In a curing process for plastic and other polymer-based materials, a plastic sheet, which initially is 25°C, is placed on a hot metal surface that is maintained at 250°C. If the back (or opposite) side of the 5-mm-thick plastic film is considered to be insulated, how long does it take for the back side to reach a temperature of 90°C. Assume the following plastic properties: c = 1450 J/kg • K, k = 0.23 W/m • K, and ρ  = 1975 kg/m³.