Powder Aging Effects on Mechanical and Fire Properties of a Flame Retardant PA12 in Laser Powder Bed Fusion

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Flame retardant polymer based powders for laser based powder bed fusion (PBF-LB/P) are currently not reusable. This leads to crucial economic and environmental disadvantages of the process, when using flame retardant powder. In this study, we demonstrate the processability and emerging part characteristics of samples manufactured from 100% reused powder, based on a flame retardant system of ammonium polyphosphate and pentaerythritol, opening the possibility to understand the aging process behind flame retardant powders for PBF-LB/P. To investigate the effect of aging during manufacturing, mixtures from virgin PA12 and flame retardant additives, aged PA12 and aged flame retardant additives and combinations of virgin and aged powder were prepared. The mixtures were processed using PBF-LB/P and investigated regarding the impact of PA12 and flame retardant additives. Our findings indicate both the formation of agglomerations and mutual chemical reactions between the flame retardant additives, particularly through esterification reactions. The agglomeration and aging of PA12 hinder the coalescence of the powder, resulting in reduced mechanical properties. In contrast to the mechanical characteristics, aged specimens demonstrate enhanced burning behavior, yielding an increase of the LOI by 1.2 pp and a V-0 rating for specimens of 3 mm thickness for aged powder compared to virgin powder. Moreover, the aging is associated with distinct influences on the burning behavior during cone calorimetry testing. While aged flame retardant additives cause a steep increase of HRR within the first seconds of ignition, followed by a decline and plateau formation, aged PA12 shows a slower increase in HRR, followed by a plateau formation on a higher HRR level. • Flame retardant additives agglomerate during aging and undergo esterification • Agglomeration of additives and PA12 aging causes reduced coalescence behavior • Mechanical properties are reduced due to worsened coalescence behavior • LOI and UL-94 performance improve for aged powder • Aging of flame retardant system and PA12 cause significant differences in HRR curve

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