Food-Grade Silicone: What the Certification Actually Covers

What food grade actually means

There is no such thing as a food-grade polymer. There are food-contact-compliant formulations, and compliance is a property of the finished article under a specific regulation, for a specific food type, at a specific temperature and time.

For silicone, the base polymer is essentially inert. What determines compliance is everything else: the filler, the curative and its residues, the pigment, the process aids, and the post-cure that drives out volatiles. A compliant base with a non-compliant pigment is a non-compliant part.

This is why the certificate matters more than the datasheet. A supplier who says 'food grade' without naming the regulation, the test conditions and the migration result is describing an intention, not a product.

FDA 21 CFR 177.2600

The American route regulates the substance. 21 CFR 177.2600 lists the permitted rubber articles intended for repeated use, and specifies which base polymers, fillers and additives may be used, at what levels, and for what food types.

The regulation is built on extraction tests rather than migration into food. A sample is extracted with distilled water for 7 hours at reflux, then with n-hexane for 2 hours at reflux, and the extractables are weighed. The limits are 20 mg per square inch for the aqueous extract and 175 mg per square inch for the hexane extract on the first test, with lower limits on subsequent tests.

Those numbers look generous, and they are. The regulation is a screening tool for repeated-use articles, not a simulation of a specific food contact. It also assumes the article is formulated only from listed substances — a novel additive makes the whole article non-compliant even if the extraction result is low.

EU 10/2011: material and migration

The European route regulates the finished article by what it releases. Regulation 10/2011 sets an overall migration limit of 10 mg/dm² or 60 mg/kg of food, and specific migration limits for individual substances.

Silicones are covered as a plastic material under 10/2011 when they are the food contact layer, and the testing uses food simulants rather than food itself: water, 3 percent acetic acid, 10 percent ethanol, olive oil, 95 percent ethanol and iso-octane, each at a temperature and time that represents the intended use.

The overall limit of 10 mg/dm² is much stricter than the FDA extraction numbers and it is the one that most silicone food articles fail on first test. The usual cause is insufficient post-cure: volatile siloxanes that were never driven out migrate into the simulant.

Table 1 — EU 10/2011 simulants and conditions. The limits are expressed per area or per kilogram of food depending on the pack geometry.

What the test actually measures

Overall migration is a gravimetric test: the simulant is evaporated and the residue is weighed. It does not identify what migrated, only how much. A part can pass overall migration and still fail a specific migration limit for a particular siloxane or additive.

For silicone, the substances that matter are the cyclic siloxanes — D4, D5, D6 — and any residual catalyst or peroxide fragments. D4 and D5 have specific migration limits in the EU and are the usual reason a silicone article fails after passing the overall test.

Sensory testing is a third requirement that is often forgotten. A silicone part can be fully compliant and still impart a taste or odour to the food, and that is a commercial failure even when it is not a legal one. Post-cure is the control.

Post-cure is the compliance step

A press-cured silicone part contains volatiles: unreacted oligomers, peroxide fragments if a peroxide cure was used, and filler treatment residues. Post-curing at 200 °C for 2 to 4 hours drives most of them out.

The effect on migration is large. An un-post-cured part can show an overall migration three to five times higher than the same part after post-cure. This is why a compliant formulation can still produce a failing part, and why the certificate must specify the post-cure condition, not just the compound.

Platinum-cure compounds need post-cure as much as peroxide-cure ones, though for different reasons. Platinum cure leaves fewer acidic fragments but still contains volatile siloxanes from the filler treatment.

Colourants, additives and the things that break compliance

Table 2 — The six ways a food-contact silicone part loses compliance after it has been qualified.

The most common is a pigment change. A moulder substitutes a colour masterbatch from a different supplier because the original is out of stock, and the new one contains a pigment that is not listed for food contact. The part looks identical and the compliance is gone.

The second most common is a change in post-cure. A part that was qualified with a four-hour post-cure is shipped with a two-hour post-cure because the oven was needed for another job. The dimensions are the same and the migration is double.

What to ask for

Ask for four documents and check that they match the part you are buying. A Declaration of Compliance naming the regulation and the food contact conditions. A migration test report from an accredited laboratory, on the finished article, not on a test sheet. A formulation statement listing the pigments and additives. And a process specification that states the post-cure temperature and time.

If any of the four is missing, the supplier is offering a material rather than a part. That distinction is the whole point of food contact regulation.