Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1999-05-18
2002-12-31
Kelly, Cynthia H. (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S394000, C525S343000, C525S342000
Reexamination Certificate
active
06500540
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a crosslinked, heat resistant elastic article having elevated temperature elasticity comprising a cured, irradiated or crosslinked ethylene polymer and a method for making a crosslinked, heat resistant elastic article. In particular, the invention relates to a shaped article (e.g. film or fiber) characterized by heat resistance and improved elasticity at elevated temperatures and comprising a substantially cured, irradiated, or crosslinked homogeneously branched ethylene polymer. The improved elastic article of the present invention is particularly suitable for use in applications where good elasticity must be maintained at elevated temperatures such as, for example, personal hygiene items and disposable infection-control garments at body temperatures of about 100° F. (38° C.).
BACKGROUND OF THE INVENTION
Materials with excellent stretchability and elasticity are needed to manufacture a variety of disposal and durable articles such as, for example, incontinence pads, disposable diapers, training pants, sport apparel and furniture upholstery. Stretchability and elasticity are performance attributes which function to effectuate a closely conforming fit to the body of the wearer or to the frame of the item. It is desirable to maintain the conforming fit during repeated use, extensions and retractions at body temperatures. Further, for incontinence articles, stretchability and elasticity are particularly desirable to ensure comfort and provide security against unwanted leaks.
Disposable articles are typically elastic composite materials prepared from a combination of polymer film, fibers, sheets and absorbent materials as well as a combination of fabrication technologies. Whereas the fibers are prepared by well known processes such as spun bonding, melt blowing, melt spinning and continuous filament wounding techniques, the film and sheet forming processes typically involve known extrusion and coextrusion techniques, e.g., blown film, cast film, profile extrusion, injection molding, extrusion coating, and extrusion sheeting.
A material is typically characterized as elastic where it has a high percent elastic recovery (i.e., a low percent permanent set) after application of a biasing force. Ideally, elastic materials are characterized by a combination of three important properties, i.e., a low percent permanent set, a low stress or load at strain, and a low percent stress or load relaxation. That is, there should be (1) a low stress or load requirement to stretch the material, (2) no or low relaxing of the stress or unloading once the material is stretched, and (3) complete or high recovery to original dimensions after the stretching, biasing or straining is discontinued.
Lycra (spandex) is a segmented polyurethane elastic material which is known to exhibit goodelastic properties. But Lycra tends to be extremely cost prohibitive for a many of applications. Also, Lycra like natural rubbers tend to exhibit poor environmental resistance to ozone, chlorine and high temperature, especially in the presence of moisture.
Natural rubbers, as discussed by Ferdinand Rodriguez in
Principles of Polymer Systems
, pp. 242-43, McGraw-Hill (1982), the disclosure of which is incorporated herein by reference, generally show decreases in elongation to break with increase in degree of crosslinking. Furthermore, at high degrees of crosslinking, even tenacity at break may decrease for natural rubbers.
Elastic materials such as films, strips, coating, ribbons and sheet comprising at least one substantially linear ethylene polymer are disclosed in U.S. Pat. No. 5,472,775 to Obijeski et al., the disclosure of which is incorporated herein by reference. But U.S. Pat. No. 5,472,775 does not disclose the performance of these materials at elevated temperatures (i.e., at temperatures above room temperature).
WO 94/25647 (Knight et al.), the disclosure of which is incorporated herein by reference, discloses elastic fibers and fabrics made from homogeneously branched substantially linear ethylene polymers. The fibers are said to posses at least 50 percent recovery (i.e., less than or equal 50% permanent set) at 100 percent strain. But there is no disclosure in WO 94/25647 regarding the elasticity of these fibers at elevated temperatures, nor is there any disclosure regarding resistance to high temperatures.
U.S. Pat. No. 5,322,728 to Davey et al., the disclosure of which is incorporated herein by reference, discloses elastic fibers comprised of single site catalyzed ethylene polymers. But polymers are not cured, irradiated or crosslinked and therefore are believed to exhibit poor elevated temperature elasticity.
WO 95/29197 (Penfold et al.), the disclosure of which is incorporated herein by reference, discloses curable, silane-grafted substantially ethylene polymers which are useful for use in wire and cable coatings, weather-stripping and fibers. WO 95/29197 reports examples which include fibers comprising silane-grafted substantially ethylene polymers having densities of 0.868 g/cm
3
and 0.870 g/cm
3
. While example fibers are shown to exhibit improved elastic recovery at elevated temperatures, there is no disclosure regarding percent stress or load relaxation performance at elevated temperatures.
U.S. Pat. No. 5,324,576 to Reed et al., the disclosure of which is incorporated herein by reference, discloses an elastic nonwoven web of microfibers of radiation crosslinked ethylene/alpha olefin copolymers, preferably having a density less than 0.9 g/cm
3
. The examples reported in U.S. Pat. No. 5,324,576 comprise ethylene polymers having polymer densities greater than or equal to 0.871 g/cm
3
which subjected to electron beam radiation. But Reed et al. provide no disclosure regarding the elastic performance of these radiated polymers at elevated temperatures.
U.S. Pat. No. 5,525,257 to Kurtz et al., the disclosure of which is incorporated herein by reference, discloses that low levels of irradiation of less than 2 megarads of Ziegler catalyzed linear low density ethylene polymer results in improved stretchability and bubble stability without measurable gelation.
U.S. Pat. No. 4,425,393 to Benedyk et al., the disclosure of which is incorporated herein by reference, discloses low modulus fibers having diameters in the range of 0.5 to 3 mils (about 1 to about 37 denier).
Canadian Patent No. 935,598 to Hardy et al., the disclosure of which is incorporated herein by reference, discloses elastic fibers comprised of various ethylene polymers wherein the fibers are post-drawn (stretched) and crosslinked while under tension.
U.S. Pat. No. 4,957,790 to Warren, the disclosure of which is incorporated herein by reference, discloses the use of pro-rad compounds and irradiation to prepare heat-shrinkable linear low density polyethylene films having an increased orientation rate during fabrication. In the examples provided therein, Warren employs Ziegler catalyzed ethylene polymers having densities greater than or equal to 0.905 g/cm
3
.
In spite of various disclosures relating to elastic ethylene polymer articles, including articles comprising curable, radiated and/or crosslinked ethylene polymers, there is a present need for cost-effective elastic articles having good heat resistance and elasticity at elevated temperatures, especially at human body temperatures of about 100° F. There is also a need for a method of making elastic articles having good elasticity at elevated temperatures. We have discovered that these and other objects can be completely met by the invention herein described.
SUMMARY OF THE INVENTION
We have discovered that elastic articles comprising a substantially cured, radiated or crosslinked ethylene polymer wherein the polymer is characterized as having a polymer density of less than 0.89 g/cm
3
, especially less than 0.87 g/cm
3
and most especially less than or equal to 0.865 g/cm
3
(or a differential scanning calorimetry (DSC) crystallinity at 23° C. of less than 26weight percent, especially 12 weight percent, and most especially less than or equal
Bensason Selim
Ho Thoi H.
Langohr Michael F.
Martin Jill M.
Patel Rajen M.
Gray J. M.
Kelly Cynthia H.
The Dow Chemical Company
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