Analysis of Contaminants in Recycled Poly ... - ACS Publications

Chapter 37

Analysis of Contaminants in Recycled Poly (ethylene terephthalate) by ThermalExtraction Gas Chromatography—Mass Spectroscopy 1,3

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Downloaded by FUDAN UNIV on February 15, 2017 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch037

D. E. Pierce , D. B. King , and George D. Sadler

1Illinois Institute of Technology, National Center for Food Safety and Technology, 6502 South Archer Road, Summit-Argo, EL 60501-1933 Ruska Instrument Corporation, 3601 Dunvale Road, Houston, TX 77063

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Thermal extraction-GC/MS has provided a means of determining the identity and origin of contaminants associated with food grade virgin and commercially recycled poly(ethylene terephthalate) PET. In order to address the problem of recycling PET for food use, a study was begun that examined a number of sources of recycled material. The effects of various stages of reprocessing and remanufacture were also examined and the material compared to virgin resin. In addition to determining purity, thermal extraction analysis could be used to model reprocessing steps involving heat (crystallization, drying, extrusion, etc.). Possible avenues for standardizing thermal extraction analysis as a simple and rapid screening method for recycled PET for food use will also be considered. The public increasingly feels that recycling demonstrates stewardship to the environment and represents a vital step in the preservation of dwindling natural resources. Federal, state, and community recycling laws have resulted from this popular conviction (7). Public understanding of recycling has largely been shaped by successful recycling programs with aluminum and glass. Extreme processing temperatures applied in recycling these materials assure that organic contaminants will not survive recycling steps to subsequently taint foods held in second generation containers. The FDA has expressed concern that compounds acquired from one recycling iteration might survive to contaminate foods held in containers madefromthese polymers (2). Even if the safety of recycled materials can be shown, many food companies are reluctant to embrace the use of recycled resins until they are certain that recyclinginduced flavor and odor properties are not imparted to their products. 3

Current address: Physical Science Directorate, U.S. Army Research Laboratory, Fort Monmoth, NJ 07703-5601 0097-6156/95/0609-0458$12.00/0 © 1995 American Chemical Society Rader et al.; Plastics, Rubber, and Paper Recycling ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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Analysis of Contaminants in Recycled PET

Thermal extraction GC/MS offers a powerful tool for identifying minute concentrations of contaminants in recycled polymers. It may, therefore, hold potential as an endpoint test for the quality and safety of recycled resins. The approach has been successfully used to identify trace volatiles in polymers, composites, elastomers, fibers, and adhesives. Although several thermal extraction strategies exist, they all rely on heat to remove volatile and semi-volatile compounds from test samples. Volatiles may be cryofocused, absorbed on resins, or introduced directly onto a GC column for separations. Separated peaks are identified by GC-MS. Contaminants can usually be detected in the low ppb range with relatively small sample sizes. The method is often more aggressive in removing contaminants than solvent extraction and does not result in solvent disposal problems. In addition to routine quality analysis, thermal extraction could help identify the general types and frequencies of contaminants in post-consumer resins. In the absence of controverting data, FDA calculations for estimated daily intake (EDI)(3) conservatively assume all containers are contaminated with hazardous materials. (2) Routine thermal extraction could help map the true occurrence of contamination and identify markers to differentiate virgin, chemically recycled, and physically recycled materials. The method might provide guidance for selection of surrogates in FDA's guidelines for testing plastics for food contact use. PET (polyethylene terephthalate) is an excellent candidate for recycling. It is readily identified with 2L soda bottles, is present in large volumes in recycle streams, has a reasonably high resale value, and has many end uses. Its high melting point and thermal stability also make it an excellent candidate for aggressive thermal extraction. The purpose of this paper was to examine thermal extractables from virgin, chemically recycled, and physically recycled resins provided by major commercial vendors, and from the data compile an emerging sketch (with reflection on relevance) of specific volatile constituents in recycled PET. Experimental Approach Sample Separation. Clear PET samples were acquiredfromvarious commercial sources of virgin and recycled materials. Recycled resins werefrom2° (physically recycled) and 3° (chemically recycled) resins. Virgin material was received either as blown bottles which were subsequently shredded into flake or in the form of pellets. Tertiary recycled/virgin blends were received in pellet form while secondary recycled material was received as washed flake or washed flake that had been further processed into pellet form. A total of 18 samples were examined. All samples were ground in a high speed stainless steel grinder which used a 1 mm stainless steel screen to classify the particles. Particles passing through the screen were used for thermal extraction analysis. In order to minimize the heating effects of the grinding process, samples were ground with solid C 0 added to the grinder. The dry ice was produced from research grade C 0 gas just prior to the grinding. After a sample was removed, remaining particles were brushed/vacuumed from the grinder surfaces. Then, a green colored virgin PET was subjected to the same cryogenic grinding procedure and subsequent cleanup steps as the clear samples. The 2

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Rader et al.; Plastics, Rubber, and Paper Recycling ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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result of this step was to reduce cross contamination between analyzed samples to an insignificant level. If green residues were observed in clear samples, the grinder was recleaned and the clear sample was reground. Thermal Extraction. The thermal extraction-GC-MS (TE-GC-MS) procedure involved a controlled heating of .l-.3g PET samples in a stream of flowing helium followed by analysis of the thermally extracted volatiles. The extraction cell was made of fused quartz and for the purposes of contaminant bakeoff could be heated to as high as 750 C with temperature ramp rates as high as 150°C/min. For PET analysis, samples were subjected to one of two procedures labelled A and B. In procedure A, the thermal extraction cell was first maintained at 50°C, then ramped to 160°C in 9 minutes, and finally held at 160°C for 3 minutes. In procedure B, the sample was held at 160°C for 15 minutes. With increasing temperature, compounds liberatedfromthe sample were swept by the helium through a fused silica transfer line heated to 350°C to the capillary chromatographic column where they were trapped at the head of the column by cryogenic focussing (s-120°C). During the thermal extraction process, the chromatographic column was held at 35 C. The column was 30m x 0.32mm ID with a 0.25 micron thick DB-5 phase. After the extraction period, the trapped volatiles were transferred to the column by heating the cryofocused region to 310°C at a ramp rate of about 60°C/sec. In procedure A, the column temperature was programmed to rampfrom40°C to 160°C at 5°C/min followed by a ramp to 285 C at 10 C/min and 1.5 min hold time. For procedure B, the ramp wasfrom35 C to 55°C at 10°C/min followed by a ramp to 310°C at 15°C/min and then a hold period of 10 min. The detector was a quadrupole mass spectrometer scanning the range 29-400 amu at a rate of 1.5 sec/scan.

Results Extractable Compounds. The chromatogram shown in Figure 1, is the summation of the total ion currentfromTE-GC-MS analysis of eight representory samples of secondary recycled PET washed flake obtainedfromlocations in six states. It should be immediately apparentfromthe shear number of peaks in the chromatogram that recycled material is a complex chemical system. Determining the identity and the effects of temperature on this considerable number of compounds presented a challenge to which the thermal extraction technique was well suited. All compounds were identified by comparing experimental mass spectra with spectra catalogued in the NBS library of compounds. In most cases, positive identification was possible; however, in a few cases ambiguity resultedfromthe similarities in mass spectra of closely related compounds. For certain compounds, it was necessary to examine mass spectralfragmentationpatterns and only a portion of the compound's chemical structure could be determined (e.g. phthalic acid, terephthalic acid, and isophthalic acid diesters). Compounds identified were loosely classified into categories associated with their chemical nature or presumed origins:



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thermally extracted compounds

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Figure 1. Sum of total ion current from thermal extraction of eight representative virgin and recycled resins.


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I. II. III. IV. V. VI. VII.

Small and Ethylene Glycol Related Flavor Compounds Benzoic Acid Related Benzene Dicarboxylic Acid Related Aliphatic Hydrocarbons and Acids Unexpected Contaminants Miscellaneous Compounds

Small and Ethylene Glycol Related Molecules (Class I): Compounds identified in this category were polar oxygenated species with retention times between 16 minutes and approximately 21 minutes as shown in Figures 2, #1-#10. Ethylene glycol (#6) was a major component in the chromatograms of all virgin, tertiary recycled/virgin blends, and secondary recycled samples. Compounds #3,#7-#10 are clearly related to ethylene glycol being condensation products of ethylene glycol with itself and with methanol, formic acid, acetaldehyde, and acetic acid. Compounds #1,#2,#4, and #5 are examples of small molecules which may be byproducts of the manufacture of some PET resins. Compounds in this category were found in various concentrations and combinations in both virgin and recycled samples. Flavor Compounds (Class II). In this category there were numerous compounds (#11#18) thermally extractedfrom2°-recycled material with retention times of 21-24 minutes. None of these compounds were found in virgin or 3°-recycled/virgin blends. The most predominant species in this category was d-limonene (#11), a volatile common to citrus peel oil. The family consists of eight closely related terpenes which are well known flavor constituents. Each compound has ten carbons and a six membered ring. Other similar compounds which appeared at minimum detection levels were not positively identified. Benzoic Acid Related Compounds (Class III). Another class of compounds were those related to benzoic acid. These include benzoic acid (#20), benzoic acid esters (#21,#22,#23), and benzaldehyde (#19) with retention times of23.15,26.5,23.3,25.5 and 20.8 minutes, respectively. The presence of benzoic acid is not surprising; its salts are widely used as preservatives in beverage products. Benzoic acid esters and benzaldehyde can be understood as flavoring agents, the most common being methyl salicylate (#22), the principal component in root beer flavor. The origin of n-propyl benzoic acid as a beverage constituent, however, is doubtful since it was also found in samples of virgin and 3°-recycled/virgin blends. Benzene Dicarboxylic Acid Related Compounds (Class IV). Terephthalic acid (i.e. 1,4-benzene dicarboxylic acid) is a main structural component of PET. As a result, it is not surprising that various terephthalic acid related compounds were thermally extracted and detected from both virgin and recycled material. The various mono (Ri=H) and diesters (R„ R = hydrocarbon chain) are represented in #24. These were identified by comparison to catalogued mass spectra or strongly indicated by the presence of characteristic mass spectralfragmentationspecies. Some virgin material showed diesters 2


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Figure 2. Compounds associated with thermally extracted PETE


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of this type, while others, evenfromthe same manufacturer, did not. On the other hand, all secondary recycled material showed a variety of this type of compound. For example, the prominent peak in the chromatogram in Figure 1 at 29.0 minutes has a mass spectrum very similar to terephthalic acid, methyl vinyl diester or terephthalic acid, dimethyl diester. The presence of terephthalic acid esters, which is particularly extensive in the recycled material, may be related to the hydrolysis of PET. Methyl and other esters may be present at the terminus of PET polymer chains. In addition to terephthalic acid derivatives, phthalic acid and isophthalic acid derivatives (#26 and #25 respectively) were found. In some virgin samples for example, isophthalic acid, decyl methyl ester (at 31.5 min.) and phthalic acid, di-n-octyl ester (at 33.4 min.) or closely related compounds were found. Compound #25 is a known chain modifier used in at least one process for PET manufacture. Compound #26 is possibly a polymer adjuvants used in the manufacture of PET. These two compounds were found in secondary recycled material (at 31.5 and 33.4 minutes) along with a number of other phthalic and isophthalic acid related compounds. Compounds having high molecular weight mass spectralfragmentsin addition to the characteristic diesterfragmentswere found at 36.0 minutes and 39.4 minutes. With the exception of the compounds found at 31.5,33.4,36.0 and 39.4 minutes, all other diesters appeared between 26 and 30 minutes in the 2° recycled material. Aliphatic Hydrocarbons and Acids (Class V). In all samples of washed flake, a plume of 12 aliphatic hydrocarbons was detected with retention times from 33 to 41 minutes. These compounds appear to be the members of a homologous series of long chain hydrocarbons. These are evidently waxy components, possibly the result of residues left from external package components such as base cup and label adhesives. One sample of washed flake was subjected to 3 minutes of thermal extraction at 160°C and the hydrocarbon plume of compounds dominated the chromatogram shown in Figure 3a. This same sample was reheated immediately after thefirstchromatographic analysis ceased. After the second heating to 160°C for 3 minutes the hydrocarbon plume was again seen with a decrease in intensity and a shift of the plume peak distribution to longer retention times. The repeated extraction of these hydrocarbons from secondary recycled flake indicated a reservoir of hydrocarbons whereas the shift in the hydrocarbons peak distribution indicated that more volatile compounds are preferentially removed. The straight chain organic acids, hexanoic acid, hexadecanoic acid, and octadecanoic acid were found in secondary recycledflakewith retention times of 21.1, 29.75, and 31.1 minutes in Figure 1, respectively. These compounds do not appear as homologous series of acid that might be expected from a natural source of fatty acid material. Instead, these compounds seem likely to be release agents and detergent residues leftfromthe cleaning solution. A few other complex organic molecules were detected but not clearly identified which may also be residualfromthe wash solution. Unexpected Contaminants (Class VI). Two examples of compounds in this category were drometrizole (Tinuvin) and nicotine. Drometrizole is a UV-absorber and is used to stabilize plastics and other organic materialsfromdiscoloration and deterioration. Though this compound was not found in any of the virgin PET samples examined, it was found


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in nearly all recycled flake at a retention time of 30.64 minutes. With some flake samples the drometrizole peak was a dominant peak. Possible sources for this compound are base cup polymers and adhesives, label components and adhesives. However, drometrizole is usually not added to food incorporated into food grade PET resins. The size of the drometrizole peak was highly variable. In the cleanest recycled flake material, no drometrizole was detected. Nicotine was also observed in recycled flake from a single recycler. This facility was located within a densely populated metropolitan area and the chromatogram of the sample is shown in Figure 3a. The nicotine peak at 28.1 minutes is comparable in size to some of the beverage related compounds (e.g. 1/2 the size of limonene). The variable presence and quantity of drometrizole and nicotine present examples of unexpected "spiking" of contaminants in recycled PET. Miscellaneous Compounds (Class VII): Other kinds of compounds could be seen that appear to be incorporated during the production of PET. Compounds (#30 and #31) were found at low levels in samples of both virgin and 3° -virgin blends. These may have originated as 1,4-cyclohexane dimethanol where dehydration has occurred at one or both of the hydroxyl groups producing methylene groups on the cyclohexane ring. Effects of Recycling Steps Moderately High Temperature (*160°C). The dramatic effect of heat on the contaminant content and distribution in recycled flake can be seen in Figure 3. The sample was heated to 160°C for three minutes and the species analyzed and then heated a second time and analyzed. The extraction temperature of 160°C is typical of the temperate at which PET is dried in recycling process and serves as a simple model for understanding some of the effects of the drying step in the recycling process. Based on the thermal extraction results, it is clear that the drying process can be effective in removing residual compounds. High Temperature (260-280°C). High temperatures are used at various stages in the manufacture of PET and in the extrusion process where temperatures near the melting point (i.e. 260-280°C) of the polymer are necessary. To examine the effects of extrusion on the contaminants, samples of extruded washed flake material were studied by thermal extraction. Figures 4a and 4b show washed flake obtainedfromthe same source before and after extrusion. A definite decrease in the quantity of compounds extracted was seen particularly for the flavor and benzoic acid related compounds such as d-limonene and methyl salicylate at 23.3 and 23.7 minutes respectively. On the other hand, a number of compounds appeared less affected by the heating process; in fact, one compound at 27.1 minutes was increased. The mass spectrum of this compound suggested dibutyl phthalate. The peak representing hexadecanoic acid was only slightly affected. Finally, the hydrocarbon plum at long retention times showed some decrease at the onset of the plume, a shift in the peak distribution, and even an increase in the trailing compounds.


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Figure 3. Total ion current chromatograms of 2° recycled PETEfromfirst(a) and second (b) thermal extractions.

a before extrusion

b after extrusion

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Figure 4. Total ion current chromatograms of 2° recycled PETE before (a) and (b) after extrusion.



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Selective Recycling Stream. The effect of using a select stream of recycled PET can be seen in Figure 5. Both samples shown in this figure werefromthe same recycler located in a "bottle state" where beverage bottles are redeemable for refund by the consumer. First, the effect of the "bottle states" policy on the quality of source material can be seen by comparing Figure 5a with Figure 1. Source material obtained through curbside collection may contain as much as 10% non-beverage standard PET. A somewhat different assemblage of contaminants can be expected for curbside than for a pure beverage bottle stream; but the similarities are certainly more obvious than the differences. A striking difference can be seen, however, when the select stream is made highly selective. An example would be the current state-of-the-art beverage bottle package that uses an integral molded base rather than a base cup and advanced labelling technology minimizing adhesives. Figure 5b is representative of the advantages of exceptional recycling efforts involving public policy and industrial innovation. By far, the most persistent and dominant contaminants in ordinary recycled flake, including extruded material was the high molecular weight hydrocarbons. This problem appears to be considerably lessened, if not totally removed, in the case of the superior source material from the most selective recycling stream. As a final comparison, thermal extraction results from samples of both virgin and the "most selective" washed flake are compared in Figure 6. With the exception of a handful of molecules from categories I, II, III, IV, and V, the chromatograms were roughly comparable. An even more favorable comparison can be made when the effects of heating in both the drying (approx.l60°C) and extrusion (260-280°C) are taken into account. In this case, most of the compounds in categories I though V will be reduced and under ideal condition even totally be removed. Discussion The thermal extraction information provides a basis for understanding some of the complex issues involved in the recycling of PET. For the sake of simplification, a schematic presentation of the secondary recycling process is given in Figure 7. A division of the steps can be made based upon the temperature at which the step in the process takes place. Steps expected to thermally activate the PET are shown in the shaded box. Low temperature steps are curbside collection and redemption, flake production, and washing and can be discussed individually. Curbside Collection. At present, this is the most common source of recycled material. This material is generally hand sorted to separate PET bottles from other recyclables. Along with hand sortation comes human error, so, it must be expected that undesirable components can be incorporatedfromother polymers, misused containers, food stuffs, domestic chemicals, etc. Additionally, contamination is expectedfromthe original stored contents. Many of these type of compounds originatingfrombeverage and condiment contents were detected by thermal extraction. Furthermore, a wide range of contaminants assumed to have originatedfromnon-food contact packaging materials were seen. The


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Figure 5. Total ion current chromatogram of conventional (a) and select stream (b) recycled PETEfromone bottle- state recycler.

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Figure 6. Comparison of virgin (a) and most selective (b) 2° recycled PETE.


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I) HAND SORTATION 2) CONTAMINATION 3) (3% ?)NON BEVERAGE PETE

MUNICIPAL CURBSIDE COLLECTION '


FLAKE PRODUCTION

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SURFACTANTS 2) ALKALI WASHING 3) HYDROLYSIS

REDEMPTION I) SELECT BOTTLE STREAM 2) BASE CUPS? 3) LABELLING ADHESIVE ? 4) PETE SCRAP

DRYING 160°Cj-4hrs

I) RECIRCULATION 2)WATER STRIPPING 3) COLD TRAP

EXTRUSION / PELLETIZATION 260-280°C

DRYING 160°q-4hrs

SOLID STATING 220C-4?hrs i

PREFORM 260-280°C

BOTTLE BLOWING^ , 100°C | i i

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Figure 7. Typical steps involved in processing recycled PETE (thermal steps shown inside the shaded envelope).


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substantial hydrocarbon plume seen with all but the most selective stream can be explained as a wax or paraffin component in base cup and label adhesive or possibly oligomers of HDPE base cups. In addition, a number of the phthalic acid esters that appeared in secondary recycled material can be possibly explained as plasticisers used in the fabrication of the common beverage bottle base cup adhesives. Redemption. Bottle redemption policy in certain states resulted in a much cleaner source material. The source could be divided into "select" stream and "highly select" stream. The "select" refers to high quality beverage bottles returned at a point of purchase by the consumer. "Highly select" refers to specialized beverage bottles, designed for ease of recycling, and collected as a uniform sub-stream. Thermal extraction showed that the "select stream" was in many respects similar to the municipal stream. Differences in the number of flavor and other compounds existed, making the "select" stream a somewhat superior source material. The waxy hydrocarbons were especially persistent, being found in both curbside collected and redeemed, washed flake and extruded pellets. From the standpoint of food safety, the waxy material may not constitute a serious threat. From the point of view of polymer engineering for food containers, the incorporation of unwanted compounds can compromises processibility. The "highly select" stream represents another significant improvement in source material. From the comparison showed in Figure 5, by selecting a sub-stream with an optimized package design (e.g. integral molded base cup), recycling problems can be avoided. Flake Production. Flake production is essentially mechanical; although, some elevated temperature may occur, causing slight chemical effects. During the shredding process, cross-contamination potentially occurs and unwanted materials may become irretrievably incorporated. Washing. Washing introduces a major chemical effect on the recycling process. Recycling facilities may use widely different washing formulations and techniques. Surfactant and detergent composition, wash temperature, washing stages, alkali content, etc. have a pronounced effect; however, it was not possible to assess these parameters in the present study. In a number of samples, it appeared that hydrolysis products of the PET resin were present. These will certainly be greatly affected by the washing step particularly the alkali content of the solutions. With the development of an accepted analysis method (such as thermal extraction or others) for comparing various washing parameters, it should be possible to tailor the washing process to the source material. Drying. Typically, washed flake is then dried at an elevated temperature (approx 160°C for 4 hours) to remove moisture. Thermal extraction at 160°C for 3 minutes of ground samples (see Figure 3), were effective in removing a considerable number of contaminants present in washed flake. The efficiency of this, depends largely on the volatility of the compound and the solubility of the compound in the polymer. Several considerations can be taken into account to maximize the beneficial effect of the drying step for cleaning recycled flake. Recirculation of the drying air is essential; otherwise,


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volatile contaminants can only redistribute, essentially smearing the contamination over the entire batch. It is essential to trap water, as well as volatile organic contaminants liberated in this step. Water can be removed with a desiccant and organics can be removed by cold trapping. Recyclers using cold trapping find a substantial quantity of resinous material. Thermal extraction analysis, performed on ground (

Analysis of Contaminants in Recycled Poly ... - ACS Publications

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