Dow Electronic Materials

Transitioning to Chromium-Free Etch Technology for Plating on Plastics

January 24, 2017

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plating_on_plastics

As described in an earlier blog, Dow has pursued a plating on plastics (POP) process free from hexavalent chromium as required by the pending REACH regulation, which refers specifically to the elimination of the use of chromium trioxide (CrO3) by the September 2017 sunset date.

Whilst several Authorization Applications have been submitted for the continued use of chromium trioxide for these POP applications and these requests have further been reviewed by the Socio-Economic Assessment Committee (SEAC) and the Risk Assessment Committee (RAC), with proposals for Authorization for four and seven years for use of mixtures (etching) and decorative plating respectively, such extensions do not impact Dow’s plans for the development and implementation of chromium free pretreatment of plastics for POP. Testing, qualifying and implementing such chromium free process technology across the industry will take time and therefore such extensions can provide the testing time required, as well as for any investment budgeting associated with implementing any required changes.

We find many acronyms as we navigate through the REACH process, but REACH is here to stay, and the model is being copied around the world. We’ll all be used to these acronyms in the years to come, as it’s the process of record for the EU and in a growing list of countries seeking to piggyback on the process.

The novel chrome free etch (CFE) has moved forward significantly since my previous blog posting. The POP process incorporating our novel CFE is in mass production, demonstrating that eliminating hexavalent chromium etchant is not only feasible, but now practiced.

There are so many variables within the POP industry that switching from the traditional chromium based etchant to a chrome free process will inevitably be a stepwise process. ABS, which accounts for the majority of the plated area worldwide, has a process of record established, providing consistent and repeatable results, assuming that the incoming material is similarly consistent and repeatable, as required by the existing chromium etch process.

The CFE oxidizes the ABS material, resulting in a roughened surface due to the selective oxidation of the butadiene areas of the ABS. The resulting surface looks almost identical to the surface achieved from a traditional chrome etch, which, unsurprisingly, delivers a similarly robust plated part.

SEM Structure of Etched ABS

CrCFE

When the MnIII oxidizes the plastic, it is in turn reduced to MnII, which, although very stable, must be regenerated to continue to provide “etching” of the plastic. This regeneration is a very simple process achieved using an in situ electrolytic cell, where the cell efficiency in production is multiple times more efficient than a traditional chrome regeneration cell, thus consuming less power.

MnIII_cycle

Further, the regeneration cell is controlled by a probe measuring the real-time MnIII concentration, so the optimum concentration of active oxidant is maintained automatically.

The mixed acid etchant is further controlled by the ratio of acid, as well as the water content. Using pre-etch and recovery baths, the proportions of acid and water drag-in are controlled, and water removal can allow for acid recovery and reuse.

Although hexavalent chromium is harmful to humans and the environment, it has the beneficial effect of inhibiting metallization on plating racks. In removing the hex-Cr, a novel solution to avoid rack plating is required and has been implemented in two forms; firstly, as a dip process, where the inhibitor is coated onto the racks prior to processing, thus preventing metallization of the rack. A longer-term solution is also shown by incorporating the inhibitor into the plastisol rack coating, thus avoiding the dip process. However, in an industry in transition, the majority of the racks are currently used for the chrome process and are expensive items; therefore, it’s envisaged that both options will coexist for some time.

Having established a baseline process for ABS, the logical progression is to establish a process for ABS-PC, used extensively in automotive applications. Applying a sweller ahead of the CFE step allows for effective etching of the ABS-PC blend, delivering consistent and repeatable adhesion as measured by peel strength and thermal cycling behavior, as required by automotive OEMs.

Learning and fine-tuning the CFE process for each condition, as well as ensuring economically viable transitions from the well-entrenched, 50-year-old chrome process, will take time, but substantial progress is evident and ongoing.

The growth in demand for selective plating, building complexity and value into the “decorative” POP product, opens new doors for innovative solutions. Future POP blogging will share the technology advances in this and other dynamic areas within the industry.