Eco-conscious consumers are so overwhelmed and confused that literally any advertisement with a capital PLASTIC-FREE statement sounds like “yes, that’s what we need!” But is that really plastic free? And anyways, is that the most sustainable choice

One common misconception is silicone. Silicone products can now be found everywhere, from bags and cookware (baking sheets, muffin trays, bottle stoppers) to baby and toddler products, including bottle teats, baby soothers and drinking cups. Silicone is ideal for kitchen use as it can be exposed to high or freezing temperatures. It is also an excellent water and oil repellent, insoluble in water, mineral oil or alcohol.

As a material scientist, I have chills every time I see a company advertising silicone as a “PLASTIC-FREE” solution. So, I decided I’d dig in for more information and make a definitive post about it. The goal is to provide an overview of whether the use of silicone in contact with food may be of concern for human health or the environment. It gets a bit technical. Bear with me, it won’t take too long!

First, the plastic definition

OECD defines plastic as a polymeric material not naturally occurring in the environment. That means: every time a polymeric material is manmade, it’s considered plastic. This also includes when feedstock (or raw material) comes from nature like sugar cane, corn, seaweed or sand.

Remember that post about bioplastics?

Second, the silicone composition

Silicone is highly versatile and can come in a variety of forms: oil, grease, lubricants, adhesives, rubber, resin, etc. I want to focus on the resin rubber versions of it, as those are the ones we often see in bags, straws, cups and food packaging, in general. Its exact composition is somewhat tricky, as it could be presented in different textures, thicknesses, shapes and applications. For this reason, I’ll keep it simple and be generic here. 

Silicone is also called polysiloxane. If you hear polysiloxane, know it is silicone.

Put in a very simple way, silicone is made of silicon, oxygen, carbon and hydrogen. Silicon is commonly bound to oxygen, and found on the beaches, in the form of sand. Carbon and hydrogen come from petroleum-based substances (it can be as simple as a vinyl or as complicated as a phenyl or a perfluoropropyl). The chemical reaction is also done in solvents like hexane, toluene and styrene (yep, all from oil).

The ratio of silicon to petroleum-based ingredients is hard to estimate but silicone is FAR from being made from sand only. 

Depending on the silicone structure complexity, it could also have another substance called side groups (or side chains), which could be other hydrocarbon (H and C) substances. The picture below is an attempt to simplify the silicone structure:

And then there’s the vulcanization process, when silicone reacts with sulfur or other curative agents to become more durable. A popular curative agent is Platinum, which is considered safer in food packaging than other options.

The several reactions of those components to make silicone is a typical industrial manufacturing process that uses petroleum and natural gas.

Bottomline: Silicone is a type of plastic made from a mix of bio-based and petroleum-based feedstock.

Third, is silicone safe?

There is limited data about silicone safety in food contact materials. An article published in 2015 by The Food Packaging Forum summarized certain silicone constituents leached into food, leading to potential exposures via ingestion. I put together below the observations that caught my eye, but feel free to screen over the references if you want to learn more about how the experiments were conducted:

• Baking molds: volatile siloxane compounds in the air during/after baking in silicone molds. When compared to metal, silicone molds emit higher levels of those particles into indoor air. Those substances were found to impair fertility and change hormone levels in animal toxicity studies. The same substances are considered as a very high concern by the European regulation REACH. Silicone baking molds can also lose substantial amounts of their mass during use at high temperatures (>0.5%). (Reference: Dossier Silicone)
• Baby bottles: a study with 5 different silicone baby bottles found migration of 31 non-silicone substances, like phthalates, benzophenone, naphthalene, and aldehydes. Those substances are most likely coming from additives or machine contamination during production. The level of migration is greater than with other common plastics, like polypropylene. (Reference: Simoneau, 2012)

As a matter of fact, the European Chemical Agency (ECHA) decided last year to add 3 silicone chemical intermediates D4, D5 and D6 to the REACH candidate list as substances of very high concern (SVHCs). Those are all cyclic silicones (cyclosiloxanes); you can find more about them and where they can be found here.

And lastly, what happens at its end of life?

Silicone products are extremely durable and long-lasting, and they should last a lot longer than other plastics, offsetting the environmental burden involved in its manufacturing. Great, right? That’s the reason why we see it in so many zero waste stores, it’s a good single-use replacement.

What happens when a silicone item is damaged, or you no longer need it? It gets a bit more complicated because silicone is not compostable – actually takes longer to break them down than other plastics – or curbside recyclable. Most likely, a silicone item will end up in the landfill or incinerator.

Most recently, some companies have been investigating chemical recycling of silicone, but it’s still in its infancy and there’s no collection schemes for consumers globally.

• So next time you see brands advertising “plastic free items made of silicone”, please speak up. It doesn’t mean the products are bad, but the company is definitely marketing a lie.
• If you’re using (or selling) silicone products for food or skin contact, reach out to the brand-owner and ask if it contains any of the substances listed above (cyclosiloxanes). Those are relatively new additions to the European regulation (REACH) and might still be around under the US regulation (FDA).