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About this book

​This volume provides perspectives on the approaches, mechanisms, test methods, durability considerations, and environmental concerns for contamination mitigating coatings and polymers with emphasis on their use in more extreme aerospace and marine terrestrial environments. Parts of the Volume are devoted to application of biomimetics to contamination mitigation polymeric coatings, low ice adhesion surfaces, insect residue adhesion resistance coatings, and marine biofouling mitigation materials. By juxtaposing ice insect, and marine mitigation approaches, researchers and users may more easily identify threads of similarity that will assist in future developments and potential applications in these areas. The volume is of interest to chemists and material scientists in providing awareness of both the need for efficacy in mitigating contamination and for appropriate coating durability; to physicists in providing better understanding of the interaction between the contaminant, the coated surface, and the surrounding environment; and to engineers in describing the need for better scale-up tests between laboratory and field environments.

Table of Contents


Aerospace and Marine Environments as Design Spaces for Contamination-Mitigating Polymeric Coatings

Aerospace and marine environments are two of the most challenging arenas for durable coatings. This introductory chapter for the Advances in Polymer Science volume “Contamination-Mitigating Polymeric Coatings for Extreme Environments” gives an overview of materials and test method advances pertaining to ice and insect mitigation for aerospace coatings and biofouling mitigation for marine coatings. Each of these topics is then discussed in greater detail by subject matter experts in the following chapters. A common challenge in these arenas is the cost, complexity, and limited availability of field measurements, necessitating the need for laboratory-scale testing and the setting of benchmarks. An example is provided showing the complexity of setting a benchmark for maximum ice adhesion strength to anti-contamination coatings allowing passive ice removal by wind or vibration. Modeling ice as a cantilever beam on a coating surface in a wind stream indicates that the benchmark value is dependent on the assumed shape of the ice that needs to be removed.
Douglas H. Berry, Christopher J. Wohl

Application of Biomimetics to Contamination-Mitigating Coatings


Influence of Topography on Adhesion and Bioadhesion

Nature, through evolution, has developed many different structured adhesive systems to create strong and reliable adhesion on various substrates, including those with rough or smooth surfaces under dry and wet conditions. However, the details of the adhesive interactions of structured or roughened surfaces are just beginning to be resolved. This chapter examines the physical principles of dry and wet adhesion of structured surfaces from simple to complex geometries. A particular emphasis is placed on bioadhesive systems that achieve an impressive level of control over adhesion via fascinating structural features such as fibrils and spines. The influence of surface morphology and roughness on adhesion is also covered. Recent studies show that the attachment abilities of bioadhesive systems are dramatically reduced below a critical roughness. Based on this and other principles borrowed from nature, strategies can be pursued to create anti-adhesive surfaces via manipulating the surface topography of the substrate.
Donglee Shin, J. Carson Meredith

Ice Contamination-Mitigating Coatings


Use of Liquid Ad(ab)sorbing Surfaces for Anti-icing Applications

Aizenberg and co-workers developed slippery liquid-infused porous surfaces (SLIPS) where a textured solid was infused with an immiscible perfluorinated lubricant with low surface free energy to create a smooth liquid over layer. The lubricant was selected to minimize ice nucleation and ice adhesion strength. Many slippery surfaces using various lubricating fluids (perfluorinated, silicone oil, hydrocarbon, and water) have been produced in the last 5 years. As the possible interactions between the lubricant and the patterned or porous solid structure would be adsorption or absorption in scientific terms, in this work the terminology commonly used to describe these systems, “infuse (or impregnate),” is replaced by defining these materials as “liquid ad(ab)sorbing surfaces” (LAAS) instead. In this review, initial discussion is on two main conditions to form ice on surfaces, that is, ice formation from impinging supercooled droplets and frost formation from atmospheric humidity. This is followed by a summary of the synthesis and properties of flat low surface energy and superhydrophobic anti-icing surfaces, the basic reference surfaces used in passive anti-icing. Then the synthesis and anti-icing performances of LAAS are reviewed in three subtopics, depending on the lubricant used in their preparation such as fluorinated lubricants, silicone liquids, and hydrophilic liquids. The factors affecting ice adhesion strength are discussed and the importance of ice accretion experiments is highlighted. The need for standardization of both measurements is stressed in order to compare the results reported from different laboratories. Finally, some promising approaches are recommended for future research such as the use of LAAS in combination with traditional anti-icing and deicing techniques, superhydrophobic surfaces, or Joule heating systems to decrease the amount of electrical energy supplied.
H. Yildirim Erbil

Ice Adhesion on Superhydrophobic Coatings in an Icing Wind Tunnel

Researchers have recently focused on superhydrophobic coatings as an ice-mitigation tool. These surfaces have a high degree of water-repellency and were shown in previous low-speed droplet studies to reduce surface ice adhesion strength. However, there has been little research regarding testing in aerospace icing conditions, that is, high-speed super-cooled droplet impact (>50 m/s) on a cold substrate in an environment where the air temperature is below freezing. A detailed set of experiments was conducted in an icing wind tunnel to measure the ice adhesion strength of various superhydrophobic coatings by subjecting the surfaces to a super-cooled icing cloud consisting of 20 μm droplets at a constant liquid water content (LWC) of 0.4 g/m3. Test conditions included air speeds of 50 and 70 m/s and in glaze (−5°C) and rime ice regimes (−15°C). The accreted ice was then removed by pressurized nitrogen in the tensile direction in an ice adhesion test. The pressure required for ice removal and the fraction of ice remaining were combined into an overall adhesion parameter (AP). Results showed that the present superhydrophobic coatings generally resulted in increased ice APs relative to the baseline titanium surface. The strongest indicator of ice adhesion performance for these coatings was found to be the surface roughness lateral auto-correlation length. Only superhydrophobic coatings with length-scales less than 40 μm reduced the ice AP. When compared to previous results, it can be seen that increased droplet impact speeds tended to increase the ice adhesion strength on the superhydrophobic coatings. This was because of the increased droplet impact Bernoulli and hammer pressures which exceeded the resistive capillary pressure of the surface features induced by large surface lateral auto-correlation lengths.
Yong Han Yeong, Jack Sokhey, Eric Loth

Icephobicity: Definition and Measurement Regarding Atmospheric Icing

Atmospheric ice that adheres to structures and accumulates is a critical issue in numerous northern areas. Even if different de-icing methods exist, they consume a great deal of energy or necessitate elaborate infrastructures. However, using coatings with icephobic properties could be the “miracle” solution. This chapter proposes a complete definition of icephobicity in line with the ice adhesion test methods used. The general way to assess this property is described using a holistic approach, the first step of which is a screening test campaign with many different candidate coatings evaluated in terms of their adhesion reduction factor (ARF). The relevance of this factor is also discussed. Further tests are recommended, after the better candidate coatings are identified, in an extensive test campaign performed under simulated icing and outdoor conditions prevailing in the real environment of the targeted application. Finally, a specific example of a test campaign in which the icephobic coatings are exposed to Arctic offshore conditions is presented.
Jean-Denis Brassard, Caroline Laforte, Frederic Guerin, Caroline Blackburn

Development and Testing of Icephobic Materials: Lessons Learned from Fraunhofer IFAM

The effective development of icephobic coatings requires test scenarios that simulate relevant icing conditions in the desired application field. Because of a lack of available standardized tests, developers rely on comparative tests, comparing results with pre-defined benchmark systems. In this context, tests need to be conducted under very stable conditions without allowance for fluctuation. This guarantees a development process with stepwise material improvement. Fraunhofer IFAM in Germany began working on icephobic materials over 10 years ago, seeking to develop not only the materials but also adequate test methods. This chapter describes the development process and results of these activities, as well as proposals to improve efficiency further in the future development of icephobic materials.
Nadine Rehfeld, Andrej Stake, Volkmar Stenzel

Ice Release Coatings of High Durability for Aerospace Applications

Current in-flight aircraft anti-icing and de-icing systems rely on active methods such as heat for ice mitigation, which tends to reduce the operating efficiency of the aircraft. Significant research is currently ongoing to develop anti-icing coatings for passive ice removal from aircraft surfaces. Although significant coating advances have been achieved in reducing ice adhesion and accretion, the majority of the developed prototypes cannot survive the harsh operating environments of an aircraft. Therefore, the goal of this work was to develop a coating with significant ice adhesion reduction and of sufficiently high durability to withstand typical aerospace operating conditions (with the exception of conditions at the wing leading edge areas). Low ice adhesion topcoats and clear coats have been developed based on qualified exterior aerospace coatings, and ice adhesion tests showed a decrease of up to 95% in the ice adhesion strength as compared to control coatings. These coatings are also as durable as current polyurethane aerospace topcoats. For example, the coatings did not sustain any damage for up to 30 min in a rain erosion test conducted at typical rain impact speeds to approximate real flight conditions. In addition, ice adhesion tests performed on degraded coatings (1,700 h of QUV Accelerated Weathering Test, UVB-313 nm lamp) showed that the ice release properties were retained.
Guangliang Tang, Yong Han Yeong, Mikhail Khudiakov

Antifogging and Frost-Resisting Polymeric Surfaces

Fogging/frosting can significantly reduce clarity of a transparent substrate, resulting in not only inconvenience but also potential danger in daily life. There has therefore been great demand for effective antifogging/frost-resisting surfaces to maintain visibility and transparency in high-humidity environments in a variety of applications. Although tremendous efforts have been made to prepare inorganic antifogging surfaces, the primary focus of this review is on polymeric antifogging/frost-resisting surfaces, as polymer-based antifogging/frost-resisting surfaces hold great promise for large-scale preparation via easily implementable techniques and, subsequently, find more practical applications. In this chapter, recent progress is reviewed in the design, preparation, and typical properties of various antifogging/frost-resisting polymeric surfaces.
Jie Zhao, Lingjie Song, Weihua Ming

Insect Contamination-Mitigation Coatings


An Overview of Insect Residue Accretion and Mitigation Strategies on Aerodynamic Surfaces

Research in the utilization of laminar flow technologies on aircraft to improve fuel efficiency (and hence be more environmentally friendly) has been ongoing since the conclusion of World War II. A persistent issue with regard to the maintenance of laminar flow, however, is insect residue accretion. The residues are recurrent, distributed randomly across the surface, and have the potential to exceed heights that can result in a premature transition to turbulent flow. Numerous approaches have been explored over the decades, with success being seen in a research setting. In general, implementation of these approaches in the commercial sector has not been realized. These approaches are briefly discussed herein, along with a general description of the relevant insect characteristics and a discussion of why the reduction of insect accretion on aircraft is such a challenging problem.
J. G. Smith, R. Robison, E. Loth

The Physics of Insect Impact and Residue Expansion

The phenomena of insect impacts on aircraft surfaces are complex; therefore, tailoring surface properties to minimize the adhesion of insect residues is particularly challenging. By using a first-level approximation model, where an insect impact can be approximated as a water droplet impact, information can be leveraged toward understanding insect residue accretion. However, an insect consists of an exoskeleton covering hemolymph, which is significantly more complex than water. Therefore, the energetic requirement for exoskeleton rupture will be briefly considered and the properties of the hemolymph will be discussed. Next, relevant phenomena from water droplet impact studies will be used to describe events that occur during an insect impact. A more detailed analysis of the particular water droplet impact phenomena needed to describe and model insect impact events will be reviewed. Based on this analysis, several conclusions can be drawn regarding the surface properties most likely to be beneficial for the prevention of insect residue adhesion. Among these are the requirements for moderate hydrophobicity and surface roughness, as well as recognition that the prevention of insect residue adhesion can be approached by targeting specific phases of the event itself.
Christopher J. Wohl, Frank L. Palmieri, John W. Connell

Laboratory Investigation into Anti-contamination Coatings for Mitigating Insect Contamination with Application to Laminar Flow Technologies

Insect contamination on aircraft leading edge surfaces can result in premature transition of the boundary layer, leading to an increase in skin friction drag and fuel consumption. An evaluation of candidate anti-contamination coatings was undertaken. Coatings were characterized before impact testing. Surface energy was quantified by dynamic contact angle analysis and surface roughness measured using a profilometer. Superhydrophobic coatings showed a reduction in contamination when compared to the higher surface energy specimens tested. The surface topography and chemistry, in particular the sliding angle of a coating, were found to have a significant influence on the effectiveness of a coating. Insect residue areas were theoretically predicted using high-speed liquid droplet theory and compared to experimentally obtained results. Tests with different insect species were conducted to investigate the effect of insect size and type on the effectiveness of the coatings and the evaluation procedure. Good correlations were obtained between the two test facilities used. The effect of substrate temperature on insect impact dynamics and adhesion was also evaluated.
Mariana Kok, Edmond F. Tobin, Pavel Zikmund, Dominik Raps, Trevor M. Young

Insect Abatement on Lubricious, Low Adhesion Polymer Coatings Measured with an Insect Impact Testing System

Insect debris disrupts laminar flow, obstructs operator vision, and degrades vehicle aesthetics. To protect vehicle surfaces, anti-contamination coatings have been under development for 70 years, but no known homogeneous coating both adequately reduces debris and survives on vehicle surfaces. Coatings with synergistic combinations of physical properties and materials, however, may enable improved anti-fouling and maintain long-term durability. Transparent, spray-on coatings were developed that contain a combination of fluorinated and hygroscopic chemistries in which the fluorinated component reduces wetting of insect debris while the hygroscopic component produces a lubricating layer of absorbed water that interferes with debris adhesion. Debris area after insect impact was approximately twice as low on these coatings as compared to homogeneous control materials. Furthermore, the sensitivity of debris accumulation to hygroscopic content, and thus lubricity, was measured. At least 13 wt% hygroscopic content in the developed coatings was required for decreased debris accumulation compared to a pure fluorinated surface. Lastly, resistance to common vehicle fluids and scribe-tape adhesion was measured on fluorinated-hygroscopic coatings as initial demonstrations of durability.
Adam F. Gross, Andrew P. Nowak, Elena Sherman, Christopher Ro, Sophia S. Yang, Maryam Behroozi, April R. Rodriguez

Requirements, Test Strategies, and Evaluation of Anti-Contamination and Easy-to-Clean Surfaces, and New Approaches for Development

The use of novel insect mitigating surfaces on leading edges of aircraft is one method to assist in preserving natural laminar conditions on selected areas such as wings or vertical stabilizers. Development of such surfaces is extremely challenging because of stringent requirements on resistance to aircraft fluids and to environmental conditions such as ultraviolet radiation, rain, and sand erosion.
For assessment of insect-mitigating properties of surfaces, new laboratory methods were developed that are suitable for comparing insect contamination behavior and cleanability of surfaces. When testing surfaces while considering the influence of topography and chemistry using these methods, it was observed that superhydrophobic surfaces with a particular topography are essential to reduce contamination significantly. For these surfaces, the roughest surfaces produced the lowest contamination. This observation confirms results recently published by other authors. However, the current study shows that roughness is a disadvantage for cleanability. Easy cleanability after contamination with flies or insect hemolymph-containing liquid was only observed for smooth, hydrophobic surfaces.
Silke Gruenke

Biofouling-Mitigation Coatings


Candy and Poisons: Fouling Management with Pharmacophore Coatings

This chapter provides a context and perspective to consider novel materials for use in fouling management. Fouling and biofouling are defined and the rationale for existing fouling management is provided. Fouling management in even benign environments is difficult because commercial approaches use broad-spectrum stable biocides with non-target effects on humans, environmental health, and food safety. These historic approaches target death as the only biological endpoint. If novel approaches are to be implemented, biological alternatives to broad-spectrum long-lived biocides must be discovered, designed, and fitted within existing business models, infrastructures, and regulatory frameworks. The title, “Candy and Poisons”, highlights the two major issues for sustainable fouling management. Potent, short-lived toxins and pharmacophores, protected in coatings and chemically and biologically destroyed, are eaten as is candy within hours to days after release. Fouling on roofs as an example of an extreme environment and speculation about the use of conserved biological pathways in fouling management are discussed. Major concerns for all fouling management approaches are humans, environmental health, and food safety. With novel approaches, health and food safety should be addressed at the beginning of the technology development. The most difficult hurdle that a new technology faces is the regulatory framework. Present regulatory structures repress new technology and are slowly reducing options for the highly toxic management approaches. Policy changes that support novel technologies and close loopholes for long-lived toxins are needed to stimulate environmentally appropriate solutions.
Daniel Rittschof

Unique Silicone-Epoxy Coatings for Both Fouling- and Drag-Resistance in Abrasive Environments

Multiple years of international trials in both oceanic and freshwater sites have led to successful easy-release coatings based on the methyl-silicone polymers now widely employed as substitutes for tributyltin- and copper-based ship bottom paints. These have been found to be too soft for harsh conditions, especially during abrasion, but do serve for useful periods in commercial and military circumstances where abrasion is not frequent. This chapter reviews abrasion-related research of the past 20 years that identifies a novel version of silicone-based coatings with a retained easy-release value of Critical Surface Tension (CST) of about 26 mN/m, compounded with a tough epoxy component that allows the two-component coating to survive and function well in extremely abrasive circumstances. This coating has been applied to a power plant’s large intake grate (“rack”) subject to debris impact and intense zebra mussel fouling, an airfoil blade coating showing significantly lower drag than competitive paints, a turbine encasement seal layer remaining functional in zebra mussel-infested waters, and an easy-release surface for flash-frozen ice, simultaneously also resisting damage by transit through ice floes. The coating is formulated using polymeric methyl-silicone granules that are dispersed within an oil-in-water multiple emulsion in an epoxy base that maintains excellent substratum adhesion while allowing the methyl-silicone-based matter to dominate and be continuously refreshed via minimum wear at the environmental interface.
Robert Baier, Mark Ricotta, Vincent Andolina, Faraaz Siraj, Robert Forsberg, Anne Meyer


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