Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1999-12-16
2001-03-20
Hampton-Hightower, P. (Department: 1711)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S357000, C428S394000, C428S401000, C525S056000, C525S062000
Reexamination Certificate
active
06203903
ABSTRACT:
FIELD OF THE INVENTION
This invention relates, in general, to thermoplastic film and fiber structures and methods of making the same. More specifically, this invention relates to substantially water-free films and fibers comprising unmodified polyvinyl alcohol.
BACKGROUND OF THE INVENTION
Polyvinyl alcohol (PVOH) is a commodity polymer that is used in a wide variety of different applications. However, PVOH is generally regarded as non-thermoplastic. PVOH has a high melting point of about 200° C. depending on the degree of hydrolysis. As PVOH is heated near its melting point, yellowing and discoloration occur. Therefore, when using PVOH as a base material for thermoplastic applications, the PVOH must usually be modified.
Modified PVOH is used in many different water-dispersible thermoformable articles, such as fibers, films and fabrics which maintain their integrity and strength when in use, but dissolve and disperse when placed in contact with water. Unmodified PVOH is used in industry for many different solution-based applications and is not generally considered to be thermoformable or melt-processable. Some such applications for unmodified PVOH include warp sizing in textiles, fabric finishing, adhesives, paper processing additives, and emulsifiers/dispersants.
The prior art has demonstrated some success in modifying PVOH for use in thermoplastic applications. By “modified” PVOH, it is meant PVOH resin which has been chemically modified, including PVOH having another compound grafted thereto, or PVOH resin which has been mixed with one or more plasticizers. In each instance, these “modifications” have been needed to permit PVOH to be used in thermoformable articles.
To overcome the thermoplastic processing problems, chemically modified PVOH has been used. Some prior art teachings have used ethers of PVOH, ethoxylated PVOH or lacton-modified PVOH to produce thermoformable articles.
The prior art has also used PVOH that has not been modified structurally by adding a plasticizing agent to the PVOH which permits the PVOH to be extruded into films and fibers. Examples of plasticizers include water, ethylene glycol, glycerin and ethanolamine.
However, there are problems associated with the addition of plasticizers to PVOH. One of the most pronounced problems during processing is the fogging of the volatile plasticizer during the melt extrusion and condensing of vapor and effects of the vapor to the operating environment. In addition, the extruded articles such as films or fibers lose the plasticizers since the plasticizer molecules diffuse out of the film or fibers. This causes the films or fibers to become brittle over time and often causes the article to fail.
Accordingly, what is needed is an unmodified PVOH that may be thermoplastically formed into films and fibers which can then be used in the production of water-dispersible, flushable articles without the use of plasticizing agents. These fibers, films and fabrics could be used in products such as personal care products, diapers, feminine napkins, wipes, incontinence products, release liners, product packaging, etc., which contain the above-mentioned fibers, films and fabrics.
SUMMARY OF THE INVENTION
One object of the present invention is to produce films and fibers comprising unmodified PVOH.
Another object of the present invention is to use unmodified PVOH in films and fibers without the use of a plasticizing agent.
These and other objects are satisfied by the present invention. The present invention discloses the selection and use of commercially-available grades of PVOH for thermoplastic applications. “Thermoplastic” is defined, herein, as a resin which can be melted and easily extruded to form a desired article, i.e., the material is melt process able.
PVOH is a commodity polymer, commonly used in solution-based applications. Since it is a commodity polymer, thermoplastic articles made using unmodified PVOH are generally less expensive than articles made using modified PVOH due to the additional process steps required to modify the PVOH. Also, unmodified PVOH is, in general, less expensive than other water-soluble polymers.
In its unmodified form, PVOH has not been used for thermoplastic applications. Typically, some modification of the PVOH, such as chemical grafting or addition of plasticizer, is necessary to achieve melt processability for PVOH. In the present invention, a window of thermoplastic processability has been discovered and defined for unmodified, commercially-available PVOH, according to: 1) the composition or % hydrolysis of the PVOH, 2) the molecular weight of the PVOH, 3) the solution viscosity of the PVOH, or 4) the melt viscosity of the PVOH. The selected grades of PVOH have demonstrated thermoplasticity, allowing for continuous, melt extrusion or conversion into thin films in a continuous, extrusion process.
These grades of PVOH are also useful for melt spinning of fibers, injection molding or other thermoplastic applications. Extruded films of the unmodified PVOH described herein have very high strength and modulus, excellent clarity, and fast crystallization and solidification rates. The advantages of melt processing a thermoplastic, unmodified PVOH into a useful, strong, clear, water-soluble article are evident. Melt processing is a desirable thermoforming process compared to solution processing. Melt processing eliminates the need to add steps such as chemical grafting, addition of a plasticizer, or other modification in order to achieve melt processability.
DETAILED DESCRIPTION
PVOH is generally produced by a two step process as shown in Scheme 1. Since vinyl alcohol is not a stable monomer, the polymerization of vinyl alcohol is not an option for making PVOH. Instead, the process utilizes a readily available monomer, vinyl acetate, as the starting point. The first step is the polymerization of vinyl acetate into polyvinyl acetate (PVA). The second step is the hydrolysis or alcoholysis of PVA into a copolymer of vinyl acetate and vinyl alcohol, or polyvinyl alcohol (PVOH). Depending on the hydrolysis level as defined in the equation in Scheme 1, a wide range of PVOH copolymers can be produced when the hydrolysis reaction is allowed to reach certain conversion levels.
For PVOH, the degree of hydrolysis is controlled during the alcoholysis reaction and is independent of the control of the molecular weight of the PVOH formed. Fully hydrolyzed PVOH is obtained if alcoholysis is allowed to go to completion. The reaction is terminated by removing or neutralizing the sodium hydroxide catalyst used in the process. Typically, a small amount of water is added to the reaction vessel to promote the saponification reaction of PVA. The extent of hydrolysis is inversely proportional to the amount of water added. The alcoholysis can be carried out in a highly agitated slurry reactor. A fine precipitate forms as PVA, which is then converted to PVOH. The PVOH product is then washed with methanol and is filtered and dried to form a white, granular powder.
The molecular weight of the PVOH is controlled by the polymerization condition of vinyl acetate. Many properties of PVOH depend on the degree of hydrolysis and the molecular weight. As the molecular weight increases, the solution viscosity, tensile strength, water resistance, adhesive strength, and solvent resistance increase. As molecular weight decreases, the flexibility, water solubility, and ease of solvation increase. As the degree of hydrolysis increases, the water resistance, tensile strength, block resistance, solvent resistance, and adhesion to polar substrates increase. As the degree of hydrolysis decreases, the water solubility, flexibility, water sensitivity and adhesion to hydrophobic substrates increase.
Due to the strong dependence of PVOH on the molecular weight and degree of hydrolysis, PVOH is typically supplied in combination of these two parameters. PVOH is classified into 1) partially hydrolyzed (87.0 to 89.0% hydrolysis); 2) intermediately hydrolyzed (95.5 to 96.5% hydrolysis); 3) fully hydrolyzed (98.0 to 98.8% hydrolysis); and 4) super
Pomplun William S.
Schertz David Michael
Wang James Hongxue
Hampton-Hightower P.
Kilpatrick & Stockton LLP
Kimberly--Clark Worldwide, Inc.
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