Binder fiber and nonwoven web

#1. Field of the Invention

#The present invention relates to a binder fiber which has improved adhesion with absorbent materials particularly at temperatures below about 140° C. The binder fibers can be in the form of low melting fibers or bicomponent fibers. Either of these fibers (or a mix of these fibers) can be used with absorbent material to create a nonwoven web. The improved binder fibers have improved adhesion at temperatures below 140° C. compared with current commercially available improved adhesion fibers. Such fibers enable the user to achieve the ideal thermal bonding at faster throughputs. Increase the z-directional web strength (thickness) for higher basis weight webs, and permit the incorporation of additional heat sensitive raw materials heretofore unusable, while retaining thermal bonding efficiency. Webs made from the binder fibers of the present invention are useful in diapers, incontinent pads, sanitary napkins and other absorbent pads for liquids.

#2. Prior Art

#FIG. 1 compares the bonding index as a function of bonding temperature of the inventive binder fiber compared to prior art binder fiber.

#Binder fibers of the present invention have a low melt portion which comprises either 100% of the binder fiber such that it is a low melting fiber, or a portion of the fiber is the low melt portion (such as bicomponent fibers). The low melt fiber and the low melt portion of the bicomponent fiber are made from polyolefin and are referred to as “base polyolefin”. The low melt portion may consist of a metallocene catalyzed linear low-density polyethylene (mLLDPE) with an adhesion promoter.

#The binder fibers of the present invention can also be base polyolefin with an adhesion promoter and an enhancement agent. Suitable base polyolefins may be high density polyethylene (HDPE), medium density polyethylene (MDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), ultra low density polyethylene (ULDPE), polypropylene (PP), or a mixture of these. These products are well known to those skilled in the art and are all commercially available from a wide variety of sources.

#LLDPE resins are copolymers of ethylene and alpha-olefins with low alpha-olefin content. The higher the alpha-olefin content the lower the density of the resin. Metallocene catalyzed linear low density polyethylene (mLLDPE) are produced by Exxon Mobil under the trade name EXCEED and Dow Chemical under the trade name “AFFINITY”. In contrast to LLDPE, produced with Ziegler-Natta catalysts, mLLDPE have a narrow molecular weight distribution and uniform composition distribution. Melting points of mLLDPE show a noticeable tendency on their composition and may very widely; for instance from 120° C. for copolymers containing 1½ mole % of alpha-olefin to 110° C. for copolymers containing 3.5 mole % alpha-olefin. In contrast, a LLDPE resin has a non-uniform compositional distribution. Melting such mixtures is dominated by the low branched fraction which is quite crystalline. As a result the melting points of LLDPE resins are not sensitive to copolymer composition and usually fall in the range of 125 to 128° C.