Investigating secondary crystallisation of polyamide-12 using fast scanning calorimetry

Abstract

Polyamide 12 (PA-12) is a strong and durable thermoplastic commonly used within advanced polymer processing techniques, such as powder bed fusion (PBF). The use of PA-12 for the fabrication of functional, end-use components is highly dependent on the morphology, size, shape, and stability of the crystalline phase. Secondary crystallisation, an aging process rarely studied in previous PA-12 Lature, can cause further developments in crystallinity that also alter the property profile of the material during PBF. To the best of the authors knowledge, this is the first in-depth investigation into the secondary crystallisation behaviour of PA-12 using fast scanning calorimetry (FSC). Results indicated that, across a wide isothermal crystallisation temperature (T_c) range, the crystalline structure, rate of secondary crystallisation, and the mechanism of lamellar thickening, are all closely correlated to crystallisation time (t_c) and temperature (T_c). At crystallisation temperatures between 100 °C and 130 °C, PA-12 crystallises into the hexagonal gamma (γ) phase, whilst T_c ≥ 140 °C, larger and more thermodynamically stable alpha-prime (α’) crystals are able to grow. Independent of crystal polymorph, there is significant evidence of secondary crystallisation. For extended t_c, the melting endotherm progressively shifts to higher temperatures, indicative of a slow, yet continuous lamellar thickening process. In γ crystals, the melting enthalpy and melting temperature increase linearly as a function of the logarithm of t_c (R² > 0.96), suggesting that solid-state diffusion processes such as chain-sliding and chain refolding are the dominant cause of lamellar thickening. However, within the α’ phase, hydrogen bonding can be more easily attained, resulting in a more rigid crystal structure that reduces chain mobility and prevents lamellar thickening via chain sliding or refolding. Thickening instead occurs through the incorporation of inter-lamellae amorphous chains, across the melt-crystal interface, via Hay's reptation-diffusion mechanism. This is evidenced by the thickening of α’ crystals becoming dependent on the square root of time (R² > 0.96). Such insight into the secondary crystallisation behaviour of PA-12 could be useful within the PBF industry in order to help predict the volume shrinkage effects associated with polymer crystallisation, allowing improvements to the dimensional precision and performance of final components. Similarly, an enriched understanding of the mechanisms and rate of secondary crystallisation could reveal more information about the thermal properties of un-sintered PA-12 powder, and its suitability for re-use in future build cycles.