Breathable film compositions and articles and method

A breathable film is composed of a uniform blend of two thermoplastic resins, one having a high moisture vapor transmission rate and the other having a low rate. The blend is melt extruded into a film having good handling characteristics and a controlled degree of moisture vapor transmission, as determined by blend ratio per unit of film thickness.

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.degree. 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.sup.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.sup.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 range is in the order of 10
 to 40 microns.
 In cases where the thickness of the film and MVTR are dictated by end use,
 the ratio of the low and high MVTR resins can be adjusted to obtain the
 desired MVTR in the film, as demonstrated in the following examples.
 Specific examples for high MVTR resins include polyurethanes, copolyesters,
 and polyester elastomers having a MVTR rating in excess of 500 for a 25
 micron film. Specific examples of low MVTR resins include ethylene
 copolymers, especially those which include units of ethyl methyl acrylate
 and ethyl vinyl acetate. The two classes of polymers have been found to
 provide a uniform blend which can be melted and passed through a heated
 extruder and through a slot die to produce a monolithic film free of voids
 or layers.
 For many end uses, the film will be laminated to a porous fabric. While the
 film may be produced separately and laminated to a fabric using an
 adhesive, extrusion coating of a molten layer directly onto a fabric is
 most preferred. This is easily accomplished by passing the fabric between
 the nip of two rotating rolls, and extruding the film onto the fabric at
 or near the nip.
 In comparison to films made solely from high MVTR resins alone, it has been
 found that the blends of the present invention adhere better to a variety
 of fabric substrates, including those based on natural and synthetic
 fibers or yarns. The fabric may be of the woven type or a nonwoven.
 Nonwovens include spunbond fabrics of continuous filaments, and webs of
 nonwoven fibers bonded together by heat fusion, adhesives, or mechanically
 bonded by hydroen tanglement or needling. A variety of nonwoven fabrics
 are available, and these are typically composed of fibers or filaments of
 polymers such as polyester, nylon and polyolefins, including polyethylene
 and polypropylene. Since the film resides as a layer on one side of the
 fabric, the basis weight of the fabric is not critical, and depends on the
 end use of the product. Composite fabrics may be employed, such as
 spunbond-meltblown-spunbond fabrics, or the film may save as a backing for
 looped fabrics.
 Additional details are provided in the following examples.

EXAMPLE I
 The resin system used in this example was a blend of a polyesterelastomer
 (Arnitel PL380 from Dutch State Mines) and ethylene vinyl acetate
 copolymer (Ateva 1815 from AT Plastics). A 25 micron film of the
 polyesterelastomer has a MVTR in excess of 2000 g/m.sup.2 /24 hr. A 25
 micron film of the ethylene vinyl acetate copolymer has a MVTR of less
 than 100.
 The PL380 was fed from sealed bags to a mixer to prevent pickup of
 moisture. The 1815 was added from open boxes to the mixer. A continuous
 gravimetric blender was used to combine 60% by weight PL380 and 40% by
 weight 1815 in a chamber directly above an extruder. The extruder was a
 standard single screw extruder followed by a screen pack metering pump and
 coat hanger slot die. The extrusion temperature ranged from 375.degree. F.
 in the second zone of the extruder to 450.degree. in the die. The blend
 was cast into a film, which is still in a molten stage upon exit from the
 die. The molten blanket of blended polymer was drawn between the nip of
 two rolls. One of the rolls was a smooth roll heated to 185.degree. F. and
 the other was a rubber coated roll maintained at a temperature of about
 50.degree. F. Upon exit from the nip, the film was in a solid state and
 was passed onto a chill roll maintained at about 40.degree. F. to complete
 the quenching process. A 25 micron film was cast in this manner and was
 found to have a MVTR of 1200 g/m.sup.2 /day.
 EXAMPLE II
 This example illustrates the on-line lamination of a blended film to the
 nonwoven fabric. The fabric was a carded, adhesive bonded nonwoven fabric
 of polyester fibers available from The Polymer Group (PGI Nonwovens) as
 Duratex 6864.
 The fabric was unwound from a roll located above the cast station and fed
 onto the heated smooth roll and through the nip. The same process
 conditions of Example I were employed, except that the heated roll was
 heated to 210.degree. F. The die can be adjusted to contact the fabric
 prior to the nip, at the nip, or the chill roll, with decreasing film
 adhesion of the three locations. In this example, the molten film was fed
 into the nip. The roll speed controls the thickness of the film, assuming
 a constant rate of extrusion from the die. In this example, the line speed
 was increased to provide a solid film coating of approximately 18 microns
 (0.7 mil) thick. The MVTR was found to be 1350 g/m.sup.2 /day.
 EXAMPLE III
 Using approximately the same processing conditions as Example II, the
 fabric was coated with 100% PL380 and 100% Ateva 1815 in two separate
 runs. The PL380 sample had a high MVTR but delaminated when handled. Also
 the coating had poor aesthetics and was stiff. The Ateva 1815 sample was
 found to have excellent adhesion to the fabric but had a MVTR rating of
 less than 100 g/m.sup.2 /day.
 EXAMPLE IV
 This example illustrates the ability to control the degree of breathability
 or MVTR of a film of given thickness using different blend ratios. Polymer
 characteristics are shown in Table I below.
 TABLE I
 Polymers Used in Prototype Preparation
 MFI, g/ Den-
 Polymer Supplier Type 10 min sity Detail
 ARNITEL PL380 DSM copolyester 25 1.16
 EM806009 Equistar ethylene methyl 6 0.94 20%
 acrylate MA
 AVETA 1815 AT ethylene vinyl 2.5 0.94 18%
 Plastics acetate VA
 Monolithic films were prepared using neat systems of each of the three
 polymers and then blends of Arnitel with either the EMA or EVA polymers.
 Blend ratios tested were 20-60% of the non-breathable constituent,
 balanced with the breathable copolyester. The final variable was film
 basis weight, held at 15 gsm or 30 gsm. Table II below provides the
 corresponding film gauge in microns (micrometers) for the various systems.
 The films were all tested in triplicate for MVTR according to ASTM E96,
 1980 revision, Procedure D, method X1.15, a water method at 90.degree. F.
 TABLE II
 Vapor Permeability if Films
 basis weight, gsm thickness, m MVTR, g/m.sup.2 /day
 100% copolyester 15 13 3441
 30 26 2232
 100% EMA 15 14.5 305
 30 29 139
 100% EVA 15 14.5 25
 30 29 0
 EMA/copolyester
 20/80 15 13.5 3362
 30 27 2115
 40/60 15 14 2139
 30 28 1719
 EVA/coployester
 20/80 15 13.5 3486
 30 27 1440
 40/60 15 14 2280
 30 28 1419