coatings

Article Stability of Aqueous Polymeric Dispersions for Ultra-Thin Coating of Bi-Axially Oriented Terephthalate Films

Petr Smolka 1,2,*, Lenka Musilová 1,2, Aleš Mráˇcek 1,2 ID and Tomáš Sedláˇcek 2

1 Department of Physics and Materials Engineering, Faculty of Technology, Tomas Bata University in Zlín, 76001 Zlín, Czech Republic; [email protected] (L.M.); [email protected] (A.M.) 2 Centre of Polymer Systems, Tomas Bata University in Zlín, 76001 Zlín, Czech Republic; [email protected] * Correspondence: [email protected]; Tel.: +420-576-035-102

Academic Editor: Stefano Farris Received: 24 August 2017; Accepted: 12 December 2017; Published: 16 December 2017

Abstract: The stability of polyacrylate and polyester based aqueous dispersions designed for ultrathin coating of extruded films, especially bi-axially oriented polyethylene terephthalate (BOPET), was studied. Also, the effect of the gemini surfactant based defoaming/wetting agent on the properties of the dispersions was examined. The addition of the defoaming/wetting agent resulted in reducing the surface free tension of the polyacrylate and polyester dispersion by 15% and 20%, respectively and the initial foam height by 60% and 15%, respectively. At the same time, the agent addition did not compromise the temperature and pH stability of the dispersions. Such modified dispersion can be utilized for ultrathin coating of plastic film used for packaging, to improve their processability, printability, and metallization.

Keywords: ultra-thin films; coating; polymeric dispersion; water based

1. Introduction have long ago become common packaging materials. They are used to protect products like food, pharmaceuticals, medical products, cosmetics, electronics, and many others [1]. Namely in the food manufacturing process, packaging of the product is one of the most important steps, as it maintains the quality of food for storage, transportation, and end use [2]. A single-layer plastic material is usually not capable of fulfilling all the requirements of all the processes in the manufacturing chain, thus multilayer flexible films are produced, mainly by in-line coating, lamination, coextrusion, etc. [3]. Coated plastic films provide many advantages over the standard material—e.g., resistance against high temperatures—which in turn allows their usage in the pasteurization and sterilization processes. Moreover, a coating can improve antibacterial and barrier properties (oxygen and vapor transmission rates), lower the coefficient of friction, allow metallization of the film, etc. Another reason for coating the substrate is bringing its surface free energy closer to the surface tension of inks (reprographic, flexographic, photosensitive, ... ), improving adhesion for lamination or making the surface antistatic. Recently, many different chemical compositions have been used for coating extruded plastic films. The film forming compound of such composition (polymeric dispersion) is usually polyester, polyurethane, or polyacrylate resin and the dispersed phase is often chemically and/or sterically stabilized. The thin film coating process can be substituted with corona treatment, to increase the substrate surface free energy. Such an approach is demanding though, because of high initial cost and process complications (possible electric interference with other equipment in the production line and the maintenance cost). Besides, corona treated polymer surfaces are known to suffer from so called “aging” of the treatment, a gradual reversal of the surface properties towards those of the untreated

Coatings 2017, 7, 234; doi:10.3390/coatings7120234 www.mdpi.com/journal/coatings Coatings 2017, 7, 234 2 of 10 surface. Aging is known to be accelerated by elevated temperature and moisture and occurs with most plasma treatment processes [4–10]. The coating process of typical polymeric foils (polyethylene, , and even polyethylene terephthalate) also meets with difficulties such as poor wetting of the substrate with the dispersion (when the surface tension of the dispersion is relatively high with respect to the surface free energy of the substrate to be coated). The dispersions themselves can perform well, however in industrial conditions (gravure cylinder coating at high rate) their foaming can be a problem, as it can compromise the surface properties of the final thin film on a substrate. Various additives can be used to overcome such problems, though they should not compromise the properties of the initial dispersion, namely its stability. This article gives information on modifying the properties of polymer dispersions that can be used for ultra-thin coating of biaxially-oriented films and the effect of the additive on the properties of the dispersions. Promoted adhesion of the substrate can significantly improve the performance of further conversion steps (metallization, SiOx, and AlOx coating) and lamination with other materials as BOPP, PE, , etc. and help in formulation of new advanced packaging materials [1,11]. Namely in the of BOPET, it is almost impossible to perform metallization of the foil without proper pre-treatment with a suitable thin film.

2. Materials and Methods

2.1. Solution Preparation Co-polyester polymer aqueous dispersion Eastek™ 1200 (Eastman, Kingsport, TN, USA), referred to as Co-PES further in the text, and acrylic polymer aqueous dispersion PRIMALTM AC-261 (Dow, Midland, MI, USA), referred to as ACR further in the text, were examined in the means of particle size (z-average) temperature and pH dependence, zeta potential temperature and pH dependence, surface free tension (SFT), foam forming, and stability. The testing solution was prepared by mixing the concentrated polymeric dispersion with deionized water so that the total solid content was approximately 6 wt.%, followed by homogenization with Teflon® coated magnetic stirrer (VWR International, Randor, PA, USA) for 15 min. Fresh solution was prepared prior to each measurement to avoid gel formation. Modified testing solutions were prepared by adding given amount of the defoaming/wetting agent (2,4,7,9-tetramethyl-4,7-decanediol) (Air Products, Allentown, PA, USA), designated as Agent 1 further in the text, followed by homogenization with a Teflon® coated magnetic stirrer for 5 min.

2.2. Surface Tension Measurement The critical aggregation concentration (CAC) and critical micellar concentration (CMC), of the gemini surfactant in polymeric dispersion were determined from equilibrium surface tension measurements as a function of surfactant concentration at 25 ◦C. The surface tension of the solutions was measured by the Wilhelmy plate method with the K100MK3 automatic tensiometer (Krüss GmbH, Hamburg, Germany), with the platinum plate at 25 ◦C. The tensiometer sample chamber was connected to the thermostat (Termostat CC-308B Pilot Huber, Offenburg, Germany), to maintain the desired temperature. Where appropriate (surface tension dynamics), the solution was stirred prior to each measurement for 30 s with a clean glass rod and the measurement itself was initiated within the 30 s period starting from the point when the stirring had finished. For each solution, at least three separate measurements were performed.

2.3. Particle Size and Zeta Potential Measurement The z-average (mean hydrodynamic diameter), zeta potential, and their temperature and pH dependencies were measured by dynamic light scattering method (DLS) with the Zetasizer Nano ZS (Malvern Instruments, Malvern, UK), coupled with the MPT-2 automatic titrator (Malvern Instruments, Malvern, UK). The scattered light was observed at a 173◦ angle. The disposable folded capillary cell Coatings 2017, 7, 234 3 of 10

(Malvern Instruments, Malvern, UK) was used for the z-average and zeta potential measurement. Each presented value is the average of five measurements with different samples.

2.4. Foam Forming The formation of foam was measured with polymeric dispersions prepared according to the Section 2.1, modified with 0–0.517 wt.% of Agent 1. Then, 50 mL of modified solution was transferred into the 250 mL graduated cylinder covered with Parafilm M® (Bemis Company, Inc., Oshkosh, WI, USA), the cylinder was agitated with both hands for 30 s and the height of the generated foam was recorded in time intervals 0, 5, and 30 min.

2.5. Thin Film Coating The BOPET polymeric substrates (FATRA) with the thickness of 50 and 12 µm were used for the laboratory and industrial scale testing, respectively. The coating of the BOPET polymeric substrates with Co-PES and ACR dispersions was performed with an adjustable Baker Film applicator 3525 (Elcometer Ltd., Manchester, UK) on the laboratory scale and also in the industrial scale in a professional production line with a gravure coating cyllinder (Brückner Group GmbH, Siegsdorf, Germany) at the cooperating company (Fatra A. S., Napajedla, Czech Republic). In the laboratory scale, the substrates were pre-treated with low temperature plasma, prior to dispersion coating. The treatment was performed in the commercial Diener PICO plasma apparatus (Diener electronic, Ebhausen, Germany), with capacitive radiofrequency coupling at the frequency 13.56 MHz and pressure 0.4 mbar. The following procedure was utilized: The substrates were placed inside the plasma chamber, then the vacuum pump was activated and after 5 min the chamber was purged with pure air at 10 sccm for another 5 min to minimze the effect of contaminants possibly present in the chamber. Subsequently, while the air flow was adjusted to 10 sccm, the glow discharge was initiated. The forward power was set to 100 W and the reflected power was kept under 10%. In the industrial process, the pre-treatment was performed with the corona discharge unit built directly in the production line.

2.6. Thin Film Characterization The surface of coated and uncoated substrates were analyzed with a scanning electron microscope (SEM) Phenom Pro (Phenom-World B.V., Eidhoven, The Netherlands) in the environmental mode (Phenom-World). The surface free energy of the coated and uncoated substrates was determined with the non-toxic testing inks (Arcotest GmbH, Moensheim, Germany).

3. Results and Discussion

3.1. Surface Tension of Pure and Modified Dispersions Surface tension of pure ACR and Co-PES dispersions was measured. The data in Figure1 suggest a similar trend for both pure dispersions—decent drop of the surface tension value over the 180 s period. Pure ACR and Co-PES dispersion start at approx. 44 mN/m and 52 mN/m, respectively. Very similar data were obtained for solutions aged for 24 h at 40 ◦C. From the SFT point of view, both dispersions remain stable enough over the typical processing period (preparation of dispersion—short-term storage—coating), even at elevated temperatures. In order to improve wetting properties, the ACR and Co-PES dispersions were modified with the Agent 1. The dependencies of surface tension versus surface active agent concentration show two breaks—at approx. 0.14 wt.% and 0.40 wt.%—which were attributed to the critical aggregation concentration and critical micellar concentration, respectively, of Agent 1 (Figures2 and3). The trend of curves seems to be unaffected by the chemical nature of dispersions and Agent 1 is highly efficient already at low concentrations. The addition of 0.14 wt.% provides reduction of SFT value by 15% and 20% for ACR and Co-PES dispersion, respectively, which is favorable for the thin-film coating process. Coatings 2017, 7, 234 4 of 10

The AgentCoatings 1 2017 (as, a7, representative234 of gemini surfactants) contains two hydrophilic and hydrophobic part Coatings 2017, 7, 234 withinwithinCoatings one single one2017 ,single 7, molecule,234 molecule, thus thus its its surface surface activeactive properties (efficiency (efficiency and and effectivity) effectivity) are remarkably are remarkably betterbetter than than would would be withbe with their their single single hydrophilic/hydrophobichydrophilic/hydrophobic analogues analogues [12–14] [12. –14]. within one single molecule, thus its surface active properties (efficiency and effectivity) are remarkably better than would be with their single hydrophilic/hydrophobic analogues [12–14].

Figure 1. Surface tension time dependence of ACR and Co-PES dispersions; fresh and aged for 24 h at 40 °C. Figure 1. Surface tension time dependence of ACR and Co-PES dispersions; fresh and aged for 24 h at ◦ 40FigureC. 1. Surface tension time dependence of ACR and Co-PES dispersions; fresh and aged for 24 h at 40 °C.

Figure 2. Surface tension of ACR dispersion vs. surface active additive (Agent 1) concentration.

FigureFigure 2. Surface 2. Surface tension tension of ACRof ACR dispersion dispersion vs.vs. surface active active additive additive (Agent (Agent 1) concentration. 1) concentration.

Figure 3. Surface tension of Co-PES dispersion vs. surface active additive (Agent 1) concentration. Figure 3. Surface tension of Co-PES dispersion vs. surface active additive (Agent 1) concentration. 3.2Figure. ParticleFigure 3. Surface Size 3. Surface and tension Zeta tension Potential of Co-PESof Co of-PES Pure dispersion dispersion Dispersions vs.vs. surface active active additive additive (Agent (Agent 1) concentration. 1) concentration.

3.2. ParticleThe measurement Size and Zeta with Potential pure ofdispersions Pure Dispersions at the temperature 25 °C revealed the z-average value 3.2. Particleof (144 Size± 4) nm and and Zeta zeta Potential potential of value Pure Dispersionsof (−20.0 ± 0.6) mV for the ACR dispersion. With the Co-PES The measurement with pure dispersions at the temperature 25 °C revealed the z-average value ◦ Theof (144 measurement ± 4) nm and withzeta potential pure dispersions value of (− at20.0 the ± 0.6) temperature mV for the 25 ACRC dispersion. revealed the With z-average the Co-PES value of 4 (144 ± 4) nm and zeta potential value of (−20.0 ± 0.6) mV for the ACR dispersion. With the Co-PES the values were (15.0 ± 0.1) nm and (−39.0 ± 1.0)4 mV, respectively. These numbers (namely the Coatings 2017, 7, 234 5 of 10 Coatings 2017, 7, 234

the values were (15.0 ± 0.1) nm and (−39.0 ± 1.0) mV, respectively. These numbers (namely the zeta- zeta-potential value) correspond with the practical experience, where the Co-PES dispersion remains potentialCoatings 2017 value), 7, 234 correspond with the practical experience, where the Co-PES dispersion remains ratherrather stable stable during during storage storage and and ultra-thin ultra-thin film film coating, while while the the ACR ACR dispersion dispersion shows shows the the tendencytendencythe values towards towards were gelation (15.0 gelation ± 0.1) of theofnm the stockand stock (− solution39.0 solution ± 1.0) withwith mV, prolonged respectively. storage storage These and/or numbers and/or exposure (namely exposure to theelevated to zeta elevated- temperaturestemperaturespotential in value) the in the coating correspond coating process. process. with the In In the practicalthe FigureFigure experience, 44,, thethe z z-average-average where temperature the temperature Co-PES dependence dispersion dependence remainsof both of both systemssystemsrather is shown. stable is shown. during The The Co-PES storageCo-PES dispersion dispersion and ultra- exhibits thinexhibits film similar coating, stability stability while in the inthe the ACRobserved observed dispersion temperature temperature shows range the range (except(excepttendency the fluctuations the towards fluctuations gelation in thein the of interval theinterval stock from from solution 10 10to to with 2020 ◦°C), prolongedC), as as the the ACR storage ACR dispersion. dispersion. and/or exposure to elevated temperatures in the coating process. In the Figure 4, the z-average temperature dependence of both systems is shown. The Co-PES dispersion exhibits similar stability in the observed temperature range (except the fluctuations in the interval from 10 to 20 °C), as the ACR dispersion.

FigureFigure4. 4. zz-average-average temperature temperature dependence dependence of ACR of dispersion ACR dispersion (squares () squaresand Co-PES) and dispersion Co-PES (triangles dispersion). (triangles). The zeta potential temperature and pH dependencies of both systems are shown in the Figures 5 and 6, respectively.Figure 4. z-average As can temperature be seen, the dependence zeta potential of ACR drops dispersion continuously (squares) withand Co increasing-PES dispersion temperature (triangles for). Thethe ACR zeta dispersion, potential though temperature for the Co and-PES pH dispersion dependencies it reaches ofa plateau both systemsat about 30 are °C.shown The total in the Figureschange5 andThe of6 zeta , zeta respectively. potential potential temperature is As about can and 12 be– pH15 seen, dependencies mV the in the zeta interval of potential both systems 5–50 drops °C are in shown continuously both systems,in the Figure withthoughs 5 and increasing the 6, temperatureabsoluterespectively. forvalue the As for ACRcan the be dispersion,Co seen,-PES the dispersion zeta though potential stays for drops above the Co-PES con −30tinuously mV dispersion at 50 with °C (stable)increasing it reaches while temperature a for plateau the ACR atfor about 30 ◦C.dispersionthe The ACR total dispersion,, it change approaches ofthough zeta −15 for potentialmV the at 50Co °C.-PES is The about dispersion ACR 12–15 dispersion it mVreaches in is theless a plateau intervalstable at elevatedabout 5–50 ◦30C temperatures °C in. bothThe total systems, change of zeta potential is about 12–15 mV in the interval 5–50 °C in both systems, though the thoughand the this absolute should be value considered for the in Co-PES the coating dispersion process stays setup above (for example−30 mV addit ational 50 ◦C cooling (stable) of whilethe for absolute value for the Co-PES dispersion stays above −30 mV at 50 °C (stable) while for the ACR the ACRstock dispersion, , if itthe approaches excess dispersion−15 mVfromat the 50 gravure◦C. The cylinder ACR refluxes dispersion there) is. From less stable the pH at point elevated ofdispersion view, on, theit approaches other hand, −15 the mV ACR at 50dispersion °C. The ACR keeps dispersion its zeta potential is less stable value at down elevated to pH temperatures 7 and then temperatures and this should be considered in the coating process setup (for example additional increasesand this shouldslowly be(IEP considered about pH in4) thewhereas coating the process Co-PES setupdispersion (for example exhibits abruptadditional change cooling already of the at cooling of the stock container, if the excess dispersion from the gravure cylinder refluxes there). pHstock 8 andcontainer, the IEP if is the approached excess dispersion around frompH 7. the gravure cylinder refluxes there). From the pH point Fromof the view, pH on point the ofother view, hand, on the the ACR other dispersion hand, the keeps ACR its dispersion zeta potential keeps value its down zeta potentialto pH 7 and value then down to pHincreases 7 and then slowly increases (IEP about slowly pH 4 (IEP) whereas about the pH Co 4)-PES whereas dispersion the exhibits Co-PES abrupt dispersion change exhibits already at abrupt changepH already 8 and the at IEP pH is 8 approached and the IEP around is approached pH 7. around pH 7.

Figure 5. Zeta potential temperature dependence of ACR dispersion (squares) and Co-PES dispersion (triangles).

5 FigureFigure 5.5. ZetaZeta potential potential temperature temperature dependence dependence of ACR dispersion of ACR (squares dispersion) and Co-PES (squares dispersion) and (triangles Co-PES). dispersion (triangles). 5 Coatings 2017, 7, 234

Coatings 2017, 7, 234 6 of 10 Coatings 2017, 7, 234

FigureFigureFigure 6. 6.ZetaZeta 6. Zetapotential potential potential pH pH pH dependence dependence dependence of of of ACR ACRACR dispersiondispersion ( (squares(squaressquares) and)) and and Co Co-PES Co-PES-PES dispersion dispersion dispersion (triangles (triangles (triangles). ). ).

3.3.3.3. Particle 3.3. Particle Size Sizeand andZeta Zeta Potential Potential of of ModifiedModified Modified DispersionsDispersions The concentration and chemical structure of used defoaming/wetting agent can have a The concentration concentration and and chemical chemical structure structure of used of defoaming/wetting used defoaming/wetting agent can agent have can a significant have a significantsignificant influ influenceence on on particle particle size size and and its its distribution, distribution, viscosity, viscosity, dispersion dispersion stability stability, or film, or film influenceformation. on particle Therefore, size and the itsparticle distribution, size and viscosity,zeta potential dispersion measurements stability, were or filmperformed. formation. Based Therefore, on formation. Therefore, the particle size and zeta potential measurements were performed. Based on the particlethe SFT size measurements and zeta potential of polymeric measurements dispersions were with performed. various concentrations Based on the SFTof surface measurements active of the SFT measurements of polymeric dispersions with various concentrations of surface active polymericadditive, dispersions the optimal with concentration various concentrations of Agent 1 was of surfaceselected active for the additive, ACR and the Co optimal-PES dispersions concentration ofadditive, Agentmodification, the 1 was optimal selected as described concentration for in the the ACRSection of Agent and 3.1. Based Co-PES 1 was on those dispersionsselected measurements, for modification,the ACR the optimaland Co as concentration- describedPES dispersions in the Sectionmodification,was 3.1 defined. Based as described as on0.14 those wt.% in measurements,ofthe the Section Agent 3.1.1. Then, Based the the optimal on same those dependencies concentration measurements, as for was thepure defined optimal dispersions as concentration 0.14 were wt.% of thewas Agentdefinedmeasured. 1. as Then, The0.14 z thewt.%-average same of thetemperature dependencies Agent 1. dependence Then, as forthe pure same(Figure dispersions dependencies 7) is similar were to as that measured.for of pure pure dispersions dispersions, The z-average were temperaturemeasured.except The that dependence zthere-average are no temperature (Figure fluctuations7) is at similardependence low temperatures, to that (Figure of pure suggesting 7) dispersions,is similar that the to stability exceptthat of thatofpure the there dispersions,system are no could even be improved by modification with the surface active agent. fluctuationsexcept that there at low are temperatures, no fluctuations suggesting at low temperatures, that the stability suggesting of the system that the could stability even of be the improved system could evenFrom be improved the zeta potential by modification point of view, with the thesituation surface is different, active agent. as in Figure 8. The zeta potential by modificationvalues of the with ACR the and surface Co-PES active dispersions agent. drop almost to zero at temperatures approaching 40 °C. From the zeta potential point of view, the situation is different, as in Figure 8. The zeta potential FromThe ACR the dispersion zeta potential keeps point the −10 of view,mV value the by situation the temperature is different, of about as in 30 Figure °C, while8. The with zeta the potentialCo- values of the ACR and Co-PES dispersions drop almost to zero at temperatures approaching 40 °C.◦ valuesPES of dispersion the ACR anda rapid Co-PES drop of dispersions zeta potential drop does almost not occur to zero until at about temperatures 37 °C. These approaching observations 40 C. The ACRACRshould dispersiondispersion be kept keeps inkeeps mind the the when− −1010 mV designingmV value value by an by the industrial the temperature temperature process of involvingof about about 30 such◦30C, °C, while dispersions while with with the with the Co-PES Co- dispersionPES dispersiondefoaming/wetting a rapid a rapid drop agent ofdrop zeta and of potential properzeta potential measures does not doesshould occur not be until occurapplied about until (e.g., 37 aboutcooling◦C. These 37 the °C. observationsstock These dispersion, observations should as be keptshould inmentioned mind be kept when above). indesigning mind The pH when anstability industrial designing at 25 °C, process anon the industrial involving other hand process such, remains dispersions involving almost unchanged, with such defoaming/wetting dispersions Figure 9. with agentdefoaming/wetting and proper measures agent and should proper be measures applied (e.g., should cooling be applied the stock (e.g., dispersion, cooling the as stock mentioned dispersion, above). as Thementioned pH stability above). at The 25 ◦ C,pH on stability the other at 25 hand, °C, on remains the other almost hand unchanged,, remains almost Figure unchanged,9. Figure 9.

Figure 7. z-average temperature dependence of modified ACR dispersion (squares) and Co-PES dispersion (triangles).

6 FigureFigure 7. z-average 7. z-average temperature temperature dependence dependence of modified of ACR modified dispersion ACR (squares dispersion) and Co (squares-PES dispersion) and Co-PES (triangles). dispersion (triangles).

6 Coatings 2017, 7, 234 7 of 10 Coatings 2017,, 7,7, 234

FigureFigure 8.8. ZetaZeta potentialpotential temperaturetemperature dependencedependence ofof modifiedmodified ACRACR dispersiondispersion ((squaressquares)) andand Co-PESCo-PES dispersiondispersion ((trianglestriangles).).

FigureFigure 9.. Zeta 9. potentialZeta potential pH dependence pH dependence of modified of ACR modified dispersion ACR (squares dispersion) and Co (squares-PES dispersion) and Co-PES (triangles). dispersion (triangles). 3.4.. Foam Forming 3.4. FoamFoaming Forming of both polymeric dispersions was determined as a function of defoaming/wetting agentFoaming concentration of both and polymeric time. The dispersions results for ACR was determineddispersion (Figure as a function 10a) show of defoaming/wetting higher value of the agentfoam concentration height of pure and ACR time. dispersion, The results compared for ACR dispersion to pure Co (Figure-PES 10 dispersioa) shown higher (Figure value 10b). of The the foamfoaminess height of of ACR pure is ACR 2.6 times dispersion, higher compared than for Co to- purePES. Co-PESThe foam dispersion height of (Figure pure ACR 10b). dispersion The foaminess was ofaround ACR (108.0 is 2.6 ± times 4.0) mm. higher The than first addition for Co-PES. of Agent The foam1 caused height lowering of pure of foam ACR height dispersion to (40.0 was ± 0.5) around mm, (108.0i.e.,, more± 4.0) than mm. 60 The%. As first can addition be seen of Agent in the 1 Figure caused 10a, lowering the foaming of foam height of ACR to decreases (40.0 ± 0.5) markedly mm, i.e., morebetween than 0. 60%.02 and As 0.08can bewt.% seen concentration in the Figure 10 of a, the the Agent foaming 1 (close of ACR todecreases the CAC markedly value). When between the 0.02concentration and 0.08 wt.% of Agent concentration 1 is close to of the the CMC, Agent the 1 (close foam to height the CAC reaches value). a plateau. When the concentration of AgentSimilar 1 is close results to the were CMC, obs theerved foam forheight Co reaches-PES, Figure a plateau. 10b. The foam height of pure Co-PES dispersionSimilar was results around were (41.0 observed ± 3.0) formm. Co-PES, The first Figure addition 10b. Theof Agent foam 1 height caused of lowering pure Co-PES of foam dispersion height wasto (35.0 around ± 0.1) (41.0 mm,± i.e.3.0),, about mm. 15 The%. When first additionthe concentration of Agent of 1 causedAgent 1 lowering exceeded of the foam Agent height 1 CAC to (35.0(0.14 ±wt.%0.1)), again mm, i.e.,the plateau about 15%. was reached. When the concentration of Agent 1 exceeded the Agent 1 CAC (0.14These wt.%), results again theindicate plateau that was Agent reached. 1 can work as defoamer for both ACR and Co-PES dispersions, thoughThese the resultsdirect impact indicate will that be Agent more 1 profound can work in as the defoamer acrylic fordispersion. both ACR and Co-PES dispersions, though the direct impact will be more profound in the acrylic dispersion.

7 Coatings 2017, 7, 234 Coatings 2017, 7, 234 8 of 10 Coatings 2017, 7, 234

Figure 10. Foam height time dependence of the ACR dispersion (a), and Co-PES dispersion (b) with differentFigure 10. concentrationsFoam height timeof the dependence Agent 1. of the ACR dispersion (a), and Co-PES dispersion (b) with Figure 10. Foam height time dependence of the ACR dispersion (a), and Co-PES dispersion (b) with different concentrations of the Agent 1. different concentrations of the Agent 1. 3.5. Thin Film Coating and Characterization 3.5. Thin Film Coating and Characterization 3.5. ThinThe Film coating Coating in the and laboratoryCharacterization scale was performed solely for the purpose of determination The coating in the laboratory scale was performed solely for the purpose of determination whether whetherThe the coating dispersions in the will laboratory wet the scalesubstrate was and performed thus would solely be forsuitable the purposefor the industrial of determin process.ation the dispersions will wet the substrate and thus would be suitable for the industrial process. Figure 11 Figurewhether 11 the displays dispersions the surface will wet of thevirgin substrate and ACR and dispersionthus would coated be suitable substrates. for the As industrial can be seen process., the displays the surface of virgin and ACR dispersion coated substrates. As can be seen, the manual coating manualFigure 11 coating displays provides the surface a relatively of virgin thick and (upACR to dispersion 10 microns) coated layer. substrates. In the case As ofcan the be industrialseen, the provides a relatively thick (up to 10 microns) layer. In the case of the industrial process, the layer is so processmanual, coatingthe layer provides is so thin a relatively (partly because thick (up of to the 10 thin microns) liquid layer. layer In deposited the case with of the the industrial gravure thin (partly because of the thin liquid layer deposited with the gravure cyllinder and also due to the cyllinderprocess, theand layer also due is so to thin the (partlybiaxial becausestretching of— thethe thindispersion liquid is layer deposited deposited before with the the transverse gravure biaxial stretching—the dispersion is deposited before the transverse direction orientation takes place), directioncyllinder and orientation also due takes to the place), biaxial it canstretching hardly— bethe analyzed dispersion with is deposited any other before method the than transverse surface it can hardly be analyzed with any other method than surface tension measurement (either inks or tensiondirection measurement orientation takes(either place), inks or it sessil can e hardly drop method). be analyzed Testing with the any substrate other methodwith ink thanrevealed surface the sessile drop method). Testing the substrate with ink−2 revealed the surface free energy of approx. 42, 40, surfacetension freemeasurement energy of (eitherapprox. inks 42, or40 sessile, and 62 drop mJ· mmethod). for virgin Testing BOPET, the substrate ACR coated with BOPET, ink revealed and Co the- and 62 mJ·m−2 for virgin BOPET, ACR coated BOPET, and Co-PES coated BOPET, respectively. PESsurface coated free BOPET, energy ofrespectively. approx. 42, 40, and 62 mJ·m−2 for virgin BOPET, ACR coated BOPET, and Co- PES coated BOPET, respectively.

(a) (b)

(a) (b) FigureFigure 11. SEMSEM images images of of the the virgin virgin ( a)) and and laboratory laboratory ACR coated ( b) substrates. Figure 11. SEM images of the virgin (a) and laboratory ACR coated (b) substrates. 4. Conclusions 4. Conclusions 4. ConclusionThe effects of the gemini surfactant based defoaming/wetting agent on the properties of aqueous The effect of the gemini surfactant based defoaming/wetting agent on the properties of aqueous acrylic and polyester dispersions was studied. The dispersions were examined from the perspective acrylicThe and effect polyester of the gemini dispersions surfactant was studied. based defoaming/wetting The dispersions were agent examined on the properties from the perspectiveof aqueous of surface tension, particle size, zeta-potential, and foaming. Pure ACR and Co-PES dispersion were acrylicof surface and tension, polyester particle dispersions size, zeta-potential, was studied. The and dispersions foaming. Pure were ACR examined and Co-PES from dispersionthe perspective were of surface tension, particle size, zeta-potential, and foaming. Pure ACR and Co-PES dispersion were 8 8 Coatings 2017, 7, 234 9 of 10 found to have the SFT about 44 mN/m and 52 mN/m, respectively and kept this value relatively well when aged for 24 h at both 25 ◦C and 40 ◦C. The addition of 0.14 wt.% of Agent 1 reduces the SFT value by 15% and 20% for ACR and Co-PES dispersion, respectively, which is favorable for the thin-film coating process. Taking the above-mentioned into account, the modified dispersions will provide better wetting of a substrate without the need for its surface treatment and will withstand short-term storing even at mildly elevated temperatures. On the other hand, according to the data obtained from the z-average and zeta-potential measurements, prolonged storage and/or exposure to elevated temperatures in the coating process can possibly bring problems with gelation in the case of ACR dispersion. The Co-PES dispersion is expected to perform better in such conditions. Besides better wetting, the addition of the Agent 1 is defoaming the dispersions significantly, though its effect is more obvious in the ACR dispersion than in the Co-PES dispersion (initial foam reduction 60% and 15%, respectively). Such modified dispersion can be utilized for Co-PES ultrathin coating of plastic film (especially BOPET) used for packaging, to improve their processability, printability, metallization, and barrier properties.

Acknowledgments: This article was written with the support of the Operational Program Research and Development for Innovations co-funded by the European Regional Development Fund (ERDF) and the national budget of the Czech Republic, within the framework of the project Center of Polymer Systems (reg. number: CZ.1.05/2.1.00/03.0111). This work was also supported by the Ministry of Education, Youth and Sports of the Czech Republic—Program NPU I (LO1504), the European Regional Development Fund (Grant No. CZ.1.05/2.1.00/19.0409). The research was performed in collaboration with the industrial partner Fatra, a. s., Czech Republic. Author Contributions: Petr Smolka conceived and designed the experiments and wrote the ; Lenka Musilová performed the experiments and performed data evaluation; Aleš Mráˇcekperformed data evaluation and literature research; Tomáš Sedláˇcekcontributed the materials and analytical apparatus. Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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