Consider an acrylic sheet of thickness L = 5 mm that is used to coat a hot, isothermal metal substrate at Th = 300 C. The properties of acrylic are ρ = 1990 kg / m3, c = 1470 J / kg K , and k = 0.21 W / m K. Neglecting the thermal contact resistance between the acrylic and the metal substrate, determine how long it will take for the insulated back of the acrylic to reach its softening tempature, Tsoft = 90 C. The initial acrylic tempature is Ti = 20 C.