Is Silicone Biodegradable?

No, silicone isn't biodegradable or compostable—at least not in the span of a normal human lifetime—but it is often touted as a healthier and more eco-friendly choice than conventional plastics, which is only partially true.

Silicone leaches fewer potentially toxic chemicals into food and drinks, which is why it's often used for food storage and cooking. It can also be reused many more times than typical plastic, particularly single-use products, which makes it less wasteful.

Overall, silicone can be a more environmentally friendly choice if it's being used instead of a disposable plastic item. However, due to its low level of recyclability, its non-biodegradability, and its possible health effects, it's not quite as safe or "green" a choice as glass, cloth, or waxed cloth bags or wraps, or stainless steel, which are easily recyclable (metal and glass), or biodegradable (cloth).

What Is Silicone?

Silicone is often called a rubber, but it isn't one, though it is rubber-like. It's technically an elastomer. Silicone is made of rearranged silicon and oxygen (like sand), but unlike sand, it also has the addition of hydrocarbons—which is exactly what gives it all those useful qualities of a plastic. Some people say that because it's based on silica, silicone is as safe as sand, but others are concerned that there are still substances that get into foods from silicone, especially as it's used in cookware, when it's heated to high temperatures.

Silicones are different from silica, which are also different from silicon. It's important to know the difference between them.

Scientifically speaking, silicone is the name for a large group of similar compounds, so there are many different kinds of silicones. They all share a main chain of alternating silicon and oxygen atoms.

Silicones are different from silica, which is one of the most common substances on Earth, found in all kinds of rock. Silica is what both quartz and most beach sand are made of. In contrast, silicon is an element you can find on the periodic table. It isn't found on its own in the natural world, but has to be created in lab. It is well-known for being the semiconductor in computer chips.

To make the silicone baking sheet in your kitchen cupboard, silica (SiO2) is heated to very high temperatures, which separates the elemental silicon atoms from the oxygen it was bonded to. What's left is silicon (just Si). That's then mixed with hydrocarbons, usually derived from fossil fuels, to create a monomer, which is then bonded into a polymer. Depending on how pure that process is, so is the quality of the silicone that you get at the end.

Is Silicone a Healthy Choice?

As we have covered on Treehugger, "Silicone is widely accepted as safe by organizations such as Health Canada and the U.S. Food and Drug Administration." Health Canada has advised that there aren't any known health hazards associated with silicone cookware and that "silicone rubber does not react with food or beverages, or produce any hazardous fumes."

However, there haven't been many studies done on silicone. One study on food additives and contaminants did show that siloxanes can leach into food, especially fattier foods with prolonged exposure and mostly at higher temperatures, above 300 degrees F. Further research has supported this finding, showing that certain types of silicone leach siloxanes into oily food. Whether those siloxanes have health effects is still being debated, and is dependent on the quality of the silicone, so it's also difficult to make a blanket statement that applies to all kinds of silicone (as some are manufactured to be more pure than others).

The authors of "Life Without Plastic: The Practical Step-By-Step Guide to Avoiding Plastic to Keep Your Family and the Planet Healthy" raise some concerns about silicone. They cite studies that reveal leaching, and point out that siloxanes (the backbone chemical of silicones) are known endocrine and fertility disruptors and possibly carcinogenic.

They write: "One study tested the release of siloxanes from silicone nipples and bakeware into milk, baby formula and a simulant solution of alcohol and water. Nothing was released into the milk or formula after six hours, but after 72 hours in the alcohol solution several siloxanes were detected."

More cautious bakers and home cooks might want to consider other types of bakeware, since plenty of time-tested alternatives exist. There's less concern about using silicone at lower temperatures and for shorter durations—like a spatula, nipples for a baby bottle, seals for bottles, or in any application where they don't come into contact with fatty or very hot foods for long periods of time.

Silicone's Main Qualities

Silicone has the advantages of its plastic cousins in that it can be shaped into a wide variety of molds of various sizes. It can be soft or hard, and tends to have a characteristic bounce and feel. It's flexible, malleable, can be translucent or take light or dark colors, it isn't affected by UV rays, and is almost waterproof. The fact that it is gas-permeable means that it's especially useful in medical equipment.

The fact that it's nonreactive is why silicone is used for breast implants, medical tubing, and menstrual cups. It's also used in personal care products and for construction sealants, like bathroom tile grout.

It's unique in that it's much more heat-resistant than most plastics. That property, along with being nonstick and easy to clean, mean that it's very popular for kitchen equipment in addition to the uses above.

Is Silicone an Eco-Friendly Choice?

It depends. If you are using silicone in place of a thin disposable plastic (like a sandwich bag) that can't be easily recycled, it's a better choice, since it can be reused many more times. Also, plastics break down into microplastics, which end up in our soil and water supplies, making their way into the ocean and into the bodies of the animals we eat, as well as human bodies. Silicones don't break down in this way and don't shed microplastics.

Still, a glass container that can be recycled easily at end of life, or a biodegradable paper bag, or fabric or waxed fabric (which can both biodegrade), are all better choices.

When it comes to a harder plastic container for food storage, you're probably best off using glass, especially for anything hot, or a no. 1 (PET) or no. 2 (HDPE) plastic container for room temp or colder stuff, both of which are more easily recycled than silicone.

For bakeware, stick to glass, ceramic, stainless steel, or iron bakeware for both environmental sustainability and health reasons. Glass and stainless steel are both recyclable (the steel won't be accepted by most curbside programs, but is acceptable via scrap metal collections) and ceramics will biodegrade, as will iron eventually—though it will take a long time.

Biodegradable Compounds

Silicone, like other human-created compounds, doesn't biodegrade because it is a very new material. It hasn't existed long enough for natural processes that break down other materials—such as yeasts, bacteria, fungi, and enzymes—to evolve to degrade them. So, like plastics, discarded silicones will sit around in a landfill, breaking into pieces over time, but not fully degrading into constituent parts that can be used again by a growing organism. We do not know how long it would take silicone to fully degrade.

Some research points to a silicone-containing polyurethane made from bio-based materials. Those materials could allow already existing bacteria to be able to digest it. So, while at this point silicone isn't a biodegradable material, if it were made from different constituent materials, there's a possibility it could be. But that's still in the future.

Can Silicone Be Recycled? 

Silicone can't be recycled at curbside recycling pick up in any U.S.-based program. That's an immediate red flag from an environmental perspective. But silicone can be recycled by some specialty recyclers, so you could get together with friends and send silicone bakeware or other items into TerraCycle's Kitchen Zero Waste Box. You might also ask around if your town or city has special recycling days where they accept materials that wouldn't get recycled curbside; sometimes these programs accept used silicone bakeware or construction materials. Usually when recycled, however, silicone is simply turned into industrial lubricant oil.

How to Reuse Silicone

While silicone isn't easily recyclable, there are a few ways to upcycle it.

Old silicone can be repurposed at home following a few steps. First you cut or grind the silicone up and then you add more fresh silicone, which can be purchased in a powder or liquid form. Knowing how much of the new silicone to mix depends on the type of silicone you are trying to recycle. Then, the silicone needs to be set in a mold and cured. The old silicone is basically a filler of sorts to bulk up the new item.

Silicone can also be used as a playground mulch, by grating it and spreading it on the ground beneath play equipment. Another way to repurpose it is simply to cut it up; an old silicone baking mat could be cut into pieces that could work as oven-mitt like hand coverings, or trivets to keep hot dishes off countertops. Silicone mats could be useful around a fireplace to keep sparks from hitting the floor, or could be used for storage of dirty items like gardening equipment, since silicone rinses off easily.

In conclusion, silicone is far from being a silver bullet solution to the plastics problem. Like so many other things, it's complicated. If you're hooked on using single-use plastic sandwich bags, then switching to a washable, zippered silicone pouch would definitely be an improvement. But for many other uses, like baking and food storage, there are better (if less exciting) options out there—the same old metal baking sheets and muffin tins and cake pans that your parents' generation used. They may be less colorful, shiny, and flexible, but they do the job well.