The melting temperature for HDPE is 266 degrees Fahrenheit. This is the point where it changes from solid form to liquid. The maximum safe working temperature is 248 degrees Fahrenheit. Above this point, it starts to soften. The minimum safe working temperature is 148 degrees below zero Fahrenheit. At less than 180 degrees below zero, it becomes brittle.
HDPE's electrical insulation strength, known as dielectric strength, is 22 kilovolts per millimeter. This means it takes 22,000 volts to penetrate one millimeter. A millimeter is about as wide as the lead in a pencil. This is a lot of voltage to penetrate such a small distance. For this reason, HDPE is considered an insulator, rather than a conductor.
The specific density of HDPE is 0.94 to 0.95 of a gram per cubic centimeter. Water is 1 gram per cubic centimeter. This means, per unit size, HDPE weighs 0.05 of a gram less than water. A solid block of HDPE will float, but just barely.
The water absorption rate for HDPE is 0.01 percent. At any given weight, HDPE will only absorb one hundredth of one percent of water. This is a microscopic amount. For all practical purposes, it is impenetrable to water. This is why it is a suitable plastic for containers, such as water bottles.
HDPE's ultraviolet resistance is poor. The plastic will break down and become weak in ultraviolet light. As sunlight is mostly ultraviolet light, this is not a good plastic to use for long-term outdoor applications, such as outdoor flowerpots.