Thermosetting starch adhesive for wood-based panel and preparation method therefor

Abstract

The present disclosure discloses a thermosetting starch adhesive for a wood-based panel and a preparation method therefor, and belongs to the technical field of preparation of adhesives. In the present disclosure, starch is used as a main raw material, and after acid hydrolysis thereof, a semi-continuous seed emulsion polymerization method is adopted to improve control of monomer polymerization stability. After grafting is completed, a cross-linking monomer with polymerizable double bonds and condensable methylol functional groups is added for copolymerization. The cross-linking monomer is also added in a semi-continuous manner. After the cross-linking reaction is completed, the reaction mixture is gelatinized and incubated, and finally a thermosetting adhesive which can be used for bonding of hot-pressed wood-based panels is obtained. The adhesive of the present disclosure has the properties of two-step cross-linking and high-temperature rapid cross-linking curing, good film formation, good heat resistance, and strong cohesion. With a hot-pressing process in an appropriate coating manner, appropriate temperature and pressure, the resultant wood-based panels have all their properties superior over those of Class II wood-based panels, and they can be widely used in decoration of hot-pressed woods, and bonding of wood-based panels and plywoods.

Claims

1. A preparation method for a thermosetting starch adhesive wherein thermosetting starch adhesive for a wood-based panel comprises, in parts by mass, 100 parts of starch, 300-500 parts of water, 6-18 parts of concentrated hydrochloric acid, 10-20 parts of sodium hydroxide, 1-2 parts of an initiator, 60-80 parts of a grafting monomer, 2-9 parts of a crosslinking monomer, 1-3 parts of an emulsifier, 0-10 parts of a diluent, 10-25 parts of an antifoaming agent, and 0.01-0.04 part of a polymerization inhibitor; and wherein the preparation method comprises: preparing a seed emulsion by stirring a part of the grafting monomer, the emulsifier, and the initiator well to be used as the seed emulsion; performing acid hydrolysis of a starch emulsion made of the starch as a main raw material, adjusting pH of the starch emulsion to pH 4-5, and then adding the seed emulsion to the starch emulsion to form a reaction mixture for starting a grafting reaction; adding a first portion of the remaining initiator and gradually adding dropwise the remaining grafting monomer to the reaction mixture, incubating the reaction mixture for 3-5 hours, and adding a second portion of the remaining initiator and the cross-linking monomer by a semi-continuous method to react for 2-6 hours; heating the reaction mixture, and incubating for 0.5-1 hour; next, removing the residual monomer; and finally, cooling down the reaction mixture, adding the polymerization inhibitor and urea, and discharging the resulting mixture after mixing well, thereby obtaining the thermosetting starch adhesive.

2. The preparation method according to claim 1, wherein the crosslinking monomer comprises one or more of N-hydroxyacrylamide derivatives of N-methylol acrylamide, N-methoxymethacrylamide, N-isopropoxymethacrylamide, and N-butoxymethacrylamide.

3. The preparation method according to claim 1, wherein the grafting monomer is one or more selected from a group consisting of vinyl acetate, acrylic acid, butyl acrylate, methyl acrylate, and acrylamide.

4. The preparation method according to claim 1, wherein the emulsifier is one or more selected from a group consisting of sodium lauryl sulfate, sodium dodecylsulfonate, cetyltrimethylammonium bromide, octylphenol polyoxyethylene ether-10 (OP-10), and phosphate emulsifiers.

5. The preparation method according to claim 1, wherein preparing the seed emulsion comprises: taking 10-20% of the initiator and 6-12% of the grafting monomer relative to their respective total addition amount, and the emulsifier to formulate the seed emulsion; wherein performing acid hydrolysis of the starch emulsion comprises formulating starch and water into the starch emulsion with a mass ratio of 25-45% based on dry starch, stirring well, and adjusting a temperature of the starch emulsion to 50-60° C., adding 0.5 mol/L hydrochloric acid for acid hydrolysis for 1.5-2.5 hours, and then adjusting pH of the starch emulsion to pH 4-5; wherein adding the first portion of the remaining initiator comprises adding 24-48% of the initiator relative to its total addition amount; wherein adding the second portion of the remaining initiator and the cross-linking monomer comprises slowly adding dropwise 20-40% of the initiator relative to its total addition amount and a cross-linking monomer solution at a mass concentration of 5-15%; wherein heating the reaction mixture comprises heating to 80-85° C.; and wherein cooling down the reaction mixture comprises cooling the reaction mixture to 30-50° C.

Description

DETAILED DESCRIPTION

(1) The present disclosure is further described below in combination with examples. It should be understood that these examples are only for the purpose of illustration, and do not limit the protection scope of the present disclosure.

Example 1

(2) Formula (Mass Percentage):

(3) TABLE-US-00002 Corn starch 100 Water 400 Vinyl acetate (monomer) 70 Ammonium persulfate (initiator) 2 Sodium lauryl sulfate (emulsifier) 6 N-hydroxymethacrylamide (crosslinking monomer) 2 High-performance silicone defoaming agent 15 Hydroquinone (polymerization inhibitor) 0.02

(4) Process Flow:

(5) (1) Corn starch was used as a raw material to formulate an emulsion with a mass concentration of 35%, which was acidified with hydrochloric acid at 60° C. for 2 h, and then adjusted to pH of about 3-5 with sodium hydroxide, heated up to 90° C., incubated for 0.5 h, and then cooled down to 70° C.

(6) (2) The pre-formulated seed emulsion was slowly added dropwise to the reactor, the speed of the stirrer was about 300 r/min, the condensation tube was observed until the reflux liquid was significantly reduced.

(7) (3) The initiator of 50% of its total addition amount was added to the reactor, the remaining monomer solution was slowly added dropwise for 3 h, the initiator of 15% of its total amount was additionally added, and the reaction mixture was allowed to further react for 1.5 h;

(8) (4) The pre-formulated N-hydroxymethacrylamide solution of 10% mass concentration and the initiator of 35% of its total addition amount were slowly added dropwise in 2 hours, and the reaction mixture reacted for 4 h.

(9) (5) After the dropwise addition of the monomer was completed, the temperature was raised to 80° C. and the reaction mixture was maintained at this temperature for 0.5 h to volatilize the residual monomers. The temperature was reduced to 50° C. and the hydroquinone polymerization inhibitor was added.

(10) The resultant product is milky white, shiny, with low initial viscosity; with hot pressing at 100° C. and 0.4 MPa, the three-layer plywood produced has a dry strength of 1.5 MPa and a wet strength of 0.8 MPa, exceeding the national standard of 0.7 MPa.

Example 2

(11) Formula (Mass Percentage):

(12) TABLE-US-00003 Corn starch 100 Water 400 Vinyl acetate (monomer) 70 Ammonium persulfate (initiator) 2 Sodium lauryl sulfate (emulsifier) 2 N-hydroxymethacrylamide (crosslinking monomer) 6 urea(diluent) 6 High-performance silicone defoaming agent 15 Hydroquinone (polymerization inhibitor) 0.02

(13) Process Flow:

(14) (1) Corn starch was used as a raw material to formulate an emulsion with a mass concentration of 35%, which was acidified with hydrochloric acid at 60° C. for 2 h, and then adjusted to pH of about 3-5 with sodium hydroxide, added the urea and heated up to 90° C., incubated for 0.5 h, and then cooled down to 70° C.

(15) (2) The pre-formulated seed emulsion was slowly added dropwise to the reactor, the speed of the stirrer was about 300 r/min, the condensation tube was observed until the reflux liquid was significantly reduced.

(16) (3) The initiator of 50% of its total addition amount was added to the reactor, the remaining monomer solution was slowly added dropwise for 3 h, the initiator of 15% of its total amount was additionally added, and the reaction mixture was allowed to further react for 1.5 h;

(17) (4) The pre-formulated N-hydroxymethacrylamide solution of 10% mass concentration and the initiator of 35% of its total addition amount were slowly added dropwise in 2 hours, and the reaction mixture reacted for 4 h.

(18) (5) After the dropwise addition of the monomer was completed, the temperature was reduced to 50° C. and the hydroquinone polymerization inhibitor was added.

(19) The resultant product is milky white, shiny, with low initial viscosity; with hot pressing at 100° C. and 0.4 MPa, the three-layer plywood produced has a dry strength of 1.4 MPa and a wet strength of 0 (The wet strength is measured after soaking for 3 h. If delamination occurs in 3 hours, the wet strength is 0. The relative wet strength is characterized by the delamination time. The longer the delamination time, the better the water resistance), and the delamination time is 1.5 h.

Example 3

(20) Formula (Mass Percentage):

(21) TABLE-US-00004 Corn starch 100 Water 400 Vinyl acetate (monomer) 70 Ammonium persulfate (initiator) 2 Sodium lauryl sulfate (emulsifier) 2 urea(diluent) 6 High-performance silicone defoaming agent 15 Hydroquinone (polymerization inhibitor) 0.02

(22) Process Flow:

(23) (1) Corn starch was used as a raw material to formulate an emulsion with a mass concentration of 35%, which was acidified with hydrochloric acid at 60° C. for 2 h, and then adjusted to pH of about 3-5 with sodium hydroxide, added the urea and heated up to 90° C., incubated for 0.5 h, and then cooled down to 70° C.

(24) (2) The pre-formulated seed emulsion was slowly added dropwise to the reactor, the speed of the stirrer was about 300 r/min, the condensation tube was observed until the reflux liquid was significantly reduced.

(25) (3) The initiator of 50% of its total addition amount was added to the reactor, the remaining monomer solution was slowly added dropwise for 3 h, the initiator of 15% of its total amount was additionally added, and the reaction mixture was allowed to further react for 1.5 h;

(26) (4) After the dropwise addition of the monomer was completed, the temperature was raised to 80° C. and the reaction mixture was maintained at this temperature for 0.5 h to volatilize the residual monomers. The temperature was reduced to 50° C. and the hydroquinone polymerization inhibitor was added.

(27) The resultant product is milky white, shiny; with hot pressing at 100° C. and 0.4 MPa, the three-layer plywood produced has a dry strength of 1.3 MPa and a wet strength of 0, and the delamination time is 0.5 h.

Example 4

(28) Formula (Mass Percentage):

(29) TABLE-US-00005 Corn starch 100 Water 400 Vinyl acetate (monomer) 70 Ammonium persulfate (initiator) 2 Sodium lauryl sulfate (emulsifier) 2 N-hydroxymethacrylamide (crosslinking monomer) 6 urea(diluent) 6 High-performance silicone defoaming agent 15 Hydroquinone (polymerization inhibitor) 0.02

(30) Process Flow:

(31) (1) Corn starch was used as a raw material to formulate an emulsion with a mass concentration of 35%, which was acidified with hydrochloric acid at 60° C. for 2 h, and then adjusted to pH of about 3-5 with sodium hydroxide, heated up to 90° C., incubated for 0.5 h, and then cooled down to 70° C.

(32) (2) The pre-formulated seed emulsion was slowly added dropwise to the reactor, the speed of the stirrer was about 300 r/min, the condensation tube was observed until the reflux liquid was significantly reduced.

(33) (3) The initiator of 50% of its total addition amount was added to the reactor, the remaining monomer solution was slowly added dropwise for 3 h, the initiator of 15% of its total amount was additionally added, and the reaction mixture was allowed to further react for 1.5 h;

(34) (4) The pre-formulated N-hydroxymethacrylamide solution of 10% mass concentration and the initiator of 35% of its total addition amount were slowly added dropwise in 2 hours, and the reaction mixture reacted for 4 h.

(35) (5) After the dropwise addition of the monomer was completed, the temperature was reduced to 50° C. and the hydroquinone polymerization inhibitor and the urea were added.

(36) The resultant product is milky white, shiny; with hot pressing at 100° C. and 0.4 MPa, the three-layer plywood produced has a dry strength of 1.5 MPa and a wet strength of 1.1 MPa.

(37) The effect of urea and cross-linking monomer on the performance of the adhesive is demonstrated through examples. The properties of the product obtained according to the formulas in the examples are shown in Table 1:

(38) TABLE-US-00006 TABLE 1 Properties of products obtained according to different formulas in the examples Example 1 2 3 4 Swelling ratio 0.74 0.95 /  0.61 Dry 1.5  1.4  1.4 1.5 strength/MPa Delamination Not Delaminated Delaminated Not time (63° C.) delaminated at 1.5 h in 1 h delaminated at 3 h at 3 h Wet 0.8 MPa / / 1.1 strength/MPa “/” Means absence of swelling ratio * When the bonding strength is about 0.8 MPa, the plywood will be severely damaged during the stretching process, that is, the actual shear strength far exceeds the measured value and exceeds the tolerance range of the panel. The swelling ratio is measured after the starch adhesive is coated and dried on a tetrafluoroethylene plate, and then immersed in excess water for 24 h at room temperature. After swelling equilibrium, the swelling ratio Q = (We − Wd)/Wd, where We and Wd are the mass after swelling equilibrium and the mass before soaking, respectively.

(39) It can be known from Table 1 that Example 1 is the thermosetting starch-based wood adhesive with the highest degree of crosslinking, Example 2 takes the second place in the degree of crosslinking, and Example 3 is an ordinary starch adhesive without the degree of crosslinking. The swelling ratio is used to characterize the crosslinking density. The swelling ratio is inversely proportional to the degree of crosslinking, so the swelling ratio of the ordinary starch adhesive without a crosslinking network in Example 3 is meaningless, that is, there is no degree of crosslinking. By comparing the above properties, it can be found that the water resistance is positively related to the degree of crosslinking. To a certain extent, the stronger the degree of crosslinking, the longer the delamination time. It can be found that urea reduces the degree of crosslinking. This is because urea added before polymerization forms oligomers with NMA, inhibiting the main reaction of NMA with starch and VAC.

Example 5

(40) This example shows the performance comparison between adhesives prepared by some traditional processes and the starch adhesives for a wood-based panel prepared by the improved process of the present disclosure.

(41) Method 1

(42) Formula (Mass/g):

(43) TABLE-US-00007 Cassava starch 100 Vinyl acetate (monomer) 80 Ammonium persulfate (initiator) 2 Sodium lauryl sulfate (emulsifier) 2 N-hydroxymethacrylamide (crosslinking monomer) 8 High-performance silicone defoaming agent 15 Hydroquinone (polymerization inhibitor) 0.02

(44) Process Flow:

(45) (1) Cassava starch was used as a raw material to formulate an emulsion with a mass concentration of 35%, which was acidified with hydrochloric acid at 60° C. for 2 h, and then adjusted to pH of about 3-5 with sodium hydroxide, heated up to 90° C., incubated for 0.5 h, and then cooled down to 70° C.

(46) (2) The pre-formulated seed emulsion was slowly added dropwise to the reactor, the speed of the stirrer was about 300 r/min, the condensation tube was observed until the reflux liquid was significantly reduced.

(47) (3) The initiator of 50% of its total addition amount was added to the reactor, the remaining monomer solution was slowly added dropwise for 3 h, the initiator of 15% of its total amount was additionally added, and the reaction mixture was allowed to further react for 1.5 h;

(48) (4) The pre-formulated N-hydroxymethacrylamide solution of 10% mass concentration and the initiator of 35% of its total addition amount were slowly added dropwise in 2 hours, and the reaction mixture reacted for 4 h.

(49) (5) After the dropwise addition of the monomer was completed, the temperature was raised to 80° C. and the reaction mixture was maintained at this temperature for 0.5 h to volatilize the residual monomers. The temperature was reduced to 50° C. and the hydroquinone polymerization inhibitor was added.

(50) The resultant product is milky white, shiny, with good fluidity; with hot pressing at 105° C. and 0.3 MPa, the three-layer plywood produced has a dry strength of 1.6 MPa and a wet strength of 0.85 MPa.

(51) Method 2

(52) Formula (Mass/g):

(53) TABLE-US-00008 Cassava starch 100 Vinyl acetate (monomer) 80 Ammonium persulfate (initiator) 2 Sodium lauryl sulfate (emulsifier) 2 N-hydroxymethacrylamide (crosslinking monomer) 2 High-performance silicone defoaming agent 15 Hydroquinone (polymerization inhibitor) 0.02

(54) Process Flow:

(55) (1) Cassava starch was used as a raw material to formulate an emulsion with a mass concentration of 35%, which was acidified with hydrochloric acid at 60° C. for 2 h, and then adjusted to pH of about 3-5 with sodium hydroxide, heated up to 90° C., incubated for 0.5 h, and then cooled down to 70° C.

(56) (2) The pre-formulated seed emulsion was slowly added dropwise to the reactor, the speed of the stirrer was about 300 r/min, the condensation tube was observed until the reflux liquid was significantly reduced.

(57) (3) The initiator of 50% of its total addition amount was added to the reactor, the remaining monomer solution was slowly added dropwise for 3 h, the initiator of 15% of its total amount was additionally added, and the reaction mixture was allowed to further react for 1.5 h;

(58) (4) The pre-formulated N-hydroxymethacrylamide solution of 10% mass concentration and the initiator of 35% of its total addition amount were slowly added dropwise in 2 hours, and the reaction mixture reacted for 4 h.

(59) (5) After the dropwise addition of the monomer was completed, the temperature was raised to 80° C. and the reaction mixture was maintained at this temperature for 0.5 h to volatilize the residual monomers. The temperature was reduced to 50° C. and the hydroquinone polymerization inhibitor was added.

(60) The resultant product is milky white, shiny, with good fluidity; with hot pressing at 105° C. and 0.3 MPa, the three-layer plywood produced has a dry strength of 1.5 MPa and a wet strength of 0.7 MPa.

(61) Method 3

(62) Formula (Mass/g):

(63) TABLE-US-00009 Waxy corn starch 100 Vinyl acetate (monomer) 60 Ammonium persulfate (initiator) 1.5 Sodium lauryl sulfate (emulsifier) 2 N-hydroxymethacrylamide (crosslinking monomer) 4 High-performance silicone defoaming agent 15 Hydroquinone (polymerization inhibitor) 0.02

(64) Process Flow:

(65) (1) Waxy corn starch was used as a raw material to formulate an emulsion with a mass concentration of 35%, which was acidified with hydrochloric acid at 60° C. for 2 h, and then adjusted to pH of about 3-5 with sodium hydroxide, heated up to 90° C., incubated for 0.5 h, and then cooled down to 70° C.

(66) (2) The pre-formulated seed emulsion was slowly added dropwise to the reactor, the speed of the stirrer was about 300 r/min, the condensation tube was observed until the reflux liquid was significantly reduced.

(67) (3) The initiator of 50% of its total addition amount was added to the reactor, the remaining monomer solution was slowly added dropwise for 3 h, the initiator of 15% of its total amount was additionally added, and the reaction mixture was allowed to further react for 1.5 h;

(68) (4) The pre-formulated N-hydroxymethacrylamide solution of 10% mass concentration and the initiator of 35% of its total addition amount were slowly added dropwise in 2 hours, and the reaction mixture reacted for 4 h.

(69) (5) After the dropwise addition of the monomer was completed, the temperature was raised to 80° C. and the reaction mixture was maintained at this temperature for 0.5 h to volatilize the residual monomers. The temperature was reduced to 50° C. and the hydroquinone polymerization inhibitor was added.

(70) The resultant product is milky white, shiny, with good fluidity; with hot pressing at 105° C. and 0.3 MPa, the dry strength is 1.7 MPa, the wet strength is 1.2 MPa.

(71) Method 4

(72) Formula (Mass/g):

(73) TABLE-US-00010 Corn starch 100 Vinyl acetate (monomer) 60 Ammonium persulfate (initiator) 1.5 Sodium lauryl sulfate (emulsifier) 2 Borax (cross-linking agents) 4 High-performance silicone defoaming agent 15 Hydroquinone (polymerization inhibitor) 0.02

(74) Process Flow:

(75) (1) Corn starch was used as a raw material to formulate an emulsion with a mass concentration of 35%, which was acidified with hydrochloric acid at 60° C. for 2 h, and then adjusted to pH of about 3-5 with sodium hydroxide, heated up to 90° C., incubated for 0.5 h, and then cooled down to 70° C.

(76) (2) The pre-formulated seed emulsion was slowly added dropwise to the reactor, the speed of the stirrer was about 300 r/min, the condensation tube was observed until the reflux liquid was significantly reduced.

(77) (3) The initiator of 50% of its total addition amount was added to the reactor, the remaining monomer solution was slowly added dropwise for 3 h, the initiator of 15% of its total amount was additionally added, and the reaction mixture was allowed to further react for 1.5 h;

(78) (4) After the dropwise addition of the monomer was completed, the temperature was raised to 80° C. and the reaction mixture was maintained at this temperature for 0.5 h to volatilize the residual monomers. The temperature was reduced to 50° C. and the hydroquinone polymerization inhibitor, the borax (5.0 wt %), the urea were added.

(79) The resultant product is milky white, shiny, with good fluidity; with hot pressing at 105° C. and 0.3 MPa, the dry strength is 1.6 MPa, the wet strength is 0, the delamination time is 1 h.

(80) Method 5

(81) Formula (Mass/g):

(82) TABLE-US-00011 Corn starch 100 Vinyl acetate (monomer) 70 Ammonium persulfate (initiator) 1.5 Sodium lauryl sulfate (emulsifier) 2 N-hydroxymethacrylamide (crosslinking monomer) 5 High-performance silicone defoaming agent 15 Hydroquinone (polymerization inhibitor) 0.02

(83) Process Flow:

(84) (1) Corn starch was used as a raw material to formulate an emulsion with a mass concentration of 35%, which was acidified with hydrochloric acid at 60° C. for 2 h, and then adjusted to pH of about 3-5 with sodium hydroxide, heated up to 90° C., incubated for 0.5 h, and then cooled down to 70° C.

(85) (2) The pre-formulated seed emulsion was slowly added dropwise to the reactor, the speed of the stirrer was about 300 r/min, the condensation tube was observed until the reflux liquid was significantly reduced.

(86) (3) The initiator of 50% of its total addition amount was added to the reactor, the remaining monomer solution was slowly added dropwise for 3 h, the initiator of 15% of its total amount was additionally added, and the reaction mixture was allowed to further react for 1.5 h;

(87) (4) The pre-formulated N-hydroxymethacrylamide solution of 10% mass concentration and the initiator of 35% of its total addition amount were slowly added dropwise in 2 hours, and the reaction mixture reacted for 4 h.

(88) (5) After the dropwise addition of the monomer was completed, the temperature was raised to 80° C. and the reaction mixture was maintained at this temperature for 0.5 h to volatilize the residual monomers. The temperature was reduced to 50° C. and the hydroquinone polymerization inhibitor was added.

(89) The resultant product is milky white, with poor fluidity, gelled, so that it cannot be industrially applied.

(90) Method 6

(91) Formula (Mass/g):

(92) TABLE-US-00012 Waxy corn starch 100 Vinyl acetate (monomer) 60 Ammonium persulfate (initiator) 1 Sodium lauryl sulfate (emulsifier) 2 N-hydroxymethacrylamide (crosslinking monomer) 2 High-performance silicone defoaming agent 15 Hydroquinone (polymerization inhibitor) 0.02

(93) Process Flow:

(94) (1) Waxy corn starch was used as a raw material to formulate an emulsion with a mass concentration of 35%, which was acidified with hydrochloric acid at 60° C. for 2 h, and then adjusted to pH of about 3-5 with sodium hydroxide, heated up to 90° C., incubated for 0.5 h, and then cooled down to 70° C.

(95) (2) The pre-formulated seed emulsion was slowly added dropwise to the reactor, the speed of the stirrer was about 300 r/min, the condensation tube was observed until the reflux liquid was significantly reduced.

(96) (3) The initiator of 50% of its total addition amount was added to the reactor, the remaining monomer solution was slowly added dropwise for 3 h, the initiator of 15% of its total amount was additionally added, and the reaction mixture was allowed to further react for 1.5 h;

(97) (4) The pre-formulated N-hydroxymethacrylamide solution of 10% mass concentration and the initiator of 35% of its total addition amount were slowly added dropwise in 2 h, and the reaction mixture reacted for 4 h.

(98) (5) After the dropwise addition of the monomer was completed, the temperature was raised to 80° C. and the reaction mixture was maintained at this temperature for 0.5 h to volatilize the residual monomers. The temperature was reduced to 50° C. and the hydroquinone polymerization inhibitor was added.

(99) The resultant product is milky white, with poor fluidity, and a small part of it gelatinizes; with hot pressing at 105° C. and 0.3 MPa, the dry strength is 1.6 MPa, and the wet strength is 0.9 MPa.

(100) Method 7

(101) Formula (Mass Percentage):

(102) TABLE-US-00013 Corn starch 100 Vinyl acetate (monomer) 80 Ammonium persulfate (initiator) 2 Sodium lauryl sulfate (emulsifier) 2 N-hydroxymethacrylamide (crosslinking monomer) 8 urea(diluent) 10 High-performance silicone defoaming agent 15 Hydroquinone (polymerization inhibitor) 0.02

(103) Process Flow:

(104) (1) Corn starch was used as a raw material to formulate an emulsion with a mass concentration of 35%, which was acidified with hydrochloric acid at 60° C. for 2 h, and then adjusted to pH of about 3-5 with sodium hydroxide, heated up to 90° C., incubated for 0.5 h, and then cooled down to 70° C.

(105) (2) The pre-formulated seed emulsion was slowly added dropwise to the reactor, the speed of the stirrer was about 300 r/min, the condensation tube was observed until the reflux liquid was significantly reduced.

(106) (3) The initiator of 50% of its total addition amount was added to the reactor, the remaining monomer solution was slowly added dropwise for 3 h, the initiator of 15% of its total amount was additionally added, and the reaction mixture was allowed to further react for 1.5 h;

(107) (4) The pre-formulated N-hydroxymethacrylamide solution of 10% mass concentration and the initiator of 35% of its total addition amount were slowly added dropwise in 2 h, and the reaction mixture reacted for 4 h.

(108) (5) After the monomers are added dropwise, the reaction mixture was heated to 80° C. and held for 0.5 h to volatilize the residual monomers. The mixture was cooled down to 50° C. and urea and hydroquinone polymerization inhibitor were added.

(109) The resultant product is milky white, shiny, with good fluidity; with hot pressing at 105° C. and 0.3 MPa, the three-layer plywood produced has a dry strength of 1.4 MPa and a wet strength of 0.85 MPa.

(110) The properties comparison of the adhesives prepared by the above seven methods is shown in Table 2:

(111) TABLE-US-00014 TABLE 2 Properties of adhesives prepared by different methods Method 1 2 3 4 5 6 7 Initial 30.13 26.15 41.56 22.24 / / 29.89 viscosity/Pa .Math. s Dry 1.6 1.5 1.7 1.6 — 1.6 1.4 strength/MPa Wet 0.85 0.7 1.2 0 — 0.9 0.85 strength/MPa “/” Indicates that the viscosity exceeds the range to form gel; “—” indicates that the adhesive cannot be applied normally.

(112) As can be seen from Table 2, the adhesives prepared by the traditional synthetic processes (Methods 5 and 6) has too high viscosity, which is not convenient for practical application. All indexes of the adhesives prepared by the improved synthetic processes of the present disclosure (Methods 1-3) can meet the requirements. Ordinary cross-linking agents such as borax (Method 4) have no significant improvement in wet strength. The addition of urea during the polymerization process will reduce the degree of cross-linking and reduce the wet strength, and the addition of urea after polymerization (Method 7) will not affect the wet strength and can improve storage stability.

(113) As a comparison of this patent, some adhesives prepared by conventional methods are provided below.

Comparative Example 1

(114) Formula (Mass/g):

(115) TABLE-US-00015 Maltodextrin (DE = 5) 170 N-hydroxymethyl acrylamide (monomer) 50 Toluene-p-sulfonic acid (catalyst) 0.1 P-methoxyphenol (stabilizer) 0.15

(116) Process Flow:

(117) (1) Maltodextrin, N-methylol acrylamide, toluene-p-sulfonic acid, and ascorbic acid were mixed to prepare a mixed liquid with a solid content of 30% w/w. p-methoxyphenol was added as a stabilizer.

(118) (2) The mixture was frozen within 8 h and store at −20° C.

(119) (3) The frozen mixture was freeze-dried.

(120) (4) The freeze-dried product was processed in a fluidized bed reactor at 150° C. to obtain a prepolymer of a methylene acrylamide ether derivative.

(121) (5) The prepolymer was dispersed uniformly in water to make a solution with a solid content of 30% w/w.

(122) (6) The prepolymer liquid was applied to the paper and a small amount of 30% H.sub.2O.sub.2 was sprayed thereon to form the final polymer.

(123) The resultant product is slightly yellow and can be used for rapid curing and bonding of paper at room temperature. However, it has been found through experiments that the polymer colloid cannot be used for the preparation of hot-pressed panels, and is not convenient for the industrial production of wood-based panels.

Comparative Example 2

(124) Formula (Mass/g):

(125) TABLE-US-00016 Vinyl acetate 50 Polyvinyl alcohol (protective colloid) 7 Alkylphenol polyoxyethylene ether (10) 0.5 Ammonium persulfate (initiator) 0.75 Sodium dodecyl sulfate 6.25 N-hydroxymethacrylamide 0.5

(126) Process Flow:

(127) (1) 25 g of vinyl acetate, 7 g of polyvinyl alcohol, 0.5 g of alkylphenol polyoxyethylene ether, 6.25 g of sodium lauryl sulfate, 0.375 g of ammonium persulfate, 0.5 g of NMA and 250 g of water were mixed to obtain a uniform emulsion.

(128) (2) The temperature was increased to 40° C. and the emulsion was incubated for 0.5 h.

(129) (3) The temperature was raised to 70° C., another 0.375 g of ammonium persulfate was added, and 20 g of vinyl acetate was slowly added dropwise to the emulsion to react for 6 h.

(130) (4) The temperature was raised to 85° C., and polymerization was further performed for 0.5 h.

(131) The temperature was reduced to 50° C. and other additives were added.

(132) The resultant product is milky white. The wooden block was bonded and cured at room temperature. The polymer colloid is not based on starch, and it is difficult to control the production process. Also, it cannot be used for the preparation of hot-pressed panels. In wood-based panels, the dry strength can reach 1 MPa, and the wet intensity is 0 MPa.