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Automotive Interior Odors: An Unavoidable Challenge for Material Engineers 2025-07-10


Odor is both a sensory experience and a perceived measure of quality.


In the confined space of a car cabin, the "new car smell" from plastics is not a symbol of luxury—instead, it often becomes a major source of consumer complaints.


This article, grounded in engineering practice, systematically explores the sources, mechanisms, analytical methods, and controllable strategies related to odor. It aims to support material engineers in reducing odor risks at the source during the design of automotive interior materials.



Where Does the Odor in Plastics Come From?
Odorous molecules in plastic materials primarily exist in the form of volatile organic compounds (VOCs), which are released into the air through three main mechanisms:

1. Diffusion: Unreacted monomers and small molecules migrate from the interior of the material to its surface. VOCs in plastics follow Fick’s second law of diffusion.
For example, in polypropylene (PP), the diffusion coefficient of aldehydes is approximately 10⁻⁹ cm²/s. At 23°C, it can take up to 48 hours to reach equilibrium surface concentration. However, when the temperature rises to 60°C—comparable to summer cabin temperatures—the diffusion rate can increase by 3 to 5 times.

2. Desorption: VOC molecules that are adsorbed on the surface of the material are released into the surrounding air.

3. Migration: VOCs can also migrate from additives such as plasticizers, lubricants, or residual solvents.



How the Human Nose Works: From Molecules to the Brain



When volatile molecules released from plastics—such as nonanal and decanal—enter the nasal cavity, a highly precise recognition process begins at the microscale. Within the olfactory epithelium (approximately 5 cm²), about 350 types of olfactory receptor proteins are densely distributed. These receptors function like “molecular locks,” each specifically recognizing certain odorant “keys.”

Take (E)-2-nonenal as an example: its hydrocarbon chain structure binds to the olfactory receptor OR51E2 with a binding energy of -8.7 kcal/mol. This specific interaction triggers the opening of ion channels, generating electrical signals. Olfactory signal transmission follows the “lock-and-key model”: once the odorant binds to a G protein-coupled receptor (GPCR) on the cilia, it activates the cAMP second messenger pathway, leading to depolarization of the cell membrane. The resulting signal is transmitted along the olfactory nerve fibers to the olfactory bulb, where mitral and tufted cells process and project it to the cerebral cortex.

Each olfactory sensory neuron expresses only one type of receptor, yet through combinatorial coding, the system can distinguish tens of thousands of different odors. For instance, a mixture of aldehydes released from polypropylene (PP) may activate receptor combinations such as OR1A2 and OR2J3.

This biological recognition mechanism provides a benchmark for evaluating material odors. For example, when the concentration of DEHP released from PVC artificial leather exceeds 2200 µg/m³, its molecules bind to OR3A4 receptors and trigger a “pungent” odor perception—precisely the kind of threshold automotive interior designers aim to avoid.

By understanding the molecular-receptor interaction mechanisms behind human olfaction, material engineers can reverse-engineer low-odor formulations based on the “human olfactory perception map.”



Typical Odors and Their Sources in Different Plastics


Polymer Type Typical Odor Description

Main Source/Substance

or Mechanism

Additional Notes
Polyethylene (PE)
Waxy, oily, mild irritation
Antioxidant degradation (e.g., BHT → phenols), oxidative cleavage (aldehydes)
Odor becomes more noticeable at higher processing temperatures
Polypropylene (PP)
Slightly sweet, light oily smell
Oxidation products (alkyl aldehydes, ketones), antioxidant residues
Typically mild odor, may worsen after modification
Polystyrene (PS)
Sweet, aromatic, strong irritation
Residual styrene monomer, decomposition products (toluene, ethylbenzene)
HIPS (rubber-modified PS) has more complex odor
Acrylonitrile Butadiene Styrene (ABS)
Pungent, spicy, slightly acidic
Residual acrylonitrile, styrene, oxidized butadiene, emulsifiers
Severe thermal degradation increases odor intensity
Polyvinyl Chloride (PVC)
Ink-like, plastic smell, irritating
Plasticizers (e.g., phthalates), decomposition of stabilizers, HCl
Poor thermal stability; odor becomes stronger after degradation
Polyurethane (PU)
Fishy, amine-like, strong irritation
Residual isocyanates, hydrolysis products (amines)
Encapsulated isocyanates can help reduce odor
Polyamide (PA6/PA66)
Roasted, ammonia-like smell
Chain-end amines, oxidation, thermal degradation (e.g., caprolactam)
Hydrolysis after moisture absorption may also produce odor
Polyester (PET/PBT)
Slight burnt smell, acidic
Decomposition products (benzoic acid, phthalic acid), residual solvents
High temperature injection molding tends to release stronger odor
Polycarbonate (PC)
Bitter, phenolic, slightly pungent
Residual BPA, decomposition of carbonate (phenol-based)
Encapsulated antioxidants can help reduce odor
Polymethyl Methacrylate (PMMA)
Slightly irritating, ester-like, acceptable
Residual MMA, thermal degradation (small esters)
High-purity PMMA is almost odorless
Polyoxymethylene (POM)
Unpleasant, irritating gases
Formaldehyde, acetal-type volatiles
Odor mainly released during high-temperature injection molding
Fluoropolymers (e.g., PTFE)
Nearly odorless, slight waxy note
Almost no VOC emissions
Very low odor, suitable for high-standard interior applications


Mechanisms of Odor Formation
The odor of plastic materials does not appear out of nowhere—it is gradually generated during processing, storage, and use.

The main mechanisms include:

1. Thermal degradation: High processing temperatures cause molecular chain scission, resulting in the formation of low-molecular-weight odorous compounds (e.g., aldehydes).


Polymer Thermal Degradation Products
Polyamide 66 (PA66)
Cyclopentanone, pyridine, cyclic imide, amides, carboxylic acids, caprolactam
Polyethylene (PE)
Ketones, carboxylic acids, furanones, keto-acids
Poly(ethylene oxide–propylene oxide–ethylene oxide)
Formate esters, acetate esters, carboxylic acids, aldehydes
Poly(L-lactide) (PLLA)
Lactide, lactic acid, lactoyl-lactic acid
Polymethyl methacrylate (PMMA)
Methyl methacrylate monomer
Silicone rubber (Polysiloxane)
Cyclic oligomers
Polystyrene (PS)
Styrene, styrene-acrylonitrile, tert-butylbenzene, α-methylstyrene, BHT (butylated hydroxytoluene)
Polysulfide rubber
1,3,6,7-dioxodithiepan, other cyclic degradation products


2. Oxidative degradation: Antioxidants or polymer oxidation produce unpleasant odors (e.g., BHT oxidation products).
Polyamide (PA66): Thermal oxidative degradation generates cyclopentanone compounds such as 2-ethylcyclopentanone, which can reach concentrations up to 0.3 μg/g after aging at 100°C for 300 hours, causing a "medicinal" odor.

3. Photoaging: UV radiation causes polymer chain scission, releasing small molecule gases.

4. Processing residues: Residual catalysts or solvents that are not fully removed.
Polyurethane (PU): Amine catalysts such as triethylamine have a very low odor threshold (0.67 μg/m³) and are the main cause of the characteristic "fishy" smell of PU foam.



How to Analyze Plastic Odors?

Common Methods for Testing and Evaluating Plastic Odors Include:


Test Method
Basic Principle
Output Results
Applications
Sensory Sniff Test
Personnel subjectively smell and score samples by nose
Odor intensity scale (e.g., 1–6 scale)
Preliminary material screening, end-user sensory reference
VDA 270 Standard Test
Sample heated under constant temperature to release odor, then sniffed
Odor rating (German scale)
Automotive interior material odor testing
GC-MS (Headspace Gas Chromatography-Mass Spectrometry)
Headspace gases collected and separated by chromatography; mass spectrometry for identification and quantification
VOC types and concentrations (μg/m³)
Accurate identification of odor sources
TD-GC-MS (Thermal Desorption GC-MS)
Sample released gases collected on adsorbent tubes, thermally desorbed into GC-MS
Gas component profiles and concentration curves
Long-term material emission testing, trace-level analysis
Chamber Test (Emission Chamber Test)
Sample placed in sealed chamber at fixed temperature to detect TVOC release
Total Volatile Organic Compounds (TVOC) levels
Odor rating for whole vehicle or parts
Gas Sensor Array (Electronic Nose)
Multiple sensors mimic human olfactory nerves to detect and digitally map odors
Digital odor profile, pattern recognition
Rapid screening, automated process odor quality control
Dynamic Olfactometry
Odor samples diluted and presented to human panelists for detection threshold and intensity statistics
Odor detection threshold, intensity index
Urban odor control, industrial odor source analysis, material selection



How Can Engineers Control Odor at the Source?
Controlling odor at the material selection stage is the most cost-effective and impactful strategy.

The following recommendations are suggested:


Method Type
Specific Technique/Method
Principle/Mechanism
Applicable Scenarios
Material Source Control
Use high-purity raw materials and improve the polymerization process Reduce residual monomers, solvents, and impurities
Raw material procurement and early-stage material formulation development
Use low-odor additives (e.g., polymeric antioxidants)
Enhance resistance to migration and oxidative degradation Engineering plastics, automotive and home appliance interiors
Raw material procurement and early-stage material formulation development
Formulation Optimization
Add adsorbents (such as activated carbon, zeolite) Capture released gases
Plastic blending and composite material systems
Add deodorants (e.g., cyclodextrins)
Include/complex odor molecules to reduce volatility
Encapsulation materials, packaging films, household plastics, etc.
Plastic blending and composite material systems
Processing Optimization
Apply vacuum degassing, secondary extrusion, and shear ventilation
Lower processing temperature/time and enhance the release of low-molecular-weight substances
Extrusion/injection-molded profiles and engineering plastics production
Equipment cleaning, prevent cross-contamination
Eliminate residual "external odor sources"
Multi-material mixed-line processing scenarios Extrusion/injection-molded profiles and engineering plastics production
Post-treatment Techniques
Use thermal treatment (aging), photo-oxidation, and UV exposure
Promote early release or decomposition of residual small molecules Automotive interior parts, composite panels, leather-like products
Surface treatment (e.g., plasma, coating)
Modify surface emission and adsorption behavior
Coated parts and textured decorative surfaces
Automotive interior parts, composite panels, leather-like products
Structural Design
Optimize material thickness and geometric structure
Reduce emission rate per unit area
Electronic housings, automotive center control panels, and other areas requiring close-range sniff testing



The selection revolution from "olfactory experience" to "molecular design"
The low-odorization of automotive interiors is not simply a matter of sensory optimization, but involves a systematic engineering approach encompassing polymer chemistry, mass transfer kinetics, and analytical chemistry.


For material selection engineers, it is essential to establish the correlation between "structure – performance – odor":
When the molecular chain regularity of PP increases by 15%, the release of aldehydes can be reduced by 38%;
When the molecular weight of PVC plasticizers rises from 300 Da to 500 Da, the migration rate decreases by 60%.


This molecular-level design logic is the key to breaking through the technological bottleneck of low-odor materials.




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