### Non-PFOS, Permanent Mist Suppressa nts for Hard Chrome Plating, Decorative Chrome Plating and Chromic Etch Applications

Neil Patton, Atotech Deutschl and GmbH, Berlin, Germany
Gene Barlowe, Atotech USA Inc., Rock Hill, SC

Mist Suppressants are an important tool for reducing exposure of workers to Cr (VI) during plating and etch ing operations. A common raw material used for efficient mist suppressants was, and still is, PFOS. PFOS is subject to legislation across the world prohibiting its use for many applications. We will present developments and experiences with non-PFOS, permanent and non-permanent mist suppressants.

For more information contact:

Gene Barlowe

Atotech USA Inc.

1750 Overview Drive Rock Hill, South Carolina, 29730-2000, USA Phone – (803) 817 3500

E-mail – gene.barlowe@atotech.comThe generation of a large amount of H 2 and O 2 gas bubbles due to electrolysis is a

characteristic of even the most efficien t chrome plating process. If no precautions are taken, these bubbles can cause a mist containing chromic acid to form above the chrome plating solution.

Fig. 1 – Typical Hydrolysis during Chrome PlatingAnodeCathode Chromic Acid Mist

Gas bubbles due to hydrolysis

The mist is due to the bubbles formed during electrolysis rising up through the solution and ‘bursting’ on the surface. Th is ‘bursting’ causes high speed droplets to be ejected from the surface of the solution. Collectively these form a mist.

Fig. 2 – Formation of Mist Droplets (Jets) from Electrolysis Gas Bubbles

The resulting speed of a droplet (jet) can be up to 10 m/s.As chromic acid is classified as toxic, there are regulations in all countries controlling the exposure of members of staf f to chromic acid and especially from this mist above the plati ng bath. There are several proven preventive measures that can be implemented to reduce exposur e. One of these measures is to use a suitable mist/fume suppressant.

The picture below shows the mist generated from a high efficiency hard chrome plating solution, in a 1 liter beaker , plating at 50 A/dm2 (3.2 A/in2).

Picture 1 –Mist Droplets from a Chrome Plating Solution without a Mist Suppressant Mist Droplets

The size of the mist droplets and the heig ht they have reached are easy to see in picture 1.

The test was performed in a fumehood a nd the surroundings covered in paper to prevent contamination. It’s easy to see th at the beaker, cathode rod and protecting paper are heavily contaminated with chro mic acid due to the mist that has been generated. This is after just 5 minutes of plating.Reduction of the mist can be made in several ways, but a popular method is to employ a mist suppressant.

Picture 2 –Hard Chrome Plating Solution without a Mist Suppressant

Picture 3–With a Foaming Mist Suppressant Picture 4 –With a Low Foaming Mist Suppressant

When either a foaming or a low-foam ing mist suppressant are used then contamination to the surroundings are si gnificantly reduce d. The beakers and surroundings tested with each version of mist suppressant showed no evidence of

contamination even after several hours of plating.

Most common mist suppressants are surf actant based and work by reducing the

surface tension of the solution. This then has a two-fold effect on the generation

of mist. A reduced surface tension reduces the size of the gas bubbles generated during electrolysis. These smaller bubbles travel slower through the solution and have less energy when they arrive at the solution’s surface. At the same time, the lower surface tension reduces the energy with which the resulting droplets are ejected into the air. Together, both of these effects can reduce the emission of droplets, and therefore mist generation, by over 98%. When a mist suppressant is properly applied, this ha s been found to be suffici ent to reduce exposure to employees around pla ting installations.Due to the aggressive chemical and el ectrochemical environment of Chrome plating solutions, most mist suppressants ar e made from highly stable surfactants. PFOS (Perfluorooctanesulphonate) was, and still is, commonly us ed as the basic surfactant in popular mist suppressant products.

Fig. 3 – PFOS Molecular Structure

PFOS is highly resistant to chemical attack and so is ideal for use in harsh environments like hot chromic acid plati ng baths. Unfortunately the extremely

robust nature of PFOS means that it is not easily biodegraded or waste treated and

so is released into the environment wh ere it builds up. PFOS has been classified as a persistent, bioaccumulative an d toxic (PBT) for some years now.

The negative health effects of PFOS on an imals and humans have been studied and were found to be unacceptable i

, ii . As a result, there ar e several legislations worldwide restricting the marketing, produc tion and sales of s ubstances, including PFOS. In the US, the EPA iii included PFOS in a new tabl e (3) of substances now subject to Significant New Use Rule s (40 CFR 721.9852). This rule became

effective on the November 11 th 2007. There is an exemption for one form of PFOS, with CAS number 56 773-42-3, when it is used for metal finishing and electroplating applic ations including hard and decorative chromium plating, chromic acid anodizing or reverse etching, et ching plastics prior to metallization, Nickel plating, etc. There is a more restrictive PFOS directive from the EU iv that

came into effect on June 27 th 2008. There is also a wo rldwide directive from the

Stockholm Convention of Persistent Or ganic Pollutants, effective August 26 th

2010, which will affect over 150 countries including Japan, implementing an effective ban of sales and production of all PFOS containing products starting from April 1 st 2010. This may need work. Not sure I understood it.

One of the main aims of these directives is to ultimately ensure the phase-out of the utilization of PFOS. There are currently exemptions in most of these directives for hard chrome plating v . However, there are several communities, organizations, and authorities that are campaigning for a complete ban on PFOS

use.Due to the increased awareness of the HES risks, the use of PFOS has declined by over 95% in the last decade, leaving only a few specialized applications. As the health and safety implications of the use of fume suppressants for Cr (VI) applications are serious, a suitable replacement for PFOS must be rigorously tested to ensure its suitability. Investiga tions into a large variety of alternative, non-PFOS candidate compounds have been going on for several years.

Typical criteria for a goo d mist suppressant are:

 Reduce surface tension to ≤ 30 Dynes/cm

 At 30 Dynes/cm – have no discernible mist

 No effect on plating qua lity or efficiency

 No effect on bath stability

 Be relatively stable - minimal dosing

 Any break down products should not influence the bath quality or functionality

 No PFOS should be generated or created

Not all candidates are successful even if th ey seem to initially meet most of the

criteria listed above. The unsuccessful ca ndidate below gave insufficient mist suppression despite having a supposedly good surface tension of 29 Dynes/cm.

Picture 5 –Unsuccessful non-PFOS Mist SuppressantPermanent and non-permanent mist suppre ssant candidate compounds were tested and it was found that the non-permanent mist suppressant compounds were basically unsuitable fo r use in the field.

Most non-permanent, non-PFOS mist suppre ssant candidates that were tested

failed the functional test as on the previous page, but even those that passed most of the criteria, failed due to bath contamination.

Typically, the non-permanent, non-PFOS co mpounds are inherently instable in standard hard chrome plating solutions and thus required constant dosing, even during idle times. When the bath was under plating conditions, even more dosing of the suppressant chemistry was generall y needed. This can cause control issues and needs constant monitoring of the dosing equipment to ensure that the

maximum surface tension limit is never exceeded.

Apart from this, the biggest issue seen with this type of suppressant chemistry was the tendency for the excessive break down of the chemistry to form an ‘oil’ or ‘scum’ on the plating bath surface that eventually interferes with the plating quality.

The candidate tested below shows a more ex treme example of this ‘oil’ or ‘scum’ formation. It is easy to imagine that pa rts plated in a solu tion containing this compound, with its surface ‘scum’ layer, would cause plating quality issues.

Picture 6 –Unsuccessful non-permanent , non-PFOS Mist SuppressantAfter extensive testing of suitable permanent, non-PFOS fume suppressant candidates, a family of products was fi nally developed, thoroughly field tested and brought to the market.

There are two versions of permanent, non- PFOS mist suppressant that have been developed and are available on the market today. One is the foaming type and the other, a low-foaming type. Both ar e based on similar non-PFOS technology and have years of proven usage in the industry. These products can fully comply with PFOS legislation.

Picture 7 – Foaming non-PFOS Mist Suppressant in a Decorative Chrome Plating Tank

Picture 8 – Low-Foaming non-PFOS Mist Suppressant in a Hard Chrome Plating Tank

The low-foaming version of fume suppressa nts is, in general, more popular due to the lack of a thick foam blanket covering the entire surface of the plating solution. This helps to minimize the issue of H 2 and O 2 gas getting trapped in the foam blanket, which can lead to hydrogen explos ions. It also means there should be less chance of foam being ‘sucked’ into the exhaust system on tanks with minimal

free-board.Comparison tests between current, leadi ng PFOS products have been made, and the newly developed non-PFOS produc ts performed just as well.

Picture 9 – A Hard Chrome Plating Solution with a

PFOS , Permanent, Low-Foaming Mist Suppressant

Picture 10 – A Hard Chrome Plating Solution with a

Non-PFOS , Permanent, Low-Foaming Mist SuppressantThe preferred method for ensuring the best operation of a fume suppressant in a plating bath is to monitor and control the surface tension.

The permanent, non-PFOS mist suppressants can easily be monitored and controlled by either stalagmometer or tensiometer surface tension measurement.

0 10 20 30 40 50 60 70 80

0 0.5 1 1.5 2 2.5 3 3.5 4

Mist Suppressant Concentration [ml/l]Surface Tension [mN/m] Tensiometer

Stalagmometer

Fig. 4 – Surface Tension vs non-PFOS Mist Suppressant Concentration

The surface tension of a ch rome plating bath can be reduced to 20 Dynes/cm using the non-PFOS products, but are co mmonly kept at an average of 30 Dynes/cm. At this level, consumption of the mist suppressant is kept to a minimum and the Cr (VI) emissions are kept under control.

When changing from a PFOS based product to a non-PFOS product there are 2 options; a completely new make-up or a slide conversion. Both methods have been employed and studied. The most common method is to perform a slide conversion of an existing solution. This is generally an easy operation. The surface tension of the bath is analyzed and then the required amount of non-PFOS product is added to attain the desire d surface tension. Th e non-PFOS product is subsequently dosed to maintain the surface tension within the desired range.When a slide conversion is made, under normal circumstances, the PFOS

concentration in the solu tion should reduce over time. One example of a slide conve rsion shows the potential decline in PFOS over time.

This was a decorative chrome bath thatwas converted to a foaming type nonPFOS mist suppressant.

0 10 20 30 40 50 60 70 80 90 100

012345678

MonthsNormalised PFOS Concentration

Fig. 5 – PFOS Concentration over Time after a Slide Conversion to a non-PFOS Product

It took 7 months to reduce the PFOS conten t to only 1% of the original level at this particular commercial plater. Diffe rent installations may yield different

results due a number of factors.

In conclusion, there are now suitable, successful a nd well proven permanent nonPFOS mist suppressants available in the market, both foaming and low-foaming,

for hard chrome and decorative chrome plating as well as chromic acid etching.

References

i “Perfluoroctane Sulfonat e (PFOS)”, Dossier prepared in support for a nomination of PFOS to the UN-ECE,LRTAP Protocol and the Stockholm Convention, dossier prepared by the Swedish Chemicals Inspectorate (KemI)and the Sw edish EPA, Sweden, August 2004

ii “Environmental Risk Evaluation Report: Perfluorooctanesulphonate (PFOS)”, D. Brooke, A. Footitt, T.A. Nwaogu, Building Re search Establishment Ltd., Risk and Policy Analysts Ltd.

iii US EPA directive under section 5 (a) (2) of the Toxic Substances Control Act amending the Significant New Use Rule at 40 CFR 721.9582 adding table 3, exclusion (4)

iv EU directive 2006/122/EC amending for the 30 th time the council directive 76/769/EEC

v Annex 3(c) of EU directive 2006/122/EC