Why transparent silicone parts turn yellow and foggy: the problem usually does not lie with the silicone itself

Transparent silicone parts are the most controversial category among all silicone products. The point of controversy is not about whether transparency can be achieved, but about to what extent transparency is considered qualified - because transparency can be judged by the naked eye, but when there are no limits on samples and judgment methods, what both parties say about "transparency" is often not the same thing. This article discusses the factors that affect transparency and common causes of yellowing and fogging separately.

What is the key to achieving transparency

Silicone itself is amorphous and theoretically transparent. The key to making transparent parts lies in the filler: it must use gas-phase white carbon black with sufficiently small particle size (nanometer level) and surface treatment to approach the refractive index of the base polymer. The products made from ordinary precipitation method white carbon black are semi transparent (commonly referred to as "semi transparent" in the industry).

So the answer to 'can we achieve full transparency?' is basically yes, but the cost has increased significantly, and the requirements for subsequent processes such as rubber mixing, filtration, molding, and vulcanization have also skyrocketed.

Yellowing: mostly due to thermal and historical issues

The vulcanization temperature is too high or the time is too long. Under high temperature, the silica gel molecular chain oxidizes to generate chromophores. Insufficient sulfur may also cause yellowing - residual decomposition products of sulfurizing agents remain inside.

Excessive sulfurization after the second stage. The post sulfurization of the main driving compound, if the temperature or time exceeds the standard, will actually cause the surface to oxidize and turn yellow. This is the reason why 'the more you dry, the more yellow you become'.

Long term exposure to ultraviolet radiation. Transparent parts exposed to outdoor or long-term light will gradually turn yellow, which is a common problem with most polymer materials. Silicone is relatively good, but not immune.

Exposure to pollution. Oil stains, sweat stains, and certain packaging materials can cause the surface of transparent parts to turn yellow, which can usually be wiped off with solvents.

Additives in the formula. Some anti-aging agents and inhibitors of certain platinum systems have color or may change color over time as by-products.

Fogging: mostly due to process and interface issues

Poor dispersion of fillers. Insufficient mixing time or insufficient shearing can cause white carbon black to aggregate and form scattering centers, giving the naked eye a foggy sensation.

Damp. White carbon black is highly hygroscopic. If the humidity in the rubber mixing workshop is not controlled properly, the moisture in the rubber material will form small bubbles or water vapor marks during high-temperature vulcanization, resulting in white mist. This type of fog is usually uniform throughout the entire area, rather than appearing only locally.

Surface condition of the mold. Whether the mold is mirrored, sandblasted, or treated with other methods, it will be written onto the surface of the product on a one-to-one basis. There are finger prints, oxidation, and residual adhesive on the mold, and the product is foggy.

Residual release agent. Some internal release agents can cause surface fogging, and a balance needs to be struck between maintaining demolding properties and transparency.

Impurities in the rubber material. The filtration is not enough, and the particles, fibers, and gel particles mixed in can be seen at a glance in the transparent pieces, so they need to be filtered strictly according to the mesh number.

Fluctuations in sulfurization parameters. Excessive temperature can form micro bubbles on the surface, presenting a fine mist.

How to evaluate with a basis

There are two commonly used indicators for transparency: transmittance and haze. Transparency measures how much light can pass through, while haze measures how much light has been scattered and blurred. The two are independent - some materials have high light transmittance but also high haze, making them appear hazy.

A practical approach in actual projects is to do three things at the same time: agree on the testing method, agree on the qualified line, and make limit samples. When there are no limited samples, any subjective judgment that is not very transparent cannot be judged.

To make transparency requirements feasible

Choose the right raw material system: transparent or semi transparent, and clarify whether platinum system is required (better yellowing control) first.

Control the rubber mixing environment: humidity and cleanliness are the lifeline of transparent parts.

Rubber material filtration: Clear by mesh, the process results are directly reflected in black spots and impurities.

The polishing grade of the mold is clear: if you cannot achieve a mirror finish, do not require optical grade transparency.

The vulcanization parameters should be stable and recorded: fluctuations within the same batch should be traceable.

Packaging and transportation should also be managed: transparent parts are most prone to scratches and contamination during the packaging process, and should be packaged independently to avoid friction with each other.

A reminder

For the project of transparent silicone parts, spending half an hour in the early stage to clarify what is considered qualified can save a lot of return disputes in the later stage. The suggestion given is to make several samples representing "acceptable" and "obviously not acceptable" during the sampling stage, and both parties should sign and keep them on file. This action is more effective than any written description.