Since their inception in the 1960s, UV-curable coatings have been increasingly adopted over solvent- and water-based coating systems, owing to their efficiency and sustainability advantages, both hallmark qualities desired in modern coating systems. Compared to solvent- and water-based systems, UV coatings cure instantaneously, eliminating the need for long and potentially energy-intensive curing procedures. UV coatings also comply with low-VOC requirements, as they do not use carriers that create VOCs or hazardous air pollutants.
The automotive industry is one area where the use of UV coatings has expanded. Recent trends pertaining to lightweighting vehicles to improve fuel economy have increased the use of low–surface energy plastic coating substrates. When UV coatings are applied on these substrates, they face significant challenges in establishing good wetting and adhesion, both critical aspects of performance. Eastman, a leading specialty chemical manufacturer in the coatings industry, has developed a portfolio of additives to address the needs of modern UV coating formulators looking to overcome these obstacles.
Eastman Adhesion Promoters and Solus™ Additives
Poor adhesion and wetting on low–surface energy plastics pose serious challenges for applying coatings and preserving their performance. Poor adhesion can result in delamination and low resistance to scratches, corrosion, and other damage. In the case of poor wetting, coatings cannot be evenly applied, creating coverage issues and surface defects.
Eastman’s adhesion promoters are additives that have long been used in many applications to increase the adhesion of various coating systems to low–surface energy plastics, such as polypropylene (PP), thermoplastic olefin (TPO), and nylon. When incorporated into primers, these adhesion promoters significantly reduce UV coating adhesion issues. Typically used as plant-based binders in solvent-based systems, Eastman’s Solus™ performance additives are powerful rheology modifiers and additives that improve wetting and leveling while imparting a minimal viscosity increase.
Improving UV-Curable System Adhesion with Eastman Adhesion Promoters
To test the adhesion improvements Eastman’s adhesion promoters can make to automotive substrates, five different acrylate-based UV-curable coating systems were prepared and applied to TPO test panels. The systems varied from having high functionalization with hard and brittle films to having low functionalization with softer films. Each system’s formulation is detailed below.
UV-Curable System Formulations
Ingredient
Formulation A
B
C
D
E
Ebecryl® 3720-HD20
40
28
0
0
0
Ebecryl® 810
0
20
0
0
0
Ebecryl® 284
0
0
38
0
0
Ebecryl® 264
0
0
0
38
0
Ebecryl® 1290
7
0
0
0
36
HDODA
36
0
40
45
22
TRPGDA
0
26
0
0
0
TMPTA-N
10
0
15
10
10
OTA-480
0
19
0
0
0
DPHPA
0
0
0
0
25
Ebecryl® P 115
4
4
4
4
4
Irgacure® 500
3
3
3
3
3
Viscosity (cPs)
106
186
140
106
500
A primer made using CP 730-1 was applied to the TPO substrates, then air dried and used as a tie coat to test the improved adhesion of the UV-curable coatings. After the coatings were applied, adhesion was evaluated through crosshatch testing using ASTM D-3359. The results below show a marked improvement in the adhesion of the UV-curable system to the TPO substrate; without the primer, no system adhered to the substrate. Two samples were tested per panel, and all systems benefitted from using the primer.
TPO Adhesion Tests
Untreated TPO Adhesion (%) TPO with 730-1 Adhesion (%)
Formulation
Sample 1
Sample 2
Sample 1
Sample 2
A
0%
0%
100%
100%
B
0%
0%
60%
80%
C
0%
0%
100%
100%
D
0%
0%
100%
100%
E
0%
0%
100%
100%
Improving UV-Curable System Wetting with Solus™ 2100
As mentioned previously, good wetting and leveling are things all coatings, including UV-curable systems, must achieve to exhibit good performance. Poor wetting will lead to premature degradation and surface defects, as coatings cannot provide level coverage across the substrate. Eastman’s Solus™ 2100 is a plant-based additive that provides the necessary rheology control and surface tension reduction to impart good wetting and leveling in UV-curable systems. Typically, rheology modifiers contribute a significant viscosity increase that prohibits good wetting and spreading of droplets during spray application of coatings. Solus™ 2100 is an extremely low molecular weight additive engineered to provide a minimal viscosity increase to avoid such issues.
The UV-curable overprint varnish (OPV) formulations detailed below were prepared with and without Solus™ 2100 to evaluate the additive's effect on wetting. Both the surface tension of the liquid formulation and the cured film of the UV-curable OPV containing Solus™ 2100 (OPV+) demonstrated decreases in surface tension. The results of drop spread tests of the two OPV formulations are shown below, with the OPV+ achieving better results than the standard OPV.
Drop Spread Tests
Leneta Paper
OPV on TPO
OPV+ on TPO
The wetting improvement was much more pronounced when the OPVs were drawn down on TPO substrates. Below, the OPV without Solus™ 2100 immediately began to draw in upon itself after application to the substrate. This occurs because the surface tension of the liquid formulation is higher than the surface energy of the TPO substrate. The number in the upper-right-hand corner of each image indicates the elapsed time since the application.
OPV Drawdown on TPO
In comparison, the OPV+ resisted drawing in upon itself over time after application, indicating better wetting performance because of Solus™ 2100.
OPV+ Drawdown on TPO
Combining Eastman Adhesion Promoters and Solus™ 2100
While the OPV formulations used in the tests above exhibited improved wetting with Solus™ 2100, they did not adhere well to the TPO substrate. To improve adhesion, Eastman’s adhesion promoters can be used as primers in the manner described previously. As UV-curable coatings are preferred for their more sustainable footprint, it is also desirable to ensure that the primers used contribute to sustainability. Eastman has developed Advantis™ 510W, a water-based, APEO-free, and halogen-free adhesion promoter that minimizes harm to workers and the environment.
The Advantis™ 510W adhesion promoter was used to create a 5-micron-thick primer that was applied to TPO test panels, and the same OPV formulations above were applied to the primed test panels. Below are surface tension measurements on various surfaces, including the primed and unprimed TPO and the OPVs.
The positive impact of the Advantis™ 510W and Solus™ 2100 was quite apparent: the surface tension of the TPO increased from 27.6 dynes/cm to 43.1 dynes/cm because of the Advantis™ 510W primer, and the surface tension of the OPV decreased from 49.3 dynes/cm to 32.9 dynes/cm with the inclusion of Solus™ 2100. The high difference in surface tension between the substrate and coating, combined with the action of the primer, facilitated excellent wetting and adhesion performance.
To visualize this performance, the left half of the TPO test panels were primed with Advantis™ 510W, then the OPV with Solus™ 2100 (OPV+) was applied. The system thoroughly wetted out the primed substrate, formed a good film, and exhibited 100% adhesion through crosshatch adhesion testing only on the left side of the panel.
The same test was repeated with the OPV that did not contain Solus™ 2100, forming droplets on the primed test panel that exhibited good adhesion.
A quantified overview of these results is presented below.
System
TPO
TPO + Eastman Advantis™ 510W Adhesion Promoter
OPV
No film, no adhesion
No film, good adhesion
OPV + Eastman Solus™ 2100
Good film, 40% adhesion
Good film, 100% adhesion
Summary
UV-curable systems face issues with adhesion and wetting on automotive substrates such as TPO. Eastman’s adhesion promoters, including water-based, APEO-free, and halogen-free Advantis™ 510W, significantly improve adhesion to low–surface energy plastics. Eastman’s Solus™ 2100 is a plant-based additive designed for modern UV-curable system formulations that helps reduce surface tension and improve wetting to these low–surface energy substrates. Click below to discuss your formulation with one of our technical experts and request samples.
