Study On Preparation And Properties Of Poplar Veneer/Polyethylene Film Composite Carbonized Plywood

In this paper,polyethylene(PE)film was used as an adhesive to manufacture thermoplastic resin plywood with poplar veneer.However,the weak interfacial compatibility between the polar poplar veneer and the non-polar PE film was insufficient to form a good bonding interface,resulting in poor properties and a limited range of applications of plywood.To enhance the physical and mechanical properties and interfacial compatibility of poplar veneer/PE film composite plywood,the plywood was hot-pressing carbonized by extending the hot-pressing time While hot-pressed,which was mainly based on the characteristic of thermoplastic resin remaining in a molten state all the time under high temperatures and curing under cooling.The effect of hot-pressing carbonization treatment on the physical and mechanical properties and interfacial compatibility of the thermoplastic resin plywood was investigated.The main research contents and results are as follows:(1)Based on the thermal properties of PE film,the poplar veneer/PE film composite plywood was carbonized(long time hot-pressed)at 160℃,180℃,and 200℃.The shear strength,modulus of rupture(MOR),modulus of elasticity(MOE),24 h water absorption(24 h WA),24 h thickness swelling(24 h TS),surface contact angle,and decay resistance of the plywood were tested.The results showed that after being carbonized at 180℃ for 2 h,the shear strength of the plywood was effectively increased from 0.49 MPa to 0.94 MPa,meeting the requirements of Type Ⅰ plywood according to the Chinese National Standard GB/T 9846-2015.Carbonization treatment can improve the flexural properties of plywood to a certain extent under temperatures of 160℃ and 180℃.The flexural properties of plywood carbonized at 180℃ for 2 h were optimal,with the MOR and MOE reaching 78.3 MPa and 8607 MPa,respectively.Carbonization treatment can improve the water resistance of the plywood.With the increase of carbonization temperature and the extension of carbonization time,the 24 h TS and 24 h WA of the plywood gradually decreased,and the surface contact angle of the plywood gradually increased.The higher the treatment temperature,the better the durability of the plywood.When carbonized at 200℃ for 2 h,the decay resistance grade of the plywood reached class Ⅱ(durable).Increasing the number of PE film layers has no significant effect on improving the Type Ⅰ shear strength and the water resistance of plywood.The optimal process parameters for carbonized plywood were hot-pressing temperature of 180℃,hot-pressing time of 2 h,and PE film dosage of 2 layers(54.6 g·m-2).(2)The effect of carbonization treatment on the chemical components,crystalline structures,and interfacial morphology of plywood was investigated with Fourier transform infrared spectroscopy(FTIR),X-ray diffraction(XRD),and scanning electron microscopy(SEM).FTIR spectra indicated that the hemicelluloses were degraded,and the number of hydroxyl groups and carbonyl groups was reduced after carbonization treatment,thus decreasing the surface hydrophilicity of poplar veneer.XRD analysis showed that the crystallinity of cellulose was increased and then decreased with the increasing carbonization temperature under the carbonization time of 2 h.When the carbonization temperature was 180℃ and the carbonization time was 2 h,the crystallinity of cellulose reached the maximum value of 51.18%,and the increased crystallinity of cellulose was beneficial to the improvement of dimensional stability and flexural properties of the plywood.SEM images showed that the carbonization treatment by prolonging the hot-pressing time can improve the interfacial compatibility between polar veneer and non-polar PE film to a certain extent.Compared with untreated plywood,the penetration of PE film into the porous structure of the veneer was better,and more continuous and uniform glueline was formed at the bonding interface of carbonized plywood.Consequently,the carbonized plywood possessed higher shear strength.(3)The effect of carbonization treatment on the combustion performance of poplar veneer/PE film composite plywood was investigated by cone calorimeter(CONE)test.It was found that carbonization treatment attenuated the pk-HRR,pk-PCO,pk-PCO2 and av-Yco of the plywood,which had a certain effect on reducing the fire hazard.However,in general,there was no substantial change in the flame retardant properties of carbonized plywood.In order to eliminate the restrictive application of flammable carbonized plywood in the interior decoration industry and furniture industry,ammonium polyphosphate(APP)flame retardant was introduced to impregnate poplar veneer to prepare carbonized flame retardant plywood.The APP flame retardant decreased the shear strength and flexural properties of plywood.When the concentration of APP impregnation solution did not exceed 15%,the shear strength of the plywood could still meet the standard requirements of Type Ⅰ plywood of GB/T 9846-2015.APP flame retardant improved the flame retardancy of plywood and promoted the dehydration of plywood into carbon.Considering the physical and mechanical properties and flame retardant properties of plywood,the optimal APP impregnating solution concentration was 15%,with the av-HRR,pk-HRR2,and THR of flame retardant plywood compared to untreated plywood effectively reducing by 37.6%,22.4%,and 43.2%,respectively.However,the APP flame retardant generated a negative effect on the smoke release performance of the plywood,and the SPR and TSP of the flame retardant plywood significantly increased.