Cyanoacrylates curing

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

Salerni, C. M., Light-Cured Cyanoacrylates An Adhesive Option for Medical device Assembly, MD DI, Jun. 2002. 

For this study, 250 p-thick circular discs, each having a diameter of 15 mm, were prepared by pouring the cured cyanoacrylate into a plastic template submerged in a phosphate buffer solution at pH = 7.4 and 37°C. The cured discs were dried and their dry weight was determined. Weight was determined periodically after incubating in a phosphate buffer at 37°C and pH = 7.4. Using titis method, it was shown that MFC and V-200 lost 55 3% and 80 4% of their initial mass at 75 days. These results are consistent with the thesis that the oxalate polymer modifier accelerates the absorption rate of polymeric MFC. 

When fully cured, cyanoacrylates will meet class VI standards for plastics, the highest class of safety and biocompatibility as defined by the United States Pharmacopoeia (USP). 

A low-toxicity, low volatility solvent for general cleaning applications. Especially suited for use in cleaning of uncured polymers, such as epoxies, urethanes, and silicones. Also very effective for dissolving cured cyanoacrylate instant glues. 

The mechanism of cure is quite different for cyanoacrylates on one hand and silicones and isocyanates on the other. In the former case, cure is by Chain polymerization initiated by water on the surface of the substrate, while in the latter, water from the atmosphere diffuses into the adhesive or sealant and then participates in a Step polymerization. One consequence of this is that only catalytic qnantities of water are reqnired to cure cyanoacrylates, while much larger, stoichiometric quantities are needed for silicones and isocyanates. 

Another way to view this phenomenon is to consider the curing cyanoacrylate as a plasticizer-polymer mixture in which the unreacted monomer is the plasticizer. The Tg of a plasticized polymer is lower than that of unplasticized polymer, and the Tg drop is governed by the amount and efficiency of the plasticizer. In the present case, even if the polymer portion of the mixture has reached its ultimate Tg, the remaining monomer acts as a plasticizer that lowers the Tg to a value about 30 C above Tcure-... 

UV curing cyanoacrylates are often an excellent alternative option if the use of activators is undesirable (see also Section 10.4.4). 

One of the major benefits of the pinch valve is that the fast curing cyanoacrylate is not in contact with any moving parts. These valves have been used in a wide range of production... 

Slow cure may also be due to a thick bond line (adhesive >0.2 mm). Cyanoacrylates are most suited to applications where the bond line is less than 0.1 mm thick although a cure through a volume up to several mm is possible using UV curing cyanoacrylates. 

Solvent resistance is typical of polar, linear high polymers. In nonpolar solvents, attack is negligible, while solvents of similar solubility parameter will weaken cured cyanoacrylate bonds slowly. (See Table 3). Moisture resistance of cyanoacrylates is not considered to be a strong point of these adhesives, however, with proper attention to adhesive and substrate composition, excellent bonds are achievable. 

UV-curing cyanoacrylates - these products cure by surface moisture. 

PVC is readily bonded by most engineering adhesives (Table 2.18) although it can be stress cracked by some slower curing cyanoacrylates. 

Most engineering adhesives will bond readily to thermoset polyesters - often achieving substrate failure on test pieces (Table 3.4). Roughening the surface is usually beneficial, especially with the faster-curing cyanoacrylate adhesives. 

In these applications a variety of adhesives are used including UV cure, cyanoacrylates and two-part adhesives (epoxies or acrylics) and the adhesive is selected for its clarity, cost, speed of cure and ease of use. 

The gap between the parts and therefore the thickness of the adhesive film has an important bearing on the characteristics of the joint [4], A thick bond line (>0.25 mm) will generally be a weakening feature for cyanoacrylates as the mechanical strength of the cured cyanoacrylate film is likely to be less than the plastic or other substrate... 

Slow cure can also be overcome by using an activator (or accelerator). The activators increase the level of initiators on the surface to negate the stabiliser and thus increase the speed of polymerisation. UV-curing cyanoacrylates have also been used in applications to accelerate the cure speed and thus eliminate the possibility of blooming. 

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. 

Shannahan M. Adhesive Innovation Light cure cyanoacrylates technology. Loctite Corporation, 2014. 

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