Overtreatment of TPO

Question: We supply solvent-borne automotive coatings, and recommend that our customers flame treat their TPO components to 48 to 60 dynes/cm for best adhesion and durability. We have seen adhesion failures at higher dyne levels. Would you expect that? And, would your test markers be able to identify overtreatment?

Answer: First, you are correct that any polymer can be over-treated by either flame or corona. The mechanistic details of just what happens to the material’s surface layers are undoubtedly different for the two treating methods. But what basically occurs in either case is the surface layer gets etched and oxidized to the point where it may be water-wettable, but it has been so decimated by the aggressive treatment environment that it no longer anchors well to the bulk of the polymer. The paint adheres well to the surface layer, but the entire paint/surface layer will easily lift away from the bulk of the polymer.

Over-treatment will not cause a decrease in dyne testing results unless too much pressure or abrasive force is used during the test – this would have the same effect as mentioned above, with the untreated underlayer exposed at the surface. So, as long as a light application pressure is used, as directed in the instructions, ACCU DYNE TESTTM Marker Pens can definitely identify the excessive levels of surface treatment that you have found to cause defects.

The problems that arise from over-treated surfaces suggest that it should be standard practice to establish a realistic maximum treatment level, as measured by the dyne test, as well as a lower one. Your suggested range of 48 to 60 dynes/cm sounds reasonable, but I would think that for most applications you’d do fine with a surface energy of 44 dynes/cm or so for solvent-based paints. The presence and concentration of additives and pigments could affect this minimum, especially with thick parts, where there is a large polymer bulk compared to surface area. And, for waterborne or energy cured coatings, the required dyne level would increase substantially.

Finally, as this is a rigorous application (automotive finishing), I would recommend that your customer consider an experimentally designed study of treatment parameters and measured dyne levels vs. end-use quality and durability metrics. Tightening the window of optimal treatment level could prove commercially beneficial.

Published by

Russ Smith

Russ Smith formed Diversified Enterprises - the first business to focus specifically on applications of the dyne test - in 1986, and has served as President of the company ever since. He has over 30 years of experience in the fields of surface treatment and analysis, and deals with technical inquiries from customers worldwide on a daily basis. Russ is a member of ASTM, the Society of Plastics Engineers, the American Chemical Society, the American Society for Quality, the American Association for the Advancement of Science, and TAPPI.

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