Preparation And Properties Of Starch-based Wood Adhesive

Currently,Urea-formaldehyde(UF)resin is commonly used in wood-baesd panel industry.It has relatively low cost,high bonding strength,but is very sensitive to hydrolysis and produce health hazards on the human health,beyond a certain concentration,because of the release of free formaldehyde.With the implementation of national mandatory standard and the improvement of people's awareness of environmental protection,the environmental requirements for wood-based panels will be more and more stringent.Meanswhile,with the consumption of fossil resources,the contrdiction of supply and demand of resource is becoming prominent.Exploring the wood adhesives with low toxicity or non-toxic and environmental friendly will be the trendency in the future.Starch is low cost,environmental friendly and biodegadable.Mixed with water and heated,starch has bonding properties and can be used as adhesive.However,there are lots of hydrophilic hydroxyl groups,which are easy associate with water molecules to form hydrogen bonding,resulting in poor water resistance,mobility and slow drying of starch adhesive.These affect its application,especially in wood-based panel industry.Thus,it is necessary to midify starch in some degree.In this study,corn starch as raw material,after composite modification,corn starch adhesive was obtained.Then a certain amout of polymethylene polyphenyl polyisocyanate(PAPI)prepolymer was added to prepare high temperature cured starch-based wood adhesive and normal temperature cured starch-based wood adhesive.The properties of adhesive were characterized by differential scanning calorimeter(DSC),thermo gravimetric analysis(TGA),dielectric analyzer(DEA),electron scanning calorimetry(SEM)and X-ray photoelectron spectroscopy(XPS).Under the optimal conditions to prepare adhesive with best performance,the cure kinetics of high temperature cured adhesive and thermal decomposition kinetics of normal temperature cured adhesive were analyzed.After acid-thinned,oxidization and crosslinking,the high temperature cured adhesive was obtained.The effects of type and addition of crosslinking agent,thickener addition on the properties of adhesive were explored.The optimal conditions to prepare adhesive with best performance were as follows:R=25,12%addition and CMC%=0.375.Under this condition,the wet bonding strength of plywood was about 1.1 MPa,meeting the requirement of plywood of National Standard of Republic of China GB/T 17657-2013.The normal cured temperature adhesive was prepared by esterification,gelinization and crosslinking.The influences of esterification agent and crosslinking agent addition were investigated.When maleic anhydride addition was 3%and prepolymer addition was 8%,the adhesive had best performance.The dry and wet shear bonding strength(about 11 MPa and 4MPa,respectively)exceeded the requirement of National Standard of Republic of China HG/T 2727-2010.The cure behavior of high temperature adhesive was studied by DSC and cure kinetics was calculated with Kissinger equation and Ozawa equation.The apparent activation energy Ea calculated by Kissinger method of NSA,CNSA,PSAopt was 53.24kJ/mol,41.98kJ/mol and 59.58kJ/mol,respectively.And the order of reaction n was 0.89,0.99 and 1.0,respectively.The Ea of them calculated by Ozawa method was 56.69kJ/mol,39.92kJ/mol and 56.92kJ/mol,respectively.The thermal decomposition kinetics of normal temperature cured adhesive was analyzed.The results showed the thermal decomposition activation enengy Ea of NS,ES,NSA,ESA and PESA was 174.13kJ/mol,171.80kJ/mol,173.86kJ/mol,170.94kJ/mol and 172.82kJ/mol,respectively.