Molecular background of the undesired odor of polypropylene materials and insights into the sources of key odorants

Screening the volatiles isolated from a standard polypropylene material consisting of a polypropylene homopolymer, the filler talcum, and a mixture of antioxidants, for odor- active compounds by application of an aroma extract dilution analysis revealed 30 odorants with flavor dilution factors ranging from 1 to 64. Eighteen odor- active compounds were subsequently quantitated by gas chromatography- mass spectrometry using stable isotopically substituted odorants as internal standards, and their odor activity values (OAVs) were calculated as ratios of the concentrations to the odor threshold values. Five odorants showed OAVs ≥1, among which were hex- 1- en-3- one (OAV 12), butanoic acid (OAV 3),


| INTRODUC TI ON
Polypropylene is the second most important technical polymer with 20% of the total plastic production. 1 It is used in different industrial sectors such as packaging, consumer products, home textiles, and automotive parts. In some applications, the smell of polypropylene material can be a severe problem, particularly in indoor environments such as buildings, cars, and aircrafts. Consumers perceive the characteristic smell as rather unpleasant and tend to imply possible health implications, which often leads to the rejection of polypropylenebased products. Thus, manufacturers are in urgent need to mitigate the undesired odor of polypropylene materials. Addition of adsorbents or absorbents can lead to a reduction in the perceived odor intensity. 2,3 Nevertheless, the odor-active compounds remain in the material. Thus, preventing odorant formation would be the better alternative. However, data on the crucial processing steps leading to the characteristic polypropylene odor as well as data on the causative compounds are currently scarce. Potential odorant sources include side reactions during polymerization, impurities in additives, reactions induced by temperature, light and/or oxygen during processing (e.g., extrusion, injection molding), storage and/or use. In any case, a targeted mitigation approach requires the knowledge of the causative compounds. 4 For this purpose, gas chromatographyolfactometry (GC-O) together with gas chromatography-mass spectrometry (GC-MS) measurements are indispensable. 5 In 2009, Tyapkova et al. 6  Further GC-O studies on odor-active compounds in polypropylene material applied simultaneous distillation/extraction (SDE) 7 and headspace-SPME 8,9 as volatile sampling approaches. The compounds reported as potent odorants included aldehydes such as heptanal, octanal, nonanal, (2E)-non-2-enal, (6E)-non-6-enal, (6Z)non-6-enal, non-8-enal, and decanal, ketones such as hex-1-en-3one, oct-1-en-3-one, and non-1-en-3-one, and carboxylic acids such as propanoic acid and butanoic acid. [7][8][9] In summary of the literature overview, there was some knowledge on odor-active compounds in polypropylene materials available, but most screening data had not been substantiated by quantitative measurements and by comparison of the obtained concentrations to odor threshold values (OTVs). Furthermore, it remained unclear how these odorants are formed and how their formation could be minimized. The aims of our study therefore were (1) to characterize the key odorants in

| Miscellaneous chemicals
Dichloromethane, diethyl ether, and pentane were purchased from VWR and freshly distilled before use. Silica gel 60 (0.040−0.063 mm) was obtained from VWR and purified as detailed previously. 27 Silicone oil was purchased from Merck, and sunflower oil was bought in a local supermarket.

| GC-O/FID system
A Trace GC Ultra gas chromatograph (Thermo Scientific) was equipped with a cold on-column injector, a flame ionization detector (FID), and a tailor-made sniffing port. 28 The fused silica column was  Before a heart-cut analysis, the retention times of the target compounds in the first dimension were determined after injection of reference compounds by using the monitor detectors. During the elution of the target compounds in the heart-cut run, the effluent of the first column was directed via the MCSS system to the column in the second dimension, and the transferred substances were cryofocused in the precooled trap. When the MCSS system had switched back to the monitor detectors, the trap cooling was turned off, and the temperature program of the second gas chromatograph was started together with the mass spectrometer. The MS Workstation software (Varian) was used for the evaluation of the mass spectra.

| GC × GC-MS system
A 6890 Plus gas chromatograph (Agilent) was equipped with a Combi The temperature of the secondary oven was 70°C for 2 min, followed by a gradient of 6°C/min to a final temperature of 250°C, which was held for 5 min. Mass spectra were generated in the EI mode at 70 eV, a scan range of m/z 35−300, and a scan rate of 100 spectra/s. The GC Image software (GC Image) was employed for data analysis.

| Isolation of volatiles
PP-1 was cut into pieces of ~4 mm diameter with a knife. The granulates of PP-2 to PP-8 were directly used for volatiles isolation. Dichloromethane (250 mL) was added to the samples (100 g). . The combined organic phases were dried over anhydrous sodium sulfate and concentrated to 1 mL (fraction AV). The fraction NBV was further fractionated. To remove dichloromethane, hexane (0.5 mL) was added and the mixture was concentrated to 0.5 mL.
The concentrate was applied onto a slurry of purified silica gel (9 g) in pentane in a water-cooled (12°C) glass column (1 cm i.d.). Elution was performed with 50 mL pentane and 100 mL diethyl ether. Both eluate portions were concentrated to 1 mL (fractions NBV1 and NBV2).

| AEDA
The   Table S2). To obtain the calibration line equation, mixed dichloromethane solutions of analyte and standard in different concentration ratios were analyzed under the same conditions followed by linear regression.

| OTVs
OTVs were determined according to the American Society for Testing and Materials (ASTM) standard practice for determination of odor and taste thresholds by a forced-choice ascending concentration series method of limits. 32 All threshold values were determined orthonasally in low-odor sunflower oil.  To assess the importance of the individual odorants for the overall odor of the polypropylene material, an OAV was calculated for each compound as ratio of its concentration to its orthonasal OTV.

| Odor reconstitution
OTVs in polypropylene were approximated by using OTVs in lowodor sunflower oil, which were taken from the LSB@TUM odorant database 33 or, for some compounds, were determined by using the same approach. Low-odor sunflower oil was considered an appropriate approximation for the nonpolar polypropylene matrix. For five odorants (3, 17, 32, 4, 36), an OAV ≥1 was determined (Table 3)      a All polypropylene odorants reported in this paper were consecutively numbered according to their retention time on the DB-FFAP column.

| Insights into the sources of the key odorants
b Each odorant was identified by comparing its retention indices on two fused silica columns of different polarity (DB-FFAP, DB-5), its mass spectrum obtained by GC-MS, as well as its odor quality as perceived at the sniffing port during GC-O to data obtained from authentic reference compounds analyzed under equal conditions. c Odor quality as perceived at the sniffing port during GC-O.    (Table 8). Despite its importance, the source of hex-1-en-3-one is yet unknown. Considering the small differences between the samples with and without antioxidants and fillers in our study, an origin from additives is unlikely. Furthermore, hex-1-en-3-one has been detected as major odorant not only in polypropylene samples, 8 but also for example in a polyvinyl chloride-based material. 39 Thus, it most likely neither originated from the polymer backbone.
Alkylphenols, among which 2-tert-butylphenol was the most potent, represented the other important odorants in the polypropylene materials. Our results suggest that they were formed via degradation of phenolic antioxidants initiated by talcum. Consequently, none of the key odorants could be linked to the methyl-branched basic polymer.
Mitigation of the phenolic odorants was possible by using wollastonite as alternative filler or by using a combination of talcum and a mixture of antioxidants including Irgafos® 168 FF, Irganox® 1010 FF, and Tinuvin® 770 DF in high amounts. As polypropylene materials are widely used in buildings, cars, and aircrafts, a reduction in odor-active compounds will ultimately contribute to improve the indoor air quality.