Driven by the 'dual carbon' goal, the recycled plastic industry has become an important pillar of the circular economy, but odor issues have always plagued the industry's development. During the recycling, processing, and storage of recycled plastics, volatile substances such as aldehydes, sulfides, and nitrogen oxides may be released due to the oxidation and breakage of polymer chains, volatilization of additives, or residual pollutants. This not only affects the expansion of product application scenarios, but may also pose a threat to human health and the ecological environment. Therefore, establishing precise and efficient odor analysis technology is crucial for upgrading the quality of recycled plastics.
Based on this, this study aims to establish an electronic nose testing method for evaluating the odor of recycled plastics, providing reference for the industry.
⦁ ⦁

01
Principles and core characteristics of electronic nose technology
⦁ AIRSENSE electronic nose

The AIRSENSE electronic nose adopts a three in one architecture of 'sensor array signal processing pattern recognition', with a core composed of highly selective metal oxide semiconductor sensors. Each sensor has exclusive sensitivity to specific functional compounds: W1C targets aromatic components and benzene, W5S responds to nitrogen oxides, W1W recognizes sulfides, W2S is sensitive to alcohols and aldehydes, and W1S responds significantly to methyl compounds. This multi-sensor collaborative working mode can form a detection network that covers the main odor components of recycled plastics.
The workflow is divided into three steps: first, the sample volatile gas is introduced into the detection chamber through the injection system, and the sensor array reacts specifically with the volatile substance, causing a change in resistance value and outputting an electrical signal matrix; Subsequently, environmental interference is removed through preprocessing algorithms to extract feature response values; Finally, by using built-in pattern recognition models such as principal component analysis (PCA) and discriminant analysis, odor fingerprint spectra are generated and qualitative and quantitative analysis is completed.

02
Experimental Design and Methods

Experimental materials and instruments
Part.1
Selecting commercially available recycled PP, PE, ABS, and HIPS as research objects, covering a total of 10 samples of first, second, and third grade recycled materials, ensuring coverage of mainstream recycled plastic categories and quality levels. Experimental equipment: AIRSENSE PEN3.5 electronic nose and electric constant temperature drying oven are used, and sample pretreatment is strictly carried out in accordance with GB/T 40006.1-2021 standard.
Optimization of experimental process
Part.2
Sample pretreatment: Weigh (20 ± 0.1) g of the sample and place it in an odorless wide mouthed bottle. Heat it at a constant temperature of 80 ℃ for 120 minutes to promote odor release. After cooling to 60 ℃, immediately test to ensure stable concentration of volatile components.
Electronic nose testing parameters: The optimal parameter combination is determined through multiple orthogonal experiments - sensor cleaning time of 80 seconds, zeroing time of 5 seconds, data acquisition time of 120 seconds, and injection flow rate of 400 mL/min. Under this parameter, the highest response intensity value of each sensor can be obtained, laying a reliable data foundation for subsequent analysis.

03
Experimental results and analysis
Analysis of Sensor Response Characteristics>>>
The response value (G/G ₀) of AIRSENSE electronic nose sensor shows a typical curve of 'rapid increase tends to stabilize' over time, reaching the peak response value at 120 seconds. Among them, the four sensors W5S, W1W, W2S, and W1S showed significant response advantages, with strong recognition ability for nitrogen oxides, sulfides, aldehydes, and methyl odor components in recycled plastics. This is highly matched with the main odor sources of recycled plastics - carbonyl and methyl compounds produced by PP and PE oxidation degradation can be captured by W2S and W1S, while sulfides and nitrogen oxides evaporated by amine antioxidants and other additives are recognized by W1W and W5S.

Construction and Application of Odor Fingerprint Map>>>
Select the peak response values of each sensor to draw a radar fingerprint spectrum and establish a visual analysis model. The results showed that the odor fingerprints of recycled PP and PE plastics exhibited significant grade differences: the response values of W2S, W1W, W1S, and W5S all showed 'third grade recycled material>second grade recycled material>first grade recycled material', while the response values of other sensors were basically stable. This characteristic difference provides a visual basis for the rapid grading of recycled plastic quality. Enterprises can screen for inferior materials during the raw material storage stage by comparing fingerprint spectra.

The odor discrimination ability of PCA model>>>
Using AIRSENSE electronic nose built-in analysis software for principal component analysis, the contribution rate of the first principal component was 75.95%, the contribution rate of the second principal component was 22.42%, and the cumulative total reached 98.37%, which can fully characterize the overall information of the sample odor. In the PCA diagram, 10 types of recycled plastic samples are distributed in different regions without overlap, achieving differentiation.

The ranking of odor intensity is: ABS (Level 1)<PP (Level 1)<PE (Level 1)<HIPS (Level 1)<HIPS (Level 2)<ABS (Level 2)<PE (Level 2)<PP (Level 2)<PE (Level 3)<PP (Level 3), which is consistent with artificial odor recognition. Among them, ABS has the weakest odor due to its strong resistance to thermal oxidative aging, while PP and PE have significantly enhanced odor with increasing regeneration times, which confirms the reliability of the model.

04

Conclusion
AIRSENSE electronic nose, with its technological advantages of fast response, precise recognition, and multi scenario adaptation, effectively solves the industry pain points of odor analysis for recycled plastics. The experiment confirmed that under the test parameters, the odor fingerprint constructed by it can intuitively present the odor differences of different grades of recycled materials. The PCA model can achieve over 98% of odor information representation and differentiation, and the analysis results are highly consistent with manual odor recognition.
The industrial application of this technology not only provides efficient quality control tools for recycled plastic enterprises, but also promotes the transformation of odor evaluation from subjective perception to objective quantification, which is of great significance for enhancing the added value of recycled plastic products and promoting the standardized development of the industry. With the continuous upgrading of technology and the expansion of application scenarios, AIRSENSE electronic nose will undoubtedly play a more critical supporting role in the field of circular economy.