Quick Detection of Aldehydes and Ketones in Automotive Textiles

1.1 Testing standards of VOC

Current studies have shown that formaldehyde, benzaldehyde, and the other aldehydes and ketones are the main harmful volatile compounds in cars [3]. When some researchers analyzed VOC in vehicles, they found that the contents of formaldehyde and other aldehydes and ketones were higher than other compounds [4,5,6]. Thus, in this study, aldehydes and ketones are the main VOC detected in automotive textiles.

The literature reviews of testing standards present that the German Automobile Industry Association has proposed several testing standards for volatile substances in-vehicle parts. According to “Determination of Organic Emissions From Non-Metallic Materials in Vehicle Interiors” VDA 277-1995, volatile compounds under a static condition can be measured by the headspace method. “Thermo-Desorption Analysis of Organic Emissions for the Characterization of Non-Metallic Automobile Interior Materials” VDA 278-2002 specifies that volatile and semi-volatile substances can be determined by thermal desorption analysis under dynamic conditions. Tests for the entire vehicle part need to be done in a sealed compartment. In addition, Japan and Russia have proclaimed related testing standards to test organic volatiles in cars [7]. In China, the State Environmental Protection Administration has declared “Evaluating Guidelines for Air Quality in Passenger Vehicles” GB/T 27630-2011, which limits the contents of formaldehyde, acetaldehyde, and acrolein separately less than 0.10, 0.05, and 0.5 mg/m³ in vehicles [8].

Current testing standards of VOC focus on quantitative analysis of each compound in the car, rather than on general qualitative analysis. In addition, in the current testing methods, the air quality is measured in whole vehicles, and it is very difficult to supervise volatile substances in different vehicle parts before assembling, so that contents of VOC released from each part cannot be detected in time. To provide good air quality in vehicles, each in-vehicle part or interior materials including automotive textiles should be detected and meet standard requirements. Generally, there is a lack of specialized testing standards for detecting VOC in automotive textiles, so this study was aimed to establish an effective and reliable testing method to detect mainly aldehydes and ketones in automotive textiles before the car assembling.

1.2 Testing methods of aldehydes and ketones

Methods for volatile substances collection and analysis include detection module, headspace method, thermal analysis method, and sampling bag method [9,10]. The detection module method is commonly used to test VOC in the whole vehicle, and it is not suitable for interior textiles. The cost and testing conditions of this method are very high, and it is often applied by motor corporations or third-party testing organizations. The headspace method is easy to implement and the cost is relatively low. In this method, the entire quantity of organic volatile substances in vehicles is tested, but each substance cannot be analyzed. The thermal analysis method is only used to detect benzene and hydrocarbons. The sampling bag is the most common method used in Japanese car corporations. Tested samples are put into a sealed bag. This method is very convenient to operate and is friendly to small sample sizes. In comparison, the sampling bag is the most feasible method to collect aldehydes and ketones in automotive textiles.

Detection methods of aldehydes and ketones include gas chromatography and liquid chromatography [11,12,13]. When the contents of aldehydes and ketones in the air are less, the sensitivity and accuracy of instruments need to be high enough. In addition, compounds such as formaldehyde and acetone do not have UV-responsible groups to be detected in the UV detection method. Aldehydes and ketones could react with 2,4-dinitrophenylhydrazine (DNPH) in a strong acid condition, and the derivatives have high sensitivity in liquid chromatography [14]. According to “Determination of Formaldehyde in Ambient Air Using Adsorbent Cartridge Followed by High Performance Liquid Chromatography (HPLC)” EPA-TO-11, liquid chromatography is commonly used to detect aldehydes and ketones organic volatiles in the air. Therefore, this study selected HPLC to test aldehydes and ketones in automotive textiles.

2.1 Simulation of the inner environment in vehicles

The inner environment of vehicles is relatively complex. When windows and doors were closed for a certain time, in addition to aldehydes and ketones, lots of nitrogen exit inside the vehicle, and harmful volatile matters are hard to spread out [15]. Referring to standards “Interior Air of Road Vehicles—Part 2: Screening Method for the Determination of the Emissions of Volatile Organic Compounds from Vehicle Interior Parts and Materials—Bag Method” CNS15676-2-2013 and “Determination of Volatile Organic Compounds and Carbonyl Compounds in Cabin of Vehicles” HJ/T400-2007, the study designed sealed bags with insert materials such as polyvinylidene fluoride to isolate outside air and to avoid reaction with organic compounds in bags. Then the sealed bags were filled with pure and dry nitrogen to simulate the enclosed inner environment of vehicles.

When the vehicle is exposed to the sun for a long time, the inner temperature increases rapidly which can accelerate the volatilization of aldehydes and ketones in a short time [16]. In this study, treated samples were placed in an oven to simulate the condition of the sun exposure. Two aldehyde reagents and two ketone reagents were used to simulate VOC evaporated from automotive textiles.

2.2 Testing method

The sampling bag method was selected to collect VOCs in textiles. In the experiment, woven fabric samples were added with a certain amount of standard aldehyde and ketone solutions and quickly put into sealed bags. Five liters of highly pure nitrogen as an insert gas were used to fill 10 L sealed bags. To accelerate the volatile of aldehydes and ketones, sealed bags with treated samples were placed in an oven at 65°C for 2 h ± 5 min.

After cooling the bags, adsorption tubes filled with DNPH were used to adsorb aldehyde and ketone volatiles in bags. Organic volatiles can react with DNPH to form a stable hydrazine derivative in acidic conditions. Then adsorption tubes were eluted with acetonitrile tetrahydrofuran. The collected 5 mL eluent was analyzed by the HPLC method to determine the contents of each aldehyde and ketone volatiles. The testing conditions of HPLC are shown in Table 1.

2.3 Material and solutions

In this study, to validate the efficiency and effectiveness of the pretreatment and the HPLC method, linear regression models of concentrations and chromatographic peaks were developed. Four aldehyde/ketone–DNPH solutions including formaldehyde, hexanal, butanone, and cyclohexanone were applied to investigate the effects of concentrations on liquid chromatography. Nine concentration levels of each standard aldehyde/ketone–DNPH solution were 15μ, 7.5μ, 3.75μ, 1.875μ, 0.9375μ, 0.4688μ, 0.2344μ, 0.0586μ, and 0.0293 μg/mL, respectively. With the lowest concentration (0.0293 μg/mL), 10 samples were treated with each mixed solution, yielding 40 samples. According to the results of liquid chromatography, limit of detection (LOD) and limit of quantitation (LOQ) in the HPLC method were identified.

Fabric samples were woven with polyester, and their size was 10 cm × 10 cm. Four types of aldehyde and ketone solutions added to woven fabrics were formaldehyde, hexanal, butanone, and cyclohexanone. Of note, 0.05 g pure solutions were mixed with acetonitrile solutions in 50 mL volumetric flask and made into 1000 μg/mL with standard aldehyde and ketone solutions. For each monomer solution, 100 μL was added to fabric samples for pretreatment and detected contents in HPLC were compared with the added amount.

3.1 Efficiency and effectiveness of the method

3.1.1 Linear analysis

To estimate the efficiency of HPLC in determining volatiles of automotive textiles, linear analysis had been done to compare characteristic peak in chromatogram with the concentration of volatiles. High correlation means the method could effectively and efficiently determine the contents of the volatiles [17].

Testing results presented that the peak area for each volatile was raised with increasing concentrations. To verify the correlation between peak area and concentration, a linear model was established in the least square method as shown in the following equation:

Ai=Ki×Cj+bi

where A_i means the peak area of aldehyde and ketone compounds in standard solutions and i = 4, representing formaldehyde, hexanal, butanone, and cyclohexanone; K_i is the slope of the linear model; C_j means the concentration of aldehyde and ketone compounds in standard solutions, and its unit is mg/mL, j = 9; and b_i is the intercept of the linear model.

Linear models for formaldehyde, hexanal, butanone, and cyclohexanone are separately illustrated in Figures 2–5.

Figure 2

Linear regression of formaldehyde.

Figure 3

Linear regression of hexanal.

Figure 4

Linear regression of butanone.

Figure 5

Linear regression of cyclohexanone.

Regression coefficients of each model are listed in Table 3, and they are >0.995, indicating that four standard curves have very high linear correlations. Therefore, the peak area in chromatogram could be used to represent the contents of aldehyde and ketone compounds in tested samples, and the method provides an efficient and effective way of determining VOC of automotive textiles.

Table 3

The linear model of four VOCs in automotive textiles

Name	Linear model	Regression coefficients (R2)
Formaldehyde	y = 746.39 × −4.82	=0.999
Hexanal	y = 196.2 × −0.23	=1.000
Butanone	y = 252.01 × +0.79	=1.000
Cyclohexanone	y = 232.9 × −0.07	=1.000

3.2 Accuracy and precision of the method

Differences between the added quantity of aldehyde and ketone volatiles and the detected contents of those volatiles in HPLC could validate the accuracy of the method to determine VOC in automotive textiles. Lower differences prove less loss of VOC in the process of testing and declare that the results in HPLC can accurately present the contents of VOC in textiles. Usually, the ratio of detected contents and added quantity should be above 50% [20]. Precision was proved by the relative standard error (RSD) as shown in the following equation:

where n equals sample repeats; X_i represents detected content of volatiles in each sample; and X̅ represents average of detected contents.

In the experiment, 100 μL standard formaldehyde, hexanal, butanone, and cyclohexanone solutions were, respectively, added to blank woven fabrics. For each treatment, six samples were pretreated and detected contents of each volatile were collected.

In Table 5, differences between the added amount and detected contents are very low. Of note, 94.48% formaldehyde, 93.23% hexanal, 98.61% butanone, and 91.12% cyclohexanone could be detected. Findings reveal that HPLC is an easy and repeatable method that can accurately and precisely determine the contents of VOC in automotive textiles.