Coating operations account for several hundred billion dollars of industrial activity per annum in the U.S. alone. Some coating operations, such as roll and curtain coating, which are used in the manufacture of photographic film and adhesives, have been studied extensively. Others, such as atomization and powder coating, despite their widespread use and ever increasing technological importance, are poorly understood. The major goal of this center is to carry out research to remedy the deficiency in understanding of such emerging coating processes that are essential to the industrial competitiveness of the U.S. A second goal of the center is to exploit similarities between situations that are clearly recognized as coating operations and ones that are not. An example is the spraying of crops, an operation that is closely related to atomization coating and painting, but which has heretofore only been treated in isolation of its closely related analogs in coating operations.

Delivering a coating on a substrate is of course only part of the picture. What happens to the coating thereafter has immense repercussions from the points of quality, long-term behavior and durability, and environmental impact. From an environmentally benign manufacturing point of view, removal of coatings is as important as deposition of coatings and will hence be a focus area in this center. Given the scientific and technological complexity of coating operations, the research in the center will be naturally divided into three primary research areas which are nevertheless intimately related. These are:

• Area 1: Flow, transport, and rheology in the efficient delivery of coatings.
• Area 2: Curing, drying, adhesion and related processes occurring after delivery.
• Area 3: Quality, surface science, durability and removal of coatings.

Research in these areas will involve a combination of fundamental science and technology development. The products of the center will include computational tools for predicting performance of spray coating operations, novel spectroscopic and imaging techniques for research and on-line process control, and ways for modifying substrates for enhanced coating deposition as well as removal.
Back to Table of Contents

Coating operations are legion in industries ranging from photographic to electronics. Some well known manufacturing processes where coating operations play a central role include

Coating operations account for several hundred billion dollars of industrial activity per annum worldwide. Some coating operations, such as roll, curtain, knife/blade, slot/extrusion, and dip coating, which are commonly called "continuous coating operations," have been studied extensively. Others, such as atomization and powder coating, which are referred to as "discontinuous coating operations," despite their widespread use and ever increasing technological importance, are poorly understood. The major goal of this center is to carry out research to remedy the deficiency in understanding of discontinuous coating processes that are essential to the industrial competitiveness of the U.S. Although discontinuous coating processes offer the means to apply coatings to complex-shaped objects that sometimes cannot be coated by any continuous coating process, there are situations where both types of operations could be used. Therefore, a goal that is complimentary to this major objective is to carry out research to explore advantages or disadvantages of discontinuous operations relative to continuous ones when they are both practically realizable.

A second major goal of the center is to exploit similarities between situations that are clearly recognized as coating operations and ones that are not. An example is the spraying of crops, an operation that is closely related to atomization coating and painting, but which has heretofore only been treated in isolation of its closely related analogs in coating operations.

Delivering a coating on a substrate is only part of the picture. What happens to the coating thereafter has immense repercussions from the points of quality, long-term behavior and durability, and environmental impact. From an environmentally benign manufacturing point of view, removal of coatings is as important as deposition of coatings and will hence receive much attention in this center.

Not only can coating operations evolve over time but so can the types of coatings being used. For example, in the early 1970s more than 90% of paints used were low solids (5-20% by weight), solventborne coatings. However, due to quality, performance, energy consumption, and environmental impact issues, a transition has begun from using such pains to more complex ones such as high solids (60% by weight), solventborne coatings and waterborne coatings. Successful application of waterborne coatings requires the use of surfactants in the coating formulation. Understanding the role of surfactants and dynamic surface tension effects that they necessarily give rise to is crucial in the application of waterborne coatings whereas it is a nonissue in that of solventborne coatings. Therefore, there is also the need to understand entire coating operations as systems, rather than focusing separately on the chemistry of coating solutions, coating flows, and curing and drying operations. Whereas this piecemeal approach has been the common mode of operation to date, the integrated systems approach will be adopted in the proposed center.

Given the scientific and technological complexity of coating operations and the need to treat them as systems, the research in the center will be naturally divided into three distinct but nevertheless intimately connected research areas:

Research in these areas will involve a combination of fundamental science and technology development. The products of the center will include computational tools for predicting performance of spray coating operations, novel spectroscopic and imaging techniques for research and on-line process control, and ways for modifying substrates for enhanced coating deposition as well as removal.
Back to Table of Contents

In any coating operation, the coating must first be delivered to some suitable substrate. Therefore, the first goal of this research area is to improve the fundamental understanding of delivery of coatings in (a) certain widely practiced but poorly understood and (b) novel coating processes. The emphasis here will be on so-called "discontinuous" rather than "continuous" coating operations. The goals of this research area will be accomplished through the following tasks and subtasks.

1. Develop a fundamental understanding of microscopic and macroscopic level processes and interactions between them in deposition of coatings on substrates. Also, develop and/or improve, when appropriate, methods for characterization and prediction of transfer efficiency. In atomization coating, spray painting, ink-jet printing, and crop spraying, for example, this will involve:
   1. Studies of axisymmetric breakup of simple jets or emission of single drops at idealized nozzles versus aerosolization into a population of drops (see items (e) and (f) below).

   2. Investigation of impact and spreading of single or few drops on well characterized substrates versus deposition of numerous drops on ideal and complex substrates (see items (e) and (f) below).

   3. Studies of complex atomization, transport, and deposition phenomena when the number of drops approaches or equals that found in practical applications.

   4. Control by electric fields of few and populations of drops during atomization at a nozzle, transport from a nozzle to a substrate, and deposition on a substrate.

   5. Experimental characterization of atomization and deposition processes through the use of a Kodak Ektapro high-speed digital imager (with the capability for recording 12,000 frames per second), a Cordin ultra high-speed digital imager (with the ability to record 8 distinct frames each separated by a time period as short as 10 ns), holography, and phase Doppler anemometry (PDA), just to name a few of the techniques that we have available to us and will be used.

   6. Theoretical and computational analysis of atomization and deposition processes.
      • For simple fluids, extend recent advances made by two of the PIs in finite element (FEM) and boundary element (BEM) analyses of simple drop formation and jet breakup processes to perform rigorous analyses of complex breakup and deposition phenomena. In doing so, take advantage of parallel architectures available on PC and supercomputer class machines.
        
      • Using the so-called fluid pseudo-density model along with the quasi-axisymmetric approximation, develop predictive tools for describing the dynamics of dense sprays that rely on a mixture of fundamental science and empiricism (that is nevertheless supported by experimental observation).
        
      • Develop theories, models, and algorithms for dealing with effects of bulk and interfacial rheology (see goal 2 below) that are prevalent in real-world applications.
        
      • Account for effects of air flow, evaporation, and applied electric fields.

   7. Use computational tools developed to design new and novel processes, nozzles, and additives (surfactants, polymeric additives, etc.).

2. Measure, modify and account for bulk and interfacial rheological properties including shear and extensional viscosity and static and dynamic surface tension.

3. Study consequences of substrate preparation and modification on delivery.
   This goal provides a direct link to Area 3.

4. Compare efficiency and quality of coatings deposited by different techniques.
   This goal provides direct links to both Areas 2 and 3.
   1. Compare:
      • Atomization coating with spin coating or dip coating when substrates to be coated have complex shapes or features on them.
        
      • Effervescent sprays with airblast atomization in automotive applications, where the latter technique is the current standard but the former one might prove superior.

   2. There are certain coating operations where the delivery is almost trivial. However, these operations and others that entail a complex delivery process share many common features during the post delivery period. These features are discussed in the following section.

5. Migrate research tools being developed or used in Area 3 in imaging/characterization to on-line process control.
   Back to Table of Contents

In any coating operation, once the coating material is delivered to the substrate in the form of drops, particles, or a film, the so-called film formation phase begins. The coated film subsequently dries, cures, or otherwise tends to a final solid coating. The film that is thereby coated must of course adhere to the substrate in order for the coating operation to be successful. The adhesion process provides a smooth transition from the second research area to the third research area (section IV). The goals of this research area will be accomplished through the following tasks and subtasks.

1. Develop a fundamental understanding of microscopic and macroscopic level processes occurring during film formation. In atomization coating, a fluid based process, and powder coating, a solid based process, for example, this will involve:
   1. Investigation of the leveling of a number of very viscous drops or "solid" particulates into a smooth surface.

   2. Studies of the effects of ever-present disturbances or vibrations on drips, runs, and surface waves on the newly formed surface.

   3. Studies of remaining residual stresses once fusion of many particles leads to an acceptable film in powder coating.

   4. Experimental observation of the leveling process in fluid systems via the imaging systems described in the previous section. Of special interest here is non-Newtonian character and high solids content of fluid systems that may give rise to a yield stress. Since theoretical analysis of such systems is beyond the capability of existing methods (see below), the research will initially emphasize exploration of the appropriate parameter space through experimental variation of the relevant dimensionless groups such as Deborah number, ratio of yield stress to viscous stress, etc.

   5. Experimental observation of the fusion of viscoelastic particles and development of a mechanistic understanding of how the microscopic stresses give rise to surface coarsening.

   6. It is well known in electrohydrodynamics that whereas normal electric fields can destabilize an interface, tangential electric fields can stabilize it. Thus, we will test the viability of using electric fields to
      • assist leveling of coated films
        
      • control onset and evolution of, or even to suppress, coating defects.

   7. Theoretical analysis of film formation processes in both fluid and solid systems requires solution of mathematically challenging free boundary problems on account of the viscoelastic nature of the coating materials and occurrence of coupled transport processes. For fluid systems, analyze effects of
      • small amplitude vibrations by extension of classical linear stability analysis to include viscoelastic effects
        
      • large amplitude deformations by FEM (see previous section).

      For solid systems, account for both viscoelasticity and nonisotermal effects in developing a theoretical description of the fusing and leveling of the particulates.

2. Develop a fundamental understanding of the drying of coated films. This will entail:
   1. Investigation of heat and mass transfer that occur during the evaporation of a volatile solvent from an applied coating.

   2. Studies of the effects of the dyring process on the surface quality, e.g.
      • do stresses generated during drying cause surface roughness?
        
      • what is the optimum surface temperature or heat flux to produce a quality surface in the least amount of time and/or with minimum energy consumption?
        
      • how does the evolution of a porous coating affect its adsorption of radiation and conductive heat transfer characteristics?
        
      • what is the optimum design of a radiant oven to deliver the desired thermal history?

   3. Experimental studies that go beyond simple determination of the rate of solvent removal by mesuring weight loss. Such gross techniques only provide information about the total amount of solvent remaining but fail to provide detailed information that is needed for a detailed mechanistic understanding. Therefore, the surface composition will be determined directly by IR to evaluate the presence/absence of composition nonuniformities on the film surface during the drying process. Depth profiling will be carried out by freezing the material, microtoming and then doing IR microscopy of the through section of the coating. This process will be repeated to obtain concentration profile information as a function of time. The radiant heat transfer profile of the drying film will be measured as well as the VLE behavior of the polymer/solvent/filler system.

   4. The theory will once again be quite challenging as it will require analysis of diffusion in a viscoelastic material which can go from a fluid to a solid. The ultimate goal is to develop a three-dimensional FEM code that can solve a coupled deformation, mass, and heat transfer problem.

3. Develop a basic understanding of curing. This will entail:
   1. Carrying out experiments to determine if
      • a temperature gradient exists between the surface of the film and the substrate that can affect the cure pathway
        
      • there is a spatial dependence of temperature and composition fields.

   2. Developing a model that can describe the cure kinetics as the material is undergoing solidification. This will require chemistry to be incorporated into a viscoelastic solidification model. This model eventually will have to be combined with the radiant heat transfer model for design of optimal curing ovens.
      Back to Table of Contents

Adhesion to the substrate surface is the most important requirement of an effective coating. If a liquid medium is used in the coating process, its wetting and spreading properties are affected by the intermolecular forces between the liquid and the substrate. It is well known that surface preparation plays a key role at the gas/liquid/solid contact line and gas/liquid, gas/solid, and liquid/solid interfaces. Moreover, modifications of a substrate with nanometer-thick layers can affect drastically its wettability and the adhesion of the final coating to it. Therefore, it is not too surprising that some of the research goals of this last focus area are intimately intertwined with those of the first two focus areas. For example, surface modifications affected through certain means described in this section will be used in the drop impact and spreading experiments of the sorts described earlier. However, this focus area also incorporates research goals that are wholly different from those discussed in the previous two sections. Among these is developing an understanding of the long-term durability and quality of a coating and how these will be affected by both its adhesive properties and its bulk qualities. The goals of this research area will be accomplished through the following tasks and subtasks.

1. Develop definitions and measurements of quality and durability of coatings. This will entail:
   1. Developing scientific measures of the quality of a coating and relating them to empirical criteria of appearance and customer acceptance. "Quality" will be defined as strong and long-lasting adhesion, good substrate protection, thickness specifications and uniformity, stability to thermal cycles, proper mechanical properties (e.g., hardness, scratch, and impact resistance), electrical, and optical properties.

   2. Using multi-angle and multi-wavelength ellipsometry and FT-IR spectroscopy to characterize average film thickness and optical properties.

   3. Developing time-resolved imaging techniques based on ellipsometry (Ellipsomicroscopy for Surface Imaging, EMSI) and FT-IR, to follow the position-dependent film structure at length-scales down to ~ 5 microns. Electron microscopy and atomic force microscopy will be used to obtain information down to the nanometer scale.

   4. Developing in situ techniques for probing the film structure and composition in the direction normal to the substrate for evaluating film adhesion and durability. The resulting information on coating quality will include surface roughness, existence of pinholes and other defects, crystallinity and orientation, and uniformity of composition.

   5. Developing scientific measures of durability, which here will mean preservation of desired properties for as long time as demanded by the application. For example, good barrier properties may protect the substrate from corrosion or other problems and ensure maintenance of film adhesion. Appropriate mechanical properties may reduce the residual stresses. New chemical structures may increase the resistance to chemical, mechanical, or thermal disturbances. The weakening of adhesion, both mechanical and chemical, is frequently poorly understood. The kinetics of weakening is mainly controlled by the rate at which moisture is delivered to the interfacial zone and the rate at which subsequent degradation occurs. Rupture of interfacial bonds will then occur in combination with micro-cracking and plastication. Plastication and change of mechanical properties of the polymer can relax the residual stresses significantly, affecting the mechanical interlocking and local residual stresses.

   6. Developing mathematical and computational models of durability that account for existence of residual stresses and intrusion of moisture into the interfacial zone between the coating and substrate.

   7. Combining of the research groups strengths in experimental techniques to study degradation mechanisms both at the macroscopic level and the molecular level using mechanical testing in shear, tension and peel, SEM, AFM, XPS and FT-IR. We will use different polymers that show varying degrees of moisture absorption in combination with an intervening Si based interface between the coating and substrate.

2. Advance the understanding of the surface chemistry of coatings. This will entail:
   1. Studying the adhesion of coatings on metals, semiconductors, non-porous substrates, and plastics. Frequently, the exact surface chemical bonding mechanisms and the influence of surface chemical composition on the adhesion strength and the film quality are poorly understood. A better understanding of coating-substrate interactions is not only important for protective coatings, but also in the area of structural adhesive bonding for metallic materials. This will be accomplished by
      • Carrying out in situ studies of the surface chemistry of coating formation with sufficient spatial and temporal resolution to understand their dynamics on both microscopic and mezoscopic length-scales.
        
      • Employing surface-sensitive spectroscopic and microscopic techniques which are well-suited for a wide variety of conditions. Light-based techniques to be applied include time-resolved Fourier transform infrared spectroscopy (TR-IRAS), EMSI, and surface second harmonic generation (SSHG).

   2. Designing surface coatings by chemically functionalizing the substrate surface in order to carry out various functions, including reversible and photo-patterned coatings. By incorporating photo- and electroactive linkers between the surface and the coating, the linker can be cleaved with either light or through an electrochemical oxidation or reduction, resulting in removal of the coating layer. Using photolithographic techniques and light-sensitive linkers, the surface can be photo-patterned. Different coatings can then be applied to the exposed areas, resulting in domains of different finishes. Attachment of transition metal complexes to a solid interface will result in functional coatings capable of carrying out and catalyzing various chemical reactions. The metal complexes can be oxidized or reduced, resulting in charged interfaces which will dramatically affect surface properties. The metal complexes can also be used as a scaffold to build three dimensional architectures on the surface.

   3. Using molecular theories to understand the properties of surfaces modified by organic molecules, ranging from short surfactants to long polymer molecules. This will result in very efficient and accurate tools to design at the molecular level surface modifiers that will provide optimal properties for adhesion.

3. Performing fundamental studies on removal of coatings. This will be accomplished by:
   1. Innovating new techniques based on the use of lasers to remove the surface coating completely and cost-effectively without damaging the underlying substrate. This approach should leave unaltered the integrity and reusability of the substrate.

   2. Demonstrating the efficiency of laser-based techniques over competing techniques. The laser-based technique offers the following advantages:
      • removal of coating without the addition of any other materials or chemicals
        
      • automated control
        
      • high surface finishing quality, i.e. leaves the substrate practically unaffected.

      
      Back to Table of Contents

The educational and technology transfer opportunities are among the primary drivers of the center. Ultimate technology transfer is the preparation of educated and trained students that will become productive employees.

Technology transfer is a daily activity; therefore, regular and ongoing communication must exist between participants and industry collaborators. It will be facilitated by annual center workshops, semi-annual technology reviews, visits to industrial collaborators and visits by industrial scientists. Multimedia tools will be used to facilitate this transfer and interchange; distance learning and video conferences will be used. Direct relationships are needed for optimum transfer; therefore, industrial scientists will be encouraged to work in the university labs.

The Coatings Systems Research Center will allow Purdue to broaden its educational opportunities. We will not only make use of historical educational activities and tools but also attempt to develop unique methods. Courses focusing on the physical science and chemistry of coatings will prepare and retrain industry researchers. Seminars and symposia will be utilized to keep industrial scientists aware of new or changes in coatings science. Purdue’s continuing education capabilities will allow us to facilitate industry training by using distance education techniques. Purdue’s ties to Instituto Tecnologico y de Estudios Superiores de Monterrey (ITESM) in Monterrey, Mexico and to the Hong Kong Polytechnic University and a consortium of 14 Chinese universities will be used to give international exposure and experience.

The center will reach out to high school students. The Chemistry Department has a ChemMobile which visits high schools to demonstrate chemistry and recruit students to science. We will develop coatings experiments to include in the demonstrations. It is anticipated that young people will be intrigued by the practical uses of science. We will focus on schools in neighborhoods with high concentrations of minority or disadvantaged students.

In order to give the center’s students "real-life" experiences, we will place them in internships and co-op opportunities with our industrial partners. Purdue has had a very strong program in this area for years. This center will facilitate the expansion of this education approach to a broader range of companies. The graduate students will also be exposed to industrial laboratories. Summer research programs will be established to recruit potential graduate students. By utilizing NSF’s Young Scholars Program, the center will expose promising high school students to research experiences.

Experts from industry and academia will be brought to the center on a regular basis to keep center participants aware of the industry’s advancements and needs.

Purdue
HomePage

General
Information

Engineering
HomePage

Computing
Network

Topic
HomePage