6.2.2 Two-Step, No-Mix Acrylics

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.

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