Polymer additive analysis by Py-GC. III Antioxidants, Artykuły naukowe, Polimery i ich analiza
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//-->Journal of Chromatography A, 891 (2000) 325–336www.elsevier.com / locate / chromaPolymer additive analysis by pyrolysis–gas chromatographyIV. AntioxidantsFrank Cheng-Yu Wang*Analytical Sciences Laboratory,Michigan Division,Building1897B,The Dow Chemical Company,Midland,MI48667,USAReceived 5 April 2000; received in revised form 30 May 2000; accepted 7 June 2000AbstractAntioxidants are important additives in polymers. Because of the low level of antioxidants normally used, they cannot beanalyzed directly by common spectroscopic or thermal chemical techniques. However, antioxidants as well as other additivesin polymers can be qualitatively analyzed by pyrolysis–gas chromatography (Py–GC) after separating the polymers andadditives. In this study, several antioxidants have been investigated to demonstrate that Py–GC is a viable tool to analyzethem. The advantages of using Py–GC in the analysis of antioxidants have also been discussed.©2000 Elsevier ScienceB.V. All rights reserved.Keywords:Pyrolysis; Polymers; Antioxidants1. IntroductionInherently, all polymeric materials will react withoxygen. The kinetics of the oxidation reaction de-pend on the type of polymers as well as theprocessing and application environment, such astemperature, humidity, etc. The oxidation reaction ispresent in every stage of the life cycle of a polymersuch as synthesis / manufacture, processing (extru-sion, molding, etc.), and final application usage bythe customers [1]. It is necessary to prevent or slowdown this oxidation activity in order to extend theuse of the polymer.The typical oxidation phenomena are generalizedin terms of ‘‘degradation’’ or ‘‘aging’’. The results ofoxidation reactions are seen as discoloration (yellow-ing), loss of gloss or transparency, chalking and*Tel.:11-908-730-2744;fax:11-908-730-3314.E-mail address:fcwang@erenj.com (F.C.-Y. Wang).surface cracks. On the other hand, the result of theoxidation reaction occurs more or less simultaneous-ly as loss of mechanical properties such as impactstrength, elongation, tensile strength, etc. The phe-nomenon of oxidation reaction also accounts for theoccurrence of two degradation processes, chainscission and cross-linking. Chain scission results inthe loss of molecular mass, increase in melt flow,and decrease in toughness. Cross-linking increasesmolecular mass, decreases melt flow and increasestoughness in the early stage.Fundamentally, there are several different ap-proaches to reduce the speed of or prevent theoxidation reaction. For example, the oxidation re-action can be slowed by structural modification ofthe polymer through copolymerization with mono-mers that have antioxidation capability. The secondway to decrease the effect of the oxidation reaction isto introduce inert compounds to cap off the reactivesites (normally, at the end of the chain) of the0021-9673 / 00 / $ – see front matter©2000 Elsevier Science B.V. All rights reserved.PII: S0021-9673( 00 )00647-6326F.C.-Y.Wang/J.Chromatogr.A891 (2000) 325–336polymer. The third way to control the oxidationreaction is to physically stabilize the polymer byorientation of the polymer such as stretching. Thefinal way to manage the oxidation reaction is to addstabilizing additives, such as antioxidants [2].Addition of antioxidants seems to be the mostconvenient way to retard the oxidation reaction in thepolymer. However, making the proper selection of anantioxidant system can be a tough task. There are analmost unlimited number of antioxidants, antioxidantcombinations, and various antioxidant concentrationsthat might provide the required antioxidant protec-tion. However, the criteria of choice can be asfollows, (1) safety, (2) physical form, (3) volatility,(4) extractability and migration, (5) color, (6) odorand taste, (7) compatibility and (8) cost and per-formance [3].Antioxidants can be divided into two major classesbased on their mechanisms of action: primary anti-oxidants and secondary antioxidants [4]. Primaryantioxidants are radical scavengers or hydrogendonors or chain reaction breakers. The major mole-cules of primary antioxidants include hinderedphenols and secondary aryl amines. Secondary anti-oxidants are peroxide decomposers. They are com-posed of organophosphites and thioesters.It is hard to see that a single antioxidant canprovide all of the varied properties required in apolymer application. Consequently, combinations ofprimary and secondary antioxidants are used to takethe advantage of synergistic effects [5]. A hinderedphenol might be used synergistically in conjunctionwith a thioester where the phenol provides the long-term stability of the polymer and the thioestersupplies the long-term stability for the hinderedphenol. In the same way, a hindered phenol might beused in combination with a phosphite to improvecolor and at the same time to provide the pro-cessibility.The concentration of antioxidants used in poly-mers is usually between 100 and 1000 ppm of eachprimary and secondary antioxidant depending on thetargeted applications and the processing conditions.The spectroscopic approach to analyze antioxidant inthe polymer is difficult because of ultra low con-centration and interference of the parent polymermatrix. The general approach of antioxidant analysisis first to separate the polymer and additives [6],followed by appropriate gas chromatography (GC)or liquid chromatography (LC) methods to separateand identify the antioxidants from the additivemixture. However, for the primary antioxidants suchas hindered phenols, not only are these rather largemolecules but also they are similar in structure. Thechoice of GC or LC to achieve the separation anddetection / identification is laborious when facing anunknown. It always requires some applicationknowledge and a library of standard spectra in orderto induce the correct identification. However, there isanother technique, pyrolysis (Py)–GC, that may beapplied to simplify this antioxidant analysis task.Py–GC [7] is an important technique for polymeras well as large molecule analysis. Py–GC is atechnique that uses thermal energy (pyrolysis) tobreak down a polymeric chain or large molecule tomonomers, oligomers and other fragments, followedby the separation of the pyrolysates with GC anddetection with appropriate detectors. Flame ioniza-tion detection (FID) is one of the most frequentlyused detection methods for quantitative analysis ofpyrolysates. Mass spectrometry (MS) is one of themost commonly used detection methods for identifi-cation. The intensities of monomers or monomer-related fragments are commonly used to obtaincompositional data [8]. The oligomers or oligomer-related fragments are used to elucidate microstruc-ture as well as compositional information [9]. Py–GC has been used in the antioxidant characterizationfor the polymer analysis, such as Irganox 1010 inpolyethylene (PE) and poly(butylene terephthalate)(PBT) in the presence of tetramethylammoniumhydroxide (TMAH) [10]. There is another Py–GCpaper for additives in rubbers and plastics studybased on the comparison of the pyrolysate’s massspectrum with an additive spectrum library [11].Direct analysis of antioxidants in the polymer byPy–GC is difficult because of the low concentrationof antioxidant and the possible pyrolysates interfer-ence from the original polymer matrix. Antioxidantsas well as other additives in the polymer can bequalitatively and quantitatively analyzed by Py–GCafter separating the polymer and additives. In thisstudy, several different types of antioxidants havebeen studied to demonstrate that Py–GC is a goodtool to investigate the antioxidants in polymers. Thediscussion of how to identify different antioxidantsF.C.-Y.Wang/J.Chromatogr.A891 (2000) 325–336327will focus on the antioxidants with slight differencesin structure. The major purpose of this approach is todemonstrate that different pyrolysates are createdfrom different antioxidants with similar structures inorder to lead to a systematic approach to identifyeach individual antioxidant. The advantages of usingPy–GC for antioxidant analysis are also discussed.and thermoplastic resin were used as they werereceived without any further purification.2.2.Py–GC conditionsSamples of antioxidant (approximately 0.5 mg)were carefully deposited into a quartz tube. Thequartz tube was inserted into a 3008C interfaceconnected to the injection port of a Hewlett-Packard(HP) Model 6890 gas chromatograph equipped withan FID system. The samples were pyrolyzed (CDS2000 Pyroprobe, Pt coil) at a calibrated temperatureof 9508C. The coil was heated to the calibratedtemperature at 208C / ms and held at the set tempera-ture for a 20-s interval. The pyrolysates were split inthe 3008C injection port, with 250:1 split ratio. TheGC system was set up with a fast flow program [15p.s.i. (1 p.s.i.56894.46 Pa) / 0.2 min, 75 p.s.i. / min, to90 p.s.i. / 8.8 min]. The separation was carried out ona fused-silica capillary column (J & W ScientificDB-5, 10 m30.10 mm I.D., 0.4mmfilm) using a fasttemperature ramping program (508C / 0.2 min,1008C / min, to 1008C / 0 min; 808C / min, to 1408C / 02. Experimental2.1.AntioxidantsIrganox 1010 (CAS No. 6683-19-8), Irganox 1076(CAS No. 2082-79-3), Irganox 1035 (CAS No.41484-35-9), Irganox MD1024 (CAS No. 32687-78-8), Irganox 259 (CAS No. 35074-77-2), Irganox3114 (CAS No. 27676-62-6), Irganox 1425 (CASNo. 65140-91-2), Irganox 565 (CAS No. 991-84-4)and Irgafos 168 (CAS No. 31570-04-4) were ob-tained from Ciba-Geigy (Tarrytown, NY, USA). AGE Cyloloy C3600 thermoplastic resin was obtainedfrom GE Plastic (Detroit, MI, USA). All antioxidantsFig. 1. The pyrogram of Irganox 1010 and its structure. The peaks labeled in the pyrogram have been identified and listed in Table 1.328F.C.-Y.Wang/J.Chromatogr.A891 (2000) 325–336Table 1Peak assignments for the pyrograms of Irganox 1010 (Fig. 1)Peak No.123456789101112Mr94108122122120136134164178218234232StructurePhenol4-Methylphenol2,4-Dimethylphenol4-Ethylphenol2,3-Dihydrobenzofuran4-Ethyl-2-methylphenol4-Vinyl-2-methylphenol2-tert.-Butyl-4-methylphenol2-tert.-Butyl-4-ethylphenol2,6-Bis(tert.-butyl)-4-methylenecyclohexa-2,5-dien-1-one2,6-Bis(tert.-butyl)-4-ethylphenol2,6-Bis(tert.-butyl)-4-vinylphenolmin; 608C / min, to 2008C / 0 min; 508C / min, to2808C / 0 min; 408C / min, to 3208C / 5.2 min).2.3.Py–GC–MS conditionsThe sample preparation and pyrolysis in the Py–GC–MS experiments were the same as in the Py–GC experiments. The GC system used was a HPModel 5890 gas chromatograph. The pyrolysis prod-ucts were split in the 3008C injection port, with 10p.s.i. head pressure, and 30:1 split ratio. Thepyrolysates were separated on a fused-silica capillarycolumn (J & W Scientific DB-5, 30 m30.25 mmI.D., 1.0mmfilm) using a linear temperature pro-Fig. 2. The pyrogram of Irganox 1076 and its structure. The peaks labeled in the pyrogram have been identified as (13) 3-[3,5-bis(tert.-butyl)-4-hydroxyphenyl]propanoic acid, (14) Irganox 1076.F.C.-Y.Wang/J.Chromatogr.A891 (2000) 325–336329gram (408C / 4 min, 108C / min, to 3208C / 18 min);and detected by a HP 5791 mass-selective detector.The GC transfer line to the mass-selective detectorwas kept at 3008C. An electron ionization massspectrum was obtained every second over them/zrange of 15 to 650. The results of Py–GC–MS wereused mainly for identification of pyrolysates.3. Results and discussionThe primary antioxidants are the family of hin-dered phenols and the secondary aryl amines. In thehindered phenol family, all the antioxidants arerelatively similar in structure. During the identifica-tion, it is easy to conclude that a hindered phenol isused as antioxidant based on the several alkylsubstituted phenol pyrolysates detected. It is hard todistinguish which specific hindered phenol relatedantioxidant has been used. However, if a specialfragment can be found that correlated to a specifichindered phenol structure, the positive identificationstill can be reached. This type of identificationrequires a good pyrolysis database of antioxidants aswell as good pyrogram interpretation skill.The most typical antioxidant to illustrate thehindered phenol functions is Irganox 1010. Furtherexamination of this structure shows that it is methanewith four hydrogens substituted by four hinderedphenol units; the hindered phenol unit is a 3,5-di-tert.-butyl-4-hydroxyhydrocinnamate.Essentially, itis a phenol unit with twotert.-butylgroups attachedat positions 2 and 6, position 4 being connected tothe carbon at position 3 of a propanoate. Fig. 1shows the pyrogram of Irganox 1010 with its chemi-cal structure. The major pyrolysates labeled in thepyrogram have been identified and are listed in Table1. All the pyrolysates are contributed from the sidechain fragmentation of hindered phenol unit. Thepyrogram of Irganox 1010 can almost be viewed asthe pyrogram of hindered phenol (3,5-di-tert.-butyl-4-hydroxyhydrocinnamate) unit. There is no quick /easy way to identify this antioxidant by Py–GC. Ifall necessary pyrolysates from hindered phenols areFig. 3. The pyrogram of Irganox 1035 and its structure. The peaks labeled in the pyrogram have been identified as (15) divinyl sulfide, (16)2,6-bis(tert.-butyl)-4-[2-(1,3-dioxolen-2-yl)ethyl] phenol, (17) 2-vinylthioethyl 3-(3,5-di-tert.-butyl-4-hydroxyphenol)propanate.
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