INCLUDING THE SAME

TECHNICAL FIELD

The present disclosure broadly relates to fluoropolymer coatings.

BACKGROUND

Fluoropolymers are well known to be an important family of organic materials in industry as they typically provide high thermal stability, excellent chemical resistance, good film strength, low absorption of UV and visible light at a very broad region and low energy surfaces.

Fluoropolymers that are perfluorinated and thermoplastic, known in the art as fluoroplastics or fluorothermoplastics, are typically inert to nearly all chemicals and solvents, even at elevated temperatures and pressures, reacting only with strong reducing agents such as molten alkali metals. In addition, their high crystallinity leads to relatively low gas and moisture permeability, so they act as excellent barrier resins. Their low surface energies and coefficients of friction against other materials lead to the well-known anti-stick applications. Their extremely low refractive indices and optical clarity make them useful in optical applications including antireflective coatings. Fluoroplastic copolymers of monomers comprising tetrafluoroethylene and a perfluoro(propyl vinyl ether) are collectively known in that art by the abbreviation "PFA". The term PFA includes copolymers of tetrafluoroethylene with a perfluoro(propyl vinyl ether) (e.g., perfluoro(methyl vinyl ether)) as well as copolymers of tetrafluoroethylene with a perfluoro(propyl vinyl ether) and one or more additional perfluorinated monomers (e.g., hexafluoropropy lene) .

Fluoropolymers have been used to coat glass substrates such as light bulbs for shattering prevention when light bulbs break, to coat metallic substrates for anti-stick properties (e.g., cookware, bakeware), and also for corrosion protection in industrial applications (chemical tanks, exhaust ducts). However, their inherent anti-stick characteristics can result in challenges when attempting to bond them to substrates. SUMMARY

In one aspect, the present disclosure provides a primer composition comprising PFA polymer particles and colloidal silica particles dispersed in a vaporizable aqueous liquid vehicle, wherein the primer composition has a weight ratio of the colloidal silica particles to the PFA polymer particles is less than or equal to 0.6, and wherein the primer composition contains less than one part by weight of: a) non- vaporizable organic compounds having at least one C-H bond; and b) inorganic particles having a mean particle size of greater than one micrometer.

In some embodiments, the weight ratio of the colloidal silica particles to the PFA polymer particles is greater than or equal to 0.3. In some embodiments, the colloidal silica particles have a mean particle size of 4 to 45 nanometers (nm) or 4 to 20 nm. In some embodiments, the primer composition consists essentially of the PFA polymer particles, the colloidal silica particles, and the vaporizable aqueous liquid vehicle. In some embodiments, the primer composition has a solids content of from one to ten percent. In some embodiments, the vaporizable aqueous liquid vehicle is free of vaporizable organic solvent.

In another aspect, the present disclosure provides an article comprising a substrate having a glass surface with a primer layer disposed on at least a portion thereof, wherein the primer layer is formed by at least partially drying a primer composition according to the present disclosure.

In some embodiments, the article further comprises a fluoropolymeric layer coated on the primer layer. In some embodiments, the substrate comprises a light bulb. In some embodiments, the substrate comprises at least one of a solar cell, a mirror, a window, or a solar thermal collector.

In another aspect, the present disclosure provides a method comprising: contacting the primer composition according to the present disclosure with a glass surface of a substrate, and at least partially evaporating the vaporizable aqueous liquid vehicle to provide a primer layer.

In some embodiments, the method further comprises coating a fluoropolymeric layer on the primer layer.

Advantageously, primer compositions according to the present disclosure have good adhesion to glass and good optical transparency, and may provide a suitable surface for further fluoropolymer protective coatings such as, for example, a PFA layer. Further, in typical embodiments the primer compositions are resistant to coloration due to thermal oxidation and/or light (e.g., weathering).

As used herein:

The term "aqueous" means containing more than an adventitious amount of water (e.g., at least about 1, 5, 10, or even at least about 20 percent by weight of water, or more).

The term "inorganic" refers to chemical compounds that are not organic; for clarity as used herein it expressly includes all elemental forms of carbon.

The term "non-vaporizable" means not vaporizable.

The term "organic" refers to chemical compounds having at least one carbon- hydrogen (i.e., C-H) bond.

The term "perfluoroplastic" means a thermoplastic perfluoropolymer.

The term "perfluoropolymer" means a polymer wherein all hydrogen atoms have been replaced by fluorine atoms.

The term "vaporizable" means evaporable at a temperature of less than 150°C at a pressure of one atmosphere (0.1 MPa).

The features and advantages of the present disclosure will be understood upon consideration of the detailed description as well as the appended claims. These and other features and advantages of the disclosure may be described below in connection with various illustrative embodiments of the disclosure. The above summary is not intended to describe each disclosed embodiment or every implementation of the present disclosure. The Figures and the detailed description which follow more particularly exemplify illustrative embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a side view of an exemplary article according to the present disclosure. While the above-identified drawing figures set forth several embodiments of the present disclosure, other embodiments are also contemplated, as noted in the discussion. In all cases, this disclosure presents the disclosure by way of representation and not limitation. It should be understood that numerous other modifications and embodiments can be devised by those skilled in the art, which fall within the scope and spirit of the principles of the disclosure. The figures may not be drawn to scale. DETAILED DESCRIPTION

The primer composition comprises PFA particles, typically dispersed in the vaporizable aqueous liquid vehicle in latex form, although other dispersions (e.g., which may be metastable) may also be used. Methods for making PFA dispersions are well known; for example, as disclosed in U.S. Pat. Nos. 3,132,123 (Harris et al); 4,029,868 (Carlson); 4,078,134 (Kuhls et al); and 4,078,135 (Sulzbach et al). Typically, the PFA fluoropolymer comprises 92-99.5 percent, more typically 96-99 percent of

tetrafluoroethylene, and 0.5-8 percent, more typically 1-4 percent of perfluoro(propyl vinyl ether). Aqueous PFA dispersions are commercially available under the trade designation TEFLON PFA from E. I. du Pont de Nemours and Co. of Wilmington, DE; for example, as TE-7224, 857N-110, 857N-210, 858N-916, and 858N-917. Combinations of two or more PFA particles may be used.

Typically, PFA dispersions are stable over a pH range of from 1 to 11 , although other pH values may also be used. Typically, the primer composition has a total PFA content of from 0.1 to 40 percent by weight, more typically 2 to 25 percent by weight based on the total weight of the primer composition, although other amounts can be used.

Optionally, the primer composition may contain one or more additional perfluoropolymer particles. Examples include polytetrafluoroethylene particles, polyhexafluoropropylene particles, and poly(hexafluoropropylene-co-tetrafluoroethylene) (FEP) particles. The optional additional perfluoropolymer particles may be provided as a latex or other type of dispersion; for example, prepared according to known methods or obtained from commercial sources.

If present, the optional perfluoropolymer particles are present in a weight ratio of PFA:optional perfluoropolymer particles in a range of from 1 :4 to 4: 1 , more typically in a weight ratio of from 3:7 to 7:3.

The silica particles typically have a mean particle size of less than about 150 nm, although larger mean particle sizes may be used if they can be suspended in the primer composition. Typically, the silica particles have a mean particle size in a range of from 4 to 45 nm, more typically in a range of from 4 to 20 nm. As used herein, the term "silica particle" refers to silica particles that do not have any covalently bound organic surface group(s). The silica particles are typically provided in a colloidal dispersion (e.g., as a sol), although this is not a requirement. Colloidal silica particles are available in acidic or basic forms, and either form may be utilized. Methods of making colloidal silica particles include, for example, partially neutralization of an alkali-silicate solution. More typically, colloidal silica dispersions are provided from commercial suppliers; for example, under the trade designation LUDOX from W.R. Grace of Columbia, MD; or as NALCO 1034A colloidal silica (NALCO 1034A), NALCO 1129 colloidal silica (NALCO 1129), NALCO 2327 colloidal silica (NALCO 2327), NALCO 2326 colloidal silica (NALCO 2326), and NALCO 1140 colloidal silica (NALCO 1140), all available from Nalco Chemical Company of Naperville, IL.

NALCO 1034A has a mean particle size of 20 nm and a silica content of approximately 34 percent by weight in water with a pH of approximately 3.1. NALCO 1129 has a mean particle size of 20 nm and an Si0 ₂ content of approximately 30 percent by weight in a solution of 40 percent isopropanol and 30 percent water. NALCO 2327 has a mean particle size of 20 nm and a silica content of approximately 40 percent by weight in water with a pH of approximately 9.3, and ammonium as the stabilizing ion. NALCO 2326 has a mean particle size of 5 nm and a silica content of approximately 14.5 percent by weight in water with a pH of approximately 9.0, and ammonium as the stabilizing ion. NALCO 1140 has a mean particle size of 15 nm and a silica content of approximately 40 percent by weight in water with a pH of approximately 9.7, and sodium as the stabilizing ion.

The vaporizable aqueous liquid vehicle comprises water and optionally one or more organic solvents, typically volatile organic solvents, in amounts such that the vaporizable aqueous liquid vehicle forms a single phase. Examples of organic solvents include methanol, ethanol, isopropanol, acetone, 2-methoxyethanol, glyme, and tetrahydrofuran. Typically, the vaporizable aqueous liquid phase is essentially free of (e.g., contains less than one percent by weight) volatile organic solvents. Typically, the concentration of organic solvent ranges from 0 to 40 percent by weight based on the weight of the vaporizable aqueous liquid vehicle.

The primer composition may have any pH that renders the composition stable enough for its intended use as a primer composition. Typically, the pH is in a range of from about 9 to about 11, although other pH values may also be useful. The primer composition may have any solids content; for example, from about 2 percent to about 20 percent by weight based on the total weight of the primer composition, although other amounts may also be used.

Optionally, a surfactant may be added to the primer composition. Useful surfactants include ionic surfactants such as, for example, sodium

hexadecylbenzenesulfonate, and nonionic surfactants.

The primer composition is typically made by combining with mixing the PFA particles and silica particles as aqueous dispersions, although other methods may be used. Typically, pH for the primer composition is kept at 9 or higher, however this may vary depending on the dispersions used.

Primer compositions according to the present disclosure are typically applied to a glass surface of a substrate and at least partially dried to form a primer layer, more typically essentially completely dried. As used herein, the term "dried" refers to removal of the vaporizable aqueous liquid vehicle as a whole, not just water. The primer composition may be disposed on the glass surface by any suitable method, such as for example, dip coating, spray coating, brushing, curtain coating, knife coating, squeegee, bar coating, or wiping. Once disposed on the substrate, the primer composition is at least partially dried to remove at least some, typically essentially all, of the vaporizable aqueous liquid vehicle. Drying may be facilitated, for example, by air flow assisted evaporation, heating, and/or reduced pressure.

A fluoropolymeric layer is then disposed on the primer layer. This is illustrated in Fig. 1, wherein exemplary article 100 comprises substrate 110 having glass surface 115, primer layer 120 disposed on glass surface 115, and fluoropolymeric layer 130 disposed on primer layer 120.

The substrate has a glass surface on which the primer composition is disposed, although not all surfaces of the substrate need be glass. The primer composition may be disposed on a portion of the glass surface or the entire glass surface. In some

embodiments, the primer composition may be deployed on at least a portion of one or more additional surface(s) of the substrate in addition to the glass surface. As used herein, the term "glass" refers to a non-crystalline transparent or translucent material formable by fusing silica, optionally with additional compounds (e.g., soda and lime, lead compounds, boron compounds, barium compounds, rare earth compounds, iron compounds, and cerium compounds) followed by rapid cooling. Exemplary glasses include soda-lime glasses and borosilicate glasses.

Exemplary substrates having glass surfaces include light bulbs, windows, cookware (e.g., saucepans, skillets), bakeware (e.g., PYREX baking dishes), solar cells, thermal solar collectors, windshields, and mirrors (e.g., solar reflectors). In some embodiments, such as, for example, as a coating for light bulbs, application of a PFA layer to the primer layer may provide shatter resistance. In the case of breakage of the light bulb, the PFA layer may serve to contain the glass shards, and mercury in the case of fluorescent tubes or compact fluorescent light bulbs.

The perfluoropolymeric layer comprises at least one perfluoropolymer. Typically, the fluoropolymeric layer is essentially free of (e.g., contains less than one percent by weight) of polymers having C-H bonds, although this is not a requirement. The perfluoropolymeric layer may include additional components such as, for example, fillers, light stabilizers, antioxidants, surfactants, and colorants. Examples of suitable

perfluoropolymers include FEP and PFA. The polymeric layer may be applied to the primer layer, for example, as a dispersion or extruded directly onto the primer layer.

Advantageously, primer compositions, articles, and methods according to the present disclosure can solve problems such as a lack of high transmission of light of existing PFA bonding primers, improve mechanical properties PFA coatings, and improve adhesion strength of perfluoroplastics to glass surfaces.

Objects and advantages of this disclosure are further illustrated by the following non-limiting examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this disclosure.

EXAMPLES

Unless otherwise noted, all parts, percentages, ratios, etc. in the Examples and the rest of the specification are by weight.

Silica nanoparticle dispersions NALCO 1115 (15 weight percent solids, 4 nanometer average particle size colloidal silica), NALCO 2326 (14.5 weight percent silica, 5 nm average particle size colloidal silica), and NALCO 2327 (40 weight percent silica, 20 nm average particle size) were obtained from the Nalco Chemical Company of Naperville, IL.

DYNEON PFA 6900 N PFA dispersion (120 nm mean particle size, 50 weight percent solids) was obtained from 3M Dyneon of Oakdale, MN.

Soda lime glass slides were obtained from VWR International of West Chester, PA, and were pretreated by gentle scrubbing with BAB-0 powdered cleanser from Fitzpatrick Bros., Inc. of Chicago, IL, and subsequently washed thoroughly with deionized water before use. EXAMPLE 1

NALCO 1115 silica dispersion (1.0 grams (g), 4 nm mean particle size, 15 weight percent solids) was diluted with 9 g of water. To the diluted dispersion was added

DYNEON PFA 6900 N PFA dispersion (1.0 g). The mixed dispersion (PRIMER

COMPOSITION 1) was stable and clear.

A commercially available light bulb (General Electric 40 watt tungsten filament) was rinsed with water and dried by air. To the surface was applied (by dip coating) the aqueous liquid primer described above except the neck area of the light bulb. This provided an area where the PFA coating would not adhere to the glass in order to create a tab for the peel test. The coated light bulb was dried in an oven at 100°C for an hour. The light bulb was then electrostatically powder coated with Dyneon PFA 6503B EPC powder (PFA powder from Dyneon LLC) using a Nordson SURECOAT manual powder coating system (from Nordson Corp. of Westlake, OH) at 70 volts, 150 kPa airflow until no bare glass was visible. The light bulb was then baked in an air circulating oven at 750°F (400°C) for 10 minutes. Upon removal from the oven, the light bulb sample was cooled to room temperature. The resulting PFA-coated light bulb was clear, the interfacial adhesion between PFA layer and primed glass surface exhibited excellent adhesion compared with the interface between PFA layer and unprimed neck area where no adhesion was observed.

EXAMPLE 2

A commercially available light bulb (General Electric 40 watt tungsten filament) was rinsed with water and dried by air. PRIMER COMPOSITION 1 was applied to the light bulb as before, except that the entire surface of the bulb was coated. The primed light bulb was dried in an oven at 100°C for an hour. The light bulb was then

electrostatically powder coated with PFA 6503B EPC powder Dyneon PFA 6503B EPC powder (PFA powder from Dyneon LLC) using a Nordson SURECOAT manual powder coating system, at 70 volts, 150 kPa airflow until no bare glass was visible. The powder- coated light bulb was then baked in an air circulating oven at 750°F (400°C.) for 10 minutes. Upon removal from the oven, the light bulb was cooled. The resulting PFA coated light bulb was just like the one obtained in the previous experiment. The PFA coated light bulb was thrown at least 3 meters high above the floor. Upon impact the light bulb was completely broken, but all the broken pieces were retained by the coated PFA layer while the PFA coating remained intact.

EXAMPLES 3 - 7 and COMPARATIVE EXAMPLES A - B Two strips each of soda lime glass 1 x 6 x 0.05 inch (2.54 x 15.2 x 0.127 cm) microscope slides were cleaned with soapy water. The strips were then rinsed several times with water, and dried in air. The strips were coated with the primer compositions reported in Table 1 , except that at the top of the glass strips over 2 inches (5 cm) of one end of each strip. This provided an area where the coating would not adhere to the glass to create a tab for the peel test. The strips were then electrostatically powder coated with PFA 6503B EPC powder Dyneon PFA 6503B EPC powder (PFA powder from Dyneon LLC) using a Nordson SURECOAT manual powder coating system, at 70 volts, 150 kPa airflow until no bare glass was visible. The strips were then baked in an air circulating oven at 750°F (400°C.) for 10 minutes. Upon removal of the strips from the oven, the strips were cooled and the edges of each strip were scraped with a sharp blade to remove any coating that may have accumulated at the edges of the specimen. The peel strength was measured by testing the samples using an Instron tensile testing apparatus equipped with a floating roller peel test fixture at a crosshead speed of 6 in/min (15 cm/min) and peeling to 3.75 inches (9.5 cm) extension generally according to test method ASTM D3167-03a (2004), "Standard Test Method for Floating Roller Peel Resistance of

Adhesives". The peel strength was calculated over 1 to 3 inches (2.54 to 7.62 cm) extension using an integrated average and reported as an average of three replicate specimens. Pencil Hardness was also measured and is reported in Table 1 (below). Percent light transmission measured using a UV-VIS spectrometer is reported in Table 2. TABLE 1

TABLE 2

All patents and publications referred to herein are hereby incorporated by reference in their entirety. All examples given herein are to be considered non-limiting unless otherwise indicated. Various modifications and alterations of this disclosure may be made by those skilled in the art without departing from the scope and spirit of this disclosure, and it should be understood that this disclosure is not to be unduly limited to the illustrative embodiments set forth herein.