Abstract: A heat-adhesive composite fiber consisting of core portion and sheath portion, the core portion being of the side-by-side type composite structure comprising two core components of different polypropylene base polymers at a composite ratio of 1:2 to 2:1, one of the core components having a Q value, expressed in terms of the weight-average molecular weight/the number-average molecular weight, equal to or higher than 6 and the other having a Q value equal to or lower than 5, and the sheath portion meeting at least the requirements that it should comprise a sheath component of a polyethylene base polymer having a melting point lower by at least 20.degree. C. than the lower one of the melting points of the two core components, and it should cover the core portion in a proportion of 25 to 55% by weight based on the total weight of it and the core portion.

Abstract: Nonwoven fabrics contain at least 30% by weight of heat-adhesive composite fibers consisting of core portion and sheath portion, said core portion being of the side-by-side type composite structure comprising two core components of different polypropylene base polymers in a composite ratio of 1:2 to 2:1, one of said core components having a Q value, expressed in terms of the weight-average molecular weight/the number-average molecular weight, equal to or higher than 6 and the other having a Q value equal to or lower than 5, and said sheath portion meeting at least the requirement that it should comprise a sheath component of a polyethylene base polymer having a melting point lower by at least 20.degree. C. than the lower one of the melting points of said two core components. The nonwoven fabrics are bulky and soft due to the crimps of the heat-adhesive composite fibers resultant from the core portion and are stabilized by the inter-fiber bonds of the sheath portion.

Abstract: Heat-adhesive side-by-side type composite fibers consisting of a component composed mainly of a crystalline polypropylene and a component composed mainly of an olefin polymer, having few naturally developed crimps and few latent crimpability and superiority in undetachability of the composite components, are produced by using two composite components having a specified relationship of melt flow ratio and by stretching unstretched composite fibers at a specified temperature range.

Abstract: A porous non-woven fabric having a good dimensional stability and uniformity is prepared from a web obtained by arranging side-by-side composite fibers alone or a blend thereof with another fibrous material into a web and heat-treating the resulting web at a specified temperature, said composite fibers having few naturally developed crimps and being obtained by stretching unstretched side-by-side composite fibers consisting of a polypropylene component and an olefin polymer component having a specified melting point and a specified melt flow rate.