| In this study,melamine,urea and formaldehyde were used as raw materials,and the brittleness of melamine-urea-formaldehyde(MUF)resin adhesive was modified by using the toughening agent polyvinyl alcohol,polyamide,polyethylene glycol 400 and polyethylene glycol 4000.The amount of the toughening agent added is 2%,4%,6%,8%,and 10% of the urea mass,respectively,and the curing agent is a self-made mixed curing agent.The structure of the resin before and after modification was characterized by infrared spectroscopy(FT-IR)and X-ray diffraction(XRD).The morphology of the resin before and after modification was observed by scanning electron microscopy(SEM);according to GB/T 14074-2006,GB/ T 26899-2011,GB/T 6344-2008 and other national standards test the basic properties,mechanical properties and toughness of the resin before and after modification.The main findings of this study are as follows:1.Polyvinyl alcohol modification: the dry strength and boiling water wet strength of the modified resin glulam are improved;the higher the addition of polyvinyl alcohol,the greater the elongation at break,indicating that the toughness improvement is more obvious;After the alcohol,the tensile strength is increased first and then decreased.Combined with the shear strength and impregnation degree of the glulam,it can be concluded that the polyvinyl alcohol addition amount is 6% of the urea mass,and the dry shear strength of the glulam is 6.71 MPa.The shear strength of boiling water for 2 h is 6.22 MPa,the shear strength of cold water for 24 h is 5.67 MPa,the tensile strength is 6.20 MPa,and the toughness is increased by 24.12%.The fracture mode of the resin before modification is brittle fracture,and the fracture mode of the modified resin is ductile fracture.After the addition of polyvinyl alcohol,the cross-section morphology of the resin model was significantly different,and the fracture mode was also changed,indicating that the brittleness of the resin was improved.2.Modification of polyamide: The higher the amount of polyamide added,the higher the dry strength and wet strength of the modified resin glulam;the elongation at break of the resin model increases first and then decreases,but both are higher than the original resin.Adding polyamide can improve the toughness of the resin;combined with the shear strength of the glulam,it can be concluded that the optimum amount of polyamide added is 8% of the urea mass,and the dry shear strength of the modified wood prepared by the modified resin is 8.08 MPa.The shear strength of boiling water is 2.75 MPa for 2 h,the shear strength for cold water is 7.12 MPa for 24 h,the tensile strength of the resin is 15.07 MPa,and the toughness is increased by 72.06%.The fracture mode of the resin before modification is brittle fracture.The fracture mode of the resin after modification is ductile fracture.The more pores indicate that the resin after curing is easy to enter by moisture,which also reflects the resin modified by polyamide.The obtained plywood immersion peeling rate was not the reason of “0”.After the addition of polyamide,the cross-section morphology of the resin model was significantly different,and the fracture mode was also changed,indicating that the brittleness of the resin was improved.3.Modification of polyethylene glycol 400: With the increase of polyethylene glycol 400,the dry strength and wet strength of the modified MUF resin glulam increased,the toughness increased,and the tensile strength increased first and then decreased.According to the shear strength results of glulam,it is considered that the optimum amount of polyethylene glycol 400 is 8% of the urea mass.At this time,the dry shear strength of the glulam is 8.41 MPa,and the shear strength of the boiling water is 6.43 MPa.The shear strength of cold water at 24 h is 6.61 MPa,the tensile strength of the resin is 9.11 MPa,and the toughness is increased by 48.24% compared with that before unmodification.The fracture mode of the resin before modification is brittle fracture.The fracture mode of the resin after modification is ductile fracture.Although the section is rough but has no pores,it indicates that the resin is easy to adhere to the surface of the glued wood and is not easy to be infiltrated by moisture.The gluing wood prepared by the modification of the ethylene glycol 400 has high bonding strength and certain water resistance.After adding polyethylene glycol 400,the cross-section morphology of the resin model was significantly different,and the fracture mode was also changed,indicating that the brittleness of the resin was improved.4.Modification of polyethylene glycol 4000: After adding polyethylene glycol 4000,the tensile strength is increased first and then decreased,combined with the shear strength and immersion peeling rate of the bonded wood,the optimal polyethylene glycol 4000 can be obtained.The amount is 8% of the urea mass,the dry shear strength of the glulam is 9.07 MPa,the shear strength of the boiling water is 2.37 MPa,the shear strength of the cold water is 6.29 MPa,and the tensile strength of the resin is 7.44 MPa.Resilience increased by 29.41%.The fracture mode of the resin before modification is brittle fracture.The fracture mode of the resin after modification is ductile fracture.Although the section is rough but has no pores,it indicates that the resin is easy to adhere to the surface of the glued wood and is not easy to be infiltrated by moisture.The gluing wood prepared by the modification of the ethylene glycol 4000 has high bonding strength and good water resistance.After adding polyethylene glycol 4000,the cross-section morphology of the resin model was significantly different,and the fracture mode was also changed,indicating that the brittleness of the resin was improved.5.Compared with polyvinyl alcohol,polyamide and polyethylene glycol 400 three kinds of toughening agents,the glued wood prepared by PEG 4000 modified MUF resin has superior dry shear strength and wetness.The shear strength and good water resistance were more pronounced in the prepared resin model “toughness fracture”. |
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