Abstract. 1. Introduction. Ana Cristina Carranquinha 1, Maria España 2, Pedro T. Gomes 1, J. C. Bordado 1

Transcription

1 Implementation of New Quantification Methods for Waterborne Paints: the determination of the contents of extenders and pigments, Ana Cristina Carranquinha 1, Maria España 2, Pedro T. Gomes 1, J. C. Bordado 1 1 DEQ, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, nº 1, Lisboa, Portugal; 2 Resiquímica Resinas Químicas, S. A., Rua Francisco Lyon de Castro, nº 28, Mem Martins, Portugal Abstract The objective of this work consists on the deformulation of waterborne paints, aiming to create alternatives to the existing methods in Resiquímica, for determination of the binder content and the extenders and pigments content and also, to develop, within Resiquímica, a new method for the determination of titanium dioxide (TiO 2 ) content. A method based on centrifugation was studied for the separation of the binder from pigments and extenders. For the determination of TiO 2 content, it was applied a method which uses aluminum as a reducing agent. The methods were tested for a set of waterborne paints with known formulation. It has been concluded that, in the centrifugation method, it is important the type of binder used, which conducted to a way of distinction from acrylic and styrene- -acrylic paints from VINA-VeoVa paints. In general, this method conducts to more accurate results when compared with the existing methods. For the determination of TiO 2 content, the best results are obtained in the application of the method to binderless solid residues. Keywords: waterborne paint, binder, extenders, pigments, centrifugation, titanium dioxide. 1. Introduction A paint comprises constituents such as solvent, binder, pigments and additives. The solvents are, generally, liquid compounds at atmospheric temperature and pressure, which can dissolve other substances without changing their chemical properties. An ideal solvent must be volatile enough so it could evaporate rapidly, allowing a quick filmification, but should not be extremely volatile so that the film stays uniform (Stoye et al. 1998). The binder is a fluid in continuous phase that makes the connection with the pigments, enabling the paint cohesion. It is the binder that determines the coatings properties, such as mechanical properties, elasticity, chemical resistance, alkalinity resistance, weathering and UV radiation resistance, gloss and tacking. Therefore, the binder must have adherence to guarantee that the paint coating does not drip, as well as hardness enough to support mechanical friction, but should also give flexibility to the coating (Marrion 1994). In waterborne paints, the binders can be based on acrylic copolymers, which are applied in the manufacture of primers, exterior coatings and enamel paints. These present a high UV, water and alkalinity resistance (Resiquímica 2010). The waterborne paints can also be based on styrene-acrylic copolymers, their main application being now the production of enamel and interior matte paints. They give rise to paints that are good water absorbers and are alkalinity and wet scrub resistant, but present low resistance to UV radiation, resulting in the yellowing of a coating (Resiquímica 2010). Finally, the binder of a waterborne paint can be based on vinyl copolymers, normally constituted of monomers such as vinyl acetate (VINA) or the vinyl ester of versatic acid (VeoVa). The VINA- -VeoVa copolymers are characterized for their high resistance to alkalinity and UV radiation, and their main applications are in matte, satin and exterior paints (Resiquímica 2010). Pigments are solid particles dispersed in paints that influence color, opacity, gloss, durability, mechanical resistance, UV and corrosion protection. To achieve these properties, the pigments must be uniformly placed in the binder. The white pigments must have a high refraction index, be stable and colorless, with an appropriate particle size (ca. 20 nm) (Talbert 2008). Titanium dioxide (TiO 2 ) is the most used pigment and it exists in three different crystalline structures: rutile, anatase and brookite, although only the first two are commercialized. Rutile is photochemically inert, meaning that the pigments protect the coatings from UV degradation. In the other hand, anatase is used in interior applications because it presents a more limpid white color. TiO 2 in the rutile form is the most used pigment in the coatings

2 Implementation of New Quantification Methods for Waterborne Paints: the determination of the contents of extenders and pigments, industry and and is embodied in almost every paint, to adjust the final color and opacity (Stoye 1998, Talbert 2008, Resiquímica 2001). Extenders are also called as auxiliary pigments, and they are characterized by having a refraction index lower than 1.7. Generally, they are inorganic substances with different compositions and, consequently, with different physical properties. The particle size is lower than that of pigments and they are present in various forms such as calcium carbonate, talc, kaolin, mica, baryte, dolomite and blanc fixe. They are added to grant the intended gloss, easy sanding, avoid the pigment sedimentation and, for economical reasons, to spatially fill in the paint, with the desired consistency. Its addition affects the impact, swelling and water vapor permeability resistance. They can enhance the coating adhesion to the substrate and the outwear resistance. Calcium carbonate (CaCO 3 ) is the most appellative extender due to its reduced price, good wetting, opacity increase and, consequently, a better coating hiding power (Resiquímica 2001, Goldschmidt et al. 2003). Additives are used to avoid coating defects, such as foam bubbles, settling, etc., or to achieve special properties, such as UV stability, etc., which cannot be achieved by the paints base components. Additives are basic constituents in all kind of paints formulations, and its right choice is an important step in formulation development. They can be classified as defoamers, wetting and dispersing agents, ph regulators and preservatives (Resiquímica 2001, Scholz 2006). In terms of its finishing, the main types of waterborne paints are: matte, satin and high gloss. The main differences in their composition rest in the binder, pigments and extenders quantity. A higher gloss paint will have more binder and pigments but, in other hand, it will have less extenders (Resiquímica 2001). The waterborne paints characteristics and performance improvement is a growing requirement, which begins with the formulation. Due to the competitiveness among coatings industries, the characterization of paints base constituents becomes indispensable, namely regarding to the binder, extenders and pigments. Nowadays, there are methods suitable for the characterization of a waterborne paint. However, there is a gap concerning the determination of the pigments content, namely TiO 2. Therefore, the goal of this study consists in the development of new methods of waterborne paint deformulation, in order to separate and quantify the binder, extenders and pigments contents. 2. Experimental In a first stage, the separation of the binder from extenders and pigments takes place by centrifugation, using a Minor brand centrifuge. In a second stage, it is applied to the solid residue, corresponding to the pigments and extenders, the titanium dioxide quantification method. All the experiments were conducted at room temperature, and the titanium dioxide quantification method was carried out in a fume cupboard Centrifugation Method This method, used for the quantification of binder, and extenders and pigments, consists in a total of five extractions by centrifugations of a certain amount of paint analyzed (2 g). The qualitative identification of the type of binder is achieved in an empirical way after three centrifugation extractions. After two centrifugations with 10 ml of distilled water, a third centrifugation with a 1:1 toluene/acetone mixture (7 ml) allows to obtain an ideal phase separation according to the type of binder used in the waterborne paint. Thereby, it is possible to identify whether the emulsion is acrylic or styrene-acrylic (Figure 1(a)) when a good phase separation is obtained, or a VINA- -VeoVa (Figure 1(b)) one when the total separation between the solid and liquid phases is not achieved. (a) (b) Figure 1 Differentiation between (a) acrylic or styrene- -acrylic and (b) VINA-VeoVa emulsions. To differentiate between acrylic and styrene- -acrylic binders, a method already existing in Resiquímica is applied, based on UV exposure of a coating film, whereby if the coating yellowing occurs the binder will be considered as styrene- -acrylic, otherwise it will be acrylic. Hence, for waterborne acrylic or styrene- -acrylic paints, the distilled water solvent used in the first two centrifugations promotes a paint dilution and, therefore, a decrease of viscosity. Thus, the pigments and extenders particles settling in the bottom of the centrifuge tube is facilitated. After the first two centrifugations, two more take place using a 1:1 toluene/acetone mixture. Toluene and acetone destroy the emulsion through the dissolution/extraction of the copolymers, promoting the settling of pigments and extenders solid particles (Figure 2). A fifth and last centrifugation is carried out using diethyl ether (7 ml) with the purpose of cleansing the residue by 2/6

3 Implementation of New Quantification Methods for Waterborne Paints: the determination of the contents of extenders and pigments, extracting the residual solvent. The liquid fractions from each five centrifugations are gathered in one erlenmeyer and, simultaneously, the corresponding solid residue collected from the centrifuge tubes, are put to rest in a stove at 105 C, until constant mass. (a) (b) (c) Figure 2 Phase separation after the fourth extraction/centrifugation for (a) acrylic, (b) styrene-acrylic and (c) VINA-VeoVa paints Regarding the VINA-VeoVa waterborne paints, the same procedure is applied, however, in the first four centrifugations a 1:1:1 toluene/acetone/ethanol mixture (7 ml) is used, in order to obtain the desired phase separation Titanium Dioxide Quantification The basis for this method is an oxidation- -reduction reaction, in which aluminum is used as a titanium reducer agent, allowing its determination through a titration process. The titrant is a solution of ammonium iron(iii) sulfate [FeNH 4 (SO 4 ) 2.12H 2 O] 0.06 M, in which the Fe 3+ ion will react with the analyte. The sample to be analyzed (0.2 g) is collected from the solid residues resulting from the centrifugation method, and attacked with an acid solution (20 ml of H 2 SO 4 and 20 ml of HCl) containing ammonium sulfate (7 to 8 g), to which is then added distilled water (120 ml) and metallic aluminum foil (1 g). Being the Al 3+ a strong reducing agent, in comparison with the other ions in solution, this will promote the reduction of all the other species, especially the reduction of Ti 4+ to Ti 3+. In order that the reaction between Ti 4+ and aluminum originates exclusively Ti 3+, the solution must be excluded from atmospheric oxygen, so that the formed Ti 3+ does not re-oxidise, leading to wrong results. Therefore, an inert atmosphere is created using an U tube submerged in a sodium bicarbonate saturated solution (Figure 3). Figure 3 Experimental setup for the reduction of Ti 4+ with metallic aluminum foil (erlenmeyer on the left) under an inert atmosphere. After the reactive dissolution of aluminun (exothermic reaction), the resulting solution containing the dissolved aluminum is heated up to ebullition during five minutes, to complete the aluminum oxidation. The solution is then cooled to 60 C, the U tube removed, and the titration must been performed immediately in order to avoid titanium oxidation with atmospheric oxygen. A solution of ammonium thiocyanate (NH 4 SCN) (2 ml, 3.22 M) is used as a redox indicator, but since it decomposes at temperatures above 70 C (Rahm 1952), the indicator is added just immediately before the titration. When no more Ti 3+ exists in solution, only existing Ti 4+, the excess of Fe 3+ of the next drop will be complexed by the thiocyanate ion (from the ammonium thiocyanate indicator), originating the iron(iii) [Fe(SCN) 4 (OH 2 )] - complex, which gives rise to a strong orange color, indicating the end of titration Calcination Method The method of determination of binder and extenders (composed by calcium carbonate), already in use in Resiquímica, consists in the calcination of the paint sample (1 g), by heating up to temperatures of 450 C and of 950 C, respectively. Knowing the solid residue content obtained at 105 C, it is possible to estimate the quantity of pigments and extenders through the subtraction of binder content from the solid residues content. 3. Results and discussion In order to verify the applicability of the developed methods, 14 different paints with different characteristics and known formulations were tested. In comparison, the calcination method was also applied to the same paint samples. The method of determination of the content of titanium dioxide was applied to the solid residues obtained 3/6

4 % Binder content average error % Pigments and Extenders content average error Implementation of New Quantification Methods for Waterborne Paints: the determination of the contents of extenders and pigments, in the centrifugation method and also to those obtained in the calcination method Binder Content For acrylic paints, the centrifugation method leads to values that are more approximated to the formulation ones in comparison with the calcinations method results (Figure 4). It can be concluded that, in the calcination method, an excess error is made because at 450 C it is very likely that, besides the binder, other substances may be eliminated. For styrene-acrylic paints, the centrifugation method leads to values more approximated to the formulation ones. However, the best results are obtained for water based enamels, lacking extenders, and also for a particular situation of a paint without pigments in its constitution. Regarding the other analyzed paints, a higher relative error is obtained, which could mean that the paint has some additive(s) in its composition that makes more difficult the separation of the binder from extenders and pigments, leading to higher values than the formulation ones. For VINA-VeoVa paints, the centrifugation method results are, in general, more accurate (in relation to the formulation values), than those obtained by the calcination method Acrylic Binder Figure 4 Binder content average relative error (in relation to the formulation values). Generally, taking the formulation values as a reference, the centrifugation method results are more accurate in comparison with the calcination method results, with 6.0 % average relative error for acrylic paints, 22.8 % for styrene-acrylic paints and 12.5 % for VINA-VeoVa paints. The centrifugation method performance is better for acrylic paints and worse for styrene-acrylic paints Pigments and Extenders Content Calcination Centrifugation Styrene-Acrylic VINA-VeoVa For acrylic paints, the centrifugation method results for the content in pigments and extenders leads, in general, to values more accurate than the calcination method, but only slightly far from the formulation values (Figure 5). For styrene-acrylic paints there is a small difference between the values of the two methods. For VINA-VeoVa paints, the calcination method leads to more accurate results. The analysis of the results of the centrifugation method indicates that part of the binder is retained within the pigments and extenders fraction, leading to higher values than those of the formulation and, consequently, to higher relative errors Acrylic Pigments and Extenders Figure 5 Pigments and Extenders content average relative error (in relation to the formulation values). Therefore, the application of the centrifugation method to determine the content in pigments and extenders from VINA-VeoVa paints, leads to higher relative errors than those of the calcination method (although reasonably low in absolute value), with an average relative error of 5.5 %, whilst for styrene-acrylic paints the performance is better when compared with the calcination method, with an average relative error of 1.6 %. The average relative error of 3.1 % for acrylic paints is slightly higher than the one obtained for the calcination method Titanium Dioxide Content Calcination Centrifugation Styrene-Acrylic VINA-VeoVa A water based enamel, lacking extenders, was tested, leading to the same average relative error of 2.9 %, either the method for the determination of titanium dioxide content is applied in the case of the solid residues resulting from the centrifugation method or from the calcination method. This low error can be due to the reduction of impurities present in the aluminum foil (Rahm 1952), or due to the presence of other substances in the water based enamel that may reduce in the presence of aluminum. For styrene-acrylic paints, the determination of the content in titanium dioxide in the solid residues applied in the case of the centrifugation method led to results that are close to the formulation ones (Figure 6). For a paint lacking of pigments, it was determined 0.1 % of titanium dioxide content, which double-checks the method. When the calci- 4/6

5 % TiO 2 content average error Implementation of New Quantification Methods for Waterborne Paints: the determination of the contents of extenders and pigments, nation method is used, the TiO 2 content obtained is significantly lower in comparison to the formulation values, suggesting that there may be losses of titanium during the calcination process. For VINA-VeoVa paints, when the calcination method is used, the TiO 2 content obtained is significantly lower in comparison to the formulation values, which suggests, once again, losses of titanium during this process. The use of the solid residues centrifugation method also leads to more accurate results, closer to the ones of the formulation Calcination Centrifugation Figure 6 Titanium Dioxide content average relative error (in relation to the formulation values). Thereby, it is visible that the use of solid residues from the centrifugation method conducts to more accurate TiO 2 content results, in relation to those obtained from the calcination method. For VINA-VeoVa paints, the average relative error obtained in the centrifugation method is 19.2 %, which is nearly twice the error obtained for acrylic paints, of 9.1 %, being the best results achieved for styrene-acrylic paints, with an average relative error of 6.1 %. 4. Conclusions TiO 2 Acrylic Styrene-Acrylic VINA-VeoVa For the determination of the contents of binder, pigments and extenders, a method was developed based on a visual observation of the paint phases separation after centrifugation with the use of a solvent that facilitates the above mentioned separation. The supernatant liquid phase resulting from the centrifugation includes the binder content, while the solid phase corresponds to the pigments and extenders content. The solvent used in the phase separation differs according to the type of emulsion present in the paint under analysis. After the third centrifugation it is possible to distinguish visually between acrylic/styrene-acrylic and VINA-VeoVa paints. Applying the existing method of UV irradiation, it is possible to distinguish between acrylic and styrene-acrylic paints. For acrylic paints, the centrifugation method leads to binder contents with lower relative errors, in relation to the formulation, in comparison with the calcination method, providing to Resiquímica a new and more accurate alternative for binder content determination. In general, the best results for the pigments and extenders content in acrylic paints come from the calcination method, although the centrifugation method results are not too different from the formulation values. For VINA-VeoVa paints, the binder content results are, generally, closer to the formulation values than the calcination method ones. In some cases the relative error obtained was more significant, which could be due to the difficulty in the separation of the binder from the pigments and extenders of the emulsion, likely indicating the retention of binder in the centrifugated solid residue. This assumption is verified by the content of pigments and extenders obtained, which revealed to be higher than that resulting from the formulation, thus indicating the binder presence. The centrifugation method best results for the binder content are obtained for acrylic paints. The best results for the pigments and extenders content are obtained for styrene-acrylic paints, but, when regarding the binder content, the results obtained are far from the formulation ones, when compared with the other emulsions. For VINA-Veova paints, the binder content obtained by the centrifugation method is more accurate than the obtained by the calcination method. However, the results for the pigments and extenders contents arising from the centrifugation method are farther apart than in the other emulsions, although relatively small absolute deviations are observed (maximum of ca. 10 %). This result could have been improved by increasing the centrifugation rotational speed, but due to the technical limitations of the equipment, this was not possible. For paints based on this type of emulsion, the calcination method is more accurate. Concerning the TiO 2 content determination method, the solid residues coming directly from a coating film cannot be used for that purpose. The test sample must come from the solid residues obtained in the centrifugation method or in the calcination method, in order to decrease the presence of other substances that might be reduced by aluminum, in acid medium. For acrylic paints, the TiO 2 content determination by the solid residues of the centrifugation method conducts to more accurate results in relation to the formulation values than those of the calcination method, although there is not a substantial difference. On the other hand, for styrene-acrylic paints, while the use of the centrifugation method leads to an average relative error of 5 % in relation to the formulation, the calcination method provides an average error of approximately 45 %, pointing 5/6

6 Implementation of New Quantification Methods for Waterborne Paints: the determination of the contents of extenders and pigments, out the advantage of the application of the centrifugation method. For VINA-VeoVa paints, the TiO 2 content obtained from the centrifugation method is higher than that of the formulation, which once more proves the existence of binder traces in the centrifugation solid residues that can be reduced simultaneously with Ti 4+, leading to higher errors. For paints composed by this type of emulsion, the TiO 2 content determination method is more accurate when applied to the solid residues of calcination method. A paint lacking pigments was tested, in a blank test, in order to verify the method applicability. In this particular case, the result obtained was 0.1 % TiO 2 content, regardless of the solid residue was coming from centrifugation or calcination method. Therefore, this test proved the reliability of the results obtained and, thus, the importance of the implementation of this new method in the characterization of waterborne paints in Resiquímica. Generally, it is also verified the importance of the centrifugation method in order to obtain more accurate results (more approximated to formulation values) in comparison with the calcination method, since in the latter one there is the possibility of loss of other substances besides the binder during the procedure. However, if the time available is short, it is still possible to use the calcination method, although it is sure that the corresponding accuracy will be lower. Therefore, we can conclude that the goal to develop for Resiquímica a new alternative method for the determination of the contents in binder and pigments and extenders was achieved, as well as the development of a new method for the determination of the contents in pigments, in the present case TiO 2. Resiquímica, Tintas Plásticas para Construção Civil, Goldschmidt, Artur; Streitberger, Hans-J; BASF Handbook of Coating Technology, Vincentz, Scholz, Wilfried; Coatings Additives: Small amount Large effect, BYK-Chemie GmbH, Rahm, Joseph A.; Determination of Titanium in Pigments and Ores Titrimetric Method, Titanium Division, National Lead Co., St. Louis, Mo., Acknowledgements This paper reflects the work carried out for over six months at Resiquímica Resinas Químicas, S. A., but more than that, it reflects a new set of experiences and knowledge, that I will never forget, enriching my personal and professional life, and I thank all the people who somehow have contributed to this amazing experience. References Stoye, Dieter; Freitag, Werner, Paints, Coatings and Solvents, 2ª Edition, Wiley-VCH, Marrion, A. R.; The Chemistry and Physics of Coatings, Royal Society of Chemistry, Cambridge, Resiquímica, Produtos Resiquímica, Talbert, Roger; Paint Technology Handbook, CRC Press, /6