Acrylate monomer-free/VOC-compliant ultraviolet-a radiation-curable technology for automotive refinish clearcoat.
UVA-curable coatings have since been experiencing growth in automotive refinish applications. They have many advantages over conventional coatings systems in that they can cure fast and are 1K. Current developments in UV-A curable technology have yielded coatings that are highly reactive and also minimize residual unsaturation in the cured coating. Despite this, the use of these resins in automotive refinish clearcoats or spot repair is still limited. The main reason has been the difficulty in the sanding and buffing of the clearcoat due to the difference in coatings properties of the refinish coatings and the OEM coating being repaired. Properties of concern are the hardness and glass transition temperature ([T.sub.g]). Since 2K polyurethane technology is widely used in the automotive refinish market, if the UV-A curable refinish coating could mimic its properties then the system will blend well. This led to the development of two acrylate monomer-free aliphatic urethane acrylate resins (Figure 1).
Urethane Acrylate Resin A was designed as a 100% solids soft resin that has a viscosity of 10,000 mPas at 25 [degrees]C. The cured film [T.sub.g] of this resin was 10[degrees]C. Urethane Acrylate Resin B is a hard resin that is 60% solids in n-butyl acetate and has a viscosity of 200 mPas at 25[degrees]C. This resin is hard, as demonstrated by having a cured film [T.sub.g] of 100[degrees]C.
By blending these two resins the desired combination of [T.sub.g] and hardness can be achieved for optimum sanding and buffing characteristics.
A matrix study was performed using these two resins to determine the optimum blend ratios to achieve superior sanding and buffing performance. The study was conducted by making variations of high and low [T.sub.g] urethane acrylate resins and blending them in five different ratios. The UVA-curable clearcoat formulations that were made were spray applied over panels prepared from commercial refinish primers and basecoats. For spot repair, the samples were applied over commercial ACT OEM finished panels. Chemical resistance (MEK double rubs), gloss, and appearance testing after sanding and buffing were determined on these samples. UVA-curable paint formulations were also spray applied over glass substrates to determine [T.sub.g] and pendulum hardness. A commercial UVA-curable refinish system and a commercial 2K polyurethane refinish system were also applied and tested in the same manner for comparison study.
The blend ratios studied are shown in Table 1 and a generic coating composition is shown in Table 2.
The sprayed clearcoats were flashed at room temperature for 5 min before being cured under an H & S AutoShot 400 UVA lamp (see Figure 2) for 4 min at a distance of 12 in. Figure 2 shows a picture of curing a clearcoat in a body-shop environment.
Chemical resistance was determined by MEK double rubs and is shown in Table 3. The results show that the lower [T.sub.g] Resin A has very little chemical resistance unblended. The much higher [T.sub.g] Resin B has very good resistance as well as all of the UA Blends. UA Blend 5 showed a drop in chemical resistance at 14 days but this blend also has the highest amount of UA Resin A in it. This phenomenon has been seen other times when high amounts of UA Resin A are used. It is thought that since UA Resin A is much softer, the polymer relaxes after its initial cure. This allows the MEK to penetrate and soften the polymer network. MEK does not dissolve the coating but softens it to the point that it is being abraded during testing.
[FIGURE 1 OMITTED]
Glass Transition Temperature ([T.sub.g])
The [T.sub.g] of a clearcoat can be an important contributing factor in the ability to buff out a coating (Table 4). During buffing the coating will begin to heat due to friction. If there is a difference between the new coating and the OEM coating, buffing out blend lines and increasing the gloss can be difficult. Typical polyurethane clearcoats have a [T.sub.g] from 58[degrees]C to 65[degrees]C.
Clearcoat hardness was determined using pendulum hardness. The clearcoat formulations were spray applied over glass panels and cured in the same manner as described previously. Hardness was measured 30 min after cure and also 1, 2, 7, and 14 days after cure. The samples were kept at a constant temperature and humidity to ensure consistency between measurements. Samples were tested according to ASTM D 44366 (Table 5).
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Current commercial 2K technology is usually sanded and buffed one day after curing to achieve optimum appearance characteristics while maintaining job efficiency. At this time the hardness is 95 sec, whereas the current commercial UV system is much harder at 207 sec. UA Blends achieved a 2K type of hardness 30 min after curing as opposed to one day using the conventional 2K system, thereby increasing efficiency.
Figure 3 illustrates the effect that varying amounts of UA Resin A have on [T.sub.g] and hardness of the clearcoat (hardness values in Table 5 are at 30 min after curing). The [T.sub.g] appears to remain stable relative to % UA Resin A until more than 30% has been added, at which time it begins to drop sharply. The hardness, however, begins to drop as soon as UA Resin A is introduced.
When refinish coatings are discussed it is very important to also discuss appearance properties such as halo and blend lines. Halo is a phenomenon that is common when sanding and buffing refinished parts. Halo is a hazy area of the coating that usually forms around the edges of the unsanded buffed area. To remove this, sometimes the entire body panel section will need to be buffed. Figure 4 illustrates the halo effect on a refinished panel. To determine how susceptible a UV coating is to forming a halo, panels were refinished using each system. Thirty minutes later they were sanded using 1000-grit sand paper. A buffer was then placed over the sanded area and that area was buffed for one minute without pressing on the buffer. Panels were ranked as to having halo, medium halo, slight halo, or no halo (Table 6). UA Resin A was not determined because by itself it could not be buffed.
Blend lines refer to the area of a refinished part where the new coating meets the OEM coating. This area will typically be sanded and buffed so that the new coating appears to be the same as the original. Figures 5 to 7 show the blend line improvements made with the UV formulations.
Gloss and DOI were determined after curing and also after sanding and buffing. Initial gloss readings of the UA Blends were comparable to the commercial 2K refinish system. After sanding and buffing there was a slight improvement over the 2K system. The commercial UVA refinish system started at a lower gloss compared to the UA Blends. However, it did improve with sanding and buffing to about the same gloss level as the UA Blends (Table 7).
Monomer-free aliphatic urethane acrylate resins with different hardness were successfully synthesized. UV cure coatings were developed by using these resin blends to match the performance of the existing OEM coating, meeting the low-VOC requirements. The coatings developed can be sanded and buffed much like a commercial 2K polyurethane system with improved blending capability and near minimal halo effect.
(1) Bach, H. et al., "UV Refinish Primer and Clearcoat," Proc. Radtech Europe, 2003.
(2) Strazisar, S., Dvorchak, M.J., and Bach, H "Development of Low Energy UV Refinish Formulations," Proc. Radtech, 2004.
(3) Subramanian, R., Gambino, C., Ratliff, K., and Dvorchak, M., "Curing at the Speed of Photons--A Major Paradigm Shift in Automotive Refinish Coatings," Proc. International Waterborne, High-Solids and Powder Coatings Symposium, 2005.
(4) Subramanian, R., Gambino, C., and Dvorchak, M., "UV Refinish of Plastic Headlamps," Proc. Coatings Show 2005, Indianapolis, IN, 2005.
by Michael Jeffries, Charles Gambino, and Joe Pierce
Bayer MaterialScience LLC*
*100 Bayer Rd., Pittsburgh, PA 19205.
This paper is reprinted from the RadTech Reporter, Vol. 20, No. 5 (September/October 2006).
Table 1 -- Urethane Acrylate Blend Ratios (Parts by Weight Based on Solids) UA Resin A UA Resin B UA Blend 1 10 90 UA Blend 2 20 80 UA Blend 3 30 70 UA Blend 4 40 60 UA Blend 5 50 50 Table 2 -- Urethane Acrylate Clearcoat Generic Formulation Raw Material Weight Volume Weight Solids Volume Solids UA Resin B 54.95 6.46 32.97 3.44 UA Resin A 14.13 1.49 14.13 1.49 Photoinitiator 0.47 0.05 0.47 0.05 Flow agent 1 0.12 1 0.12 Photoinitiator 1.88 0.21 1.88 0.21 Light stabilizer 0.47 0.47 0.47 0.47 Light stabilizer 1.18 1.17 0.94 1.13 Solvent 1 5.76 0.78 0 0 Solvent 2 20.16 3.01 0 0 Total 100 13.75 51.87 6.9 Calculated Properties Weight solids 51.87 Wt/gal 7.27 Volume solids 50.19 Theoretical VOC 3.5 Table 3 -- Chemical Resistance as Measured by MEK Double Rubs Resin 30 M 1 Day 7 Day 14 Day UA Resin A 14 9 5 15 UA Resin B 100 100 100 100 UA Blend 1 100 100 100 100 UA Blend 2 100 100 100 100 UA Blend 3 100 100 100 100 UA Blend 4 100 100 100 100 UA Blend 5 100 100 100 77 Commercial UV Refinish 100 100 100 100 Commercial 2K Refinish NA 100 100 100 Table 4 -- Clearcoat Glass Transition Temperature ([T.sub.g]) Resin [degrees] C UA Resin A 10 UA Resin B 104 UA Blend 1 103 UA Blend 2 105 UA Blend 3 106 UA Blend 4 84 UA Blend 5 74 Commercial UV Refinish 101 Commercial 2K Refinish 62 Table 5 -- Pendulum Hardness (seconds) Resin 30 Min 1 Days 2 Days 7 Days 14 Days UA Resin A 21 18 20 17 17 UA Resin B 139 167 171 167 181 UA Blend 1 130 158 169 189 193 UA Blend 2 115 115 115 137 140 UA Blend 3 102 105 108 112 113 UA Blend 4 95 98 99 104 104 UA Blend 5 92 95 99 102 101 Commercial UV Refinish 207 207 210 221 218 Commercial 2K Refinish N.D 95 105 112 127 Table 6 -- Halo Effect in Various UV-Curable Resins Resin Halo Effect UA Resin A Not Tested UA Resin B Halo UA Blend 1 Medium Halo UA Blend 2 Slight Halo UA Blend 3 Slight Halo UA Blend 4 No Halo UA Blend 5 Slight Halo Commercial UV Refinish No Halo Commercial 2K Refinish No Halo Table 7 -- DOI and Gloss Before and After Sanding and Buffing Initial Resin DOI 20 [degrees] 60 [degrees] UA Resin A ND ND ND UA Resin B 97 86 90 UA Blend 1 97 86 91 UA Blend 2 97 86 91 UA Blend 3 97 87 91 UA Blend 4 97 87 91 UA Blend 5 97 86 91 Commercial UV Refinish 79.1 71 89 Commercial 2K Refinish 96.7 89 93 After Sanding, Buffing Resin DOI 20 [degrees] 60 [degrees] UA Resin A ND ND ND UA Resin B 92.2 60 80 UA Blend 1 91.9 75 85 UA Blend 2 89.3 75 86 UA Blend 3 91.1 80 89 UA Blend 4 94 81 89 UA Blend 5 92.4 80 88 Commercial UV Refinish 95.5 82 91 Commercial 2K Refinish 89.8 65 77 % Retention After Buffing Resin % DOI % 20 [degrees] % 60 [degrees] UA Resin A ND ND ND UA Resin B 95.1 69.8 88.9 UA Blend 1 94.7 87.2 93.4 UA Blend 2 92.1 87.2 94.5 UA Blend 3 93.9 92.0 97.8 UA Blend 4 96.9 93.1 97.8 UA Blend 5 95.3 93.0 96.7 Commercial UV Refinish 120.7 115.5 102.2 Commercial 2K Refinish 92.9 73.0 82.8
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|Title Annotation:||Technology Today|
|Author:||Jeffries, Michael; Gambino, Charles; Pierce, Joe|
|Article Type:||Cover story|
|Date:||Apr 1, 2008|
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