CA2175541A1 - Pvc-free coil-coated steel having excellent cut-side corrosion resistance - Google Patents

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

i~ 217534 PATENT
Docket No.: F-648 PVC-FREE COII,C~ED STI~EL HA~rlNG EXCELL~NI
CUT-SIDE CORROSION RESISTANCE, 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 5 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 esse-nti~lly in building uses, and to a lesser extent in white goods (refrigelalol~, washing m~hin~s and the like).
The best system ~lcselllly 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 10 as cllvi~ lly friendly for such use. An ~ live solution is thus desired in the art, which should at least have equal les;.~l~.u e to cut-side corrosion.
It is an objective of the invention to provide a process for ...~....r;~ PVC-free coil-coated steel having excellent cut-side corrosion l~;~ e. ..
Another object of the invention is to provide a primer composition for a PVC-free coating 15 system ~a.li~g excellent cut-side corrosion le-~ e 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.

-- 2175~41 '-SUMMARY OF THE INVENTION
The primer composition of the invention essentially consists of a binder system, one or more anti-corrosive pigrnent~ 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 polyiso~;y~tes (preferably having an isocyanide functionality of about 2), melamine 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 essenlially linear polyester (i.e., prepared using less than 2% branched monomers. Raw materials selection and reactant amounts calculated as a function of desired pr~lies is known in the art. (see e.g., Vol. III, pp. 81 ff.
in "A Manual of Resins for Surface Coatings", M.J. Hllsb~n-ls 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-lin~g agents is of from 0.8 to 4.0, preferably of from 1.0 to 2Ø Blocked cross-linking agents are preferably used, to prevent possible instability of the composition and/or interactions with pigments/fillers; the ~
cure temperature of blocked cross-linking agents is preferably between 50 and 150~C, most 2175~41 i;~

preferably between 60 and 80~C. The most ple~ d cross-linking agents is blocked di-isocyanide.
As known in the art, an appLop~ cross-linlcing catalyst must be used, in an amount which must be su~ficient to provide for complete reaction during the short heating time of coii 5 coating processes.
The anti-collosive pigmPnt~ must be of the chrollldl~ type. The pr~r~lled chrolllat~s are sLIollliulll chromate (which is most pl~r~lled) and zinc chromate (or related compounds such as zinc tetroxy chromate). Calcium, ~LlollliUlll and zinc molybdates are less favoMble 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, prefeMbly of more than 95% thereof; all~lllalively, the pigment to resin weight Mtio must be of 0.5 to 1.2, preferably of about 1. - -Usual solvents are also used, geneMlly in an amount of 30 to 70 wt. % of the total composition, prefeMbly 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 geneMlly used. The boiling range of the solvent must be suitable for the coil coating process; a boiling ~lu~l~.lul~ or range between 120 and 220~C, preferably 150 to 200~C is generally used. The amount of solvent is selected to 20 obtain a primer having an a~lopliate 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 mm. 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 pre~lled to apply a layer of primer on both faces of the steel coil, the use of a top c02t on the back side being optio~al.
S The primer composition of the invention is a universal primer composition, i.e., it can be use~ on substantially all pretreated steel substrates and can be covered with substantially all top coating compositions while having excellent a&esion and merh~nic~l 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 thil~kn~ ss Gl 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.

li,Y~mple 1 The following primer paint composition was prepared:
-polyester binder: , j 19.5 parts by weight (pbw) -monomer composition: 20.93 pbw terephthalic acid 2.11 pbw trimellitic anhydride 52.08 pbw isphthalic acid 14.46 pbw ethylene glycol 23.53 pbw neopentyl glycol 2.86 pbw 1,6-hexanediol -properties:
-hydroxyl number : 15 21 7534 1 ( '!

-acid number : 5 -number average molecular weight: 6,000 -blocked diisocyanate: 2.3 pbw -e~uivalent weight : 378 S -minimllm cure temperature: 333~C
-dibutyltin learate: 0.2 pbw -~llollLiulll chromate: 20.0 pbw -polyacrylate additive (leveling agent): 0.25 pbw -polyisobutylacrylate -properties:
-acid number: max. 5 -viscosity (23 ~C): 47 dPa.s -ultrafine silicon dioxide powder (average 7 nm): 0.3 pbw -solvents: 57.4 pbw of which -commercial hydrocarbon solvent boiling at 150~C : 19.5 -id. at 200~C : 15.0 -alcohols (isobutanol, isopropanol, butylglycol, propylenemonoglycolether) -esters (n-butyl and 1-methoxy-2-propyl acetates) : ~.2 Example 2 In a coil coating line operating at 100 m/min, there were slcces~ively applied the following, each layer being cured by passing in an over at ajPMT of 245~C:
(a) on ~e front side - a 0.015 mm dry thickness layer of the primer of Example l; then - a 0.02 mm dry thickness layer of a commercial polyester for outdoor use (SIGMATOP sold by SIGMA COATINGS);

-' 21 75~;41 , ~, _ (b) on the back side - a 0.015 mm dry thickness layer of the primer of Example 1.
The substrate was 0.6 mm thi.,k hot dip galvanized steel (HDG~ with Bonder 1300 pretrea~nent.
The properties of the coating coil were evaluated as follows:
'' 1. Erichsen stamping resistance: the plate is progressively stamped using a 20 mm m~ter ball pressing from the uncoated side up to 8 mm diameter cracking of the coating is evaluated visually and using a lOx magnifying lens, and adherenceis evaluated using TESA 4104 adhesive tape (TESA is a tr~clem~rk of BDF-Beiersdorf):
- notation 10: no cracking, no adhesion failure;
- notation 9 : cracking invisible with naked eyes, no a&esion failure;
- notation 8 : slight cracking visible with naked eyes, no a&esion failure;
- notation 7 : large cracking, no a&esion failure.
2. Zero T a&erence: 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 a&esion: the coating is scraped usi~g a coin, and the a&erence on the primer is evaluated using the subjective notations 0 (bad), 5 (average) or 10 ~good);

; ' 2175~

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 mm.
The results are mentioned in Table 1.

5 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 thickn~.ss layer of commercial epoxy coating composition.
The results are mentioned in Table 1.

Ex~~ lcs 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 15 Example B was essentially identical to that of Example 1, except for the pigment composition which was a follows: , ~llollliulll chromate 11.4 wt%
tit~nil-m dioxide 88.6 wt%

Ex 2 Ex 3 Ex 4 Ex 5 Ex 6 Comp.A Comp.B
Front side primer Ex. 1 id. id. id. id. prior art id.
dry thickness (mm) 0.015 0.015 0.015 0.005 0.005 0.005 o.oo5 top coating polyester id. id. id. id. PVC polyester dry thickness (mm) 0.02 0.02 0.02 0.02 0.02 0.1 0.02 Back side primer 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 0 007 Properties Erichsen 8mm 10 10 10 10 10 10 10 OT adhesion 10 10 10 10 10 10 10 intercoat adhesion 10 10 10 10 10 10 10 ~~
cut-side corrosion 0 0 2 4 9 10 15

Claims (9)

1. Primer coating composition essentially consisting of (i) a binder system consisting essentially of:
(a) an essentially linear polyester resin having a number-average molecular weight of 3,000 to 15,000, a acid number of 0 to 10 and a hydroxyl index of 10 to 25;
(b) a cross linking agent selected from block polyisocyantes, melamine and urea-formaldehyde adducts, the equivalent ratio of polyester resin to cross linking agent being of from 0.8 to 4.0; and (c) a suitable cross linking catalyst;
(ii) at least one anti-corrosive pigment of the chromate type, the amount of anti-corrosive pigment being at least 50 weight percent of all pigments in the composition, and the pigment volume concentration being 90 to 99% of the critical pigment volume concentration; and (iii) at least one solvent.

2. The primer according to Claim 1, wherein the polyester has a number-average molecular weight of 4,500 to 10,000.

3. The primer according to Claim 1, wherein the polyester has an acid number of about 5 and a hydroxyl number of about 15.

4. The primer according to Claim 2, wherein the polyester has an acid number of about 5 and a hydroxyl number of about 15.

5. The primer according to any one of Claims 1-4, wherein the cross linking agent is a blocked diisocyante.

6. A primer according to any one of Claims 1-4, wherein the equivalent ratio of polyester resin to cross linking agent is about 1 to 2.

7. A primer according to any one of Claims 1-4, wherein strontium chromate is used as the anti-corrosive pigment.

8. A primer according to any one of Claims 14, wherein anti-corrosive pigments represent at least 90 weight percent of all pigments in the composition, the pigment to resin weight ratio being about 1.

9. A process for coil coating steel, characterized by the use of a coating composition according to Claims 1-4.