Jana Mikešová

Jaromír Šňupárek

Jiří Horský

Otakar Quadrat

Comptes Rendus Chimie

Full paper / MémoireViscoelastic behaviour of butyl acrylate/styrene/2-hydroxyethylmethacrylate/acrylic acid latices thickened with associativethickenersOtakar Quadrat a,*, Jana Mikešová a, Jiří Horský a, Jaromír Šňupárek ba Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 16206 Prague 6, Czech Republicb Institute of Polymeric Materials, University of Pardubice, 532 10 Pardubice, Czech RepublicReceived 25 February 2003; accepted 22 September 2003AbstractThe rheological behaviour of butyl acrylate/styrene/acrylic acid latices thickened with a hydrophobically modified ethoxy-lated polyurethane (HEUR) or hydrophobically modified alkali-soluble polyacrylate emulsion (HASE) was investigated. Whilethe pseudoplastic character of frequency dependence of complex viscosity was similar for both thickeners, viscoelasticbehaviour, expressed as the ratio of loss and storage moduli, significantly differed, indicating that the HEUR molecules, unlikeswollen HASE particles, create a viscoelastic space structure. The increase in hydrophilicity of the particle surface, achieved byincorporation of 2-hydroxyethyl methacrylate (HEMA) monomer into the latex copolymer reduced the viscoelasticity of laticesthickened with HEUR, but not of those thickened with HASE. This confirms that adsorption of hydrophobic end-groups onparticle surface is important for thickening of latices with HEUR and that a physical network of latex particles interconnected bythe thickener macromolecules is formed. To cite this article: O. Quadrat et al., C. R. Chimie 6 (2003).© 2003 Académie des sciences. Published by Éditions scientifiques et médicales Elsevier SAS. All rights reserved.RésuméOn a étudié le comportement rhéologique de latexes de copolymères styrène/acrylate de butyle/acide acrylique, épaissis parun polyuréthane éthoxylé hydrophobiquement modifié (HEUR) ou une émulsion de polyacrylate soluble dans l’alcali, égalementhydrophobiquement modifié (HASE). Alors que le caractère pseudo-plastique de la dépendance en fréquence de la viscositécomplexe était similaire dans les deux cas, le comportement viscoélastique, exprimé comme le rapport des modules de perte etde conservation, diffère de manière significative, indiquant que les molécules de HEUR, à l’inverse des particules de HASE, quisont gonflées, créent un espace structuré. L’augmentation de l’hydrophilie superficielle des particules de latex copolymère, dueà l’introduction du monomère 2-hydroxyethyl méthacrylate (HEMA), induit une diminution de la viscoélasticité dans le cas de* Corresponding author.E-mail address: quadrat@imc.cas.cz (O. Quadrat).C. R. Chimie 6 (2003) 1411–1416© 2003 Académie des sciences. Published by Éditions scientifiques et médicales Elsevier SAS. All rights reserved.doi:10.1016/j.crci.2003.09.004HEUR, mais pas dans le cas de HASE. Ceci confirme que l’adsorption de groupements terminaux hydrophobes sur la surface desparticules joue un rôle important dans le phénomène d’épaississement dans le cas des HEUR, où un réseau physique de particulesde latex est formé par les macromolécules d’épaississant. Pour citer cet article: O. Quadrat et al., C. R. Chimie 6 (2003).© 2003 Académie des sciences. Published by Éditions scientifiques et médicales Elsevier SAS. All rights reserved.Keywords: latices; thickening; acrylic copolymers containing 2-hydroxyethyl methacrylate groups; associative thickeners; viscoelasticityMots clés : treillis ; épaississement ; copolymères acryliques contenant des groupements 2-hydroxyéthyl méthacrylate ; épaississants associatifs ;viscoélasticité1. IntroductionRheological characterisation is one of the most im-portant methods providing information on suitabilityof latex materials for practical use. Reliable knowledgeof flow parameters is especially required to set theoptimum flow properties of thickened latices [1]. Atpresent, excellent results in latex thickening are ob-tained with associative thickeners based on hydropho-bically modified ethylene oxide urethane block co-polymers (hydrophobic ethoxylated urethanes,HEUR) or hydrophobically modified alkali-solubleemulsion (HASE) [2,3]. It is assumed that hydropho-bic segments of those materials adsorb on the latex andpigment particle surface [4–11]. Consequently, theparticles are bridged by thickener macromolecules andthe system forms a physical network, which is gradu-ally destroyed after a shear field is applied. Due to thenetwork structure, elasticity of a thickened latex maybe expected, which may significantly influence appli-cation of this material.Latices of acrylic copolymers carrying hydrophilicreactive groups are increasingly important as promis-ing materials in the painting industry. Even a smallamount of these groups may significantly improveproperties of coating films if additional crosslinkingwith various resins or polyisocyanate proceeds afterapplication. Incorporation of a hydrophilic monomerinto the latex copolymer may change adsorption activ-ity of associative thickener and, consequently, the flowor elastic behaviour of the thickened material. In thisstudy, viscoelastic properties of model acrylic laticesof copolymers, both with and without 2-hydroxyethylmethacrylate (HEMA), thickened with commercial as-sociative thickeners (HEUR and HASE) produced byRohm & Haas, are investigated.2. Experimental2.1. laticesTwo model latices of copolymers of butylacrylate/styrene/acrylic acid with 0 and 25 wt % of2-hydroxyethyl methacrylate denoted L0 and L25 witha fraction of solids of ca 50 wt % were prepared by anon-seeded semicontinuous emulsion copolymerisa-tion with a monomer emulsion feed. The polymerrecipe and the composition of the monomer feed aregiven in Tables 1 and 2. Technical-grade monomersbutyl acrylate (Rohm, Darmstadt, Germany), styrene(Kaučuk Kralupy, Czech Republic), 2-hydroxyethylmethacrylate (Rohm, Darmstadt, Germany) andacrylic acid (Eastman Sokolov, Czech Republic) wereused. Polymerisations were carried out in a glass reac-tor under nitrogen atmosphere using an ammoniumperoxydisulphate–sodium disulphite initiator systemand Disponil AES 60 (sodium polyglycol alkylarylTable 1Polymerisation recipe for latices (in grams)Reactor chargeWater 200Sodium disulphite 4Monomer emulsion feedWater 200Disponil AES 60 52Ammonium peroxydisulphate 8Monomers 800Table 2Composition of monomer feed (wt %)Latex ButylacrylateStyrène HEMA Acrylic acidL0 49 49 0 2L25 36.5 36.5 25 21412 O. Quadrat et al. / C. R. Chimie 6 (2003) 1411–1416ether sulphate, Henkel, Germany) in an amount of2 wt% of the active component (relative to the mono-mers) as an emulsifier. Such process guarantees statis-tical composition of the dispersed copolymer particles[12–14].Low-molecular-weight salts were removed from la-tex samples by dialysis (Dialysierschlauch Kalle AG,Wiesbaden, Germany) against ultra-filtrated water(Milli Q) carried out until a constant conductivity wasattained.2.2. ThickenersThe effect of two commercial associative thickenersfrom Rohm & Haas (Germany), RM 12 W (HEUR)and TT 615 (HASE) has been investigated. No otherinformation was provided by the manufacturer, exceptthe expected rheological properties of the resultingthickened systems given in Table 3.Capillary viscometry of dilute alkalinized thicken-ers (Fig. 1) revealed a great difference in intrinsicviscosity [g] of these materials (Table 3). The relativelylow value of this quantity for HEUR indicates well-dissolved HEUR macromolecules, possibly assembledin flower-like micelles [15]. In contrast, a high intrinsicviscosity of HASE is a result of highly swollen par-ticles. A higher value of the Huggins constant kH pre-dicts a steeper increase in viscosity with the HASEconcentration and the difference between the twothickeners becomes even more pronounced after alka-linization (Fig. 2).2.3. Latex particle characterisationThe effective hydrodynamic volume of latex par-ticles alkalinized with ammonium hydroxide to pH8.5–8.8 at infinite dilution (intrinsic viscosity [g],Table 4) was obtained by linear extrapolation of U/ lngr to the zero volume fraction U of the latex (Fig. 3)according to the Mooney equation [16]:(1) gr = exp {[g]U/(1 – U/Uc)}where Uc is the volume fraction of particles at whichviscosity reaches the infinite value (the volumefraction at maximum packing of particles). The relativeviscosity gr =g/gs, the ratio of the viscosity of latex gand that of the dispersion medium gs, was measuredwith an Ostwald capillary viscometer at a constantlow-shear stress s ≅ 0.8 Pa.Table 3Characteristics of Rohm & Haas thickenersType Product Producer’s description [g](dl g–1)kHHEUR RM-12 W The most pseudoplastic of theHEUR‘s, provides excellent lowshear thickening efficiency0.9 1.2HASE TT 615 Very efficient at low shear butvery thinning at high shear8.0 2.5Fig. 1. The dependences of reduced specific viscosity gsp/c on thethickener concentration c. ([) HEUR, (C) HASE.Fig. 2. The dependence of viscosity g of thickener solutions on theconcentration c. Points denoted as in Fig. 1.Table 4Characteristics of latex particles. Particle diameter of acid latices(da) and intrinsic viscosity [g], maximum packing volume concen-tration Uc and particle diameter of alkalinized latices (db)Latex da (nm) [g] Uc db (nm)L0 124 3.14 0.63 124L25 128 5.58 0.52 1341413O. Quadrat et al. / C. R. Chimie 6 (2003) 1411–1416Hydrodynamic diameters of particles of originalacid and alkalinized latices were measured by dynamiclight scattering (Auto-Sizer LoC, Malvern Instru-ments, UK); the difference between L0 and L25 wasless than 10 % in both cases (Table 4).2.4. Sample preparationWe investigated latices of a medium concentration(30 wt % of solids), similar to the particle concentra-tion in a water-borne painting material, thickened with0.5 wt % of the thickener. The pH of the samples wasadjusted by adding dilute aqueous ammonia to a mix-ture of dialysed latex with the thickener until pHreached the required value (8.5–8.8).pH measurements were carried out using a digitalpH-meter (Radiometer Copenhagen) with a combinedelectrode GK 2321 C. All measurements, includinglatex characterisation, were performed at 25 °C, withsamples aged one day after pH adjustment.2.5. Rheological measurementDynamic oscillatory experiments were performedon a rheometer ARES (Rheometrix Scientific, USA)equipped with a cone-plate geometry of 50 mm diam-eter and 0.02 rad cone angle. Measurements were car-ried out at room temperature in the region of linearviscoelasticity.3. Results and discussionThe latex characteristics showed (Table 4) that theHEMA presence in the latex copolymer significantlyincreases the effective hydrodynamic volume of alka-linized latex particles due to increased swelling andextension of ionic atmosphere [17]. Consequently,both complex viscosity g* and the storage and lossmoduli, G′ and G′′ , of 30 wt % latex L25 also exceedthose of L0 (Fig. 4). In both cases, the loss modulus ismuch higher than the storage modulus, which reflectslow elasticity of latices. The decrease in complex vis-cosity g* with frequency indicates a slight pseudoplas-tic character of both latices.Dynamic properties of latex L0 thickened withHEUR differed markedly from those of latex L0 thick-ened with HASE. For both thickeners, the low-frequency dynamic moduli increased by more than oneorder of magnitude; however, the storage componentwas dominant in the whole frequency range of HEURspectra (Fig. 5a) whereas it exceeded the loss compo-nent only slightly and only at the highest frequenciesused with HASE (Fig. 5b). This behaviour may beascribed to the adsorption activity of the hydrophobicsegment of the thickeners on the latex particle surface.The HEUR thickener may be assumed to adsorb byhydrophobic ends to particles of latex L0 and formingFig. 3. The Mooney plot of U/ln gr vs. U, for alkalinized latices L0([) and L25 (C).Fig. 4. The frequency dependence of the storage modulus G′ ([),loss modulus G″ (C) and complex viscosity g* (_) of latex L0 (a)and latex L25 (b).1414 O. Quadrat et al. / C. R. Chimie 6 (2003) 1411–1416a relatively strong physically bonded highly elasticnetwork of bridged latex particles. In contrast, bridgingby HASE is weak, and the somewhat higher complexviscosity of L0 thickened with HASE is caused byexcluded volume of swollen HASE dispersion par-ticles.Fig. 6 illustrates the effect of the HEMA inclusionin the latex copolymer on the viscoelasticity of thick-ened material. With both thickeners, storage and lossdynamic moduli were higher for L25 than for L0. Inthe case of HASE, the storage modulus at low frequen-cies was lower than the loss modulus; with increasingfrequency, this difference diminishes similarly tothickened L0. This indicates that the viscous characterdominates in both HASE-thickened latices and, there-fore, is not influenced by the HEMA present in latexparticles.On the other hand, if the latex L25 was thickenedwith HEUR thickener, the loss modulus at low fre-quencies increased more than the storage modulus andthe viscous component prevailed. At medium frequen-cies, the moduli became identical and at higher fre-quencies, the loss modulus decreased and reached alimit value. The different viscoelastic character ofthickened latices is clearly characterised by the fre-quency dependence of the phase angle as the ratio ofthe loss and storage moduli (tan d) in Fig. 7.If HEMA is incorporated in the latex copolymers,the particle surface becomes more hydrophilic and itsFig. 5. The frequency dependence of the storage modulus G′ ([),loss modulus G′′ (C) and complex viscosity g* (_) of latex L0thickened with 0.5 wt % of HEUR (a) or HASE (b) thickener.Fig. 6. The frequency dependence of the storage modulus G′ ([),loss modulus G″ (C) and complex viscosity g* (_) of latex L25thickened with 0.5 wt % of HEUR (a) or HASE (b) thickener.Fig. 7. Frequency dependence of tan d. Latex L0 (C) and L25 (● )thickened with HEUR, latex L0 (e) and L25 (◆ ) thickened withHASE.1415O. Quadrat et al. / C. R. Chimie 6 (2003) 1411–1416association with the hydrophobic segments of thethickener is lower. Consequently in the case of HEUR,the latex–thickener network, responsible for the elas-ticity of the material, is weaker and the loss modulusprevails especially at low frequencies corresponding tohigh relaxation times. At higher frequencies, however,the response to the frequency is too low and the storagemodulus becomes dominant.In the latices thickened with HASE, the thickeningdue to hydrodynamic interaction of swollen latex andthickener particles is more important than the adsorp-tion mechanism. The swollen physical network isloose and its elasticity is lower in comparison withHEUR-thickened system, which results in higher lossmodulus. In this case, the HEMA on the surface oflatex particles has a much lower effect on the structureof the thickened system. After thickening, the coursesof the dependence of loss and storage moduli of laticesL0 and L25 thickened with HASE on the frequency aresimilar.It has to be stressed that the description of theHEMA effect on viscoelasticity applies to HEUR andHASE at concentrations used and generalizationshould by made with regard to the predominant thick-ening mechanism (particle bridging vs. excluded vol-ume increase) rather than to the thickener type (HEURvs. HASE), because it is possible that after equalizing,for example, the low frequency viscosity, either byincreasing the HEUR concentration or by decreasingthe HASE concentration, the thickening mechanismand therefore the HEMA effect will become similar.4. ConclusionThe results demonstrated that the presence of hy-drophilic comonomer, such as 2-hydroxyethyl meth-acrylate, in the copolymer of acrylic latices thickenedwith hydrophobically modified urethanes (HEUR)might significantly influence the elasticity of the prod-uct. In contrast, a much lower elasticity and higherviscosity of latices thickened with hydrophobicallymodified alkali-soluble emulsion (HASE) are not af-fected.AcknowledgementsThe Grant Agency of the Czech Republic and theMinistry of Education of the Czech Republic are grate-fully acknowledged for supporting this work by grantNos. 104/02/1360 and MSM 2531 00001.References[1] H.L. Beeferman, D.A. Bergren, J. Paint. Technol. 38 (1966) 9.[2] P.R. Howard, J. Coatings Technol. 64 (1992) 87.[3] R.D. Hester, D.R. Squire Jr, J. Coatings Technol. 69 (1997)109.[4] J.H. Bieleman, F.J.J. Riesthuls, P.M. van der Velden, Polym.Colour J. 176 (1986) 1.[5] P.A. Reynolds, Prog. Org. Coatings 20 (1992) 393.[6] K. Alahapperuma, J.E. Glass, Prog. Org. Coatings 21 (1992)53.[7] Z.Y. Ma, M. Chen, J.E. Glass, Colloids Surfaces A 112 (1996)163.[8] T. Svanholm, F. Molenaar, A. Toussaint, Prog. Org. Coatings30 (1997) 159.[9] T. Svanholm, B. Kronberg, F. Molenaar, Prog. Org. Coatings30 (1997) 167.[10] Q.T. Pham, W.B. Russel, W. Lau, J. Rheol. 42 (1998) 159.[11] B. Emelie, U. Schuster, S. Eckersley, Prog. Org. Coatings 34(1998) 49.[12] J. Šňupárek, F. Krška, J. Appl. Polym. Sci. 20 (1976) 1753.[13] J. Šňupárek, Makromol. Chem. Suppl. 10–11 (1985) 129.[14] J.W. Morgan, D.P. Jensen, Makromol. Chem. Suppl. 10–11(1985) 59.[15] M.A. Winnik, A. Yekta, Curr. Opin. Colloid Interface Sci. 2(1997) 424.[16] M.J. Mooney, J. Colloid Sci. 6 (1951) 162.[17] J. Horský, O. Quadrat, L. Mrkvičková, B. Porsch, J. Šňupárek,Colloids Surf. 180 (2001) 75.1416 O. Quadrat et al. / C. R. Chimie 6 (2003) 1411–1416

Viscoelastic behaviour of butyl acrylate/styrene/2-hydroxyethyl methacrylate/acrylic acid latices thickened with associative thickeners

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Viscoelastic behaviour of butyl acrylate/styrene/2-hydroxyethyl methacrylate/acrylic acid latices thickened with associative thickeners

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