Abstract:
This invention relates to substrates for paperboard packaging having a press-applied barrier coating. Such structures of this type, generally, reduce the edgewick in the paperboard package such that catastrophic failure of the paperboard package is substantially reduced.

Description:
This application is a division of application Ser. No. 08/503,175, filed Jul. 17, 1995, now U.S. Pat. No. 5,747,141, patented May 5, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to substrates for paperboard packaging having a press-applied barrier coating. Such structures of this type, generally, reduce the edgewick in the paperboard package such that catastrophic failure of the paperboard package is substantially reduced. 
     2. Description of the Related Art 
     It is known, in paperboard packaging, to produce a paperboard carton with a press-applied barrier coating on the nonclay-coated side. Due to the lack of hold-out, severe pin-holing may be observed. On a clay-coated side, a nearly pin-hole free, press-applied film may be achieved. However, during folding and scoring operations, pin holes develop in the press-applied barrier film at corners and seams and may result in the food product attacking the paperboard layer resulting in catastrophic failure. Also, if a clay-coated surface is required beneath the press-applied film, this may require exterior printing on a nonclay-coated side if C1S (coated one side) board is used. This may dramatically reduce the print quality of the package. Consequently, a more advantageous substrate, then, would be presented if such edgewicking could be substantially reduced. 
     It is also known, in paper coating systems, to make use of styrene maleic anhydride as a coating binder for mineral coated paper. Exemplary of such prior art is U.S. Pat. No. 2,577,624 (&#39;624) to George E. Niles, entitled &#34;Mineral-Coated Paper and Process of Producing Same&#34;. While the styrene maleic anhydride is used as a coating binder for the mineral coated paper, its presence in the coating does not contribute to any edgewick resistance of the substrate. Thus, it would not adequately prevent edgewicking. Consequently, a still further advantageous substrate, then, would be presented if a suitable sizing compound were employed that prevented edgewicking. 
     It is apparent from the above that there exists a need in the art for a substrate for paperboard packaging which is capable of reduced edgewicking and Cobb measurements, but which at the same time is able to have a press-applied barrier placed on one side of the paperboard. 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. 
     SUMMARY OF THE INVENTION 
     Generally speaking, this invention fulfills these needs by providing a substrate for paperboard packaging, comprising a first layer of particulate minerals, a first layer of a sizing material including a copolymer binder for substantially preventing edgewicking located interior to the first layer of particulate minerals, a paperboard layer located interior to the first layer of the sizing material, a second layer of a sizing material located interior to the paperboard layer, and a second layer of particulate minerals located interior to the second layer of the sizing material. 
     In certain preferred embodiments, the particulate minerals are clay. Also, the copolymer binder is styrene maleic anhydride. 
     In another further preferred embodiment, essentially all of the edgewicking is reduced within the substrate such that the paperboard package should not catastrophically fail. 
     The preferred substrate, according to this invention, offers the following advantages: ease of assembly; excellent stability; excellent durability; good economy and excellent edgewicking characteristics. In fact, in many of the preferred embodiments, these factors of ease of assembly, stability, durability and edgewicking are optimized to the extent that is considerably higher than heretofore achieved in prior, known substrates. 
     The above and other features of the present invention, which will become more apparent as the description proceeds, are best understood by considering the following detailed description in conjunction with the accompanying drawings, wherein like characters represent like parts throughout the several views and in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical, cross-sectional view of the coated two-side base stock, according to the present invention; and 
     FIG. 2 is a top plan view of the coated two-side base stock, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference first to FIG. 1, there is illustrated substrate 2 for paperboard packaging. Substrate 2 includes, in part, conventional exterior particulate coating 4, sizing layer 6, paperboard substrate 8, surface sizing layer 10, and conventional particulate coating 12. 
     In particular, particulate coatings 4 and 12, preferably, are constructed of clay or calcium carbonate. Surface sizing layers 6 and 10, preferably, employ the use of styrene maleic anhydride binder (SMA) which is applied at a rate of around 4% concentration, by weight. Substrate 8 is, typically, constructed from a 0.018 inch thick sheet of solid bleached sulfate (SBS), solid unbleached sulfate (SUS) or clay coated newsback (CCNB). The term &#34;paperboard&#34; describes paper within the thickness range of 0.007 to 0.028 inches. The invention is relevant to the full scope of such a range, as applied to packaging and beyond. 
     The present invention incorporates a combination of various concepts in manufacturing ovenable paperboard substrates. These concepts are the use of coated two-sided board with a specific sizing method that is measured using a nontraditional method of measuring Cobb (Z-direction water penetration). Coated two-side board, according to the present invention, is manufactured without using the conventional high internal wet-end alum/size recipe for conventional C1S ovenable paperboard, but the more typical recipe used for standard folding carton solid bleached sulfate (SBS) in order to enhance runnability. Additional sizing is achieved using a size press application of a surface size such as styrene maleic anhydride. This allows the Cobb value to be reliably obtained below 120 without the difficulties in manufacturing a highly sized, coated two-side board. 
     Since the board of the present invention is clay-coated on both sides, Cobb cannot be measured using the standard apparatus that measures four square inches. A smaller Cobb device, which measures one square inch, is utilized and the measurement is taken at the edge strip where the base stock 2 is not clay coated (FIG. 2). This is a less desirable measurement than the TAPPI standard method, but surface size has been demonstrated to more reliably size to a particular Cobb than internal sizing, especially when coated two-side broke is used in the furnish. The size press application of the surface size (layer 6) also penetrates the board sufficiently such that edgewick is reduced sufficiently. 
     The following Example is provided to further illustrate the present invention and is not to be construed as limiting the invention in any manner. 
     EXAMPLE 1 
     Coat Weights of Particulate 
     Mineral Coating: 10 lbs/ream on each side 
     Coat Weights of Sizing Material: approximately 2 lbs/reams 
     Concentration by weight of SMA Copolymer in Size Press Final Solids: 4% 
     Board Caliper:0.018 
     TAPPI Cobb (Triton) 45 seconds: 120 maximum 
     This method of manufacturing yields a substrate that can be converted into cartons, trays and lids for holding oily and aqueous food products. Pin holing was not encountered and minimal effect was observed in the runnability of the paper machine. Finally, proper selection of the press-applied barrier coating may yield a carton that is suitable for cooking in the microwave or conventional oven. 
     Once given the above disclosure, many other features, modifications or improvements will become apparent to the skilled artisan. Such features, modifications or improvements are, therefore, considered to be a part of this invention, the scope of which is to be determined by the following claims.