A method for forming biodegradable fibers is provided. The method includes blending polylactic acid with a polyepoxide modifier to form a thermoplastic composition, extruding the thermoplastic composition through a die, and thereafter passing the extruded composition through a die to form a fiber. Without intending to be limited by theory, it is believed that the polyepoxide modifier reacts with the polylactic acid and results in branching of its polymer backbone, thereby improving its melt strength and stability during fiber spinning without significantly reducing glass transition temperature. The reaction-induced branching can also increase molecular weight, which may lead to improved fiber ductility and the ability to better dissipate energy when subjected to an elongation force. Through selective control over this method, the present inventors have discovered that the resulting fibers may exhibit good mechanical properties, both during and after melt spinning.

Provided is an extra-fine fiber sheet including an extra-fine fiber assembly including extra-fine fibers having an average fiber diameter of 500 nm or smaller. The extra-fine fiber sheet includes an extra-fine fiber assembly. The assembly includes a solvent-spinnable polymer (A) having a weight average molecular weight of 5,000 to 100,000 as a main component and a polymer (B) having a weight average molecular weight equal to or more than 10 times as large as that of the polymer (A) as an accessory component; and the assembly includes constituent fibers having an average fiber diameter of 10 to 500 nm. The polymer (A) may be a non-conductive polymer, and the polymer (B) may be a thickening polymer.

The present invention is directed to articles comprising nanofibers. The nanofibers, having a diameter of less than 1 micron, may comprise a significant number of the fibers in one layer of the web contained by the article. Preferably, the nanofibers are produced in a melt film fibrillation process. The articles include diapers, training pants, adult incontinence pads, catamenials products such as feminine care pads and pantiliners, tampons, personal cleansing articles, personal care articles, and personal care wipes including baby wipes, facial wipes, and feminine wipes.