Preparation method of phenolic resin adhesive for bamboo oriented strand board

Abstract

The invention relates to the field of phenolic resin adhesives, and particularly discloses a preparation method of a phenolic resin adhesive for a bamboo oriented strand board, which comprises the following steps: (1) phenol activation: adding phenol and a basic catalyst aqueous solution into a reaction kettle, and activating to form an intermediate state of phenate negative ions; (2) addition stage: adding a high-concentration formaldehyde reactant into the reaction kettle, and carrying out heat preservation reaction to generate hydroxymethyl phenol; (3) heating and replenishing: heating to 80 + / -2 DEG C, keeping the temperature, replenishing the formaldehyde aqueous solution into the reaction kettle, and keeping the temperature for 20-25 minutes; (4) polycondensation stage: adding a basic catalyst aqueous solution, heating to 90 + / -2 DEG C, and carrying out heat preservation reaction; and (5) terminating the reaction: after the heat preservation reaction, detecting the viscosity, cooling, and discharging to obtain the adhesive. The phenolic resin with the solid content of 60% or above can be prepared, the adhesive has excellent bonding performance, and the prepared shaving board is excellent in mechanical strength.

Description

Technical Field

[0001] The present invention relates to the field of phenolic resin adhesives, and more specifically, to a method for preparing a phenolic resin adhesive for bamboo oriented strand board. Background Art

[0002] Currently, phenolic resin adhesives are generally prepared using 37 wt % formaldehyde as a raw material, and have a low solid content. Without dehydration, it is difficult to obtain a phenolic resin with a solid content of more than 60%.

[0003] Bamboo is difficult to drain and has a low compression ratio. If it is sprayed with a low-solid content resin containing a lot of water and then directly hot-pressed, it will cause the board to burst and seriously affect the performance. The usual method of use is to dilute the phenolic resin adhesive with water (because it needs to be diluted, there is no strict requirement for the solid content), soak it, dry it, strictly control the moisture, and then hot-press it. The production process is relatively cumbersome.

[0004] High solid content resin is usually obtained by dehydration, which will make the product have a shorter shelf life and higher viscosity, which is not conducive to industrial production. The dehydration process has high requirements on equipment, requires a higher vacuum degree, and has poor stability.

[0005] Currently, phenolic resin adhesives in this direction all use low-solid content dipping, and direct spraying almost uses MDI as an adhesive. This application can be directly sprayed, with better performance than MDI and less than 1 / 3 of its cost. Summary of the invention

[0006] The present application provides a method for preparing a phenolic resin adhesive for bamboo oriented particle board, which can not only prepare a phenolic resin with a solid content of more than 60%, but also the adhesive has excellent bonding properties, and the prepared particle board has excellent mechanical strength.

[0007] The present application provides a method for preparing a phenolic resin adhesive for bamboo oriented strand board, which adopts the following technical scheme:

[0008] A method for preparing a phenolic resin adhesive for bamboo oriented strand board comprises the following steps:

[0009] (1) Phenol activation: Phenol and an alkaline catalyst aqueous solution are added to a reactor to activate and form an intermediate state of phenolate anion;

[0010] (2) Addition stage: Add high-concentration formaldehyde reactant into a reactor at a temperature of 45°C ± 2°C and keep the temperature to react to generate hydroxymethylphenol; wherein the molar ratio of the effective content of formaldehyde in the high-concentration formaldehyde reactant to phenol is (1.2-1.5):1, and the water content in the reactor does not exceed 55.75% of the mass of phenol.

[0011] (3) Heating and adding: Raise the temperature to 80±2°C, keep warm for 10-12 minutes, add formaldehyde aqueous solution to the reactor, and keep warm for 20-25 minutes; the concentration of the added formaldehyde aqueous solution is not less than 37 wt%;

[0012] (4) Polycondensation stage: adding an aqueous solution of an alkaline catalyst, heating to 90±2° C., and maintaining the temperature for reaction; wherein the mass ratio of phenol to the alkaline catalyst added in step (4) is 140:(3-4);

[0013] (5) Reaction termination: After the reaction is kept warm for 30 to 35 minutes, the viscosity is detected to reach 15 to 16 seconds, and then the temperature is lowered at 1±0.2°C / min to obtain the adhesive.

[0014] Furthermore, in the step (1), the mass ratio of phenol to alkaline catalyst is 140:(3-3.5).

[0015] Furthermore, in step (2), the high-concentration formaldehyde reactant is a formaldehyde aqueous solution, and the concentration of the formaldehyde aqueous solution is not less than 48wt%, and in step (3), the concentration of the added formaldehyde aqueous solution is not less than 48wt%. In step (2), the formaldehyde aqueous solution is added dropwise within 50 to 55 minutes.

[0016] Furthermore, in the step (2), the high-concentration formaldehyde reactants are paraformaldehyde and formaldehyde aqueous solution, wherein the concentration of the formaldehyde aqueous solution is not less than 37wt%, and in the step (3), the concentration of the added formaldehyde aqueous solution is not less than 37wt%. In the step (2), the paraformaldehyde solid and the alkaline catalyst are added to the reaction kettle, and the mixture is stirred at 45°C±2°C until the paraformaldehyde solid is dissolved to be clear and transparent, and then the formaldehyde aqueous solution is added dropwise. In the step (2), the formaldehyde aqueous solution is added dropwise within 38 to 53 minutes. In the step (2), the mass ratio of the paraformaldehyde solid to the alkaline catalyst added in the step (2) is 1:(0.07-0.11). In the step (2), the mass ratio of the paraformaldehyde to the phenol is (0.2 to 0.25):1.

[0017] Furthermore, in the step (2), the reaction is kept warm for 30 to 35 minutes.

[0018] Furthermore, in the step (3), the mass ratio of the added formaldehyde to the added phenol is (15.54-19.2):140.

[0019] Furthermore, in step (5), when the temperature reaches below 60°C, 4±1% of the total mass of diluent is added, and the material is discharged after the temperature drops to below 30°C; the diluent is at least one of methanol, ethanol, and isopropanol. Adding the diluent can play a dilution role, reduce the viscosity, and passivate the phenolic resin, so that the viscosity growth decreases and the product's shelf life is extended. At the same time, the diluent needs to be added at a suitable temperature, because adding it at too low a temperature may lead to insufficient mixing.

[0020] Furthermore, the alkaline catalyst includes at least one of sodium hydroxide, potassium hydroxide and barium hydroxide, wherein the alkaline catalyst aqueous solution is a saturated aqueous solution.

[0021] Furthermore, in step (5), when the temperature reaches below 40°C, the alkaline solution is slowly added dropwise, and the material is discharged after the temperature drops to below 30°C.

[0022] In summary, this application has the following beneficial effects:

[0023] 1. The present application can obtain an adhesive product with excellent performance through a simple process flow, and can obtain an adhesive product with a high solid content without the need for dehydration, urea modification and other operations; it avoids the problems of increased viscosity of the adhesive due to dehydration, long production cycle, high energy consumption and high cost, and also avoids the problem that the addition of urea will lead to the introduction of a large number of flexible segments in the molecular chain, increased brittleness and adverse effects in application.

[0024] 2. Because the performance of the adhesive will be greatly affected by insufficient depolymerization, paraformaldehyde is mostly used in other fields. In addition, because high-temperature depolymerization will cause the simultaneous occurrence of polycondensation reaction, or disproportionation reaction, formaldehyde consumption and the production of products that are not conducive to resin synthesis, it is not suitable for high-performance adhesives. Moreover, paraformaldehyde is usually fed in batches multiple times to control the reaction temperature to avoid excessive temperature rise, which is relatively cumbersome. However, under the production process of the present application, only one feeding can make paraformaldehyde fully depolymerize at low temperature, and the slow depolymerization method is used to play the role of formaldehyde slow release. The released formaldehyde will immediately react with phenol to promote the depolymerization reaction balance to move towards the product direction. At the same time, the depolymerization rate of paraformaldehyde is controlled by a lower catalyst amount and a lower temperature, and the reaction heat is controllable, and there is no risk of accelerated polycondensation and formaldehyde disproportionation.

[0025] 3. This application utilizes high-concentration formaldehyde and paraformaldehyde, and the solid content can be adjusted between 60% and 80%. It is suitable for fields that require low water volume, such as bamboo oriented strand board. No dipping is required, and it can be used by direct spraying and hot pressing, thus avoiding the problems of long production cycle, complicated process, high cost, high three wastes, etc. caused by dipping.

[0026] 4. The present application suppresses the occurrence of polycondensation by using a low amount of catalyst and low temperature in the early stage, and controls the reaction conditions after sufficient addition, so that the polycondensation occurs in the same stage and the molecular growth is basically the same. The polycondensation product at 90°C has the characteristics of high molecular weight and narrow distribution; the reaction is carried out by adopting a phenol-formaldehyde feed ratio that can obtain the highest cross-linking density, and more hydroxymethylphenol is obtained by adding formaldehyde twice. Formaldehyde is added at the beginning of the polycondensation reaction, and the initial steric hindrance is small, which can enable more sufficient addition and increase the content of hydroxymethyl; the phenolic resin condensed at high temperature has a high molecular weight and high viscosity, but the present application adds a small amount of methanol as a diluent. While introducing diluents such as methanol for dilution, it plays a role in passivating the phenolic resin and greatly extends the product's applicability period.

[0027] 5. The addition of paraformaldehyde and the use of high-concentration formaldehyde solution will cause the reaction to have obvious reaction heat that is difficult to control. The present application uses secondary polycondensation (secondary addition of formaldehyde) and secondary addition of catalysts. In the early stage, only sufficient catalysts for activation and addition are added to disperse the exothermic process of the reaction, reduce the pressure of heat dissipation of the system, make the reaction stable, and facilitate production.

[0028] 6. The water tolerance of the present application, i.e. the dilution multiple of water, is basically 300%-400%. At the same time, in the cooling stage after the reaction, when the temperature drops below 40°C, the polycondensation reaction is inhibited, and the cooling is started and the alkaline solution, such as sodium hydroxide, is slowly added to adjust the water tolerance of the product. The maximum adjustment can be almost infinite, such as increasing to 4000%. The water tolerance is proportional to the amount of sodium hydroxide added. Therefore, the adhesive of the present application can also be used in the dipping process, and has a wide range of applications. DETAILED DESCRIPTION

[0029] The present application is further described in detail below with reference to the embodiments.

[0030] The present application embodiment provides a method for preparing a phenolic resin adhesive for bamboo oriented strand board, comprising the following steps:

[0031] (1) Phenol activation: Phenol and an alkaline catalyst aqueous solution are added to a reactor to activate and form an intermediate state of phenolate anion; in this step, the mass ratio of phenol to alkaline catalyst is 140:(3-3.5).

[0032] (2) Addition stage: Add high-concentration formaldehyde reactant into a reactor at a temperature of 45°C ± 2°C, and keep the temperature to react for 30 to 35 minutes to generate hydroxymethylphenol; wherein the molar ratio of the effective content of formaldehyde in the high-concentration formaldehyde reactant to phenol is (1.1 to 1.6):1, and the water content in the reactor does not exceed 55.75% of the mass of phenol, preferably 49.3%.

[0033] (3) Heating and adding: raising the temperature to 80±2°C, keeping the temperature for 10-12 minutes, adding formaldehyde aqueous solution to the reactor, and keeping the temperature for 20-25 minutes; the concentration of the added formaldehyde aqueous solution is not less than 48wt%; in step (3), the mass ratio of the added formaldehyde to the phenol is (15.54-19.2):140.

[0034] (4) Polycondensation stage: adding an aqueous solution of an alkaline catalyst, heating to 90±2° C., and maintaining the temperature for reaction; wherein the mass ratio of phenol to the alkaline catalyst added in step (4) is 140:(3-4);

[0035] (5) Reaction termination: After keeping the reaction warm for 30 to 35 minutes, after the viscosity reaches 15 to 16 seconds, cool down at 1±0.2°C / min. When the temperature reaches below 60°C, add 5±1% of the total mass of diluent, and wait until the temperature drops to below 30°C before discharging. The diluent is at least one of methanol, ethanol, and isopropanol.

[0036] In the preparation process of the present application, the sources of the high-concentration formaldehyde reactant can be divided into two categories, specifically: ① the high-concentration formaldehyde reactant is a formaldehyde aqueous solution, and the concentration of the formaldehyde aqueous solution is not less than 48wt%; ② the high-concentration formaldehyde reactant is paraformaldehyde and a formaldehyde aqueous solution, wherein the concentration of the formaldehyde aqueous solution is not less than 37wt%. Depending on the source of the high-concentration formaldehyde reactant, the specific operation of step (2) is different, as follows:

[0037] (i) When the high-concentration formaldehyde reactant in step (2) is a formaldehyde aqueous solution, and the concentration of the formaldehyde aqueous solution is not less than 48wt%, the operation of step (2) is as follows: adding the formaldehyde aqueous solution to a reaction kettle at a temperature of 45°C±2°C, adding the formaldehyde aqueous solution dropwise within 50-55 minutes, and then keeping the temperature to react for 30-35 minutes to generate hydroxymethylphenol.

[0038] (ii) When in step (2), the high-concentration formaldehyde reactants are paraformaldehyde and formaldehyde aqueous solution, wherein the concentration of the formaldehyde aqueous solution is not less than 37wt%. At this time, the operation of step (2) is: at a temperature of 45°C±2°C, add paraformaldehyde solid and alkaline catalyst into a reaction kettle, keep warm and stir at 45°C±2°C until the paraformaldehyde solid is dissolved to be clear and transparent, then add the formaldehyde aqueous solution dropwise within 38-53 minutes, and then keep warm and react for 30-35 minutes to generate hydroxymethylphenol. The mass ratio of the paraformaldehyde solid to the alkaline catalyst added in step (2) is 1:(0.07-0.11), and the mass ratio of the paraformaldehyde to the phenol is (0.2-0.25):1.

[0039] In addition, in the preparation process of the present application, the alkaline catalyst used includes at least one of sodium hydroxide, potassium hydroxide, and barium hydroxide, wherein the alkaline catalyst aqueous solution is a saturated aqueous solution.

[0040] The following is an explanation through specific embodiments.

[0041] Example 1

[0042] A method for preparing a phenolic resin adhesive for bamboo oriented strand board comprises the following steps:

[0043] (1) Phenol activation: 140 parts of molten phenol were poured into a reaction kettle, and 3 parts of a 40 wt% aqueous solution of an alkaline catalyst were added. There was obvious heat release, and the mixture was stirred for 20 minutes and the temperature was controlled at 45° C.±2° C. In this step, the mass ratio of phenol to alkaline catalyst was 140:3.

[0044] (2) Addition stage: strictly control the temperature at 45°C ± 2°C, add 124 parts of high-concentration formaldehyde aqueous solution dropwise over 50 minutes, and keep warm for 30 minutes after the addition. At this time, the concentration of the high-concentration formaldehyde aqueous solution is 48%, and the molar ratio of the effective content of formaldehyde participating in the reaction to phenol in the high-concentration formaldehyde reactant is (1.33):1. At this time, the water content in the reactor is 49.3% of the mass of phenol.

[0045] (3) Heating and adding: Raise the temperature to 80°C and keep it for 10 minutes. Add 34 parts of 48% formaldehyde aqueous solution to the reactor over 38 minutes. Keep it for 20 minutes after the addition. At this time, the mass ratio of added formaldehyde to phenol is 16.32:140.

[0046] (4) Polycondensation stage: Pour 3 parts of a 40 wt% aqueous solution of an alkaline catalyst, raise the temperature to 90° C., and keep the temperature for reaction.

[0047] (5) Termination of reaction: After keeping warm for 30 minutes, test the viscosity every 2 minutes. When the viscosity reaches 15.5 seconds using a 4-cup tester, turn off the heating and cool at a rate of 1°C / min. When the temperature reaches below 60°C, add 5% of the total weight of methanol and wait until the temperature drops to below 30°C before discharging.

[0048] Example 2

[0049] A method for preparing a phenolic resin adhesive for bamboo oriented strand board comprises the following steps:

[0050] (1) Phenol activation: 140 parts of molten phenol were poured into a reaction kettle, and 3 parts of a 40 wt% aqueous solution of an alkaline catalyst were added. There was obvious heat release, and the mixture was stirred for 20 minutes and the temperature was controlled at 45° C.±2° C. In this step, the mass ratio of phenol to alkaline catalyst was 140:3.

[0051] (2) Addition stage: Add 28 parts of polyformaldehyde solid and 2 parts of alkaline catalyst into the reaction kettle, keep warm and stir at 45℃±2℃ for 1 hour until the polyformaldehyde solid is dissolved and becomes clear and transparent, then add 88 parts of formaldehyde aqueous solution dropwise within 50 minutes, and keep warm and react for 30 minutes to generate hydroxymethylphenol. At this time, the concentration of the formaldehyde aqueous solution is 37%, and the molar ratio of the effective formaldehyde content participating in the reaction in the high-concentration formaldehyde reactant to phenol is (1.36:1). At this time, the water content in the reaction kettle is 42.9% of the mass of phenol.

[0052] The mass ratio of the polyformaldehyde solid to the alkaline catalyst added in step (2) is 1:0.07, and the mass ratio of the polyformaldehyde to the phenol is 0.2:1.

[0053] (3) Heating and adding: Raise the temperature to 80°C and keep it for 10 minutes, add 40 parts of 48% formaldehyde aqueous solution to the reactor over 45 minutes, and keep it for 20 minutes. At this time, the mass ratio of added formaldehyde to phenol is 19.2:140.

[0054] (4) Polycondensation stage: Pour 3 parts of a 40 wt% aqueous solution of an alkaline catalyst, raise the temperature to 90° C., and keep the temperature for reaction.

[0055] (5) Termination of reaction: After keeping warm for 30 minutes, test the viscosity every 2 minutes. When the viscosity reaches 15.5 seconds using a 4-cup tester, turn off the heating and cool at a rate of 1°C / min. When the temperature reaches below 60°C, add 5% of the total weight of methanol and wait until the temperature drops to below 30°C before discharging.

[0056] Example 3

[0057] A method for preparing a phenolic resin adhesive for bamboo oriented strand board comprises the following steps:

[0058] (1) Phenol activation: 140 parts of molten phenol were poured into a reaction kettle, and 3 parts of a 40 wt% aqueous solution of an alkaline catalyst were added. There was obvious heat release, and the mixture was stirred for 20 minutes and the temperature was controlled at 45° C.±2° C. In this step, the mass ratio of phenol to alkaline catalyst was 140:3.

[0059] (2) Addition stage: Add 28 parts of polyformaldehyde solid and 2 parts of alkaline catalyst into the reaction kettle, keep warm and stir at 45℃±2℃ for 1 hour until the polyformaldehyde solid is dissolved and becomes clear and transparent, then add 88 parts of formaldehyde aqueous solution dropwise within 50 minutes, and keep warm and react for 30 minutes to generate hydroxymethylphenol. At this time, the concentration of the formaldehyde aqueous solution is 37%, and the molar ratio of the effective formaldehyde content participating in the reaction in the high-concentration formaldehyde reactant to phenol is (1.36:1). At this time, the water content in the reaction kettle is 42.9% of the mass of phenol.

[0060] The mass ratio of the polyformaldehyde solid to the alkaline catalyst added in step (2) is 1:0.07, and the mass ratio of the polyformaldehyde to the phenol is 0.2:1.

[0061] (3) Heating and adding: Heating to 80°C and keeping the temperature for 10 minutes, adding 42 parts of 37% formaldehyde aqueous solution to the reactor over 45 minutes, and keeping the temperature for 20 minutes. At this time, the mass ratio of added formaldehyde to phenol is 15.54:140.

[0062] (4) Polycondensation stage: Pour 3 parts of a 40 wt% aqueous solution of an alkaline catalyst, raise the temperature to 90° C., and keep the temperature for reaction.

[0063] (5) Reaction termination: After keeping warm for 30 minutes, test the viscosity every 2 minutes. When the viscosity reaches 15.5 seconds using a 4-cup tester, turn off the heating and cool at a rate of 1°C / min. When the temperature reaches below 60°C, add 3% of the total weight of methanol and wait until the temperature drops to below 30°C before discharging.

[0064] Example 4

[0065] This embodiment provides a method for preparing a phenolic resin adhesive for bamboo oriented strand board, comprising the following steps:

[0066] (1) Phenol activation: Phenol and a 40 wt % aqueous solution of an alkaline catalyst are added to a reactor to activate and form an intermediate state of a phenolate anion; in this step, the mass ratio of phenol to the alkaline catalyst is 140:3.5.

[0067] (2) Addition stage: strictly control the temperature at 45°C ± 2°C, add 112 parts of high-concentration formaldehyde aqueous solution dropwise over 50 minutes, and keep warm for 35 minutes. At this time, the concentration of the high-concentration formaldehyde aqueous solution is 48%, and the molar ratio of the effective content of formaldehyde participating in the reaction to phenol in the high-concentration formaldehyde reactant is (1.2):1. At this time, the water content in the reactor is 45.35% of the mass of phenol.

[0068] (3) Heating and adding: the temperature was raised to 80°C, kept warm for 12 minutes, and formaldehyde aqueous solution was added to the reactor, and kept warm for 25 minutes; the concentration of the added formaldehyde aqueous solution was 48 wt %; in step (3), the mass ratio of the added formaldehyde to the phenol was 19.2:140.

[0069] (4) Polycondensation stage: adding an aqueous solution of an alkaline catalyst, heating to 92° C., and maintaining the temperature for reaction; wherein the mass ratio of phenol to the alkaline catalyst added in step (4) is 140:4;

[0070] (5) Reaction termination: After keeping the temperature for 35 minutes, the viscosity is detected after 15 seconds, and the temperature is lowered at 1.2°C / min. When the temperature reaches below 60°C, 5% of the total weight of diluent is added, and the material is discharged after the temperature drops to below 30°C. The diluent is methanol.

[0071] Example 5

[0072] This embodiment provides a method for preparing a phenolic resin adhesive for bamboo oriented strand board, comprising the following steps:

[0073] (1) Phenol activation: Phenol and an alkaline catalyst aqueous solution are added to a reactor to activate and form an intermediate state of phenolate anion; in this step, the mass ratio of phenol to alkaline catalyst is 140:3.5.

[0074] (2) Addition stage: strictly control the temperature at 45°C ± 2°C, add 140 parts of high-concentration formaldehyde aqueous solution dropwise over 50 minutes, and keep warm for 35 minutes after the addition. At this time, the concentration of the high-concentration formaldehyde aqueous solution is 48%, and the molar ratio of the effective formaldehyde content participating in the reaction to phenol in the high-concentration formaldehyde reactant is (1.5):1. At this time, the water content in the reactor is 55.75% of the mass of phenol.

[0075] (3) Heating and adding: the temperature was raised to 80°C, kept warm for 10 minutes, and formaldehyde aqueous solution was added to the reactor, and kept warm for 20 minutes; the concentration of the added formaldehyde aqueous solution was 48 wt %; in step (3), the mass ratio of the added formaldehyde to the phenol was 16.32:140.

[0076] (4) polycondensation stage: adding an aqueous solution of an alkaline catalyst, heating to 90° C., and maintaining the temperature for reaction; wherein the mass ratio of phenol to the alkaline catalyst added in step (4) is 140:4;

[0077] (5) Reaction termination: After the reaction is kept at room temperature for 35 minutes, the viscosity is detected to reach 16 seconds, and the temperature is lowered at 1°C / min. When the temperature reaches below 60°C, 5% of the total weight of diluent is added, and the material is discharged after the temperature drops to below 30°C. The diluent is methanol.

[0078] Example 6

[0079] A method for preparing a phenolic resin adhesive for bamboo oriented strand board comprises the following steps:

[0080] (1) Phenol activation: 140 parts of molten phenol were poured into a reaction kettle, and 3 parts of a 40 wt% aqueous solution of an alkaline catalyst were added. There was obvious heat release, and the mixture was stirred for 20 minutes and the temperature was controlled at 45° C.±2° C. In this step, the mass ratio of phenol to alkaline catalyst was 140:3.

[0081] (2) Addition stage: Add 35 parts of polyformaldehyde solid and 3.85 parts of alkaline catalyst to the reactor, keep it warm and stir at 45℃±2℃ for 1 hour until the polyformaldehyde solid is dissolved and becomes clear and transparent, then add 60 parts of formaldehyde aqueous solution dropwise within 50 minutes, and keep it warm and react for 30 minutes to generate hydroxymethylphenol. At this time, the concentration of the formaldehyde aqueous solution is 37%, and the molar ratio of the effective formaldehyde content participating in the reaction to phenol in the high-concentration formaldehyde reactant is 1.28:1. At this time, the water content in the reactor is 34.3% of the mass of phenol.

[0082] The mass ratio of the polyformaldehyde solid to the alkaline catalyst added in step (2) is 1:0.11, and the mass ratio of the polyformaldehyde to the phenol is 0.25:1.

[0083] (3) Heating and adding: Raise the temperature to 80°C and keep it for 10 minutes, add 40 parts of 48% formaldehyde aqueous solution to the reactor over 45 minutes, and keep it for 20 minutes. At this time, the mass ratio of added formaldehyde to phenol is 19.2:140.

[0084] (4) Polycondensation stage: Pour 3 parts of a 40 wt% aqueous solution of an alkaline catalyst, raise the temperature to 90° C., and keep the temperature for reaction.

[0085] (5) Termination of reaction: After keeping warm for 30 minutes, test the viscosity every 2 minutes. When the viscosity reaches 15.5 seconds using a 4-cup tester, turn off the heating and cool at a rate of 1°C / min. When the temperature reaches below 60°C, add 5% of the total weight of methanol and wait until the temperature drops to below 30°C before discharging.

[0086] Comparative Example

[0087] Comparative Example 1

[0088] Comparative Example 1 is an MDI adhesive, which is purchased from Wanhua with a brand name of CW20.

[0089] Comparative Example 2

[0090] Comparative Example 2 is a commercially available high-solid phenolic resin adhesive, specifically Hangmo New Material Y6450.

[0091] The difference between Comparative Example 3 and Example 1 is that in step (2), the temperature of the reactor is 70° C., the color changes during the addition stage of the reaction, and polycondensation starts in advance.

[0092] The difference between Comparative Example 4 and Example 2 is that: solid paraformaldehyde and alkaline catalyst are added to the reaction kettle, the mixture is kept warm and stirred at 45°C±2°C for 30 minutes, and then 88 parts of formaldehyde aqueous solution are added dropwise within 50 minutes.

[0093] The difference between Comparative Example 5 and Example 1 is that in step (3), the temperature is raised to 92° C. and kept warm for 10 min.

[0094] The difference between Comparative Example 6 and Example 1 is that in step (3), the temperature is raised to 80° C. and kept for 30 min, and then kept for another 20 min.

[0095] The difference between Comparative Example 7 and Example 1 is that in step (4), 6 parts of a 40 wt% aqueous solution of a basic catalyst are poured in, the temperature is raised to 90°C, and the reaction is kept warm. The temperature is out of control and gelation occurs prematurely.

[0096] The difference between Comparative Example 8 and Example 1 is that in step (4), 3 parts of a 40 wt % aqueous solution of an alkaline catalyst are poured into the mixture, the temperature is raised to 95° C., and the mixture is kept warm for reaction.

[0097] The difference between Comparative Example 9 and Example 1 is that in step (5), the temperature is lowered at 2°C / min.

[0098] Comparative Example 10

[0099] On the basis of Example 1, in step (4), the temperature is raised to 90° C., urea with a mass percentage of 10% of the mass of phenol is added, and then the temperature is kept for reaction.

[0100] Comparative Example 11

[0101] On the basis of Example 1, in step (5), when the temperature reaches below 60°C, soluble starch with a mass of 10% of phenol is added, and the material is discharged after the temperature drops to below 30°C.

[0102] The difference between Comparative Example 12 and Example 2 is that:

[0103] (2) Addition stage: within 50 minutes, add 88 parts of formaldehyde aqueous solution dropwise; then add 28 parts of polyformaldehyde solid and 2 parts of alkaline catalyst into the reaction kettle, keep stirring at 45℃±2℃ for 1 hour until the polyformaldehyde solid is dissolved and becomes clear and transparent, then keep warm for 30 minutes to generate hydroxymethylphenol.

[0104] The difference between Comparative Example 13 and Example 2 is that:

[0105] Addition stage: add 28 parts of polyformaldehyde solid to the reaction kettle, keep stirring at 45℃±2℃ for 1 hour, until the polyformaldehyde solid is dissolved to become clear and transparent, then add 88 parts of formaldehyde aqueous solution dropwise within 50 minutes, and then keep reacting at this temperature for 30 minutes to generate hydroxymethylphenol.

[0106] Performance Testing

[0107] The adhesive obtained in the examples and comparative examples was applied to bamboo shavings by atomizing spraying, stirred, oriented, paved, and hot pressed to obtain a bamboo oriented strand board. In comparative example 1, the amount of MDI adhesive used was 3.3% of the mass of the shavings, and the amount of adhesive used in the other comparative examples and examples was 10% of the mass of the shavings.

[0108] During the process, the glue is sprayed with atomization and oriented paving, and then hot pressed at 160℃ and 4MPa pressure for 90s / mm. After the pressure is released to 0.1-0.2MPa, the pressure is maintained for 2min. The performance test of the particleboard is carried out in accordance with GBT17657-2013. The test results are shown in Table 1

[0109] Table 1 Particleboard performance test table

[0110]

[0111]

[0112] The bamboo oriented strand board for the container bottom plate refers to the requirements of LYT3226-2020. The main indicators are as follows: static bending strength 75MPa, elastic modulus 9000MPa, and internal bonding 1.7MPa.

[0113] Generally, a product that meets the standards can be obtained by using MDI adhesive or a phenolic resin adhesive with a low solid content for impregnation. The performance of the product using MDI adhesive has certain advantages, so this application is compared horizontally with MDI adhesive, and a parallel experiment comparative example 1 is set. At the same time, in order to adopt the direct spraying method to avoid the environmental pollution problems caused by the dipping process, the phenolic resin adhesive is required to have a higher solid content under this requirement, so it is compared with the commercially available high solid content phenolic resin adhesive, that is, comparative example 2.

[0114] It can be found from the data that the particleboard prepared by MDI adhesive in Comparative Example 1 can meet the standard, and the bonding strength of commercially available phenolic resin adhesive is slightly weaker, and it has advantages over MDI in terms of elastic modulus and water absorption expansion rate. This is caused by the structure of phenolic resin, which has better performance in waterproof performance and hardness. However, due to the solid content of commercially available phenolic resin increased by the dehydration process, it will lead to molecular growth during the dehydration process. The wettability of macromolecules is slightly worse than that of MDI with lower molecular weight, making it difficult to penetrate wood to form glue nails, which is ultimately reflected in the bonding strength that cannot meet the standard. And because dehydration and concentration have high requirements on equipment, high vacuum degree, and high process risk, the cost is also higher, which is 150% of the cost of this application. Therefore, the price advantage of this application is obvious.

[0115] The high solid content phenolic resin directly obtained by the process of this application has significant performance advantages over the phenolic resin obtained by the conventional process. Referring to Examples 1-5, it can be found that all indicators have been improved. Under the most preferred process conditions, the static bending strength can reach 105MPa, the elastic modulus can reach 13000MPa, the 24h absorption thickness can reach 2%, and the bonding strength can reach 1.75MPa, which can meet the standard requirements. While giving full play to the advantages, it makes up for the natural disadvantages of phenolic resin compared to MDI, and achieves a bonding strength level close to that of MDI. Moreover, at the same cost, MDI glue only has a static bending strength of 78MPa and an elastic modulus of 8500MPa. In particular, the water absorption expansion rate of MDI reaches 5.2%. Although it meets the standard, as a container bottom plate that may be used in a humid environment for a long time and has high moisture-proof requirements, the phenolic resin of this application can have great advantages.

[0116] With respect to the preparation of the adhesive of the present application, analysis of the performance of comparative example 3 shows that when the reaction temperature of step (2) is too high, polycondensation will start prematurely, affecting product quality.

[0117] Analysis of the performance of Comparative Examples 4, 12 and 13 revealed that, since paraformaldehyde is used to further increase the solid content of the adhesive, two main reactions occur in step (2), namely, the depolymerization of paraformaldehyde and the addition of activated phenol and free formaldehyde. At this time, the control of step (2) is very important. First, for step (2) as a whole, the order of adding paraformaldehyde first and then adding formaldehyde solution should be adopted. In addition, for the depolymerization of paraformaldehyde, its stirring time has a certain influence on the product performance. In addition, the alkaline catalyst added with paraformaldehyde is very important. If it is not added or too little, the depolymerization and addition will be insufficient, and the depolymerization process will be extended to the subsequent reaction. Of course, the amount of alkaline catalyst should not be too much. Too much will lead to excessive total alkali content, too fast reaction, difficult temperature control, faster reaction, and earlier formation of more polycondensation products. Both of the above situations will affect the molecular weight and molecular weight distribution of the phenolic resin, and the bonding performance of the phenolic resin is closely related to the molecular weight and molecular weight distribution, which will lead to a certain degree of performance degradation.

[0118] Analysis of the performance of Comparative Examples 5 and 6 revealed that in step (3), the purpose of heat preservation is to stabilize the temperature. The temperature at this stage should not be too high. Polycondensation has already begun at this stage. If the temperature is too high, as in Comparative Example 5, the polycondensation will be too fast. At the same time, the heat of the polycondensation reaction will be difficult to control, which will reduce the product quality. At the same time, the heat preservation time should not be too long, as in Comparative Example 6. If it is too long, the polycondensation will proceed too much. At this time, the addition of formaldehyde will not have a sufficient reaction effect, which will still affect the product quality. In addition, the subsequent slow addition of formaldehyde is to make full use of the reaction sites of phenol, reduce free phenol and increase the crosslinking density of the adhesive during curing to improve the performance.

[0119] Analysis of Comparative Examples 7 and 8 revealed that in step (4), the catalyst was added at this stage to increase the subsequent polycondensation reaction rate to narrow the molecular weight distribution of the phenolic resin, reduce production time, and fully react to reduce free aldehyde and free phenol. At the same time, the reaction temperature should be strictly controlled at 90±2°C to obtain a phenolic resin with a narrow distribution and a high molecular weight. The high molecular weight phenolic resin can reduce the internal stress after hot pressing.

[0120] Analysis of the performance of Comparative Example 9 revealed that the cooling rate in step (5) should not be too fast, as this would lead to unevenness and thus affect product quality.

[0121] In addition, in combination with Comparative Example 10, it is found that the addition of urea will lead to the introduction of a large number of flexible segments, increase brittleness, and produce adverse effects in application, such as a significant decrease in elastic modulus. However, the present application can obtain an adhesive product with excellent performance through a simple process flow, as long as the relevant process parameters are strictly controlled, and an adhesive product with a high solid content can be obtained without the need for dehydration, addition of urea modification, and other operations; it avoids the problem of increased viscosity of the adhesive, long production cycle, high energy consumption, and high cost caused by dehydration, and also avoids the problem that the addition of urea will lead to the introduction of a large number of flexible segments, increased brittleness, and adverse effects in application. Analysis of the performance of Comparative Example 11 shows that, like urea modification, as a common modification method for increasing solid content, soluble starch modification will also result in performance loss. Among them, urea mainly affects the elastic modulus, while starch has a significant decrease in bonding strength while maintaining a high elastic modulus. Both modifications have a greater impact on waterproof performance.

[0122] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make modifications to the present embodiment without any creative contribution as needed, but such modifications are protected by the patent law as long as they are within the scope of the claims of the present application.

Claims

1. A method for preparing a phenolic resin adhesive for bamboo oriented strand board, characterized in that: The following steps are involved: (1) Phenol activation: Phenol and an alkaline catalyst aqueous solution are added to a reactor to activate and form an intermediate state of phenolate anion; (2) Addition stage: Add high-concentration formaldehyde reactant into a reactor at a temperature of 45°C ± 2°C and keep the temperature to react to generate hydroxymethylphenol; wherein the molar ratio of the effective content of formaldehyde in the high-concentration formaldehyde reactant to phenol is (1.2-1.5):1, and the water content in the reactor does not exceed 55.75% of the mass of phenol. (3) Heating and adding: raising the temperature to 80±2°C, keeping the temperature for 10-12 minutes, adding formaldehyde aqueous solution to the reactor, and keeping the temperature for 20-25 minutes; the concentration of the added formaldehyde aqueous solution is not less than 37 wt%; (4) Polycondensation stage: adding an aqueous solution of an alkaline catalyst, heating to 90±2° C., and maintaining the temperature for reaction; wherein the mass ratio of phenol to the alkaline catalyst added in step (4) is 140:(3-4); (5) Reaction termination: After the reaction is kept warm for 30 to 35 minutes, the viscosity is detected to reach 15 to 16 seconds, and then the temperature is lowered at 1±0.2°C / min to obtain the adhesive.

2. The method for preparing a phenolic resin adhesive for bamboo oriented strand board according to claim 1, characterized in that: In the step (1), the mass ratio of phenol to alkaline catalyst is 140:(3-3.5).

3. The method for preparing a phenolic resin adhesive for bamboo oriented strand board according to claim 1, characterized in that: In the step (2), the high-concentration formaldehyde reactant is a formaldehyde aqueous solution, and the concentration of the formaldehyde aqueous solution is not less than 48 wt %. In the step (3), the concentration of the added formaldehyde aqueous solution is not less than 48 wt %.

4. The method for preparing a phenolic resin adhesive for bamboo oriented strand board according to claim 3, characterized in that: In the step (2), the formaldehyde aqueous solution is added dropwise within 50 to 55 minutes.

5. The method for preparing a phenolic resin adhesive for bamboo oriented strand board according to claim 1, characterized in that: In the step (2), the high-concentration formaldehyde reactants are paraformaldehyde and formaldehyde aqueous solution, wherein the concentration of the formaldehyde aqueous solution is not less than 37 wt %; and in the step (3), the concentration of the added formaldehyde aqueous solution is not less than 37 wt %.

6. The method for preparing a phenolic resin adhesive for bamboo oriented strand board according to claim 5, characterized in that: In the step (2), the polyformaldehyde solid and the alkaline catalyst are added to the reaction kettle, and the mixture is stirred at 45°C±2°C until the polyformaldehyde solid is dissolved and becomes clear and transparent, and then the formaldehyde aqueous solution is added dropwise.

7. The method for preparing a phenolic resin adhesive for bamboo oriented strand board according to claim 6, characterized in that: In the step (2), the formaldehyde aqueous solution is added dropwise within 38 to 53 minutes.

8. The method for preparing a phenolic resin adhesive for bamboo oriented strand board according to claim 6, characterized in that: In the step (2), the mass ratio of the polyformaldehyde solid to the alkaline catalyst added in the step (2) is 1:(0.067-0.11).

9. The method for preparing a phenolic resin adhesive for bamboo oriented strand board according to claim 6, characterized in that: In the step (2), the mass ratio of the paraformaldehyde to the phenol is (0.2-0.25):

1.

10. The method for preparing a phenolic resin adhesive for bamboo oriented strand board according to claim 1, characterized in that: In the step (2), the reaction is kept warm for 30 to 35 minutes.

11. The method for preparing a phenolic resin adhesive for bamboo oriented strand board according to claim 1, characterized in that: In the step (3), the mass ratio of the added formaldehyde to the added phenol is (15.54-19.2):

140.

12. The method for preparing a phenolic resin adhesive for bamboo oriented strand board according to claim 1, characterized in that: In the step (5), when the temperature reaches below 60° C., 4±1% of the total mass of a diluent is added, and the material is discharged after the temperature drops to below 30° C.; the diluent is at least one of methanol, ethanol, and isopropanol.

13. The method for preparing a phenolic resin adhesive for bamboo oriented strand board according to claim 1, characterized in that: In the step (5), when the temperature reaches below 40°C, the alkaline solution is slowly added dropwise, and the material is discharged after the temperature drops to below 30°C.

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