Cyanoacrylates excess adhesive

Goatskin Model A layer of paraffin wax was molded into the bottom of polypropylene containers. The goatskin was dissected from the subcutaneous tissue, stretched to its original dimensions, and pinned onto the wax layer. The container was then filled with a saline solution (0.9% w/vol. NaCl and 0.05% w/vol. NaNg) until the skin was totally submerged. The sample was stored in a freezer at -10°C. Prior to use, the sample was removed from the freezer and thawed at room temperature. The saline was poured from the container. A lint-free tissue was used to remove excess moisture. A 19-cm incision was made down the length of the sample. The skin around the incision was approximated and 400 pL of a cyanoacrylate tissue adhesive... 

Cellulosic materials can be mechanically fastened by a number of methods. However, their rigidity and propensity to have internal molding stresses must be carefully considered. The adhesives commonly used are epoxies, urethanes, isocyanate-cured polyesters, nitrile-phenolic, and cyanoacrylate. Only cleaning is required prior to applying the adhesive. A recommended surface cleaner is isopropyl alcohol. Cellulosic plastics may contain plasticizers. The extent of plasticizer migration and the compatibility with the adhesive must be evaluated. Cellulosics can be stress cracked by uncured cyanoacrylate and acrylic adhesives. Any excess adhesive should be removed from the surface immediately. 

Application of cyanoacrylates to the bonding surface is simple, for these are one-part, 100% reactive adhesives. The adhesive is applied as a drop or bead to one surface, then the other adherend is used to spread the adhesive in a thin film. Due to the volatility of the monomer and the fast cure, no more than three or four square inches of bondline should be open at one time. Enough adhesive should be applied to provide a slight fillet when the adherends are mated. This will ensure that the adherends are thoroughly wetted and that any air bubbles are removed. On impervious, well-fitted substrates, two drops (25 mg) of adhesive per square inch is sufficient coverage. Too much adhesive will create a weak bond, as the surface-initiated cure may not extend throughout a thick adhesive layer. Also, excessive adhesive can wash the initiators out of the bondline, causing a slow or incomplete cure. 

If excessive adhesive is applied, there is insufficient surface moisture and the cyanoacrylate will be slow to cure and may bloom (see Section 10.12.1). The adhesive should be applied... 

When excess adhesive has been applied, the surface to volume-of-adhesive ratio is too low and the moisture on the surface will be insufficient to neutralise the stabiliser in the adhesive. The cyanoacrylate vapour will escape and fuming will occur. Reduce the quantity of the adhesive by using fine bore application nozzles and/or dispensing equipment. 

In the same way as excess adhesive can cause blooming, a slow cure may give a similar result. The cyanoacrylate at the periphery of the joint will search for available moisture from the surrounding air and may then cure as a white powder on the adjacent surface. A slow cure may be the result of excess adhesive, but is also likely to be caused by acidic deposits on the substrate. These acidic deposits can cancel ont the nentralising effect of the initiators (moisture) and result in very slow polymerisation or in some cases inhibition of cure completely (see also Section 10.4.1). 

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. 

Ethicon, Inc., (Somerville, NJ) vendors Dermabond Topical Skin Adhesive for closing surface or skin type wounds. This material is not intended for internal use. Closure Medical Corporation manufactures the actual adhesive. Ethicon, Inc. is a Johnson Johnson company. Dermabond is chemically 2-octylcyanoacrylate and it cures on contact with water, ethanol and others (i.e., addition polymerization and catalysis by weak bases). For internal tissues, the excessive moisture can overcure cyanoacrylate by saturating its surface with moisture, cause disbondment from the substrate, and render it useless as an adhesive. The cured polymer possesses a hard and brittle consistency. 

The modification of cyanoacrylate with other polymers and/or plasticizers provided more flexible adhesives, but the lack of bond and disbondment from excessive moisture continued to float off the adhesive without making firm contact with the tissue. 

The most suitable in any particular instance is best established by trials in advance. Sometimes, fairly severe abrasion (by increasing the surface area available) is the most successful. On the other hand, excessive roughening may weaken the substrate and so give inferior results. With fast-setting adhesives like the cyanoacrylates, microscopic bubbles of air may be trapped between adhesives and substrate and so impair bonds. 

Another reason for adhesive failure might be excessively fast cure. Cyanoacrylates will sometimes cure so rapidly on an alkaline surface that they polymerise before they have a chance to properly adhere to the surface. A glazed or glossy appearance to the failed cyanoacrylate is often an indication that the adhesive has cured too quickly. Plated metals sometimes have traces of alkalinity remaining on the surface and washing with an aqueous cleaner can rectify the situation. 

---