Preparation And Properties Of Fire-retardant Eucalyptus Plywood

Rapid development of the eucalyptus plywood industry in China has brought thewood-based panel industry new hope and energy during the last decade. However, theeucalyptus plywood production is still at an early development stage, such as low productivityand added value. Design fire-retardant eucalyptus plywood is one way to facilitate the progressof this field. Fire-retardant eucalyptus plywood will not only increase its added value, thusimproving the economic efficiency of the enterprise, but also extend its application.In this study, FR-2fire retardants were first prepared to impregnate eucalyptus veneers.Then the treated veneers were processed into fire-retardant plywood in a conventional way.Next the properties of the plywood were measured by equipments like as cone calorimeter(CONE), thermal gravimetry (TG) and compared to that of commercial flame retardant FRSand poplar plywood. The results were as follows:(1)Formula optimization of FR-2fire retardants and performance comparison betweenPNB-1and FRSThree formulations of FR-2fire retardants were prepared according to the AWPAP50-2010standard. Among the three FR-2retardants, the PNB-1flame retardant, composed ofmonoammonium phosphate (MAP), diammonium phosphate (DAP) and boric acid (BA),showed the best performance in retarding fire and suppressing smoke. The effects on class IIbonding strength and hygroscopicity of plywood were nearly the same for these three fireretardants. Thus we conclude the PNB-1is a preferable FR-2fire retardant for eucalyptusplywood.Using the same impregnation process, the veneer treated by FRS flame retardant has2-3%higher retention rate than that treated by PNB-1fire retardant, but the plywood has similarphysical and mechanical properties. However, the PNB-1flame-retardant plywood showedsmaller heat release rate (HRR) and total heat release (THR), longer time to ignition (TTI).Also the char yield for PNB-1fire retardant was higher during thermal gravimetric analysis. So the PNB-1fire retardant is better in retarding fire than FRS flame retardant. However, the totalsmoke production (TSP) of PNB-1was much higher than that of FRS, which suggests FRSsuppresses the smoke more effectively than PNB-1. Through the thermodynamic calculation,we found that PNB-1reduced the activation energy of eucalyptus wood by11.44kJ·mol-1,while FRS increased that by6.08kJ·mol-1. The results indicate that flame retardant mechanismof PNB-1and FRS may be different.(2)Manufacturing process of fire-retardant eucalyptus plywoodUnder atmospheric pressure, higher fire retardant retention rate of eucalyptus veneer couldbe obtained by either increasing the concentration or raising the temperature of the solution orelongating the impregnation time. As expected, increasing the concentration from10%to30%significantly increased the retention rate in the orthogonal test results (α=0.05). However,raising the temperature from30℃to70℃and elongating the time from3hr to12hr showed nosignificant increase. Hence, concentration of the solution should be adjusted to achieve the fireretardant retention rate required for specific flame retardant rating.According to the National Center for Quality Supervision and Testing of Wood andBamboo Products, commercially manufactured fire-retardant eucalyptus plywood, the E.saligna, which showed the best bonding performance, and the E. dunnii, which had the lowestbonding performance, were met the criteria of GB/T9846-2004“plywood” standard. Therefore,we believe that, with additional veneer impregnation devices, the fire-retardant eucalyptusplywood could be manufactured in conventional eucalyptus plywood production line.(3)Effect of tree species on properties of fire-retardant plywoodThe bonding strength of FRS fire-retardant E. saligna and E. urophylla×grandis plywoodreached1.5MPa, which were higher than that of the FRS fire-retardant E. grandis, E. cloezianaand E. dunnii plywood. The MOR and MOE of E. dunnii and E. urophylla×grandisfire-retardant plywood were significantly lower than the others.Normally the retention rate of eucalyptus veneer was4-6%lower than that of the poplarveneer under the same impregnation process. With comparable retention rate (8.38%) as poplar plywood (7.42%), eucalyptus plywood treated with PNB-1fire retardant showed lowerbonding strength and wood failure percentage. It also had higher density and MOE. Howeverthe MOR was similar. The treated eucalyptus plywood showed better flame retardant properties.It had lower peak of HRR in second burning stage, later appearance of the peak value andlower THR in10min. However, when it came to later stage of burning (about5min later),treated eucalyptus plywood showed higher smoke production rate (SPR), and its TSP graduallysurpassed that of treated poplar plywood.(4)Effect of PNB-1fire-retardant treatment on properties of plywoodPNB-1fire retardant treatment increased the pH value and acid buffering capacity ofeucalyptus veneer. It also shortened the curing time of the adhesive and reduced the bondingstrength of plywood. However, it had no significant impact on MOR and MOE of plywood.Nonetheless, we showed that it improved flame retardant properties of plywood. On the onehand, it greatly reduced the HRR and peak of HRR so that THR in10min dropped from63.32MJ·m-2to17.79MJ·m-2. On the other hand, it reduced the weight loss at the main thermaldecomposition stage from61.42%to36.44%and increased the char yield from3.13%to14.22%. Moreover, it decreased the amount of smoke at the early stage of combustion, eventhough the SPR is higher at the later stage than the control group and the TSP exceeded thecontrol group in380s. In addition, it reduced CO2generation rate while increasing the rate ofCO generation.