Rutherford backscattering analysis of contaminants in PET

Nuclear Instruments and Methods in Physics Research B 124

(I 997)

575-578

Beam Interactions with Materials 6 Atoms

ELSEVIER

Rutherford backscattering analysis of contaminants in PET D.E. Pierce a** , R.L. Pfeffer a, G.D. Sadler b a U.S. Army Reseurch Lahoramry. h Nutionul

Centerfiw

Sensors and Elecrron

Food Sufeg and Technology,

Illinois

Devices Direcrorute. Insrirute

Fort Monmouth,

of’ Technology. Summir-Argo.

NJ 07703-5601, IL 60501-1933.

USA 1JSA

Received 28 October 1996

Abstract Rutherford Backscattering Spectrometry (RBS) was used to understand the sorption and desorption of organic contaminants in the polymer Poly(ethylene terephthalate), or PET. Samples were exposed to a range of organics to simulate contamination of PET that can take place in the post-consumer waste stream. From RBS analysis, concentration depth profiles were shown to vary from a monolayer regime surface layer to a saturation level, depending on the contaminant. Heat treatments were also applied to contaminated polymer to simulate thermal processing steps in the recycling of PET. Heating caused a dramatic decrease in contaminants and in some cases a complete removal of contamination was achieved to the limit of RBS detectability. PACS:

82.8O.Yc,

Rutherford backscattering; 61.41 + e, Polymers, elastomers and plastics; 81.05.L.

Keyworck: Recycling; Pesticides; Polfiethylene

1. Introduction There is considerable interest in the quality of polymeric material that is retrieved from the recycling stream and reprocessed. Of particular concern is the presence of deleterious contaminants in such food grade material as Poly(ethylene terephthalate), or PET. This polymer is extensively used as a beverage container and in other foodrelated products. It is the sole constituent of virtually all plastic carbonated beverage containers as well as those for many condiments, cooking oils, liquid cleaning agents, and a large variety of miscellaneous domestic and industrial packaging. PET from the post-consumer waste stream contains a broad spectrum of contaminant compounds whose origin is adsorption and absorption of components in the original contents, cross contamination by contact and mixing with commingled materials in the waste stream, and misuse of the containers [I]. As an example of misuse, PET bottles are commonly used to mix and store household chemicals such as insecticides and automotive fluids such as used motor oil. The misused material is discarded and mixed

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with less adulterated material. To effectively recycle PET it is important to know the extent of contamination, the way contaminants permeate into the polymer, and methods to remove them. Rutherford backscattering spectrometry (RBS) provides a unique vantage point from which to view these concerns. It has been shown to be an effective tool for observing organic contamination in polymer thin films [2-41. In this study RBS was used to examine the concentration depth profiles of a restricted number of surrogate contaminants which serve to represent classes of compounds that might be expected. Among the contaminants we have investigated are xylene and benzene, which are found in automotive fluids such gasoline and motor oils. These are of concern because of their high affinity for PET and their deleterious effects on health (e.g., carcinogenesis and liver damage). We also studied benzoic acid, hexanoic acid, dodecane, butyric acid and benzaldehyde, which are constituents variously of food additives, flavors, fats, waxes and oils and decaying milk products. All of these can spoil the integrity of the polymer for further use. Finally we studied the widely used insecticide malathion, a toxin which interferes with nerve function. In recycling, collected PET material is sorted, shredded/ground, then washed in one or more stages in aqueous cleaning solutions which contain surfactant for removal of contamination at the surface [ 1,5]. The effective-

1997 Elsevier Science B.V. All rights reserved

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ness of this cleaning step depends on the nature of the compounds that are being removed. Solution cleaning has severe limitations, particularly in removing hydrophobic contamination and compounds that have migrated into the polymer matrix [6]. In PET recycling there are several stages of heating in the process of bringing the polymer into an appropriate state for re-extrusion and bottle blowing. Thermal treatments can be particularly effective in reducing and even removing contamination [4,5], and thermal processes can be tailored to serve as cleanup steps. In this paper we report on the application of RBS to determine the extent of contamination and the effectiveness of thermal treatments to eliminate it.

tn Phys. Rrs. B 124 (1997) 575-578

compressed between stainless steel plates, each of which had a central I” hole, exposing both surfaces to the ambient while preventing excessive deformation that can occur during heat treatment. The heat treatments, performed in a vacuum oven at reduced pressure, ranged from 100°C to 165°C for durations of I to I3 h. Then, in preparation for RBS, a thin (30-150 Al Au film was deposited on one surface of each sample in a commercial planar magnetron triode sputtering system with a Au target operating in a 0.02 Torr Ar ambient at 3 kV. The film served to prevent surface charge buildup and to reduce ion-induced desorption. For the RBS analysis it also functioned as an energy calibration point and as a stable reference to determine the composition shift of the polymer upon ion bombardment.

2. Experimental For this study, brominated analogs of hydrocarbon compounds were obtained from commercial sources. These served as surrogates for the RBS analysis of contamination in PET. Brominated analogs had a single bromine atom substituted for a hydrogen in each molecule, (e.g., benzene, C,H,, became C,H,Br). This provided some key advantages. First, by tracking the bromine, contaminant levels could be measured down to a considerable depth (approximately I.9 pm) with a high sensitivity. RBS sensitivity for elemental detection is proportional to the square of the atomic mass, so the bromine atom in brominated analogs provided a sensitive measure of the presence of organic molecules to which they were attached. In addition to brominated analogs, the common household insecticide malathion was studied. Both undiluted commercial malathion solution and the same solution with the recommended dilution in water (I : 5) were used. Malathion represents a class of compounds containing phosphorous which inhibit the enzyme acetylcholinesterase. The molecular formula is C,,H ,,O,S,P and the sulfur and phosphorous serve as tracers for RBS detection. Since the atomic masses are close, the signals from these two elements are not separable but combine to increase sensitivity. It may be noted that nearly all insecticides contain one or more atoms (typically P, S, Cl and Br) that can function as tracers in RBS analysis. Samples were prepared by cutting 2” X 2” pieces from unused (i.e., never filled) PET beverage containers, obtained direct from bottle manufacturers. These were exposed to neat (i.e., undiluted) brominated liquid, brominated solid (benzoic acid only). and malathion solution. Samples were exposed for 2 weeks at 40°C according to FDA protocol for contaminant studies [7]. After contamination, samples were given IO washes in pure ethanol at room temperature to eliminate gross surface contamination (for malathion, 3 washes in pure methanol preceded the ethanol). The samples were then blown with dry nitrogen and stored in air in closed teflon-sealed glass containers. Samples that were further treated were mechanically

3. RBS analysis Each flat polymer sample was clamped under a stainless steel annulus mounted at the center of the stage of a 2-axis goniometer in the channeling beamline of a General Ionex Model 41 I7 Tandetron accelerator. The 2 MeV He’+ beam was slightly defocused and offset from the center of rotation of the stage, which was rotated under the beam. This served to reduce the ion damage by lowering the maximum fluence; the resulting beam spot was an annulus IO mm OD X 4 mm ID. The total beam charge deposited on the sample was measured using a ratemeter and digital current integrator; secondary electron emission was suppressed by a small high-field permanent magnet in the stage immediately below the sample. Each sample was analyzed with the ion beam only once, and discarded afterward. Gold coated PET samples which had no contamination or post-manufacture heat treatment were used to determine ion beam-induced alteration of stoichiometry. It was necessary to determine sample doses which could be kept low enough to maintain sample integrity, while furnishing enough data for reasonable statistical accuracy. This was done by recording spectra after successive I6 pC doses up to 96 pC, and simulating data using the RUMP software package [8]. The repeatability of the beam current integration was checked by normalizing all spectra to the integral under the Au peak. The repeatability turned out to be within 0.8%. Fits of the successive spectra showed that PET lost oxygen at the rate of 0.24% per pC of ions. The oxygen loss was found to be linear with dose by observing the C: Au and 0: Au ratios and extrapolated from zero dose, where the stoichiometry of PET is C,,H,O,. By limiting the exposure to 8 pC per sample, we were able to minimize damage and maintain the polymer composition to within 1.9% that of virgin PET. A spectrum was recorded for each sample, and the concentrations of bromine. phosphorus and sulfur (depending on sample) versus depth were determined.

D.E. Pierce et ul./Nucl.

4. Results and discussion :,I

We observed that the different contaminants showed strikingly different initial distributions as well as different responses to thermal treatments. A representative initial spectrum of contaminated sample appears in Fig. I. The raw data shows bromoxylene contaminated gold coated PET that has not received heat treatment. In addition to raw data, a RUMP simulation of the sample is also shown. As is evident, the constituents observed were C (edge at channel 13 I), 0 (channel 1891, Br (channel 432) and Au (channel 487). The simulation yielded a fit of representing a contaminant level of C,,.sH~.,O,Bro.,s,, one molecule of bromoxylene (C,H,Br) per 5.4 units of PET (C ,,,H,O,). This contaminant level was uniform down to a depth of at least 1.9 pm, indicating that the bromoxylene enters the PET readily and diffuses rapidly, establishing a uniform equilibrium concentration deep into the material. Also evident is a peak in the bromine signal at the bromine backscatter edge. The simulation indicates that this bromine peak represents 3 X lOI atoms/cm’, or a few monolayer equivalents. The origin of the surface peak can be threefold: the polymer may be modified near the surface, the contaminant may be adsorbed on the surface, and the contaminant may be entrapped in cracks and crevices at the surface. Fig. 2 shows the response of the bromoxylene to thermal treatments. The three uppermost curves represent data from samples that were respectively unheated, heated at 1 10°C for 1 h, and heated at 1 15°C for 13 h. The bottom curve is from uncontaminated PET. The figure shows that

Energy

517

Instr. und Mcth. in Phys. Res. B 124 (1997) 575-578

(MeV)

,.,,,,*~,

1. A representative initial RBS spectrum of bromoxylene contaminated gold coated PET that has not received heat treat-

Fig.

ment. The solid curve is data, the dashed curve a RUMP simulanon indicating the constituents observed; the best fit was C 11.s%.704Br0 ]s5. The spectrum shows that the contaminant level, one molecule of bromoxylene (C,H,Br) per 5.4 units of PET (C ,0 H,O,), was uniform down to a depth of at least I .9 pm.

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Fig. 2. Response of bromoxylene to thermal treatments. The three uppermost curves represent data from samples that were respectively, unheated, heated at 110°C for I h, and heated at I 15°C for I3 h; the bottom curve is from uncontaminated PET. Upon heating, the concentration profile, initially uniform with depth, rapidly depletes with no significant surface barrier. In the 115°C heated sample, small peaks are apparent around channel 400 and 310; these are probably surface aluminum and copper deposited during gold sputtering.

the concentration profile, initially uniform with depth, rapidly depletes upon heating through what is likely Fickian diffusion with no significant surface barrier [9]. A number of high temperature steps are normally carried out in the processing of recycled PET. In particular, the postwash drying step of 160°C for 4 h [I] would appear sufficient to eliminate the bromoxylene. In the 115°C heated sample, small peaks are apparent around channel 400 and 310. These appear to be aluminum and copper deposited on the surface in minute quantities during gold deposition. Bromobenzoic acid behaves much differently, as shown in Fig. 3. The upper curve represents data from a sample that was unheated, and the lower curve from one that was heated at 160°C for 1 h. The upper curve indicates that the diffusion of this contaminant into PET was much slower than that of bromoxylene. The volume concentration, even at its peak value near the PET surface, was only one molecule per 50 units of PET, and fell to a third of this value at a depth of about 1.2 urn. Below 1.2 pm the concentration was relatively uniform. The lower curve, from a sample after heat treatment, shows only a small residue of bromobenzoic acid near the surface, with an area1 density of 0.2 X IO” atoms/cm’. Again, the small peak around channel 305 indicates a bit of surface aluminum cosputtered with gold. Other contaminants had various behaviors. Bromobenzene behaved similarly to bromoxylene, having a nearly flat concentration profile with an effective formula of or one molecule for 6.25 PET units. Cr0.$fs.a04Br0.,~

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Fig. 3. Response of bromobenzoic acid to thermal treatments. The upper curve represents data from a sample that was unheated, and the lower curve one that was heated at 160°C for 1 h. The upper curve indicates that the diffusion of this contaminant into PET was much slower than that of bromoxylene. The volume concentration, even at its peak value near the PET surface, was only one molecule per 50 units of PET, and declined to a third of this value at a depth of about I .2 pm. The heat treated sample shows only a

small residue of bromobenzoic acid near the surface, with an areal density of 0.2X IO” atoms/cm’.

Rutherford backscattering spectrometry proved a useful tool in understanding the nature of contamination in PET. Most importantly, RBS could determine the concentration distribution of contamination both before and after heat treatment. Knowing a contaminant’s distribution within the material is of critical value in defining the impact of contamination and developing recycling processes. Other methods involving thermal extraction [I] or chemical extraction [6] of the polymer combined with chromatography (and often mass spectrometry) have been particularly useful in determining which compounds are actually present in recycled material. These, however, offer little or no information about the contaminant distribution which RBS so easily provides. Although the high energy ion beam technique does suffer the limitations of requiring a tracer atom. causing sample damage, and having limited depth sensitivity and detectability, in the present case it provided valuable information that is not available otherwise.

References could be easily driven out to baseline concentrations by heating at 160°C for I h. The malathion contamination was less extensive, giving or effectively one molecule for 18.2 C ,o.,H,O,.,Po.ossSo.~ I PET units. This contaminant could also be completely removed after I h at 160°C. Bromohexanoic acid began with a uniform stoichiometry of C ,0,8sH9,5404~28BrO_,~ (1 : 7.1 units PET) and was reduced to C,0~,Hs,,804,03Bro.o,ti (1 : 62.3 units) by I h at 160°C. This reduction factor of nearly nine suggests further heating could affect a complete removal of this contaminant. Bromododecane. a twelve carbon long chain molecule, CHs(CH,),,CH,Br, had minimal penetration into the PET and was confined to within a few monolayers of the surface. Although heating reduced this layer, a 0.2 X lO”/cm’ sub-monolayer residue remained. Extended heating would likely remove this small residue completely. Bromobutyric acid substantially modified the polymer resulting in a complex RBS spectrum which was difficult to interpret. Bromobenzaldehyde reacted with PET and visibly modified the material, Additionally,

bromobenzene

[II D.E. Pierce, D.B. King and G.D. Sadler, in: Plastics, Rubber, and Paper Recycling A Pragmatic Approach, eds. C.P. Rader, S.D. Baldwin, D.D. Cornell, G.D. Sadler and R.F. Stockel (American Chemical Society, Washington, DC, 199.5). [21 E.J. Kramer. Mater. Res. Sci. Bull. 21 (1996) 37. 131 T.P. Gall, R.J. Lasky and E.J. Kramer, Polymer 31 (1990) 1491. [41 P.J. Mills and E.J. Kramer, J. Mater. Sci. 21 (1986) 4151. [51 L.D. Tacito, in: Plastics, Rubber, and Paper Recycling - A Pragmatic Approach, eds. C.P. Rader, S.D. Baldwin, D.D. Cornell, G.D. Sadler and R.F. Stockel (American Chemical Society, Washington, D.C., 1995). 161 V. Komolprasert and A. Lawson, Proc. Annual SPE Meeting, (SPE. San Francisco. 1994). [71 U.S. Food and Drug Administration. Points to Consider for Use of Recycled Plastics in Food Packaging: Chemistry Considerations, HFS-245, (FDA Center for Food Safety and Applied Nutrition, Washington, DC, 1992). [81 L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. [91 I. Crank. The Mathematics of Diffusion (Clarendon Press, Oxford,

1975).