Slow Cure

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 may also be caused by acidic deposits on the substrate. These acidic deposits can cancel out the neutralising effect of the initiators (moisture) and result in very slow polymerisation or in some cases inhibition of cure completely.

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.

Slow cure may also be due to a thick bond line (>0.2 mm). Cyanoacrylates are most suited to applications where the bond line is less than 0.1 mm thick, although a deeper cure depth is possible using activators (or through the use of UV-cure grades).

Plasma treatment of a plastic surface can remove all the surface moisture and parts may need to be left to re-acclimatise prior to bonding with cyanoacrylates. Low Relative Humidity

Low relative humidities (less than 20% RH) will also encourage the formation of blooming and irritant fumes because there is less moisture on the surface to initiate the cure.

Best results are obtained when the relative humidity is between 40% and 60% RH. Higher humidity will accelerate the cure process but could affect the final bond strength and in some cases increase the risk of blooming. The use of activators can considerably assist the cure but placing a container of water adjacent to the workstation has been known to increase the local relative humidity.

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 semi­sealed) container immediately after cyanoacrylate bonding as there is a risk that the cyanoacrylate will bloom in the box, resulting in a poor appearance to the end-product. Blooming does not affect the strength of the bonded assembly and is usually only aesthetic. However, in some applications, e. g., the bonding of a rubber ring seal near an infrared sensor housing, blooming may occur on the surface of the sensor thus rendering it inoperative. In this case the use of a low-bloom product should eliminate the possibility of blooming.

In summary therefore there are three methods of overcoming blooming:

• Avoid excess adhesive

The use of dispensing equipment and ensuring that the minimum quantity of adhesive required to fill the joint is applied will give best results.

• Ensure fast cure

The longer the cyanoacrylate remains liquid the greater the risk of blooming. Designing assemblies with close-fitting parts and therefore achieving thin bond lines will increase the speed of polymerisation and will decrease the risk of blooming. The use of activators or a UV-curing grade is recommended for the fast cure of exposed fillets of cyanoacrylates after assembly.

• 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 unventilated 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.

The third (and probably the most common) failure mode is when the adhesive does not adhere properly.

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