The Multifunctional Siloxane-based Gemini Surfactant TEGO Twin Technology

Transcription

1 The Multifunctional Siloxane-based Gemini Surfactant TEGO Twin Technology Authors: Susanne Struck Kathrin Lehmann Kai Steenweg Petra Hinrichs Company: Tego Chemie Service GmbH Goldschmidtstrasse Essen Germany Tel.: Fax:

2 Introduction Substrate wetting and flow is a well known problem in the industry especially to the formulators of water-based s and inks which have already a market share of more than one third of the world production. [3-6] Creating a closed, defect-free film is not only a problem on the common low-energy substrates like plastics. But wetting on metal can be difficult, too, since it is not always perfectly degreased. Wetting on wood can be critical, since wood needs to be penetrated well and often contains oil. And printed paper or cardboard often creates major wetting problems, since the actual substrate for the ink or to apply is not paper but wax-containing printing ink. And last but not least any substrate can be contaminated by dirt, dust, finger-prints and more. To overcome these wetting problems substrate wetting additives are commonly being used. They are special s that modify interfacial tensions. Generally they reduce the surface tension of a or ink to a homogeneously low level, enabling the or ink to wet the above mentioned substrates. Very strong surface tension reduction is required to wet oily and greasy contaminations. Existing technologies and their property profiles A lot of different technologies [1,2] is currently being used to supply the s and inks industry with effective substrate wetting additives. Different property profiles make the different classes more or less suitable for different applications. There are additives that provide a very strong surface tension reduction, enabling substrate wetting even on contaminated substrates. At higher dosages however, they tend to stabilise foam. This is especially true for fluorocarbon s but may to a lesser degree also apply for siloxane-based s. There are some organic s that show hardly any foaming or are even slightly defoaming. But these are not very effective surface tension reducers and tend to fail in the case of very low energy substrates or contaminations. Table 1: Different classes of substrate wetting additives and their general properties Surfactant Substrate wetting Defoaming Effectiveness at low properties properties dosages oligo siloxane Special branched polyether O + O sulfosuccinate + -- O fluorocarbon acetylene diol derivative O + O Development of a defoaming and highly surface active substrate wetting additive When discussing the targets for our development the following criteria were considered most important: The new structure should provide a strong surface tension reduction resulting in good substrate wetting properties be effective at very low addition levels provide a distinct defoaming effect in most water-based s and inks be non water-hazardous, having the German Wassergefährdungsklasse 1 We knew: a very strong reduction of surface tension can be achieved by the use of siloxane-based s. Studying literature about behaviour we found that so-called Gemini structures first named by Menger [8] in can show high effectiveness at very low dosages and potentially contribute to low foaming properties. [9,10] And finally the selection of relatively hydrophobic structures could let us expect additional defoaming properties. The idea of the siloxane-based Gemini was born. When selecting potential raw materials and resulting structures we paid special attention to ecological aspects, such as water hazard.

3 Selecting the right structure Gemini s in literature are also called bis- or double-tailed s. [7,8,10] They are characterized by at least two hydrophobic chains and two ionic or polar groups and the presence of a spacer. The spacer can be of various chemical natures; such as a simple alkyl chain, a polyoxyalkylene segment or an aromatic unit. Most Gemini s are symmetrical structures. Figure 1: Comparison of a conventional single-tailed with a Gemini Compared to conventional s Gemini s display very different aggregation behaviour. This results in a significantly lower critical micelle concentration (CMC) and a far higher surface activity (factor up to 100x/1000x) making them quiet efficent. [11-13] Depending on their structure they can also show a significantly reduced foam stabilisation. Combining different in-house technologies we were able to synthesize first structures of siloxanebased Gemini s for evaluation given in the figure below. Varying the structural parameters, the nature of the polar groups and the overall molecular weight a number of different species was obtained that showed distinctly different performance. For screening purposes all structures were tested in three water-based binders with regards to the desired properties. The following table (Table 2) gives a few examples summarising the obtained performance of the species. Table 2: Comparison of different siloxane-based Gemini s in water-based binder Surface tension reduction (Du Nouy) Substrate wetting Effectiveness Defoaming properties Structure A Structure B O - Structure C + O + O Structure D It is very obvious that structure D provided properties in our screening that perfectly fit our original idea. A similar molecule was finally chosen to become the first commercially available siloxane-based Gemini. First test results Comparing the characteristics of this with other existing technologies impressive data were obtained that underline the special behaviour of this new technology. Table 3: Properties of different s in water in water Static σ CMC [mg/l] [mn/m] siloxane-based Gemini very turbid oligo siloxane very turbid Polyether turbid sulfosuccinate clear fluorocarbon clear Acetylene diol derivative turbid Appearance 0,5% in water The siloxane-based Gemini shows a very low critical micelle concentration plus the lowest surface tension reading of all s tested. (Table 3) However, these data are just suitable to characterise the s behaviour being the only ingredient in the water. But being an effective substrate wetting agent in formulations means that the has to show the same properties even in the presence of other surface active ingredients such as emulsifier or dispersing agents. This is especially true since some of the tested

4 s including the siloxane-based Gemini s are hydrophobic and not homogenously soluble in pure water. What finally counts is the performance in practical application. Surface tension measurements in binders and s Comparing the surface tension reduction of a siloxane-based Gemini to other commercially available substrate wetting additives it becomes obvious how effective this new is. mn/m siloxane-based Gemini classical siloxane acetylenic diol AC/PU wood Styr/AC overprint v. Aqueous UV wood PU plastic Mod. AC ar. Coating organic Gemini no Figure 3: Static surface tension of various water-based s Throughout a range of water-based s it provides an extraordinarily strong reduction of static surface tension (measured at an addition level of 0.2 % by the DuNouy ring method) (Figure 3). When varying the addition levels the extreme effectiveness becomes even more obvious Siloxane-based gemini Siloxane comp. Acetylenic diol mn/m ,05 0,1 0,2 0,3 0,5 % Figure 4: Static surface tension in water-based acrylic/pu The tested siloxane-based Gemini creates a very strong surface tension reduction already at addition levels below 0.1 % (Figure 4). And finally the siloxane-based Gemini could prove very good wetting properties in numerous application tests. Practical findings - wetting properties For spray application it is beneficial if a forms a closed film already at low film thicknesses. This ensures a defect-free film and maximises the application window of the. This can be achieved by the use of a substrate wetting additive that ensures fine atomization when the leaves the spray nozzle and good wetting of the droplets on the substrate once they hit the surface. When performing a spray application of a metallic base coat on filler-coated panes by spray robot at a very low film thickness (approx. 15 µm dry), interesting results were obtained. (Figure 5) As can be

5 seen in the pictures below the siloxane-based Gemini manages to create a completely closed film at very low film thickness. control acetylenic diol siloxane-based gemini Figure 5: Microscopic view on the base-coat (applied on white-coloured filler) Closed film! Another impressive example is the wetting of the before mentioned styrene-acrylic overprint varnish on fresh litho ink. Freshly applied litho ink contains mineral oil and waxes and is very difficult to wet. Standard in the industry is to use sulfosuccinate wetting agents at very high dosages. Other organic s and commercially available siloxane-based wetting agents fail in this application (Figure 6). control (without additive) with acetylenic diol with siloxane Gemini Figure 6: Draw-down of water-based overprint varnish on fresh litho ink Practical findings - defoaming properties A quick test on the foaming properties of a or ink is the Tego standard stir test. 50 g of are stirred at 3000 rpm for 1 minute by a 3 cm dissolver blade. Immediately afterwards 45 g of are poured into a graduated cylinder and the volume is read. The lower the reading the less foam has been entrained. Performing this stir test in a water-based wood based on a PU/acrylic emulsion impressive results were obtained. The control without any additives had a reading of 76 ml/45 g, whereas the sample containing 0.2 % of siloxane-based Gemini had a reading of 46 ml/45 g. Having the density of the formulation in mind it was nearly foam-free. complete wetting Here the siloxane-based Gemini provided effective foam prevention comparable to a powerful defoamer (Figure 7). control 0.2 % Gemini s. Figure 7: Defoaming properties Depending on the binder-system this effect could be reproduced or was significantly weaker. But in our tests the siloxane-based Gemini was never foam-neutral or even foam stabilising. The following chart displays its influence on the foaming behaviour of various water-based s in comparison to an oligomeric classical siloxane, an acetylenic diol and an organic Gemini (Figure 8).

6 mn/m siloxane-based Gemini 60 classical siloxane 55 Acetylenic diol organic Gemini 40 AC/PU wood Styr/AC overprint v. Aqueous UV wood PU Plastic Mod. AC ar. no Figure 8: Results of the Tego foam test in various aqueous binders The tested Gemini did also prove to provide excellent foam prevention under high shear conditions. When performing a high shear test (15 min stirring at 3000 rpm) with an aqueous overprint varnish the control built up a lot of microfoam, literally turning into whipped cream. The test had to be stopped before time. The sample containing the siloxane based Gemini only showed moderate foam build-up after 15 min (Figure 9). control (without any additives) sample with siloxane-based Gemini Figure 9: Illustration of foam prevention after 15 min of high shear stirring Another interesting property with regards to foaming is the much faster foam break-down that could be observed in water-based s and inks containing the siloxane-based Gemini. Concluding we can state that the tested siloxane-based Gemini significantly reduces the build-up of macro- and microfoam and speeds up the foam-break and deaeration in water-based s and inks. Further comments The described siloxane-based Gemini has recently been introduced into the market and is enjoying great success. It shows much better overall compatibility than conventional defoamers. Surface defects were hardly ever obtained only in a few formulations with very low solids content. For the testing of this it is highly recommended to leave out any substrate wetting additive and defoamer in the beginning and just add this one product. Further addition of defoamer may or may not be necessary. When using the siloxane-based Gemini an overall reduced additive

7 amount in the or ink formulation can be expected due to the two-in-one character of this additive class. First tests in radiation-curing and rather polar solvent-based s indicate that the siloxane-based Gemini acts as an amphiphilic in these formulations as well. Even though the surface tension reduction is significantly lower than in water-based formulations it contributes to improved substrate wetting and especially flow. Additional advantages for this modern are the ecological aspects. The product is free of organic solvents and classified as non water-hazardous (WGK 1). Conclusion and outlook The tested siloxane-based Gemini combines very high surface activity, good substrate wetting and effective defoaming in one molecule. It is most interesting for all water-based formulations that would normally require the use of a substrate wetting additive and a defoamer. With this unique and patented technology property profiles can be obtained that were impossible to achieve with any other common technology. It is highly probable that the siloxane Gemini technology will enable us to develop further powerful s with unique property profiles. References [1] H.-D. Dörfler, Grenzflächen und kolloid-disperse Systeme, Springer-Verlag Berlin Heidelberg 2002, , [2] Ingenieurbüro Kern, Das Wasser, Infos zu Tensiden [3] Freedonia, Phänomen Farbe 4 (2001) [4] J.P. Monteny, Farbe & Lack 4 (2002), 18 [5] G. König, P. Lazerme, Farbe & Lack 11 (2002), [6] H.-J. Streitberger, A. Goldschmidt, Besser Lackieren 19 (2002) [7] Yong-Keun Kim, Dissertation an der Universität Freiburg/Breisgau, 2001 [8] F. M. Menger, C.A. Littau, J. Am. Chem. Soc. 113 (1991), [9] M. J. L. Castro, J. Kovensky, A. F. Cirelli, Tetrahedron Letters Vol. 38 (1997), [10] M. J. Rosen, L. Lio, JAOCS Vol 73 (1996), [11] J. M. Pestman, H. A. van Doren, et al, Langmuir 13 (1997), [12] P. Renouf, C. Mioskowski, L. Lebeau, Tetrahedron Letters Vol. 39 (1998), [13] F.M. Menger, J.S. Keiper, Angew. Chem. 112 (200),