Method for making a graphic representation on objects made of a polyolefin polymer and object made according to said method

Abstract: The method for making a graphic representation on objects made of a polyolefin polymer, consists of printing a graphic representation on a face of a first film made of a stretched polymer, chosen among a polyester polymer and an acrylic polymer, associating the first film with a second film in order to obtain a membrane formed by the associated first and second films containing the graphic representation between them, transferring the membrane inside an injection mold and injecting a polyolefin polymer in the mold for forming the object, obtaining at least a partial melting of the second film so that the membrane forms one body with said object.

Description: The present invention relates to a method for making a graphic representation on objects made of a polyolefin polymer, and to the obtained molded objects. As is known, currently, for reproducing a graphic representation on a plastic object, for example on an object made of a polyolefin polymer, such as polyethylene or polypropylene, a film is printed with a known printing system (rotogravure or flexographic printing). After the printing operation, the film is either manually or mechanically introduced into a mold. Subsequently, in a per se known manner, plastics is injected into the mold (polyethylene or polypropylene) for producing the desired object. Drawings, signs and similar, are currently printed by this method on small objects of common use. This method has a considerable disadvantage: the film perfectly integrates in the object on which it is applied only in the regions which are not printed, while on the printed film surface, the film only rests on the object surface because of the inks that constitute a "barrier" to the molecular penetration between the film and the material constituting the object. The above is further complicated by the fact that the film, in order to be printed, requires an adapted flame surface treatment or a high frequency electric discharge (crown effect) surface treatment, before printing with ink. For improving the adhesion of the inks it is also possible to use a hot chemical etching with a highly oxidant solution, such as chromic acid, oleum, and chlorosulphonic acid. As readily understood, this inconvenience causes the graphic representation to detach especially when the printed portion of the film is subjected to an abrasive action or denting. Furthermore, the described method only allows to print small graphic designs on small objects. In fact, the greater is the size of the design, the greater are the chances of its detachment from the object. It is not to be overlooked the fact that the bigger is the design, the more difficult is to arrange it inside the mold, especially if this operation is performed mechanically. In order to obviate to the above inconveniences, another type of transfer is used for transferring a design on a plastic object. To this purpose, by means of a calender for thermal printing, the colors are sublimated from the paper on the surface of the object to be printed. This system allows to print a design only on small objects and on substantially plane surfaces. It is also so expensive that its use is limited to selected and limited cases. FR 2150 209 discloses a method to incorporate a graphic representation into an object made of a termoplastic polimer. The incorporation is carried out directly during hot moulding avoiding the formation of bubbles during moulding. European Patent application No. 94108168.9 discloses the use of an external polypropylene film, a graphic representation printed between the external film and an internal film such that the internal film partially melts during injection molding of an object on the film. It has been found however, that this system has a limited duration, namely the adhesion between the layers diminishes after a while. In this situation the technical aim of the present invention is to obviate to the above inconveniences. Within this technical aim, an important object of the invention is to devise a method for making a graphic representation on objects made of a polyolefin polymer, wherein the graphic representation (printed and unprinted portions) is totally integrated with the material of the object on which the design has to be printed by an effective molecular integration. A further important object of the invention is to devise a method for making a graphic representation on objects made of a polyolefin polymer, which are extremely economical and which ensure a long durability of the representation. A further object of the invention is to provide a method for making a graphic representation on objects made of a polyolefin polymer, which can be used, without any problem, also for large graphic representations adapted to be transferred on, for example, toys, outdoor seats and tables, etc. A further object of the invention is to provide a method and an apparatus for making a graphic representation on objects made of a polyolefin polymer, which can be used in existing productions lines without their substantial modifications. A further object of the invention is to obtain four-colour process prints of the best quality through successive printing passages with different colouring agents. Not least object of the invention is to provide a method and an apparatus for making a graphic representation on objects made of a polyolefin polymer, which allow an easy and durable application on an object, for example made of polypropylene, in order to endure thermic, mechanical and physical actions without spoiling even after a long time. The above technical aim and objects are substantially achieved by a method for making a graphic representation on objects made of a polyolefin polymer, according to claim 1. Preferably, said acrylic polymer is a polyalkyl methacrylate, more preferably is a polymethil methacrylate or a polydimethyl methacrylate. Said method comprises the use of grip means for transferring said perfectly tense membrane at said mold, and means for electrostatically charging said membrane for a perfect adhesion of said membrane to said mold. Conveniently, the laminate for making a graphic representation on objects made of a polyolefin polymer, is characterized in that it comprises a first film is an acrylic polymer, said graphic representation being printed on one face of said first film, said first film being associated, through an adhesive, with a second film, in unstretched polypropylene, so that said graphic representation is visible exclusively through said transparent first film. Preferably, said first film has adapted additives for increasing its resistance to ultraviolet rays. The present invention relates to an injection molded object according to claim 11. It has been verified in practice that the choice of said first film, being an acrylic polymer has advantages over the case where said first film is of polypropylene. In particular, the advantages are a greater transparency, an increased hardness and therefore a better resistance to scratches and wear and a greater brilliancy. This is especially true for a polymetyl methacrylate film. Preferably, the melting point of the second film is comprised between 120° and 190°C and more preferably between 135° and 170°C and still more preferably between 145° and 150°C. Preferably the polymethyl methacrylate is formed by 80% in weight of methyl acrylate and up to 20% of acrylic esther or other monomers. The first film is soluble in the solvent of the ink used for printing said graphic representation. According to the invention the solvent is ethyl acetate and the first film is formed preferably by polymethyl methacrylate. Further characteristics and advantages of the invention will be more apparent by the description of a preferred but not exclusive embodiment of a method for making a graphic representation on objects made of a polyolefin polymer, wherein the method, according to the invention, is performed by the following succession of steps. First, the starting material is extruded. The starting material is constituted by a granulated material being an acrylic polymer plus a per se known additive for increasing the resistance to ultraviolet rays for protecting the polymer from the action of atmospheric oxygen and solar light. Thanks to the additives the resistance to thermal aging and to the light improves from about 1-2 months to about 2-3 years of continuous use. A film of selected height and size is thus obtained which is subsequently flame treated in order to ensure the adhesion of ink and/or adhesives. Then the graphic representation, which will eventually be transferred to the object to be decorated, is printed on the first film. This printing is performed in a per se known manner by rotogravure or flexographic printing. By means of a polyurethane adhesive, which is also protected from ultraviolet rays for preventing it from yellowing, the first film is associated with a second unstretched polypropylene film. In particular, the polyurethane adhesive is protected from UV rays thanks to the insertion of specific additives belonging to the monomeric HALS class and preferably to the monomeric methyl-hindered HALS class. Preferably, the adhesive contains a solvent wherein the first film is soluble. In case the first film is in PMMA the solvent is preferably ethyl acetate, even though it is possible to use ethyl alcohol. The PMMA is rapidly attacked by ethyl acetate and just slightly attacked by ethyl alcohol. It is however possible to use also so called "solvent free" polyurethane adhesives. In this manner, the associated first and second films form a laminate containing the graphic representation between the films, the graphic representation being thus perfectly protected both above and below. The application of the second polypropylene film to the first film provides two advantages. On one hand, makes the printing ink adhere to and coverthe object completely, eliminating the inconvenience of the prior art in that region where the polypropylene film did not adhere to the object. The second advantage is that, during injection of the material (for example polypropylene) for forming the object, the second film tends to at least partially melt thereby providing a perfect integration of the second film molecules with the polypropylene. In particular, afterthe laminate has been obtained in the above described manner, the laminate is then grabbed and kept perfectly stretched and, before being applied to the mold, it is electrostatically charged. The electrostatic charge makes the laminate adhere inside the mold perfectly, so that it assumes the same shape, even if the mold does not have a perfectly plane surface, but rather has lowered and raised regions according to the object to be molded. Of course, the laminate has the first film against the mold while the second film is inside the mold so that it is affected by the injected polypropylene; not directly on the laminate but indirectly in order not to spoil or move the laminate from its position. The second film has a melting point substantially lower than the temperature of the polyolefin polymer forming the object on which the graphic representation will be applied, when it goes into contact with the second film during the injection molding. Comparative Examples 1-5 The first film is constituted by a high rigidity bioriented polypropylene having a melting point of 163°C and conveniently stabilized with a monomeric and polymeric HALS mixture. The first film is superficially flame treated for rendering it printable and is printed by a flexographic or rotogravure process and with inks having high resistance to the light. In the examples 1-4 the ink solvent is ethyl acetate. In example 5 the ink solvent is ethyl alcohol. In all the examples the first film is not soluble in the ink solvent. The first film is spread with polyurethane adhesive which, in examples 1, 2 and 5 has an ethyl acetate based solvent, while in examples 3 and 4 is "solvent free". The polyurethane adhesive added with a monomeric methyl-hindered HALS mixture for improving the resistance to light. The first film, printed and glued, is then glued to the second film constituted by random copolymer polypropylene having a melting point of 148°C. The membrane, thus formed, after a time sufficient for the adhesive reticulation, is cut for obtaining the exact shapes to be inserted into the mold, in order to obtain, by injection molding, the object on the surface thereof the graphic representation has to be visible. Example 6 comparative Example 1 is repeated with the exception that the first film, instead of polypropylene, is constituted by high rigidity bioriented polyethylenetherefthalate having a melting point of 256°C and conveniently stabilized with a monomeric and polymeric HALS mixture. The ink solvent is ethyl acetate. The adhesive solvent is ethyl acetate. Examples 7-40 according to the invention Example 1 is repeated with the exception that the first film, instead of bioriented polypropylene, is constituted by polymethyl acrylate. The ink solvent is ethyl acetate. The adhesive solvent is ethyl acetate. The polymethyl acrylate is soluble in the ethyl acetate. Resistance tests The results of examples 1-40 have been subjected to two series of tests for evaluating the adhesion of the first and second films. In the first series of tests the sample obtained with the above examples is subjected to a series of rather drastic thermal cycles for verifying after how many cycles the films separate. In each thermal cycle the sample is treated at variable temperature: immersed in water at 23°C for eight hours, for 16 hours treated with air at 100°C. The number of thermal cycles required for separating the films is indicated in the enclosed table. In the second series of tests, the sample obtained with the above examples is subjected to a series of radiation cycles for verifying after how many total hours the films separate. In each radiation cycle the sample is subjected to 8 hours of ultraviolet radiations UVB and to 4 hours of condensate at 40°C, wherein the lamps are turned off and water evaporates from the bottom condensating on the samples. The number of hours required for separating the films is indicated in the enclosed table. example thermal cycle number of cycles radiation cycle hours 1 1-2 45-60 2 20 260 3 20 300 4 3-6 150 5 27 800 6 6 100 7 >30 (interrupted) 4000 8-31 >30 (interrupted) >2700 (in course) 32-40 >30 (interrupted) >1900 (in course) It is apparent from the table that the samples produced according to the invention achieve a resistance to separation which is more than ten times that of the average duration of the comparative examples. It has then been verified that by repeating the above examples with the addition of a further protection against aging process of the polyurethane adhesive the is an improvement in the resistance to the solar light. Such further protection is obtained during the printing step of the graphic representation, by covering all the remaining regions of the first film which are not affected by the graphic representation, with an ink having the same color of the object on which the membrane is applied. In this manner, the polyurethane adhesive, between the two films, is not directly exposed to the solar light. The application of the above described lamimate is performed with an apparatus comprising grip means, for example holes or suckers for sucking air, for grabbing the perfectly stretched laminate and moving it to the mold. The same support plate of the grip means, cinematically operated by numeric control, according to the known art, advantageously comprises means for electrostatically charging the laminate so that the laminate may perfectly adhere to the mold where it is arranged. Also the means for electrostatically charging may be any as far as they charge the laminate sufficiently for the above described requirements. The present invention also relates to the laminate for implementing the above described method. The invention achieves the intended aims and has important and considerable advantages. It has in fact being devised a new method for making a graphic representation on objects made of a polyolefin polymer; the method achieves a perfect integration of the graphic representation support laminate with the object surface. Furthermore, the laminate applied in this manner is considerably durable and is resistant to possible scratches and abrasions. The invention thus conceived may have numerous modifications and variations, all within the inventive concept. Furthermore, all the details may be substituted with technically equivalent elements. The materials employed, as well as the dimensions, may be any according to the specific needs and the state of the art.

Claims: Method for making a graphic representation on objects comprising: printing a graphic representation on a face of a first transparent film of an acrylic polymer, associating said first printed film with a second film by an adhesive layer in order to obtain a laminate formed by said associated first and second films containing said graphic representation between them, transferring said laminate inside an injection mold and injecting a polymer in said mold for forming said object, so that said graphic representation is visible exclusively through said transparent first film, characterized in that it comprises obtaining at least a partial melting of said second film in order that said laminate forms one body with said object; said first film being soluble in a solvent of the ink used for printing said graphic representation; said second film having a melting point lower than the melting point of said polymer injected into said mold; said second film having a melting point comprised between 120°C and 190°C and being a polyolefine polymer; said injected polymer being a polypropilene polymer, said solvent being ethyl acetate. Method, according to claim 1, characterized in that said second film is substantially unstretched polypropylene. Method, according to at least one of the preceding claims, characterized in that at least said first film is first stabilized with additives for improving its resistance to the atmospheric oxygen and to solar light. Method, according to at least one of the preceding claims, characterized in that an adhesive is protected from the ultraviolet rays. Method, according to at least one of the preceding claims, characterized in that said laminate is electrostatically charged before being applied to said mold. Method, according to at least one of the preceding claims, characterized in that said second film is produced with an unstretched polyolefin polymer. Method, according to at least one of the preceding claims, characterized in that said adhesive has a polyurethane base preferably added with a a monomeric HALS mixture and more preferably added with a methyl-hindered monomeric HALS mixture. Method, according to at least one of the preceding claims, characterized in that the melting point of said second film is comprised between 135°C and 170°C and preferably between 145°C and 150°C. Method, according to at least one of the preceding claims, characterized in that , during the printing step of said graphic representation on said first film, the regions of said first film which are not affected by said graphic representation are covered by an ink having the same color of said object on which the laminate is applied. Method, according to at least one of the preceding claims, characterized in that said acrylic polymer is a polyalkyl methacrylate, more preferably is a polymethil methacrylate or a polydimethyl methacrylate. Injection molded object comprising an external layer formed by an acrylic polymer, a first lower layer formed by at least one ink adapted to define a graphic representation, a second lower layer formed by an adhesive polymer, a third lower layer formed by a polyolefin polymer layer; characterized in that the melting point of the body of said molded object is higher than the melting point of said third layer; said external layer being soluble in a solvent of the ink used for printing said graphic representation; said third layer adhering by melting to the body of said object and having a melting point comprised between 120°C and 190°C; said injection molded object having the body made of a polypropilene polymer, said body having a core made of said polypropilene polymer, said solvent being ethyl acetate. Molded object, according to claim 11, wherein the melting point of said third layer is comprised between 135°C and 170 °C and preferably between 145 °C and 150°C. Molded object, according to at least one of the preceding claims, wherein the melting point of the body of said molded object is higher than the melting point of said third layer. Molded object, according to at least one of the preceding claims, wherein said acrylic polymer is a polyalkyl methacrylate, more preferably is a polymethyl methacrylate or a polydimethyl methacrylate. Molded object according to at least one of the preceding claims in which said third layer is made of a polypropilene polymer.

Inventors: GRAZIOLI VITTORIO, GRAZIOLI, VITTORIO
Applicants: GRAND SOLEIL S P A, GRAND SOLEIL S.P.A.
Application year: 1995
Main classification: B29C45
IPC codes: B29C45