Protocol To Verify Contaminant Removal from Postconsumer Poly

Chapter 33

Protocol To Verify Contaminant Removal from Postconsumer Poly(ethylene terephthalate) Downloaded by NORTH CAROLINA STATE UNIV on January 2, 2018 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch033

1

2

E. N. Nowak and R. M. Oblath

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1Product Safety and Compliance, Polyester Business, and Polyester Technical Center, Shell Chemical Company, 130 Johns Avenue, Akron, OH 44305 This paper discusses a test protocol used to verify that model contaminants were efficiently removed from intentionally contaminated PET by Shell's process for manufacturing REPETE polyester resin. Thefirststep of the protocol consisted of developing a model for postconsumer contamination. The model included: identification of contamination routes, identification of potential contaminants, selection of model contaminants, and estimation of the level of contamination. An experimental protocol was then developed to simulate the model and data was developed to assess process efficiency.

In December, 1991, Goodyear's Polyester Division, now Shell Chemical's Polyester Business, obtained from FDA a "letter of no objection" to the use of REPETE polyester resin for manufacturing food packaging (Rulis, Alan M , FDA, personal communication, December 6, 1991). REPETE is a registered trademark for PET made from post-consumer recycled PET via Shell's tertiary recycle process. FDA's decision was based on their review of data generated at our facilities over a two year period through a protocol developed to validate the safety of our process for manufacturing REPETEfrompost-consumer recycle. Shell's process takes advantage of the unique chemistry of PET. Before discussing our protocol, it will be useful to briefly review the chemistry and manufacturing process for PET made by the acid route (Figure 1). In a vessel heated to greater than 260°C and operated under positive pressure, the acid and glycol are reacted to form low molecular weight, low viscosity oligomers containing from two to ten repeat units. Water coproduced in the esterification is taken overhead. The oligomers are then conveyed to a second vessel where, over several hours at greater than 270°C in the presence of catalysts, the molecular weight is increased by polycondensation. The reaction is driven by removal under high vacuum of ethylene glycol and small amounts of water.

0097-6156/95/0609-0404$12.00/0 © 1995 American Chemical Society Rader et al.; Plastics, Rubber, and Paper Recycling ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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

ESTERIFY >260°C pos. pressure several hours

2

SOLID PHASE POLYMERIZE >200°C atm pressure >6 hours dry N purge

2

Figure 1. Shell's PET Polymerization Process (Reproduced with Permission from Reference 5).

MELT POLYMERIZE >270°C high vacuum >2 hours

2

EG+N +H0

Downloaded by NORTH CAROLINA STATE UNIV on January 2, 2018 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch033

BOTTLE POLYMER

BLEND SILO


406

PLASTICS, RUBBER, AND PAPER RECYCLING

The resulting polymer, containing from 100 to 120 repeat units, is discharged, cooled to ambient temperature and cut into pellets. The pellets are fed to a final polymerization vessel operated at temperatures below the melt point of the polymer. In this vessel, which may be operated either under vacuum or with a dry nitrogen purge, the molecular weight of the polymer is increased to the desired level over a sbchour period. Thefinalpolymer is then discharged to a certification vessel, subjected to quality assurance tests and then sent to inventory. An inherent characteristic which makes PET uniquely suitable for closed-loop tertiary recycling is that the polycondensation (polymerization) reaction (Figure 2) is reversible. That is, if PET is heated in the presence of TPA and/or EG, the condensation reaction will reverse (depolymerize) with the monomers progressively cleaving the polymer chains, ultimately to low molecular weight oligomers containing from two to ten repeat units. This ability to depolymerize PET is the basis of Shell's process for manufacturing PET resinsfrompost-consumer recycle. Shell's process is shown, in part, in Figure 3. Although Figure 3 shows the fundamental depolymerization/repolymerization steps used in Shell's process, it should be noted that our process contains additional proprietary steps not shown in this schematic. The principal difference between the typical PET polymerization process depicted in Figure 1 and Shell's process for manufacturing REPETE polyester shown in Figure 3 is that our recycle process includes cleaned post-consumer flake as an additional raw material. During esterification, in the presence of TPA and EG, flake is depolymerized to low molecular weight oligomers. Any contaminants which may have absorbed into the polymer matrix due to consumer misuse are thereby released for facile removal during subsequent processing, including the proprietary elements of Shell's process. The mixture of oligomers is fed to the melt polymerization vessel where, under the conditions of high temperature and vacuum, REPETE resin is produced. The resulting purified melt polymer is finished through the normal processing steps. When we began to design a protocol to evaluate tertiary recycle processes for producing PET suitable for food packaging, no guidelines were available to aid in protocol development. We believed that an appropriate protocol should be based on a holistic approach. That is, the recycle process was considered to begin when the container is delivered to the consumer and to include collection, sortation and flake cleaning as well as the subsequent tertiary processing. The first consideration, then, was to identify, by broad classification, the contaminants most likely to be found in the recycle loop. These contaminants, and the various steps in the recycle loop where they are removedfromthe stream, are shown in Figure 4. Microbiological contamination is effectively removed by the hot caustic wash during flake cleaning. Any trace microbiological contamination is destroyed during subsequent processing in polymerization reactors which are operated at temperatures greater than those typically encountered in an autoclave. Foreign materials, such as wood from pallets, aluminum from bottle caps, and glass are removed through the handling practices used in the collection, sorting and cleaning operations. Shell specifications for flake also limit these materials. Proprietary steps in Shell's process also effectively remove foreign materials.



TPA + EG

HOCH2CH2OH

2-10

PET

Figure 2. Reversible Polycondensation of TPA and EG to PET (Reproduced with Permission from Reference 5).

OLIGOMER

J


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

>200°C atm pressure

high vacuum >2 hours

pos. pressure

several hours

2

purge

Figure 3. Shell's Tertiary Recycle Process (Reproduced with Permission from Reference 5).

dry N

>6 hours

SOLID PHASE POLYMERIZE

>270°C

H?0

2

>260°C

c

9

c—

A 2

EG+N +H 0

MELT POLYMERIZE

2

Ae .

ESTERIFY/DE-ESTERIFY

EC

TPA

RECYCLE PET


POLYMER

BOTTLE

1

SILO

BLEND

C o

©

>

w w w

3

GO

o

© 00

33. NOWAK AND OBLATH

Contaminant Removal from Postconsumer PET

CONTAMINANT Microbiological

Foreign material

REMOVAL STEP Hot caustic wash, Shell process Handling practices,

Wood

Specifications, Shell

Aluminum

process


Glass Mixed plastics

Sorting, handling, flotation,

PP

Specifications, Shell

HDPE

process

PVC

Non-food residues

Sorting, washing, Shell process

Contaminants from consumer misuse

Shell process

Pesticides Petroleum products Metal Compounds

Figure 4. Removal of Potential Contaminants in Post-Consumer PET (Reproduced with PermissionfromReference 5). Mixed plastics, such as polypropylenefrombottle caps, high density polyethylene from base cups and PVC from cap liners are removed by sorting, handling and flotation procedures. These materials are also limited by our specifications and are removed in the proprietary steps of our process. Non-food residues, e.g., detergents, are removed by washing and in the depolymerization/repolymerization phase of our process. Contaminants resulting from consumer misuse of containers present perhaps the greatest concern associated with use of post-consumer recycle in food packaging. Pesticides and heavy metal compounds, because of toxicological considerations, and petroleum products, because of broad accessibility to consumers, are of particular concern. These materials also present the greatest challenge to any recycle process. For these reasons it was felt necessary to clearly demonstrate, through validation studies, that our process has both the capacity and the efficiency to remove contaminants which may be present in the recycle stream as a result of consumer misuse. In order to design an appropriate experimental program to validate a recycle process, it is necessary to first develop a model for post-consumer contamination. Development of a suitable model consists of four steps. First, the routes and scenarios


409


410


by which contamination is most likely to occur are identified. Second, the universe of potential compounds which may enter the recycle stream via those routes are identified. Third, model compounds representative of that universe are selected. Finally, the likely level of contaminants in the recycle stream is estimated. Based on this model, an experimental protocol can then be developed to simulate the model It is generally believed that by far the most likely scenario for contamination of the recycle stream is that involving secondary storage in beverage bottles. Both pesticides and petroleum products sold in bulk are likely to enter the stream via this route. In this scenario, a consumer would purchase a pesticide concentrate from a local garden supply store, make up a dilution for application to his garden and save the unused portion in a two liter PET bottle for application the following week. The implications of this scenario to further model development are that only pesticides sold as concentrates are included in the universe of possible contaminants and that time be allowed for contaminants to absorb into the matrix of the polymer. Because of this "most likely" scenario, the universe of potential contaminants was limited to pesticides and petroleum products. The universe of potential pesticide contaminants was developed for the model from several EPA documents, marketing research reports, and from information obtained from the National Pesticide Clearing House (Hankes, Jill, Texas Tech, personal communication, 1990) (1,2,3). From these documents, some 56 pesticides were identified. This list of 56 candidate compounds was narrowed to approximately 24 by applying the following selection criteria: • Pesticides either banned by EPA or voluntarily withdrawn by the manufacturer were eliminated. • Pesticides not generally available to the consumer (those regulated for use by professional applicators only) were eliminated. • Pesticides sold as dusts or aerosols were eliminated. Only those sold as concentrates and requiring dilution for use were considered. • Pesticides for which no sufficiently sensitive analytical methods existed (and were not likely to be developed) were eliminated. This list of 24 compounds was loosely grouped by chemical class and further narrowed by considering: • Toxicity. From among a group of similar compounds, the most toxic compound was selected. • Stability. Likewise, the most stable among a group of structurally similar compounds was selected. • Removal difficulty. The compound judged to be the most difficult to remove, based on boiling point, was selected from each group of structurally similar compounds. • Breadth of usage. The compound which enjoyed the greatest usage, based on market volume and number of approved uses, was selectedfromits grouping. This tentative final list was then reexamined to assure that any individual class of compounds was not unduly over-represented. Through this selection process, the model compounds chosen from the identified universe of pesticides were those which met the usage criteria, were the most toxic



33.

NOWAK AND OBLATH

Contaminant Removal from Postconsumer PET

and most stable, were the most difficult to remove, and for which suitably sensitive analytical methods either existed or could be developed. While developing the model and test protocol, ongoing discussions were held with FDA (April 25, 1990; September 5, 1990; December 20, 1990), and it was concludedfromthose discussions that the Agency's general approach to validating a recycle process contained the following elements: • 100% of the post-consumer flake should be contaminated with model contaminants. • The contamination level should be 100 ppm. • Model contaminants should represent: • Nonvolatile, nonpolar compounds; nonvolatile, polar compounds; volatile, nonpolar compounds; volatile, polar compounds; heavy metal compounds. • The detection limits should be 50-100 ppb. Although FDA's general approach recommends the inclusion of a heavy metal surrogate, and, while this may be appropriate for other plastics, our model for postconsumer contamination indicated this was not the case for PET. Exclusion of a heavy metal surrogate from validation studies of recycle processes for PET is further supported by migration data. In migration studies conducted at 120°F for extended periods (30 days) with the very aggressive solvent, 95% ethanol, no migration of metallic PET catalysts (antimony and cobalt) was detected using a method having a lower detection limit of 3 ppb. In evaluations of cleaned PETflakesupplied to our plant over a three-year period, no heavy metal contamination was detected by X-ray fluorescence having a detection limit of 2-15 ppm. FDA's general approach to evaluation of recycling processes calls for measurement of residual contamination in the processed polymer using an analytical method having a lower detection limit of 50-100 ppb. To achieve a level of contamination in the incomingflakeof 50 ppb would require a "spike" of contamination consisting of more than 1000 bottles, each contaminated with ca 200 ppm metal, a highly unlikely event. Lastly, consumer accessibility to heavy metal containing products is extremely limited. Only two metal containing pesticides were registered for use in the US in 1987, and the registration was for use as a herbicide on cotton crops. Lead and mercury containing paints are being phased out and chromium based pigments are used only in industrial paints. After consideration of all the above, including FDA's general approach, the following model compounds were selected for use in our validation studies: Lindane, representing a nonvolatile, nonpolar compound; Diazinon, representing a nonvolatile, polar compound; toluene, representing a volatile, nonpolar compound; chloroform, representing a volatile, polar compound; motor oil, representing a mixture of nonvolatile, nonpolar compounds; and gasoline, representing a mixture of volatile, nonpolar compounds. Having selected surrogates for the universe of materials which could enter the recycle stream through consumer misuse, the next step was to design experiments to simulate our model and evaluate the efficiency and capacity of our process to remove these surrogates. Two separate sets of experiments were conducted. In one set, Lindane, Diazinon, motor oil and gasoline were stored in PET bottles for two weeks at 40°C to allow ample time for absorption of the contaminants.


411


412


The bottles were then rinsed briefly with distilled water to remove gross surface contamination. The bottles were ground to flake and subjected to a washing sequence typical of that used in the industry. After washed flake was air dried at ambient temperature it was blended with uncontaminated flake at the 2-4% level. Blend levels were chosen to represent maximum levels of contamination estimated to be likely in post-consumer recycle using marketing data and other considerations which are beyond the scope of the present discussion. The blends were then subjected to our REPETE polyester process. Samples were analyzed for contaminants after the water rinse, after washing and after processing into REPETE polyester. Data are shown in Table I. Whereas washing in 2% caustic resulted in an approximate 50% reduction in contaminant level, the complete depolymerization/repolymerization process reduced the contaminant levels to below the detection limits shown in Table I. Because of the multicomponent nature of motor oil and gasoline, analysis of these materials at extremely low levels presents a major challenge to analytical chemists. Although the analytical methods for motor oil and gasoline developed for these studies represented significant improvements in sensitivity over previously published methods, the limits of detection were not sufficiently low to provide adequate assurance of safety. The data for gasoline and motor oil however indicate the process has the capacity to remove fairly high levels of complex mixtures of volatile and nonvolatile compounds. According to the principles discussed in FDA's 'Guidelines'; toluene (nonpolar, volatile) and Lindane (nonpolar, nonvolatile) can be considered as surrogates for gasoline and motor oil, respectively (4). Based on this consideration, a separate series of experiments were conducted in which neat Lindane, Diazinon, toluene, and chloroform were individually and quantitatively added to separate samples of ground PET flake to produce 100 ppm in the flake. [Appropriate amounts of each contaminant were added directly to the flake at the time flake was introduced into the depolymerization vessel to assure that all the contaminant was introduced into the process.] The contaminated flake was then individually processed into REPETE polyester containing 50% recycle. Data from analysis of the resins after repolymerization, are shown in Table II. It should be noted that analyses were conducted after the melt polymerization step, not after the final solid-state polymerization step. In all cases, no residual contaminant was detected in the polymer after processing. The detection limits of the analytical methods used are also shown in Table II. It can be seen that the detection limits of the methods were as low as, and in some cases significantly lower than, those recommended by FDA. Figure 5 describes the totality of our results, showing the range of volatilities on the Y axis and the reduction of contamination (on a log scale) along the X axis. Superimposed is the "de minimis" level of contamination which would result in a level of 0.5 ppb in the diet. This chart clearly shows both the capacity of our process to remove fairly high levels of contaminant (motor oil) and its efficiency in removing a broad range of compounds of varying volatility and polarity. These data clearly demonstrate the ability of this process to remove unrealistically high levels of model contaminants and its ability to produce for food packaging, suitably pure PET containing up to 50% post-consumer recycle. During the course of these studies, data were developed to permit preliminary evaluation of four technologies of varying complexity: simple washing, with no further



10 ppb 5 ppb

ND ND

23 ppm 120 ppm 13,000 ppm

40 ppm

670 ppm

25,000 ppm

Diazinon

Gasoline

Motor Oil

25 ppb

25 ppb

ND

187 ppm

280 ppm

Lindane

ND

LOP

Wash

Blending

Storage

After

Compound

After

After

Model

Table I. Concentrations of Contaminants in PET at Various Stages of Shell*s Depolymerization/Repolymerization Process



100 ppm 100 ppm 200 ppm 200 ppm

Diazinon

Toluene

Chloroform

Contamination Level

Lindane

Model Compound

Table IL Residual Contaminant Concentration in REPETE

10 ppb 100 ppb 50 ppb

ND ND

25 ppb

LOP

ND

ND

After Repolvmerize



PPM in PET - Log Scale

100

"hv JX^

10

0.1

0.0005 ppm in Diet

1.0

De Minimis

5.0 ppm Limit of Detecti m

25.0 ppm Limit of Detection

>


0.01

^^).025 ppm Limit of Detection

>


> > 0.050 ppm Limit of Detection

Figure 5. Summary of Efficiency and Capacity for Contaminant Removal (Reproduced with PermissionfromReference 5).

1000

Lindane (Non-Polar)

Motor Oil (Mixture)

Diazinon (Polar)

Gasoline (Mixture)

Toluene (Non-Polar)

Chloroform (Polar)



13300 187

24600

281

Motor Oil

Lindane

2

333

BLENDING

AFTER

All Values in ppm in PET SOURCE: From Proceedings of Recyclingplas VII Conference: Plastics Recycling as a Business Opportunity, Table II.

WASH

WASH

AFTER

COMPOUND

BEFORE

SOLID PHASE 45 0.6

270 2

AFTER

REPELLETIZE

AFTER

Table HI. Concentrational Selected Contaminants in PET Following Processing Operations



33.

NOWAK AND OBLATH

Contaminant RemovalfromPostconsumer PET

417

process; washing followed by melt extrusion into pellets; washing, pelletizing and solid state polymerizing; and, finally, washing followed by our depolymerization/repolymerization process. The results of these limited studies using motor oil and Lindane as examples are shown in Table DDL In this case, bottles were exposed to the contaminants for two weeks at 40°C. The bottles were washed, ground, dried in air at ambient temperature and then blended with uncontaminated flake at a 2-4% leveL The blendedflakewas then extruded and repelletized and,finally,solid stated. As can be seen from Table m, extrusion/pelletization was not effective in removing these contaminants. Although solid-stating did effect a fourfold reduction in these contaminants, the washing, pelletizing and solid-stating sequence used in this study was not adequate for producing resin of suitable purity for use in food packaging. In summary, we have developed a model for post-consumer contamination and designed experiments to simulate the model. Through these experiments, data was developed which demonstrate the suitability of Shell's process for manufacturing for food packaging, polyester resin containing post-consumer recycle. Literature Cited (1) Nonoccupational Pesticide Exposure Study (NOPES), EPA/600/3-90/003, 1990. (2) National Household Pesticide Usage Study, 1976-1977, EPA 540/9080-002,1980. (3) Caldereni, Phil, Pesticide Industry Overview, CEH Marketing Research Report, Chemical Economics Handbook - SRI International, 1988. (4) Points to Consider for the Use of Recycled Plastics in Food Packaging: Chemistry Considerations, Division of Food Chemistry and Technology, HFF-410, Center for Food Safety and Applied Nutrition, NS Food and Drug Administration, 1992. (5) Plastics Institute of America, Inc, Recyclingplas VII Conference, Plastics Recycling as a Business Opportunity, May 20-21, 1992. RECEIVED April 14, 1995


Protocol To Verify Contaminant Removal from Postconsumer Poly

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