US3574669A - Nonblocking coated sheet material - Google Patents

Abstract

A FLEXIBLE PACKAGING SHEET MATERIAL BEARING A TACKY WAX COMPOSITION COATING AND A NONBLOCKING OVERCOATING OF A POLYAMIDE.

Description

United States Patent O 3,574,669 NONBLOCKING COATED SHEET MATERIAL Arthur Ruthven Chase, Neenah, Wis., assignor to American Can Company, New York, N.Y. No Drawing. Filed July 22, 1968, Ser. No. 746,241 Int. Cl. B32b 23/08 U.S. Cl. 11776 1 Claim ABSTRACT OF THE DISCLOSURE A flexible packaging sheet material bearing a tacky wax composition coating and a nonblocking overcoating of a polyamide.

BACKGROUND OF THE INVENTION This invention relates to a heat-scalable wrapping material and in particular to such material having a nonblocking surface overcoating applied thereto. More specifically, the invention relates to a wrapping material Including a flexible packaging material substrate such as paper or cellophane coated on one side with a thermoplastic heat-sealable wax and elastomer composition which is normally somewhat tacky at room temperature and finally a nonblocking overcoating composition superposed on the surface of the heat-scalable wax composition.

Flexible packaging sheet materials employing a surface coating of a composition including a substantial percentage of petroleum waxes of both paraffin and microcrystalline types have long been utilized in the packaging industry because of their excellent combination of protective properties, low cost, freeness from taste and odor, heat sealability and excellent appearance In terms of gloss and luster. Wax compositions utilized n many packaging applications include various modify ng additives such as natural and synthetic rubbers, reslns, polymers and the like, which alter many of the physical and protective properties of the coating in adapting it to specific applications. of particular lnterest in the present invention are wax compositions adapted to the manufacture of coated wrappers for cheese and cheese products, the wrappers bearing coatings comprising petroleum waxes, primarily of the microcrystalline type, and a small percentage of an elastomer such as polyisobutylene, generally ranging from 3 to of the total coating weight. These coatings, when applied to a flexible base sheet such as cellophane, metal foil or combinations of these ma terials with other sheet materials, yield heat-scalable Wrappers having excellent protective properties for cheese packaging. The surfaces of the coatings are, however, slightly tacky at room temperature and the wrappers tend to stick together when sheets are stacked one on another or when a coated sheet material is stored in roll form. This sticking or blocking of the coated sheet materials presents a severe problem in the use of these wrappers and has necessitated the application to the coating surface of a release-treated interleave sheet or a dusting of a tack-relieving material. The use of interleaving sheets is inconvenient and expensive and, although many materials, including starch, salt, hard surfaced waxes, silicones and the like have been applied to coated cheesewrapper surfaces to relieve blocking, all have had adverse effects on either the heat sealing qualities or the transparency of the wrapper, or have been unacceptable because of transfer to the surface of the cheese when the wrapper is removed from the product. Nonblocking coatings containing salt crystals also tend to accumulate water on the surface of the cheese in the vicinity of the salt, giving an undesirable appearance to the product.

It is therefore an object of the present invention to provide an improved cheese packaging material having a nonblocking overcoating material on the surface of a wax composition coated flexible packaging base sheet.

It is a further object to provide a flexible, wax composition coated cheesewrapper having a nonblocking overcoating of improved transparency and heat sealability, and highly resistant to transfer of any component of the wrapper to the packaged cheese product when the wrapper is removed therefrom.

U.S. Pat. 2,833,671 describes a nonblocking coating composition for overcoating wax composition coatings which comprises a normally solid, straight chain fatty acid, a solid, hydrogenated castor oil or solid polyethylene glycol as a nonblocking agent. The selected material is applied to the tacky surface of the wax composition in an organic solvent containing a small percentage of a film-forming binder material, cellulose ethers, polyvinyl acetate and polyamides being cited as satisfactory binders for this purpose. U.S. Pat. 3,340,089 discloses a nonblocking overcoating comprising salt crystals as a nonblocking agent suspended in an organic solvent containing a binding agent such as polyvinyl acetate, a polyacrylate, a polyamide, shellac or a cellulose ether. In each of these patents, the primary consideration is to cover the surface of the tacky wax composition with a multitude of dis crete solid particles or granules of material which serve to prevent contact of the tacky coating with a superposed sheet and thus avoid the problem of blocking.

SUMMARY OF THE INVENTION The present invention discloses a nonblocking overcoating composition of improved transparency, which does not interfere with the heat sealing qualities required of the wax composition and which does not transfer to the surface of the packaged product, the purpose of the invention being achieved Without the necessity of a'pplying granules of non-sticky materials to the surface of the coated sheet. It has now been found that a completely satisfactory nonblocking overcoating for tacky wax composition coated sheets may be achieved by applying on the surface thereof a thin film of a polyamide dissolved in a suitable organic solvent, followed by evaporative removal of the solvent. A suitable polyamide for this purpose is Versamid 940, which is a polyamide formed by reaction of a dimerized and trimerized fatty acid with ethylene diamine and is sold by General Mills, Inc., under the above trade name. This material is readily soluble in volatile organic solvents such as isopropyl alcohol and methyl ethyl ketone which wet, but do not dissolve to an appreciable extent the wax composition coatings normally utilized in food packaging at the temperatures at which the overcoating is normally applied.

DETAILED DESCRIPTION OF THE INVENTION To illustrate the principles of this invention, a cellophane sheet material coated with a normally tacky Wax composition coating comprising microcrystalline wax, 7% petrolatum and 15% polyisobutylene was overcoated with a polyamide, such as Versamid 940, sold by General Mills, Inc., dissolved in isopropyl alcohol to form solutions ranging in solids content from 2 to 15%. All percentages given above are on a weight basis. The solutions were applied to the wax composition surface by means of an etched roll partially immersed in a bath of the polyamide solution, the excess solution being doctored from the roll in conventional manner. The solvent was removed by evaporation to leave a thin, continuous film of the polyamide on the surface of the wax composition. The weight of polyamide applied per ream (3,000 sq. ft.) of sheet material was controlled by the concentration of the solution applied although it is quite possible also to obtain comparable control by varying the depth and pat- TABLE I Pounds of Overcoating composipolyamide per tion in percent solids of roam of coated Blocking Heat sealing polyamide sheet resistance quality Excellent. 0.3 cod... Do. 0. 4 Excellcn Do. 0.7 do... Do. 1. 2 do Poor to fair. 2.0 do Poor.

It is evident from the above data that overcoating compositions of the present invention when applied to the surface of a tacky, wax composition coated sheet material in amounts ranging from about 0.2 lb. to about 1.0 lb. per ream of such coated sheet impart substantially improved blocking resistance while retaining the desired degree of heat scalability. Preferably, the coating is applied in an amount between about 0.3 lb. and about 0.7 lb. per

ream.

In contrast to the above results, ethyl cellulose, when applied in like amounts as an overcoating, seriously affects the heat sealability of the coated sheet. Polyvinyl acetate, however, although not deleteriously affecting the heat seal characteristics of the sheet, gave a somewhat tacky overcoated surface and was therefore not satisfactory from a blocking prevention standpoint. Among the several materials previously utilized as the binding agent component in nonblocking coating compositions, polyamides were found to be unique in contributing all of the necessary characteristics of such coatings without the addition of further components such as salt, a fatty acid, starch or other similar materials known to assist in blocking prevention.

The polyamide coating, which was homogeneous and continuous, showed no transfer to the surface of cheese vacuum packaged in the overcoated wrappers described above, the cheese surface remaining completely free of any foreign material when the packages were opened after six months of storage under refrigerator conditions. All packages maintained a satisfactory vacuum for the test period. The protective properties of the wax composition coated sheet were unimpaired and the wrappers also exhibited low surface friction and superior performance in machine packaging operations.

The polyamide overcoated sheets are also quite satisfactory for vacuum packaging of sliced luncheon meats and similar food items, heat sealing characteristics, vacuum retention and protective properties having been am ply demonstrated in six weeks packaging tests with 6 to 8 ounce portions of thin sliced spiced luncheon meat.

In summary, this invention encompasses the discovery that polyamides soluble in the lower aliphatic alcohols or ketones or similar volatile organic solvents are uniquely capable of forming a completely satisfactory non-blocking overcoating on the surface of tacky wax composition coated sheet materials without the necessity of any additional adhesion-preventing materials. The resulting overcoated sheets show superior heat sealability, machine packaging characteristics, transparency and protective properties.

It is thought that the invention and its attendant advantages will be understood from the foregoing description and it will be evident that modifications may be made within the spirit of the invention, the embodiments herein described being exemplary rather than limiting.

I claim:

1. A flexible packaging sheet material coated with a heat scalable, normally tacky wax composition and overcoated with between about 0.2 lb. and about 1.0 lb. per ream of a continuous evenly distributed film of a nonblocking composition consisting essentially of a polyamide, wherein said polyamide is formed by reaction of a dimerized and trimerized fatty acid with ethylene diamine.

References Cited UNITED STATES PATENTS 2,728,737 12/1955 Wittcoif 161--235X 2,833,671 5/1958 Funk et al. ll792X 3,340,089 9/1967 Bougie 117-76(F) WILLIAM D. MARTIN, Primary Examiner R. HUSACK, Assistant Examiner US. Cl. X.R.