Greg Monaghan and Roderick Marshall, EPS Engineered Polymer Solutions04.11.23
Abstract
Many tennis and pickleball court coatings are based on conventional exterior acrylic emulsion polymers, but it is possible to engineer the polymers specifically to meet higher performance requirements in these coatings. In addition to having excellent UV resistance to withstand exposure to full sun, the coatings must stand up to abrasion from foot traffic and have good adhesion to the sand filler that is used for texture on the courts. They also must be able to be applied in thick films but still dry quickly enough that they will not blister if the coatings get wet a few hours after application. A crosslinking acrylic polymer which is designed for good abrasion resistance and water resistance was evaluated for potential use in court coatings. The performance of this crosslinking acrylic polymer is compared to a conventional acrylic used in sports court paints.Introduction
Sports court coatings are expected to meet several critical performance requirements. Good abrasion resistance is important so that the coatings will withstand the abrasion from the running and abrupt stops in tennis and pickleball games. The coatings need to have good early water resistance, so blisters don’t form if the coatings are exposed to water before they are fully cured. The coatings also need to have good adhesion to the substrates (usually concrete or asphalt) to resist the forces that create blisters and for good long-term durability.Finally, the polymers used in the coatings must have good adhesion to the sand used to give the coatings texture. The adhesion to the sand is important because the playing speed of the court can change as the sand used in the coating is lost from abrasion. The loss of sand will give a smoother surface that will cause the ball to bounce lower and will give a faster playing speed. As sand is usually lost unevenly on the surface, areas in the court can develop where the playing speed is faster than the rest of the court. A sports court coating with good sand adhesion will require less frequent recoats to maintain the desired playing speed.
Acrylic polymers could be formulated with several refinements to try to improve the performance when used in a sports court coating. Ambient temperature self-crosslinking could be built into the acrylic polymer since crosslinking has been shown to give improved abrasion resistance1 and tensile yield strength2. The acrylic polymers could also incorporate adhesion modifiers since these can help to give improved adhesion to the sand and the aggregate in the substrate3,4. Dirt pick up resistance could be improved through several strategies including using crosslinkers5, UV absorbers6, or silanes7. Finally, the amount of surfactant used in the emulsion polymers has been shown to affect water resistance8, so the acrylic would need to be formulated with the minimum amount of surfactant consistent with good polymer stability.
Materials and Methods
An experimental all-acrylic crosslinking polymer was compared to a variant of that polymer without the crosslinker. The two polymers had a Tg of 7°C, an MFFT of <0°C and were both 50% solids. In a second round of experimentation, the crosslinked experimental polymer was compared to a competitive acrylic polymer commonly used for tennis and pickleball court coatings.A sports court color topcoat was formulated with the different polymers at 20.6% PVC, 32 gm/Liter VOC and 36% volume solids. Polymer levels were adjusted on a volume solids basis and 4% of a hydrophobic coalescent based on polymer weight solids was included. The color topcoat formulation had 5% chrome green pigment and 12% nephylene syenite extender pigment on total formula weight. A polyacid dispersant was used and the formula was thickened with a combination of HEC thickener and attapulgite clay. No fibers were used in the experimental color topcoat formulation so that tensile and elongation could be run. Some of the testing was run using the diluted color topcoat with added 80 mesh sand to simulate the sand filled textured topcoat.
Test Methods
The two experimental polymers with and without the crosslinker were evaluated for the following tests.Early Blister Resistance: A 20 mil wet film was drawn down on a vinyl scrub chart, dried for 4 hours at room temp and 50% relative humidity, then half submerged the panels in water and soaked overnight. The amount of blistering was recorded using a 1 -10 scale for average blister size (10 = no blistering) and density of blisters (None, Few, Moderate, Dense).
Abrasive Scrub Resistance: Paints without added sand were drawn down side-by-side using a 7 mil Dow blade in a vinyl scrub chart. The panels were aged for one week, then the abrasive scrub resistance was checked on a Gardner scrub Tester using a scrub brush and SC-2 Leneta abrasive scrub testing media. Testing was run over a shim and the number of cycles to first break and cru through across the shim were recorded.
Tensile / Elongation: Elongation was tested by first drawing down each paint with a 15 mil block bar onto release paper. After aging overnight, a second layer was applied directly atop the aged paint and allowed to age for two weeks, peeling the paint and flipping it over after one week. Ten strips were then punched out from each film, measuring 0.5” x 3”, and stretched on a United Model STM-10 kN Tensile Tester using a 10 lb load cell at a rate of 2”/min. Average Tensile and elongation for the 10 samples were recorded.
Dirt Pick Up: Dirt pick up resistance was performed by drawing down each paint onto untreated aluminum QUV panels with a 15 mil block bar. Each panel aged for one week, then exposed for 18 hours to QUV-A bulbs. Half the panel was coated with an 83% red iron oxide solution and allowed to dry overnight. The following day, the dirt solution was wiped off using a damp paper towel, removing any residue. ∆E was recorded of the clean area vs. the wiped area.
In the second part of the study, the competitive polymer and the crosslinking acrylic were evaluated for the same tests as well as the following additional tests.
Taber Abrasion Resistance: Paints were tested with and without added sand using a Taber Abrader to evaluate sand retention. Each paint was brushed onto a wooden panel with a single coat and allowed to age overnight, followed by a second coat. The panels were then allowed to age for one week. Panels were weighed prior to abrasion, then abraded for 100 or 200 cycles using CS17 Wheels and 500 grams weights on a Model 5135 Taber Rotary Platform Abrasion Tester. Once completed, samples were weighed again to record weight loss.
Water Swell: Water swell was run by casting films on release paper, then cutting a 1 inch square section. Sections were weighed, then soaked for one week. Sections were dried quickly on a paper towel to remove water from the surface, then the sections were reweighed.
Low Temperature Flex: To test low temperature flexibility, paints were drawn down on a treated aluminum QUV panel with a 40 mil block bar and aged for three days, followed by five days in a 120˚F oven. The panel was then cooled to -15˚F. While still at -15˚F, the panel was bent over the ½” mandrel. Samples were assessed for cracks and other flaws in the film along the bend.
Pull Off Adhesion to Concrete and Asphalt: Coatings were brushed out in two coats on sections of aged concrete or aged asphalt in the parking lot. The dry adhesion to concrete and asphalt was tested using an Elcometer 106 / 6 with 2” dollies. Dollies were glued to the coating using a 2 part epoxy adhesive and cured overnight. The force to pull the dolly and the failure type was recorded.
QUV-A: For accelerated weathering, each paint was drawn down on untreated aluminum panels using a 15 mil block bar and allowed to age for one week. Once aged, each panel ran for 1000 hours in a QUV with according to ASTM G154 cycle 1. ∆E and gloss change was measured for the exposed area compared to the initial color and gloss of the panel.
Results: Effect of crosslinking
The results of the lab evaluation of the acrylic without crosslinking compared to the crosslinked analogue in the sports court coating are given in Table 1.Since sports court paints are subject to abrasion from foot impacts while running and from ball strikes, the toughness and abrasion resistance of the film is important.
One measure of the film toughness is the abrasive scrub resistance of the polymer, although relative water and alkali resistance of the polymers probably also play a role in this test. The paint made with the crosslinked acrylic was found to have much better abrasive scrub resistance than the paint made with the acrylic analogue without the crosslinking. Improvements in scrub resistance have been reported for several types of crosslinked polymers9,10 and it is likely that the higher abrasive scrub in this study was related to the intra particle crosslinking of the acrylic polymer. The increased toughness of the crosslinked film was shown in the tensile and elongation measurements. The crosslinked film had higher tensile strength (582 psi) than the uncrosslinked paint (249 psi) (Figure 1). The increase in mechanical properties after crosslinking has also been reported in several studies11,12 for different kinds of crosslinking. The improved scrub resistance and a higher tensile strength may indicate that the crosslinked acrylic would stand up well to the abrasion from the impacts from the tennis ball or foot traffic in a tennis or pickleball game.
If the sports court paints are too soft, they can collect dirt from foot traffic or from the environment, which can lead to discoloration. In a lab test for dirt pick up where iron oxide slurry is applied to the panel, softer films often retain more of the iron oxide pigment, giving a higher Delta E color measurement. The lower Delta E value for crosslinked film in the laboratory dirt pick up test may indicate that the crosslinked film may have improved dirt pick up when used on sports courts. This improvement in dirt pick up resistance may lead to improved appearance as the court ages.
In tennis and pickleball court coatings, water resistance is also very important because blistering of the coating is a frequent cause of complaints. Blistering can occur when a partially cured coating is exposed to ponding water or rainfall. Since the coatings are typically applied in two relatively thick coats in a single day, it can be difficult to get good blister resistance if the coatings are exposed to an unexpected rainfall after only a few hours of dry. In this study, both paints had very good blister resistance if they were allowed to cure overnight, but the paint with ambient temperature crosslinking had a significant improvement in the early blister resistance compared to the uncrosslinked control (Fig 2). With the improved early blister resistance, the crosslinking technology may help prevent blistering complaints if there is a sudden rainstorm after only a few hours of dry.
Ambient humidity was found to be important in the development of early blister resistance, however. At 65% humidity, the crosslinked paint required longer (6 hours instead of 4 hours) to reach blister resistance. Even with that 6 hour time to develop blister resistance, the crosslinked acrylic was still much better than the non-crosslinked control since the curing of that control paint was also slowed.
The self-crosslinking polymer was better than the uncrosslinked control for scrub, tensile strength, dirt pick up resistance and early blister resistance, all of which may indicate improved performance in sports court paints. To determine if the performance would be commercially acceptable, the crosslinked control was compared to a competitive acrylic polymer which is commonly used in sports court coatings.
Results: Comparison to competitive polymers
The crosslinking polymer was compared to a competitive acrylic polymer which is used in sports court applications in Table 2. Polymer 1 is the experimental self-crosslinking acrylic, with 50% solids, 7°C Tg and 0°C MFFT. Polymer 2 is also a competitive all acrylic used in sports court applications at 65% solids, 12 Tg and 9 MFFT. The sports court formulation was also used in this part of the testing, replacing the polymers on a volume solids basis, and some of the testing was done with added sand.Abrasive scrub resistance is often considered a measure of abrasion resistance. The scrub was run side by side with the crosslinked control in formulations without added sand. The paint with the ambient temperature crosslinking had much higher abrasive scrub resistance than the competitive polymer.
Another test of abrasion resistance is the Taber abrasion test. In this test painted panels are exposed to a rotating abrading wheel and weight loss is measured. Taber testing was run on paints formulated with and without sand added. There wasn’t much difference in weight loss without the sand, but in the testing with the sand (Figure 5), the crosslinking acrylic had much lower weight loss than the competitive acrylic. One possible reason for the lower weight loss for the crosslinked polymer in the formulation with sand might be that the polymer has better adhesion to the sand, either because of the crosslinking or because of the adhesion modifiers that were used. This may indicate the sand retention of the crosslinked acrylic is better than the competitive polymer and that the crosslinked acrylic will have less sand loss on sports courts. Since the sand loss can lead to a degradation of playing properties which can only be remedied by recoating, the crosslinked polymer may require fewer recoats.
The early blister resistance of the paints made with the two polymers is shown in Figure 5. In this test where a thick film is dried for 4 hours at 50% relative humidity, the crosslinked acrylic was much better for early blister resistance than competitive polymer #2. The early blister resistance of the crosslinked acrylic may help prevent blistering failures if the fresh film is exposed to rain after only a few hours of dry. The better blister resistance of the paint made from the crosslinked polymer might be due to a less water sensitive film.
One other test of water resistance is a water swell test. In this test, free films of the paints are immersed in water for a week and the weight change is measured to indicate how much water has been taken up by the dry film. The results of the water swell test are shown in Figure 6. Ambient temperature self-crosslinking has been shown to reduce water uptake in clear films13, although to a relatively small degree. In this case the improved water swell compared to the competitive polymers may be due to a combination of crosslinking, monomer selection and surfactant differences between the polymers.
The sports court paints also need a balance of flexibility and toughness. Flexibility is needed so that the films will not crack if the substrate expands or contracts because of changes in temperature. Film toughness contributes to good abrasion resistance. The tensile and elongation numbers indicate that the crosslinked polymer had higher tensile strength, indicating more film toughness, but lower elongation than the competitive polymers. If the elongation is too low, the paint film might be expected to crack at low temperatures, so a low temperature flexibility test was also run. In this test the panels are aged for one week then bent over a quarter inch mandrel after equilibrating the panels at -15 degrees F. The amount of cracking where the paints were stretched at the bend is considered a measure of how much the film can move with the substrate at low temperature. The panels from the low temperature flex test are shown in Figure 7. Despite the lower elongation, the crosslinked polymer had better crack resistance than the competitive polymer and would be expected to flex even with the stresses due to extreme temperature changes in
the field.
The panels from the dirt pick up resistance test are shown in Figure 8. The crosslinked acrylic had less discoloration and better dirt pick up resistance than the competitive polymer.
Adhesion to the substrate is also important. Paints with good adhesion will not become disbonded as easily if the substrate expands or contracts, and they can better resist the forces that can cause blistering if the film is exposed to water. Both paints had good adhesion with substrate failure in a pull off test that was run on aged concrete and asphalt parking lots and would be expected to have good durability in sports courts built on concrete or asphalt substrates.
Conclusion
When compared to an uncrosslinked acrylic in the same formulation, the experimental crosslinked acrylic had several significant improvements in performance when formulated as a sports court coating. The crosslinked acrylic had better early blister resistance, scrub resistance, dirt pick up resistance and highertensile strength.
The advantages of the self-crosslinking and overall polymer design were also seen in the comparison to the competitive commercial acrylic polymers used for sports court paints. Compared to the competitive polymers, the crosslinked acrylic had better scrub, dirt pick up resistance and higher tensile strength. In addition, the crosslinking acrylic had much better Taber abrasion resistance in the formulation with sand, which could indicate better sand retention. Finally, the early blister resistance of the crosslinked polymer was significantly better than one of the competitive products and equal to the other.
The balance of lab tested properties of the crosslinked acrylic was very good overall and the performance when applied to sports courts was also expected to be very good. In practice, this is what was found. This technology has been commercialized and is being used successfully on sports courts.
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