Abstract:
PVC-free coil coated steel having excellent cut-side corrosive resistance can be prepared by using on one or both sides a primer composition essentially consisting of a linear polyester of Mn=3,000-15,000, of acid number 0-10, and hydroxyl index 10-25; a blocked polyisocyanate, melamine or an urea-formaldehyde adduct, in an equivalent ratio of 0.8-4.0 to polyester; a catalyst; anti-corrosive pigments of the chromate type, being at least 50 weight percent of all pigments; and solvents.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the prevention of corrosion of coil-coated steel. More particularly, it relates to the prevention of cut-side corrosion using PVC-free coating compositions. 
     Coil-coated steel (often called pre-coated steel) is not coated on the cut sides, because it is cut after being coated. Corrosion can thus initiate at the cut sides, and progressively creep under the coating. Cut-side corrosion is a problem essentially in building uses, and to a lesser extent in white goods (refrigerators, washing machines and the like). 
     The best system presently known to prevent cut-side corrosion of coil-coated steel is based on polyvinylchloride (PVC) plastisol, and it is known that PVC is no longer considered as environmentally friendly for such use. An alternative solution is thus desired in the art, which should at least have equal resistance to cut-side corrosion. 
     It is an objective of the invention to provide a process for manufacturing PVC-free coil-coated steel having excellent cut-side corrosion resistance. 
     Another object of the invention is to provide a primer composition for a PVC-free coating system imparting excellent cut-side corrosion resistance of coil-coated steel. 
     Yet another object of the invention is to provide PVC-free coil-coated steel having excellent cut-side corrosion resistance. 
     These and other objects are provided by the invention as disclosed hereinafter. 
     SUMMARY OF THE INVENTION 
     The primer composition of the invention essentially consists of a binder system, one or more anti-corrosive pigments and one or more solvents. 
     The binder system consists of (a) a polyester resin having a number-average molecular weight of 3,000-15,000 (preferably 4,500-10,000, most preferably about 6,000), an acid index of 0 to 10 (preferably about 5) and a hydroxyl index of 10 to 25 (preferably about 15); (b) a cross-linking agent selected from blocked polyisocyanates (preferably having an isocyanide functionality of about 2), reelamine and urea-formaldehyde adducts; and (c) a suitable cross-linking catalyst. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The polyester resin used in the primer is an essentially linear polyester (i.e., prepared using less than 2% branched monomers). Raw materials selection and reactant amounts calculated as a function of desired properties is known in the art. (see e.g., Vol. III, pp. 81 ff. in &#34;A Manual of Resins for Surface Coatings&#34;, M. J. Husbands et al., SITA, London, 1987) and need not be described herein. 
     Suitable cross-linking agents are known in the art and need also not be described herein. 
     The equivalent ratio of polyester resin to cross-linking agents is of from 0.8 to 4.0, preferably of from 1.0 to 2.0. Blocked cross-linking agents are preferably used, to prevent possible instability of the composition and/or interactions with pigments/fillers; the minimum cure temperature of blocked cross-linking agents is preferably between 50° C. and 150° C., most preferably between 60° and 80° C. The most preferred cross-linking agents is a blocked diisocyanate. 
     As known in the art, an appropriate cross-linking catalyst must be used, in an amount which must be sufficient to provide for complete reaction during the short heating time of coil coating processes. 
     The anti-corrosive pigments must be of the chromate type. The preferred chromates are strontium chromate (which is most preferred) and zinc chromate (or related compounds such as zinc tetroxy chromate). Calcium, strontium and zinc molybdates are less favorable alternatives. 
     At least 50 weight percent of all pigments in the primer must be of the chromate type, preferably at least 90 weight percent and most preferably 100 weight percent. Moreover, the pigment volume concentrate must be of 90 to 99% of the critical pigment volume concentration, preferably of more than 95% thereof; alternatively, the pigment to resin weight ratio must be of 0.5 to 1.2, preferably of about 1. 
     Usual solvents are also used, generally in an amount of 30 to 70 wt. % of the total composition, preferably 40 to 60 wt %, most preferably about 55 wt. %. The nature of the solvent is selected to have a good solvent power towards the polyester resin, the cross-linking component and the catalyst; a hydrocarbon solvent is generally used. The boiling range of the solvent must be suitable for the coil coating process; a boiling temperature or range between 120° C. and 220° C., preferably 150° C. to 200° C. is generally used. The amount of solvent is selected to obtain a primer having an appropriate viscosity for use in coil coating lines. The primer composition is then spread on a steel coil substrate to provide when cured, a film thickness of 0.005 to 0.03 mm, preferably about 0.015 min. The coating is cured by cross-linking, typically at a peak metal temperature (PMT) of from 220° C. to 280° C. The substrate speed is usually in the range of 100 to 200 m/min, typically of about 120 m/min. 
     It is preferred to apply a layer of primer on both faces of the steel coil, the use of a top coat on the back side being optional. 
     The primer composition of the invention is a universal primer composition, i.e., it can be used on substantially all pretreated steel substrates and can be covered with substantially all top coating compositions while having excellent adhesion and mechanical properties. More particularly, the primer composition of the invention can be used in combination with PVC-free top coating compositions to form PVC-free coating systems having excellent cut side corrosion resistance. 
     The inventors unexpectedly found that, by using the PVC-free coating systems of the invention, the top coating layer no longer had to have a 0.1-0.2 mm thickness as PVC plastisol did, but could have a thickness of about 0.02 mm while still having excellent or even improved cut side corrosion properties. 
     The invention will now be illustrated by the following examples. 
     EXAMPLE 1 
     The following primer paint composition was prepared: 
     
         __________________________________________________________________________polyester binder:           19.5                          parts by weight (pbw)monomer composition:        20.93 pbw terephthalic acid        2.11 pbw trimellitic anhydride        52.08 pbw isophthalic acid        14.46 pbw ethylene glycol        23.53 pbw neopentyl glycol        2.86 pbw 1,6-hexanediolproperties:hydroxyl number: 15acid number: 5number average molecular weight: 6,000blocked diisocyanate:       2.3                          pbwequivalent weight: 378minimum cure temperature: 333° C.dibutyltin stearate:        0.2                          pbwstrontium chromate:         20.0                          pbwpolyacrylate additive (leveling agent):                       0.25                          pbwpolyisobutylacrylateproperties:acid number: max. 5viscosity (23° C.): 47 dPa.sultrafine silicon dioxide powder (average 7 nm):                       0.3                          pbwsolvents:                   57.4                          pbw of whichcommercial hydrocarbon solvent boiling at 150° C.: 19.5id. at 200° C.: 15.0alcohols (isobutanol, isopropanol, butylglycol, propylenemonoglycolether)esters (n-butyl and 1-methoxy-2-propyl acetates): 22.2__________________________________________________________________________ 
    
     EXAMPLE 2 
     In a coil coating line operating at 100 m/min, there were successively applied the following, each layer being cured by passing in an over at a PMT of 245° C.: 
     (a) on the front side 
     - a 0.015 mm dry thickness layer of the primer of Example 1; then 
     - a 0.02 mm dry thickness layer of a commercial polyester for outdoor use (SIGMATOP sold by SIGMA COATINGS); 
     (b) on the back side 
     - a 0.015 mm dry thickness layer of the primer of Example 1. 
     The substrate was 0.6 mm thick hot dip galvanized steel (HDG) with Bonder 1300 pretreatment. 
     The properties of the coating coil were evaluated as follows: 
     1. Erichsen stamping resistance: the plate is progressively stamped using a 20 mm diameter ball pressing from the uncoated side up to 8 mm diameter cracking of the coating is evaluated visually and using a 10× magnifying lens, and adherence is evaluated using TESA 4104 adhesive tape (TESA is a trademark of BDF-Beiersdorf): 
     - notation 10: no cracking, no adhesion failure; 
     - notation 9: cracking invisible with naked eyes, no adhesion failure; 
     - notation 8: slight cracking visible with naked eyes, no adhesion failure; 
     - notation 7: large cracking, no adhesion failure. 
     2. Zero T adherence: the plate is bent back upon itself so that no space remains between the bent layers of substrate, and the bent film is evaluated according to the above notation; 
     3. Intercoat adhesion: the coating is scraped using a coin, and the adherence on the primer is evaluated using the subjective notations 0 (bad), 5 (average) or 10 (good); 
     4. Cut side corrosion: the coated plate is submitted to 1500 hours salt mist, then the extent of corrosion is evaluated according to ECCA-T8 standard method and the corroded distance from the cut edge is expressed in min. 
     The results are mentioned in Table 1. 
     COMPARATIVE EXAMPLE A 
     A pre-coated steel plate was prepared as in Example 2, except as follows: 
     (a) on the front side 
     - a 0.005 mm dry thickness layer of commercial PVC-plastisol primer; 
     (b) on the back side 
     - a 0.007 mm dry thickness layer of commercial epoxy coating composition. 
     The results are mentioned in Table 1. 
     EXAMPLES 3 TO 6 AND COMPARATIVE EXAMPLE B 
     Pre-coated steel plates were prepared as in Example 2, except as mentioned in Table 1, wherein the test results are also mentioned. The primer coating composition of Comparative Example B was essentially identical to that of Example 1, except for the pigment composition which was a follows: 
     strontium chromate 11.4 wt. % 
     titanium dioxide 88.6 wt. % 
     
                                           TABLE 1__________________________________________________________________________     Ex.2 Ex.3              Ex.4                 Ex.5                    Ex.6                        Comp.A                             Comp.B__________________________________________________________________________Front sideprimer    Ex.1 id. id.                 id.                    id. prior art                             id.dry thickness (mm)     0.015          0.015              0.015                 0.005                    0.005                        0.005                             0.005top coating     polyester          id. id.                 id.                    id. PVC  polyesterdry thickness (mm)     0.02 0.02              0.02                 0.02                    0.02                        0.1  0.02Back sideprimer    invention          id. id.                 id.                    --  --   --dry thickness (mm)     0.015          0.015              0.005                 -- --  --   --top coating     --   epoxy              -- -- epoxy                        id.  id.dry thickness (mm)     --   0.007              -- -- 0.007                        0.007                             0.007PropertiesErichsen 8 mm     10   10  10 10 10  10   10OT adhesion     10   10  10 10 10  10   10intercoat adhesion     10   10  10 10 10  10   10cut-side corrosion     0    0   2  4  9   10   15__________________________________________________________________________