1 9 2 ICGCNOflTHAMERrCA VOLUME 2S NUMBER 2 FEBfiUAHY 2007 www. chromatographyonllne.com

Application of SEC-MALS inPaint Manufacturing

Stepan Podzimek and DanrelaVareckova

SYNPO, 532 07 Pardubice, CzechRepublic

Please direct correspondence to StepanPodzimek at stepan.podzimek@synpo. cz

Paints are an important part of our everyday life. The averageconsumption of paint per year, that is the amount of paint used in agiven country divided by number of people living in a country, isseveral tons. Paint has not only a decorative purpose, but also a veryimportant protective effect. Paint typically consists of polymer binders,solvents, pigments, fillers, and additives. The polymer binder is a film-forming component and gives the paint film good mechanicalproperties. Molar mass distribution and often, branching, are keyproperties of polymer binders influencing not only the viscosity andsolids of paints, but the properties of final paint films as well. Sizeexclusion chromatography (SEC) has been used for the characterizationof paint binders and paint-related compounds for several decades (1,2).In the beginning, SEC analysis was used typically for simple graphicalfingerprint comparison of chromatograms of similar samples with nopossibility of determining true molar mass or gaining informationabout the molecular architecture. The development of improved light-scattering detection, methodology, and data interpretation (3,4) hasallowed detailed characterization of polymers used in the paintindustry.

S EC in its convenrional mode istypically performed with refrac-tive index (RI) detection, and to

determine molar mass it is necessary tofind the relationship between the elutionvolume and molar mass of eluied mole-cules. This calibration procedure typicallyis carried out by the measurement of aseries of standards of known molar massand narrow molar mass distribution.However, such standards are availableonly for a very limited number of poly-mers. In the area of synthetic polymers,narrow polystyrene standards are widelyused for establishing the SEC calibration.The obtained calibration relation can betransferred to a calibration of anotherpolymer using a so-called universal cali-bration procedure thai requires knowl-edge of constants of the Mark-Houwinkequation. It is necessary to point out thatneither suitable calibration standards northe constants of the Mark-Houwinkequation are available for most of thepolymer binders used in paint manufec-turing. The column calibration by poly-

styrene standards is employed prlmarik-for the characterization of polymerbinders in paint research and develop-ment and manufacturing. The obtainedresults can be used solely for comparisonof moiar mass distribution of similar sam-ples, but it is necessary to emphasiTe thaithey are often markedly far from the trin.'values with no ability to reveal branchingor non-SEC separation. Unfortunately,many SEC users are not aware uf this faciand the results of conventional SEC oftenare misinterpreted.

The calibration problem can be solvedmo5C effectively by using mukiangle lightscattering (MALS) detection capable ofon-line measurement of molar mass ofmolecules eluted from SEC columns.SEC-MALS is actually the only routineanaJytical technique that can yield truemolar mass information for various poly-mer binders used in paint manufacturing.The application of MALS detection notonly eliminates the need for column cali-bration, but also brings other advantages.The SEC-MALS method is not only

1 9 4 LCGC NORTH AMERICA VOLUME 25 NUMBER 2 FEBRUARY 2007 ivvvw. chroma tographyonline.com

o I.OxiO''

1000.012.0 14.0 16.0 18.0 20.0

Volume (mL)

Figure 1: SEC calibration curves of polystyrene (red) and alkyd resin (blue).

independenr of rhe calibration standards,but It is also significantly less sensitive toflow-rate variations, temperature varia-tions, injected mass, SEC column per-formance, and non-SEC separationmechanisms. Consequently, a typical fea-ture of SEC-MALS is vety good long-term repeatability, which makes it an idealtool for production quality control. Inaddition, data generated in various labora-tories using different SEC-MALS setupsare very reproducible. SEC-MALS alsocan provide additional information aboutthe polymers, such as the root meansquare (RMS) radius (also called radius ofgyration), detection and characterization

of branching, ot detection of even traceamounts of fractions with extremely highmolar mass. By plotting the molar massagainst the elution volume, SEC-MALScan reveal non-SEC separation mecha-nisms and, thus, help to find appropriateseparation conditions. Ir is also veryimportant that MALS detection is readilyadaptable, that is, it can be added easily toany current SEC sj'stem without the needfor any special arrangements.

There is a general demand for volatileorganic compound (VOC) reduction,which can he achieved by decreasing thesolvent content. However, the reducedsolvent content results in an undesirable

10.0 12.0 14.0 16.0 18.0

Volume (mL)20.0 22.0

Figure 2: RMS radius versus elution volume plots for three samples taken from the batchduring the synthesis of alkyd resin. The samples have identical chemical composition, but dif-ferent reaction conversions, vi/hich increases in the order red, black, and green.

increase of paint viscosity. The problemcan be solved pardy by the application ofbranched or hyperbranched polymers thatcan be well characterized by SEC-MALS.Branching influences not otjy the solu-tion viscosity, but also other importantpolymer properties including mechanicalproperties and chemical resistance. Bothmolar mass and molecular size must bemeasured to get branching characteriza-tion. Unlike classical light-scatteringmeasurement in a batch mode, the com-bination of SEC-MALS yields molarmasses and sizes for almost monodispersepolymer fractions separated by SEC, andthe polymer conformation can be revealedfrom the relation between the RMS radiu-sand molar mas.s. The sensitivity of state-of-the-art MALS detection allows thedetermination of molar mass as low as afew hundreds. However, the RMS radiascan be measured from about 10 nm,which means for molecules having amolar mass of about 10^ g/mol and more.The limitation comes from the fact thatmolar mass is determined from the inten-sity of scattered light extrapolated to zeroangle, whereas the RMS radius is deter-mined from the slope of the angular vari-ation of scattered-light intensity. Smallpolymer molecules scatter light with iden-tical intensity in all directions and theirRMS radius cannot be determined.Because many polymer molecules usa.1 inpaint manufacturing have a molar massbelow 10^ g/mol, another type of detec-tion must be used for the determinationof size. On-line viscometry can be usedtogether with the SEC-MALS setup tomeasure molar mass and size simultane-ously. The size is expressed as the intrinsicviscosity and the experimental data allowdetermination of the so-calledMark-Houwink plot. The slope of theplot (an exponent of the Mark-Houwinkequation) for linear polymers in thermo-dynamically good solvents is typicallyaround 0.7 and decreases with increasingdegree of branching. The method doesnot require data fot the corresponding lin-ear polymer, which is very importantbecause many branched polymer binderscannot be prepared in a purely linear con-formation.

Environmental regulations result ingrowing production of waterborne paints.These paints often are based upon poly-mers containing polar funaional groups

1 9 6 LCGC NORTH AMEffiCA VOLUME 25 NUMBER2 FEBRUAflV 2007 www, chroma tographyon line.com

fluss..dvma

3-63-85-76-810-610-7

390043004400470050005000

59,000334,00083,000177,00090,000105,000

140,000

1,546,000255.000

759,000272,000334,000

Three couples of different alkyd resins. The couples of a given alkyd are samples taken from the batch at different conversion.

i/Veight-average molar masses (M J determined by conventional SEC are markedly underestimated. The error increases with the sample conversion and canachieve hundreds percent.

Number-average molar masses IMn) are fairly comparable, because Mp in contrast with M^̂ counts mainly low molar mass fractions and the polystyrene andalkyd calibrations cross in the region of lower molar masses.

thai increase the probability of strongenthalpic interactions with SEC columnpackings. SEC itstlf generally cannotreveal these interactions, but the obtainedresults often suffer poor reproducibility.1'hf results generated by N4ALS detectionare independent of SEC separation and.thus, superior reproducibilit)' over con-ventional SEC is achieved. In addition,the interaction of the polymer with thecolumn packing is revealed readily fromthe molar mass versus elution volumeplot.

ExperimentalThe SEC-MALS setup consisted of aWaters Alliance 2695 Separations Module(Waters, Milford, Massachusetts), aminiDAWN photometer (Wyatt Technol-ogy Corporation. Sanra Barbara, Califor-nia), a ViscoStar viscometer (Wyatt Tech-nology Corporation), and an RI 2414detector (Waters). Two Pl^el Mixed-C orMixed-E coltimns (Varian, P;ilo Alto, Cal-ifornia) were used for SEC measurements,Tctrahydrofuran at a flow rate of 1ml./min was tised as a mobile phase. Sam-ples were injected as solutions in tetrahy-drofuran in a 100-|xL volume and variousconcentrations in the range of about0.2-2% (w/v). ASTRA software (WyattTechnology) was u.sed for data collectionand processing. Polymer binders wereeither commercially available or preparedin the author's laboratory (SYNPO. Par-dubice, Czech Republic). Specific refrac-tive index increments of the analyzedbinders, which are needed to determinedmolar mass from the intensity of scatteredlight, were calculated by ASTRA softwarefrom the injected mass and responseobtained from the RI detector.

l.OxiO" 1.0x10^ 1.0x10^

Molar mass (g/mol)

Figure 3: Mark-Houwir>k plot of alkyd resin.

100,013.0 14.0 15.0 16.0

Volume (mL)17.0

Figure 4: Molar mass versus elution volume plot and RI chromatogram for hexamethoxymethyl meiamine resin.

Results and DiscussionSeveral applications of SEC-MALS forthe characterization of various paintbinders are demonstrated in this article todemonstrate how SEC-MALS can pro-

vide es.sential information in the paintresearch and manufacturing process.

Alkyd resins are polymers used almostexclusively in paint manufacturing. Alkydresins c;m differ not oniy in molar mass

1 9 S LCGC NORTH AMERICA VOLUME 25 NUMBER! FEBRUARY 2007 www, chramatographyonline. com

1000.010.0 12.0 14.0

Volume (mL)16.0 18.0

Figure 5: Molar mass versus elution volume plots for tvi/o cationic polymers {green, blue)compared with the calibration curve of polystyrene (red). RI chromatograms are superim-posed in the plot.

distribution, but also in the degree ofbranching. The characterization ofbranching is complicated by the fact thatit is impossibile to prepare linear alkyd ofihe same composition. Alkyds are also agood example of polymers for which nei-

ther suitable calibration standards norparameters of universal calibration areavailable. Alkyds represent a traditionalpaint binder that has been replaced inmany application area.s by other polymersyielding better p;unt properties. However,

the great advantage of alkyds can be fbtmdin the tact that they arc made partly fromnatural oils, that is, renewable raw materi-als and, thus, continuous interest in thesepolymers can be expected. Figure 1 com-pares molar mass versus elution volumeplots (that is, SEC calibration curves) ofalkyd resin and polystyrene. Both plotsintersect, and toward Iower elution vol-umes, the plot of alkyd .shifts markedly tohigher molar masses, which is given by thebranched (that is. more compact) struc-ture of alkyd molecules. Possible errorsacquired by conventional SEC with poly-st)'rene calibration are shown in Tlible I.

Figure 2 depicts the RMS radius versusthirion volume plots of three samplestaken from the baicb during the synthesisof alkyd resin. The plot fbr the lowest con-version sample decreases evenly with theincreasing elution volume as can beexpected for the separation by the mecha-nism of size exclusion, while the plots ofthe other two samples rise at higher elu-tion volumes. This abnormal SEC elutionbehavior is caused by entanglement (5) oflarge, highly branched molecules in thecolumn packing and their delayed elution

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2 0 0 LCGC NORTH AMEf i ia VOLUME 25 HUMBER2 FEBRUARY 2007

together with smaller molecules separatedby pure size exclusion. As a consequenceof the delayed elu[ion, the slices at higherclution volumes are contaminated bysmall amounts of high molar mass frac-tions, and MAI^ detection measures theweight-average molar masses and the a-average RMS radii. The z-averages aremore sensitive to the high molar massfractions than the weight-averages, whichis the explanation of the abnormalupswing.

The Mark-Houwink plot of an alkydresin is shown in Figure 3. TheMatk-Houwink plot was obtained bycompleting the SEC-MALS setup withan on-line viscometer. The slope of theplot of 0.34 is well below the values forlinear random coils in thermodynamicallygood solvents (around 0.7) and gives evi-dence about the presence of branchedmolecules.

Paint binders often consist ofoligomcric compounds that can increasemolar mass by further reaction.s duringpaint application. An example of theSEC-MALS analysis of an oligomericpaint binder is shown in Figure 4. This

example proves the ability of modetnlight-scattering detection to measuremolar masses down to a few hundreds. Itis worth mentioning that a part of the sci-entific community still considers lightscattering capable of characterizing onlypolymers having molar mass above10,000 g/mol, which is certainly not true,as seen from Figure 4.

Figure 5 presents moiar mass versus eiu-tion volume plots of two cationic poly-mers containing neutralized aminegroups. Such polymers typically interactwith the column packing strongly, whichresults in poor separation according to thehydrodynamic size. SEC-MALS canreveal these interactions readily and, inaddition, provide at least correct weight-average molat mass (A/J, which is deter-mined by the MALS detector independ-ently of separation. The ability to measurethe molar mass as a function of elutionvolume can be further used for the modi-fication of separation conditions to elimi-nate non-SEC effects.

ConclusionsSEC-MALS can be used routinely for the

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analysis of various paint binders. Themethod is able to provide correct molarmass distribution, including samples ofvery low molar mass. In addition, it canreveal undesirable non-SEC mechanismsand yield at least correct M^., even in thecase of poor SEC resolution. A veryimportant feature of SEC-MALS is itsability to characterize branching, whichcan be enhanced tiirther by the additionof an on-line viscometer.

References(1) C,-Y. Kuo, X Provdcr. and A.F. Kah, Ch^anic

Coatings. Science and Technology, vnl. 6. C D .

Parfitc and A,V. Paisis, Eds. {Marcd Dekkcr.

Inc., New York, 1984). pp, 101-124,

(2) L. Mandik. Proff'. Organic Coatings 5.

131-198(1977),

(3) RJ, Wyan, Annlyiica Chlm. Acta 111, 1^0

(1993),

(4) S. Podzimek./ Appl Pnlym. Sci. M. ')i-l().i

(1994).

(5) S. Podzimck, T, Vicck. and C. Johann. ,/.

App/. Polym. Sci. 81. 1588-1594 (2001). •

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