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Authors: Wang Hongdi1,2, Zhao Xiu1,2, Liu Yinan1,2, Jia Xiaoran1,2
Affiliation: 1. Wood Science Research Institute, Heilongjiang Academy of Forestry; 2. Key Laboratory of Timber Processing Research, State Forestry Administration
Funded project: "Research on health and safety performance testing and evaluation technology of wooden home furnishing materials" (2016YFD0600706), a national key R&D program project during the 13th Five-Year Plan period.
Author profile: Wang Hongdi (1961-), male, researcher at the Institute of Wood Science, Heilongjiang Academy of Forestry.
Source: Wood Industry, Issue 3, 2018
Introduction
This paper summarizes the existing formaldehyde detection methods for wooden floors and their advantages and disadvantages. Based on the characteristics of the currently commonly used quantitative formaldehyde analysis methods for artificial panels, it is proposed that the climate chamber method and the chromotropic acid method are more suitable for the detection of formaldehyde emissions from wooden floors used for floor heating. It is also suggested that the equilibrium treatment conditions for the sample and the parameters for adding a simulated geothermal device in the climate chamber should be determined according to the ambient temperature characteristics of the floor heating system.
Studies have shown that ambient temperature significantly affects formaldehyde emissions. Wood floors in underfloor heating environments often emit higher amounts of formaldehyde than floors in normal use.
In the existing standard LY/T 1700-2007 "Wooden Floors for Underfloor Heating", the formaldehyde emission detection method is carried out in accordance with the provisions of GB 18580. Its formaldehyde emission limit and detection method are only applicable to wooden floors used at room temperature. With the continuous expansion of the application area of floor heating systems, it is necessary to study the detection method of formaldehyde emission limit applicable to wooden floors for underfloor heating.
1. Formaldehyde detection and quantitative analysis methods for wood products
1.1 Formaldehyde detection method
Among the current flooring standards, only a very small number of standards separately propose formaldehyde emission limit values. Most standards require that the formaldehyde emission limit "should comply with the requirements of national standard GB 18580."
GB 18580-2017, "Limits of Formaldehyde Emission from Artificial Boards and Their Products for Indoor Decoration and Renovation," stipulates that the formaldehyde emission limit test method for all flooring products follows the 4.60 formaldehyde emission per 1 m³ climate chamber method specified in GB/T 17657-2013, "Test Methods for Physical and Chemical Properties of Artificial Boards and Finished Artificial Boards." Gas analysis, desiccators, or perforated extraction methods all have limitations and are only useful for reference during production control.
1) Gas analysis method
The disadvantage 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 a small space, which seriously deviates from the actual use conditions of the floor. In addition, only the formaldehyde emission of the specimen within 24 hours is measured, rather than the total emitting amount.
3) Perforation extraction method
Through high temperature and extraction, all the formaldehyde contained in the board can be measured. Therefore, this method measures the total content of free formaldehyde in the artificial board, and it is impossible for all the free formaldehyde in the artificial board to be emitted. There is no corresponding relationship between the two. It is not objective to use it to evaluate the formaldehyde release of the floor, and the high temperature detection environment can affect the physical and chemical properties of the board.
4) Climate chamber method
The climate chamber method provides a large test space, with settings for an air exchange rate of (1.0±0.05) h⁻¹, an air velocity over the specimen surface of (0.1-0.3 m/s), and a load factor of (1.0±0.02) m⁻²/m³. These test conditions most closely resemble the actual use of wood-based panels.
1.2 Formaldehyde quantitative analysis method
Spectrophotometry is widely used to quantify formaldehyde concentrations. It offers low investment costs, simple operation, and precise measurement, making it widely adopted both domestically and internationally. The acetylacetone method, chromotropic acid method, and phenol reagent method are commonly used to determine formaldehyde content (or emissions) in artificial boards, furniture, and indoor environmental indicators.
1) Acetylacetone method
The color developer of this method is relatively stable, with a minimum detection concentration of 0.25 mg/L and a wide linear range, making it suitable for the detection of high-content formaldehyde.
2) Chromotropic acid method
By changing the chromotropic acid concentration and 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 rapidity and sensitivity, but its disadvantages are that it must be carried out in concentrated sulfuric acid, has high safety requirements, is cumbersome to operate, and is susceptible to interference from phenols, olefin compounds, and NO2.
3) Phenol reagent method
The phenol reagent method is simple to use and highly sensitive, with a minimum detection concentration of 0.015 mg/L, making it suitable for determining trace formaldehyde content in wood panels. However, the presence of acetaldehyde, propionaldehyde, and sulfur dioxide can interfere with the determination, potentially causing the result to be biased low. The phenol reagent is also less stable, and the absorbance stability after color development with the color developer is not as stable as with the acetylacetone method. It is also susceptible to interference with time and temperature.
2. Discussion on the detection method of formaldehyde emission from floor heating
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 external temperature; as time goes on, the formaldehyde release rate gradually slows down and tends to be flat, 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 by plywood increases only slightly with increasing humidity. However, when the relative humidity exceeds 50%, the amount of formaldehyde released increases significantly with increasing humidity.
2.2 Detection method research
LY/T 1700-2007 stipulates that the formaldehyde emission detection method for floors adopts the climate chamber method, 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 take into account the impact of ambient temperature on the formaldehyde emission of floors, nor does it stipulate the limit for floor heating floors used at higher temperatures. It cannot accurately evaluate the formaldehyde emission of floors under floor heating conditions.
Current domestic research on formaldehyde emission testing methods for wood flooring used in underfloor heating primarily utilizes heating dryers and climatic chamber heating to simulate underfloor heating environments. Both methods achieve uniform heating of the ambient temperature, and while their results can be used to simulate indoor radiator heating, they are not suitable for floor heating environments using radiant floor heating. In reality, heat in underfloor heating environments radiates and conducts upward, requiring a thermal environment that creates a temperature gradient from bottom to top for testing underfloor heating wood flooring.
2.3 Future research priorities
1) Floor specimens must be subjected to constant temperature and humidity treatment before testing. Changes in relative humidity and temperature within the chamber will affect the time it takes for specimens to reach a constant mass and formaldehyde emissions. Therefore, the constant temperature and humidity treatment method in GB 18580-2017 (i.e., 23±1°C and 50±3% relative humidity for 15±2 days) should not be simply applied to floor heating flooring. Research is needed to determine appropriate constant temperature and humidity treatment parameters for floor heating floor specimens.
2) The formaldehyde diffusion process of floor heating wood flooring in a geothermal environment is a non-equilibrium diffusion process, with a temperature gradient between the upper and lower surfaces of the floor. Therefore, a simulated geothermal device should be incorporated into the climate chamber. For example, a heating plate using water as the heat medium could be placed under the floor, with the plate temperature controlled at 30°C and the air temperature inside the chamber at 23°C, to examine the formaldehyde release from the floor.
3) In a geothermal environment, the formaldehyde emission of the floor will increase. It is necessary to establish the relationship between geothermal temperature and floor formaldehyde emission through experiments, and to reasonably formulate the formaldehyde emission limit for wooden floors used for floor heating.
4) Quantitative analysis of formaldehyde release should adopt the chromotropic acid method or other methods with lower detection limits.
Conclusion
1) A unique formaldehyde emission detection method for wooden floors used for floor heating should be established. For example, a device simulating floor heating should be installed in a climate chamber. This method should be able to accurately and objectively simulate the floor heating environment and detect the actual formaldehyde emission of the floor in the floor heating environment.
2) When using the climate chamber method to test the formaldehyde emission of wooden floors for floor heating in a geothermal environment, it is necessary to determine the constant temperature and humidity treatment conditions based on the specific test environment parameters, and adopt a more sensitive and accurate analysis method for formaldehyde quantitative analysis.