Mechanical behavior of polyamide-6 after combined photo-oxidative and hygrothermal aging

Abstract

The effect of moisture on the photo-oxidative degradation of polyamide-6 (PA-6) was studied by analyzing the mechanical response after two different accelerated aging procedures. In the first aging procedure, the PA-6 was only exposed to ultra-violet (UV) radiation at 60 $^\circ$C. In the second procedure, the same duration of UV radiation was periodically interrupted while the relative humidity was raised to 100%. Diffusion-limited and nominally homogeneous degradation conditions were investigated using bulk and film specimens, respectively. Accelerated UV aging reduced the ductility of PA-6, but the additional hygrothermal exposure had no effect on the ductility or strength, indicating that humidity did not influence the photo-oxidation of PA-6. This finding contrasts with previous studies that found thermo-oxidation of PA-6 was accelerated by moisture.

Graphical abstract

Appendix: Hygroscopic aging of PA-6

PA-6 is hygroscopic, meaning it readily absorbs moisture [41, 42]. The absorbed moisture acts as a plasticizer that reduces the glass transition temperature and strength while increasing the ductility [41]. For this reason, the experimental protocol included a vacuum drying procedure as described in Section 2.3. The vacuum drying procedure was chosen based on preliminary studies on round notched bars loaded to failure after various storage times in a desiccant, but without any pre-test vacuum drying. The round notched bars had a notch root diameter and notch radius of 3.90 mm, a notch shoulder diameter of 7.00 mm, a gauge length of 6.22 mm, and a total length of 57.50 mm. The notched bar geometry was chosen to study triaxiality effects in photo-oxidized PA-6 for a publication in preparation and can be designated as RN10 specimens following the nomenclature for round notched bars used in [51, 52]. These specimens were stored in a cabinet with a tray of calcium sulfate (CaSO$_4$) desiccant after being machined from a cast plate of PA-6. When the desiccant was saturated, indicated by a change in color from light blue to purple, it was changed. The ambient humidity in the laboratory where the cabinet was located was typically $60\pm 5$%, although measurements as low as $50\pm 5$% and as high as ${70\pm 5}{\%}$ were recorded. These specimens were not aged by either UV or hygrothermal aging, so any changes in their mechanical properties with time are an effect of hygroscopic aging, i.e., changes caused by the absorption of moisture under ambient conditions. The specimens were loaded in displacement control mode on an MTS Insight Electromechanical Testing System. The test frame was instrumented with a 30-kN load cell and a laser extensometer. The nominal axial stress was calculated as $\sigma =F/A_0$, where F is the force measured by the load cell and $A_0$ is the initial cross-sectional area at the notch root (narrowest point in the notch). The nominal strain was calculated as $\Delta L/L_0$, where $\Delta L$ is the displacement measured by the laser extensometer and $L_0$ is the initial gauge length, defined as the axial length of the notch from shoulder to shoulder.

Fig. 9

Nominal stress–strain curves for PA-6 RN10 bars that were loaded after being stored in a chamber with a desiccant (CaSO$_4$) or were loaded immediately after drying in a vacuum oven. Markers represent the strain-to-fracture. Moisture absorbed during storage increased the ductility and decreased the strength

Figure 9 shows nominal stress–strain curves up to the strain-to-fracture for RN10 bars that were loaded without being dried. A single realization of a bar that was dried prior to loading is also shown for reference. The storage time in the legend is measured from the receipt of the raw material. For increasing storage times, specimens show increasing ductility and decreasing strength, which are the anticipated signatures for moisture absorption [43]. It is notable that the ambient temperature and humidity were able to effect significant changes in the mechanical behavior of PA-6 over a time period of only 60 days. When the specimens were dried prior to loading, the reverse trends were observed (ductility decreased, strength increased), indicating that moisture was removed. The stress–strain curve of the material dried for 72 h was repeatable regardless of the storage time, so that drying time was adopted in the experimental procedure.

Fig. 10

An undeformed PA-6 RN10 bar (left). A PA-6 RN10 bar loaded past the peak nominal stress after storage for 217 days with a desiccant (CaSO$_4$) (center). A PA-6 RN10 bar loaded to fracture after storage for 217 days with a desiccant (right). After absorbing ambient moisture during storage, the material was ductile enough that the notches in the specimen became locally cylindrical during loading

Figure 10 shows images of RN10 bars that were loaded after storage for 217 days. An undeformed RN10 bar is also shown for reference. These images show that the increased ductility allowed the undried notched bars to deform until the notch became smooth, i.e., locally cylindrical. The transition from a shallow notch to a smooth notch represents a significant change in triaxiality: from 0.56 to 1/3. In this case, round notched bars are clearly not suitable as constant triaxiality specimens, complicating their use with ductile polymers to explore the effects of the stress state on the mechanical behavior and damage.

Rozanski and Galeski [43] reported that moisture and other low molecular weight penetrants could suppress cavitation in PA-6. This must be at least partially responsible for the increased ductility in the undried specimens. However, since the necked regions of the specimens in Fig. 10 exhibit stress-whitening, cavitation must have still occurred [53, 54]. Therefore, constitutive models seeking to predict fracture in PA-6 will need to include mechanisms for how moisture affects both the mechanical behavior and damage evolution.