In light of the recent global environmental problems, it is a target of attention to use biodegradable (microorganism-degradable or naturally degradable) materials in order to prevent environmental pollution caused by industrial waste. Recently, voluntary restraint of CO2 emissions is strongly demanded in order to deal with the exhaustion of earth resources and the global warming In such a situation, naturally occurring materials as opposed to petroleum-derived materials, and materials which require a small amount of heat or emit a small amount of CO2 when being incinerated, are paid attention to.
It is conventionally known that polymers having an aliphatic ester structure are biodegradable. Representative examples of such polymers include poly-3-hydroxybutyrate (PHB) produced by microorganisms, polycaprolactone (PCL) which is a synthetic polymer, polybutylene succinate (PBS) or polybutylene succinate adipate (PBSA) each containing succinic acid and butanediol as main components, polyester carbonate, polylactic acid (PLA) obtained from L-lactic acid and/or D-lactic acid produced by fermentation as a main starting material, and the like. Among these, PLA, for example, is a naturally occurring material.
These polymers having an aliphatic ester structure, except for PLA, generally have properties similar to those of polyethylene and have good moldability and biodegradability. However, such polymers are not sufficiently strong in a field requiring rigidity or in a field requiring tensile strength. The rigidity of these polymers may be improved using a filler such as talc or the like or using a nanocomposite forming technology. However, there are problems including reduction of fluidity, and improvement on this point has been desired. Regarding PLA, improvement in thermal resistance and toughness has been strongly desired.
Conventionally, there have been several studies by which a core-sheath composite fiber is formed of a biodegradable material and is used as a raw cotton of a thermally bonded nonwoven fabric. For example, Patent Documents 1 and 2 disclose using biodegradable polymers having different melting points as thermoplastic biodegradable fibers for a core and a sheath. Patent Document 3 discloses using a high melting point L-polylactic acid for a core and a copolymer of L-polylactic acid and D-polylactic acid for a sheath. Patent Document 4 discloses a composite fiber in which at least one of the components of a core and a sheath is a biodegradable polymer. The components are different in the melting point by 20 to 80° C., and the melting point distribution of the components is sharp. Patent Document 5 discloses an interior finishing material obtained from a composite fiber, which is formed of polylactic acid covered with another thermoplastic resin.
Patent Document 1: Japanese Laid-Open Patent Publication No. H7-133511
Patent Document 2: Japanese Laid-Open Patent Publication No. H8-260320
Patent Document 3: Japanese Patent No. 3355026
Patent Document 4: Japanese Laid-Open Patent Publication No. 2006-97148
Patent Document 5: Japanese Laid-Open Patent Publication No. 2008-57095