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Authors: Wang Hongdi 1, 2, Zhao Xiu 1, 2, Liu Yinan 1, 2, Jia Xiaoran 1, 2
Institutions: 1. Institute of Wood Science, Heilongjiang Academy of Forestry Sciences; 2. Key Laboratory of Timber Research, State Forestry Administration
Funding Project: National Key Research and Development Program of China during the 13th Five-Year Plan Period, “Research on Health and Safety Performance Testing and Evaluation Technology of Wood Furniture Materials” (2016YFD0600706).
About the author: Wang Hongdi (1961-), male, researcher at the Institute of Wood Science, Heilongjiang Academy of Forestry Sciences.
Source: Timber Industry, Issue 3, 2018
Introduction
This paper summarizes existing formaldehyde detection methods for wood flooring and discusses their advantages and disadvantages. Based on the characteristics of the commonly used quantitative analysis methods for formaldehyde in engineered wood products in China, it proposes that the climate chamber method and the chromotropic acid method are more suitable for detecting formaldehyde emissions from wood flooring used for underfloor heating. It also suggests that the equilibrium treatment conditions of the samples and the parameters of the simulated geothermal device in the climate chamber should be determined according to the environmental temperature characteristics of underfloor heating.
Existing studies have shown that ambient temperature has a significant impact on formaldehyde release. Wooden flooring installed under underfloor heating systems often releases more formaldehyde than wooden flooring installed under normal usage conditions.
The existing standard LY/T 1700-2007 "Wooden Flooring for Underfloor Heating" specifies the formaldehyde emission test method according to GB 18580. However, its formaldehyde emission limits and test methods are only applicable to wooden flooring used at normal temperatures. With the continuous expansion of the application area of underfloor heating systems, it is necessary to discuss test methods for formaldehyde emission limits applicable to wooden flooring used for underfloor heating.
1. Formaldehyde Detection and Quantitative Analysis Methods for Wood Products
1.1 Formaldehyde Detection Methods
In the current flooring standards, only a very few standards specify formaldehyde emission limits separately. Most standards require that the formaldehyde emission limits "should comply with the requirements of the national standard GB 18580".
GB 18580-2017, "Formaldehyde Emission Limits in Wood-based Panels and Their Products for Interior Decoration and Renovation," specifies that the test method for formaldehyde emission limits in all flooring is based on the method described in GB/T 17657-2013, "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels," specifically the method using a climate chamber with a formaldehyde emission rate of 1 m³. Gas analysis, desiccator methods, or perforation extraction methods all have limitations and should only be used as a reference for production control.
1) Gas analysis method
The drawback of this method is that the test temperature is high and the relative humidity is low. The high temperature and low humidity test environment is contrary to the actual use environment of the floor, which will inevitably lead to deviations in the test results.
2) Dryer method
The dryer used in this method is a closed space with no air exchange and is relatively small, which is far from the actual usage conditions of the floor. In addition, it only measures the formaldehyde emission of the specimen within 24 hours, rather than the total emission.
3) Perforation extraction method
The formaldehyde contained in the board can be completely determined by high temperature and extraction. Therefore, this method measures the total content of free formaldehyde in the board. However, it is impossible for all the free formaldehyde in the board to be released. There is no corresponding relationship between the two. Therefore, it is not objective to use this method to evaluate the formaldehyde release of flooring. In addition, the high temperature test environment can affect the physical and chemical properties of the board.
4) Climate Chamber Method
The climate chamber method offers a large testing space, with settings including an air exchange rate of (1.0±0.05) h⁻¹, an air velocity on the specimen surface of (0.1～0.3 m/s), and a load-bearing capacity of (1.0±0.02) m²/m³. The testing conditions in the climate chamber most closely resemble the actual usage conditions of engineered wood panels.
1.2 Quantitative Analysis Methods for Formaldehyde
Spectrophotometry is widely used for the quantitative determination of formaldehyde mass concentration due to its low investment cost, ease of operation, and high accuracy. It is widely adopted both domestically and internationally. The determination of formaldehyde content (or release) in engineered wood products, furniture, and indoor environmental indicators commonly employs the acetylacetone method, chromotropic acid method, and phenol reagent method.
1) Acetylacetone method
The colorimetric reagent used in this method is relatively stable, with a minimum detectable concentration of 0.25 mg/L and a wide linear range, making it suitable for detecting high levels of formaldehyde.
2) Chromotropic acid method
By changing the concentration of chromotropic acid and the sampling method, the detection needs of formaldehyde at different concentrations can be met. Using 0.1% chromotropic acid and 86% sulfuric acid solution as the absorption liquid, the detection limit can reach 0.020 mg/L.
The advantages of this method are its speed and sensitivity, but its disadvantages are that it must be carried out in a concentrated sulfuric acid medium, which places high demands on safety factors, is relatively cumbersome to operate, and is easily affected by phenols, alkenes and NO2.
3) Phenol reagent method
The phenol reagent method is simple to operate, highly sensitive, and has a minimum detection concentration of 0.015 mg/L, making it suitable for determining trace formaldehyde content in wood-based panels. However, the presence of acetaldehyde, propionaldehyde, and sulfur dioxide can interfere with the determination, potentially leading to lower results. Furthermore, the phenol reagent has poor stability, and the absorbance stability after color development is not as good as the acetylacetone method, and it is easily affected by time and temperature.
2. Discussion on methods for testing formaldehyde emission from underfloor heating flooring
2.1 Study on the Influence of Temperature and Humidity
Increasing the temperature will promote the release of formic acid from the board, especially the formaldehyde release rate in the initial stage. The release amount can reach more than 80% of the total release within 8 hours, indicating that the formaldehyde release in this stage is most sensitive to changes in the external temperature. As time goes on, the formaldehyde release rate gradually slows down and tends to level off, reaching a dynamic equilibrium.
Increased relative humidity also promotes formaldehyde release. Studies have found that when the humidity is below 50%, the amount of formaldehyde released from plywood only increases slightly with increasing humidity, but when the relative humidity exceeds 50%, the amount of formaldehyde released increases significantly with increasing humidity.
2.2 Research on Detection Methods
LY/T 1700-2007 stipulates that the formaldehyde emission test method for flooring adopts the climate chamber method, and the formaldehyde emission index refers to the limit of GB 18580. The test temperature is (23±1)℃, and the formaldehyde limit value is (E1≤0.124 mg/m³). It does not consider the influence of ambient temperature on the formaldehyde emission of flooring, nor does it specify the limit for flooring under underfloor heating under high temperature conditions. Therefore, it cannot accurately evaluate the formaldehyde emission of flooring under underfloor heating conditions.
Current domestic research on formaldehyde emission testing methods for wood flooring used in underfloor heating mainly employs methods such as heated dryers and climate chamber heating to simulate the underfloor heating environment. Both methods uniformly heat the environment, and the test results can be used to simulate the state when there are indoor heating radiators, but they are not suitable for underfloor heating environments with radiant floor heating. This is because, in reality, heat in an underfloor heating environment radiates and is conducted upwards, requiring the creation of a thermal environment that forms a temperature gradient from bottom to top for testing wood flooring used in underfloor heating.
2.3 Future Research Focus
1) Flooring samples must undergo constant temperature and humidity treatment before testing. Changes in relative humidity and temperature within the chamber will affect the time it takes for the sample quality to reach a constant level and the formaldehyde release. Therefore, the constant temperature and humidity treatment method of GB 18580-2017, i.e., placing the samples at (23±1)℃ and (50±3)% relative humidity for (15±2) days, should not be simply applied to the flooring samples for underfloor heating. Further research is needed to reasonably set appropriate constant temperature and humidity treatment parameters for the flooring samples for underfloor heating.
2) The formaldehyde diffusion process of wood flooring used for underfloor heating in a geothermal environment is a non-equilibrium diffusion process, with a temperature gradient existing between the upper and lower surfaces of the floor. Therefore, a simulated geothermal device should be added to the climate chamber. For example, a water-based heating plate can be placed under the floor, with the plate temperature controlled at 30 ℃ and the gas temperature inside the chamber controlled at 23 ℃, to examine the formaldehyde release from the floor.
3) In a geothermal environment, the formaldehyde emission of flooring will increase. It is necessary to establish the relationship between geothermal temperature and formaldehyde emission of flooring through experiments, and to reasonably formulate the formaldehyde emission limit for wood flooring used for underfloor heating.
4) For quantitative analysis of formaldehyde release, the chromotropic acid method or other methods with low detection limits should be used.
Conclusion
1) A unique method for testing the formaldehyde emission of wooden flooring used for underfloor heating should be established. For example, a device simulating geothermal heat should be installed in a climate chamber. This method should be able to accurately and objectively simulate the underfloor heating environment and test the actual formaldehyde emission of the flooring under the underfloor heating environment.
2) When testing the formaldehyde release of wooden flooring for underfloor heating in a geothermal environment using the climate chamber method, it is necessary to determine the constant temperature and humidity treatment conditions according to the specific test environment parameters, and to use a more sensitive and accurate analytical method for quantitative formaldehyde analysis.