in Hard, Scratch-Resistant Coating Applications
Hard, scratch-resistant, acrylate-based components are used in coatings for plastic substrates in a broad range of applications, from electronics, communications, semiconductor and data storage to optics, automotive, aerospace and medical devices. Just as the applications are wide and diverse, the variety of plastics that may be used has increased over time, and now includes materials such as polyethylene teraphthalate (PET), poly(methyl methacrylate) (PMMA) and polycarbonate (PC).
This article describes a range of high-functionality UV-curable products, including both 100%-solids oligomers and waterborne polyurethane dispersions, which can be formulated for excellent scratch and abrasion properties as well as weathering performance, supporting their use in exterior coating applications.
Highly Functional Urethanes
Aliphatic urethane acrylates (UA) having a polyester backbone structure were investigated. This type of oligomer has long been recognized as having a chemical structure that exhibits excellent durability when subjected to harsh environmental exposure, in either naturally occurring or accelerated weathering conditions. A higher crosslink density generally increases a coating’s hardness and scratch resistance. Thus the oligomers selected for this study ranged in acrylate functionality from 6 to 9 units per molecule and, for the purpose of this work, are identified as CN9006, CN9026, CN9025 and CN9013.
Data is also presented for a family of UV-curable polyurethane dispersions, providing a comparison of physical properties as they relate to wear resistance performance. These dispersions are identified as PRO12874, CN9500 and CN9501.
Urethane Acrylate Hard Coats
The first part of this investigation focuses on a series of highly functional aliphatic urethanes and assesses their protective coating properties on a variety of substrates. Table 1 provides a brief description of the oligomers and their attributes.
Tabered Haze Testing
Initially the performance of the neat oligomers was quantified by tabered haze testing, using a Taber wheel, which assesses the effect of surface abrasion on the change in haze for a clear coating. A description of the application conditions and performance results is provided in Figure 1.
As these oligomers vary widely in viscosity, acetone was added to each to allow good film thickness control. A photoinitiator (PI) was added to allow for UV curing. Each mixture was applied to a transparent substrate and UV cured after removal of the solvent, yielding a dry film thickness of 75 microns. An epoxy acrylate (CN120) oligomer was also tested for comparative purposes.
The light transmission properties were then measured before and after Taber testing. The decrease in percent of light transmission is reported as the Delta haze. The results demonstrate a dramatic shortfall in abrasion resistance of the epoxy acrylate (CN120) as compared to the urethane acrylate (UA) family of oligomers. This result is due in part to the higher functionality of the urethane acrylates, which results in improved surface cure. However, better flexibility of the urethane acrylate materials is also a contributing factor.
Other tests were conducted to quantify the hardness characteristics of each oligomer, and results are listed in Table 2. The glass transition temperature (Tg) of a cured film is a fairly good indicator of hardness. Typically, a higher glass temperature yields a film with higher surface hardness. However, high hardness does not always equate to good abrasion resistance. The steel wool resistance data demonstrate this effect. Oligomers having lower Tg values passed the steel wool test while those with higher Tg values failed.
Taber Testing by Weight Loss
Whereas the Tabered haze test quantifies surface scratch and abrasion resistance qualities of a coating or film, the Taber abrasion resistance test measures the bulk properties. The test is typically conducted on a thicker coating using a more aggressive Taber abrasion wheel (CS17 instead of CS10). The oligomers were tested in a formulation that included a low-functionality monomer to better control film thickness and to improve ductile properties. Figure 2 summarizes the base formulation, the Taber test conditions and the Taber weight loss results for each oligomer formulation. The tests were conducted on 50-micron-thick cured films applied to aluminum test panels.
Again, poor abrasion resistance of the epoxy acrylate oligomer was noted, with a weight loss of 140 mg after 500 Taber cycles. By contrast, the urethane acrylate oligomers exhibited substantially lower weight loss. It is also noteworthy that the highest-functionality oligomer (CN9013) had poorer results than those having a functionality of 6. This would suggest that high hardness and crosslink density do not always drive good abrasion resistance because the resiliency and ductile properties of the cured film also influence the abrasion performance. In other words, the ability to resist abrasion is controlled both by hardness and lack of brittleness. This concept is also supported by the excellent performance of CN9026, which was chemically designed for enhanced flexibility.
Weathering Resistance Testing
To be effective as a protective barrier a coating must have excellent scratch and abrasion resistance, and must not degrade when exposed to harsh environmental conditions. When high-functionality oligomers are used for enhanced scratch resistance, cracking of the coatings may result, especially in thicker film sections. The UA oligomers were applied to a substrate at film thicknesses of 5, 10 and 15 microns. The cured samples were then subjected to accelerated laboratory weathering in a QUV® test chamber. The QUV test conditions were varied to include a cycle of 8 h of UV radiation at 60 °C, followed by 4 h of dark condensation at 40 °C. The test chamber was fitted with UVA 340 lamps. The UVA 340 lamp spectral output ranges from 300-400 nm and is centered at 340 nm. This lamp most closely replicates the emission spectrum of sunlight. Both the degree of yellowing and the gloss retention of the film were measured as functions of time and the results were recorded.
Since the UA oligomers tested have an aliphatic structure, no change in color or increased yellowness was detected with time. The gloss of each coating was also measured, and differences were noted between the oligomers. In each case a loss of gloss was observed, which is due to micro-fracturing of the hard coat. The severity of fracturing is related to both acrylate functionality and film thickness. As oligomer functionality and the thickness of the coating are increased, the extent of fracturing increases. The exceptions to this are the oligomers that were modified to have improved flexibility in combination with high acrylate functionality, specifically CN9026 and CN9025.
No fracturing of the CN9025 oligomer was observed after 5000 h of exposure at a 5 micron film thickness. Fracturing was noted after 1500 h at a 10 micron thickness and after only 700 h exposure at 15 microns. The CN9026 oligomer performed well regardless of film thickness, and no fracturing was observed for any film thickness after 5000 h of exposure, the full duration of the test. These results are shown in Table 3.
Urethane Acrylate Hard Coat Summary
Aliphatic urethanes with high acrylate functionality result in cured films having excellent abrasion resistance properties. Higher functionality yields a higher crosslink density, which, in general, is desirable. However, the downside of increased crosslink density can be a decrease in abrasion resistance and an increase in micro-cracking of the cured film during weathering exposure, particularly when coating thickness exceeds 5 microns. These limitations can be controlled by modification of the backbone structure to impart increased flexibility or by the addition of lower-functionality acrylates to the formulation to reduce crosslink density. These adjustments improve the ductile properties of the film, making it more malleable, resulting in cured film with better abrasion resistance and a lower propensity for micro-cracking during weathering.
Substrate Weathering Protection
Protection of the Coated Substrate
An important attribute of a barrier coating is the ability to prevent degradation of the end product onto which the coating is applied. To examine this feature a scratch-resistant hard coat was applied to a polyester (PET) film and exposed in the QUV for 2000 h. Where noted, a light stabilizer was added to the hard coat formulation. The difference between this testing and the previous weathering data is that ?E is tracked with QUV exposure instead of degree of yellowness (YI). This is a measurement of the total color change. The readings were taken on 250-micron-thick PET with and without the hard coat applied. In this way the degradation of the PET can be monitored and compared to the coated stock to demonstrate the effectiveness of the coating for protecting the PET substrate. The base formulation, and the cure and application parameters are detailed in Table 4.
?E Test Results
The coated PET samples performed well during this testing regardless of the level or type of added light stabilizer. All of the coated PET samples had a maximum ?E of approximately 2.0 after 2000 h of QUV exposure. By contrast, the ?E of the uncoated PET sample was 4.5 after 1300 h and reached a value of 8.5 after 2000 h of exposure, which indicates significant discoloration. The detailed results are presented in Figure 3.
The gloss retention of the same PET films (coated and uncoated) was also monitored as a function of QUV exposure time. The effectiveness of the light stabilizers can be observed in Figure 4.
The coated PET without light stabilizers showed gloss retention of 70% of its original value, while the PET coated with the light stabilizer-containing formulation retained 86% of its initial gloss after exposure. Finally, the uncoated PET performed very poorly and maintained only 3% of its initial gloss value.
The light transmittance properties of uncoated PET and coated PET, with and without added hindered amine light stabilizer (HALS), were measured using UV/Vis as a function of time during 2000 h of QUV exposure. The test results (Figure 5) demonstrate the ability of the UV-cured coating formulation to protect the PET substrate from degradation. The protective properties are somewhat enhanced by the addition of a light stabilizer.
Substrate Performance Observations
Certain grades of PET will degrade when exposed to heat, light and moisture for extended periods. A properly formulated coating can lessen the degradation and provide a protective wear layer for the PET. The addition of HALS and light absorbers can further enhance protection.
UV-Cured Polyurethane Dispersion Hard Coats
Waterborne Oligomer Testing
A series of UV-curable polyurethane dispersions (UV-PUD) was evaluated for scratch and abrasion resistance performance in a range of application-related tests. Table 5 lists the waterborne UV-PUD oligomers tested along with the corresponding liquid properties for each. The typical solids content for many UV-PUD materials on the market is 35% or less, and typical viscosities are greater than 20 cps. It should be noted that the oligomers tested have an average solids content of 40% with a viscosity range of 8-15 cps. The higher solids content coupled with a lower viscosity offers more latitude in formulating industrial coatings.
Application Properties of Waterborne UV-PUD Oligomers
Each of the UV-PUD oligomers was formulated using standard industry additives to give good film forming properties and was tested over a variety of application parameters, including flexibility, hardness, abrasion resistance, stain resistance, water resistance and weathering resistance. The solids content of each was adjusted to 35% to offer a consistent basis of comparison.
Each formulated UV-PUD oligomer was applied to the substrate at a wet film thickness of 150 microns, which resulted in a dry film thickness of approximately 50 microns. Laboratory drying procedures (30 min at room temperature followed by 30 min at 60 °C) ensured that all water was removed before UV curing. Drying conditions for production situations would vary depending on the equipment used, but typically 10 min at temperature in the range of 50-60 °C is sufficient.
Figure 6 shows the experimental approach used for each UV-PUD. A relative ranking of end-use performance for each of the properties tested is displayed as well. All of the oligomers performed well in the stain resistance, water resistance and solvent resistance testing. The control UV-PUD (a widely used commercial UV-PUD product) was found to be lacking in flexibility and QUV yellowing resistance, but was acceptable in terms of hardness and abrasion resistance.
The UV-PUD oligomers show excellent water resistance. For example, none of the cured films whitened after direct exposure to hot water. Also of note is that the QUV accelerated weathering results are comparable to those for an aliphatic urethane.
Surface Scratch Testing
Given that CN9501 (PUD 3) shows good abrasion resistance, stain resistance and superior moisture resistance, it was chosen for comparison to the competitive control for surface scratch resistance.
Figure 7 describes the surface scratch test method used. The gloss retention was measured for loads from 100 g to 500 g in 100-g increments. At each load the sample received 50 abrasion cycles with a white abrasive pad (3M ScotchBrite). The control material had much poorer resistance to abrasion and showed a much more significant loss in gloss than the CN9501 sample. This PUD maintained a relatively high level of gloss for the duration of the test.
Whereas the surface scratch test provides information about the surface properties of the cured film, the Taber abrasion resistance test assesses the bulk abrasion resistance properties of the coating. CN9501 and the control UV-PUD were tested by Taber abrasion using a CS17 Taber wheel, with weight loss measured every 200 cycles (up to 1000 cycles total). This protocol employs a much more aggressive abrasive medium compared to the abrasive pad used in the surface scratch test, which makes this test significantly harsher. Based on the weight loss results shown in Figure 8, CN9501 is substantially more resistant to abrasive wear than the control material.
Scratch and Abrasion Resistance
- Aliphatic urethanes with high acrylate functionality result in cured films having excellent abrasion resistant properties.
- High oligomer functionality results in a high crosslink density, which increases hardness. However, depending on the flexibility of the formulated coatings, high functionality may result in micro-cracking of the cured film during weathering, particularly when coating thickness exceeds 5 microns.
- Two approaches can be taken to improve film flexibility and ductile properties to improve abrasion resistance and address micro-cracking for high-functionality oligomers: modification of the backbone structure to impart better flexibility; and/or addition of lower-functionality acrylates to the formulation to reduce film crosslink density.
- Certain grades of PET will degrade when exposed to heat, light and moisture for extended periods.
- A properly formulated UV coating can dramatically lessen the PET film degradation by providing a protective wear layer. This protective function can be applied to other substrates as well.
- The addition of HALS and light absorbers to the UV coating can further enhance weathering protection.
- The family of UV-PUD materials examined offers a higher reactive solid content at a lower viscosity when compared to typical UV-PUDs on the market. These features provide greater formulating latitude for industrial coatings.
- When compared to the control UV-PUD, enhanced surface scratch resistance, flexibility, yellowing upon QUV exposure and abrasion resistance are noted for the UV-PUDs studied.
- The UV-PUD oligomers show excellent water resistance. For example, no whitening of any of the cured films was noted after surface exposure to hot water.
- The QUV accelerated weathering results for the UV-PUD oligomers are excellent; these materials have comparable weathering resistance to those of an aliphatic urethane.
This paper was originally presented at the 2014 RadTech UV & EB Technology Expo and Conference.
By William Schaeffer, Steven Tyson, Indu Vappala and Robert Kensicki | Sartomer Americas, Exton, PA