China's wood-based panel production enterprises are numerous and widely distributed. The enterprises with a small scale have their own quality inspection departments and laboratories. There are also a large number of full-time wood-based panel quality inspection institutions. In the large provinces (regions) of several wood-based panel industries, there are full-time quality inspection institutions at the provincial, prefecture, city and even county levels. The sum of the quality of quality inspection of artificial boards implemented in China may be greater than the sum of all other wood-based panel producing countries. The large amount of inspection data obtained by this method provides abundant materials for scientific research work, and with the improvement of the quality of the quality inspection personnel. The accumulation of empirical knowledge and the ability to discover, analyze and solve problems in quality inspection work are also strengthening year by year.
A few years ago, the amount of formaldehyde released from artificial boards was measured for the widely used perforation extraction method at that time. Many research reports have been published, which have played a very good role in how to correctly understand and implement the standard of the measurement method. The dryer method has become the most widely used and most popular method for the determination of formaldehyde emission in artificial wood panels due to its simple equipment, convenient operation, low detection cost and no toxicity to operators. There are several versions of the standard for this method. Japan's JAS and JIS each have a dryer method standard. After careful comparison, there are two differences that will affect the test results. The difference between the two is the number of test pieces and the pretreatment of the test piece. The rules of time are inconsistent. ISO , ASTM and GB/T also have a dryer method standard. The ISO and GB/T dryer methods are basically the same as JIS , but there are some slight differences in data processing and specimen pretreatment conditions. The ASTM dryer method is very different from JIS , such as the number of test pieces, the time and temperature of the test piece placed in the dryer, the pretreatment method of the test piece, the size of the absorption dish, the volume of the absorption liquid (distilled water), and the test piece. Whether the edge sealing treatment or the quantitative analysis of the absorption liquid and the chromotropic acid method are different from the JIS , ISO and GB/T regulations, the detection result naturally cannot adopt the same limit value index.
Since it seems that no one in the country has used the ASTM dryer method to determine the amount of formaldehyde released (the comparison test between the individual quality inspection units and the acetylacetone method), there has not been any dispute concerning the method. But the JIS , ISO , JAS, and GB/T dryer methods have different regulations, and even just a very small difference has a big impact on the test results? Is there any rule to follow? What is the correspondence between the results of the dryer method and the environmental impact of the use of wood-based panels? At present, there are only a handful of senior quality inspectors who insist on the front line for a long time to judge. For this reason, the relevant research reports should not only stay on the list of some test data and simple analogy, but through theoretical analysis, and finally Get a reliable mathematical model with sufficient basis to improve the readers' ability to identify false and true.
In view of this purpose, the author intends to provide the readers with the theoretical knowledge that has been determined by the senior experts and their own thoughts and thoughts, in order to inspire others. Two problems are discussed below: one is the mechanism problem of formaldehyde release from wood-based panels; the other is the absorption of formaldehyde by the absorption liquid (distilled water) on the artificial board in the state of the dryer.
1 Several mechanism problems of formaldehyde release from wood-based panels
Why does the wood-based panel made of phenolic resin glue do not need to consider the formaldehyde release problem? Why does the wood-based panel produced using urea-formaldehyde resin glue have a formaldehyde release problem? These issues have been finalized and need not be reconsidered. What I want to say here is why the formaldehyde release from wood-based panels is a long process. The first thing to say before this is that the wood-based panels must first have adsorption to formaldehyde, and then there is a problem with the release of formaldehyde.
1.1 formaldehyde adsorption
Why is the wood-based panel first adsorbing formaldehyde? This is because any molecule or atom in a solid is bound by chemical bond or intermolecular attraction between each other, but the molecule or atom on the solid surface is not present due to the presence of molecules or atoms of the substance on the outside. The residual valence or intermolecular attraction, so that molecules in the fluid outside it can be trapped, which is the cause of the adsorption. The same is true for the surface of wood and internal voids.
The literature has roughly classified the adsorbents into three categories. One is an adsorbent with a strongly charged adsorption site, the second is an adsorbent that has no strongly charged sites but has a polarity, and the third is a non-polar adsorbent. The generation of adsorption is due to the ubiquitous intermolecular dispersion force. The surface of wood and internal pores is generally non-polar and does not belong to the first category. Although the surface may have polarity locally, which may affect the surface adsorption performance, these parts may have a small proportion, so the wood or The generation of artificial board adsorption is mainly due to the dispersion force between molecules, which is not objectionable. However, the dispersive force is the smallest among the intermolecular gravitational forces, and the adsorption energy generated is much smaller than that of the first and second types of adsorbents. The carbonyl group on the formaldehyde molecule is polarized, the oxygen atom is partially negatively charged, and the carbon atom is partially positively charged. If the formaldehyde molecules are adsorbed due to the polarity of the surface of the wood and the artificial board, it is difficult to cause desorption, and if the adsorption due to the dispersive force is relatively easy to desorb.
1.2 formaldehyde desorption
Why is formaldehyde desorption and release is a long process, the reason is roughly the same: the presence of formaldehyde molecules in the internal voids of the wood-based panel is a physical adsorption, and the attraction between the molecules (and mainly the dispersive force) is temporarily captured. On the surface, when the adsorbed formaldehyde molecules obtain the necessary energy, they will be detached from the adsorbent (ie, the surface of the void), ie, desorption occurs, adsorption and desorption are reversible, and the velocity distribution of gas molecules is determined by Maxwell. - Boltzmann's law of distribution is similar to the probability density curve of a normal distribution. There are very few molecules moving at very high or very low speeds. Most gas molecules are near average speed (can pass The theoretical calculation of gas molecular motion), only those formaldehyde molecules that reach a certain speed (that is, a certain kinetic energy) can be desorbed and released. In order to maintain the balance of the system, there must be some formaldehyde molecules in a very high speed state, and continue to desorb and be released. In addition, when the pore surface is filled with a layer of formaldehyde molecules, the residual force field of the adsorbent (ie, formaldehyde molecule) or the new force field formed by the formaldehyde molecules occupying the surface can continue to adsorb, and the adsorbed formaldehyde continues. Molecules are also more susceptible to detachment, which is one of the reasons why the duration of formaldehyde release can be very long. Another reason is that the desorbed formaldehyde molecules need to be routed through different lengths to be released into the air, and new adsorption may occur during the release process after desorption. The last reason is that everyone knows that urea-formaldehyde resin wood-based panels will also produce new free formaldehyde molecules during use, and will experience a long process of adsorption and desorption.
Many years ago, some companies tried to prevent the release of formaldehyde by sealing the surface of the artificial board, but this has great risks. Once the sealing material is damaged, many formaldehyde gases can be released inside the board. Therefore, even if the damage is small, the short-term release rate of formaldehyde is far greater than the same type of sheet that is not sealed. Some companies have introduced the material according to the data, and the sheet is treated with ammonia gas to eliminate the release of formaldehyde, but the ammonia adsorbed by the sheet is also the same. Will be released slowly, this method is not advisable, this phenomenon can be confirmed by experiments.
1.3 External environment
The above analysis is the internal cause of formaldehyde release. The effect of external environment on formaldehyde release from wood-based panels is analyzed below. First, the influence of formaldehyde concentration in the external ambient air. The higher the concentration, the slower the release. When the concentration reaches a certain value, the plate may also adsorb formaldehyde. Of course, release and adsorption always exist at the same time, but the usual release is always Far more than adsorption, the combined effect is that the concentration of formaldehyde in the air is increasing, but it can not rule out that the adsorption is greater than the release. Therefore, some standards emphasize the pretreatment of the test for formaldehyde emission, and do not make the difference greatly. The test pieces are placed in the same pretreatment chamber to avoid interference. The second is the influence of ambient temperature. All the measurement method standards stipulate temperature requirements. It is easy to understand that the two are positively correlated. Third, the influence of environmental humidity, according to the US large and small climate box method to determine the amount of formaldehyde emission, are required to consider the impact of the relative humidity ( RH ) of the environment, when the actual RH is less than the specified, multiplied by a coefficient greater than 1 ; Conversely, multiply by a factor less than 1 , which means that the RH of the environment is positively correlated with the release of formaldehyde. However, the author believes that if the environmental RH causes an increase in the moisture content of the test piece, it will definitely reduce the amount of formaldehyde released. The effect of RH is not considered in the US dryer method, which is discussed below. The fourth is the influence of the ambient airflow speed. When the airflow velocity of the environment is parallel to the plane of the board, the higher the speed, the more the release will increase. This is unquestionable. Therefore, when the climate chamber method measures the amount of formaldehyde released, an airflow velocity is specified. The range is not considered in the dryer method.
2 General rule of absorption of formaldehyde by absorption liquid (distilled water) in the dryer method
The gas is absorbed by water or escapes from water and is a function of the following three factors: the solubility of the gas, the rate of dissolution and escape of the gas, and the rate of dispersion of the gas in the water. For theoretical analysis, please refer to the relevant literature. When the amount of formaldehyde is measured by the dryer method, since the volume of the dryer is small and the container is sealed, the process of absorbing formaldehyde by the distilled water is in addition to the general rule, and there are some special factors that need to be carefully studied.
It can be assumed that the air in contact with the water is quickly saturated with water vapor, which is different from the climate chamber method, which is why the relative humidity correction coefficient is not specified in the US dryer method; secondly, it is polar. Formaldehyde molecules are easily absorbed by water, and at the end of the measurement, the concentration of formaldehyde in the water is far from reaching a saturated concentration, so the escape of formaldehyde in the absorption liquid can be ignored. Based on this, the general process of formaldehyde migration in the dryer can be described. In a closed dryer, the effect of temperature, relative humidity and air flow on the release of formaldehyde from the test piece can be disregarded. The only thing that can affect the rate of formaldehyde release is the concentration of formaldehyde in the air in the dryer. As time increases, the concentration of formaldehyde in the air in the dryer will also increase, but due to the thermal motion of the molecules, the concentration of formaldehyde in the air in the dryer can be considered as uniform; the absorption at the bottom of the dryer The water in the dish also continuously absorbs formaldehyde. Due to the limitation of the absorption area, the amount of formaldehyde initially absorbed by the water must be less than the amount of formaldehyde released from the test piece. During this time, the concentration of formaldehyde in the air will gradually increase, which in turn will result in an increase in the absorption of formaldehyde by the absorption liquid (Henley's Law).
The question is whether the balance between release and absorption can be achieved? That is, the amount of formaldehyde released by the test piece is exactly equal to the amount absorbed by the water in the absorption dish, ensuring that the concentration of formaldehyde in the air in the dryer remains unchanged. Another question is, how long does it take to achieve a balance between release and absorption? Can balance be achieved within the specified 24 h ? The answers to these questions may be different for specimens with a large difference in formaldehyde emission. Some thoughtful thinkers have done a lot of useful work and have come to a reliable and credible conclusion. If some research reports confirm that the area of ​​the absorption dish is proportional to the formaldehyde concentration of the absorption liquid (the volume of the absorption liquid is constant), the volume of the absorption liquid is inversely proportional to the concentration of formaldehyde (the area of ​​the absorption dish is constant). This conclusion is undoubtedly correct for the very low concentration of absorbing liquid, and it seems that the establishment of the release and absorption equilibrium does not take too long.
Some full-time inspectors told the author that when using the dryer method, the first 24 h result is the same as the 24 h test again. If this conclusion is confirmed to be correct, the balance between release and absorption in the desiccator can be determined. The establishment is relatively fast. Some studies have confirmed that the volume change of the dryer is 1~2 L , which does not affect the test results (the standard also stipulates), which also seems to indicate that the release and absorption equilibrium states are relatively fast. For specimens with different release amounts, if the time between equilibrium establishments is too large, the comparability of the test results will decrease. However, what are the uncertainties in the dryer method when the formaldehyde emission of the wood-based panels is getting lower and lower? It is still difficult to conclude.
3 Conclusion
In summary, the dryer method is different from the 1 m3 climate chamber, and is different from the perforation extraction method. Its physical process and how it is related to the environmental impact are still clearly clarified. I sincerely hope that the majority of inspectors will work hard to solve these doubts.
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