FR2978970A1 - METHOD OF APPLICATION OF UV-POLYMERIZING POWDER PAINT TO METAL PIECE - Google Patents

Abstract

The present invention relates to a method for applying a UV-curing powder paint to a metal part, characterized in that it comprises the following successive stages: the mechanical treatment of the surface of the metal part by impact and / or the chemical treatment of the surface of the metal part, said chemical treatment comprising a step of chemical conversion of the metal surface, the application of the photocurable thermosetting powder paint to the surface of the metal part, and the baking of the paint, said baking comprising: a melting of the powder with infrared rays (IR), then a polymerization of the powder melted by ultraviolet radiation (UV).

Description

The present invention relates to the technical field of applications of paints and coatings in the form of powder on metal parts. In particular, the invention relates to a method for applying thermosetting powder paint with UV curing on a metal part. The painting of metal parts must meet certain specific constraints, particularly in areas such as the automotive industry. In addition to the decorative function, paints or coatings applied to metal parts must protect the parts against aging, corrosion, and must adhere to metal parts over a long period of time, typically for at least 5 years. Today, the usual techniques used for the application of paints on metal parts are unsatisfactory because they generally involve cooking the paint by convection and thus require the heating of the core parts, typically at a temperature of 1 ° C. order of 140 to 220 ° C for 7 to 20 min, may cause a change in the structure of the metal. Moreover, these processes require many manual operations, can hardly be automated, and cause difficulties in obtaining a constant and optimal quality level. They also require very long implementation times, particularly because of the surface treatment time and firing metal parts which have a very high thermal inertia. Such processes are thus expensive and unprofitable because they do not allow to treat many parts in a day and there are particularly high energy and labor costs of use since they require in particular the use of temperatures. important heating. Therefore, there was a need to develop a new method for applying paint to metal parts not having the disadvantages of the prior art and to respect the constraints imposed by certain technical fields including in terms of adhesion of paint film, protection and resistance to aging and corrosion.

The present invention fills this need. The Applicant has thus discovered a new method of application to metallic powder paint parts in the form of ultraviolet radiation curing powder, requiring a very short processing time and low temperatures, which makes it possible to avoid heating the parts. at heart and thus allowing the maintenance of the hardness of the metal. Furthermore, the method according to the present invention makes it possible to process many parts per day and to achieve high productivity, requires little energy in comparison with conventionally used techniques, and is an environmentally friendly process since it leads to a release of volatile organic compounds almost zero and a decrease in the release of CO2 in the atmosphere. Such a process also makes it possible to treat different types of rough surfaces and to respond to a particularly wide and high resistance to corrosion, which can vary from 96 to 1000 hours in salt spray.

The subject of the present invention is thus a method for applying a UV-curing powder paint to a metal part, characterized in that it comprises the following successive stages: the mechanical treatment (1) of the surface of the metal part by impact and / or the chemical treatment (2) of the surface of the metal part, said chemical treatment (2) comprising a step of chemical conversion of the metal surface, - the application of the paint (3) in the form of a photo-curable thermosetting powder on the surface of the metal part, and - the baking of the paint (4), said baking comprising: (a) a melting of the powder with infrared rays (IR), then (b) a polymerization of the powder melted by Ultraviolet (UV) radiation.

The metal parts that can be treated or coated in the context of the present invention are very diverse parts that can come from all sectors, such as the transport sector, for example the automotive, railway, aeronautical or nautical, or the wind sector or building or general industry. Advantageously according to the present invention, the metal parts are automotive parts, such as automobile transmission parts, such as bowls, shafts, bars, transmission tulips, chassis parts, torsion bars, brake calipers, shock absorber tubes. The metals of the parts used in the context of the invention are typically made of steel or cast iron.

In a particular embodiment of the present invention, the method comprises a step of mechanical treatment (1) of the surface of the metal part, as well as a step of chemical treatment (2) of the surface of the metal part, the two stages being operated one after the other. In another particular embodiment of the present invention, the method comprises a step of mechanical treatment (1) of the surface of the metal part, as well as a step of chemical treatment (2) of the surface of the metal part, wherein these two steps are operated simultaneously.

Typically, the surfaces of the parts to be coated are rough surfaces, having on their surface calamine, rust, hardening or other pollutions such as grease. Advantageously according to the present invention, the method comprises a step of mechanical treatment (1) by impact allowing in particular to clean the surface of the workpiece and / or to increase the contact surface of the workpiece.

According to a particular characteristic of the present invention, the mechanical treatment (1) is carried out by mechanical stripping, advantageously by shot blasting or sanding. Typically, at least one substrate, such as microbeads, for example steel or glass balls, or a biological substrate (nuts, wheat, etc.), is projected onto the surface of the metal part, creates impacts on the surface of the metal and thus modifies the surface structure of the part, advantageously by improving the roughness of the part.

The projected substrate is an abrasive from attacking the metal part and is chosen according to the metal part to be treated. The mechanical treatment (1) thus substantially eliminates pollution on the surface, such as scale rust or grease. This results in a bare metal. In a particularly advantageous manner according to the present invention, following the mechanical treatment (1) of the surface of the metal part by impact, the metal part has a degree of care (Ds) of between 2.5 and 3 and a roughness ( Ra) of between 0.7 and 1.3 μm, advantageously between 0.8 and 1 μm, for example of the order of 0.9 μm.

The mechanical treatment (1) by impact according to the invention can also be achieved by other technologies such as laser stripping, cryogenics treatment, ...

For some very clean metal parts, it is not necessary to mechanically treat the surface of these parts. One can then operate a chemical treatment (2) directly on the surface of these parts.

Advantageously according to the present invention, the method comprises a step of chemical treatment (2) of the surface of the metal part. In a particular embodiment, the chemical treatment (2) is carried out as a result of, simultaneously with or prior to the mechanical treatment step (1) as described above. Typically, the chemical treatment (2) according to the invention is carried out as a result of the mechanical treatment operation (1). In the context of the present process, the chemical treatment (2) comprises a step of chemical conversion of the surface of the metal part to be coated. According to a particular characteristic of the present invention, the chemical conversion of the surface is carried out by spraying at least one chemical product such as nanomaterials. The chemical conversion can also be carried out by chromating, phosphating for example with iron or zinc, by trication or by alkaline oxidation.

Spraying is generally carried out in a tunnel provided with rings with nozzles projecting the chemical, typically in liquid form, onto the metal part to be treated. Advantageously according to the present invention, the conversion chemical product protects the part from corrosion, such as rust, and also facilitates subsequent attachment of the powder paint to the metal part. Thus, particularly advantageously according to the invention, the conversion chemical is an anticorrosion agent which also makes it possible to increase the contact area of the metal part, typically by creating roughnesses on the surface of the part. In a particular embodiment of the present invention, during the chemical treatment (2) of the surface, a step of degreasing the surface is performed prior to the chemical conversion step. Such a degreasing step thus makes it possible to remove the oils, dusts, salts, and other polluting substances present on the surface of the part, and to obtain a clean room. The degreasing of the surface may be carried out with a liquid degreasing agent composed in particular of at least one surfactant. According to a particular feature of the present invention, the chemical treatment (2) of the surface of the metal part comprises degreasing, rinsing, chemical conversion and drying steps.

At least one rinsing step is typically carried out after the degreasing step, for example with water such as demineralized water, and eliminates the active degreaser molecules from the surface of the workpiece. A rinsing step can also be carried out as a result of the chemical conversion, for example with water such as demineralised water, in order to remove the excess of chemical on the surface of the pieces and to obtain a clean surface. Final drying is generally carried out at the end of the chemical treatment, after the chemical conversion step and a possible rinsing step. Drying is generally carried out in a heated tunnel allowing the residual water to be removed from the metal parts, advantageously at a temperature of the order of 90 to 150 ° C., typically 95 to 125 ° C., in particular 100 to 100 ° C. 120 ° C, for example at 110 ° C.

In a particular embodiment of the present invention, the chemical treatment (2) of the surface of the metal part the following successive stages: - degreasing of the surface with the aid of at least one degreasing agent, - rinsing, typically with demineralised water, - chemical conversion, - rinsing, typically with demineralized water, and then - drying, typically at a temperature of the order of 90 to 150 ° C, preferably 100 to 120 ° C.

Advantageously according to the present invention, the application of the powder (3) is by electrostatic spraying without solvent, preferably by Corona effect. The powder used in the context of the present invention is a thermosetting, photocrosslinkable, UV-curing, solvent-free powder.

In particular, the powder may be a powder based on polyesters, epoxies, mixed epoxy polyesters, polyurethanes, or even acrylics.

According to a particular characteristic of the present invention, protection is provided for certain zones of the metal part not to be painted. This protective step is advantageously carried out before application of the powder (3) or following the application of the powder (3) and prior to the step of firing the paint (4).

The baking of the powder (4) contains a first step of melting the powder with infrared rays (IR).

Advantageously according to the present invention, the fusion or preheating with infrared rays makes it possible to melt the powder paint so that it spreads evenly over the support constituted by the metal surface of the part, and to obtain a film of paint on the surface of the room. In a particularly advantageous manner, the infrared ray melting is carried out at a temperature of between 100 and 150 ° C., typically between 120 and 130 ° C., for example of the order of 125 ° C., advantageously for a period of between 30 seconds. and 3 min, typically between 1 min and 2 min, for example of the order of lmin30.

The baking of the powder (4) contains a second stage of polymerization of the ultraviolet (UV) radiation melted powder following the melting of the powder with the infrared (IR) rays. During this step, the melted powder is crosslinked under the action of UV rays. The paint film is thus frozen and solidified on the surface of the metal part. The UV radiation used is advantageously UV A, UVB and UV V radiation, in particular with a UV A + B dose of between 1.5 and 4 Joules / cm 2, typically between 2 and 3 Joules / cm 2, for example from 2.5 Joules / cm 2, and with a UV V dose of between 1 and 2.5 Joules / cm 2, typically between 1.5 and 2 Joules / cm 2, for example of the order of 1.7 Joules / cm 2 . The UV doses applied must be sufficient to properly polymerize and crosslink the paint film. Advantageously according to the present invention, the UV radiation is applied for a period of between 20 s and 1 min, typically between 20 s and 30 s. The time of application of the UV radiation which makes it possible to bake the paint on the part is very short.

During the cooking step (4) according to the present invention, the parts are not heated to heart and the skin temperatures are not very high.

In a particularly advantageous manner according to the invention, following the polymerization step, a film of paint is obtained on the metal part with a film thickness of between 60 and 120 μm, advantageously between 80 and 100 μm.

Advantageously, the paint film adheres perfectly to the metal support. In addition, such a film typically has: - an anticorrosion behavior can vary from 96 to 1000 hours, in particular from 400 to 800 hours in salt spray and from 35 to 45 cycles in ECC1, - a post-grid adhesion advantageously of aaa after immersion 7 days in engine oil at 100 ° C or 250 hours in a FORD tank, and / or - a good resistance to gravel advantageously with less than 5 impacts to the support (typically between 2 and 5 impacts) in tests of projection of 500g of shot at a pressure of 1 bar for 10 seconds.

In a particular embodiment, the metal part is an automobile transmission part and, following the implementation of the method, a metal part coated with a paint film having the following resistance properties is obtained: an absence of corrosion and blistering on all samples tested both on solid surfaces and on edges, and in particular complies with the requirements of the ECC1 test (standard RENAULT D172028 / C (10/2007)) after 42 cycles, - an absence of corrosion in full panel and adhesion after grid of AAA, and is in particular in conformity with the requirements of the test of corrosion in artificial atmospheres (Brouillards Salins - standard NFENISO9227 (03/2007)) after 440 h, - a loss of hardness included between 10 and 15% and an adhesion after grid of aaa, and is in particular in accordance with the requirements of the test of immersion in the engine oil (norm PSA B155220 / C (02/2008) code 250) after 7 days at 100 ° C, - a blistering score of 0 and a bond rating after gridding of aaa, and in particular complies with the requirements of the liquid resistance test (immersion method - EN ISO standard 2812-2 (01/1995)) after 250 h, - less than 5 impacts on the support (between 2 and 5 impacts) during tests of projection of 500g of shot at a pressure of 1 bar for 10 seconds, and in particular complies with the requirements of the gravel test (PSA standards D24 1312 / D (05/2003) and PSA B15 5220 / C (02/2008)).

The following example is given without limitation and illustrate the present invention.

Example 1: A metal automotive part of the shaft type, bowl or tulip steel is mechanically treated by shot blasting using steel microbeads sprayed on the surface of the workpiece.

The metal part then has a clean surface with a degree of care of 2.5 and a roughness of the order of 0.9 microns. The part is then subjected to a chemical treatment comprising the following steps: degreasing of the surface with the aid of at least one degreasing agent such as surfactants, rinsing with demineralized water, chemical conversion with solutions enriched in nanomaterials, - rinsing with demineralized water, then - drying at a temperature of the order of 110 ° C.

UV curable photocurable thermosetting powder is sprayed onto the part by Corona effect, then the powder-coated part is heated at a temperature of about 125 ° C. for 1 min under infrared rays, which makes it possible to obtain a piece covered with a film of liquid paint. The piece is then subjected to a UVA, UVB and UVV radiation polymerization step, with a UV A + B dose of 2.5 Joules / cm and a UV V dose of 1.5 Joules / cm 2 for 30 s, which allows to crosslink and solidify the paint film. This gives a piece covered with a paint film having a thickness of about 80 microns and having the following resistance performance: - Absence of corrosion and blistering both on solid surfaces and on edges after 42 cycles of the ECC1 test, - Absence of corrosion in full panel and adhesion after grid of AA-A, after 800 h in artificial atmospheres (Brouillards Salins), - Loss of hardness of 12.8% and adhesion after grid of AAA after 7 days of immersion in the engine oil at 100 ° C., 30 - blistering rating of 0 and a grip score after gridding of aaa after 250 h of immersion in water at 40 ° C. in a Ford container,

Claims (14)

REVENDICATIONS1. Method for applying a UV-curing powder paint to a metal part, characterized in that it comprises the following successive stages: the mechanical treatment (1) of the surface of the metal part by impact and / or the chemical treatment (2 ) of the surface of the metal part, said chemical treatment comprising a step of chemical conversion of the metal surface, - the application of the paint (3) in the form of thermosetting powder photocrosslinkable on the surface of the metal part, and - the cooking the paint (4), said cooking comprising: a. a fusion of the powder with the infrared rays (IR), then b. a polymerization of the powder melted by ultraviolet (UV) radiation. 2. Method according to claim 1, characterized in that the metal part is an automobile part, such as an automobile transmission part. 3. Method according to claim 1 or 2, characterized in that the mechanical treatment (1) of the surface increases the contact surface of the workpiece and is advantageously achieved by shot blasting or sanding. 4. Method according to claim 3, characterized in that the mechanical treatment of the surface (1) provides a metal part with a degree of care of between 2.5 and 3 and a roughness of between 0.7 and 1 , 3 μm, advantageously between 0.8 and 1 μm. 5. Method according to any one of the preceding claims, characterized in that, during the chemical treatment (2) of the surface, the conversion of the surface is carried out by spraying at least one chemical such as nanomaterials. 6. Method according to claim 5, characterized in that the conversion chemical is an anticorrosion agent which also increases the contact area of the metal part. 7. The method of claim 5 or 6, characterized in that, during the chemical treatment (2) of the surface, a degreasing step of the surface is performed prior to the chemical conversion step. 8. Method according to claim 7, characterized in that the chemical treatment (2) of the surface of the metal part comprises the following successive steps: - degreasing of the surface with at least one degreasing agent, - rinsing typically with demineralised water, chemical conversion, rinsing, typically with deionized water, followed by drying, typically at a temperature in the range of 90 to 150 ° C, preferably 100 to 120 ° C. . 9. Method according to any one of the preceding claims, characterized in that the application of the powder (3) is by electrostatic spraying without solvent, preferably by Corona effect. 10. Process according to any one of the preceding claims, characterized in that, during the firing of the paint (4), the melting of the powder with IR rays is carried out at a temperature of between 100 and 150 ° C., typically between 120 and 130 ° C, advantageously for a period of between 30s and 3 min, typically between 1 min and 2 min. 11. Process according to any one of the preceding claims, characterized in that, following the melting of the powder, the melted powder is polymerized by UV radiation A, UVB and / or UV V, advantageously with a single dose. UV A + B between 1.5 and 4 Joules / cm 2, typically between 2 and 3 Joules / cm 2, and / or with a UV V dose of between 1 and 2.5 Joules / cm, typically between 1.5 and 2 joules / cm2. 12. The method of claim 11, characterized in that the UV radiation is applied for a period of between 20 s and 1 min, typically between 20 s and 30 s. 13. Method according to any one of the preceding claims, characterized in that, following the polymerization step, a paint film is obtained on the metal part with a film thickness between 60 and 120 μm, advantageously between 80. and 100 μm. 14. Process according to any one of the preceding claims, characterized in that, following the polymerization step, a metal part coated with a paint film having the following properties is obtained: an anticorrosion behavior that can vary from at 1000 hours, in particular from 400 to 800 hours in salt spray and from 35 to 45 cycles in ECC1, - adhesion after gridding of aaa after immersion for 7 days in engine oil at 100 ° C. or 250 hours in a FORD tank, - good grit resistance with less than 5 impacts to the support during projection tests of 500g of shot at a pressure of 1 bar for 10 seconds.