The increasing adoption of advanced driver assistance systems (ADAS) and autonomous vehicles (AVs) demands highly reliable sensors that can operate effectively in adverse environmental conditions. Fog, rain, mud, and debris accumulation can compromise sensor performance, posing challenges for safe and efficient operation. To address these issues, UV-durable hydrophobic (UVH) coatings designed to improve sensor reliability have been developed.

The effectiveness of UVH coatings was evaluated under both simulated and realworld conditions. The study assessed vision cameras, IR cameras, LiDAR, and radar sensors exposed to various environmental stressors, including fog, rain, mud, insect splatter, and off-road terrain while operating the vehicle at representative speeds. The results demonstrated that UVH coatings effectively reduced environmental contamination on sensors, enhancing imaging, target recognition, and signal clarity, though the impact on LiDAR was less significant.

These findings suggest that UVH coatings can enhance ADAS and AV sensor reliability by reducing maintenance requirements and improving sensor functionality, particularly in wet and debris-prone environments. This article details the coating formulation, testing methodology, results, and broader implications for ADAS and AV systems, highlighting their potential for both commercial and military applications.

Introduction

Advanced driver assistance systems (ADAS) and autonomous vehicles (AVs) depend on a suite of sensors to interpret and navigate the driving environment. Cameras, LiDAR, radar, and infrared sensors provide critical data for object detection, environmental perception, and decision making. However, environmental contaminants such as rain, fog, mud, and debris on sensor covers can significantly degrade sensor performance, potentially compromising both functionality and safety. As a result, maintaining clean and dry sensor surfaces is essential to ensuring the reliability of ADAS and AV technologies.

To address these challenges, several automatic cleaning systems have been developed. For instance, Valeo¹ and Continental² have introduced fully automatic cleaning systems that employ retractable liquid-jet nozzles to spray cleaning fluid onto sensor lenses, effectively removing debris. Ford’s³ AV development team has designed aerodynamic shields to divert airborne contaminants away from rooftop-mounted LiDAR sensors, reducing the accumulation of dirt and moisture. While these active cleaning solutions enhance sensor reliability, they introduce additional costs related to installation, maintenance, and long-term repairs, making them less desirable for large-scale implementation.

A promising alternative to active cleaning mechanisms is the application of hydrophobic self-cleaning coatings that passively repel water, dirt, and environmental contaminants. Such coatings could significantly reduce the need for frequent cleaning cycles, minimize the consumption of cleaning fluids, and lower overall energy requirements. However, the development of hydrophobic coatings suitable for sensor applications presents several challenges, particularly in achieving high optical transparency while maintaining essential properties such as scratch resistance, UV durability, self-cleaning capabilities, and stain resistance.

Several fabrication methods have been explored to produce transparent nonwettable surfaces, including self-assembly techniques, sol-gel processes, micro-phase separation, templating, and nanoparticle assembly.^{4, 5} Additionally, mold transfer methods such as nanoimprint lithography have been employed, utilizing hydrophobic transparent elastomer precursors or UV-induced polymerization.^{6, 7, 8} Some ceramic precursor-based transparent hydrophobic films have also been proposed for applications in windows and solar energy conversion surfaces.⁹ However, many of these techniques lack the mechanical durability and weatherability necessary for long-term use in automotive sensor applications, as they suffer from poor adhesion to substrates and inadequate scratch resistance, leading to rapid degradation under environmental stressors.

To address these limitations, a fluorinated silane-based coating system was developed as a UV-durable hydrophobic (UVH) coating with enhanced UV stability, optical transmission, and mechanical durability. The coating was designed to maintain high hydrophobicity over extended periods while withstanding environmental exposure and mechanical wear. Initial performance validation was conducted in a stationary testbed under simulated weathering conditions, including fog, rain, mud, and insect splatter, confirming both the coating’s durability and its ability to improve sensor performance.¹⁰ To further evaluate real-world effectiveness, PPG Industries, Inc., in collaboration with the Nevada Automotive Test Center (NATC), conducted comprehensive field tests. The results demonstrated that UVH coatings significantly enhanced sensor clarity and functionality, reinforcing their potential as a cost-effective and durable alternative to active sensor cleaning systems.

Experimental Design

UV-Durable Hydrophobic (UVH) Coatings

Hard coat-coated borosilicate glass, as well as hard-coated polycarbonate (PC) and silicon substrates, were pretreated with air plasma for 15 min using an ATTO™ plasma treater (Diener Electronics, Germany). Proprietary UVH coatings, formulated with fluorinated silane from PPG Industries, Inc., were subsequently applied to the pretreated substrates via an ultrasonic spray coating technique. The coatings were deposited using a Prism Ultra-Coat ultrasonic spray system (Ultrasonic Systems, Inc., Haverhill, MA).

The coated samples were cured at 100 °C for a duration of 10 to 15 min. Post-curing, the static water contact angle (sWCA) was measured by placing three 2.0 μL droplets of deionized water onto the surface of each sample using Kruss Drop Shape Analyzer DSA100. The average sWCA was determined using ADVANCE software in conjunction with a Kruss Drop Shape Analyzer DSA100 Instrument. Due to the inherent hydrophobic properties of the UVH coatings, the sWCA was measured to be 115° ± 1°.

Automotive manufacturers are increasingly interested in drop-ondemand (DOD) inkjet technology for its potential to enable high-resolution, on-demand customization while improving paint shop efficiency and sustainability. Compared to traditional spray application methods, DOD technology offers significant benefits in reducing volatile organic compound (VOC) emissions, minimizing paint waste, and eliminating the need for masking and demasking processes. However, applying automotive coatings via DOD presents unique challenges due to the high solids content, viscosity, and shear-thinning behavior required for environmental compliance, application feasibility, and appearance quality.

Unlike conventional inkjet inks, which are Newtonian and low viscosity (<20 cP), automotive paints require specialized formulation and process adaptations to ensure reliable jetting. This article explores the integration of piezoelectric DOD technology with a circulation system to address key challenges such as maintaining jetting stability, achieving clean droplet break-off, preventing nozzle drying, and controlling undesirable wetting behavior. Here, we demonstrate successful jetting of waterborne and solventborne automotive coatings, highlighting the role of rheological control, droplet throw distance management, and defect mitigation strategies. Through collaboration with industry partners, these key learnings were implemented in trials printing on an automotive body. Results confirm the feasibility of this approach for OEM-scale implementation, bringing DOD technology closer to practical automotive manufacturing applications.

Introduction

The demand for personalized car designs has been increasing in recent years, with the “Tutone” two-tone color scheme (Figure 1a) experiencing a resurgence. This design, popular in the 1950s, has made a comeback as customers are willing to pay a premium for customization.¹ However, manufacturing a Tutone vehicle presents several challenges, including increased volatile organic compound (VOC) emissions from additional paint consumption, waste from masking and demasking, higher labor costs, and a more complex, time-consuming production process.

Another popular but currently non-OEM-compatible customization method is graphic design (Figure 1b) or even logos. This is typically achieved using decals, which are labor-intensive to apply. More intricate graphic designs are executed using vinyl wraps.

Drop-on-demand (DOD) inkjet technology has been used for complex graphics for years, and, in 2022, Boeing demonstrated its potential on the 737 Max 9.² Given this precedent, can a similar approach be applied to the automotive industry? The answer is yes, but with key differences. Inkjet printing traditionally relies on depositing individual pixels of cyan, magenta, yellow, and black inks. Such an approach does not result in a continuous protective coating required for automotive application.

OEM-Compatible Processes for DOD

Figure 2a illustrates how current Tutone vehicles are manufactured and the masking and demasking steps are wasteful and labor intensive. With DOD, we propose two OEM-compatible options: one in-line and one end-of line process (Figure 2b). Both would work for Tutone as well as graphic designs.

FIGURE 1 Examples of Tutone (a) and graphic (b) design.

FIGURE 2 (a) Current OEM process for Tutone vehicles. (b) Proposed in-line and end-of-line processes.

Piezoelectric DOD Inkjet and Circulation System

DOD technology generates droplets only when needed, achieved through various methods. Common techniques include (1) thermal, which is suitable only for waterborne (WB) inks; (2) valvejet, which offers low resolution and flow rate; and (3) piezoelectric, which is compatible with both WB and solventborne (SB) formulations and is the preferred method.³

In piezoelectric DOD, the printhead walls are composed of piezoelectric materials that deform when a voltage is applied.⁴ This deformation generates a pressure wave that ejects a droplet from the chamber behind the nozzles. We found that combining piezoelectric DOD with a circulation system significantly enhances paint jetting performance.

Figure 3 presents the system diagram. The paint circulates between the reservoir and printhead, driven by two pumps. A differential pressure of approximately 100–200 mbar is maintained to ensure a consistent flow rate, keeping the printhead chamber adequately supplied. A final-stage filter is installed before the printhead to capture large particles that could clog nozzles and channels. A mild negative pressure (meniscus pressure) of approximately -10 to -20 mbar at the nozzle prevents paint oozing and nozzle plate flooding while still allowing droplet ejection when triggered by the piezo-induced pressure wave. Precise and stable control of paint supply and meniscus pressure of the printhead is critical to good jetting.

FIGURE 3 Piezoelectric DOD with circulation system. (a) Setup of circulation system. (b) Cross section of the printhead to illustrate the paint circulating behind the nozzles.

Shear Rate Considerations

Paint experiences a wide range of shear rates throughout the system. Shear rate is near zero in the reservoir, around 100 s–1 in tubing, increases to ∼10,000 s–1 in printhead channels, and reaches ∼100,000 s–1 or higher at the nozzles (10–50 μm in diameter). Traditional piezo DOD inks are low-solids, low-viscosity (<20 mPa·s), and Newtonian, whereas paints have significantly higher solids content, viscosity, and exhibit shear-thinning behavior.

Figure 4 illustrates shear rates at various locations and presents examples of jettable WB formulations. Unlike inks, paints require higher pressure to achieve comparable flow rates due to their higher viscosity. Additionally, shear-thinning paints are often thixotropic. Without a circulation system, inkjet operation involves on-off cycles, leading to fluctuations in viscosity, pump demand, and system pressure, which can negatively impact jetting quality. Continuous circulation mitigates these fluctuations, stabilizing jetting performance while preventing aggregate and air bubble formation. Since automotive paints often contain fast-drying solvents, circulation also prevents nozzle drying.