Silicone Pinch Test: when you pinch, twist, or pull a silicone product if it turns “white”, then it contains fillers and is therefore not safe or a good quality silicone.
It is a common misconception that the presence of white fillers in silicone kitchenware indicates that the silicone is not safe or of good quality. However, this is not necessarily true. Silicone does have fillers, but the fillers≠bad quality. To figure it out, first, we need to know how silicone is made.
1. What is silicone? how silicone is made?
In another post: What Is Silicone? & How Is Silicone Made? we discussed the origin of silicone. The silicone manufacturer giant Shin-Etsu Silicone has an explanation of what silicone is made of as below :
In short, silica is not used directly in the production of silicone, but it is a source of silicon that can be extracted and purified to create the silicon that is used in silicone production. Silica, also known as silicon dioxide (SiO2), is a chemical compound that is made up of silicon and oxygen.
Silicone rubbers often possess poor mechanical properties thermal, and electrical properties and must be reinforced with fillers. To be effective, the reinforced fillers must have inherently high mechanical properties and create a strong interaction with the silicone rubber. Reinforcement of silicone enhances properties, for example, tear strength, tensile strength, hardness, and abrasion resistance. Silica is one of the most common fillers for silicone reinforcement. Fumed silica and precipitated silica are two forms of silica that are produced through different manufacturing processes. They get their name from how they are produced.
2. Precipitated Silica VS Fumed Silica
a. Method of production
Precipitated Silica is produced by reacting a silicon-containing compound with an acid to form a precipitate of silica, which is then collected, washed, dried, and ground to produce a fine, white powder. The idealized equation involving a trisilicate and sulfuric acid is:
Fumed silica is produced by heating a silicon-containing compound to high temperatures in an oxygen-free environment, which breaks down the compound into its individual elements releasing silicon, oxygen, and other gases.The silicon reacts with the oxygen in the air to create a fine, white powder that consists of microscopic particles of silica (SiO2).The reaction equation is as follows:
b. Difference Between Precipitated Silica and Fumed Silica
One of the main differences between fumed silica and precipitated silica is their particle size. Fumed silica is made up of extremely small particles that are less than 100 nanometers in diameter, while precipitated silica is made up of larger particles that are typically in the range of 10 to 50 micrometers in diameter. This difference in particle size gives fumed silica and precipitated silica different physical and chemical properties, which can make them more suitable for different applications.
Another difference between fumed silica and precipitated silica is their purity. Fumed silica is typically composed of 99.8% silica, while precipitated silica is typically composed of around 90% silica. Fumed silica also has a low moisture content, while precipitated silica has a high moisture content.
The last difference between fumed silica and precipitated silica is the cost. Fumed silica is more expensive than precipitated silica due to the differences in their production processes and the resulting properties of the silicone. In the rubber industry, fumed silicas are limited due to expense, mainly used as reinforcing fillers for silicone rubbers. As silicone rubber fillers, precipitated silica is an alternative to fumed silica in silicone rubber compounds by reducing cost and process in efficiencies for silicone rubber compounders.
3. The different impacts of Precipitated Silica and Fumed Silica on silicone
Precipitated silica and fumed silica can both be used as reinforcement fillers for silicone to improve its strength and durability. However, they have different impacts on the properties of silicone, due to the differences in their composition and physical properties.
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Silicone with Fumed silica as filler has much better tensile strength, abrasion resistance, and Elongation than Precipitated silica.
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Silicone with fumed silica as a reinforcement filler can have higher transparency. Fumed silica can produce translucent or even clear, non-yellowing silicone with minimal aging effect. silicone with precipitated silica as a reinforcement filler may have low transparency.
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Silicone with precipitated silica as filler will turn white when stretched or twisted, but fume silica won’t.
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Silicone with fumed silica as filler will have better quality than precipitated silica.
Why silicone with precipitated silica as the filler will turn white when pinched or twisted but fumed silica won’t?
Silicone products that are produced with precipitated silica as a filler will turn white when they are stretched or deformed. This is known as strain whitening, and it is a normal phenomenon that occurs when the outer layer of the silicone becomes strained and becomes more transparent, resulting in a white appearance. This is different from fumed silica, which does not typically cause strain whitening when used as a filler in silicone.
The reason for this difference is that precipitated silica has a relatively low surface area and a narrow range of particle sizes, which can make it more susceptible to strain whitening. Fumed silica, on the other hand, has a very high surface area and a wide range of particle sizes, which can help to prevent strain whitening.
Overall, strain whitening is not a problem with silicone products that are produced with fumed silica as a filler. However, it is something to be aware of when using silicone products that are produced with precipitated silica, as they may turn white when stretched or deformed.
4. Misconceptions on silicone pinch test
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One common misconception about the silicone pinch test is that if silicone turns white, it is of bad quality that is unsafe.
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Use pinch test to tell if the silicone is food-grade or not. If the silicone turns white, it is not food-grade.
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Take pinch test as a way to tell “food grade” or“medical grade”.
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Misleading that if silicone changes to a whiteish color, then it most likely has plastic fillers.
Please note that “Turn white≠ Not safe”. FDA food-grade silicone will turn white, but it does comply with FDA standards. You can’t tell if the silicone is safe or food-grade by looking at the silicone. The only way to verify is to simply ask for the testing result document with the picture of the product tested. There are third-party organizations such as the FDA or NSF International that have established safety standards for materials that come into contact with food, and they will issue a certification if a silicone product meets their standards.
There are professional companies that provide inspection, verification, testing, and certification services. Such as SGS,Bureau Veritas, Intertek, and TÜV SÜD. Some silicones can be certified for both medical and food uses depending on the testing they have been subjected to. Like there are different tests for “medical” certifications, and there are also different tests, and levels of certification, for food applications. The two main tests for silicone are FDA & LFGB testing.
Conclusion
The silicone pinch test is not an official method for determining compliance with FDA standards, and it should not be relied upon as the sole basis for determining the safety of a silicone product. Therefore, the best way to determine the safety of silicone kitchenware is to look for products that are certified by a reputable agency, such as the FDA, rather than relying on the presence or absence of white fillers. If you do worry about the safety of silicone products, you can look for products with LFGB test reports. It is a more strict standard than FDA. Products in LFGB food-grade silicone won’t turn white, and of course, the cost will be higher.
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