FR2488676A1 - PROCESS FOR PROCESSING PRE-COATED SHEETS BEFORE SHAPING - Google Patents

Abstract

Treatment of precoated sheet metal (e.g. steel or Al) to improve the behaviour of the coating during working of the metal is effected by applying a layer of water-soluble lubricant to form an adherent film without altering the underlying coating. The lubricant film protects a wide range of coatings (e.g. Zn-based anticorrosion coatings, polymer films, etc.) from damage during working (e.g. stamping or folding). The lubricant can be a polyethylene glycol, polypropylene glycol, polyoxymethylene or -ethylene polymer, polyurethane, polyester, alkyd resin, acrylic resin or epoxy-ester resin. It is pref. solid at ambient temp. The lubricant film is pref. applied by spraying the hot metal with an aq. soln. of the lubricant. The coating wt. is pref. 0.5-20 (esp. 1.5-4) g/square m.

Description

 The present invention relates to a method for treating pre-coated sheets in order to improve the resistance of the coating during shaping.

 Steel and aluminum, in particular, in most of their uses in fields such as construction, household appliances, office equipment, automotive or planking, must be coated to be protected against corrosion .

 The use of pre-coated sheets is experiencing strong development due to the multiple advantages brought over the methods of applying coatings to the part after shaping. It is, in fact, much easier to deposit a uniform coating on a flat surface than on a surface which has already undergone shaping.

 However, this use of pre-coated sheets has not yet been able to have its full development due to the problems of damage to the coating during shaping, which leads, of course, to insufficient protection against corrosion in the damaged areas.

 This is why treatment methods have already been proposed intended to improve the resistance of the coating during shaping, although to date none of the solutions proposed has been found to be satisfactory.

 It has, first of all, been proposed to apply various lubricants to the coating to be protected during the actual shaping operation by spraying the sheet with oils or emulsions. These solutions have not always been satisfactory. Indeed, the shapers are reluctant to use liquid lubricants, in particular when the protective films thus proposed do not remove or very badly and prove to be harmful in the operations which follow the shaping or even at the level of the protective effect. of the coating.

Thus, in the case of oils, most can only be difficult to remove and are likely thereafter to harm the adhesion of the layers of paint which may be deposited on the sheets after shaping. The presence of water in the emulsions is often the source of a corrosion problem during storage.

 We have also tried to make so-called "self-lubricating" coatings such as, for example, the coatings described in French patent 74 35671 which relates to a system consisting of a cover composed of an organic binder and a lubricating substance.

 However, this type of coating has the same drawbacks as those mentioned above in the sense that the film formed cannot be removed and is therefore liable to have an unfavorable action in the use of the sheet after shaping.

 In any event, it has been found that none of the solutions currently in existence makes it possible to satisfactorily eliminate the problems of damage to the coating during shaping.

 Surprisingly, the Applicant has discovered that this shaping problem can be resolved by applying a film of water-soluble lubricant to the pre-coated sheet.

 This is why the present invention relates to a process for treating pre-coated sheets in order to improve the resistance of the coating during shaping, a process characterized in that a layer of a coating is applied to the pre-coated sheet before shaping. water-soluble lubricant to obtain a water-soluble film of lubricant adhering without altering it to the underlying coating.

 The method according to the present invention can be used on any type of coating, that is to say in fields as different as the building industry in which the pre-coated sheets carry coatings of the plastisol, polyester silicone, acrylic type, alkyd, polyester, in household appliances in which the coatings can be polyurethane coatings and especially in the automobile industry with vinyl coatings but also anti-corrosion coatings: zinc-rich primer, galvanized, electro-galvanized, etc.

 Although the method is more particularly usable for steel or aluminum sheets, it can also be used for metal sheets having another composition.

 Finally, we generally mean by "shaping" all mechanical forming operations without removing material such as stamping and bending.

 Among the water-soluble lubricants which can be used in implementing the process according to the present invention, mention should be made more particularly of polyethers, in particular polyethylene glycols, polypropylene glycols, mixtures of polyethylene glycol and of polypropylene glycol, polyoxymethylenes, grafted polyoxyethylenes, as well as other large families of lubricating polymers such as polyurethanes which can be synthesized from polyethylene glycols and isocyanates, water-soluble polyesters, alkyl, acrylic and epoxy polymers water-soluble and neutralized esters.

 It is, of course, also possible to use mixtures of these different lubricants.

 Among the lubricants mentioned above, it is particularly preferred to use lubricants that are solid at room temperature (that is to say at about 200 ° C.) such as, for example, polyethylene glycols with a molecular mass greater than about 2000.

 The weight of the lubricant film deposited is not a critical parameter of the process, however, it is preferred to use a film weight of between 0.5 and 20 g / m2 and preferably between 1.5 and 4 g / m2.

 The application of the layer of water-soluble lubricant can be very easily carried out by spraying the pre-coated sheet with an aqueous solution of the water-soluble lubricant. In this case, the lubricant layer will preferably be applied to the hot sheet leaving, for example, the cooling section of the last baking oven, which will ensure rapid drying and almost instant film formation. Spraying can be carried out by any known means such as a spraying ramp, which will be followed by a roller, the pressure of which can be adjusted on the strip in order to ensure the uniform distribution of the layer and the adjustment of thickness.

 It is of course possible to envisage other modes of application of the layer of water-soluble lubricant, in particular it can be applied by spraying the pre-coated sheet with the molten water-soluble lubricant, under these conditions it is preferable to apply the lubricant layer on a cold sheet.

 In certain cases it may be advantageous to introduce into the aqueous solution of water-soluble lubricant additional components intended to improve the conditions of application of the lubricant layer, among these components mention may be made of thickening agents known in the field of paints such as polysaccharides, ionic or nonionic surfactants or alcoholic cosolvents such as alcohols or ether-alcohols.

 The following examples are intended to illustrate the present invention but should not limit it in any way.

Example 1
A pre-coated steel sheet is used for the automotive industry by a two-layer system consisting of a surface treatment based on hexavalent chromium and a zinc-rich primer with a zinc content of 80 to 95 t.

In order to demonstrate the advantages of the process according to the present invention, three different lubricant systems are used.
a solution A according to the present invention,
- a WEDOLIT 1011 aqueous emulsion
(marketed by the company DIETZ)
(emulsion B),
- a RATAK FW 2 mineral oil (sold
by the company FUCHS) (oil C).

Solution A has the following composition - deionized water .................... 69.7 parts - non-ionic wetting agent
(TRITON CF 54 -ROHM & HAAS) ....... 0.01 part - polysaccharide .................... 0.3 part - polyethylene glycol 6,000 ......... 30, - parts
100, - parts
A non-ionic wetting agent is incorporated, a thickening agent of the polysaccharide type to facilitate the application of this lubricating composition using a roller (the polyethylene glycol of molecular mass 6,000 used is that sold by the company PCUK ).

These three compositions are applied to the hot sheet at 70-1000C with a layer weight of 2 of the order of 1.5 to 2.5 g / m
In the case of solution A, a dry film is immediately obtained. In the case of solutions
B and C, an oily layer forms on the surface of the sheet.

We control the stamping behavior
of pre-coated sheet on a laboratory device, the "LUB TESTER" marketed by the company
DETROIT TESTING MACHINE COMPANY, 9390 Grinnell Avenue,
Detroit, Michigan L 8213 (USA)
This device reproduces certain conditions of sliding of the sheet at the level of the rods on the press during stamping.

 The results are established by assessing the alteration of the coating on the part which has undergone the deformation. We note good - fair - bad, depending on the case.

 Compared under the same operating conditions, on the one hand, lubricated test pieces according to the invention, on the other hand, test pieces which were coated before the test with conventional lubricants. For this purpose, a mineral oil, RATAK FW 2 (sold by the company FUCHS) and an aqueous emulsion, WEDOLIT Z 1011 (from the company DIETZ) are used.

Under the conditions of the test: - oil-coated test pieces
mineral (C) are noted: bad - test specimens coated with emulsion
aqueous (B) are noted: fair - test pieces lubricated according to
the invention (A) are noted: good
The coating was completely protected by the layer
of polyethylene glycol.

 Under the same conditions as above, the control of the corrosion resistance of the parts after deformation by the salt spray test (standard ASTM B 117) shows a much better behavior of the sheets lubricated according to the present invention compared to those having undergone a treatment with oils or with emulsions.

 Test pieces B and C, the coating of which has been altered, show the onset of red rust after 24 hours.

 Test specimens A only show corrosion after 96 hours.

Example 2
An aluminum sheet pre-coated for the building industry is used by a paint system (SUPERCLAD 630 sold by the company ASTRAL), the binder of which is based on a mixture of polyester and polyvinyl chloride.

 Solution.solution A is applied to the hot sheet 2 at 70-100 C a layer weight of 1.5 to 3 g / m2.

 A lubricated coating is thus obtained which can undergo folding operations in an automatic machine without damage for the production of the tubular blades of the roller shutters.

Claims (10)

 1) Process for treating pre-coated sheets in order to improve the resistance of the coating during shaping, characterized in that a layer of a water-soluble lubricant is applied before shaping on the pre-coated sheet in order to obtain a water-soluble film of lubricant adhering without altering it to the underlying coating.  2) Method according to claim 1, characterized in that the water-soluble lubricant is chosen from: polyethylene glycols, polypropylene glycols, polyoxymethylenes, polyoxyethylenes, polyurethanes, water-soluble polyesters and alkyd, acrylic and epoxy polymers -soluble and neutralized esters and mixtures of these compounds.  3) Method according to claim 2, characterized in that the water-soluble lubricant is a polyethylene glycol.  4) Method according to one of claims 1 to 3, characterized in that the water-soluble film of lubricant is solid at room temperature.  5) Method according to one of claims 1 to 4, characterized in that the weight of the lubricant film is between 0.5 and 20 g / m.  6) Method according to claim 5, characterized in that the weight of the film is between 1.5 and 4 g / m2.  7) Method according to one of claims 1 to 6, characterized in that the lubricant layer is applied by spraying the pre-coated sheet with an aqueous solution of the water-soluble lubricant.  8) Method according to claim 7, characterized in that the lubricant layer is applied to the hot sheet.  9) Method according to one of claims 1 to 6, characterized in that the lubricant layer is applied by spraying the pre-coated sheet with the molten water-soluble lubricant.  10) Method according to claim 9, characterized in that the lubricant layer is applied to the cold sheet.