Waterborne UV-Curable Coatings
February 2016 www.coatingsworld.com Coatings World | 45
Yung-Chi Yang, Ming-hua Lai, Yu-Shu Sung, Yin-Ting Lai, Chung-Han Lu, Hsiao-Fang Chiu,
Chih-Hsien Chein, Steven Lee and Dr. Yao-Hsing Huang, Everlight Chemical Industrial
Corporation, Taipei, Taiwan R.O.C.
Waterborne UV-curable coatings –
Eco-Friendly coatings - have been
developed to replace conventional UV-curable coatings that emit VOCs. The
challenges of waterborne UV-curable
coatings have two aspects. One is that
discoloration may occur when resins
are exposed to ultraviolet (UV) light
from natural sunlight. Therefore, waterborne UV-curable light stabilizers
are incorporated in the surface coating for protection. The other challenge
is that yellowing can occur during the
curing process. This problem will affect product quality and, potentially,
increase product cost.
A novel light stabilizer (NLS) was de-
veloped specifically for waterborne UV-
curable coatings to reduce yellowing
during the curing process. The Design of
Experiments, or Experimental Design,
(DOE) was used in this study. Test data
showed that the NLS in a waterborne,
clear UV-curable coating would not
negatively impact cure speed. The re-
sults confirmed that increasing the con-
centration of NLS in the topcoat can
effectively reduce yellowing during the
curing process. Furthermore, data also
showed that by increasing the coating
thickness, yellowing during the curing
process cannot be improved (decreased)
to any appreciable degree.
Introduction
UV-curing processes have been used extensively in many different industries
such as packaging, printing inks and protective coatings for wood, plastic, metal,
etc. However, traditional UV-curable
coatings may emit monomers, have an
odor, and are not zero-VOC. On the other hand, UV-curable waterborne coatings
have both the advantages of waterborne
coatings and UV-curable coatings such
as being non-toxic, pollution–free, and
are generally safe to use. In recent years
research and development has grown
quickly in these areas.
The challenges of waterborne UV-
curable coatings are twofold. One is that
discoloration may occur when resins
are exposed to UV light from natural
sunlight. Waterborne UV-curable light
stabilizers can be incorporated in the
surface coating for further protection.
The other is yellowing that occurs during
curing. This problem will affect product
quality and cost.
Light stabilizer packages typically include UV absorbers (UVAs) and hindered
amine light stabilizers (HALS) and are
known to prevent coating damage caused
by UV radiation. On the other hand, photoinitiators absorb UV energy to initiate
curing processes. UVAs absorb UV energy
to prevent coating damage. As a result,
UVAs and photoinitiators are competing
with each other for UV energy and create
a negative impact on the cure speed.
In this article, we demonstrate our
recent development on a liquid NLS for
waterborne UV-curable coatings. Our
objectives were to effectively reduce yellowing during the curing process and to
eliminate any negative impact on the curing speed.
Light stabilizers enhance color protection during cure.
Waterborne
UV-Curable Coatings
Table 1: Formulation used in this study.
Ingredient Characteristic
NLS Blend of UVA and HALS (for water-based system)
Polyurethane dispersion Water-based system
Photoinitiator α-Hydroxyketone (for water-based system)
