Propylene Based Meltblown Nonwoven Layers and Composite Structures

Abstract

The present invention relates to propylene-based nonwoven layers made by the meltblown process, and laminates incorporating such layers. The meltblown layers of the present invention comprise propylene copolymers characterized by having less than 50 percent crystallinity. The meltblown layers of the present invention show an improved combination of extensibility and tensile strength. The laminate structures of the present invention are characterized by a combination of low bending modulus with high peel strength.

Description

FIELD OF THE INVENTION[0001]The present invention relates to propylene-based nonwoven layers made by the meltblown process, and laminates incorporating such layers. The meltblown layers of the present invention comprise propylene copolymers characterized by having less than 50 percent crystallinity. The meltblown layers of the present invention show an improved combination of extensibility and tensile strength and also exhibit markedly better bonding strength when bonded to spunbond layers, particularly spunbond layers made from fibers in which a polyethylene based material comprises at least a portion of the surface.BACKGROUND AND BRIEF SUMMARY OF THE INVENTION[0002]Nonwoven webs or fabrics are desirable for use in a variety of products such as bandaging materials, garments, disposable diapers, and other personal hygiene products, including pre-moistened wipes. Nonwoven webs having high levels of strength, softness, and abrasion resistance are desirable for disposable absorbent gar...

AI Technical Summary

Benefits of technology

[0008]Fiber extensibility / elasticity is another important criteria for nonwoven structures, particularly those used in hygiene and medical applications, because the characteristic translates to a better comfort and fit as the article made from the fiber will be able to be more body conforming in all situations. Diapers with elastic components will have less sagging in general as body size and shape and movement vary. With improved fit, the general well being of the user is improved through improved comfort, reduced leakage, and a closer resemblance of the article to cotton underwear.

[0009]A nonwoven meltblown layer which exhibits a combination of high tensile strength, good elongation and adequate hand feel is therefore desired and is an aspect of the present invention. It has been discovered that such extensible / elastic meltblown fabrics can be made from a particular class of polypropylene, known as propylene based plastomers and elastomers, without the need for blending substantial amounts of higher tenacity materials such as hPP. The propylene based plastomers and elastomers can be characterized by one or more of the following traits: crystallinity less than 50 percent; flex modulus less than 50 kpsi; melting point less than about 140° C. (and even less than about 130° C.); and / or heat of fusion less than 80 J / g. The propylene based polymer preferably comprises copolymers of propylene and an alpha-olefin, and the alpha olefin is preferably ethylene. The ethylene in the preferred embodiment is preferably present in an amount of from 3 to 20 percent by weight of the propylene based polymer. Propylene based polymers having ethylene in an amount of from 9 to 20 percent by weight of the propylene based polymer are more elastomeric. Such polymers may be referred to as propylene-based elastomers (PBE). The preferred propylene based polymer has an MWD of from 2 to 4. The propylene based polymer typically may have a melt flow rate (prior to any rheology modifier) in the range of from 1 to 100 g / 10 min. Nonwoven meltblown layers made from propylene based plastomers and elastomers, without substantial amounts (such as more than about 10 percent by weight (10 wt. percent )) of hPP is therefore another aspect of the present invention. All percentages specified herein are weight percentages unless otherwise specified.

[0010]By themselves, for example, these nonwovens may advantageously be used in filtration applications, or they may be combined with other materials, including other nonwoven materials. For structures can have hydrohead performance from 100, preferably 200 to 800 mm H2O for a 25 gsm basis weight. Higher basis weights may be able to achieve higher hydrohead performance.

[0013]It has been observed, however, that poor bonding strength between layers is particularly problematic when polyethylene materials (including bicomponent fibers where polyethylene forms at least part of the surface of the bicomponent fibers) are used to make one layer, and propylene materials such as homopolymer polypropylene (“hPP”) or random copolymer polypropylene (“RCP”) are used for an adjacent layer. It would therefore be particularly desirable to improve the bonding strength between polyethylene-based layers and polypropylene-based layers in a nonwoven composite structure.

[0014]It has also been discovered that meltblown fabrics made from this particular class of polypropylene based materials offers superior bonding strength to spunbond layers made from fibers having surfaces comprised of polyethylene based materials. In addition to providing superior bonding strength between the meltblown and spunbond layers, the use of these polymers for use in the meltblown nonwoven webs has been observed to increase the overall softness as compared to composites in which the meltblown layers comprise substantial amounts of hPP or RCP.

Problems solved by technology

Fabrics made from hPP, particularly nonwoven fabrics, exhibit high modulus but poor elasticity and softness.

In comparison, polyethylene-based elastomers, and the fibers and fabrics made from these polymers, tend to exhibit low modulus and good elasticity, but they also tend to have low tenacity, stickiness and exhibit a hand feel which is generally considered as unacceptable for many applications.

In contrast, a fabric made from a higher modulus fiber, for example, hPP, will feel harsher (stiffer) and will drape less well resulting in a poorer fit.

Fabrics made from polyethylene-based elastomers also tend to lack adequate hand feel as they tend to have an undesirable feel to the skin commonly characterized by descriptors such as tacky, sticky, clammy, rubbery, or wet.

It has been observed, however, that poor bonding strength between layers is particularly problematic when polyethylene materials (including bicomponent fibers where polyethylene forms at least part of the surface of the bicomponent fibers) are used to make one layer, and propylene materials such as homopolymer polypropylene (“hPP”) or random copolymer polypropylene (“RCP”) are used for an adjacent layer.

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