1636 POLYMER ENGINEERING AND SCIENCE, SEPTEMBER 2004, Vol. 44, No. 9

Removal of Ungrafted Monomer From Polypropylene-Graft-Maleic Anhydride via Supercritical Carbon Dioxide Extraction

KELLY CLARK and SUNGGYU LEE*

Department of Chemical EngineeringUniversity of Missouri-Columbia

Columbia, MO 65211

The removal of spent initiator, excess initiator and unbound monomer from newlysynthesized polypropylene-graft-maleic anhydride (PP-g-MA) has typically beencarried out using Soxhlet extraction. Standard solvents used in the Soxhlet processare xylene, toluene, acetone or methanol. These chemicals are not only hazardous tosome degree, but also lead to undesirable maleic anhydride (MA) ring opening. Thispaper establishes that supercritical carbon dioxide (scCO2) extraction of residualsprevents anhydride ring opening and eliminates the need for post-extraction vacuum-drying of the copolymer product. An added benefit of this innovative process is thatenvironmentally friendly carbon dioxide (CO2) is used in place of harmful solvents.The successful extraction of the residuals is established by thermogravimetric anal-ysis (TGA), whereas FTIR spectroscopy is used to compare the anhydride content ofSoxhlet-extracted PP-g-MA to that of scCO2-extracted copolymer. Polym. Eng. Sci.44:1636–1641, 2004. © 2004 Society of Plastics Engineers.

INTRODUCTION

The use of supercritical fluids as an extraction me-dium has been successful in many applications as

a replacement for liquid solvents and as a means of per-forming difficult separations. Supercritical carbondioxide has replaced methylene chloride as the solventchoice for caffeine extraction from coffee beans (1),whereas otherwise difficult polymer fractionation iscarried out in supercritical fluids to isolate relativelymonodisperse polymer samples (1�3). The utility of su-percritical fluids in extraction processes relies on theunique properties of the supercritical-fluid state. Thesolvating power of a substance increases approxi-mately with the density (1). Liquids have higher densi-ties than gases and generally are better solvents. Su-percritical fluids have intermediate densities that canbe tuned by pressure and temperature changes andthe solvating power is thus controlled through thesemanipulations of process conditions. Supercritical flu-ids also exhibit a near-critical phenomenon, density

augmentation, whereby the local density around dis-solved solute molecules is greater than the bulk density(4, 5). This clustering can enhance the solvating powerof the fluid at relatively mild extraction temperatures,which is beneficial when processing polymers andchemicals that degrade at elevated temperatures. Inaddition to solvating power, mass transport is impor-tant in extractions. Diffusivities of supercritical fluidsare orders of magnitude higher than liquid diffusivities(1). In polymer applications, reduced surface tension inaddition to polymer swelling in an appropriately chosensupercritical fluid facilitates the transport of small mol-ecules into and out of the polymer matrix. A final ben-efit of using scCO2 or other compressed gases as ex-traction solvents is that the ultimate separation ofsolute and solvent is accomplished by simple depres-surization.

Graft copolymerization processes typically involveentrapment of unreacted monomer, unspent initiatoror its fragments, and undesirable homopolymer of re-active monomer, all of which are considered process re-siduals. For product purification of PP-g-MA copolymers,irrespective of grafting process, methanol Soxhlet ex-traction has frequently been used for residual removal(6�10). However, methanol Soxhlet extraction resultsin the unwanted esterification of pendant succinic an-hydride groups (10). Recovery of some of the anhydridefunctionality is possible after lengthy vacuum-drying of

*To whom correspondence should be addressed. E-mail: leesu@missouri.edu© 2004 Society of Plastics EngineersPublished online in Wiley InterScience (www.interscience.wiley.com).DOI: 10.1002/pen.20161

the copolymer; however, the costly solvent recovery andthe additional drying time remain barriers to a moreprofitable industrial synthesis process. Further envi-ronmental concerns include the use of harsh solventsin alternative extraction techniques. Toluene and ace-tone are examples of harsh solvents commonly usedwhen washing PP-g-MA.

Supercritical fluid extraction of residuals is a usefulalternative to the Soxhlet-extraction process. Super-critical fluid extraction of polymers has found use inthe analysis of polymer additives used in both qualitycontrol and in the evaluation of a competitor’s polymerproduct (11). In both applications, additives are ex-tracted from the polymer matrix in order to identify theformulation, or, in the case of quality control, how closethe actual composition is to a given specification. Therehave been several examples of supercritical fluid ex-tractions of plasticizers, stabilizers, and oligomers frompolymers in the literature (12�14). The purpose of thispaper is to show the capability of scCO2 extraction forpreserving the anhydride bond of succinic anhydridependant groups in PP-g-MA copolymer. The copolymerstructure is characterized by solid-phase 13C NMRspectroscopy, and the degree of functionalization (i.e.,graft level) is determined by alkali titration. Thermo-gravimetric analysis is performed to examine differ-ences in residual content between scCO2-extracted andunextracted PP-g-MA samples. FTIR spectroscopy isused to determine the existence of ring-opened struc-tures in the extracted copolymer samples, providing acomparison between the methanol-Soxhlet and scCO2extractions.

EXPERIMENTALMaterials

Copolymer is synthesized from Pro-Fax (PH020) poly-propylene (PP) homopolymer. The PP, obtained fromMontell Polyolefins, was determined to be isotactic byidentification of characteristic IR absorption bands at810, 842, 900, 1000, 1105, 1170, 1260 and 1308 cm�1.Weight-average molecular weight and polydispersityindex were determined by gel permeation chromatog-raphy to be 155,000 and 5.20, respectively. Benzoylperoxide (BPO) initiator, maleic anhydride monomer,triallylcyanurate coagent, methanol solvent (99% pure),sodium hydroxide titrant (0.0987 N), p-xylene (99%pure) and phenolphthalein indicator (0.5 wt% in eth-anol) were obtained from the Aldrich Chemical Com-pany. Nitrogen (N2 ) purge gas and instrument-gradecarbon dioxide were obtained from Praxair, Inc.

Copolymer Synthesis

The PP-g-MA was synthesized in this investigationusing the patented solid-phase grafting process (15) ina 2-L Processall industrial mixer. The mixer wascharged with 1 kg of PP, heated to 100°C and ther-mostated at that temperature. The vessel was purgedwith N2 during the inductive-heating period. Monomer(5.0 wt%) and initiator (10.0 wt%) were added in four

equal aliquots to the thermostated PP at 15-min inter-vals. The coagent (0.5 wt%) was charged to the mixerwith the first addition of MA and BPO. Two impellersagitated the mixer during the heating, synthesis andcooling steps. Total reaction time of 1 h was used, fol-lowed by quenching to room temperature prior to thestorage of the graft copolymer. The PP-g-MA productwas stored in a desiccator away from heat and lightsources.

Supercritical Carbon Dioxide Extraction

The scCO2 extractions were carried out in a stainless-steel, 300-cm3, bolt-closure vessel. A 500-W furnacewas used to heat the vessel. For each set of extractionconditions, a 20-g sample of unextracted PP-g-MA wascharged to the reactor, which was subsequently sealedand purged with CO2. Extraction conditions were ob-tained by heating the extraction vessel and by introduc-tion of CO2 from the upstream storage vessel. Oncesteady state was attained, a soaking period started,after which a control valve was opened and the CO2 anddissolved residuals were vented over a 1-h period. Eachsample was removed and stored until FTIR analysis wasperformed. Samples treated in scCO2 were not vacuum-dried. The scCO2 extractions were carried out at operat-ing pressures of 8.91, 10.30, 12.34 and 14.39 MPa andoperating temperatures of 62°C, 79°C, 104°C, 129°Cand 146°C. Contact time at the experimental conditionswas 3, 10, 20, 30 or 37 min.

Soxhlet Extraction

A 20-g sample of newly synthesized PP-g-MA wasplaced in a Soxhlet apparatus containing methanol andallowed to reflux for 24 h. The sample was then vac-uum-dried at 85°C over a 96-h period and FTIR spec-tra were obtained after 0, 48 and 96 h of drying. TheSoxhlet extraction and subsequent vacuum-drying wasperformed in order to provide samples to compareagainst the scCO2 treated samples.

Graft Level Determination

A titrimetric graft level determination was carried outon the raw PP-g-MA product as well as on both themethanol Soxhlet extracted and an scCO2-extractedPP-g-MA sample. For the graft level determination, 1 gof sample was dissolved in 200 ml of refluxing p-xylenewith 4 drops of phenolphthalein indicator. Dropwiseaddition of sodium hydroxide was performed until acolor change lasting for 1 min occurred. The colorchange was taken as the end point.

13C NMR Analysis

Solid-phase 13C NMR analysis was performed to ver-ify successful grafting of maleic anhydride monomeronto the homopolymer, as succinic anhydride pendantgroups. The cross-polarization and magic-angle samplespinning techniques were used because of their successin identifying PP-g-MA grafting (9). 13C NMR spectra

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POLYMER ENGINEERING AND SCIENCE, SEPTEMBER 2004, Vol. 44, No. 9 1637

were obtained from both an ungrafted-PP sample and aSoxhlet-extracted PP-g-MA sample. Each polymer sam-ple was individually packed in a thimble and placedinto the spectrometer for analysis. Each sample wasrotated at 5000 Hz, and signal transients for the male-ated polymer sample were collected for 12 h to increasethe signal-to-noise ratio of the Fourier-transformedspectrum. An scCO2-extracted sample was not analyzedusing 13C NMR because there was not expected to be adiscernible difference between the NMR spectra of theSoxhlet and supercritical-fluid treated samples at thisresolution. The use of 13C-doped MA in the grafting re-action would be necessary to enhance the resonancesignals and resolve differences between the spectra of aPP-g-MA sample that has undergone ring opening anda PP-g-MA sample devoid of ring opening.

FTIR Analysis

FTIR analysis was performed to identify which treat-ment methods (i.e., Soxhlet or scCO2 extraction) andwhich treatment conditions preserved the MA func-tionality of the copolymer. Specimens were prepared forFTIR analysis by hot-pressing 5 g of PP-g-MA sample ina Carver press between sheets of Kaladex 2000/500-gauge film (obtained from DuPont). Samples of un-grafted PP, Soxhlet-extracted PP-g-MA, and scCO2 ex-tracted PP-g-MA (for each treatment condition) werepressed at 175°C and 69 MPa for 1 min. The sampleswere quenched by internally cooling the platens withtap water. Samples were removed from the press at85°C, verified for 40�60-�m thickness, cut into 3-cm� 3-cm squares, and immediately analyzed with theFTIR apparatus. A Thermo Nicolet spectrometer wasused for the FTIR analysis. A spectrum for each samplesquare was collected for the range of 400�4000 cm�1.A data spacing of 0.482 cm�1 was used to collect the 32scans for each spectrum. The carbonyl groups of inter-est absorb IR in the range of 1650�1900 cm�1. It is im-portant to note that the copolymer samples extractedwith scCO2 above 79°C formed extremely brittle films,impractical for FTIR analysis. The BPO remaining inthe copolymer after solid phase grafting has a half-lifeof less than 19 min at temperatures above 100°C andprobably contributed to the degradation of these co-polymer samples during scCO2 extraction. Spectra forthe remaining samples are reported here.

Thermogravimetric Analysis

Thermogravimetry was utilized to verify residual re-moval from the PP-g-MA samples via the scCO2 ex-traction process, relative to the unextracted control.Thermograms were obtained on PP-g-MA samples foreach scCO2 treatment and for an unextracted copoly-mer sample. The 1-mg samples were individuallyloaded into a platinum thimble and placed on the mi-crobalance. Each suspended, loaded thimble washeated in air at 10°C/min and the relative weight ofthe sample was recorded for the temperature rangeof 100°C�500°C.

RESULTS AND DISCUSSION

Graft Level Determination

Titrimetry indicates that MA content decreases from4.8 wt% in unextracted PP-g-MA to 4.0 wt% in scCO2-extracted copolymer and 3.8 wt% in Soxhlet-extractedsamples. This result is expected because residualmonomeric MA is present in the PP-g-MA samples andits presence results in artificially high graft levels for un-extracted samples. Previous studies have also showndifferences in titrimetrically determined graft levels be-tween untreated PP-g-MA and Soxhlet-extracted copoly-mer from commercially available copolymer (6). The dif-ferences in graft level between the scCO2-extracted andthe Soxhlet-extracted samples are due to the presenceof ring-opened succinic anhydride pendant groups inthe Soxhlet samples. Mono-esterification of succinic an-hydride with methanol occurs during Soxhlet extrac-tion, lowering the titrimetrically observed graft level (5).Typically, the anhydride functionality is not fully recov-ered after vacuum-drying.

13C NMR Analysis

Figure 1 shows the 13C NMR spectra of a neat PPsample and a Soxhlet-extracted PP-g-MA sample. Theungrafted PP sample has strong 13C NMR resonancesignals at 20, 25 and 45 ppm (Fig. 1a), whereas addi-tional peaks located at 130 and 174 ppm are observedin the spectrum of the Soxhlet-extracted PP-g-MA sam-ple (Fig. 1b). The peaks located at 20, 25 and 45 ppm inboth spectra are attributed to each of the three carbonsin the PP backbone. The peaks at 130 and 174 ppm arediagnostic of MA grafting, but they cannot be used todistinguish between acid and anhydride forms (9).

FTIR Analysis

Literature sources indicate that the IR-absorbancebands near 1860 and 1776 cm�1 are attributed to car-bonyl stretching in 5-member-ring anhydride groups(6, 10, 16). The 1860-cm�1 band is due to asymmetricstretching, whereas the 1776-cm�1 band is due to sym-metric carbonyl stretching in this structure. The 1860-cm�1 absorbance band is characteristically smallerthan the 1776-cm�1 band produced by pendant suc-cinic anhydride groups. Absorbance bands near 1740cm�1 are attributed to carbonyl stretching in ester unitsand an absorbance near 1721 cm�1 results from car-bonyl stretching of carboxylic acid and ketone func-tional groups (10, 16). Figure 2a shows an absence ofabsorbance bands in the spectrum of neat PP indicat-ing that ungrafted samples do not contain anhydride,ester or carboxylic acid linkages. The spectrum of theunextracted PP-g-MA is shown in Fig. 2b and containsabsorbance bands at 1847, 1774 and 1720 cm�1 witha shoulder at 1699 cm�1. The 1847- and 1774-cm�1

bands indicate the presence of both pendant MA andunbound monomer residues. The 1720-cm�1 band and1699-cm�1 shoulder are larger than the 1774-cm�1

peak, indicating the presence of large quantities of

Kelly Clark and Sunggyu Lee

1638 POLYMER ENGINEERING AND SCIENCE, SEPTEMBER 2004, Vol. 44, No. 9

Removal of Ungrafted Monomer

POLYMER ENGINEERING AND SCIENCE, SEPTEMBER 2004, Vol. 44, No. 9 1639

unbound monomer, spent initiator (benzoic acid) andother carbonyl-containing impurities. The presence ofmonomer residues in unextracted PP-g-MA has previ-ously been established (6, 8, 10) and is a likely con-tributing factor.

Figure 3 compares the IR spectrum of an scCO2-extracted PP-g-MA sample with the IR spectrum of aSoxhlet-extracted sample that has been vacuum-driedfor 96 h. The spectra of Fig. 3 have been scaled relativeto the absorbance band at 1776 cm�1 by dividing theabsorbance at a given wavenumber by the absorbanceat 1776 cm�1 and thus generating a plot of relative

intensities. Both spectra have absorbance bands at1863 and 1776 cm�1 indicating the presence of graftedsuccinic anhydride. The 1776-cm�1 peak is also largerthan the neighboring peaks of either sample. This con-trasts with the unextracted sample of Fig. 2b that hasa much larger absorbance at 1720 cm�1 compared withthe 1776-cm�1 band. This indicates a decrease in car-bonyl containing species after both extraction meth-ods, relative to the unextracted copolymer samples.The shape and location of the peak near 1721 cm�1

also is different for the samples treated with the differ-ent extraction methods. The Soxhlet-extracted samplespectrum (Fig. 3a) has a peak centered at 1723 cm�1

with a shoulder at 1740 cm�1, whereas the scCO2-extracted sample spectrum (Fig. 3b) has a peak at 1720cm�1 and no 1740-cm�1 shoulder. The 1740-cm�1 ab-sorbance of the Soxhlet-treated sample spectrum is the result of ester groups in the sample. The superpo-sition of the 1740- and 1720-cm�1 peaks results in the4-cm�1 difference between the locations of the 1720-cm�1 bands of the two spectra. From these peak loca-tions, it is concluded that scCO2 treatment does notdestroy MA functionality, whereas Soxhlet extractionwith methanol results in ring opening. Vacuum-dryingof the Soxhlet-extracted sample for 96 h prior to FTIRanalysis does not recover all of the anhydride function-ality. Therefore, an scCO2 extraction process is suc-cessful at maintaining MA functionality while the Soxh-let method fails in this respect.

Fig. 1. 13C NMR spectra of a) an ungrafted-PP sample, and b) a Soxhlet-extracted PP-g-MA sample.

Fig. 2. FTIR spectra of a) a PP homopolymer sample, and b) anunextracted PP-g-MA sample.

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1640 POLYMER ENGINEERING AND SCIENCE, SEPTEMBER 2004, Vol. 44, No. 9

Further evidence of the ester content of Soxhlet-ex-tracted samples is shown in Fig. 4, which comparesthe spectra of Soxhlet-extracted samples after 0, 48and 96 h of vacuum-drying. The height of the 1723-cm�1 absorbance decreases, relative to the 1776-cm�1

absorbance, with increasing drying time. This indicatesthat some recovery of anhydride functionality is possi-ble after vacuum-drying; however, an inordinateamount of drying time is required.

Thermogravimetric Analysis

Figure 5 compares the TGA thermogram for anscCO2-extracted PP-g-MA sample with the thermogram

of an unextracted PP-g-MA sample for the same heat-ing profile. At each temperature of the temperatureramp, the relative weight of the scCO2-treated sampleis higher than the relative weight of the untreated co-polymer control. The difference in relative weights is at-tributed to the differences in residual content betweenthe two samples. Residuals are low molecular weightspecies that vaporize early in the TGA temperature pro-file. The lower relative weight of the unextracted TGAthermogram indicates higher residual content prior toextraction. This supports the FTIR spectra finding oflower residual content in the extracted samples relativeto the unextracted samples.

Fig. 3. FTIR spectra of a) Soxhlet-extracted PP-g-MA after 96-h drying, b) scCO2 extracted PP-g-MA.

Fig. 4. FTIR spectra of Soxhlet-extracted PP-g-MA after a) air-drying, b) vacuum-drying for 48 h, c) vacuum-drying for 96 h.

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POLYMER ENGINEERING AND SCIENCE, SEPTEMBER 2004, Vol. 44, No. 9 1641

Operating Conditions

Operating temperature, operating pressure and ex-traction time did not affect the MA ring structure of PP-g-MA in this investigation. Similar absorbance bandswere recorded for each of the FTIR spectra of the scCO2samples. However, polymer degradation becomes anissue as the treatment temperature increases. The in-ability to press films from the samples treated withscCO2 above 79°C is evidence that the temperaturerange is too broad in this exploratory investigation. Thesuccess of the process at lower temperatures leaves aregion between the critical temperature of carbon diox-ide (31.4°C) and the 79°C treatment for optimization ofthe extraction process.

CONCLUDING REMARKS

This investigation has established the following re-sults:

1) Residual removal from PP-g-MA is feasible usingscCO2 in place of the traditional Soxhlet-extractiontechnique.

2) The scCO2 process avoids ring-opening reactions,which remove anhydride functionality from the co-polymer in favor of ester and acid groups.

The results were confirmed using 13C NMR, FTIR andTGA analyses.

ACKNOWLEDGMENTS

NSF Grant CHE-95-31247 and NIH Grant1S10RR11962-01 provide partial funding of the 300-MHz NMR facility.

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Fig. 5. TGA thermogram comparing the relative weight of an scCO2-treated copolymer sample to an unextracted copolymer sample.