Effects of Ultraviolet (UV) on Degradation of Irgafos ... - ACS Publications

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Effects of Ultraviolet (UV) on Degradation of Irgafos 168 and Migration of Its Degradation Products from Polypropylene Films Yueping Yang,†,∥ Changying Hu,*,†,‡,∥ Huaining Zhong,§ Xi Chen,Δ Rujia Chen,† and Kit L. YamΔ †

Department of Food Science and Engineering, Jinan University, Guangzhou, Guangdong 510632, China Key Laboratory of Product Packaging and Logistics of Guangdong Higher Education Institutes, Jinan University, Zhuhai, Guangdong 519070, China § Inspection and Quarantine Technology Center, Guangdong Entry−Exit Inspection and Quarantine Bureau, Guangzhou, Guangdong 510623, China Δ Department of Food Science, Rutgers University, New Brunswick, New Jersey 08901, United States ‡

ABSTRACT: The effects of ultraviolet (UV) irradiation on the degradation of Irgafos 168 and the migration of its two degradation products, 2,4-di-tert-butylphenol and tris(2,4-di-tert-butylphenyl)phosphate, from polypropylene (PP) were investigated. A blown film machine was used to extrude PP films containing Irgafos 168, the films were stored in the dark for 45 days, two UV treatments and sunlight exposure were applied to the films, and GC-MS was used for degradation and migration studies. Extrusion, storage, UV treatments, and sunlight exposure significantly affected concentrations of Irgafos 168 and the degradation products. 2,4-Di-tert-butylphenol was the major degradation product produced by UV irradiation, but tris(2,4-di-tert-butylphenyl)phosphate was the major degradation product produced by extrusion, storage, and sunlight exposure. The degradation products have no or little health risk, because migration study and threshold of toxicological concern (TTC) analysis show that experimental maximum migration of 2,4-di-tert-butylphenol and tris(2,4-di-tert-butylphenyl)phosphate are only 2 and 53% of the theoretical maximum migration amounts, respectively. KEYWORDS: UV irradiation, Irgafos 168, degradation, migration, threshold of toxicological concern

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but there is limited information about DP2. Simoneau et al.9 studied 449 baby bottles and found that 90% of them had a DP1 level >400 mg kg−1. Hirata-Koizumi10 reported that the revealed no-observed adverse-effect levels (NOAEL) of DP1 were 5 and 20 mg kg−1 for newborn and young rats, respectively,10 indicating that DP1 is more toxic than Irgafos 168 as shown by the acute toxicity of Irgafos 168 on rats that there was no obvious adverse effect even at the concentration of >2000 mg kg−1. With regard to regulation of Irgafos 168 and its degradation products, the specific migration limit (SML) of Irgafos 168 is 60 mg kg−1 based on EU 10/2011;7 however, there are no SML specifications for DP1 and DP2. Despite the limited regulation on the two degradation products, it is important to investigate the degradation of Irgafos 168 and the migration of Irgafos 168 and these two degradation products because of their potential health risk. Additives in food packaging may migrate to food.11 As a common antioxidant, the detection and migration of Irgafos 168 itself were studied,12,13 but only limited publications studying its degradation were found, and these studies focused on identifying the degradation products under different conditions, such as UV irradiation,4 gamma irradiation,14 pulsed light irradiation,15 and electron beam irradiation,16,17 or on migration of additives and their degradation products from polymer under

INTRODUCTION Ultraviolet (UV) irradiation is a food safety intervention technology approved by the U.S. Food and Drug Administration (FDA), which can be used to decontaminate food and food contact surfaces.1,2 In recent years, the food industry has expressed great interest in using this nonthermal technology for decontamination while minimizing flavor, color, nutrient, and other quality losses often associated with the traditional thermal processing methods.3 However, there is also a concern that UV irradiation may cause formation of degradation products from packaging additives,4 which are considered as nonintentionally added substances (NIAS) that may migrate into food.5 The migrated degradation products can cause undesirable flavors or even toxicity in food.6 The recent Regulation on Food Contact Materials7 recognized that any potential health risk in the final material or article arising from NIAS should be assessed by the manufacturer in accordance with internationally recognized scientific principles of risk assessment. Irgafos 168 (tris(2,4-di-tert-butylphenyl)phosphite) is a widely used antioxidant in the polyolefin industry, but it may degrade and generate NIAS during polymer processing. Studies from our group8 and the literature4 have shown that under UV irradiation Irgafos 168 can degrade into several degradation products; two of them are 2,4-di-tert-butylphenol and tris(2,4-di-tert-butylphenyl)phosphate, via the chemical reaction shown in Figure 1. For simplicity, hereafter these two degradation products, 2,4-di-tert-butylphenol and tris(2,4-ditert-butylphenyl)phosphate, are abbreviated DP1 and DP2, respectively. DP1 has been subjected to several toxicity studies, © 2016 American Chemical Society

Received: Revised: Accepted: Published: 7866

July 13, 2016 September 16, 2016 September 23, 2016 September 23, 2016 DOI: 10.1021/acs.jafc.6b03018 J. Agric. Food Chem. 2016, 64, 7866−7873

Article

Journal of Agricultural and Food Chemistry

Figure 1. Degradation reactions of Irgafos 168: (A) degradation to 2,4-di-tert-butylphenol; (B) degradation to tris(2,4-di-tert-butylphenyl)phosphate.

Table 1. LOD, LOQ, Linear Equation, and Correlation Coefficient Irgafos 168 in n-hexane Irgafos 168 in isooctane 2,4-di-tert-butylphenol (DP1) in n-hexane 2,4-di-tert-butylphenol (DP1) in isooctane tris(2,4-di-tert-butylphenyl) phosphate (DP2) in n-hexane tris(2,4-di-tert-butylphenyl)phosphate (DP2) in isooctane

LOD (μg L−1)

LOQ (μg L−1)

5.00 6.00 0.20 0.30 25.00 20.00

12.00 15.00 0.45 0.55 60.00 53.00

microwave treatment,6,18 r-irradiation,19 or simulated the worstcase scenario,20,21 which did not represent the real migration after the polymer had been treated by UV irradiation. No publication was found on the quantification of the degradation products under UV irradiation and on the migration of Irgafos 168 and its degradation products. The objective of this study was to investigate the effects of UV irradiation on the degradation of Irgafos 168 and migration of Irgafos 168 and its degradation products from polypropylene (PP) pouches to isooctane; however, the effects of extrusion, storage, and sunlight exposure were also investigated because they were also involved in the experiment. This study addresses three aspects barely covered in the literature: (1) quantification of degradation products of Irgafos 168, because most published studies reported only the identification of those degradation products; (2) effects of different UV treatmentshere, migration was studied for pouches under two UV treatments to simulate different processing methods, UVPP in which empty PP pouches were treated by UV and UVPPFI in which PP pouches filled with isooctane were treated with UV; and (3) estimation of potential health risk based on an independent migration studyhere, an independent migration study was conducted using 0.22% Irgafos 168 and 100 μm PP film at 25 °C.

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linear equation y y y y y y

= = = = = =

6.356 3.876 1.077 0.972 4.975 3.892

× × × × × ×

103x 103x 105x 105x 102x 102x

correlation coefficient

− 1.048 × 104 + 1.656 × 104 + 1.388 × 104 + 1.097 × 104 − 4.725 + 9.830

0.9990 0.9967 0.9989 0.9973 0.9955 0.9956

(Irgafos 168) by reacting 2 mL of hydrogen peroxide (30%) with 6 mL of H2O in a microwave digestion system (Bergamo, Italy), and the conversion ratio of this reaction was determined to be 100% by GC-MS analysis.22 Pellets of polypropylene resin (PP-R) were purchased from Borealis (Vienna, Austria). Food Simulant for Migration Study. Because Irgafos 168 and its degradation products have very low water solubilities,18,21 a hydrophobic system is more appropriate than an aqueous system for migration study. Here isooctane was selected as fatty food simulant due to the stability of Irgafos 168 in this solvent.23−26 Olive oil was not used because Irgafos 168 in this oil can be converted into phosphate by oxidation process.21 PP Film and Pouch Production. PP films were produced in three steps: (1) Irgafos 168 and PP pellets were dry mixed by hand in a container, (2) the mixture was extruded in a twin-screw extruder (Kebeilong Machinery Limited Co., Nanjing, China) and the extrudant was cooled in water and then pelletized, and (3) a blown film machine (Kebeilong Machinery Limited Co.) was used to produce films with the newly formed pellets. Two films of different thicknesses and concentrations were produced. The first film was 30 ± 1 μm thick, had 2.29% Irgafos 168 concentration (about 10 times the maximum allowable concentration27), and was used to prepare pouches for degradation and migration analyses. The second film was 100 ± 5 μm thick, had 0.22% Irgafos 168 concentration (maximum allowable level is 0.25%27), and was used to prepare pouches for threshold of toxicological concern (TTC) analyses. Empty PP pouches were produced from the films by heat sealing. The effective inner surface area of each pouch was 10 cm × 10 cm. A corner of each pouch was cut to create a small opening for filling according to the procedure described in GB/T 23296.1-2009.25 Each pouch was filled with 100 g of isooctane through the opening, and the opening was then closed by a paper clip. UV Irradiation and Sunlight Exposure. UV irradiation was conducted in a sterilized operating environment (Sujingantai Co.,

MATERIALS AND METHODS

Materials. Analytical grade n-hexane, isooctane, and hydrogen peroxide were purchased from Guangzhou Chemical Reagent Factory (Guangzhou, China). Irgafos 168 was purchased from Shanghai Tixiai Co., Ltd. (Shanghai, China). 2,4-Di-tert-butylphenol was purchased from Dr. Ehrenstorfer GmbH (Augsburg, Germany). Tris(2,4-di-tertbutylphenyl)phosphate was prepared from the antioxidant precursor 7867

DOI: 10.1021/acs.jafc.6b03018 J. Agric. Food Chem. 2016, 64, 7866−7873

Article

Journal of Agricultural and Food Chemistry

Table 2. Recoveries and RSDs of Irgafos 168 and Its Two Degradation Products Spiked on n-Hexane and Isooctane (n = 6) n-hexane substance Irgafos 168

2,4-di-tert-butylphenol (DP1)

tris(2,4-di-tert-butylphenyl)phosphate (DP2)

isooctane

spiked concentration

mean recovery (%)

RSD (%)

mean recovery (%)

RSD (%)

25.00 75.00 125.00 0.04 5.00 15.00 0.10 1.00 5.00

97.98 104.50 100.61 106.46 106.16 90.00 90.00 83.50 89.30

3.68 3.89 2.35 4.52 3.81 4.54 4.54 2.23 2.41

109.21 101.22 98.38 103.23 105.89 98.98 96.68 89.26 95.63

3.98 3.67 3.07 4.89 4.21 3.98 4.32 3.09 2.89

Table 3. Concentrations of Irgafos 168 and Its Degradation Products at Each Step of the Process Irgafos 168 2,4-di-tert-butylphenol (DP1) tris(2,4-di-tert-butylphenyl)phosphate (DP2) a

before extrusion

after extrusion

initial concentration (after 45 days in the dark)

UVPP

3.00% nda nd

2.29% 0.04‰ 0.07%

2.20% 0.09‰ 0.12%

2.03% 0.07% 0.17%

nd, no detection.

Table 4. Degradation of Irgafos 168 and Generation of DP1 and DP2 under 520 nm Green Light and 405 nm Purple Lighta wavelength

decrease of Irgafos 168 in PP film (%) (×10−3)

increase of DP1 in PP film (%) (×10−4)

increase of DP2 in PP film (%) (×10−3)

520 nm 405 nm

40 ± 0.23a 160 ± 0.52b

0.40 ± 0.01a 5.80 ± 0.03a

4.28 ± 0.03a 51.94 ± 0.67b

a

Concentrations of Irgafos 168 and its degradation products in this table were compared with their initial concentrations, respectively.

Table 5. Degradation of Irgafos 168 and Generation of DP1 and DP2 under 365 and 254 nm UV Lighta

Figure 2. Degradation of Irgafos 168 under UV irradiation treatment.

wavelength

decrease of Irgafos 168 in PP film (%) (×10−3)

increase of DP1 in PP film (%) (×10−4)

increase of DP2 in PP film (%) (×10−3)

365 nm 254 nm

180 ± 0.31a 390 ± 0.43b

44.10 ± 0.32a 1720.58 ± 10.78b

63.00 ± 0.89a 98.42 ± 5.49b

a

Concentrations of Irgafos 168 and its degradation products in this table were compared with their initial concentrations, respectively.

Sunlight exposure was also studied as a comparison to UV light treatment. The treatment was conducted by placing the PP film in an indoor environment near a window with natural sunlight exposure at room temperature (20−25 °C). Each sampling was prepared in triplicate. To investigate the generation mechanism of DP1 and DP2, PP films were treated by laser beam (Lasever Inc., Ningbo, China) of 405 nm purple light and 520 nm green light (100 mW) for 15 min and treated by UV transilluminators (Baoshanggucun Electro-Optics Installation Factory, Shanghai, China) with 254 nm and 365 nm UV light (12 W × 6 lamps) for 3 h, respectively. Extraction Irgafos 168 and Its Degradation Products from PP Film. PP film was cut into small pieces, approximately 0.5 cm × 0.5 cm, of which 100 mg was immersed in 10 mL of n-hexane for 24 h at 60 °C in an HH-4 thermostat water bath (Honghua Instrument Plant, Jiangsu, China) and then diluted to 25 mL with n-hexane for GC-MS analysis. The 24 h extraction time was based on the literature15 and our earlier experiments. Analyses of Irgafos 168 and Its Degradation Products. The identification and quantification of Irgafos 168 and its degradation products in n-hexane and isooctane were carried out by a 7890A gas chromatograph coupled with a 5795C mass selective detector

Figure 3. Degradation of Irgafos 168 under sunlight exposure. Suzhou, China) at 25 °C under a 20 W UV lamp (Hailian Limited Co., Qidong, China) at 253.7 nm wavelength. For degradation study, the films were treated up to 80 h. For migration study, samples from 30 min of treatment were used, because this treatment time is commonly used in the industry. For migration study, two conditions were studied: (1) UVPP was the condition in which PP pouches made from the film were first treated with UV and then filled with isooctane to simulate UV sterilization of the pouches alone, and (2) UVPPFI was the condition in which PP pouches made from the film were first filled with isooctane and then treated with UV to simulate UV sterilization of the pouches and the food product together. Each sampling was prepared in triplicate. 7868

DOI: 10.1021/acs.jafc.6b03018 J. Agric. Food Chem. 2016, 64, 7866−7873

Article

Journal of Agricultural and Food Chemistry

Figure 4. Migration of Irgafos 168: (A) 4 °C; (B) 25 °C; (C) 40 °C. with a DB-5MS 30 m × 0.25 mm × 0.25 μm capillary column (Agilent Technologies, Santa Clara, CA, USA). The GC-MS operating conditions were as follows: helium (carrier gas) flow rate, 2 mL min−1; oven program started at 50 °C (2 min hold), ramp rate 20 °C min−1 for 8 min, 300 °C final temperature; solvent delay, 6 min; splitless injection; for detection, electron ionization at 70 eV and selective ion monitoring, 441, 191, 316 used for Irgafos 168, DP1, and DP2, respectively.8 Migration of Irgafos 168 and Its Degradation Products from PP Pouches. PP films were stored for 45 days under darkness before migration study to simulate the situation that usually they are not used

immediately after extrusion. Then pouches made from the films were UVPP and UVPPFI treated, and no UV treatment was used as control. Migration study was conducted under 4, 25, and 40 °C. An aliquot of 0.6 mL of isooctane was withdrawn from the pouches at time intervals based on the migration rates at different temperatures. Each sampling was made in triplicate. Method Validation. The analytical method was validated on the basis of the limit of detection (LOD, S/N = 3), limit of quantification (LOQ, S/N = 10), linearity, recovery, and repeatability. To establish linearity, six standard solutions were prepared with n-hexane and isooctane each, and the solutions were then analyzed by GC-MS. 7869

DOI: 10.1021/acs.jafc.6b03018 J. Agric. Food Chem. 2016, 64, 7866−7873

Article

Journal of Agricultural and Food Chemistry

Figure 5. Migration of 2,4-di-tert-butylphenol, DP1: (A) 4 °C; (B) 25 °C; (C) 40 °C. thresholds obtained from TTC analyses. The method of calculation was modified from the literature29 as

To check the accuracy of the proposed method, a spiked recovery study was performed at three concentrations. The repeatability was validated by spiking pure n-hexane and isooctane standard solutions with three levels of the Irgafos 168 and its two degradation products in six replicate experiments. Threshold of Toxicological Concern Analysis. TTC was used to assess the potential health risk of migrated degradation products because it is commonly used to evaluate substances with insufficient toxicological data and lower exposure level.28 An independent migration study was conducted using 0.22% Irgafos 168 and the 100 μm PP film at 25 °C. The analysis was based on a TTC decision tree approach proposed by the European International Life Sciences Institute. Theoretical maximum migration amounts (mg/kg) for the two degradation products were calculated on the basis of their safety

EDI = migration × food intake × CF

(1)

where EDI is estimated daily intake in milligrams per person per day, migration of degradation products is in milligrams per kilogram, food intake is 3 kg per person per day, and CF is the fraction of daily diet expected to be in contact with a specific packaging material (0.04 for PP).

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RESULTS

Method Validation. The LOD, LOQ, linear equation, and correlation coefficient are shown in Table 1. The recoveries 7870

DOI: 10.1021/acs.jafc.6b03018 J. Agric. Food Chem. 2016, 64, 7866−7873

Article

Journal of Agricultural and Food Chemistry

Figure 6. Migration of tris(2,4-di-tert-butylphenyl)phosphate, DP2: (A) 4 °C; (B) 25 °C; (C) 40 °C.

During the 45 days of storage in the dark, Irgafos 168 concentration decreased from 2.29 to 2.20% (a loss of 4% in amount). DP1 and DP2 increased by 125 and 70% in amount, respectively, and DP2 was about 13 times the amount of DP1. The UVPP treatment caused Irgafos 168 to lose 8% in amount. DP1 and DP2 increased by 656 and 45% in amount, respectively, and DP2 was about 2.43 times the amount of DP1. The above data show that different forms of energy and environments (i.e., heat and shear energies in extrusion, thermal energy

and RSDs are shown in Table 2. These two tables indicate good LOD, LOQ, linearity, repeatability, and recovery. Degradation of Irgafos 168 and Generation of Degradation Products in PP Film. Table 3 shows the concentrations of Irgafos 168 and its degradation products at each step of the process. Immediately after extrusion, Irgafos 168 concentration decreased from 3.00 to 2.29% (a loss of 24% in amount) due partly to degradation and partly to loss in the extruder. DP2 generation was about 18 times the amount of DP1. 7871

DOI: 10.1021/acs.jafc.6b03018 J. Agric. Food Chem. 2016, 64, 7866−7873

Article

Journal of Agricultural and Food Chemistry Table 6. Risk Assessment

max migration amount (mg/kg) treatment

compound

no UV

Irgafos168 2,4-di-tert-butylphenol (DP1) tris(2,4-di-tert-butylphenyl)phosphate (DP2) Irgafos168 2,4-di-tert-butylphenol (DP1) tris(2,4-di-tert-butylphenyl)phosphate (DP2) Irgafos168 2,4-di-tert-butylphenol (DP1) tris(2,4-di-tert-butylphenyl)phosphate (DP2)

UVPP

UVPPFI

Cramer classification

safety threshold (μg/person/day)

I III

1800 90

I III

1800 90

I III

1800 90

theor

exptl

60 15 0.75 60 15 0.75 60 15 0.75

3.48 0.07 0.32 2.27 0.28 0.40 2.62 0.26 0.39

generation of DP2 as shown in Figure 2. However, the effect was more obvious for DP1. Figure 5 shows that UV irradiation increases not only the initial rate of migration but also the amount released at equilibrium. The effect is obvious for UVPP pouches, which release 4−5 times the amount of DP1 at equilibrium compared to pouches without UV treatment. The effect is less obvious for UVPPFI pouches because isooctane adsorbed some UV energy during the treatment and reduced the generation of degradation products. Thus, UVPPFI may be a better treatment than UVPP to inhibit the migration of DP1. The maximum amounts released (worst-case scenario) for DP1 and DP2 are 0.13 and 0.56 mg, respectively, which occurred at 40 °C and for UVPP. The initial amounts of DP1 and DP2 for UVPP are 0.38 and 0.95 mg, respectively (calculated using the values in Table 3). The maximum migration rates for DP1 and DP2 under this worst-case scenario are 34 and 58%, respectively. Threshold of Toxicological Concern Analysis. The toxicities of DP1 and DP2 are classified as Cramer I and Cramer III of the Cramer toxicity, respectively. Their maximum values recommended for human exposure are 1800 and 90 μg/person/day, indicating that DP1 is less toxic than DP2. These values were used for the EDIs of DP1 and DP2 in eq 1 to calculate the theoretical maximum migration amounts. Table 6 shows that the experimental maximum migration amounts of Irgafos 168 and its two degradation products are below the theoretical maximum migration amounts established by TTC analysis, as the initial concentration of Irgafos 168 in the film was 0.22% in our experiments, which was close to the regulatory maximum allowable level, 0.25%,25 indicating that there was limited health hazard from Irgafos 168 and its degradation products associated with UV irradation. However, it is better to avoid exposing PP packaging to UV for a long time and to store PP away from sunlight. More toxicity data are needed to confirm this conclusion.

in dark storage, and UV energy) can affect the degradation of Irgafos 168 and the generation of its degradation products differently. Under UV irradiation, the concentration changes of Irgafos 168 and its two degradation products in PP film are shown in Figure 2. Irgafos 168 concentration decreases sharply with complete degradation after 24 h, which is consistent with the finding of the literature4 that the intensity of m/z 647 (Irgafos 168) ion was very low after 24 h of UV irradiation. The concentration of DP1 increased to a maximum value of 0.57% at 12 h and decreased afterward. Similarly, the concentration of DP2 increased to a maximum value of 0.42% at 24 h and decreased afterward. The decreases in concentration suggest that DP1 and DP2 may degrade to some unknown products at long UV irradiation times. Under sunlight exposure, a much slower degradation reaction occurred and the major degradation product was DP2 (Figure 3), compared to UV irradiation, which gave a major degradation product of DP1 (Figure 2). Because UV irradiation has higher energy than sunlight exposure, this may suggest that DP1 needs higher energy to form than DP2. The possible explanation for less DP2 being generated than DP1 for UV irradiation and for more DP2 being generated by sunlight exposure (0.66% concentration after 105 days) than the maximum 0.42% concentration under UV irradiation is because DP1 is also an antioxidant30,31 that can inhibit the generation of DP2. The additional experiments revealed the influence of different wavelengths for the generation of DP1 and DP2. Table 4 shows the concentration of DP1 was limited and apparently lower than that of DP2 when the wavelength was more than 405 nm. However, under UV irradiation, Table 5 shows the concentration of DP1 significantly increased with decreasing wavelength, and DP1 was more than DP2 under 254 nm UV light. These results can prove that DP1 needs higher energy to be generated than DP2. Migration of Irgafos 168 and Its Degradation Products from PP Pouches. Figures 4−6 show the migration of Irgafos 168 and its degradation products from PP pouches at 4, 25, and 40 °C. The pouches were UV irradiated for 30 min prior to migration study. Pouches without UV irradiation were used as control. As expected, the initial migration rates and the total migration amount increased with temperature. Due to its largest molecule, DP2 migrated more slowly and took up to twice the time to reach equilibrium compared to Irgafos 168 and DP1. Different UV treatments did not significantly affect the migration of Irgafos 168 and DP2, probably because 30 min of UV treatment caused only small degradation of Irgafos 168 and

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AUTHOR INFORMATION

Author Contributions ∥

Y.Y. and C.H. contributed to the work equally and should be regarded as co-first authors Funding

This study was supported by the National Natural Science Foundation of China (Grant 31571762, 21277061) and the General Administration of Quality Supervision Inspection and Quarantine Science-Technology Programs (Grant 2014IK078). Notes

The authors declare no competing financial interest. 7872

DOI: 10.1021/acs.jafc.6b03018 J. Agric. Food Chem. 2016, 64, 7866−7873

Article

Journal of Agricultural and Food Chemistry

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DOI: 10.1021/acs.jafc.6b03018 J. Agric. Food Chem. 2016, 64, 7866−7873