NEW PROCESS FOR THE PRE-TREATMENT OF NON-CONDUCTIVE MATERIAL SUBSTRATES IN PROCESSES FOR POLYMER POWDERS ELECTROSTATIC COATING DESCRIPTION

The present invention refers to an electrostatic painting process. In particular, it refers to a process for the electrostatic painting of non- conductive material surfaces. According to the same inventive concept, the present invention further refers to an apparatus for the electrostatic painting and a device for the pre- treatment of surfaces to be painted by the same electrostatic coating technique. In the electrostatic painting field it is known that thermo-sensitive substrates of non-conductive material, in particular of plastics material, cannot be electrostatically coated with paints (e.g., polymer powders) without the aid of specific pre-treatments.

Known pre-treatments generally use rather complex techniques, like, e.g., flaming, surface ionization, plasma, or, as an alternative, a conductive primer in order to obtain surfaces of non-conductive material (e.g., plastics) conductive, semi-conductive or antistatic materials allowing to apply the electrostatic painting process.

However, said pre-treatments have several drawbacks. Among those, it should be mentioned the negative environmental impact due to the use of pollutants like, e.g., solvents or aggressive substances.

Further drawbacks are represented by low operative flexibility, need of skilled labor and high costs.

Lastly, processes for the electrostatic painting of non-conductive surfaces overall have not yet attained a market-required specialization and quality level.

The technical problem underlying the present invention is that of providing an electrostatic painting process, in particular for non-conductive material

surfaces, allowing to overcome the drawbacks mentioned above with reference to the known art.

Such problem is solved by a process according to claim 1.

According to the same inventive concept, the present invention further refers to an electrostatic painting apparatus according to claim 17, and to a device for the pre-treatment of non-conductive surfaces to be painted by electrostatic coating according to claim 28,

Preferred characteristics of the present invention are present in the dependent claims thereof. It will be understood that in the present context by 'paint' it is meant any substance to be applied on a surface with the function of protecting and/or coloring the same. Those encompass liquid or powder paints anyhow identified.

The present invention provides several relevant advantages. The main advantage of the process according to the present invention consists in making paintable, by non-toxic material and steps void of liquid residues to be disposed of, surfaces otherwise refractory (impervious) to the electrostatic coating process.

Other advantages, characteristics and manner of using the present invention will be made apparent in the following detailed description of some embodiments thereof, given by way of example and not for limitative purposes. Reference will be made to the figures of the annexed drawings, wherein:

^■ Figure 1 shows a perspective view of an embodiment of an electrostatic painting apparatus according to the present invention;

^■ Figure 2 shows a perspective and partially sectional view of a preferred embodiment of a device for the pre-treatment of the apparatus of Figure l;

^■ Figure 3 shows a side section of the device for the pre-treatment according to Figure 2;

^■ Figure 4 shows the growth as a function of time of the copper mass coated on a plastics material (polyamide: PA66) surface;

^■ Figure 5 shows the surface resistivity of a plastics material (polyamide: PA66) surface subjected to a copper powder coating;

^■ Figure 6A shows the interface between a zone of a surface pre-treated by process according to the present invention, in which thin copper films are visible, coated and embedded on the surface of a plastics material (polyamide: PA66), and a zone of the same surface, untreated;

^■ Figure 6B shows thin copper films coated and embedded on a plastics material (polyamide: P66) surface after having been subjected for 1 min to a pre-treatment step. Referring to the above-mentioned figures, on the basis to a preferred embodiment of the process according to the invention, such a process envisages the electrostatic painting, in particular of products having non- conductive material surfaces, by means of an apparatus globally designated by 1, comprising: a device 2 for the pre-treatment, apt to coat on said surfaces a conductive material powder; a coating device 3 for electrostatic coating of paints; means 4 for drying the paint products; and means 5 for moving the products to be painted, as it is shown specifically in Figure 1. Referring to Figures 1 and 3, the non-conductive material surface to be painted, e.g. of plastics material, is subjected, by said device 2 for the pre- treatment, to a preliminary step of coating of a conductive material in form of powder, with a predetermined average grain size.

Preferably, said conductive material is Copper powder having average grain size ranging from 160 to 200 μm, preferably of about 178 μm. As it is shown in detail in Figures 2 and 3, in the present embodiment said device 2 for the pre-treatment is a fluid-bed one, comprising: at least one fluidizing column 21 for said conductive material powder; compressing means 22 for inletting air into said fluidizing column; means 23 for positioning said products to be treated, and powder recovery means 24 for recovering the powders of said conductive material. Specifically, said fluidizing column 21 comprises at least a first 211 and a second 212 chamber partitioned by a porous septum 213 apt to allow air flow transit from said first 211 to said second chamber 212.

In particular, said porous septum may be made of sintered bronze. In the present embodiment the first chamber 211, also called homogenizing chamber, is arranged below said second chamber 212, also called fluidizing chamber. The homogenizing chamber 211 is connected and in communication with said compressing means 22 for air feeding.

The fluidizing chamber 212 is arranged in advance by placing therein a conductive material powder, preferably said Cu powder, to form initially a fixed bed. The conductive material powder fills said fluidizing chamber 212 up to a predetermined level, generally corresponding to half-column.

The powders recovering means 24 comprises at least one hood 241 and at least one collecting filter 242. Said hood 241 is generally arranged above said fluidizing chamber 212 so as to collect and recover the powders escaping therefrom during the pre-treatment. As it is shown in Figure 3, said positioning means 23 work between said second chamber 212 and said hood 241.

As mentioned in the foregoing, the electrostatic painting process underlying the present invention comprises a pre-treatment, in particular for non- conductive material surfaces. In the present embodiment, given by way of example and not for limitative purposes, such a pre-treatment is carried out in said fluidizing column 21, whose fluidizing chamber 212 is filled with Cu powder (fixed bed) up to a predetermined level. When the device 2 for the pre-treatment is activated, the compressing means 22 for air feeding begin working, sending air into the homogenizing chamber 211.

Then, the homogenizing chamber conveys air to the fluidizing chamber 212 thereabove. The conveyed air provides a thrust apt to counterbalance the weight of the fixed bed of Copper, thereby forming a fluid bed, i.e. an air- conductive material powder emulsion.

A sample of non-conductive material, e.g. of plastics material, carried into the fluidizing chamber 212 by said positioning means 23, is subjected to a

high number of impacts with the copper powder. Said impacts result in the coating (deposition) and growth of a thin copper layer (film), owing to an embedding phenomenon (inclusion) into the plastics material surface. Through this pre-treatment, the surface changes its electrical properties so as to be suitable for the subsequent step of painting by electrostatic coating, carried out in a conventional manner.

Both the devices and the processes for electrostatic painting and any implementation thereof are known to a person skilled in the art, therefore a further description thereof will be omitted. Hereinafter there will be specifically described the parameters and processes intervening in the pre-treatment step associated to the process underlying the present invention.

During the pre-treatment, the particles forming the conductive material powder (Cu in the example) impact against the surface of non-conductive material (plastics material in the example) at a predetermined speed such as to allow their fragmenting and coating, substantially by inclusion on the non-conductive surface itself.

Preferably, said predetermined speed is less than or equal to 5 m/s. Moreover, the pre-treatment is carried out at a predetermined temperature such as to foster inclusion of particles of said conductive material powder on the surface.

As it is shown in Figure 3, in the case at issue, in which the conductive material powder is copper powder and the non-conductive surface is of plastics material (PA66), the surface is subjected to the pre-treatment for a time preferably ranging from about 40 to 70 seconds.

In fact, in Figure 4 there can be seen the pattern of the Cu mass growth on a polyamide (PA66) surface as a function of time of exposition to the pre- treatment. As it may be observed, in the first seconds of the pre-treatment there is a rapid increase of the Copper deposited onto the plastics material substrate. After about 60 s, it is reached a sort of asymptotic level in the amount of deposited Cu.

When it is desirable to expose the substrate to a longer pre-treatment, there may be provided only small increases of the deposited mass. This behaviour is explained since during the first part of the pre-treatment the plastics material film is directly subjected to Cu powder impacts. Since Cu powders are much harder than the plastics material substrate, and are projected thereon at a sufficiently high speed (about 5 m/s), rapid Cu film growth is attained.

Subsequently to this step, Cu powders impacting onto the substrate meet a surface having different properties, in which the already coated Cu film forms a sort of barrier against the coating of further Cu slivers, said film being much harder than the original plastics material. The pre-treated surface exhibits no wear nor damage and the Cu film coated thereon is homogeneous, i.e. dense and compact, as it is shown in Figures 6A and 6B, exhibiting thickness differences generally lower than or equal to about 3 μn.

With reference to the same example, in Figure 5 there are shown surface resistivity measuring of the plastics material surface after the pre-treatment. As it may be observed, surface resisitivity values comprised in the range of about 2- 10 ⁵ to 7- 10 ⁵ ohm/cm have been measured. Therefore, it will be evident that the hereto-described pre-treatment provides to non-conductive material surfaces surface resistivity values such as to allow, during the electrostatic painting step, the electrical discharging of powders after their impact onto the sample, making it possible for other powder to come and deposit onto the plastics material substrate. Moreover, it will be understood that the present invention is susceptible of several embodiments alternative to the hereto-described ones, some of which will briefly be illustrated hereinafter with reference to the sole aspects differentiating it from the hereto-considered first embodiment. According to an alternative embodiment, the device for the pre-treatment may be of spray gun type instead of fluid-bed type.

In this case the device comprises a chamber apt to contain the conductive material powders, a feed hopper, a gun apt to project said powders onto the surface to be treated and related control means.

In this device, powders move from the feed hopper to the tip of the gun thanks to the aerodynamic thrust of an air flow under moderate pressure, preferably ranging from about 8 to 10 bar. Then, the powders are projected toward the surface (substrate) for a short time interval, preferably of about 60 s per each sample face, and then hit the plastics surface with a moderate impact speed, preferably lower than or equal to about 5 m/s. Thus, Cu powders embedding into the plastics substrate is carried out. Following the impact, powders bounce back, are captured by the powder recovery system and fed into the hopper by the latter.

It will be appreciated that the powder recovery means, both in the fluid-bed and in the spray gun device for the pre-treatment, allow continuity of the process subj ect-matter of the present invention.

From the detailed description it will be evident that the apparatus for the electrostatic painting may be completely automated and sized depending on the product to be painted, at reasonable costs. The present invention has been hereto described with reference to preferred embodiments thereof. It is understood that other embodiments might exist, all falling within the concept of the same invention, and all comprised within the protective scope of the claims hereinafter.