WO2006047238A2

WO2006047238A2 - Liquid colorant dispersions for powder coatings

Info

Publication number: WO2006047238A2
Application number: PCT/US2005/037799
Authority: WO
Inventors: David Lee; Corey King
Original assignee: Degussa Corporation
Priority date: 2004-10-21
Filing date: 2005-10-21
Publication date: 2006-05-04
Also published as: WO2006047238A3

Abstract

A method is provided to obtain powder coating having improved color strength at similar pigment loadings, which maintain their physical properties, by the use of a liquid pigment dispersion.

Description

LIQUID COLORANT DISPERSIONS FOR POWDER COATINGS

FIELD OF THE INVENTION This invention relates to powder coatings and, more particularly, to a method for the incorporation of a liquid pigment dispersion into a powder coating base resin, and tinted or colored powder coatings incorporating a pigment. BACKGROUND OF THE INVENTION

It is well-known in the art to mix one or more dry powdered pigments with other solid raw materials used for powder coatings and to heat the mixture to a molten state in an extruder. The molten material is then cooled and pulverized to form fine particles. The foregoing process, which typically includes organic polymers, fillers, pigments and additives, is referred to as powder coating. In this process, the coating is fusible with the substrate onto which it is sprayed to form a protective layer. Currently, those skilled in the art of powder coatings add dry pigments to the powder coating formulation, employing a variety of mixing/blending operations. The number of dry pigments employed can vary from at least one pigment to as many as five or more in order to attain the tint which is desired.

One of the negative issues associated with the use of dry pigments is the handling of large quantities of very fine powders, which presents physical, environmental, and logistical problems. Furthermore, the inhalation of these fine powders by plant workers represents a serious health hazard in the workplace requiring special handling equipment, the need for special clothing for the workers, and safety equipment to avoid illness and disease.

Other factors which mitigate against the use of dry pigment dispersions are the difficulty of accurately weighing the pigment, the difficulty of obtaining reproducible tint strength, significant losses of valuable pigment occasioned by the small particle sizes involved, and the need for improved color strength. strength, significant losses of valuable pigment occasioned by the small particle sizes involved, and the need for improved color strength. BRIEF SUMMARY OF THE INVENTION

It has now been found that powder coatings having improved color strength at similar pigment loadings, while maintaining their physical properties, are obtained by the use of a liquid pigment dispersion in accordance with the present invention in place of the conventional dry pigment compounds and compositions. Liquid pigment dispersions provide excellent batch-to-batch uniformity.

In one embodiment of the present invention, a premix is prepared by mixing a pigment dispersion comprising a liquid resin containing one or more pigments, with other powder coating raw materials in a disperser operating at a speed of about 100 rpm to about 6000 rpm for about 1 minute to about 120 minutes. The liquid pigment dispersion is typically from 3% to about 10%, by weight, of the premix.

The premix is then charged to an extruder and processed at an elevated temperature until such time as a uniform melt of the premix is formed.

The pigmented melt is then extruded onto a chill roll, followed by grinding and sieving of the pigmented extrudate into powdered form suitable for use as a powder coating. When the powder coating is ready for use, it is placed in a suitable device, such as a corona gun, for spraying onto various substrates, for example, cold-rolled steel panels, to provide an adherent coating having good physical properties and color strength which, in its physical properties, is comparable to, or better than, that obtained using a dry pigment or pigments for tinting the coating material. DETAILED DESCRIPTION OF THE INVENTION

This invention relates to pigment dispersions comprising a dispersing resin, with or without reactive diluent and with or without solvent(s), (including organic and inorganic solvents). Additionally, pigment dispersions of the invention optionally contain a surfactant(s) and an anti-oxidant(s). This invention utilizes dispersing resins which are soluble and compatible in powder coating systems. This invention introduces liquid colorants, with or without organic solvents or inorganic solvents, to the above powder coating systems. The present invention can also employ aliphatic, aromatic, unsaturated and/or oxygenated non-aqueous carrier solvent(s). These pigment dispersions can contain none, one or more surfactants. Any organic and inorganic pigments can be used in this invention to prepare colorant dispersion products. Any solvents, organic or inorganic, are removed in the powder making process or before the process by spray drying or other novel methods. In order to improve the powder flowability, additives such as glass transition temperature (Tg) modifiers, i.e., materials with a higher Tg, or processing aids, such as polymers, calcium carbonate, silica and talc, can be used.

General compositions of the invented pigment dispersions are shown below. The amounts are % by weight.

Polyester Resin with or without reactive diluent 1-95 Solvent(s) 0

Surfactant(s) 0-10

Anti-settling agent (optional) 0-5

Anti-oxidant (optional) 0-5

Pigment(s) 1-85 Flow Improvement Additives 0-50

Other dispersing Resin 1-45

Solvent(s), Organic or Inorganic 10-80

Surfactant(s) 0.1-10

Anti-settling agent (optional) 0-5 Pigment(s) 5-75

Flow Improvement Additives 0-50 In a preferred embodiment, the following are the ranges of compositions. The amounts are % by weight.

Polyester Resin with or without reactive diluent 1-30

Solvent(s) 0 Surfactant(s) 0-10

Anti-oxidant (optional) 0-5

Pigment(s) 5-75

Flow Improvement Additives 0-50 Pigment dispersions without any solvents and highly concentrated pigment dispersions can be prepared from the present invention. These dispersions contain very high amount of pigments and still have the reasonable viscosity for easy handling. Example 1 : Yellow iron oxide dispersion

The following example contains 40% by weight of pigment and 59.99% Polyester dispersing resin (about 85 to 90% Polyester and 10 to 15% reactive diluent).

Polyester Resin with or without reactive diluent 59.99

Anti-oxidant (optional) 0.01

Pigment - Yellow Oxide 40.0

Example 2: Red iron oxide dispersion

The following example contains 43% pigment and 56.99% Polyester dispersing resin (about 85 to 90% Polyester and 10 to 15% reactive diluent).

Polyester Resin with or without reactive diluent 56.99

Anti-oxidant (optional) 0.01 Pigments - Red Oxides 43.0

Example 3: Organic blue dispersion

The following example contains 11.2% pigment and 87.79% Polyester dispersing resin (about 85 to 90% Polyester and 10 to 15% reactive diluent). Polyester Resin with or without reactive diluent 87.79

Anti-oxidant (optional) 0.01

Pigment - Phthalo Blue 11.2 Example 4: Organic red dispersion

The following example contains 14.5% pigment and 85.49% Polyester dispersing resin (about 85 to 90% Polyester and 10 to 15% reactive diluent).

Polyester Resin with or without reactive diluent 85.49 Anti-oxidant (optional) 0.01

Pigment -Red 170 14.5

Example 5: Mixture of yellow, red and blue dispersions

The following example contains a blend of 50% of Example 1 dispersion, 25% of Example 2 dispersion and 25% of Example 3 dispersion.

The preceeding examples define pigment dispersions that are yellow, red, and blue, which were tested in powder coatings as reported in the examples that follow. However, there are many other colors that can be incorporated in the liquid pigment dispersions of the invention, including, carbon black, phthalo green, titanium dioxide, pigment yellow 74, pigment red 170 and others.

There are also other modifiers that can be incorporated into the liquid pigment dispersions of the invention to improve certain properties of the final powder coating. These modifiers include, but are not limited to, ketone-aldehyde resins for Tg improvement, anti¬ oxidants for improved UV stability, additives for improved flow, oils for better wetting of high-oil absorptive pigments, and surfactants (both ionic and nonionic) for improved dispersion of high oil-absorption pigments. Modifiers that can be used in the example formulations are as follows: soya lecithin, nonoxynol-9 phosphate, mineral spirits and propylene glycol monomethyl ether acetate. Anti-settling additives, fumed silica and organoclay can be used to provide better suspension of heavy inorganic pigments. Some aldehyde-ketone resins are Laropal® from BASF, Krumbhaar™ from Eastman Chemicals and Synthetic Resin from Degussa. Some available distilled tall oils are Westvaco® from MeadWestvaco Corporation, Sylvatal® from Arizona Chemical, PAMAK from Eastman and XTOL® from Georgia-Pacific. Resins can also be selected for specific formulations with the object of lowering the formulated cost. Preparation of Pigment Dispersions

The method for the preparation of the pigment dispersions of this invention uses standard dispersers and milling equipment. Special devices, such as sealed microfluidizer or high pressure milling devices are required to prepare inorganic solvent-based pigment concentrates, such as supercritical liquid CO₂, NH₃, SO₂, SO₃, H₂O pigment concentrates and others.

The process consists of two principal steps: Step 1 : Preparation of Premix: A premix is prepared using a high speed disperser in an open or a sealed environment. The mixing time is usually 30 minutes and a typical speed is 3000 rpm.

Step 2: Milling of the Premix: The premix can be milled utilizing traditional milling equipment that is commercially available or a special pressurized milling equipment. The milling time is usually 20 minutes in a traditional Chicago Boiler bead mill or a sealed mircofluidizer. Some inorganic pigment dispersions can be prepared using only high speed disperser, and do not require milling, such as titanium dioxide dispersions.

Preparation of Powder Coatings Using Pigment Dispersions The pigment dispersions formulated from the present invention useful in industrial powder coating systems as color tinting components. They were tested for compatibility, adhesion and other properties in industrial powder coating bases of the polyester-tri- glicidyisocyanate (TGIC), polyester-polyurethane (PUR) and epoxy-polyester (Hybrid) types. The results obtained demonstrate that the liquid dispersions have broad compatibility, minimal or no adverse effects on physical properties, and in most cases, actually improved color development as compared to dry pigments. The coatings were tested in TGIC, PUR, and Hybrid bases. The pigment dispersions can be used in other bases commonly used by the powder coating industry. These additional bases include epoxies, phenolics, novalac-epoxies, silicones, polyester-hydroxyalkylamides, acrylic-hydroxyalkylamides, GMA-acrylics, acrylic-polyurethanes and polyester- tetramethoxymethylglycolurils.

The following is a general composition of a powder coating that contains a pigment dispersion (the amounts are by weight percent):

Resin(s) 1-95

Crosslinker(s) 0-40

Hiding and extender pigments 0-50

Pigment dispersion 0.5-15

Flow and leveling additives 0.1-4

Degassing agent 0.1-2

Gloss modifiers (optional) 0-10

Anti-oxidants (optional) 0-2

Other powder coating additives (optional) 0-5

In a preferred embodiment, the following is the range of compositions. The amounts are by weight percent.

Resin(s) 30-70

Crosslinker(s) 10-25

Hiding and extender pigments 10-30

Pigment dispersion 3-6

Flow and leveling additives 1-2

Degassing agent 0.5-1

Gloss modifiers (optional) 0-10

Anti-oxidants (optional) 0-2

Other powder coating additives (optional) 0-5 Powder coatings incorporating pigment dispersions can be manufactured using traditional methods of pre-mixing all the raw material components together, melt-mixing them in an extruder, grinding them in a mill, sieving them to remove large particles, electrostatically spraying them onto a substrate, then curing them for a sufficient time in an oven. The specific steps of the powder coating preparation is as follows: Step 1. Preparation of the pre-mix. The powder coating components are weighed slowly into a paper bucket and stirred briefly with a tongue depressor. The components are then added to a high-speed blade mixer and blended at 1500rpm for 2 minutes.

Step 2. Extrusion of pre-mix. The pre-mix is poured into a hopper that feeds a Werner & Pfleiderer ZSK^® 30 twin-screw extruder. The zone 1 and 2 temperatures were set at 90⁰C and 105⁰C, respectively. The hot extrudate was fed directly onto a chill-roll unit which cooled the extrudate to room temperature and pressed it into a thin sheet.

Step 3. Grinding and sieving the extrudate. The extrudate was pounded into chips then placed into the hopper of a Bantam^® Hammermill to grind the extrudate chips into a fine powder. The powder was then sieved using a Retsch vibratory sifter using a 140 mesh screen (105μm). The sieved powder was collected in a plastic bag.

Step 4. Application and cure of the powder coating. The powder was placed in the hopper of a Nordson Sure Coat^® corona gun. The powder was applied to cold-rolled steel panels by spraying, to provide an approximate thickness of 2 mils (50μm). These panels were placed in an oven at 200⁰C and baked for 12 minutes.

Step 5. Testing the powder coating. The powder coatings were tested for impact resistance, slow deformation resistance (Erichsen cupping), pencil hardness, cross-hatch adhesion, gel time reactivity and color development.

The following powder coating samples were prepared and tested using the prescribed method, with comparable formulations for coatings formulated using the liquid pigment dispersions of the invention, and those formulated with dry pigments ("comparative"). The first three powder coating examples detail the use of pigment dispersions in Hybrid, TGIC and PUR bases, and the amounts are given in weight of the formula. All of the examples listed below contain the following compounds: Titanium dioxide 270.0

Degassing agent (benzoin) 5.0

Flow agent on silica carrier 10.0

Dibutyltin dilaurate catalyst on silica carrier 10.0 Example 6 (A. B, C. D) - PUR base

Polyester resin, OH value: 50 mg KOH/g polymer 550.3 ε-caprolactam blocked polyisocyanate, NCO content 15.3% 134.7

Pigment dispersion (A, B, C, D use Examples 1, 2, 3 and 5, resp.) 30.0

Example 7 (A. B, C. D) - Hybrid base

Polyester resin, Acid value: 71.0 mg KOH/g polymer 337.5

Epoxy resin, EEW 675-760 (Type II) 337.5

Pigment dispersion (A, B, C, D use Examples 1, 2, 3 and 5, resp.) 30.0

Example 8 (A. B, C. D) - TGIC base

Polyester resin, Acid value: 35.0 mg KOH/g polymer 637.0

Triglycidylisocyanurate 48.0

Pigment dispersion (A, B, C, D use Examples 1, 2, 3 and 5, resp.) 30.0

Depending upon the type of pigment dispersion used (red iron oxide, yellow iron oxide, organic blue), comparative formulations based on dry pigment blends were prepared using the identical pigment and the same quantity (pigment solids) used in the corresponding pigment dispersion. The grey coatings were prepared to show that mixtures of dispersions can be prepared to make other colors, and that one dispersion can be used to eliminate the addition of several dry pigments to a powder coating formulation to produce the desired color.

Example 9 (Comparative) - Yellow iron oxide with PUR base

Polyester resin, OH value: 50 mg KOH/g polymer 564.8 ε-caprolactam blocked polyisocyanate, NCO content 15.3% 138.2

Solid pigment- yellow iron oxide 12.0

Example 10 (Comparative^*) - Red iron oxide with PUR base

Polyester resin, OH value: 50 mg KOH/g polymer 564.1 ε-caprolactam blocked polyisocyanate, NCO content 15.3% 138.0

Solid pigment - red iron oxides 12.9

Example 11 (Comparative) - Organic Blue with PUR base

Polyester resin, OH value: 50 mg KOH/g polymer 571.6 ε-caprolactam blocked polyisocyanate, NCO content 15.3% 139.8

Solid pigment- phthalo blue 3.6

Example 12 (Comparative) - Grey with PUR base

Polyester resin, OH value: 50 mg KOH/g polymer 566.3 ε-caprolactam blocked polyisocyanate, NCO content 15.3% 138.6

Solid pigment - yellow iron oxide 6.0 Solid pigment - red iron oxides 3.3

Solid pigment- phthalo blue 0.8

Example 13 (Comparative) - Yellow iron oxide with Hybrid base

Polyester resin, Acid value: 71.0 mg KOH/g polymer 346.5

Epoxy resin, EEW 675-760 (Type II) 346.5 Solid pigment- red iron oxides 12.0

Example 14 (Comparative) - Red iron oxide with Hybrid base

Polyester resin, Acid value: 71.0 mg KOH/g polymer 346.1

Epoxy resin, EEW 675-760 (Type II) 346.0 Solid pigment- red iron oxides 12.9

Example 15 (Comparative) - Organic blue with Hybrid base

Polyester resin, Acid value: 71.0 mg KOH/g polymer 350.7

Epoxy resin, EEW 675-760 (Type II) 350.7 Solid pigment-phthalo blue 3.6 Example 16 (Comparative^') - Grey with Hybrid base

Polyester resin, Acid value: 71.0 mg KOH/g polymer 347.5

Epoxy resin, EEW 675-760 (Type II) 347.4

Solid pigment - yellow iron oxide 6.0 Solid pigment - red iron oxides 3.3

Solid pigment-phthalo blue 0.8

Example 17 (Comparative) - Yellow iron oxide with TGIC base

Polyester resin, Acid value: 35.0 mg KOH/g polymer 653.8 Triglycidylisocyanurate 49.2

Solid pigment-yellow iron oxide 12.0

Example 18 (Comparative) - Red iron oxide with TGIC base

Polyester resin, Acid value: 35.0 mg KOH/g polymer 653.0 Triglycidylisocyanurate 49.1

Solid pigment-red iron oxides 12.9

Example 19 (Comparative) - Organic blue with TGIC base

Polyester resin, Acid value: 35.0 mg KOH/g polymer 661.6 Triglycidylisocyanurate 49.8

Solid pigment-phthalo blue 3.6

Example 20 (Comparative) - Grey with TGIC base

Polyester resin, Acid value: 35.0 mg KOH/g polymer 655.6 Triglycidylisocyanurate 49.3

Solid pigment-yellow iron oxide 6.0

Solid pigment-red iron oxides 3.3

Solid pigment-phthalo blue 0.8

The preceding examples 6-20 were tested for typical cured powder coating properties, such as impact resistance, solvent (methyl ethyl ketone) resistance, cross-hatch adhesion and pencil hardness, as well as for color strength (development).

The tables in the following pages describe the properties determined for these coatings (Tables 1-3). Each table separates the chemistries - PUR-Hybrid and TGIC, respectively. Each coating chemistry has different properties associated with it, e.g., level of hardness, chemical resistance and gel time reactivity. There were no observed effects of pigmentation (red, blue, yellow or gray) on coating properties in any of the coating chemistries, nor were there any effects of use of the pigment dispersions instead of the dry pigments. In each of these systems, the properties of each coating were essentially the same, whether dry pigments or liquid dispersions were used.

Table 4 specifically compares the color strength of the coatings prepared using dry pigments and liquid dispersions. The color strength is listed as a percentage, and the higher the percentage, the better the dispersion of pigment in the coating. The color strength of the liquid dispersions varied between the coating systems and the color used, though the improvement was independent of the specific color and coating chemistry. Overall, 8 of the 12 comparative systems showed an improvement in color strength when liquid dispersions were used. In general, the lighter colors (yellow and red), showed greater color strength than the darker colors (blue and gray).

The coating properties data in Tables 1-3, and color strength data in Table 4, demonstrate that liquid dispersions can be used in powder coatings without adverse effects on the coating properties. Furthermore, an increase in color strength indicates that some of these systems can be formulated with less pigment to achieve the desired color. Color dispersions can also be formulated to reduce the amount of pigments weighed and added by the user (e.g., each of the gray coatings) to produce the desired colors, thus reducing production time on the part of the coating formulator.

The color dispersions of the invention can be used to replace dry pigments in powder coatings to produce desired colors, while maintaining coating properties, reducing production time, and in most cases, improving color strength. able 1. Polyurethane Formulations Comparing Pigment Dispersions and Dry Pigments in Yellow, Red, Blue and Grey

*MEK rating after 100 double rubs: 10 - no mar, 8 - trace mar, 6 - slight mar, 4 - moderate mar, 2 - severe mar, 0 - complete

Table 2. Hybrid Formulations Comparing Pigment Dispersions and Dry Pigments in Yellow, Red, Blue and Grey

*MEK rating after 100 double rubs: 10 - no mar, 8 - trace mar, 6 - slight mar, 4 - moderate mar, 2 - severe mar, 0 - complete removal

Table 3. TGIC Formulations Com arin Pi ment Dis ersions and Dr Pi ments in Yellow, Red, Blue and Gre

Table 4. Color Strength Difference of Formulations containing Pigment Dispersions versus Dry Pigments

Claims

WE CLAIM

1. A method of tinting a powder coating resin with a liquid pigment dispersion, which comprises: a) preparing a tinted premix by mixing a liquid pigment dispersion that includes a resin and at least one pigment into a powder coating base in a disperser for a period of from about 1 minute to about 120 minutes at a speed of about 100 rpm to about 6000 rpm; b) adding the tinted premix to an extruder and extruding the tinted premix at an elevated temperature as a tinted melt onto a moving belt to cool the extruded tinted melt and form a tinted sheet; and c) grinding and sieving the tinted sheet into a powder suitable for use in a powder coating apparatus.

2. The method of claim 1 wherein the powder coating base is selected from the group consisting of polyester-TGIC, polyester-polyurethane, epoxy-polyester, epoxies, phenolics, novalac-expoxies, silicones, polyester-hydroxyalkylamides, acrylic- hydroxyalkylamides, GMA-acrylics, acrylic-polyurethanes and polyester- tetramethoxymethylglycolurils .

3. The method of claim 1 , wherein the liquid pigment dispersion is from about 3% to about 10%, by weight, of the premix.

4. The method of claim 1 , wherein the disperser is operated at about 1500 rpm for about 2 minutes.

5. The method of claim 1, wherein the extruder temperature is about 90⁰C to about 105°C.

6. The method of claim 1, wherein the hot extruded tinted melt is fed onto the moving belt which is mounted about a chill roll.