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

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).

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:

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:

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.