanoacrylates, are hard glassy resins that exhibit excellent adhesion to a wide variety of materials. The polymers are spontaneously formed when their liquid precursors or monomers are placed between two closely fitting surfaces. Tile spontaneous polymerization of these very reactive liquids and the excellent adhesion properties of the cured resins combine to make these compounds a unique class of single-component, ambient-temperature-curing adhesive of great versatility (Table 3). The materials that can be bonded run the gamut from metals, plastics, most elastomers, fabrics, and woods to many ceramics. 

Both the liquid and cured 2-cyanoacrylic esters support combustion. 

Gelatin-resorcin-formaldehyde adhesives (GRF) have been studied 38-40>. These adhesives compared with cyanoacrylates have an advantage in that they join tissues reliably under conditions of a moist operative area and undergo rapid degradation in the organism. The rate of curing is adjusted easily by changing the ratio of components. 

Fig. 2.13 The three basic steps in producing a polymer from the cyanoacrylate monomer via anionic polymerization (moisture curing)...

Polycyanocrylate - 2-Octyl-cyanoacrylate plasticized and self-stabilized is the slower, but slightly more flexible (octyl- vs. ethyl-substituent group) ionically curing adhesive that is catalyzed by weak bases including water. The liquid and nonsolvated DermabondR product forms a continuous film in -3-5 min that is continuous and very adherent to tissue. 

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