Buffer substrate and use thereof

A buffer substrate comprising a nonwoven fiber assembly which comprises a fiber comprising a moistenable-thermal adhesive fiber and a conjugated fiber comprising a plurality of resins which are different in thermal shrinkage and form a phase separation structure, in which the fibers constituting the nonwoven fiber assembly are entangled with each other and bonded at contacting points by melting the moistenable-thermal adhesive fiber to distribute the bonded points approximately uniformly,

• wherein a bonded fiber ratio is from 3 to 30% in each of three areas and the proportion of the minimum value relative to the maximum value among the bonded fiber ratios in each of the three areas is not less than 50%,
• providing that the three areas are obtained by cutting the buffer substrate in the thickness direction to give a cross section and dividing the cross section in a direction perpendicular to the thickness direction equally into three,
• wherein the bonded fiber ratio represents the proportion of the number of cross sections of bonded fibers relative to the total number of cross sections of fibers as determined from a microphotograph of a cross section with respect to the thickness direction of the nonwoven fiber assembly,
• wherein the bonded fibers means fibers which are still contacted with each other as observed on said microphotograph,
• wherein a curved ratio of the conjugated fiber is not less than 1.3 in each of three areas and the proportion of the minimum value relative to the maximum value among the curved ratios in each of the three areas is not less than 75%,
• providing that the three areas are obtained by cutting the buffer substrate in the thickness direction to give a cross section and dividing the cross section in a direction perpendicular to the thickness direction equally into three,
• wherein the curved ratio represents a ratio (L2/L1) of a fiber length (L2) of the crimped fiber relative to a length between the both ends of the crimped fiber (L1).