Preparation Process And Properties Of Layered Flame-retarded Strandboard

In the context of the national"double carbon"strategy,strand boards(such as oriented strand board)are promising green building materials.The improvement of fire safety is a crucial factor for their use.Currently wood-based panels are commonly produced by the addition of flame retardants during the manufacturing process.Although this process is convenient,it will lead to problems such as low flame-retardant efficiency,poor ignition inhibition and significant reduction of physical and mechanical properties of the strandboard,especially in terms of internal bonding strength.How to improve the flame-retardant efficiency,and how to take into cognizance the flame-retardant performance and the mechanical properties of wood-based board,is the key.In this paper,by combining the difference between the burning phenomena of surface layers and the core layer during laminated strandboard combustion,the effect of laminated flame-retardant strandboard manufacturing process on the flame-retardant performance was investigated.The non-directional flame-retardant strandboard was prepared by using strands derived from the small-diameter poplar strandboard which was supplied by the industrial production line of oriented strandboard,using isocyanate resin as the adhesive and ammonium polyphosphate(APP)and tris(2,3-dibromopropyl)isocyanate(TBC)as the flame retardants.To determine the overall performance of strandboard and study the mechanism of structural design on the mechanical and flame-retardant properties,the three-layer structure of"top surface layer/core layer/lower surface layer"(the mass ratio of each particle layer is 1:1:1)was applied with different mass ratios of APP and/or TBC.The results of the study are presented in the following section:(1)The three-dimensional distribution characteristics of the flame retardant inside the strand boards are revealed by X-ray computed tomography(X-CT).During the hot-pressing process,the flame retardant is relocated.Due to its low melting point,TBC was moved downward during hot pressing.Further,APP was agglomerated during hot pressing.The density gradient of the strandboard can be controlled by the distribution of the flame retardants.(2)The APP flame retardant treatment had a significant effect on the performance of strandboards.The mechanical properties and wet strength of all flame-retardant strandboards treated with APP were better.However,the performance of the flame-retardant strandboards produced via the laminated application process was better.The maximum increase in internal bond strength was 0.15 MPa and the maximum increase in module of rupture was 8.59 MPa.The best performance was obtained from APP424(the ratio of APP flame retardant layer application was 40%/20%/40%).The amount of inorganic flame retardant APP applied to the surface layer affects the module of ruoture of strandboard,and the amount applied to the core layer affects the internal bond strength;The layered application of flame retardant significantly improved the flame retardant performance of strandboard,and the second peak of heat release rate was delayed by 255～435 s,and the fire hazard was significantly reduced,among which APP343 had the best fire safety.(3)The effect of TBC on board properties differed significantly from APP.(b)After applying a total of 10 phr TBC,the flexural properties of flame retardant large chipboard were lower than those of pure PMDI strandboard,but the internal bond strength and 24-hour thickness swelling rate were even optimized;The TBC lamination process further improved the internal bonding strength(maximum 0.52 MPa)and module of rupture(18.35 MPa-30.01 MPa)of flame retardant strandboards;TBC 424 was the best(i.e.,the ratio of flame retardant TBC lamination was 40%/20%/40%).The highest performance of flame retardant was achieved by TBC343,which showed a significant decrease in FGI value and a remarkable increase in FPI compared with other strandboards;the second peak of HRR was delayed by 200～330 s and reduced by 18.7～48.4 k W/m~2.(4)The combined APP/TBC/APP laminated flame retardant(ATA)process further improves the flame-retardant performance and the physical and mechanical properties of the strandboards.Among them,ATA343(i.e.3phr APP top layer/4phr TBC core layer/3phr APP bottom layer)has the best performance.The internal bond strength increased by 0.49MPa,0.39MPa and 0.57MPa compared to the PMDI group,10phr TBC flame-retardant board and 10phr APP flame-retardant board,respectively.The modulus of elasticity increased by 2349MPa and 1512MPa compared to10phr TBC and 10phr APP.The modulus of rupture increased by 22.83 MPa and 17.81 MPa respectively;The oxygen index reached 38.19%,which was significantly higher than PMDI group and 10phr TBC or 10phr APP;The FGI values decreased significantly and the FPI values increased significantly;The second peak of HRR was delayed by 280～360 s.Compared with the APP and TBC groups,the second peak of HRR of ATA343 and ATA262 was significantly delayed,and the peak HRR decreased to about 160～204 k W/m~2.In conclusion,the laminated application of flame retardants effectively regulated the mechanical properties and wet stability of strandboards and significantly improved the flame-retardant properties of strandboards.The mechanical properties of the ATA group were better than those of the APP group and the flame-retardant properties were better than those of the TBC group,which indicated that the designment of APP/TBC/APP layered flame-retardant structure fully combined the advantages of APP and TBC and was worthy of application in production.