Leading Edge Erosion. Effect of droplet impact frequencies and dry intervals on incubation times of polyurethane coatings

Abstract

Rain erosion of leading edges in wind turbines caused by high-velocity droplet impacts is one of the current problems in wind energy that affects their energy production and maintenance costs. The most widespread solution for this problem is the application of viscoelastic coatings (generally, polyurethane) along the leading edge of the wind blade, the area that first meets the raindrop. The focus of the present study is to assess experimentally the effect of the frequency of droplet impacts and the performance of dry intervals (this is, intervals of time at which the test stops) on the incubation times of industrial viscoelastic polyurethane PU coatings. The coated samples were tested in a Pulsating Jet Erosion Tester (PJET) facility. The incubation times, defined as the period of time until damage is observed in the material, were measured in number of impacts and in global testing time (time that the test was kept running) until damage. The two effects that are aimed to investigate, the droplet impact frequency and the dry intervals, represent different relaxation times that the coating may undergo under real-life rainfall conditions: time intervals between individual raindrops (between raindrops) and between rain events (between rainfalls). Higher relaxation times are expected to have a beneficial effect on the rain erosion performance, as the viscoelastic coating would have time to recover itself to the original stress and strain state. The effects of test conditions that may influence the PJET, such as the water cushion effect, temperature and humidity were considered and evaluated. It was found that droplet impact frequency has an effect on the number of impacts until incubation: the lower the impact frequency, the lower the number of impacts until incubation. This effect has been linked to the longer and more energetic droplets produced by the PJET for these low frequencies and is not related to the viscoelastic behaviour of the coating. The existence of dry intervals when testing significantly increased the incubation times of PU coatings. This increase was larger when the dry intervals had a longer duration and when they were performed earlier during the tests. The effects of dry intervals in incubation times have been linked to the viscoelastic behaviour of the polyurethane coating. These results are expected to shed light on the effects of testing variables (impact frequency and dry intervals) that have historically been ignored in rain erosion tests and will help the design of new test parameters for erosion testing. The conclusions of the work will help to understand the importance of relaxation times and viscoelasticity in the incubation time of PU coatings and reveal that these effects will help to refine models that aim to predict the lifetime of polyurethane coatings in wind blade turbines based on real-life rainfall data. While investigating the influence of impact frequency, effects that may affect the PJET results were investigated. The effect of the water cushion was evaluated. This phenomenon consists of a water film that remains on the coating and that affects the posterior droplet impact. Water cushion effects were found to slightly increase incubation times for the highest impact frequencies. Their effects were counteracted with an air supplier aimed at the tested sample which removed the water cushion. Humidity and temperature were found to have an effect on the incubation times: in general, the higher the temperature and the relative humidity, the shorter the incubation times. The incubation times of saturated samples which went through humidity treatments were tested: their incubation times were lower than for those samples which did not undergo that treatment. DMA analysis was performed on PU samples, but the change in mechanical properties of the coating could not be directly linked to the drop in incubation times. It is suspected that plasticization is affecting the mechanical properties of the coating. The change in the environmental conditions may also be affecting the adhesion between the coating and substrate, resulting in lower incubation times.