Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1998-09-29
2001-02-20
Short, Patricia A. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S173000, C525S176000
Reexamination Certificate
active
06191221
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to breathable films, or films capable of transmitting water vapor but generally resistant to the passage of water in a liquid form.
Breathable films have been known and used for many years in the production of, for example, water proof clothing, to allow escape of water vapor from the surface of the skin of the wearer outwardly. Vapor breathable films can be broadly classified as being a microporous film or a monolithic film.
A microporous film has a large number of pores formed in the film using special processing conditions. One method of obtaining microporsity is described in U.S. Pat. No. 3,870,593. A quantity of powdered inert material such as calcium carbonate is incorporated into the film prior to extrusion. After extrusion, the film is drawn, causing small pores to be established at the sites of the filler particles. As described in U.S. Pat. No. 4,308,303 after production of the microporous film, the film may be coated with fibers to produce a composite. The lamination of microporous films to fabrics is also well known.
Microporous films are not suitable for certain end uses and are not desirable for others due to their nature of indiscriminately passing all gases, vapors and other agents such as pathogens. These films, for example, are generally not suitable as viral barriers, and garments and other articles having a viral barrier yet good breathability are highly desirable in the medical and other industries where exposure to blood is commonplace.
A second class of breathable films can be referred to as monolithic films, in which the film is continuous and free of pores. Monolithic breathable films are capable of allowing the transfer of certain gases and liquid vapors due to chemical absorption, transfer through the film thickness and release on the opposite surface. For films having a high rate of moisture transmission, the rate of transfer is very rapid, and is driven by the relatively high concentration and pressure of vapor on one side of the film. This mechanism of transfer is described in U.S. Pat. No. 5,445,874, which discloses thin films of certain polyurethanes which possess moisture vapor transmission rates (MVTR) higher than the human skin, allowing the film to be used as an outer layer in burn dressings.
Several thermoplastic resins are currently available which allow the extrusion of films having a high MVTR. Examples include polyurethanes, copolyesters and polyester elastomers. These resins can be extrusion coated on a porous support substrate such as a woven or nonwoven fabric which is used to make protective clothing and other articles.
Apart from being relatively expensive in comparison to other film forming thermoplastic polymers, the above noted polymers are not suitable for certain types of end uses. The only way to change the MVTR of the film is to increase or decrease the thickness of the film, with thinner films providing higher MVTR's. Some films cannot be laminated directly to certain fabrics, for example, by extrusion coating, or the bond between the fabric and the film is unduly weakened by use. Also, these breathable films tend to be harsh and noisy when combined with fabrics and may not have an attractive visual appearance. Since all these properties are important from the viewpoint of manufacture and successful end use, additional improvements in the field of breathable monolithic films are needed.
SUMMARY OF THE INVENTION
In accordance with the present invention, a vapor breathable monolithic film is provided, which film can be continuously extruded as a hot melt. The hot melt can be extruded into a film or extruded directly onto a substrate, such as a porous fabric, with excellent adhesion.
The film is composed of a homogeneous and uniform blend of at least two thermoplastic polymers. One of the polymers, if used alone and formed into a thin film, exhibits a high moisture vapor transmission rate. The second polymer, if used alone and formed or extruded into a thin film, exhibits a low MVTR. When blended together at varying ratios, the blend can be hot melt extruded into a film having desirable properties and characteristics beyond either of the individual components. The film has a softer feel and better adhesion to fabrics in comparison to high MVTR films alone. Notwithstanding the inclusion of substantial amounts of low MVTR components, the composite film will still have a high MVTR, suitable for use on breathable articles.
Whereas the degree of breathability of prior art films was dictated primarily by film thickness, the MVTR rating of the monolithic film of the present invention can be fixed per unit of thickness by adjusting the ratio of the two components in the blend. This feature offers benefits to a variety of end use applications, where, for example, a specific film thickness is designated.
One of the components is selected from the group consisting of polyester resins, copolyester elastomers and polyurethanes, which are capable of being extruded into a film having a high MVTR. The second component is selected from a group of ethylene esther copolymers, such as ethylene vinyl acetate and ethylene methyl acrylate. Those polymers, when used alone, provide a film having a very low MVTR. The weight of the second component in the blend, based on total weight, is from 10% to 80%.
DETAILED DESCRIPTION
As used herein, the term moisture vapor transmission rate or MVTR is a value expressed in terms of grams of transmitted moisture per square meter of film over a 24 hour or one day period as determined by standard test procedures known to those skilled in the art. The standard procedures employed herein is known as ASTM E96, 1980 revision, Procedure D, method X1.15, a water method at 90° F.
The term “blend” as used herein refers to a compatible homogeneous mixture of two thermoplastic polymers, which can be melted together and cooled to form a monolithic structure, such as a thin, continuous film. Many thermoplastic polymers cannot be successfully blended together and tend to separate when heated and passed through an extruder.
The term “low MVTR polymer or film” means a thermoplastic polymer, when extruded into a film having a thickness of one mil or about 25 microns, will have a MVTR of less than 350 g/m
2
/day. The term “high MVTR polymer or film” means a thermoplastic polymer, when extruded or cast in a film having a thickness of one mil or about 25 microns, will have a MVTR of greater than 500g/m
2
/day.
The present invention contemplates the use of blends of high and low MVTR thermoplastic resins, which can be melted together and extruded or cast into thin films either as a film alone, or as a layer on a fabric. If high MVTR resins are used alone to form films, in many cases the film will have an excessive MVTR, and the only solution is to increase the thickness of the film, and hence the cost. By using the blends of resins disclosed herein, the ratio of the polymers may be adjusted to obtain a desired MVTR rating at a specified thickness.
In the preferred embodiment, the low MVTR resin is incorporated into the blend in the amount of 10% to 90% by weight, with the remainder as the high MVTR resin. It has been found that if the level of high MVTR resin falls significantly below 10%, the MVTR of the blend becomes too low. Also, as a general rule, the MVTR of the film increases as the thickness decreases. The thickness is dictated by practical and cost considerations. Within the blend parameters set forth above, some of the practical considerations include durability of the film when used alone or as a layer with a fabric, and the minimum thickness at which a film can be formed using conventional equipment. The minimum thickness is in the order of five to ten microns, and the maximum thickness can extend up to 75 microns. A thin film having a higher amount of low MVTR resin might be used, for example, as an outer barrier for single use garments such as diapers. Thicker films might be employed in articles where durability is a more important factor. A preferred thickness rang
Huskey Timothy
McAmish Larry
Polymer Group Inc.
Pyle & Piontek
Short Patricia A.
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