US3403045A - Coated release paper - Google Patents

Description

United States Patent 3,403,045 COATED RELEASE PAPER Richard C. Erickson and Robert C. Williams, Richmond,

Va., assignors to Albemarle Paper Company, Richmond, Va., a corporation of Virginia No Drawing. Continuation-impart of application Ser. No. 286,453, June 10, 1963. This application Mar. 3, 1967, Ser. No. 620,252 7 Claims. (Cl. 11768) ABSTRACT OF THE DISCLOSURE This invention is concerned with the provision of paper sheets having markedly enhanced release properties by virtue of having a highly effective lamellar coating affixed thereto, said coating consisting of a silicone polymer coating which is carried by a polyethylene or other comparable plastic undercoating.

CROSS REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 286,453, filed June 10, 1963, now abandoned.

BACKGROUND OF THE INVENTION As is well known, the paper industry is continually called upon to provide paper products of enhanced utility for specific applications. In the packaging of tacky materials, such as rubber, asphalt, and the like, as well as in the production of pressure sensitive tapes, it is necessary to provide paper sheets having good release properties. In the absence of suitable release properties, the paper would adhere tenaciously to the tacky substance and cause obvious disadvantages in use.

It is common practice to utilize organo polysiloxane coatings upon paper in order to provide suitable release properties. These organo polysiloxanes (commonly referred to as silicone polymers such as poly dimethyl siloxane) have very good release properties and are essentially free from the tendency to migrate to the tacky surfaces; however, these silicone coatings are very expensive. Furthermore, there is a limit to the ability of organo polysiloxane coatings to afford optimum release properties. Experience has shown that the application of commercially feasible amounts of silicone coatings to kraft paper provides at best a Keil release value of 30 grams. Generally speaking, silicone coatings on kraft paper produce Keil release values ranging from 30 upwards to about 300 grams. A significant contribution to the art would be the provision of a kraft sheet having a Keil release value lower than 30 grams so that the article could be considered as a super release sheet. In this connection the standard Keil test measures release values according to a standard scale such that the lower the number the lower the adherence on or the better the release from the surface. Such a contribution would be particularly noteworthy if it could be accomplished through the use of reduced amounts of the expensive silicone coatings.

Accordingly, it is an object of this invention to provide paper sheets of enhanced release properties. Another object is to provide a super release sheet, that is, kraft paper sheets having markedly enhanced release properties by virtue of having affixed thereto a highly effective lamellar coating which is capable of providing Keil release values lower than 30 grams. A still further object is to provide novel articles and methods for their manufacice ture characterized by excellent release properties achieved at relatively low costs. A more specific object is to provide means by which the amount of conventional silicone release coatings for kraft paper and the like can be substantially reduced while at the same time obtaining better release properties than heretofore feasible on a commercial basis. Other important objects will be apparent from the ensuing description.

SUMMARY OF THE INVENTION In accordance with this invention, a lamellar coating is applied to kraft paper and the like and especially to kraft paper having a Gurley densometer value of at least 20 seconds per 100 cc. Upon this paper is afiixed a coating of polyethylene, preferably in an amount equivalent on a dry basis to from about 1 to about 25 pounds per 3000 square feet. Superimposed and afiixed upon this polyethylene coating is a cured organo polysiloxane coating, preferably in an amount equivalent on a dry basis to from about 0.05 to about 1 pound per 3000 square feet. Methods and techniques for applying these respective coating layers are available in the art.

A highly significant feature of this invention is the discovery that the above described lamellar coated kraft paper often possesses a Keil release value in the range of 10 to 15 grams even though only about 20 percent as much silicone coating is used as compared with that amount heretofore necessary to obtain a Keil release value of 30 grams. Further, polyethylene itself has a Keil release value in the order of 100' grams or more.

Hence, this invention may be viewed as interposing a layer of a material having a relatively high Keil release value between kraft paper and a silicone coating which normally has at best a Keil release value of 30 grams and thereby achieving a release value significantly lower than can be achieved from practical amounts of either coating material when used alone.

DESCRIPTION OF THE PREFERRED EMBODIMENT The porosity of the kraft paper used as the substrate material for the production of the release paper of this invention may vary over the wide range of porosities generally found in the production of papers. Release papers having the desired properties may be prepared from papers having a wide range of Gurley densometer values; however, as stated previously, kraft papers having a Gurley densometer value of at least 20 seconds per 100 cc. are preferred. For best results, the kraft paper should have a Gurley densometer value of at least about 50 seconds per 100 cc., and paper with a densometer value of from about 50 to about 200 seconds per 100 ccs. is especially preferred. It is also highly desirable to utilize kraft paper having a basis weight of from about 30 to about pounds per 3000 square feet, although other grades may be utilized for specific applications. It will be understood that kraft paper is utilized primarily because of its desirable physical characteristics and relatively low cost. However, the principles of this invention may be applied if desired to the utilization of other fibrous cellulosic sheet material, such as bleached paper and the like as well as papers made from pulps prepared by chemical, mechanical or chemical-mechanical processes other than the kraft process.

In particularly preferred embodiments of this invention the intermediate polyethylene coating is affixed to the paper substrate in an amount equivalent on a dry basis to from about 2 to about 8 pounds per 3000 square feet as this amount provides the best results at the lowest cost. For the same reasons the polysiloxane coating superimposed thereon is most preferably utilized in an amount equivalent on a dry basis to from about 0.1 to about 0.2 pound per 3000 square feet.

Examples of particularly suitable articles provided by this invention include kraft paper having a Gurley densometer value of from about 50 to about 200 seconds per 100 cc., having aflixed thereto a coating of polyethylene in an amount equivalent on a dry basis to from about 2 to about 8 pounds per 3000 square feet, upon which coating is affixed a cured organo polysiloxane coating in an amount equivalent on a dry basis to from about 0.1 to about 0.2 pound per 3000 square feet, especially when the kraft paper has a basis weight of from about 30 to about 70 pounds per 3000 square feet.

To prepare the novel lamellar coated papers provided by this invention, the appropriate amount of polyethylene is applied to the kraft paper substrate and thereafter an appropriate amount of the coating of the organo polysiloxane is applied to the polyethylene layer and subjected to a cure, preferably a heat cure. Depending upon the end use of the coated article, the kraft paper substrate may be coated on one or both of its sides in the manner described above.

To apply the polyethylene to the kraft paper, use may be made of such techniques as emulsion coating, solution coating, and like techniques, or, if desired, the polyethylene layer may be extruded upon the paper surface. Conventional commercially available grades of polyethylene, both high density and low density, may be employed. The main features of this step are to apply the polyethylene resin substantially uniformly over the paper surface to be coated and to utilize for this purpose the preferred amounts described above.

Application of the silicone coating likewise involves use of techniques known to those skilled in the paper coating arts. Hence, the material-usually in monomeric or partially polymerized form-can be applied by such means as air-knife coating, trailing blade, hair brush coater, and roll coater. For best results, the silicone coating which is thereby applied substantially uniformly upon the surface of the polyethylene is dried in contact with a polymerization catalyst and then cured as quickly as possible so that the desired amount of polymerized silicone is held on and tightly aflixed to the surface. This is readily accomplished by drying the silicone coating-usually, but not necessarily, applied as an emulsion containing the polymerization catalystby the use of drier ovens, hot air ovens, high velocity air driers, infrared heaters, infrared lamps, or suitable combinations of these. When utilizing common commercially available silicone coatings in emulsion form an especially suitable cure condition is to expose the same for 10 seconds to a temperature of 350 F. The use of higher temperatures will give shorter cure times. Further information and details relative to silicone coatings are well known to the art and reported in the literature, for example, in Rochow Chemistry of The Silicones, second edition, John Wiley and Sons, Publisher (1951).

Generally speaking, the silicone bath formulations contain from about 1 to about 10 percent by weight of silicone, preferably 7-10 percent. Examples of the silicones are dialkyl polysiloxanes, diaryl polysiloxanes, alkyl-aryl polysiloxanes, and mixtures thereof. The formulations will further contain a conventional quantity of a polymerization catalyst to effect polymerization or further polymerization of the silicone contained in the bath. Exemplary of such catalysts are zinc and tin soaps (such as zinc and tin octoate, zinc and tin stearate, zinc and tin oleate, etc.), alkyl tin carboxylates (such as dibutyltin dilaurate, etc.), mixed stannous and stannic polymerization systems, organic amines, or other soaps (e.g., iron octoate, etc.), or the like. An example of such a formulation is as follows:

Pounds Methyl silicone emulsion (40% solids) 65 Silicone polymerization catalyst (24% solids) 13 Car-boxymethyl cellulose as a viscosity modifier 10 Acetic acid to prolong pot life 4 Water 273 Exemplary of the practice and advantages of this invention, sheets of various types of kraft paper as described above were coated with polyethylene (2 pounds per 3000 square feet). This was accomplished by dissolving commercially available polyethylene in hot toluene (approximately 190 F.) and applying this'solntion uniformly'over the surface of the paper sheets. After evaporating the toluene a roll coater and a rod doctor were.used to apply the silicone from a 7 percent bath having the composition corresponding to that given above. This coating was subjected to a cure temperature of 550-600 F. for 4 seconds. The resultant lamellar coated kraft sheets were found to possess Keil release values of 10-15 grams.

On a production basis the coating sequence of this invention may be effected utilizing the same coating machine or the paper stock may be passed into a first coating apparatus for application of the polyethylene coating and thereupon be passed into a second coater for application of the silicone laminate.

While this invention has been discussed with particular reference to the utilization of polyethylene as the intermediate layer, the principles and practice of this invention extend to the utilization of other comparable plastics, such as polyvinyl chloride, polyvinylidene chloride, polypropylene, polyvinyl alcohol, polyvinyl acetate, or any solutions, emulsions, or molecular dispersions which contain such plastic materials. Likewise copolymers of ethyl- P ene, propylene, vinyl chloride, vinylidene chloride, vinyl alcohol, vinyl acetate, and the like may be used. Mixtures of the foregoing may also be used. For most applications polyethylene is most suitable because of its abundance, ready availability, relatively low cost, and excellent release properties when utilized pursuant to this invention. As will be apparent to those skilled in the art, the coating solutions, emulsions, or molecular dispersions may also contain extenders, such as clay, pigment, or fillers, the use of which will be dependent to some extent on the ultimate use to which the end product is to be put.

Accordingly, the novel and useful contribution constituting the present invention is as defined in the ensuing clfaims and extends to the full range of equivalents there- 0 What is claimed is:

1. In a release paper having a release type silicone polymer coat1ng imparting release characteristics to the paper, the mprovement according to which the silicone polymer coating is carried by a polyethylene undercoating weighmg about 1 to about 25 pounds per 3000 square feet of the paper.

2. The combination of claim 1 in which the paper is kraft paper.

3. The combination of claim 1 in which the paper has a basis weight of from about 30 to about 70 pounds per 3000 square feet.

4. The combination of claim 1 in which the paper has a Gurley densometer value of at'least 20 seconds per cc.

5. The combination of claim 1 in which the pair of coatings is duplicated on'both faces of the paper.

6. The combination of claim 1 in which the polyethylene weighs from about 2 to about 8 pounds per 3000 square feet and the silicone polymer coating Weighs from about 0.1 to about 0.2 pound per 3000 square feet.

7. The combination of claim 6 in which the paper is kraft paper having a Gurley densometer value from about 50 to about 200 seconds per 100 cc. and a basis weight from about 30 to about 70 pounds per 3000 square feet.

(References on following page) References Cited UNITED STATES PATENTS Lewis et a1. 117155 Meyer-Jagenberg 117155 X Wilkins 117155 X Eder et a1. 117155 X 6 OTHER REFERENCES Meals et a1.: Silicones, Reinhold Publishing Corp., New York, 1959, TP 248.55 M4 (pages 128-134 relied on).

5 WILLIAM D. MARTIN, Primary Examiner.

R. HUSACK, Assistant Examiner.