ITRM20090327A1 - MULTILAYER PAINTING PROCEDURE - Google Patents

Description

Description for invention entitled:

MULTILAYER PAINTING PROCESS

Description

The present invention relates to a process for painting a metallic substrate, in which the curing of a multilayer powder paint, in particular a double layer, of the metallized and transparent type is carried out. This kind of process is well known in the art and is used to paint a wide range of items, such as household appliances, where a high resistance of the painted layer to scratches and corrosion and a high quality of the surface finish is required.

The known techniques of metallized powder painting on metal substrates involve the use of a multi-step process, in which the substrate is initially prepared by washing and pre-treatment cycles aimed at improving the corrosion resistance and adhesion of the paint.

Then a deposition of a layer of metallized powder paint is carried out electrostatically, followed by a phase of cross-linking of this layer by physical and / or chemical way, for example in a convective oven for 15-20 minutes at 160-170 ° C, and from a cooling phase down to room temperature.

On the first layer, a second layer of transparent powder paint is then deposited on the metallized and cross-linked layer, again electrostatically and for substantially protective purposes.

This second layer is also subjected to a cross-linking phase in a convective oven, similar to the previous one, obviously followed by a second cooling phase.

It is therefore understood that this succession of depositions, cross-linking in the oven and cooling considerably lengthens the times of

process, even over an hour.

The technical problem underlying this

invention is to provide a method of

painting capable of shortening the times of

process.

This problem is solved by a procedure such as

specified above, which includes the phases of:

• electrostatic deposition of a first layer

powder paint;

• crosslinking of this first layer by means of

irradiation in the infrared field up to

obtain a thickness of the crosslinking

between 0.5 and 4.0 µm:

• electrostatic deposition of one second

transparent powder paint layer on

said first layer; And

• crosslinking of said first and second layers

in a convective oven.

The double layer painting process

defined above allows you to completely eliminate the

firing cycle of the first layer, currently

necessary to avoid pigment migration

from the metallic decorative layer to the layer

transparent protective during the reticulation of the latter. This allows to reduce the time

total production of about 40% and of

check the layering of the material.

This procedure is therefore usable in the

decorative powder coating sector

of the layer by layer type, and find his

ideal application in the field of

appliances, fixtures and objects

ornaments and furniture decorated with paint

metallized.

Further details of the procedure according to the

present invention will appear from the description

of his favorite execution example, provided

by way of non-limiting example with

reference to the attached drawings, in which:

Figure 1 shows a diagram illustrating i

spectrophotometric test results,

conducted on a coating obtained by

the second painting procedure

<the invention; and>

Figure 2 shows a diagram illustrating

the result of a scratch test conducted a

20 N with a Rockwell C tip, conducted on a

coating obtained by the process

of painting according to the invention.

The metallized powders used in this process generally consist of:

• an organic phase with a binder function, • a pigment with a decorative function,

• a catalyst that allows the cross-linking of the organic phase at high temperature, • a blocking agent that prevents the low-temperature cross-linking of the organic phase and allows the preservation of the powder at room temperature, and

• a series of additives that increase the chemical and mechanical resistance properties of the paint.

This paint composition is intended to be applied on a metal substrate, for example a galvanized sheet, which is previously subjected to a sequence of washing and pre-treatment stages such as to confer high corrosion resistance properties.

A first phase of deposition of said metallized powder paints is then provided according to a work cycle consisting of a first electrostatic spraying, which allows to obtain a metallized powder layer of about 60-80 µm.

After this electrostatic application, the depsited paint layer is only lightly adhered to the metal substrate as a result of the electrostatic interaction phenomena activated during the deposition process, in particular polarization phenomena.

Therefore, a consolidation step of the metallized powder layer is required by means of cross-linking activated by the presence of said catalyst in the organic phase, in order to give the metallized powder layer the expected aesthetic characteristics and mechanical resistance properties.

Furthermore, during the cross-linking phase the pigment contained in the metallized layer remains locked in its position, giving stability to the aesthetic characteristics of the paint which therefore appears solid, smooth and well adhered to the metal substrate.

In the process according to the present invention it acts on the first crosslinking cycle, i.e. on the crosslinking cycle of the metallized layer, the aim is to reduce the working times of the painting process and to control the stratification by adjusting the thickness of the crosslinked metallized powder paint by means of flashes of IR radiation.

Then, in the subsequent crosslinking phase, the electrostatically deposited metallized powder is exposed to infrared radiation by means of infrared (IR) lamps with powers ranging from 1 kW to 3 kW and exposure times ranging from 3 sec to 30 sec depending on the chosen powers.

The IR radiation allows to obtain a localized heating of the outermost layers of the metallic powder paint.

The grains of metallized powder contained in the outermost layers, subjected to heating, tend to become fluid and then to coalesce, giving rise to a controllable layer, the thickness of which advantageously assumes values between 0.5 and 4.0 µm, of cohesive thickness and in which the crosslinking processes are initiated.

This layer is also characterized by a smooth and uniform morphology, the result of the ability to flow of the powder coatings subjected to heating.

Instead, the underlying layers are not affected by the short irradiation phase, remaining, in fact, with unaltered properties. Therefore, at the end of the irradiation cycle with IR radiation, the metallized powder deposit consists of a first substrate, adjacent to the metal substrate, with unchanged characteristics and a second substrate, superimposed on the first, already cohesive and with chemical, physical and mechanics developed.

In particular, the energy provided by the IR radiation is such as to block the pigment present in this second layer in its positions and to act as a diffusive barrier against the pigment present in the underlying layer of material and not affected by the heating phenomena.

The painted surface treated with infrared is immediately cold when the radiation is deactivated, as the thickness of the affected layer and the characteristics of the irradiation process mean that very little energy remains accumulated in the material.

Therefore, this surface is immediately suitable for the subsequent deposition phase with transparent powders and it is not necessary to carry out a dedicated cooling phase.

In fact, the cross-linked metallic paint layer would still have a short life, as the pigment used to obtain the particular decorative characteristics is not very resistant to external agents and direct contact with the atmosphere would cause it to deteriorate rapidly.

To overcome this drawback, a further electrostatic deposition phase is provided for a second layer consisting of a transparent powder paint that has such characteristics as to protect the metallized layer without altering its aesthetic characteristics, indeed emphasizing them. Note that this layer with transparent powders cannot be deposited on the layer with metallized powders before the latter is cross-linked. In fact, if this were the case, it would be necessary to reticulate the two layers together and this process would determine migration of the pigment from the metallized layer to the transparent layer with consequent compromise of the aesthetic characteristics of the paint.

Therefore, the transparent powder layer is deposited on the metallized paint layer by electrostatic spraying until a thickness of about 40-60 µm is obtained.

The transparent powder layer also needs to be subjected to a cross-linking phase which, however, can be carried out in a conventional manner, ie in a convective oven at 160-170 ° C for 15-20 min.

This cycle allows the simultaneous crosslinking of both the metallized layer, whose crosslinking is then brought to completion, and the transparent layer, ensuring the establishment of the functional properties of the paint.

The migration effect of the pigments from the metallized to the transparent layer is prevented thanks to the IR cross-linking cycle of the metallized layer which, as mentioned, creates a thin 'pre-cross-linked' layer with a diffusive barrier function.

In this way, even the aesthetic characteristics of the double-layer paint are preserved. The evaluation of the two painting techniques, conventional with double passage in a convective oven and according to the present invention, was carried out through a sequence of quantitative tests that made it possible to evaluate the mechanical strength and the aesthetic appearance.

The tests used to evaluate the mechanical performance of the paints were the scratch test, in micro and macro mode, and wear with a ball-on-disk device.

The spectrophotometric test, the scanning electron microscope and the metric profile analysis were instead used to evaluate the aesthetic and morphological characteristics of the paints.

The results showed substantial equivalence of the performances of the manufactured articles painted with the two techniques, thus confirming the feasibility of the process according to the present invention. In other words, by replacing the curing cycle of the metallized layer in the oven with a so-called infrared 'flash' curing and eliminating the post-curing cooling phase, it is possible to obtain painted products with similar performance to the painted products obtained with the traditional technique. Stratification is controllable by scanning electron microscopy and differential scanning calorimetry.

The procedure was tested on galvanized sheet metal, and the results of the spectrophotometric test are shown in Figure 1, which highlights the results for painting with different IR lamp powers (1500 W and 2000 W) and times (30 s and 60 s) compared to a conventional treatment.

Note how it is necessary to reach a minimum level of power to obtain a spectrum comparable to the spectrum of oven-cured paints. In particular, the peak of the spectrum obtained by infrared 'pre-crosslinking' of the metallized layer is very close to that obtained with the conventional process for the power of the IR radiation set to 2000 W and exposure times of 30 s. Longer times or powers below 2000 W lead to shifted color ranges.

Powers lower than 2000 W cause a widening of the chromatic range and, therefore, a less defined color.

Long times tend to narrow the chromatic range and make it possible to obtain a more defined color at the same radiated power.

The scratch test resistance of paints can be expressed in terms of depth of penetration measured during the execution of the test and under load conditions.

In Figure 2, the result of a scratch test conducted at 20 N with a Rockwell C tip is shown in which the results for paints with different IR lamp powers (1500 W and 2000 W) and times (30 s and 60 s) are highlighted. ) compared to a conventional treatment.

The test shows how the mechanical performance of the coatings is substantially independent of the infrared pre-crosslinking mode of the metallic paint layer and independent of the crosslinking technique chosen, i.e. traditional or innovative process.

However, it is possible to arrive at a classification of the various scenarios examined by the penetration depth curves. It can be concluded that the 'pre-crosslinked' IR coatings at a power of 2000 W and for irradiated for a time of 30s or 60s allow to obtain the best scratch performance.

The traditional process and the pre-crosslinked at 1500 W for 60 s show a slightly lower resistance to penetration. The pre-crosslinked at 1500 W for 30 s shows an even lower resistance to penetration.

Clearly, the interpretation of these phenomenologies is extremely complex and requires the understanding of often crossed mechanisms. In this context, it is useful to state that a good 'pre-crosslinking' of the metallized layer can ensure a correct crosslinking process of the double layer in the oven and the consequent establishment of high mechanical properties, even better than in the traditional case.

The limit of the traditional process is linked to the adhesion of the transparent layer on a metallized layer already completely cross-linked and therefore not very prone to adhere with the overlying layer. The limit of low power pre-crosslinking is instead due to the presence of an outer layer of metallized paint not sufficiently processed to guarantee advantages in the overall crosslinking of the coating in its entirety and, therefore, a potential source of loss of functional properties of the paint.

In order to satisfy further and contingent needs, a person skilled in the art may make numerous further modifications and variants to the above described multilayer painting process, all of which however fall within the scope of protection of the present invention, as defined by the attached claims.

Claims (7)

CLAIMS 1. Multilayer painting process of a metal substrate, which involves a double layer, respectively of the metallized and transparent type, which includes the steps of: • electrostatic deposition of a first layer of powder paint; • crosslinking of this first layer by irradiation in the infrared field to obtain a crosslinking thickness between 0.5 and 4.0 µm: • electrostatic deposition of a second layer of transparent powder paint on said first layer; And • crosslinking of said first and second layers in a convective oven. 2. Process according to claim 1, wherein said first layer of powder paint comprises: • an organic phase with a binder function, • a pigment with a decorative function, • a catalyst that allows the cross-linking of the organic phase at high temperature, • a blocking agent that prevents the low-temperature cross-linking of the organic phase and allows the preservation of the powder at room temperature, and • a series of additives that increase the chemical and mechanical resistance properties of the paint. 3. Process according to claim 1 or 2, in which the metal substrate, before the electrostatic deposition of a first layer of powder paint, is previously subjected to a sequence of washing and pre-treatment steps such as to confer high resistance properties to the corrosion. 4. Process according to any one of the preceding claims, wherein said first metallized powder layer has a thickness of about 60-80 µm. 5. Process according to any one of the preceding claims, wherein said crosslinking of said first layer by irradiation in the infrared field takes place by exposure to infrared (IR) lamps with powers ranging from 1 kW to 3 kW and exposure times included between 3 sec and 30 sec depending on the chosen powers. 6. Process according to any one of the preceding claims, wherein said second transparent powder layer has a thickness of about 40-60 µm. 7. Process according to any one of the preceding claims, wherein crosslinking of said first and second layer in a convective oven takes place at 160-170 ° C for 15-20 min, allowing the simultaneous crosslinking of both the metallized layer, the crosslinking of which is then brought upon completion, and of the transparent layer.