TECHNOLOGY FOCUS - POLYMERS - Preferred Sheet: The back sheet has a maintenance ratio of elongation (%) (100x elongation after heat-treatment/elongation before heat-treatment) of greater than or equal to 80% both in the longitudinal and transverse directions, when measured after heat-treatment for 75 hours using pressurized water under 2 atm at 120 degrees C. The back sheet is prepared by drying a polyester resin which comprises at least one repeating unit of trimethylene naphthalate and trimethylene terephthalate in an amount of greater than or equal to 85 wt.% to reduce the moisture content to less than 50 parts per million (ppm) before melt-extrusion. Preferred Method: In the method, the trimethylene terephthalate repeating unit is prepared by polymerizing 1,3-propanediol with terephthalic acid or its derivative; and the trimethylene naphthalate repeating unit is prepared by polymerizing 1,3 -propanediol with naphthalene dicarboxylic acid or its derivative. The step a) further involves drying the polyester resin to reduce the moisture content to less then 50 ppm before the melt-extrusion and quenching steps. The step c) further involves coating one side of the final sheet with an ethylene vinyl acetate (EVA) layer and the other side of the sheet with a fluorine resin layer. Preferred Components: The polyester further comprises at least one of UV stabilizer and UV absorbent in an amount of 0.01-1 wt.%. The UV stabilizer is benzotriazole-based compound or hindered amine light stabilizer (HALS) compound and the UV absorbent is hydroxybenzophenone or hydroxyphenyl benzotriazole. The polyester further comprises inorganic particles (0.01-15 wt.%). The polyester comprises at least one additional repeating unit prepared by polymerizing at least one dibasic acid selected from isophthalic acid (IPA), succinic acid, glutaric acid, adipic acid, suberic acid, azelaic acid, sebacic acid, and its ester derivative; with at least one diol (0.01-15 wt.%) selected from ethylene glycol (EG), diethylene glycol (DEG), neopentyl glycol (NPG), propylene glycol (PG), 1,4-butanediol (1,4-BDO)5 pentanediol, hexanediol, 2,2-butylethyl-1,3-propanediol (BEPD), 2-methyl-1,3-propanediol (MPDiol), and 1,4-cyclohexanedimethanol (1,4-CHDM). EXAMPLE - A terephthalic acid was added to the first reactor, and 1,3-propanediol and tetrabutoxy titanate (TBT) diluted in n-butanol as a catalyst were added to it in an amount of 120 parts by weight (pbw) and 0.03 pbw, respectively. The resulting mixture was allowed to react under 1.2 kg/cm2 at 260 degrees C for 4 hours with removing by-product, i.e. water. After the esterification was complete, triethyl phosphate (TEP) as a stabilizer and silica particles having an average particle size of 2.5 mu m as a slip agent were added to it in an amount of 0.045 pbw and 0.07 pbw, respectively, then the whole was stirred for 5 minutes. The resulting mixture was transferred to the second reactor and allowed to polymerize at 270 degrees C under reduced pressure until the agitation motor reach to the predetermined electricity, to obtain polytrimethylene terephthalate (A1) as a pellet having an intrinsic viscosity (IV) of 0.870 dl/g. The polymers (A1) (100 wt.%) obtained were mixed in various ratios. The resulting mixture was subjected to crystallization at 120 degrees C for 2 hours using a paddle dryer, and then dried at 165 degrees C for 5 hours to reduce the moisture content to 50 ppm. After work up, biaxially oriented sheet was obtained.