Abstract:
A nontoxic anticorrosive pigment composition free from toxic heavy metals comprises a condensed phosphoric acid aluminum salt, typically aluminum dihydrogen tripolyphosphate, and magnesium silicate, typically magnesium hexasilicate. The anticorrosive pigment composition enhances the thermal resistance of paint films when formulated in a coating composition containing a chlorine-containing vinyl polymer such as vinyl chloride-vinylidene chloride copolymer emulsion paints.

Description:
BACKGROUND OF THE INVENTION 
     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. 
     SUMMARY OF THE INVENTION 
     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. 
     DETAILED DESCRIPTION OF THE INVENTION 
     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 2 .nH 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° 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. 
    
    
     EXAMPLE 
     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. 
     Example 1 
     Anticorrosive Pigment Compositions 
     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. 
     
                       TABLE 1______________________________________Pigment  MaterialComposition    ATP.sup.1)             Mg hexasilicate.sup.2)                         ZnO.sup.3)                                CaSiO.sub.3.sup.4)______________________________________A        20       180         --     --B        40       160         --     --C        60       140         --     --D        100      100         --     --E        160      40          --     --F        180      20          --     --G        195      20          --     --H        140      --          --     60I        140      --          60     --______________________________________ .sup.1) KFresh #100P, Tayca Corp. .sup.2) KYOWAAD 600, Kyowa Chemical Industry Co. .sup.3) Mitsui Mining &amp; Smelting Co. .sup.4) Calcium metasilicate, NYCO 
    
     Example 2 
     Anticorrosive Paints and Test Panels 
     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. 
     
                       TABLE 2______________________________________Material             %______________________________________Water                11.22Thickening agent.sup.1)                0.14Defoaming agent.sup.2)                0.22Wetting agent.sup.3) 3.67Wetting agent.sup.4) 0.34Dispersant.sup.5)    0.57Anticorrosive pigment of Table 1                6.67Talc.sup.6)          20.62Titanium dioxide.sup.7)                2.30Butyl cellosolve     1.58Acryl-modified vinyl chloride                51.28vinylidene chloride copolymeremulsion .sup.8)28% ammonia          0.5010% sodium nitrite.sup.9)                0.80Total                100.00______________________________________ .sup.1) Benton LT, NL Industries Inc. .sup.2) Foamaster S, Diamond Shamrock Corp. .sup.3) Simplonic (30%), ICI Chem. &amp; Poly. .sup.4) Surfinol 104H, Air Products Chemicals .sup.5) BYK 155, BYK Chemie .sup.6) Simgon, Nippon Talc Co. .sup.7) JR701, Tayca Corp. .sup.8) Haloflex 202, ZENECA Resins, 60% solids .sup.9) Spot rust preventing agent 
    
     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° C. for 10 minutes. 
     Example 3 
     Storage Stability Test 
     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° C. for 28 days. The change of viscosity with time was observed at several intervals at a sample temperature of 20° C. using a B-type viscometer. The results are shown in Table 3. 
     
         ______________________________________   Viscosity, mPa · sPigment DaysComposition   Initial 1       7      14    21    28______________________________________A       1400    2850    Gelled --    --    --B       1050    2160    3260   3580  3880  3980C       890     1100    2230   2950  3320  3410D       990     1050    2830   3240  3080  2810E       610     610     1990   2290  2870  2860F       400     450     810    1880  1930  2150G       400     410     760    1400  1430  1510H       870     2060    4200   4400  4750  6500I       550     560     1140   1320  1590  1480______________________________________ 
    
     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. 
     Example 4 
     Film Heat Resistance 
     Each test panel prepared in Example 2 was placed in a hot air-circulating oven maintained at 140° C. and 170° 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. 
     
                                           TABLE 4__________________________________________________________________________Pigment  Initial    140° C. × 1 hr.                        170° C. × 1 hr.Composition  L* a* b*   L* a* b*   L* a* b*__________________________________________________________________________A      78.9     -2.2        +4.5 76.0                -1.5                   +9.4 59.9                           +3.8                              +14.1B      79.1     -2.4        +4.7 76.1                -1.5                   +9.5 57.1                           +3.1                              +10.5C      78.8     -2.4        +4.6 75.2                -1.8                   +9.5 54.2                           +4.1                               +9.8D      79.0     -2.4        +4.8 75.1                -1.6                   +9.7 52.0                           +3.9                              +14.4E      78.8     -2.0        +4.3 69.9                -0.4                   +13.9                        49.7                           +5.2                              +15.1F      76.2     -2.1        +4.2 48.1                +1.2                   +8.0 32.0                           +1.4                               +3.1G      74.5     +0.8        +11.2             43.1                +0.4                   +4.2 31.0                           +0.5                               +1.4H      75.4     -2.4        +4.2 56.0                +2.0                   +12.2                        37.4                           +2.2                               +5.9I      79.7     -2.2        +1.2 28.1                -0.1                   +0.4 30.2                           -0.1                               +0.4__________________________________________________________________________ 
    
     In this test, the lightness (L * ) values upon exposure the film at 140° 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. 
     Example 5 
     Anticorrosive Test 
     Test panels prepared in Example 2 were cut in a cross with knife reaching the substrate and subjected to the following test methods. 
     Method 1 
     The panel was continuously sprayed with a 5% NaCl aqueous solution for 24 hours at a spray pressure of 1 kg/cm 2  at a chamber inner temperature of 35° C. 
     Method 2 
     The panel was sprayed with an aqueous solution containing 0.35% ammonium sulfate and 0.05% NaCl for 1 hour at 25° C. and then dried at 35° 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:&lt;0.2% ; 3:&lt;0.5% ; 2:&gt;1% 1:&gt;3% ; 0:&gt;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:&lt;3% ; 1:&lt;10% ; 0:&gt;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:&lt;1 mm ; 4:1-2 mm ; 3:2-4 mm; 2:4-7 mm ; 0:&gt;10 mm. 
     Comprehensive Evaluation 
     Total points of all items. 
     The results are shown in Table 5. 
     
                       TABLE 5______________________________________Method 1            Method 2Pigment Flat area Cut         Flat area                                 CutComposition   Bliter Rust   area Total                           Bliter                                Rust area Total______________________________________A       2      4      0    6    5    4    1    10B       5      5      1    11   5    5    3    13C       5      5      2    12   5    5    3    13D       5      5      4    14   5    5    4    14E       5      5      3    13   5    5    4    14F       5      5      1    11   5    5    3    13G       0      5      0    5    5    5    2    12H       0      5      0    5    5    5    2    12I       5      5      4    14   5    5    3    13______________________________________ 
    
     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. 
     
                       TABLE 6______________________________________   Anticorrosive  Heat      StoragePigment Property       Resistance,                            StabilityComposition   Method 1 Method 2  at 140° C.                              40° C. × 28______________________________________                              daysA       Bad      Bad       Good    BadB       Good     Good      Good    GoodC       &#34;        &#34;         &#34;       &#34;D       &#34;        &#34;         &#34;       &#34;E       &#34;        &#34;         &#34;       &#34;F       &#34;        &#34;         &#34;       &#34;G       Bad      Fair      Fair    GoodH       Bad      Fair      Good    BadI       Good     Good      Bad     Good______________________________________ 
    
     Example 6 
     Analogous to the preceeding examples, the effects of exposure of coated panels to heat at 177° 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° C. for 240 hours and evaluation was made as in Example 5. The results are shown in Table 7 and Table 8. 
     
                       TABLE 7______________________________________Heat ResistancePigmentComposition L*           a*     b*______________________________________Mg tetrasilicate       42.8         +0.1   +3.1D           75.2         -0.1   +8.3H           28.7         -0.2   +0.3SZP-391     29.1         -0.3   +0.4______________________________________ 
    
     
                       TABLE 8______________________________________Anticorrosive TestPigment      Flat area         CutComposition  Blister  Rust     area Total______________________________________Mg tetrasilicate        1        3        0     4        (1)      (5)      (3)   (9)D            5        5        3    13        (5)      (5)      (3)  (13)H            0        4        0     4        (5)      (5)      (0)  (10)SZP-391      0        3        3     6        (3)      (4)      (0)   (7)______________________________________ Note: Values in parentheses indicate those before exposure to heat. 
    
     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. 
     Example 7 
     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° 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. 
     
                       TABLE 9______________________________________         Pigment Composition                           MgItem            D        H      tetrasilicate______________________________________HCl concentration, ppm           1.0      4000   500Binder resin in paint film, g           0.58     0.86   0.67mg HCl/g Resin  0.01     22.7   3.6______________________________________ 
    
     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. 
     Example 8 
     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. 
     
         ______________________________________Pigment Composition           Loading level in film______________________________________Pigment Composition D           10%Zinc Phosphate  10%SZP-391         10%______________________________________ 
    
     As a control, a corresponding coating composition free from any anticorrosive pigment was prepared. 
     
         ______________________________________Coating Composition Formulation:               Control Test CompositionLoading level       0%      10%______________________________________Anticorrosive pigment               0       6.7BaSO.sub.4          21.7    20.4Talc                10.6    5.2TiO.sub.2 (JR-701, Tayca)               10.6    10.6EPIKOTE 1001 (70% nonvolatiles,               20.4    20.4Shell Chemicals)Versamid 230 (60% nonvolatiles,               15.8    15.8Henkel Hakusui)Solvent (xylene/n-butanol = 1/1)               20.9    20.9Total               100.0   100.0______________________________________ 
    
     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° C. for 1440 hours and evaluated as in Example 5. The results are shown in Table 10. 
     
                       TABLE 10______________________________________     Flat areaPigment     Blister Rust       Cut area                                Total______________________________________Control     3       5          0     8D           5       5          3     13Zinc Phosphate       2       5          0     7SZP-391     3       5          0     8______________________________________ 
    
     As shown in Table 10, the anticorrosive pigment composition of this invention was effective when formulated in epoxy-based primer coating compositions.