0. Structure-property relations for polyamide 6
PolymerTechnology
Structure-property relations
for polyamide 6
E. Parodi, L.E.Govaert, G.W.M. Peters
Introduction
Polymers are getting more and more importance in load-bearing applications. In this, high-performance is the key word. Optimization of processing is required to achieve as best performance as possible.
Polyamide 6 (PA6) is an aliphatic polyamide and, despite its strong hydrophilic character (which tend to strongly decrease its mechanical properties), it is largely used in several high performance applications.
The aim of this research is to investigate the mechanical properties of different phases in different hydration level.
Polymorphism in polyamide 6
PA6 has a stable crystalline phase called α-phase (monoclinic) which is obtainable by slow cooling from melt; however, upon fast cooling from the melt the γ mesophase (pseudo-hexagonal) may be formed. Upon very fast cooling (quenching), it is even possible to obtain a fully amorphous phase.
Sample preparation
Plates containing only a single phase were made by compression molding following these production protocols:
Alpha: cooling in air (in between of hot steel plates, thickness 3mm)
Gamma: cooling in “cold” press at 80ºC, force < 10 KN (in between of hot steel plates, thickness 3mm)
Amorphous: quenching in ice-water bath (in between of aluminium sheets)
For all phases: Tm at 265ºC for 5min and Fpressure 100 KN.
Crystallization kinetics – TTT curves
The crystallization kinetics was investigated by Flash-DSC. Both crystallization from melt and from amorphous (annealing) have been performd, and TTT curves (temperature-time-transformation) have been obtained (Figure 3). With these results it is possible to determine the time to crystallization at several temperatures.
In the case of crystallization from melt, the different kinetics between α and γ phase are easily recognizable as shown in Figure 3(left), and α kinetics dominates at high temperature whereas gamma kinetics start to govern at temperature of about 80°C. Instead, the TTT curve obtained from amorphous (cold crystallization), has revealed a similar kinetics for both the phase as shown in Figure 3.
Mechanical properties – Tensile test
Stress-strain curves for the 3 phases are shown in Figure 4 for both dry and wet conditions. In Figure 5 the rate dependence of the yield stress. For the wet samples the yield stress is much lower and the strain rate dependence less pronounced.
Conclusion
In this research has been demostrated that each phase of PA6 has different stress-strain response, but a similar strain rate dependence.
Figure 1 – The first transmission cross beam made of nylon 6, is standard equipment in the new BMW M5.
Figure 3 – TTT curves for polyamide 6 crysatllized from melt (left), and crystallized from amorphous (right).
Polyamide 6
DRY
WET
Figure 2 – Schematic representation of alpha (left) and gamma (right) structures.
Figure 4 – Stress-strain curves for samples in dry and wet condition.
Figure 5 – Eyring plot for dry and wet samples.
/ department of mechanical engineering