The "Why": Surface Energy Explained
The core of the problem lies in a property called surface energy. Think of it as the molecular-level "grip" that a surface possesses. For a liquid adhesive to form a strong bond, it must be able to spread out and make intimate contact with the plastic's surface—a process known as "wetting out."
LSE plastics like polypropylene (PP), polyethylene (HDPE), nylon (PA), and acetal have very low surface energy. Most adhesives are repelled by these surfaces, causing them to bead up like water on a freshly waxed car. To create a strong bond, the surface energy of the plastic must be raised to a level higher than the surface tension of the adhesive.