Bicomponent fibers in nonwovens manufacture.

In Japan, increased interest has been seen recently in many types of bicomponent fibers for nonwoven fabrics. Typical bicomponent fibers for us in making nonwovens--classified in four different types--are shown in Table 1.

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The first type, usually called "sheath/core," is made of a sheath and a core polymer, with the sheath having a melting point lower than the core. The core is situated either concentrically or eccentrically with the sheath. This type of bicomponent fiber, which was the first introduced in the manufacture of nonwovens, is widely used in thermal bonded nonwovens such as coverstock, wipes, wrapping material and floppy disk liners. Production of this type exceeds 30,000 tons annually in nonwovens.

Type B, usually called "side by side," has its cross-section made to show two polymers bound together symmetrically or asymmetrically. The two polymers are different in thermal contractibility. The nonwoven fabric made of this type of bicomponent fibers is heat-treated to turn stretchable and bulky, since the high contractile component contracts much larger than the lower contractile one. The resulting stretchable and bulky nonwovens are used as poultice bases and in apparel insulation.

Type C bicomponent fiber, which has been applied in the field of nonwovens from a relatively early time, is made of a polystyrene matrix with very thin polyester or nylon fibers embedded within for the purpose of obtaining microfiber nonwovens. The bicomponent fibers of this type are first needlepunched, then subjected to a process of dissolving and removing the polystyrene matrix to obtain a nonwoven fabric with only microfibers of 0.001 to 0.2 denier left undissolved. The microfiber nonwovens manufactured in this manner are used to make man-made leathers.

Type D, which is now being most actively researched, is also manufactured to make microfiber nonwovens and composed of two kinds of polymer microfibers equally bound together in one bicomponent fiber. After the nonwoven is made of this type of bicomponent, it is subjected to water jets to make the constituent bound microfibers disengage from one another. The original nonwoven then turns into a microfiber nonwoven fabric composed of two microfibers from 0.1 to 0.3 denier. It is only relatively recently--around 1988--that this type of

bicomponent began to be practical in the field of nonwovens. In contrast with the previous Type C bicomponent fiber, which is used only in man-made leathers, Type D fiber has a variety of applications including wipes, filters and battery electrode separators. For this reason, enthusiastic research and development activities are being made on this type of bicomponent fiber and various polymer combinations and cross-sectional patterns have been devised. Of all types of bicomponent fibers, this Type D seems to be most promising.

Although many kinds of fibers, bicomponents included, are used in nonwovens, the number of types is not as large as in woven fabrics. Therefore, it is doubtless that further kinds of fibers will come to be used in nonwovens, in combination with further diversification in the technology of web forming and bonding, causing many new types of nonwovens to be marketed in the near future.