Shrinkage during UV curing of monomer mixtures is more complex than previously thought, to the extent that the theoretical calculations currently used to predict shrinkage in single monomer mixes by manufacturers formulating their own liquids may no longer apply.

This is according to a paper, Evaluation of UV Curing Properties of Mixture Systems with Differently Sized Monomers published in the journal Materials, by a Korean research team, Park et al. The findings will enable greater control over curing formulations and the resulting physical properties of the polymer.

It is the latest in a series of papers published over the last quarter century that address monomer shrinkage, which is one of the core issues with all multifunctional (meth)acrylate free radical polymerisations. 

Why shrinkage patterns matter

Shrinkage is a common phenomenon in commercial polymerisation applications, exacerbated when monomer mixes are used. Shrinkage occurs during UV curing, which according to Park et al is because “the sum of free volumes of unit molecules is larger than the free volume of a polymer formed from those unit molecules.” 

As the resulting polymer framework shrinks during UV curing, multiple issues arise that vary according to the application. If the monomer mix is designed to be an adhesive, the physical reduction in volume results in adhesion loss and an uneven surface. 

Under the surface, internal stress is generated during shrinkage which goes on to change the shape of the polymer over time. A practical example of this potentially becoming problematic would be dental acrylics, where denture bases or implants can cause irritation if the fit or feel is not perfect and it could eventually lead to infection.

Overall, the combination of volume change and internal stress means short and long-term quality issues in materials produced through additive manufacturing techniques (3D printing). 

Single monomer mechanisms versus complex composite formulations

The relevant historical literature provides detailed coverage of the mechanisms at work during the curing of single monomer systems, as Park et al explain: “Shrinkage is generally known to be inversely proportional to the molecular weights of the unit molecules and proportional to the number of functional groups. Furthermore, it also varies with molecular mobility and the environmental conditions of the curing system.”

However, many applications use formulations with not just multiple monomers, but many other constituents: monomers and oligomers, initiators and additives such as crosslinkers and fillers. The composition of fillers can also vary. The established models for single molecule systems do not apply to these composite formulations, as shown by the experimental results from Park et al’s research.

Park et al note that the composite systems contain too many variables to accurately model, and that by generating experimental results from many simplified systems, a more robust model can be created for predicting how blends used for commercial applications will behave.

Composite formulations have multiple mechanisms at work within the system 

Park et al’s research highlights not only that the shrinkage of complex composite formulations does not fit single monomer mixture models, it also provides insights into why. One of the significant mechanisms at play in the binary-monomer systems that is driving the counter-intuitive shrinkage is the nano-porous effect.

Essentially, the impact of this mechanism is that “reactivity is maximised when small molecules are located between large molecules”. So, in the context of blends of monomers with differently sized molecules, this additional energy is what impacts on the liquidity and density of the acrylate.

The intensity of the UV also increases the molecular activity and reactivity, which changes the conversion ratio. But the impact of the nano-porous effect decreases the conversion ratio and increases the cure rate.

Predicting physical properties of multifunctional (meth)acrylate polymerisations

The net impact of these multiple mechanisms resulting from the composite nature of the formulation are behaviours that cannot be modelled by conventional single monomer calculations. 

Significantly, an understanding of the mechanisms at work will enable more controlled curing formulations that will suffer less from dimensional instability, loss of adhesion, incomplete curing and other effects. 

For companies formulating their own liquids, this is relevant to the prediction of physical properties derived from the amount of crosslinker in the formulation, especially matters of dimensional control and stress related brittleness.