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Extraction of plasticizers: An entire and reproducible quantification method for historical cellulose acetate material
Polymer Testing 80 (2019) 106096
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Test Method
Extraction of plasticizers: An entire and reproducible quantification method for historical cellulose acetate material
T
Benjamin Kempera,∗, Dirk Andreas Lichtblaub a b
University of Fine Arts Dresden, Güntzstraße 34, 01307, Dresden, Germany Lichtblau e.K., Loschwitzer Str.15a, 01309, Dresden, Germany
ARTICLE INFO
ABSTRACT
Keywords: Cellulose acetate Plasticizers Extraction Ultrasonic sound Quantification Degradation
Plasticizers from historical artefacts made of cellulose acetate were extracted and quantified. In compare to common procedures the presented method led to an entire and reproducible extraction which can even be performed in a much shorter time. The success of the improved procedure is based on the use of ultrasonic sound, methanol as solvent and multiple extraction steps for a complete removal of the plasticizers. The procedure was developed for small amounts of different types of products made from cellulose acetate.
1. Introduction Cellulose acetate (CA) is one of the most astonishing and versatile semi-synthetic polymers which embossed a bigger part of the 20th century. It was used for various applications and purposes. Best known is probably the replacement of the highly flammable cellulose nitrate (CN) for photographic films, the so-called safety film. Equally important in the film industry is its use as animation cels [1,2]. However, CA was also used as textile fibres [3] or for the production of daily use objects like combs, spectacles or toothbrushes. In addition, art objects or teaching material were made of CA, too. Even today it is still used in high quantities, mainly as the fleece in cigarettes filters. On the other hand, the material that has been used so widely in industry, art and culture has proved to be a great challenge in preserving these artefacts nowadays [4]. The main problems are shrinkage and increasing brittleness, the main reasons are hydrolysis, glycoside bond cleavage and loss of plasticizers [4–7]. Hydrolysis of the ester bonds as the main degradation process causes the deacetylation of the acetyl groups. This is affected by humidity and leads to an acetic acid release (“Vinegar Syndrome”). Acetic acid in turn catalyzes the glycoside bond cleavage, further deacetylations and can affect other materials in the vicinity of the polymer as well [4,5,8–10]. Another phenomenon in the degradation of CA is the loss of plasticizers. They are added to impart flexibility and softness and to facilitate processability by reducing viscosity. The plasticizers diffuse between the polymer chains and increase intermolecular space which is
∗
necessary for changes in shape, flexing and moulding of the material [7,11]. If the plasticizers migrate out of the polymer, it becomes brittle. The loss of plasticizers along with deacetylation results in loss of mass and shrinkage of the polymer [12]. This degradation process is affected by many factors as relative humidity or temperature [13,14]. To understand these complex processes, it is necessary to understand the influence of these factors on the loss of plasticizers. Furthermore, the content of plasticizers can be an indicator for the degradation status of the polymer as well. Whitnack and Gantz [15] published an extraction for plasticizers from CA by using a Soxhlet extractor. Best results they obtained after a 20 hours extraction by using a 1:1 mixture of hexane and ethanol. However, they tested several solvents as methylene chloride, benzene, ethanol and hexane before. Giachet et al. [1,2]. worked with micro sampling and modified the extraction procedure by Whitnack and Gantz by omitting the use of Soxhlet extractor. Since they extracted the sample at 38 °C under reflux only, they increased the extraction time to 48 hours. The use of these time-consuming procedures was found to be ineffective for the authors’ ambition to determine the plasticizers content of hundreds of samples in an adequate time. In addition, it was expected that a simple reflux would lead to an incomplete extraction which would not be acceptable to achieve the quantification of the plasticizers. Instead of a 20 hours extraction by Soxhlet or a 48 hours single extraction under reflux, multiple extraction steps with ultrasonic sound were applied due to its advantages of being an effective, time-saving
Corresponding author. E-mail addresses: [email protected] (B. Kemper), [email protected] (D.A. Lichtblau).
https://doi.org/10.1016/j.polymertesting.2019.106096 Received 2 July 2019; Received in revised form 22 August 2019; Accepted 3 September 2019 Available online 03 September 2019 0142-9418/ © 2019 Elsevier Ltd. All rights reserved.
Polymer Testing 80 (2019) 106096
B. Kemper and D.A. Lichtblau
and manageable method for mixing, solving and extraction [16]. The authors therefore present an entire and reproduceable method for plasticizers extraction in this paper. On that base, quantification of plasticizers from historical CA will be performed by gas chromatography and mass spectroscopy (GCMS).
Table 1 Plasticizers identified in the test sample, abbreviation, CAS-number and retention time (RT).
2. Materials As reference material for the development of the extraction method, a CA sheet from the German Hygiene-Museum in Dresden has been selected. It was produced around 1980 and was supposed to be used for thermoforming of components for the ‘Transparent Figures’. Such figures were manufactured in the in-house workshop of the German Hygiene-Museum in Dresden to impart the knowledge about human anatomy since the Second International Hygiene Exhibition in 1930 [17,18]. The production of such figures was stopped in 2000 but the existing cellulose acetate sheets were repealed in closed containers over almost four decades until now. During this time, the sheets have aged, and their plasticizers have already partially leaked and formed a puddle. This material was chosen as reference because it is very application oriented. For the extraction different solvents were used: methanol (VWR SupraSolv® for GC), ethanol (Roth, GC-grade) and hexane (Roth, GCgrade). For the calibration curves in chapter 5 dimethyl phthalate (DMP, Acros Organics, 99%) and triphenyl phosphate (TPP, Acros Organics, 99+ %) were chosen as standards.
Common name
Abbreviation
CAS number
RT (min)
Dimethyl phthalate Benzenesulfonamide Diethyl phthalate Tris(chloroethyl)phosphate Triphenyl phosphate
DMP
131-11-3 5183-78-8 84-66-2 115-96-8 115-86-6
9.295 10.710 11.144 13.347 20.817
DEP TCEP TPP
first extraction step only. After each extraction step, resulting extract was analysed by GCMS for remaining plasticizers. This was considered to be important for a reliable quantitative analysis. The analysis was performed by a Shimadzu GCMS-QP2020 with direct inlet and a DB5 column (30 m x 0.25 mm x 25 μm). GC oven program: 80 °C for 1 minute, 12 °C/minute to 140 °C, 8 °C/minute to 270 °C, 40 °C/minute to 320 °C, isothermal for 7 minutes. Helium flow rate at 1.2 mL/minute. Injection at 280 °C with a 1:10 split ratio or splitless; injection volume 1 μL. MS interface at 290 °C. The software Lab Solutions GCMS solution Version 4.45 by Shimadzu Corporation was used for evaluation. 4. Results The CA test sample contains five different plasticizers as mentioned in Table 1, whereas Fig. 1 shows the total ion chromatogram which sharp and well separated peaks for all five plasticizers. Since the peak area depends on the concentration in the extract, the large differences between the five plasticizers can be observed in the chromatograms already visually. After peak area integration it could be shown that TPP is the main plasticizer in the test sample. Whereas the above mentioned, not shredded sample could not be extracted finally within seven extraction steps, the smaller fragments could be extracted completely. The number of required steps depends on the used solvent as listed in Table 2. With a 1:1 (v:v) mixture of ethanol and hexane as described in the literature, six extractions were necessary. The same number of extraction steps was needed for pure ethanol as solvent. However, pure ethanol was at least more efficient for the phthalates than the 1:1 (v:v) mixture of ethanol and hexane. The most efficient extraction was achieved with methanol as solvent. After three extraction steps no plasticizers were detectable anymore in the less sensitive split mode. Due to the higher sensitivity, GCMS analysis in splitless mode is required to proof the totality of the extraction. However, only extraction steps were analysed in addition in splitless mode which have not shown any plasticizers content in split mode before. In Table 3 the results are listed. The limits of detection (LOD) for the plasticizers in this method are listed in Table 4. For calculation the signal to noise ratio (S/N ratio) was used [19]. In summary, even after seven extraction steps it was not possible to remove the plasticizers completely with ethanol or with a mixture of 1:1 (v:v) ethanol and hexane. Only with methanol it was possible to remove all plasticizers with three extractions. In the fourth extraction no plasticizers could be detected anymore. Methanol is therefore an efficient solvent for plasticizers extraction. The extraction with methanol was validated by repeating the procedure several times with different CA materials. Three extractions with methanol led in each case to a total extraction of plasticizers. To be always sure that the plasticizers extraction of various CA samples is total, four extractions with methanol were chosen for all further quantifications.
3. Experimental 3.1. Sample preparation Small samples were taken of a cellulose acetate sheet with a thickness of 1 mm. First results showed that extractions for sample pieces with a thickness of 1 mm were quite inefficient and time-consuming. In conclusion, small chips of a maximum size of 0.1 x 0.5 mm were scraped off by using a scalpel. The shredded sample was weight in a 5 mL welted glass and dried for 3 hours at 60 °C in a drying chamber. A 12 mL threaded vial was also dried this way. For cooling and potential storing over night the samples were transferred into an exsiccator with dry beads. This was necessary to get the oven-dry mass of the CA and to avoid weighting errors of the vials. The extraction method is applied for an oven-dry mass of 20–25 mg. This mass was chosen to get the weighing error below 0.5 % on scales with an accuracy of ± 0.1 mg. The sample was weighed into the dried 12 mL threaded vial with an accuracy of ± 0.1 mg. 3.2. Extraction A VWR USC300D ultrasonic cleaning device with a maximum peak power of 160 W was used. To avoid a strong increase of the bath temperature while extracting, only 80 % (128 W) of the available peak power were used. The bath temperature was warmed up to 30 °C but should not exceed 40 °C. The duration of ultrasonic sound treatment was set to 15 min. The dried sample was weighed in a 12 mL threaded vial and 5 mL of solvent were added. As solvents were tested: methanol, ethanol and a 1:1 (v:v) mixture of ethanol and hexane as well. For a more effective extraction the sample was swelled in the solvent for 30 min and then treated by ultrasonic sound under the above-mentioned conditions. The resulting extract was removed with a microliter pipette; the sample was washed with 1 mL fresh solvent and removed again. Both extracts were combined. To obtain the number of required extraction steps for removing all contained plasticizers completely, the extraction procedure was repeated several times. However, the swelling was performed prior the
5. Quantification Following the extraction method above, the four extracts of a 2
Polymer Testing 80 (2019) 106096
B. Kemper and D.A. Lichtblau
Fig. 1. Total Ion Chromatogram (TIC) of CA sample after extraction. Correlation to plasticizers via retention time in Table 1.
sample were collected in a 25 mL volumetric flask and filled up with methanol to the calibration mark. This is necessary to compensate any evaporation of the methanol during the extraction steps and to obtain a defined volume. Although 24 mL methanol was used for the extractions, about 1.5–2 mL was required to fill the volumetric flask to the calibration mark. As external calibration standards DMP and TPP were used. They were chosen because they are most abundant in the CA sample used for the extraction method. Furthermore, they are very common in different CA materials [7,20]. Certainly other standard plasticizers can be used for calibration and quantification as well. 25.6 mg TPP and 25.3 mg DMP were each weighed in a 50 mL volumetric flask and filled up to 50 mL with methanol. These stock solutions were diluted with methanol to five different concentrations between 50 and 250 mg/L. Quantification was performed by GCMS under the conditions mentioned before. The peak areas were integrated and calibration curves were generated and forced through zero. The calibration curves are shown in Fig. 2. For the evaluation of the quantification after extraction; three
samples from the same CA test sheet were taken (A, B and C), extracted and each extract was analysed two times (A1/A2, B1/B2, C1/C2). Table 5 summarizes the oven-dry masses, peak areas and calculated percentages of DMP and TPP. The limits of quantification (LOQ) for DMP and TPP are 77 tsd. [UA] and 67 tsd. [UA]. They were calculated by S/N ratio [19]. To calculate the percentage of TPP and DMP, the linear equations of the calibration curves were used (y = ax) and expanded by the volume of the extracts (0.025 L) and the oven-dry mass. For calculation equations (1) and (2) were used. x=y / a
(1)
Percentage plasticizer % = [(x ∙ L volume extracts) / (mg oven-dry mass) ∙ 100] (2) The low standard deviations ± 0.31% and ± 0.35% demonstrate the good reproducibility of the developed extraction and quantification method. This was also reached by avoiding potential errors. By drying the sample a deviation of about 2–3% was avoided. By defining the initial weight to 20–25 mg the weighing error is below 0.5%. The
Table 2 Integrated peak areas for DMP, Benzenesulfonamide, DEP, TCEP and TPP after each extraction in thousand unit areas [UA]. Split ratio 1:10. Samples were cut into small fragments. Solvent
Sample size
Extraction
Split mode, peak area in tsd. [UA] DMP
Benzene-sulfon-amide
DEP
TCEP
TPP
Ethanol:Hexane (1:1, v:v)
Small fragments
1 2 3 4 5 6 7
12685 2309 415 160 77 0 0
5683 802 68 0 0 0 0
11399 1986 327 168 84 0 0
779 80 0 0 0 0 0
15333 3343 674 341 323 0 0
Ethanol
Small fragments
1 2 3 4 5 6 7
12597 1390 99 0 0 0 0
5973 308 0 0 0 0 0
12028 1251 113 0 0 0 0
877 56 0 0 0 0 0
17446 2604 434 148 64 0 0
Methanol
Small fragments
1 2 3 4 5 6 7
13848 53 0 0 0 – –
9164 0 0 0 0 – –
11302 60 0 0 0 – –
1497 0 0 0 0 – –
26019 325 0 0 0 – –
3
Polymer Testing 80 (2019) 106096
B. Kemper and D.A. Lichtblau
Table 3 Integrated peak areas for DMP, Benzenesulfonamide, DEP, TCEP and TPP after each extraction in thousand unit areas [UA]. Splitless mode. Solvent
Sample Size
Extraction
Splitless mode, peak area in tsd. [UA] DMP
Benzene-sulfon-amide
DEP
TCEP
TPP
Ethanol:Hexane (1:1, v:v)
Small fragments
6 7
2359 2139
1233 1110
1485 1415
81 65
1369 1656
Ethanol
Small fragments
6 7
0 0
0 0
0 0
0 0
30 11
Methanol
Small fragments
3 4
0 0
0 0
0 0
0 0
206 0
Table 4 Limits of detection for plasticizers in this method in tsd. area units [UA]. Plasticizer
DMP
Benzenesulfonamide
DEP
TCEP
TPP
LOD in tsd [UA]
23
16
24
25
20
Table 6 Previously identified plasticizers in photo negatives and motion picture films. Common name
Abbreviation
CAS number
RT (min)
Content (%)
Dimethyl phthalate Diethyl phthalate Dibutyl phthalate Diisobutyl phthalate Triphenyl phosphate Cresyl diphenyl phosphate
DMP DEP DBP DIBP TPP CDP
131-11-3 84-66-2 84-74-2 84-69-5 115-86-6
Dicresyl phenyl phosphate
DCDP
4-Biphenylyl diphenyl phosphate
BDP
9.282 11.132 15.788 16.293 20.798 21.405 21.635 21.924 22.427 22.663 22.868 24.657
0–2.5 Traces 0–10.5 Traces 0.2–16.2 Traces Traces Traces Traces Traces Traces 0–3.5
17269-997
Using the above described extraction and quantification, a set of 15 film bases with different grade of degradation from 1948 to 2016 was analyzed. The set contains sheet films but also roll films and X-rays. The type of plasticizers as well as their content in Table 6 differs from the CA sheet from the German Hygiene-Museum in Dresden as shown in Table 1. TPP was found in all film bases from this sample set and was the plasticizer with the largest proportion in most of the samples. In some films TPP was associated with traces of CDP and DCDP. The most recent film base was taken from a disposable camera in 2016 and delivered with BDP an alternative plasticizer from the phosphate type. It should be mentioned that phenol was found in several samples which is most probably a degradation product of the phenyl phosphates. Phthalates were found so far in film bases from the 1940ties till the 1960ties. However, the phthalates DMP, DEP, DBP and DIBP were always combined with the phosphate TPP. In addition to the CA sheets and film bases, the applicability of the procedure to animations cells was also proofed successfully. However, TPP was found to be the only plasticizers in the available animation cell which was donated by the Deutsches Institut für Animationsfilm (DIAF). Its content was 8.6 %.
Fig. 2. Calibration curve of TPP and DMP solved in methanol and analysed by GCMS.
temperature of the ultrasonic bath was kept under 40 °C to reduce evaporation of the methanol. While collecting the extracts the same pipette tip was used for the same sample to prevent loss of extract. The total content of all five plasticizers was calculated to be about 25–30%. 6. Applicability Above, it could be shown that the extraction method is applicable to CA from 3D-Objects. It should be useable for CA from other 3D-Objects like spectacles or combs as well. Another highly important application belongs to photo negatives and motion picture films. Loss of plasticizers causes shrinkage, distorts the images or reduces the distance of the perforation and leads to film breaks when playing the film. Table 5 Percentage of DMP and TPP in the CA sample. Calculation with calibration curves. Sample
Oven-dry mass CA (mg)
Peak area DMP in tsd [UA]
Percentage DMP (%)
Peak area TPP in tsd. [UA]
Percentage TPP (%)
A1 A2 B1 B2 C1 C2 Mean value Standard deviation
26.4 26.4 25.1 25.1 25.2 25.2
3178 3242 3230 3098 3169 3001
10.79 11.01 11.54 11.06 11.27 10.67 11.06 ± 0.31
3040 2850 2932 3010 2815 3027
8.43 7.90 8.55 8.78 8.18 8.79 8.44 ± 0.35
4
Polymer Testing 80 (2019) 106096
B. Kemper and D.A. Lichtblau
7. Conclusion
10.1016/j.polymdegradstab.2014.03.009. [3] M. Lewin, Handbook of Fiber Chemistry, third ed., CRC/Taylor & Francis, Boca Raton, 2007. [4] D. Littlejohn, R.A. Pethrick, A. Quye, J.M. Ballany, Investigation of the degradation of cellulose acetate museum artefacts, Polym. Degrad. Stab. 98 (1) (2013) 416–424 https://doi.org/10.1016/j.polymdegradstab.2012.08.023. [5] B. Lavédrine, A. Fournier, G. Martin, Preservation of Plastic Artefacts in Museum Collections, (2012). [6] J. Puls, S.A. Wilson, D. Hölter, Degradation of cellulose acetate-based materials: a review, J. Polym. Environ. 19 (1) (2011) 152–165 https://doi.org/10.1007/ s10924-010-0258-0. [7] Y. Shashoua, Conservation of Plastics: Materials Science, Degradation and Preservation, Routledge, London, New York, 2016. [8] G. Mitchell, C. Higgitt, L.T. Gibson, Emissions from polymeric materials: characterised by thermal desorption-gas chromatography, Polym. Degrad. Stab. 107 (2014) 328–340 https://doi.org/10.1016/j.polymdegradstab.2013.12.003. [9] J.-L. Bigourdan, Stability of acetate film base: accelerated-aging data revisited, J. Imaging Sci. Technol. 50 (5) (2006) 494 https://doi.org/10.2352/J.ImagingSci. Technol.(2006)50:5(494). [10] K. Curran, A. Možir, M. Underhill, L.T. Gibson, T. Fearn, M. Strlič, Cross-infection effect of polymers of historic and heritage significance on the degradation of a cellulose reference test material, Polym. Degrad. Stab. 107 (2014) 294–306 https:// doi.org/10.1016/j.polymdegradstab.2013.12.019. [11] F. Toja, D. Saviello, A. Nevin, D. Comelli, M. Lazzari, M. Levi, L. Toniolo, The degradation of poly(vinyl acetate) as a material for design objects: a multi-analytical study of the effect of dibutyl phthalate plasticizer. Part 1, Polym. Degrad. Stab. 97 (11) (2012) 2441–2448 https://doi.org/10.1016/j.polymdegradstab.2012.07.018. [12] A. Benazzouz, E. Dudognon, N.T. Correia, V. Molinier, J.-M. Aubry, M. Descamps, Interactions underpinning the plasticization of a polymer matrix: a dynamic and structural analysis of DMP-plasticized cellulose acetate, Cellulose 24 (2) (2017) 487–503 https://doi.org/10.1007/s10570-016-1148-y. [13] E. Richardson, M. Truffa Giachet, M. Schilling, T. Learner, Assessing the physical stability of archival cellulose acetate films by monitoring plasticizer loss, Polym. Degrad. Stab. 107 (2014) 231–236 https://doi.org/10.1016/j.polymdegradstab. 2013.12.001. [14] J.-L. Bigourdan, P. Adelstein, J. Reilly, Use of microenvironments for the preservation of cellulose triacetate photographic film, J. Imaging Sci. Technol. 42 (2) (1998) 155–162. [15] G.C. Whitnack, E. St.C. Gantz, Extraction and determination of plasticizers from cellulose acetate plastics, Anal. Chem. 24 (6) (1952) 1060–1061 https://doi.org/10. 1021/ac60066a054. [16] C. Bendicho, I. Lavilla, Ultrasound-assisted metal extractions, in: I.D. Wilson (Ed.), Encyclopedia of Separation Science, Academic Press, San Diego, 2000, pp. 4421–4426. [17] J. Radtke, Transparent Figures: exhibition icons of the 20th century. An interdisciplinary research platform on the long-term preservation of objects made of plastic, Plastiquarian 57 (2017). [18] K. Vogel, The Transparent Man - Some Comments on the History of a Symbol, (1999), pp. 31–61. [19] A. Shrivastava, V. Gupta, Methods for the determination of limit of detection and limit of quantitation of the analytical methods, Chronicles Young Sci. 2 (1) (2011) 21 https://doi.org/10.4103/2229-5186.79345. [20] M. Schilling, M. Bouchard, H. Khanjian, T. Learner, A. Phenix, R. Rivenc, Application of chemical and thermal analysis methods for studying cellulose ester plastics, Acc. Chem. Res. 43 (6) (2010) 888–896 https://doi.org/10.1021/ ar1000132. [21] W. Diemair, Analytik der Lebensmittel Nachweis und Bestimmung von Lebensmittel-Inhaltsstoffen, Springer Berlin Heidelberg, Berlin, Heidelberg, 1967.
A new method to extract plasticizers from cellulose acetate could be developed. Using ultrasonic sound leads to significant time savings compared to previous extraction methods. Cutting the sample into small chips leads to an increase of extraction efficiency. The sample mass is specified up to 20–25 mg which reduces weighing errors and increases the accuracy. It could be found out that methanol is in combination with ultrasonic sound the most efficient solvent. Four extraction steps are sufficient to achieve a total extraction of the plasticizers. The ultrasonic sound extraction is a simple method which can be used in laboratories and workshops without larger investments. The results of this method are reproducible and can be used for quantification of the plasticizers, e.g. with GCMS which is also a standard method in many laboratories. The extract can also be used to analyse the presence of plasticizers with Thin Layer Chromatography (TLC) [21]. It could also be shown that the extraction method is not only applicable to CA sheets but also to photographic films and animation cells. It will be shown if the extraction method is also applicable to other polymers. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Acknowledgments Funding: This work was supported by the Volkswagen Foundation, the Swiss National Library, the Swedish National Archives and the Atelier Michael Rothe GmbH Bern. The applicability was tested with reference materials provided by the German Hygiene-Museum Dresden (DHMD), Lichtblau e.K. and the Deutsches Institut für Animationsfilm (DIAF). References [1] M.T. Giachet, M. Schilling, J. Mazurek, E. Richardson, C. Pesme, H. Khanjian, T. Learner, K. McCormick, Characterization of chemical and physical properties of animation cels from the walt disney animation research library, ICOM-CC 17th Triennial Conference Preprints, Melbourne, 15-19 September, 2014. [2] M.T. Giachet, M. Schilling, K. McCormick, J. Mazurek, E. Richardson, H. Khanjian, T. Learner, Assessment of the composition and condition of animation cels made from cellulose acetate, Polym. Degrad. Stab. 107 (2014) 223–230 https://doi.org/
5
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Test Method
Extraction of plasticizers: An entire and reproducible quantification method for historical cellulose acetate material
T
Benjamin Kempera,∗, Dirk Andreas Lichtblaub a b
University of Fine Arts Dresden, Güntzstraße 34, 01307, Dresden, Germany Lichtblau e.K., Loschwitzer Str.15a, 01309, Dresden, Germany
ARTICLE INFO
ABSTRACT
Keywords: Cellulose acetate Plasticizers Extraction Ultrasonic sound Quantification Degradation
Plasticizers from historical artefacts made of cellulose acetate were extracted and quantified. In compare to common procedures the presented method led to an entire and reproducible extraction which can even be performed in a much shorter time. The success of the improved procedure is based on the use of ultrasonic sound, methanol as solvent and multiple extraction steps for a complete removal of the plasticizers. The procedure was developed for small amounts of different types of products made from cellulose acetate.
1. Introduction Cellulose acetate (CA) is one of the most astonishing and versatile semi-synthetic polymers which embossed a bigger part of the 20th century. It was used for various applications and purposes. Best known is probably the replacement of the highly flammable cellulose nitrate (CN) for photographic films, the so-called safety film. Equally important in the film industry is its use as animation cels [1,2]. However, CA was also used as textile fibres [3] or for the production of daily use objects like combs, spectacles or toothbrushes. In addition, art objects or teaching material were made of CA, too. Even today it is still used in high quantities, mainly as the fleece in cigarettes filters. On the other hand, the material that has been used so widely in industry, art and culture has proved to be a great challenge in preserving these artefacts nowadays [4]. The main problems are shrinkage and increasing brittleness, the main reasons are hydrolysis, glycoside bond cleavage and loss of plasticizers [4–7]. Hydrolysis of the ester bonds as the main degradation process causes the deacetylation of the acetyl groups. This is affected by humidity and leads to an acetic acid release (“Vinegar Syndrome”). Acetic acid in turn catalyzes the glycoside bond cleavage, further deacetylations and can affect other materials in the vicinity of the polymer as well [4,5,8–10]. Another phenomenon in the degradation of CA is the loss of plasticizers. They are added to impart flexibility and softness and to facilitate processability by reducing viscosity. The plasticizers diffuse between the polymer chains and increase intermolecular space which is
∗
necessary for changes in shape, flexing and moulding of the material [7,11]. If the plasticizers migrate out of the polymer, it becomes brittle. The loss of plasticizers along with deacetylation results in loss of mass and shrinkage of the polymer [12]. This degradation process is affected by many factors as relative humidity or temperature [13,14]. To understand these complex processes, it is necessary to understand the influence of these factors on the loss of plasticizers. Furthermore, the content of plasticizers can be an indicator for the degradation status of the polymer as well. Whitnack and Gantz [15] published an extraction for plasticizers from CA by using a Soxhlet extractor. Best results they obtained after a 20 hours extraction by using a 1:1 mixture of hexane and ethanol. However, they tested several solvents as methylene chloride, benzene, ethanol and hexane before. Giachet et al. [1,2]. worked with micro sampling and modified the extraction procedure by Whitnack and Gantz by omitting the use of Soxhlet extractor. Since they extracted the sample at 38 °C under reflux only, they increased the extraction time to 48 hours. The use of these time-consuming procedures was found to be ineffective for the authors’ ambition to determine the plasticizers content of hundreds of samples in an adequate time. In addition, it was expected that a simple reflux would lead to an incomplete extraction which would not be acceptable to achieve the quantification of the plasticizers. Instead of a 20 hours extraction by Soxhlet or a 48 hours single extraction under reflux, multiple extraction steps with ultrasonic sound were applied due to its advantages of being an effective, time-saving
Corresponding author. E-mail addresses: [email protected] (B. Kemper), [email protected] (D.A. Lichtblau).
https://doi.org/10.1016/j.polymertesting.2019.106096 Received 2 July 2019; Received in revised form 22 August 2019; Accepted 3 September 2019 Available online 03 September 2019 0142-9418/ © 2019 Elsevier Ltd. All rights reserved.
Polymer Testing 80 (2019) 106096
B. Kemper and D.A. Lichtblau
and manageable method for mixing, solving and extraction [16]. The authors therefore present an entire and reproduceable method for plasticizers extraction in this paper. On that base, quantification of plasticizers from historical CA will be performed by gas chromatography and mass spectroscopy (GCMS).
Table 1 Plasticizers identified in the test sample, abbreviation, CAS-number and retention time (RT).
2. Materials As reference material for the development of the extraction method, a CA sheet from the German Hygiene-Museum in Dresden has been selected. It was produced around 1980 and was supposed to be used for thermoforming of components for the ‘Transparent Figures’. Such figures were manufactured in the in-house workshop of the German Hygiene-Museum in Dresden to impart the knowledge about human anatomy since the Second International Hygiene Exhibition in 1930 [17,18]. The production of such figures was stopped in 2000 but the existing cellulose acetate sheets were repealed in closed containers over almost four decades until now. During this time, the sheets have aged, and their plasticizers have already partially leaked and formed a puddle. This material was chosen as reference because it is very application oriented. For the extraction different solvents were used: methanol (VWR SupraSolv® for GC), ethanol (Roth, GC-grade) and hexane (Roth, GCgrade). For the calibration curves in chapter 5 dimethyl phthalate (DMP, Acros Organics, 99%) and triphenyl phosphate (TPP, Acros Organics, 99+ %) were chosen as standards.
Common name
Abbreviation
CAS number
RT (min)
Dimethyl phthalate Benzenesulfonamide Diethyl phthalate Tris(chloroethyl)phosphate Triphenyl phosphate
DMP
131-11-3 5183-78-8 84-66-2 115-96-8 115-86-6
9.295 10.710 11.144 13.347 20.817
DEP TCEP TPP
first extraction step only. After each extraction step, resulting extract was analysed by GCMS for remaining plasticizers. This was considered to be important for a reliable quantitative analysis. The analysis was performed by a Shimadzu GCMS-QP2020 with direct inlet and a DB5 column (30 m x 0.25 mm x 25 μm). GC oven program: 80 °C for 1 minute, 12 °C/minute to 140 °C, 8 °C/minute to 270 °C, 40 °C/minute to 320 °C, isothermal for 7 minutes. Helium flow rate at 1.2 mL/minute. Injection at 280 °C with a 1:10 split ratio or splitless; injection volume 1 μL. MS interface at 290 °C. The software Lab Solutions GCMS solution Version 4.45 by Shimadzu Corporation was used for evaluation. 4. Results The CA test sample contains five different plasticizers as mentioned in Table 1, whereas Fig. 1 shows the total ion chromatogram which sharp and well separated peaks for all five plasticizers. Since the peak area depends on the concentration in the extract, the large differences between the five plasticizers can be observed in the chromatograms already visually. After peak area integration it could be shown that TPP is the main plasticizer in the test sample. Whereas the above mentioned, not shredded sample could not be extracted finally within seven extraction steps, the smaller fragments could be extracted completely. The number of required steps depends on the used solvent as listed in Table 2. With a 1:1 (v:v) mixture of ethanol and hexane as described in the literature, six extractions were necessary. The same number of extraction steps was needed for pure ethanol as solvent. However, pure ethanol was at least more efficient for the phthalates than the 1:1 (v:v) mixture of ethanol and hexane. The most efficient extraction was achieved with methanol as solvent. After three extraction steps no plasticizers were detectable anymore in the less sensitive split mode. Due to the higher sensitivity, GCMS analysis in splitless mode is required to proof the totality of the extraction. However, only extraction steps were analysed in addition in splitless mode which have not shown any plasticizers content in split mode before. In Table 3 the results are listed. The limits of detection (LOD) for the plasticizers in this method are listed in Table 4. For calculation the signal to noise ratio (S/N ratio) was used [19]. In summary, even after seven extraction steps it was not possible to remove the plasticizers completely with ethanol or with a mixture of 1:1 (v:v) ethanol and hexane. Only with methanol it was possible to remove all plasticizers with three extractions. In the fourth extraction no plasticizers could be detected anymore. Methanol is therefore an efficient solvent for plasticizers extraction. The extraction with methanol was validated by repeating the procedure several times with different CA materials. Three extractions with methanol led in each case to a total extraction of plasticizers. To be always sure that the plasticizers extraction of various CA samples is total, four extractions with methanol were chosen for all further quantifications.
3. Experimental 3.1. Sample preparation Small samples were taken of a cellulose acetate sheet with a thickness of 1 mm. First results showed that extractions for sample pieces with a thickness of 1 mm were quite inefficient and time-consuming. In conclusion, small chips of a maximum size of 0.1 x 0.5 mm were scraped off by using a scalpel. The shredded sample was weight in a 5 mL welted glass and dried for 3 hours at 60 °C in a drying chamber. A 12 mL threaded vial was also dried this way. For cooling and potential storing over night the samples were transferred into an exsiccator with dry beads. This was necessary to get the oven-dry mass of the CA and to avoid weighting errors of the vials. The extraction method is applied for an oven-dry mass of 20–25 mg. This mass was chosen to get the weighing error below 0.5 % on scales with an accuracy of ± 0.1 mg. The sample was weighed into the dried 12 mL threaded vial with an accuracy of ± 0.1 mg. 3.2. Extraction A VWR USC300D ultrasonic cleaning device with a maximum peak power of 160 W was used. To avoid a strong increase of the bath temperature while extracting, only 80 % (128 W) of the available peak power were used. The bath temperature was warmed up to 30 °C but should not exceed 40 °C. The duration of ultrasonic sound treatment was set to 15 min. The dried sample was weighed in a 12 mL threaded vial and 5 mL of solvent were added. As solvents were tested: methanol, ethanol and a 1:1 (v:v) mixture of ethanol and hexane as well. For a more effective extraction the sample was swelled in the solvent for 30 min and then treated by ultrasonic sound under the above-mentioned conditions. The resulting extract was removed with a microliter pipette; the sample was washed with 1 mL fresh solvent and removed again. Both extracts were combined. To obtain the number of required extraction steps for removing all contained plasticizers completely, the extraction procedure was repeated several times. However, the swelling was performed prior the
5. Quantification Following the extraction method above, the four extracts of a 2
Polymer Testing 80 (2019) 106096
B. Kemper and D.A. Lichtblau
Fig. 1. Total Ion Chromatogram (TIC) of CA sample after extraction. Correlation to plasticizers via retention time in Table 1.
sample were collected in a 25 mL volumetric flask and filled up with methanol to the calibration mark. This is necessary to compensate any evaporation of the methanol during the extraction steps and to obtain a defined volume. Although 24 mL methanol was used for the extractions, about 1.5–2 mL was required to fill the volumetric flask to the calibration mark. As external calibration standards DMP and TPP were used. They were chosen because they are most abundant in the CA sample used for the extraction method. Furthermore, they are very common in different CA materials [7,20]. Certainly other standard plasticizers can be used for calibration and quantification as well. 25.6 mg TPP and 25.3 mg DMP were each weighed in a 50 mL volumetric flask and filled up to 50 mL with methanol. These stock solutions were diluted with methanol to five different concentrations between 50 and 250 mg/L. Quantification was performed by GCMS under the conditions mentioned before. The peak areas were integrated and calibration curves were generated and forced through zero. The calibration curves are shown in Fig. 2. For the evaluation of the quantification after extraction; three
samples from the same CA test sheet were taken (A, B and C), extracted and each extract was analysed two times (A1/A2, B1/B2, C1/C2). Table 5 summarizes the oven-dry masses, peak areas and calculated percentages of DMP and TPP. The limits of quantification (LOQ) for DMP and TPP are 77 tsd. [UA] and 67 tsd. [UA]. They were calculated by S/N ratio [19]. To calculate the percentage of TPP and DMP, the linear equations of the calibration curves were used (y = ax) and expanded by the volume of the extracts (0.025 L) and the oven-dry mass. For calculation equations (1) and (2) were used. x=y / a
(1)
Percentage plasticizer % = [(x ∙ L volume extracts) / (mg oven-dry mass) ∙ 100] (2) The low standard deviations ± 0.31% and ± 0.35% demonstrate the good reproducibility of the developed extraction and quantification method. This was also reached by avoiding potential errors. By drying the sample a deviation of about 2–3% was avoided. By defining the initial weight to 20–25 mg the weighing error is below 0.5%. The
Table 2 Integrated peak areas for DMP, Benzenesulfonamide, DEP, TCEP and TPP after each extraction in thousand unit areas [UA]. Split ratio 1:10. Samples were cut into small fragments. Solvent
Sample size
Extraction
Split mode, peak area in tsd. [UA] DMP
Benzene-sulfon-amide
DEP
TCEP
TPP
Ethanol:Hexane (1:1, v:v)
Small fragments
1 2 3 4 5 6 7
12685 2309 415 160 77 0 0
5683 802 68 0 0 0 0
11399 1986 327 168 84 0 0
779 80 0 0 0 0 0
15333 3343 674 341 323 0 0
Ethanol
Small fragments
1 2 3 4 5 6 7
12597 1390 99 0 0 0 0
5973 308 0 0 0 0 0
12028 1251 113 0 0 0 0
877 56 0 0 0 0 0
17446 2604 434 148 64 0 0
Methanol
Small fragments
1 2 3 4 5 6 7
13848 53 0 0 0 – –
9164 0 0 0 0 – –
11302 60 0 0 0 – –
1497 0 0 0 0 – –
26019 325 0 0 0 – –
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B. Kemper and D.A. Lichtblau
Table 3 Integrated peak areas for DMP, Benzenesulfonamide, DEP, TCEP and TPP after each extraction in thousand unit areas [UA]. Splitless mode. Solvent
Sample Size
Extraction
Splitless mode, peak area in tsd. [UA] DMP
Benzene-sulfon-amide
DEP
TCEP
TPP
Ethanol:Hexane (1:1, v:v)
Small fragments
6 7
2359 2139
1233 1110
1485 1415
81 65
1369 1656
Ethanol
Small fragments
6 7
0 0
0 0
0 0
0 0
30 11
Methanol
Small fragments
3 4
0 0
0 0
0 0
0 0
206 0
Table 4 Limits of detection for plasticizers in this method in tsd. area units [UA]. Plasticizer
DMP
Benzenesulfonamide
DEP
TCEP
TPP
LOD in tsd [UA]
23
16
24
25
20
Table 6 Previously identified plasticizers in photo negatives and motion picture films. Common name
Abbreviation
CAS number
RT (min)
Content (%)
Dimethyl phthalate Diethyl phthalate Dibutyl phthalate Diisobutyl phthalate Triphenyl phosphate Cresyl diphenyl phosphate
DMP DEP DBP DIBP TPP CDP
131-11-3 84-66-2 84-74-2 84-69-5 115-86-6
Dicresyl phenyl phosphate
DCDP
4-Biphenylyl diphenyl phosphate
BDP
9.282 11.132 15.788 16.293 20.798 21.405 21.635 21.924 22.427 22.663 22.868 24.657
0–2.5 Traces 0–10.5 Traces 0.2–16.2 Traces Traces Traces Traces Traces Traces 0–3.5
17269-997
Using the above described extraction and quantification, a set of 15 film bases with different grade of degradation from 1948 to 2016 was analyzed. The set contains sheet films but also roll films and X-rays. The type of plasticizers as well as their content in Table 6 differs from the CA sheet from the German Hygiene-Museum in Dresden as shown in Table 1. TPP was found in all film bases from this sample set and was the plasticizer with the largest proportion in most of the samples. In some films TPP was associated with traces of CDP and DCDP. The most recent film base was taken from a disposable camera in 2016 and delivered with BDP an alternative plasticizer from the phosphate type. It should be mentioned that phenol was found in several samples which is most probably a degradation product of the phenyl phosphates. Phthalates were found so far in film bases from the 1940ties till the 1960ties. However, the phthalates DMP, DEP, DBP and DIBP were always combined with the phosphate TPP. In addition to the CA sheets and film bases, the applicability of the procedure to animations cells was also proofed successfully. However, TPP was found to be the only plasticizers in the available animation cell which was donated by the Deutsches Institut für Animationsfilm (DIAF). Its content was 8.6 %.
Fig. 2. Calibration curve of TPP and DMP solved in methanol and analysed by GCMS.
temperature of the ultrasonic bath was kept under 40 °C to reduce evaporation of the methanol. While collecting the extracts the same pipette tip was used for the same sample to prevent loss of extract. The total content of all five plasticizers was calculated to be about 25–30%. 6. Applicability Above, it could be shown that the extraction method is applicable to CA from 3D-Objects. It should be useable for CA from other 3D-Objects like spectacles or combs as well. Another highly important application belongs to photo negatives and motion picture films. Loss of plasticizers causes shrinkage, distorts the images or reduces the distance of the perforation and leads to film breaks when playing the film. Table 5 Percentage of DMP and TPP in the CA sample. Calculation with calibration curves. Sample
Oven-dry mass CA (mg)
Peak area DMP in tsd [UA]
Percentage DMP (%)
Peak area TPP in tsd. [UA]
Percentage TPP (%)
A1 A2 B1 B2 C1 C2 Mean value Standard deviation
26.4 26.4 25.1 25.1 25.2 25.2
3178 3242 3230 3098 3169 3001
10.79 11.01 11.54 11.06 11.27 10.67 11.06 ± 0.31
3040 2850 2932 3010 2815 3027
8.43 7.90 8.55 8.78 8.18 8.79 8.44 ± 0.35
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7. Conclusion
10.1016/j.polymdegradstab.2014.03.009. [3] M. Lewin, Handbook of Fiber Chemistry, third ed., CRC/Taylor & Francis, Boca Raton, 2007. [4] D. Littlejohn, R.A. Pethrick, A. Quye, J.M. Ballany, Investigation of the degradation of cellulose acetate museum artefacts, Polym. Degrad. Stab. 98 (1) (2013) 416–424 https://doi.org/10.1016/j.polymdegradstab.2012.08.023. [5] B. Lavédrine, A. Fournier, G. Martin, Preservation of Plastic Artefacts in Museum Collections, (2012). [6] J. Puls, S.A. Wilson, D. Hölter, Degradation of cellulose acetate-based materials: a review, J. Polym. Environ. 19 (1) (2011) 152–165 https://doi.org/10.1007/ s10924-010-0258-0. [7] Y. Shashoua, Conservation of Plastics: Materials Science, Degradation and Preservation, Routledge, London, New York, 2016. [8] G. Mitchell, C. Higgitt, L.T. Gibson, Emissions from polymeric materials: characterised by thermal desorption-gas chromatography, Polym. Degrad. Stab. 107 (2014) 328–340 https://doi.org/10.1016/j.polymdegradstab.2013.12.003. [9] J.-L. Bigourdan, Stability of acetate film base: accelerated-aging data revisited, J. Imaging Sci. Technol. 50 (5) (2006) 494 https://doi.org/10.2352/J.ImagingSci. Technol.(2006)50:5(494). [10] K. Curran, A. Možir, M. Underhill, L.T. Gibson, T. Fearn, M. Strlič, Cross-infection effect of polymers of historic and heritage significance on the degradation of a cellulose reference test material, Polym. Degrad. Stab. 107 (2014) 294–306 https:// doi.org/10.1016/j.polymdegradstab.2013.12.019. [11] F. Toja, D. Saviello, A. Nevin, D. Comelli, M. Lazzari, M. Levi, L. Toniolo, The degradation of poly(vinyl acetate) as a material for design objects: a multi-analytical study of the effect of dibutyl phthalate plasticizer. Part 1, Polym. Degrad. Stab. 97 (11) (2012) 2441–2448 https://doi.org/10.1016/j.polymdegradstab.2012.07.018. [12] A. Benazzouz, E. Dudognon, N.T. Correia, V. Molinier, J.-M. Aubry, M. Descamps, Interactions underpinning the plasticization of a polymer matrix: a dynamic and structural analysis of DMP-plasticized cellulose acetate, Cellulose 24 (2) (2017) 487–503 https://doi.org/10.1007/s10570-016-1148-y. [13] E. Richardson, M. Truffa Giachet, M. Schilling, T. Learner, Assessing the physical stability of archival cellulose acetate films by monitoring plasticizer loss, Polym. Degrad. Stab. 107 (2014) 231–236 https://doi.org/10.1016/j.polymdegradstab. 2013.12.001. [14] J.-L. Bigourdan, P. Adelstein, J. Reilly, Use of microenvironments for the preservation of cellulose triacetate photographic film, J. Imaging Sci. Technol. 42 (2) (1998) 155–162. [15] G.C. Whitnack, E. St.C. Gantz, Extraction and determination of plasticizers from cellulose acetate plastics, Anal. Chem. 24 (6) (1952) 1060–1061 https://doi.org/10. 1021/ac60066a054. [16] C. Bendicho, I. Lavilla, Ultrasound-assisted metal extractions, in: I.D. Wilson (Ed.), Encyclopedia of Separation Science, Academic Press, San Diego, 2000, pp. 4421–4426. [17] J. Radtke, Transparent Figures: exhibition icons of the 20th century. An interdisciplinary research platform on the long-term preservation of objects made of plastic, Plastiquarian 57 (2017). [18] K. Vogel, The Transparent Man - Some Comments on the History of a Symbol, (1999), pp. 31–61. [19] A. Shrivastava, V. Gupta, Methods for the determination of limit of detection and limit of quantitation of the analytical methods, Chronicles Young Sci. 2 (1) (2011) 21 https://doi.org/10.4103/2229-5186.79345. [20] M. Schilling, M. Bouchard, H. Khanjian, T. Learner, A. Phenix, R. Rivenc, Application of chemical and thermal analysis methods for studying cellulose ester plastics, Acc. Chem. Res. 43 (6) (2010) 888–896 https://doi.org/10.1021/ ar1000132. [21] W. Diemair, Analytik der Lebensmittel Nachweis und Bestimmung von Lebensmittel-Inhaltsstoffen, Springer Berlin Heidelberg, Berlin, Heidelberg, 1967.
A new method to extract plasticizers from cellulose acetate could be developed. Using ultrasonic sound leads to significant time savings compared to previous extraction methods. Cutting the sample into small chips leads to an increase of extraction efficiency. The sample mass is specified up to 20–25 mg which reduces weighing errors and increases the accuracy. It could be found out that methanol is in combination with ultrasonic sound the most efficient solvent. Four extraction steps are sufficient to achieve a total extraction of the plasticizers. The ultrasonic sound extraction is a simple method which can be used in laboratories and workshops without larger investments. The results of this method are reproducible and can be used for quantification of the plasticizers, e.g. with GCMS which is also a standard method in many laboratories. The extract can also be used to analyse the presence of plasticizers with Thin Layer Chromatography (TLC) [21]. It could also be shown that the extraction method is not only applicable to CA sheets but also to photographic films and animation cells. It will be shown if the extraction method is also applicable to other polymers. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Acknowledgments Funding: This work was supported by the Volkswagen Foundation, the Swiss National Library, the Swedish National Archives and the Atelier Michael Rothe GmbH Bern. The applicability was tested with reference materials provided by the German Hygiene-Museum Dresden (DHMD), Lichtblau e.K. and the Deutsches Institut für Animationsfilm (DIAF). References [1] M.T. Giachet, M. Schilling, J. Mazurek, E. Richardson, C. Pesme, H. Khanjian, T. Learner, K. McCormick, Characterization of chemical and physical properties of animation cels from the walt disney animation research library, ICOM-CC 17th Triennial Conference Preprints, Melbourne, 15-19 September, 2014. [2] M.T. Giachet, M. Schilling, K. McCormick, J. Mazurek, E. Richardson, H. Khanjian, T. Learner, Assessment of the composition and condition of animation cels made from cellulose acetate, Polym. Degrad. Stab. 107 (2014) 223–230 https://doi.org/
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