Cyanoacrylates bloom

Figure 10.4 When a cyanoacrylate blooms , the adhesive monomer escapes from the bond line and cures on an adjacent surface...

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 have been used in industry for many years with an excellent safety record. Vapour problems will not normally be of concern in a well ventilated working area, and can be eliminated with the use of low odour/low bloom cyanoacrylates. 

Sometimes cyanoacrylates will form a white powder adjacent to the bond line. This is known as blooming. This section explains the reason for blooming and suggests various options to eliminate it from the production line. Also included are general guidelines for troubleshooting an application. 

Blooming occurs when cyanoacrylate molecules escape from the main body of the adhesive and react with water vapour in the surrounding air. The molecules of cyanoacrylate cure and then fall to the adjacent surface as a white powder. Even with rubber toughened cyanoacrylates, (which are often black in colour) it is the monomer which escapes and the deposit will still be white (see Figure 10.18). 

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). 

Blooming does not always occur during the first few seconds, indeed it is more likely that parts will bloom some hours after assembly (up to 24 - 48 hours later). A bonding application is often one of the last operations in a production cycle and care is required to ensure that parts are not put straight into a sealed (or semi-sealed) container immediately after cyanoacrylate bonding as there is a risk that the cyanoacrylate will bloom in the box resulting in a poor aesthetic appearance to the end product. Trials should be conducted accordingly. 

Use a heavy molecular weight cyanoacrylate (low bloom product)... 

The heavy molecular weight cyanoacrylates are ideal for applications where bonds must be cosmetically perfect, or for delicate electrical and electronic assemblies. The additional low odour characteristic of these cyanoacrylates is ideal where operators are required to work in confined imventilated spaces. The low bloom cyanoacrylates are also slower curing than standard ethyl or methyl grades which means that more time is available to assemble parts, where careful alignment is required. 

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

Cyanoacrylate One-component, very fast curing, low viscosity, high strength on most plastics Poor gap filling, cannot autoclave, blooming, can stress crack some plastics... 

These monomers are virtually odorless and have much lower vapor pressures so that the blooming characteristics are greatly reduced or eliminated. Performance is similar but not equivalent to that of the lower methyl and ethyl esters. On metals or rubber the cute speeds are quite fast, but the cure speeds on plastics were slower than conventional cyanoacrylates. Updated, faster curing versions of these products are becoming available. As costs of production for these adhesives are reduced, and the curing properties improved, they are expected to replace the lower esters in many applications. 

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). 

Cyanoacrylates should be cured quite quickly otherwise this may result in blooming (see Section 10.2.6). 

Figure 10.5 On the left-hand plastic extrusion, the cyanoacrylate has bloomed...

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. 

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