Effect of a layered combination of APP and TBC on the mechanics and flame retardancy of poplar strandboards

Abstract

How to balance the flame-retarding and physico-mechanical properties of wood composites treated with chemicals remains a big challenge. Herein, a layered pattern of bi-component flame-retarding agent in strandboards was established by applying tris (2,3-dibromo-propyl) isocyanate (TBC) in the core and ammonium polyphosphate (APP) in the surface layers. The mechanical and flame retardant properties of treated strandboards were controlled by adjusting the addition ratio of APP to TBC. X-Ray computed tomography was applied to disclose the on-site distribution of APP and TBC chemicals in the strandboard matrices. Both oxygen index meter and cone calorimeter were used to characterize the burning behavior of the strandboards. Addition of TBC resulted in a higher-density core of strandboards. Compared to the untreated and evenly-distributed strandboards, internal bond (IB) strength and modulus of elasticity (MOE) were enhanced by up to 6% and 62.9%, respectively. Meanwhile, the peak heat release rate (pk-HRR) of strandboards after APP/TBC/APP treatment decreased by 33.6%, and the limited oxygen index (LOI) reached a maximum value of 38.2%. When exposed to combustion, the core layer of strandboard treated with TBC generates non-combustible gas and expands the surface char barrier established by APP, thereby enhancing flame retardancy. Compared to traditional methods, a lamination pattern of flame retardants in strandboards exhibits improved bonding performznd satisfied fire resistance. This innovative approach holds great potential for application in the fire safety wood composites industry.