Paper, in the form of "boxboard" or "paperboard", has been widely used for such disposable items as drinking cups and containers for liquids, such as milk and fruit juices.
For these applications, paper has the advantages of low toxicity, low cost, printability, biodegradability, and the ease with which it may be formed into the required shapes.
However, untreated paper is not suitable for the above-mentioned applications, because it is permeable to water and other aqueous liquids, and because, in the thickness desirable, it lacks adequate rigidity.
In order to overcome the above-mentioned disadvantages of untreated paper, efforts to reduce its water permeability and increase the stiffness by various methods have been taken, such as coating it with various waxes or plastics such as polyethylene, or by laminating the paper with plastic film. However, these materials do not accept ordinary printing inks, so it has been necessary to either use special, more expensive, inks on the coated material, or to print the paper before coating it and, therefore necessarily, to use a transparent coating.
Furthermore, the above-mentioned past efforts to solve the problems of high water permeability and low rigidity have rendered the resultant paper product non-biodegradable. Accordingly, there is the need for a multiply paper product having the above-mentioned desired characteristics, which is yet biodegradable.
U.S. application Ser. No. 08/640,401 describes paper made with cellulose acetate staple fiber. In addition, cellulose acetate fibers have been combined with cellulosic pulp in the manufacture of paper products as disclosed in U.S. Pat. Nos. 3,057,755, 4,731,091, 4,040,856 and 5,102,501 and Chemical Abstracts 73:121691s.
U.S. application Ser. No. 08/506,986 describes cellulosic paper made with fibers having an inner core of cellulose acetate. U.S. Pat. No. 4,040,856 to Litzinger contains an extrusion process for making cellulose acetate fibers for papermaking. The fibers are suitable for direct addition to conventional papermaking methods. Two related patents, U.S. Pat. Nos. 4,047,862 and 4,460,647 by Keith, disclose a cellulose acetate fiber for use in paper applications which is produced by precipitation of cellulose acetate from a dope under high shear conditions. In U.S. Pat. No. 5,213,883 by Mehta, a decorative sheet is disclosed having 1 to 20 weight percent cellulose acetate fibers added to cellulose pulp and TiO.sub.2. Japanese Patent. Nos. 52,096,208 and 52,096,231 to Shiyuuichi et al. are for another form of cellulose acetate fibers which are mixed with staple fibers for making into paper.
U.S. application Ser. No. 08/550,474 describes films made from paper containing cellulose ester fiber. The films of that invention are made from cellulose ester-containing paper such as that described in U.S. application Ser. No. 08/640,401 and U.S. application Ser. No. 08/506,986 discussed above which has been treated with a sufficient amount of a cellulose ester plasticizer, then subjected to heat and pressure.
All of the foregoing references deal with the preparation of single-ply sheets composed of cellulose ester containing paper, which usually include plasticizer.
It is therefore an object of the present invention to provide a multiply paper comprising at least one ply made of a mixture of cellulose fibers and cellulose ester fibers, having imparted softening characteristics, and a method of making the same.
It is another object of the present invention to provide a multiply paper in which the plies are integral with the paper web. That is, the multiply paper is formed from multiple plies of wet fibers integrally connected in the wet state, and prepared on conventional paper-making machines which are used for making multiply paper, rather than being laminates in which two or more separate materials are combined by adhesives or thermal or purely mechanical means (i.e., rather than a multiply product formed from dry sheets).
It is another object of the present invention to provide a multiply paper product that can be printed either on the all-cellulose plies or on the cellulose ester-containing ply, using ordinary inks.
It is another object of the present invention to provide a multiply paper product having the above-mentioned characteristics while having excellent biodegradability. The multiply paper of the present invention is also impermeable to penetration by aqueous liquids at ambient temperatures.