Release of silver from silver doped PET bottles

Highlights

• Production of silver-doped PET containers.

• Release of Ag⁺ from them was depended on percentage of antimicrobial filler, temperature and acidic strength of simulant.

• Ag⁺ release into distilled water was not detected for nanosilver-doped PET containers.

• Diffusion coefficients of Ag⁺ in PET were observed to be between 3.2 × 10⁻¹⁷ and 1.7 × 10⁻¹⁵ cm²/s.

• By enhancing the ratio of surface to filling volume, a higher silver content in the solution was achieved.

Abstract

Development of active packaging based on polymers doped with silver has gained increasing interest. PET bottles were produced by injection and stretch blow molding under addition of silver phosphate (Ag₃PO₄) glass at 3 % and 10 % (w/w) and of AgPURE masterbatch at 10 % (w/w) which contained 6500 ppm silver nanoparticles (AgNPs). These bottles underwent release tests. Silver released into distilled water, 0.5 % and 3 % (w/w) acetic acid was quantified using inductively coupled plasma mass spectrometry (ICP-MS). The amount of silver released ranged from 2 μgL⁻¹ to 72 μgL⁻¹ after 10 days at either 21 °C or 43 °C. For PET bottles containing AgNPs no release of silver was detected. Release was dependent on percentage of antimicrobial filler in the polymer, temperature, and acidic strength. Diffusion coefficients of silver ion (Ag⁺) in PET were observed to be between 3.2 × 10⁻¹⁷ and 1.7 × 10⁻¹⁵ cm²/s. Bottles containing 3% (w/w) did not exceed the proposed specific migration level of 0.05 mg Ag per kg food, whereas bottles containing 10 % (w/w) did.

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.