Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
2000-09-01
2002-09-03
Tarazano, D. Lawrence (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S304400, C428S910000, C428S523000, C525S240000, C525S241000, C264S288800, C264S290200
Reexamination Certificate
active
06444302
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a method for producing breathable films from blends of synthetic polymers.
BACKGROUND OF THE INVENTION
Multi-layer films have been traditionally used to provide semi-permeable or selectively permeable properties in applications where breathability is desired. By breathable, it is meant that the film will allow transmission of water vapor and oxygen but not micro-organisms or bulk solids or liquids. Such properties are useful in a variety of applications, including fresh produce and flower packaging, disposable personal garments, house wrap, and membranes for various separation processes. Breathable films are especially useful for fabrication of clothing items where it is important to protect the wearer from environmental exposure or to prevent the escape of human waste while allowing the wearer greater comfort than permitted by an impermeable material. Examples of such products include, but are not limited to, surgical and health care related products, disposable work wear, and personal care absorbent products. Surgical and health care related products include surgical drapes and gowns, and the like. Disposable work wear includes coveralls and lab coats, and the like. Personal care absorbent products include diapers, training pants, incontinence garments, sanitary napkins, bandages, and the like.
Primary functions of such breathable films are to provide liquid barrier properties and/or block the passage of micro-organisms, yet allow the transmission of moisture, air, other gases, or combinations thereof Apparel made from breathable and/or microporous films are more comfortable to wear by reducing the moisture vapor concentration and the consequent skin hydration underneath the apparel item. However, the pore size in breathable films cannot be too large, especially in protective apparel and personal care applications, such as industrial or medical garments, diapers, etc., where liquid penetration presents a contamination risk. Moreover, films containing pores that are too large may allow passage of liquids and/or viruses and thereby reduce the effectiveness of the protective apparel.
The conventional process for obtaining a breathable microporous film for commodity markets has been to stretch a thermoplastic film typically containing inorganic fillers. Microvoids are created by separation at the interface between filler particles and the polymer matrix containing the filler, when the film is stretched or drawn. The film is usually heated prior to these drawing processes to optimize the malleability of the film during stretching. This drawing or stretching also orients the molecular structure within the film, which increases its strength and durability with respect to forces applied in the stretch direction. Stretching can be in the machine direction (MD), the transverse (cross-machine) direction (TD), or biaxially (both MD and TD). Uniaxial or otherwise unbalanced stretching typically results in unbalanced properties (e.g. films tend to split more easily upon applied forces transverse to the direction of a uniaxial stretch). Regardless of the selected drawing process, there are inherent difficulties in processing filled polymers.
First, uniform dispersion of a filler, such as calcium carbonate, in a polymer requires a separate compounding step and substantial mechanical work. Second, the higher melt temperatures sometimes required during compounding can cause discoloration of the polymer and sometimes even polymer degradation. Third, additional equipment is required in order to assure efficient vacuum stripping to remove water released from the filler during the compounding step. The vacuum vents can plug and this leads to water in the compounded product and ruins the cast film. Fourth, die drool and smoking can occur as it is increasingly difficult to process filled polymer as the filler particle size decreases, especially below 2 microns. Consequently, the calcium carbonate must be screened to remove particles smaller than 2-3 microns. Smaller particles raise the viscosity of the compounded polymer so high that mechanical failures of the equipment are common. Finally, the calcium carbonate filled breathable films can have a gritty feel and tend to feel heavier since the CaCO
3
is more dense than the polymer matrix. Therefore, it would be highly desirable to have a method for producing breathable films that avoids the difficulties associated with hard filler substances, such as calcium carbonate.
Other methods to produce or enhance breathability include mechanical puncture of a film (see U.S. Pat. No. 4,747,895) or extraction of a co-continuous phase of a film formed from blended polymers (see U.S. Pat. No. 4,804,472). A third method uses a crystalline polymer or a mix of amorphous and crystalline polymers that create regular row-lamellar structures which result in uniform pores in the polymer when prepared and stretched under controlled conditions (see U.S. Pat. Nos. 3,843,761, 3,801,404, 4,138,459, and 4,620,956).
Multi-layer films, produced by either lamination or coextrusion of single-layer films have also been used to create breathable materials that are both impervious to liquids and have a durable cloth-like appearance and texture. The outer covers on disposable diapers are but one example. In this regard, reference may be had to U.S. Pat. Nos. 4,818,600 and 4,725,473. Surgical gowns and drapes are other examples. See, U.S. Pat. No. 4,379,102. U.S. Pat. No. 5,914,184 discloses a breathable multi-layer film laminate including microporous filled film bonded to continuous film. A support layer, such as a fibrous web, can be adhered to the film laminate on one or both surfaces. Lamination of multiple layers of film also requires additional processing steps and is therefore subject to more potential processing difficulties.
Typically, as in the above references, films are made breathable by the addition of fillers and subsequent stretching of the films and durability is improved through lamination of the film to other layers of film having properties lacking in the first layer. Using filler to produce breathable film and improving the toughness of a breathable film by lamination with other polymer layers are both time consuming and costly additional steps. There is therefore a need for films providing both breathability and durability without the need for either or both fillers and lamination of multiple layers.
SUMMARY OF THE INVENTION
The present invention relates to a cold-drawn film formed from a blend of a soft polymer component and a hard polymer component. The soft polymer component (SPC) is a copolymer of a major olefinic monomer and a minor olefinic monomer. The major olefinic monomer is either ethylene or propylene and forms the majority of the SPC. The minor olefinic monomer forms the remainder of the SPC and is a linear, branched, or ring-containing C
2
to C
30
olefin, capable of insertion polymerization, and is different from the major olefinic monomer.
In one embodiment, the SPC has a melting point greater than 25° C. and a flexural modulus at ambient temperature less than 100 Mpa, and the hard polymer component (HPC) has a flexural modulus at ambient temperature greater than 200 MPa.
In another embodiment, the SPC has a melting point greater than 25° C. and a secant modulus at ambient temperature less than 350 Mpa, and the hard polymer component (HPC) has a secant modulus at ambient temperature greater than 400 MPa.
The SPC and HPC can be blended as co-continuous phases, but preferably the HPC is a dispersed phase in a continuous phase of the SPC.
An initial film formed from a blend of the SPC and HPC is cold drawn at a temperature below the highest transition temperature, either melting or glass transition, of the HPC. Preferably, the drawing temperature is also higher than the temperature at which the first crystalline melting can be detected and lower than the maximum temperature at which the final crystalline melting can be detected in the SPC. The cold drawing is uniaxial or biaxial with a draw-down
Brant Patrick
Chambon Francois Henri
Srinivas Srivatsan Watson
Stokes James Peter
ExxonMobil Chemical Patents Inc.
Reid Frank E.
Runyan, Jr. Charles E.
Tarazano D. Lawrence
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