Degradation Effects on Mechanical Behaviour of 3DPrinted, Reinforced and Recycled Polypropylene

Abstract

The growing amount of plastic waste generated by the consumer goods industry has led to excessive landfilling and increased greenhouse gas emissions, contributing significantly to the greenhouse effect. Government initiatives have identified the potential for a more sustainable approach to managing this homogeneous plastic waste stream. The Netherlands Circular Economy Program 2023-2030 (NCEP) envisions the MEGA project as a solution to repurpose plastic waste by replacing aging pedestrian infrastructure. Polypropylene (PP), known for its chemical stability and weather resistance, can be reinforced with glass fibers (rPP-G) to meet the requirements of such applications. Additionally, 3D printing is recognized as a promising method to reduce plastic waste during production, making it a valuable component of this initiative.

Nevertheless, outdoor infrastructural applications are subjected to load and temperature cycling due to their exposure to varying weather conditions, static loads, and pedestrians walking at different frequencies. This study aimed to assess and evaluate the effects of hygrothermal aging on the mechanical behavior of rPP-G. Three conditions were studied: a control condition not subjected to the aging environment, an extreme condition subjected to 95 % relative humidity (R.H.) and 80 ◦C, and a condition that approximated real application humidity levels of 77 % R.H. at the same temperature.

The tensile properties were evaluated by applying the load at a speed of 1 mm/min. The dynamic mechanical response was investigated at frequencies of 1 Hz and 2 Hz to simulate a crowd walking and a crowd brisk-walking on the material. The creep response was examined through short-term creep tests at elevated temperatures, and a methodology was derived based on the Time-Temperature Superposition (TTS) principle to extrapolate the behavior at ambient temperatures for stress levels of 8 and 10 MPa. For further analysis of the results, Scanning electron microscopy (SEM) captures were acquired, and the IR spectrum and diffraction pattern of rPP-G under different aging conditions was measured. The tensile properties suggested increasing degradation with hygrothermal effects, leading to a 14.3 % reduction in ultimate tensile strength (UTS), a 16 % reduction in Young’s modulus, and a 93.3 % increase in strain at break. This indicated weakening of the fiber/matrix interface, verified by increased fiber pull-out. However, no hydrolytic degradation was observed, as no differences in chemical compounds and crystallinity were detected. The dynamic mechanical response suggested the existence of two mechanisms: water acting as a stiffener at cryogenic temperatures, and induced plasticization and fiber/matrix weakening from the hygrothermal effects. However, fiber/matrix weakening dominated under extreme aging conditions, hindering the stiffening effects of water on the molecular structure of rPP-G. This effect was more pronounced under median aging conditions for the 1 Hz loading, as more time allowed for molecular relaxation. Increasing frequencies appeared to shift the glass transition temperature (Tg) to higher temperatures, highlighting the material’s elastic character. The creep response of rPP-G revealed an exponential relationship between temperature and time to fracture, allowing the use of Time-Temperature Superposition (TTS) with the Arrhenius equation to predict its behavior at ambient temperatures. The derived methodology suggested that hygrothermal effects leading to the degradation of tensile properties are significantly more apparent under long-term load applications, revealing a 282.6% reduction of time to fracture at 8 MPa under extreme conditions.

This thesis demonstrated the significance of dynamic mechanical analysis in understanding the effects of degradation on reinforced thermoplastic polymers. Additionally, it highlighted the importance of creep testing for such applications, revealing crucial insights into the long-term load application effects on the service life of thermoplastic polymers.