Thermoplastic elastomer copolymer films

Abstract

Disclosed are (1) a thermoplastic elastomeric film comprised of an elastomeric arene-diene block copolymer and particular ethylene / .beta.-olefin copolymers having low ethylene crystallinity and (2) the process of preparing films thereof. The disclosed films have superior strength and elasticity characteristics which render them particularly useful in apparel and healthcare items such as disposable diapers.

Description

FIELD OF INVENTIONThis invention relates to thermoplastic elastomer blends of elastomeric block copolymers and certain olefin copolymers and, more specifically, to the fabricated films of the blends which exhibit high tensile strength and elasticity.BACKGROUND OF THE INVENTIONThermoplastic elastomers are readily commercially available and are fabricated as elastomeric films in products such as disposable diapers, leggins, waistbands and gloves. Typically, these elastomeric films have been made of elastomeric block copolymers, combinations of elastomeric block copolymers and liquid butadienes, combinations of elastomeric block copolymers and ethylene vinyl acetate copolymers, thermoplastic urethanes, ethylene-propylene rubbers (EP or EPR), including ethylene-propylene diene terpolymers (EPDM), natural rubbers and combinations of polyester copolymers and ethylene-vinyl acetate copolymers. Each of these materials has suitable properties for elastomeric films, such as low tensile set, l...

AI Technical Summary

Benefits of technology

The LLDPE plastomer copolymers to be used in accordance with the present invention have very low ethylene crystallinity. The films made from blends of these low crystalline ethylene copolymers and elastomeric copolymers have the characteristics of high elasticity and low blocking tendency. These benefits occur because the instant film blends contain low crystalline plastomeric ethylene copolymers having certain characteristics of molecular weight distributions (Mw / Mn) -(MWD), melt index (MI), density, and crystallinity.

The films of the present invention have improved elastic recovery over those films of the prior art and provide benefits and applications where both low modulus and excellent elastic recovery are necessary as in the case of elastic components of disposable diapers. The instant films exhibit a low blocking tendency and consequently can be manufactured more efficiently and less expensively than other elastomeric films which require significantly more slip and antiblocking additives for their processed blends.

A process for producing the elastomeric film of this invention comprises the steps of (a) providing a thermoplastic elastomeric film blend of (1) a block copolymer elastomer and (2) an LLDPE having a density of from about 0.86 to 0.900 g / cm.sup.3, a MWD in the range of 2 to about 3.5, a melt index of less than 1000 dg / min, and an ethylene crystallinity of less than 35%; (b) extruding the blend composition through a die having a gap greater than about 5 mils and less than about 120 mils; (c) expanding the extruded film to a thickness of between 0.5 mils and about 15 mils by application of differential pressure as exerted by a gas and (d) rapidly cooling the expanded extruded film.

Problems solved by technology

For example, elastomeric films containing thermoplastic urethanes, copolyesters and / or ethylene vinyl acetate copolymers are expensive due to the cost of raw materials.

While these films demonstrate suitable properties for most elastomeric film applications, the shortcomings of these films are (1) a significantly reduced elasticity of the patented blend as compared to the elastic property of the pure elastic S-B-S copolymer; (2) difficulty in processing of the blend; and (3) typically excessive blocking of the S-B-S elastomeric copolymer.

These films generally exhibit good strength and elastic properties but also undesirable excessive blocking.

When films made from block copolymers of monoalkenyl arenes and conjugated dienes are stored on a roll or in stacks, the film layers become difficult to unroll or unstack over a period of time due to their blocking tendency.

Such blocking tendency can cause the film to tear when it is being unwound from a roll or being unstacked.

The addition of these materials is undesirable because such material can accumulate on the film fabrication apparatus, particularly the rollers, and this accumulation can result in significant down time for machine cleaning.

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