US 2793966 A
Description (OCR text may contain errors)
NON-CURLING GUMMED PAPER, METHOD AND' COMPOSITION FOR MAKING SAME No Drawing. Application January 11, 1954, 1
Serial No. 403,433
6 Claims. (Cl. 117-122) This invention relates to gummed paper, that is, paper having adherent thereto a film of dry adhesive, commonly referred to as gum, which when moistened with water, develops an adhesive tackiness permitting the application of the paper to another article in a position of use. Printed labels are articles consisting essentially of pieces of such paper, and perhaps the most familiar example of such a label is the ordinary postage stamp. Adhesive of the character referred to is commonly called a remoistenable adhesive. The thickness of the coating film is usually specified in terms of adhesive to ream of paper. This is the equivalent of a statement of so many pounds to so many units of area. A fifteen to twentyfive pound coat of glue is common. The coating of dextrin on a postage stamp is thinner. Very roughly a fifteen-pound coating is about a mil thick. Herein in referring to a film I refer to a coating of the kind usual on gummed labels and similar products. The thickness is at most a few mils.
The object of the present invention is to provide such a gummed paper which will lie flat and will continue to lie flat under varying conditions of atmospheric humidity to which the product may be exposed.
Gummed papers are commonly produced by preparing a solution of the chosen gum in a coatable consistency, applying a film thereof to paper, and evaporating the volatile ingredient to yield a dry, non-tacky, continuous film of potentially adhesive material.
Adhesive coated papers produced in this manner are troublesome in practical use because of the tendency .of the gummed sheets or strips to curl or roll up. This effect is due to the fact that the paper backing and the adhesive layer respond differently to changes in atmospheric moisture content. Although both the paper backing and the adhesive layer expand as they absorb moisture from the atmosphere and contract as they release moisture to the atmosphere, normally this behavior is much more pronounced in the adhesive layer and thus it expands or contracts to a greater degree than the paper backing. Thus if such a sheet is subjected to an atmosphere of high moisture content, the adhesive layer will expand more than the paper backing, causing the sheet to curl away from the adhesive side. If such a sheet is subjected to an atmosphere of low moisture content, the adhesive layer will contract more than the paper backing.
The most important and effective method resorted to by manufacturers in an attempt to counteract these disadvantages in producing what the trade refers to as gummed flats is the use of breakers. This involves passing the coated sheet over a series of bars set at various angles causing the adhesive layer to be broken into small sections. Theoretically, each small section thus produced reacts to variations in moisture content of the atmosphere independent of all other sections thus lessening the tendency of the'sheet to curl. In actual practice, this technique is only partially .efiective since that portion of the adhesive layer in contact with the paper surface is reinforced by paper fibers and is particularly resistant to rupture. Thus, though the surface United States Patent I ice of the adhesive layer is broken, that portion of the film in contact with the paper remains continuous and reacts to variations of atmospheric moisture content with suflicient force to cause curl. Furthermore, if such sheets so processed are subjected to an atmosphere of high moisture content, each section of the adhesive layer will expand and unite with adjacent areas tending to reform into a continuous layer. For this reason, manufacturers of this type of product usually specify that finished material be stored in an atmosphere of low moisture content. The breaking is an added operation. It is hard to control. Damage to the sheet may be caused and, because of the high tension used, rupture of the sheet is not uncommon.
In accordance with the present invention a paper is provided coated with a solid remoistenable film which to the naked eye (megascopically) is continuous and homogeneous, but it is composed in major proportion of an aggregation of minute discrete particles of solid but remoistenable adhesive which particles are adhered to the paper and to each other by a minor proportion of the film-forming adhesive which itself is water soluble. Such a product may be prepared by mixing the primary adhseive ingredient in its minute particle form as a solid with a solution of the second or binder ingredient in a volatile liquid which does not dissolve the primary ingredient, the resulting mixture being a fluent one of coatable consistency. This is coated on the paper in accordance with the usual practices of the coating art and the volatile liquid ingredient evaporated.
I have not attempted to illustrate by a drawing the nature of the film. Megascopically (to the naked eye) it appears homogeneous and continuous. A drawing would have to be on an enormously exaggerated scale and in detail arbitrary and diagrammatic. Thus, it might be misleading rather than informative. The individual discrete particles presumably differ markedly in size and shape on the microscopic scale although in bulk and statistiaclly they are a uniformly fine powder. Whether in a given formulation the discrete particles which constitute the major proportion of the volume are immersed in a substantially continuous matrix of material cementing them to one another and to the paper, or whether individual particles are coated and thereby secured atpoints of contact to one another and to the paper is a question of microscopic or submicroscopic structure which might depend on the particular formulation. It may well be that both states might locally occur simultaneously, although from the point of view of the label maker or the label user the applied film would be homogeneous and uniform.
It may be desirable at this point to illustrate what has been said ,by giving an example and a counter-example. The following is an example of the invention:
Example A Material: Parts by weight Dextrin Polyvinyl methyl ether 20 Toluol 45 .and consisting of separate finely divided particles of dextrin bound together by the polyvinyl-methyl ether.
Patented May 28, 1957 ditioning, etc.) is required.
Material: Parts. by weight Dextrin 8O Polyvinyl methyl ether. 20 Water 150 The dextrin and polyvinyl methyl ether are dissolved together inthe water, and agitated until a homogeneous solution is obtained. This makes a coatable composition which when applied to paper and dried by evaporation of the water, produces. a transparent homogeneous layer.
Unless theproduct of Example B is dried. slowly under controlled conditions of humidity and without mechanical restraint the sheet will form a, tube during drying and be impossible to handle. If mechanically restrained until wound into rolls it will dry in an unbalanced condition and, for example, a sheet severed from the roll will immediately curl. If conditioned, on breakers in such manner as to exhibit no curl at 50% relative humidity and 70 degrees F., an increase in atmospheric moisture content to 62% relative humidity or higher will cause a definite curl away from the adhesive face. If the atmospheric humidity is reduced from 50% relative humidity to 33% relative humidity or lower, a definite curl toward the glue surface results.
In the description of Example A above reference is made to the dextrin as being finely ground. At an earlier point I have expressed the idea by referring to it as forming a. fine powder. Since, as I have stated, the film is megascopically homogeneous some idea of what is meant by fineness will be obvious. If the film coating is, for instance, to have an apparent thickness of only a mil, obviously, except for scattered instances, the particles must be less than a mil in mean diameter. In fact, preferably the great majority of them are much finer than that and their size could more suitably be expressed in terms of microns than in terms of mils. The fineness is, of course, statistical. It is not to be assumed that they will be of identical sizes or shapes if accurately measured in microns. It would of course be meaningless to say that they are as fine as possible, but in general, the finer they are, short of colloidal size, the better the product. By minute I mean that the great majority of all the particles are smaller than the thickness. of the film. As an aggregation they are desirably as fine as it is commercially feasible to produce them.
The use of dextrin as the solid ingredient is an advantageous choice because it is available initially in the form of a very fine powder, presumably reflecting the size of the starch granules from which it is produced. In the case of bone glue or other potentially adhesive materials not commercially available in the particle size desired, such material may be dry ground or ground in dispersed form in the solution of the other component until a particle size is obtained which provides a satisfactory gummed product.
The particular materials given in Example A are not the only ones which might be used. For instance, the solid particles might be of ordinary animal glue instead of dextrin or of vinyl methyl ether-maleic anhydride copolymer or of carboxy methyl cellulose. The binder ingredient might be vinyl methyl ether-maleic anhydride copolymer or hydroxy-propyl methyl cellulose ether.
Suitable solvents, depending on the nature of the materials utilized as particles and, as binder, may be chosen from in addition the binder ingredient must be soluble in a solvent in which the primary minute particle ingredient is not soluble.
The proportions of the two may also be varied within considerable limits, except that the solid particle component is the major proportion of the solid content of the film. Theoretically one might postulate that the solid particles should constitute the adhesive layer subject to the requirement that they should be essentially isolated one from another and secured by the secondary binding component and, conversely, that the latter should be present in the minimum quantity necessary to secure the particles together and to the paper. Assuming this to be an ideal, close approximationthereto is not required in practice, particularly when the binder used is of a very flexible nature. Satisfactory practical formulations may readily be determined by empirical methods.
It is apparent also that more than one material could be used in discrete particle form and more than one material as the binder ingredient provided the ingredients respectively satisfied the primary conditions above referred to.
The results cited under Example A are characteristic of those obtained by the practice of the invention. As they are empirically demonstrated I deem it unnecessary to theorize as to the physical action which occurs when humidity conditions to which the product is subjected vary, particularly since on account of the minute scale of the components of the film the action cannot conveniently be directly observed, although what takes place may be inferred with considerable confidence.
I am aware that the invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and I therefore desire the present embodiment to be considered in all respects as illustrative and not restrictive, as is in fact clear in several matters from the description itself. Reference is to be had to the appended claims to indicate those principles of the invention exemplified by the particular embodiment described and which I desire to secure by Letters Patent.
1'. Remoistenable gummed paper comprising a paper base having adherent to a face thereof a potentially adhesive solid fihn which when moistened by water develops in the film an adhesive tackiness permitting the securing of the material in a position of use, the film comprising a multiplicity of minute, discrete, solid particles ofwater soluble gum intimately mechanically admixed as a dispersed phase in the film in substantially uniform proportions throughout the mass of the film, and constituting the major portion of its volume, the film also including as an external phase a different gum, likewise water soluble and also soluble in a solvent in which the first named gum is insoluble, coating the particles and cementing them together and to the base, the paper and the adherent film being as a whole substantially flat and remaining so under varying conditions of humidity to which they maybe exposed.
2. Remoistenable gummed paper as set forth in claim 1 wherein the particles are of dextrin.
3. Remoistenable gummed paper as set forth in claim 1 wherein the particles are of glue.
4. Remoistenable gnmmed paper as set forth in claim 1 wherein the other gum is polyvinyl methyl ether.
5. The method of making non-curling gununecl paper which comprises dispersing a multiplicity of minute discrete solid particles of water-soluble gum in a solution, inlasolvent inwhich the aforesaid gum is insoluble, of
6 homogeneous and wherein the discrete particles as such form the major portion of the volume.
References Cited in the file of this patent UNITED STATES PATENTS 2,125,241 Humphner July 26, 1938 2,477,344 Neumann July 26, 1949 FOREIGN PATENTS France Jan. 5, 1945