In 2010 a new concept of branched short chain fluorosurfactants

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1 High-speed fluorosurfactants Excellent balance of product performance and ecotoxicological impact * Contact: Steffen Schellenberger Merck KGaA PM-AB Performance Additives steffen.schellenberger@ merckgroup.com Steffen Schellenberger Jörg Pahnke Reiner Friedrich Gerhard Jonschker A new class of more environmentally-friendly fluorosurfactants based on C2 perfluoralkyl chains has been developed. The branched short-chain structure yields a low static surface tension and exceptional dynamic properties. In waterborne coatings wetting and leveling properties are improved whereas cratering defects and foam generation are reduced effectively. Additionally, a synergetic effect with C6 based fluorosurfactant is presented. In 2010 a new concept of branched short chain fluorosurfactants with exceptional properties and an improved ecotoxicological profile was presented [1]. Following this approach, a series of fluorosurfactants with a remarkable combination of speed and low static surface tension has been synthesised and investigated in typical coating applications. Below, the structure-property relationships of various surfactants are analysed, and the ability of the new fluorosurfactant to satisfy the requirements of various applications is considered. An introduction to fluorosurfactants Fluorosurfactants are indispensable for achieving surface tensions below 20 mn/m, as they have the highest potential to reduce interfacial tensions. The fluorophobic effect [2], which implies minimum interactions with Figure 1: Minimum surface tensions achievable with different surfactant technologies other components in complex coating formulations, is the main reason for the much higher effectiveness and efficiency of fluorosurfactants compared to other surfactant technologies. The basis of every fluorosurfactant is a hydrophobic moiety consisting of a perfluorinated alkyl chain. Recent concerns about the environmental persistence and bioaccumulative behaviour of longer perfluorinated chains and their derivatives [3] have led to a ban on compounds with chains containing more than six perfluorinated carbon atoms. Reducing the length of the perfluorinated chain appears to be the next logical step, and a great variety of fluorosurfactants based on perfluorinated C6, C4 and even C2 chains has been introduced to the market. Reducing the perfluorinated chain length, however, also reduces the fluorophobic effect, which in some cases leads to disappointing performance. Obviously, simply cutting down the chain length does not lead to satisfactory results in balancing ecotoxicological impact and performance. The importance of molecular packing density In general, a high interfacial packing density of the perfluorinated groups correlates with a low surface tension. Linear long-chain fluorosurfactants such as C8-based materials are known to have a dense (crystal-like) packing of hydrophobic groups resulting in static surface tensions that can reach minimum values as low as 16 mn/m in water. In contrast to this, common theory indicates that shortchain fluorosurfactants based on linear perfluorinated C2 chains cannot provide a high molecular packing density, because their hydrophobicity is too low to shield the interfacial surfactant layer against polar water molecules. This would lead to an enlargement of the intermolecular surfactant distance (distance between individual surfactant molecules) with a lower packing density, which results in a higher surface tension of mn/m (see Figure 1). So, what options exist to design surfactants with ecofriendly C2 or C3 perfluorinated chains that have low surface tensions comparable to long-chain fluorosurfactants? In the case of perfluorooxetanes, many C2 groups are attached to an oligomer backbone [4] which leads to an increase in packing density and a lower surface tension of 23 mn/m compared to linear C2 perfluorinated surfactants (see Figure 1a). However, surfactants based on this concept cannot achieve a very dense packing because the distance between perfluorinated groups is limited by the oligomeric backbone, leading to performance inferior to that of C6-based materials. To improve the packing density, a different structural concept is needed. With regard to branched surfactant 32 European Coatings JOURNAL 11 l

2 structures [5] the intramolecular distance (distance within a surfactant molecule) between the perfluorinated groups can be adjusted by the choice of molecule segments between the hydrophobic groups and their connecting points. Hence, the total packing density of perfluorinated groups per surface unit can be increased with branched surfactants because the distance between perfluorinated groups given by covalent binding is lower than in the case of a self-assembly of linear fluorosurfactants, where the packing is simply given by the intermolecular distance (see Figure 1b). This can lead to lower surface tensions. New structure is effective and environmentally safe Confi rmation that the branched structure is beneficial is given by the static surface tension of a branched shortchain surfactant based on three perfluorinated C2 groups linked to one hydrophilic moiety, which has been introduced to the market as Tivida FL This molecule can reduce aqueous surface tension to 20 mn/m, a value not previously reported for a C2-based surfactant. In addition, no bio-accumulation could be determined according to the OECD 305 guidelines (for bio-concentration in fish), making this a very attractive alternative for ecofriendly formulations regulated by governmental guidelines that exclude long-chain fluorosurfactant (for example, the Nordic Swan system). Additionally, no signs for toxicity could be found in relevant test studies and for this reason "Tivida 2300" will receive no classification for oral and inhalative toxicity. Results at a glance Fluorosurfactants are able to reduce the surface tension of aqueous formulations to values lower than can be achieved with other products. However, concern over the environmental effects of long-chain (C8) fluorosurfactants has led to a ban on their use. Shorter-chain types are less bioaccumulative but in general cannot achieve such low surface tension values as the longer-chain products. A new class of fluorosurfactant has been developed based on a very short C2 chain but with a branched structure. It is shown that this product can achieve high packing densities and so a relatively low surface tension, combined with a very rapid dynamic surface tension reduction. Combining this with a C6 fluorosurfactant leads to a synergistic combination with very fast surface tension reduction and low final values. Results are presented to show that this is very effective in reducing cratering defects and foam generation. MicrobialControl Anzeige 86 x 250 (Satzspiegel) oder 105 x 297 (randabfallend) Materials Protection Sustainable Solutions to Support Your Global Needs The Materials Protection business of Lonza Microbial Control provides innovative and cost-effective solutions that bring your products up to today s sustainability standards, while providing the protection you ve come to rely upon. The new organization, formed by Lonza s acquisition of Arch Chemicals, is now the largest microbial control business in the world. The benefits to our customers include the availability of a complementary range of products and actives in both established and emerging markets and an expansive service portfolio with solutions for your every need. Our portfolio includes: Preservation Dry-film Protection Marine Antifouling Plant Hygiene Visit us at Chinacoat 2012, Booth #10.2H09-12 and learn how we can become a responsible part of your environmentally-preferred solution l 2012 European Coatings JOURNAL E: microbialcontrol@lonza.com T: l T: Use biocides safely. Always read the label and product information before use.

3 The fast diffusion of branched structures can be explained by reference to properties that are already known from hydrocarbon Gemini type surfactants. These structures form unstable aggregates in solution, and these are easier to dissolve than micelles of common linear surfactants. Therefore Gemini amphiphiles are more quickly available to occupy newly formed interfaces (see Figure 3). The hypothesis can be formulated that a similar effect could be the cause of the exceptional speed of the new fluorinated surfactant. After 100 ms, it shows even better dynamic behaviour than one of the fastest acetylenic diol Gemini types in the market, which can only reach an equilibrium surface tension of 26 mn/m. Figure 2: Dynamic surface tension measurements of surfactants with different structures: dynamic surface tension curves compared at 0.1 wt % active surfactant concentration in water (maximum bubble pressure method) Dynamic performance is exceptional However, the static surface tension in water alone is not sufficient to describe a surfactant. In the final application, the most important property may be the time the surfactant needs to reduce the surface tension of a liquid. Applications are not static, they are dynamic processes, and thus it makes sense to investigate the dynamic properties of surfactants. This is usually done with a bubble pressure tensiometer which measures the time dependence of the reduction in surface tension. The branched structure is also beneficial for improving the dynamic surface tension of fluorosurfactants. The speed of the new surfactant is shown by the comparison of dynamic surface tension curves in Figure 2. At a concentration of 0.1 wt%, the surface tension of water can be reduced to < 30 mn/m within 100 ms. This shows significant advantages compared to all other fluorosurfactant technologies, which typically need 10 s to reach this value. Figure 3: Formation of stable micelles is inversely correlated with the speed of covering new surfaces: linear surfactants diffuse slowly from micelles (a) while unstable aggregates give branched surfactants rapid diffusion (b) Speed and final performance relationship examined One conclusion of these findings is that the dynamic properties of different fluorosurfactant building blocks differ to such a large extent that this cannot be disregarded when a fluorosurfactant is described. Therefore a new type of diagram is proposed, in which static surface tension values are combined with the dynamic surface tension. In Figure 4 the most effective surfactant is located as close as possible to the upper right corner (first quadrant), which means it is capable of reducing the surface tension very quickly and to very low values. From Figure 4 it becomes clear that the branched fluorosurfactant is not only fast, but it effectively outperforms every other surfactant. After 100 ms, it lowers the surface tension to 30 mn/m, while conventional linear surfactants still show high values of up to 70 mn/m. It is expected that this speed advantage should give the product a decisive head start in fast-drying applications such as printing or coil coatings. In Figure 4 the relationship between dynamic and static surface tension of all relevant types of fluorosurfactant is illustrated. This shows that today s most widely used types need quite some time to lower the surface tension, which might be the reason why in some cases they do not show satisfactory effects. Surfactant combination shows synergistic effects In some cases, however, surface tensions of < 20 mn/m might be needed, so it is useful to investigate whether the exceptional speed of branched short-chain fluorosurfactants can be combined with the very low surface tension of slower linear longer-chain molecules. Figure 4 shows that an 80:20 mixture of the branched fluorosurfactant with a C6-based fluorosurfactant does not simply show a volume-dependent mixture of the properties of both ingredients but a significant synergistic effect (shift from second to first quadrant). The static surface tension of the mixture is even lower than that of the product based on C6 perfluorinated groups alone, combined with the exceptional speed of the branched surfactants. Hence, branched short-chain fluorosurfactants can be used as a tool to speed up slower fluorosurfactants and achieve synergistic effects. Thermodynamics indicate that the faster surfactant reaches its final interfacial packing first, before it is partially or fully substituted by a slower surfactant with a lower g static. If a partial substitu- 34 European Coatings JOURNAL 11 l

4 tion takes place, certain interfacial mixtures can result in a very dense packing that leads to a synergistic effect. The surface tension that can be achievable with such a mixture would be lower than using the single surfactants alone, with the additional speed of the branched surfactant. Mixture gives best anti-cratering performance Next, the correlation between aqueous surface tension measurements and the effect of speed and low surface tension in water-based coating systems will be considered. For this purpose a 1-K PUR / melamine plastic base coat was formulated containing a 10-fold overdose of SiO 2 /silicone based defoamer. The coating drawdown shows extreme cratering caused by the incorporation of low-energy contaminating particles. The reason for choosing this system was to show a differentiation of de-cratering properties of different surfactants due to the reduction of interfacial tension between the particles in the water-based coating. The result in Figure 5 shows the very good anti-cratering performance of the branched-chain surfactant in this application. Only the C6 telomer can compete with its effectiveness. Interestingly, in this application it is not the low static surface tension of the formulation that is important for effectiveness, but a combination of surface Figure 4: Dual surface tension relationship of various fluorosurfactant technologies showing the minimum static surface tension (g static ) vs. surface tension reached after 100 ms (g dynamic ) at 100 ms) B-Tough TM A _ Tough enough to deal with the unexpected NEW B-Tough A, reactive novel epoxy functionalised toughening agents for high performance structural adhesives. Are you looking for performance improvements of your epoxy formulations such as: 4 excellent impact & shock resistance 4 improved adhesion......then it s time to discover how B-Tough A outperforms conventional toughening agents. To download the B-Tough A brochure and to request a product sample we invite you to register for Croda Direct at Croda Coatings & Polymers your natural choice 11 l 2012 European Coatings JOURNAL 35

5 tension reduction and speed that cannot be described by the static surface tension alone. Although the polymer based on C4 perfluorinated groups in Figure 5 achieves a static surface tension of 20 mn/m, it does not show a convincing anti-cratering effect due to its inferior dynamic properties. This proves the hypothesis that fast dynamic surface tension is necessary for good surfactant application performance. Nevertheless, an extremely low surface tension in the 1K basecoat is possible with a combination of the branched fluorosurfactant with a C6 telomer. This surfactant mixture combines the lower static surface tension of the telomer with the faster speed of the branched structure and delivers a superior de-cratering effect. It should be noted that the additive performance can differ significantly from formulation to formulation and the optimum mixture has to be found in application-specific trials. Foam formation significantly reduced Figure 5: Anti-cratering effect of different fluorosurfactants in a 1K PUR/melamine coating (Bayer recipe; layer thickness 50 µm; 0.15 wt % active surfactant concentration) A branched structure should also show advantages in foaming behaviour [5]. This can be proved impressively by using the product in floor polish applications where a polymer latex is applied by wiping with a soaked cloth, a process in which a lot of foam may be generated. The task of the fluorosurfactant is to assist wetting, levelling and gloss improvement of the dried latex film on the floor. Foam formation is strongly correlated to the stability of foam lamellae. Current surfactant theory shows that linear surfactant molecules tend to stabilise the foam lamellae, thus being prone to foam. Branched surfactants, however, tend to destabilise the foam lamellae, thus avoiding foam formation. Figure 6 shows the excellent results of the branched fluorosurfactant in comparison to a C6 perfluorinated linear fluorosurfactant. For this reason, the product has been recommended in floor polish application since the beginning of 2012 by major producers. To summarise, the concept of branched short-chain fluorosurfactants not only leads to more environmentally friendly products, but also to one of the fastest-acting surfactants ever measured. The outstanding dynamic properties of the concept in combination with a very low static surface tension could be achieved with a new structural principle. A new diagram (Figure 4) for visualising static and dynamic surfactant properties was proposed and its relevance could be proved in practical coatings applications. The product can be used to improve the dynamic properties of other fluorosurfactants and achieve additional synergistic effects in mixed surfactant systems. í REFERENCES Figure 6: Tivida FL 2300 shows lower foaming and better levelling in a polymer emulsion than C6 telomer fluorosurfactants [1] Jonschker G. et al, Europ. Coat. Jnl., 2010, No. 7-8, pp [2] Farn R. J., Chemistry and Technology of Surfactants, 2006, Blackwell Publishing, pp [3] Environmental Protection Agency (EPA), 2010/2015 PFOA Stewardship Program. [4] Ashwin R. et al, Langmuir, (2006), Vol. 22, No. 10, pp [5] Menger F., Keiper J., Angew. Chem., 1989, Vol. 112, No. 2000, pp [6] Schwarz J., Jnl. Coat. Techn., 1992, Vol. 67, pp European Coatings JOURNAL 11 l

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