The ForaSolv process for Cr(VI)-free surface modification of plastics for chemical metal plating
We meet plated plastics all days, e.g. at inner and outer parts of vehicles. With their aesthetics they contribute to the acceptance of plastics, which is the pre-condition for light-weight solutions in automotic engineering, aircrafts and spacecrafts. The metallic layer, besides the design, contributes to protection against aggressive media (chemicals, irradiation) and mechanical strain. Moreover, plated plastics are applied in electronics. With changing to electro mobility, a demand arises for parts that are electrically insulation, but have an electromagentic shielding shell.
A cost-effective process is the chemical etching with following chemical and galvanic plating. Etching is done in chromosulphuric acid (dichromate process). This etching works only for ABS, a blend of acrylonitrile, butadien, and styrene polymer. ABS contains poly butadiene (ca. 10 vol %) domains that are dissolved during etching under formation of micro cavities. The inner surfaces of these cavities are chemical modified due to the oxidative effect of Cr(VI). Size and shape of the micro cavities is defined by the poly butadiene domains. The etching state depends on the etching conditions and the time. Pd nanoparticles are then deposited in the cavities as catalytic seeds, which then initiate the reductive deposition of a first metal layer, mostly Ni or Cu. This layer is mechanically anchored in the micro cavities, which ensures high adhesion. Further layers are deposited galvanically. A thin and hard Cr layer is often the finish.
The Cr(VI) compounds in the etching solution is registered in the REACH (Restriction of Hazardous Substances (RoHS)) since 2017. Its use depends on authorisation and is limited in time. Thus, there is a demand on an alternative process of plastic part surface modification for plating. The ForaSolv process may be such an alternative. It is explained in figure 1.
Figure 1. Processing steps of the ForaSolv process.
A) Starting point is particle made of a sacrificial core and a shell of a functional and reactive polymer.
1. The particles are applied onto the mould of an injection moulding machine, e.g. by spray-coating;
2. On contact of the melt with the particles, two processes proceed: (i) the particles are embedded by the melt, (ii) the shell is chemically bonded to the melt surface;
3. The particles are dissolved in the first bath of the galvanic processing chain under formation of chmeically functionalised mico cavities. Further processing is equivalent to the conventional process:
4. Deposition of catalytic Pd seeds;
5. Chemical deposition of a Ni layer, followed by deposition of further metal layers.
C) Example: Formed structure on a plastic surface with many cavities after step 3.
The advantages of this process are:
- High process integration is expoilted
- Cr(VI)-free and REACH conform
- Size and shape of the cavities can be controlled by the particle core
- Chemical functionality can be controlled by the particle shell
- Applicable to any plastic, not only ABS
- Site selectivity easy to realise by selective application of the modifier, instead of using two-component injection moulding
- Saving of many galvanic baths
- Cost-effective, compared to stand dichromate or alternative processes
The process is beeing validated for industrial use.