Surface treatment of plastic is not an uncommon procedure in the industrial world. Innumerable plastics are treated each and every day for improved wettability leading to proper adhesion of paints, inks, coats etc.
Although, the problem of attaining good quality adhesion of coatings, glues, sealants and paints as compared to other conventionally difficult to bond materials e.g. propylene has confronted engineers since the 1950’s where it all began.
Adhesion problem is common in materials that possess low surface energies. Examples can include High-Density Polyethylene, Polypropylene, EPDM, and Polyethylene etc. With the surface energy ranging anywhere from twenty nine to thirty six Dyne/cm or mN/m (milinewton per meter). Obtaining a level of adhesion on any materials with low surface energy i.e. Polypropylene or Polyethylene etc. can sometimes become extremely difficult because of their slickness. Thus, this adhesion problem leads to the printed ink rubbing off, coatings or paint not sticking on the surface along with failed gluing and weak sealing.
Industries dealing with medical devices or automotives are generally subject to the adhesion problem as many others that coat plastics, print on them or apply bonding techniques. And due to the increased use of Polyolefin, surface treatment of plastic is becoming more and more difficult. Normally, polyolefin has a non-polar surface or low surface energy which is unresponsive to printing, bonding, coating or painting etc. unless you’re willing to apply treatments that are damaging to the environment and require a lot of effort.
RotoTEC - plastic treater
Our RotoTEC product for example, offers a cost-effective method, of treating plastic materials. It is an example of a machine which treats plastic materials such as PP, PE, PEEK, ABS, PC, PS and others. Click here to see the machine.
Surface tension and the comparative surface energy of a material determine the potency of a bond existing between a 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, producers are often challenged with the task of increasing the surface’s energy level making it greater than the surface tension of the coating, printing ink, and paint or adhesive so as 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.
Improving Adhesion and Corona treatment for plastics
Ideally, a surface to be bonded should be dust free, wettable i.e. high surface energy, clean, smooth, dry and non-porous. There are numerous methods available for increasing the polarity and surface energy of plastics. These include treatments involving high temperature, wet chemicals, etc. which are not so friendly for the environment. Other methods consist of treating the plastic surface to a high voltage corona and plasma surface activation which is friendlier towards the environment than wet chemicals and high flames.