Support substrate for separation membrane

A separation membrane support substrate composed of a laminated nonwoven fabric comprising a front layer as a coating layer to be coated with a resin, a middle layer, and a back layer, which are integrally formed by heat bonding and are subjected to calender treatment after the integral formation by heat bonding, wherein said laminated nonwoven fabric satisfies the following conditions (1) to (6);

• (1) the front layer has at least one layer comprising thermoplastic resin filaments with a single filament diameter of 7-30 μm;
• (2) the middle layer has at least one layer comprising melt blown fibers with a single fiber diameter of no greater than 5 μm, and a fiber basis weight of at least 1 g/m2 and comprising no more than 30 wt % of the total fiber basis weight;
• (3) the back layer has at least one layer comprising thermoplastic resin filaments with a single filament diameter of 7-20 μm, and has a fiber basis weight of 3-40 g/m2;
• (4) the laminated nonwoven fabric has an apparent density of 0.67-0.91 g/cm3;
• (5) the laminated nonwoven fabric has a thickness of 45-110 μm;
• and
• (6) the smoothness of the coating layer surface is 0.2-2 μm in terms of the KES surface roughness SMD,
• wherein the thermoplastic resin in the filaments of the front and back layers is the same, and
• wherein a flat roll is used for said heat bonding at a temperature of 50-120° C. below the melting point of the thermoplastic resin filaments, after which said calender treatment is carried out at a temperature of at least 10° C. higher than the temperature of said heat bonding and 10-100° C. lower than the melting point of the thermoplastic resin filaments.