Cyanoacrylate acid stabilizers

To prevent premature polymerization, a strong protic or a Lewis acid is added to the distilled monomer and to adhesive formulations. A wide variety of materials have been utilized as acidic stabilizers in the alkyl cyanoacrylate monomers. A list of some of these materials is shown in Table 1 [2,11-14]. 

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. 

Alkyl cyanoacrylate monomers have been copolymerized with a variety of monomers, both by radical and anionic initiation. The radical-initiated copolymerization with acrylic monomers was performed with a sufficient amount of an acid stabilizer present to suppress polymerization by anionic means [19]. This investigation has been covered extensively elsewhere. 

Cyanoacrylates undergo easily anionic polymerization in the presence of weak bases, including H2O. They are used in commercial products (Super glue) mixed with traces of acidic stabilizers, which are neutralized in contact with various surfaces during the use. 

Cyanoacrylate monomers are generally low viscosity, colorless liquids that are unstable in the absence of acidic stabilizers. The commercially important materials are lower alkyl esters of 2-cyanoacrylic acid including methyl, ethyl, n-propyl, al-lyl, n-butyl, isobutyl, 2-methoxyethyl, 2-methoxypropyl, and n-octyl derivatives. The structures of some of these monomers are shown in Figure 1. The physical properties of these and related monomers are reported in detail in several previous... 

Cyanoacrylate monomers and polymers are routinely characterized by all common chromatographic and spectroscopic techniques including ir, nmr, ms, gc, sec, and uv analytical techniques. The levels of acid stabilizers in monomers are usually determined by potentiometric titration. Microstructural assignments of an-ionically and zwitterionically polymerized poly(ethyl 2-cyanoacrylate) have been unambiguously determined using DEPT and HETCORnmr analysis (59). 

For rapid fixture times, the adhesive used should be as fresh as possible. Old adhesive tends to be slower, as acidic stabilizers gradually hydrolize the monomer, forming carboxylic acids. To maintain the quality of the adhesive, bottles should be kept tightly closed and stored in a cool, dark location. Cyanoacrylates can be safely stored in most polyethylene, polypropylene, and aluminum containers. 

A cyanoacrylate adhesive is a very rapid curing adhesive, also from the acrylic family tree but having a completely different cure system. Cyanoacrylate monomer is made from a eomplex ehemical process. The monomer produces a very reaetive polymerization. The reaction or polymerization process is stabilized and the monomer kept in the liquid state by the addition of a small amount of an acid stabilizer material. ... 

When a drop of cyanoacrylate adhesive is put on the surface of a part, the acid stabilizer molecules react with the water molecules present on the surface of the part from the relative humidity in the air. The reaction of the water and acid causes the acid stabilizer to be neutralized. The cyanoacrylate molecules then react with each other and form polymer chains without cross-linking [13]. 

Cyanoacrylate adhesives undergo anionic polymerization in the presence of a weak base, such as water, and are stabilized through the addition of a weak acid. The stabilizer is usually in the form of a weak acidic gas such as SO2, NO, or BF3. An essential function of the stabilizer is to prevent polymerization in the container, which is usually made of polyethylene. When the adhesive contacts a slightly alkaline surface, trace amounts of adsorbed water or hydroxide ions (OH ) that are present on the substrate s surface neutralize the acidic stabilizer in the adhesive, resulting in rapid polymerization as shown in Figure 10.2. 

In order to become useful dmg delivery devices, biodegradable polymers must be formable into desired shapes of appropriate size, have adequate dimensional stability and appropriate strength-loss characteristics, be completely biodegradable, and be sterilizahle (70). The polymers most often studied for biodegradable dmg delivery applications are carboxylic acid derivatives such as polyamides poly(a-hydroxy acids) such as poly(lactic acid) [26100-51-6] and poly(glycolic acid) [26124-68-5], cross-linked polyesters poly(orthoesters) poly anhydrides and poly(alkyl 2-cyanoacrylates). The relative stabiUty of hydrolytically labile linkages ia these polymers (70) is as follows ... 

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-... 

Manufacture and Processing. The cyanoacrylic esters are prepared via the Knoevenagel condensation reaction, in which the corresponding alkyl cyanoacetale reacts with formaldehyde in the presence of a basic catalyst to form a low molecular weight polymer. The polymer slurry is acidified and the water is removed. Subsequently, the polymer is cracked and redistilled at a high temperature onto a suitable stabilizer combination to prevent premature repolymerization. Strong protonic or Lewis acids arc normally used in combination with small amounts of a free-radical stabilizer. 

The bonding of 2-cyanoacrylates to mineralized tissues In an aqueous environment appears to be superior to that of other adhesives. The higher homologues of 2-cyanoacrylates may be useful clinically where an intermediate-term bone adhesive Is desired. The isobutyl ester Is the most effective 2-cyanoacrylate for bonding dentin to acrylic resin. Pretreatment of the dentin with dilute acid, addition of 2-cyanoacrylate polymer to the adhesive or application of a protective coating to the bonded surface increases the hydrolytic stability of the bond. 

Since bases are catalysts for the curing reaction and acids are stabilizers for the cyanoacrylates, the pH value of the surface will control the cure speed. Surfaces that tend to be acidic will cure slowly compared to a neutral surface, which in turn will cure more slowly than an alkaline surface. In most applications the objective is to speed the cure therefore, all the commercially available activators are weak bases dissolved in a volatile carrier. Applying an activator to a surface places a layer of the weak base in position to initiate the cure. Since they are stronger bases than moisture, they are able to neutralize the stabilizer systems in the adhesive more effectively, and thus they can tolerate larger gaps than would be possible with moisture alone. In general, the fixturing time is 10 times faster with activator than without it. Even with activator, the effect of the gap is clear (see Table 4). 

Cyanoacrylates are mostly supplied in polythene containers as pure monomer, with a small amount of an acidic gas (e.g. SO2) added as stabilizer. Glass is not nsed as a container... 

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. 

One method to reduce the volatility of sulfur dioxide is to complex it with imidazole and then add the complex to the adhesive. Chelates of boric acid derivatives with polyhydroxy compounds have also been prepared as anionic polymerization inhibitors. The chelates can be preformed or prepared in situ by adding the boric acid compound and the polyhydroxy compound to the adhesive. Sulfamides have been used to stabilize cyanoacrylates. The sulfamides (24) in question are prepared by reacting sulfuryl diisocyanate (23) with active hydrogen compounds such as carboxylic acids, as shown in Eq. (7). ... 

Obviously a great deal of research effort has been directed toward developing stabilizers for cyanoacrylate adhesives. Two major types have evolved the volatile acid gases and the nonvolatile acids. These are used in varying concentrations, depending upon the type of stabilizer. Table III is a summary of the types of stabilizers used and typical concentrations. The end use of the adhesive also dictates the type and level of inhibitor used a fast-curing industrial ethyl cyanoacrylate adhesive will tolerate much... 

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