Demethylation Of Alkali Lignin And Its Preparation Of Formaldehyde Free Adhesives

Urea and phenolic resins are the main adhesives used in the wood industry.The use of formaldehyde in wood adhesives will cause risk of carcinogenic to people's health;at the same time,the price of the adhesives is also affected by the non-renewable and the fluctuations of petrochemical raw materials.Lignin,as one of the important components in plant cell wall and is the most abundant phenolic compound.It has advantages in the preparation of formaldehyde free adhesives.However,industrial lignin is mainly produced by pulp mnd paper mills and biorefinery eompanies,and there is a common disadvantage of low reactivity.Therefore,it is of great significance to improve the reactivity of lignin based on chemical modification.We choose alkali lignin from con cob as material that was modified by two demethylation methods that using hydrobromic acid(HBr,40%)and sodium sulfite(Na2SO3).In the HBr method,it studied the conventional method of heating and microwave assisted heating on the demethylation effect of alkali lignin.IH-NMR results showed that the intensity of the methoxyl signal was significantly attenuated by the conventional heating time of lh in the HBr method,indicating successful demethylation.The results showed that the effect of CTAB on demethylation was better than SDBS.In the microwave assisted heating experiment,add hexadecyl trimethyl ammonium bromide(CTAB)and sodium dodecyl benzene sulfonate(SDBS)respectively as surfactant to investigate the stimulative effect of the two kinds of surfactants on the reaction.In the experiment of sodium sulfite method,the effect of conventional method and microwave assisted heating was also investigated.It shows that under the condition of reducing the reaction time by 40 minutes,the demethylation effect of microwave heating is better than the conventional method.Then we mixed the demethyl alkali lignin that by HBr and polyethyleneimine(PEI)to prepare wood formaldehyde free adhesives.The effects of lignin in different conditions and the ratio of lignin/PEI on the shear strength were investigated.At the same time,the curing process of the adhesive was studied by means of FTIR and TG-DSC.We found that:Compared with FTIR of the curing process of adhesives,it showed the weakening of amino signal intensity and the enhancement of carbonyl signal intensity,indicating that lignin and PEI may react through Schiff base formation reaction or Michael addition reaction.The lignin/PEI adhesive made from alkali lignin after CTAB treatment was the best in terms of wet strength(type ? plate treatment).(the average strength was 0.85 MPa).When the ratio of lignin/PEI to 2:1,it exhibited the largest wet strength CSmax=1.3 MPa.