Theory of surface treatment of plastics

Significance:

It is required for improved wet ability for proper adhesion of paints, inks, coats, glues, sealants etc. poor adhesion result in to rubbing off, coatings or paint not sticking on the surface, weak sealing and failed gluing.

Some Plastic material that requires surface treatment:

·         Polyethylene (PE)

·         Plexiglas (PMMA)

·         Polypropylene (PP)

·         Teflon (PTFE)

·         Polystyrene (PS)

·         Polycarbonate (PC)

·         EPDM-rubber

·         Polyurethane (PUR)

·         ABS etc.

Technical reasons for poor adhesion:

The materials that possess low surface energies (between 29-36 dyne/cm) or non- polar surface, exhibit adhesion problem. (e.g. HDPE, PP, EDPM, PE, Polyolefins etc.). Due to their slickness they are unresponsive to printing, bonding, coating, or painting. Surface tension and the comparative surface energy of a material determine the potency of a bond existing between the coating and the material itself. If a solid possesses high levels of surface energy as compared to the surface tension of a liquid, there will be increased molecular attraction drawing closer together the adhesive and the ink or paint etc resulting in superior bond strength. Similarly, if the solid’s surface tension is lower than that of the liquid, then the attractive forces will weaken thus, resulting in repelling of the coat. During the surface treatment of plastic, the surface’s energy level is made greater than the surface tension of the coating, printing ink, and paint or adhesive, to increase the chemical attraction. This results in proper adhesion due to improved wettability of the surface. Generally, a substrate’s surface energy should range at least 5 mN/m (dyn/cm) above the surface tension of the adhesive, paint, coating or ink to be used on the surface.

A surface to be bonded should be dust free, clean, smooth, dry, non-porous and wettable i.e. having high surface energy and increased polarity.

Wettability of surface depends on the surface energy (surface tension) of the surface. It is measured in mN/m. it measured in terms of contact angle.

Contact angle:

 

It is the angle between the tangent line at the contact point and the horizontal line of the solid surface. When a liquid droplet is set on a smooth solid horizontal surface, it may spread out over substrate and the contact angle will approach zero if complete wetting takes place. If the wetting is partial the angle will be between 0 – 180 and vice versa. If the surface energy of the solid substrate is higher than the surface tension of the liquid, the wettability is better; it is verified by the smaller contact angle. In order for a proper bond to exist between a liquid and a substrate surface, the substrate’s surface energy should be at least 2-10 mN/m higher than that of the liquid’s tension.