Fire retardancy improvement of high‐density polyethylene composites based on thermomechanical pulp treated with ammonium polyphosphate

Thermomechanical pulp (TMP) was pretreated with ammonium polyphosphate (APP) to improve the fire retardancy of extruded thermoplastic composites for facades. In addition, APP was also added during compounding to protect the polymer matrix. The limiting oxygen index (LOI) value of the composites was significantly increased when APP was added during compounding. The additional TMP pretreatment leads to a further increase of the LOI, which resulted in a maximum value of 35% indicating the self‐extinguishing behavior of the material. Compared to untreated TMP, thermal stability of pretreated TMP was lower up to a temperature of ~250 °C which was attributed to the formation of polyphosphoric acid, which catalyzes the degradation of cellulose. Upon formation of a char layer, thermal stability of pretreated TMP was increased compared to untreated TMP. Tensile strength of composites based on pretreated TMP was slightly reduced compared to untreated TMP while tensile modulus of elasticity (MOE) was unaffected. Swelling and water uptake of the composites was increased due to the use of APP during compounding but the fiber pretreatment did not lead to any further increase. The single flame source test showed that the formulations which included APP were self‐extinguishing even when an extended flame treatment of 300 s (normally: 30 s, according to EN ISO 11925‐2) was applied. The fire shaft test was not passed for extruded profiles based on pretreated TMP; however, its conditions are more severe than those of the single‐burning item (SBI) test. POLYM. COMPOS., 40:2410–2423, 2019. © 2018 Society of Plastics Engineers