SURFACE TREATMENT COMPOSITIONS AND COATED ARTICLES PREPARED THEREFROM

Abstract

Surface treatment compositions are provided comprising: (a) a fluorine-containing polymer; (b) a solvent containing at least one C—F bond; and (c) a rheology modifying component. Also provided are coated articles, comprising: (i) a substrate; and (ii) a self-assembled coating layer applied to at least one surface of the substrate. The self-assembled coating layer is formed from the surface treatment composition above.

Description

FIELD OF THE INVENTION

The present invention relates to surface treatment compositions that demonstrate thixotropy and to coated articles prepared therefrom.

BACKGROUND OF THE INVENTION

The use of coatings to protect electronic circuitry from external environments is widely known and many different treatments exist to perform that function. However, there are currently no liquid, solution-based coatings that can provide a uniform coating thickness over the complex geometries of printed circuit boards. Chemical vapor deposition (e. g., vacuum-applied technologies) such as the application of parylene can make a true conformal coating. Unfortunately, vacuum-deposited coatings such as parylene-based treatments usually require labor-intensive masking of “keep-out” areas.

Conventional conformal coating solutions used on circuit boards will flow and level to some degree without being under external shear forces besides gravity. This causes the corners, edges, sidewalls to have a much thinner coating than the flat/recessed areas of the coated substrate, and when the coated circuitry is exposed to water while power is on, failure occurs at these areas. Therefore, in order for a printed circuit board to survive immersion in water while being powered up, a coating must be applied that will have uniform coverage regardless of surface topology.

It would be desirable to provide surface treatment compositions that deliver the benefits of conformal coatings yet eliminate the need for costly capital investment and avoid the bottlenecking caused by vacuum-based batch manufacturing processes or masking operations.

SUMMARY OF THE INVENTION

Surface treatment compositions are provided comprising:

(a) a fluorine-containing polymer;
(b) a solvent containing at least one C—F bond; and
(c) a rheology modifying component. Also provided are coated articles, comprising:
(i) a substrate; and
(ii) a self-assembled coating layer applied to at least one surface of the substrate; wherein the coating layer is formed from the surface treatment composition above.

DETAILED DESCRIPTION OF THE INVENTION

Other than in any operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between (and including) the recited minimum value of 1 and the recited maximum value of 10, that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10.

As used in this specification and the appended claims, the articles “a,” “an,” and “the” include plural referents unless expressly and unequivocally limited to one referent.

The various aspects and examples of the present invention as presented herein are each understood to be non-limiting with respect to the scope of the invention.

The present invention provides surface treatment compositions that form self-assembled structures upon application to a substrate, and result in coatings of uniform thickness regardless of the surface topology of the article being coated. It has been discovered that particular combinations of polymer, solvent and rheological modifier can yield a coating composition that provides the correct balance of the delicate intramolecular interactions between the various components in order to generate self-assembled structures. These self-assembled structures provide thixotropy to the composition and thus allow it to demonstrate dramatic changes in viscosity as a response to external stimuli, particularly the application of shear forces. Most importantly, these compositions will rapidly increase in viscosity when shear forces are removed, which allows for the coating composition to adhere to complex shapes without running (sag) or flowing. The resultant dried film will have a much greater uniformity in thickness compared to a coating composition that does not form such structures.

The surface treatment compositions of the present invention comprise (a) a fluorine-containing polymer. The polymers may be any polymers that contain fluorocarbon (i. e., C—F) units, such as —C(F)₂—, —C(F)(H)—, and/or terminal units such as —C(F)_x(H)_y, wherein x is greater than or equal to 1 and x+y=3. Note that the phrase “and/or” when used in a list is meant to encompass alternative embodiments including each individual component in the list as well as any combination of components. For example, the list “A, B, and/or C” is meant to encompass seven separate embodiments that include A, or B, or C, or A +B, or A +C, or B +C, or A +B +C. The fluorine-containing polymer (a) is most often a (meth)acrylic polymer. By “(meth)acrylic” is meant polymers prepared from monomers having acrylic functional groups, polymers prepared from monomers having methacrylic functional groups, and/or polymers prepared from both types of monomers. The polymers are typically prepared from acrylic and methacrylic monomers such as acrylic acid, methacrylic acid, and esters thereof. Useful alkyl esters of acrylic acid or methacrylic acid include aliphatic alkyl esters containing from 1 to 30, and often 4 to 18 carbon atoms in the alkyl group. Non-limiting examples include methyl methacrylate, ethyl methacrylate, butyl methacrylate, ethyl acrylate, butyl acrylate, and 2-ethyl hexyl acrylate. Suitable other copolymerizable ethylenically unsaturated monomers include vinyl aromatic compounds such as styrene and vinyl toluene; nitriles such as acrylonitrile and methacrylonitrile; vinyl and vinylidene halides such as vinyl chloride and vinylidene fluoride, vinyl esters such as vinyl acetate or ethers such as vinyl trifluoromethyl ether. Fluoro-functional monomers such as monofluoroethylene, difluoroethylene, trifluoroethylene, and tetrafluoroethylene are suitable for imparting fluoro functionality to the polymer.

(Meth)acrylic polymers can be prepared via known organic solution polymerization techniques. Generally any method of producing such polymers that is known to those skilled in the art utilizing art recognized amounts of monomers can be used.

The fluorine-containing polymer (a) typically demonstrates a weight average molecular weight (Mw) of 5,000 to 500,000; usually 25,000 to 250,000 Daltons as measured by gel permeation chromatography using a polystyrene standard. Particularly suitable methacrylic polymers include I500101, commercially available from Aculon, Inc.

The fluorine-containing polymer (a) is usually present in the surface treatment composition in an amount of 5 to 25 percent by weight, more often 12.5 to 17.5 percent by weight, based on the total weight of the surface treatment composition.

The surface treatment compositions of the present invention further comprise (b) a solvent containing at least one C—F bond. Examples include hydrofluoroether (HFE) solvents. Such solvents were developed originally as replacements for CFCs, HFCs, HCFCs, and PFCs. An advantage of using an HFE solvent is its short atmospheric lifetime and zero ozone depletion potential compared to alternative chemicals.

Examples of particular hydrofluoroether solvents include 1,1,1,2,2,3,3,4,4-nonafluoro-4-methoxybutane and/or 1,1,1,2,2,3,3,4,4-nonafluoro-4-ethoxybutane, commercially available from 3M Corporation as NOVEC 7200. Other exemplary solvents include 3-ethoxyperfluoro(2-methylhexane) (HFE 7500, also available from 3M Corporation); 1H,1H,5H-Octafluoropentyl-1,1,2,2-tetrafluoroethyl ether (HFE 6512, available from Fuxin Hengtong); and/or 1,1,1,2,3,4,4,5,5,5-Decafluoropentane (Vertrel XF, available from E. I. DuPont de Nemours).

The solvent (b) is usually present in the surface treatment composition in an amount of 75 to 90 percent by weight, more often 75 to 89 percent by weight, based on the total weight of the surface treatment composition.

The surface treatment compositions of the present invention further comprise (c) a rheology modifying component. Fumed silica that is surface modified with hydrophobic functional groups is a particularly suitable rheology modifying component. The fumed silica contributes to the thixotropic nature of the surface treatment composition of the present invention. By “hydrophobic” is meant that the functional groups have non-polar properties and have a tendency to interact with, be miscible with, or be dissolved by non- polar solvents such as alkanes and oils.

Fumed silica is formed from microscopic droplets of amorphous silica fused into branched, chainlike, three-dimensional secondary particles which then agglomerate into tertiary particles. It is manufactured by flame pyrolysis of silicon tetrachloride or from quartz sand vaporized in a 3000° C. electric arc. The resulting powder has an extremely low bulk density (often 160-190 kg/m3) and high surface area (usually 50-600 m2/g). Hydrophobic functional groups with which the silica particle surfaces may be modified include hexamethyldisiloxane, dichlorodimethylsilane, trimethylchlorosilane, octadecyldimethylchlorosilane, chlorinated poly(dimethylsiloxane), and the like. Particularly suitable fumed silica is commercially available from Cabot Corporation and Evonik Industries.

The rheology modifying component (c) is usually present in the surface treatment composition in an amount of 1 to 5 percent by weight, more often 1.5 to 2.5 percent by weight, based on the total weight of the surface treatment composition.

The surface treatment compositions of the present invention typically demonstrate a thixotropic index greater than 2. Thixotropic index is defined as the ratio of the viscosity under slow speed shear to the viscosity under high speed shear of a non-Newtonian fluid. It is used as a tool to define how well the fluid will hang or resist sagging under gravity. The thixotropic index is obtained by measuring the viscosity of a fluid at ambient temperature. Ambient temperature typically ranges from 60 to 90° F. (15.6 to 32.2° C.), such as a typical room temperature, 72° F. (22.2° C.). A viscometer is used to measure the viscosity at two speeds (one is a multiple or a factor of 10 of the other). The index is calculated by dividing the viscosity at the lower speed shear by the viscosity at the higher speed shear.

In a particular example of the present invention, the surface treatment composition comprises (a) a (meth)acrylic fluorine-containing polymer; (b) a hydrofluoroether solvent; and (c) fumed silica that is surface modified with hydrophobic functional groups.

The surface treatment compositions of the present invention are suitable for use in preparing coated articles such as coated circuit boards. The coated articles of the present invention comprise a substrate (i). Substrates suitable for use in the preparation of the coated articles of the present invention can include a metal such as copper or steel, or any substrate commonly used in the preparation of circuit assemblies, such as polyepoxides, including fiberglass reinforced polyepoxides, polyimides, phenolics, and fluorocarbons. The polymeric substrates may comprise an electrically conductive circuit pattern imprinted in (such as in the case of a blind via) or on the substrate surface. Often the substrate is an electronic component of a circuit assembly, such as a printed circuit board.

The substrate may take any shape as desired for the intended application, such as flat, curved, bowl-shaped, tubular, or freeform. For example, the substrate may be in the form of a flat plate having two opposing surfaces, such as would be suitable for use in an electronic circuit assembly as a circuit board.

Prior to application of any coatings, the substrate may be cleaned such as by argon plasma treatment or with a solvent such as lonox 13416 or Cybersolv 141-R, both available from Kyzen.

The coated articles of the present invention further comprise a self- assembled coating layer (ii) applied to at least one surface of the substrate. The coating layer comprises any of the surface treatment compositions described above.

The coating layer may be applied to the substrate by one or more of a number of methods such as spraying, dipping (immersion), spin coating, or flow coating onto a surface thereof.

The self-assembled coating layer typically demonstrates a dry film thickness (DFT) of 5 to 100 microns. The coating layer demonstrates a very uniform thickness over the surface of the substrate and serves as a conformal coating. The dry film thickness of the coating layer over the surface of the substrate typically varies by no more than 50 percent.

Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. A surface treatment composition comprising:

(a) a fluorine-containing polymer;

(b) a solvent containing at least one C—F bond; and

(c) a rheology modifying component.

2. The surface treatment composition of claim 1 wherein the fluorine-containing polymer (a) is present in an amount of 10 to 20 percent by weight, based on the total weight of the surface treatment composition.

3. The surface treatment composition of claim 1 wherein the fluorine-containing polymer (a) comprises a (meth)acrylic polymer.

4. The surface treatment composition of claim 1, wherein the fluorine-containing polymer (a) has a weight average molecular weight of 25,000 to 500,000.

5. The surface treatment composition of claim 1 wherein the solvent (b) is present in an amount of 75 to 89 percent by weight, based on the total weight of the surface treatment composition.

6. The surface treatment composition of claim 1, wherein the solvent (b) comprises 3-ethoxyperfluoro(2-methylhexane), 1,1,1,2,2,3,3,4,4-nonafluoro-4-methoxybutane, 1,1,1,2,2,3,3,4,4-nonafluoro-4-ethoxybutane, 1,1,1,2,3,4,4,5,5,5-Decafluoropentane and/or 1H,1H,5H-Octafluoropentyl-1,1,2,2-tetrafluoroethyl ether.

7. The surface treatment composition of claim 1 wherein the rheology modifying component (c) is present in an amount of 1 to 5 percent by weight, based on the total weight of the surface treatment composition.

8. The surface treatment composition of claim 1 wherein the rheology modifying component (c) comprises fumed silica that is surface modified with hydrophobic functional groups.

9. The surface treatment composition of claim 1 wherein said surface treatment composition demonstrates a thixotropic index greater than 2.

10. A coated article comprising:

(i) a substrate; and

(ii) a self-assembled coating layer applied to at least one surface of the substrate; wherein the coating layer is formed from a surface treatment composition comprising:

(a) a fluorine-containing polymer;

(b) a solvent containing at least one C—F bond; and

(c) a rheology modifying component.

11. The coated article of claim 10 wherein the substrate (i) comprises a printed circuit board.

12. The coated article of claim 10, wherein the coating layer (ii) demonstrates a dry film thickness of 5 to 100 microns.

13. The coated article of claim 12, wherein the dry film thickness of the coating layer over the surface of the substrate varies by no more than 50 percent.

14. The coated article of claim 10 wherein the fluorine-containing polymer (a) is present in the surface treatment composition in an amount of 10 to 20 percent by weight, based on the total weight of the surface treatment composition.

15. The coated article of claim 10 wherein the fluorine-containing polymer (a) comprises a (meth)acrylic polymer.

16. The coated article of claim 10 wherein the fluorine-containing polymer (a) has a weight average molecular weight of 25,000 to 500,000.

17. The coated article of claim 10 wherein the solvent (b) is present in the surface treatment composition in an amount of 75 to 89 percent by weight, based on the total weight of the surface treatment composition.

18. The coated article of claim 10 wherein the solvent (b) comprises 3-ethoxyperfluoro(2-methylhexane), 1,1,1,2,2,3,3,4,4-nonafluoro-4-methoxybutane, 1,1,1,2,2,3,3,4,4-nonafluoro-4-ethoxybutane, 1,1,1,2,3,4,4,5,5,5-Decafluoropentane and/or 1H, 1H,5H-Octafluoropentyl-1,1,2,2-tetrafluoroethyl ether.

19. The coated article of claim 10 wherein the rheology modifying component (c) is present in the surface treatment composition in an amount of 1 to 5 percent by weight, based on the total weight of the surface treatment composition.

20. The coated article of claim 10 wherein the rheology modifying component (c) comprises fumed silica that is surface modified with hydrophobic functional groups.