Biaxially oriented polyester film

A membrane switch sheet comprising two substrate films for membrane switches and a spacer sheet provided between the two substrate films for membrane switches, wherein the substrate films for membrane switches each independently comprise a laminated biaxially oriented polyester film that satisfies the following (1) to (10) and a conductive layer on at least one surface of the laminated biaxially oriented polyester film:

• (1) a content rate of an ester constituting unit derived from an isophthalic acid component is 0.5 mol % or more and 5.0 mol % or less with respect to 100 mol % of all ester constituting units in all polyester resins constituting the laminated biaxially oriented polyester film;
• (2) an amount of change in haze of the laminated biaxially oriented polyester film, Δhaze {Δhaze=(haze after heating)−(haze before heating)} is 5.0% or less when the film is heated at 150° C. for 30 minutes;
• (3) an intrinsic viscosity of the laminated biaxially oriented polyester film is 0.59 dl/g or more and 0.65 dl/g or less;
• (4) a storage modulus of the laminated biaxially oriented polyester film at 150° C. is 5.0×108 [Pa] or more and 7.6×108 [Pa] or less as an average value of storage moduli in a machine direction and a transverse direction of the film when a polyester film having a width of 5 mm is subjected to measurement at a grip interval of 30 mm using a dynamic viscoelasticity measuring instrument under conditions of a tensile mode, a frequency of 10 Hz, and a rate of temperature increase of 5° C./min;
• (5) an acid value of the laminated biaxially oriented polyester film is 40 eq/ton or more and 60 eq/ton or less;
• (6) polyethylene terephthalate is a main constituent of the polyester resin constituting the laminated biaxially oriented polyester film;
• (7) the thickness of the laminated biaxially oriented polyester film is 25 μm or more and 75 μm or less;
• (8) (Ymax−Ymin) is 0 or more and 0.005 or less;
• (9) the following relational expression is satisfied:
• X
• ×
• (
• Y
• max
• 3
• -
• Y
• min
• 3
• )
• ≤
• 0
• .
• 0
• ⁢
• 5
• ⁢
• 9
• ⁢
• 1
• ×
• (
• X
• ×
• Y
• min
• 3
• )
• +
• 0
• .
• 0
• ⁢
• 0
• ⁢
• 5
• wherein X is a storage modulus at 170° C., and Y is a film thickness per 1000 mm in a transverse direction with Ymax as a maximum value and Ymin as a minimum value;
• and
• (10) the laminated biaxially oriented polyester film comprises a first skin layer, a core layer, and a second skin layer in that order.