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The weathering of epoxy based coating is known to be a challenge for many applications with the field of protective coatings. In this study, the objective was to develop the next generation of light stabilizer additives that would enhance the outdoor durability of epoxy coatings.

The use of light stabilizer additive in epoxy coating is known to slow the high yellowing that is exhibited quite rapidly in such coatings. However, the type and performance from current product offering has been quite limited.

Indeed, the challenge in using any type of additives in an epoxy coating is the adverse reactions that can occur such as change in initial color, haziness, partial cure, change in shelf and physical property of the coating after cure such as hardness.

In this study, we first evaluated a broad range of light stabilizers additives to determine their influence in a standard epoxy coating formulation. After we identified the chemistries of light stabilizers most suitable in epoxy coatings, we performed several accelerated and outdoor weathering tests to determine the weathering performance of these UV stabilized epoxy coatings. Finally, we compared the weathering results with a novel type of epoxy resin developed for improved weathering resistance.

 

Experiment

As a 2K epoxy clear coating system 100% solid, we used in this study CETEPOX 293R as the epoxy resin (BPA/BPF in reactive diluent) and CeTePox 1537H as the polyamine hardener (accelerated modified polyamine) – both products supplied by CTP GmbH (Germany).

Based on the Epoxy Equivalent Weight (EEW) and the amine equivalent weight, we used a ratio of 2:1 for the formulation of the epoxy coating.

After mixing the light stabilizer additives with the amine harder, we checked the solubility and the compatibility of each additive on the total coating system (hardener + epoxy resin) and the appearance, gloss and shore hardness after 7 days.

 

Selection of the light stabilizer additives

In this section, we evaluated most light stabilizers additives currently used in the overall broad coating industry (UV Absorbers and radical scavengers) for their compatibility in a 2K clear epoxy coating.

As shown in figure 1 below, we found that certain UV absorber and radical scavenger additives were either incompatible, not soluble or had adverse effect on the gloss or hardness of the 2K epoxy coating system.

Adverse effects of several UV Absorbers and Radical Scavengers in an epoxy coating system
FIGURE 1 » Adverse effects of several UV Absorbers and Radical Scavengers in an epoxy coating system

 

Weathering performance results

For the light stabilizer additives that were found to be compatible with the clear epoxy coating system and having no adverse effect on physical properties of the coating, accelerated and natural outdoor weathering test were performed.

Below are the standard used for the accelerated test:

  • QUV-A exposure test and reference standard : ASTM G154

  • Xenon-Arc exposure test and reference standard ISO 4892-2 E2013 Wet/Dry

We also performed outdoor weathering tests in Bangkok, Thailand.

Appearance, gloss, color change and cracks evaluations were performed at regular interval.

 

Analysis of QUV-A Results

Based on the performance results in graph 1, we can conclude the following:

  • An increase dosage of light stabilizer additives shows an increase in performance (1.5% LS2 vs. 3.0% LS2)
  • Each type of light stabilizer additives performs differently (LS1, LS2, LS3 etc...
  • Taking in consideration the approximate cost of each light stabilizer additives, an optimized solution cost vs. performance is shown to be possible (LS4 vs. LS7)
Yellowing Index Measurement Epoxy Coating
GRAPH 1 » Yellowing Index Measurement Epoxy Coating

 

Interestingly in graph 2, it is LS5 and not LS7 that performed the best in overall least color change (delta E measurements).

The cause for the change in ranking DE vs. YI performance is not yet fully understood. However, it is known the chemistry of the UV Absorber in LS 5 exhibits longer photo-permanence and has broader absorbance spectra.

Delta E Measurements Epoxy Coating
GRAPH 2 » Delta E Measurements Epoxy Coating

An analysis of the weathering results QUV-A vs. Outdoor vs. Xenon/WoM confirms the conclusion previously stated. In graph 3, the best performance independently of the type of weathering is obtained with the light stabilizer solution LS5.

Delta E (Color Change) Measurement Epoxy Coating
GRAPH 3 » Delta E (Color Change) Measurement Epoxy Coating

In regards to the Yellowing Index, again independently of the weathering exposure, the light stabilizer solutions LS4, LS5 and LS 7 show almost the same performance results. However, it is the solution LS4 that is most cost optimized.

Yellowing Index Measurement Epoxy Coating
GRAPH 4 » Yellowing Index Measurement Epoxy Coating

 

Conclusions

For epoxy coatings, improvement in weathering and especially color change and yellowing is a constant market requirement. Currently, the only solutions available are either using a light stabilizer additive based on a unique UV absorber chemistry, or by modification of the epoxy resin itself. We found in this study that not all UVA and /or HALS can be used in a 2K clear epoxy coating. For the additives showing full compatibility with an epoxy system, we also found that an increase of dosage leads to an increase in performance. Finally, we found for yellowing index, a cost improve solution can be used for similar performance as best known additive for epoxy coating. For an overall color change improvement (DE), the unique and novel light stabilizer solution L5 exhibits the best performance in both type of accelerated weathering (QUV-A) and in natural outdoor weathering.