A novel technology-the siloxane-based multi-functional additive.
Printed paper or cardboard frequently creates major wetting problems, because the actual substrate for the ink or coating is not paper, but wax-containing printing ink. And last, but certainly not least, any substrate can be contaminated by dirt, dust, finger-prints and more.
Substrate wetting additives are commonly used to overcome these problems. They are special surfactants that modify interfacial tensions. Generally, they reduce the surface tension of a coating or ink to a homogeneously low level, enabling it 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
Currently, many different technologies (5,6) are being used to supply the coatings and inks industry with effective substrate wetting additives. Different property profiles make the various 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 addition levels, however, they tend to stabilize foam. This is especially true for fluorocarbon surfactants, but may, to a lesser degree, also apply for siloxane-based surfactants. There are some organic surfactants 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.
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 coatings and inks; and
* Be non water-hazardous.
We knew that a very strong reduction of surface tension could be achieved by the use of siloxane-based surfactants. Studying literature about surfactant behavior, we found that so-called Gemini structures, first named by Fredric Menger (8) in 1991, can show high effectiveness at very low levels and potentially contribute to low foaming properties. (9,10) In addition, the selection of relatively hydrophobic structures allowed us to expect additional defoaming properties.
As a result, the idea of the multi-functional additive based on a combination of siloxane and Gemini technology was born. When selecting potential raw materials and resulting structures, we paid special attention to ecological aspects, such as water pollution.
Selecting the Right Structure
Gemini surfactants in literature are also called bis-or double-tailed surfactants. (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 surfactants are symmetrical structures.
[FIGURE 1 OMITTED]
Compared to conventional surfactants, Gemini types display very different aggregation behavior. This results in a significantly lower critical micelle concentration (CMC) and a far higher surface activity (factor up to 100x/1000x), making them quite efficient. (11-13) Depending on their structure, they can also show significantly reduced foam stabilization.
By combining different in-house technologies, we were able to synthesize structures of siloxane-based multi-functional surfactants for evaluation as shown in Figure 2.
[FIGURE 2 OMITTED]
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 regard to the desired properties. Table 2 presents examples that summarize the obtained performance of the species.
It is clear that structure D provided specific properties in our screening that perfectly matched our original idea. A similar molecule was finally chosen to become the first commercially available siloxane-based multi-functional surfactant.
Initial Test Results
A comparison of the characteristics of this surfactant with other existing technologies produced impressive data that underline the special behavior of this new technology.
The siloxane-based multi-functional surfactant shows a very low critical micelle concentration plus the lowest surface tension reading of all surfactants tested (Table 3).
However, these data are suitable only for characterizing the surfactant's behavior when it is the only ingredient in the water. But being an effective substrate wetting agent in coating formulations means that the surfactant has to show the same properties even in the presence of other surface active ingredients, such as an emulsifier or a dispersing agent. This is especially true since some of the tested surfactants, including the siloxane-based multi-functional types, are hydrophobic and not homogenously soluble in pure water. What finally counts is the performance in practical application.
Surface Tension Measurements In Resins and Coatings
A comparison of the surface tension reduction of a siloxane-based multi-functional surfactant with other commercially available substrate wettings clearly demonstrates how effective this new surfactant is.
It provides an extraordinarily strong reduction of static surface tension throughout a range of water-based coatings (measured at an addition level of 0.2% surfactant by the Du Nouy ring method) (Figure 3).
[FIGURE 3 OMITTED]
When the addition levels are varied, the extreme effectiveness becomes even more obvious. The tested siloxane-based multi-functional surfactant already creates a very strong surface tension reduction at addition levels below 0.1% (Figure 4). And finally, the siloxane-based multi-functional surfactant could improve wetting properties in numerous application tests that would be conducted.
[FIGURE 4 OMITTED]
Practical Findings--Wetting Properties
For spray application, it is beneficial if a coating forms a closed film already at low film thicknesses. This ensures a defect-free film and maximizes the application window of the coating. This is achieved by the use of a substrate wetting additive that ensures fine atomization when the coating leaves the spray nozzle and good wetting of the coating droplets on the substrate once they hit the surface.
Interesting results were obtained 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 [micro]m dry) (Figure 5). As is demonstrated in the pictures below, the siloxane-based multi-functional surfactant manages to create a completely closed film at very low film thickness.
[FIGURE 5 OMITTED]
Another impressive example is the wetting of the abovementioned styrene-acrylic overprint varnish on fresh lithographic ink, which contains mineral oil and waxes and is very difficult to wet. Standard procedure in the industry is to use sulfosuccinate wetting agents at very high addition levels. Other organic surfactants and commercially available siloxane-based wetting agents fail in this application (Figure 6).
[FIGURE 6 OMITTED]
Practical Findings--Defoaming Properties
A quick test on the foaming properties of a coating or ink is the Tego standard stir test. 50 g of coating are stirred at 3000 rpm for 1 minute by a 3 cm dissolver blade. Immediately afterward, 45 g of coating 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 coating based on polyurethane/acrylic emulsion produced impressive results. The control without any additives had a reading of 76 ml/45 g, whereas the sample containing 0.2% of siloxane-based multi-functional surfactant had a reading of 46 m/45 g. Considering the density of the formulation, it was nearly foam-free.
Here the siloxane-based multi-functional surfactant provided effective foam prevention, comparable to a powerful conventional defoamer (Figure 7).
[FIGURE 7 OMITTED]
Depending on the resin system employed, this effect could be reproduced or was significantly weaker. But in our tests the siloxane-based multi-functional surfactant was never foam-neutral or even foam stabilizing.
The following chart displays its influence on the foaming behavior of various water-based coatings in comparison to an oligomeric classical siloxane surfactant, an acetylenic diol and an organic Gemini surfactant (Figure 8).
[FIGURE 8 OMITTED]
The candidate multi-functional surfactant also proved to provide excellent foam prevention under high shear conditions. When performing this test (15 min stirring at 3000 rpm) with an aqueous overprint varnish, the control built up a large amount of micro-foam, literally turning into "whipped cream." The test had to be stopped in less than 15 minutes. The sample containing the siloxane based multi functional additive only showed moderate foam build-up after 15 minutes. (Figure 9).
[FIGURE 9 OMITTED]
Another interesting property with regard to foaming is the much faster foam break-down that can be observed in water-based coatings and printing inks containing the siloxane-based multifunctional surfactant.
In conclusion, we can state that the tested siloxane-based multi-functional surfactant significantly reduces the build-up of macro- and micro-foam and speeds up the foam-break and deaeration in water-based coatings and inks.
The described siloxane-based multi-functional surfactant has recently been introduced and is receiving good results in commercial applications. It shows significantly better overall compatibility than conventional defoamers. Surface defects were hardly ever obtained; and those in a few formulations with very low solids content.
To evaluate this surfactant, it is highly recommended to add this product to the coating system without any additional substrate wetting additive and defoamer. Further addition of defoamer may or may not be necessary. When using the siloxane-based multi-functional surfactant, an overall reduced additive amount in the coating or printing ink formulation can be expected due to the "two-in-one" character of this additive class.
First tests in radiation-curing and somewhat polar solvent-based coatings indicate that the siloxane-based multifunctional surfactant acts as an amphiphilic surfactant 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 characteristics.
Additional advantages for this modern surfactant are the ecological aspects. The product is free of organic solvents and therefore, will not contribute to VOC of the coating system.
Conclusion and Outlook
The tested siloxane-based multi-functional surfactant combines very high surface activity, good substrate wetting and effective defoaming in one molecule. It is most promising 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 conventional technology. It is highly probable that the combination of siloxane and Gemini technology will enable us to develop further powerful surfactants with unique property profiles.
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Susanne Struck received her degree in chemistry and coatings technology in Krefeld, Germany. She began working for Tego in 1990 in the technical service department as an engineer for coatings technology. Her primary focus was on glide additives and deaerators. Later, she specialized in additives for printing ink applications. Currently, Ms. Struck is group leader for printing inks and UV applications and deputy director of the technical service department. She has published numerous articles on various additives and presented several papers at international congresses.
Susanne Struck, Kathrin Lehmann, Kai Steenweg, Petra Hinrichs Degussa, Tego Coating & Ink Additives, Germany
Table 1: General properties of classes of substrate wetting additives. Substrate Wetting Defoaming Effectiveness Surfactant Properties Properties at Low Levels Oligomeric siloxane Excellent Inferior Good Special branched Neutral Good Neutral polyether Sulfosuccinate Good Very Poor Neutral Fluorocarbon Excellent Very Poor Excellent Acetylenic diol Neutral Good Neutral derivative Table 2: Comparison of silo.vrrne-based multi-functional additives in a water-based resin. Surface Tension Reduction Substrate Deforaming (Du Nouy) Wetting Effectiveness Properties Structure Good Inferior Good Excellent Structure B Excellent Excellent Neutral Inferior Structure C Good Neutral Good Neutral Structure D Excellent Good Good Good Table 3: Properties of surfactants in water. Static Appearance Surface 0.5% Surfactant Tension CMC Surfactant tmN/m] [mg/l] in Water Siloxane-based 20.4 89.4 very turbid Multi-functional Oligomericsiloxane 24.2 848.5 very turbid Polyether 29.4 370.8 turbid Sulfosuccinate 25.6 1194.3 clear Fluorocarbon 22.3 31.6 clear Acetylenic diol derivative 29.2 547.5 turbid
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|Author:||Struck, Susanne; Lehmann, Kathrin; Steenweg, Kai; Degussa, Petra Hinrichs|
|Date:||Jun 1, 2006|
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