Use of acoustic emission in the analysis of polypropylene failure caused by photodegradation

Abstract

Photodegradation is among the most frequent causes for the low performance of polypropylene (PP) products during service. This type of failure has been investigated extensively throughout the years using conventional methods including infrared spectroscopy and molar mass measurements. The use of acoustic emission, conversely, is not a common technique to evaluate polymer behavior, even though showed to be very useful to detect and locate damage during loading, contributing to the understanding and, hence, to the prevention of failure. In this investigation, injection molded tensile bars were exposed to the ultraviolet radiation in the laboratory for periods of up to 35 weeks and then characterized by Fourier transform infrared spectroscopy, X‐ray diffraction, tensile properties, and scanning electron microscopy. During tensile testing, the acoustic emission technique was applied to monitor parameters like intensity of hits, events, and energy released during deformation. The results confirmed the high efficiency of hindered amines light stabilizers to inhibit chemical degradation of PP and the high sensitivity of the neat polymer toward UV exposure. The several steps of polymer failure were detected by acoustic emission, being a very important tool to understand the differences in mechanical behavior of the types of samples under investigation. It showed the beginning of the failure process as well as significant events that occurred during the test, such as the formation of shear bands, cracking, and material fracture. These observations, combined with the traditional degradation monitoring procedures, allowed a good understanding of the effect of UV radiation on PP failure mechanism. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 46943.

Chemical degradation and failure analyses by acoustic emission of PP/EOC blends exposed to ultraviolet radiation

“…However, a major different in relation to the unstabilized blends was the absence of visible cracks on the nonexposed faces of the samples with the UVA additive, even at 40 weeks exposure. This was also observed with neat PP [15] and it is supposed to be a consequence of the absorption of ultraviolet radiation at the proximity of the exposed side, making more difficult the UV path to the interior of the sample bar and to the back side. Actually, measurements of the UV intensity at the unexposed side showed that the amount of ultraviolet light at the opposite side of the sample was only 6% of the total UV radiation.…”

“…This is very similar to what was shown before for neat PP exposed under the same conditions. [15] Besides reducing the mechanical properties, these cracks also damage the physical appearance of the products, turning then useless to the consumer, depending on the application. The cracks are not formed randomly, but show a pattern mainly perpendicular to the flow direction during injection moulding.…”

“…Actually, the intensity UV light at the non-exposed face was about 40% of the emitted UV radiation, according to measurements done with the sensor placed on the back side of the samples. [15] F I G U R E 1 Scanning electron microscopy morphologies and particle size distribution of PP/EOC blends without and with photo stabilizers. EOC, ethylene-octene copolymer; HALS, hindered amine light stabilizers; PP, polypropylene; UVA, ultraviolet absorber [Color figure can be viewed at wileyonlinelibrary.com]…”

“…Considering that, the presence of EOC induce a toughening mechanism in PP, this can be related to the acoustic emission activities that, together to other characterizing procedures, can contribute to understand the blend's failure process. The current work follows a previous investigation, [15] using the same PP, photo stabilizers and procedures, and it is a step forward, in the sense of optimizing the composition of PP to achieve better engineering properties. A major difference in failure mechanism must be expected when impact modifiers are added and, hence, this can be monitored by acoustic emission measurements.…”

“…Even though acoustic emission measurements are widely used with metals and composites, [6][7][8][9][10] very few work was done with polymers. [11][12][13][14][15] In studies of polymer degradation, for example, acoustic emission was used, to the best of our knowledge, in just one study, [15] conducted by the authors of the current article. The study showed that the acoustic emission was very useful as a tool to locate the failure origin as well as the main events that take place during the fracture process.…”