| Lignin-phenol-formaldehyde(LPF)resin can effectively reduce the amount of toxic phenol used during the synthesis of phenolic resin,reduce the harm to human health or environment during the preparation and use process of phenolic resin,improve the curing rate of phenolic resin,and reduce the production costs of phenolic resin as well as its products.However,lignin has low reactivity,high molecular weight,and large steric resistance,leading to poor adhesion property of LPF resin and low additive amount of lignin.In addition,traditional lignin activation methods are difficult to realize industrial application because of their harsh reaction conditions,tedious reaction process,and low activation efficiency.In order to address the abovementioned problems,simple and effictive methods of lignin activation were used to improve the reactivity of alkali lignin(AL),and the actived AL was used to prepare lignin-based phenolic resin.The effects of activation of AL using demethylation reagents at low temperature and atmospheric pressure,degradation of AL using NaOH/urea aqueous solution at low temperature and atmospheric pressure,and oxidative biomimetic modification of AL using sodium periodate under mild conditions on the chemical structures and reactivity of AL were studied;The effects of demethylated lignin,degradated lignin,or oxidized lignin-modified phenolic resins on the adhesion and curing properties of lignin-based phenolic resins were also investigated.The activation mechanisms of AL activated under mild conditions,and the curing and adhesion mechanisms of activated lignin-based phenolic resins were analyzed.The main conclusions are summarized as follows:(1)The demethylation activation of alkali lignin can be realized under mild conditions by using sodium sulfite et al.as the demethylation reagent.Different kinds of demethylation reagents were used to modify AL under low temperature and atmospheric pressure.Compared with unactivated AL,the activated AL had decreased content of methoxy group,increased content of phenolic hydroxyl groups,and enhanced reactivity.When sodium sulfite was used as the demethylation reagent,reaction temperature was 80?,reaction time was 60 min,and the dosage of demethylation reagent was 15%,the effect of lignin demethylation was the best.Under the optimum reaction conditions,the obtained activated AL had the highest reactivity,the content of methoxy groups decreased by 47.5%,and the content of phenol hydroxyl groups increased by 39.2%.When sodium sulfite was used as the demethylation reagent,sulfite ion attacked the methoxy groups in alkali lignin,which resulted in the nucleophilic substitution reaction of methoxy groups and the transformation of methoxy groups into high reactive phenolic hydroxyl groups.(2)Modified phenolic resins with high performances can be prepared by using different demethylated AL.The demethylation reaction of alkali lignin increased its reactive site content,thus,compared with the alkali lignin-phenol-formaldehyde(ALPF)resin,the demethylated AL-modified phenolic resins had high polymerization degree,fast curing rate,high adhesion strength,low formaldehyde emission,and good thermal stability.The gel time of sodium sulfite-demethylated AL-modified phenolic resin(DLPF-Na2SO3)was half of the phenol-formaldehy(PF)resin,which had the fastest curing rate.The bonding strength of DLPF-Na2SO3 resin was above 0.70MPa,the formaldehyde emission was less than 0.50 mg/L,and its thermal stability were higher than the rest demethylated lignin-based phenolic resins.When phenolic resin was modified by lignin demethylated under the optimum conditions(DL-Y1),the adhesion strength of the modified phenolic resin increased from 0.91 MPa to 1.07MPa,and the formaldehyde emission decreased from 0.58mg/L to 0.22mg/L.(3)Alkali lignin can be degraded and activated by using NaOH/urea aqueous solution under low temperature and atmospheric pressure.The polymerization degree of the degraded AL(DAL)decreased and the content of active sites significantly increased.During the degradation process,sodium hydroxide can promote the break of ?-O-4,and urea hydrate can stabilize the degradation solutions.Compared with undegraded AL,the degraded AL decreased from phenylpropane octamer to phenylpropane trimer,the content of active site increased by 767%,the content of phenolic hydroxyl group increased by 71.4%.When the degradation temperature is 25?,NaOH/urea aqueous solution had the best degradation effect on AL.Compared with undegraded AL,the content of phenolic and alcohol hydroxyl group of the degraded AL obtained under the optimum reaction conditions increased by 143%and 81.9%,respectively,exhibited the highest reactivity.(4)Lignin-based phenolic resins with high performances can be prepared by using degraded alkali lignin.The degraded AL had low molecular weight,increased reactive site content,and enhanced reactivity,thus,compared with ALPF resin,phenolic resin modified by degraded AL had high degree of hydroxymethylation and polymerization,short gel time,and fast curing rate.In addition,the bonding strength increased from 0.90MPa to 1.06MPa,and the formaldehyde emission decreased from 0.56mg/L to 0.38mg/L.Compared with other degraded alkali lignin-based phenolic resins,lignin obtained under optimum conditions modified phenolic(DALPF-3)resin had high adhesive strength and thermal stability.The gel time decreased from 530s to 428s,the bonding strength increased from 0.91 MPa to 1.13MPa,and the formaldehyde emission decreased from 0.58mg/L to 0.34mg/L.(5)The oxidation and activation of alkali lignin can be realized by using sodium periodate under mild conditions,and the oxidized lignin can be used to modify phenolic resin.Compared with unoxidized alkali lignin,the methoxy groups of the oxidized AL(OAL)decreased from 0.3168mmol/g to 0.0013mmol/g,and the methoxy groups of the oxidized DAL(ODAL)decreased from 0.8042mmol/g to 0.0069mmol/g.The oxidized lignin had enhanced reactivity,and the phenolic rings of guaiacyl unit were transformed into ortho-quinone and/or catechol.Compared with PF resin,the gel time of OALPF and ODALPF resins decreased from 635 s to 548s and 439s,the bonding strength increased from 0.90MPa to 1.02MPa and 1.16MPa,and the formaldehyde emission decreased from 0.59mg/L to 0.48mg/L and 0.35mg/L,respectively.The reverse dismutation reactions between the catechol and the ortho-quinone in oxidized AL increased the crosslinking density of the modified phenolic resin,leading to the improvement of the comprehensive properties of the activated alkali lignin-based phenolic resin.(6)Demethylation reagents such as sodium sulfite can cleave methoxy groups in alkali lignin and convert them into high active phenolic hydroxyl groups,increase the content of reactive sites in alkali lignin and enhance its reactive activity.Thus,the synthesized demethylated alkali lignin-based phenolic resins had good curing and adhesive properties.NaOH/urea aqueous solution can realize the degradation and activation of alkali lignin,decrease the molecular weight of alkali lignin,increase the content of reactive sites,and enhance the reaction activity.Thus,the degraded alkali lignin-based phenolic resins had excellent curing and adhesive properties.Sodium periodate can realize the oxidative activation of alkali lignin or the degraded alkali lignin by converting the methoxy groups into ortho-quinone and/or catechol,and increase the reactivity of alkali lignin.Thus,the oxidized alkali lignin-based phenolic resins had excellent curing and adhesive properties.Compared with lignin demethylation and oxidation,lignin depolymerization using NaOH/urea aqueous solution had the advantages of simple operation,high utilization rate of raw materials,and no need for separation and purification of the depolymerized lignin,therefore,it has great potential for industrial applications. |
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