Triggering Systems

In polymer science, initiation is the process of bringing monomers into reaction to create polymer chains or networks. In the manufacture of bulk polymers or plastics by experts, this initiation is readily controlled, but for adhesives, sealants and coatings, it is in the hands of non-experts or end users, who just need the product to work.

In these situations, ensuring that initiation happens in the way intended every time is determined by careful tailoring of the ‘cure engine’ – a combination of small molecules such as redox pairs, acid–base pairs, and photoinitiator–synergist combinations. The cure engine is at the heart of formulation design, since it determines how a reactive composition can remain dormant, yet suddenly (and easily) be triggered into action on demand, often to the delight of the unconcerned end-user.

I have a lot of experience constructing these cure engines, and producing very appealing product functions and properties as a result. I hope the examples below will help to explain their value.

Cure engines that promote surface-based cures are dominated by acrylic applications, largely because the process of generating initiators is well-understood. But could an acrylic cure engine be ‘tricked’ into curing epoxies? 

During my time at Henkel, the solution we came up with was to introduce an intermediate step, in which the radical-based acrylic polymerisation initiator reacts with a ‘transformation initiator’ (an example of which is shown in the diagram) to trigger the release of a cationic initiator that is suitable for epoxies. The result is surface-initiated epoxy polymerisation, described in this patent. A paper in Polymer describes a similar system, in which the initial radicals are generated by light rather than from a surface.

Designing the cure engine is an important aspect of creating new polymerisation systems. For example, the light-curing cyanoacrylates (CAs) described earlier tend to be rather slow to work at room temperature with radicals, especially with visible light.

The solution is to use a formulation-specific redox-active component (sometimes referred to as a synergist) to speed up the process, and in this case while I was working at Afinitica we used a metallocene to produce an initiating species from the initially produced radicals. Further details can be found in this Macromolecules  article.

Even if formulations don’t (for example) polymerise, they can still respond to a stimulus in other ways. For such non-reactive formulations, controlling physical properties can be important, with the rheology of cosmetic creams and gels being a prime example that impacts end-use, shelf-life stability, and even manufacturing sub-processes.

These properties can be controlled on-demand too, with changes in colour being particularly interesting. Such changes can be reversible or irreversible, while the colour itself can be a consequence of the formations of ions or radicals, or even a change in temperature (all of which can also be used in cure systems).

Many clever products have emerged that exploit such responsive systems – such as thermochromic freeze–thaw indicators on temperature-sensitive goods, self-warming shaving foams, and emergency light glow sticks. I can help you explore options in these interesting fields, and suggest approaches that take advantage of them.

Whether reactive or non-reactive formulations are being considered, the beauty of many innovations related to triggering systems is that the underlying molecular machinery can often be transposed to unrelated applications to create novelty.

This concept is shown operating in the example below, which additionally uses a triggering system to tailor the structure of the formulation components. This is also valuable commercially, because by imparting unique properties to the product, such components can create product differentiation and erect insurmountable barriers to competitors.

‘Living polymerisation’ allows the structure of a growing polymer chain to be customised to the application. But although controlled living radical polymerisation is well-used for regular acrylates, applying the concept to anionic polymerisation of cyanoacrylates (CAs) is technically challenging and rarely seen.

While at Henkel, I realised that recent developments in ‘frustrated Lewis pairs’ would allow the development of a system (shown) that could provide both initiation and (temporary) termination of the growing chain. Henkel supporting a team at Universitat Autònoma de Barcelona, developed the chemistry, and consequently were the first ever to report controlled living anionic co-polymerisation of highly reactive CAs at room temperature. For more information, see this article in Chemical Science.