Fraunhofer Institute for Microstructure of Materials and Systems IMWS
Fraunhofer Institute for Microstructure of Materials and Systems IMWS

Degradation tests on paint systems

Who hasn't experienced it: garden furniture that was freshly painted a rich brown in spring looks dull, brittle and cracked after just a few summers. The once smooth layer of paint begins to peel off. Weathering processes are to blame: Moisture from rain, snow and dew affects the surfaces just as much as solar radiation. In particular, the ultraviolet (UV) component of the sun's spectrum breaks down the chemical bonds in the paint over time, a process known as photodegradation.

© Fraunhofer IMWS
Fig. 1: Schematic representation of a pigmented coating layer on a substrate. The talcum pigments within the coating are functionalized with organic and/or inorganic UV protection layers.

But what exactly happens at the molecular level? And how can we develop coatings that last longer and better withstand the effects of weathering? These questions were addressed by a research project involving the Fraunhofer Institute for Microstructure of Materials and Systems IMWS. Other project partners in the CORNET (Collective Research Networking) initiative, an international network of funding organizations for research projects benefiting small and medium-sized enterprises, were the Fraunhofer Institute for Manufacturing Engineering and Automation IPA and the Belgian research institutes Materia Nova A.S.B.L. and Belgian Building Research Institute. Together, they analyzed the mechanisms of photodegradation in paints - and investigated the extent to which the photodegradation resistance of coatings can be enhanced with UV protection particles.

© Fraunhofer IMWS
Fig. 2: Exemplary representation of the intensity distribution of component-specific mass signals (here: Maprenal and Uralac) in the ToF-SIMS depth profile of the non-pigmented 2K polyester system without (blue) and with UV exposure (different exposure times). Depth-dependent changes that indicate the photodegradation are clearly visible, and are different for the different components and UV exposure times.

In the "PhotoProtect" project, which was completed in 2024, the team at the Fraunhofer IMWS primarily contributed to a better understanding of the degradation processes. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis was used to investigate the depth-dependent change in various coating systems (1K acrylate, 2K polyurethane, 2K epoxy primer and 2K polyester), which were subjected to an accelerated ageing process through artificial weathering, known as suntest ageing. 

Usually, methods based on infrared or Raman spectroscopy (determination of the so-called photooxidation index, POI) are used for such investigations of the degradation behavior. These methods offer comprehensive insights in the degradation processes, but lack the possibility to get  thickness-dependent results, due to an insufficient depth resolution. In comparison, the ToF-SIMS technique available at the Fraunhofer IMWS offers the possibility of gently removing the coatings layer by layer from the surface to the substrate and recording a mass spectrum of each "freshly" uncovered layer, using a gas cluster ion source. This makes it possible to precisely analyze the changes that occur to the polymer groups, depending on the coating thickness.

That proved to be useful for analyzing the coatings depth-dependent photodegradation of the different components of the model systems. This was particularly evident in the two-component polyester system used with the main components Uralac (polyester) and Maprenal (melamine-formaldehyde resin). 

© Fraunhofer IMWS
Fig. 3: Exemplary representation of the intensity distribution of component-specific mass signals (here: for Maprenal and Uralac) in the ToF-SIMS depth profile of the pigmented 2K polyester system without (blue) and with UV exposure (different exposure times).

The ToF-SIMS depth profiles obtained show that UV- and temperature-induced degradation (decomposition of long-chain polymer components of the coating and oxidation) proceeds differently for these components: After 500 hours of Suntest exposure, the Uralac component is hardly affected, whereas Maprenal shows a strong degradation effect in the area close to the surface. Only after prolonged weathering (1.600 hours, 2.800 hours), a continuous degradation across the coating depth for Maprenal (with a decreasing tendency towards the substrate) and also stronger, but depth-homogeneous degradation for Uralac can be seen. The fact that this effect is particularly noticeable in the uppermost, surface-near area of the coating and decreases with increasing depth or thickness of the coating is proof of the depth dependence of the process: the degradation starts, as expected, "from the top".   

Based on these results, talcum particles were incorporated into the paint formulations. These were functionalized with organic (Tinuvin, HALS) and/or inorganic (TiO2, CeOx) thin coatings with UV protection effect, using special coating processes at Fraunhofer IPA and Materia Nova. Figure 3 shows an example of photodegradation in pigmented polyester coatings as a function of coating thickness. Looking again at the two main components of the 2K polyester coating, it can be seen that the degradation trends are basically the same - i.e. a stronger degradation of the Maprenal component compared to the Uralac component, as well as a recognizable depth or coating thickness dependence (stronger degradation in the area close to the surface). However, the use of UV protection pigments significantly delays the onset of degradation: after 500 hours of weathering, there is still virtually no recognizable degradation in the pigmented systems, and even after longer weathering (1.600 hours), initially only a near-surface degradation effect is recognizable in the pigmented system. Only after very long weathering times, a clear degradation effect can be witnessed - again mainly of the Maprenal component of the coating.

The example shows how gentle, high-resolution polymer analysis using ToF-SIMS can help break down degradation processes in coatings, thereby making a valuable contribution to the development of UV-resistant protective coating systems.

(May 19, 2025)

In Germany, Cornet project participants are funded by the Industrial Collective Research (IGF) program sponsored by the Federal Ministry for Economic Affairs and Climate Protection (BMWK). IGF project no. 01IF000320C, applicant research association: Forschungsgesellschaft für Pigmente und Lacke e.V. – FPL