1, free formaldehyde source
In the wood-based panels and all other products bonded with formaldehyde-based adhesives, there is a problem of free formaldehyde release. The source of formaldehyde has three aspects:
a. It is related to the manufacture of the adhesive and the bonding conditions. Adhesives for bonding wood fibers are most typical of amino formaldehyde. The ratio of urea to formaldehyde is about 1:1. However, in order to obtain a good performance resin, formaldehyde is usually put a little more. Therefore, after the hot-pressed curing of the glued product in the production process, there must be excess unbonded residual formaldehyde, which is then released.
b. Free formaldehyde produced by the degradation of wood-based panels during use. Degradation is a process in which the acid in the wood fiber reacts with the adhesive after the water reacts with the adhesive to cause the resin to decompose water and formaldehyde. The rate of degradation is accelerated by high temperatures and high humidity.
c. Free formaldehyde contained in the wood itself. Both wood and unprocessed foods have a certain amount of formaldehyde, usually at levels of 1-90 mg/kg. These free formaldehydes are slowly and continuously released, and their release rate is related to temperature, humidity and air circulation conditions.
The release of formaldehyde is usually considered in the first aspect, that is, the development and use of high-quality adhesives, and the search for reasonable bonding conditions. As the world's largest producer of wood adhesives, Taier is focusing on the development of high-quality adhesives, and actively cooperates with adhesive manufacturers to develop the best process.
2. Factors affecting MDF free formaldehyde
Based on the experience of Taier Global and Chinese companies, I introduce the following effects of adhesive and wood-based panel production parameters on the free formaldehyde of wood-based panels. In the production of wood-based panels, there are a series of problems related to the amount of formaldehyde released. Ask these questions based on the actual situation to explore how to properly produce MDF. It can be seen from the analysis that there are many factors affecting the amount of formaldehyde released, but it can be divided into three main aspects: wood itself, adhesive and processing technology.
2.1 The effect of formaldehyde on the adhesive
The relationship between the molar ratio and the amount of formaldehyde released is very obvious, which is also the main aspect to solve the amount of formaldehyde released. In the manufacture of MDF, the molar ratio of the gum (F:NH2) is decreased by 0.01, the formaldehyde emission is reduced by 1.0 mg/100 g, and the drying method is lowered by 0.1 mg/Ltr. This indicates that if the molar ratio of the 16 mm thick plate is F:NH2=0.47, the formaldehyde emission is 12 mg, but to produce a 16 mm plate with a formaldehyde emission of 8 mg, the molar ratio should be reduced to F:NH2=0.43. This can only be achieved under conditions in which the amount of sizing and the hot pressing conditions are stable, but in practice, when a low molar adhesive is used, the hot pressing time is slightly extended and the amount of sizing is also increased.
2.2 Influence of moisture content of slab before hot pressing: Low moisture content, less formaldehyde emission
During hot pressing, the higher humidity inside the board accelerates the hydrolysis of the adhesive, especially at higher temperatures, and the hydrolysis of the adhesive produces free formaldehyde.
2.3 Influence of hot pressing conditions
2.3.1 Adhesive dosage
Most people realize that reducing the molar ratio of the adhesive, the amount of formaldehyde released from the board will decrease linearly, and that the same rule applies to the amount of sizing. However, contrary to expectations, when the amount of sizing increased, the amount of formaldehyde released from the board decreased nearly linearly. A rough estimate is that the amount of sizing is doubled and the amount of formaldehyde released from the board is reduced by half.
The fiber is a porous material, and formaldehyde can be released through the pores. As the amount of sizing increases, the increase in the area of ​​the gel-coated fiber will reduce the formaldehyde release channel, so formaldehyde is also reduced in release. The above theory has been verified in the laboratory. Three kinds of plates were produced with the same adhesive and hot pressing time. The amount of the plate A was 8%, the plate B was 18%, the plate C was impregnated with 10% of the glue, and dried at 105 ° C for 24 hours. The gum was heat-pressed at 8%. As a result, the amount of formaldehyde released from the plate A was 12 mg, the plate B was 7 mg, and the plate C was 6 mg.
From the density distribution of MDF, it can be seen that the increase in the amount of sizing can lead to an increase in thermal energy, because the density curve is the result of the temperature acting on a given hot pressing cycle. We observe these two cases, the difference is only the amount of sizing, The increase in the amount of sizing results in an increase in the density of the core layer, since a large amount of adhesive releases more heat, and more heat causes an increase in the temperature of the core layer.
Another phenomenon with this conclusion is the situation when thin sheets are stacked. One plate has a sizing amount of 8% and the other is 17%. After two days of sanding, the 8% sizing amount of the board is not abnormal. When the 17% sizing amount of the board is sanded, the board is very hot and emits hot air. Since the adhesive releases more heat, the formaldehyde is released. the amount. For particleboard and plywood, this experience is not suitable.
2.3.2 Effect of hot pressing temperature on formaldehyde emission
The hot pressing temperature and hot pressing time have a significant effect on the amount of formaldehyde released from the board. When the hot pressing temperature is increased or the hot pressing time is extended, the amount of formaldehyde released from the board is lowered. The same as reducing the hot pressing temperature or shortening the hot pressing time, the formaldehyde release of the board increases. This principle is very useful when analyzing the cause of low board performance. If the amount of formaldehyde released in the measuring plate is increased than expected, it may be due to the lack of complete curing of the adhesive resulting in a decrease in the performance of the board.
2.5 Relationship between plate thickness and formaldehyde emission
Practice has shown that the release of the board increases as the sheet thickness decreases. The shape of Figure 2.5 is likely to be related to the heat absorbed by the board. In particular, when producers use thinner temperatures to produce sheets, there is a linear relationship between formaldehyde emission and sheet thickness. This may be related to the amount of formaldehyde absorbed or released by the fiber.
The above can be explained that the more the thermal energy in the board, the lower the amount of formaldehyde released. Since the temperature is higher when the thick plate is produced, the heat energy is absorbed more than the thin plate, resulting in a lower amount of formaldehyde emission.
In production, choose the right adhesive. Generally speaking, the production of 2.5-6mm board with F:NH2 is 0.35 adhesive, 7-12mm should be about 0.37, 12-25mm is about 0.4, 25-30mm is 0.45, 30mm or more board application F:NH2 is 0.5 Adhesive.
2.6 Thermal stacking
By cold-rolling and then storing at 700C for more than 2 hours, the amount of formaldehyde released from the board will be reduced by more than half. This situation was confirmed in the production of MUF-bonded boards with a formaldehyde emission reduction from 12 mg/100 g to 4 mg/100 g. Once the time and temperature have caused such a consequence, the thickness expansion is also greatly reduced.
3. Issues to be aware of when addressing formaldehyde release issues
a. The adhesive properties of the adhesive decrease as the molar ratio decreases. Therefore, when the method of reducing the molar ratio is adopted to reduce the release of free formaldehyde, the performance of the sheet is weighed. In the high molar ratio, that is, the high release interval, the same molar ratio of rubber produced by different factories has little effect on the performance of the board. However, in the low molar ratio range, the same molar ratio of rubber produced by different factories may have a great impact on the performance of the board.
b. The moisture content of the slab affects the release of formaldehyde while affecting the heat transfer efficiency. The moisture content of the slab can lead to an increase in the release of formaldehyde, but a certain moisture content should be maintained. By means of the action of water vapor, the heat transfer can be accelerated to accelerate the curing speed of the rubber compound and improve the production efficiency.
c. Produce different thicknesses of the board using different adhesives. It is expected that the performance of the adhesive will also decrease as the thickness of the sheet increases and the amount of formaldehyde released decreases. This means that a low molar ratio adhesive can be used very well for thin sheets but not for thick boards. It is not possible to have a higher moisture content when using a low molar ratio adhesive. In most cases, low molar adhesives are used at standard hot pressing speeds and temperatures. This means that if the choice of adhesive is reasonable, the factory needs to increase the amount of glue and reduce the water content to produce E2 board to E1 board.
4. Particularity of free formaldehyde wood-based panel products
4.1 Process comparison of low and high aldehyde glue in the production process of wood-based panels The manufacturer of wood-based panels must understand the different processes in the production process of low- and high-aldehyde wood-based panels:
a. More stringent production process
b. Equipment support capability
c. High-quality technical personnel
4.2 The best wood-based panel production process
The best process for board production should:
a. Guarantee the physical and mechanical properties of the product
b. Meet national standards for low free formaldehyde
c. Consider the highest overall economic benefits: low sizing, small pre-solidified layer, low defect rate, high production efficiency
5. in conclusion
a. The problem of formaldehyde release has become a key issue in the environmental pollution of the living room and should be taken seriously.
b. The release of formaldehyde from wood-based panels will be a mandatory standard, and the resolution of this problem will directly affect the survival and benefits of enterprises.
The main advantage to using Kitchen Scissors is that they make quick work of slow or tedious tasks - think cleaning shrimp and removing the stems from kale. they also eliminate the need for a Cutting Board and are often safer than using a knife because your hands are away from the blade - they`re even great for kids learning to cook.
Kitchen Scissors
Kitchen Scissors,Kitchen Shears,Stainless Steel Kitchen Scissors,Cooking Scissors
Yangjiang Superwins Trade Co., Ltd. , https://www.steelkitchenware.nl