HK1117864A - Powder coating base coat - Google Patents

Description

Powder coating base coat

CROSS-REFERENCE TO RELATED APPLICATIONS

The priority of this application is based on provisional application serial No. 60/678908, filed on 6/5/2005, which is incorporated herein by reference.

Background

Technical Field

The present invention relates to the application of non-stick coatings to substrates.

Prior Art

It is known from publications such as WO 2005/58389 to mix together powders of a fluoropolymer and a thermoplastic polymer and an inorganic filler of a pigment, thereafter to apply the powder mixture to a metal substrate and to melt the applied powder to form a uniform coating on the substrate.

Multilayer application of various fluororesins and thermoplastic polymers is also known from publications such as WO 2003/015935.

The present invention is based on the discovery of a process comprising a unique combination of sequential steps wherein multiple layers of a mixture of fluoropolymer and thermoplastic polymer, inorganic filler and perfluoropolymer are applied to a substrate.

Summary of the invention

Accordingly, the present invention includes a method of coating a substrate with a coating comprising a fluoropolymer. The method comprises the following continuous steps:

a. preparing a solid mixture comprising one or more fluoropolymers and one or more thermoplastic polymers that are thermally stable at temperatures in excess of 400 ℃;

b. melt mixing and extruding the solid mixture at a temperature of about 250 ℃ to about 400 ℃ to achieve homogeneity;

c. subjecting the extrudate to a mechanical process to obtain a powder having an average particle size of up to about 100 microns;

d. mixing the resulting powder with an inorganic filler having an average particle size of about 5-100 microns, and a perfluoropolymer powder having a particle size of about 5-100 microns to the blend of step c to obtain a powder coating; wherein the inorganic filler and perfluoropolymer may be mixed with the powder of step c simultaneously, or with the inorganic filler prior to the perfluoropolymer, or with the perfluoropolymer prior to the inorganic filler;

e. applying a powder coating to a substrate; and

f. the substrate is heated to a temperature sufficient to cause partial agglomeration of the powder.

Other embodiments of the invention relate to details regarding the methods and materials used.

Detailed Description

The process of the present invention is based on a unique combination of steps, materials and conditions that produce a multi-layer non-stick coating with excellent adhesion between the base coat and the top coat.

The present invention advantageously provides a VOC-free coating system. The liquid primers currently in commerce are solvent based and therefore have an undesirably high VOC content. Even water-based systems contain significant levels of VOCs.

The invention also simplifies the process to a fully powdered process. In addition to reducing VOC, this also reduces the amount and complexity of the device that the paint applicator must hold.

The powder primer of the non-stick powder coating of the present invention is based on powders of fluoropolymers, thermoplastic polymers and perfluoropolymers, and inorganic fillers. This is to be distinguished from commercially used powder coatings based on FEP (copolymer of hexafluoropropylene and tetrafluoroethylene) or PFA (copolymer of perfluoropropyl vinyl ether and tetrafluoroethylene) with solvents. The materials of the invention are converted into powder coatings by mixing the raw materials, followed by coextrusion and subsequent grinding to the desired particle size.

With respect to the primer layer, and the use of fluoropolymers and thermoplastic polymers, the present technology is similar to but different from that described in patent application US 2004/0253387 a1, which is incorporated herein by reference. However, if the invention described in that patent application is used as an undercoat of a powder coating, there will be a lack of intercoat adhesion between the undercoat and the perfluoropolymer powder topcoat. The topcoat, typically PFA, FEP and/or MFA (copolymer of tetrafluoroethylene and perfluoromethyl vinyl ether), will delaminate completely when tested by the base adhesion test (cross-hatch adhesion and nail adhesion).

The coatings of the present invention can be used with substrates of any desired hardness. The type of substrate to which the coating is applied and the shape formed are not limiting to the scope of the invention. In a preferred embodiment, the non-stick coating of the present invention is used to coat a metal substrate, such as steel or aluminum, which forms cookware. It is highly preferred to pretreat the substrate, for example by chemical etching or grit blasting, to improve the adhesion of the primer coating. Sandblasting is preferred.

The present invention uses two approaches to improve intercoat adhesion. One way to improve intercoat adhesion is to create a rough surface on the basecoat which creates a mechanical bond between the basecoat (primer) and the topcoat, the smaller the particle size, the less rough the basecoat surface, and the poorer the intercoat adhesion. This rough surface and improved mechanical adhesion can be obtained by mixing an inorganic filler such as a pigment having a particle size that will produce a rough surface when the primer is applied and baked into an extruded and ground powder coating. An example of such a pigment is an aluminum flake that has been bonded to a powder primer. Thus, when the resulting powder primer is applied to a metal substrate and cured, followed by application of a powder topcoat and curing, the bond between the basecoat and topcoat is strong enough to withstand boiling water, cross-hatch, and tape adhesion tests. However, the intercoat adhesion of the coatings prepared in this way will fail on their own when tested by the standard post boiling water nail adhesion test.

A second way to improve intercoat adhesion is to mix powders comprising a base coat, which may comprise FEP, PFA and MFA having a particle size similar to the topcoat to be applied over the base coat. This improves the tighter adhesion between the base coat and the top coat by chemical and mechanical adhesion. Improving the adhesion of the base coat to the top coat to the extent that the resulting coating passes the boiling water, cross hatch and tape tests as well as the nail test is very important for most industrial applications such as the coating of commercial bakeware.

One or both of the above approaches can be used, but the best adhesion is obtained by using both.

As used herein, the term "similar" with respect to particle size means that the average diameter of the particles is no more than 50% greater or less than the average diameter of the comparative particles.

To form the primer layer, the present invention performs melt mixing of the fluoropolymer and thermoplastic polymer at a temperature of about 250 ℃ to about 400 ℃ to achieve homogeneity. Preferably, the amount of fluoropolymer in the resulting solid mixture is from about 5 to about 50 weight percent and the amount of thermoplastic polymer in the solid mixture is from about 50 to about 95 weight percent, with the solid mixture being mixed at the preferred temperature of about 250-400 ℃ and extruded with a twin screw extruder. The extrudate can be ground in air at a temperature of about-10 ℃ to 20 ℃ to a powder of up to about 100 micron average particle size.

The powder is then mixed with an inorganic filler, such as a pigment, having an average particle size of about 5 to 100 microns, and a perfluoropolymer powder having an average particle size of about 5 to about 100 microns to provide a powder coating. The inorganic filler and perfluoropolymer may be mixed with the milled extrudate powder simultaneously, or with the inorganic filler prior to the perfluoropolymer, or with the perfluoropolymer prior to the inorganic filler. Preferably, the particle size of the inorganic filler powder is similar to the particle size of the polymer powder.

The inorganic filler/polymer mixture may be bound prior to step e whereby the mixture is heated until the polymer particles become sufficiently sticky to bind the inorganic filler particles.

Preferred fluoropolymers for use in the present invention may be selected from PTFE (polytetrafluoroethylene), TFE (tetrafluoroethylene) and copolymers of such comonomers as PMVE (perfluoromethyl vinyl ether), PPVE (perfluoropropyl vinyl ether), HFP (hexafluoropropylene), ethylene, CTFE (chlorotrifluoroethylene), and combinations of the above comonomers.

The thermoplastic polymer preferably used in the present invention may be selected from the group consisting of polyether sulfone (PES), polyaryl sulfone (PAS), polyphenylene sulfide (PPS), polyether ether ketone (PEEK), Polyimide (PI) and Polyamideimide (PAI). PPS is most preferred.

Inorganic fillers useful in the present invention include mica methyl silicate (methasilicate) or wollastonite (wollostonite), talc and aluminum flakes. Most preferred is aluminum flake. One example is aluminum flake manufactured by Alcon-Toya as known from PCF 7130. It is advantageous to coat the aluminum flakes with an acrylic resin.

Preferably, the average particle size of the inorganic filler is equal to or greater than the thickness of the coating on the substrate in step f below.

The powder mixture is then applied to a substrate, preferably in a layer of about 20-60 microns thick, but may be up to 100 microns thick, depending on the application, electrostatically, heating the substrate to a temperature sufficient to cause partial coalescence of the powder (melting only to the extent that the powder particles will adhere to each other), typically from about 370 ℃ to about 415 ℃.

To form the topcoat, the perfluoropolymer powder is then applied to the coated substrate, which is again heated at a temperature typically ranging from about 370 ℃ to about 415 ℃ to produce a perfluoropolymer powder that becomes sufficiently fluid and adheres to the first layer. As previously mentioned, the powder forming the top coat is preferably of a similar particle size to the powder forming the base coat.

The perfluoropolymers preferably used in the primer and topcoat layers of the present invention are PFA, FEP and MFA, with PFA being most preferred due to its physical properties (toughness, release properties, etc.).

Once the base coat is applied, it may be flashed at about 370 ℃ and 400 ℃ for 2-3 minutes. The top coat may then be applied and post-cured at 400 ℃ for about 10 minutes thereafter. If flash evaporation is not used, the post-cure may be carried out at about 400 ℃ and 425 ℃, preferably about 415 ℃ for 10 minutes.

In the following non-limiting examples, substrates were applied with only one of the various base coats prepared according to the present invention and subjected to three tests to determine how well the perfluoropolymer topcoat would adhere to the base coat. The first test was a pencil test in which pencils of various hardness were drawn on the coated surface and observed whether the top coat delaminated from the base coat upon cutting. In a second test, ball point pens were pushed into the coating with various pressures to make the same measurements. The third test was a nail touch test to determine if the topcoat could be peeled off with the tester's nail, the first was easy to peel off with the nail, and the second was peeled off with the nail after the coated panel was immersed in boiling water for 30 minutes.

Example 1

All of the requirements of the present invention are met in this example except that no filler is used and no perfluoropolymer is mixed with the powdered extrudate.

Example 2

All of the requirements of the present invention are met in this example except that the perfluoropolymer is not mixed with the powdered extrudate.

Example 3

This example illustrates the invention in all respects.

The following table summarizes the starting materials used in the above examples:

	raw materials (% by weight)	Example 1	Example 2	Example 3
					a. Mixing the above raw materials and extruding.
	Ryton V1	75.99	72.3	68.76
						PTFE TL-1	19.93	18.96	18.03
	Channel Black (Channel Black)	2.39	2.27	2.16
						Neocryl BT-44	1.56	1.43	1.36
b. The following raw materials were mixed with the extruded chips and ground.
						Fumed silica (Fume silica)	0.19	0.19	0.18
c. The crumbs were combined with the following aluminum flakes by the Benda-Lutz method.
						PCF7130 non-floating sheet aluminum powder		4.85	4.76
d. Mixed with the following powders.
						Hyflon®PFA 7010			4.76
						Total of	100	100	100

Description of raw materials:

ryton V1 is a low viscosity polyphenylene sulfide prepared by Chevron-Phillips of Bartlesville, Oklahoma.

PTFE TL 10 is a fluoropolymer, known as polytetrafluoroethylene. It is manufactured by AGCCchemicals America, Inc of Downningtown, Pennsylvania.

Channel blacks are micronized channel blacks sold by Keystone Aniline Corp.

Neocryl BT-44 is a 45% solids, water-based acrylic latex manufactured by Neoresins Inc., a business unit for DSM.

Fumed silica, produced by Cabot corp. or Degussa, is an additive used to improve powder coating spray.

PCF7130 is a non-leafing flake aluminum pigment produced by Toyal America inc. The D50 particle size of this pigment was 23 microns.

Hyflon^®PFA 7010 is a copolymer of tetrafluoroethylene and perfluoropropyl vinyl ether with a D50 particle size of 38 microns.

In examples 1-3, the resulting crumb after step c had a D50 particle size (approximately 50% of the particles having the indicated diameter) of 22 microns, an extrusion temperature of 300 ℃ and an air temperature of-1 ℃. The particle size of the topcoat layer comprising perfluoropolymer is about 20-25 microns.

With respect to the pencil test, the hardness of the lead was B, HB and F, with the hardness increasing from small to large.

For the ball point pen test, pressures of 20, 50 and 70psi were used.

The following table summarizes the results:

	example 1	Example 2	Example 3
				Pencil test	Only the hardness B passes	Hardness B and HB only pass	B. HB and F through
Ball-point pen test	Passing only up to 20psi	Passing only up to 50psi	At 70psi through
				Nail test	Failure of	By passing	By passing
Nail followed by boiling water test	Failure of	Failure of	By passing

Only the coating of example 3 passed all tests, the top coat did not delaminate or peel.

Claims (9)

1. A method of coating a substrate with a coating comprising a fluoropolymer, said method comprising the sequential steps of:

a. preparing a solid mixture comprising one or more fluoropolymers and one or more thermoplastic polymers that are thermally stable at temperatures in excess of 400 ℃;

b. melt mixing and extruding the solid mixture at a temperature of about 250 ℃ to about 400 ℃ to achieve homogeneity;

c. subjecting the extrudate to a mechanical process to obtain a powder having an average particle size of up to about 100 microns;

d. mixing the resulting powder with an inorganic filler having an average particle size of about 5-100 microns, and a perfluoropolymer powder having a particle size of about 5-100 microns to the blend of step c to obtain a powder coating; wherein the inorganic filler and perfluoropolymer are mixed with the powder of step c simultaneously, or with the inorganic filler before the perfluoropolymer, or with the perfluoropolymer before the inorganic filler;

e. applying the powder coating to a substrate; and

f. heating the substrate to a temperature sufficient to cause partial agglomeration of the powder.

2. The method according to claim 1, wherein the blend of inorganic filler powder and polymer powder of step d is bonded.

3. The process according to claim 1, wherein a perfluoropolymer powder coating is applied to the coated substrate from step f; the matrix is reheated to cause the powder to become sufficiently fluid and adhere as a second layer to the first layer.

4. The method according to claim 1 wherein said powder coating of step e is applied electrostatically in a layer of about 20 to 100 microns thick.

5. The method according to claim 4, wherein the average particle size of the inorganic filler is equal to or greater than the thickness of the coating layer on the substrate in the following step f.

6. The method of claim 1, wherein the inorganic filler comprises a pigment.

7. The method of claim 6, wherein the pigment comprises an aluminum flake.

8. The method according to claim 1, wherein the particle size of the powder of step c is similar to the particle size of the inorganic filler of step d.

9. A method according to claim 3, wherein the particle size of the powder forming the second layer is similar to the particle size of the powder forming the first layer.