MXPA01008801A - Printed thermoplastic materials and process for providing same - Google Patents

Abstract

A printed packaging material and a method for making the same is described. On a primary surface of a thermoplastic flexible packaging material is disposed a printed image. That image includes two primary components. The first is at least one marking containing a pigment. The second is a pigment-free coating which overlies the outermost marking. The coating is made from materials which can polymerize and/or crosslink when exposed to ionizing radiation. After the film is exposed to such radiation, the coating hardens to form a protective layer over the printed markings.

Description

PRINTED THERMOPLASTIC MATERIALS AND PROCESS TO PROVIDE THEM BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention relates to the printing of thermoplastic packaging materials, particularly to printing techniques that involve the use of radiation curable coatings employed to protect underlying layers of printed markings. 2. BACKGROUND OF THE INVENTION Even though printing techniques have become very specialized and well defined over the years, the printing of thermoplastic packaging films has remained somewhat like black magic. Only recently packers have required film makers to provide packaging films with printed images with photo quality. This represents a challenge in itself, but the use to which some packers submit these films makes a difficult situation even more complex. Packaging applications that require shrink films present especially complex problems for film makers. This is due to the need for the printing ink (s) to have sufficient flexibility to not crack or peel off once the film has been subjected to thermal shrinkage. Thermal shrinkage applications involving significant amounts of heat, friction, and / or contact between film and metal amplify the problem. Films intended for cooking applications can be subjected to all these conditions and provide film manufacturers and converters with some of their most complex printing challenges. To prevent the formation of cracks and / or detachment of printed images, film makers have developed several strategies. In most cases, these strategies involve the use of new ink formulations. Standard inks used in the printing of thermoplastic films involve pigments carried in a resin (eg, nitrocellulose, polyamide, etc.) soluble in vehicle solvent (eg, an alcohol). Once the ink is applied to the film, the solvent evaporates, leaving behind the resin-pigment combination. More recently, more exotic formulations have involved two-part polyurethane resin systems as well as solvent-free systems where the resin (s) can be cured by ultraviolet (UV) light. These new approaches are not without drawbacks, however, mainly due to concerns regarding operator exposure (due to the fact that the components cause short-term effects such as nausea, headaches, nosebleeds, etc.) and due to the need to ensure that the components are sufficiently cross-linked to ensure that the system complies with applicable government regulations on food safety. The components used in the system of two parts of the first are often not approved for use with films to pack food while the second requires the presence of photoinitiators that migrate in the packaged product. Both are unacceptable for the careful film maker. What the technique has not taught and that is still desirable is a printing technique that allows the use of standard ink formulations but that avoids the problems of cracking and / or detachment that these types of ink have presented in the conditions of effort presented for the applications of thermal shrinkage. SUMMARY OF THE INVENTION In summary, the present invention offers a printed thermoplastic flexible packaging material that includes a coating of a material that protects the printed image. The packaging material includes at least two primary surfaces. On at least one of these surfaces, a printed image is applied. The image includes a mark that contains at least one pigment derived from a solvent-based ink and a pigment-free coating that covers the outermost pigment-containing mark. The coating includes one or more polymerizable materials, one of which can be cured by ionizing radiation. When printed packaging material is exposed to ionizing radiation, the coating hardens to form a protective layer over the pigment-containing marks of the printed image. In another aspect, the present invention provides a method for printing a packaging material. This method includes (a) the application of one or more solvent-based inks on a flexible thermoplastic packaging material and the fact of allowing or causing the applied ink (s) to be fixed on it. packaging material in order to create a brand that contains pigment in the packaging material; (b) applying on the marked packaging material, in a manner that substantially covers all of the pigment-containing marks, a pigment-free coating that includes one or more polymerizable materials; and (c) exposing the labeled packaging material to ionizing radiation for the purpose of polymerizing and, optionally, crosslinking the polymerizable material or the various polymerizable materials in the pigmentless coating. When more than one ink is applied on the packaging material, each ink is preferably applied only after sufficient fixing of the previous ink or the previous inks on the packaging material in such a way as to avoid staining. The method of the present invention offers a clear and significant advantage compared to previously described printing methods insofar as it allows the use of standard solvent-based inks, even when the final use of the printed film includes physical abuse and / or important chemical. By using an extremely resistant coating on these inks, these inks are protected even when subjected to severe handling and processing conditions. This avoids the need for exotic ink systems and / or an impairment of handling and processing conditions. The following definitions apply in this document unless expressly stated otherwise: "including" means that it includes at least the named materials (in relation to an article or composition), parts (in relation to a machine), or good steps (in relation to a method) but not limited to them; "positioned in" in relation to the location of an ink relative to the surface layer of the printed film means coated or applied such that it is in intimate contact with a primary surface of the film; "flexible" means capable of deformation without catastrophic failure; "packaging" means one or more packaging materials (eg, a film) configured around a product; "polymer" means the polymerization product of one or more monomers and includes monopolymers, copolymers, and interpolymers as well as mixtures and modifications thereof; "mer unit" refers to the portion of a polymer derived from a single reactive molecule; for example, a mer unit of ethylene has the general formula -CH2CH2-; "homopolymer" refers to a polymer that consists essentially of a single type of repeating mer unit; "copolymer" means a polymer that includes mer units derived from two reactants (usually monomers) and includes random, block, segmented, graft copolymers, etc; "interpolymer" refers to a polymer that includes mer units derived from at least two reagents (usually monomers) and includes copolymers, terpolymers, tetrapolymers, and the like; "polyolefin" means a polymer wherein some mer units are derived from an olefinic monomer which may be linear, branched, cyclic, aliphatic, aromatic, substituted or unsubstituted (eg, olefin homopolymers, interpolymers of two or more olefins, copolymers of one olefin and a non-olefinic comonomer such as for example vinyl monomer, and the like; "(meth) acrylic acid" refers to acrylic acid and / or methacrylic acid; "(meth) acrylate" refers to an ester of (meth) acrylic acid "grafted with anhydride" refers to a group containing an anhydride portion, such as for example the portion derived from maleic acid, fumaric acid, etc., which has been chemically fixed on a polymer given or affiliated with a given polymer; permeation "(in the packaging industry the term" permeation "often refers to" transmission speed ") refers to the volume of gas (eg, 02) that passes through a given cross section of film (or film layer) at a particular temperature and in a particular relative humidity condition when measured in accordance with a standard test such as for example ASTM D 1434 or D 3985; "curable" means that it can be polymerized and / or crosslinked; "photoinitiator" refers to a substance that, when exposed to actinic radiation or specific wavelengths (for example, polymerization), forms a reactive species that initiates a reaction in one or several other substances in its vicinity; "solvent-based ink" refers to an ink wherein a pigment is dispersed in a polymer vehicle which, in turn, is solvated in a liquid medium such as water, alcohol, ester or the like; "corona treatment" or "corona discharge treatment" refers to a process in which one or both primary surfaces of a thermoplastic film are subjected to the ionization product of a gas (e.g., air) in the vicinity of ( s) film surface (s) for the purpose of causing oxidation and / or other changes to the film surface (s); "cooking" refers to the heating of a food product thereby effecting a change as to one or more chemical or physical properties of said product (eg, color, texture, flavor, and the like) "longitudinal direction" refers to direction along the length of a film, that is, in the direction of the film as formed during extrusion and / or coating; "transverse direction" refers to the direction through the film and perpendicular to the direction of the machine; "free shrink" refers to the change in dimension in percent, in accordance with that measured by ASTM D 2732 (incorporated herein by reference) in a 10 cm x 10 cm sample of film when subjected to heating; "Shrinkage stress" refers to the force per average cross-sectional area developed in a film, in a specific direction and at a specific elevated temperature, as the film tries to shrink at this temperature while it is being held (in accordance with what was measured according to ASTM D 2838, which is incorporated herein by reference); as a verb, "laminar" refers to fixing or adhering (for example through adhesive bonding, pressure bonding, crown lamination, and the like) two or more film articles made separately between them in order to form a structure of multiple layers; as a noun, "laminate" refers to a product produced by the fixation or adhesion that we have just described; "directly adhered" as applied to film layers, refers to the adhesion of the subject film layer on the subject film layer, without a bonding layer, adhesive, or another layer between them; "between", as applied to the film layers, means that the subject layer is placed between two object layers, independently of whether the subject layer is directly adhered to the object layers or if the subject layer if it is separated from the layers. object layers by one or several additional layers; "inner layer" refers to a layer of a film having each of its principal surfaces directly adhered to another layer of the film; "outer layer" refers to a layer of a film having less than both principal surfaces directly adhered onto other layers of the film; "inner layer" refers to the outer layer of a film where a product is packed that is closer, relative to the other layers of the film, to the packaged product; "outer layer" or "surface layer" refers to the outer layer of a film in which a product is packaged that is further away, relative to the other layers of the film, of the packaged product; "Swept layer" refers to a film layer that can exclude one or more gases (e.g., O;); "Abuse layer" refers to an outer layer and / or an inner layer that resists abrasion, perforation, and other potential causes of packaging integrity reduction and / or appearance quality; "tie layer" refers to an inner layer having the primary purpose of providing adhesion between layers to adjacent layers that include otherwise non-adherent polymers; and "volume layer" refers to any layer that has the purpose of increasing resistance to abuse, hardness, modulus, etc., of a multilayer film and usually comprises polymers that are economical as compared to other polymers in the film that provide a certain specific purpose not related to resistance abuses, module, etc. DETAILED DESCRIPTION OF ILLUSTRATIVE MODALITIES The flexible packaging films of thermoplastic packaging find widespread use in the industry and are found in various forms and characteristics of end use. If the film contains one layer or more than one layer it is not important insofar as the film remains satisfactory for the particular end use for which it is contemplated. Such films often contain at least one layer that includes a polymer that includes mer units derived from ethylene. Even when certain ethylene homopolymers are employed, interpolymers are often preferred. Examples of interpolymers include interpolymers including mer units derived from one or more C3-C20 α-olefins, vinyl acetate, (meth) acrylic acid, and C?-C 2o esters of (meth) acrylic acid. The isomers may also be useful. Preferred interpolymers are ethylene / α-olefin copolymers. The relatively recent emergence of single site type catalysts (eg, metalócenos) requires further clarification in terms of definition when discussing ethylene homopolymers and copolymers. Heterogeneous polymers are polymers that have a relatively wide variation in molecular weight and composition distribution. The polymers prepared, for example, with conventional Ziegler Natta type catalysts are heterogeneous. Such polymers can be used in the outer layer of the film, as well as numerous other layers of the film when it has several layers. On the other hand, homogeneous polymers have a relatively narrow distribution of molecular weights and composition. Homogeneous polymers differ structurally from heterogeneous polymers insofar as they present a relatively regular sequencing of comonomers within a chain, a mirror of sequence distribution in all chains, and a similarity of chain lengths, ie a distribution narrower molecular weights. Homogeneous polymers are typically prepared using metallocene or other single site type catalysts. The homogeneous copolymers can also be employed in the printed film of the present invention. The term "ethylene / α-olefin interpolymer" as used herein refers to both heterogeneous materials such as low density polyethylene (LDPE), medium density polyethylene (MDPE), linear low density polyethylene.

(LLDPE), and very low density and ultra low polyethylene (VLDPE and ULDPE), as well as homogeneous materials that, in general, are prepared by the copolymerization of ethylene and one or several α-olefins. Preferably the comonomer (s) is (are) one or more C4-C20 α-olefins, more preferably one or more C4-C12 α-olefins, and more preferably one or more C4-C8 α-olefins. . Particularly preferred α-olefins include 1-butene, 1-hexene, 1-octene and mixtures thereof. In general, from about 80 to 99% by weight of ethylene and from 20 to 20% by weight of α-olefin, preferably from about 85 to 95% by weight of ethylene and from 5 to 15% by weight of α-olefins are copolymerized in the presence of a single site catalyst. Examples of commercially available homogeneous materials include the metallocene-catalyzed Exact ™ resins (Exxon Chemical Co., Baytown, Texas), the substantially linear Affinity ™ and Engage ™ resins (Dow Chemical Co., Midland, Michigan), and the Tafmer linear resins. ™ (Mitsui Petrochemical Corporation, Japan). Homogeneous ethylene / α-olefin interpolymers can be characterized by one or several methods known to those skilled in the art such as, for example, molecular weight distribution (M "/ Mn), composition distribution width index (CDBI), narrow range of melting points, and unique melting point behavior. The molecular weight distribution, which is also known as polydispersity, can be determined, for example, by gel permeation chromatography. The homogeneous ethylene / α-olefin copolymers to be employed in a layer of the film of the present invention preferably have a Mw / Mn distribution of less than 2.7; more preferably from about 1.9 to 2.5; preferably even greater than about 1.9 to 2.3. The CDBI of homogeneous ethylene / α-olefin interpolymers is generally greater than about 70%. CDBI is defined as the weight percentage of polymer molecules having a comonomer content within 50% (ie, ± 50%) of the average total molar comonomer content. The CDBI can be determined by fractionation of temperature rise elution in accordance with that described, for example, in Wild et al., J. Poly. Sci. Poly. Phys. Ed. Vol. 20, 441 (1982). Linear polyethylene, which does not contain comonomer, is defined as having a 100% CDBI. The CDBI determination clearly distinguishes homogeneous copolymers (CDBI values generally greater than 70%) from the currently available VLDPEs (CDBI values generally lower than 55%). Homogeneous ethylene / α-olefin interpolymers typically also exhibit an essentially unique melting point with a peak melting point (Tm), as determined by differential scanning calorimetry (DSC), of about 60 ° C to 105 ° C , more precisely, a DSC peak Tm of about 80 ° C to 100 ° C. As used herein, the term "essentially unique melting point" means that at least about 80% (by weight) of the material corresponds to a single Tm at a temperature within a range of about 60 ° C to 105 ° C and essentially no substantial fraction of the material has a peak melting point that exceeds about 115 ° C in accordance with that determined by DSC analysis (e.g. in a System of Perkin Elmer ™ System 7 Thermal Analysis). The presence of higher melting peaks is detrimental to film properties such as seal start and overcast temperature. Regardless of the type (s) of polymer (s) containing (n) mer units derived from ethylene that are employed in the outer layer, other layers may be present in the film. For example, the film may include a layer having a low oxygen permeation, preferably an oxygen permeation at a temperature of about 23 ° C and 0% relative humidity not greater than about 150 cm3 / m2.atm.24 hours, more preferably not greater than about 100 cm3 / m2.atm.24 hours, and preferably even greater not more than about 50 cm3 / m2.atm.24 hours, and especially not more than about 20 cm3 / m2. atm.24 hours Said barrier layer to 02 preferably has a thickness from about 0.001 to about 0.05 mm, more preferably from about 0.002 to about 0.0075 mm, and especially from about 0.0025 to about 0.005 mm. Said barrier layer to 02 may include one or more of EVOH, PVDC, polyalkylene carbonate, polyamide and polyester. Preferably any barrier layer a 02 is an inner layer of a film used in accordance with the present invention. When the film includes two or more layers, one or more tie layers may be employed to provide increased adhesion between the other layers. Such layers often have a comparatively high degree of compatibility with polymers employed in barrier layers a2 (e.g. EVOH or polya ida) as well as with polymers employed in other non-barrier layers (e.g., polyolefins). When said tie layer is present, it is preferably placed on one or both primary sides of the barrier layer a 02, more preferably it is directly adhered on one or both primary sides of the barrier layer 02. Such layers of The linkers may include one or more polymers containing mer units derived from at least one of the following elements: C2-C12 α-olefin, styrene, amide, ester and urethane, preferably one or more of the following: grafted ethylene interpolymer with anhydride / α-olefin, ethylene interpolymer grafted with anhydride / ethylenically unsaturated ester in ethylene interpolymer grafted with anhydride / ethylenically unsaturated acid. The film can also include one or several other layers that can serve as internal or external layers and can be classified as volume capable, abuse layers, etc. Said layer may include one or more polymers including mer units derived from at least one of the following: C2-C2-styrene α-olefin, amides, esters and urethanes. Among these homopolymers and interpolymers those which include mer units derived from ethylene, propylene, and 1-butene are preferred, with an even greater preference an ethylene interpolymer such as for example ethylene / C3-C8-ethylene interpolymer, ethylene / ester interpolymer ethylenically unsaturated 8 for example, ethylene / butyl acrylate copolymer), ethylene / ethylenically unsaturated acid interpolymer 8 for example, ethylene / methacrylic acid copolymer), and ethylene / vinyl acetate interpolymer. Preferred ethylene / vinyl acetate interpolymers are the interpolymers which include from about 2.5 to about 27.5% (by weight), preferably from about 5 to about 20% (by weight), preferably even greater than from about 5 to about 17.5 % (by weight) of mer units derived from vinyl acetate. Said polymer preferably has a melt index of from about 0.3 to about 25, more preferably from about 0.5 to about 15, preferably even more from about 0.7 to about 5, and especially from about 1 to about 3. The film may include a layer derived at least in part from a polyester and / or a polyamide. Examples of suitable polyesters include amorphous (co) polyesters, (poly) ethylene / terephthalic acid, and (poly) ethylene / naphthalate, although they may be preferred for certain applications (poly) ethylene / terephthalic acid with at least about 75 mole%, more preferably at least about 80% mole Molar% of its mer units derived from terephthalic acid. Examples of suitable polyamides include polyamide 6, polyamide 9, polyamide 10, polyamide 11, polyamide 12, polyamide 66, polyamide 610, polyamide 612, polyamide 61, polyamide 6T, polyamide 69, interpolymers made from any of the monomers used to form two or several of the above homopolymers, and mixtures of any of the above homopolymers and / or interpolymers.

Preferably, a film employed in accordance with the present invention includes from 2 to 20 layers; more preferably, from 2 to 12 layers; more preferably from 2 to 9 layers; especially from 3 to 8 layers. Several combinations of layers can be used in the formation of multiple layer film layers. For illustrative purposes, only modalities of 2 to 9 layers are provided here; however, a film in accordance with the present invention may include more layers. Following are examples of preferred combinations where the letters are used to represent film layers: A / B, A / B / A, A / B / C, A / B / D, A / B / C / A, A / B / C / D, A / C / B / C / A, A / B / C / D / A, A / D / B / A, A / B / C / D / C, A / B / D / C, A / B / D / C / D, A / C / B / D, A / D / C / D, A / B / D / C / C, D / C / D / C / D / C / A, D / C / D / C / A, D / C / A / C / D / B / D / C / A, A / C / D / B / D / C / A where A represents a a layer including a polymer including mer units derived from ethylene (in accordance with that described above); B represents a layer including a polymer having a low oxygen permeation (according to that described above); C and C represent layers that include one or more polymers including mer units derived from at least one of the following: C2-C2-α-olefin, styrene, amide, ester and urethane; and D represents a layer including a polyester or polyamide. Obviously, one or more tie layers can be used in any of the aforementioned structures. In accordance with the previously described, the film of the present invention has printing on one of its primary surfaces, preferably in its outer layer. The outer layer preferably includes one or more of the following: (poly) C2-C2-polyamide / polyamide, polyester, (poly) vinylidene chloride, and ethylene / vinyl alcohol copolymer. Regardless of the number and order of the layers, one or more conventional packaging film additives may be included here. Examples of additives that may be incorporated include, but are not limited to, antiblocking agents, anti-clouding agents, slip agents, colorants, flavors, antimicrobial agents, preservatives for meat, and the like. (The person with ordinary skill in the art is aware of numerous examples of each of the above). When the film is to be processed at high speed, the inclusion of one or more antiblock agents in and / or on one or both outer layers of the film structure may be preferred. Examples of antiblocking agents useful for certain applications are about 15% starch. The measurement of the optical properties of plastic films, including the measurement of total transmission, overcast, clarity and brightness is discussed in detail in Pike, LeRoy, "Optical Properties of Packaging Materials," Journal of Plastic Film & Sheeting, vol. 9, no. 3, pages 173-80 (July 1993), which is incorporated herein by reference. Specifically, the cloudy is a measurement of the transmitted light scattered more than 2.5% of the axis of the incident light. The clouding of a particular film is determined by the analysis of compliance with 1990 Annual Book of ASTM Standards section 8, vol. 08.01, ASTM D 1003, "Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics" pages 358-63, which is incorporated herein by reference. Cloudy results can be obtained using instruments such as an XL 211 HAZEGARD ™ system (Gardner / Neotec Instrument Division); Silver Spring, Maryland), which requires a minimum sample size of approximately 6.5 cm2. A film used in accordance with the present invention preferably has a clouding less than about 20%, more preferably less than about 15%, preferably still greater than less than about 10%, preferably even less than about 7.5%, and especially less than about 5%.

A film used in accordance with the present invention may have an intrinsic brightness value (ie gloss before printing) to the extent that the film remains adequate for the intended end use). Intrinsic brightness values of preferred films for use in accordance with the present invention are within a range of about 25 to about 75%. The gloss can be measured with the procedure described in ASTM D 2457, which is incorporated herein by reference. Useful films can have any desired total thickness insofar as they offer the desired properties, for example, optical properties, modulus, seal strength, etc., for a given packaging operation. However, the films to be used according to the present invention preferably have a total thickness of about 0.0075 to about 0.25 mm, more preferably about 0.0125 about 0.125 mm, preferably even greater than about 0.025 to about 0.1 mm, and especially from about 0.045 to about 0.075 mm. Packaging films can be irradiated and are frequently irradiated; which involves the fact of subjecting a film material to radiation such as treatment with high-energy electrons. This can alter the surface of the film and / or induce crosslinking between molecules of the polymers contained therein. The use of ionizing radiation for crosslinking polymers present in a film structure is disclosed in U.S. Patent No. 4,064,296 (Bornstein et al), the teachings of which are incorporated herein by reference. If desired or if necessary, for example increase the adhesion on a wrapped meat product, all or a part of the film can be treated in a crown and / or plasma. These types of surface oxidant treatment involve carrying a film material in the vicinity of a gas containing 02 or N2 (eg, ambient air) that has been ionized. Exemplary techniques are described, for example, in U.S. Patent Nos. 4,120,716 (Bonet) and 4,879,430 (Hoffman), the disclosures of which are incorporated herein by reference. Some end-use applications may require films of surface energies of at least about 0.034 J / m2, more preferably at least about 0.038 J / m2, and especially of at least about 0.040 J / m2. Regardless of whether an oxidizing treatment is used to achieve these levels, the films that have them may be preferred for end-use applications of this type. A film for use in the present invention can be used to pack several products, although it is preferable to use it to pack a food substance, especially meat products, cheeses and vegetables. Examples of meat products that can be packaged include, but are not limited to, poultry (eg, chicken or turkey breast), Bologna sausage, beef, pork, lamb, and whole muscle products such as roast beef. Examples of vegetables that can be packaged include, but are not limited to lettuce, carrots, radishes, celery and the like cut and uncut. The packing of fluids or materials that can flow is also a desirable end use. A bag can be made from a film by sealing the outer layer on itself, whereby the layer becomes the outer layer of the bag or by stapling at at least one end. The bag can be a sealed bag at the ends, a bag sealed laterally, a bag with L seal (that is, sealed at the bottom and on one side with an open top), or a bag (ie, sealed on three sides with an open top). In addition, splice seals can be used. After the formation of a bag, a product can be introduced into the bag and the open end of the bag can be sealed. Alternatively, a film can be wrapped substantially completely around a product and then heat sealed in order to form a package, 27 The aforementioned inks involve pigment (s) dispersed in one or more standard carrier resins. The pigment can be 4B Toner (PR57), 2B Toner (PR48), Lake Red C (PR53), red lithol (PR49), iron oxide (PR101), permanent red (PR4), permanent red 2G (P05), orange pyrazolone (P013), diaryl yellows (PY12, 13, 14), monoazo yellows (PY3,5,98), phthalocyanine green (PG7), phthalocyanine blue, form β (PB15), ultramarine (PB62), violet permanent (PV23), titanium dioxide (P 6), carbon black (furnace / channel) (PB7), pink PMTA, green, blue, violet (PR81, PG1, PB1, PV3), copper ferrocyanide dye complexes (PR169, PG45, PB62, PV27), or the like.

(Identifications in the above refer to the generic color index prepared by the Society of Dyers and Colourists). Such pigments and combinations thereof can be used for various colors including, but not limited to, white, black, blue, violet, red, green, yellow, cyan, magenta or orange. Examples of typical carrier resins used in standard inks include those having nitrocellulose, amide, urethane, epoxide, acrylate, and / or ester functionalities. Standard carrier resins include one or more of the following: nitrocellulose, polyamide, polyurethane, ethyl cellulose, cellulose acetate propionate, (meth) acrylates, (poly) vinyl butyral, acetate (poly) vinyl, (poly) vinyl chloride, and the like. Typically, such resins are mixed, with widely used mixtures including nitrocellulose / polyamide and nitrocellulose / polyurethane. The latter mixture is preferred in the present invention since it can resist the penetration of monomers and / or oligomers existing in the coating layer (discussed below). Resin (s) of ink are usually solvated or dispersed in one or more solvents. Typical solvents employed include, but are not limited to, water, alcohols (e.g., ethanol, 1-propanol, isopropanol, etc.), acetates (e.g., n-propyl acetate), aliphatic hydrocarbons, aromatic hydrocarbons (e.g. toluene) and ketones. Such solvents are typically incorporated in sufficient amounts to provide inks having viscosities in accordance with that measured in a Zahn number 2 cup as is known in the art of at least about 15 seconds, preferably at least about 20 seconds, with greater preference for at least about 25 seconds, and especially about 25 to about 35 seconds. Preferably, each of the inks used to make the printed markings on the surface of the film is essentially free of photoinitiators, thus eliminating the possibility that such materials may migrate towards the film. product packed and inside the packaged product. Also, the ink (s) is (are) preferably essentially free of waxes which can prevent uniform distribution and adhesion of the coating layer (discussed below). Once a first layer of ink is applied on the film, the evaporation of the solvent contained therein is allowed or caused. When a printing system as described in the aforementioned patent is employed, the evaporation of the solvent is preferably caused by heating or by forced air in order to reduce the amount of time before the application of the following layers of paint. ink. Once the first ink layer is applied, all subsequent ink layers (if any) are applied in a similar standard way). Any number of inks to create the printed image. However, cost and space limitations usually impose a certain practical limit. In the case of printing systems employing eight printing stations, more than one and up to seven different inks are preferably used to apply marks containing pigment on the film. The use of up to seven inks allows the eighth print station to be reserved for the pigmentless coating material, described infra. Alternatively, the eight print stations can be reserved for 30 Inks and a pigment-free coating material, described infra, can be applied downstream (preferably in the same printing system). This can allow complete air drying of the solvents in the inks prior to sealing with the coating material. Once all the various layers of ink have been applied on the film surface. A coating without pigment is applied over substantially the entire surface of the film that has been printed. This coating is the coating that can provide protection to the printed image during processing, treatment and subsequent use. This coating is preferably essentially transparent in such a way that the underlying printed marks are as visible as possible. Preferably, a coating material is essentially free of photoinitiators, which eliminates the possibility that said materials migrate towards the product to be packaged and into said product to be packaged. The coating includes one or more polymers or oligomers, optionally mixed with one or more copolymerizable monomers, which polymerize and / or crosslink when exposed to ionizing radiation. These materials may be monofunctional or have two or more terminal polymerizable ethylenically unsaturated groups per molecule. The polymerizable compounds or precursors with energy include, not limited to, reactive vinyl monomers, including esters of (meth) acrylic acid, such as for example (meth) acrylate of beta-carboxyethyl; hexandiol di (meth) acrylate; ethoxylated hexandiol di (meth) acrylate; di-, tri-, and / or polypropylene glycol diacrylate; isobornyl (meth) acrylate; propoxylated glycerol triacrylate; tri (methyl) trimethylolpropane tri (meth) acrylate; ethoxylated trimethylolpropane tri (meth) acrylate; propoxylated trimethylolpropane tri (meth) acrylate; polyether diacrylates; bisphenol A diacrylate; aminoplast (meth) acrylates. Other polymerizable compounds include (meth) acrylamides, vinyl acetate, polythiols, and the like. Oligomers include, but are not limited to, (meth) acrylated epoxides, (meth) acrylated polyesters, (meth) acrylated urethanes / polyurethanes, (meth) acrylated polyethers, and (meth) acrylic acrylic oligomers. When oligomers are combined with one or more monomers, the viscosity of the mixture is preferably such that it can be printed / applied in a material similar to solvent-based inks. Typical concentrations of monomer (s) and reactive oligomer (s) and / or polymer (s) may vary from about 5 to about 95% monomer (s) and from about 95 to about 5% oligomer (s) reagent (s) and / or polymer (s). When copolymerizable components are included in the composition, the amounts used are depend on the total amount of the ethylenically unsaturated component (s) present; for example, in the case of polythiols, from 1 to 98% of the stoichiometric amount can be employed (based on the ethylenically unsaturated component (s)). These types of materials typically contain small amounts of polymerization inhibitors, processing aids, and other additives. Such additives are preferably reactive in order to be incorporated in the polymer matrix of the coating or have a sufficiently high molecular weight such that the possibility of migration in the film or the film is reduced or eliminated. Preferred materials include those containing (meth) acrylate functionalities, especially acrylate functionalities.) The material (s) from which the coating is formed can be applied using the same techniques as those described. previously in relation to the ink (s). Exemplary techniques include, but are not limited to, screen, engraving and flexographic techniques. Although the application of the coating may occur separately at the time and / or location of the application of the ink (s), it is preferable that it occur in line with the application of the ink (s). Regardless of the selected application technique, 33 the thickness of the resulting coating is preferably sufficient to provide good scratch resistance (during handling and film processing) and resistance to chemical agents, eg, fatty acids, oils, processing aids, etc., but not so thick as to prevent the coating layer from shrinking or bending with the film as required by the application (s) of the film. In general, useful coating thicknesses can be within a range of about 0.5 to about 12 μm, preferably from about 1 to about 10 μm, more preferably from about 1.5 to about 8 μm, and especially from about 2 to about 5. μm. Once the coating is applied, the printed film is exposed to ionizing radiation. This polymerizes and / or cross-links the materials in the coating layer, thereby providing a hardened "wrap" over the underlying printed markings. Useful types of ionizing radiation include electron beams (e-rays), X-rays, corona discharge and the like, with the former being preferred. Regardless of the source, the dose of ionizing radiation is preferably sufficiently high to polymerize and crosslink the coating layer however not so high as to degrade the underlying printed marks or the surface of the film. In general, useful radiation dosages 34 they can be located within a range of about 50 to about 250 keV, preferably about 55 to about 200 keV, and more preferably about 60 to about 150 keV. (Radiation units with conventional beams operate at higher voltages and are considered to produce electrons that pass through the coating without effectively and efficiently curing the total coating.) Even though it has not been scientifically proven to date, new lightning irradiation units are available. Low voltage (60-100 keV) such as those commercially available from Applied Advanced Technologies (Winchester, MA) are considered to incorporate one or more materials in the unit window that allow electrons to traverse at a lower speed and provide more effective curing of the coating surface.) If the processing techniques employed allow the use of a low oxygen content environment, the coating and irradiation steps occur preferably in such an atmosphere. A standard nitrogen stream can be used to achieve an atmosphere of this type. The oxygen content of the coating environment is preferably not greater than about 300 ppm, more preferably not more than about 200 ppm, preferably even higher is not greater than about 100 ppm. ppm, with still greater preference is not greater than about 50 ppm, and especially is not greater than about 25 ppm with a totally oxygen-free environment being the ideal environment. Regardless of the intrinsic brightness of the film used, the printed film preferably exhibits a brightness of at least about 50%, more preferably at least about 65%, and especially at least about 75% after application. irradiation of the coating layer. In addition, the gloss level of the coating layer itself is preferably at least about 75%. The techniques described above can be employed with various packaging materials, including materials used to pack beef, pork, poultry, cheese, vegetables, liquids, pet foods, and the like. A preferred application includes packaging materials used in combination with processed food products in thermoplastic film packaging by subjecting the packaged product to elevated temperatures (e.g., hot water or steam), i.e. cooking. Various meat products, such as pork, sausage, poultry, bologna, Bologna sausage, beef, etc., are prepared as cooked products; certain products that contain non-meat proteins such as soybean can be processed from 36 Similarly. In all cases, obtaining adequate adhesion between film and food and providing a cozy package may be necessary for an acceptable aesthetic appearance. The packaging materials for use in cooking applications are typically produced in the form of rolls and then, after printing, converted into canes, bags and the like gathered for the end user. Therefore, a cooking film must be able to withstand exposure to solvents (eg mineral oil), mechanical stresses (eg bending), high temperatures, high pressure, abrasions, etc., for long periods of time without compromising its capacity to contain the food product or its flexibility. During a typical conversion process, approximately 75 m of film are mechanically compressed in approximately 0.75 m. Frequently the speed, pressure, and mineral oil process cause a failure of adherence of standard ink systems in relation to the underlying film. In cooking applications, the packaging material is typically segmented and filled with a paste of meat product. The package is pushed into a stainless steel mold and immersed in a cooking tank, usually during a relatively long cooking cycle. Immersion in water 37 hot (i.e., a temperature of about 55 ° C to about 65 ° C), is usually for a period of about 4 hours; Immersion in water of 70 ° C to 100 ° C or exposure to steam for up to 12 hours is common, even though most cooking procedures do not normally exceed temperatures of approximately 90 ° C. After the cooking process, the film or the package is preferably adapted, if not completely, then at least substantially, to the shape of the contained food product. The printed film of the present invention retains at least about 80%, preferably at least about 85%, more preferably at least about 90% of its printed markings even after being subjected to elevated temperatures such as example 70 ° C for long periods of time such as one hour or more. Objects and advantages of this invention are further illustrated through the following examples. The particular materials as well as their amounts and other conditions and details mentioned in these examples should not be considered as limiting this invention. EXAMPLES Several coating formation formulations were evaluated to determine if they could improve the 38 resistance to heat / scratches from a nitrocellulose / polyurethane ink system. The outer surface of a tube made from a mixture of LLDPE and ethylene / vinyl acetate copolymer was treated by corona discharge at a level of 0.042 J / m2 and then printed on a central printing flexographic printing press with inks white, red and blue. The tube was then cut into numerous film segments. A film, used as a control, was coated on its ink marks with a nitrocellulose / solvent based polyurethane coating. This coating was dried through hot air. A series of radiation curable coating mixtures was applied to the printed surfaces of other films using a manual tester with a cell configuration of 360 lines by 2.54 cm (inch) and 6.2 x 109 mm 3 (hereinafter "bcm"). The coating materials used and the suppliers of each of them are listed below: (a) MiraGloss® 9100 polyacrylate (Morton International, Chicago, IL) (b) acrylated polybutadiene PR01598 (Sartomer Co, Inc., Exton, PA) (c) alkoxy-functionalized triacrylate SR415 (Sartomer) (d) CRODAMER® 215 polyester acrylate (Croda, Inc .; York, NY) (e) TRPGDA-DEO diacrylate (UCB Chemicals Corp.; Smyrna, GA) (f) oligomer / acrylate monomer mixture SARCRYL® CN 818 (Sartomer) (g) silicone functionalized with EBERCRYL® 350 acrylate (UCB) Four coating mixtures were prepared from the above materials. The composition of each coating was as follows: (1) 95% (a), 3.5% (b), 1.5% (c) (2) 85% (a), 15% (f) (3) 49.5% (d) ), 49.5% (e), 1% (g) (4) 49.5 (e), 49.5% (f), 1% (g) (5) 24.8% (d), 49.5% (e), 24.7% ( f), 1% (g) The coated films were then loaded onto a tray that was passed under an 80-keV electron beam radiation unit until exposed to a dose of 3.0 megarads. Before use, the radiation unit was purged in such a way that the oxygen concentration of the work zone was about 300 ppm. Coated samples were then subjected to a free shrink test and a product simulation test. In the first test, 3.8 cm x 5.1 cm portions of each tube were placed in water at a temperature of 85 ° C for 40 minutes. about 5 minutes during said period each film material shrunk approximately 20% both widthwise and lengthwise. Each sample was removed and cooled to room temperature (approximately 23 ° C). In the second test, the printed tubes were placed against a stainless steel mold and cooked at a temperature of 85 ° C for about 6 hours. During the cooking process, the tubes were shrunk and moved through the hot stainless steel surface. Each test sample, from both tests, was evaluated based on the following rating scale: ++: perfect +: no loss - perhaps a point loss when kept light + -: small and non-obvious loss points - deserves a second chance -: significant loss -: significant losses and the results are summarized in the following table. Table 1 - tests of free shrinkage and resistance to scratches with application of heat shrinkage free scratches with application of heat control ++ 41 1 + ++ 2 ++ ++ 3 + ++ 4 ++ ++ 5 ++ ++ In general, both the ink-derived markings and the coating varnishes are fractured and separated from a film when they do not shrink in a proportion equal to or greater than the shrinkage ratio of the film. Solvent-based inks generally shrink in the same proportion as heat shrink tubing, while most radiation curable coatings are crosslinked and tend not to shrink as much. However, the data in Table 1 show that certain radiation curable formulations can offer excellent resistance to scratching and peeling. Various modifications and alterations that do not go beyond the scope and spirit of the invention will be apparent to those skilled in the art. This invention is not limited to the illustrative modalities specifically described.

Claims (25)

1. 42 CLAIMS A method for forming a printed packaging material, comprising: supplying a flexible thermoplastic packing material; applying one or more layers of solvent-based ink on the packaging material to form a pigment-containing mark; subsequently applying, through a solvent-based ink application method, a pigment-free coating on the pigment-containing mark, the pigment-free coating comprising one or more radiation-curable materials; and subsequently exposing the pigmentless coating to ionizing radiation to provide a cured non-pigmented coating having a brightness of at least about 50%. The method according to claim 1 wherein said application step includes the application of a solvent-based ink selected within the group consisting of an alcohol-based ink, an acetate-based ink, and a water-based ink, and mixtures thereof. The method according to claim 1 wherein said exposure step offers a coating 43 Cured transparent. The method according to claim 1 wherein the step of supplying includes the supply of a packaging material having a surface energy of at least about 0.040 J / m2. The method according to claim 1 wherein said application step includes the application of a solvent-based ink having a color-forming pigment selected from white, black, blue, violet, red, green, yellow, cyan, magenta and orange The method according to claim 1 wherein said application step includes the sequential application of at least two layers of solvent-based ink to form the pigment-containing mark. The method according to claim 1 wherein said supply step includes the supply of a flexible thermoplastic film. The method according to claim 1 wherein said delivery step includes the supply of a heat shrinkable packing material having a total free shrink at a temperature of 85 ° C of at least about 5%. The method according to claim 1 wherein said subsequent application step includes the step of application of a curable material with radiations that has an acrylate portion. The method according to claim 1 wherein the exposure step offers a cured coating having a brightness of at least about 65%. The method according to claim 1 wherein the exposure step offers a cured material having a brightness of at least about 75%. The method according to claim 1 wherein said subsequent application step includes the application of a pigment-free coating comprising from about 5 to about 95% by weight of monomer components. The method according to claim 1 wherein said subsequent application step includes the application of the solvent-free coating through a solvent-based ink application method selected from the group consisting of screen, etch, and flexographic techniques. . The method according to claim 1 wherein the packaging material has two primary surfaces, and said application step includes the application of one or more layers of ink based on solvent on only one of said primary surfaces. The method according to claim 1 wherein said delivery step includes the supply of a flexible packaging material having a thickness from about 0.0075 to about 0.125 mm. 16. The method according to claim 1 wherein said supply step includes the supply of a flexible packaging material having a thickness from about 0.0125 to about 0.125 mm. The method according to claim 1 wherein said delivery step includes the supply of a flexible packaging material having a thickness from about 0.025 to about 0.1 mm. 18. The method according to claim 1 wherein said exposure step offers a cured non-pigmented coating having a thickness of about 0.5 to about 12 μm. The method according to claim 1 wherein said exposure step offers a cured non-pigmented coating having a thickness of about 1.5 to about 8 μm. The method according to claim 1 wherein said supply step includes the supply of a thermoplastic flexible tube. 46 21. The method according to claim 1 wherein said application step includes the application of one or more layers of solvent-based ink having a resin capable of resisting the penetration of monomers. 22. The method according to claim 1 wherein said delivery step includes the application of one or more layers of solvent-based ink having a urethane resin. 23. The method according to claim 1 wherein said application step includes the application of one or more layers of solvent-based ink having a nitrocellulose / polyurethane resin. The method according to claim 1 wherein said exposure step includes exposing the pigmentless coating to a source of electron beam ionizing radiation. 25. The method according to claim 1 wherein said exposure step includes exposing the pigmentless coating to a source of ionizing X-ray radiation. The method according to claim 1 wherein said exposure step includes exposing the pigmentless coating to a radiation dosage of about 50 to about 250 47 keV. The method according to claim 1 wherein said exposure step includes exposing the pigmentless coating to a radiation dosage of 60 to 100 keV. The method according to claim 1 wherein: the application step includes the application of one or more layers of solvent-based ink using a solvent-based ink printing system of multiple stations; and the subsequent application step includes the application of the pigmentless coating using the last station of the multi-station solvent-based ink printing system. The method according to claim 1 wherein: the step of supplying includes the supply of a multilayer packaging film having an outer layer comprising a polyamide; and the step of supplying includes the supply of at least one layer of solvent-based ink on the polyamide layer of the packaging film.