Global and Local Aging in Differently Stabilized Polypropylenes Exposed to Hot Chlorinated Water without and with Superimposed Mechanical-Environmental Loads
Sprache des Titels:
Englisch
Original Kurzfassung:
The influence of chlorinated water on the global and local aging behavior of polypropylene
(PP) was investigated for three differently stabilized PP grades consisting of the same PP base polymer.
While one of the PP grades contained only a processing stabilizer (PP-S0), the other two were modified
with a primary phenolic antioxidant (PP-S1) and a combination of a primary phenolic antioxidant
and a hindered amine stabilizer (PP-S3). To study global aging effects, micro-sized specimens were
pre-exposed to chlorinated water (5 mg/L free chlorine) at 60 °C for up to 750 h. Over the entire
exposure period, significant material aging was detected by monitoring a continuous decrease in
stabilizer content, oxidation induction temperature, mean molar mass, and mechanical strain at
break. In terms of aging resistance and ultimate mechanical performance, PP-S1 was found to
outperform the other two material formulations under these test conditions. Moreover, superimposed
mechanical-environmental fatigue tests with cracked round bar specimens were carried out with the
three PP grades in non-chlorinated (0 mg/L free chlorine) and chlorinated (5 mg/L free chlorine) water
at 80 °C and 95 °C to study local crack tip aging effects. While the fatigue crack growth resistance
substantially deteriorated for all three materials in chlorinated water, a significantly stronger effect
was found for the higher temperature, with crack growth rates at a given stress intensity factor range
in chlorinated water being ca. 30 to 50 times faster than in non-chlorinated water, depending on the
material. Molar mass measurements of material samples taken from various positions of the tested
CRB specimens provided clear evidence of enhanced local crack tip aging due to the chlorinated
water environment.