38 June 2005 JCT CoatingsTechJCT CoatingsTech

INTRODUCTIONLiquid chromatography (LC) is prob-

ably the most commonly used methodof separation and analysis of chemicalcompounds and ions in solution.Increased understanding of liquid chro-matography since its invention in 1906has led to fascinating and dramatic de-velopment and usage of the techniquein a variety of applications.1-7 For exam-ple, instrumental advances, automation,and miniaturization of the techniquehas brought about the development ofsignificant methods utilizing complexseparation mechanisms, such as elec-trophoresis, supercritical fluid chro-matography, microscale separations, andnumerous powerful hyphenated tech-niques such as online and off-line liq-uid chromatography/nuclear magneticresonance (LC-NMR), and very efficientmulti-detector analysis systems for ex-tremely complex analyses.

High pressure liquid chromatogra-phy (HPLC) is an analytical techniqueused for separation of low-to-moder-ate molecular weight compounds ofresins. The instrumentation for HPLC

*Spring House Technical Center, 727 Norristown Rd., Spring House, PA 19477.

by Francis V. Acholla, Rohm and Haas Company*

High Pressure Liquid Chromatography in Coatings Analysis

and size exclusion (SEC) or gel-per-meation chromatography (GPC) issimilar, but the columns differ. Aschematic diagram of HPLC compo-nents is shown in Figure 1.

HPLC components consist of the fol-lowing: (a) solvent and/or solvent mix-ture, (b) solvent delivery system (pumpsand controller), (c) sample delivery sys-tem (manual or automatic injection),

(d) a column suitable for the separationof interest, (e) a detector system, and (f)a data system.

TYPES OF SEPARATIONMECHANISMS IN LIQUIDCHROMATOGRAPHY

There are numerous types of separa-tion mechanisms in liquid chromatog-raphy. The most important ones in-clude: affinity chromatography, ionchromatography, capillary electrophore-sis, normal phase chromatography, re-verse phase chromatography, supercriti-cal chromatography, and microscalechromatography.

Affinity ChromatographyAffinity chromatography is a tech-

nique that uses like-like interactions for

High pressure liquid chromatography (HPLC) is an established standard analytical tool

for the analysis of coatings and related materials. An extensive body of literature has been

published in various analytical and related fields’ journals on HPLC methods. These meth-

ods have been developed and used in the determination of small to very large molecules

ranging from small organic impurities to polymer resins. In addition, fundamentals of HPLC

and related methods have also been extensively covered in the literature and the reader is en-

couraged to review them as needed. The objective of this article focuses mainly on the appli-

cations of HPLC and gel permeation chromatography (GPC) methods in the analysis of

coatings.

Figure 1—Schematic diagram of HPLC components.

effective separation and analysis of sam-ple components in solution. The col-umn is designed to retain specific orsimilar analytes while allowing every-thing else through the column. The re-tained species can then be selectivelyeluted from the column by appropriatesolvent. A schematic example of affinitychromatography is shown in Figure 2.

Ion ChromatographyIon chromatography is a liquid chro-

matography method that can be usedfor the separation and quantification ofanions and cations. For example, anionsand cations that be separated and quan-tified are given in Table 1. A schematicanion separation and identification isshown in Figure 3.

In the context of this article, ionicchromatography is used in the determi-nation of acid species in the acid/ther-mal decomposition of polymeric mate-rials and ionic contamination inpolymers and paints.

Capillary ElectrophoresisThe capillary electrophoresis tech-

nique is based on the migration ofcharged molecules in an electrical field.The units are composed of a powersource with a voltmeter and voltage reg-ulator, an electrophoresis tank thatholds the electrophoresis buffer, an an-ode and a cathode connected to thepower source, a glass plate that holdsthe gel which is submerged into theelectrophoresis buffer, and a comb sam-pler. These units are based on the sim-ple relationship between the appliedelectric field (V), applied current (I),and resistance (R)—the Ohm’s law:

V = I R

The applied electric field is measuredin volts (V) and is proportional to the

current (I) measured in milliamps. For agiven voltage, a constant amount of cur-rent flows through the circuit and thevoltage decreases across any element asits resistance increases. Molecules whichhave electrical charge due to groups ca-pable of dissociating electrolytically (forexample proteins), will migrate in anelectrical field with a certain velocityand direction, which will depend ontheir electrical mobility. It has beenshown that electrical mobility of a mol-ecule is dependent on a variety of fac-tors, such as electrical charge, coefficientof friction, shape, and molecularweight.

While capillary electrophoresis is avery important tool in the separationand identification of bio-molecules, italso finds important application inpolymer analysis.8

Normal Phase LiquidChromatography

Normal phase liquid chromatogra-phy (NPLC) a powerful separation tech-nique, relies on the usage of polar sta-tionary phase and nonpolar liquidmobile phases for the separation of po-lar species. See the Type of ColumnPhases section below.

Reverse Phase LiquidChromatography

Reverse phase liquid chromatogra-phy (RPLC) is the most popularmethod of liquid chromatography. Thename is derived from the reverse elu-tion order of most solutes when com-pared to NPLC. It relies on the usage ofnonpolar stationary phases with polarmobile phases. Typical mobile phasesmay include water with added polar or-ganic solvent, such as acetonitrile. Seethe Type of Column Phases section.

Supercritical FluidChromatography

Supercritical fluid chromatography(SFC) is a technique that uses mobilephases as either gases or liquids takenabove their critical point. The advan-tages of this approach include the sol-vating power of liquid mobile phasesand diffusion coefficients closer togases. The lower viscosity of SFC allowsfor higher flow rates compared to LC,and the solvating power can be adjustedby changing the pressure. A supercriticalfluid chromatograph consists of a gassupply (CO2), a pump, a column in atemperature controlled oven, a restrictorto maintain high pressure in the col-

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Figure 2—Schematic separation scheme in affinity liquid chromatography.

Anions Cations

Bromide ....................Br– Ammonium................NH4+

Chloride ....................Cl– Calcium.....................Ca2+

Fluoride ....................F– Lithium.....................Li+

Nitrate......................NO3– Magnesium ................Mg2+

Nitrite ......................NO2– Sodium .....................Na+

Phosphate .................PO43– Zinc .........................Zn2+

Sulfate......................SO42–

Table 1—Anions and Cations that can beSeparated and Quantified

Abbreviations and AcronymsFTIR - - - - - - - - - -Fourier Transform InfraredGPC - - - - - - - - - -Gel Permeation ChromatographyHPLC - - - - - - - - -High Performance Liquid ChromatographyLC - - - - - - - - - - - -Liquid ChromatographyMS - - - - - - - - - - -Mass SpectrometryMWT - - - - - - - - - -Molecular WeightNMR - - - - - - - - - -Nuclear Magnetic ResonanceRPLC - - - - - - - - - -Reverse-Phase Liquid ChromatographyPyro-GC/MS - - - -Pyrolysis Gas Chromatography Mass SpectrometrySEC - - - - - - - - - - -Size Exclusion ChromatographyTHF - - - - - - - - - - -TetrahydrofuranUV - - - - - - - - - - -Utraviolet

which consists of a polar column materialand relatively nonpolar solvents, and (2)reversed phase HPLC which consists of anonpolar column material and relativelypolar solvents.

Reversed phase HPLC is the preferredtype of HPLC as it has proven to be thebest for analysis of many types of resins.The most common column packing ma-terials are nonpolar hydrocarbons, C8,C18 and phenyl bonded phasescolumns. The most common polar sol-vents for resins include acetonitrile(ACN)/water, methanol/water, andtetrahydrofuran (THF)/water. They arenormally used in the gradient elutionmode because they allow one to startwith low content of organic componentin the eluent (ACN or THF) then thestrongly retained components will movefaster for improved resolution. Perfor-

40 June 2005 JCT CoatingsTech

umn, and a detector. The flame ioniza-tion detector (FID) is most common,but other GC or LC detectors can alsobe used. SFC finds many applications inpolymer additives analysis.9

Microscale LiquidChromatography

Microscale liquid chromatography issimply the miniaturization of liquidchromatography technique. The mainadvantages are the significant reductionof solvents, shorter analysis times, andoverall improved sensitivities.

TYPE OF COLUMN PHASESThere are two main types of phase sep-

aration: (1) the normal phase HPLC

mance of gradient elution is stronglydependent on the instrumentation.Solubility of the material to be sepa-rated is a very important considerationin HPLC. For example, ACN/water isgood for low molecular weight resinsand is limited by solubility in highermolecular weight resins. THF/water isbetter and sometimes a mixture ofACN/THF/water may be the best for an-alyzing low to high molecular weightresins.

INJECTORS FOR HPLC Samples are injected into the HPLC

via an injection port. The injection portof an HPLC commonly consists of aninjection valve and the sample loop.The sample is typically dissolved in themobile phase before injection into thesample loop. The sample is then drawninto a syringe and injected into the loopvia the injection valve. A rotation of thevalve rotor closes the valve and opensthe loop in order to inject the sampleinto the stream of the mobile phase.Loop volumes can range between 10 µlto over 500 µl. In modern HPLC sys-tems, the sample injection is typicallyautomated.

DETECTORSDetection methods are critical to the

successful application of HPLC methodsin separation. Some of the conventionaland newer detection methods are:UV/Vis, fluorescence, conductivity, andluminescence, Fourier transform in-frared spectroscopy (FTIR), mass spec-trometry (MS), and nuclear magneticresonance (NMR). Exhaustive discus-sion of these newer detection methodshas been reviewed in the literature.10

An example of HPLC Run conditionsfor polymer emulsion and/or paints canbe as follows:

SAMPLE PREPARATION: The sample canbe dissolved in THF at roughly 1%(wt/wt) and centrifuged at 40,000 rpmfor 15 minutes. The supernatant is thenfiltered through 0.2 µm filters prior toinjection on the HPLC system.

CHROMATOGRAPHIC CONDITIONS:

Column: PLRP-S 4000Å 8µ analyticalcolumn (Polymer Labs), 4.6 mm × 15cm, 8 µm dp.

Mobile Phase: 25:75 THF/methanolinitially. Linear gradient to 75:25

Figure 3—Schematic representation of anion separation and identification by ion chromatogra-phy. The identification of the species is made against known references. Detection limits can beas low as parts per billion (ppb) levels.

Figure 4—HPLC analysis of two batches of surfactants for residual al-cohol impurities. Surfactant B is clearly purer than surfactant A(printed with permission from Dan Dohnmier, unpublished).

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THF/methanol over 35 min. Back to initial conditions in 5min. Equilibrate for 10 min.

Flow Rate: 1.0 ml/minInjection Volume: 10 µlColumn Temperature: AmbientDetector 1: UV @270 nmIDetector 2: Evaporative Light Scatter-ing Detector (ELSD)

APPLICATIONSRaw materials that go into the manufacture of paints are

sometimes more complex than the supplier or formulators re-alize. Components such as surfactants may be sold as “pure”or within certain specifications but may often contain severalby-products that may play an important role in the chemistryof the formulated paint. The following examples illustratehow HPLC can be used for raw materials and fully formu-lated paint analyses.

ANALYSIS OF RESIDUAL ALCOHOLS AND ALKANES IN

SURFACTANTS: Figure 4 shows HPLC analysis of two batches ofsurfactants for residual alcohol impurities. In Figure 5, theHPLC of two batches of a sulfated surfactant that are sup-posed to be chemically identical are presented.

ANALYSIS OF BINDER TYPE IN FULLY FORMULATED PAINT

SYSTEMS: Compositional analysis of paints can be a dauntingtask for analysts. The paint components can be dissolved inappropriate solvent (THF) and separated by either normalphase HPLC (polar column and nonpolar solvents) or re-verse phase HPLC (nonpolar column and polar solvents).For example, a failed coating can be extracted by THF; how-ever, depending on the composition of the formulatedpaint, analysts may encounter very challenging situationswhere answers to specific questions may depend very muchon many variables. For example, the use of organic opaci-fiers has many benefits including increased opacity, lowbinder demand, and partial TiO2 replacement, which signifi-

Figure 5—HPLC of two batches of a sulfated surfactant that are sup-posed to be chemically identical. Surfactant A appears to have signif-icantly higher levels of C10-C14 and <C8 hydrocarbons when comparedto Surfactant B (printed with permission from Dan Dohnmier, unpub-lished).

Figure 6—HPLC chromatogram of polybutylacrylate binder (top) andopaque polymer (paint additives to complement TiO2 pigment) (bot-tom). Polybutylacrylate (pBA) has one main peak labeled B, at a re-tention time of about 9-15 minutes, while the opaque polymer has amain peak labeled D, at a retention time of about 22-31 minutes.Peaks labeled A and C are probably due to impurities or some low MWTcomponents in the system (printed with permission from Haojie Yuan,unpublished).

Figure 7—HPLC chromatogram of two paints: A styrenic primer (top)and an all-acrylic copolymer/opaque polymer (bottom). The primer(top) shows four peaks labeled: A—an impurity or low mwt compo-nent; B—polymer component #1; C—polymer component #2; and D—opaque polymer. The all-acrylic paint (bottom), on the other hand,shows three peaks labeled: E—an impurity or low MWT component;F—the main polymer component (all acrylic copolymer); and G—opaque polymer. Thus, HPLC can be used to determine the composi-tion of fully formulated paint (printed with permission, Haojie Yuan,unpublished).

cantly reduces cost. The challenge ofdetermining binder types and compo-sition in fully formulated paints whichmay or may not contain organic opaci-fiers (mainly styrene, such as Ropaquesfrom Rohm and Haas Company), canbe very difficult. We have demon-strated in our labs that such systemscan be analyzed by HPLC and/orHPLC/FTIR and/or off-lineHPLC/Pyrolysis-GC/MS methods (seeFigures 6 and 7).

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Figure 8—Schematic representation of theseparation mechanism in a GPC column.

ANALYSIS OF ANTIFOULING ADDITIVES IN

PAINTS: Antifouling biocides are impor-tant in paints and coatings. Some ofthese preservatives include chlorothano-lin, dichlorofluanid, diuron, Irgarol1051, and SeaNine 211. These are rela-tively small molecules and the mostcommon method for analysis of theirpurity, efficacy, level, and fate in the en-vironment when associated with paintparticles is by HPLC.11 HPLC allows forthe separation, identification, andquantitative analysis.

ANALYSIS OF EPOXY COATINGS RESINS:Sheih and Benton have provided an in-depth overview of how HPLC methodscan be used in the analysis of epoxycoatings resins.12 Epoxy resins are usedwidely in coatings due to their superiorbalance of properties such as good ad-hesion, impact resistance, chemical re-sistance, and flexibility.

GEL PERMEATIONCHROMATOGRAPHY

Gel permeation chromatography(GPC), or size exclusion chromatogra-phy (SEC), is a branch of HPLC whoseprimary role is to provide molecularweight data on polymers and resins.GPC allows polymer synthesis chemistsand paint formulation and applicationscientists to isolate and characterize lowmolecular weight additives andmedium-to-high molecular weight poly-mers and resins in polymeric systems.

The two most common detectors are ul-traviolet (UV) and refractive index (RI).Only samples that absorb in the UV re-gion (with UV chromophore) can bedetected by UV detection; for example,aromatic rings, or conjugated doublebonds. If a polymer has no UV chro-mophore or is unknown, the RI detec-tor is the best choice. The RI detectorfunctions by monitoring the change inrefractive index between the pure mo-bile phase and that containing the sam-ple. The difference is the signal of thepeak(s) of interest.

In GPC, the column is packed with asemi-rigid gel or small organic particles,which function as a physical barrier forseparating molecules of different molec-ular weights. The key function of thestationary phase is to allow the polymermolecules to interact with the solventmolecules in such a way that those mol-ecules that have more physical interac-tion with the pores of the column pack-ing will take longer to elute than thosethat have no physical interaction withthe pores. For example, high molecularweight polymers will elute faster. Theseparation mechanism in a GPC col-umn is represented in Figure 8.

It can be simplistically shown thatthe smaller molecules can fit into thesmall pores and therefore take longer toelute through the column. The largermolecules, on the other hand, due totheir big size, are unable to enter thepores and simply travel around the gelparticles on their way out. Thus, thelarge polymer molecules are the first toelute out, followed by the medium size,and finally by the smaller molecules. Itis important to point out that GPC alsoseparates molecules according to theirhydrodynamic volume.13

In order for the GPC data to be use-ful, it must be calibrated with polymersof known molecular weight. Polystyrenestandards are often used because theyare readily available over a wide rangeof molecular weights.

APPLICATIONS IN FAILUREANALYSIS

Coatings scientists are always inter-ested in determining the compositionof polymers as they largely determinethe chemical resistance properties ofcoatings. GPC is one of the best meth-ods that can be used to measure themolecular weight of polymers in coat-ings. In order to accomplish this, the

Figure 9—GPC profile of THF extract of an unknown paint. Using poly-styrene standard, the molecular weight of the polymer was determinedto be about 437,000 with a polydispersity of 6.25.

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polymer must be isolated from thepaint by using an appropriate solvent.In the example shown in Figure 9, thepolymer was isolated from a paint byextraction in THF followed by centrifu-gation to remove the pigment and otherTHF insolubles.

In coatings failure analysis, GPC canprovide data to show whether the fail-ing paints conform to specification bycomparing and finger-printing failedpaint to a control paint. Sometimes,failures due to polymer decompositioncan easily be detected. For example,GPC performed on a failing and oncontrol paints showed a dramatic differ-ence in molecular weight, with the fail-ing samples being lower than the con-trol.14 There are numerous examples inthe literature of how GPC can be usedto investigate coating failures.15 It is im-portant to point out that GPC has twomajor limitations in coating analysis: itcannot be used on insoluble coatingsamples and it cannot provide chemicalinformation. However, the chemical in-formation limitation can be overcomeby interfacing GPC to a variety of chem-ically sensitive tools, such as FTIR,NMR, and MS.7,12

SUMMARYA general overview of the application

of HPLC techniques for analysis ofpaints and related materials has beenpresented. We have discussed how

HPLC can be used for raw materials andfor fully formulated paint analyses. GPChas been demonstrated as one of thebest methods that can be used to meas-ure the molecular weight of polymers incoatings and coatings failure analyses.However, these methods require thatthe polymer or isolated componentsmust be soluble in appropriate sol-vent(s).

References(1) Dorsey, J.G., “Liquid Chromatography”

and “Column Theory and Resolution inLiquid Chromatography” Encyclopedia ofAnalytical Chemistry, Vol. 15, 11231-11233 and 11334–1134 (2000).

(2) Hage, D.S., “Affinity Chromatography,”Encyclopedia of Analytical Chemistry, Vol.15, 11233-11251 (2000).

(3) Dolan, J.W. and Snyder, L.R., “GradientElution Chromatography,” Encyclopediaof Analytical Chemistry, Vol. 15, 11342-11360 (2000).

(4) Pietrzyk, D.J., “Ion Chromatography,”Encyclopedia of Analytical Chemistry, Vol.15, 11360-11382 (2000).

(5) Cooper, W.T., “Normal PhaseChromatography,” Encyclopedia ofAnalytical Chemistry, Vol. 15, 11428-11442 (2000).

(6) Lough, W.J., “Reverse Phase LiquidChromatography,” Encyclopedia ofAnalytical Chemistry, Vol. 15, 11442-11450 (2000).

(7) Provder, T., Barth, H.G., and Urban,M.W. (Eds.), in ChromatographicCharacterization of Polymers Hyphenatedand Multidimensional Techniques,Advances in Chemistry Series 247,American Chemical Society,Washington, D.C. (1995).

(8) Simio, C., Cottet, H., Vayaboury, W.,Giani, O., Pelzing, M., and Cifuentes,A., “Nonaqueous CapillaryElectrophoresis-Mass Spectrometry ofSynthetic Polymers,” Anal. Chem., 76(2) pp 335-344 (2004).

(9) Daimon, H. and Hirata, Y., “DirectlyCoupled Supercritical FluidChromatography for Analysis ofPolymer Additives,“ Chromatographia,32, 549-554 (1991).

(10) Patony, G. (Ed.), in HPLC DetectionNewer Methods, VCH Publishers, Inc.,New York, NY (1992).

(11) Thomas, K.V., McHugh, M., Hilton, M.,and Waldock, M., “Increased Persistenceof Antifouling Paint Biocides WhenAssociated with Paint Particles,”Environmental Pollution, 123 (1), 153-161 (2003).

(12) Sheih, D.P. and Benton, D.E., “HPLCAnalysis for Epoxy Coatings Resins,”Analysis of Paints and Related Materials:Current Techniques for Solving CoatingsProblems, ASTM STP 1119, AmericanSociety for Testing and Materials,Philadelphia, 41-46, (1992).

(13) Snyder, L.R. and Kirkland, J.J.,Introduction to Modern Chromatography,John Wiley, New York, NY, 512-514(1979).

(14) Weldon, D.G., in Failure Analysis ofPaints and Coatings, John Wiley andSons, New York, NY, pp. 206, 2001.

(15) Kuo, C. and Provder, T., “HighPerformance Gel PermeationChromatography Characterization ofOligomers Used in Coatings Systems,”JOURNAL OF COATINGS TECHNOLOGY, 53,No. 683, 69 (1981).

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