Plating on Plastic Using Gas
GM-driven process increases materials that can be chrome plated.
Plating on plastic isn’t new in the finishing world, but a Fridley, Minn., company says it is about to improve on the process by using reactive gases to activate the surface and get adhesion strength four times better than conventional methods.
Incertec—which until recently was known as Spec Plating—says it will be the first to use a proprietary process developed by a Michigan firm and MacDermid Enthone that was initiated by General Motors to improve the chrome on trim parts.
Surface Activation was developed by Surface Activation Technologies (SAT; Troy, Mich.), which worked with MacDermid, GM and several other vendors perfecting the technology. MacDermid markets the process chemistries under the name of Macuplex.
Robert Bess, director of sales and marketing for Incertec, says his company has been testing the Surface Activation process with several of its customers and is pleased with the results. Some of its clients include Honeywell, Goodrich, Medtronic, Boston Scientific, General Dynamics and Lockheed Martin.
“The gases cause the atomic structure of the polymer to change to a depth of 25 microns, creating a polar and hydrophilic surface,” Bess says. “The plastic then becomes plateable without chromic or sulfuric etching, allowing for a permanent and stable treatment to a wider variety of polymers than ever before.”
Robert Hamilton, MacDermid’s technology manager for molded interconnect devices, was the lead researcher on the project and presented his first report on the process at the 2009 NASF Sur/Fin conference, along with GM’s Charles Buehler and Stuart Brown. John Wallace, Tom Schmoyer and Don Corning joined from SAT.
Hamilton says that normal plating on plastic surfaces for automotive parts is almost always done on acrylonitrile-butadiene-styrene (ABS) or polycarbonate- acrylonitrile-butadiene-styrene (PC/ABS) substrates. The mixtures of chromic acid and sulfuric acid are used to selectively attack the butadiene and etch the plastic blend.
But the new process increases the types of materials that can be chrome plated, and eliminates the need for chromic acid and the environmental and safety problems that come with it.
“Up to now, there’s been an insurmountable technological barrier related to the etching of the plastic because, despite considerable research, no viable alternative to chromic acid etching of ABS and PC/ABS has been found,” Hamilton says. “Chromic acid etching isn’t much good for most other plastics. It will attack many of them, but produces adhesion-promoting topography only on a few.”
Working with the researchers at SAT, Hamilton and others knew going in that polyolefins such as polyethylene can sulfonate using oleum—fuming sulfuric acid —with one result being that the polymer becomes hydrophilic and wettable.
What they didn’t know was whether the plastic could then effectively be plated without using a chromic acid etch. They also worried about the oleum, which is very hazardous to handle and extremely difficult to maintain in a chemical-steady state.
But SAT researchers had already developed a vapor-phase sulfonation process—which Incertec is using as Surface Activation—that was reacting with polymers in a controlled, consistent and repeatable manner.
“With the improved consistency, it was then possible to screen electroless plating processes against varied reaction conditions of the activation process, and develop a plating process which is broadly applicable to sulfonated polymers,” Hamilton says.
GM joined the research group in 2007 with MacDermid, SAT and Lyondell Basell to work on the surface activation process, and later they started to look at acryolonitrile-styrene-acetate (ASA) as well.
Hamilton says the conditioner is a mild alkaline cleaner, run at 140˚F with a typical immersion times of 1-5 minutes. In addition to cleaning, he says the alkaline ensures that the parts are uniformly wetted and are receptive to the catalyst.
The activator is an alkaline, ionic palladium, rather than an acidic, colloidal tin-palladium. Typical conditions are 110˚F for four minutes of immersion time. The accelerator is acidic and strongly reducing in nature. The usual conditions are 120˚F for three minutes.
Hamilton says the metallization process can be either electroless copper or electroless nickel. The chromic acid etched ABS is much rougher than the PC/ABS because the PC does not etch much at all. Adhesions are much better on ABS as a result, typically 6-8 lb/inch vs. 2-3 lb/inch on PC/ABS.
“There is some roughening on the activated ABS because the treatment will attack butadiene, but it is much smoother than the chromic etch,” he says. “There is almost no roughening on PC/ABS and none at all on TPO, which has some existing surface roughness due to the presence of fillers.”
The researchers plate tested pieces with full decorative chrome plating per the GM 14668 spec. (See chart.) The relevant test results are shown in the table above, along with some results for PC/ABS for comparison purposes and some engineering resins they also tested.
“The results show that both TPO and ASA can be plated successfully to GM exterior specifications, using the new process,” Hamilton says. “In particular, both resins show much higher adhesion values than what can be achieved with the conventional process on ABS-based resins, which offers a reliability improvement.”
To contact Incertec, please call 763-717-2810 or visit incertec.com; to contact MacDermid, please call 720-479-3060 or visit macdermid.com. To contact Surface Activation Technologies, please call 248-273-0037, or visit surfaceactivation.net.
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