WO2010016075A1

WO2010016075A1 - Ultraviolet/thermal mixed painting composition (dual cure) to be used as a pre-metalli zation primer under vacuum

Info

Publication number: WO2010016075A1
Application number: PCT/IT2008/000538
Authority: WO
Inventors: Schiava Oscar Della
Original assignee: Quantum Technologies S.R.L.
Priority date: 2008-08-07
Filing date: 2008-08-07
Publication date: 2010-02-11

Abstract

The present invention concerns a single -component UV transparent primer paint with consecutive mixed UV and oven cross -linking, comprising: a) at least one acrylic resin crosslinkable through the combined action of UV radiation and heat; b) one or more acrylic resins crosslinkable by means of the sole exposure to UV radiation; c) at least one acrylic monomer crosslinkable through exposure to UV radiation; d) one or more photoinitiators; e) one or more additives; f) one or more solvents; g) one or more adhesion promoters; h) at least one catalyst. The invention also concerns the application and cross- linking process of the primer paint on highly three- dimensional plastic or metal parts.

Description

DESCRIPTION

ULTRAVIOLET/THERMAL MIXED PAINTING COMPOSITION (DUAL CURE) TO BE USED AS A PRE-METALLIZATION PRIMER UNDER VACUUM

The present invention refers to a single-component UV transparent paint to be used as a primer on plastic and metal supports to be subsequently subjected to metallization processes under vacuum.

The present invention also concerns an application method of such pre-metallization UV primer which allows greater transfer efficiencies and improvement of the final aspect of the film with regard to extension.

Vacuum metallization techniques have seen an increasingly important development in recent years especially regarding the need for replacing electrolytic bath treatments or galvanic treatments, which present technical and environmental disadvantages that can no longer be overlooked. One just has to think that in the case of chromium-plated finishes, used as a protective layer against corrosion and also for decorative purposes, chemical baths used hexavalent chromium, a product classified as carcinogenic. The replacement of the latter with trivalent chromium has not however satisfactorily solved the technical and environmental problems. In order to solve these problems both from a technical and an environmental standpoint, various vacuum metallization techniques have been developed, which provide for the conversion of a solid material into vapor through thermal evaporation, through cathode arc (PVD) or through ion bombardment (sputtering) . The evaporation process must generally be carried out in a so-called "bell jar" in which controlled vacuum conditions are reached (ICT³ ÷ 10^"4 mbar) in order to avoid interactions between the metal vapors and impurities contained in the air. The material to be evaporated or target may be of different types depending on the final effect that one wants to obtain, targets in aluminum, chromium, nickel, etc. are particularly used. The target, once in the vapor form, is then deposited by condensation on the samples previously introduced in the bell jar by forming a thin metal layer endowed with a high brightness and completely similar to a galvanic treatment .

In combination with the vacuum deposition techniques, a whole range of painting products or primers came into being, to be applied to the various articles to be metalized before their introduction into the bell jar. The main function of these products is to serve as an interspace between the substrate's surface and the metal layer applied by evaporation. Such interspace, being a polymer-nature layer, will provide for leveling possible surface defects of the substrates by forming a homogeneous layer which, after metallization, will certainly see its brightness properties improve. Another important function carried out by the primer is to improve the adhesive properties of the layer deposited by evaporation. Last but not least, the primer, once cross- linked, will have the important function of serving as a barrier for every possible degassing, in vacuum step, of the support to which it was applied, as the occurrence of such phenomenon leads to the appearance of metal surfaces with an opaque and iridescent aspect.

A whole range of primers for vacuum metallization are already commercially available and are covered by patent, either of conventional or ultraviolet cross- linking type.

Conventional or thermal-type cross-linking primers were the first to become widespread, especially for their close relationship with conventional baked paints. Their widespread diffusion has however run into obvious environmental and process problems. The high content of solvents used for their application, especially of the heavy aromatic type, is actually quite unavoidable. It should then be added that the need for having a particularly hard layer, in order to favor the mechanical and chemical performance of the layer deposited by evaporation, has led to the use of dual-component products to be mixed before use, with the unavoidable handling of hazardous substances from the environmental and operator health standpoint. Cross-linking times are then extremely long as, even if the product is cross- linked in oven for times that range from thirty to sixty minutes, it is necessary to wait for at least twenty-four hours to be able to introduce the article thus primerized into the bell jar. Its introduction immediately after the oven baking process would actually give rise to degassing processes by the polymer film itself once a vacuum has been formed in the bell jar, with the known consequences of having to discard the parts thus obtained.

In order to meet the increasingly pressing needs for higher mechanical performance, productivity and low or non-existent solvent content, various types of products have been developed.

One of these development trends, from acrylic-nature raw materials, has led to the creation of acrylic- radical-nature primers or photo-crosslinkable products through Ultra Violet radiation.

From a polymer standpoint, the raw materials used in a coating crosslinkable by means of UV radiation (or photo-crosslinkable coating) allow, following a correct photo-exposure of the polymer film, instantaneously reaching particularly high mechanical surface performances. Such performance is however matched by quite high limits from an industrial application standpoint which results, mainly by limits due to the size of the parts, in a certain lack of flexibility of the pure Ultra Violet cross-linking products.

The very nature of such products actually provides that, once applied, the beginning of catalysis (and as a result its propagation) may efficiently occur only if the thus painted surface is uniformly hit by an adequate Ultra Violet radiation. The latter will actually have to possess a certain wavelength (or, conveniently, comprise a wavelength interval) of suitable intensity and hit the polymer film for a certain time period. The three- dimensional surfaces of particularly complex geometry will therefore create an obstacle to the uniform distribution of the UV radiation.

The impossibility for the Ultra Violet radiation to uniformly and adequately reach the painted surface leads to the simultaneous presence of areas of optimal cross- linking and others with incomplete cross-linking. The latter, as logical in a product whose cross-linking occurs only after exposure to an adequate UV source, not having been correctly exposed will exhibit an imperfectly cross-linked surface and therefore will have a performance which is lower than the required standards . The non-irradiated surfaces are still wet and sticky. All this then results in rather marked phenomena of degassing in the bell jar, also by this primer type. The problem addressed by the present invention is to make a primer formulation available which, while maintaining all the advantages inherent to a UV formulation, allows for obtaining a uniform cross-linking over the entire surface of the manufactured article . The problem discussed above is solved by a dual cure UV primer, by a painting method and by a UV and heat (or IR radiation) mixed cross-linking procedure as outlined in the attached claims, whose definitions are an integral part of the present description. It has been seen that the possibility of completing the cross-linking, even via thermal treatment, of those areas in which the applied polymer film has not been adequately cross-linked by the sole UV radiation, ensures that the subsequent under vacuum operations are not invalidated by possible degassings of the areas not completely cross-linked.

In a first aspect, the present invention therefore concerns a dual cure UV transparent paint comprising: a) at least one acrylic resin crosslinkable through the combined action of the UV radiation and heat; b) one or more acrylic resins crosslinkable by means of the sole exposure to UV radiation; c) at least one acrylic monomer crosslinkable through exposure to UV radiation; d) one or more photoinitiators; e) one or more additives; f) one or more solvents; g) one or more adhesion promoters h) at least one catalyst. The dual cure UV primer of the present invention is preferably sprayed through electrostatic cups on highly three-dimensional metal or plastic parts obtaining, after post-baking in oven, a painting product which is completely and uniformly cross-linked, as explained hereinafter.

Acrylic resin crosslinkable with UV/heat The acrylic resin crosslinkable by means of the combined action of the heat and UV radiation is an oligomer acryl acrylic resin, preferably diluted with 45% of the butyl acetate diluent, and endowed with a viscosity preferably comprised between 2800 and 3200 mPa.s, at 25°C (measured according to method ASTM D 2196). A commercial example of this oligomer is CYTECS EBECRYL^® 1200. In the formulation of the invention, this resin is comprised between 2 and 20% by weight, and it is preferable to use it in percentages comprised between 8 and 12% by weight.

Such resin possesses hydroxyl terminals capable of, in the presence of suitable catalysts, reacting by action of the heat as conventional thermo-crosslinkable resins. Such resin actually possesses both -OH (hydroxyl) terminals crosslinkable by action of polyisocyanates and double unsaturated bonds (C=C) crosslinkable via UV- formed radicals. It further gives the coating properties of excellent hardness, flexibility and excellent resistance to chemical agents.

UV crosslinkable acrylic resin

One of the acrylic resins crosslinkable by means of the sole exposure to UV radiation is an oligomer urethane acrylic resin of aliphatic type, preferably endowed with a molecular weight comprised between 700 and 1500, more preferably between 900 and 1100, and endowed with a viscosity advantageously comprised between 1800 and 2200 mPa.s, at 60⁰C (measured according to method ASTM D 2196) . It has further been found that the urethane functionality of this oligomer may be equal to one or more, even if the preferred functionality is equal to 6.

A commercial example of the described oligomer is CYTECS EBECRYL^® 1290; Bencryl 655 of Benasedo S.p.A. is alternatively used. In the formulation of the invention, this compound may be comprised between 2 and 10% by weight, it is preferable to use it in percentages comprised between 4 and 8% by weight.

Such oligomer gives the coating the properties of desired hardness, high resistance to solvents and brightness .

One of the other acrylic resins crosslinkable by means of the sole exposure to UV radiation is an oligomer epoxy acrylic resin of aliphatic type, preferably endowed with a molecular weight comprised between 400 and 1200, more preferably between 700 and 900, and endowed with a viscosity advantageously comprised between 800 and 1200 mPa.s, at 25°C (measured according to method ASTM D

2196) . It has further been found that the epoxide functionality of this oligomer may be equal to one or more, even if the preferred functionality is equal to 2.

A commercial example of the described oligomer is CRAY VALLEY'S CRAYNOR^® 132. In formulation, this compound may be comprised between 4 and 16% by weight; it is preferable to use it in percentages comprised between 6 and 12% by weight.

Such oligomer gives the coating the desired flexibility, high reactivity and brightness properties.

One of the subsequent acrylic resins crosslinkable by means of the sole exposure to UV radiation is an oligomer urethane acrylic resin of aliphatic type, preferably endowed with a molecular weight comprised between 5200 and 6000, more preferably between 5400 and 5800, and endowed with a viscosity advantageously comprised between 23 and 27 Pa. s, at 50⁰C (measured according to method ASTM D 2196) . It has further been found that the urethane functionality of this oligomer may be equal to one or more, even if the preferred functionality is equal to 2.

A commercial example of the described oligomer is CRAY VALLEY'S CRAYNOR^® 965.

In formulation, this compound may be comprised between 1 and 10% by weight, it is preferable to use it in percentages comprised between 2 and 6% by weight.

Such oligomer gives the coating the desired flexibility and brightness properties.

One of the acrylic resins crosslinkable by means of the sole exposure to UV radiation is made up of an oligomer urethane acrylic resin of aliphatic type, preferably endowed with a molecular weight comprised between 8800 and 9700, more preferably between 9100 and 9500, and endowed with a viscosity advantageously comprised between 21 and 25 Pa. s, at 50⁰C (measured according to method ASTM D 2196) . It has further been found that the urethane functionality of this oligomer may be equal to one or more, even if the preferred functionality is equal to 2. A commercial example of the described oligomer is CRAY VALLEY'S CRAYNOR^® 966H90.

In formulation, this compound may be comprised between 1 and 10% by weight; it is preferable to use it in percentages comprised between 2 and 6% by weight. Such oligomer gives the coating the desired flexibility and brightness properties. Acrylic monomer

The acrylic monomer crosslinkable through exposure to UV radiation is a multifunctional acrylic monomer, preferably with a molecular weight comprised between 300 and 700, preferably between 400 and 60.0, and endowed with a viscosity advantageously comprised between 4 and 10 Pa. s, at 25⁰C (measured according to method ASTM D 2196) . It has further been found that the acrylic functionality of this monomer may be equal to one or more, even if the preferred functionality is equal to 5.

A commercial example of the described monomer is CRAY VALLEY'S SARTOMER^® 399 LV (dipentaerythritol penta acrylate) . In formulation, this compound may be comprised between 2 and 15% by weight; it is preferable to use it in percentages comprised between 5 and 9% by weight.

It is inserted in formulation to give the coating the properties of high reactivity, increase of cross- linkage density, excellent hardness and for the presence of free hydroxyl groups (-OH) crosslinkable by action of polyisocyanates .

Photoinitiator agent

The photoinitiator agent of the dual cure UV primer of the invention carries out an essential function to obtain a rapid and efficient cross-linking of the components of the composition once exposed to the adequate Ultra Violet radiation.

Suitable photoinitiators comprise UV radiation photosensitive compounds like free radical sources such as, for example: hydroxyketones , aminoketones and ketosulphones , benzyldimethylketal , benzophenones , acylphosphines , bisacylphosphines and thioxantones .

It has further been found that the presence of several photoinitiators in the formulation of the dual cure UV primer, besides increasing the polymerization speed of the acrylic-based polymeric mixture, determines a balancing of the curing degree of the paint both superficially and in depth. According to a preferred embodiment, a mixture of two photoinitiators conveniently made up of a bisacylphosphine oxide and an alpha-hydroxy alkyl phenyl ketone is used.

For example, a particularly suitable mixture of photoinitiators is made up of a mixture of a bisacylphosphine oxide of the type IRGACURE^® 819 of Ciba Speciality Chemicals, conveniently present in a quantity comprised between 0.5 and 4% by weight, preferably between 1.5 and 2.5% by weight and an alpha-hydroxy alkyl phenyl ketone of the type DAROCUR^® 1173 of Ciba Speciality Chemicals, conveniently present in a quantity comprised between 2 and 6% by weight, preferably between 3.5 and 5.5% by weight.

The photoinitiators are typically present as a mixture in the additive of the invention in a quantity varying between 5 and 10 % by weight. Additives

The dual cure UV primer of the invention also comprises one or more silicone-type additives to increase its properties of extension and wettability of the substrate, and one or more additives to safeguard the conservation of the painting product, still in liquid phase, during the storage period.

Suitable silicone additives comprise siloxane copolymers, polydimethylsiloxane polyethers or modified polydimethylsiloxane polyethers with acrylic functionality or hydroxyl functionality.

It has further been found that the simultaneous presence of polydimethylsiloxane polyethers modified and not determines the right degree of balancing between the properties of wettability of the substrate and those of good superficial leveling of the polymeric film. The union of these properties ensures that the layer of applied dual cure primer is free of surface defects (craters, orange skin, etc.), having, with a uniform and bright surface, a positive influence on the subsequent vacuum metallization process.

A particularly suitable mixture of the two above- mentioned compounds has been the union of a polyether in solution with polydimethylsiloxane modification of the type BYK^® 306 of BYK-CHEMIE GmbH, conveniently present in a quantity comprised between 0.2 and 1.2% by weight, preferably between 0.4 and 0.8% by weight, and a polyether/polyester copolymer in solution with polydimethylsiloxane modification and functional hydroxyl groups of the type BYK^® 375 of BYK-CHEMIE GmbH, conveniently present in a quantity comprised between 0.2 and 1.0% by weight, preferably between 0.4 and 0.6% by weight . The additives are typically present as a mixture in the dual cure UV primer in a quantity varying between 0.5% and 1.5% % by weight.

The second type of additives comprised in the dual cure UV primer of the present invention concerns an additive capable of safeguarding the stability of -the product still in liquid phase during the storage period. The nature of the various raw materials used in the invention, especially the various types of acrylic resins, requires conservation at temperatures comprised between +5 and +35⁰C. In case such temperatures are exceeded for long periods of time, problems of block gelation of the preparation may be encountered, if not even the complete solidification inside the storage container . An additive particularly suitable for the resolution of such phenomena has been the polymerization inhibitor of the type Genomer^® 20 of RAHN AG, conveniently present in a quantity comprised between 0.05% and 0.7% by weight, preferably between 0.1 and 0.5% by weight. Solvents The dual cure UV primer of the invention also comprises one or more solvents in order to reduce the viscosity of the preparation and facilitate the application process. Suitable solvents are represented by alcohols (Ethyl Alcohol, Isopropyl Alcohol, Isobutyl Alcohol, etc.) , esters (Isobutyl Acetate, Ethyl Acetate, Ethόxypropylacetate , etc . ) and glycol ethers (Methoxy Propanol Acetate, 2-Butoxyethanol, etc.) .

According to another aspect of the present invention, the mixture of solvents was selected in order to obtain a total formulation flash point greater than 30-35⁰C. Such data is important for the application of the dual cure UV primer with electrostatic cups: if, indeed, the flash point of the applied composition were lower, serious fire hazards inside the application booths would be encountered, the cups being subjected to an electric voltage gradient .

According to an experimentally tested embodiment, a mixture of two solvents conveniently made up of an alcohol and a glycol ether is used.

For example, a particularly suitable mixture of solvents is made up of a mixture of isobutyl alcohol, conveniently present in a quantity comprised between 25 and 40% by weight, preferably between 30 and 35% by weight, and a 2-butoxyethanol type glycol ether, conveniently present in a quantity comprised between 0.5 and 10% by weight, preferably between 2 and 5% by weight. The solvents are typically present as a mixture in the dual cure UV primer of the invention in a quantity varying between 30 and 40.5% by weight. Adhesion promoters

The dual cure UV primer of the present invention also comprises one or more adhesion promoters to improve its adhesive properties to the metal or plastic support onto which it was applied. The adhesive properties, in the case of a UV formulation, are sometimes weaker compared to a conventional product either for the different nature of the resins used or for the use of less aggressive but more manageable solvents . Suitable adhesion promoters to overcome such disadvantage are resins of acrylic or methacrylic origin with acid modification or tetramercaptan derivatives .

It has further been found that the combination of the two families, acid resins and tetramercaptan derivatives, in the formulation of the dual cure UV primer, offers a guarantee of adhesion both for metal substrates and for plastic ones.

A particularly suitable mixture of the two above- mentioned compounds has been the union of a methacrylate resin with acid modification of the type EBECRYL^® 171 of CYTEC, conveniently present in a quantity comprised between 0.5 and 10% by weight, preferably between 2 and 6% by weight, and a tetramercaptan derivative of the type EBECRYL^® 375 of CYTEC, conveniently present in a quantity comprised between 0.5 and 5% by weight, preferably between 1 and 4% by weight. Catalyst

The catalyst of the dual cure UV primer of the invention carries out an essential function for obtaining a rapid and efficient cross-linking of the components of the composition endowed with a double thermo- and photo- crosslinkable nature, which in the areas not adequately exposed to the UV radiation, thanks to the presence of hydroxyl groups, will give rise, once exposed to an adequate source of thermal energy, to the formation of molecular bonds through condensation reaction with the - NCO terminals of the polyisocyanate catalysts.

It has further been found that the presence of polyisocyanate catalysts of the blocked type, namely capable of starting the condensation reaction only above certain temperatures, allows for their incorporation directly into the preparation steps of the formulation composition by avoiding subsequent additions prior to in situ application operations. Such prerogative constitutes an advantage either from the standpoint of avoiding handling hazardous substances in the application environment, or avoiding the use of automatic compounding and dosage machines, which entail a significant extra cost. Depending on the post-baking temperatures to which the painted substrates may be subjected, it is also possible to choose a polyisocyanate with a certain unblocking temperature .

It has further been found that the use of polyisocyanates, in the formulation of the dual cure UV primer, besides the thermal cross-linking of the polymeric film underexposed to the UV radiation, leads to an increase of the properties of adhesion to the support, either metal or plastic, and to an increase of general flexibility which constitutes an advantage in case of need for processing following the vacuum metallization (folding, etc. ) .

Suitable- catalysts comprise polyisocyanate compounds, blocked or not, of aromatic or aliphatic nature, of the type: toluene diisocyanate (TDI) , diphenylmethane diisocyanates (MDI) and hexamethylene diisocyanate (HDI) .

According to a preferred embodiment, for a metal substrate, a blocked aliphatic polyisocyanate based on HDI of the type Desmodur^® BL 3175 SN of BAYER MATERIAL SCIENCE AG is used, conveniently present in a quantity comprised between 4 and 12 % by weight, preferably between 6 and 10 % by weight and, for a plastic substrate, a blocked aromatic polyisocyanate based on TDI of the type Desmodur^® BL 1265 MPA/X of BAYER MATERIAL SCIENCE AG, conveniently present in a quantity comprised between 4 and 12 % by weight, preferably between 6 and 10 % by weight .

The dual cure UV formulation of the invention is applied as a pre-metallization primer on a metal or plastic substrate. The metallization primers with only UV- formed radicals, as already explained, although now widespread, exhibit high limits from the standpoint of the cross-linking where the shape and the dimensions of the object to be cross-linked become complex. The present invention addresses and solves this problem from a different standpoint compared to the prior art.

Indeed, ^• the UV-formed radical formulations are compositions whose chemical nature is based only on the presence of oligomers and monomers endowed with C=C double bonds, namely capable of reacting, through photoinitiators, only when exposed to an adequate source of UV radiation.

That entails that their application is aimed at those manufactured articles which, by shape and size, may be uniformly exposed to a source of Ultra Violet radiation. In the case of manufactured articles with complex geometries, even if there were no problems at the application level reaching and covering in the same way the surface of the manufactured article with a primer layer, the complication might stem, in UV cross-linking step, from reaching the areas "in the shade" with an adequate cross-linking energy. The latter is a fundamental parameter for the subsequent vacuum metallization operation as the "under-cross-linked" or even non-exposed areas will be subjected to negative pressure values up to 10^"3 ÷ 10^"4 mbar. Under these conditions, the components of the formulation not adequately incorporated into the polymeric network would give rise to a phenomenon called "degassing" of the film. The phenomenon just described mostly results in damage to the extraction pumps for the creation of the vacuum, which suffer • irreversible damage as time goes by, but immediately results in the obtaining of a metal film with a hardly bright aspect and with iridescent lines. By providing a dual cure UV formulation specific to the use as a pre-metallization primer, the present invention allows, by exploiting of all the performance and economic advantages of the UV technology, extending the number of possible applications, without problems related to the conformation of the substrates . That is possible as the present invention comprises an acrylic resin having functionalities that cross-link by means of ultraviolet radiation, namely double unsaturated bonds, and functionalities that thermally cross-link, namely hydroxyl terminals. The application of an adequate UV radiation ensures that the main components of the UV primer, namely the various acrylic resins and the acrylic monomer which possess only functionalities crosslinkable by means of UV, namely double bonds, react via radicals with the double bonds of the resin having double functionality. Such resin, thanks to the hydroxyl groups, once the primer is subjected to an adequate thermal energy, will be capable of reacting through condensation with the -NCO groups of the polyisocyanates unblocked as a result of the heat.

In a further embodiment of the invention, the primer may contain various polyisocyanate groups depending on the type of final support to which it will be applied. In particular for the thermoplastic-nature supports, the type of polyisocyanate used will have an "unblocking" temperature comprised between 70 and 80 ⁰C, such supports, especially at relatively low thicknesses (a few mm) , not being able to withstand higher temperatures without deformations and morphologic-nature changes. For the metal-type supports, it is possible to use polyisocyanates capable of being unblocked at temperatures between 110 and 180⁰C, significantly- reducing the post-baking time. In particular, times will range from 20-30 minutes for the polyisocyanates that can be unblocked at 70-80⁰C, to 10÷15 min for the polyisocyanates that can be unblocked between 120 and 180⁰C.

The end user will, however, have at disposal a finished and ready for use formulation without the need for possible additions before the application process. Painting process

According to another aspect of the present invention, a process is provided for the painting of a metal or plastic substrate, comprising the following passages: a) Degreasing the substrate; b) Applying the primer through various transfer techniques; c) Evaporating the residual solvent; d) Cross-linking through adequate UV radiation; e) Completing the cross-linking of the film through thermal post-baking.

In passage a) , substrate means, depending on the cases, a thermoplastic-nature support, namely made up of one or more resins whose polymeric nature is of thermoplastic type. Examples of a few materials taken into consideration by the following invention will be:

Acrylonitrile Butadiene Styrene (ABS) , Polycarbonate

(PC) , Polyamide (PA) , Polypropylene (PP) , Polyvinyl Chloride (PVC) and copolymers of the ABS-PC or PA-ABS type. In the same way, metal-type substrate will be referred to when the article onto which the invention will be applied is made up of a metal or metal alloy, preferably steel, aluminum, copper and their possible alloys.

Depending on the substrate, in the case of a thermoplastic-nature surface, degreasing with adequate organic solvent (isopropyl alcohol, ethyl alcohol, etc.) will be carried out in order to remove possible particles from the surface while, in the case of a metal surface, the manufactured article will be subjected to acid degreasing with subsequent rinsing cycles with normal and demineralized water, followed by a drying process in oven at temperatures higher than 110÷120°C in order to eliminate possible residual water traces.

In passage b) , the application step of the invention may take place by following a few known transfer techniques, such as, for example, conventional spraying, electrostatic spraying, flow coating, rain and spreading. Advantageously, the transfer technique of the dual cure UV primer especially on parts with a complex geometry has been, once adjusted the properties of the painting product, that with the use of electrostatic cups. The latter, indeed, unite two fundamental principles for high transfer efficiency painting or, in other words, centrifugal force and electrostatic attraction. In electrostatic painting, "the electrostatic effect" is obtained by taking advantage of the physical principle of ionization. An electrode placed on the cup at the outlet of the paint creates a negative current potential (0÷130 kv max) which gives rise to an electric field with field lines converging towards the earthed part to be painted. The atomized paint, by passing through the force field, acquires the potential (it is charged negatively, namely it is ionized) , being thus attracted by the substrate.

Moreover, because equal charges mutually repel each other

(Coulomb's La-w), the atomization of the paint spray is considerably improved, giving the part a superior finish.

The action of the cup that rotates at speeds comprised between . 15,000 and 30,000 rpm permits an optimal atomization of the paint without the use of compressed air and allows a product flow without excessive dispersions in the ambient air by being conveyed towards the part through the product cone generated by the centrifugal action. Thanks to the combination of these two principles, with this application system it is possible to obtain a high transfer efficiency of the paint applied on the manufactured article which may reach 90% and which allows for obtaining a substantial paint saving with lower environmental pollution and higher execution speed.

Advantageously, step c) of residual solvent evaporation takes place in a ventilated oven at temperatures comprised, regardless of the type of support, between 50 and 80⁰C for a time period comprised between 5 and 10 minutes. In a further embodiment of the invention, the solvent evaporation step may take place through the use of IR lamps with powers varying between 1000 and 2000 Watt for a time period varying between 2 and 5 minutes.

In cross-linkage passage d) , mercury vapor arc lamps and electrodeless microwave lamps are preferably used. The UV cross-linking technique uses a radiation emission source comprised between the wavelengths of 200 and 450 nm. The energy necessary to the complete cross-linking of the exposed area (also said DOSE and measured in mj/cm²) will have to be comprised between 2 and 6 J/cm² preferably between 3 and 4 J/cm², in the emission portion of the ultraviolet spectrum of the UVA band (Region of the UV-A spectrum: between 320 and 390 nm; Region of the UV-B spectrum: between 280 and 320 nm; Region of the UV-C spectrum: between 250 and 260 nm; Region of the UV-Vis spectrum: between 395 and 445 nm) .

Advantageously, post-baking step e) takes place in an oven, according to the cycle of the blocked polyisocyanate catalyst. As previously described, for thermoplastic substrates on which, given their nature, compositions containing polyisocyanate catalysts with lower unblocking temperatures are applied, post-baking conditions are obtained with times from 20÷30 min and temperatures comprised between 70 and 80 ⁰C, while for metal substrates, on which compositions containing polyisocyanate catalysts with higher unblocking temperatures are applied, post-baking conditions are obtained with times from 10÷15 min and temperatures comprised between 120 and 180 ⁰C.

The process according to the invention allows achieving a uniformly cross-linked and optimally spread coating, which finds a specific use indication in the sector of primerization systems for the subsequent application of metallizing coatings through the vacuum vapor deposition process, which constitutes a further object of the invention.

The following examples are provided for merely illustrative purpose of the present invention and must therefore not be intended as limiting of the protection scope, as defined by the attached claims.

EXAMPLE 1. PRIMER FORMULATION APPLICATION ON METAL SUBSTRATE

BAC = butyl acetate EXAMPLE 2 . PRIMER FORMULATION APPLICATION ON PLASTIC SUBSTRATE

Claims

1. Dual cure UV transparent primer paint comprising: a) at least one acrylic resin crosslinkable through the combined action of UV radiation and heat; b) one or more acrylic resins crosslinkable by means of the sole exposure to UV radiation; c) at least one acrylic monomer crosslinkable through exposure to UV radiation; d) one or more photoinitiators; e) one or more additives; f) one or more solvents; g) one or more adhesion promoters h) at least one catalyst .

2. Primer paint according to claim 1, wherein said at least one acrylic resin crosslinkable through the combined action of UV radiation and heat is an oligomer acryl acrylic resin.

3. Primer paint according to claim 2 , wherein said oligomer acryl acrylic resin is diluted with 45% of the butyl acetate diluent and is endowed with a viscosity comprised between 2800 and 3200 mPa.s, at 25⁰C (measured according to method ASTM D 2196) .

4. Primer paint according to claim 2 or 3 , wherein said oligomer acryl acrylic resin is comprised between 2 and 20% by weight.

5. Primer paint according to claim 4 , wherein said oligomer acryl acrylic resin is comprised between 8 and 12% by weight.

6. Primer paint according to any one of the claims 1 to 5, wherein said one or more acrylic resins crosslinkable by means of the sole exposure to UV radiation is an oligomer urethane acrylic resin of aliphatic type.

7. Primer paint according to claim 6, wherein said oligomer urethane acrylic resin of aliphatic type has a molecular weight comprised between 700 and 1500, or comprised between 900 and 1100, and is endowed with a viscosity comprised between 1800 and 2200 mPa.s, at 60⁰C (measured according to method ASTM D 2196) , and has a urethane functionality equal to one or more, preferably equal to 6.

8. Primer paint according to claim 6 or 7 , wherein said oligomer urethane acrylic resin of aliphatic type is comprised between 2 and 10% by weight or between 4 and 8% by weight .

9. Primer paint according to any of claims 1 to 5, wherein said one or more acrylic resins crosslinkable by means of the sole exposure to UV radiation is an oligomer epoxy acrylic resin of aliphatic type.

10. Primer paint according to claim 9, wherein said oligomer epoxy acrylic resin of aliphatic type has a molecular weight comprised between 400 and 1200 or comprised between 700 and 900, and is endowed with a viscosity comprised between 800 and 1200 mPa.s, at 25°C (measured according to method ASTM D 2196) , and has an epoxide functionality equal to one or more, preferably equal to 2.

11. Primer paint according to claim 9 or 10, wherein said oligomer epoxy acrylic resin of aliphatic type is comprised between 4 and 16% by weight or between 6 and 12% by weight.

12. Primer paint according to claim 6 , wherein said oligomer urethane acrylic resin of aliphatic type has a molecular weight comprised between 5200 and 6000, or comprised between 5400 and 5800, and is endowed with a viscosity comprised between 23 and 27 Pa. s, at 50⁰C

(measured according to method ASTM D 2196) , and has a urethane functionality equal to one or more, preferably equal to 2.

13. Primer paint according to claim 12, wherein said oligomer urethane acrylic resin of aliphatic type is comprised between 1 and 10% by weight or between 2 and 6% by weight .

14. Primer paint according to claim 6 , wherein said oligomer urethane acrylic resin of aliphatic type has a molecular weight comprised between 8800 and 9700 or comprised between 9100 and 9500, and is endowed with a viscosity comprised between 21 and 25 Pa. s, at 50⁰C (measured according to method ASTM D 2196) , and has a urethane functionality equal to one or more, preferably equal to 2.

15. Primer paint according to claim 14, wherein said oligomer urethane acrylic resin of aliphatic type is comprised between 1 and 10% by weight or between 2 and 6% by weight .

16. Primer paint according to any of claims 1 to 15, wherein said at least one acrylic monomer crosslinkable through exposure to UV radiation is a multifunctional acrylic monomer, having a molecular weight comprised between 300 and 700 or comprised between 400 and 600, endowed with a viscosity comprised between 4 and 10 Pa. s, at 25°C (measured according to method ASTM D 2196) , and having an acrylic functionality equal to one or more, preferably equal to 5.

17. Primer paint according to claim 16, wherein said multifunctional acrylic monomer is dipentaerythritol penta acrylate .

18. Primer paint according to claim 16 or 17, wherein said multifunctional acrylic monomer is comprised between 2 and 15% by weight or between 5 and 9% by- weight .

19. Primer paint according to any one of the claims 1 to 18, wherein said one or more photoinitiators are chosen among UV radiation photosensitive compounds like free radical sources such as hydroxyketones , aminoketones and ketosulphones, benzyldimethylketal , benzophenones, acylphosphines , bisacylphosphines and thioxantones .

20. Primer paint according to any one of the claims 1 to 19, comprising a mixture of two photoinitiators chosen between a bisacylphosphine oxide and an alpha- hydroxy alkyl phenyl ketone .

21. Primer paint according to claim 20, wherein said bisacylphosphine oxide is present in a quantity comprised between 0.5 and 4% by weight, or between 1.5 and 2.5% by weight, and said alpha-hydroxy alkyl phenyl ketone is present in a quantity comprised between 2 and 6% by weight, or between 3.5 and 5.5% by weight.

22. Primer paint according to any one of the claims 1 to 21, wherein said one or more photoinitiators are present as a mixture in the additive of the invention in a quantity varying between 5 and 10% by weight.

23. Primer paint according to any one of the claims 1 to 22, wherein said one or more additives are chosen between silicone additives of the following type: siloxane copolymers, polydimethylsiloxane polyethers or polydimethylsiloxane polyethers modified with acrylic functionality or with hydroxyl functionality.

24. Primer paint according to any one of the claims 1 to 23, comprising a mixture of a polyether in solution with polydimethylsiloxane modification in a quantity comprised between 0.2 and 1.2% by weight, or between 0.4 and 0.8% by weight, and a polyether/polyester copolymer in solution with polydimethylsiloxane modification and functional hydroxyl groups in a quantity comprised between 0.2 and 1.0% by weight, or between 0.4 and 0.6% by weight .

25. Primer paint according to any one of the claims 1 to 24, wherein said one or more additives are present as a mixture in the dual cure UV primer in a quantity varying between 0.5% and 1.5% by weight.

26. Primer paint according to any one of the claims 1 to 25, wherein said one or more additives comprise a polymerization inhibitor in a quantity comprised between 0.05% and 0.7% by weight, or between 0.1 and 0.5% by weight .

27. Primer paint according to any one of the claims 1 to 26, wherein said one or more solvents are chosen between alcohols such as ethyl alcohol, isopropyl alcohol, isobutyl alcohol, esters such as isobutyl acetate, ethyl acetate, ethoxypropylacetate, and glycol ethers such as methoxy propanol acetate, 2-butoxyethanol.

28. Primer paint according to any one of the claims 1 to 27, comprising a mixture of solvents capable of giving the total formulation a flash point higher than

30-35⁰C, said mixture being made up of an alcohol and a glycol ether .

29. Primer paint according to claim 28, wherein said mixture of solvents is made up of a mixture of isobutyl alcohol in a quantity comprised between 25 and

40% by weight, or- between 30 and 35% by weight, and 2- butoxyethanol , in a quantity comprised between 0.5 and 10% by weight, or between 2 and 5% by weight.

30. Primer paint according to any one of the claims 1 to 29, wherein said one or more solvents are present as a mixture in the dual cure UV primer of the invention in a quantity varying between 30 and 40.5% by weight.

31. Primer paint according to any one of the claims 1 to 30, wherein said one or more adhesion promoters are chosen between resins of acrylic or methacrylic origin with acid modification or tetramercaptan derivatives or their mixtures.

32. Primer paint according to claim 31, wherein said one or more adhesion promoters are made up of a mixture of a methacrylate resin with acid modification, in a quantity comprised between 0.5 and 10% by weight, or between 2 and 6% by weight, and a tetramercaptan derivative in a quantity comprised between 0.5 and 5% by weight, or between 1 and 4% by weight.

33. Primer paint according to any one of the claims 1 to 32, wherein said at least one catalyst is chosen between polyisocyanate compounds, blocked or not blocked, of aromatic or aliphatic nature, of the type: toluene diisocyanate (TDI) , diphenylmethane diisocyanates (MDI) and hexamethylene diisocyanate (HDI) .

34. Primer paint according to claim 33, wherein, for a metal substrate, said catalyst is a blocked aliphatic polyisocyanate based on HDI, that can be unblocked at 120-180⁰C, in a quantity comprised between 4 and 12% by weight, or between 6 and 10% by weight, and, for a plastic substrate, a blocked aromatic polyisocyanate based on TDI, that can be unblocked at 70- 80⁰C, in a quantity comprised between 4 and 12% by weight, or between 6 and 10% by weight.

35. Process for painting a metal or plastic substrate, comprising the following passages: a) Degreasing the substrate; b) Applying the primer paint according to any one of the claims 1 to 34, through a suitable transfer technique ; c) Evaporating the residual solvent; d) Cross-linking through adequate UV radiation; e) Completing the cross-linking of the film through thermal post-baking.

36. Process according to claim 35, wherein:

- said step b) of primer paint application takes place by following a transfer technique chosen between conventional spraying, electrostatic spraying, electrostatic cups, flow coating, rain and spreading; and/or

- said step c) of residual solvent evaporation takes place in a ventilated oven at temperatures comprised between 50 and 80⁰C for a time period comprised between 5 and 10 minutes; or through the use of IR lamps with powers varying between 1000 and 2000 Watt for a time period varying between 2 and 5 minutes; and/or

- said cross-linking step d) takes place by means of mercury vapor arc lamps or electrodeless microwave lamps, with a radiation emission source ^v comprised between the wavelengths of 200 and 450 nm, wherein the energy necessary for complete cross-linking of the exposed area is comprised between 2 and 6 J/cm², or between 3 and 4 j/cm², in the emission portion of the ultraviolet spectrum of the UVA band; and/or - said post-baking step e) takes place in an oven, according to the cycle of the blocked polyisocyanate catalyst .

37. Metallization process of a plastic or metal substrate via vacuum vapor deposition, comprising the coating of said substrate with a primer paint as outlined in any one of the claims 1 to 34 and according to the method outlined in any one of the claims 35 or 36, and the subsequent metallization step of said substrate coated with said primer paint via deposition of metal vapors under vacuum.