Control of Protective Coating Performance Aspects Through Manipulation of Polymer Fragility

Liquid applied, high performance protective coatings applied under ambient conditions must undergo film formation to deliver a coherent, adherent and functioning coating film to deliver the necessary performance. The rate and extent of film formation is of the highest importance both in terms of the process (how quickly a coated article can be handled or stored) and also in terms of coating performance (what properties have developed). Ensuring the coating has a workable pot-life, is dried in an economic timeframe and that coating performance is adequate when placed into service are vital components of successful film formation.
Two component thermosetting epoxy-amine formulations represent the majority of ambient cured coating technology where corrosion protection or chemical resistance is required. Complete conversion of all reactive groups in the polymer binder is rarely achieved under ambient curing conditions before vitrification to the glassy state occurs and, following a period of diffusion controlled processes, reaction essentially stops. Consequently the coating must often achieve necessary performance in an under-cured state.
In order to improve the desired balance of application productivity alongside final performance properties, greater understanding of the inter-relationships between cure temperature, extent of conversion of reactive groups, choice of polymer architecture and polymer glass transition (Tg) on coating film performance is required to guide design of coating formulations.
The concept of polymer glass fragility is associated with the extent that polymer mobility changes with temperature close to Tg. "Fragile" systems are described as those that have large changes in polymer mobility close to Tg (accompanied by large heat capacity change) whereas "strong" systems have lower mobility (and lower change in heat capacity).
Fragility quantifies how quick a material changes from liquid to solid and is related to polymer segment packing efficiency which, in turn, is believed correlated with various key performance criteria (of particular relevance is mechanical properties and molecular diffusion barrier properties). Through choice of reactive components, manipulation of polymer fragility may thus provide a route to manipulate both drying processes and final coating performance criteria.
We propose to examine the hypothesis that polymer fragility can be manipulated through polymer architecture brought about through choice of reactive components. Influence of main epoxy-amine binder components and other common reactive additives used in modern epoxy-amine coating formulations including tertiary amine accelerators will be explored.
Using model formulations, application of state-of-the-art Modulated DSC methodology will be explored to interpret complex glass transitions regions. Assessment of changes in heat capacity of starting materials and corresponding fully cured films will allow determination of differences in fragility. Application of Infra-Red techniques will quantify reaction conversion and allow relationships between polymer architecture, conversion and fragility to be explored.
The implications of polymer fragility on key film performance attributes will be explored. Primarily drying processes, but also those associated with through-film diffusional molecular transport properties of cured films using Positron Annihilation Spectroscopy (PALS) or Dynamic Vapour Sorption (DVS) and polymer film aging processes using Dynamic Mechanical Analysis (DMA).