The present invention relates to a thermobondable, hydrophilic bicomponent synthetic fiber for use in the blending of fluff pulp, and to a process for producing the fiber.  More specifically, the invention relates to a fiber comprising an outer sheath component and an inner core component, the core component having a higher melting point than the sheath component.  The fiber is permanently substantially hydrophilic.  The term "hydrophilic" refers to the fact that the fiber has an affinity for water, and thus is easily dispersed in water or aqueous mixtures.  This affinity may be ascribed to the presence of polar groups on the fiber's surface.  The term "permanently" substantially hydrophilic refers to the fact that the fiber will retain its hydrophilic properties after repeated dispersions in water.  This is obtained by incorporating a surface active agent and optionally a hydrophilic polymer or copolymer into the sheath component of the fiber.  The fiber of the present invention is useful in the preparation of "fluff", which is a fluffy fibrous material used as an absorbent and/or liquid-conducting core in the production of hygiene absorbent products such as disposable diapers.  Fluff is produced by defibrating and dry forming so-called "fluff pulp", which is comprised of natural and/or synthetic fibers. 
There has been a trend in recent years towards stronger, thinner and lighter weight disposable diapers, and other disposable hygiene absorbent products.  One factor in this trend has been the development of a number of synthetic fibers, notably heat-adhesive (thermobondable) synthetic fibers, which have been used to replace at least some of the natural cellulose fibers in these products.  Such thermobondable synthetic fibers are typically used to bond the cellulose fibers together, thereby achieving an absorbent material with improved strength and allowing the production of thinner and lighter weight products.  Examples of patents describing such fibers, or their use or production, are U.S. Pat.  Nos.  4,189,338 (non-woven fabric comprising side-by-side bicomponent fibers), 4,234,655 (heat-adhesive composite fibers), 4,269,888 (heat-adhesive composite fibers), 4,425,126 (fibrous material using thermoplastic synthetic fibers), 4,458,042 (absorbent material containing polyolefin pulp treated with a wetting agent) and 4,655,877 (absorbent web structure containing short hydrophilic thermoplastic fibers), and European patent application No.  0 248 598 (polyolefin-type nonwoven fabric). 
However, the use of these synthetic fibers in absorbent products has not been without problems.  One problem which may be encountered is that it can be difficult to distribute the synthetic fibers into fluff pulp produced by a wet process, since these synthetic fibers are generally of a hydrophobic nature.  Such hydrophobic fibers repel water, and therefore have a tendency to form conglomerations in the fluff pulp or to float at the surface of the wet fluff pulp if they are lighter than water.  If the synthetic fibers are also distributed unevenly in the fluff, than barriers which hinder the transport of moisture may be created in the absorbent product, due to the fusion of the thermobonded fibers to each other in areas where there is a conglomeration of such fibers.  Furthermore, the synthetic fibers currently used in the production of fluff are generally quite short, i.e. normally shorter than the cellulose fibers which typically comprise a substantial portion of the fluff.  The supporting structure of the absorbent material is therefore formed by the cellulose fibers in the material, and since absorbent cores of such natural cellulose fibers have a tendency to break under the stress and bending to which, for example, diapers are subjected, wicking barriers are easily formed.  Absorbent cores which consist only of natural cellulose fibers, i.e. which do not contain any synthetic fibers, may likewise also be subject to breakage and formation of wicking barriers due to stress and bending. 
Hygiene absorbent products often include a so-called super absorbent polymer, in the form of a powder or small particles, which is incorporated into the material in order to achieve a weight reduction.  However, the super absorbent polymer in these materials often has a tendency to sift out of the position in which it was originally placed, due to the lack of a structure which can effectively retain the small particles. 
The long bicomponent synthetic fiber of the present invention addresses the problems mentioned above.  The bicomponent fibers of the present invention are substantially longer than other fibers typically used in the preparation of fluff.  During the production of absorbent products from fluff containing the bicomponent fiber, the fluff is subjected to a heat treatment (thermobonding), in which the sheath component of the bicomponent fiber is melted, while the high melting core component of the fiber remains intact.  The core component of the long bicomponent fibers are thus fused together by the melting of the sheath component, forming a strong uniform supporting three-dimensional matrix in the absorbent material.  The absorbent material is thus able to withstand flexing without developing wicking barriers due to breakage of the absorbent core.  In addition, the matrix structure formed by the bicomponent fibers gives the material improved shape retention under dynamic stress during use of the absorbent product. 
The three-dimensional mesh-like structure formed by the high melting component of the bicomponent fibers in the thermobonded material enables the super absorbent polymer to be held in the desired position.  This is a further advantage, giving a more efficient use of the super absorbent polymer and helping to increase porosity, as well as giving the possibility of producing lighter weight absorbent materials. 
In addition, the low melting sheath component has been made permanently substantially hydrophilic, thus allowing the fibers to be distributed homogeneously in the wet-processed fluff pulp which is typically used in the preparation of absorbent material.  It is also desirable that the fibers in the finished product are hydrophilic, so that the product's absorbent and liquid-conducting properties are not impaired, as may be the case in a product with a substantial content of hydrophobic fibers.