1. Chemistry and properties
Silicones and plastics are both highly versatile groups of polymers used in food contact materials (FCMs), but silicones have certain characteristics that distinguish them from plastics. The backbone of any silicone is based on alternating silicon (Si) and oxygen (O) atoms. Two organic groups are bound to each Si atom and the most common silicone is polydimethylsiloxane (PDMS). Depending on the length of the polymer chains and the degree and nature of crosslinking, silicones are present as fluids, rubbers or resins. The terms “siloxanes” and “silicones” are often used as synonyms, whereby for oligomers the terms “siloxane” or “siloxane oligomers” are generally preferred and polymers are mostly designated as “silicones” or “polysiloxanes”. Cyclic and linear siloxane oligomers are common by-products during the production of silicones. Silicones are generally water- and oil-repellent, gas-permeable, and insoluble in water, mineral oil and alcohols. PDMS can behave like an elastic solid and a viscous liquid.
2. Types and applications
- Silicone fluids are generally based on PDMS. In food packaging and processing plants, they are broadly used as release agents on a wide variety of materials and equipment. PDMS is also added directly to food during production and processing (e.g. as anti-foaming agent during sugar production, processing of beverages and washing of vegetables).
- Silicone greases are prepared by dispersing silica fillers and soaps into the PDMS formulation. They are used as lubricants on rubber parts and in bearings and gears with rolling friction to maintain the functioning of machines.
- Silicone rubbers are produced on the basis of silicone fluids by cross-linking the linear PDMS molecules. In a process called high-temperature vulcanization (HTV) the cross-linking reaction is catalyzed by the addition of peroxides at high temperatures. HTV generates by-products, which can be removed by heating the final product before use. Alternatively, silicone rubbers are synthesized by cross-linking linear silicones containing functional groups (e.g. vinyl or hydroxyl) in the presence of catalysts (e.g. tin- or platinum based) and suitable reagents. These processes are named room temperature vulcanization (RTV). Depending on the underlying chemistry, one- and two-component systems are commercially available (RTV-1 and RTV-2). Many household utensils which are in direct food contact, e.g. baking molds, spoons, spatula, containers, gaskets and ice cube trays, are composed of silicone rubber. Baby soothers and feeding teats are also commonly made of silicone rubber.
- Silicone resins are made by cross-linking linear silicones, which provide additional OH-groups in the backbone as branching points. Silicone resins are commonly used as anti-stick coatings on kitchen utensils and in food processing factories.
- Silicones are further used as additives in thermoplastic polymers to enhance the flow during manufacturing, the fire resistance and the surface finish of the product. Further, silicones find application in the pulp and paper industry as defoamers, additives and for de-inking. Silicone-containing plastics, paper and board is commonly used in food packaging.
In the EU, silicone materials are included on the list of FCM articles and materials for which specific measures shall be established (Framework Regulation (EC) 1935/2004), but no harmonized legislation exists to date for silicone materials used as FCMs. In 2004, the Council of Europe published a resolution on the use of silicones in food contact applications. In Germany and France, the use of silicones is regulated on a national level by the Recommendation XV from the BfR and by Arrêté du 25 Novembre 1992, respectively. In the U.S., silicones used in FCMs are generally regulated as indirect food additives by the U.S. Food and Drug Administration (FDA) under C.F.R., Title 21 on Food and Drugs, parts 170 to 199. A database providing yearly updates can be accessed and searched on the website of the FDA. In the production of silicones, prior sanctioned ingredients and substrates generally recognized as safe (GRAS) are also legally allowed.
4. Migration and exposure
Substances that may migrate from silicone-based materials into food include oligomers, additives, catalysts, and breakdown and reaction products. In the last ten years, several scientific studies investigated chemical migration from silicones into food (for a list of references see below). Analytical methods for the quantification of siloxane oligomers and one predictive algorithm were described to assess overall and/or specific migration from silicones. FCMs under investigation included baking molds, kitchen utensils, nipples and baby bottles, and popcorn packaging. For repeated-use articles, the migration of siloxane oligomers sometimes exceeded the legal limits especially during the first cycles of use. A high fat content of the food was described to increase migration. Substances related to printing inks (e.g. benzophenone and diisopropyl naphathalene), phthalates and aldehydes were reported to migrate from silicone baby bottles into food simulant, used to resemble migration into milk.
Data describing the food-related exposure to silicones, including cyclic and linear siloxane oligomers, are scarce. Most of the studies report on occupational exposure and the exposure from personal care products and indoor air. Recently, inhalation followed by skin absorption was judged to be the most prevalent exposure pathway (Rücker and Kümmerer, 2015).
5. Toxicology and risk assessment
Siloxane oligomers are the most typical and largest group of migrants from all types of silicones. Hence this paragraph only focusses on the toxicology and risk assessment of siloxane oligomers; other migrants (e.g. catalysts, additives) are not covered here.
In 2015, three linear siloxane oligomers (octamethyltri-, decamethyltetra- and dodecamethylpentasiloxane) were listed on the Community Rolling Action Plan (CoRAP) of the European Chemicals Agency (ECHA) due to their potential bioaccumulative, persistent and toxic properties. In 2013, hexamethyldisiloxane was already added to CoRAP based on its proposed carcinogenic, mutagenic and reprotoxic properties. The evaluations of all four linear siloxanes are currently ongoing. In 2015, Health Canada and Environment Canada concluded in a screening assessment that only limited empirical health effect data was available for hexamethyldi-, octamethyltri- and decamethyltetrasiloxane, but effects on the liver, kidney and lung, and on body weight gain have been reported.
In 2007, several peer-reviewed studies investigated endocrine disrupting effects and reproductive toxicity of the cyclic siloxane octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5). D4 was described to be a weak estrogen and to cause impaired fertility in rats after exposure to 700 ppm by whole-body vapor inhalation. No reproductive toxicity was reported for D5 from one oral and two inhalation studies in rats, but a two-year toxicity study resulted in a significant increase of uterine tumors in rats after exposure to 160 ppm of D5. Further organs effected by D4 and D5 include the liver, lung, adrenal, thymus and kidney. In the EU, D4 is labelled as being toxic to fertility (category III). D4 and D5 were both judged to be very persistent and very bioaccumulative (vPvB) by the ECHA PBT expert group. In addition, D4 is also persistent, bioaccumulative and toxic (PBT). D4 is on the Danish list of PBT and vPvB substances and shall be phased out in Sweden. In 2008 Environment Canada and Health Canada stated that D4, D5 and dodecamethylcyclohexasiloxane (D6) may have an immediate or long term harmful effect on the environment or its biological diversity, but not on human health.
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