MXPA99009838A - Transparent powder coating compositions for protecting surfaces - Google Patents

Abstract

Powder coating compositions are described containing a blend of ionomers and polyethylene copolymers which fuse at temperatures of about 150°C or below and are durable, weatherable, solvent-resistant, and thereby provide transparent protective coatings on substrates such as signs, particularly traffic signs and license plates.

Description

COMPOSITIONS FOR TRANSPARENT POWDER COATING FOR PROTECTION OF SURFACES Field of the Invention This invention relates to powder coating compositions that are capable of being fused to surfaces at relatively low temperatures to provide protection against environmental conditions, solvents and abrasion.

Background of the Invention Powder coatings are a segment of the industry of industrial coatings that have undergone rapid development. There are two broad categories of powder coatings - thermosetting and thermoplastics. By far the main portion of the market is for thermosetting powders. The first powder coatings were thermoplastic coatings that formed films by coalescing without crosslinking. Thermoplastic coatings have several disadvantages compared to REF .: 31427 thermosetting coatings. These are generally difficult to spray at small particle sizes. In this way, they are generally applied in relatively thick films. Due to the high molecular weights of the binders required, even at high baking temperatures, these are viscous and frequently give poor flow and leveling. A widely recognized problem in the powder coating industry is finding powders that melt at relatively low temperatures, for example around 150 ° C. These low temperatures for powder coating melting are necessary for heat sensitive substrates such as plastics, painted surfaces, and wood. Melting powders at lower temperatures also offer energy savings even for metal substrates that can use conventional powder coatings, which fuse at around 200 ° C. Powders that melt around 150 ° C are not commonly available in thermoplastic formulations because the resulting fused coatings have severe deficiencies in 'one or more properties, such as solvent resistance, abrasion resistance, durability and chemical resistance. A few powders with thermosetting chemistries that fuse around 150 ° C are available or are being developed within the powder coatings industry. These coatings require long times to cure at these low temperatures, thus limiting their utility and increasing costs, or require radiation curing equipment that also increases the cost of these coatings. These powder coatings, thermosetting, melting at low temperature, are also likely to be deficient in the resistance to external environmental conditions and flexibility. The reactivity of thermosetting powder coatings also raises safety issues for workers. Polymer blends generally do not lead to coatings with good clarity. Turbid coatings often result due to some incompatibility of the polymers that are mixed.

Brief Description of the Invention In spite of the previously recognized problems, for example, the lack of success with the thermoplastic polymers and clear coatings with blends, a thermoplastic powder coating composition has been found which comprises a compatible mixture of polymers where the powder composition is fuses at a relatively low temperature, approximately 150 ° C or lower. The new powder compositions are transparent after fusing, for example, approximately equal to or greater than 90% as determined directly by percentage transmittance measurements or by retroreflection of the light going through the coating. The present powder compositions after melting are also durable, resistant to environmental conditions, resistant to solvents, flexible and strongly adherent, and are useful as protective coatings, for example top coatings, for license plates, traffic signs, laminates. retroreflectors, painted surfaces, and plastic surfaces.

Accordingly, the present invention is a transparent, adherent powder coating composition comprising a compatible mixture of (a) about 50 to 85 parts by weight (based on the total polymer content) of an ionomeric resin; and (b) about 15 to 50 parts by weight of a low molecular weight copolymer of ethylene and methacrylic or acrylic acid. More preferred compositions include about 60 to 80 parts by weight of an ionomeric resin and about 20 to 40 parts by weight of the low molecular weight copolymer of ethylene and acrylic acid. The compositions may also include stabilizers and additives in a minor amount, for example about 1 to 5 parts by weight. A second aspect of the present invention is a method for protecting a signal comprising the steps of (a) applying to a surface of the signal having a plurality of image-possessing areas and non-image areas, a powder composition such as described above, and (b) fusing said powder composition to form a transparent protective layer over image-bearing areas and non-image areas.

The clear powder coatings of this invention offer improved protection to coated substrates against: mechanical damage; such as abrasion or impact; high pressure washing; attack with solvent; detergents; acid rain; hydrolysis; degradation by ultraviolet light; discoloration; attack by fungi or microbes; disturbance; and vandalism. One important application is the protection of printed information on the reflective laminate, such as license plate registration numbers, validation information, printed structures and graphic designs.

Detailed description The powder coatings of the present invention as well as the resulting protective coating after melting, on a surface or substrate, includes a compatible polymer blend. The main component of the mixture is an ionomeric resin which is a copolymer of monomers comprising by weight a higher proportion of at least one of ethylene and propylene and a minor proportion of an acid functional group monomer such as acrylic acid or methacrylic acid, whose polymer has been crosslinked by cations such as zinc, sodium or potassium. Zinc is preferred, because it provides good resistance to environmental conditions. Ionomeric resins are defined, for example, in U.S. Patent No. 4,109,037. Preferred commercially available ionomeric ethylene copolymers are polymers of the so-called "SURLYN" series from EI duPont deNemours &Company A particularly preferred SURLYN copolymer is ABCITE AB1060N which is a copolymer of a higher proportion by weight of ethylene and a minor proportion of methacrylic acid ionically crosslinked with zinc ions The second component of the composition is a low molecular weight copolymer of ethylene and acrylic or methacrylic acid Such polymers are preferably low molecular weight copolymers of ethylene and acrylic acid, for example those found under the trade name PRIMACOR obtained from Dow Chemical.For low molecular weight, these polymers include those having high melt index values.A particularly preferred copolymer is PRIMACOR 59901 which has a melt index of about 1300 dg / minute under ASTM D1238 Condition (B), 125 ° C / 2.1 6 kg The compositions may also contain stabilizers. These stabilizers are so-called stabilizers against environmental conditions, and are generally included in polymeric films, and include ultraviolet light absorbers, antioxidants, and materials that purify or inhibit the action of chemical radicals within the films. Antioxidants such as hindered phenols are particularly useful as stabilizers., UV stabilizers such as hindered amines, and ultraviolet light absorbers. The compositions may also contain additives that aid in the flow of the powder compositions. The additives include dry flow and flow additives in molten form. Dry flow additives include, for example, fine particles of silica, treated and untreated fumed silica, finely divided aluminum oxide, feldspar, calcium silicate, and the like. Flow additives in molten form include, for example, benzoin, low molecular weight acrylics, microcrystalline waxes, and the like.

The powder coatings of the present invention are manufactured by well-known methods, basically through the steps of premixing the ingredients, melt extrusion of the mixture and spraying. The extruder is preferably a twin screw extruder for this process. For example, the ingredients included in the composition of the present invention can be mechanically mixed using a twin screw extruder (eg, a variable speed twin screw extruder, such as that produced by Baker Perkins or Werner Pfleiderer). Preferably, the extruder generates a melting temperature of about 150 ° C to about 225 ° C during extrusion. The extruded product can be milled in hammer mills and cryogenically shredded to generate a mixture having particle sizes in the range of about 5 to about 200 microns, preferably with an average size of about 5 to about 100 microns, and more preferably of about 10 to about 60 microns. The resulting material can be added to the organic pigment hopper of a powder coating application system, for example, a Tribomatic II Powder Coating System available from Nordson Company of Amherst, OH, and then applied to the desired surface. Alternatively, the material can be added to the organic pigment hopper of a laser printer, for example, the Siemens Laser Printer model 2090 available from Siemens Nixdorf, Poing, Germany. The powder coating compositions of the present invention are especially useful for signal protection. Signals such as traffic signals and license plates characterize a reinforcement or backup in which a plurality of images, eg, alphanumeric characters, symbols, artwork, and the like (the "areas possessing images") is placed. against a background (the 'areas that do not have images'). The characters can be clear, in which case the background is colored. Alternatively, the signal may consist of colored characters placed against a light background. Characters are typically formed using techniques such as embossing (in which characters are mechanically defined) or printing (in which characters are defined electronically, digitally or mechanically). Frequently, the signals are provided with a retroreflective property to improve visibility. Typically, the backing or backing itself can be a retroreflective sheet adhesively bonded to substrates such as aluminum, steel or plywood. To protect the signal for example from environmental conditions and from abrasion, a clear "top coat" may also be included, as used in the present "image possessing area" refers to images such as alphanumeric characters, symbols, artistic work, graphics, and the like that appear on the surface of the sheet. The "imageless" area refers to any area of the sheet that lacks said images The above-mentioned protective coating or "top coat" can be applied, preferably, by deposition of the powder composition essentially fof solvent, of the present invention, to the surface of the laminate, followed by heating to fuse the powder particles in the form of a continuous film. Electrostatic spray is the main process for the application of powder coatings. An electrostatic spray gun consists essentially of a tube to carry dust carried by air to an orifice with an electrode located in the hole. The electrode is connected to a high voltage (approx. 5-100 kv), low amperage power supply. As the dust particles leave the hole they pass through an ion cloud, called a corona and collect a negative or positive electrostatic charge. The object to be coated is electrically crushed. The difference in the potential attracts the dust particles towards the surface of the part. These are more strongly attracted to areas that are not already covered, forming a reasonably uniform layer of dust even on irregularly shaped objects. The particles adhere to the surface sufficiently hard and long enough for the object to be transported to an oven, where the powder particles melt to form a continuous film, flow, and optionally crosslink. The powder particles of the present invention are capable of being melted at an oven temperature of about 150 ° C or less (but not below about 100 ° C). Dust particles that do not adhere to the object to be coated (over-spray) can be recovered and recycled, typically by mixing with virgin powder. Almost 100% is used sooner or later - a major advantage over liquid coatings applied by spray. Other conventional techniques can be adopted to include: electrostatic or thermal fluidized bed, blade coating, gravity feed and dust spray suspended in water. The powder coatings of this invention have essentially no volatile organic content in a manner contrary to solvent-borne or water-borne coating alternatives, and the over-spray or over-spray can be reused instead of being a hazardous waste. The powder coatings of this invention also have advantages over the laminated film alternatives. No adhesive or liner is required with the powder coatings, as would be the usual situation with a film lamination. The powders also conform to the etched or irregular surfaces better than what a film could do. A film based on the same composition as the powders of the invention is, however, useful for lamination to flat or gently curved substrates, and should be considered as part of the present invention. The invention will now be further described by way of the following examples. All quantities are given in parts by weight, unless noted otherwise.

EXAMPLES Glossary AbciteMR AB 1060N Copolymer of ethylene and methylacrylic acid cross-linked with zinc ions (EI duPont deNemours &Co.) PrimacorMK 59901 Copolymer of ethylene-acrylic acid of low molecular weight (Dow) TinuvinMR 900 2- [2-hydroxy-3,5 -di- (1, 1-dimethylbenzyl) -phenyl] -2H-benzotriazole (Ciba-Geigy) TinuvinM 144 Bis (l, 2,2,6,6-pentamethyl-4-piperadinyl) propane (3-7) di-t-butyl-4-hydroxybenzyl) butyl (Ciba Geigy) IrganoxMK 1010 Tetrakis [methylene (3,5-di-t-butyl-4-hydroxy idrocinnamate) methane (Ciba-Geigy) TinuvinMK 328 2- (2'- idroxy-3,5'-di-t-amylphenyl) benzotriazole (Ciba-Geigy) ChimassorbMR 944LD Bis-tetramethylpiperadinylhexandiamine polymer, trichlorotriptan, trimethylpentanamine (Ciba-Geigy) PrimacorMR 59801 AcryloidMK ethylene-acrylic acid copolymer, higher molecular weight (Dow) AcryloidMK A-l l Methyl methacrylate copolymer (Rohm &Haas) AcryloidMR B-66 Methyl methacrylate / butyl copolymer (Rohm &Hass) V3M30 Ethylene glycol / isophthalate polyester (Plast-o-meric Inc.) Morthane1 ^ L425.91 Aliphatic Polyurethane Thermoplastic Resin (Morton International) THV 200 Fluoropolymer Resin (Dyneon) Example 1 71. 4 parts of AbciteMR AB1060N EMA A / zinc ionomer 28.6 parts of PrimacorMR copolymer 59901 EAA 3.0 parts of stabilizers (1 part each) Tinuvin ™ 900, Tinuvin ™ 144, Irganox ™ 1010) Example 2 70. 0 parts of AbciteMR AB1060N 30.0 parts of PrimacorMR 59901 3.0 parts of stabilizers (1 part each) Tinuvin ™ 328, Chimassorb ™ 944LD, Irganox ™ 1010) .. Example 3 60. 0 parts of AbciteMR AB1060N 40.0 parts of Primacor ™ 59901 3.0 parts of stabilizers (1 part each) Tinuvin ™ 328, Chimassorb ™ 944LD, Irganox ™ 1010) Example 4 70. 0 parts of AbciteMR AB1060N 30.0 parts of Primacor ™ 59901 The above compositions of Examples 1-4 were mixed in a small twin-screw mill and pressed into thin films (from 25.4 to 50.8 microns (1-2 mils)) between the release liners using a platen press with both plates at 135 ° C. These films were pressed against aluminum license plate blanks that had reflector laminate applied to the front sides, using the same platen press. The resulting samples were fused using infrared heaters to simulate the fusion of the powder coatings. The melting furnace was operated to provide a temperature of about 150 ° C for about 1 minute. The resulting coatings had good adhesion to the Reflector License Plate Laminate No. 4770, Scotchlite brand of 3M, had good clarity and good resistance to gasoline (soaking for 1 minute), to toluene (soaking for 1 minute), and to alcohol methyl (10 minutes soak). Small samples of the compositions of Examples 1, 2 and 3 were cryogenically ground. These powders were splashed on a retroreflective laminate on aluminum license plate primordia, fused and tested as were the previous laminated film samples, also showing good adhesion, clarity and solvent resistance.

Comparative Example 1 71. 4 parts of Primacor ™ 59801 EAA copolymer (highest molecular weight) 28.6 parts of Primacor ™ 59901 EAA copolymer (low molecular weight) 3.0 parts of stabilizers (1 part each of Tinuvin ™ 900, Tinuvin ™ 144 and Irganox ™ 1010) Comparative Example 2 71. 4 parts of V3M30 ethylene glycol / isophthalate polyester 28.6 parts of MorthaneMR L425.91 thermoplastic polyurethane, aliphatic 3.0 parts of stabilizers (as in Comparative Example 1) Comparative Example 3 71. 4 parts AcryloidMR A-ll 28.6 parts polyester V3M30 3.0 parts stabilizers (as in Comparative Example 1) Comparative Example 4 71. 4 parts of DuPont Nucrel ™ 699 EMAA 28.6 parts of polyurethane Morthane ™ L425.91 3.0 parts of stabilizers (as in Comparative Example 1).

Comparative Example 5 71. 4 parts AcryloidMR A-ll 28.6 parts polyurethane MorthaneMR L425.91 3.0 parts stabilizers (as in the Example Comparative 1).

Comparative Example 6 80. 0 parts AcryloidMR B66 acrylic copolymer 20.0 parts polyurethane MorthaneMR L425.91 3.0 parts stabilizers (1 part each of Tinuvin ™ 328, ChimassorbMR 944LD, IrganoxMR 1010) Comparative Example 7 70. 0 parts of AcryloidMR B66 30.0 parts of fluoropolymer THV 200 3.0 parts of stabilizers (as in the Example Comparative 6) The compositions of Comparative Examples 1-7 were each blended in a twin screw mill. None of them had good clarity. The composite samples by extrusion of Comparative Examples 4,6 and 7 were made into films and laminated to retroreflective sheets on aluminum license plate primers using the platen press and the melting furnace as described for Examples 1-4 . These samples had all poor clarity and poor resistance to gasoline and toluene.

Comparative Example 8 90. 0 parts of AbciteMR AB1060N EMAA / zinc ionomer . 0 parts of copolymer PrimacorMR 59901 EAA 3.0 parts of stabilizers (as in the Example Comparative 6) Example 5 80. 0 parts of AbciteMR AB1060N 20.0 parts of PrimacorMR 59901 3.0 parts of stabilizers (as in Comparative Example 6) Comparative Example 9 50. 0 parts of AbciteMR AB1060N 50.0 parts of PrimacorMR 59901 3.0 parts of stabilizers (as in the Example Comparative 6) These three compositions (Comparative Examples 8 and 9 and Example 5) are different mixing ratios of the materials previously observed to give good results. These samples were each mixed in a small twin-screw mill to give strands that were pressed into films and then laminated and fused over retroreflective laminate onto aluminum license or circulation plate blanks, as described above for Examples 1- Four. The compositions of Comparative Example 8 had poor adhesion to each of the 3M Scotchlite Reflective License Plate Laminates, Nos. 3750, 4750, and 4770 that were tested. The sample of Comparative Example 9 had good adhesion to laminates Nos. 3750 and 4770 but had poor resistance to gasoline and methanol. Example 5 gave good results, for example good adhesion and resistance to solvents. Adhesion is reported in the subsequent tables from 'Poor' to 'Good' and was observed using the 'Adhesion of Tape Pressure Adjustment' test or the ASTM D 3359-93 Cross Cut Tape test.

Example 6 The following examples were each stabilized with 1.0 pph of Tinuvin ™ 328, 1.0 pph of Chi assorb ™ 944LD and 1.0 pph of Irganox ™ 1010. Each composition was mixed in a small twin-screw mill and pressed into small films between the linings of release using a platen press with both plates at 135 ° C. These films were pressed, using the same platen press and the same conditions, and then laminated and fused on the reflector laminate in aluminum circulation plates.

Example 7 The stabilizers were reduced from the levels used in the previous examples. The following examples were each stabilized with 0.5 pph of Tinuvin ™ 328, 0.5 pph of Chimassorb ™ 944LD and 0.05 pph of Irganox ™ 1010. Each composition was mixed in a small twin screw mill, and pressed into thin films between the liners of release using a platen press with both plates at 135 ° C. These films were pressed, using the same platen press under the same conditions, and then laminated and fused onto the reflector laminate on aluminum license or circulation plates.

Example 8 These same compositions were tested on the following combinations of ink and laminate, roller coating.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (9)

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. A thermoplastic powder coating composition, characterized in that it comprises a compatible mixture of: (a) 50 to 85 parts by weight (based on the total polymer content) of a ionomeric resin crosslinked with a cation; and (b) 15 to 50 parts by weight of a low molecular weight copolymer, of ethylene and acrylic or methacrylic acid; wherein the mixture is fused at a temperature of 150 ° C or less, and where, when fused in this way, said composition is equal to or more than 90% transparent as determined by the measurements of percent transmittance.

2. The composition according to claim 1, further characterized in that it comprises 1 to 5 parts by weight of stabilizers.

3. The composition according to claim 1, characterized in that it comprises a mixture of: (a) 60 to 80 parts by weight of the ionomer; and (b) 20 to 40 parts by weight of the low molecular weight copolymer of ethylene and acrylic or methacrylic acid.

4. The composition according to claim 1, characterized in that the ionomer resin (a) is a copolymer of ethylene and methacrylic acid, wherein the copolymer is crosslinked with zinc cations.

5. The composition according to claim 1, characterized in that (b) is a low molecular weight copolymer of ethylene and acrylic acid.

6. A method for protecting a signal, characterized in that it comprises the steps of: (a) applying to a surface of said signal, having a plurality of image-carrying areas and non-image areas, of a powder composition in accordance with claim 1; and (b) melting the powder composition to form a transparent protective layer on the image-bearing and non-image-containing layers.

7. He . method according to claim 6, characterized in that the powder composition is applied by electrostatic deposition.

8. The method according to claim 6, characterized in that the signal is a license plate or license plate.

9. A circulation or license plate, characterized in that it comprises a protective coating fused thereon, comprising the composition according to claim 1.