Preparation Process Optimization And Mechanism Study Of Thermosetting Starch-based Adhesive

Starch is a kind of green,environment-friendly and renewable resource with large yield,wide source,low price and biodegradability.Wood adhesives prepared from starch as the main raw material have the potential to meet people’s demand for environment-friendly formaldehyde free adhesives.Starch-based adhesives with good application performance and thermosetting properties had been prepared from corn starch in the previous laboratory,which could be applied to the bonding of hot-pressing Ⅱ type wood-based panels,but there was still room for improvement in the distribution uniformity of cross-linking monomers and reaction stability.In this paper,on the basis of the existing thermosetting starch-based adhesive reaction system,the monomer distribution uniformity was further improved to improve the stability of the preparation process.Meanwhile,the mechanism of vinyl acetate(VAc)in the reaction system was studied in depth.The main research contents are as follows:Firstly,in view of the problem of poor stability of the existing preparation process of thermosetting starch-based wood adhesives,based on the existing acidolysis pretreatment,the pre-gelatinization temperature,pH value of grafting,grafting time,N-hydroxymethyl acrylamide(NMA)solution dropping time,NMA solution concentration were optimized,and the optimal preparation process conditions were obtained as follows:starch solution which pre-gelatinized at 82.5℃ grafted with VAc in the pH of 4-5 for 30 min,then dropped NMA solution of 7%(w/w)at a constant speed within 3 h,the thermosetting starch-based wood adhesive with good fluidity and storage stability was successfully prepared,and the stability of the adhesive was effectively improved to avoid the formation of gel in the process of preparation.In this paper,the thermosetting starch-based adhesive was used to replace urea-formaldehyde adhesive.After optimizing the hot pressing process,the dry and wet strength of three-layer wood-based panels prepared reached 1.58 MPa and 1.39 MPa,respectively,which exceeded the standard of 0.7MPa for Ⅱ type plywood in the national standard.Secondly,the effects and action mechanism of monomers on the application properties of thermosetting starch-based adhesives were studied by adjusting the addition amount of VAc and NMA.The effects of VAc and NMA on the dry and wet strength of the adhesive were investigated.Through process optimization,the dry and wet strength were increased to 1.67 MPa and 1.45 MPa,respectively.At the same time,it was found that VAc had little effect on adhesive strength,while NMA had a greater effect on wet strength.Excess NMA would increase the hydrophilicity of adhesive particles and decrease the wet strength.The performance of adhesives were characterized by scanning electron microscopy,Raman mapping and contact angle,respectively.It proved that too much NMA led to monomer on starch granule surface gathering,and reduce water resistance and uniformity of monomer distribution in adhesive,and VAc could effectively adjust the density of NMA,avoid the happening of local system of overreaction and gel;Differential scanning calorimetry(DSC)was used to analyze the mechanism of VAc improving the freeze-thaw stability.The results showed that VAc could effectively inhibit starch retrogradation by grafting starch particles to block hydroxyl groups.The coefficient of variation of hot-pressing time and the analysis of opening time proved that VAc could regulate the distribution of NMA in the adhesives,which further improved the physical properties,chemical properties and application properties of thermosetting starch-based adhesives.Finally,thermogravimetric analyzer(TGA),Fourier transform infrared spectrometer(FT-IR),Raman Mapping and nuclear magnetic resonance(NMR)were used to explore the mechanism of VAc in improving the distribution uniformity of NMA in the preparation of thermosetting starch-based adhesives.FTIR and TGA showed that VAc could appropriately reduce the grafting density of NMA and avoid the formation of gel by local excessive grafting of NMA.Raman Mapping proved that VAc could effectively improve the uniformity of NMA and the distribution of NMA on the surface of starch particles.The chemical shift in the 1H-CPMAS spectrum and the shoulder peak in the 13C-CPMAS spectrum demonstrated that both VAc and NMA were more inclined to react with-OH at the C6 sites of starch,and they formed a competitive reaction.VAc improved the distribution uniformity of NMA in adhesives by inhibiting the overreaction between NMA and starch,so that the crosslinking monomer NMA had to react at other active sites to improve the reaction stability and application performance of adhesives.