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ethylene emulsions and paints
Progress in Organic Coatings 35 (1999) 69–77
Influence of nonionic emulsifiers on the properties of vinyl acetate/VeoVa10 and vinyl acetate/ethylene emulsions and paints Carsten Heldmann*, R. Ivan Cabrera, Bernhard Momper, Rolf Kuropka, Klaus Zimmerschied Clariant GmbH, G832, 65926 Frankfurt am Main, Germany Received 15 July 1998; accepted 10 December 1998
Abstract This paper presents results on the influence of the nonionic surfactant on the properties of vinyl acetate/VeoVa10 and vinyl acetate/ ethylene emulsions and paints made thereof. Emulsions were prepared in which the concentration of the nonionic surfactant and its degree of ethoxylation were varied. An increase of the nonionic emulsifier concentration and of the length of the ethylene oxide chain leads to dispersions with smaller particles and higher viscosities. Using these emulsions as binders in high pigmented paints, it was observed that the pigment binding capacity of the interior paints goes through a maximum which is located at a emulsifier concentration of about 2–3% and at a degree of ethoxylation of 17–28 mol ethylene oxide (EO). In (semi-) gloss paints, the gloss of the paint films improves with an increase of the emulsifier concentration and reaches a constant value at around 4% or a degree of ethoxylation of ca. 17 EO-moieties per molecule. The blocking of the films shows a drastic increase at a concentration above 4% and at a chain length of greater than 17 EO-moieties. 1999 Elsevier Science S.A. All rights reserved. Keywords: Nonionic surfactant; Vinyl acetate ethylene emulsion; Vinyl acetate VeoVa10 emulsion; Paint; Coating
1. Introduction Copolymer dispersions of vinyl acetate (VA) are a widely used type of synthetic latexes in the coatings industry. They comprise several different types including copolymers with ethylene, versatic acid 10 esters (VeoVa10), acrylates, maleinates, etc. each with its own field of application. For many of these applications very distinct requirements have to be met. This refers to the colloidal state of emulsion (particle size distribution, shear stability, viscosity etc.) as well as the properties of the final polymer film. Generally, the colloidal and polymer properties are determined by monomer composition, crosslinking and molecular weight of the polymer, surfactant system, initiators and by the polymerisation process [1]. All of these topics have been a matter of intensive research in our laboratories and many of the results including influence of monomer varia-
* Corresponding author.
0300-9440/99/$ - see front matter PII: S03 00-9440(99)000 25-9
tion, type of crosslinking agent and of stabilising system, have already been published [2,3]. In particular, the influence of stabilisers on the properties of polymer dispersions and/or paints is often a matter of major concern since polymer dispersions used as binders for interior paints have to fulfil strong requirements concerning mechanical (e.g. shear) stability. It has been shown that combinations of protective colloids with nonionic emulsifiers function satisfactorily with regard to these requirements. Very often, ethoxylated fatty alcohols are employed as nonionic emulsifiers. The mechanism by which they work is based on steric stabilisation. The water-soluble segments of the surfactant molecule extend into the water phase to form a viscous barrier layer inhibiting the approach of another particle. In this paper we would like to give more details about the influence of the concentration and variation of the ethylene oxide chain on the properties of the polymer dispersion. Moreover, we would also like to demonstrate that some of the most important properties of a paint, e.g. the pigment binding capacity of an interior paint, or
1999 Elsevier Science S.A. All rights reserved.
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C. Heldmann et al. / Progress in Organic Coatings 35 (1999) 69–77
Table 1 Basic recipe for the preparation of the VA/E and VA/VeoVa10 Surfactant/protective colloid
1.5–5.0% nonionic emulsifier C11H23O- X EO (X = 3–50) 1.0–2.0% cellulose ether (Tylose H300) 0.5% sodium vinyl sulphonate
Initiator
t-butyl hydroperoxide/Rongalit C; sodium peroxodisulphate (after treatment)
Polymerisation temperature (°C)
60
the blocking resistance and gloss of (semi-) gloss paints depend very much on a proper choice of the nonionic emulsifier.
3. Results
2. Experimental
An increase of the concentration of the emulsifier from 1.5 to 5.0% results in a decrease of the particle size and a narrower particle size distribution. The viscosity of the dispersions measured at a shear velocity gradient of D = 18 s−1 increases drastically (Fig. 1). Similar results were obtained by varying the degree of ethoxylation of the emulsifier (Fig. 2): an increase of the ethylene oxide chain length leads to latices with smaller particles and higher viscosities. This can be explained by taking into account that the surface area stabilised by the emulsifier increases with an increase of the concentration of the surfactant resulting in smaller particles due to mass balance considerations. A dispersion with a large number of small particles exhibits a higher viscosity than one with a small number of large particles. As shown in Fig. 3, the glass transition temperature of the VA/VeoVa10 copolymers decreases with an increase of the concentration of the emulsifier. This effect is even more pronounced for the VA/E copolymers where an increase of the concentration of the nonionic emulsifier by 3.5% results in a depression of the glass temperature by 9°C (Fig. 4). The same trend is observed if the ethylene oxide
The general features of the basic recipe of the VA/E and VA/VeoVa10 dispersions are given in Table 1. Ethoxylated C-11 oxoalcohols with a degree of ethoxylation varying from 3 to 50 mol of ethylene oxide (EO) units were used as surfactants. The compounds were synthesised by reacting undecyl alcohol with various amounts of ethylene oxide. The degree of ethoxylation (X in Table 1) given in the text should only be understood as average figures. Two series of experiments were carried out. In the first one, the amount of the C-11 oxoalcohol with 28 mol EO (C11-28 EO) was varied from 1.5 to 5.0% relative to the amount of monomers. In the second one, the influence of the degree of ethoxylation was studied: relative to a concentration of 3.0 parts by weight (pbw) of the non-ionic emulsifier C11-28 EO, molar equivalents of the other compounds with 3–50 mol EO were used. As shown in Table 1, the temperature of polymerisation was 60°C and a redox system comprising t-butyl hydroperoxide/Rongalit C (sodium formaldehyde sulphoxylate) was employed as initiator.
3.1. Influence of the emulsifier on the properties of the polymer dispersions
Fig. 1. Viscosity and particle size as a function of the nonionic emulsifier.
C. Heldmann et al. / Progress in Organic Coatings 35 (1999) 69–77
Fig. 2. Viscosity and particle size as a function of the degree of ethoxylation.
Fig. 3. Glass transition temperature as a function of the concentration of the emulsifier (VA/Veo VA).
Fig. 4. Glass transition temperature as a function of the concentration of the emulsifier (VA/E)
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Fig. 5. Glass transition temperature as a function of the degree of ethoxylation (VA/E).
chain gets larger (Fig. 5). Since the ethoxylated C-11 oxoalcohols used here are wax-like compounds, it is likely that they act as internal plastisisers. However, it can not be ruled out unambiguously that the stronger effects observed with the VA/E copolymers may partially be due to a slightly higher built-in of ethylene during the polymerisation. Moreover, it is known that the emulsifiers affect the way the initiator or the redox systems are decomposed. This results in a change of the polymerisation rate, which in turn influences the amount of ethylene incorporated in the copolymers. 3.2. Influence of the emulsifier on the paint properties One of the most important practical properties of a binder for interior paint applications with a high pigment volume concentration is the pigment binding capacity often referred to as scrub resistance, scrubability, or washability. Therefore, paints were made according to the paint recipe given in the appendix in order to evaluate the performance of the various emulsions described above. The pigment binding power was tested according to DIN 53778: paint films were casted onto black Leneta-Foils
using a 4 cm wide doctor blade so that the final thickness of the dry films was ca. 100 mm. The paint is then dried for 28 days at a temperature of 23°C and a relative humidity of 50%. Before and during the measurements, the paint is wetted with Marlon A solution (0.5% in water) and the panel is scrubbed with a standardised bristle brush until the black Lenta becomes visible (counted in brush cycles). It should be mentioned that very special further measures have to be taken to ensure reproducibility according to the DIN 53778 method which is not the scope of this paper. As shown in Figs. 6 and 7, for both the VA/E and the VA/ VeoVa10 systems, the scrub resistance of the paint films as a function of the concentration and the degree of ethoxylation, respectively, goes through a maximum, which is located at a concentration of ca. 2–3% and an ethylene oxide chain length of 17–28 mol EO. This result can be explained by two counteracting effects: the first effect was already described in the previous section, i.e. the decrease of the particle size by the increase of the concentration and degree of ethoxylation of the emulsifier. In Fig. 8, a very simplified illustration of a paint film is given with the circles representing the latex particles and the unevenly shaped objects the pigments. It can be deduced
Fig. 6. Pigment binding capacity as a function of the concentration of the emulsifier (interior paint with a pigment volume concentration of 78.3% (VA/E) and 82% (VA/VeoVa10)).
C. Heldmann et al. / Progress in Organic Coatings 35 (1999) 69–77
73
Fig. 7. Pigment binding capacity as a function of the degree of ethoxylation of the emulsifier (interior paint with a pigment volume concentration of 78.3% (VA/E) and 82% (VA/VeoVa10)).
Fig. 8. Simplified illustration of the interactions between latex and pigment particles in a paint film depending on particle size of the polymer latex.
from Fig. 8 that a latex with smaller particles should give a more thorough interconnection of the binder and pigments. In other words, the paint made with such a latex should exhibit stronger mechanical properties resulting in a better scrub resistance. Unfortunately, the increase of the concentration of the emulsifier has also a detrimental effect: it lowers the molecular weight of the polymer. This is schematically depicted in Fig. 9. Probably, a chain transfer reaction takes place involving the hydrogen atoms of the
methylene groups connected to oxygen in the EO-moieties. It can be expected that a reduction of the molecular weight deteriorates the mechanical properties of the polymer (toughness/hardness) resulting in the lower wet-srub resistance. Matt paints for interior application contain only very little emulsion. Higher contents of binder are usually employed in semi-gloss or even gloss paints. Consequently, the performance of semi-gloss or gloss paints should be even more
Fig. 9. Schematic representation of a process leading to a reduction of molecular weight due to a chain transfer reaction involving the emulsifier.
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Fig. 10. Blocking and gloss as a function of the concentration of emulsifier (VA/E: semi-gloss paint (PVC: 33%), VA/VeoVa10: gloss paint (PVC: 26%)).
influenced by the properties of the emulsion than in matt paints. It was, therefore, very interesting to investigate the structure-property relationship between emulsifier type and dispersion properties in paints with a higher content of binder. Semi-gloss paints with a pigment volume concentration of 33% were prepared using the VA/E dispersions whereas gloss paints with a pigment volume concentration of 26% were used in case of the VA/VeoVa10 dispersions (see appendix for recipes). The dependency of the gloss and blocking resistance on the amount of the nonionic emulsifier is shown in Fig. 10. The influence of the type of emulsifier is illustrated in Fig. 11. In general, the gloss of the films improves with an increase of the emulsifier concentration and its degree of ethoxylation and reaches a plateau value at around 4% and an ethylene oxide chain length of 17. Quite the opposite is observed with regard to blocking: the blocking of the films remain nearly constant at the beginning and then shows a
drastic increase at a concentration of 4% and at a chain length of 17 EO-moieties. The interpretation of these results is rather difficult considering the fact that the properties observed are a result of complex interactions among pigments, fillers and polymer dispersion. Moreover, the test method employed for measuring ‘blocking’ (force expressed in grams to bring two paint films apart from having been pressed together for 2 h at room temperature with a load of 2 kg) is not very well suited to theoretical analysis. Our interpretation of these results is as follows: first of all, the particle size of the latex influences the gloss of a paint film. The smaller the size of the polymer dispersion particles the better the gloss of a paint made from it. Since an increase of the degree of ethoxylation and concentration of the emulsifier leads to smaller particles, this should be one reason to explain the improvement in gloss of the paint films. Another factor to be considered might be the migration of the emulsifier to the surface. This migration predominantly takes
Fig. 11. Blocking and gloss as a function of the degree of ethoxylation (VA/E: semi-gloss paint (PVC: 33%), VA/VeoVa10: gloss paint (PVC: 26%)).
C. Heldmann et al. / Progress in Organic Coatings 35 (1999) 69–77
75
Fig. 12. Blocking as a function of the glass transition temperature (3.0% C11–28 EO) (semi-gloss paint with a PVC of 33%).
place into the interstices between the individual latex particles when film formation starts, thus lowering surface roughness and improving gloss. This theory is supported by electron microscopy. The results concerning blocking can be understood if one considers that a reduction of the glass transition temperature of the polymer must lead to an increase in blocking (recall that an increase of the concentration or degree of ethoxylation results in a depression of the glass transition temperature). This assumption is supported by the following experiments: polymer dispersions with different contents of ethylene in the copolymer and consequently different glass transition temperatures were prepared by changing the ethylene pressure during polymerisation. The concentration of the nonionic emulsifier (C11–28 EO) was kept constant. Semi-gloss paints were then prepared using these dispersions. As shown in Fig. 12, the blocking of such paint films is definitely a function of the glass transition temperature of the polymer.
In the first part of this paper it was demonstrated that an increase of the concentration of the nonionic emulsifier leads to a drastic increase in the viscosity of the dispersions. Some customers are concerned about changes of the viscosity of the dispersion since this might have a strong influence on the rheological behaviour of their paints. In a previous publication [3] we have shown that the viscosity of interior paints prepared with VA/E dispersions is practically not influenced by the rheological properties of the dispersion. The same is true if VA/VeoVa10 dispersions are used. In Fig. 13 the increase of the viscosity of the dispersions upon variation of the concentration of the emulsifier is shown over a wide range of shear rates. However, when these dispersions are used for the preparation of high filled interior paints, the viscosity curves of the corresponding paints practically coincide to form a master curve (Fig. 14). This shows that the viscosity of the dispersion has only a minor influence on the rheological behaviour of interior paints with a high pigment volume concentration. The
Fig. 13. Viscosity of VA/VeoVa10 dispersions as described above over a wide range of shear rates.
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Fig. 14. Viscosity of high PVC interior paints prepared with the VA/VeoVA10 dispersions over a wide range of shear rates.
rheology is mostly governed by the cellulose ether or other rheology modifiers used to prepare the paint. The situation is slightly different for (semi-) gloss paints since the binder content is higher. However, the difference in the viscosities of the paints due to the different rheological behaviour of the dispersion is not significant (Fig. 15).
4. Conclusions The influence of nonionic emulsifiers on the properties of VA/E and VA/VeoVa10 emulsions and paints has been studied. It was observed that an increase of the nonionic emulsifier concentration and its degree of ethoxylation lead to dispersions with smaller particles and higher viscosities. When these dispersions are used to prepare paints
with a high pigment volume concentration the pigment binding capacity of the paint goes through a maximum which is located at a emulsifier concentration of about 2– 3% and a degree of ethoxylation of 17–28 mol EO. The gloss of the paint films in semi-gloss or gloss paints, respectively, improves with an increase of the emulsifier concentration and reaches a constant value at around 4%. Gloss improves first and reaches a constant value at degrees of ethoxylation higher than 17. The blocking of the films remains nearly constant at the beginning and shows a drastic increase at a concentration of greater than 4% and at a chain length of 17 EO-moieties. In summary, VA/E and VA/VeoVa10 dispersions have served to study structure-property relationships between-on one hand-emulsifiers and dispersions and, -on the other hand-emulsifiers and paints. The investigation of similar
Fig. 15. Viscosity of gloss paints prepared with the VA/VeoVa10 over a wide range of shear rates.
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relationships in styrene/acrylate systems is currently in progress.
Recipe for a semi-gloss paint; PVC: ca. 33.4%
pbw
Acknowledgements We gratefully acknowledge financial support by the BMBF, Contract No. 03N3018A. Appendix. Recipe for a high filled interior paint, PVC: ca. 78.3% pbw Water Cellulose ether (Tylose H 6000 yP) Dispersing agent (Lopon 894, 45%) Dispersing agent (Calgon N, 10%) NaOH (10%) Preservative (Mergal K 9N) Defoamer (Agitan 310) Micro talc AT 1 Titaniun dioxide (rutile, treated) Calcium carbonate (2 mm) Calcium carbonate (5 mm) Calcium carbonate (ppt) Emulsion (approximately 55%) Paint
314.4 6.0 2.0 5.6 2.0 2.0 2.0 56.0 70.0 160.0 120.0 150.0 110.0 1000.0
Water Cellulose ether (Tylose H 15000 yP) Dispersing agent (Dispex G 40, 40%) Dispersing agent (Calgon N, 10%) NaOH (10%) Preservative (Mergal K 9N) Defoamer (Agitan 310) Pole star 200 Titaniun dioxide (rutile, treated) Calcium carbonate (ppt) Emulsion (approximately 50%) Paint
164.0 3.0 3.0 12.0 2.0 2.0 4.0 60.0 220.0 70.0 460.0 1000.0
References [1] J. Snuparek, Prog. Org. Coat. 29 (1996) 225. [2] H. Rinno, XIX Fatipec Congress, Aachen, 1988. [3] B. Momper, R. Kuropka, XXIII Fatipec Congress, Brussels, 1996.
Influence of nonionic emulsifiers on the properties of vinyl acetate/VeoVa10 and vinyl acetate/ethylene emulsions and paints Carsten Heldmann*, R. Ivan Cabrera, Bernhard Momper, Rolf Kuropka, Klaus Zimmerschied Clariant GmbH, G832, 65926 Frankfurt am Main, Germany Received 15 July 1998; accepted 10 December 1998
Abstract This paper presents results on the influence of the nonionic surfactant on the properties of vinyl acetate/VeoVa10 and vinyl acetate/ ethylene emulsions and paints made thereof. Emulsions were prepared in which the concentration of the nonionic surfactant and its degree of ethoxylation were varied. An increase of the nonionic emulsifier concentration and of the length of the ethylene oxide chain leads to dispersions with smaller particles and higher viscosities. Using these emulsions as binders in high pigmented paints, it was observed that the pigment binding capacity of the interior paints goes through a maximum which is located at a emulsifier concentration of about 2–3% and at a degree of ethoxylation of 17–28 mol ethylene oxide (EO). In (semi-) gloss paints, the gloss of the paint films improves with an increase of the emulsifier concentration and reaches a constant value at around 4% or a degree of ethoxylation of ca. 17 EO-moieties per molecule. The blocking of the films shows a drastic increase at a concentration above 4% and at a chain length of greater than 17 EO-moieties. 1999 Elsevier Science S.A. All rights reserved. Keywords: Nonionic surfactant; Vinyl acetate ethylene emulsion; Vinyl acetate VeoVa10 emulsion; Paint; Coating
1. Introduction Copolymer dispersions of vinyl acetate (VA) are a widely used type of synthetic latexes in the coatings industry. They comprise several different types including copolymers with ethylene, versatic acid 10 esters (VeoVa10), acrylates, maleinates, etc. each with its own field of application. For many of these applications very distinct requirements have to be met. This refers to the colloidal state of emulsion (particle size distribution, shear stability, viscosity etc.) as well as the properties of the final polymer film. Generally, the colloidal and polymer properties are determined by monomer composition, crosslinking and molecular weight of the polymer, surfactant system, initiators and by the polymerisation process [1]. All of these topics have been a matter of intensive research in our laboratories and many of the results including influence of monomer varia-
* Corresponding author.
0300-9440/99/$ - see front matter PII: S03 00-9440(99)000 25-9
tion, type of crosslinking agent and of stabilising system, have already been published [2,3]. In particular, the influence of stabilisers on the properties of polymer dispersions and/or paints is often a matter of major concern since polymer dispersions used as binders for interior paints have to fulfil strong requirements concerning mechanical (e.g. shear) stability. It has been shown that combinations of protective colloids with nonionic emulsifiers function satisfactorily with regard to these requirements. Very often, ethoxylated fatty alcohols are employed as nonionic emulsifiers. The mechanism by which they work is based on steric stabilisation. The water-soluble segments of the surfactant molecule extend into the water phase to form a viscous barrier layer inhibiting the approach of another particle. In this paper we would like to give more details about the influence of the concentration and variation of the ethylene oxide chain on the properties of the polymer dispersion. Moreover, we would also like to demonstrate that some of the most important properties of a paint, e.g. the pigment binding capacity of an interior paint, or
1999 Elsevier Science S.A. All rights reserved.
70
C. Heldmann et al. / Progress in Organic Coatings 35 (1999) 69–77
Table 1 Basic recipe for the preparation of the VA/E and VA/VeoVa10 Surfactant/protective colloid
1.5–5.0% nonionic emulsifier C11H23O- X EO (X = 3–50) 1.0–2.0% cellulose ether (Tylose H300) 0.5% sodium vinyl sulphonate
Initiator
t-butyl hydroperoxide/Rongalit C; sodium peroxodisulphate (after treatment)
Polymerisation temperature (°C)
60
the blocking resistance and gloss of (semi-) gloss paints depend very much on a proper choice of the nonionic emulsifier.
3. Results
2. Experimental
An increase of the concentration of the emulsifier from 1.5 to 5.0% results in a decrease of the particle size and a narrower particle size distribution. The viscosity of the dispersions measured at a shear velocity gradient of D = 18 s−1 increases drastically (Fig. 1). Similar results were obtained by varying the degree of ethoxylation of the emulsifier (Fig. 2): an increase of the ethylene oxide chain length leads to latices with smaller particles and higher viscosities. This can be explained by taking into account that the surface area stabilised by the emulsifier increases with an increase of the concentration of the surfactant resulting in smaller particles due to mass balance considerations. A dispersion with a large number of small particles exhibits a higher viscosity than one with a small number of large particles. As shown in Fig. 3, the glass transition temperature of the VA/VeoVa10 copolymers decreases with an increase of the concentration of the emulsifier. This effect is even more pronounced for the VA/E copolymers where an increase of the concentration of the nonionic emulsifier by 3.5% results in a depression of the glass temperature by 9°C (Fig. 4). The same trend is observed if the ethylene oxide
The general features of the basic recipe of the VA/E and VA/VeoVa10 dispersions are given in Table 1. Ethoxylated C-11 oxoalcohols with a degree of ethoxylation varying from 3 to 50 mol of ethylene oxide (EO) units were used as surfactants. The compounds were synthesised by reacting undecyl alcohol with various amounts of ethylene oxide. The degree of ethoxylation (X in Table 1) given in the text should only be understood as average figures. Two series of experiments were carried out. In the first one, the amount of the C-11 oxoalcohol with 28 mol EO (C11-28 EO) was varied from 1.5 to 5.0% relative to the amount of monomers. In the second one, the influence of the degree of ethoxylation was studied: relative to a concentration of 3.0 parts by weight (pbw) of the non-ionic emulsifier C11-28 EO, molar equivalents of the other compounds with 3–50 mol EO were used. As shown in Table 1, the temperature of polymerisation was 60°C and a redox system comprising t-butyl hydroperoxide/Rongalit C (sodium formaldehyde sulphoxylate) was employed as initiator.
3.1. Influence of the emulsifier on the properties of the polymer dispersions
Fig. 1. Viscosity and particle size as a function of the nonionic emulsifier.
C. Heldmann et al. / Progress in Organic Coatings 35 (1999) 69–77
Fig. 2. Viscosity and particle size as a function of the degree of ethoxylation.
Fig. 3. Glass transition temperature as a function of the concentration of the emulsifier (VA/Veo VA).
Fig. 4. Glass transition temperature as a function of the concentration of the emulsifier (VA/E)
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Fig. 5. Glass transition temperature as a function of the degree of ethoxylation (VA/E).
chain gets larger (Fig. 5). Since the ethoxylated C-11 oxoalcohols used here are wax-like compounds, it is likely that they act as internal plastisisers. However, it can not be ruled out unambiguously that the stronger effects observed with the VA/E copolymers may partially be due to a slightly higher built-in of ethylene during the polymerisation. Moreover, it is known that the emulsifiers affect the way the initiator or the redox systems are decomposed. This results in a change of the polymerisation rate, which in turn influences the amount of ethylene incorporated in the copolymers. 3.2. Influence of the emulsifier on the paint properties One of the most important practical properties of a binder for interior paint applications with a high pigment volume concentration is the pigment binding capacity often referred to as scrub resistance, scrubability, or washability. Therefore, paints were made according to the paint recipe given in the appendix in order to evaluate the performance of the various emulsions described above. The pigment binding power was tested according to DIN 53778: paint films were casted onto black Leneta-Foils
using a 4 cm wide doctor blade so that the final thickness of the dry films was ca. 100 mm. The paint is then dried for 28 days at a temperature of 23°C and a relative humidity of 50%. Before and during the measurements, the paint is wetted with Marlon A solution (0.5% in water) and the panel is scrubbed with a standardised bristle brush until the black Lenta becomes visible (counted in brush cycles). It should be mentioned that very special further measures have to be taken to ensure reproducibility according to the DIN 53778 method which is not the scope of this paper. As shown in Figs. 6 and 7, for both the VA/E and the VA/ VeoVa10 systems, the scrub resistance of the paint films as a function of the concentration and the degree of ethoxylation, respectively, goes through a maximum, which is located at a concentration of ca. 2–3% and an ethylene oxide chain length of 17–28 mol EO. This result can be explained by two counteracting effects: the first effect was already described in the previous section, i.e. the decrease of the particle size by the increase of the concentration and degree of ethoxylation of the emulsifier. In Fig. 8, a very simplified illustration of a paint film is given with the circles representing the latex particles and the unevenly shaped objects the pigments. It can be deduced
Fig. 6. Pigment binding capacity as a function of the concentration of the emulsifier (interior paint with a pigment volume concentration of 78.3% (VA/E) and 82% (VA/VeoVa10)).
C. Heldmann et al. / Progress in Organic Coatings 35 (1999) 69–77
73
Fig. 7. Pigment binding capacity as a function of the degree of ethoxylation of the emulsifier (interior paint with a pigment volume concentration of 78.3% (VA/E) and 82% (VA/VeoVa10)).
Fig. 8. Simplified illustration of the interactions between latex and pigment particles in a paint film depending on particle size of the polymer latex.
from Fig. 8 that a latex with smaller particles should give a more thorough interconnection of the binder and pigments. In other words, the paint made with such a latex should exhibit stronger mechanical properties resulting in a better scrub resistance. Unfortunately, the increase of the concentration of the emulsifier has also a detrimental effect: it lowers the molecular weight of the polymer. This is schematically depicted in Fig. 9. Probably, a chain transfer reaction takes place involving the hydrogen atoms of the
methylene groups connected to oxygen in the EO-moieties. It can be expected that a reduction of the molecular weight deteriorates the mechanical properties of the polymer (toughness/hardness) resulting in the lower wet-srub resistance. Matt paints for interior application contain only very little emulsion. Higher contents of binder are usually employed in semi-gloss or even gloss paints. Consequently, the performance of semi-gloss or gloss paints should be even more
Fig. 9. Schematic representation of a process leading to a reduction of molecular weight due to a chain transfer reaction involving the emulsifier.
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Fig. 10. Blocking and gloss as a function of the concentration of emulsifier (VA/E: semi-gloss paint (PVC: 33%), VA/VeoVa10: gloss paint (PVC: 26%)).
influenced by the properties of the emulsion than in matt paints. It was, therefore, very interesting to investigate the structure-property relationship between emulsifier type and dispersion properties in paints with a higher content of binder. Semi-gloss paints with a pigment volume concentration of 33% were prepared using the VA/E dispersions whereas gloss paints with a pigment volume concentration of 26% were used in case of the VA/VeoVa10 dispersions (see appendix for recipes). The dependency of the gloss and blocking resistance on the amount of the nonionic emulsifier is shown in Fig. 10. The influence of the type of emulsifier is illustrated in Fig. 11. In general, the gloss of the films improves with an increase of the emulsifier concentration and its degree of ethoxylation and reaches a plateau value at around 4% and an ethylene oxide chain length of 17. Quite the opposite is observed with regard to blocking: the blocking of the films remain nearly constant at the beginning and then shows a
drastic increase at a concentration of 4% and at a chain length of 17 EO-moieties. The interpretation of these results is rather difficult considering the fact that the properties observed are a result of complex interactions among pigments, fillers and polymer dispersion. Moreover, the test method employed for measuring ‘blocking’ (force expressed in grams to bring two paint films apart from having been pressed together for 2 h at room temperature with a load of 2 kg) is not very well suited to theoretical analysis. Our interpretation of these results is as follows: first of all, the particle size of the latex influences the gloss of a paint film. The smaller the size of the polymer dispersion particles the better the gloss of a paint made from it. Since an increase of the degree of ethoxylation and concentration of the emulsifier leads to smaller particles, this should be one reason to explain the improvement in gloss of the paint films. Another factor to be considered might be the migration of the emulsifier to the surface. This migration predominantly takes
Fig. 11. Blocking and gloss as a function of the degree of ethoxylation (VA/E: semi-gloss paint (PVC: 33%), VA/VeoVa10: gloss paint (PVC: 26%)).
C. Heldmann et al. / Progress in Organic Coatings 35 (1999) 69–77
75
Fig. 12. Blocking as a function of the glass transition temperature (3.0% C11–28 EO) (semi-gloss paint with a PVC of 33%).
place into the interstices between the individual latex particles when film formation starts, thus lowering surface roughness and improving gloss. This theory is supported by electron microscopy. The results concerning blocking can be understood if one considers that a reduction of the glass transition temperature of the polymer must lead to an increase in blocking (recall that an increase of the concentration or degree of ethoxylation results in a depression of the glass transition temperature). This assumption is supported by the following experiments: polymer dispersions with different contents of ethylene in the copolymer and consequently different glass transition temperatures were prepared by changing the ethylene pressure during polymerisation. The concentration of the nonionic emulsifier (C11–28 EO) was kept constant. Semi-gloss paints were then prepared using these dispersions. As shown in Fig. 12, the blocking of such paint films is definitely a function of the glass transition temperature of the polymer.
In the first part of this paper it was demonstrated that an increase of the concentration of the nonionic emulsifier leads to a drastic increase in the viscosity of the dispersions. Some customers are concerned about changes of the viscosity of the dispersion since this might have a strong influence on the rheological behaviour of their paints. In a previous publication [3] we have shown that the viscosity of interior paints prepared with VA/E dispersions is practically not influenced by the rheological properties of the dispersion. The same is true if VA/VeoVa10 dispersions are used. In Fig. 13 the increase of the viscosity of the dispersions upon variation of the concentration of the emulsifier is shown over a wide range of shear rates. However, when these dispersions are used for the preparation of high filled interior paints, the viscosity curves of the corresponding paints practically coincide to form a master curve (Fig. 14). This shows that the viscosity of the dispersion has only a minor influence on the rheological behaviour of interior paints with a high pigment volume concentration. The
Fig. 13. Viscosity of VA/VeoVa10 dispersions as described above over a wide range of shear rates.
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Fig. 14. Viscosity of high PVC interior paints prepared with the VA/VeoVA10 dispersions over a wide range of shear rates.
rheology is mostly governed by the cellulose ether or other rheology modifiers used to prepare the paint. The situation is slightly different for (semi-) gloss paints since the binder content is higher. However, the difference in the viscosities of the paints due to the different rheological behaviour of the dispersion is not significant (Fig. 15).
4. Conclusions The influence of nonionic emulsifiers on the properties of VA/E and VA/VeoVa10 emulsions and paints has been studied. It was observed that an increase of the nonionic emulsifier concentration and its degree of ethoxylation lead to dispersions with smaller particles and higher viscosities. When these dispersions are used to prepare paints
with a high pigment volume concentration the pigment binding capacity of the paint goes through a maximum which is located at a emulsifier concentration of about 2– 3% and a degree of ethoxylation of 17–28 mol EO. The gloss of the paint films in semi-gloss or gloss paints, respectively, improves with an increase of the emulsifier concentration and reaches a constant value at around 4%. Gloss improves first and reaches a constant value at degrees of ethoxylation higher than 17. The blocking of the films remains nearly constant at the beginning and shows a drastic increase at a concentration of greater than 4% and at a chain length of 17 EO-moieties. In summary, VA/E and VA/VeoVa10 dispersions have served to study structure-property relationships between-on one hand-emulsifiers and dispersions and, -on the other hand-emulsifiers and paints. The investigation of similar
Fig. 15. Viscosity of gloss paints prepared with the VA/VeoVa10 over a wide range of shear rates.
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relationships in styrene/acrylate systems is currently in progress.
Recipe for a semi-gloss paint; PVC: ca. 33.4%
pbw
Acknowledgements We gratefully acknowledge financial support by the BMBF, Contract No. 03N3018A. Appendix. Recipe for a high filled interior paint, PVC: ca. 78.3% pbw Water Cellulose ether (Tylose H 6000 yP) Dispersing agent (Lopon 894, 45%) Dispersing agent (Calgon N, 10%) NaOH (10%) Preservative (Mergal K 9N) Defoamer (Agitan 310) Micro talc AT 1 Titaniun dioxide (rutile, treated) Calcium carbonate (2 mm) Calcium carbonate (5 mm) Calcium carbonate (ppt) Emulsion (approximately 55%) Paint
314.4 6.0 2.0 5.6 2.0 2.0 2.0 56.0 70.0 160.0 120.0 150.0 110.0 1000.0
Water Cellulose ether (Tylose H 15000 yP) Dispersing agent (Dispex G 40, 40%) Dispersing agent (Calgon N, 10%) NaOH (10%) Preservative (Mergal K 9N) Defoamer (Agitan 310) Pole star 200 Titaniun dioxide (rutile, treated) Calcium carbonate (ppt) Emulsion (approximately 50%) Paint
164.0 3.0 3.0 12.0 2.0 2.0 4.0 60.0 220.0 70.0 460.0 1000.0
References [1] J. Snuparek, Prog. Org. Coat. 29 (1996) 225. [2] H. Rinno, XIX Fatipec Congress, Aachen, 1988. [3] B. Momper, R. Kuropka, XXIII Fatipec Congress, Brussels, 1996.