METHOD FOR PRETREATING METAL SHEETS TO BE PAINTED

Abstract: A method for pretreating metal sheets to be painted, the particularity of which consists of the fact that degreasing and phosphating of the metal sheets are performed simultaneously in a single step, assisting the chemical action of a degreasing-phosphating solution with the application of mechanical energy.

Description: METHOD FOR PRETREATING METAL SHEETS TO BE PAINTED DESCRIPTION The present invention relates to a method for pretreating metal sheets to be painted. It is known that metal sheets used for the production of many components, such as panels of metal furniture, cai doors and hoods, as well as structures of electrical household appliances such as washing machines or air conditioners, require treatment processes as a preliminary to painting, so as to bring the surface to the best conditions to allow optimum adhesion of the paint. In the cm rent state of the art, a typical treatment for preparing the metal sheet for painting mainly entails peitorming degreasing, in order to eliminate solid particles, residues of rolling- mill oils and other contaminants, and phosphating (conversion) in order to form a layer of phosphor salts on the surface of the sheets; these salts facilitate the adhesion of the paint and provide protection against corrosion. In conventional-type treatments, degreasing and phosphating are performed in separate and mutually successive steps with the addition of intermediate and final rinses. For example, one of the most classic methods for pretreating metal sheets entails, as a whole, eight separate steps: pie minaiy degreasing in hot water or in alkaline solution, alkaline degreasing, two intermediate rinses, activation (weakly alkaline), conversion with iron phosphate (pH 3.5-5) or zinc phosphate (pH 2.8), (also known as phosphating), and two rinses, the latter using demineiahzed water. The overall duration of a treatment of this type for die-cut metal sheets is approximately 10 minutes. As regards degreasing in particular, in the case of continuous metal sheets in rolls the surface to be tieated is subjected to a mechanical brushing action so as to remove the outer layer constituted by oxidized metal, residues of rolling-mill oil and so forth. This allows extremely rapid degreasing treatments and subsequent phosphating. generally performed at line speeds of 20-50 m/min and requiπng times on the order of one minute In this case, howevei , when the metal sheet is subsequently die-cut, the edges of the cut remain untieated and constitute corrosion starting points, leading to consequent technical diawbacks On the other hand, brushing can be difficult to perform in the case of metal sheets that aie die-cut beforehand and entirely impossible in the case of pre-drawn or pre-folded metal sheets In oidei to obviate these drawbacks, in metal sheets of this kind degreasing is generally peifoimed by essentially chemical methods, using a suitable detergent solution which is spi ayed at veiy low pressure onto the suiface of the metal sheet to be tieated oi in which the metal sheet is immeised In this manner, however, the degieasing action is considerably slowei , accoidingly, the line speed must be reduced, consequently leducing the productivity of the plant, oi the length of the treatment tunnel must be incieased in ordei to have a sufficient letention time. Accordingly, typical complete treatment times are on the order of ten minutes Anothei diawback of known kinds of method resides in the fact that the intermediate rinses between degieasing and phosphating require a considerable execution time and use a large amount of watei , this clearly has a negative effect on operating and plant costs The aim ot the piesent invention is therefore to piovide a method foi preti eating metal sheets to be painted, in which the treatment times aie reduced considerably with lespect to conventional methods. Within the scope of this aim, an object of the present invention is to provide a method foi pi eti eating metal sheets to be painted which reduces the consumption of watei used foi washing the metal sheets during the pretreatment cycle. Anothei object ot the present invention is to provide a method foi pretreating metal sheets to be painted which increases plant productivity Another object of the present invention is to provide a method for pretreating metal sheets to be painted in which the plant occupies less space that plants for performing known kinds of method. Another object of the present invention is to provide a method for pretreating metal sheets to be painted in which the treatment is performed with a higher degree of uniformity than treatments performed with conventional methods. Not the last object of the present invention is to provide a method for pretreating metal sheets to be painted which is highly reliable, relatively easy to provide and at competitive costs. This aim. these objects and others which will become apparent hereinafter are achieved by a method for pretreating metal sheets to be painted, characterized in that it comprises the steps of: a) simultaneous degreasing and phosphating the metal sheets by means of the chemical action of a degreasing and phosphating solution assisted by the application of mechanical energy; b) final rinsing of said metal sheets. Further characteristics and advantages of the invention will become apparent from the description of preferred but not exclusive embodiments of the method according to the invention, illustrated only by way of non-limitative example. The method according to the invention comprises, as a whole, a first step in which metal sheet degreasing and phosphating are performed simultaneously and a second rinsing step for final cleaning of the metal sheet. In particular, simultaneous execution of degreasing and phosphating is performed by assisting the chemical action of a suitable degreasing- phosphating solution by means of the application of mechanical energy. In a preferred embodiment of the method according to the invention, said mechanical energy is provided by spraying the degreasing-phosphating solution at high pressure onto the surface of the metal sheet to be treated; advantageously, it is possible to add to said solution solid particles constituted foi example by abrasive powders or glass beads or polymeric powders or any othei kind ot paiticle, piovided that they are compatible with the application The mechanical blushing action is therefoie no longei lequired, since it is leplaced by the high-piessuie spiaying of a solution meant to simultaneously produce degreasing and phosphating of the metal sheet As an alternative the initial degreasing and phosphating step might be obtained by lmmeising the metal sheet to be treated in a deteigent solution and by supplying the necessaiy mechanical eneigy by means of ultrasonic waves, said ultrasonics technique pioduces high-fiequency vibrations of the solution and of the surface to be treated which assist the chemical action of the degieasing-phosphating solution If the mechanical energy is supplied by high-pressure spraying, the working pressure can vaiy conveniently ovei a wide range whose limits are set simply on the one hand by the need to have an effective degreasing action and on the other hand by the convenience of using plants which aie as simple and low-cost as possible, at the same time avoiding excessive evapoiation of the solution used Foi example, it has been found experimentally that effective lesults aie achieved with piessuies vanable between 3 and 80 bar, moie preferably between 5 and 60 bar, and even moie piefeiably between 10 and 30 bar, in particulai. the pressure is to be understood as being measuied at the spiay nozzle The spiayed solution, in contact with the metal sheet to be treated, comprises a surfactant which is suitable to woik in an acid environment, in paiticular. the surfactant used may be the substance known commercially as Gaidoclean G 501 , manufactuied by Bondei S p A The degieasing-phosphating solution furthermoie comprises phosphoric acid and/oi deπvatives theieof, zinc oxides and/or salts, and/or iron oxides and/or salts, and optionally nickel and/oi man g anese salts It is tuitheimoie possible to include amounts of hydrofluonc acid, sodium nitrate and sodium niti ite allowing to acceleiate the reaction, and of caustic soda in order to adjust the acidity of the solution The deteigent-phosphating solution to be spiayed is drawn from a suitable containei and can be at a tempeiatuie between 30 and 80°C, prefeiably between 40 and 70°C, moie piefeiably between 50 and 60°C, in paiticulai, said temperature limits aie dictated on the one hand by the need to avoid excessive evaporation of the detergent solution and on the othei hand by the need to have an effective degreasing action in any case The type of nozzle used foi spiaying the solution against the metal sheet to be pretreated piovides both foi solid-cone nozzles and flat-spraying nozzles oi foi a combined solution of the two types ot nozzle The advantage of flat-spraying nozzles is the fact that foi an equal pressure of the supply fluid they piovide a higher impact pressure of the jet against the metal sheet, in this manner the mechanical action of said jet is enhanced, making the detergent action moie effective The final step foi rinsing the metal sheet is performed in demineiahzed water at ambient tempei atuie and piessuie, as an alternative, said final rinsing can be pei formed at high piessuie and/oi at highei temperatures or can be split into two πnses In piactice it has been found that the method according to the invention fully achieves the intended aim and objects, since it allows to combine in a single step the degieasing and phosphating tieatments, consequently reducing the cycle times, fuithermore, the space occupied by the plant for peiforming the pietreatment according to the invention is teduced with lespect to known types of plant, since as the parts to be tieated aie in constant motion the leduction in cycle times allows to shorten the tieatment tunnel Shortening the tunnel allows savings in teims of stiuctural work and movement systems It is fuitheimoie possible to use less polluting degreasing agents, foi example without alkaline deteigents. since the highei effectiveness of the combined degreasing and phosphating pioduct is achieved by applying mechanical energy, for example due to the high- piessuie jet, which assists the chemical action, instead of using moie aggiessive chemical pioducts as occuis in conventional pretreatment methods Anothei considei ble advantage of the method according to the invention is the fact that the continuous leplacement of fiesh solution on the metallic surface of the metal sheet to be pietieated ensuies constant uniformity of the action of the product and therefore prevents the foi mation of stieaks and bands, which instead occur in conventional treatments In conventional methods the solution in contact with the metal sheet in fact tends to saturate eai liei at ceitain aieas of the suiface of the metal sheet which aie moie affected than otheis by the piesence of oils and the like and replacement of the deteigent pioduct occuis very slowly It is noted that the method thus conceived also allows to treat metal sheets which are not peitectly flat, that is to say, already drawn or even folded, foi example by fitting the spray nozzles on the head of a robot which allows to access all the vaiiously oiientated surfaces of the metal sheet The method thus conceived is susceptible of numerous modifications and variations, all of which aie within the scope of the inventive concept; thus, foi example, it can be convenient to intioduce a piehminaiy wash in water or to perfoim phosphating with different phosphoi salts, foi example non phosphates Finally, all the details may be any accoidmg to the requπements and the state of the ait

Claims: CLAIMS 1. A method for pretreating metal sheets to be painted, characterized in that it comprises the steps of: a) simultaneous degreasing and phosphating the metal sheets by means of the chemical action of a degreasing and phosphating solution assisted by the application of mechanical energy; b) final rinsing of said metal sheets. 2. The method according to claim 1 , characterized in that said step a) is provided by high- pressure spraying of said degreasing and phosphating solution. 3. The method according to claim 2, characterized in that abrasive powders or polymeric powders or glass beads are added to said degreasing and phosphating solution. 4. The method according to one or more of the preceding claims, characterized in that said degreasing and phosphating solution comprises a surfactant. 5. The method according to one or more of the preceding claims, characterized in that said degreasing and phosphating solution furthermore comprises zinc oxides and/or salts, and/or iron oxides and/or salts, and phosphoric acid and/or derivatives thereof. 6. The method according to one or more of the preceding claims, characterized in that said degreasing and phosphating solution is sprayed at a pressure between 3 and 80 bar, preferably between 5 and 60 bar, more preferably between 10 and 30 bar. 7. The method according to one or more of the preceding claims, characterized in that said degreasing and phosphating solution has a temperature which can vary between 30 and 80°C. preferably between 40 and 70°C, more preferably between 50 and 60°C. 8. The method according to claim 1 , characterized in that said step a) is performed by using ultrasonic waves. 9. The method according to one or more of the preceding claims, characterized in that it compiises a predegreasing step before said step a). 10 The method according to one or more of the preceding claims, characterized in that said step b) is performed in deminerahzed water at ambient temperature and pressure. 1 1 The method accoiding to one oi more of claims 1 to 9, characterized in that said step b) is pei foi med in deminerahzed water at ambient temperature and at high pressure. 12. The method according to claim 1 , characterized in that said metal sheets comprise die- cut, drawn or pie-folded metal sheets.

Inventors: ANTONIAZZI ANTONELLO, RINALDI FRANCESCO DI, ANTONIAZZI, ANTONELLO, RINALDI, FRANCESCO DI
Applicants: ABB FLEXIBLE AUTOMATION SPA, ANTONIAZZI ANTONELLO, MARCON MARIA RAFFAELLA HF, RINALDI CHIARA HF, RINALDI MICHELA HF, ANTONIAZZI, ANTONELLO, MARCON, MARIA, RAFFAELLA HF, RINALDI, CHIARA HF, RINALDI, MICHELA HF
Application year: 1999
Main classification: C23C22
IPC codes: C23C22