This paper discusses different detection methods for formaldehyde content and free formaldehyde in water-based architectural coatings, and evaluates the uncertainty of both.

Comparison of Acetylacetone and HPLC Methods for Detecting Formaldehyde in Coatings

Formaldehyde is colorless with an irritating odor and can irritate the eyes, nose, and other parts of the body. At low temperatures, formaldehyde polymerizes to form paraformaldehyde, which, upon heating, escapes as free formaldehyde and slowly releases into the air at room temperature, causing indoor air pollution. On October 27, 2017, the World Health Organization's International Agency for Research on Cancer (IARC) listed formaldehyde as a Group 1 carcinogen.

With increasing environmental awareness and concern for personal health, attention to formaldehyde content in coatings is rising, leading to stricter limits on formaldehyde content and higher demands on the accuracy of its determination. The national mandatory standard GB 18582-2020 "Limits of Hazardous Substances in Wall Coatings for Buildings" reduces the formaldehyde limit from 100 mg/kg to 50 mg/kg compared to the previous version; according to GB/T 35602-2017 "Green Product Evaluation Coatings," water-based building coatings, including interior and exterior wall coatings and interior wall putty, have limit requirements for formaldehyde content and free formaldehyde, with corresponding different detection methods.

I. Comparison of two commonly used formaldehyde detection methods in coatings is shown in the table below.

Table 1: Comparison of different formaldehyde detection methods

Formaldehyde Detection Method Standard	Detection Principle	Detection Limit
GB/T 23993-2009 - Acetylacetone Spectrophotometry	1. Sample distillation 2. Acetylacetone color reaction 3. Spectrophotometric measurement	5mg/kg
GB/T 34683-2017 - High-Performance Liquid Chromatography (HPLC)	1. Acetonitrile extraction of the sample 2. Derivatization reaction with 2,4-dinitrophenylhydrazine 3. Measurement using a liquid chromatograph.	Detection limit: 1.5 mg/kg

The main influencing factors in formaldehyde determination are the sample pretreatment method and the quantitative detection method. Sample pretreatment is the process of extracting formaldehyde, which directly affects the accuracy and precision of formaldehyde determination in the sample. In the quantitative detection method, the chromogenic agent, working curve, and instrument sensitivity have a greater impact. Tables 2 and 3 list the comparisons of different methods.

II. Uncertainty evaluation of different detection methods

Table 2: Comparison of different pretreatment methods

Pretreatment Method	Advantages	Disadvantages
Distillation method (GB/T 23993-2009)	1. Classic method 2. Simple equipment required 3. Wide range of applications	1. Time-consuming 2. Distillation time and temperature affect the stability of the distillate 3. Substances that interfere with the determination may be distilled out, or the sample may coke and decompose to release formaldehyde
Extraction method (GB/T 34683-2017)	1. Efficient 2. Eliminates interference from the complex matrix of the coating, improving selectivity and accuracy	Large amount of organic solvent used

Table 3: Characteristics of different quantitative methods

Quantitative Detection Method	Characteristics
Acetylacetone spectrophotometry	1. Acetylacetone color development is stable, but the color reaction is greatly affected by temperature and time 2. Larger operational error at low formaldehyde concentrations 3. Wide linear range, wide test range 4. Minimum allowable value of repeatability precision: 10 mg/kg
High-performance liquid chromatography (HPLC)	1. High sensitivity, less interference 2. High probability of separating substances with similar properties and structures 3. Linear range: 30-300 mg/L 4. Minimum allowable value of repeatability precision: 0.5 mg/kg

In summary, different methods for detecting formaldehyde content in coating products have their own characteristics. Acetylacetone spectrophotometry is suitable for products with high formaldehyde content, has a wide measurement range, but has more interfering factors and higher uncertainty in the detection results. For samples with low free formaldehyde content or coating raw materials that are easily decomposed or volatilized by high-temperature distillation, HPLC will be more accurate. In comparison, the acetylacetone spectrophotometric method, due to the use of the distillation method to treat the coating sample, increases the uncertainty of the detection results. Coatings or certain raw materials (such as surfactants and biocides) are mixtures of multiple substances. When implementing the GB 18582 standard, the detection of formaldehyde content uses the distillation method to treat the sample, which may distill out interfering substances or decompose to produce formaldehyde, resulting in unsatisfactory detection data.