Silicone Surface-Active Agents. By Donna Perry Dow Corning Corporation

Transcription

1 Silicone Surface-Active Agents By Donna Perry Dow Corning Corporation

2 Dow Corning Silicone Solutions As the pioneer of silicon-based technology, Dow Corning has been improving customers products and profitability for more than 60 years. With a full range of product and application solutions, reliable supply, world-class manufacturing, and global reach, Dow Corning can meet virtually any silicone-related need through our total solution offering and technology leadership. For more information, call Dow Corning ( provides performance-enhancing solutions to serve the diverse needs of more than 25,000 customers worldwide. A global leader in technology and innovation, offering more than 7,000 products and services, Dow Corning is equally owned by The Dow Chemical Company and Corning Incorporated. More than half of Dow Corning sales are outside the United States.

3 Silicone Surface-Active Agents by Donna Perry Dow Corning Corporation Introduction Surface-active agents are chemicals that reduce the surface energy of a material to which they are added by absorption at the air/liquid interface. The increasing popularity of waterborne coatings presents a continuing challenge for formulators. There is the need to formulate more environmentally acceptable products to comply with government regulations on VOCs, to reduce flammability and to generally reduce the hazard profile of coating systems. In comparison with solvent-borne systems, waterborne coatings exhibit a much higher surface tension. This results from water having a surface tension of 72.4 mn/m. As a result, waterborne coatings exhibit poor substrate wetting capabilities when compared to solventbased equivalents. The high surface tension of water is due to its polarity and the cohesive hydrogen bonding between the water molecules. Additives or surface-active agents, such as silicone polyethers, are used to modify the surface properties of waterborne coatings to achieve the desired flow, levelling and wetting performance. Silicone polyethers have traditionally been used in the coating area for surface modification, but along with these types of materials, Dow Corning has introduced novel functionalities to broaden applications and improve surfactant performance. This paper covers the traditional performance attributes of the silicone polyether additive class, while also introducing novel carbinol functional materials and their differences. Surface Tension Effects in Waterborne Coatings Surface tension can be considered in various ways, but perhaps best as a result of the forces of attraction existing between the molecules of a liquid. It is measured by the force per unit length acting in the surface at right angles to an element of any line drawn in the surface (mn/m). Wetting is the effect of displacing one fluid from the substrate surface by another liquid. In general, the liquid must have the same or a lower surface tension than the substrate; otherwise, no wetting will be achieved. This will produce a positive spreading coefficient and lead to a good coating. This can be best expressed in equation (1). By addition of the correct additive, the surface tension of the coating can be modified to improve substrate wetting. The additive acts by lowering the surface tension of the coating, enabling it to wet-out over the substrate. Levelling is the process of eliminating the surface irregularities of a continuous liquid film under the influence of the liquid s surface tension. Levelling is an important step in obtaining a smooth and uniform film. Concentration and surface Spreading wetting: S (Spreading coeff.) tension gradients in coatings are the main causes of poor levelling, which develop as the coating dries, due to variations in drying rates. Addition of the appropriate additive will modify the surface tension. For instance, a silicone additive will migrate to the liquid/air interface, giving an even surface tension across the film during the drying process, thus reducing surface tension gradients. Wetting and levelling are therefore totally different processes. Though the surface tension of the coating plays an important role in both processes, the effects of surface tension on spreading is opposite to that observed in levelling. Substrate wetting can be subdivided into two distinct categories: spreading wetting and adhesion wetting. Spreading wetting is necessary for even coverage of applied films, in particular, for the low-surface-energy substrates of many plastics. Adhesion wetting is defined in equation (2). It is important, for example, in printing processes in which it is undesirable for the applied ink to spread across a surface and then merge with other colors. = - G Spreading / A = γ Substrate - γ Substrate/Liquid - γ Liquid = γ Liquid (cos θ - 1) (1) When γ Substrate/Liquid > 0, spreading will occur spontaneously. Adhesion wetting: W A(Work of adhesion) = - G Adhesion / A = γ Substrate + γ Liquid - γ Substrate/Liquid = γ Liquid (cos θ +1) (2) (γ (gamma) = Surface tension) (θ (theta) = Contact angle)

4 Silicones as Surface-Active Agents Of the various surface-active chemistries currently available, this paper will mainly concentrate on a class of materials called silicone polyethers and the introduction of novel carbinol materials based on similar structures. This family of copolymers is used to provide multifunctional benefits in waterborne systems. The main uses of silicone polyethers in inks and coatings include de-foaming, de-aerating, improved substrate wetting and levelling, and enhanced slip properties 1,2. The three most common molecular structures for silicone surfactants are rake-type copolymers, ABA copolymers and trisiloxane surfactants. These are illustrated in Figures 1, 2 and 3 Figure 1: Molecular structure of a rake-type silicone surfactant. ( Si O ( Si O ) x -( Si O ) y Si ( (CH 2 (OCH 2 CH 2 ) n OR Figure 2: Molecular structure of an ABA-type siloxane surfactant. Figure 3: Molecular structure of a trisiloxane surfactant. ( Si O Si O - Si ( (CH 2 RO (CH 2 CH 2 O) n (CH 2 Si O ( Si O ) x Si (CH 2 (OCH 2 CH 2 ) n OR (OCH 2 CH 2 ) n OR respectively, and the performance of these structures will be described in two types of coatings. Silicones are highly surface active due to their low surface tension caused by the large number of methyl groups and due to the small intermolecular attractions between the siloxane hydrophobes. The siloxane backbone of the molecule is highly flexible, which allows for maximum orientation of the attached groups at interfaces. Silicone polyethers are non-ionic and have both a hydrophilic part (lowmolecular-weight polymer of ethylene oxide or propylene oxide or both) and a hydrophobic part (the methylated siloxane moiety). The new carbinolfunctional materials also incorporate a hydrophilic species on them and function under the same principles in which the traditional silicone polyethers do. The polyether groups are either ethylene oxide or propylene oxide, and are attached to a side chain of the siloxane backbone through a hydrosilylation or condensation process. They can form a rake-like, comb structure, or linear structure. Silicone polyethers are stable up to F ( C). There is a great degree of flexibility in designing these types of polymers. A very wide variety of co-polymers is possible when the two chemistries are combined. They can be varied by molecular weight, molecular structure (pendant/linear) and the composition of the polyether chain (EO/PO), and the ratio of siloxane to polyether. The molecular weight influences the rate of migration to the interface. The increased molecular weight of a polymer typically leads to an increased viscosity, but may also give greater substantivity to surfaces and improved shine level. Other variables include absence or presence of functionality or end groups on the polyether fragments. Depending on the ratio of ethylene oxide to propylene oxide, these molecules can be water soluble, dispersible or insoluble; obviously, for efficient wetting, these surfactants need to have good solubility in solution. As surfactants, they have the ability to produce and stabilize foam depending on their structure. Conversely, if their solubility parameters are low, they behave as antifoaming agents. The different structures affect how the molecules can pack at an interface. With pendant types in aqueous media, the silicone backbone aligns itself with the interface, leaving the polyalkylene oxide groups projecting into the water. Linear types form a very flattened W alignment where the central silicone

5 portion of the molecule aligns with the interface and the terminal groups are in the aqueous phase. The amounts to be added vary between 0.01% and 0.5% of the total formulation. This paper will now discuss the behavior of silicone polyethers, along with comparisons to silicones with novel carbinol functionalities in water, together with some studies in coating formulations. The results are compared with other additives that are currently used in coatings. Silicone Polyethers as Wetting Agents In this study, the three different siloxane surfactants described above were evaluated in aqueous solution, in a water-based printing ink, and in a waterbased polyester coating, in comparison with fluorosurfactants and acetylenic glycols, which are also used as wetting agents. To achieve good wetting and a positive spreading coefficient, the surface tension of the coating must be lower than the critical surface tension of the substrate. Water has a typical surface tension of 72 mn/m and, as can be seen from Table I, all the surfactants tested reduced the surface tension of the system, and, as a result, the aqueous medium wetted more efficiently. For the silicone surfactants, the best results were achieved with product A, a low-molecular-weight material. Trisiloxane A gave a very low figure that was improved by the fluorosurfactant. The critical micelle concentration (CMC) is the required level of product to initiate the formation of micelles in the bulk of a liquid. Up to this point, the surfactant added to the water migrated to the liquid/air interface to form a film that reduced the surface tension. The low CMC for A showed its high packing efficiency at the interface, in the much lower level required in comparison to the other products. Table I: Equilibrium surface tension with a Krüss K10T tensiometer and a platinum Wilhelmy plate, additive concentrations 0.1%. Surfactant Equilibrium Surface Tension (nm/m) CMC (%) Trisiloxane A ABA Siloxane B Rake Siloxane C Fluorosurfactant Ethoxylated Acetylenic Diol In this context, it is important to understand the difference between equilibrium and dynamic surface tension. For an equilibrium surface tension measurement, a platinum plate is immersed in a test solution and then slowly withdrawn. The force required to remove the plate from the solution is a measure of the surface tension of that liquid. Dynamic surface tension can be measured by an instrument that bubbles air through the test liquid at an increasing rate, during which the maximum pressure that is required to form a bubble is measured. As the bubble rate increases from 1 bubble per second to 10, the time to create the new interface (liquid/air) decreases. This is effectively measuring how quickly the surfactant lowers the surface tension. M.M The dynamic method is more representative of the coating application, for example, spraying. Under these circumstances, it is important to know how quickly a surfactant can migrate to newly formed interfaces. An ideal surface-active agent would provide excellent surface tension reduction under both equilibrium and dynamic conditions. Figure 4 shows dynamic surface tension results for the above materials at 0.1% in water. Trisiloxane A gives better performance than the other silicone polyethers, but does not give the very low values achieved by either the fluorosurfactants or ethoxylated acetylenic glycols. However, it must be remembered that dynamic surface tension represents only one aspect of the total phenomenon. Figure 4: Dynamic surface tension, maximum bubble pressure (Krüss BP1), 0.1% in water. "UBBLE &REQUENCY (ERTZ #!DDITIVE "!DDITIVE!!DDITIVE & 3URFACTANT %THOXYLATED!CETYLENIC $IOL

6 In practice, not only the conditions at the air/liquid interface are of interest but also the conditions at the liquid/substrate interface. In general, the smaller the contact angle produced by a system, the better the substrate wetting. The equipment used to measure the contact angle, theta (θ), was a VCA 2000 instrument that automatically dispensed a minute droplet of the liquid to be measured and then photographed the droplet after a set period, or could be programmed to take a number of photographs and measure corresponding contact angles after regular time intervals. This technique enabled the monitoring of droplets spreading on non-porous surfaces. When θ is greater than 90 degrees, as in Figure 5, then beading or non-wetting is occurring, as is shown by water on a plastic surface such as high-density polyethylene. If θ is less than 90 Figure 5: Contact angle measurement. degrees, as shown in Figure 5, then wetting is occurring; the smaller the angle, the better the wetting process. Spontaneous wetting occurs when θ=0, the droplet immediately spreading to Figure 6. Contact angle measurement with a VCA 2000 video contact angle equipment, additive concentration at 0.1% in water. #ONTACT!NGLE 3URFACE %NERGY OF 3UBSTRATE M.M form a very thin continuous film on the substrate with effectively no contact angle capable of being determined. Figure 6 shows the behavior of the same surfactants on a range of low-energy substrates such as polyethylene and polypropylene. For wetting, it has been shown that the most efficient silicone polyether is trisiloxane A, which has three Si atoms with pendant ethylene oxide. The low molecular weight and size give greater mobility in solution and allow for very efficient packing at interfaces. For critical low-energy substrates, only the trisiloxane surfactant A allows perfect (spontaneous) wetting and excellent spreading. #ONTROL "!DDITIVE #!DDITIVE %THOXYLATED!CETYLENIC $IOL!!DDITIVE & 3URFACTANT Extending this performance behavior now into practical life applications, the effects of these surfactants in a waterbased flexographic ink formulation are shown in Figure 7. The addition of siloxane surfactant A shows excellent substrate wetting performance on plastic foil, followed by the fluorosurfactant. The siloxane surfactants B and C, which have a different structure and a higher molecular weight, do not improve the wetting behavior. It can be seen that the acetylenic glycol has only a small impact on the wetting behavior. Any surfactant molecule has the potential to generate foam that is undesirable in many applications. Figure 8 shows the density measurements after a high shear stir test, which give an indication of air entrapment in the ink. The siloxane Figure 7: Flexographic printing ink, additive concentration 0.5%, wetting/appearance performance after application on plastic foil (surface energy 34 mn/m) by draw down 12 micron, rated on a scale from 1 5. ANCE7ETTING PPEAR! #ONTROL 4RISILOXANE!!"! " 2AKE #!CETYLENIC & 3URFACTANT 'LYCOL 2ATING FROM EXCELLENT Figure 8: Flexographic printing ink, additive concentration 0.5%, density measurement after dissolver stirring test (5 minutes at 2800 rpm) as a measure of rate of foam development. $ENSITY IN GCM #ONTROL 2AKE!"! " 4RISILOXANE!!CETYLENIC & 3URFACTANT 'LYCOL $ENSITY WITHOUT STIRRING GCM

7 surfactant C (rake) acts only as a lowto-moderate foam enhancer compared to the fluorosurfactant. What is also interesting is that siloxane surfactant B, the silicone product with the highest molecular weight, is performing as a de-aerator. Silicones as Slip Additives Slip represents the lubrication of a dry coating surface. Given that the function of a coating is often to protect the underlying surface from damage while maintaining a satisfactory appearance, it is clear that the coating itself must be capable of resisting mechanical damage. This is where slip is important. It is often referred to as mar resistance, rather than abrasion resistance. In the latter, the bulk mechanical properties of the film are important, in addition to the surface lubricity. Slip additives must reduce friction at the coating surface. A thin layer of a material with low inter-molecular forces is capable of achieving this. The slip additive should be sufficiently compatible with the coating before application to avoid separation. After application, it should not cause fisheyes or other defects associated with nonwetting of the additive by the coating. However, there has to be some degree of incompatibility to drive the additive to the surface of the coating film during drying. Silicone polyethers are important as slip agents due to their structure. The ratio of the EO/PO segments has to be carefully controlled to achieve the required compatibility balance. Too low a polyether content may lead to the de-wetting defects already mentioned. On the other hand, high polyether content can render the copolymer too soluble, with no driving force to get it to the coating surface during drying. Figure 9: Effect of molecular weight on slip performance of silicone-polyether copolymers in water-reducible polyester stoving paint (A: low molecular weight; B: high molecular weight). 3LIP!NGLE Table II: Contact angle data. Sample #ONTROL 4RISILOXANE! 2AKE " As previously described for wetting, the architecture of the copolymer has a profound effect on its behavior as a slip additive. Optimized structures have been identified by designed experimentation to give the desired combination of compatibility and slip performance. Compatibility is particularly important in clear coatings, where gloss reduction or haze is not acceptable. Figure 9 shows slip angle results for silicone-polyether copolymers A and B in water-reducible stoving paint. The main difference between the copolymers is overall molecular weight. Both products contain pendant polyether groups. It can be seen that trisiloxane A has almost no impact on slip. This is believed to be due to the very short nature of its silicone chains. A minimum amount of dimethylsiloxy units is required to give noticeable changes in slip. This requirement is met in rake structure B, which shows an improved slip. Water Contact Angle Acrylic Binder 43 Binder with 1% ABA 34 Binder with 1% Novel ABA <15 Binder with 1% Novel Resin <15 Novel Carbinol Function Silicones in Comparison to Traditional SPEs in Architectural Coatings The purpose of looking at new functionalities was to determine if we could improve hydrophilicity beyond what we were seeing in the traditional silicone polyethers that would lend themselves to more easy-clean surface development. In this study, two novel carbinol functionality materials were compared to a traditional silicone polyether of similar degree of polymerization (Dp) and its performance toward dirt release in an architectural binder. Each sample was doped into an acrylic-based binder by weight of resin, and surface appearance and properties were characterized. With the incorporation of the novel carbinol functionality, the data shows an increase in the hydrophilicity of the coating as compared to the traditional SPE (Table II). This yields what was seen as an

8 Table III: Dirt-release characteristics through simple spray method. Sample Dirt Release Properties Binder 0 Binder with 1% ABA + Binder with 1% Novel ABA ++ Binder with 1% Novel Resin ++ though with the addition of the traditional ABA materials there was a slight improvement over the baseline but not as great as the unaged samples. The novel ABA materials still showed significant improvement, while the resinous materials showed excellent dirt-release characteristics (Table V). Table IV: Dirt pickup and pencil hardness. Pencil Formulation Dirt Pickup Hardness Binder 0 3B Binder with 1% ABA - 4/5B Binder with 1% Novel ABA Binder with 1% Novel Resin - - 4/5B 4B Table V: Dirt release characteristics after outdoor aging. Sample SG 30 Neat - SG 30 with 1% ABA 0 SG 30 with 1% Novel ABA + Dirt Release Properties after 1008 Hours Weathering SG 30 with 1% Novel Resin ++ increase in the dirt-release ability of the coating through simple water spraying. This increase in hydrophilicity was seen in both the ABA-structured materials as well as resinous-based siloxanes. After a simple draw down and cure time, the coatings were dusted with a layer of dirt and sprayed with water to note the dirt-releasing properties. With the addition of the ABA silicone polyether structure, there was an enhancement of the dirt-release properties, but this enhancement was even greater in those samples incorporating the new carbinol functionality (Table III). One thing to be noted is that the addition of the silicone surfactants, either with the new carbinol material or the traditional polyethers, does cause a slight softening of the coating itself, which actually correlates into an initial dirt pickup that is slightly enhanced over the neat acrylic binder (Table IV). Durability of this hydrophilicity is a known deficiency, given the current additives function by migrating to the surface and can often wash away over an extended time with exposure to the environment. After subjecting the coatings to external weathering exposure for 1008 hours, the dirt-release perfor mance decreased. The neat SG 30 sample no longer released dirt as easily, Figure 10: Mar resistance as a function of gloss. 'LOSS )NITIAL "ASE 2UBS 4 RISILO XANE 4 RISILO XANE WITH.OVEL Novel Carbinol Functional Silicones in Comparison with Traditional SPEs in Overprint Varnish Applications A baseline formulation for a generalpurpose overprint varnish was obtained by Johnson Polymers. The formulation was run with and without the Aerosol OT-75, noting no differences in the wet-out performance, and was omitted for all the runs with the addition of the additives. The appearance and wet-out of the OPV were not affected by the incorporation of the siloxane additive, either standard or with the novel functionality. Samples were coated onto Lentea charts and run for 250 cycles on a Sutherland rub tester with gloss being documented prior and after rubs to determine each formulation s resistance to mar. The initial point to be noted was an increase in the initial gloss with siloxane incorporation, excluding the standard ABA and the BYK material benchmarked against. Enhancement was higher in those containing the novel functionality. There was also better mar resistance overall with all these samples, versus the baseline material.!"! &ORMULATION!"! WITH.OVEL 2ESIN WITH.OVEL #OMPETITOR 3AMPLE

9 Figure 11: Coefficient of friction data on OPV samples. #O& "ASE 4RISILOXANE 4RISILOXANE WITH.OVEL &ORMULATION 3TATIC #O& Coefficient of friction data (CoF) indicates that the mar resistance improvements were not due to increasing the slip of the surface. By industry standards, the slip change that was documented is within error. This indicates a possibility for applications such as floor varnish where mar resistance needs to be increased with no effect on the CoF or slipperiness of the surface. The CoF data demonstrates that the additive is not residing on the surface but possibly allowing the PE wax to better orientate at the surface and yield higher mar resistance. In looking at the surface tension data of the materials used in this study there is actually very little difference that would!"!!"! WITH.OVEL +INETIC #O& 2ESIN WITH.OVEL #OMPETITOR 3AMPLE indicate this is the reason why the material performs this way. Work continues to optimize the structures and find application in which they can be utilized. Conclusions Silicone-based surfactants offer many benefits in waterborne coatings. By the addition of the correct silicone surfac tant, the coating surface tension can be Table VI: Surface energy of standard and modified siloxanes. Surface Energy at 1% (Dynes/cm) Trisiloxane 20.1 Trisiloxane with Novel 21.2 ABA 24.6 ABA with Novel 23.8 Resin not soluble Resin with Novel 26.4 modified to improve substrate wetting. In particular, the trisiloxane structure gives the best equilibrium surface tension reduction and excellent wetting to plastic surfaces and other low-energy substrates. This material has the greatest capacity for lowering the liquid/solid interfacial tension. The higher-molecular weight siloxane surfactants, with rake and linear structures, give moderate wetting. However, materials of these types can be used to provide other benefits, for instance mar resistance, slip and de-aeration. The incorporation of a resinous material with functionality can also yield a higher substantivity in a coating that is likely desired when a coating is being exposed to long-term weathering effects. The addition of novel functionalities in place of the linear polyethers also can enhance the proper ties of the silicone polyether materials; research is being done to fine-tune these functionalities to the most appropriate applications.

10 References 1 Schlachter, I., and Feldmann-Krane, Georg, Silicone Surfactants, Surfactants Sci. Ser., 1998, 74, p Scholz, W., Verkroniek, 10, 13, 1995.

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