FIELD OF THE INVENTION
This invention relates, in general, to blend compositions of an unmodified poly vinyl alcohol and a thermoplastic elastomer and thermoplastic film and fiber structures comprising these blend compositions. More specifically, this invention relates to substantially water-free films and fibers comprising unmodified polyvinyl alcohol and a thermoplastic elastomer.
BACKGROUND OF THE INVENTION
Personal care articles are widely used in today's society. Many of these articles use films and fibers that are thermoplastic. Additionally, these articles use films and fibers that have different properties, depending on their location in the product. For example, some films and fibers are elastomeric. Others are breathable while still others act as liquid barriers. Finally, some of the films and fibers, especially those in contact with the wearer of the product, are designed to be softer to the touch. These different films typically comprise polymers or polymer blends that, when processed, form a film or fiber having the desired characteristic or characteristics.
Additionally, in an attempt to deal with decreasing land-fill and solid waste disposal many of these films and fibers are designed to be water-dispersible such that the product will partially or completely disperse in water, thereby allowing the product to be disposed of without dumping or incineration. These products may be placed in sewage systems or may be flushed down a conventional toilet. To produce these water-dispersible products, the films and fibers used in the products will typically use blend compositions that include a water-dispersible polymer such as polyethylene oxide or polyvinyl alcohol.
Polyvinyl alcohol (PVOH) is a commodity polymer that is used in a wide variety of different applications. Many of these applications are thermoplastic. However, PVOH is generally regarded as a non-thermoplastic polymer. PVOH has a high melting point of about 200° C. depending on the degree of hydrolysis. Accordingly, 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 that 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.
Additionally, films and fibers including modified PVOH or PVOH and a plasticizer may be limited in their utility. These films and fibers may be too stiff to be used for certain applications. Additionally, the texture of the films may not be soft enough for comfortable contact with the skin of an individual. Finally, these films and fibers may be too “noisy” such that bending or flexing of the film or fiber causes an audible sound that may be distracting to the user of the product.
Accordingly, what is needed is an unmodified PVOH that may be used in blend compositions that are thermoplastically formed into films and fibers. These films and fibers may 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 and pads, training pants, wipes, adult incontinence products, release liners, product packaging, etc., which contain the above-mentioned fibers, films and fabrics. Additionally, what is needed are thermoplastically formed films and fibers that have enhanced softness and ductility and produce less noise when bent or flexed.
SUMMARY OF THE INVENTION
Accordingly, the present invention desires to produce films and fibers including blend compositions having unmodified PVOH and a thermoplastic elastomer.
Another desire of the present invention is to use unmodified PVOH and a thermoplastic elastomer in films and fibers without the use of a plasticizing agent.
These and other desires 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 processable. These commercially-available grades of PVOH are combined with a thermoplastic elastomer to provide a blend composition useful in the production of films and fibers that have enhanced softness and ductility and produce less noise.
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/thermoplastic elastomer blends 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.
These grades of PVOH may be mixed with additional polymers, such as thermoplastic elastomers, to provide desired characteristics to the films and fibers, such as enhanced ductility, enhanced softness and lower noise generation.
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 hydrolyzed (>99.3% hydrolysis). Within each category of PVOH, the resin is differentiated by solution viscosity, measured at 4% solution in water at 20° C. in centipoise. The viscosity is used as a molecular weight measure since solution viscosity is typically related to the molecular weight by the well known Mark-Houwink equation:
η=KM v a
K=constant (dependent upon the polymer)
a=factor based on the rigidity of the polymer chains and is dependent on the polymer.
For unmodified PVOH, it was known that higher molecular weight grades were not thermoplastic. It was surprising that unmodified PVOH at lower molecular weights would be thermoplastic based on the non-melt processability of higher molecular weights grades. Unmodified PVOH with weight average molecular weight as low as 8750 g/mole was discovered to be thermoplastic and melt processable, with high melt strength, excellent film strength and great clarity. Typically, a polymer with such a low starting molecular weight would not be expected to be melt processable into a useful material.
Additionally, it was discovered that the melt viscosity of the PVOH grades could be used to determine which grades of PVOH were thermoplastic. In general, those grades having a melt viscosity less than about 1500 Pa·s at a shear rate of 500 s−1 were determined to be melt processable.
Not all grades of PVOH were discovered to be thermoplastic. The PVOH grades useful in this invention desirably have a solution viscosity of less than about 10 cp in a 4% water solution at 20° C. and a hydrolysis of less than about 90%. Examples of commercially-available grades of PVOH useful in this invention are ELVANOL® 51-05 from DuPont (Wilmington, Del.), AIRVOL® 203 and 205 from Air Products and Chemical, Inc. (Allentown, Pa.), and GOHSENOL® KP-06 from Nippon Gohsei (Japan). PVOH is typically sold in powder or granule form, however pellets or other forms of resin can be used in this invention since the physical form of PVOH does not affect melt processability.
Additionally, depending on the type of blend application for which the PVOH will be used, films or fibers, the exact processing characteristics may vary. For example, some of the thermoplastic grades may be better suited for the production of thermoplastic films while other grades may be more useful for the production of fibers. The exact grade to use will depend upon the item being made and the elastomer that is blended with the PVOH.
The present invention uses these thermoplastic PVOH grades with an additional compound to form blend compositions. These blend compositions may then be formed into thermoplastic articles such as films and fiber. The additional compound is used to enhance the properties of the resulting films and fibers. In the present invention, a thermoplastic elastomer is used to help produce films that are softer, more ductile and less noisy than films comprising PVOH alone.
The blends including thermoplastic PVOH grades and a thermoplastic elastomer may be extruded using most known extruding devices. In general, while a thermoplastic film may be extruded at extrusion temperatures above the melting point of the PVOH/elastomer blend, it is preferred to use extrusion temperatures near the melting point as the resulting films and fibers are generally clearer, have fewer imperfections, are more ductile and stronger, and can be drawn into much thinner films.
As discussed earlier, the films and fibers of the present invention can be extruded from unmodified PVOH/elastomer blends without the use of a plasticizer. Many different plasticizers are known, including, for example, ethylene glycol, glycerines and ethanolamine. In addition to these plasticizers, water is also known to be used as a plasticizer in the production of PVOH films and fibers. However, these plasticizers, including water, have several disadvantages when used in the production of films and fibers. In general, plasticizers, including water, will slowly diffuse out of a PVOH film or fiber causing the film or fiber to become lucid and brittle and therefore more likely to break or shatter.
Additionally, plasticizers, including water, added to PVOH may cause bubbling of the film during the extrusion process. This is especially true with water. Therefore, care must be taken prior to the blending with an elastomer and production of the film to ensure that the PVOH powder or pellets remain substantially water-free. This helps to ensure that the films and fibers produced are also substantially water-free. By “substantially water-free” it is meant that the films and fibers produced using the unmodified PVOH/elastomer blends contain less than about 2.0 percent by weight of water. Desirably, the films and fibers contain less than about 1.0 percent by weight of water. More desirably, the films and fibers contain less than 0.5 percent by weight of water.
There are three basic types of thermoplastic elastomers: 1) styrenic thermoplastic elastomers, 2) hard/polymer elastomer combinations, and 3) multi-block polymers with crystalline hard segments.
For styrenic thermoplastic elastomers, the elastomer may be an A-B-A block polymer where “A” is a polystyrene and “B” is an elastomer segment. The elastomeric segments may be selected from polybutadiene, polyisoprene, poly(ethylene-butylene), and poly(ethylene-propylene). Accordingly, these polymers may be referred to as S-B-S, S-I-S, S-EB-S, and S-EP-S, respectively, where “S” refers to polystyrene, “B” refers to polybutadiene, “I” refers to polyisoprene, “EB” refers to poly (ethylene-butylene), and “EP” refers to poly (ethylene-propylene).
Hard/polymer elastomer combinations include a hard thermoplastic polymer, such as polypropylene, in a fine dispersion within a matrix of an elastomer. The elastomer may be selected from ethylene-propylene-diene monomer (EPDM) or ethylene-propylene copolymer (EPR). Other elastomers that may be used include nitrile, butyl, and natural rubbers.
Multi-block polymers with crystalline hard segments generally include multi-block (A-B)n structures, wherein “A” is a crystalline thermoplastic, while “B” is a softer, elastomeric segment that is amorphous. Examples of hard segments include, but are not limited to, thermoplastic polyurethanes, thermoplastic polyesters, and thermoplastic polyamides. Examples of soft segments include, but are not limited to, polyesters.
The importance of this invention is that PVOH/elastomer blends have been discovered that may be directly extruded into a water-soluble, thin film without the need for any chemical modification of the PVOH or the addition of a plasticizer. The elimination of any chemical modification of the PVOH eliminates the labor intensive step of chemically modifying or grafting the PVOH. The elimination of a plasticizer admixed with the PVOH relieves the common problems involved with plasticizers as previously discussed. The water-soluble film of the present invention will keep its original properties and in-use performance unlike a PVOH/elastomer film containing a plasticizer which will become brittle over time.
One additional advantage in the production of water-soluble products from the PVOH/elastomer films and fibers of the present invention is in the product converting stage. PVOH has a higher melting point than many other water-soluble polymer systems used for making water-dispersible, flushable articles, including, for example, polyethylene oxide-based materials. PVOH film can withstand heat from a hot-applied melt adhesive which may be used during product construction. In contrast, PEO-based materials have limitations in this aspect due to the low melting temperature of the PEO of about 60 to 70° C. Therefore, the PVOH/elastomer films and fibers of the present invention have great usefulness in the production of water-dispersible, flushable products.
The PVOH/elastomer blends, films and fibers of the present invention include a thermoplastic elastomer that enhances certain characteristics of the films and fibers when compared to films and fibers comprising only unmodified PVOH. The elastomer imparts improved softness and ductility to the film while reducing the amount of noise the film makes when manipulated. These features are very useful for films that are used in a personal care article, such as a diaper, feminine article, incontinence device, among others.
The present invention uses a thermoplastic elastomer. Suitable thermoplastic elastomers include, but are not limited to, KRATON® polymers from Shell, such as Kraton D, a S-B-S or S-I-S polymer, and Kraton G, a S-EB-S or S-EP-S polymer, elastomeric polyurethanes, ethylene-octene copolymers, polyester polyurethane, natural rubber, nitrile rubber, butyl rubber, ethylene-propylene terpolymers, silicone rubber, polyurethane rubber, thermoplastic rubbers, elastomeric block copolymers, copolymers of polyethylene oxide and polybutylene terephthalate, polyamide-polyether block copolymers, styrenic block copolymers, elastomeric polypropylene, or mixtures thereof. The amount of thermoplastic elastomer that may be used is in the amount of from about 1 to about 99% by weight of the PVOH/elastomer blend. Desirably, the blend comprises from about 50 to about 90% by weight PVOH and from about 50 to about 10% thermoplastic elastomer. Even more desirably, the blend comprises from about 65 to about 80% by weight PVOH and from about 35 to about 20% thermoplastic elastomer.
The relative amounts of PVOH to that of thermoplastic elastomers determine the water-responsiveness of the resulting PVOH/thermoplastic elastomer films. When the PVOH is a volumetric majority component, the resulting article is water-dispersible or water-disintegratable as defined by standard test methods. As used herein, the term “water-responsive” includes articles that are water-dispersible, water-disintegratable and water-weakened. “Water-dispersible” is used herein to describe a 5 mil (0.005 of an inch) film that, under the water-responsiveness test described below, dissolves or breaks into pieces smaller than a 20 mesh screen.
“Water-disintegratable” describes a 5 mil film that, under the water-responsiveness test, breaks into multiple pieces after two minutes with some of the pieces caught by a 20 mesh screen. “Water-weakened” describes a 5 mil film that, under the water-responsiveness test, remains intact, but loses rigidity and becomes drapable, i.e., will bend with an external force applied to the film when it is held by one corner at a substantially horizontal position.
The present invention is further illustrated by the following examples, which are not to be construed in any way as imposing limitations upon the scope thereof. On the contrary, it is to be clearly understood that resort may be had to various other embodiments, modifications, and equivalents thereof which, after reading the description herein, may suggest themselves to those skilled in the art without departing from the spirit of the present invention and/or the scope of the appended claims.