Artwork, archeological materials, and ethnographic objects often require the use of protective coatings to stabilize them against degradation. For indoor storage conditions (i.e., within a museum), stabilization is often achieved through effective climate control, such as humidity and temperature control of the microenvironment within a transparent vitrine. For pieces that are displayed outdoors or are monumental in scale, the challenge of providing a stable noncorrosive environment is nontrivial; and it is often necessary to apply protective coatings that act as chemical and physical barriers. The types of materials used as coatings vary widely, and there are several critical features that must be considered to select an appropriate coating. First, the interaction between the substrate and the coating must be thoroughly understood. Adhesion at the substrate-coating interface often limits the effectiveness of the coating, where delamination provides a region for water/electrolyte accumulation and subsequent damage. Second, the material properties of the substrate must bear on coating selection. In the case of porous materials, such as stone or concrete, water vapor is drawn into or expelled from the material due to capillary action, and the coating should allow water vapor to pass or the damaging effects of efflorescence usually result. In the case of metals (e.g., bronze, brass, mild steel, Cor-Ten, and aluminum), impenetrable barrier films are thought to be the most effective means of achieving corrosion control. Third, the effects from the environment: ultraviolet light, daily and seasonal temperature and humidity fluctuations, pollutants, and physical abrasion (from windborne particulates or vandalism) should be considered. While numerous scientific studies exist that demonstrate the efficacy of a coating or groups of coatings on specific substrate(s), the sensitivity of cultural heritage objects necessitate that collections care personnel undertake their own testing to demonstrate compatibility between a specific substrate and coating.

Within the considerations of pairing an appropriate substrate and coating there exists additional constraints that affect the design of intelligent coatings for corrosion control for the conservation of material cultural heritage. The coating must be optically Transparent; it must be Reversible (removable) without harming the underlying artwork; and it should be easily Applied, requiring only a brush or pressurized air sprayer. Practically speaking, the coating must be commercially available, which typically requires that the price per gallon is acceptable for other applications where cost of materials is a critical deciding factor. The impact on the Environment and health of those applying the coating should be minimized. Taking account these five issues (TRACE) reduces the coatings options that otherwise would be acceptable in other fields when selecting coatings for conserving objects of cultural heritage. For example, curing coatings or those that require melting in an oven are not viable coatings for material cultural heritage.

Intelligence of protective coatings for use on material cultural heritage is therefore defined having at least one advanced chemical or physical property in addition to meeting the basic TRACE criteria. Advanced chemical or physical properties include the rational design of the polymer to achieve a specific improvement or the addition of components into the formulation to achieve marked increases in performance. Described below is our investigation of both chemical and physical modifications to coatings compatible with the TRACE requirements that are expected to improve the weatherability and water barrier properties of coatings, with the goal of increasing their working lifetime and corrosion protection for use in the conservation of material cultural heritage.

19.1.2 Defining intelligence: chemical and physical

Generally speaking, coatings having chemical intelligence will be able to be responsive to applied chemical stress (such as that produced by oxygen or radicals). Likewise, systems that can reduce physical stress by an induced response are considered to have physical intelligence. There are three broad categories of methods for conferring intelligence: (1) in resin, (2) in formulation, and (3) in encapsulation. First, the in resin approach involves incorporating intelligence into the polymer chain targeting specific functional groups that may respond to stresses. For example, incorporating fluorine groups close to the polymer chain backbone, such as in polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE), will reduce chain-scission by re-forming homolytically cleaved bonds.^1–4 Or through the use of copolymers, functionalities having a range of glass transition temperatures (T_g) may confer flexibility to the polymer under a wider thermal range,⁵ or monomers that may act as corrosion inhibitors may be copolymerized with those that act as barriers.⁶ Second, the in formulation approach involves two differently functioning parts of a single mixture. For example, ultraviolet absorbers may be incorporated into the formulation of a polymeric suspension, allowing the polymer to act as a chemical and physical barrier against corrosion, while the UV absorber reacts with radicals formed to prevent chain-scission or side-group cleavage.⁷ Third, rather than uniformly distributing intelligence throughout the polymer or dispersion, others cleverly encapsulate moieties that are released by a stressor and upon release act to mitigate that stress. An example of this approach is in self-healing coatings that, ideally, would fill-in cracks by the release of encapsulated chemicals due to an external stimulus such as pH, chloride ions, water, light, temperature, or other mechanical or physical changes.^8–12 Combinations of any of the three approaches and layering different coatings are commonly done.¹³ In developing intelligent coatings for the prevention of corrosion on material cultural heritage, usually the simplest approach is preferred because that is expected to have the fewest unanticipated side-reactions over its lifetime, which reduces the risk of the coating damaging the very work being protected.

19.1.3 Commonly used coatings in the conservation of cultural heritage

It is useful to consider the history of protective coatings on material cultural heritage before discussing how improvements may be made. Since the Roman times, oils and resins were used as protective coatings on top of naturally formed metal oxides. In the first century, Pliny the Elder noted: “Things made of copper or bronze get covered with copper-rust more quickly when they are kept rubbed clean than when they are neglected, unless they are well greased with oil. It is said that the best way of preserving them is to give them a coating of liquid vegetable pitch.”¹⁴ Starting in the nineteenth century plant oils, oil lacquers, and paraffinic waxes were used to protect outdoor metals.¹⁵ In the 1960s, a specialized lacquer (polymer dissolved in solvent) was developed to prevent copper corrosion by the International Copper Research Association (INCRA), which was called Incralac.¹⁶ In 2008, we conducted an online survey sent to material cultural heritage conservators (with ~ 120 respondents) to learn more about how coatings are used in conservation today. To protect iron from corrosion, 47 respondents had used waxes, 23 had used solvent-borne acrylics, 45 had used pigmented coatings, 15 had used other polymeric coatings and only three had used acrylic emulsions (see Figure 19.1a). On bronze and brass, 84 had used waxes, 58 had used acrylic lacquers, 35 had used pigmented coatings, and only two used acrylic emulsions (see Figure 19.1b). These responses indicated that wax and paint are the most commonly used coatings for iron substrates and wax and acrylic lacquers are the most common for bronze protection. Despite the fact that conservators expected nonwax coatings, such as acrylic lacquers, to last longer, some even indicating that they expected a lifetime of up to 20 years, their use trailed that of the wax. Acrylic emulsions were rarely used by conservators. For acrylic lacquers, three specific resins are commonly used: Paraloid™ B-44, B-48, and B-72. Conservators reported having used one of those acrylic lacquers and benzotriazole (BTA), a corrosion inhibitor,¹⁷ with 18 reporting having mixed BTA into Incralac (which is composed of Paraloid™ B-44), eight having used BTA as a pretreatment and 12 having used it as both a pretreatment and as a mix into the coating. In the survey we also asked conservators what was the expected lifetime of protective coatings (see Figure 19.1c). Fifty-four percent of respondents expected waxes to have a brief lifetime of 1 year or less. For nonwaxes, the lifetime expectancy was longer: with 39% expecting a lifetime of 3-5 years (see Figure 19.1d). The responses from the survey clearly indicated that although chemical advances in the past 100 + years have provided alternatives, wax is preferred by the conservation community as a protective coating and is expected to have a lifetime of 5 years or less.

19.1.4 Weathering studies of common coatings used in conservation

To obtain a quantitative understanding of the performance of some of the commonly used protective coatings in the conservation of material cultural heritage, a 4-year study was undertaken in 2009 during which bronze panels were coated and weathered under different conditions: 18 months in southern Florida exposure, under QUV-B light for 1000 h and outdoors for 18 months in Portland, Oregon. Tables 19.1–19.3 show the 60° gloss and film thickness changes of various coatings on bronze panels as measured using a Gardco TRI-gloss meter for each of the three different exposure situations. Notably, all panels reflected significant decreases in gloss, with losses ranging from 88% to 11%. Over time, the wax coated panels showed the greatest change in gloss, however gloss of waxed panels was the lowest at the start, having only 4-5% reflectance. During outdoor weathering waxed panels lost the most film thickness (− 5.8 μm in South Florida and − 5.3 μm in Portland), and significant corrosion/substrate darkening was observed. However likely because of the absence of ablative particulates within of the QUV chamber, weathered wax panels only lost 0.6 μm of film thickness. The coatings made from solvent-based resins, Incralac and Paraloid™ B-44, also showed overall losses in gloss and thickness upon exposure in each of the three weathering environments, and the addition of corrosion inhibitors and/or a light stabilizer (Tinuvin, added at 1% w/w) did not improve gloss or thickness retention over time. The resin in waterborne Incralac contains both acrylic and urethane, which is different from the acrylic-only solvent-based version; the performance of the waterborne coating was significantly different: it cracked and yellowed, corrosion formed underneath and, aside from the wax, showed the most film loss under South Florida exposure. For its poor initial performance, waterborne Incralac was omitted from further study along with a number of commercially available materials marketed for conservation use. These results demonstrate that although the coatings included in this study were commonly used by conservators in our 2009 survey, they noticeably aged in every one of the conditions to which they were exposed. The results point to the need to develop higher-performing coatings with long working lifetimes that are also compatible with the TRACE requirements described in Section 19.1.1.

19.1.5 Our method of approach in developing intelligent coatings for material cultural heritage

We aim to confer both chemical and physical intelligence to coatings for the conservation of material cultural heritage by using a highly weatherable resin and incorporating nanoclays into the coating formulation. In the work that follows we describe separate studies, testing first chemically intelligent coatings then coatings having both physical and chemical intelligence. We utilize a waterborne resin that contains PVDF and polyacrylic acid in a 70:30 wt. ratio, which is available commercially as Kynar Aquatec® RC-10206 or as a 50:50 ratio, available as Kynar Aquatec® FMA 12 (Arkema Inc.). Being hydrophobic, PVDF is copolymerized with polyacrylic acid to improve its dispersibilty in water (yielding waterborne latex suspensions having a lower environmental impact than solvent-based coatings). PVDF-containing coatings are known to be highly resistant to degradation by UV light, having protective properties lasting for 30 years and longer.^3,18–20 Their long lifetime is due to their chemically inert structure, which in turn has the undesirable consequence of having poor adhesion to metals. A solvent-based coating of the highly UV transparent resin Paraloid™ B-44 (Dow Inc.) was used as a basecoat to adhere strongly to the metal substrate and to provide good interlayer adhesion to the weatherable topcoat of Kynar Aquatec® RC-10206. With the goal of conferring additional resistance to hygroscopic swelling, silicate nanoclays (Laponite RD, Rockwood Inc.) have been incorporated into the topcoat. An overview of our method of approach is shown in Figure 19.2. This coating system meets the TRACE requirements: (1) the coating is transparent—even with the inclusion of the nanoclays, their size is too small to cause visible light scatter; (2) the coatings are reversible—both the solvent-based primer and waterborne topcoat are removable with acetone (and many other solvents); (3) the coatings may be applied by brush or spray gun (with the basecoat thinned to 20% wt. and the topcoat to ~ 35% wt.); (4) the resins are commercially available through Dow and Arkema Inc.; and (5) the coating system has a reduced environmental impact because it includes a waterborne component (Kynar Aquatec® RC-10206).

19.1.6 Anticipated challenges with this coating system

This coating could be further improved if the adhesion of the topcoat to the metal was increased so that the use of a basecoat was not required. Increasing the adhesion of the topcoat might be achieved by the incorporation of monomers known to interact with metals, such as ureido methacrylate,²¹ or through modification of the resin to reduce its minimum film formation temperature (MFFT). (Kynar Aquatec® FMA 12 has a lower MFFT than Kynar Aquatec® RC-10205, at 12 °C compared to 22 °C, and has increased adhesion, although not sufficiently high to exclude the need for a basecoat.) An additional complication of the use of waterborne coatings comes from the differences in the mechanism of film formation from a latex suspension, which is considerably more complex and prone to film defects than the same process from solvent-based polymeric coatings. We have characterized these processes for our selected coating system²² and have determined that coalescent migration is responsible for whitening films of Kynar Aquatec® RC-10206 if they are exposed to water before the film has completely formed.²³ In our previous work we determined that annealing of Kynar Aquatec® RC-10206 by an infrared heating lamp for 4 h reduced the whitening from 25% to 5%, which is comparable to controls of visually transparent sprayed films (where 0% is completely transparent and 100% is white).

19.1.7 Using electrochemical impedance spectroscopy to characterize barrier properties of protective films

Perhaps more important than monitoring visual changes (gloss, thickness, color, adhesion, whitening, etc.) of coated substrates during weathering is characterizing and monitoring their barrier properties: the ability of the protective coating to block the passage of corrosive ions and water. Measurements of films’ barrier properties are assessed by numerous methods, such as voltammetry and potentiometry; however, those are generally not amenable to making measurements on sculptures or architectural elements as the measurement method itself must be nondestructive and noninvasive, methods that generally require applied voltages or current flow to induce electrochemical reactions.²⁴ Electrochemical impedance spectroscopy (EIS) has emerged as a useful method for coating analysis that, depending on the exact configuration of the setup, may be nondestructive because there is no net current applied and measurements can be made at the open circuit potential.^20,25–28 EIS can be operated as a two-electrode measurement system where a small AC signal (~ 10-30 mV) is applied to the working electrode and the magnitude and phase of the current is measured at the counter electrode. Measurements are made as the frequency is varied in a range from ~ 1 to 1 MHz, with high frequencies limited by instrumental capabilities and noise limiting low-frequency measurements. Changes in either magnitude (Z) or phase measured at the counter compared to the working electrode are associated with resistive and/or capacitive characteristics of the system, because capacitors give rise to a − 90 ° phase and their impedance is frequency dependent while resistors are in phase and are frequency independent, as shown in Equation (19.1).

$Z_R=R;\text{}{Z}_C=\frac{1}{j\omega C}$

R is the resistance, C is the capacitance, ω is the angular frequency, j is the complex number.

For a coated metal substrate exposed to electrolyte there are several circuit elements present, such as the bulk electrolyte, the capacitance of the protective coating, the resistance of the protective coating, and the resistance at each material interface. When there is a coating defect other circuit elements then present, such as the charge transfer resistance and double-layer capacitance. The values of each of these circuit elements can be determined by fitting the experimental data to equivalent circuit models. When monitored for change over time, the values of the individual elements give insight into how protective coatings change and eventually fail. Actual coated systems often do not behave like ideal capacitors and so constant phase elements (represented in Equation 19.2) are used to more accurately describe the observed data.

$Z_{\text{CPE}}=\frac{1}{{\left(j\omega \right)}^{\alpha }C}$

α is an integer that varies between 1 and 0.5 (however α is usually between 1 and 0.9 for a dry/intact film).

Bronze substrates (2.54 × 7.62 cm; 90% Cu 10% Sn, TB Hagstoz & Son Inc.) were treated to prevent flash rust with a 24 h immersion of 17 mM BTA (Alfa-Aesar) in ethanol. Basecoat primers were either resin dissolved in solvent [Paraloid™ B-44 resin, principally composed of poly(methyl methacrylate-co-ethyl acrylate), referred to as SB Acrylic, in 20% (w%/w toluene), Dow, Inc.]. The topcoat used was waterborne Kynar Aquatec® RC-10206 latex, a poly(vinylidene fluoride) and poly(methyl methacrylate) blend, Arkema, Inc. Substrates were spray-coated with a Fuji HVLP Super 4 XPC™ for a dry film thickness of ~ 10-15 μm for each layer (i.e., primer base coats or WB PVDF topcoat) for a total dry film thickness of ~ 30 μm. Samples were annealed (heated) in an oven for 12 h at 60 °C.

19.2.2 Experimental details for weathering studies of coated panels

The Florida exposures of coated panels were conducted according to ASTM D6675 with salt spray exposure at Q-Labs in southern Florida at a 45° incline facing South with evaluation after hand washing with water every three months. The conditions for exposure to ultraviolet light were established according to ASTM G154 Cycle 2: a 500 h cycle consisting of 4 h of UV-B exposure at 60 °C, irradiance = 0.78 and 4 h of condensation at 50 °C. The coatings’ thicknesses ranged between 25 and 30 μm for each panel and were applied by spray-coating or, for waxes, applied using a cloth and remelting under low, evenly applied heat.

19.2.3 Experimental details for characterizing substrates

Gloss and thickness measurements were acquired as an average of five trials with a Gardco μ-Tri-Gloss meter. EIS measurements were collected with Gamry Reference 600 Potentiostat and Gamry Framework software. EIS measurements were performed with a standard three-electrode Gamry Paint cell setup: a glass cell filled with 3% NaCl was clamped to a coated bronze panel acting as the working electrode, and the Ag/AgCl reference electrode and graphite counter electrode were immersed in the electrode solution with a 4-cm distance between the counter and working electrodes. An area of exposure of the working electrode equal to 14.6 cm² was used for analysis from 1 MHz to 0.1 Hz with AC voltage 20 mV rms, and DC voltage 0.0 V versus open circuit potential 0 ± 200 mV. Spectra were interpreted and modeled against equivalent electrical circuits (EECs) with Scribner Associates ZView. Errors in fit between EEC models and the data were calculated as the sum of residual error where the fit differed from the experimental data. Average potentiostat instrument error ranged from 10% at 0.1 Hz to < 0.5% at 1 MHz.

Fourier transform infrared microspectroscopy (FTIR-m) measurements were made using a ThermoScientific iS10 infrared spectrometer with a Nicolet Continuμm FTIR microscope, and a 50-μm MCT (mercury telluride/cadmium telluride) detector was used to acquire each spectrum from 4000 to 650 cm^− 1 with 4 cm^− 1 resolution using Omnic software. Spectra of microsampled cross-sections were acquired in transmission mode for 128 scans on a diamond slide. Data was transformed using an N-B strong apodization function and Mertz phase correction. All X-ray studies were acquired with a Rigaku Ultima IV Multipurpose X-ray diffractometer with a Cu-kα radiation source (λ = 1.542 Å) and step size = 0.002° for 12 s. Powder samples were ground finely with an agate motor and pestle and pressed into a random orientation on a mirrored slide. For scattering studies in transmission mode, thin-walled glass capillary tubes (1.5 mm, Charles-Supper, Co.) were filled with either the aqueous clay dispersions or wet resin and attached to a 20-mm sample window fixed in place adjacent to the vacuum path window.

To test the weatherability of chemically intelligent coatings compared to common acrylic coatings, bronze or brass substrates coated with a primer of Paraloid™ B-44 and a topcoat of Kynar Aquatec® RC-10206 or just layers of Paraloid™ B-44 were prepared and weathered in South Florida for 18 months and in a QUV-B chamber for up to 2750 h. The changes in their barrier properties were monitored by EIS and changes in chemistry monitored by FTIR-m.

19.3.2 Characterization of weathered coated substrates by EIS

Before weathering, a simple model best fit both types of coatings, which can be seen in Figure 19.3, where R_b describes the bulk solution resistance and the remaining resistor and capacitor describe the coating. The simplicity of the circuit model implied that there was only a single pathway for current to travel along, due to the resistance of the coating and capacitance of the coating. However, after extensive weathering, the equivalent circuit model best fitting the data became more complex with the addition of either one or two pairs of parallel resistor and constant phase element. We observed that QUV-B films of Paraloid™ B-44 weathered for 2250 h were the most degraded because they required the complete set of three time constants, which are due to (1) the regions of dry/intact coating, (2) the water-swollen/defective layer of the coating, and (3) the exposed metal substrate (having an electrical double- layer capacitance and charge transfer resistance from the growing oxide layer). We attributed R₁ to the resistance of the coating itself, which was 4.2 × 10⁹ Ω before weathering. After weathering in the QUV-B chamber, the value of R₁ decreased to 1.21 × 10⁵ Ω, and after weathering in Florida, it decreased to 3.12 × 10⁵ Ω, making the reduction in resistance over four orders of magnitude. Comparatively, after weathering for 2750 h in the QUV-B chamber, the films with Kynar Aquatec® RC-10206 showed an increase in the total impedance at frequencies between 10 and 0.1 Hz, likely due to the completion of film formation assisted by the gentle heating (annealing) in the QUV-B weathering cycles. After the initial annealing increase (which actually improved the barrier properties of that coating), the film showed no measurable change in impedance. Before weathering, R₁ had a value of 2.76 × 10⁸ Ω, and after weathering in Florida, that value decreased to 2.02 × 10⁷ Ω. These data suggest that coatings with Kynar Aquatec® RC-10206 have longer working lifetimes both in South Florida and in accelerated weathering than do films of Paraloid™ B-44 alone.

19.3.3 Characterization of weathered coated substrates by FTIR

During weathering, coatings may undergo a variety of chemical changes that should be detectable. We monitored changes in chemistry of weathered films using FTIR-m. Shown in Figure 19.4 is the IR spectrum for a solid film of Paraloid™ B-44 before weathering, as well as the subtraction results after weathering in Florida for 18 months and after weathering in a QUV-B chamber for 2250 h. The spectra were normalized to the most intense peak (the carbonyl peak at 1732 cm^− 1 before subtraction). A partial structure of Paraloid™ B-44 is also shown as a reference aid in correlating the IR band and chemical changes during weathering. Relative to the intensity of the carbonyl peak, the subtraction spectra after both types of weathering show decreases in bands associated with COC stretching, CO bond motions, CH₂ and CH₃ deformation, and rocking and stretching motions as can be seen in the spectra by the negative peaks. For example, the intense broad collection of peaks between 1140 and 1430 cm^− 1 that are associated with CO bond motions decreased in intensity after weathering, as can be seen by the negative peaks in that region. The spectral changes indicated that one of the primary chemical changes associated with weathering is loss of the pendant methoxy (OCH₃) or ethoxy (OCH₂CH₃) groups. It is interesting to note that the intensity of the observed band decrease was not large. It has been observed that polymer degradation can lead to the loss of low molecular weight/volatile compounds that are not detected after weathering.²⁹ Similarly, while large peak decreases were not observed in our data, it is likely that volatile material was lost and thus not observable in our spectra (given that intensity normalization was necessary). Upon coating failure after QUV-B weathering, visual inspection of the panel showed puckering and shrinking of the film, suggestive of significant chemical and physical property changes in the film due to weathering.

Because Kynar Aquatec® RC-10206 contains PVDF and acrylic, preferential loss of the acrylic over the more weatherable PVDF components was expected and was observed in the data shown in Figure 19.5. The spectra were normalized to the most intense peak in the spectrum, which was the CO stretch at 1149 cm^− 1. Relative to that peak a decrease was observed in the carbonyl band, which is consistent with the loss of acrylic components. Although it is likely that CO bond intensities decreased with the loss of carbonyl groups, interpretation of the 1140-1430 cm^− 1 region was complicated by the spectral overlap of the CF₂ and CO bond motions. The films of Kynar Aquatec® RC-10206 remained flexible without cracks or puckers after weathering.

The studies described above effectively incorporated chemical intelligence into coatings for outdoor metals and demonstrated longer working lifetimes than commonly used coatings to protect material cultural heritage. But, those studies did not take advantage of strategies to confer physical intelligence. We focus on incorporating optically transparent inorganic materials, such as the synthetic clay, Laponite, to produce films having reduced susceptibility to hygroscopic swelling. Large aspect ratio particles, such as clays, accomplish this by forcing water molecules to travel a tortuous path around them. Laponite is a synthetic silicate in the smectite family whose chemical formula Na_0.7[(Si₈ Mg_5.5Li_0.3)O₂₀(OH)₄] creates a 2:1 particle with one octahedrally coordinated magnesium/lithium oxide sheet sandwiched between two tetrahedrally coordinated silica sheets and one interlayer cationic sheet of Na⁺ ions and water molecules for charge balance. Laponite has discoid single crystals with a high aspect ratio of 25, specific surface area (350 m²/g), and charged surfaces, where the face has an induced negative charge from the expansion of sodium cations out of the interlayer space, and the edge of the platelet is positively charged at near-neutral pH due to protonated hydroxyl groups along the magnesium-lithium octahedral sheet, as illustrated in Figure 19.6a.³⁰ Like many clays, Laponite is able to undergo chemical and exchange reactions because it has accessible hydroxyl groups available for silanation on the sheet’s broken edge and exchangeable sodium cations connecting the face of particle-to-particle.^31–34 And like most clays, Laponite forms multilayered stacks of clay sheets that may be hundreds of nanometers in thickness, which, if left intact, are large enough to be visible to the eye and to cause significant disruptions in film formation when incorporated into coatings. An idealized picture of Laponite is shown in Figure 19.6a with a layer thickness of 1.08 nm and diameter of 25 nm. The size and shape of the individual platelet has been well characterized by powder diffraction as a disc shaped with a thickness of 1.29 nm in ambient conditions, a discrepancy with the smaller chemical structure that is caused by water sorption within the clay.³⁵ The swelling ability of Laponite aids in exfoliation, that is, separating the layers and stabilizing the individual platelets in solution, which can be a significant hurdle in successfully incorporating these nanoparticles into coatings. Once exfoliated, the platelets must be stable in solution so that they do not aggregate nor induce coagulation of the latex particles in the wet coating suspension. Stabilization of these anisotropically charged particles can be achieved in a number of ways including balancing the positive charges on the rim with a peptizing agent, inducing an inverse double-layer formation through cation exchange with surfactants, or covalently attaching any number of siloxane molecules. The selection of which method or combination thereof is based mostly on the specifics of the system and can also be used to confer specific chemical properties.

19.4.2 Modification of nanoclays to increase compatibility with the coating

To exfoliate and stabilize Laponite platelets and produce dispersed colloidal suspensions in Kynar Aquatec® RC-10206, two basic strategies were used: (1) covalent modification and (2) cation exchange. To accomplish that and to increase compatibility between the nanoclay and latex binder, Laponite was covalently modified by a silanation reaction with the accessible hydroxyl groups on the edge of the clay sheet. Two different silane molecules were used: 3-acetoxypropyltrimethoxysilane (Lap-APTMS), where its polar groups were expected to interact with the acrylic portion of the resin, or tridecafluoro-1,1,2,2-tetrahydrooctyl triethoxysilane (Lap-FOTES), where its hydrophobic, fluorinated groups were expected to interact with the PVDF portion of the resin. To encourage exfoliation after covalent modification, cation exchange of sodium atoms at the clay surface was conducted using phosphorylcholine tetrahydrate (PC), thus producing nanoclays that were both covalently and charge exchanged, referred to as Lap-APTMS + PC and Lap-FOTES + PC. The types of modification are shown in Figure 19.6b.

19.4.3 Experimental procedures for modification of the nanoclay

Laponite (RD, Southern Clay Products, Inc.) and silylating agents 3-acetoxypropyltrimethoxysilane (APTMS, Gelest, Inc.) and (tridecafluoro-1,1,2,2-tetrahydrooctyl) triethoxysilane (FOTES, Gelest, Inc.) were used as supplied without further drying or purification. Toluene was distilled directly into a flask containing Laponite RD (1 g nanoclay/100 mL solvent) under anhydrous conditions in a closed system (δ⁺ N₂ flow). After 30 min of heating with stirring at 35 °C, the siloxane was added to the toluene clay mixture (1 mmol/g clay) and the contents in the flask were left to stir an additional 4 h at the same temperature. The covalently modified Laponite was isolated for diffraction analysis by vacuum filtration with a 0.2 μm Nylon membrane, washed extensively with toluene, and dried overnight at 65 °C. The aqueous dispersions of modified Laponite were obtained by quenching the grafting reaction with 50 mL of water and isolating the water-stable nanoclay by liquid-liquid extraction in a separatory funnel, the final percent solids being ~ 2.5% (w clay/%w H₂O). The dispersions were then either sonicated with tetrasodium phyrophosphate (TSPP) for post-let down addition to the resin or prepared for further modification. For cation exchange the modified nanoclay (LR-APTMS and LR-FOTES) was diluted to 100 mL and heated to 50 °C after which one equivalent (CEC_LR = 0.75 mmol/g clay) of phosphorylcholine chloride calcium salt tetrahydrate (PC) was added and stirred for 12 h. The exchanged/grafted clay was vacuum filtered, washed with ethanol, and immediately redispersed in DI water for post-let down addition to the resin, as detailed in the formulation.

19.4.4 Characterization of modified Laponite by FTIR

Fourier transform infrared spectra of Lap-APTMS with and without PC were collected and shown in Figure 19.7. With Laponite present, a number of IR bands were produced by the hydroxyl groups and sorbed water in the clay, where OH stretching peaks appeared from 3600 to 3400 cm^− 1 and an HOH deformation peak was visible near 1632 cm^− 1, as can be seen in the spectrum of unmodified Laponite (Figure 19.7e). After grafting with APTMS additional peaks were observed: aliphatic CH stretching (3000-2800 cm^− 1 region), carbonyl stretching at 1709 cm^− 1, and CH rocking at 1285 and 1259 cm^− 1. No increase in the SiO stretch (1012 cm^− 1) was observed postmodification because the vast majority of the band’s intensity was produced by Laponite itself and the silane functionalization contributed only slightly. The Lap-APTMS spectrum (Figure 19.7d) was subtracted from the Lap-APTMS + PC (Figure 19.7c) spectrum to observe small changes in the spectrum after cation exchange with PC and is shown in Figure 19.7b. A reference spectrum of PC only is shown in Figure 19.7a. Apparent in the subtraction spectrum are asymmetric and symmetric stretching PO₃ bands at 1151 and 1092 cm^− 1, respectively, the COP stretching band at 967 cm^− 1, and an additional CH bending peak (δ CH) near 1478 cm^− 1; all are spectral features of the negatively charged phosphate head group because the quaternary amine is not infrared active. Given the relatively small amount of both phosphoryl choline on the surface and APTMS covalently bound to the edge compared to intense vibration of the nanoclay lattice structure, it is not surprising that the peaks corresponding to modification were of low intensity, as depicted in Figure 19.7c. Presence of the expected IR bands after each modification step in the production of Lap-FOTES and Lap-FOTES + PC were also verified by infrared analysis although the data is not shown. Negative peaks from the subtraction of Lap-APTMS from the Lap-APTMS + PC spectrum occur near the CO and CH bending regions of the spectrum, indicating a slight loss of the silane after cation exchange, likely due to loss of adsorbed silane.

19.4.5 Characterization of modified Laponite by X-ray methods

Both wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS) were utilized in characterizing particle sizes and inter- and intraparticle spacing in the sub-500 nanometer range. In general, the sharpening of a diffraction peak indicates an increase in short- or long-range order in the material. The WAXD patterns of Laponite before and after modification are shown in Figure 19.8. The most significant difference between spectra occurs at low angles and is the interlayer spacing (d₀₀₁) of the clay, that is, the distance from one cationic sheet to the next. Full characterization of the diffractogram of the unmodified sodium Laponite (labeled Na-Lap in Figure 19.8) indicated the basal spacing was 1.23 nm, calculated by Bragg’s law from a broad peak occurring at 7.1 2θ, with additional peaks at 20.0 2θ (d_02,11 = 0.44 nm), 27.8 2θ (d₀₀₅ = 0.32 nm), and 35.3 2θ (d_20,13 = 0.25 nm).³⁶ When covalently modified by APTMS the basal spacing increased: the d₀₀₁ peak shifted to 5.6 2θ (d₀₀₁ = 1.57 nm). Laponite modified with FOTES produced a broader peak with its maximum at 6.38 2θ, corresponding to a 001 spacing of 1.38 nm. Cation exchange did not appreciably change the basal plane spacing in either case, with a Lap-APTMS + PC peak at 5.59 2θ (d₀₀₁ = 1.58 nm) and a Lap-FOTES + PC peak at 6.33 2θ (d₀₀₁ = 1.39 nm), respectively (data not shown). These correspond well to established literature values where diffraction analysis of dry Laponite in ambient conditions provided a d₀₀₁ peak occurring near 6.7° 2θ that corresponds to an interlayer spacing of 1.29 nm, where upon edge modification of the interlayer space increased to 1.41 nm.³⁷

Further inspection of the undiluted aqueous dispersion of only covalently modified clays by SAXS suggested that two distinct sizes were present in the Laponite dispersions, with one corresponding to single, completely exfoliated platelets having thicknesses between 1.26 and 1.45 nm after covalent modification (1.11 nm before modification) and diameters between 25.22 and 28.92 nm (22.11 nm before modification); see Table 19.4 for a complete list of the values. These measurements show that on average with covalent modification, particle diameter increased, which is consistent with clay edge modification rather than random or face modification.

Table 19.4

WAXD and SAXS Characterization Data Table of Unmodified (Na-Lap) and Modified Laponite

	WAXD $T_{d_{001}}$ (nm)	SAXS D (nm) in H2O Cylinder	SAXS T (nm) in H2O Cylinder	SAXS T (nm) in H2O Spheroid		SAXS D (nm) in H2O Spheroid	SAXS T (nm) in FMA 12 Spheroid		SAXS D (nm) in FMA 12 Spheroid
Na-Lap	1.23	22.11	1.11	A	1.84	27.59	A	1.66	A	24.84
				B	22.84	22.84	B	158.20	B	158.20
Lap-APTMS	1.57	25.22	1.26	A	1.18	23.50	A	1.29	A	25.86
				B	45.84	45.84	B	158.20	B	158.20
Lap-APTMS + PC	1.58	35.42	1.77	A	1.75	34.98	A	1.33	A	26.58
				B	36.91	36.91	B	158.20	B	158.20
Lap-FOTES	1.38	28.92	1.45	A	1.59	23.87	A	1.56	A	23.34
				B	104.59	104.59	B	152.44	B	152.44
Lap-FOTES + PC	1.39	35.56	1.78	A	2.00	29.94	A	1.55	A	23.19
		73.30	73.30	B	105.83	105.83	B	153.26	B	153.26
	Near Neighbor (nm) in H2O Cylinder	Volume Fract. (%) in H2O Cylinder	Near Neighbor (nm) in H2O Spheroid		Near Neighbor (nm) in FMA 12 Spheroid		Volume Frac. (%) in H2O Spheroid		Volume Frac. (%) FMA 12 Spheroid
Na-Lap	15.28	100.00	A	1.06	A	1.93	A	95.27	A	13.59
			B	12.30	B	10.99	B	4.73	B	86.41
Lap-APTMS	1.82	100.00	A	1.33	A	1.33	A	98.30	A	22.33
			B	10.02	B	12.73	B	1.70	B	77.67
Lap-APTMS + PC	1.78	100.00	A	1.26	A	1.32	A	96.60	A	20.00
			B	22.73	B	12.62	B	3.40	B	80.00
Lap-FOTES	1.55	100.00	A	1.39	A	1.37	A	96.83	A	40.89
			B	24.68	B	13.79	B	3.17	B	59.11
Lap-FOTES + PC	2.19	93.58	A	1.94	A	1.51	A	92.41	A	25.05
	65.22	6.42	B	68.55	B	13.53	B	7.59	B	74.95

SAXS data was taken in both aqueous solution and in Kynar Aquatec® FMA 12 and fit using either or both cylinder or spheroid models, as labeled in the column headings. Nearest neighbor and volume fraction estimations were calculated by fitting SAXS data. Spheroid data fitting allowed two distributions to be fit, which are labeled as A and B within the data table.

19.4.6 Fitting SAXS data

The SAXS curve of Lap-APTMS + PC was fit with two different particle geometries in Figure 19.9: spheroid (left) and cylindrical (middle). A platelet model, which might be the most suitable for Laponite, has not yet been developed. The spectrum fit with a spheroid model in neutral deionized water showed two distinct populations, one with an average platelet thickness of 1.75 nm and diameter of 34.98 nm and a second, more symmetric particle with average thickness and diameter of 36.91 nm (diameters were determined by geometrical calculations based on the spheroid and particle aspect shape). The 1.75 nm population consists of an estimated 96.6% of the scattering solids and likely represents disperse but intercalated platelets due to an estimated nearest neighbor value of 1.26 nm. The second set of particles, while larger and more uniform in shape, compose only 3.4% of the solids in the aqueous suspension, but are still involved with particle-particle interactions at a distance of 22.73 nm. For comparison, the SAXS curves of Lap-APTMS + PC were fit with a cylindrical model that yielded one population with an average diameter of 35.42 nm and a layer thickness of 1.77 nm. Other variables in the model assigned a nearest neighbor value equal to 1.78 nm, indicating that in both models the closest platelet was within the thickness of perpendicular travel through one hydrated particle. While both models provided good fits to the data, the results were slightly different, and AFM was utilized to provide an additional means for measuring particle sizes and to monitor the uniformity of the hydrated particles.

19.4.7 Characterization of modified Laponite by AFM

To provide a direct method of measuring particle sizes, a droplet of Lap-APTMS at a concentration of 0.025% w/w in water was allowed to dry on a freshly cleaved mica substrate and characterized by tapping mode atomic force microscopy (AFM). Shown in Figure 19.10 is a 500 × 500 nm height image. Many completely exfoliated Laponite platelets, having approximate dimensions of 35 × 1.5 nm, can be observed. The line sections of the image were taken to obtain particle height measurements. Particles 1 and 2 showed heights of ~ 1.5 nm, which are similar to thickness measurements obtained by cylindrical fitting of SAXS data. Particle 3 appears to be larger and thicker, with a height of ~ 2.4 nm and diameter of ~ 55 nm. While small platelets were commonly observed, there were also aggregations of a few platelets (having dimensions of 125 × 7 nm), one of which can be seen in the lower left quadrant of Figure 19.10. Our observations by AFM, which compare well to other’s,³⁸ and our values obtained by SAXS-fitting showed a small range of diameters that are comparable in size to the theoretical crystal model of Laponite. Observation of correctly sized particles provided confirmation that our analytical tools and data analysis were sensitive to Laponite dispersed in aqueous solutions or as a dry film or powder.

19.4.8 Incorporating modified Laponite into coatings

After having ensured that the platelets existed as mostly exfoliated small particles in aqueous dispersions as characterized by AFM, SAXS and having observed the optical clarity of the solutions, the stability of the clay in the waterborne PVDF resin was explored with the goal of limiting particle aggregation. It was found that at even low loading levels (< 0.1% Laponite w/w% Kynar ARC) upon drying, the normally flexible film showed small cracks after spray-coating on bronze that may have been induced by aggregation of clay particles during drying. Control over platelet motion and charge was tuned by adjusting the rheology of the bulk matrix, and subsequently, a more viscous coating, Kynar Aquatec® FMA 12 was used. Films of that coating did not crack after drying, possibly because limiting particle diffusion may reduce nanoclay aggregation. The undiluted wet Kynar Aquatec® FMA 12 with Lap-APTMS + PC nanocomposite resin was analyzed by SAXS and fit to a two-dispersion spheroid model shown in Figure 19.11 in an effort to determine if either particle shape or interactions changed in the aqueous latex suspension compared to water alone (as shown in Figure 19.9). While the aqueous dispersions fit with a cylinder geometry model were found to give the most accurate data when compared with AFM data, spheroid fits allow for multiple populations with slightly different shapes in the dispersion, which certainly exist for Laponite (one distribution) in a polymer latex (the second distribution) suspension. Fitting the scattering curves obtained by SAXS to the spheroid model, one distribution had a size similar to that obtained in the aqueous SAXS data, having a diameter of 26.6 nm, 1.33 nm in thickness and a nearest neighbor at 1.32 nm, which likely corresponded to the Lap-APTMS + PC particle itself (labeled A in Table 19.4). A second distribution existed and was assigned to the blended PVDF/acrylic latex resin having a median size near 158.2 nm with a nearest neighbor of 12.6 nm (labeled B in Table 19.4). The small and broad diffraction peak occurring at 5.9 2θ (d₀₀₁) is an indication of low crystallinity, suggesting that there may be very short scale ordering in both distance and time in the suspension.

In our previous work we have shown that the barrier properties of the waterborne PVDF resin improved after exposure to QUV-B weathering cycles, due to the annealing effects of those cycles,²³ and we again investigated that effect for the nanocomposite films. Shown in Figure 19.12a and b are EIS spectra of films with the fluorinated nanoclay, Lap-FOTES + PC, incorporated into Kynar Aquatec FMA 12 as a topcoat with a solvent-based Paraloid™ B-44 primer on bronze before annealing, after annealing, and after weathering in a QUV-B chamber for 250 h. There are two major changes in the EIS spectrum after annealing: (1) the total impedance of the system increased by approximately two orders of magnitude at low frequencies and (2) because the phase angle was close to − 90° at all frequencies, capacitive properties of the system became dominant. After 12 h of annealing at 65 °C, every coated substrate with covalently modified Laponite (including those having undergone the additional cation exchange step) incorporated into the topcoat displayed a total impedance greater than 100 MΩ at 0.1 Hz, as well as a phase angle higher than − 70° at all frequencies (one example data set is shown in Figure 19.12). Using the same method outlined in a previous publication,²³ the EEC shown in Figure 19.12c was fit to EIS data taken before and after films of Kynar Aquatec® FMA 12 with Lap-FOTES + PC incorporated were soaked in water for 4 h in a standard electrolyte. The three elements in the model fit to the data were assigned: R_bulk corresponded to the bulk electrolyte resistance, CPE_Layer2 and CPE_Layer1 corresponded to the electrolyte swollen (more permeable) portion of the coating and dry (less permeable, more protective, higher impedance, lower capacitance) layer of the coated bronze. As the CPE_Layer1 was fit with a phase parameter nearly 0.95 in all cases, it is most likely associated with the drier portion of the topcoat due to its low capacitance and nearly ideal phase. In contrast, CPE_Layer2 was assigned a phase parameter (ϕ) close to 0.85 in all cases, indicating that layer of the coating exhibited more water transport through the film.

In general, the permeability of a coating to electrolyte causes an increase in capacitance, which is not a desirable feature of protective coatings as it may precede corrosion. Capacitance can be determined from a constant phase element value and those values for films with and without nanoclays incorporated are shown in Table 19.5. Circuit model fitting provided an initial capacitance of 3.49 and 3.62 nF for Lap-FOTES + PC nanocomposites, which were higher than the 3.31 nF for Layer 1 and 2.14 nF for Layer 2 for the film containing Lap-APTMS + PC. The noncation exchanged Lap-FOTES was extremely difficult to disperse in a homogenous mixture and also had the highest modeled capacitance of 8.65 and 4.53 nF for the first and second layers, respectively.

Table 19.5

Capacitance and Volume Fractions of Polymer Nanoclay Films

	Capacitance (nF) Before	Capacitance (nF) After	${\text{VF}}_{{\text{H}}_2\text{O}}$ (%V/V)	Total ${\text{VF}}_{{\text{H}}_2\text{O}}$(%V/V)
Kynar Aquatec ARC*	C1: 3.86	C1: 5.80	9.22	9.22
Kynar Aquatec FMA-12*	C1: 3.43	C1: 5.33	10.1	10.1
Na-Lap in FMA-12	C1: 1.57	C1: 1.89	4.21	8.90
	C2: 5.66	C2: 6.95	4.69
Lap-APTMS in FMA-12	C1: 0.723	C1: 0.76	1.20	7.33
	C2: 2.50	C2: 3.28	6.13
Lap-APTMS + PC in FMA-12	C1: 3.31	C1: 6.97	16.9	18.0
	C2: 2.14	C2: 2.24	1.06
Lap-FOTES in FMA-12	C1: 8.65	C1: 1.06	4.55	7.02
	C2: 4.53	C2: 5.05	2.47
Lap-FOTES + PC in FMA-12	C1: 3.49	C1: 3.96	2.90	3.45
	C2: 3.62	C2: 3.71	0.55

C1 and C2 were calculated using fit values from circuit modeling. Because resistors did not fit the EIS data, capacitance values of films containing Laponite were calculated using C = CPE × ω^a − 1 , while capacitance values from circuit models where resistors fit data (films denoted by * where no nanoadditive was used) were calculated using C = (CPE × R)^1/a/R The values of C1 and C2 were then used to calculate volume fractions using: $\text{VF}=log\left(C_t/C_0\right)/log{\epsilon }_{{\text{H}}_2\text{O}}\times 100\%$ where VF is the volume of water uptake in percent, C_t is the capacitance after soaking, C₀ is the initial dry capacitance, and ${\epsilon }_{{\text{H}}_2\text{O}}$ dielectric constant of water (80.1 at 20 °C).

19.5.2 Calculating capacitances and volume fractions of water in electrolyte swollen films

The resistance of films with and without having nanoclays incorporated was challenged by immersing coated metal substrates in water. Because water/electrolyte ingress causes capacitance to increase, calculated capacitance values may be used to determine volume fraction of water using the Brasher-Kingsbury equation.³⁹ Capacitance values and volume fractions of water occupying the coating before and after soaking in electrolyte were determined and are listed in Table 19.5. Films with not only the lowest volume fraction, but also the lowest initial capacitance should offer the best barriers to water transport through films and best corrosion protection. Kynar Aquatec® RC-10206 and FMA-12 films without nanoclay additives absorbed the most water (nearly 10% volume fraction of water) as calculated by their changes in capacitance after immersion. The Lap-APTMS film in FMA-12 absorbed 7.33% total volume fraction of water, while the film containing Lap-APTMS + PC absorbed 18.0%. For comparison, unmodified Laponite was also incorporated into films and found to uptake a total volume of water of 8.90%. The Lap-FOTES + PC dispersed easily in water, and showed the least total volume fraction uptake of 3.45%.

19.5.3 Assessment of the effect of intelligent coating

Comparing the performance of these intelligent coatings to our prior work on common waterborne and solvent-based acrylic coatings provides a useful metric for evaluating the effect of our method of approach. We previously determined that waterborne coatings of Rhoplex™ WL-81 adsorbed a water volume fraction of ~ 14% and solvent-based coatings of Paraloid™ B-44 absorbed ~ 5%.²² In the studies described here, we observed that waterborne films having only chemical intelligence had a water volume fraction of ~ 10%, considerably less than traditional waterborne coatings previously measured. Most interestingly, the best-performing films had both chemical and physical intelligence and resulted in a water volume fraction of ~ 3.5%, lower than solvent-based coatings and, in fact, were the lowest water volume fraction for protective coatings for corrosion control that we have measured to date.