Vulcanized rubber features low surface energy, weak surface activity, low polarity and poor wettability. In addition, small organic molecules inside vulcanized rubber tend to migrate and accumulate on the surface, forming a weak interfacial layer that reduces bonding strength. To enhance adhesion, surface treatment is required to increase surface roughness and introduce polar groups. The mainstream surface treatment methods for vulcanized rubber include sulfuric acid treatment, chlorination, plasma treatment, corona discharge, UV treatment and ozone treatment.

Plasma Cleaning Technology

Plasma refers to gas with an ionization degree above 0.1%, consisting of ions, electrons and neutral particles (atoms and molecules). It is electrically neutral as a whole, yet contains abundant electrons and ions, and presents unique electromagnetic properties. Charged particles inside plasma undergo thermal motion and diffusion, and migrate under the action of an electric field. Regarded as the fourth state of matter, plasma is distinct from solids, liquids and gases, and is the most common state of matter in the universe.

Plasma contains various active species such as ions, electrons, free radicals, excited molecules and ultraviolet rays. When a material is exposed to plasma, a series of physical and chemical reactions will occur on its surface. The surface morphology and chemical structure will be altered: the surface can be etched to become rougher, a dense cross-linked layer can be formed, or oxygen-containing polar groups can be grafted. These modifications improve the material's hydrophilicity, adhesion, dyeability, biocompatibility and electrical properties, while leaving its bulk properties basically unchanged.

Plasma cleaning can greatly raise the surface energy of rubber, effectively boosting the bonding between vulcanized rubber substrates, as well as between vulcanized rubber and metals. In this process, polymer materials are placed in non-polymerizable gas plasma. Bombardment by plasma species changes the surface structure and realizes surface modification.

The energy of plasma particles generally ranges from several to tens of electron volts (eV). For typical chemical bonds in vulcanized rubber, the bond energy is 4.3 eV for C-H, 4.4 eV for C-F and 3.4 eV for C-C. Most plasma particles carry higher energy than these chemical bonds, which is sufficient to break and reconstruct chemical bonds on the rubber surface.

Oxygen plasma generated by ionizing air is commonly adopted for rubber surface cleaning. A large number of active species in oxygen plasma, including O⁺, O⁻, O₂⁺, O₂⁻, oxygen atoms, ozone, ozone ions, metastable O₂ and free electrons, interact with the rubber surface via physical and chemical effects. A great many polar groups are formed on the surface of fluororubber, converting C-H and C-F bonds into carbonyl, hydroxyl, carboxyl and other functional groups. This increases surface energy, modifies surface chemical properties, and ultimately enhances surface adhesion and bonding performance.