6.2.1 Cyanoacrylates

Cyanoacrylate (CA) adhesives were first created in 1958 by Eastman Kodak, but were not fully commercialised until the 1970s. The delay in commercialisation was due to the difficulty in manufacturing and packaging such reactive materials in a way that ensured that the product remained liquid. Eastman Kodak licenced the technology to LOCTITE^® in the 1970s, and with some development work on stability to improve upon the technology, a product with commercially viable shelf life was released.

The cure reaction associated with these adhesives is so rapid that they are referred to as ‘instant adhesives’ or ‘superglue’. When pressed into a thin bond line between two surfaces, CAs cure rapidly to form rigid thermoplastics with excellent adhesion to most substrates.

6.2.2 Two-Step, No-Mix Acrylics

Two-step or no-mix acrylic adhesives consist of a resin and an activator. The resin component is a solvent-free, high-viscosity liquid, while the activator is a low-viscosity liquid catalyst. Typically, the activator is applied to one of the substrates to be bonded while the resin is applied to the other. Upon mating the two substrates, the activator meets the adhesive and catalyses the breakdown of peroxide in the adhesive to form free radicals. These free radicals then cause the adhesive to polymerise to a thermoset plastic. The key here is that the resin and activator do not start reacting until the two substrates are mated, highlighting the key attribute of this adhesive system: practically unlimited open time.

Considerations for ‘activator’-only selection

There are different types of activators available for use with 2C and two-step no-mix acrylic adhesive systems. Generally, activator selection is based on four criteria:

• Fixture time (for two-step no-mix acrylics) – a measure of how quickly the adhesive cures.
• Bond strength – the ultimate strength of the bond, which is heavily affected by the type of activator chosen (the activator can also affect the environmental durability of the bond).
• On-part life of activator – the useful life of an activator once it has been applied to a part. On-part life can range from 30 minutes to 30 days. The longer the on-part life of the activator, the easier it is to integrate its use into the manufacturing process.
• Activator form – one of the three forms in which activators are supplied:
  • Active ingredient dispersed in a volatile solvent
  • Active ingredient dispersed in a non-volatile solvent
  • 100% solids formulations containing no solvents.

Each form has unique processing and economic demands that must be considered to identify the optimum solution for each application.

Considerations for using volatile activators

These activators are typically applied to one surface, the solvent evaporates and the activated surface is mated with the surface which has resin dispensed on it. This rapid evaporation minimises the time required between the activator dispensing step and the parts mating step. Additionally, since the active ingredient is present at low levels, it is very difficult to apply too much of it.

The main limitation of these systems is the flammability of the carrier solvent. There are activators formulated with non-flammable carrier solvents. However, the cost of these activators can be prohibitive.

Proper precautions must be taken while using volatile activators. In some cases, explosion-proof dispensing equipment must be used. Irrespective of flammability, local regulations may consider the solvent as VOCs and limit their release to the environment. Additionally, proper ventilation is needed to ensure that the solvent level in the work environment does not present a health hazard.