Factors Affecting the Formaldehyde Emission from Wood-Based Panel
Orinal source https://www.hindawi.com/journals/ijps/2018/9349721/
Formaldehyde emission from wood-based panel can be a complicated process, which can be affected by (1) factors related to the materials, such as type of panel, wood species, adhesive, and overlay used for the panels; (2) factors related to the environment, such as temperature, humidity, air velocity, and air exchange rate; (3) factors related to treatment; and (4) factors related to panel fabrication process, such as resin content, moisture content of the panel, and others.
3.1. Factors Related to the Materials
Formaldehyde emission from the wood-based panels is affected by the physical and chemical properties of the products, such as formaldehyde content, component structure, chemical composition, density, thickness, and surface properties of the material.
The main source of formaldehyde is the adhesive. For example, the molar ratio (formaldehyde to urea, f/u) for the urea formaldehyde adhesive is an important factor affecting the formaldehyde emission. The higher the molar ratio (f/u), the greater the formaldehyde emission. Resin with a higher molar ratio will have more free formaldehyde, from which a large number of formaldehyde monomer are generated after consolidation of the panels. However, reducing the f/u molar ratio will lower the viscosity of the resin, and the activity and stability of adhesive can be affected. The amount of resin content significantly affects the formaldehyde emission of wood-based panels, especially during hot pressing. He found that the content of formaldehyde in the adhesive is the main factor affecting the formaldehyde emission of wood-based panels. A correlation was obtained between the formaldehyde emission of the adhesive and the formaldehyde emission factor of the wood-based panel. That is, the higher the content of formaldehyde in the adhesive, the higher the level of formaldehyde emission from wood-based panel, and there is a good linear relationship between them.
Chemical composition of wood will change during cutting, drying, hot pressing, and other processing of wood-based panel, which may affect the formaldehyde emission of wood-based panels. Generally, the formaldehyde emission of PB made from low-density wood is higher than those made from high-density species .
MDF uses more adhesive than PB, which causes higher initial formaldehyde emission of MDF than PB. Li tested the amount of volatile organic compound (VOC) emission and its composition of six kinds of commercial wood-based panels in a period of time for 28 days. It was found that the formaldehyde emission level from high to low was high-density fiberboard, medium-density fiberboard, PB, plywood, veneer MDF, and oriented strand board (OSB).
For the compression molded panels, less resin consolidation happens at the inner layers than that on the outer layers due to the lower temperature, higher moisture content, and lower pH value, which is easier to produce formaldehyde by hydrolysis, and a greater amount of free formaldehyde emission is from the core of the panel. Kim et al. found that the amount of formaldehyde released from the edge of PB was significantly higher than that from the MDF because of a greater porosity of PB. Wang et al. analyzed the influencing factors of formaldehyde emission from PB and concluded that hot pressing parameters and environmental factors had significant effects on the formaldehyde emission. The higher the moisture content of raw material, the greater the formaldehyde emission. Higher temperature or longer hot pressing time will reduce the amount of formaldehyde emission, but increase the cost. Usually, the formaldehyde emission of thicker panel is lower than the thinner one because of more energy absorption.
3.2. Factors Related to the Environmental Conditions
Many scholars have studied the influence of environmental factors on the formaldehyde emission from wood-based panel, such as temperature, relative humidity, and ventilation rate, and tried to establish various mathematical models and formulas.
3.2.1. Ambient Temperature
Lin et al. found that formaldehyde emission rate and its concentration increased 1.5–12.9 times when the temperature was raised from 15°C to 30°C. Chi tested the formaldehyde emission of plywood, MDF, block board, and laminate at different temperatures and loading rates using the 1 m3 small chamber. The results showed that the higher temperature accelerated the formaldehyde release. The higher the temperature, the faster the initial growth rate and the greater the final concentration. It was found that the formaldehyde emission would increase 10%–30% if the temperature was increased by 5°C . Temperature increases the kinetic energy and speeds up the diffusion rate of formaldehyde molecules. In the meantime, high temperature leads to decomposing adhesive, which increases the formaldehyde release. However, these methods that they used cannot estimate the emission under other temperature. Therefore, the correlation equation will have more practical value. According to the studies carried out by Mayers , the effect of temperature on the indoor formaldehyde concentration showed an exponential relationship. The diffusion coefficient (), partition coefficient (), and the initial emittable concentration () are the three key parameters used to predict the formaldehyde emissions. Zhang et al. found that and may be strongly affected by temperature, the partition coefficients () decrease while the diffusion coefficients () increase with increasing temperature, and they developed a formula to study the influence of temperature on by experiments and theoretical analysis, which is as follows:where and are constant for a given adsorbent and adsorbate.
Deng et al. derived a new correlation between and based on the assumption that molecular diffusion is dominant. The equation is as follows:where and are constants for a given adsorbent and adsorbate.
Huang et al. derived the relationship between and temperature:where , is the total concentration, , and is a constant. The derived correlations (3) quantitatively establish the relationship between , , and for formaldehyde emissions from building materials. When the parameters and in (3) are obtained from available results, the derived correlations can be used to predict the emittable ratio and at other temperatures. This is very helpful for predicting the emission characteristics of pollutants at various temperatures.
Mayers showed that the higher the humidity, the more the formaldehyde emission. A linear relationship between humidity and formaldehyde concentration in the test chamber was found for the wood-based panel. Lin et al. found that the formaldehyde concentration and release rate increased up to 32 times, when the relative humidity in the chamber was increased from 50% to 80%. Frihart et al. tested the formaldehyde emissions from urea formaldehyde- (UF-) bonded PB and found that the emission rate increased 6–9 times when the RH was increased from 30% to 100%. Parthasarathy et al. tested the steady formaldehyde concentration in the chamber and found that the formaldehyde release rate increased 1.8–3.5 times when the relative humidity increased from 50% to 85%, because the reaction of weak acidic steam in the air and the free dimethylol oligomer in the UF resin can generate formaldehyde, which can be hydrolyzed to release formaldehyde. It should be pointed out that these studies mainly focused on the analysis of formaldehyde concentrations or emission rates at steady or equilibrium conditions, and the results would not be appropriate to be applicable to actual indoor spaces with variable environmental conditions.
3.2.3. Combined Environmental Factors
Li studied the formaldehyde emission of wood-based panel at different environmental factors, such as temperature, relative humidity, and gas exchange rate. The results showed that temperature affects the formaldehyde emission rate interacting with the vapor pressure of the compound inside the panel. Relative humidity affects the formaldehyde emission rate interacting with the evaporation of water vapor inside the panel. Gas exchange rate affects the formaldehyde emission rate interacting with the concentration gradient of boundary layer of the panel. Therefore, the formaldehyde emission rate of panel can be increased by raising the temperature, relative humidity, and gas exchange rate, especially at the early stage when the panel is used.
Guo et al. studied the relationship of the mean formaldehyde concentration () and potential factors, such as temperature (), relative humidity (RH), time duration of the windows and doors being closed before sampling (DC), time duration from the end of decoration to sampling (DR), and source characteristics (). A model correlating the indoor average formaldehyde concentration to these five factors (, RH, DC, DR, and ) was established based on 298 samples (). The relationship among five dominant factors can be expressed as
Each factor contributes to the formaldehyde concentration in the following order: , 43.7%; , 31.0%; DC, 10.2%; DR, 8.0%; and RH, 7.0%. Specifically, meteorological conditions (i.e., RH plus ) accounted for 50.7%. The coefficient of and RH, RTH, was proposed to describe their combined influence on formaldehyde emission, which also had a linear relationship () with formaldehyde release in a simulation chamber test. In addition, experiments confirmed that it is a synergistic action as and RH accelerated the release of formaldehyde and that it is a significant factor influencing indoor formaldehyde pollution. These achievements could lead to reference values of measures for the efficient reduction of indoor formaldehyde pollution.
Yang et al. studied the combined effects of relative humidity and temperature on the formaldehyde emission and the emission parameters (, , and ), and a theoretical model of the emission parameters , , and , relative humidity, and temperature was developed.where , , , , , and .where , , , and .where , , , , and .
These models can predict the emission parameters of formaldehyde in different temperatures and humidity and predict the equilibrium concentration of formaldehyde in the air by combining with the panel release model.
Huang et al. developed a theoretical model (8) using indoor temperature, humidity, and air change rate, to predict indoor formaldehyde concentrations.where is the ambient formaldehyde concentration (μg/m3), is the indoor formaldehyde concentration, is the volume of the room (m3), is the ventilation rate of the room (m3/h), is the formaldehyde emission rate from the indoor sources per unit area (μg/(m2·h), is the emission area of the sources (m2), is the adsorptive rate of indoor sinks (μg/h), and is the emission time (h).
Analysis using this equation showed that the indoor formaldehyde concentrations in northern China were 4.0 times greater than those in southern China. This result was indirectly affected by China’s heating policy and building energy efficiency standards.
The above studies show that the higher the temperature and humidity, the better ventilation and the more conducive to the formaldehyde emission. So during the interior decoration, increasing the environment temperature, humidity, and air change rate can accelerate formaldehyde emission.