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Controlled polymers for pigment dispersants

Clemens Auschra, Ernst Eckstein, Ralf Knischka, FrankPirrung, Peter Harbers.Techniques based on controlling free radical polymerizationhave recently been commercialized for pigment dispersions.Such systems based on acrylic block copolymers showgood dispersant properties and could help to meet thechallenges of future coatings.Polymeric pigment dispersants are an indispensable class ofadditives to realize high quality pigment dispersions for usein modern paints [1-3]. A uniform and stable dispersion stateof the pigments is necessary to achieve good colouristicproperties and high gloss in the final coating. In addition,well-selected pigment dispersants can contribute toincreased production economics by allowing higher pigmentloads and faster grinding. A prominent example of advancedproduction concepts is the use of resin minimal or resin freepigment concentrates.New developments in pigment dispersants are gearedtowards more efficient products which offer superiorrheological performance and which can be applied broaderin different coating systems. The polymer architecture andthe type of pigment affinic anchoring groups are keyparameters to tailor dispersant performance [4,5]. Goodcontrol on polymer architecture requires appropriateadvanced polymerization techniques. Only very recently hasthe novel pioneering technique of controlled free radicalpolymerization (CFRP) been commercialized in the field ofpigment dispersants [6].In this contribution acrylic block copolymer type "controlledpigment dispersants" are presented based on controlled freeradical polymerization technology. Examples are given ofhow the polymer design capabilities of CFRP can betranslated into improved solutions in pigment dispersion.

Novel polymerization regulators for CFRPAmongst the different methodologies for controlled freeradical polymerization which have been studied in academiaand industrial research, the nitroxyl-mediated controlled freeradical polymerization has proven very useful for thesynthesis of defined block copolymers [7, 8]. The chemicalmechanism of nitroxyl-mediated controlled free radicalpolymerization is based upon the reversible capping of agrowing polymer chain radical by a stable nitroxyl radical(Figure 1). During polymerization, only a very smallconcentration of active polymer chains is present inequilibrium with "dormant" polymer chains. This reducesunwanted side reactions and, if certain kinetic conditions arefulfilled, a "controlled" polymerization results. This leads towell-defined polymers with narrow molecular weightdistribution [9]. The "living" character of the controlledpolymerization also enables the synthesis of blockcopolymers by sequential addition of different monomers.New classes of sterically hindered alkoxyamine compounds,like special open chain NOR [10], piperazinone-type NOR[11], piperidine-type NOR [12] and 7-ring heterocyclic NOR[13] have been developed. These NOR polymerizationregulators represent a versatile and robust toolbox for thesynthesis of functional polyacrylates with controlledstructure. Some representative examples of NORcompounds, which have been shown to be especially usefulfor the synthesis of polyacrylate dispersants are shown(Figure 2).The reaction conditions for the use of such NOR regulators

are compliant with the basic requirements of industrialpolymer production, e.g. there are no stringent requirementson the purity of the raw materials and the process does notrequire special precautions for the handling of very reactiveor toxic compounds.A representative example for the controlled polymerizationof butylacryate using the regulator NOR 3 is shown (Figure3). The GPC-analysis of the polymer formed at differentpolymerization times displays all the elements of awell-controlled polymerization process: The molecularweight Mn continuously increases with monomer conversionand the molecular weight distribution stays narrow withoutbroadening towards the low molecular weight side.

Synthesis of controlled block copolymer dispersantsControlled polymerization using the specific NORcompounds shown in figure 2 enables the synthesis ofacrylic block copolymers by sequential polymerization ofdifferent monomers or monomer compositions. A series ofdifferent AB-type dispersants containing an aminicanchoring block was synthesized by using the regulators inFigure 2; Table 1. All samples are very comparable inmolecular weight in the range of 10000 to 12000. Effects ofthe variation of the molecular weight of similar AB-type blockcopolymers have been described elsewhere [14]. The stericstabilizer block of the block copolymers tested was alwaysselected from monomers of medium polarity. The blockcopolymers BC-3 to BC-5 differed primarily by the type andamount of pigment anchoring groups in the B-block.Different degrees of cationic charges, as well asmodification with special acidic anchoring groups were used.

Application testing of controlled dispersantsThe block copolymer dispersants were evaluated ondifferent pigments in comparison to with convetionalbenchmark dispersants R-1 to R-5. In one set ofexperiments resin minimal pigment concentrates (RMPC)were prepared and let down into different industrial coatingsystems including white reductions (Table 2).

Carbon black pigments have high specific surface areaCarbon black pigments, especially the type HCC are difficultto disperse, because most grades have extreme highspecific surface area. High concentrations of activedispersant of typically 30 - 70% relative pigment are neededto achieve acceptable stabilization and reduction of millbaseviscosities.Compared to the references R1, R2 and R5, the controlleddispersant BC-1 combined excellent millbase rheology anda fully stable behavoir in the paint. At use levels of 50% orhigher, very low viscosity with almost ideal Newtonian flowprofile is achieved (Figure 4). The masstone pour outs alsoshowed very high gloss and perfect transparency,demonstrating complete flocculation free behaviour in thepaint.

Performance of controlled dispersants in RMPCResin minimal pigment concentrates (RMPC) is a conceptby which pigment concentrates with low resin content canbe used in different coating systems. The controlleddispersant BC-2 is a product that is optimized for broadapplicability in RMPC. Pigment concentrates according toTable 2 were compared in rheology and concerning the

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quality of colouration in typical industrial coating systems.Out of the group tested pigments, the controlled dispersandBC-2 provided superior milbase rheology with mostpigments as exemplified with the iron oxide red (Figure 5).Only in the case of the tested blue pigment concentrate, theblock copolymer BC-2 could not provide the same strongviscosity reduction, but nevertheless it gives excellentuniform colouration in different coating systems, what is aneven more important criterion for this application.The blue pigment concentrate containing the controlleddispersant BC-2 provided very uniform colouration in thedifferent coating systems under conditions of low shearincorporation (Figure 6a, b). The colour positions are muchcloser together than in the case of the reference dispersantsR-4 and R-5.The dispersant BC-2 also provided similar advantagesconcerning uniform colouration with other pigments such asthe iron oxide red. These results demonstrate that thecontrolled dispersant BC-2 offers a very efficient solution forresin minimal pigment concentrates with broad applicabilityto different coating systems.

Controlled dispersants for specific organic pigmentsFor very demanding applications like those with transparentorganic pigments, it can be beneficial to optimize thedispersant structure for a specific pigment. In particular,pigments that have not been surface treated with polarderivatives can cause severe dispersion problems becausethey do not allow good interaction with common anchoringgroups. In this section an example is given for theoptimization of AB-type controlled pigment dispersants forthe non-surface treated pigment PR 254-I, which belongs tothe diketo-pyrrolo-pyrrole (DPP) class and has a very highspecific surface area of BET = 94 m2/g.Typical commercial dispersants and also BC-1 did notprovide significant lowering of the millbase viscosity, even atvery high addition levels. Also the paints derived from thesedispersions showed poor transparency and flocculation.These dispersants did not perform well because they do notcontain the right anchoring groups. Only the blockcopolymer BC-3, which was modified with special aromaticacidic anchoring groups, showed somewhat improvedrheology and good transparency.Starting from BC-3 the anchoring block was furtheroptimized with special chemical groups which haveimproved adsorption capability on PR 254-I. The blockcopolymers BC-4 and BC-5 have the same structure asBC-3 except that the B-Block contains different anchoringgroups and different degrees of cationic charges. BC-4 andBC-5 gave very strong viscosity reduction even at muchhigher pigment load of 18% wt (Figure 7).The paints that were derived from the concentrates usingBC-4 and BC-5 showed excellent transparency and veryhigh gloss. This proves that the dispersion of the nano-sizedpigment particles was very stable. Such an improveddispersion of transparent pigments like PR 254-I allows formore brilliant colouration, for example in automotive metallicbase coats.

Conclusions and OutlookSpecial designed NOR-polymerization regulators allow forthe synthesis of acrylic copolymers with very well definedstructures. Based on the NOR-technology, novel AB-typeblock copolymer dispersants have been developed fordifferent solvent-based paint applications. Key parametersfor the optimization of dispersant performance are theselection of the steric stabilizer chain and the type ofanchoring groups.Novel "controlled" pigment dispersants have been

developed. These include BC-1, which offer excellentperformance on difficult pigments like carbon black, andBC-2, which is an ideal dispersant for multipurpose resinminimal pigment concentrates for industrial coatings. Byoptimizing the pigment anchoring chemistry, AB-typedispersants can be tailor-made for specific pigments. Thishas been demonstrated on the example of a transparentDPP pigment.In the area of acrylic chemistry, NOR-technology gives forthe first time access to advanced polymer architecturesunder viable conditions of industrial polymer production.Controlled pigment dispersants are the first example of thecommercialization of functional polymers made by controlledfree radical polymerization. The versatility of acrylicchemistry combined with the polymer design possibilities ofNOR-technology forms a powerful technical platform torealize novel functional materials. Development workcontinues to expand the product offering on controlledpolymer pigment dispersants and will be extended to othertypes of coating additives. It can be expected that newmaterials based on controlled polymerization will make asignificant contribution to meet the challenges of futurecoating technologies.

AcknowledgementsThe authors would like to thank all their colleagues at CibaSC and EFKA Additives for their contributions andcooperation, especially: Peter Nesvadba and AndreasMuehlebach for their pioneering research work onpolymerization regulators and controlled polymers; AlmutStaniek and Werner Steiner for assistance in polymersynthesis; Tissa Rebmann, Matthias Graber and Piet vander Steeg for assistance in paint applications; and MartinPhilipoom for application expertise and discussions.

Literature[1] J. Bielemann in : 'Lackadditive', Ed. J. Bieleman,Wiley-VCH Verlag GmbH, Weinheim, 1998, p.67[2] J. D. Schofield, in L.J. Calbo (Ed.), 'Handbook of CoatingAdditives', Vol.2, Marcel Decker, New York, 1992, pp.71-104[3] F.O.H. Pirrung, P.H. Quednau, C. Auschra, Chimia, 56,(2002), 170[4] H.L. Jakubauskas, J. Coat. Techn. 58 (736), (1986), 71[5] H.J.W. van den Haak, J. Coat. Techn., 69 (873), (1997),137[6] P. Harbers, product presentation EFKA-4300 andEFKA-4330, European Coatings Show, Nuernberg, April2003[7] K. Matyaszewski, J. Xia, Chem. Rev., 101, (2001), 2921[8] D.H. Solomon, G. Waverly, E. Rizzardo, P. Cacioli, US 4581 429, 1986[9] H. Fischer, J. Polym. Sci.: Part A: Polym. Chem., 37,(1999), 1885[10] M.O. Zink, A. Kramer, P. Nesvadba, Macromolecules,33, (2000), 8106[11] P. Nesvadba, A. Kramer, M.O. Zink, GB 2 342 649,2000[12] A. Kramer, P. Nesvadba, GB 2 335 190, 2000[13] P. Nesvadba, A. Kramer, M.O. Zink, US 6 479 608,2002[14] C. Auschra, E. Eckstein, A. Mühlebach, M.O. Zink, F.Rime, Prog. Org. Coat., 45, (2002), 83

Results at a glance- Special designed NOR-polymerization regulators allowacrylic copolymers with very well defined structures to bemade.- Novel AB-type block copolymer dispersants have been

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developed for different solvent-based paint applications. Keyparameters for the optimization of dispersant performancewere the selection of the steric stabilizer chain and the typeof anchoring groups.- The novel "controlled" pigment dispersant BC-1 offersexcellent performance on difficult pigments like carbonblack.- The novel "controlled" pigment dispersant BC-2 is an idealdispersant for multipurpose resin minimal pigmentconcentrates for industrial coatings.- By optimizing the pigment anchoring chemistry, AB-typedispersants can be tailor-made for specific pigments.

The authors:> Clemens Auschra received his Ph.D in polymer chemistryfrom University of Mainz in 1992 and afterwards worked forRöhm GmbH and RohMax GmbH. After joining CibaSpecialty Chemicals in 1998, he has been engaged in thedevelopment of coating additives, now responsible as R&Dmanager for Polymer Specialties.> Ernst Eckstein received the BS degree (chemicalengineering) from the Basle Institute of Technology in 1987.He has been working in R&D for Dynamit Nobel and later intechnical service and R&D for Ciba Specialty Chemicals.Since 1998 he has been responsible for the applicationlaboratory for Polymer Specialties.> Ralf Knischka received his PhD in MacromolecularChemistry from the University of Freiburg in 2000. Afterworking for mnemoScience GmbH, he joined Ciba SpecialtyChemicals in 2001. He has been engaged in R&D ofPolymer Specialties as coating additives.> Frank Pirrung received his Ph.D in organic chemistry fromthe University of Amsterdam in 1995. He joined EFKAAdditives as development chemist in the field of polymers.Since 2000, he has been Head of R&D at EFKA, beingresponsible for the R&D programs for coating, graphic artsand plastic additives.> Peter Harbers worked since 1988 for Chemie Gro, laterknown as Exachem and Beer Lakfabrieken, responsible fordeveloping automotive and car refinish paints. In 2000 hejoined EFKA Additives as an area manager in technicalservice, now responsible as Head Application Development.Comité of scientists awarded Clemens Auschra and hisco-authors with the "Fatipec 2004 Prize for Excellence".

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Figure 1: Mechanism of nitroxyl-mediated controlled free radical polymerization.

Figure 2: New NOR-polymerization regulators for acrylic monomers.

Figure 3: GPC-analysis of the controlled polymerization of n-butylacrylate in bulk withNOR 3.

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Figure 4: Millbase viscosities with carbon black.

Figure 5: Viscosities of red pigment concentrates.

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Figure 6a: Colour measurement on white reductions of blue pigment concentrates;shaded area as guide for the eye to compare the spread of data.

Figure 6b: Colour measurement on white reductions of blue pigment concentrates;shaded area as guide for the eye to compare the spread of data.

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Figure 7: Millbase viscosity of pigment concentrates with 18% pigment load of PR 254-I.

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Table 1: Overview on the dispersants used in this study; *) estimate.

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Table 2: Characteristic data of resin minimal pigment concentrates; "Laropal A81" isused as grinding resin.

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