Consumers often report a "rubber smell" from newly purchased silicone kitchen utensils, pacifiers, or phone cases. Although non-toxic, this odor can detract from the user experience. In medical applications, automotive interiors, and high-end electronics, being "odorless" is a strict requirement for market entry. So, where does the odor of silicone rubber come from? Does it indicate harmful substances? How can we achieve true "low odor" or even "odor-free"? Behind these questions lies an intricate engineering effort ranging from raw material purification to advancements in vulcanization processes.

I. Main Sources of Odor

Highly chemically inert polydimethylsiloxane (PDMS), the base material of silicone rubber, is almost odorless. The so-called "rubber smell" mainly comes from residual small molecules and byproducts:

Unreacted Monomers and Cyclic Oligomers

D3, D4, D5, and other cyclic siloxanes are difficult to completely remove during polymerization.

D4, listed as a Substances of Very High Concern (SVHC) by the EU, has a faint gasoline-like odor and can volatilize at high temperatures, contributing significantly to the "new product odor."

Byproducts from Vulcanization Systems

Peroxide vulcanization (e.g., DCP, 2,4-DTBP):

Produces acetone, acetophenone, tert-butanol, etc., with strong odors and residues reaching hundreds of ppm.

Condensation-type room temperature vulcanization (RTV):

Releases acetic acid (acidic odor), oximes (ammonia-like odor), or alcohols, which are irritating.

Additives and Impurities

Fillers like silica adsorb moisture, releasing water vapor carrying odors when heated.

Colorants, lubricants, antistatic agents, and other additives may also volatilize.

Residual metal catalysts (e.g., tin) from raw materials can catalyze side reactions.

II. Paths to Achieving Low Odor

Adopting Addition-Cure Platinum Vulcanization Systems

This system involves vinyl and Si-H addition without producing small molecule byproducts.

It results in significantly lower odors compared to peroxide systems and is preferred for medical and food-grade applications.

Strict control over inhibitors (such as acetylene alcohol) is necessary to prevent unpleasant odors at high temperatures.

High Vacuum Devolatilization and Post-Treatment

Raw rubber undergoes thin-film evaporation or multi-stage vacuum extraction (<1 mbar, 150–200°C).

Over 90% of D4/D5 and low molecular weight compounds are removed.

Total volatiles (TV) <0.5%, approaching odorlessness.

High-Purity Raw Materials

Use of high-purity D4 monomers (>99.99% purity).

Hydrophobic treatment of silica reduces hydroxyl content.

Platinum catalysts supported on carriers reduce free metal ions.

Optimized Formulations

Avoid using low molecular weight plasticizers.

Select high-boiling point, low-VOC additives.

Adding odor-masking agents (like cyclodextrin inclusion complexes) offers a temporary solution but isn't a fundamental fix.

III. Odor Evaluation Methods

Sensory Testing (Sniff Test)

Trained panels evaluate samples in a 60°C oven on a scale of 0–5 (0 = odorless, 5 = strongly uncomfortable).

GC-MS (Gas Chromatography-Mass Spectrometry)

Quantitatively analyzes volatile organic compounds (VOCs) such as D4, acetone, and aromatic compounds.

VDA 270 (German Automotive Standard)

Automotive interior materials must have an odor rating ≤3 to be used.

FDA/LFGB Sensory Requirements

Extracts should not impart detectable odors to food.

IV. Application Scenario Grading Requirements

表格

Application Field Odor Requirement     Technical Solution

General Kitchenware  No strong odor, disappears after boiling      Peroxide vulcanization + post-baking

Baby Pacifiers      Odorless (LFGB sensory approved) Platinum vulcanization + high-vacuum devolatilization

Automotive Interiors  VDA 270 ≤3 Low-cyclic LSR + solvent-free formulations

Implantable Medical Devices    TV<0.1%, GC-MS undetectable Medical-grade platinum rubber + ultra-pure raw materials

V. Industry Trends: From "Low Odor" to "Zero Perception"

Closed-Loop Production Processes

Reduce exposure of materials to air to minimize oxidative byproducts.

Bio-Based Siloxanes

Exploring glycerol routes for monomer synthesis to reduce impurities from the source.

Online Monitoring

Near-infrared (NIR) real-time monitoring of volatile content ensures batch consistency.

Conclusion

The "odor issue" of silicone rubber is not just about sensory experience; it reflects the purity of materials and process control. From kitchens to operating rooms, from cockpits to infants' mouths, every promise of "odorlessness" represents a silent commitment to health and quality. When technology can refine industrial materials to near-natural purity, we truly understand that the highest level of safety often lies in the subtlest breaths.



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