Light Stabilizers Make The UV Protection Of Waterborne UV-curable Coatings Easier

Abstract
UV curing processes have been used extensively in many different industries such as packaging, printing inks and protective coatings for wood, plastics and metal. Advantages of UV curing are instant drying, low operating cost, improved quality and reduced space.
Waterborne UV curable coatings – green coatings – have been developed to replace conventional UV curable coatings with VOC emissions. It can be a challenge when light stabilizers are required to protect coatings to UV degradation.
The novel light stabilizer (NLS) is developed specifically for waterborne UV curable coats. This article is a comparison study of three different light stabilizers to disperse in water and waterborne UV curable coating. Test data showed that NLS in a waterborne clear UV curable coating would not have negative impacts to cure speed. The results confirmed that by increasing the concentration of NLS in the top coat can effectively protect from UV light degradation.
Introduction

UV curable coatings require direct UV light to initiate monomers and oligomers. It is converted from a liquid into a solid film. UV curing coatings feature speed in production and are faster in drying. However, most polymers are susceptible to degradation by UV and visible light. It requires light stabilizers to enhance the durability from sunlight degradation. To understand more on the light stabilizers, it can be divided into two groups: UV absorbers (UVA) and the Hinder Amine Light Stabilizers (HALS). UVA’s function is absorbing UV light and transforming it into heat and the HALS is best known to capture free radical to prevent material degradation.

It is important to know UV curable coatings especially in dry faster system. Photo initiator absorbs UV energy to initiate curing processes. UV absorber absorbs UV energy to prevent coatings damages. As a result, UV absorber and photo initiator are competing for UV energy to cause negative impacts about cure speed. The NLS can meet this problem.

UV radiation is a well-accepted technology. However the UV curable coatings have odor, emission monomers and not zero VOC emission .
The UV curable waterborne coating has both the advantages of waterborne coating and UV curable coating  such as non toxic, pollution-free, and safety in use. In recent years the research and development has grown faster.
This subject of this article is to show the use of newly developed liquid NLS, designed for waterborne UV curable coatings, is effectively to protect from UV light degradation and would not have negative impacts to cure speed.

Experiment
The model formulation used in this study was classification of light stabilizers and compositions of waterborne UV curable coating. Their detail compositions are shown in Table 1 and Table 2. Design of Experiment (DOE) was used to design test runs as well as to analyze test data. DOE technique allows us to validate the data.


Dispersion Tests
Waterborne UV curable coatings are based on water soluble or water dispersed resins. It is non-toxic, odorless and nonflammable. Liquid light stabilizers are normally not easy to disperse in water-based UV curable coatings. Adding UV stabilizers directly to water-based paint system will cause condensation in coatings and make it difficult to use. After mixing together, we use filter paper to test.
If the residual is less, the performance is better. On the other hand, it can be used directly and homogenously disperse in waterborne UV Curable resin. According to dispersion test, NLS is the best performance in both systems. Photos of 10% different types of light stabilizers in waterborne UV curable coating is filtrated tested (as shown in Figure 2).

Minimal Energy to Reach Curing Tests
Waterborne based UV curable coatings were made with wire-wound rod on coated Leneta charts. The dry film thickness (DFT) was 15μm and heated in an oven at 60˚C for 2 min. The next were cured under a 1 x 150w/cm high-pressure Hg lamp. Cure speed was recorded when coating left no visible marks after a finger nail scratched across the coating surface a few times. The result shows that NLS would not have negative impacts to cur speed (as shown in Figure 2).

Weathering Test
Before and after exposure of delta Y measurements (as shown in Figure 3) of adding NLS, coatings would receive different degrees of the protection according to each consumption level. Evidently, a higher use level provides a better protection.
Experiment procedure:w
Model: Q-U-V Accelerated Weathering Tester 
Test Model: ASTM G154-1(QUV with UVB-340 light bulb)
Test Time: 120 hours
Material: Plastic

Results
Our data confirmed that NLS can be used directly and homogenously disperse very well in waterborne UV curable coating and water systems. Test data showed that NLS in a waterborne clear UV curable coating would not have negative impacts to cure speed.
The increasing the concentration of NLS in the waterborne UV curable coating can effectively protect from UV light degradation.

Conclusion
Environmentally friendly coatings are the trend for future development in the coating industry. In this article we highlighted the benefits of using NLS in waterborne UV-curable coating system.
Coating manufactures may still need to run the experiments to confirm their coating systems are compatible. The company who can offer tailor UV absorber and light stabilizer products will have the key to providing UV protection in the manufacturer’s coating products.


Everlight Chemical Industrial Corporation
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Tel: +886-3-4837682; Fax: +886-3-4837683; gigi@ecic.com.tw;
www.everlight-uva.com