Adhesives, by definition, contain reactive polar groups within their molecular structures. These groups are used to form adhesive bonds with substrate surfaces. In their pre-cured state, adhesives usually absorb microwave energy because their dipoles are free to move. After curing, dipole mobility is often reduced and absorption of microwave energy is usually reduced too. In certain cases, where the net product of the molecular dipole contributions is approaching zero, adhesives will absorb less microwave energy, making them more transparent.