Discussion on Low-Odor Carbon Black for Automotive Interior Materials-Blog-Anhui Black Cat

【Abstract】 This paper primarily introduces the development of odor testing for automotive interior materials, while also sharing a simple and versatile application testing method for carbon black odor. This aims to promote the improvement of odor measurement standards for carbon black in the field of modified plastics.

【Keywords】 Automotive Interior Materials; Low Odor

I. Sources and Impact of Odor

The odor of plastic materials originates from three stages: polymerization, modification, and molding. Residues such as monomers, catalysts, emulsifiers, by-products, and oligomers during raw material synthesis; volatile small molecule compounds produced by decomposition reactions of raw materials, additives, and fillers in the formulation; and small molecules generated from material degradation caused by screw shear during the molding process—all these are sources of material odor and VOC (Volatile Organic Compounds).

Odor control involves every aspect, from resin selection, filler choice, combination of processing aids, to molding and post-treatment processes. Selecting base resins with few residues, choosing high-purity fillers like talc, and rationally combining additives can all reduce the generation of small molecules, thereby controlling odor.

The amount of plastic used in automotive interiors is increasing. Interior components such as pillars, door panels, and instrument panels in many car models utilize plastics to varying degrees. Low-odor plastics refer to a class of plastics used in automotive interior parts or materials that release relatively low levels of odor, complying with relevant testing standards for automotive interior components. VOC is a significant source of odor in automotive interior parts. On one hand, some interior materials exposed to sunlight reach high temperatures and can volatilize VOCs with strong odors. On the other hand, materials may generate odorous small molecules during processing, and the release of these small molecule substances is a slow, long-term process of VOC emission. When the concentration of VOCs inside a car reaches a certain level, people can experience headaches, nausea, vomiting, and limb weakness in a short time. In severe cases, adverse reactions such as convulsions, coma, and memory loss may occur. Furthermore, VOCs inside cars can also damage the human liver, kidneys, brain, and nervous system, and may even be carcinogenic. Therefore, car manufacturers have requirements for material odor, necessitating passage of odor tests that meet specific standards.

Figure 1: Automotive Interior Materials

Different car manufacturers implement different standards. Currently, China, Europe, America, and Japan lack national standards or regulations for in-vehicle air quality. However, some information indicates that major foreign automotive companies control in-vehicle air quality mainly through the management of supporting components. In the automotive field, various series of materials used in vehicles, such as modified PP, modified ABS, PC/ABS alloys, and modified PA, need to comply with EU requirements for hazardous substances like RoHS and REACH. These types of materials are characterized by balanced mechanical properties, excellent processability, high heat resistance, low odor (e.g., PP material odor < level 2.5, PC/ABS and PA material odor < level 3), and low emissions.

II. Odor Detection Methods

Odor is a subjective issue that varies from person to person, making it challenging to develop a systematic scale for odor. The first step in objective odor confirmation is to have a group of people identify and classify whether the odors they smell are pleasant or unpleasant. Based on the panel's responses, standard instrumental analysis methods such as gas chromatography, solid-state spectroscopy, and GC/MS are then used to determine the level of plastic volatiles. However, these earlier instrumental analysis systems required careful analysis and evaluation by experts to draw accurate conclusions.

For example, European and American car series often use Headspace Gas Chromatography: A certain amount of sample is placed in a headspace vial, heated at a specific temperature, and the volatilized odor is measured using gas chromatography-mass spectrometry. This method is relatively simple but has poor reproducibility. Japanese car models, on the other hand, more frequently use the Sampling Bag-Gas Chromatography Mass Spectrometry method: The sample to be tested is placed in a special sampling bag of a certain specification. The sealed sampling bag is then placed in an oven and heated at 65°C for 2 hours. Sampling is performed using a Tenax adsorption tube and a sampling pump. The sampling tube can be analyzed using thermal desorption-gas chromatography-mass spectrometry, or the adsorbent can be eluted with an organic solvent, and the eluate tested using gas chromatography-mass spectrometry.

The latest plastic odor detection instrument is a device known as the "electronic nose," which relies on electronic probe arrays and pattern recognition technology. The instrument operator heats the sample and guides the released volatiles into the probe array for detection. The results obtained are displayed in a digital format, making it easy to correlate with the conclusions of the sensory panel. Depending on specific requirements, the odor of volatiles can be classified as pleasant, neutral, or unpleasant, or reported in terms of intensity and molecular aggregation. In the competitive plastics market, pungent odors can significantly hinder sales, while neutral or pleasant odors are welcomed by customers. Employing odor removal technologies might increase the cost of plastic products in the short term, but they are sure to yield good returns in the long run.

The following shares a simple internal odor measurement method used in the Anhui Black Cat laboratory.

III. Experimental Section

(I) Main Raw Materials
Carbon Black, Low-odor Polypropylene Resin (CNOOC Shell EP648U), Antioxidant 1010, Antioxidant 168.

(II) Main Equipment and Instruments
Twin-screw Extruder, Oven.

(III) Sample Preparation

1. Masterbatch Preparation:
Premix according to the formulation in Table 1 using a high-speed mixer. After mixing, feed the material into a twin-screw extruder (processing temperature approx. 200°C) for extrusion and pelletizing to produce the odor masterbatch.

Table 1: Odor Masterbatch Formulation

2.Testing Procedure
(1) Clean a 1L odor bottle with anhydrous ethanol and dry it in an oven at 100°C for 30 minutes (keep the bottle open, separate the bottle and lid).
(2) Place 20g of the sample material into a clean odor test bottle. Place in an oven at 85°C. After 2 hours, remove the odor bottle (as shown in Figure 4).
(3) Allow the odor bottle to cool to around 60°C (hot to the touch but tolerable). Members of the odor evaluation panel individually loosen the bottle cap, independently smell the odor, and assign a score according to the standard in Table 2. Record each person's data and calculate the average as the final odor result (refer to Table 3).

Table 2: Odor Evaluation Criteria

Table 3: Odor Rating Score Sheet

Figure 4: Laboratory Odor Bottle

(4) Result Analysis
By having multiple people smell the odor and assign scores, the strength of the odor can be quantitatively compared.

(5) Another Simple VOC Test Method for Automotive Interior Materials (Condensate Weighing Method)
a. Thoroughly dry 20-50g of sample material and place it in a clean, dry glass test bottle. Cover the bottle with a pre-weighed glass plate. Install a movable condensation device (temperature controlled at 38°C) on the glass plate.
b. Place the sealed test bottle in a 102°C oil bath and heat for 16 hours.
c. Compare the weight change of the glass plate before and after heating. The weight of volatile condensate can thus be calculated. Divide this by the original material weight to obtain the VOC content. Take the average of multiple measurements as the final test result.

Conclusion

With people's pursuit of a high quality of life, cars are continuously entering more and more households. According to statistics from the Ministry of Public Security, China's car inventory reached 336 million in 2023, with 24.56 million newly registered cars, a figure that has exceeded 20 million for 10 consecutive years. The smell inside cars has long been criticized by users. Besides extensive work carried out on main materials like polyurethane and polypropylene, black, as the mainstream color for automotive interiors, has a very broad market volume. Carbon black is the primary coloring material for black automotive interiors. Due to its production process and raw materials, the surface of carbon black can retain polycyclic aromatic hydrocarbons (PAHs) and contain trace halogens, which also pose a threat to the environment and human life and health. Therefore, research and development of low-odor carbon black is becoming increasingly important and aligns with current environmental policy requirements. Concurrently, China produces over 30 million cars annually, requiring carbon black for interior plastic parts, leather goods, structural adhesives, sealing strips, etc. Calculated at 0.5kg of carbon black per vehicle, the required amount is 15,000 tons. At 25 RMB/kg, the value of carbon black usage alone amounts to 375 million RMB, representing an extremely substantial potential market scale.