Preparation And Characterization Of Polyisocyanate PAPI Microcapsules For Plywood

With the development of society and economy,people’s living standard has been gradually improved,so the requirements of living environment have also gradually improved,which makes the demand of wood furniture and decorative materials gradually increase.In order to meet the needs of wood materials,wood-based panels have been developed gradually.Wood-based panel is a panel or profile formed by pressing under certain conditions,which is mainly made of small diameter wood,branches and branches,and processed by mechanical processing and other processes.As the necessary raw material of wood-based panels,the properties of adhesives seriously affect the quality of wood-based panels.Isocyanate adhesives have been gradually applied because of their excellent bonding properties,excellent water resistance and chemical properties.Because of its highly unsaturated isocyanate group,isocyanates are extremely chemically active and can react with anything containing active hydrogen,including water.Therefore,the working life of isocyanate in common water-based wood adhesive system is relatively short,and the common method is chemical protection.However,the application of blocked isocyanate in wood processing industry is limited due to a series of problems such as high temperature of unsealing,certain toxicity of blocking agent and residue of blocking agent in the system.In this study,microcapsule technology was used to protect high activity isocyanate,improve the stability of isocyanate and prolong the working life of isocyanate in water-borne wood adhesive.In other words,isocyanate microcapsules were prepared by different methods,and the preparation process was optimized according to the characteristics of each method.Then,the isocyanate microcapsules prepared were characterized by a series of test methods.Finally,the prepared isocyanate microcapsules were applied in the water-borne wood adhesive with polyvinyl alcohol(PVA-1788)as the main agent to prepare plywood.The working life of isocyanate microcapsules in the system was studied by the viscosity of wood adhesive and the shear strength of plywood.The main research is as follows:(1)In situ polymerization method was used to prepare isocyanate microcapsules with urea-formaldehyde resin as shell and PAPI as core in aqueous oil emulsion.Different kinds of emulsifiers were used to emulsify and disperse isocyanates.It was found that when the emulsifiers were Tween 80,gum Arabic,PVA-1788 and sodium carboxymethyl cellulose,the emulsion or suspension was stable and isocyanate microcapsules could be successfully prepared.In addition,when F/U=1.75,the temperature was 55℃,and the core-shell ratio was 3:1,the preparation time of isocyanate microcapsules was short,and the activity content of isocyanate microcapsules prepared was about 23.5%.It was found that the size and distribution of isocyanate microcapsules gradually decreased and narrowed with the increase of stirring speed.The results showed that the isocyanate microcapsules were orange spherical or similar spherical,with rough outer surface,smooth inner surface and complete shell layer,which provided a basis for increasing the stability of isocyanate.However,isocyanate microcapsules can be damaged and release isocyanate with light pressure,which provides a prerequisite for the release of microcapsules.Thermogravimetric analysis showed that the fastest decomposition temperature of isocyanate microcapsules was about 240℃,indicating that the microcapsules prepared had good thermal stability.After 8 weeks of storage,the content of isocyanate microcapsules decreased from 23.5%to 20%,indicating that the prepared isocyanate microcapsules had good storage stability.The prepared isocyanate microcapsules were applied to the wood adhesive to prepare plywood.When the amount of isocyanate microcapsules was 15wt%,the prepared plywood could meet the requirements of national classⅡ and Class Ⅰ standards.Due to the shielding effect of the shell,microencapsulation also increased the working life of isocyanates to nearly 5 hours,significantly improving the stability of isocyanates.(2)An isocyanate microcapsule with gum Arabic/gelatin as shell and isocyanate as core was prepared in water-oil emulsion by complex coagulation method,in which the wall material was used as emulsifier and the outer layer of isocyanate as cross-linking agent.The optimized preparation of isocyanate microcapsules by complex coagulation method showed that when gelatin:gum Arabic =1:1.2,pH was about 4,and wall material concentration was 2%,the system had equal opposite charge and charge density was also appropriate,which made gelatin and gum Arabic condense to the maximum extent to cover isocyanate.Isocyanate microcapsules with Arabic gum/gelatin as the wall and PAPI as the core were prepared by complex coagulation in water in oil emulsion.The preparation method uses gum Arabic as the emulsifier and the outer layer of isocyanate as the crosslinking agent.When the reaction temperature was 50℃,the cross-linking curing rate of isocyanate was fast,and few side reactions occurred.When the core-shell ratio was 3:1,the content of active groups and the core ratio of isocyanate microcapsules were high.It was found that the particle size of the microcapsules could be controlled by stirring rate.In addition,the principle of isocyanate microcapsules prepared by this method was also discussed.The gum Arabic adsorbed on the surface of the isocyanate droplet condenses with the gelatin in the system and then encapsulates the isocyanate.In addition,isocyanate can also be encapsulated by the dispersed Arabic gum and gelatin after condensation.It was found that the isocyanate microcapsules prepared were translucent orange spherical particles with slightly rough surface.The liquid isocyanate was released when pressed lightly under the cap-glass of the microcapsule,which indicated that the core of the prepared isocyanate microcapsule was liquid and provided a basis for the release of the microcapsule.Thermogravimetric analysis showed that the initial decomposition temperature of isocyanate microcapsules was 217℃,higher than 177℃ of isocyanate microcapsules,indicating the thermal stability of isocyanate microcapsules prepared.After 8 weeks of storage,the content of active groups in the microcapsules only decreased by less than 4%,indicating that the microcapsules had good stability.The isocyanate microcapsules were used to prepare plywood in wood adhesives.The water resistance of plywood was obviously improved,and the working life of isocyanate was also increased to more than 5 hours after the microencapsulation.(3)An isocyanate microcapsule was prepared by interinterface polymerization in water-oil emulsion,with a small part of the isocyanate reacting with chain extender as the wall and the isocyanate not involved in the reaction as the core.It was found in the preparation process that when the reaction temperature was 60℃,pH=10,and the chain extender was small molecule diol or water,the content of active group in isocyanate microcapsules prepared was relatively high,about 25%.In addition,the size of isocyanate microcapsules was controllable,and the size and distribution of isocyanate microcapsules narrowed with the increase of stirring speed.Under the best conditions,the isocyanate microcapsules prepared showed translucent orange-yellow spherical or similar spherical shape with good dispersibility.The shell generated by polymerization at the oil-water interface has good compactness.After half a year’s storage,the active group of the microcapsule only decreased by 5%,showing good stability.The shielding effect of the capsule wall also improves the thermal stability of isocyanate.The prepared isocyanate microcapsules can release liquid core-isocyanate by light pressure.Plywood was prepared by blending isocyanate microcapsules with PVA solution(15%)as a wood adhesive.In order to make the best value of isocyanate microcapsules,the efFects of the particle size and addition on the bonding strength were studied.The results show that when the amount of isocyanate microcapsules wass 15%and the particle size was 1200 RPM,the isocyanate microcapsules prepared by hot pressing were released and cross-linked with the active hydrogen of the system,and the working life of isocyanate was increased to nearly 6 hours.By observing the bonding layer,it was found that the bonding of plywood was the result of intermolecular force,hydrogen bond force,interwoven network structure of transition zone,covalent bond and "glue nail" action.(4)Isocyanate microcapsules with polyurea/urea formaldehyde resin as composite wall were prepared by interfacial polymerization/in-situ polymerization.The degree of interfacial polymerization reaction was controlled by the reaction time of interface polymerization,and then the ratio of thickness of two wall materials was controlled.The results showed that the stability of isocyanate microcapsules was the best when the interfacial polymerization time was 90 min and then in-situ polymerization.The results of IR and morphology analysis showed that the microcapsules had two different wall components.The prepared isocyanate microcapsules were orange spherical,and the core was liquid.The isocyanate group content was about 25%and the particle size was about 200 μM.after 8 weeks of storage,the active group content only decreased by 3%,which showed good storage stability.The prepared isocyanate microcapsules were used as cross-linking agent in the preparation of plywood with wood adhesive.The results showed that the working life of isocyanate was further increased to nearly 7 hours by the prepared isocyanate microcapsules,indicating that the stability of isocyanate was further improved by the composite microcapsules.