| Fast-growing poplar is the most important timber species of wood-based panel in northChina for the rapid growing wood-based panel industry.With the rising of environmental safetyawareness, people pay more attention to environmental performance of man-made panelproduct. Through improving environmental performance, increasing competitiveness andvalue-added of poplar plywood products, it has become an important way to promote sustainedand healthy development of poplar plywood industry.In this paper, high-density polyethylene (HDPE) film was selected as an adhesive materialto maunfacture formaldehyde free poplar/HDPE composite plywood based on a specificalhot–cold pressing process.In order to establish poplar/HDPE composite plywood formationmechanism, as well as to the HDPE penetration and crystallization properties of bondinginterface status, screening and optimization of hot composite factors with bonding strength byResponse Surface Methodology, effect of hot and cold pressing composite on bonding strength,compression ratio, water absorption (WA) and thickness swelling (TS), HDPE penetration inbonding interface influenced by composite factors, the crystallization behavior of HDPE inplywood interface system were studied.The main contents and conclusions are as follows:(1) Bonding strength as the response value, Response Surface Methodology was used tooptimize the processing factor. Results showed that bonding strength was significantly affectedby HDPE dosage, hot-pressing time, hot-pressing, temperature and pressure. HDPE dosagereached the very significant level as well as interaction effects between pressing temperatureand time；hot-pressing time reached a significant level.When plywood was manufactured under following conditions: HDPE dosage of276g/m2,hot-pressing temperature of161℃，hot-pressing pressure of1.05MPa, hot-pressing time of0.83mm/min, the bonding strength reached1.68MPa which met the requirement of Type IIplywood according to the National Standard of GB/T9846.3-2004. (2) Based on the resluts of Response Surface Methodology, experiments were carried outto analyze the effect of factors (such as moisture content of veneer, HDPE dosage, hot-pressingtemperature, hot-pressing pressure, cold-pressing temperature and cold-pressing pressure) onphysical and mechanical properties (such as bonding strength, compression ratio, waterabsorption(WA),thickness swell (TS)).With increasing of veneer moisture content, bonding strength and compressionperformance declined but WA and TS gradually increased; meanwhile, the factor of HDPEdosage showed an oppsite influence. Under the optimal parameter(hot-pressing temperature at160℃, hot-pressing pressure at1.0MPa), bonding strength increased but then declined as theincreasing of hot pressing temperature and pressure. Compression ratios decreased gradually;the value of WA also rised firstly and then decreased, both of them also reached the minimumlevel in the optimal factor levels.However, the effect trend of TS was diffient between them,theformer increasing firstly and then decreased with the lowest value presenting on the160℃level,while the effect of latter factor showed a increasing trend. Within the temperature rangefrom30℃to90℃，compression ratio went down with cold-pressing temperature rising，whileTSincreasedfirstlyandthendecreased,bothofthemreachedthemaximumvalueon120℃level.Inparticular,coldtemperaturewasnotasignificantfactorforbondingstrengthandWA.Inordertostudysynergisticeffectbetweenhot-pressingandcold-pressingpressure,theeffectsofcold-pressingpressureonpanelperformanceunderdifferenthot-pressingconditionswerecompared.ReslutsshowedIncresingcold-pressingpressurewasbenefittoreducethedifferenceofplywoodperformancecausedbyhot-pressingpressure.There wasasynergisticeffectbetweenthem.(3)Byopticalmicroscopeandscanningelectronmicroscopy(SEM),permeabilitydistributionandpathofHDPEinbondinginterfacewerestudied.ThereslutsshowedtherewasagapinbondinginterfacebetweenHDPEandpoplarveneer,theircombinationdependedonmechanicalinterlockentirely.Using guleline width,average penetration depth (AP) and effective permeability (EP) toevaluate the penetration status of HDPE in the composite interface, the impacts of composite factors on penetration position were analyzed. As the veneer moisture content increasing,glueline width increased,but the value of AP and EP gradually decreased. With the increase ofHDPE dosage, value of all these three indicators increased.Effects of hot-pressing temperature and pressure on penetration in the bonding interfaceare similar: with hot-pressing time and pressure increasing, the glueline width decreased, APand EP increased gradually. However cold-pressing temperature only affected AP significantly.Under high pressure conditions, effects of cold-pressing pressure on AP and EP reachedextreme significant level,while the impact on glueline width only reached significant level.With the increasing of cold-pressing pressure, both of AP and EP went up under differenthot-pressing pressure conditions. The levels of penetration depth in low hot-pressing pressurewere lower than high hot-pressing pressure conditons. With cold-pressing pressure decreasing,the level difference between different hot-pressing pressure declined.Comparison with effects of composite factors on AP,EP and bonding strength, the effectsof veneer moisture content, HDPE dosage, cold-pressing temperature and cold-pressingpressure on the performance of bonding strength and penetration depth had the similar trend.But there were an obvious difference of factor’s impact on them between hot-pressing andcold-pressing pressure. The result indicated that increasing penetration depth couldeffectively enhance bond strength,however excessive penetration would cause the decrease ofbonding performance.By comparing the correlation of bonding strength and permeability, EPwas more suitable as an interface penetration indicator of poplar/HDPE composite plywood.（4）HDPE crystallization behavior in bonding interface system of composite plywood wasstudied by DSC.Using crystallization temperature(Tc), melting temperature(Tmpeak) andcrystallinity(Xc) to evaluate HDPE crystallization performance. Based onAvaramicrystallization kinetics equation, the crystallization processs was analyzed by Avaramiindex (n), crystallization rate constant (logK (T)) and half of crystallization time (t1/2).Effects of cold-pressing temperature, pressure and the dosage on the crystallizationbehavior in interface system in preparation process were studied. Results showed increasing of HDPE dosage and cold-pressing temperature or reducing cold-pressing pressure could increasethe crystallinity of HDPE,while there was little effect on Tcby composite factors. With theincreasing of HDPE dosage and cold-pressing temperature, the value of n increasedgradually,while the value of n decreased by the improvement of cold-pressing pressuredecreased. As the crystal growth of HDPE had been in small dimension mode.Effect of cooling rate on the non-isothermal crystallization in plywood sample preparedunder the optimal parameters was studied by DSC. Results showed the cooling rate had littleeffect on Tc, while Xcdeclined gradually with improving the cooling rate. With increasingcooling rate, crystal growth of HDPE was restricted, so crystal size was smaller andcrystallization rate slowed down.Isothermal crystallization of pure HDPE and plywood samples with different HDPEdosage were studied. In a full isothermal crystallization, Tc, Tmpeakand Xcof HDPE in bondinginterface system were lower than pure HDPE, expect the level of1layer HDPE, the value of nand logK (T) were also lower than the latter,the number of t1/2increasdgradully.It was obviousthat bonding interface system of plywood would hinder HDPE crystallization behavior innucleation, stability and regularity of crystallization. |
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