CN111393593B

CN111393593B - Demethylated lignin liquid modified phenolic resin, preparation method thereof and application thereof in phenolic foam

Info

Publication number: CN111393593B
Application number: CN202010139653.3A
Authority: CN
Inventors: 朱晨杰; 胡有钦; 李明; 应汉杰; 庄伟�; 高成; 陈勇; 柳东; 牛欢青; 杨朋朋
Original assignee: Nanjing Tech University
Current assignee: Nanjing Tech University
Priority date: 2020-03-03
Filing date: 2020-03-03
Publication date: 2020-09-25
Anticipated expiration: 2040-03-03
Also published as: CN111393593A

Abstract

The invention discloses a demethylated lignin liquid modified phenolic resin, a preparation method thereof and application thereof in phenolic foam, wherein the modified phenolic resin has a pH value of 8.3-8.6, a solid content of 75-80%, a viscosity of 1800-4000 cp and a gel time of 80-100 s. The invention has the following advantages: (1) the lignin is demethylated by halogen acid through microwave assistance, the reaction sites of the demethylated lignin are increased, and the solvent used for lignin modification is phenol, so the demethylated lignin liquid does not need to be further treated, and can directly replace phenol to prepare phenolic resin and phenolic foam, thereby simplifying the production flow of the resin and the foam. (2) The phenolic foam prepared by the method has lower heat conductivity coefficient, stronger mechanical property and higher oxygen index than unmodified lignin-based phenolic foam, and has market application prospect.

Description

Demethylated lignin liquid modified phenolic resin, preparation method thereof and application thereof in phenolic foam

Technical Field

The invention belongs to the field of heat insulation materials, and particularly relates to demethylated lignin liquid modified phenolic resin, a preparation method thereof and application thereof in phenolic foam.

Background

The phenolic foam is thermosetting rigid foam plastic which is prepared by taking phenolic resin as a main raw material and performing crosslinking, foaming and curing in the presence of a foaming agent, a surfactant, a curing agent and other auxiliaries. The phenolic foam plastic has the unique advantages of excellent flame retardance, thermal stability, heat insulation, low combustion toxicity, corrosion resistance, high modulus and the like, is widely applied to the adjacent areas of buildings, petrochemical industry, vehicles, ships, aerospace industry and the like, and is known as the king of heat preservation due to the performance characteristics of the phenolic foam plastic.

The lignin is an aromatic high polymer with a three-dimensional network structure formed by connecting S, G, H three phenylpropane units through C-O-C and C-C bonds, is a unique high-capacity renewable raw material consisting of aromatic hydrocarbons in nature, has high similarity with a molecular structure of phenolic foam (resin), contains a large amount of phenol structures, can replace part of phenol to prepare lignin-based phenolic resin, and achieves the purposes of reducing the production cost and reducing the use of fossil fuel resources.

In chinese patent application CN110041481, a demethylated lignin phenolic adhesive modified by halogen acid is disclosed, alkali lignin is activated by halogen acid, and a part of phenol is replaced, so as to prepare the phenolic adhesive. The temperature required in the lignin modification process is high (120-130 ℃), the required reaction time is long (12-20 h), the solvent required for modification is DMF, and the adhesive can be prepared by adding acid, filtering and washing after modification. In chinese patent application CN102250363B, a lignin modification method is disclosed, in which lignin is demethylated by organic solvent, mercaptan and alkaline catalyst in a certain mass ratio, and then the demethylated lignin is obtained by centrifugation, washing and drying. The temperature required in the lignin modification process is higher (more than or equal to 100 ℃), and the mercaptan and the organic solvent required for modification can be removed after the modification is finished to prepare the subsequent phenolic resin, so that the environment and the production cost are influenced to a certain extent.

According to the microwave-assisted heating mode of the Xiayinglong, lignin is subjected to demethylation modification in an HBr/hexadecyl tri-n-butyl phosphonium bromide (HBr/TBHDPB) system, and compared with the phenolic hydroxyl group-containing raw material lignin of the prepared modified lignin, the activity of the lignin in the reaction with formaldehyde is improved by 32.71 percent and 18.15 percent. The hexadecyl tri-n-butyl phosphorus bromide used in the modification has higher price, and the modification needs to be carried out in DMF, so the industrial application is difficult.

Disclosure of Invention

The purpose of the invention is as follows: the invention aims to solve the technical problem of providing a demethylated lignin liquid modified phenolic resin aiming at the defects of the prior art.

The technical problem to be solved by the invention is to provide the preparation method of the modified phenolic resin.

The invention finally aims to solve the technical problem of providing the application of the modified phenolic resin in phenolic foam.

The invention idea is as follows: untreated lignin is less reactive and poorly polymerizes with formaldehyde due to the hindrance of the methoxy groups on the aromatic rings. The phenolic resin prepared from the unmodified lignin has the defects of low activity and high viscosity, greatly hinders the stability and uniformity of foaming of the phenolic resin, and further reduces the mechanical property, the heat insulation property and the flame retardant property of the foam. The existing reported lignin demethylation needs higher temperature and pressure and longer reaction time, and needs to modify lignin by taking an organic solvent as a solvent, and the modified lignin can be obtained by filtering and washing for downstream phenolic resin preparation, so that the preparation process is complex and the industrial application is difficult.

In order to solve the technical problems, the invention discloses a demethylated lignin liquid modified phenolic resin, which is prepared by demethylated lignin liquid; the pH value of the demethylated lignin liquid modified phenolic resin is 8.3-8.6, the solid content is 75 wt% -80 wt%, the viscosity is 1800-4000 cp, and the gel time is 80-100 s.

The demethylated lignin liquid modified phenolic resin is prepared by reacting demethylated lignin liquid with paraformaldehyde, an alkali catalyst and water.

The demethylated lignin liquid is prepared by the following method: mixing lignin, phenol and aqueous solution of halogen acid, reacting, and cooling to obtain demethylated lignin solution.

Wherein the lignin is any one or a combination of more of enzymolysis lignin, black liquor lignin, lignosulfonate and organic solvent lignin.

Wherein, the halogen acid is one or the combination of two of hydrobromic acid and hydroiodic acid, and the mass fraction of the halogen acid in the aqueous solution of the halogen acid is 45-50 percent (preferably 48 percent); the mass ratio of the lignin to the phenol is 1: 2-10; the mass ratio of the aqueous solution of the halogen acid to the lignin is 0.7: 1-1; the reaction is carried out in microwaves at 80-120 ℃ (the highest power of the microwaves is 1000W, and the instrument can automatically adjust the power according to the set temperature) for 1-4 h; cooling to 40-60 ℃.

The demethylated lignin liquid modified phenolic resin is prepared from the following raw materials in parts by weight:

wherein, the alkali catalyst is any one or a combination of more of sodium hydroxide, potassium hydroxide, magnesium hydroxide, triethylamine, magnesium oxide and calcium oxide.

The preparation method of the demethylated lignin liquid modified phenolic resin is also within the protection scope of the invention, and comprises the following steps:

(1) mixing phenol, lignin and a halogen acid aqueous solution according to parts by weight, reacting for 1-4 hours at 80-120 ℃ in a microwave, and cooling to 40-60 ℃ to obtain demethylated lignin liquid; adjusting the pH value of the demethylated lignin liquid to be 8.5-9.0:

(2) adding part of paraformaldehyde, part of base catalyst and part of water into the liquid with the pH adjusted in the step (1) for reaction;

(3) and (3) adding the residual paraformaldehyde, an alkali catalyst and water into the reaction liquid obtained in the step (2), reacting, and cooling to obtain the demethylated lignin liquid modified phenolic resin.

In step (1), the pH adjusting agent is an aqueous solution of sodium hydroxide (30 wt%).

In the step (2), the dosage of the partial paraformaldehyde is 40-60% (preferably 50%) of the total mass of the paraformaldehyde; the dosage of the partial alkali catalyst is 40-60% (preferably 50%) of the total mass of the total alkali catalyst; the dosage of the part of water is 40 to 60 percent (preferably 50 percent) of the total mass of the water; the reaction temperature is 80-90 ℃, and the reaction time is 1-2 h.

In the step (3), the reaction temperature is 70-90 ℃, and the reaction time is 0.5-1 h; cooling to 40-70 deg.C (preferably 65 deg.C).

The application of the demethylated lignin liquid modified phenolic resin in the preparation of phenolic foam is also within the protection scope of the invention.

The phenolic foam comprises the following components in parts by weight:

wherein, the foaming agent is any one or the combination of more of n-pentane, n-butane, isopentane, petroleum ether, polyvinyl alcohol aqueous solution and diisopropyl ether.

Wherein, the curing agent is any one or the combination of a plurality of sulfuric acid, hydrochloric acid, phosphoric acid, hydrobromic acid, benzenesulfonic acid, p-toluenesulfonic acid and naphthalenesulfonic acid.

Wherein the surfactant is any one or combination of more of polysiloxane, polyoxyethylene ether, polyoxypropylene, polyoxyethylene polyoxypropylene, polyethylene sorbitan fatty acid, polydimethylsiloxane and tween series.

Preferably, the phenolic foam further comprises any one or a combination of several of a toughening agent, a plasticizer and a flame retardant.

The toughening agent is any one or a combination of more of polyurethane prepolymer, polyethylene glycol, polyvinyl alcohol, polyester polyol, nitrile rubber and epoxy resin.

Wherein, the flame retardant is one or the combination of more of brominated cresol glycidyl ester, magnesium hydroxide, aluminum hydroxide, halogenated phosphate ester and zinc borate.

The preparation method of the phenolic foam comprises the steps of uniformly mixing all the components, pouring the mixture into a mould, and heating and foaming the mixture.

The uniformly mixing means that the demethylated lignin liquid modified phenolic resin and the surfactant are uniformly mixed at the rotating speed of 500-2500 rpm, then the foaming agent is added into the mixed system and uniformly mixed at the rotating speed of 500-1500 rpm, and finally the curing agent is added into the mixed system and uniformly mixed at the rotating speed of 1000-2500 rpm.

Wherein the foaming temperature is 70-90 ℃ and the time is 0.3-3 h.

Has the advantages that: compared with the prior art, the invention has the following advantages:

(1) according to the invention, under the microwave-assisted condition, the lignin is demethylated by using the halogen acid, the reaction sites of the demethylated lignin are increased, and the solvent used for lignin modification is phenol, so that the demethylated lignin liquid does not need to be further treated, and can directly replace phenol to prepare the phenolic resin and phenolic foam, thereby simplifying the production process of the resin and the foam.

(2) The microwave can also reduce the activation energy of the reaction and change the reaction kinetics, thereby improving the reaction rate and reducing the temperature and time required by the reaction.

(3) Due to the increase of the reaction activity of the demethylated lignin, the gel time of the prepared demethylated lignin liquid modified phenolic resin is reduced, the resin activity is improved, the content of free formaldehyde is low, and the stability of the subsequent resin foaming is enhanced.

(4) The phenolic foam prepared by the method has lower thermal conductivity (reduced by 19-57%), stronger mechanical property (improved by 12-82% in compressive strength and improved by 13-67% in tensile strength) and higher oxygen index (improved by 7-25%) than that of unmodified lignin-based phenolic foam, and has market application prospect.

Detailed Description

The invention will be better understood from the following examples. However, those skilled in the art will readily appreciate that the description of the embodiments is only for illustrating the present invention and should not be taken as limiting the invention as detailed in the claims.

The detection method of each parameter in the following implementation is as follows:

gel time: the national standard GB/T33315-2016;

pH: the national standard GB T32364-2015;

free formaldehyde: the national standard GBT 32684-2016;

solid content, viscosity: GBT 14074-;

properties of the foam: national standard GBT 20974-2014.

Example 1

Taking 100 parts of phenol, adding 20 parts of lignin, adding 8 parts of hydrobromic acid aqueous solution with the mass fraction of 48%, placing the mixture in a normal-pressure microwave reactor to react for 2 hours at the temperature of 80 ℃, and cooling to 50 ℃ to obtain demethylated lignin liquid; adding 30 wt% of sodium hydroxide aqueous solution into the demethylated lignin liquid, and adjusting the pH value to 8.5-9.0; adding 45 parts of paraformaldehyde, 8 parts of an alkaline catalyst (sodium hydroxide) and 35 parts of water in two batches based on 100 parts of phenol, adding 50% by mass of paraformaldehyde, 50% by mass of the alkaline catalyst and 50% by mass of water for the first time, heating to 80 ℃, reacting for 1h, adding the rest paraformaldehyde, the alkaline catalyst and water into the reaction solution again, reacting for 0.5h at 80 ℃, and cooling to 65 ℃ to obtain the demethylated lignin liquid modified phenolic resin; the pH value of the phenolic resin is 8.4, the solid content is 76 wt%, the viscosity is 2920cp, and the gel time (150 ℃) is 85 s.

And (3) taking 100 parts of the hairstyle phenolic resin and 3 parts of polydimethylsiloxane (DC-193), and stirring at 1500rpm of a mechanical stirrer for 60 seconds until the mixture is uniformly stirred. 9 parts of n-pentane was added thereto, and the mixture was stirred with a mechanical stirrer at 1000rpm for 30 seconds until the mixture was uniformly stirred. Adding 17 parts of curing agent (p-toluenesulfonic acid: sulfuric acid: 1), 10 parts of polyethylene glycol and 10 parts of brominated cresol glycidyl ester, stirring for 30s at 2000rpm of a mechanical stirrer until the mixture is uniformly stirred, introducing the mixture into a mold, and curing for 2h at 70 ℃ to obtain the lignin-based phenolic foam material.

Example 2

Taking 100 parts of phenol, adding 30 parts of lignin, adding 7 parts of hydrobromic acid aqueous solution with the mass fraction of 48%, placing the mixture in a normal-pressure microwave reactor to react for 2 hours at the temperature of 90 ℃, and cooling to 50 ℃ to obtain demethylated lignin liquid; adding 30 wt% of sodium hydroxide aqueous solution into the demethylated lignin liquid, and adjusting the pH value to 8.5-9.0; adding 42 parts of paraformaldehyde, 8.5 parts of an alkaline catalyst (sodium hydroxide) and 40 parts of water in two batches based on 100 parts of phenol, adding 50% by mass of paraformaldehyde, 50% by mass of the alkaline catalyst and 50% by mass of water for the first time, heating to 90 ℃ for reaction for 1 hour, adding the rest paraformaldehyde, the alkaline catalyst and water into the reaction solution again, reacting at 90 ℃ for 0.5 hour, and cooling to 65 ℃ to obtain the demethylated lignin liquid modified phenolic resin; the pH value of the phenolic resin is 8.5, the solid content is 78 wt%, the viscosity is 3280cp, and the gel time (150 ℃) is 82 s.

100 parts of the above hairstyle-changeable phenolic resin and 3.5 parts of polydimethylsiloxane (DC-193) are taken, and stirred for 60 seconds at 1500rpm of a mechanical stirrer until the mixture is uniformly stirred. 7 parts of n-pentane is added and the mixture is stirred for 30s at 1000rpm by a mechanical stirrer until the mixture is uniformly stirred. Adding 16 parts of curing agent (p-toluenesulfonic acid: sulfuric acid: 1), 10 parts of polyethylene glycol and 10 parts of brominated cresol glycidyl ester, stirring for 30s at 2000rpm of a mechanical stirrer until the mixture is uniformly stirred, introducing the mixture into a mold, and curing for 2h at 75 ℃ to obtain the lignin-based phenolic foam material.

Example 3

Taking 100 parts of phenol, adding 40 parts of lignin, adding 6 parts of hydrobromic acid aqueous solution with the mass fraction of 48%, placing the mixture in a normal-pressure microwave reactor to react for 1.5h at 100 ℃, and cooling to 50 ℃ to obtain demethylated lignin liquid; adding 30 wt% of sodium hydroxide aqueous solution into the demethylated lignin liquid, and adjusting the pH value to 8.5-9.0; adding 38 parts of paraformaldehyde, 9 parts of an alkaline catalyst (sodium hydroxide) and 42 parts of water in two batches based on 100 parts of phenol, adding 50% by mass of paraformaldehyde, 50% by mass of the alkaline catalyst and 50% by mass of water for the first time, heating to 85 ℃, reacting for 1h, adding the rest paraformaldehyde, the alkaline catalyst and water into the reaction solution again, reacting for 0.5h at 85 ℃, and cooling to 65 ℃ to obtain the demethylated lignin liquid modified phenolic resin; the pH value of the phenolic resin is 8.6, the solid content is 80 wt%, the viscosity is 3920cp, and the gel time (150 ℃) is 80 s.

And (3) taking 100 parts of the hairstyle phenolic resin and 4 parts of polydimethylsiloxane (DC-193), and stirring for 60 seconds at 1500rpm of a mechanical stirrer until the mixture is uniformly stirred. 8 parts of n-pentane is added, and the mixture is stirred for 30 seconds at 1000rpm of a mechanical stirrer until the mixture is uniformly stirred. Adding 17 parts of curing agent (p-toluenesulfonic acid: phosphoric acid: water: 7: 3: 3), 10 parts of polyethylene glycol and 10 parts of aluminum hydroxide, stirring for 30s at 2000rpm of a mechanical stirrer until the mixture is uniformly stirred, introducing the mixture into a mold, and curing for 2h at 80 ℃ to obtain the lignin-based phenolic foam material.

Comparative example 1

Heating 100 parts of phenol, 10 parts of an alkaline catalyst (sodium hydroxide) and 20 parts of lignin at 90 ℃ for 2h, then adding 150 parts of 37 wt% formaldehyde aqueous solution in three batches, heating at 90 ℃ for 2h, then cooling to 65 ℃ and reacting for 30 min. Finally, adding dilute hydrochloric acid with the pH value of 1 to adjust the pH value of the mixed solution to 7, and performing reduced pressure dehydration until the solid content is 75% to obtain the expandable phenolic resin; the pH value of the phenolic resin is 7.0, the solid content is 75 wt%, the viscosity is 3050cp, and the gel time (150 ℃) is 110 s.

And (3) taking 100 parts of the hairstyle phenolic resin and 3 parts of polydimethylsiloxane (DC-193), and stirring at 1500rpm of a mechanical stirrer for 60 seconds until the mixture is uniformly stirred. 9 parts of n-pentane was added thereto, and the mixture was stirred with a mechanical stirrer at 1000rpm for 30 seconds until the mixture was uniformly stirred. Adding 17 parts of curing agent (p-toluenesulfonic acid: sulfuric acid: 1), 10 parts of polyethylene glycol and 10 parts of brominated cresol glycidyl ester, stirring for 30s at 2000rpm of a mechanical stirrer until the mixture is uniformly stirred, introducing the mixture into a mold, and curing for 2h at 80 ℃ to obtain the lignin-based phenolic foam material.

Comparative example 2

Heating 100 parts of phenol, 10 parts of an alkaline catalyst (sodium hydroxide) and 30 parts of lignin at 85 ℃ for 2h, then adding 140 parts of 37 wt% formaldehyde aqueous solution in three batches, heating at 80 ℃ for 2h, then cooling to 70 ℃ and reacting for 30 min. Finally, adding dilute hydrochloric acid with the pH value of 1 to adjust the pH value of the mixed solution to 7, and performing reduced pressure dehydration until the solid content is 75% to obtain the expandable phenolic resin; the phenolic resin has a pH value of 7.0, a solid content of 75 wt%, a viscosity of 3460cp and a gel time (150 ℃) of 95 s.

100 parts of the above hairstyle-changeable phenolic resin and 3.5 parts of polydimethylsiloxane (DC-193) are taken, and stirred for 60 seconds at 1500rpm of a mechanical stirrer until the mixture is uniformly stirred. 9 parts of n-pentane was added thereto, and the mixture was stirred with a mechanical stirrer at 1000rpm for 30 seconds until the mixture was uniformly stirred. Adding 17 parts of curing agent (p-toluenesulfonic acid: sulfuric acid: 1), 10 parts of polyethylene glycol and 10 parts of brominated cresol glycidyl ester, stirring for 30s at 2000rpm of a mechanical stirrer until the mixture is uniformly stirred, introducing the mixture into a mold, and curing for 2h at 70 ℃ to obtain the lignin-based phenolic foam material.

Comparative example 3

Heating 100 parts of phenol, 10 parts of alkaline catalyst (sodium hydroxide) and 40 parts of lignin for 1.5h at 90 ℃, then adding 145 parts of 37 percent by weight of formaldehyde aqueous solution in three batches, heating for 2h at 85 ℃, then cooling to 70 ℃, and reacting for 30 min. Finally, adding dilute hydrochloric acid with the pH value of 1 to adjust the pH value of the mixed solution to 7, and performing reduced pressure dehydration until the solid content is 75 wt% to obtain the expandable phenolic resin; the pH value of the phenolic resin is 7.0, the solid content is 75%, the viscosity is 3990cp, and the gel time (150 ℃) is 102 s.

And (3) taking 100 parts of the hairstyle phenolic resin and 4 parts of polydimethylsiloxane (DC-193), and stirring for 60 seconds at 1500rpm of a mechanical stirrer until the mixture is uniformly stirred. 9 parts of n-pentane was added thereto, and the mixture was stirred with a mechanical stirrer at 1000rpm for 30 seconds until the mixture was uniformly stirred. Adding 17 parts of curing agent (p-toluenesulfonic acid: sulfuric acid: 1), 10 parts of polyethylene glycol and 10 parts of brominated cresol glycidyl ester, stirring for 30s at 2000rpm of a mechanical stirrer until the mixture is uniformly stirred, introducing the mixture into a mold, and curing for 2h at 75 ℃ to obtain the lignin-based phenolic foam material.

Comparative example 4

Taking 100 parts of phenol, adding 30 parts of lignin, adding 7 parts of hydrobromic acid aqueous solution with the mass fraction of 48%, placing the mixture in a four-neck reaction bottle, reacting at 90 ℃ for 2h, and cooling to 50 ℃ to obtain demethylated lignin liquid; adding 30 wt% of sodium hydroxide aqueous solution into the demethylated lignin liquid, and adjusting the pH value to 8.5-9.0; adding 42 parts of paraformaldehyde, 8.5 parts of an alkaline catalyst (sodium hydroxide) and 40 parts of water in two batches based on 100 parts of phenol, adding 50% by mass of paraformaldehyde, 50% by mass of the alkaline catalyst and 50% by mass of water for the first time, heating to 90 ℃ for reaction for 1 hour, adding the rest paraformaldehyde, the alkaline catalyst and water into the reaction solution again, reacting at 90 ℃ for 0.5 hour, and cooling to 65 ℃ to obtain the demethylated lignin liquid modified phenolic resin; the pH value of the phenolic resin is 8.5, the solid content is 78 wt%, the viscosity is 3660cp, and the gel time (150 ℃) is 90 s.

Various properties of the lignin-based phenolic foam prepared in examples 1-3 and comparative examples 1-4 were characterized as shown in Table 1.

TABLE 1

By observing examples 1 to 3 and comparative examples 1 to 4, it is found that compared with unmodified lignin and lignin modified directly with halogen acid (without microwave), the phenolic foam prepared by using microwave-assisted halogen acid demethylated lignin as a raw material has obviously improved compressive strength and tensile strength, and certain improvement on thermal conductivity and oxygen index, and meanwhile, the content of free formaldehyde in resin prepared by using microwave-assisted halogen acid demethylated lignin as a raw material is also obviously reduced, which indicates that the active sites of lignin are increased and the reactivity with formaldehyde is enhanced.

The method for preparing the phenolic resin and the phenolic foam by taking the microwave-assisted halogen acid demethylation lignin as the raw material increases the content of phenolic hydroxyl groups of the lignin and improves the reaction activity of the lignin. And the phenolic resin prepared from the demethylated lignin has short gelling time and high reactivity, and the phenolic foam prepared from the demethylated lignin has excellent mechanical property, fireproof property and heat preservation property, and has a prospect of large-scale application in the market.

Claims

1. The demethylated lignin liquid modified phenolic resin is characterized in that the pH value of the modified phenolic resin is 8.3-8.6, the solid content is 75-80 wt%, the viscosity is 1800-4000 cp, and the gel time is 80-100 s;

the demethylated lignin liquid modified phenolic resin is prepared by reacting demethylated lignin liquid with paraformaldehyde, an alkali catalyst and water;

the demethylated lignin liquid is prepared by the following method: mixing lignin, phenol and aqueous solution of halogen acid, reacting, and cooling to obtain demethylated lignin solution;

the halogen acid is any one or combination of hydrobromic acid and hydroiodic acid, and the mass fraction of the halogen acid in the aqueous solution of the halogen acid is 45-50%; the mass portion ratio of the phenol, the lignin and the aqueous solution of the halogen acid is 100 portions: 10-50 parts of: 2-10 parts; the reaction is carried out in microwave at 80-120 ℃ for 1-4 h; cooling to 40-60 ℃.

2. The demethylated lignin liquid modified phenolic resin of claim 1, wherein the demethylated lignin liquid modified phenolic resin is prepared from the following raw materials in parts by mass:

100 portions of phenol

10-50 parts of lignin

2-10 parts of aqueous solution of halogen acid

40-50 parts of paraformaldehyde

2-8 parts of alkali catalyst

20-35 parts of water.

3. The method for preparing the demethylated lignin liquid modified phenolic resin according to claim 1 or 2, comprising the following steps:

(3) and (3) adding the residual paraformaldehyde, the residual alkali catalyst and the residual water into the reaction liquid obtained in the step (2), reacting, and cooling to obtain the demethylated lignin liquid modified phenolic resin.

4. The method for preparing the demethylated lignin liquid modified phenolic resin as claimed in claim 3, wherein in the step (2), the amount of the part of paraformaldehyde is 40-60% of the total mass of paraformaldehyde; the dosage of the partial alkali catalyst is 40-60% of the total mass of the alkali catalyst; the using amount of the partial water is 40-60% of the total using mass of the water; the reaction temperature is 80-90 ℃, and the reaction time is 1-2 h.

5. The method for preparing the demethylated lignin liquid modified phenolic resin according to claim 3, wherein in the step (3), the reaction temperature is 70-90 ℃ and the reaction time is 0.5-1 h; cooling to 40-70 ℃.

6. Use of the demethylated lignin liquid-modified phenolic resin of claim 1 in the preparation of phenolic foam.

7. The use of claim 6, wherein the phenolic foam comprises the following components in parts by weight:

100 parts of demethylated lignin liquid modified phenolic resin;

5-15 parts of a foaming agent;

15-25 parts of a curing agent;

2-10 parts of a surfactant.