Adsorption studies of associative interactions between thickener and pigment particles

PROGRESS IN ORGANIC COATIRGS ELSEVIER

Progress in Organic Coatings 30 (1997) 167-171

Adsorption studies of associative interactions between thickener and pigment particles T. Svanholm a'*, B. Kronberg a, F.

Molenaar b

"YKI, P.O. Box 5607, S-114 86 Stoekhohn, Swedel~

bTNO C,:qltl'cfiat Coatings Research. P.O. Box 6034, NL-2600 JA Dell?. Netherland~

Received 3 July 1996: received in revised form l0 August 1996: accepted 26 August 1996

Abstract Adsorption of a HEUR thickener and a HMHEC thickener have been studied on two titanium dioxide pigment grades with and without dispersant previously adsorbed. One of the pigments had an acidic surface, mainly silica, and the other a more basic alumina/zirconia surface. Both associative thickeners adsorbed on pigments having a partially hydrophobic (c~-olefinic-maleic acid co-polymer) dispersant preadsorbed on the surface. No adsorption takes place when sodium hexametaphosphate, a highly charged low molecular weight inorganic dispersant, was used. The c~-olefinic-maleic acid co-polymer dispersant is shown to adsorb on a negatively charged silica surface. This is assigned to hydrogen bonding between unionised carboxylic groups from the dispersant and oxide ions on the pigment surface. © 1997 Elsevier Science S.A. K e v w o r d s : Thickeners; Pigments: Surfacc adsorption; Dispersants

1. Introduction Associative thickeners improve several properties for latex coatings such as gloss, levelling and flow when properly formulated. The interactions with (stabilised) pigments are often overlooked but have a substantial influence on the final coating properties. Both presence and absence of pigmerit-thickener interactions have been reported as beneficial for achieving high gloss latex coatings. Lundberg et al. [1] have reported an increased gloss when diisobutylenemaleic acid copolymer (D1BMA) dispersants were used in combination with hydrophobically modified ethoxylated urethane (HEUR) thickeners. Kroon [2] showed higher gloss when hydrophobically modified hydroxyethyl cellulose (HMHEC) thickeners are used in combination with dispersants that prohibit interactions with pigments, thus favouring interactions with latex. Johnson 13] assigns lower gloss for formulations containing HEUR thickeners and highly charged dispersants due to poor solubility for HEUR polymers at high ionic strength. A careful selection of the dispersant to be combined with the thickener is thus important for utilising the potential of associative thickeners. In this paper, the adsorption mechanisms for disper-

* Corresponding author. Present address: Casco Products AB, PO Box 11538, S-100 61 Stockhohn, Sweden. 0300-9440/97/$17.(10

('~ 1997 Elsevier Science S.A. All rights reserved

sants will be discussed as well as how adsorption of associative thickeners can be regulated by the choice of dispersants. Pigments treated with two different mineral oxides, giving one acidic and one more basic surface are used in combination with a highly charged low molecular weight inorganic dispersant (sodium hexametaphosphate) and a low charged partially hydrophobic oe-olefinic co-maleic acid polymeric dispersant. The associative thickeners used are a low molecular weight HMHEC and a HEUR. The interactions between thickener and titanium dioxide pigments are studied by investigating the adsorption of dispersants and thickeners individually as well as by studying the behaviour of thickeners when dispersants already have been adsorbed on the pigment.

2. Materials 2.1. P i g m e n t s

Two commercial titanium dioxide pigments with different surface treatments were chosen. A purely silica-treated laboratory product was also used, kindly supplied by Kemira TiO~. The surface composition of the different pigments, measured by ESCA, is shown in Table 1.

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T. Svanholm et al. /Progress in Organic Coatings 30 (1997) 167 171

Table 1 Pigment composition Pigment

Compound Ti O C AI Si Ca Zr S Na

Al_~O~/ZrO:-coatedcommercial product

SiO:-coated commercial product

SiO:-coated laboratory product

Mass (f/~)

Atomic (~/c)

Mass (%)

Atomic (%)

Mass I%)

Atomic (%)

I 1.7 52.2 5.7 22.9 0.6 0.7 5.0 1.3

4.9 65.7 9.6 17.1 0.4 0.4 I. I 0.8

3.2 53.2 2.6 15.1 24.0

1.3 65.1 4.3 10.9 16.7

15.1 49.5 5.0

6.4 62.8 8.6

28.5

20.6

1.9

1.6

1.9

1.6

2.2. Dispersants

Sodium hexametaphosphate (SHMP), a low molecular weight highly charged inorganic dispersant, and a diisobut y l e n e - m a l e i c acid copolymer (DIBMA) - a low-charged alternating copolymer with hydrophobic segments (diisobutylene) - were used. 2.3. Associative thickeners

Two different types of associative thickeners have been used: An aliphatic HEUR thickener (Hydrophobically modified Ethylene oxide URethane block copolymer) and An HMHEC thickener (Hydrophobically Modified HydroxyEthylCellulose). The thickeners are characterised as follows:

,Q, M, Hydrophobe length

HEUR

HMHEC

26 000 17 00(1 C12 CI4

27 000 (2716

3. Methods 3.1. Adsorption isotherms

Adsorption isotherms were determined by a batch procedure. The pigments were mixed with solutions of varying adsorbate concentrations and dispersed ultrasonically, before achieving 24 h equilibration time. The pigment concentration was 30% (w/w) and the pH was adjusted to 9.0 with NaOH. The concentration of the adsorbate was determined in the supernatant after centrifugation. The concentration of the D I B M A dispersing agent was determined by potentiometric titration.

SHMP concentrations were determined by atomic emission spectroscopy, where the concentration of phosphorus was measured. The concentrations of both thickeners were determined spectrophotometrically after forming complexes [4].

3.2. Zeta-potential

As the determination of adsorption isotherms is time-consuming, we used electrokinetic measurements to achieve qualitative information on the adsorption behaviour [5]. When a nonionic associative thickener adsorbs on a charged particle, the plane of shear is moved outwards and hence, the (absolute) zeta-potential decreases. In addition, thickener adsorption may desorb anionic emulsifier, which also would contribute to a decreased zeta-potential. Monitoring the zeta-potential as a function of associative thickener concentration thus gives information on the occurrence of adsorption. The zeta-potential was determined by measuring the Doppler effects in scattered light using a Malvem Zetasizer 4 instrument. The pigments were mixed with thickener or dispersing agent in 0.01 M NaCI at pH 9 and dispersed ultrasonically, before achieving 24 h equilibration time. In the case of competitive adsorption, the pigments were stabilised by adsorbing dispersants at concentration high enough to ensure total surface coverage. The slurry was ultrasonically dispersed and left overnight before the addition of the thickener. The measurements were performed alter 24 h to reach a steady state.

4. Results and discussion The isoelectric points (IEP) from electrophorefic mobility measurements on the two commercial pigments were 3 and 8, indicating predominantly silica respectively alumina- or zirconium-coated pigments. However, substantial amounts of alumina were found on the silica-treated pigment by

E S,'a#l/u,hn et al. / Progress" in Organic Coalings 30 (/997) 167 171

ESCA measurements, suggesting also some aluminium oxide on the surface. In order to study adsorption on a pure silica-treated TiO: pigment, a laboratory product was used. Titanium signal was found in the surface region of all pigments, as was also found by Johansson [6]. This can be due both to the penetration depth of ESCA (2-5 nm) being higher than the coating thickness, or to a patchy surface treatment. Higher adsorption of the dispersants was found at pH 9 for both dispersants on the Al:OdZrO2 coated pigment. This is in line with previous results where adsorption of dispersants onto TiO2 pigments is considered to be due mainly to Br6nsted acid-base interactions (Schr6der [7], M~ikinen [8] and Hulddn 191). After full adsorption the electrophoretic mobility was the same for both commercial pigments, independent of dispersant. This indicates that charge repulsion determines the dispersant adsorption. A higher mobility (-5.0 mV) was shown for SHMP than for DIBMA dispersant (-3.9 mV). This is expected, as SHMP has a higher charge density and is likely to provide the plane of shear closer to the particle than the polymeric dispersant. SHMP adsorbed to a larger extent than the DIBMA dispersant on both commercial pigments, as seen in Figs. 1 and 2. At pH 9 a silica surface can be considered as purely negatively charged. If acid-base interactions should be the only driving force for adsorption, the dispersants should not adsorb on the silica surface due to charge repulsion. To investigate the adsorption characteristics, isotherms were made for both dispersants on the silica-treated laboratory pigment. No adsorption was found for SHMP, supporting acid-base interactions as the only driving force for the adsorption. The adsorption seen at the silica coated commercial pigment can be explained from the ESCA results showing the presence of alumina on the surface. For the DIBMA, dispersant adsorption takes place on the silica surface, showing the presence of another driving forces apart from acid-base interactions. Morrison [10] sug-

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Fig. 2. Adsorption isotherms for SHMP on different TiO: pigments, showing the highest adsorption on the more basic alumin~zirconium-treated pigment. SHMP do not adsorb on a negatively charged silica surface, suggesting acid-base interaction as the only cause for adsorption. O: silica-treated commercial pigment; O: alumina-zirconia-treated commercial pigment: D: pure silica-treated pigment.

gests hydrogen bonding between residual hydroxyl groups and surface oxide ions to be responsible for adsorption above the isoelectric point. The pK~ for the second acid group in maleic acid contiguous hydrophobes is reported to be 10.8 [1]. At a pH of 9, there are thus unionised carboxylic groups available for hydrogen bonding with the surface. The results are thus in line with the mechanism proposed by Morrison. Polyacrylic acid was found not to adsorb on silica-treated TiO_, [8,9], showing that the need for unionised carboxylic groups as PAA is fully ionised at pH 9 [111. The HEUR thickener did not adsorb on either the S i O 2 - o r the A12OdZrO2-coated pigment. The HMHEC thickener, however, adsorbed on the Al2OjZrO,-coated pigment but not on the SiO2-coated one, shown by electrokinetic measurements in Fig. 3. The strong reduction of the absolute value of the zeta-potential reflects adsorption of the nonionic thickener to the AleOJZrOx-coated pigment, thereby

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H M H E C C o n c e n t r a t i o n / [g/l] Fig. 1. Adsorption isotherms for DIBMA dispersant on different TiO, pigments, showing the highest adsorption on the more basic alumina\tqzirconium-treated pigment. The dispersant adsorbs also on a negatively charged silica surface suggesting an additional mechanism for adsorption next to acid base interactions. O: silica-treated commercial pigment: @: alumina-zirconia-treated commercial pigment: D: pure silica-treated pigillell[.

Fig. 3. Electrokinetic measurements on HMHEC thickener/pigment mixtures, showing adsorption on the more basic alumina-zirconium-treated pigment. The thickener does not adsorb on the more negatively charged silica-treated pigment, as seen from the linear change of zeta-potential which is due to a viscosity effect. O: silica-treated commercial pigment; Q: alumina-zirconia-treated commercial pigment.

T. Sr..hohn el ,I. /Prod, rep's" ht Organic ('o,titLg,~ 30 tl997) 167 /71

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Fig. 4. Elcctrokinetic measurements showing adsorption of bolh thickeners on silica-uvatcd commercial pigment stabiliscd with D I B M A dispersanl. In the case of the S H M P dispcrsant, 11o thickener adsorption occurs. @: D I B M A and H M H E C thickener: D: D I B M A and H E U R lhickener: *: S H M P and H M H E C lhickener: <>: S H M P and H E U R thickener.

moving the plane of shear outwards from the particle surface. The small linear (absolute) decrease in zeta-potential for the SiO,-coated pigment is an effect of increasing viscosity of the aqueous phase only [9J. The pigment dispersions are normally stabilised with some dispersant. Thickener adsorption is thus more interesting to study on stabilised pigments. Both thickeners adsorb to pigments stabilised with the DIBMA dispersant that has hydrophobic parts, as seen in Figs. 4 and 5, although the adsorption on the SiO,-coated pigment stabilised with DIBMA dispersant is found to be lower, in line with the lower dispersant adsorption onto this pigment. No adsorption occurs when the pigment is stabilised with SHMP. The small linear change for the HEUR thickener is purely a viscosity effect. This is not seen for the more low-viscosity HMHEC thickener. Rheology measurements in Fig. 6 show a strong interaction between the HMHEC thickener and the

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Fig. 5. ElectrokinctJc measulvlncnts showing adsorption of both thickeners on alumina zirconia-treatcd c o m m e r c i a l pigment slabilised with D I B M A dispersant. In the case of the S H M P dispcrsant, no thickener adsotT~tion occurs. In the case of D I B M A dispersant, more thickener adsorbs corn pared to the silica-treated pigment, as seen from the stronger reduction of the absolute value of the zeta-polential. @: D I B M A and H M H E C thick ener: U: D I B M A and H E U R thickener: ~: S t t M P and H M H E C thickencr: e>: S H M P and H E U R thickener.

Fig. 6. Rhcology ineasnrements showing interactions between both /hickeners and D I B M A dispersaill. A surfactant-likc bchaviour is seen with addition of D I B M A dispersant tu the H M H E C thickener. @: H M H E C thickener: - : H E U R Ihickcncr.

DIBMA dispersant. The viscosity increase is due to the increase in the number of associative aggregates by adding more dispersants containing hydrophobic groups. The decrease in viscosity for the HMHEC thickener occurs as the functionality of hydrophobic groups from the thickener in the aggregates falls below 3 at increasingly higher dispersant concentrations I121. As seen from the electrokinetic studies, the dispersant interacts with associative thickeners also when adsorbed onto the pigments. The HMHEC thickener seems to interact more strongly with the DIBMA stabilised pigment than the HEUR thickener, suggested by a more rapid decrease in (the absolute) zeta-potential and the rheology measurements.

5. Conclusions

Electrokinetic measurements have been shown here to be a powerful tool 1o1 studying interactions between associative thickeners and pigments. Higher amounts of SHMP and DIBMA dispersants adsorb to the more basic mineral oxide-treated TiO2 pigment, as compared to the pigment having an acidic (silica) surface. This suggests acid base interactions to have a major influence on the adsorption characteristics of SHMP and DIBMA dispersants. However, acid-base interaction is not the only driving force for adsorption; hydrogen bonding also has an influence for dispersants having unionised carboxylic groups. No adsorption wits flmnd to mineral oxide surfaces by the HEUR thickener in contrast to the HMHEC thickener, which adsorbed onto AleOJZrO:-coated but not onto the SiO2-coated TiO2 pigment. More importantly, associative thickeners adsorb cooperatively onto pigments by interactions between thickener hydrophobes and hydrophobic groups in the dispersants such its diisobutylene. Thus, the presence or absence of interaction between associative thickener and pigment can be controlled by the selection of dispersing agent.

7", Svanhohn et al. /Progress in Organic Coatings 30 (1997) 167 171

Acknowledgements This w o r k was supported by the Swedish National Board for Industrial and T e c h n i c a l D e v e l o p m e n t ( N U T E K ) , the E u r o p e a n U n i o n (Brite Euram), A k z o Nobel Surface C h e m istry AB, Alcro Beckers AB, K e m i r a Oy and Neste Oy Chemicals. W e w o u l d also like to thank Ms. Maria TOrnudd for skilfully p e r f o r m i n g most o f the laboratory work. W e also gratefully a c k n o w l e d g e Mr. T u o m o Losoi for supplying the m o d e l pigment.

References [I] D.J. Lundberg and J.E. Glass, J. Coat. Technol., 64 (1992) 5361.

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[2] [3] [4] [5]

G. Kroon, in PRA 14th International Conf, Nov. 1994, paper 5. E. Johnson, Farbe + Lack. 9 (1994) 759-764. M.B. Baleux, C.R. Acad. Se. Paris. t. 274, May 1972. R.J. Hunter, Zero Potential in Colloid Science. Academic Press, London. 1981. [6] E.-S. Johansson and T. Losoi, Surf Interface Anal., 17 (199t) 230236. [7] J. Scbr6der, Prog. Org. Coat.. 19 (1991) 227-244. [8] P.O. Mfikinen, T. Losoi and A. Kohonen, in 12th Congress of the Federation c~f Scandinavian Paint and Varnish Technologists, Congress Book, May 9-11 1988, pp. 8.1-8.18. [9] M. Huld6n and E. Sj6blom, Prog. Colloid Polymer Sci, 82 (1990)

28-37, [10] W.H. Morrison Jr., J. Colloid Interface Sei.. 100 (1984) 121-127. [11] M. Mandel, Eur. Polym. J., 6, (1970) 807. [12] T. Svanholm, F, Molenaar and A. Toussaint, Prog. Org. Coat., 30 (1997) 159.