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Free from fluorine-and tin-based compounds, this coating allows graffiti tag removal using high-pressure water without relying on harsh solvents.

By Forough Zarean, Amanda Schmotzer, and Timothy McCormack Wacker Chemical Corporation, United States

Graffiti has emerged as a pervasive issue worldwide, impacting communities on multiple fronts. It contributes to financial losses by causing damage to local businesses, declining ridership on transportation systems, and reducing property values. The removal of graffiti exacerbates the problem, as it is not only a costly process but often requires the use of harsh solvents or chemical cleaners that can adversely affect the environment by emitting VOCs and HAPs.

Over the years, several types of anti-graffiti coatings have been developed to address the persistent issue of graffiti. However, a significant number of these coatings fall into the sacrificial category. Sacrificial coatings either establish a weak bond with the substrate, facilitating the removal of both the coating and the graffiti tags, or incorporate fluorine-based additives. These additives can migrate to the surface, reducing surface energy and making graffiti removal easier. Since these additives deplete over time, periodic reapplication of the coating becomes necessary. Conversely, commonly available permanent anti-graffiti coatings come with their own set of limitations. Many are presented as two-component (2K) systems, which may not always align with the preferences of end-users. Additionally, some one-component (1K) permanent options include fluorine-based additives, making them susceptible to evolving regulations and legislations imposed by environmental organizations.

We have developed a new 1K permanent anti-graffiti coating using elastomeric polysiloxanes, ideal for architectural substrates such as concrete, masonry, and dry walls. As a single-component moisture-cure system, it eliminates the inconvenience of mixing multiple components. Free from fluorine- and tin-based compounds, this coating allows graffiti tag removal using high-pressure water without relying on harsh solvents. Moreover, it maintains anti-graffiti properties even after exposure to artificial and natural weathering.

Introduction

Graffiti, defined as unauthorized writing or drawing on a public surface,¹ poses an escalating issue in urban areas globally. graffiti is commonly encountered in public places, such as walls facing streets, traffic signs, statues and monuments, bridges, parking garages, park benches, and transportation systems, including trains, subways, buses, and transit stations. Regardless of its message—political, social, religious, personal, or gang related—if defaced without the consent of the property owner or official authority, graffiti is considered an act of vandalism.

Graffiti exacerbates financial losses by harming local businesses, diminishing ridership on transportation systems, and devaluing property. Furthermore, graffiti has a pronounced cumulative impact; its initial manifestation in a location tends to attract additional instances. In cases where a sizable portion of the graffiti is associated with gangs or offensive content, the affected area is more prone to an increased gang presence, leading to heightened crime rates and turf conflicts.

Graffiti vandalism imposes a significant financial burden on communities worldwide.² In the United States, the annual expense for graffiti removal averages around $12 billion. The removal process is intricate, often necessitating the use of potent solvents that can contribute to the emission of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). Simultaneously, the act of tagging graffiti itself generates VOC emissions. It is reported that approximately 4,862 tons of VOCs were emitted from the graffiti sector in the US in 2008.³

Anti-graffiti coatings fall into three categories: sacrificial, semi-permanent, and permanent. sacrificial coatings can be removed using a high-pressure washer taking the graffiti with it. These single-use coatings are usually wax-based and form weak bonds with the substrate to allow for easy removal. sacrificial coatings require frequent reapplication to protect the surface. Semi-permanent coatings are usually acrylic or alkyd-based and can withstand a few subsequent graffiti-removal processes before deterioration by the cleaning solvents. Lastly, permanent coatings are usually based on crosslinked polymeric networks such as epoxies, acrylics, and polyurethanes, where sufficient crosslinking reduces the chance of the polymer to swell and absorb graffiti paint.

In most cases, these coatings contain silicone or fluorine-based additives that migrate to the surface and reduce the surface energy, minimizing the adherence of graffiti paints to facilitate the removal. Despite their good initial performance, after multiple graffiti removals, these additives will deplete, reducing the effectiveness of anti-graffiti paint drastically. Moreover, emerging regulations and legislation prohibiting the use of fluorine-based additives are changing the market trends and customer preferences toward fluorine-free coatings. Furthermore, the traditional two-component (2K) nature of these coatings can lead to application errors. Therefore, there is a growing need for one-component (1K) permanent coatings that are fluorine-free and ensure long-term anti-graffiti performance.

Elastomeric silicones offer high water repellency, high gas permeability, and outstanding release properties due to their low surface energy (20 MJ/m2) and high flexibility of the silicone network. This outstanding flexibility is due to larger bond angles than their carbon counterparts, which enables a high degree of segmental movement and large void volume (The Si–O–Si angle ranges from about 130° to 180°, whereas the C–O–C angle is typically 107–113°).⁴ Additionally, Si–O bonds forming the siloxane backbone are stronger than C–O bonds in conventional organic backbones (452 vs. about 360 KJ/mol), resulting in superior exterior durability, gloss and color retention, and heat resistance. These unique properties make elastomeric silicones an exceptional option for the development of durable permanent anti-graffiti coatings that enable graffiti removal with high-pressure water only without the need to use harsh solvents.