Forming Mechanism And Properties Of Melamine-Urea-Formaldehyde Resin Microcapsules

The wood industry in China is experiencing a critical development period of transformation and upgrading. Functionalization has become an important way to increase added value of wood products, to extend their application areas and to enhance competitiveness of the industry. Microencapsulation is a protective technology of encapsulating solid, liquid or gas materials into micron particles, and has been widely used in the field of functional materials. The core material of microcapsules retains its properties during encapsulation, and could be durable or release either slowly or instantly in application. Functional units are separated into stable microparticles by microencapsulation, which greatly increases their specific surface area and thus enhances functional effects. After being insulated from the external environment by shell layers, the functional units become more stable, and their compatibility with substrate materials could be obviously improved. It has important theoretical and practical value in expanding application areas of wood materials to conduct researches on microencapsulation mechanism and its application in wood functional materials basing on characteristics of wood materials.In this study, melamine-urea-formaldehyde(MUF) resin with excellent mechanical properties and low cost was used as the shell material, while styrene maleic anhydride(SMA) with high emulsifying activity and extensive applicability was used as the emulsifier. By selecting micro glass beads and tetrachloroethylene as representative solid and liquid core materials, effects of emulsifying processes, shell-forming conditions and post-drying parameters on the formation, morphology and performance of microcapsules were investigated using scanning electron microscopy(SEM), fourier transform infrared spectrometer(FTIR), thermogravimetric analyzer(TGA) and nano-indenter. The effect of cellulose nanocrystals(CNC) modification on morphologies and properties of microcapsules were discussed. The optimal synthetic technologies were applied to encapsulate reversible thermochromic mixture, ammonium polyphosphate(APP) and paraffin, in order to discover the applicability of microencapsulation mechanism for different core materials. The main results are summarized as follows:(1) The formation of MUF microcapsules can be divided into three stages. In the first stage of emulsification(or dispersion), core materials were dispersed into stable micron particles by SMA. In the next stage of shell-forming, MUF prepolymer polymerized to form a preliminary weak shell layer around the core by effects of electrostatic attraction and deposition, under acidic catalyzing and heating. In the final stage of cross-linking and solidification, MUF particles continued to accumulate on water/oil interface, and were cross-linded and cured to form microcapsules with tough resin shells.(2) During the microencapsulation, curing agent and reaction temperature determined polycondensation rate and cross-linking degree of MUF molecules, while emulsifier determined deposition rate of MUF particles onto the surface of core material. The optimal synthetic technologies for tetrachloroethylene microcapsules were as follows: reaction temperature > 70℃, SMA concentration of 1.2% to 2.0%, curing agent(potassium acid phthalate) concentration of 1%, MUF concentration > 5%, reaction time >120min. Both the decrease of volume ratio of oil phase to water phase and the increase of MUF concentration, reaction time as well as drying temperature and time would enhance the thermal stability of microcapsules.(3) Under the adsorption effect of SMA, MUF combined with CNC to generate compound shell. With the increase of SMA concentration, the shell of microcapsules tended to be smoother and thinner. When the weight ratio of CNC/MUF was less than 1:60, CNC modified microcapsules had good morphology and thermal stability. CNC modification obviously improved micromechanical properties of MUF resin, and the reduced elastic modulus and hardness for modified resin with CNC/MUF of 1:60 were 10.4% and 14.8% higher than that for control sample, respectively.(4) Thermochromic, flame retardant and phase change microcapsules were successfully prepared following optimum reaction parameters. It is suggested that the microencapsulation mechanism summarized from micro glass beads and tetrachloroethylene was also suitable for encapsulating other core materials. After being covered by MUF resin, the thermal stability of both thermochromic mixture and paraffin obviously improved, while the water solubility of APP decreased. The MUF shell is confirmed to be an effective barrier for isolating and protecting core materials.(5) Compared with tetrachloroethylene, paraffin was easier to be emulsified and encapsulated to particles less than 5μm, while thermochromic mixture was more difficult to be emulsified into stable emulsions. Nevertheless, thermochromic microcapsules with particle size of less than 10μm were prepared by further reducing volume ratio of oil phase to water phase or by increasing emulsifying speed. Both volume ratio of oil phase to water phase and emulsifying speed showed no evident effect on cracking temperature of thermochromic and phase change microcapsules, and the thermal stability of the microcapsules was obviously improved by heat treatment at 120℃ for 30 min, which is consistent with the effects of synthetic technologies and drying processes on thermal stability of tetrachloroethylene microcapsules.(6) Thermochromic veneers, which was prepared by incorporating microcapsules into coatings, showed sensitive thermochromic characteristic and good colour-changed stability. The reversible colour-changed phenomenon appeared within a temperature range of 16~32℃, and the maximum colour difference(△E) for the thermochromic veneers was larger than 20. The compatibility between APP and adhesives was improved by microencapsulation. Modulus of rupture, modulus of elasticity and internal bond strength of particleboards treated by APP microcapsules were 59.6%, 41.5% and 7.4% higher than that treated by APP.