Cyanoacrylate adhesive

These adhesives became generally known as superglues, since they cure within a very short time (within a range of seconds). Even if the syllable cyano leads to the assumption that something toxic is involved, this is by no means the case owing to the chemical structure of these substances. However, as applies to all adhesives, certain precautionary measures in application should be taken, which will be separately discussed in Section 7.5. 

The consequence of the quick reaction with moisture is the adhesive s need to be stored in absolutely leakproof containers, mostly plastic bottles, to prevent a curing reaction in the bottle. 

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. 

Cyanoacrylate adhesives are applied in various fields of industry, for example, 

Cyanoacrylate adhesives are based on a special type of acrylic monomer. They are nominally esters of 2-cyanoacrylic acid  

Where R = an alkyl group, e.g., methyl, ethyl, or butyl, octyl, allyl, ethoxyethyl 

The electron-withdrawing (electrophilic) cyano group makes these monomers very susceptible to polymerisation by nucleophilic species, i.e., they polymerise anionically. Even very weak nucleophiles such as the OH ions from atmospheric moisture on surfaces are sufficient to polymerise the monomers in seconds if they are confined between two surfaces  

The polymerisation does not normally have a termination step. This is in contrast to most other anionic polymerisations, which are very sensitive to trace impurities such as components of the atmosphere or moisture (only strong acids are terminating agents) [46]. Other initiators such as tertiary amines and phosphines have been postulated to polymerise via polymeric zwitterions. This mechanism has been proved by the isolation of stable zwitterions [47]. 

Methyl cyanoacrylate was the first monomer to be used and was particularly useful for bonding metal substrates. It was largely superseded by ethyl cyanoacrylate in industrial and consumer products because of the versatility of the ethyl monomer in bonding a wide range of metals and plastics. 

The chemistry of cyanoacrylate adhesives contains no co-reactants but can polymerise at room temperature on any substrate that is exposed to atmospheric moisture or alkaline surfaces. Synthesised cyanoacrylate esters can be methyl, ethyl, n-propyl, n-butyl, allyl, ethoxyethyl and methoxyethyl. The basic structure of the cyanoacrylate monomer is  

The initiation reaction is the nucleophilic attack at the /i-carbon of the monomer to generate a carbanion which during the propagation stage reacts with the monomer and continues to chain transfer to termination  

Unreactive modifiers can be added provided they are very low in moisture or base interferences and in some cases are added as stabilisers. Other additives include inhibitors, thickeners, plasticisers, dyes or colorants and adhesion promoters. 

To monitor these concentrations of stabilisers for precise, accurate measurements a method must be available that must detect levels as low as 600 ngg-1 and as high as 5000 ngg-1. The allowed tolerance can vary between 20% for low levels to 10% for higher levels without affecting the performance of the product. The lower concentration would be expected to cure faster than those containing higher concentrations of stabiliser. The different level of stabiliser is dictated by the application of these adhesives and must be formulated under very controlled conditions. ICP-OES is an ideal tool to monitor the metal type stabilisers using sample dilution, bomb combustion or microwave acid digestion methods. 

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The diverse nature of the cyanoacrylate adhesives appHcations illustrates vividly that there is no truly typical appHcation. The number of appHcations ia which these adhesives are used is being expanded daily as technological improvements continue to broaden their capabiUties. 

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. 

Acryhc stmctural adhesives have been modified by elastomers in order to obtain a phase-separated, toughened system. A significant contribution in this technology has been made in which acryhc adhesives were modified by the addition of chlorosulfonated polyethylene to obtain a phase-separated stmctural adhesive (11). Such adhesives also contain methyl methacrylate, glacial methacrylic acid, and cross-linkers such as ethylene glycol dimethacrylate [97-90-5]. The polymerization initiation system, which includes cumene hydroperoxide, N,1S7-dimethyl- -toluidine, and saccharin, can be apphed to the adherend surface as a primer, or it can be formulated as the second part of a two-part adhesive. Modification of cyanoacrylates using elastomers has also been attempted copolymers of acrylonitrile, butadiene, and styrene ethylene copolymers with methylacrylate or copolymers of methacrylates with butadiene and styrene have been used. However, because of the extreme reactivity of the monomer, modification of cyanoacrylate adhesives is very difficult and material purity is essential in order to be able to modify the cyanoacrylate without causing premature reaction. 

Cyanoacrylate adhesives are particularly valuable because of their speed of action, which allows the joining of intricate parts without the need for complex jigs and fixtures. Within very broad limits the more monomer that is used to make a joint the less will be the strength. These adhesives have in fact no gapfilling ability, nor can they be used on porous substrates. Whilst they have good heat and solvent resistance their weathering behaviour is limited and joints should not be in frequent contact with water. 

Primers for cyanoacrylate adhesives, chlorinated polyolefin primers... 

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. 

Crosslinking has been claimed to improve thermal resistance of the cyanoacrylate adhesive [18]. However, in other reports [6], little or no improvement in thermal resistance of the adhesive was demonstrated by the addition of a difunctional monomer. As seen in Fig. 2, the addition of varying amounts of crosslinker 7 provided no improvement in the tensile adhesive strength of ethyl cyanoacrylate on steel lapshears after thermal exposure at 121 °C for up to 48 h. 

To minimize the gradual embrittlement that can occur on aging of cyanoacrylate adhesives, plasticizers are added. Some of the materials, which have been used as plasticizers, include phthalates, phosphonates, acyl esters, succinates, and cyano-acetates. The use of allyl, methallyl, and crotyl phthalates is also claimed to improve thermal resistance properties in addition to plasticizing the adhesive [23]. 

An unexpected benefit of the rubber addition to alkyl cyanoacrylate adhesives is... 

Care must also be taken in the choice of rubber to insure that the rubber, or one of its additives, does not initiate the premature polymerization of the monomer. Even very low concentrations of a basic or nucleophilic material in the rubber or elastomer will cause the premature polymerization of an alkyl cyanoacrylate adhesive formulation. 

Interestingly, this same effect has been observed for the addition of a rubber toughening agent to ethyl cyanoacrylate-based adhesives, as was reported previously. The rubber must contain enough latent acid functionality on the polymer backbone or in an additive to inhibit the thermally activated decomposition of the alkyl cyanoacrylate adhesive polymer. 

Because of the need for basic initiators, cyanoacrylate adhesives do not perform well on acidic surfaces, such as wood. However, the addition of sequestering agents, such as crown ethers [30], 10, or calixarenes [31], 11, and others [32] to the adhesive improves the reactivity of the adhesive on less active surfaces. 

Cyanoacrylate adhesives are ubiquitous with several major manufacturers around the globe. They include, along with their band names. Alpha Giken (Alpha Ace and Alpha Techno), Henkel/Loctite (Prism, Sicomet, Superbonder, and Quicktite), National Starch (Permabond), Three Bond (Super Three), and Toa Gosei (Aron Alpha and Krazy Glue). 

Despite the universal use of sutures for wound closure, there is a need to utilize adhesives instead, because of their ease of use and the reduced risk of infection. Alkyl cyanoacrylate adhesives have been studied extensively for this use, and a significant amount of research has been performed to evaluate their interaction with living tissue [40,41 J. They have been approved for external use only, because of concerns with the fact that the polymers do not readily biodegrade and can cause inflammation around the area to which it was applied. However, these concerns are reduced for -butyl cyanoacrylate, as compared to the ethyl cyanoacrylate. There is even some evidence that their use as liquid sutures actually reduces the rate of infection around the healing wound or surgical incision [42J. 

The author wishes to acknowledge his Loctite colleagues, many of whom have spent much of their careers advancing cyanoacrylate adhesive technology, for their help in the preparation of this chapter, especially John O Connor, John Woods, Shabbir Attarwala, and David Melody. He also wishes to thank librarian Margaret Orszak for her assistance with the literature search. 

O Connor, J. and Zimmerman, W., Factors affecting adhesion of cyanoacrylate adhesive to bright, anodized surfaces. Paper to American Electroplaters Society, Denver, CO, 1976. 

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. 

Cyanoacrylate adhesives, the famous consumer Super glue , is a monomer that polymerizes when it comes in contact with moisture, even with atmospheric moisture. 

The largest application of methyl and ethyl cyanoacetale is the production of the cyanoacrylate adhesives widely used within the ear and electronic industries. 

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