Release of silver from silver doped PET bottles
Introduction
Active packaging aim to increase the shelf life of packed products as well to maintain and improve its quality. Its function is based on interactions between the filled product and the packaging material; while substances are principally released or removed from the packaging material to the filled product or to the headspace (Pant, 2016). A form of active packaging- antimicrobial packaging, intends to reduce or inhibit growth of microorganisms in the packaging material or in the packed food. There are several forms of antimicrobial packaging. A special form is the incorporation of antimicrobial agents into polymers (Appendini & Hotchkiss, 2002).
Among metallic cations, silver ion (Ag+) is known to have the best antimicrobial effect against a huge range of microorganisms. Thermal polymer processing methods like injection molding is suitable for silver since it can withstand very high temperatures (up to 800 °C). Thus, active packaging based on release of silver ions has gained increasing (Llorens, Lloret, Picouet, Trbojevich, & Fernandez, 2012). The most widely used polymer additives for food applications are silver substituted zeolites (Appendini & Hotchkiss, 2002). Moreover, silver nanomaterials have attracted increasing attention (see Table 1).
Migration, i.e., the mass transfer of a substance from a package into the foodstuff, is one of the most important safety concerns in food packaging with regard to the consumer. Migration tests should be performed in a worst case scenario, i.e., migration level should be equal or higher than those expected during real food storage (Begley et al., 2005). European Commission (EC) Regulation No 10/2011 on plastic materials and articles intended to come in contact with food sets out conditions of migration testing for the introduction of new packaging material (European Commission Regulation, 2011). The European Food Safety Authority (European Food Safety Authority (EFSA), 2011) released a positive opinion concerning the use of a certain silver (Ag) zeolite in food contact surfaces with a general specific migration limit of 0.05 mg Ag per kg food. Therefore, the maximum oral intake was limited to less than 13 % of the No Observed Adverse Effect Level (NOAEL), which is about 10 g over a total lifetime intake for humans (World Health Organization (WHO) (2003)). Mass transfer of substances from plastic into food simulants usually follows Fick´s law of diffusion (Begley et al., 2005) which is dependent on factors such as temperature, time, amount of substance in the polymer, type of substance and solubility in food (Welle & Franz, 2011).
Migration of silver from food contact materials (FCMs) has gained increasing interest. Table 1 shows a literature overview of the release of silver from FCMs. In the following, two examples were selected. Von Goetz et al. (2013) studied the migration of silver from commercially available polypropylene silver doped food containers and a polyethylene plastic bag into food simulants. The food storage container with an initial silver content of 11.9 ± 2.4 μg g−1 plastic released 9.5 ng cm−2 Ag into 3 % (w/v) acetic acid at 20 °C after 10 d. Migration into distilled water was approximately half as much. The migration from the plastic bag with an initial silver content of 37.1 ± 1.2 μg g−1 plastic was 0.5 ng cm−2 after 10 d. Bott (2017) showed that the amount that migrated from PE-LD nanocomposites containing 250 mg kg−1 silver was 1010.9 ng dm−2, into 3 % (v/v) acetic acid after 10 d at 60 °C. He found out that the silver that migrated was only present in the form of silver ions and not nanoparticular. Moreover, it became clear that targeted analytics tailored to the nanomaterial were decisive for the identification of the silver species.
Antimicrobial agents based on silver like silver zeolites or silver nanoparticles have been incorporated into polymers such as polyethylene, low density polyethylene, polypropylene, polyamide and polylactic acid (Cushen, Kerry, Morris, Cruz-Romero, & Cummins, 2014; Damm & Münstedt, 2008; Fernández, Soriano, Hernández-Muñoz, & Gavara, 2010; Jokar & Abdul Rahman, 2014; Pehlivan, Balköse, Ülkü, & Tihminliogˇlu, 2005). However, data regarding silver incorporated in polyethylene terephthalate (PET) are lacking, despite PET is one of the most common packaging material for beverages. Data on the release of silver from PET would be beneficial for health safety policies and food packaging industry.
The objectives of this study were to evaluate the release of silver from silver doped PET bottles into food simulants and to investigate the parameters such as time, temperature, medium (in terms of acidity strength), percentage of filler in the polymer and type of antimicrobial filler on the release of silver and its diffusion coefficients. Furthermore, the influence of the geometry was investigated and the mechanism of action on the release of silver ions from silver phosphate glass out of the bottle was postulated. To the best of the author´s knowledge, no publication describing the incorporation of silver into PET bottles, nor the migration of silver from them existed.
Section snippets
Manufacturing of the silver doped bottles and bellows
The preforms were made of PET resin (Selenis MASTER 10, Ribeira de Nisa Portalegre, Portugal) with an intrinsic viscosity of 0.86 ± 0.02 dl/g. IONPURE WPA < 10 μ (Ishizuka Glass Co., Ltd, Iwakura City, Aichi, Japan) containing silver phosphate (Ag3PO4) glass as active substance was incorporated into the PET in powder form at 3 and 10 % (w/w). According to the manufacturer, the silver content in the silver phosphate glass powder was 1.6 % (w/w). From the obtained mixture, the preforms were
Release of silver from silver phosphate glass powder into distilled water and distilled water supplemented with 0.028 M NaCl
It was ensured that the pure silver phosphate glass powder in general releases silver ions before incorporating it into the polymer (Fig. 2). The experiments showed that Ag+ was released over time out of the silver phosphate glass matrix. After 24 h at 43 °C, 10.3 ± 1 mg/L or 5.15 mg Ag+ was released from 0.5 g of silver phosphate glass into 0.5 L distilled water. The total concentration of silver in the dispersed powder corresponded to 16 mg/L that is 8 mg in 0.5 L in total. This means that 64
Conclusions
The experiments showed that the release of silver from silver doped PET bottles depends on the percentage of antimicrobial filler in the polymer, temperature, and acidic strength of the simulants. The type of antimicrobial filler was also a decisive parameter. However, the amount of silver released partly exceeded proposed migration limits from EFSA. For PET bottles containing nanosilver no release was detected. Further study will show if migrated silver concentrations can lead to antimicrobial
CRediT authorship contribution statement
Sabrina Braun: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Visualization, Supervision, Project administration. Vladimir Ilberg: Writing - review & editing, Supervision. Uwe Blum: Methodology, Investigation. Horst-Christian Langowski: Writing - review & editing, Supervision.
Declaration of Competing Interest
None.
Acknowledgments
The Federal Ministry for Economic Affairs and Energy financially supported this research project on the basis of a resolution of the German Bundestag and RAS AG provided AgPURE. The authors would like to thank Josef Gleixner from Inotech for producing the bottles and bellows and Indicator GmbH for ICP-MS analysis. TUM proofreading support is gratefully acknowledged.
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