Manufacturing of polyethylene thin films for high-altitude balloons

A balloon envelope for use in a stratospheric balloon, the balloon envelope comprising:

• a plurality of gores;
• and
• a plurality of tendons, each tendon of the plurality of tendons arranged at a respective one or more of the plurality of gores;
• the balloon envelope including a cross-linked film comprising an extruded polyethylene mixture having oriented molecules and crosslinked polymer chains, wherein the balloon envelope is formed by a process of:
• heat bonding the cross-linked film to form the plurality of gores;
• and
• bonding each of the plurality of tendons to each of the respective one or more of the plurality of gores,
• wherein crystallinity of the balloon envelope is greater than 50% and less than 62% after the process,
• wherein transverse direction (“TD”) tensile strain is less than 2 mm/mm for a tensile stress of 150 MPa at −60° C. with a 50 mm/min pull rate, and
• wherein meridional direction (“MD”) tensile strain is less than 2 mm/mm for a tensile stress of 150 MPa at −60° C. with a 50 mm/min pull rate.