Chemical resistance of silicone: Which media cannot be used with silicone

When selecting sealing material, the first question is whether it can withstand this medium. The chemical resistance of silicone has its own strengths and blind spots - it is not a universal corrosion-resistant material and can quickly fail in the wrong scenario. This article explains clearly what silicone is resistant to and afraid of.

The strengths of silicone

Water resistance and steam resistance - Long term exposure to hot water and steam (disinfection conditions) results in stable performance, which is the advantage of silicone over many rubbers. EPDM is also water-resistant, but its temperature resistance is not as good as silicone.

Resistant to weak acids and bases - dilute hydrochloric acid, dilute sulfuric acid, weak base solutions, and silica gel has almost no reaction at room temperature.

Resistant to ozone and climate aging - the saturated molecular chains of silicone do not react with ozone, which is why silicone is used for outdoor and corona environments (insulation components).

Resistant to vegetable and animal oils - the foundation for establishing food contact scenarios (baking molds, food machinery seals).

High temperature resistant mineral oil - can be used for a short time, but it is recommended to choose fluorosilicone gel for long-term exposure to high temperature oil.

The blind spots of silicone

Long term immersion in strong acids and alkalis - concentrated sulfuric acid, concentrated nitric acid, and strong alkalis can cause degradation of silica gel in high-temperature environments.

Petroleum based mineral oil and fuel - limited impact at room temperature, but significant swelling of silica gel (volume expansion, mechanical decrease) in high-temperature fuel environment. The sealing of fuel and lubricating oil circuits is the domain of fluorosilicone (FVMQ) and fluororubber (FKM).

Aromatic solvents such as toluene and xylene have strong swelling properties on silica gel.

Some hydraulic oils and lubricants - oils containing extreme pressure additives (including sulfur and phosphorus compounds) in the formula can corrode silicone, and sealing failure cases of steam traps and certain industrial equipment are common in these media.

Strong oxidizing agents - high concentration hydrogen peroxide, sodium hypochlorite, long-term contact, silica gel surface pulverization.

Steam itself is not a problem, but physical abrasion should be taken into account in the flushing conditions of hot water and high-speed fluid (spray, cavitation) - the wear resistance of silicone is generally average, and the abrasion environment should consider polyurethane or other materials.

Method for judging material selection

Step 1: Clarify the true composition of the medium. The customer says' oil resistant '- what kind of oil? Mineral oil, synthetic ester, silicone oil, and vegetable oil are completely different tests for materials. Synthetic ester lubricants (POE, PAG) are relatively mild to silicone, while mineral oils depend on temperature. If unsure, ask the customer to provide a Safety Data Sheet (SDS) for the medium.

Step 2: Immersion test. Standard practice: Soak the standard test piece of vulcanized silicone in the target medium (common conditions: 23 ℃× 72h or 100 ℃× 24h, simulate according to the working conditions), and measure the volume change rate, hardness change, and tensile retention before and after soaking.

Judgment reference (volume change): Change rate within ± 5% - good compatibility; +5%~+15% - slight swelling, acceptable for most sealing applications but subject to long-term evaluation; Exceeding+15% or significant negative value - incompatible, change material.

Step 3: Overlay of working conditions. The combination of medium, temperature, pressure, and dynamic motion is the true working condition. Static seals have a high tolerance for swelling (swelling actually compensates for permanent compression deformation), while dynamic seals may experience increased friction and accelerated wear after swelling, and dynamic components with the same medium may not be usable.

List of alternative materials

In situations where silicone is not suitable, conventional alternatives include: Fuel, high-temperature oil circuit → Fluorosilicone (FVMQ), Fluororubber (FKM) Strong acid strong alkali, strong oxidation → PTFE, EPDM (weak acid weak alkali), perfluororubber (FFKM, expensive) High pressure steam+amine corrosion inhibitor (power plant) → EPDM Wear resistant dynamic seal → Polyurethane (PU), filled with PTFE

A reminder for procurement and design

In the analysis of seal failure, "incompatible medium" ranks among the top three causes of failure. And the commonality of these cases is that only vague words such as "oil resistant" and "corrosion resistant" were written when selecting materials, and no one checked the specific composition of the medium. Write specific information about the medium (product name, concentration, temperature) and have the supplier perform immersion verification - the cost of this step is several hundred yuan in testing fees, saving hundreds of thousands of yuan in production line downtime.