Adhesion Promotion: Now Plasma-Treat the Resin, Not the Molded Part

A new approach to plasma treating applies the treatment to resin pellets or powder in order to achieve enhanced adhesion of paints, glues, or foamed-in-place urethanes to the molded part. This new RTS technology from Lectro Engineering Co. reportedly has already been proven effective on rotomolding powders and injection molding pellets of PE, PP, and TPO.

The system is a breakthrough because olefinic materials, due to their inertness, have relied almost exclusively on post-molding secondary operations such as plasma, corona, or flame treatments to obtain adhesion promotion by oxidizing the plastic surface. Some polyolefin applications have required expensive spray-on chlorinated adhesion promoters. The new resin treatment eliminates these secondary costs. The patent-pending technology elevates the adhesion of polyolefins to the level of more costly engineering resins such as ABS and PC, says Lectro Engineering president Lee Williams.

 

Modified equipment

The company has modified its Lectro-Treat LT2000 machine, an open-air plasma-discharge system for molded parts. The high-voltage system can use different gas mixtures to treat the surface of pellets or powder particles. Unique to the system is a special glass-like tube through which material is conveyed, distributed evenly, and treated.

Lectro Engineering’s first success was with rotomolded HDPE and LDPE powders. Previously, the company offered the LT Internal Plasma Treater to treat the interior of rotomolded parts to permit good foam adhesion. Such a treatment was labor intensive. The RTS treatment raised the surface energy (an indicator of adhesion potential) of HDPE powder from 30 dynes/cm2 (untreated) to 48, and LLDPE from 28-30 dynes to 50, which is high enough to promote good adhesion of interior foam.

Thousands of pounds of powder have been treated and numerous test parts have been rotomolded by processors, according to Williams. One rotomolder found that parts could contain as little as 30% treated resin plus 70% untreated resin to produce parts with good foam adhesion.

 

Paint adhesion improved

Lectro Engineering is also working with injection molded TPO and TPU resins for painted auto interior parts. Advanced testing shows that treated pellets achieve excellent paint adhesion with no degradation of material properties. He says that in most cases it is necessary to treat 100% of the pellets used in molding. The technology has also been adapted as a dispersion aid for additives and pigments. RTS-treated carbon black and color pigments reportedly provide more uniform dispersion and eliminate the need for dispersing agents.

Williams says the RTS treatment system currently has a capacity of 20,000 lb/day and adds only 1¢ to 2¢/lb to the material cost. What’s more, the resin treatment reportedly can last more than four months. A similar resin treatment system that was developed in Europe several years ago was prohibitively costly and lasted just a few days, according to Williams.
So far, no RTS applications are fully commercial. The company has built one machine for commercial use and is pursuing licensing agreements with material suppliers and molders.