| Moisture curing hot melt coating is a new application of reactive polyurethane(PUR).Similar to the technical scheme of PUR hot melt adhesive,it has the characteristics of environmental protection,high efficiency and good film performance.It is especially suitable for wood coatings.In order to obtain a PUR hot-melt wood coating with low melt viscosity,rapid surface drying and excellent film performance,the effects of polyisocyanate,reaction temperature,reaction time,catalyst,polyol and additives on the synthesis and some properties of PUR hot-melt coating were studied.The results of infrared spectrum analysis show that the synthetic products are PUR and have wet curing behavior.The results of other experiments show that the PUR hot-melt coating prepared by high glass transition temperature(T_g)amorphous polyester polyol(Dynacoll7150)with two different crystalline polyester polyols(PBA3000 and PHA3500)and hexamethylene diisocyanate(HDI)reacts completely at 95℃,and then adding 1‰dibutyltin laurate has better performance.In the above system,the effects of R value(the amount ratio of-NCO to-OH),the mass fraction of Dynacoll7150,the mass fraction ratio of PBA3000 to PHA3500 on the storage stability,melting viscosity,surface drying time,crystallization,heat resistance,tensile properties and various film properties of PUR hot-melt coatings were further studied.The results of storage stability test show that most of PUR hot-melt coatings can be sealed and stored at room temperature for 6 months.The test results of melt viscosity and surface drying time show that when the R value is 1.8 and the mass fraction of Dynacoll7150 is 20%,the surface drying time of PUR hot melt coating can be adjusted between 21 s～27 s and the melt viscosity at 130℃can be adjusted between 5442 m Pa·s～6865 m Pa·s.The results of differential scanning calorimetry and X-ray diffraction show that the relative crystallinity of PUR hot-melt coating is between 23.94%～40.47%,the R value and the mass fraction of Dynacoll7150 have a great influence on the crystallization properties.The results of thermogravimetric analysis showed that the heat resistance of PUR hot melt coating was improved after wet curing and began to decompose at 326.5℃,in which the mass fraction of Dynacoll7150 had a great influence on the heat resistance.The tensile test results show that the elongation at break of PUR hot-melt coating is between 447%～856.3%and the breaking strength is between 20.11 MPa～41.77 MPa,in which R value and Dynacoll7150 mass fraction have a great influence on the tensile properties.The test results of scanning electron microscope show that the paint film of PUR hot-melt coating shrinks due to cooling crystallization.Most samples can better modify the defects of wood surface and closely combine with white beech substrate.The results of other paint film tests show that when the R value is 1.8 and the mass fraction of Dynacoll7150 is 20%,the adhesion of PUR hot-melt coating can reach grade 0,the flexibility can reach the diameter of steel rod 1 mm,the impact resistance can reach 50 cm,the cold liquid resistance can reach grade1,the surface hardness can reach grade B,the shore hardness is about 56,the wear value is about 3.2 g,have the yellowing resistance,no paint film bubbling pinholes and matte surface.In this study,a series of PUR hot melt wood coatings with low melt viscosity and short surface drying time were successfully synthesized,and they have good storage stability,heat resistance,tensile properties and paint film properties.There is value for further research to be transformed into practical application. |
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