Thermo-Hydro-Glycol Ageing of Polyamide 6,6: Microstructure-Properties Relationships

Abstract

The microstructural evolutions occurring during the thermo-hydro-glycol ageing of an injection molded PA66 were studied. They were correlated to the evolutions of its mechanical properties. The aged samples were immersed in an antifreeze fluid-mainly composed of water and ethylene glycol-at varying times and temperatures. The aim was to combine an as exhaustive as possible microstructural investigation and a rigorous mechanical analysis. Consequently, the microstructure of the aged and unaged PA66 was assessed through the average molar mass, the diameter of the spherulites, the lamellae thickness, the crystallite's apparent size, a crystal perfection index, and a crystallinity index. Moreover, a core-skin approach was set up. The mechanical consequences of the microstructural changes were investigated by DMA and tensile testing. The local true strain fields were measured with a digital image correlation system. The temperatures and strain rates of the tests were chosen by referring to the time-temperature superposition principle. It is concluded that the water and ethylene glycol intake resulted in an intense plasticization, the loss of the molar mass resulted in the embrittling of the polymer, and finally, it was identified that the changes of the crystalline structure have an influence on the stiffness of PA66.

Keywords: PA66; mechanical behavior; microstructure; thermo-hydro-glycol ageing.

Figure 1

Illustration of the skin-core sampling strategy. The core and the skin samples were tooled from the whole thickness of the plates with a milling machine.

Figure 2

Typical decomposition of a PA66 diffractogram, with the identified plan families.

Figure 3

Typical analysis of a tensile test. PA66 taken at the skin of the plaque, aged during 168 h at 120 °C; (a) Strain field measured by DIC, and location of the tracking point; (b) Measurement of the elasticity modulus and the stress for a 0.2 strain. The strain was measured on the tracking point.

Figure 4

Reduction of the average molar masses due to the hydrolysis and/or oxidation reaction during the ageing processes.

Figure 5

Polarized light microscopy observation of the spherulites for a 140 °C 168 h aged PA66. The picture on the left depicts half of the thickness of the plaque.

Figure 6

Spherulite diameters were measured from microscopy observations.

Figure 7

Typical fusion peaks of unaged and 168 h aged PA66.

Figure 8

Influence of the aging conditions on the lamellae thickness at the core and the surface of the PA66 plaques.

Figure 9

Typical diffractograms of unaged and aged PA66 during 168 h.

Figure 10

Influence of the ageing conditions on the CAS at the core and at the surface of the PA66 plaques.

Figure 11

Influence of the aging conditions on the crystal perfection index: the angular gap between the (100) peak and the (010),(001) peak of WAXS diffractograms.

Figure 12

Influence of the ageing conditions on the crystallinity index at the core and the skin of the PA66 plaques.

Figure 13

(a) Decrease of the damping factor peak to lower temperature after aging; (b)Typical storage modulus curves before and after aging.

Figure 14

Illustration of some frequency scans used for building a master curve. Example of a dry unaged PA66. For better readability, only a few temperatures are displayed.

Figure 15

Superposition of two typical master curves. The dry unaged PA66 and the aged one are both loaded on their rubbery plateau. The corresponding strain-stress curves are plotted on the right.

Figure 16

Strain-stress curves of PA66 aged during 168 h, and a dry unaged PA66. Specimens taken at the core of the plaques.

Figure 17

Evolution of the true strain at the break with the decreasing molar mass. Measurements and tendencies: (a) at the core; (b) at the skin.

Figure 18

Illustration of the gradient of stiffness between the core and the skin material. Two curves are displayed for every aging condition: one for the core and one for the skin.

Figure 19

Comparison of the dimension of the crystalline entities at the core and at the surface: Lamellae thickness and crystallites apparent size.

Figure 20

Influence of the lamellae thickness on the PA66 stiffness: Elastic modulus and true stress at a 0.2 strain.