Calcium Carbonate as a Functional Filler in Polyamide 12 – Manipulation of the thermal and mechanical properties

Abstract

Controlling and adjusting the thermal response properties of a polymeric compound is a key driver for improving its usability for an additive manufacturing process such as selective laser sintering, optimising the final part density as well as hardness. Adding mineral fillers to polymers was originally aimed at cost reduction. However, fillers are often used to fulfil a functional role, such as improved thermal or mechanical properties of the polymer composite. The influence of particle size distribution, filler morphology and filler amount of calcium carbonate as a mineral functional filler particle on the thermal properties in a compression-moulded polyamide 12 matrix was investigated. The possibility to combine precisely defined particle size distributions, thereby combining the benefits which each particle size range within the chosen material size distribution contributes to the matrix, was evaluated. The loss of ductility, occurring as a common downside of the introduction of a non-flexing mineral filler, has been investigated. The effect of an optimised coupling agent on the mechanical properties of a compression-moulded compound, containing polyamide 12 filled with surface modified calcium carbonate was also determined. Mineral filler modifier, namely 6- -caprolactam, L-arginine and glutamic acid, were chosen to investigate their coupling effect. The melt flowability at 190°C, the melting speed, melting and crystallisation point, degradation temperature, crystallisation time at 170 and 172°C, as well as the elongation at break were analysed. The melt properties of a polyamide 12 matrix show a significant dependency on the filler volume-based particle size. The finer and the narrower the particle size distribution of the calcium carbonate particles in the polymer matrix become, the less influence the filler has on the melting properties, influencing the melt flow less significantly than the same surface amount of broad size distribution coarse calcium carbonate filler particles. However, due to increased nucleation, the crystallisation behaviour on cooling showed a markedly more rapid onset in the case of fine sub-micrometre filler particle size. The overall thermal and flow properties can be optimised, by adjusting the morphology and particle size distribution of a coarse and fine calcium carbonate filler, blended in an optimised functional filler ratio. With an optimised surface modifying agent and/or filler morphology, the tensile strength as well as elongation at break can be improved in comparison with uncoated filler implementation, such that up to 60% of the loss of ductility and toughness of a final part when using an untreated filler could be regained using the correct amount of an optimised surface modifier. The improvements were demonstrated with a twin-screw extruded compound, indicating the potential for the usage of a functional filler within a polymer matrix as an optimised composite for additive manufacturing.