Process for manufacturing a flushable fibrous sheet material for use in sanitary products

Abstract

A process for producing a fibrous sheet material suitable for use in sanitary products and wherein there is provided in the sheet material a high molecular weight water soluble polymeric material which serves to effect bonding together of the fibers to avoid disintegration of the sheet through normal use in sanitary products, but which permits flushing of the sanitary products after use by dispersing the polymeric particles and the fibers in the flushing water. In accordance with the process, the polymeric material, in finely ground form, is introduced into the white water system of the paper machine and permitted to remain therein for a limited time, sufficient to permit hydration and swelling of the polymeric particles to assure their retention in the fibrous sheet, but insufficient for the polymer to dissolve in the water or be lost through the paper machine wire.

Parent Case Text

This application is a continuation of our earlier-filed copending application Ser. No. 95,388, filed Dec. 4, 1970, now abandoned.

Description

The present invention relates to an improved process for manufacturing a flushable fibrous sheet material which is suitable for use as a cover sheet in sanitary products and which is resistant to disintegration in normal use.

In U.S. Pat. No. 3,370,590, issued Feb. 27, 1968 to Kenneth C. Hokanson and Donald K. George, there is disclosed a process for preventing undesirable loosening and matting on the surface of paper products when subjected to friction; the particular paper product being used as a cover sheet in flushable sanitary products such as disposable diapers, sanitary napkins, etc. The improved non-matting or non-loosening properties of the paper sheet in that case were obtained by forming the sheet of special rayon fibers on a paper making machine and impregnating the sheet by spraying or the like, before drying or complete drying, in situ, on the machine with a high molecular weight water soluble polymeric material, such as polyvinyl alcohol, pressing the sheet after impregnation and then drying of the sheet.

In the improved process of the present invention which uses high molecular weight water soluble polymeric chemicals, we have found that it is not necessary to apply the chemicals to the preformed moist sheet by spraying or the like, as specified in the above prior Hokanson et al patent.

Moreover, we have discovered that a more efficient impregnation of the sheet may be obtained by introducing the polymeric chemicals, e.g., polyvinyl alcohol, directly into the white water circulating on the paper machine on which the paper sheet is produced. In this improved process, the polymeric material we employ is in finely divided particulate forms and is dispersed in the white water before or after it is mixed with the fiber stock and before formation of the paper sheet.

In U.S. Pat. No. 2,402,469, issued June 18, 1946 to William Craig Toland and Benjamin B. Burbank, a process is disclosed wherein particles of polyvinyl alcohol polymer are mixed with pulp fibers and thereafter formed into a fibrous sheet. However, the process disclosed is for the purpose of providing wet strength properties in the sheet rather than providing a water dispersible or flushable sheet, and the polymer particles employed are previously washed in cold water and added to the pulp in a hydrated state.

In accordance with the present invention, a fibrous sheet which is resistant to disintegration in normal use and which has dispersible or flushable properties in water is obtained by adding the finely ground polymer material to the white water system of the paper machine and by allowing the same to remain in the white water for a prescribed limited amount of time, sufficient to effect hydration and swelling of the polymeric particles but avoiding an excessive residence time in the white water sufficient to dissolve the water soluble polymer components or permit loss of the hydrated particles through the paper machine wire.

The polymers which are useful in the process of this invention for producing the improved flushable sheet product comprise high molecular weight polymers that swell rapidly in cold water as they hydrate but have limited solubility in cold water. By the term limited cold water solubility, it is understood that such polymers are soluble to a limited extent in cold water, having components or fractions which are insoluble in cold water as well as components or fractions which are cold water soluble. An illustrative example is a high molecular weight polyvinyl alcohol, e.g., Air Reduction Company's Vinol 325, or National Starch and Chemical Covol 9835. These polymers, when added to the white water, or to the white water-stock mixture before the sheet is formed, swell rapidly and become tacky so that when the sheet is formed on the wire, the polymer particles have become too large to pass through the screen and will stick to the fibers and are carried on to the drying section of the paper machine. As the temperature of the paper is raised on the dryers and the water begins to evaporate, the swollen, hydrated polymer particles shrink and bond the fibers together very effectively at fiber crossover points.

For the process of the present invention, the high molecular weight polymers should be finely ground before being used. In practice, the polymer particles should be ground sufficiently to pass a 50 mesh screen and a more desirable degree of fineness would be a polymer powder, of which approximately 80% will pass through a 100 mesh screen and 20% will pass through an 80 mesh screen.

It is also important that the polymer powder be introduced into the white water system at the proper place so that sufficient contact time is allowed in the white water for the water soluble polymer particles to hydrate and become swollen. Furthermore, it is important that an excessive residence time in the white water be avoided so that the polymer cannot dissolve. We have found that when the polymer is allowed a minimum residence in the white water system of three seconds, the particles will swell sufficiently to be substantially retained on the sheet and will do a satisfactory bonding job. When the particles are allowed to remain in the white water system for times exceeding approximately 90 seconds, the particles will tend to become so soft that the hydrated particles will be substantially lost through the wire and will no longer efficiently bond the sheet together. A preferable time of contact for the particles in the white water would be between five and forty seconds.

The finely divided polymer may be added alone directly to the white water or it may be added as a slurry in a watermiscible solvent in which the polymer is not itself soluble. In practice, the polymer may be conveniently added as a slurry in isopropanol or ethylene glycol by metering the slurry through a positive displacement pump into the white water trays on the paper machine.

When a sheet of flushable paper is formed in this manner and bonded together according to the new process, the resulting sheet can be easily redispersed in water. When such a sheet is immersed in cold water, such as in a toilet, and is agitated, the high molecular weight water soluble polymer softens as it hydrates and swells and will allow the fiber-to-fiber bonding to be weakened so that the fibers can slide apart and the sheet can disperse when slightly agitated as, for example, when it is flushed through the trap of the toilet.

The process of our invention yields a sheet in which the polymeric material is more uniformly distributed since the polymeric material is thoroughly mixed with the fiber before the sheet is formed. Also, it allows more economical operation of the paper machine since no additional water has to be added to and removed from the sheet.

The following illustrative Examples 1 and 2 will serve to explain the process of this invention more fully, and to illustrate its advantages over processes (Examples 3 to 7) which are not in accordance with the invention:

EXAMPLE 1

To the circulating white water system of a paper machine to which sixty pounds per minute of 1.5 denier, 12 millimeter self-bonding hollow rayon fiber is being metered, 2.4 pounds per minute of a 25% by weight slurry of isopropanol and a high molecular weight polyvinyl alcohol ground to pass an 80 mesh screen is added at a point where eight seconds of contact results before sheet formation. The resulting dried sheet is strong and extremely abrasion resistant but can be easily softened by crushing, embossing, or creping to yield a drapable, cloth-like material which retains a high percentage of its dry strength when wettted, but which disperses rapidly into component fibers when immersed in a large excess of water and agitated.

EXAMPLE 2

To the white water trays of the paper machine of Example 1 above, is added 1.2 pounds per minute of a 50% by weight slurry in ethylene glycol of a 60 mesh carboxy methyl cellulose powder. Contact time prior to sheet formation is approximately 35 seconds. The resulting dried sheet is similar in properties to that formed in Example 1.

EXAMPLE 3

To the white water trays of the paper machine of Example 1 is added 2.4 pounds per minute of a slurry containing 25% by weight of an 80 mesh completely cold water soluble low molecular weight guar gum. The resulting paper shows little or no dry abrasion resistance and disintegrates when subjected to creping.

EXAMPLE 4

The procedure in Example 1 is repeated using a 40 mesh polyvinyl alcohol otherwise identical to that of Example 1. The resulting paper is grainy and cockled due to uneven drying and exhibits areas of tight and loose bonding so that a harsh, weak product unsuitable for use in flushable sanitary products results when the sheet is creped.

EXAMPLE 5

To the wet pressed sheet formed on the paper machine of Example 1 without the binder slurry addition, is added 60 pounds per minute of a 1% solution of polyvinyl alcohol in water by spraying the sheet just after contact with the first drying can. The resulting sheet is seen to be less evenly bonded than that of Example 1 and exhibits streaks corresponding to light and heavy areas of the spray application. Since one additional pound of water must also be evaporated for each pound of paper produced, the drying section is seen to be considerably less efficient than in Example 1.

EXAMPLE 6

Example 1 is repeated except that the polymer slurry is added to the stock chest from which the stock slurry is fed to the machine. The resulting paper exhibits practically no dry abrasion resistance.

EXAMPLE 7

Example 1 is repeated except that the polymer solution is added to the paper machine headbox where contact time is approximately 1.5 seconds. The resulting paper is very weakly and unevenly bonded, showing areas which are very stiff and boardy and areas having almost no abrasion resistance.

Various modifications and changes may be made in the above-described procedure and chemicals for carrying out the process of the present invention, the scope of which is defined in the appended claims.

Claims

That which is claimed is:

1. A process for producing a cellulosic sheet suitable for use in flushable sanitary products and being resistant to disintegration in use, and in which the sheet is formed on the machine wire of a paper machine, comprising incorporating in the white water system of the paper machine a high molecular weight water soluble polymeric material in finely ground particulate form which hydrates and swells rapidly in cold water and which has cold water soluble components, said polymeric material being sufficiently finely ground to pass through a 50 mesh screen, permitting said polymeric material to remain in the white water for about three to 90 seconds to effect hydration and swelling of the polymeric particles but avoiding an excessive residence time in the white water sufficient to dissolve the water soluble polymer components or permit loss of the hydrated particles through the paper machine wire, forming on the paper machine wire a cellulosic fiber sheet containing the swollen polymeric particles, and subsequently drying the cellulosic sheet produced on the paper machine with the water soluble polymeric particles contained therein effecting bonding together of the cellulosic fibers in a sufficiently strong manner to prevent breaking of the bond through normal use in the sanitary products, but permitting flushing of the sanitary products after use by dispersing the polymeric particles and the cellulosic fibers in the flushing water.

2. A process as defined in claim 1 and in which the polymeric material is permitted to remain in the white water system between about five seconds and 40 seconds.

3. A process as defined in claim 1 and in which the polymeric material is polyvinyl alcohol.