In order to solve the problem of human body health caused by free formaldehyde pollution of indoor decoration and decoration materials, China has implemented the mandatory national standard “Deletion Limit of Formaldehyde in Interior Decoration Materials and Its Products†from January 1 this year. The implementation of the standard has aroused widespread attention from wood-based panel manufacturers, building materials market and consumers, and has sought to produce and adopt methods to reduce the content of free formaldehyde in wood-based panels. In view of the reasons for the release of free formaldehyde from wood-based panels, this paper briefly introduces some technical measures to reduce the release of free formaldehyde from artificial boards at home and abroad, and analyzes and discusses several common measures. Finally, it proposes to solve this problem fundamentally. Suggestions for the issue for reference.
1. Reasons for free formaldehyde release in the board
The reasons for the release of formaldehyde from wood-based panels are as follows:
(1) free formaldehyde remaining in the reaction when the resin is synthesized;
(2) The resin has been involved in the reaction of formaldehyde during the synthesis, and due to the formation of unstable groups, they release the bound formaldehyde during the hot pressing and the use of the board;
(3) During the plate making process, due to the addition of the solid agent, under the action of the electrolyte, the stable adsorbed double ion layer formed around the resin colloid is destroyed, thereby releasing formaldehyde and causing the resin to solidify.
As can be seen from the above, the cause of formaldehyde release is complicated and unavoidable.
2. Some technical measures to reduce the free formaldehyde content.
There are many people engaged in research in this area in China, and the following mature prevention measures have been formed:
(1) The use of low molar ratio urea-formaldehyde resin glue free formaldehyde mainly comes from the rubber compound. The low molar ratio of urea-formaldehyde resin has a low formaldehyde content, so that the amount of formaldehyde emission during the production and use of the wood-based panel is greatly reduced. However, excessive reduction of the formaldehyde content in the resin causes deterioration of the bonding property inside the artificial board and lowers the internal bonding strength of the board. The recommended molar ratio is urea: formaldehyde = 1: 1.2 ~ 1.3 [2]. When producing low-toxic urea-formaldehyde resin glue [3], formaldehyde is added once, and the reaction is carried out in three batches at a certain pH and temperature to obtain a man-made product with stable performance, good product quality and high strength. Adhesive for board.
(2) The modified urea-formaldehyde resin urea-formaldehyde resin glue can be modified by various methods, for example, adding an appropriate amount of sulfite pulp waste liquid, adding starch, melamine and the like. It is well known that uremic resin modified with melamine has both waterproof and low toxicity effects.
(3) Adding a formaldehyde trapping agent to the adhesive (such as melamine, urea, tannin, bark powder, etc.), the method is simple, the effect is obvious, and does not have a negative effect on the physical and mechanical properties of the board. Since the price of the capture agent is low, it does not impose an excessive burden on the product cost.
(4) The adsorbent adsorption method uses an aqueous solution containing three components of an acrylamide-based polymer compound, an acid-acid hydrazine-based polymer compound and an organic ammonia compound as an adsorbent, and is applied to a wood-based panel by spraying, coating or dipping. On the surface of the plate, after the gelatinization or drying of the aqueous solution, the free formaldehyde in the plate is completely absorbed.
(5) Appropriate control of the amount of sizing, the amount of sizing also has an effect on the amount of formaldehyde released from the artificial board. According to some experiments, the amount of formaldehyde released by the particle board increased by 1.5%, and the amount of formaldehyde released increased by 1.88mg/100g. Therefore, it is recommended to minimize the amount of glue applied to ensure the quality of the board.
(6) Adjusting the hot pressing process parameters, the hot pressing temperature, the hot pressing time, the water content of the shavings and other process parameters, the amount of formaldehyde released from the particle board is greatly affected. As the hot pressing temperature increases, the hot pressing time is prolonged to reduce the moisture content of the shavings, and the formaldehyde emission of the sheet also decreases linearly.
(7) Post-processing the board. For example, the board can be vacuum-treated, that is, after the wood-based board is hot pressed, it is pushed into the vacuum tank in piles, and the temperature inside the tank is kept at 40. Around C, the vacuum is pumped into a vacuum and kept for a period of time. After the board is treated, the free formaldehyde content can be reduced by 20%.
(8) Coating sealing method, coating a layer of 15% paraffin wax and ethylene oxide on the surface of the particle board to reduce the emission rate of free formaldehyde and reduce the emission of formaldehyde per unit time. .
(9) Strengthen natural ventilation. Studies have shown that within a certain range, the increase in temperature or the increase in humidity will cause the release of formaldehyde from artificial boards to be released. Therefore, when using wood-based panels indoors, natural ventilation should be strengthened; Should be more suitable for drying at low temperatures.
(10) Use environmentally-friendly flame retardant adhesive This adhesive is a kind of adhesive that combines the advantages of adhesiveness, environmental protection, flame retardancy and low cost. It can be used together with urea glue. Whether the board made of the glue or the board made of the glue and the urea glue, the content of free formaldehyde can meet the national standard of formaldehyde emission.
Foreign research in this area is relatively early, especially in Japan, where there are ten research reports in this area published in the journal. At present, there are mainly the following methods:
(1) Low-aldehyde wood glue of stone particles, adding calcium carbide fine powder with a particle size of 0.9-100 to formaldehyde resin, and reacting hydrogen hydrolyzed with free formaldehyde to convert formaldehyde with strong toxicity and odor into toxicity. Weak methanol, which reduces the amount and release of formaldehyde. For example: take 100 parts of formaldehyde adhesive, 13 parts of starch, 1~10 parts of calcium carbide ultrafine powder of 0.9~100, 1 part of hardening agent, 10 parts of water, mixed adhesive, adhesion is 0.83~0.88Mpa The plywood made of it has a formaldehyde emission reduction of 90% compared with the absence of calcium carbide.
(2) A method of producing a formaldehyde trapping layer on a paper substrate of a decorative paper using a decorative sheet having a formaldehyde scavenger, and a method of adding a formaldehyde trapping agent to the surface protective layer for effectively capturing a material from the inner layer thereof (plywood) The formaldehyde released in the particleboard and MDF) also captures formaldehyde released from other items in the room. Formaldehyde scavengers are primarily amino or amine organic compounds. According to one experiment, the amount of formaldehyde released was 1.14 mg/L for the decorative board without the formaldehyde-trapping material, and the formaldehyde release amount of 0.53 mg/L was reduced by 53%.
(3) The use of a thermosetting resin containing an acetoacetyl group capable of absorbing an aldehyde has a strong absorption aldehyde effect on a wood product using a formaldehyde-based adhesive. The content of acetoacetyl groups is 0.01% to 40%, which is too low to achieve the effect of absorbing formaldehyde, and too high, the storage stability of the resin is not good. The resin can also be used in combination with urea, thiourea, etc., and is suitable for surface coating and impregnation of formaldehyde-based resin wood products.
(4) The aldehyde-removing materials extracted from natural plants mainly include terpene-containing essential oil capture agents and plant polyphenol-containing capture agents.
3. Discussion and recommendations
In summary, both domestic and foreign issues of formaldehyde pollution are highly valued, and the preventive measures taken have certain value and significance. At present, the most effective measure in China is to reduce the molar ratio of F/U, but this method also has certain limitations. If the molar ratio is too low, the internal bonding of the rubber will be poor, causing the flat tensile strength of the plate to decrease, thus affecting Board performance. The method of using formaldehyde scavenger to reduce the content of free formaldehyde, since the scavengers are all chemical agents, which are harmful to human health, the method of using formaldehyde scavenger to reduce the formaldehyde content may cause secondary pollution. Another method is to reduce the free formaldehyde by adjusting the hot pressing process parameters, and the productivity is lowered due to the prolonged hot pressing time, and the product cost is increased. The post-treatment vacuum processing method has the advantages of simple and convenient implementation, but the expensive vacuum equipment increases the cost of the product. As for several foreign treatment methods, there is currently no practical experience in China.
The above various treatment methods all reduce the free formaldehyde content from a certain aspect, and cannot solve the problem fundamentally. In order to completely eliminate the harm of free formaldehyde, it is recommended to use aldehyde-free release and adhesive, such as the environmentally-friendly flame retardant adhesive mentioned above, which was introduced from Russia and mainly used in the aviation industry in Russia. After being introduced into China, it has been successfully used in MDF on the MDF board after digestion, absorption and innovation. It is a promising glue for wood-based panels.
1. Reasons for free formaldehyde release in the board
The reasons for the release of formaldehyde from wood-based panels are as follows:
(1) free formaldehyde remaining in the reaction when the resin is synthesized;
(2) The resin has been involved in the reaction of formaldehyde during the synthesis, and due to the formation of unstable groups, they release the bound formaldehyde during the hot pressing and the use of the board;
(3) During the plate making process, due to the addition of the solid agent, under the action of the electrolyte, the stable adsorbed double ion layer formed around the resin colloid is destroyed, thereby releasing formaldehyde and causing the resin to solidify.
As can be seen from the above, the cause of formaldehyde release is complicated and unavoidable.
2. Some technical measures to reduce the free formaldehyde content.
There are many people engaged in research in this area in China, and the following mature prevention measures have been formed:
(1) The use of low molar ratio urea-formaldehyde resin glue free formaldehyde mainly comes from the rubber compound. The low molar ratio of urea-formaldehyde resin has a low formaldehyde content, so that the amount of formaldehyde emission during the production and use of the wood-based panel is greatly reduced. However, excessive reduction of the formaldehyde content in the resin causes deterioration of the bonding property inside the artificial board and lowers the internal bonding strength of the board. The recommended molar ratio is urea: formaldehyde = 1: 1.2 ~ 1.3 [2]. When producing low-toxic urea-formaldehyde resin glue [3], formaldehyde is added once, and the reaction is carried out in three batches at a certain pH and temperature to obtain a man-made product with stable performance, good product quality and high strength. Adhesive for board.
(2) The modified urea-formaldehyde resin urea-formaldehyde resin glue can be modified by various methods, for example, adding an appropriate amount of sulfite pulp waste liquid, adding starch, melamine and the like. It is well known that uremic resin modified with melamine has both waterproof and low toxicity effects.
(3) Adding a formaldehyde trapping agent to the adhesive (such as melamine, urea, tannin, bark powder, etc.), the method is simple, the effect is obvious, and does not have a negative effect on the physical and mechanical properties of the board. Since the price of the capture agent is low, it does not impose an excessive burden on the product cost.
(4) The adsorbent adsorption method uses an aqueous solution containing three components of an acrylamide-based polymer compound, an acid-acid hydrazine-based polymer compound and an organic ammonia compound as an adsorbent, and is applied to a wood-based panel by spraying, coating or dipping. On the surface of the plate, after the gelatinization or drying of the aqueous solution, the free formaldehyde in the plate is completely absorbed.
(5) Appropriate control of the amount of sizing, the amount of sizing also has an effect on the amount of formaldehyde released from the artificial board. According to some experiments, the amount of formaldehyde released by the particle board increased by 1.5%, and the amount of formaldehyde released increased by 1.88mg/100g. Therefore, it is recommended to minimize the amount of glue applied to ensure the quality of the board.
(6) Adjusting the hot pressing process parameters, the hot pressing temperature, the hot pressing time, the water content of the shavings and other process parameters, the amount of formaldehyde released from the particle board is greatly affected. As the hot pressing temperature increases, the hot pressing time is prolonged to reduce the moisture content of the shavings, and the formaldehyde emission of the sheet also decreases linearly.
(7) Post-processing the board. For example, the board can be vacuum-treated, that is, after the wood-based board is hot pressed, it is pushed into the vacuum tank in piles, and the temperature inside the tank is kept at 40. Around C, the vacuum is pumped into a vacuum and kept for a period of time. After the board is treated, the free formaldehyde content can be reduced by 20%.
(8) Coating sealing method, coating a layer of 15% paraffin wax and ethylene oxide on the surface of the particle board to reduce the emission rate of free formaldehyde and reduce the emission of formaldehyde per unit time. .
(9) Strengthen natural ventilation. Studies have shown that within a certain range, the increase in temperature or the increase in humidity will cause the release of formaldehyde from artificial boards to be released. Therefore, when using wood-based panels indoors, natural ventilation should be strengthened; Should be more suitable for drying at low temperatures.
(10) Use environmentally-friendly flame retardant adhesive This adhesive is a kind of adhesive that combines the advantages of adhesiveness, environmental protection, flame retardancy and low cost. It can be used together with urea glue. Whether the board made of the glue or the board made of the glue and the urea glue, the content of free formaldehyde can meet the national standard of formaldehyde emission.
Foreign research in this area is relatively early, especially in Japan, where there are ten research reports in this area published in the journal. At present, there are mainly the following methods:
(1) Low-aldehyde wood glue of stone particles, adding calcium carbide fine powder with a particle size of 0.9-100 to formaldehyde resin, and reacting hydrogen hydrolyzed with free formaldehyde to convert formaldehyde with strong toxicity and odor into toxicity. Weak methanol, which reduces the amount and release of formaldehyde. For example: take 100 parts of formaldehyde adhesive, 13 parts of starch, 1~10 parts of calcium carbide ultrafine powder of 0.9~100, 1 part of hardening agent, 10 parts of water, mixed adhesive, adhesion is 0.83~0.88Mpa The plywood made of it has a formaldehyde emission reduction of 90% compared with the absence of calcium carbide.
(2) A method of producing a formaldehyde trapping layer on a paper substrate of a decorative paper using a decorative sheet having a formaldehyde scavenger, and a method of adding a formaldehyde trapping agent to the surface protective layer for effectively capturing a material from the inner layer thereof (plywood) The formaldehyde released in the particleboard and MDF) also captures formaldehyde released from other items in the room. Formaldehyde scavengers are primarily amino or amine organic compounds. According to one experiment, the amount of formaldehyde released was 1.14 mg/L for the decorative board without the formaldehyde-trapping material, and the formaldehyde release amount of 0.53 mg/L was reduced by 53%.
(3) The use of a thermosetting resin containing an acetoacetyl group capable of absorbing an aldehyde has a strong absorption aldehyde effect on a wood product using a formaldehyde-based adhesive. The content of acetoacetyl groups is 0.01% to 40%, which is too low to achieve the effect of absorbing formaldehyde, and too high, the storage stability of the resin is not good. The resin can also be used in combination with urea, thiourea, etc., and is suitable for surface coating and impregnation of formaldehyde-based resin wood products.
(4) The aldehyde-removing materials extracted from natural plants mainly include terpene-containing essential oil capture agents and plant polyphenol-containing capture agents.
3. Discussion and recommendations
In summary, both domestic and foreign issues of formaldehyde pollution are highly valued, and the preventive measures taken have certain value and significance. At present, the most effective measure in China is to reduce the molar ratio of F/U, but this method also has certain limitations. If the molar ratio is too low, the internal bonding of the rubber will be poor, causing the flat tensile strength of the plate to decrease, thus affecting Board performance. The method of using formaldehyde scavenger to reduce the content of free formaldehyde, since the scavengers are all chemical agents, which are harmful to human health, the method of using formaldehyde scavenger to reduce the formaldehyde content may cause secondary pollution. Another method is to reduce the free formaldehyde by adjusting the hot pressing process parameters, and the productivity is lowered due to the prolonged hot pressing time, and the product cost is increased. The post-treatment vacuum processing method has the advantages of simple and convenient implementation, but the expensive vacuum equipment increases the cost of the product. As for several foreign treatment methods, there is currently no practical experience in China.
The above various treatment methods all reduce the free formaldehyde content from a certain aspect, and cannot solve the problem fundamentally. In order to completely eliminate the harm of free formaldehyde, it is recommended to use aldehyde-free release and adhesive, such as the environmentally-friendly flame retardant adhesive mentioned above, which was introduced from Russia and mainly used in the aviation industry in Russia. After being introduced into China, it has been successfully used in MDF on the MDF board after digestion, absorption and innovation. It is a promising glue for wood-based panels.
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