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More environmentally friendly alternatives to PFOS-compounds and PFOA

5 Technical assessment of alternatives

• 5.1 General information on surfactants for coatings
• 5.2 Conventional fluorinated surfactants
• 5.3 The "new generation" of fluorinated surfactants
• 5.4 Non-fluorinated surfactants
  • 5.4.1 Silicone polyethers
  • 5.4.2 Sulfosuccinate
  • 5.4.3 Aliphatic alcohols
  • 5.4.4 Propylated aromatics
• 5.5 Discussion of alternatives
  • 5.5.1 Paint and coatings
  • 5.5.2 Floor polish, waxes and cleaning agents
  • 5.5.3 Impregnation of textiles, leather, and carpets.
  • 5.5.4 Fire-fighting foam
  • 5.5.5 General comment
• 5.6 Economical assessment of alternatives

This assessment of alternatives is based on information from technical data sheets describing the usage of the compounds. Furthermore, the comments and discussion are mostly based on experience with surface coatings.

The alternatives identified in this project are described shortly in Table 5.1, and in more details in Appendix F.

Table 5.1: Alternatives to PFOS-compounds identified in this project.

5.1 General information on surfactants for coatings

A coating needs to fulfil a number of technical demands. The demands depend both on the method of application, the substrate and the demands of the coated surface. To achieve certain technical properties a number of additives are used especially within water-borne coatings. Water-borne systems including both emulsions (binder) and dispersions (e.g. pigment) normally contain several surface-active agents like emulsifiers, wetting agents, defoamers etc.

It should also be noted that water-borne systems generally are more demanding with regard to wetting and adhesion than solvent-based systems basically due to the fact that water has a very high surface tension. Normally the binder system will, at least to some extent, determine the surface tension of the coating as water-borne binder systems include emulsifiers that are surface active.

Non-adsorbing substrates like metal and plastics are a challenge for water-borne systems, as the surface tension needs to fit to the surface tension of the substrate to get proper wetting as well as adhesion. For this reason the surface tension of the coating needs to be in the same order or lower than that of the substrate. This can be achieved by using surfactants as they lower the surface tension of the coating. But it should be noted that different substrates demand different surface tensions of the coating system, and that substrates, which do not adsorb the coating, are more demanding compared to substrates that adsorb the coating.

To get a proper wetting and adhesion of a new coating layer at already coated surfaces, the surface tension of the new coating needs to be in the same order of the already coated layer or even lower. Therefore, the surfactants present in a coating may influence the possibility to recoat the already coated surface, especially if the coating has a very low surface tension, as it often is the case when fluorinated surfactants are used.

The presence of surfactants in water-borne coating may cause problems with regard to keeping the raw materials compatible. If the surfactants are not compatible with each other, the system may become unstable due to competitive adsorption. Other surfactant-induced problems may occur during levelling and drying of the coating. Examples are craters and orange peel. Foaming is a well-known problem for water-borne coating systems. In some cases extremely stable foam can be produced especially during mixing and applications like spraying. Too stable foam in a coating inevitably leads to surface defects in the coating, as for instance craters. Foaming is counteracted in coatings by adding defoamers, which are another category of surface-active agents. Generally a low content of surfactant is to be preferred, if possible, to avoid surface defects.

It is also necessary that the needed surface tension can be achieved quickly especially in industrial processes where speed is a parameter. Some industrial applications may even demand instant decrease in the surface tension. This is why the static surface tension is not an accurate prediction on flow and levelling. Dynamic processes are thus better described by the dynamic surface tension.

Often industrial processes have very high demands for the application of coatings and the surface quality. Examples are coating of airplanes, cars and corrosion protection. The same accounts for printing on plastics at high speeds or inkjet applications, where low viscosity systems are used. In inkjet applications the surface tension for application is even dependent on the type of used printing head. It is also claimed that surface tension gradients during levelling of the coating can form surface defects, due to a dynamic behaviour.

Solvent-based products do also include surface-active compounds, but the sensitivity of these coatings towards application methods and substrate is much less and therefore the number of surfactants used in a solvent-based system is normally lower.

5.2 Conventional fluorinated surfactants

In fluorinated surfactants, the hydrophobic part of the surfactant molecule contains fluorine. In perfluorinated surfactants all hydrogens in the hydrophobic segment have been replaced by fluorine. In partially fluorinated surfactants the hydrophobic part of the surfactant molecule contains both fluorine and hydrogen atoms. The hydrophobic part of the fluorinated surfactant does not only repel water but repels oil and fat as well. Fluorinated surfactants are much more surface-active than their hydrocarbon counterparts. Fluorinated surfactants exhibit surface activity in organic systems and are stable to heat, acids, bases, as well as oxidising agents. The achievable minimum surface tension is much lower for fluorinated surfactants than for non-fluorinated surfactants (Kissa 2001)

The conventional fluorinated surfactants are by producers of new alternatives (OMNOVA) claimed to have some basic problems involving foaming, too low surface tension, incompatibility with other components and lack of UV curing capability. On the other hand it should be noted that foaming also is dependent on the formulation itself, and therefore it is not necessarily due to the fluorinated surfactant. In general, non-ionic fluorinated surfactants, like hydrocarbon-type non-ionic surfactants, produce less foam and less persistent foam than ionic fluorinated surfactants. It should be noted that even though foaming should be minimised in water-borne coatings to avoid surface defects, foaming might be important in other application, e.g. fire-fighting foams.

The level of the surface tension needed in a coating is decided both by the application procedure and the type of substrate that should be wetted. This means that a broad product range of surfactants need to be available to cover the application needs ranging from very low and to moderate surface tensions. The fluorinated surfactants are mainly used for coatings where there is a demand for extremely low surface tension.

5.3 The "new generation" of fluorinated surfactants

The "new generation" of fluorinated surfactants is basically built up of a polymer and small fluorine molecules, which probably are very quick with regard to orientation towards a surface interface.

The Novec products are polymeric anionic fluorinated surfactants, which are based on perfluorobutane compounds (perfluorobutane sulfonates). These compounds are claimed to have a low dynamic surface tension or rather a rapid surface migration, which is important in high speed coating processes or in low viscosity systems. Generally, this surfactant has a lower surface tension than hydrocarbon silicone surfactants. It can also be used in a less amount than the hydrocarbon surfactants. The compounds are said to influence the second layer coating adhesion less than silicon or conventional fluorinated surfactants.

Besides the paint and coatings industry the Novec products should also be useful as surfactants in electronic coating, in industrial commercial cleaning, and cleaners for solder flux residue. They have also been identified as alternatives in connection with impregnation of textiles, leather and carpets.

The Polyfox products are polyethers with a short perfluoroalkyl side chain (C2 or C₃). It seems that these surfactants have a moderate surface tension, which is not quite as low as the conventional fluorinated surfactants. The new surfactants are claimed to have a broad processing window, where less interference with other compounds is experienced. Coating quality is improved as reduced foaming is achieved. The last item is an important factor in producing and processing water-borne coatings.

Polyfox products have been tested in a high solid 2-component acrylate urethane for automotive topcoats with success. This surfactant has also potential for use in polyester/melamine coatings where it can achieve higher solids and reduce VOC use. It can also be used in pigment dispersions, inks lacquers etc. It should be noted that there also are Polyfox surfactants for a number of applications with non-aqueous systems including P_owder coatings as well as UV curing systems. As the end group chemistry can be variated, tailor made substances with different affinity can be achieved for different applications. Three examples from the technical data sheets are given below:

• Fluorinated polyether salt in solution for usage in aqueous coatings and floor polish
• Hydroxyterminated fluorinated polyether for usage in aqueous and non-aqueous systems
• Acrylate terminated fluorinated polyether for usage in UV-cure systems

Polyfox products are available with the following end group functionality: hydroxyl, carboxylic acid, sulfate, and acrylate.

DuPont™ Forafac® products with C₆ fluorinated compounds based on perfluorohexylethyl sulfonamide betaines are used for fire extinguishing formulations.

There has not been identified any non-fluorinated alternatives.

5.4 Non-fluorinated surfactants

5.4.1 Silicone polyethers

A low viscous non-ionic special modified silicone polyether from Worlée is suggested for improvement of surface wetting of aqueous systems also on difficult substrates like polyethylene and polypropylene or contaminated substrates. It has a low surface tension and is claimed to be highly efficient improving wetting, spreading and levelling of water-borne coatings and eliminating surface defects without foam stabilising. It is further claimed that the compound normally has no negative effect on recoating. Another alternative within this group is a surfactant that combines silicone polyether and dioctylsulfosuccinate. This surfactant can be used to improve wetting properties of aqueous coatings for different substrates where the penetration into absorbing surfaces also is improved.

5.4.2 Sulfosuccinate

A sodium salt of di(2-ethylhexyl) sulfosuccinate manufactured by Cognis can be used as a wetting agent in aqueous coating systems and is particularly suitable for difficult-to-wet substrates like plastics, metal, cellulose film, silicone treated papers and glass. This surfactant may also be used as an emulsifier for emulsion polymerization. Another area where it can be used as an alternative to fluorinated surfactants is in optimising the colour acceptance of aqueous pigment concentrates in different coatings. The product has a medium foam formation.

A sulfosuccinate from Münzing Chemie, which can be used as wetting agent for aqueous systems is claimed to have good wetting properties, no increase in foam and good recoatability. The surface tension is claimed to be moderate. Application areas are decorative paint, wood and furniture coatings, automotive and repair coating, industrial coatings, printing inks and overprint varnishes.

In formulations of detergents and cleaners, anionic surfactants like sodium dioctylsulfosuccinate dissolved in a glycol and water are used as wetting agents. Sulfonates can also be used. Basically, some of the same chemistry as in coatings can be used.

5.4.3 Aliphatic alcohols

Esters of fatty alcohol ethoxylates are also used in the area of industrial cleaners. BASF produces a range of aliphatic alcohols, both anionic and non-ionic surfactants. These substances are effective wetting agents. The anionic surfactants can be used as detergent and cleaning agent for the chemical industry. Non-ionic surfactants can be used as defoamers or emulsifiers.

5.4.4 Propylated aromatics

Different isopropylnaphthalenes and isopropylbiphenyls are very hydrophobic substances that are compatible with almost all raw materials as follows: Epoxy resins, polyurethane resins, resin esters, hydrocarbon resins, polystyrene, elastomers, dispersions, emulsions, styrene-acrylate-copolymers, vinyl acetate and ethylene vinyl acetate polymers, mineral oils, bitumen, etc.

The compound can be used as water repelling agents for different applications. For example rust protection systems, marine paints, resins, printing inks, coatings, electrical applications, electronically and mechanical applications. They may also act as plasticizers and film forming aids in emulsion paints and adhesives.

5.5 Discussion of alternatives

5.5.1 Paint and coatings

In the future it will still be very attractive (or even unavoidable) to use fluorinated compounds as ingredients in specific applications. It can be assumed that especially the new fluorinated surfactants with small low surface tension groups that can orientate very quickly upon application, together with a polymeric chain that can immobilise the additive after drying, will be a very effective combination. The biggest advantage is probably due to the fact that they can be tailor-made for specific applications, where the affinity towards specific molecules and the dynamic surface tension is crucial. It should be noted that for different applications different surface tension is needed. Furthermore, it is claimed that these alternative fluorinated compounds can reduce foaming, which is important especially in water-borne coating applications.

Silicone products (also combined with other molecules) may give problems when recoating is necessary, but this may be a question about a low or a moderate surface tension.

There are alternatives to fluorinated compounds based on sulfosuccinate, which are very versatile, but they are not claimed to have the same ability to reduce foam in the same manner as the new alternative fluorinated compounds.

It seems that a number of versatile solutions are available for the coatings industry. It should be noted that in general it is very difficult to substitute substances where the demands on the dry coating are very high. It is also important to remember that it may include major formulation work when a surfactant is substituted, as the use of surfactants is system dependent also with regard to compatibility towards other raw materials.

It is probably possible to use the "new generation" of fluorinated surfactants instead of the conventional ones. It will probably depend on the demand for a low surface tension, if it is possible for the sulfosuccinates to substitute the old generation of fluorinated surfactants. As a comparison, even though volatile organic compounds are unwished, they are still used for specific industrial applications where the technical demands are high.

5.5.2 Floor polish, waxes and cleaning agents

There are alternatives within the range of the "new generation" of fluorinated surfactants that can be used within the area of floor polishing. Of course it is possible to formulate products where the fluorinated surfactants may be avoided due to the use of softer waxes or acrylics in floor polishes, but it must be noted that specific harsh demands (resistance to wear) may make fluorinated surfactants unavoidable in some cases.

Even though conventional fluorinated compounds may be used extensively in traditional cleaning agents it is probably possible to use the experiences from the coatings area to determine which substitution possibilities being available.

5.5.3 Impregnation of textiles, leather, and carpets.

There are alternatives within the range of the "new generation" of fluorinated surfactants that can be used within the area of impregnation. Furthermore, silicones combined with stearamidomethyl-pyridine-chloride may do the job. It is very possible that substitution in impregnation of paper and board may be very similar to impregnation of textiles. So the first choice would be to use telomer-based alternatives.

5.5.4 Fire-fighting foam

No non-fluorinated compounds have been identified for use in fire-fighting foam. The "new generation" of fluorinated compounds (C₆ or lower compounds) are though already in use in fire-fighting foam today.

Of course, it is of course possible to go back and use previous technology such as protein-based foams. But the disadvantages with this previous technology (the cause of technology change) should then be taken into account.

5.5.5 General comment

It is not possible to conclude that we can substitute all PFOS compounds in near future. But at the same time it is clear that material development can make it possible to more or less avoid the conventional PFOS products entirely. Basically, the development in the future will probably go towards more and more tailor-made products, where different molecules with different properties are combined. This can be achieved with many compounds but the effect both technically and environmentally will depend on the molecular size and shape.

5.6 Economical assessment of alternatives

According to an article on perfluorinated surfactants (Moody et al. 2000), the cost of fluorinated surfactants is higher than that of hydrocarbon surfactants. Within a specific application, fluorinated surfactants are typically cost effective, as their relatively high price is offset by the low concentrations needed to achieve the desired effect. Nevertheless, the authors of the article assess, that due to the high prices of fluorinated surfactants, fluorosurfactant applications are limited to problems that the conventional, lower-priced surfactants cannot address. Kissa (2001) makes the same statement, and DuPont also confirms this statement – fluorinated compounds are only used, if no other alternatives are found fit for use (Personal communication DuPont, 2004a).

According to the UK risk reduction strategy on PFOS (RPA 2004) PFAS substances have a limited use in the UK paper industry because they are considered to be very costly. The same survey states that for fire-fighting foams the fluorine-free alternatives are approximately 5-10% more expensive than the fluorosurfactant based foams. According to a manufacturer of the fluorine-free alternatives the price would fall if the market size increased. A more deliberately shift towards fluorine-free fire-fighting foam alternatives will probably eliminate the difference in cost.

In general, very little information about the price of the alternatives was found, even though the producers of alternative products were asked specifically about this information. However, the sparse information received suggests that the alternatives are about the same price as the PFOS-related compounds or even cheaper. One company mentioned in particular that the price of the alternatives intentionally is kept at the same level as the PFOS-related compounds.

The information received from different suppliers specifically within the paint and varnish industry suggests that fluorinated surfactants in general are much more expensive alternatives compared to other surfactants. Therefore, fluorosurfactants are only used for special purposes in paint and varnishes, where it is necessary to gain such a low surface tension, which no other (non-fluorinated) alternatives can achieve, e.g. in product where an extremely smooth surface is necessary.

Furthermore, it must be assumed that the "new generation" of fluorinated compounds are at the same price level as the PFOS-related compounds.

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Version 1.0 June 2005, © Danish Environmental Protection Agency