Polyolefins are everywhere in manufactured parts. Polypropylene, polyethylene, and TPO are cheap, tough, light, and chemically resistant, which is why they show up in automotive trim and bumpers, packaging film, appliance housings, and countless molded components. They share one inconvenient property: paints, inks, and adhesives do not want to stick to them.
This article covers why that happens and how a chlorinated polyolefin adhesion promoter fixes it chemically, often without the surface pre-treatment step the problem usually demands.
Why coatings will not bond to polyolefins
The issue is surface energy. Polypropylene and polyethylene are non-polar, semi-crystalline plastics with low surface energy. A liquid coating or adhesive needs to wet out and form bonds with the surface it sits on, and a low-surface-energy substrate gives it very little to work with. The coating beads up rather than wetting, and even where it does spread, there are no chemical anchor points for it to grab. The film may look fine when it dries and then peel, flake, or fail a tape or crosshatch test in service.
The standard industrial answer is to change the surface physically. Flame treatment and corona treatment both oxidize the top layer of the plastic, raising its surface energy and adding polar groups that a coating can bond to. These methods work and are widely used. They also add equipment, a process step, and a time window, because a treated surface relaxes over time and loses some of the effect if it sits too long before coating.
What a chlorinated polyolefin promoter does
A chlorinated polyolefin adhesion promoter solves the same problem with chemistry instead of energy. The workhorse versions are maleic-anhydride-grafted chlorinated polypropylene, and they are designed to be compatible with both sides of the bond.
The chlorinated polyolefin backbone is similar enough to the untreated polyolefin surface that it anchors into it and stays put. Grafted onto that backbone are polar groups from the maleic anhydride, and those groups face outward to give the next layer something to bond to. In effect the promoter becomes a tie-coat: one side keyed into the plastic, the other side presenting a polar surface that a paint, ink, or adhesive can grip.
The practical result is the same across the line. Bond strength comes up, tape-test and peel failures come down, and in many applications the flame or corona step can be reduced or removed.
Two ways to apply it
There are two common ways a promoter goes into a process. The first is as a thin primer or tie-coat, sprayed or coated onto the polyolefin part before the topcoat or ink. This is the typical route for painting automotive PP and TPO parts. The second is to blend the promoter directly into the coating or ink, so the adhesion chemistry travels with the film and keys into the substrate as the film forms. This is common in flexible-packaging inks printed on polypropylene and polyethylene film.
Matching the carrier to the system
A promoter only helps if it is compatible with the system it goes into, which is why these products come in several forms:
- Solid resin that the formulator dissolves into a carrier of their choice
- Solvent-borne solutions ready to use in solvent coatings, inks, and primers
- Waterborne dispersions for waterborne and low-VOC systems
- UV-curable grades carried in a reactive monomer for energy-cure inks and coatings, with no solvent
The form is the first decision, set by your coating or ink chemistry. After that the grade is narrowed to the substrate and the bond you need.
A quick diagnostic
If a part is made of polypropylene, polyethylene, or TPO, and you are seeing paint, ink, or adhesive peel, tape-test or crosshatch failures, or you have been told the part needs flame or corona treatment, that is a chlorinated polyolefin adhesion-promoter application. Where a line is moving to waterborne or to UV cure, the promoter form follows the same move.
Where to start
P.A.T. Products distributes Tramaco Trapylen chlorinated-polyolefin adhesion promoters across North America in solid, solvent-borne, waterborne, and UV-curable forms. Tell us the substrate, the coating or ink it has to hold, and the carrier you want, and we will recommend a grade and send a sample so you can run it on the actual part.
See CPO adhesion promoters