Low energy UVA radiation-curable refinish primers and clearcoats.

In recent years, the major emphasis in the coatings industry has been adapting to environmentally friendly technology that has all the key performance attributes of traditional solventborne systems. Lately, automotive OEMs have suggested the use of "greener coatings technologies" for the "end of line" repair and body-shop refinish applications. This "market pull" effect has rejuvenated interest among innovative resin manufacturers to bring novel concepts into this field. One such approach is the use of Ultraviolet-A (UVA) radiation-curable raw materials for primers and clearcoats.

[ILLUSTRATION OMITTED]

Various aliphatic urethane acrylates were screened for the right combination of coatings properties using high throughput screening. The few predicted formulations were subjected to rigorous performance tests to identify combinations that have fast dry time, adhesion, and sanding characteristics. The details of the formulations, evaluation methods, and the coatings properties will be presented.

INTRODUCTION

The automotive refinish industry is poised for a radical change. Of the major technological changes in the last three decades, the adaptation of 2K-urethane technology from lacquer technology is noteworthy. This big leap revolutionized the coatings industry, which transformed from the old simple polyester technology to the superior performance urethanes. Subsequently, the 2K-urethane system has been accepted as the industry standard. Another innovative concept that was launched a few years ago was the use of IR lamps for refinish cure. This fast-cure technology had a mixed response with end users due to the requirement of new capital investment and its justification in terms of the advantages seen.

The launch of UVA curable refinish primer by a major coatings manufacturer a few years ago was a milestone in the automotive refinish business. (1) Its unique advantages motivated various resin and coatings manufacturers to work in this area. During that period, the lamp technology was not as advanced as the resin and coatings technology. A significant breakthrough in this field was achieved when a range of low intensity UVA lamps (radiation output in the range of 320 nm-400 nm) with different design and power output started emerging into the market. This kindled interest in the "refinish" and "end of line" market segments that helped steer UVA technology toward commercialization. The recent developments like the introduction of 400 to 450 watts/in. lamps with easy to use assembly will encourage more end users to evaluate and adapt this technology.

The driving factors of UVA technology are:

* Faster cure time and increased productivity

* Reduced waiting time between coats (primer, basecoat, and topcoat)

* Operator safety

* Lower VOC and VHAPs wherever possible

These advantages encouraged the OEM and coating manufacturers to adapt to this environmentally friendly greener technology as opposed to the traditional high VOC and VHAPs solventborne systems. In response to these market developments, major innovative resin and coatings suppliers focused their research efforts on developing products that were robust and could cure under low intensity UVA radiation.

With these requirements in the background, an extensive investigation was conducted in our laboratory using the combinatorial approach. (2-3) Miniaturization, Automation, and Parallelization (MAP) is a wonderful tool to screen different resins and formulations for optimum properties. A Doptical design with second order interaction was used to evaluate ~25,000 coatings in a period of three months versus a two-year timeframe required for traditional testing. This initial study gave an excellent boost to our developmental efforts and provided the capability to select resin combinations that are beneficial for refinish applications. Our prior knowledge of the 2K-urethane refinish market also helped us compare this new technology with the current industry standard. This article deals with the recent developments and new opportunities available in the emerging market.

UV-CURABLE 100% SOLIDS AND SOLVENTBORNE PRIMERS

Preparation and Evaluation of Optimized Formulations

The MAP study pointed toward a few formulations that could yield good coatings properties. Our further investigation and testing using the traditional test protocol resulted in a formulation that had the best overall performance. It was a combination of an aliphatic urethane acrylate and an epoxy acrylate. The details of the formulation are shown in Table 1.

The formulations were prepared both as 100% solids and also in a solvent depending on the type of application. The resins (urethane acrylate and epoxy acrylate) were mixed together along with the photoinitiator using a mixer. The additives and pigments were then added slowly with continued stirring. The formulations prepared were kept overnight to de-aerate. The coatings were then applied on the substrate by using either a drawdown bar, putty knife, squeegee, or sprayed to a wet film thickness of ~5 mils.

Curing the formulations under low intensity UVA light sources, e.g., H & S Autoshot UVA-400 lamp at a distance of 10 inches for two minutes, resulted in a tack-free surface. The primer had excellent adhesion to cold rolled steel. It can be sanded with a 320-grit sandpaper and then base-coated right after curing. The primer exhibited excellent hiding.

The primer formulation F was then evaluated for salt-fog and humidity resistance. A commercial epoxy primer was used as the control. The test protocol consists of applying the primer followed by a commercially available basecoat (white and yellow) and 2K-urethane top clearcoat. The evaluation was done according to ASTM D 714-87 and the details are shown in Table 2 (with higher ratings indicating better coatings performance).

Test results from Tables 2-4 show that there are differences in the performance based on the type and color of the basecoat selected for the evaluation, which is typical even in a 2K-urethane system. The tests were based on a standard textbook primer formulation without any special pigments or anticorrosion pigment package. This was done mainly to study the effect of resins in the formulation. The coatings performance could be significantly improved by adjusting the formulation further. Salt-fog and humidity tests show that the UV-curable primer developed performed very well compared to the standard epoxy primer system.

UV-CURABLE SOLVENTBORNE CLEARCOAT

Aliphatic urethane acrylates are ideal resins for UV-curable clearcoat applications. They have superior weathering characteristics along with excellent chemical resistance and toughness. The development efforts were focused on achieving the right hardness and flexibility along with good solvent resistance. The urethane acrylate clearcoat formulations are shown in Table 5. The formulation A was based on a hard urethane acrylate that had eight years of proven outdoor weathering data, formulation B was based on a flexible soft urethane acrylate, and formulation C was a combination of hard and flexible urethane acrylates.

UV Clearcoat Evaluation

The clearcoats were evaluated for their solvent resistance and hardness. Table 6 shows the resistance to solvent as determined by MEK double rubs and Table 7 shows the hardness of the coatings as determined by Konig pendulum hardness.

The test data show that the hard urethane acrylate coating A had good solvent resistance and pendulum hardness. The soft urethane acrylate coating B was able to develop the initial MEK resistance but over a period of time became soft. However, by using both hard and soft urethane acrylates (formulation C), the resulting coating had good hardness along with excellent chemical resistance.

CONCLUSION

An ultraviolet-A curable refinish solventborne primer was successfully developed and evaluated. The primers have fast cure time combined with good solvent resistance and sanding characteristics. The initial salt-fog and humidity resistance evaluation show excellent performance compared to the traditional epoxy primer system. A combination of hard and soft urethane acrylates resulted in a UV-curable refinish clearcoat with good coating performance.

Table 1 -- UV-Curable Primer Formulation

                                        Weight (g)
Pigmented Formulation       A      B      C      D      E      F

Component A -- urethane
  acrylate                  50     50     50     50     50     50
Component B -- epoxy
  acrylate                  50     50     50     50     50     50
Pigment -- Ti[O.sub.2]       3.4    3.4    3.4    3.4    3.4    3.4
Calcium carbonate filler    41.1   41.1   41.1   41.1   41.1   41.1
Extender pigment -- talc    59     59     59     59     59     59
Pigment -- Bayferrox 318M    0.58   0.58   0.58   0.58   0.58   0.58
Adhesion promoter
(Trifunctional acid
  ester) -- Sartomer
  CD9052                     0     10     30      0     10     30
Photoinitiator -- Irgacure
  2020                       7.8    7.8    7.8    7.8    7.8    7.8
Solvent -- butylacetate      0      0      0     30     30     30

Table 2 -- Salt-Fog Evaluation of UV-Curable Primer

                        500 hr
Treated       Creepage from scribe
Steel Panel   (mm) read in 5 places    Average  Rating

UV Control W  0.1  1    0.5  5.5  4    2.22     6
#1 white      5    3.5  3.5  5    6    4.6      5
#2 white      0.5  0.5  3    3.5  3.5  2.2      6
#3 white      0.5  4    4    5    4    3.5      6
#4 white      1    2.5  3    3    3.5  2.6      6
EpoxyW        0.1  0.1  0.5  0.8  3    0.9      8

Untreated     Creepage from scribe
Steel Panel   (mm) read in 5 places    Average  Rating

UV Control Y  1.2  2    1    0.5  1    1.14     7
#1 yellow     2    2    2    0.5  0.3  1.36     7
#2 yellow     1    1    7    9    8    5.2      4
#3 yellow     1    0.3  0.3  0.3  1.1  0.6      8
#4 yellow     1.2  0.5  1.2  0.5  0.5  0.78     8
EpoxyY        0.2  0.2  0.2  0.3  0.3  0.24     9

Rating based on standard test for evaluation degree of blistering in
paints ASTM D 714-87.

Table 3 -- Salt-Fog Evaluation of UV-Curable Primer

                                       1000 hr
Treated Steel Panel  Creepage from scribe (mm) read in 5 places  Average

UV Control W          9     9    8  11  12                         9.8
#1                   12    11    7  13  16                        11.8
#2                    5     6    7   7  10                         7
#3                    4.5   3    4   6   8                         5.1
#4                    5     6    8  10  22                        10.2
Epoxy                 4.5   5.5  5   5   5                         5

Treated Steel Panel  Rating

UV Control W         3
#1                   2
#2                   4
#3                   4
#4                   2
Epoxy                5

Table 4 -- Humidity Resistance of UV-Curable Primers

         500 hr Cleveland Condenser
                        ASTM D 1654-79A
Untreated Steel         Rating Based on Blistering

Panel
UV Control W            3
#1 White                4
#2 White                6
#3 White                7
#4 White                6
EpoxyW                  9

Panel
UV Control Y            6
#1 Yellow               6
#2 Yellow               2
#3 Yellow               8
#4 Yellow               6
Epoxy Y                 9

Table 5 -- UV-Curable Clearcoat Formulation

                                    Weight, g
Raw Material                     A       B       C

Urethane acrylate -- hard      350.47  --      104.82
Urethane acrylate -- soft      --      350.47   83.86
Pentaerythritol tetraacrylate  --      --       25.16
Sanduvor 3058 liquid             3.5     3.5     1.93
Sanduvor 3206 liquid             8.76    8.76    4.82
Irgacure 184                     3.5     3.5     1.93
Darocur 4265                    21.03   21.03   11.57
Baysilone OL 44                  0.35    0.35    0.96
Butyl Acetate, n-              348.25  348.25  164.95
Total                          735.86  735.86  400.00

Table 6 -- Solvent Resistance of Clearcoats

                                                MEK Rubs
Systems                              Initial  Day 1  Day 7  14 Days

Urethane acrylate -- hard (A)        100      100    100    100
Urethane acrylate -- soft (B)         90       70     60     20
Urethane acrylate (hard + soft) (C)  100      100    100    100

Table 7 -- Hardness of Clearcoat

                                            Pendulum Hardness
Systems                              Initial  Day 1  Day 7  14 Days

Urethane acrylate -- hard (A)         72.8     91    110.6  100
Urethane acrylate -- soft (B)         49       30.8   19.6   19.6
Urethane acrylate (hard + soft) (C)  147      159.6  170.8  163

ACKNOWLEDGMENT 

The authors would like to thank Ms. Christine Mebane for her technical assistance.

Presented at the 82nd Annual Meeting of the Federation of Societies for Coatings Technology. October 27-29, 2004, in Chicago, IL.

References

(1) ICI Autocolor P110-5000 UV-SpeedPrime--Part of PPG Industries in USA.

(2) Bach, H. et al., "UV Refinish Primer and Clear Coat," Proc. Radtech Europe Conf., 2003.

(3) Bach, H., Dvorchak, M., and Strazisar, S., "Development of Low Energy UV Refinish Formulations: Utilizing High-Throughput Experimentation as a Tool in the Discovery Process," Proc. RadTech America, Conf., 2004.

by Ramesh Subramanian, Katherine Ratliff, Charles Gambino, and Michael Dvorchak

Bayer MaterialScience LLC*

* 100 Bayer Rd., Pittsburgh, PA 15205-9741.