Polymer additive analysis by Py-GC. IV Plasticizers, Artykuły naukowe, Polimery i ich analiza
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//-->Journal of Chromatography A, 883 (2000) 199–210www.elsevier.com / locate / chromaPolymer additive analysis by pyrolysis–gas chromatographyI. PlasticizersFrank Cheng-Yu WangAnalytical Sciences Laboratory,Michigan Division,The Dow Chemical Company,Midland,MI48667,USAReceived 21 January 2000; received in revised form 14 March 2000; accepted 16 March 2000AbstractPlasticizers are widely used in thermoplastic polymers to modify their physical properties and processibility. Plasticizersas well as most of the other additives in the polymer can be qualitatively analyzed by pyrolysis–gas chromatography(Py–GC) simultaneously with the polymer composition. The key to the successful analysis of plasticizers not only requires acomprehensive understanding of commercial plasticizers but also requires knowledge of the polymer and its applications, aswell as the Py–GC technique. In this study, several plasticizers in different polymeric systems were studied to demonstratethe utility of Py–GC as a good tool for the characterization of these systems. The advantages of using Py–GC for plasticizeranalysis are also discussed.©2000 Elsevier Science B.V. All rights reserved.Keywords:Pyrolysis; Polymers; Plasticizers1. IntroductionA plasticizer is defined by the International Unionof Pure and Applied Chemistry (IUPAC) as asubstance or material incorporated in a material(usually a plastic or an elastomer) to increase itsflexibility, workability or extensibility. A plasticizermay reduce the melt viscosity, lower the temperatureof a second-order transition or lower the elasticmodulus of the product. Most plasticizers are liquidsof low volatility. In many circumstances, they areblended to produce a wide range of physical prop-erties from a single parent polymer.The critical considerations for selection of aplasticizer for the modification of the properties of apolymeric system are compatibility, permanence,aging and its effects on the other properties [1]. Aplasticizer must be capable of being mixed uniformlyand homogeneously and remain blended whencooled to room temperature and throughout theuseful life of the plastic product. The stability of theplasticizer in its mixed state must be maintainedduring the useful life of the product. A plasticizermay migrate from the material to cause imperma-nence because of volatility, exudation, extraction orother influences in the application environment. Itmay degrade by chemical, radiation, or other con-ditions that exist in the application surroundings. Aplasticizer may affect other physical properties of thepolymer such as adhesion, electrical properties,flammability and toxicity. The choice of plasticizermust consider its efficiency for modification of thedesired properties, as well as optimization of itseffect on other properties.There are several major theories have been pro-posed to explain the mechanism of plasticization.There are the lubricity theory, the gel theory and thefree volume theory. The lubricity theory is based on0021-9673 / 00 / $ – see front matter©2000 Elsevier Science B.V. All rights reserved.PII: S0021-9673( 00 )00346-0200F.C.-Y.Wang/J.Chromatogr.A883 (2000) 199–210the fact that since the major force resisting deforma-tion of a polymer is intermolecular friction, theplasticizer would act as a lubricant, separating thepolymer chains and facilitating their movement overeach other. The gel theory depended on the fact thatsince the intermolecular force resulted from those‘‘points of interaction’’ such as dipole–dipole inter-action or hydrogen bonding, when plasticizer sol-vates the polymer, it destroys many of these ‘‘pointsof interaction’’ by replacing polymer–polymer inter-actions with polymer–plasticizer interaction. Thefree volume theory originated from consideration ofthe nature of the glass transition in super-cooledliquids and amorphous polymers. Plasticizers aremuch smaller molecules and have much greater freevolume. The addition of plasticizer has the effect ofincreasing the free volume of the plasticized polymerthat let the movement of the polymer chain easier.Because the role and purpose of a plasticizer in apolymer system are well defined, it is easy tounderstand which polymeric systems often utilizeplasticizers to modify their physical properties tomatch the applications’ needs. Polyvinyl chloride(PVC) and its copolymers [2] are the most importantpolymers that rely on plasticizers to extend theirphysical properties to fulfill a board range of applica-tions. Cellulose derivatives [3] depend on plasticizersto bring the processing temperature (melt flowtemperature) down below the polymer decompositiontemperature. Other thermoplastic polymers may addspecific types of plasticizers for a well-definedapplication. However, their magnitude of dependenceon plasticizer level is much reduced relative to thetwo polymeric systems mentioned above.There are many ways to classify plasticizers. Theymay be categorized by molecular mass, molecularstructure, compatibility, cost efficiency or purpose ofapplication. From an analytical point of view, plas-ticizers may be classified by their molecular structure[4] because the molecular structure is directly relatedto polarity and molecular flexibility, which willaffect their properties. Based on the molecularstructure, plasticizers can be divided into severalmajor groups [5]: (1) phthalic acid esters; di(2-ethylhexyl) phthalate (DOP), butyl benzyl phthalate(BBP) and dibutyl phthalate (DBP) are severalimportant examples; (2) phosphoric acid esters;trioctyl phosphate (TOP), diphenyl 2-ethylhexylphosphate and tri(2-ethylhexyl) phosphate are typicalcompounds used; (3) polyfunctional fatty acid esters;butyl and octyl adipate, sebacate, citrate andmaleates are commonly used plasticizers; (4) poly-meric plasticizers; polymeric plasticizers are oligo-mers or polymers of molecular mass above 500 usedfor plasticizers. They include polyglycols, polyesters,polyepoxides and chlorinated polyolefins.The most common way to incorporate plasticizersinto polymers is by blending, that is, by physicallymixing with the polymer molecules. When analyzingthe plasticizers, it is relatively straightforward toseparate them just by solvent extraction. Afterseparating the plasticizer from the polymer, theplasticizer-containing portion can be further sepa-rated / analyzed by proper chromatographic methodswith the appropriate detection [6,7]. Because theseprocedures involve a series of wet-chemistry steps aswell as chromatographic procedures, it is a time andlabor intensive operation. However, some polymersare insoluble in most solvents, such as the nylonfamily. Although there may be a solvent that candissolve them, the conditions required are often athigh temperatures or the viscosity of final solution isso high that the extraction is practically impossibleor the solvent is very corrosive and presents safetyhazards.Pyrolysis–gas chromatography (Py–GC) [8] is animportant technique for polymer analysis. Py–GC isa technique that uses thermal energy (pyrolysis) tobreak down a polymeric chain to monomers, oligo-mers and other fragments, followed by the separationof the pyrolysates with GC and detection withappropriate detectors. Flame ionization detection(FID) is one of the most frequently used detectionmethods for quantitative analysis of pyrolysates.Mass spectrometry (MS) or mass-selective detectionis one of the most commonly used detection methodsfor identification. The intensities of monomers ormonomer-related fragments are commonly used toobtain compositional data [9]. The oligomers oroligomer-related fragments are used to elucidatemicrostructure as well as composition information[10].Plasticizers, as well as most other additives in thepolymer, can be qualitatively and quantitativelyanalyzed by Py–GC simultaneously with the poly-mer composition and microstructure. The key to theF.C.-Y.Wang/J.Chromatogr.A883 (2000) 199–210201successful analysis of plasticizers not only requires acomprehensive understanding of commercial plas-ticizers but also requires knowledge of the polymerand its applications, as well as the Py–GC technique.In this study, several plasticizers in different poly-meric systems were studied to demonstrate theutilities of Py–GC as a good tool for the characteri-zation of these systems. The advantages of usingPy–GC for plasticizer analysis are also discussed.interval. The pyrolysis products were split in the3008C injection port, with 250:1 split ratio. The GCsystem was set up with a fast-flow program (15p.s.i. / 0.2 min, 75 p.s.i. / min, to 90 p.s.i. / 8.8 min) (1p.s.i.56894.46 Pa). The separation was carried outon a fused-silica capillary column (J & W ScientificDB-5, 10 m30.10 mm I.D., 0.4mmfilm) using afast-temperature ramping program (508C / 0.2 min,1008C / min, to 1008C / 0 min; 808C / min, to 1408C / 0min; 608C / min, to 2008C / 0 min; 508C / min, to2808C / 0 min; 408C / min, to 3208C / 5.2 min).2. Experimental2.3.Py–GC–MS conditions2.1.PolymersPVC flexible tubing (Tygon tubing) was made byNorton Performance Plastic (Arkon, OH, USA) andwas purchased from Fisher Scientific (Pittsburgh,PA, USA). A cellulose propionate polymer (catalogNo. 45,490-7) was purchased from Aldrich (Mil-waukee, WI, USA). A vinyl chloride–vinylidenechloride copolymer film was obtained from Asahi(Tokyo, Japan). An impact-modified polystyreneblended with polycarbonate (PC-HIPS) polymer,grade Daicel X7200L, was obtained from Daicel(Tokyo, Japan). A polyurethane adhesive / sealantwas obtained from Sika (Madison Heights, MI,USA). The butadiene–acrylonitrile copolymer(catalog No. 530) and PVC (catalog No. 355) werepurchased from Scientific Polymer Products (On-tario, NY, USA). The styrene–butyl acrylate co-polymer was synthesized in the laboratory with atextbook method [11]. The butadiene–acrylonitrilecopolymer and PVC blend polymer were made in thelaboratory. All polymers purchased were used asreceived without any further purification.The sample preparation and pyrolysis in the Py–GC–MS experiments were the same as the for thePy–GC experiments. The GC used was a HP Model5890 gas chromatograph. The pyrolysis productswere split in the 3008C injection port, with 10 p.s.i.head pressure, and 30:1 split ratio. The pyrolysisproducts were separated on a fused-silica capillarycolumn (J & W Scientific DB-5, 30 m30.25 mmI.D., 1.0mmfilm) using a linear temperature pro-gram (408C / 4 min, 108C / min, to 3208C / 18 min) anddetected by a HP 5791 mass-selective detector. TheGC output region to the mass-selective detector waskept at 3008C. An electron ionization mass spectrumwas obtained every second over the molecular massrange of 15 to 650. The results of Py–GC–MS wereused mainly for component identification.2.4.Evolved gas analysis conditionsA sample of polymer (approximately 0.5 mg) wasdeposited into a Pt cup and loaded on top of amicro-oven type pyrolyzer (Frontier Lab., ModelPy-2010D). The pyrolyzer is operated in the evolvedgas analysis mode. The pyrolyzer is directly con-nected to the injection port of a Hewlett-Packard(HP) Model 5890 gas chromatograph equipped witha FID system. The micro-oven is temperature pro-grammed at 1008C / 5 min, 208C / min, to 7008C / 5min. The evolved gas products were split in the3008C injection port, with 15 p.s.i. head pressure,and 30:1 split ratio. The evolved gas products floweddirectly through a deactivated fused-silica capillarycolumn (approximately 50 cm) which was directlyconnected from injection port to the detector. The2.2.Py–GC conditionsSamples of polymer (approximately 0.5 mg) werecarefully deposited into a quartz tube. The quartztube was inserted in a 3008C interface connected tothe injection port of a Hewlett-Packard (HP) Model6890 gas chromatograph equipped with a FID sys-tem. The samples were pyrolyzed (CDS 2000 Pyrop-robe, Pt coil) at a calibrated temperature of 7008C.The coil was heated to the calibrated temperature at208C / ms and held at the set temperature for a 20-s202F.C.-Y.Wang/J.Chromatogr.A883 (2000) 199–210GC oven was kept at 3008C, and the detector wasalso kept at 3008C.3. Results and discussionFig. 1 shows a pyrogram of flexible PVC tubing(Tygon tubing) which is commonly used in thechemical laboratory for water and gas lines. Allmajor peaks labeled in the figure have been identifiedand listed in the figure’s caption. The plasticizer usedin this tubing material is di(2-ethylhexyl) phthalate.The mass spectrum is shown in Fig. 2. In non-foodrelated flexible PVC applications, the family of alkylsubstituted phthalate esters has been widely used.However, the identification of those alkyl-substitutedphthalate esters through their electron ionizationmass spectra is not as simple as a library search andmatch, because the mass spectra for this phthalateester family are all very similar. One reason for thesimilarity is most phthalate esters do not give anintense parent ion in their mass spectra. In addition,all phthalate esters have common major fragments.As shown in Fig. 2, the major ions such asm/z 167and 149 correspond to phthalate acid and phthalateanhydride. The key to identifying them has todepend on the minor fragments (for example ionmass 279 in this case) as well as the retention timethrough GC separation.Most cellulose and its derivative polymers arehighly polar and have little chain flexibility. Inaddition, they have rigid ring structures in thebackbone. Thus, it is necessary to plasticize them toimprove melt processibility and impact resistance inorder to provide some flexibility in film and sheetapplications. Fig. 3 shows a pyrogram of cellulosepropionate with a dioctyl adipate plasticizer. UnderPy–GC conditions, the cellulose backbone does notFig. 1. The pyrogram of PVC tubing (Tygon tubing) with di(2-ethylhexyloctyl) phthalate as plasticizer. The identification of majorpyrolysates is (1) benzene, (2) octene isomers, (3) 2-ethylhexyl aldehyde isomers, (4) 2-ethylhexyl alcohol isomers, (5) di(2-ethylhexyl)phthalate.F.C.-Y.Wang/J.Chromatogr.A883 (2000) 199–210203Fig. 2. The mass spectrum of di(2-ethylhexyl) phthalate.Fig. 3. The pyrogram of cellulose propionate with dioctyl adipate as plasticizer. The identification of major pyrolysates is (1) propanoicacid, (2) acetyl propanonate, (3) propanoyl propanonate, (4) dioctyl adipate.
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