US2917396A - Flat gummed paper - Google Patents

Description

' United States Patent No. 2,793,966.

United States Patent FLAT GUMMED PAPER Arnold Agulnick, Worcester, Mass., assignor to Dennison Manufacturing Company, Framingham, Mass, a corporation of Massachusetts No Drawing. Application October 25, 1957 Serial No. 692,252

2 Claims. 01. 106-128 This invention relates to gummed sheet material, that is, sheet material having a coating of dry adhesive which can be moistened with water to develop an adhesive tackiness which permits the sheet to be attached to another surface. Such coated sheets are common articles of commerce, gummed labels and postage stamps being common examples. Adhesives of this nature are commonlyv called remoistenable adhesives, glue or gum and these terms are herein used to include any normally nontacky adhesive which can be activated with water.

Gummed sheet materials of this character have been troublesome in use because of their tendency to curl or roll up with changes in relative humidity due to the unequal expansion and contraction of the adhesive coating and of the backing sheet. This problem is particularly troublesome in gummed sheets which receive printing for labels and the like, which have to be run through printing presses in controlled registration therewith. This dilficulty has necessitated printing rooms with controlled relative humidity.

A number of expedients have heretofore been attempted to correct this problem but with only partial success. Recently a new expedient has been disclosed in This method involves the dispersion of minute particles of solid waterreactivatable adhesives in a. solvent solution of a second gum which is also water-soluble. The solvent used for the dispersion dissolves the second or carrier gum which is pres'ent in a minor proportion when compared with the dispersed water-reactivatable adhesive, but in which the latter is not soluble. After coating and drying, the minute solid particles of water-activatable adhesive are presumably discontinuous in a matrix of the second gum. This method produces a gummed sheet of superior flatness but introduces its own problems. An important problem with this method is the necessity for obtaining the waterreactivatable adhesive in a very fine particle size, usually less than one mil. Such materials, being adhesive and moisture-sensitive, are extremely difficult to grind and maintain in this small particle size. Also this method requires substantial amounts of a second or carrier gum which is quite expensive when compared with the watersensitive adhesive. Also, where the water-reactivatable adhesive is included as solid particles rather than deposited from a water solution, the glue is not as readily reactivated as is desirable.

The principal object of this invention is to provide superflat gummed sheet material, together with a method and coating composition for making the same which will remain substantially flat even after repeated changes in relative humidity. Further objects are to provide superfiat gummed sheet material having an adhesive layer which is firmly bound to the carrier sheet, which will not block or adhere to superposed sheets when stacked with other sheets of like material, which is readily activated with water, which is easily prepared, and which is relatively inexpensive when compared with previous methods.

In accordance with this invention the improved superflat gummed sheet material of the water-remoistenable type comprises a sheet having on one side thereof a'po tentially adhesive dry film which when moistened with Water will adhere to a surface, said film comprising in discontinuous phase a multiplicity of minute particles of water soluble gum, said particles adhesively contacting the surrounding particles and the sheet, and a continuous phase of a polymeric film-forming binder joining said par ticles together and to the sheet, said binder being soluble in water and also soluble in another solvent in which said water-soluble gum is not soluble, said film being sulficiently open to permit rapid activation by water. While the water-activatable gum is held in substantially discontinuous phase, the gum particles or agglomerates do con;

tact and adhere to a slight extent to each other and to the support, thus contributing to film strength and to adhesion to the support. Indeed, if the binder be extracted with solvent, the gum remains as a porous, self-supporting film. The particles of water-reactivatable gum should comprise a major portion of the total mass of the film.

Suitable binder resins include polyvinyl methyl ether,

i The above superflat gummed sheet material is made by a method which comprises dissolving a water-soluble binder in an incompletely water-miscible solvent, dissolving water-activatable gum in water, dispersing the.

water solution in the binder solution, coating the resulting composition on the sheet, and drying. After thecoat; ing has been dried the exposed surface of the carrier sheet should be treated under tension according to techniques, well known in the prior art, to equalize thecont'raction caused by wetting the coated side.

In a further aspect the present invention includes coatable composition for preparing water-remoistenable gummed sheet material comprising a dispersion of a water solution of a water-activatable gum dispersed in a water-immiscible solvent solution of a water-soluble; resinous binder, said gum exceeding by weight the amount;

of the binder.

To the naked eye the coatings of the present invention appear smooth and substantially homogeneous. However, microphotographs show that the coatings, after drying, have particles of water-reactivatable adhesive embedded in a water-soluble binder matrix. Both the binder and the gum are water-soluble, the gum particles are available for remoistening as is illustrated by the ease with which the film is made strongly tacky by moistening.

As an example of this invention the following example is given:

Example I Polyvinyl methyl ether 7.5 Toluene 32.5

Animal glue (50% in water) Example 11 Polyvinyl methyl ether 7.5 Toluene 32.5 Animal glue (60% in water) Example III N-butyl amine salt of polyvinyl acetate-maleic' anhydride copolymer 7.5 Toluene 75 Animal glue (50% in water) 75 The binder is dissolved in the toluene and a low viscosity animal glue is separately dissolved in water and P ten ed. Dec- 15,1259.

maintained at a temperature of approximately 150 F. The glue solution at 150 F. is added to the solvent solution of the binder with vigorous agitation.

The dispersion technique is critical in making coating according to this'invention. The water-activatable adhesive or gum must make up a major portion of the dried film and must be dispersed as a water solution in a solvent solution of the water-soluble binder. The dispersion must be a dispersion of the water-in-oil type with the solvent phase as the continuous phase and the water as the discontinuous. For this purpose the solvents used to dissolve the binder must not dissolve the water-activatable gum and must be incompletely miscible with water. Whether the solvent comprises a single liquid or a mixture of liquids the only requirement is that it be sufficiently immiscible with water to keep the water solution in dispersed phase. In addition to the toluene shown above, benzene, n-butyl alcohol, cycloxhexanone, ethyl acetate, nitroethane, and nitropropane may be used as binder solvents. The solvent or solvent mixture used to dissolve the binder must show preferential solubility for the particular binder used; that is, in the presence of the solvent-water mixture a greater portion of the binder must dissolve in the solvent. Various starches, starch ethers, dextrins and polysaccharides and mixtures thereof may be used in place of or with the animal glue. It is desirable that the water solution contain the highest possible solids commensurate with ease of preparation so that a minimum amount of water will be required. For this reason it is desirable to use low viscosity glues and proteins. Various materials such as solvent-soluble dispersing agents may be added to improve the stability of the mixture.

The continuous phase binder must adhere to the gum and to the base on which it is coated. Since both the binder and the gum are water-soluble the gum is available to the water used in remoistening. Additionally it isbelieved that the film structure is open, porous, and sponge-like due to the evaporation of the water and solvents as the coating is dried. This sponge-like structure is an additional aid in making the water-activatable gum available for remoistening. In addition to aiding and providing the sponge-like structure of a coating the deposition of the gum from a water solution dispersed in the binder solution has a number of other advantages. Gum which has been thoroughly digested and dissolved in water is more readily activated by remoistening and more quickly develops good tack. The wet gum, being adhesive as it dries, gives better internal binding of the film and to the support since it adhesively contacts the particles in association with the gum particles. While the gum particles may contact one another to some degree they are still present in a substantially discontinuous phase. This binding action resulting from the wet glue permits a higher percentage of the desired gum to be used, making a better and cheaper product. Again, the use of a wet gum obviates the need for grinding gum into very fine particles as taught in the prior art. When the water solution is heated and then added to the solvent solution with vigorous agitation, the gum will be dispersed with a very fine particle size and the resulting coating will be glossy. On the other hand, when the water solution is added cold to the solvent solution, the resultant coating will have a dull mat finish due to larger particles of gum. By this means controlled particle size can be obtained in the final coating without the need for grinding dry gum.

It should be understood that this disclosure is for the purpose of illustration only and that the invention includes all equivalents and modifications which fall within the scope of the appended claims.

I claim:

1. The method of making a substantially non-curling gummed sheet of the water-remoistenable type which comprises dissolving a water-soluble binder in a solvent, dissolving water-activatable gum in water, dispersing the water solution in the binder solution, coating the resulting composition on the sheet, and drying, the solvent being sufi'iciently immiscible with water to keep the water solution is dispersed phase and possessing preferential solubility for said hinder, the weight of the gum exceeding the weight of the binder.

2. A coating composition for preparing water-remoistenable gummed sheet material comprising a dispersion of a water solution of a water-activatable gum dispersed in a solvent solution of a water-soluble resinous binder, the solvent being sufficiently immiscible with water to keep the water solution in dispersed phase and possessing preferential solubility for said binder, said gum exceeding by weight the amount of the binder.

References Cited in the file of this patent UNITED STATES PATENTS 2,215,848 Bauer Sept. 24, 1940 2,365,020 Stillwell Dec. 12, 1944 2,577,821 Smith et a1 Dec. 11, 1951 2,793,966 Davis May 28, 1957

Claims (1)

1. THE METHOD OF MAKING A SUBSTANITALLY NON-CURLING GUMMED SHEET OF THE WATER-REMOISTENABLE TYPE WHICH COMPRISES DISSOLVING A WATER-SOLUBLE BINDER IN A SOLVENT, DISSOLVING WATER-ACTIVATABLE GUM IN WATER, DISPERSING THE WATER SOLUTION IN THE BINDER SOLUTION, COATING THE RESULTING COMPOSITION ON THE SHEET, AND DRYING THE SOLVENT BEING SUFFICIENTLY IMMISCIBLE WITH WATER TO KEEP THE WATER SOLUTION IS DISPERSED PHASE AND POSSESSING PREFERENTIAL SOLUBILITY FOR SAID BINDER, THE WEIGHT OF THE GUM EXCEEDING THE WEIGHT OF THE BINDER.