The purpose of the current study is to evaluate the ageing of poly(ethylene-co-propylene) fittings produced by several manufacturers. The fittings were aged for varying times as follows: 0, 2, 13, 23, 45, 106, 225 and 391 days at 100 °C. In a previous paper, the oxidation induction time (OItime) and oxidation induction temperature (OItemp) were evaluated with respect to every single step of the ageing process [1]. Here, the Vickers microhardness and wide-angle X-ray line width were used for characterization of the oxidative stability of poly(ethylene-co-propylene) pipes. Both the Vickers microhardness and wide-angle X-ray line width as evaluated from a certain reflex of the diffractogram generally increase with ageing for most samples. The higher microhardness is most likely related to higher fragility of the pipes. The increase of the X-ray line width with ageing indirectly proves that reducing the crystallite size as well as increase of the crystallite defectiveness take place.
During the last few decades, the old iron pure water pipes and fittings are being replaced with plastic ones. Usually the polymer used is poly(ethylene-co-propylene) where the ethylene content is in much smaller quantity. Together with a few advantages, these materials suffer from thermooxidative degradation that is the main factor that substantially decreases the service life of the piping systems [
A total of 7 commercial specimens of poly(ethylene-co-propylene) were taken “as they are” from the respective fitting that was believed to be representative for the manufacture typical production run. The specimens (designated P1, P2, P3, …, P7) were supplied by various producers. The main material information in our case is the oxidation induction time, but as stated and shown in [
Thermal ageing was conducted at 100˚C in air in an oven for 0, 2, 13, 23, 45, 106, 225 and 391 days, respectively. These numbers do not bear any special meaning but are chosen on the assumption that in the beginning of the ageing changes take place more quickly and the maximum duration should also by reasonable from a practical viewpoint. Several pieces from any single specimen were put initially into the oven. Periodically, at each time interval, one of them was taken out of the oven and cooled. Several samples were taken from various parts of each piece and characterized.
The microhardness H of the samples was derived with an apparatus for Vickers microhardness measurements (Leica VMHT, Leica Mikrosysteme GmbH, Wien, Austria) at three different loads. 30 indentations were made for each load and the results obtained for both diagonals of the indentation marks were averaged. The microhardness was received following Krumova et al. in order to minimize the elastic recovery error [
A Siemens D500 diffractometer, Germany, with a secondary monochromator and CuKα radiation was used to scan the 040 reflection, situated at 16.92˚ 2θ over the range 13˚ - 21˚ 2θ. Lorentzian function was used to fit this reflection in order to get the line width at half height β1/2.
The DSC curves, obtained in [
The oxidative degradation was estimated by measuring the microhardness of the specimens after ageing. The dependence of the microhardness vs. the aging time for all specimens P1 to P7, is shown in
As the antioxidant concentration is generally very low―in the order of parts of a percent [
microhardness values within each specimen. Generally, the increase of the microhardness is not very high in comparison to the statistical error but for five out of the seven specimens a tendency of increasing the microhardness may be noticed (
In order to check if the microhardness increase with ageing originates from additional crystallization, the enthalpy of melting vs. the aging time was also drawn (
H P P = w c H P P c + ( 1 − w c ) H P P a (1)
where H P P c and H P P a are the microharnesses of the crystalline and amorphous PP phases, respectively. The volume degree of crystallinity w c was calculated on the basis of the weight degree of crystallinity (by DSC), the overall density and the density of the crystalline phase [
Strictly speaking, w c should be calculated as:
w c = Δ H m C ⋅ H 0 (2)
where C is the PP concentration in the copolymer. However, as it is not known exactly, we applied here the conclusions drawn by Wening and Schoeller from their study of a similar to our system. They have investigated blend of PE and PP and have shown that for concentration of the PE below 10%, its maxima do not appear on the diffractograms [
The calculated after Equation (1) microhardness of PP is between 57 и 82 МРа varying with the degree of crystallinity. This is in accordance with the value between 30 and 88 МРа, measured by other authors, e.g. Martinez-Salazar et al. [
For the first time the oxidative degradation was estimated by measuring the width of a certain diffraction line of the specimens after a prolonged ageing. It is well known that the diffraction lines width might be used for evaluation of the crystallite size and microstress. The former generally is expected to increase during the ageing due to the eventual additional crystallization taking place in the process of annealing at 100˚C. For instance, for polyehtyleneterephthalate the degree of crystallinity reaches maximum values for only 6 hours due to an annealing process [
The width does not change unambiguously mainly due to the small sample size, irregular shape of the samples (they were cut from tubes and fittings) and uneven sample surface. Since the chain length decreases with the ageing, not all molecules enter the crystallites, so the crystallite size also decreases and the defectiveness increases. As a result the line width increases and this is clearly seen as a tendency for most samples. Thus, the decreasing of the crystallite size is generally registered. As already stated, an increasing of this size due to the annealing and subsequent recrystallization is not probable due to the fast PP crystallization.
In order to check if any additional crystallization takes place (irrespective of the ageing time) the melting temperature of the specimens was followed with the ageing (
so short that they remain as defects in the PP crystallites. This was confirmed by X-ray investigations of the line width (see above) whence it can be concluded that even though larger PP crystallites are formed with the ageing time, they still appear imperfect. The observed changes in the melting temperature are comparable with the experimental error, hence this temperature cannot be taken as a parameter, describing the ageing.
In summary, the fact that the melting temperature does not depend on the antioxidant distribution and changes only a little within each specimen once again indirectly proves that the high scattering of OItime (
1) The oxidative degradation was estimated by measuring the Vickers microhardness in a long lasting ageing experiment. The microhardness generally increases with the process of ageing. The observed higher microhardness is most probably connected to higher fragility of the fittings.
2) The oxidative degradation was estimated for the first time by measuring the width of a certain diffraction line of the specimens in a long lasting ageing experiment. This width generally increases with the process of ageing. The higher width indirectly proves a reducing of the crystallite size as well as an increase in their defects, which both are expected to result in poorer mechanical properties.
3) The constancy of the melting temperature with the ageing indirectly proves that no additional crystallization takes place during the ageing.
A. A. Apostolov thanks the “Materials Networking”, European Union’s Horizon 2020 research and innovation programme, Grant agreement No. 692146.
The author declares no conflicts of interest regarding the publication of this paper.
Apostolov, A.A. (2019) On the Vickers Microhardness and Wide-Angle X-Ray Line Width for Characterization of the Oxidative Stability of Poly(ethylene-co-propylene) Pipes. Materials Sciences and Applications, 10, 25-32. https://doi.org/10.4236/msa.2019.101003