Biaxially oriented polyester film and magnetic recording tape

Abstract

A biaxially oriented polyester film which has a Young's modulus (YMD) in the film machine direction (YMD) of 6.5 GPa or more and a Young's modulus in the width direction (YTD) of 8.2 to 9.8 GPa and is used as a base film for magnetic recording tapes of linear recording system, and a magnetic recording tape of linear recording system comprising the same as a base film. There are provided a biaxially oriented polyester film which has excellent dimensional stability, especially dimensional stability in the width direction and is suitable for use as a base film for magnetic recording tapes of linear recording system and a magnetic recording tape comprising the same.

1 . A biaxially oriented polyester film which has a Young's modulus in the film machine direction (YMD) of 6.5 GPa or more and a Young's modulus in the width direction (YTD) of 8.2 to 9.8 GPa and is used as a base film for magnetic recording tapes of linear recording system. 
   
   
       2 . The biaxially oriented polyester film according to  claim 1 , wherein the polyester is polyethylene-2,6-naphthalene dicarboxylate. 
   
   
       3 . The biaxially oriented polyester film according to  claim 1 , which has a thickness of 3 to 6 μm. 
   
   
       4 . The biaxially oriented polyester film according to  claim 1 , wherein the total of YMD and YTD is 15 to 18 GPa. 
   
   
       5 . The biaxially oriented polyester film according to  claim 1 , wherein YTD is equal to or lager than YMD. 
   
   
       6 . The biaxially oriented polyester film according to  claim 1 , which has a temperature expansion coefficient in the film width direction of −8 to +1 ppm/° C. 
   
   
       7 . The biaxially oriented polyester film according to  claim 1 , which has a humidity expansion coefficient in the film width direction of 5 to 10 ppm % RH. 
   
   
       8 . The biaxially oriented polyester film according to  claim 1 , which has a heat shrinkage factor (105° C.×30 minutes) in the film width direction of 0.8% or less. 
   
   
       9 . The biaxially oriented polyester film according to  claim 1 , which has a rupture elongation in the film width direction of 45% or more. 
   
   
       10 . The biaxially oriented polyester film according to  claim 1 , which has a crystallinity of 28 to 33%. 
   
   
       11 . The biaxially oriented polyester film according to  claim 1 , which has a dimensional change in the film width direction of more than 0.3% and 1% or less when a load of 32 MPa is applied thereto in the film machine direction in a 40° C. and 90% RH atmosphere. 
   
   
       12 . The biaxially oriented polyester film according to  claim 1 , wherein the surface roughness (WRa) of at least one surface is 1 to 10 nm. 
   
   
       13 . The biaxially oriented polyester film according to  claim 1 , which is a single-layer film and has a surface roughness (WRa) of 1 to 10 nm. 
   
   
       14 . The biaxially oriented polyester film according to  claim 1 , which is a laminate film consisting of two polyester film layers A and B. 
   
   
       15 . The biaxially oriented polyester film according to  claim 14 , wherein the layer A is formed on the side on which a magnetic layer is to be formed and has a surface roughness (WRa(A)) of 0.5 to 4 nm and the layer B is formed on the side on which no magnetic layer is to be formed and has a surface roughness (WRa(B)) of 5 to 10 nm. 
   
   
       16 . The biaxially oriented polyester film according to  claim 14 , wherein the layer A contains inert particles A having an average particle diameter of 0.01 to 0.20 μm in an amount of 0.01 to 0.15 wt % based on the weight of the layer A and the layer B contains inert particles B 2  having an average particle diameter of 0.2 to 0.4 μm in an amount of 0.01 to 0.2 wt % based on the weight of the layer B. 
   
   
       17 . The biaxially oriented polyester film according to  claim 16 , wherein the layer B contains inert particles B 1  having an average particle diameter of 0.01 to 0.20 μm in an amount of 0.01 to 0.3 wt % based on the weight of the layer B besides the inert particles B 2 . 
   
   
       18 . The biaxially oriented polyester film according to  claim 16 , wherein the average particle diameter of the inert particles A is 0.01 to 0.18 μm, the thickness (t) of the entire film is 3 to 6 μm, and the thickness (tB) of the layer B accounts for 50 to 90% of the thickness (t) of the entire film. 
   
   
       19 . The biaxially oriented polyester film according to  claim 16 , wherein the thickness (t) of the entire film is 3 to 6 μm, the thickness (tB) of the layer B accounts for more than 20% and less than 50% of the thickness (t) of the entire film, and the value (tB/dB) obtained by dividing tB by the average particle diameter (dB) of all the inert particles contained in the layer B is 2 to 30. 
   
   
       20 . The biaxially oriented polyester film according to  claim 16 , wherein the thickness (t) of the entire film is 3 to 6 μm, the thickness (tB) of the layer B accounts for 10 to 20% of the thickness (t) of the entire film, and the value (tB/dB) obtained by dividing tB by the average particle diameter (dB) of all the inert particles contained in the layer B is 0.5 to 15. 
   
   
       21 . The biaxially oriented polyester film according to  claim 16 , wherein the inert particles A are spherical silica particles or heat-resistant polymer particles. 
   
   
       22 . The biaxially oriented polyester film according to  claim 17 , wherein the inert particles B 1  have an average particle diameter 0.1 μm or more smaller than that of the inert particles B 2 . 
   
   
       23 . The biaxially oriented polyester film according to  claim 17 , wherein the inert particles A and the inert particles B 1  are of the same chemical species. 
   
   
       24 . The biaxially oriented polyester film according to  claim 16 , wherein the inert particles B 2  are crosslinked organic particles. 
   
   
       25 . A magnetic recording tape of linear recording system comprising a biaxially oriented polyester film having a Young's modulus in the film machine direction (TMD) of 6.5 GPa or more and a Young's modulus in the width direction (YTD) of 8.2 to 9.8 GPa and a non-magnetic layer and a magnetic layer formed on one side of the film and a back coat layer formed on the other side. 
   
   
       26 . The magnetic recording tape according to  claim 24 , wherein the total thickness of the non-magnetic layer, magnetic layer and back coat layer is 0.2 to 0.8 based on the thickness of the biaxially oriented polyester film.