There is provided a powder coating material, comprising a copolymer comprising tetrafluoroethylene unit and perfluoro(propyl vinyl ether) unit, wherein the copolymer has a content of perfluoro(propyl vinyl ether) unit of 5.0 to 6.2% by mass with respect to the whole of the monomer units, the copolymer has a melt flow rate at 372 C. of 20.0 to 35.0 g/10 min, and the copolymer has the number of functional groups of 100 or less per 10.sup.6 main-chain carbon atoms.

A brightness enhancement film coating material and an application thereof, where preparation raw materials for the coating material, in parts by weight, at least comprise the following components: 20-76 parts a monofunctional photocuring monomer, 20-76 parts a bifunctional photocuring monomer, 1-10 parts a photoinitiator, and 0-4 parts an auxiliary agent. No white streaks are produced after a brightness enhancement film formed by means of the present coating material undergoes a high-temperature high-humidity reliability test, and the adaptability of the brightness enhancement film and a polarizer is improved. Also, a reliability test is able to reflect, to a certain extent, a long-term use situation, and an excellent reliability test result can ensure long-term use stability of a display panel, and fragments produced during a cutting process are prevented from contaminating a film sheet and thereby affecting a use effect thereof.

Provided is a double layer tube including an inner layer (A) and an outer layer (B) disposed directly on the inner layer (A), wherein the inner layer (A) comprises a copolymer (a) that comprises tetrafluoroethylene unit and perfluoro(propyl vinyl ether) unit, and the outer layer (B) comprises a polymer (b) that comprises at least chlorotrifluoroethylene unit; wherein the copolymer (a) has a rate of decrease in weight of 0.05 mass % or less after heat treatment at 380 C. for 1 hour; and wherein a ratio between a thickness of the inner layer (A) and a thickness of the outer layer (B), ((A)/(B)), is 30.0/70.0 to 67.0/33.0.