Polyester film and envelope
View Patent ↗The embodiments relate to a polyester film, which has a high tensile strength in the transverse direction and a low tensile strength in the longitudinal direction, while satisfying the condition of 0.5<A<6, A being calculated by Equation 1. A polyester film according to the embodiments can easily be cut in the transverse direction.
1. A polyester film, which has a thickness of 5-50 μm, a tensile strength of 5-20 kpsi in the longitudinal direction (MD), a tensile strength of 15-40 kpsi in the transverse direction (TD) substantially perpendicular to the MD, and a light transmittance of 80% or more, wherein the polyester film has been stretched about 2-3.5 times in the MD and about 2.5-4.5 times in the TD, and wherein A calculated by Equation 1 is greater than 0.5 and less than 6:
TS
1
-
TS
2
TS
1
=
A
Equation
1
wherein TS1 and TS2 are tensile strengths in the MD and in the TD, respectively;
wherein the polyester film has a tear strength of 5-85 kgf/μm in the MD and a tear strength of 0-15 kgf/μm in the TD; and
wherein the polyester film consists of polyethylene terephthalate.
2. The polyester film of claim 1 , wherein the polyester film has a haze of 2.0-30.
3. The polyester film of claim 1 , wherein the polyester film has a tensile strength of about 8-20 kpsi in the MD.
4. The polyester film of claim 1 , wherein A is greater than 0.6 and less than 3.
5. The polyester film of claim 1 , wherein A is greater than 0.7 and less than 3.
6. The polyester film of claim 1 , wherein A is greater than 0.7 and less than 2.
7. A mail envelope comprising:
a paper envelope with a window; and
a polyester film covering the window,
the polyester film having a thickness of 5-50 μm, a tensile strength of 5-20 kpsi in the MD, a tensile strength of 15-40 kpsi in the TD substantially perpendicular to the MD, and a light transmittance of 80% or more, wherein the polyester film has been stretched about 2-3.5 times in the MD and about 2.5-4.5 times in the TD, and wherein A calculated by Equation 1 is greater than 0.5 and less than 6:
TS
1
-
TS
2
TS
1
=
A
Equation
1
wherein TS1 and TS2 are tensile strengths in the MD and in the TD, respectively;
wherein the polyester film has a tear strength of 5-85 kgf/μm in the MD and a tear strength of 0-15 kgf/μm in the TD; and
wherein the polyester film consists of polyethylene terephthalate.