Cyanoacrylate slow cure

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... 

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). 

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. 

Slow cure speed. Use low bloom grade of cyanoacrylate (see 10.12.1). 

Slow cure. Check the cure time - cyanoacrylates require between 4 -24 hours to achieve full cure. 

ManuhKturers Comments Methyl cyanoacrylate. High viscosity. Slow cure. 

Manufecturers Comments Flexible filled cyanoacrylate. High viscosity. Slow cure. Meets the requirements of (USA) MIL-A-46050B. 

The cure speed of a cyanoacrylate, if left open on a surface (as on the left-hand side of Figure 1.2), will be relatively slow (several hours) because there is insufficient moisture (although the cyanoacrylate will cure at the surface interface). When the adhesive is between two close fitting surfaces (right-hand side of Figure 1.2), there is moisture on both surfaces and the cyanoacrylate will cure rapidly. 

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. 

Polystyrene Although polystyrene is usually bonded by solvent cementing, it can be bonded with vinyl acetate/vinyl chloride solution adhesives, acrylics, polyurethanes, unsaturated polyesters, epoxies, urea-formaldehyde, rubber-base adhesives, polyamide (Versamid-base), polymethylmethacrylate, and cyanoacrylates. The adhesives should be medium-to-heavy viscosity and room-temperature and contact-pressure curing. An excellent source is a Monsanto Company technical information bulletin which recommends particular commercial adhesives for bonding polystyrene to a number of different surfaces. Adhesives are recommended in the fast-, medium-, and slow-setting ranges (10). 

A variety of external plasticizers have also been suggested for cyanoacrylates. Among these are alkyl esters of aliphatic monocarboxylic acids such as cyanoacetates aliphatic dicarboxylic esters, such as malonates, adipates, and sebacates triaromatic phosphates such as tricresyl phosphate acyl triesters of glycerine dialkyl alkylphosphonates and alkyl phthalates. Joyner and Coover state that limiting the plasticizer concentration to less than 5% will not retard the cure speed, from 5 to 20% will retard the cure rate, and over 20% plasticizer concentration seriously slows the cure rate of the adhesive. 

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... 

Cyanoacrylates contain an acidic stabiliser, so if they come into contact with an acidic surface the stabiliser is not neutralised and the cyanoacrylate remains liquid. Some materials are naturally slightly acidic, e.g., paper, cardboard, cork and leather, and a standard cyanoacrylate will either be slow to cure or be completely inhibited from curing. 

Cyanoacrylate adhesives are extremely sensitive to traces of impurities, and must be manufactured, stored and used under controlled conditions. Basic impurities or contaminants can seriously affect the shelf-life or stability of the adhesives conversely, acidic materials can slow down or completely inhibit curing. Peroxides or free-radical stabilisers in a potential additive can also seriously affect performance. Because of the sensitivity to contaminants, it is not possible to formulate cyanoacrylates with the wide range of thickeners, fillers and other additives available to formulators of other adhesive systems. 

Increased thermal resistance was claimed via a two-stage cure mechanism, whereby thermally induced crosslinking occurs after anionic polymerization of the cyanoacrylate double bond. In practice, however, the crosslinking is very slow, and assembled parts might have to be supported until it takes place. 

The sensitivity of the cure of cyanoacrylates to various substrates, notably acidic surfaces which inhibit or slow the anionic cure, has also been a recurrent problem. This had been overcome by the use of various basic surface acti-... 

Cyanoacrylates are best suited where the gaps are small (<0.15 mm) and although some grades will fill bigger gaps there is always a risk of blooming (see Section 10.2.6) if the cyanoacrylate is slow to cure. 

Solvent cleaning with solvents such as isopropyl alcohol is generally a very acceptable cleaning method although it should be noted that sometimes these solvents can stress crack or craze some amorphous thermoplastics [7]. They can also remove all traces of moisture from the surface and this can slow down or even inhibit the cure of cyanoacrylates. 

Cyanoacrylates are very dependent on the presence of small amounts of moisture on the surface and if the relative humidity in the working area is less than 25% RH, the cyanoacrylate will be slow to cure. Surface acidity can also interfere with the curing of cyanoacrylates (see Section 1.2). 

Cyanoacrylate adhesive can be made from different acrylate monomers such as methyl, ethyl, butyl, isopropyl, and so on. These molecules differ in size and adhesives and exhibit different physical properties. Methyls are the smallest molecule and seem to work best on metal and rubber parts while ethyls work best on plastic parts. Many modifications can be made to the monomers to alter or improve their properties as adhesives. They can be toughened with rubber or formulated to have low odor, resistance to thermal cycling, or less sensitivity to surface conditions which tend to stabilize the adhesive and slow down the cure. ... 

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