Accelerators cyanoacrylate adhesives

Cyanoacrylate adhesives will bond a wide variety of substrates with the exception of polyolefins (unless pre-treated). Teflon and highly acidic surfaces. Porous substrates such as wood, paper and leather require the use of products containing accelerators. Formulations are now appearing that when used in conjunction with a so-called primer can give high bond strength on polyethylene and polypropylene. See Industrial applications of adhesives. 

More recently, the monofunctional cyanopentadienoates (2) were shown to improve the heat resistance of cyanoacrylate adhesives. In one case, when more than 50 weight % of 2 was combined with a standard cyanoacrylate monomer, the impact, peel, heat, and water resistance were greatly improved. " The improved heat resistance was attributed to the crosslinking of the residual olefin (28) after the anionic polymerization of 2 (see Eq. 8). The cyanopentadienoate was used in 50-90% concentration. The slow cure rate of the dienoate was overcome by the addition of an accelerator, in this case, a small quantity of an alcohol. Improvements in the heat, water, and water vapor resistance of alkyl cyanoacrylates can also... 

These acids can be used alone or as mixtures. It is especially advantageous to use a mixture of liquid and gaseous acids. The gaseous acid will stabilize free monomer in the headspace of a container, while the liquid acid will prevent premature polymerization of the bulk monomer or adhesive. However, it is important to use only a minimum amount of acid, because excess acid will slow initiation and the formation of a strong adhesive bond. It can also accelerate the hydrolysis of the alkyl cyanoacrylate monomer to 2-cyanoacrylic acid, which inhibits the polymerization of the monomer and reduces molecular weight of the adhesive polymer. While carboxylic acids inhibit the polymerization of cyanoacrylate monomer, they do not prevent it completely [15]. Therefore, they cannot be utilized as stabilizers, but are used more for modifying the reactivity of instant adhesives. 

Early demonstration that the absorption of poly(methoxypropyl cyanoacrylate) can be accelerated in the presence of hquid absorbable oxalate polymers led Shalaby to develop a new family of methoxypropyl cyanoacrylate (MFC)/polyester formulations as tissue adhesives with a broad range of properties. These formulations were tailored to produce absorbable tissue adhesives with a range of adhesive properties and compositionally controlled compliance depending on the type and content of the absorbable polyester component in the formulation. 

Cyanoacrylates are rapidly polymerized by nucleophilic species to give linear polymers with high molecnlar weights. Unlike other adhesives, cyanoacrylates consist of pure monomer in admixture with low levels of stabilizers, accelerators and modifiers. The patent literature abounds with references to many different monomers, but only a restricted group are used in connnercial products. The structures of the most common monomers are shown below... 

Initiators, accelerators, and inhibitors of cyanoacrylate polymerization are used to modify the cure speed and storage stability of these adhesives. They can also be used to broaden the range of materials which can be bonded with cyanoacrylates. Initiators are those materials which are capable of polymerizing cyanoacrylate esters upon contact. These are, therefore, applied either to the substrate surface ( surface primers ), or mixed with the adhesive just prior to application. Accelerators are materials which do not cause polymerization on contact with monomer, but which increase the cure rate once the adhesive is applied. These chemicals are most often compounded with the monomer in the adhesive formulation. The distinction between these two classes can be blurred, as some additions will not cause immediate polymerization on contact but will shorten shelf life in the long run. Anionic polymerization inhibitors are Lewis or Bronsted acids which retard or completely inhibit anionic polymerization. Radical inhibitors prevent polymerization by adventitious, radical sources and are used to prolong the storage stability of the adhesive they generally do not affect cure speed. 

Accelerators are substances which increase the rate of polymerization but are not initiators in their own right. Typically, accelerators are compounded in the adhesive, and do not significantly detract from storage stability. The first accelerators described in the patent literature were polyethylene oxides of degree of polymerization (DP) greater than three, and nonionic surfactants having a polyethylene oxide moiety, also with DP greater than three. These ethers are added to the cyanoacrylate in 0.5-10% concentration. The same authors have also patented the use of crown ethers such as 18-crown-6, typically in concentrations of 10 to 1000 ppm, as polymerization accelerators. A variation on the use of crown ethers involves the combination of a substituted furan (0.1-1% concentration) (17,18) with the crown ether (0.05-0.1% concentrations). ... 

Cyanoacrylates bond well to wood despite the porous and acidic nature of the material. A viscous adhesive cured with a surface activator will prevent the adhesive from wicking away from the bondline and overcome the natural acidity of the wood. The nature of the initiator influences the durability of the cured bond. Strongly alkaline initiators promote hydrolysis of the adhesive, thus shortening the useful lifetime of the bonded joint. The non-basic initiators described by Robins or the accelerators described in Section II.E.2. should be employed in wood bonding. The wood surface should be freed from grease and loose dirt before bonding. 

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