Surface-treated fillers for biaxially oriented polyester films

A process for producing a mono or multi-layer biaxially oriented polyester film comprising the steps of:

• a) providing a composition comprising at least one polyester and a surface-treated filler material product, and
• b) forming a film from the composition of step a), and
• c) stretching the film obtained in step b) in machine direction (MD) and transverse direction (TD) in any order, wherein the stretching in machine direction (MD) and transverse direction (TD) is carried out sequentially or simultaneously, wherein the surface-treated filler material product comprises
• A) at least one ground calcium carbonate-comprising filler material having a weight median particle size d50 in the range from 0.5 μm to 2.5 μm, and
• B) a treatment layer on the surface of the at least one ground calcium carbonate-comprising filler material comprising
• i. a phosphoric acid ester blend of one or more phosphoric acid mono-ester and salty reaction product thereof and/or one or more phosphoric acid di-ester and salty reaction product thereof, and/or
• ii. at least one saturated aliphatic linear or branched carboxylic acid and salty reaction product thereof, and/or
• iii. at least one aliphatic aldehyde and/or salty reaction product thereof, and/or
• iv. at least one mono-substituted succinic anhydride consisting of succinic anhydride mono-substituted with a group selected from a linear, branched, aliphatic and cyclic group having a total amount of carbon atoms from at least C2 to C30 in the substituent and/or salty reaction product thereof, and/or
• v. at least one polydialkylsiloxane, and/or
• vi. mixtures of the materials according to i. to v.,
• wherein the surface-treated filler material product comprises the treatment layer in an amount of from 0.1 wt.-% to 2.3 wt.-%, based on the total dry weight of the at least one ground calcium carbonate-comprising filler material.

A method of providing a voiding agent in a mono or multi-layer biaxially oriented polyester film, wherein the voiding agent comprises a surface-treated filler material product comprising

• A) at least one ground calcium carbonate-comprising filler material having a weight median particle size d50 in the range from 0.5 μm to 2.5 μm, and
• B) a treatment layer on the surface of the at least one ground calcium carbonate-comprising filler material comprising
• i. a phosphoric acid ester blend of one or more phosphoric acid mono-ester and salty reaction product thereof and/or one or more phosphoric acid di-ester and salty reaction product thereof, and/or
• ii. at least one saturated aliphatic linear or branched carboxylic acid and salty reaction product thereof, and/or
• iii. at least one aliphatic aldehyde and/or salty reaction product thereof, and/or
• iv. at least one mono-substituted succinic anhydride consisting of succinic anhydride mono-substituted with a group selected from a linear, branched, aliphatic or cyclic group having a total amount of carbon atoms from at least C2 to C30 in the substituent and/or salty reaction product thereof, and/or
• v. at least one polydialkylsiloxane, and/or
• vi. mixtures of the materials according to i. to v.,
• wherein the surface-treated filler material product comprises the treatment layer in an amount of from 0.1 wt.-% to 2.3 wt.-%, based on the total dry weight of the at least one ground calcium carbonate-comprising filler material.