Creation of the Syracuse Chemistry of Artifacts Project (SCOAP) plastics reference database
Spectra were obtained for a large variety of reference plastics, for a total of 134 different samples that are identified in the Supplementary information section. These specific reference plastics were chosen to comprise those used in manufacturing from the late 1800s through the early 2000s, which is the manufacturing time span of the artifacts in the PAC. The reference spectra were compiled using BWID to form the SCOAP plastics reference database.
After constructing the database, we tested it using spectra of plastic marketing samples produced by the Chroma Color Corporation and some discarded consumer items marked with recycling codes. The analyses produced correlations to the database references of 93–99+% match quality, and the results can be seen in Table 2. We generally consider correlations greater than 90% match quality to be successful. Even when obtaining high correlations, complete analysis of the artifact spectrum includes looking for anomalies both within the 200–2600 cm− 1 spectral window that BWID uses for matching and also outside of that window. The presence of all spectral fingerprint features is verified in the artifact spectrum. Any anomalies are carefully considered, and best attempts are made for attributions to sample-specific aspects such as the presence of pigments.
Table 2
Samples used to test the SCOAP plastics reference database and their correlations to reference plastics.
Sample ID
|
Known composition (pigment)
|
Correlation match quality (reference)
|
ChromaAC24030
|
Acrylonitrile-butadiene-styrene terpolymer (25.5% TiO2)
|
97.0% (acrylonitrile-butadiene-styrene)
|
ChromaHC2200B
|
Polystyrene (26.5% TiO2 + 13%CaCO3)
|
99.0% (polystyrene)
|
ChromaNC2667A
|
Nylon (32% TiO2)
|
93.3% (nylon-6)
|
ChromaZC2606B
|
Polycarbonate (23% TiO2)
|
97.0% (polycarbonate)
|
RecycleCode1 (bottle)
|
Polyethylene terephthalate (colorless)
|
96.6% (polyethylene terephthalate)
|
RecycleCode2 (tub)
|
High-density polyethylene (white pigment)
|
98.8% (high-density polyethylene)
|
RecycleCode5 (bottle)
|
Polypropylene (colorless)
|
98.9% (polypropylene)
|
When testing and using the SCOAP database, we expect some variations in the correlation between the references and test samples. The presence of additives or impurities, which may have been incorporated during manufacture or as a result of degradation may reduce correlation quality. Additionally, a lower correlation quality is expected for composite materials, chemically modified polymers (e.g., CA), and copolymers due to their innate variations in composition. Finally, lower correlation qualities are seen when the baseline of a spectrum is not flat, such as when fluorescence occurs.
Successful matching of the database references to artifact spectra demands a high correlation between the sample and reference spectra. Mathematically, obtaining a high correlation is in part reliant on having a spectrum with a high signal-to-noise ratio, which is generally maximized at higher laser powers. Yet when an artifact is darkly pigmented, aged, or degraded, it is prudent to use a reduced laser power to avoid surface damage. We tested the BWID software with spectra of CA, a plastic commonly found in older artifacts, taken with reduced laser powers. This analysis (Fig. 1, Table 3) shows that the match correlations remain above 91% until the laser power is reduced from 20–10% (approximately 40 mW to 20 mW) at which point the correlation to the reference drops to 27%. At the same time, the signal-to-noise ratio for the 1729 cm− 1 peak drops from 31 to 15. This result shows that we are able to perform reliable spectra matching when the signal-to-noise ratio exceeds approximately 30:1, and in the case of CA, we can use laser powers reduced to 20% and still achieve a high correlation match.
Table 3
Effects of laser power on the Raman spectral match quality and signal-to-noise ratio for CA.
Laser power (%)
|
Correlation
match quality (%)
|
Signal-to-noise ratioa (1729 cm− 1 peak)
|
100
|
98.1
|
121
|
80
|
98.0
|
96
|
60
|
97.9
|
74
|
40
|
97.1
|
53
|
20
|
91.0
|
31
|
10
|
27.0
|
15
|
a SNR = \(\frac{\stackrel{-}{E}}{s}\) where \(\stackrel{-}{E}\)is the mean value of the peak height and s is the standard deviation of the noise.
|
Work on the SCOAP plastics reference database is an ongoing project. We intend to increase the variety of both modern and historical references with the goal of enhancing our ability to identify the composition of artifacts in future studies of the PAC. As synthetic chemistry and plastics formulation is an evolving and creative science, we need to prepare for studies which uncover artifacts with unusual polymer compositions. A further goal of expanding the database is to incorporate degraded samples and better identify the chemical fingerprints of degradation products. Finally, the incorporation of known additives such as plasticizers and pigments would benefit our work.
Analysis of the artifacts
The clear, colorless handbag covers, Artifact A, are identified in the SCRC-SUL records as being composed of polystyrene (PS). This information is confirmed by Raman analysis with a 99+% correlation to the PS reference. In Fig. 2, the artifact spectra are dominated by the aromatic ring-breathing mode band at 1000 cm− 1. As expected for PS, the peaks associated with in-plane ring deformation (620 cm− 1) and in-plane ring stretching (1455 cm− 1, 1600 cm− 1) are present as well. Beyond the BWID search region, we can identify the characteristic aromatic CH stretching at 3060 cm− 1 and aliphatic CH stretching at 2854–2923 cm− 1.(19–21) The BWID analysis combined with our peak inspection confirms that Artifact A is PS, as was listed in the SCRC-SUL artifact description.
Artifact B is a purse with a top, bottom, and handle made of colorless, clear plastic and metal sides. Its plastic composition was previously unidentified in the SCRC-SUL records. The three plastic parts were analyzed individually by Raman spectroscopy and examined with BWID software. Spectra matching revealed that all three parts were manufactured using an acrylic polymer, which is chemically known as polymethyl methacrylate (PMMA), with a 94+% correlation match quality to the PMMA reference. The spectra for the three parts of Artifact B as well as that of the reference plastic from the database are shown in Fig. 3. An inspection of the traces shows that they all have similar features. Within the BWID analysis range we can see vibrational modes characteristic of the ester functional group including the carbonyl stretch at 1735 cm− 1. Also, there is the spectrally dense region from 1100–1525 cm− 1 containing combination modes of C-O stretching and CH bending and rocking. At approximately 800 cm− 1, a sharp peak is observed, which is a C-C stretch and C-O stretch combination band.(20, 22) Outside of the BWID analysis range, we observe a broad group of bands from 2850–3040 cm− 1 indicating the presence of both methyl and methylene groups. Thus, the previously unknown plastic used in manufacturing all three parts of the handbag is identified as PMMA.
Artifact C is a clear, colorless plastic purse with glitter embedded in the plastic, and it was constructed of three molded plastic pieces: the top, bottom, and handle. The SCRC-SUL artifact description did not contain information about the plastic composition used in the purse manufacture. The Raman spectra and BWID analyses of all three pieces identify the plastic as PS, all having a 99+% correlation to the reference. The artifact spectra are shown in Fig. 4 along with the PS reference spectrum. An inspection of the data in the BWID search range shows that the purse spectra contain the characteristic modes for aromatic groups as seen for Artifact A including the intense aromatic ring-breathing mode band (1001 cm− 1), the in-plane ring deformation (620 cm− 1), and in-plane ring stretching (1455 cm− 1, 1600 cm− 1). Likewise, beyond the BWID search region, we can identify the characteristic aromatic CH stretching at 3060 cm− 1 and aliphatic CH stretching at 2854–2923 cm− 1.(19–21) This combined evidence allows us to positively identify the plastic used to manufacture Artifact C as PS.
The colorless and orange plastic bamboo-like handles (Artifacts Da and Db) were previously identified as PS in the SCRC-SUL records. Spectra shown in Fig. 5 and BWID analysis show that both of the handles are actually composed of CA with a 90+% correlation to the reference. Clearly the artifact spectra do not match the PS reference spectra since they are devoid of the characteristic aromatic peaks for PS around 620 cm− 1, 1000 cm− 1, and 3055 cm− 1. Rather, the spectra for Artifacts Da and Db have spectral features consistent with the presence of the ester group of CA including the C = O stretching (1728 cm− 1) and C = O in-plane rocking (648 cm− 1). Also, we see a spectrally dense region of 1060–1200 cm− 1 consistent with peaks arising from C-O stretching due to the ester, the cellulose rings and (non-acylated) hydroxyl groups.(20, 23) The chemical composition identities of these handles have been corrected by this study, showing that Artifacts Da and Db are composed of CA rather than PS.
The polymer composition of Artifact E, the clear, colorless clamshell purse was identified in the SCRC-SUL records as PMMA. However, Raman analysis by the BWID software indicates that this information is incorrect and that Artifact E is composed of PS with a 99+% correlation to the reference spectrum. A comparison was made of the PMMA and PS reference spectra with the artifact spectrum, all shown in Fig. 6. The artifact spectrum is dominated by the strong aromatic peak centered at approximately 1000 cm− 1 that is characteristic of PS. The artifact spectrum also contains the other characteristic peaks for PS as described in the analysis of Artifact A.(19–21) In contrast, the PMMA reference contains the characteristic carbonyl stretch at approximately 1730 cm− 1, which is lacking in the artifact spectrum. This analysis shows that Artifact E had been previously misidentified as PMMA, and we are able to definitively show that this purse is manufactured from PS.
Artifact F, the flattened cylindrical purse, is constructed of four parts: the lid, the base, the handle, and the feet. No information about the materials used in manufacturing this artifact was provided in the SCRC-SUL artifact description. Each part of this artifact was analyzed separately, and unlike other purses we studied, two different polymers were used in its construction (Fig. 7). The lid, base, and handle were all determined to be PMMA by BWID analysis with correlations to the reference Raman spectrum exceeding 96%. The similarities between the spectra for the handle, top, base and PMMA reference are readily observed with the characteristic C = O stretch at approximately 1730 cm− 1. The spectra of the handle, base, and top contain the other peaks characteristic of PMMA described in the analysis of Artifact B.(20, 22) In contrast, the feet of the purse have a different spectral fingerprint, which matches the CA reference spectrum with 94+% correlation. Like the spectra for Artifacts Da and Db, this complicated spectrum shows features originating from the C = O and C-O stretching of the ester and cellulose rings.(20, 23) Thus, the feet are determined to be constructed from CA, unlike the other parts of the purse.
The results of this Raman spectroscopic study of plastic purses and purse parts are summarized in Table 4. These results provide definitive identifications of the polymer compositions of all seven artifacts, with correlations ranging from 90–99+%. Those with the lower correlations, Artifacts Da, Db, and F, are composed of CA. These lower correlations are to be expected since CA is a chemically modified polymer that is vulnerable to degradation. Chemical modifications can be accomplished to different degrees and degradation can add impurities, both causing anomalies in the sample spectrum. The artifact spectra with the highest correlations are Artifacts A, C, and E, all of which are composed of colorless PS. This is due in part to the sample spectra having flat baselines and lacking interference peaks resulting from additives or impurities. Yet in all cases, we carefully analyzed the Raman fingerprints of the artifacts to ensure they contained the important characteristic peaks resulting from the functional groups of the identified polymer.
Table 4
Summary comparing SCRC-SUL catalog descriptions of polymer composition with Raman spectroscopy results.
Artifact
|
SCRC-SUL accession number
|
SCRC-SUL catalog description of polymer composition
|
Polymer composition determined by Raman spectroscopy
|
Correlation to SCOAP database reference
|
A
|
2010_055.147
|
PS
|
PS
|
99+ %
|
B
|
2005.17
|
Unidentified
|
PMMA
|
94+%
|
C
|
2003.207
|
Unidentified
|
PS
|
99+%
|
Da (colorless)
Db (orange)
|
2010_055.016
|
PS
|
CA
|
90+%
|
E
|
2003.206
|
PMMA
|
PS
|
99+%
|
F
|
2003.208
|
Unidentified
|
PMMA (body, handle)
CA (feet)
|
96+%
94+%
|