A lid having a cured overprint varnish

Abstract: A composite lid having an overprint varnish, wherein said composite package is comprised of: a paperboard substrate having first and second sides; a coating of particulate minerals located exterior to said first side of said substrate; a layer of printed graphics located exterior to said coating of particulate minerals; a coating of a radiation-cured, heat resistant varnish located exterior to said layer of printed graphics; and a layer of a heat sealable barrier material located exterior to said second side of said substrate.

Description: Field of the Invention This invention relates to paperboard lids having a radiation cured overprint varnish. Such structures of this type, generally, protect the graphics which are printed upon the paperboard lid such that the graphics are not distorted and/or marred. Description of the Related Art Platen sealing is gaining popularity for attaching paperboard lids to trays following product filling. The benefits of this method include more uniform seals, simpler equipment operation, and the ability to seal through food contamination. A major disadvantage with this method is that the heated platen can blemish the outside surface of the lid which contains important sales graphics and, in some cases, UPC symbols. The blemished or marred surface results in an unsatisfactory package appearance and, in some cases, UPC symbols which cannot be scanned. It is known, in coating systems, to make use of a varnish layer to protect a printed surface. Exemplary of such prior art is U.S. Patent No. 4,170,681 ('681) to J. R. Edwards et al., entitled "Method of Applying a Varnish Layer to a Printed Surface and Product Made Thereby". While the '681 reference teaches the use of a varnish layer to provide a smooth protective surface for the printed graphics, the nature of the varnish layer is such that it is not heat resistant (mass stable) to temperatures above 163°C (325°F) and also the varnish is not radiation cured. Consequently, an advantageous overprint varnish would be one which is both heat resistant and radiation cured. U.S. 5228272A discloses a lid comprising a paperboard substrate bearing on one surface a particulate mineral coating suitable as a printing surface, and on the other side of the substrate a heat sealable barrier layer. JP 06336091A describes a paper base sheet bearing on one surface a pigment coating, and a radiation-cured layer thereon. JP 55041917B, GB 2063767A and JP 06047873A disclose paper substrates bearing printing thereon and covered by a protective radiation-cured layer. It is apparent from the above that there exists a need in the art for an overprint varnish which is capable of protecting the graphics printed upon the lid, but which at the same time is heat resistant and radiation cured. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure. According to the present invention there is provided a composite lid having an overprint varnish, wherein said composite lid is comprised of: a paperboard substrate having first and second sides; a layer of particulate minerals located exterior to said first side of said substrate; a layer of printed graphics located exterior to said coating of particulate minerals; a layer of a radiation-cured, heat resistant varnish located exterior to said layer of printed graphics; and a layer of a heat sealable barrier material located exterior to said second side of said substrate; characterised in that said radiation-cured varnish is mass stable above 163°C and said varnish prevents said layer of printed graphics from being distorted during a pressure platen heat sealing of said composite lid to a vessel and said varnish does not adhere to said platen during said heat sealing. Also according to the present invention there is provided a method of constructing a composite lid having an overprint varnish wherein said method comprises the steps of: coating a first side of a paperboard substrate with a layer of particulate minerals; printing a layer of graphics substantially over said particulate mineral layer; coating said printed graphics layer with a layer of a heat resistant, radiation-curable varnish and curing said varnish layer by electron beam of ultraviolet radiation; and coating said second side of said substrate with a heat sealable barrier layer; characterised in that said varnish is mass stable above 163°C and said varnish substantially prevents said layer of printed graphics from being distorted during a pressure platen heat sealing of said lid to a vessel and said varnish does not substantially adhere to said platen during said heat sealing. In certain preferred embodiments, the particulate minerals are clay. Also, the varnish coating is cured by either electron beam or ultraviolet radiation. In another further preferred embodiment, the radiation-cured varnish overcoat protects the printed graphics from distortion and/or marring. The preferred lid, according to this invention, offers the following advantages: lightness in weight; ease of assembly; good stability; good durability; excellent printed graphics protection; and excellent economy. In fact, in many of the preferred embodiments, these factors of ease of assembly, graphics protection, and economy are optimized to the extent that is considerably higher than heretofore achieved in prior, known composite lids. BRIEF DESCRIPTION OF THE DRAWING The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing FIGURE which is a schematic illustration of a composite lid with a cured overprint varnish, according to the present invention. DETAILED DESCRIPTION OF THE INVENTION Cartons which are used for distributing, marketing, and, in some cases, heating portions of prepared foods are fabricated with a paperboard structural substrate. The basic carton design consists of a vessel to hold the product and a lid which is at least partially sealed to peripheral flanges extending from the vessel sidewalls. See, for example, commonly assigned U.S. Patent No. 5,356,070 ('070) to W. R. Rigby, entitled "Partitioned Paperboard Food Tray." The lid may also be integral with the vessel in some cases. See, for example, commonly assigned U.S. Patent No. 5,228,272 ('272) to B. G. Calvert et al., entitled "Product and Process for Heat Sealing a Paperboard Carton Having Polymer Coating on One Side Only,"(which is incorporated in its entirety by reference). The basic substrate material for the present invention is machine-made paperboard which may range in thickness from 0.178 mm to 0.889 mm. The paperboard substrate may be coated on one or both sides with a layer of particulate minerals as mentioned in claim 1. The surface of the lid next to the food product must be coated with a material which provides a barrier and is heat sealable to the vessel flanges. Coatings which may provide this are, typically, extruded polymers such as polyethylene terephthalate (PET), polypropylene (PP), and polyethylene (PE). Finally, the exterior surface of the lid may be printed with sales graphics such that these graphics are printed onto the particulate mineral layer located on the base substrate. With this background, and with reference to the FIGURE, there is illustrated composite lid structure 2. Lid 2 includes in part, conventional paperboard substrate 4, conventional barrier coating 6, conventional particulate mineral layer 8, conventional printed graphics layer 10, and radiation-cured overprint varnish layer 12. As discussed earlier, barrier coating 6 must provide a barrier and be heat sealable to the flanges of the vessel (not shown). See, for example, Figures 1 and 3 of the above-identified ('070) and ('272) patents, respectively. Preferably, the barrier is constructed of either PET, PP, or PE. Particulate mineral layer 8, preferably, is constructed of clay. Finally, the graphics are printed upon particulate mineral layer 8 by any conventional printing techniques. It is imperative that during attachment or sealing of the lid to the vessel that the sales graphics 10 should not be distorted or marred. In order to avoid this, radiation-cured overprint varnish layer 12 is placed over printed graphics layer 10 such that layer 12 does not stick to the heated platen and remains mass stable above 163°C. Two processes, namely, Electron Beam (EB) and Ultraviolet Curing (UV), may be used to produce the radiation-cured overprint varnish layer 12. In the EB process, a coating (having 100% solids) is applied to the substrate in a liquid state following printing of sales graphic layer 10. The coating is then exposed to highly accelerated electrons and reaction occurs in which the chemical bonds in the coating are broken and a new-modified overprint varnish layer 12 is formed. This process is referred to as polymerization. The polymerization causes significant physical changes in the product being treated and results in many desirable characteristics such as heat and scuff resistance. The UV process is similar in that the coating (having 100% solids) is applied in a liquid state over the printed graphics layer 10. The coating is then exposed to a UV light source. Photochemical initiators in the coating form free radicals. The free radicals initiate the cross linking of monomers and oligomers, which result in a rapid curing of the overprint varnish layer 12. As with the EB process, a durable overprint varnish layer 12 suitable for platen heat sealing systems is produced. With respect to the use of a radiation-cured varnish, the information below clearly indicates the superior performance of the coatings of the present invention (RAD.) as opposed to the conventional coatings (STD.). The temperature range covered is typical for the applications previously described. The sealing pressure can be much greater. A higher pressure would intensify the problem indicated with the standard coating. The dwell times cover a typical range depending upon the polymer being sealed and the board caliper (thickness). TEST CONDITIONS Equipment Sentinel Bar 25.4 mm Sealer Position of Sample Printed surface next to heated bar Samples Tested STD. = Standard water-based ink and varnish RAD. = Radiation-cured ink and varnish Pressure 0.414 MPa Dwell Time 1 and 3 seconds Grading 1 - No indication of marring 2 - Slight dulling of surface 3 - Dulling of surface and slight ink picking 4 - Dulling of surface, ink picking, and slight discoloration 5 - Severe discoloration, ink picking, and dulling GRADE 1,0-Second Dwell Time 3.0-Second Dwell Time Temp. °C STD. RAD. STD. RAD. 163 2 1 2 1 176 2 1 3 1 190 3 1 3 1 204 3 1 3 1 218 3 1 4 1 232 4 1 5 1 246 4 1 5 1 260 5 1 5 2

Claims: A composite lid (2) having an overprint varnish, wherein said composite lid (2) is comprised of: a paperboard substrate (4) having first and second sides; a layer (8) of particulate minerals located exterior to said first side of said substrate (4); a layer (10) of printed graphics located exterior to said coating (8) of particulate minerals; a layer (12) of a radiation-cured, heat resistant varnish located exterior to said layer (10) of printed graphics; and a layer (6) of a heat sealable barrier material located exterior to said second side of said substrate; characterised in that said radiation-cured varnish is mass stable above 163°C and said varnish prevents said layer (10) of printed graphics from being distorted during a pressure platen heat sealing of said composite lid (2) to a vessel and said varnish does not adhere to said platen during said heat sealing. A composite lid as claimed in claim 1, characterised in that said lid (2) comprises a layer of particulate minerals located exterior to said second side of said substrate (4) and interior to said barrier layer (6). A composite lid as claimed in claim 1 or 2, characterised in that said particulate minerals are comprised of clay. A method of constructing a composite lid (2) having an overprint varnish, wherein said method comprises the steps of: coating a first side of a paperboard substrate (4) with a layer (8) of particulate minerals; printing a layer (10) of graphics substantially over said particulate mineral layer (8); coating said printed graphics layer (10) with a layer (12) of a heat resistant, radiation-curable varnish and curing said varnish layer (12) by electron beam or ultraviolet radiation; and coating said second side of said substrate (4) with a heat sealable barrier layer (6); characterised in that said varnish is mass stable above 163°C and said varnish prevents said layer (10) of printed graphics from being distorted during a pressure platen heat sealing of said lid (2) to a vessel, and said varnish does not adhere to said platen during said heat sealing. A method as claimed in claim 4, characterised in that said varnish coating is radiated with an electron beam. A method as claimed in claim 4, characterised in that said varnish coating is radiated with an ultraviolet beam.

Inventors: CALVERT BARRY GENE, CALVERT, BARRY GENE
Applicants: WESTVACO CORP, WESTVACO CORPORATION
Application year: 1996
Main classification: B41M7
IPC codes: D21H19, B41M7, B65D43, B65D77