Quantification of VOCs in PLA

To quantify the VOCs present into PLA 2003D by MHS-SPME, we have firstly demonstrated that an exponential decrease of GC peak areas is achieved with a good correlation coefficients (R² ≥ 0.99) for some volatile organic compounds and secondly it needs to obtain calibration curves of the VOCs with the same procedure. After optimization phase, four compounds were

154 selected as markers because they presented good correlation coefficients of the exponential decay during optimization phase: acetaldehyde; 2-methyl-2-Propanol; 2,3-Pentanedione and 2,4-dimethyl-2-Pentanol; however, the standard of the last one is not commercialized and needed to be dropped. In the case of the three other compounds, calibrations curves of VOC standards in hexadecane were carried out according to the testing conditions selected previously.

Table 5.8 shows the characteristics of method using standard solutions. We originally assumed that all compounds selected would present larger linear ranges in the calibration curves, however only acetaldehyde and 2-methyl2-Propanol, presented a linear response to ratio between total peak area and compound mass whereas 2,3-Pentanedione was co-eluted with a minor compound present in hexadecane. For that, new solutions of 2,3-Pentanedione in propyleneglycol (>99.5%, Fluka) were carried out and a good linearity in decay for 2,3-Pentanedione was obtained under chromatographic conditions already used.

Table 5.8. Features of MHS-SPME method using standard solutions.

Compounds Studied

Since the calibration curves were obtained, their quantity in extruded films and thermoformed samples of PLA2003D was obtained. The analyses were performed in triplicate using samples of 1000 mg each one. Results showed that, in the case of acetaldehyde, total peak areas obtained using 1000 mg of sample were above the linear range of the calibration curve of this compound. Consequently, with the aim of quantifying acetaldehyde, we used PLA samples of 200 mg each one during MHS-SPME. Additionally, the change in VOCs of PLA2003D as

155 film or thermoformed samples was assessed after six months of storage at room temperature in capped vials. The VOCs identified in these samples are shown in Table 5.9 and the concentrations of three of them are presented in Table 5.10. The results of VOCs identified in PLA2003D samples after storage (Table 5.9) showed the presence of eight compounds, all of them already identified in samples before storage (see Table 5.7, 30ºC), with exception of 3-methyl 2,4-Pentanedione. The later molecule would be produced during storage at room temperature; however it is difficult to suggest a pathway of production among primary or secondary reactions of PLA degradation. Also, the peak areas of the VOCs identified after storage were lower than the peak areas determined in samples before storage. In contrast, the results found in thermoformed samples after storage showed the presence of only three compounds (ethanol, acetone and 2-methyl 2-Propanol) of eight compounds identified in extruded films. So, on the basis of these results, it is clear that the storage of samples decreased the presence of VOCs in PLA samples.

Table 5.9. Peak area of VOCs identified in PLA2003D samples (200 mg) after 6 months of storage in capped vials using MHS-SPME.

Compounds KIc* (DB5) KI ref^a (DB5) Mw CAS Extruded film

Thermoformed

Acetaldehyde nd ~427 44 75-07-0 2675215 no present

Ethanol nd - 46 64-17-5 357882 304495

Acetone nd 500 58 67-64-1 2080827 715924

2-methyl-2-Propanol 525 526 74 75-65-0 1979250 540379

2-methyl-3-Buten-2-ol 617 614 86 115-18-4 223863 no present

2,4-dimethyl 2-Pentanol- 804 - 116 625-06-9 212373 no present 3-methyl 2,4-Pentanedione - 902 903 114 815-57-6 448309 no present 2,2,4,6,6-pentamethyl-Heptane 1005 997 170 13475-82-6 172611 no present

* Kovats index calculated; ^a from NIST available in http://webbook.nist.gov/chemistry/cas-ser.html; nd = no determined.

For freshly new thermoformed PLA samples (Table 5.10), the concentration of acetaldehyde decreased and the 2,3-Pentanedione was no present in comparison with extruded film. The process of thermoforming is carried out at 90 ºC during 15 seconds that could cause the loss

156 of these two compounds which cannot contribute to produce further degradation of PLA.

After storage, the acetaldehyde concentration was lower than LOQ in extruded films and was not at all found in thermoformed samples. 2,3-Pentanedione was no present in all PLA samples after storage. Hence, it seems that these molecules were degraded during storage at ambient conditions.

In the case of 2-methyl-2-Propanol, before storage, no significant difference was found between the concentration determined in films and thermoformed samples (Table 5.10). After 6 months of storage in capped vials, the concentrations were lower than the LOQ of the method. Under our experimental conditions it is however difficult to propose a PLA degradation pathway in extrusion and thermoforming that explains the production of this molecule or that allows knowing if this molecule is produced from the secondary reactions of primary products.

From a commercial perspective these results are of interest because they show that there is not VOCs production in PLA samples during storage at room temperature, but during the forming at higher temperatures.

Table 5.10. Concentrations of VOCs (µg of compound/g of sample) in PLA2003D samples (200 mg) obtained by MHS-SPME before and after storage at room temperature in capped vials.

!Compounds! ^LOQ*! Before storage After 6 months

extruded film thermoformed extruded film thermoformed Acetaldehyde 0.385 1.40 ± 0.18 < LOQ < LOQ not present 2-methyl 2-Propanol 0.080 0.39 ± 0.08^a 0.48 ± 0.04^a < LOQ < LOQ 2,3-Pentanedione 0.069 0.074 ± 0.003^** Not present Not present Not present

* LOQ = limit of quantification (µg of compound/g of sample); ** determinated in a PLA samples of 1000 mg; ^a different letters indicate significant differences at p< 0.05 (Duncan).

Concerning the acetaldehyde content in PLA, to the best of our knowledge, no data were reported about it. However, large literature reported values of acetaldehyde content in PET (Mutsuga et al. 2006, Redzepovic et al. 2012). Mutsuga et al. (2006) reported acetaldehyde

157 values in PET bottles from Europe, North America and Japan, in ranging from 5.0 to 13.1 mg/kg, from 9.1 to 18.7 mg/kg and from 8.4 to 25.7 mg/kg, respectively. Redzepovic et al.

(2012) found acetaldehyde levels in mineral water after filling in PET bottles from 1.62 to 2.22 ppm and from 13.94 to 17.68 ppm, after 1 day and 90 days of storage at room temperature, respectively. PLA samples studied in our work showed acetaldehyde concentrations lower than PET. So, from the residual acetaldehyde content determined in this work, it seems that PLA could present minor problems of migration related to this molecule and VOCs than PET.

158

Conclusions