Anticorrosive Pigment Composition and Coating Compositions Containing the Same

1. Field of the Invention

This invention relates to a novel anticorrosive pigment composition and coating compositions containing the same. More particularly, this invention relates to an anticorrosive pigment composition which does not adversely affect, when formulated in an anticorrosive paint, particularly of vinyl chloride-vinylidene chloride copolymer emulsion type, the storage stability of paints and the thermal resistance of paint films.

2. Brief Description of the Prior Art

It is well-known that Pb or Cr based anticorrosive pigments present serious environmental problems. Accordingly, several types of nontoxic or low toxic anticorrosive pigments have been proposed and emerged on the market. U.S. Pat. No. 4,966,630 assigned to the assignee of this application discloses an anticorrosive pigment free from toxic heavy metals comprising aluminum dihydrogen tripolyphosphate and zinc oxide. However, it has been pointed out that when formulated in paints with a vinyl chloride-vinylidene chloride copolymer vehicle resin, this pigment adversely affects the thermal stability of the paint films.

To overcome this disadvantage, JP-A-08283619 discloses a nontoxic anticorrosive pigment composition comprising a condensed phosphate salt and calcium metasilicate. JP-A-06101075 discloses a similar pigment composition comprising a condensed phosphate salt and magnesium oxide. Other known attempts include to combine a condensed phosphate salt with calcium ion exchanged silica.

Unfortunately, these attempts have been proven to be not fully successful in terms of anticorrosity activity and storage stability of paint compositions incorporating the pigment. Therefore, a need exists for a nontoxic pigment composition which will eliminate or ameliorate disadvantages associated with known nontoxic pigment compositions.

According to the present invention, the above need can be met by providing an anticorrosive pigment composition comprising a condensed phosphoric acid aluminum salt and magnesium silicate.

In a preferred embodiment of the invention, the pigment composition comprises aluminum dihydrogen tripolyphosphate and magnesium hexasiliate at a weight ratio from about 90:10 to about 20:80, more preferably from about 80:20 to about 30:70.

In another aspect, the present invention provides an anticorrosive paint composition comprising the anticorrosive pigment composition of the present invention. The paint may be formulated in water-based-, solvent-, air drying- or baking type depending upon the type of particular vehicle resins used. In a preferred embodiment of the invention, the vehicle is a vinyl chloride-vinylidene chloride copolymer emulsion. The anticorrosive pigment of the present invention, when formulated with this emulsion, rather enhances the thermal stability of the paint films while exhibiting remakable anticorrosive activity. Also, the resulting emulsion paint exhibits a satisfactory storage stability for a long period of time.

Other features and advantages of the present invention will become apparent as the following detailed description proceeds.

The condensed phosphoric acid aluminum salt which can be used in the present invention is preferably hardly soluble in water and has an acidity as solid acid as high as about 2 to about 6 meq/g. Typical examples thereof include aluminum dihydrogen tripolyphosphate and aluminum metaphosphate. Aluminum dihydrogen tripolyphosphate is particularly preferred.

Several varieties of magnesium silicate are known including magnesium metasilicate, magnesium trisilicate, magnesium tetrasilicate and magnesium hexasilicate, all of which may be used in the present invention to advantage. Only requirement is that they are of pigment grade. Magnesium hexasilicate of the formula: 2MgO.6SiO.sub.2.nH.sub.2 O is, among others, particularly preferable. This salt is amphoteric and acts as a solid acid and also as a solid base. Accordingly, this salt can adsorb both acids and bases.

The condensed phosphoric acid aluminum salt and magnesium silicate are combined at a weight ratio from 90:10 to about 20:80, preferably from about 80:20 to about 30:70. If the proportion of condensed phosphoric acid aluminum salt is too excessive, then the thermal stability of chlorine-containing polymers present in the paint film will be not reach a satisfactory level. Conversely, if the above proportion is two small, the pigment is incapable of forming a passive layer on the substrate and thus exhibits poor anticorrosive activity.

The above two ingredients may be combined by any desired process, namely dry process or wet process. For use in stability-sensitive formulations or vehicle resins, the pigment composition produced by the wet process conducted for a period from 30 minutes to 3 hours at a temperature from room temperature to 90.degree. C. is preferable.

The anticorrosive pigment composition of the present invention may be formulated into an anticorrosive paint composition using any conventional vehicle resin or polymer known in the art including boiled oil, oily varnish, phenol resins, aminoplast resins, epoxy resins, polyester resins including alkyd resins, polyurethanes, vinyl resins, acrylic resins, fluororesins, chlorinated rubber, cyclized rubber, cellulose derivatives or a combination thereof. Preferred vehicle resins are air drying epoxy resins, thermosetting epoxy resins, air drying alkyd resins and vinyl chloride-vinylidene chloride copolymers. Vinyl chloride-vinylidene chloride copolymer emulsions are most preferable. The emulsion may be modified with, for example, an acrylic monomer or may be used in combination with a melamine resin, phenol resin, epoxy resin or alkyd resin provided that the characteristic performance of the vinyl chloride-vinylidene chloride copolymer is not impaired seriously. The amount of the anticorrosive pigment composition of the present invention may vary within a wide range depending upon the particular application of the paint composition but generally ranges between about 5% and 20% by weight of vehicles resin as solids.

The following examples are intended to illustrate this invention in further detail and should by no means be construed as being limitative. All parts and percents therein are by weight unless otherwise indicated.

Anticorrosive pigment compositions A to G according to this invention were prepared by dry blending varying amounts (parts) of aluminium dihydrogen tripolyphosphate (ATP) and magnesium hexasilicate in a laboratory mixer (Matsushita Electric Industrial Co., Ltd. Model MX-60) for 5 minutes. Similarly, anticorrosive pigment compositions H and I were produced by replacing Mg hexasilicate with zinc oxide or calcium silicate and used for comparative purposes.

Water-based anticorrosive paint compositions were prepared according to the formulation of Table 2. To a 200 ml stainless steel beaker provided with double walled water jacket were added water, thickening agent, defoaming agent, wetting agend and dispersant. The mixture was stirred at a low speed to make a dispersion. To this were added the anticorrosive pigment composition of Table 1, talc and titanium dioxide while stirring by a high speed disperser (VMA -Getzmann AG) for 40 minutes under cooling. Then acryl-modified vinyl chloride-vinylidene chloride copolymer emulsion containing ammonia, butyl celloselve and sodium nitrite solution were added and mixed under low speed stirring for 5 minutes. The anticorrosive pigment composition content was 11.0% on solid basis, PVC was 35.0% and pigments/binder resin ratio was 1.0.

Test panels were prepared by applying the paint onto a mild steel plate (SPCC-SB available from Nippon Test Panel Co.) using a bar coater to a dry film thickness of 40.+-.5 microns and then air dried at 83.degree. C. for 10 minutes.

A sample of each paint compositions prepared in Example 2 was placed in a 250 ml polyolefin bottle and hermetically sealed with a cap. The bottle was placed in an isothermal chamber maintained at a constant temperature of 40.degree. C. for 28 days. The change of viscosity with time was observed at several intervals at a sample temperature of 20.degree. C. using a B-type viscometer. The results are shown in Table 3.

As shown in Table 3, paint compositions containing the anticorrosive pigment compositions of this invention exhibited higher storage stability than that of the paint composition containing pigment composition H in this accelerated storage stability test. This confirms that the paint compositions are acceptable in terms of storage stability in practice.

Each test panel prepared in Example 2 was placed in a hot air-circulating oven maintained at 140.degree. C. and 170.degree. C. respectively. One hour after the panel was removed from the oven and measured color change of the coating film in terms of L * , a * and b * values using a standard calorimeter. The results are shown in Table 4.

In this test, the lightness (L * ) values upon exposure the film at 140.degree. C. for 1 hour are practically significant. As is well-known, chlorine-containing vinyl polymers are susceptible to dehydrochlorination upon heating with darkening. Therefore, decrease in the lightness value upon heating is as a rule inversely proportional to the thermal stability of paint films. A lightness value L * in the order of 70-80 represents white in color while a lightness value L * in the order of 30 represents black in color. Lightness values higher than about 40 have been considered to be acceptable in practice. As shown in Table 4, the paint films containing anticorrosive pigment compositions A to G, particularly A to E, exhibited satisfactory heat resistance whereas the paint film containing ZnO-containing anticorrosive pigment composition I exhibited poor heat resistance.

Test panels prepared in Example 2 were cut in a cross with knife reaching the substrate and subjected to the following test methods.

The panel was continuously sprayed with a 5% NaCl aqueous solution for 24 hours at a spray pressure of 1 kg/cm.sup.2 at a chamber inner temperature of 35.degree. C.

The panel was sprayed with an aqueous solution containing 0.35% ammonium sulfate and 0.05% NaCl for 1 hour at 25.degree. C. and then dried at 35.degree. C. for 1 hour in one cycle. This cycle was repeated 207 times (504 hours).

The panel was then evaluated in terms of blister and rust according to the following schedule.

Blister in Flat Area

Percents of blister area in the total area according to ASTM D714-56.

5:0% 4:<0.2% ; 3:<0.5% ; 2:>1% 1:>3% ; 0:>3%

Rust in Flat Area:

Percents of rust in the total area according to ASTM D714-56.

5:0% ; 4:0.03-0.1% 3:0.3-1%; 2:<3% ; 1:<10% ; 0:>16%;

Width of Corrosion:

The maximum width of corrosion developed along the cut line in a perpendicular direction was measured and evaluated according to the following schedule.

5:<1 mm ; 4:1-2 mm ; 3:2-4 mm; 2:4-7 mm ; 0:>10 mm.

Comprehensive Evaluation

Total points of all items.

The results are shown in Table 5.

As shown in Table 5, paint films containing anticorrosive pigment compositions C to F exhibited remarkable anticorrosive performance (11 point or higher) in both Method 1 and Method 2.

Summining up the results shown in Tables 3-5, pigment compositions B to F satisfy with all of anticorrosive property, heat resistance and storage stability requirements as shown in Table 6.

Analogous to the preceeding examples, the effects of exposure of coated panels to heat at 177.degree. C. for 15 minutes on the heat resistance and the subsequent salt spray testing were tested.

The anticorrosive pigment compositions used in this test were pigment compositions D and H as well as Ca and Sr modified zinc phosphosilicate available from Halox under the name of SZP-391 and magnesium tetrasilicate. The paint formulation and coating conditions were same as in Example 2. The anticorrosive test was conducted after exposing coated panels prepared under the above conditions by spraying a 5% NaCl aqueous solution at 35.degree. C. for 240 hours and evaluation was made as in Example 5. The results are shown in Table 7 and Table 8.

As shown in Tables 7 and 8, the paint film containing pigment composition D was not adversely affected in the anticorrosive property by the exposure to heat.

The heat stablity of paint films prepared as in Example 2 was evaluated in terms of the quantity of HCl generated upon heating. The coated panel was placed in a 3 liter gas sampling bag, purged with nitrogen gas, and heated at 120.degree. C. for three hours. The concentration of HCl gas in the bag was determined using a hydrogen chloride dector tube Model No. 14L available from Gas Tech Company. The results are shown in Table 9.

As shown in Table 9, the amount of HCl generated from the paint film containing pigment composition D is very low. This suggests that the pigment composition D would be effective as a heat stabilizer of chlorine-containing vinyl polymers.

The anticorrosive activity of the pigment composition of this invention was tested in an epoxy resin primer coating composition in comparison with zinc phosphate and Ca/Sr modified zinc phosphosilicate.

Anticorrosive pigment compositions and loading levels thereof are as follows.

As a control, a corresponding coating composition free from any anticorrosive pigment was prepared.

The above coating compositions were applied on a galvanized steel plate to a dry film thickness of about 30 microns and dried at room temperature for 7 days. Then, the coated test panels were sprayed with 5% aqueous solution of NaCl at 35.degree. C. for 1440 hours and evaluated as in Example 5. The results are shown in Table 10.

As shown in Table 10, the anticorrosive pigment composition of this invention was effective when formulated in epoxy-based primer coating compositions.