Correlating different techniques in the thermooxidative degradation monitoring of high-density polyethylene

• 05 Mar 2023

High-density polyethylene (HDPE) is widely used in the manufacture of many products, including plastic bags. These single-use
plastics have a relatively small shelf life and, after use, are frequently
disposed of in the environment, both terrestrial and marine . Prodegrading additives are commonly added to HDPE to accelerate the
oxidative abiotic degradation and thereby promote biodegradation of
the material after its disposal . However, the pro-degradant
should not impair the physical, chemical and mechanical properties of
the product during its service life. Using these additives creates a
conflict between optimum product performance during service life and
efficiency of the biodegradation process after disposal. Nevertheless, a
balance between these opposite forces can be achieved with the use of
antioxidants, which, when properly formulated, can guarantee the
performance of the product during its service life without interfering
with the oxidative abiotic degradation of the polymer after product
disposal . Hence, the main function of combining antioxidant
and pro-degradant is to delay, but not cancel the abiotic degradation
process of polymers .
The conflict between antioxidant and pro-degradant additives occurs because the chemical species involved in the respective reaction
mechanisms are the same. The action of pro-degradants, based on
transition metals, is satisfactorily explained by the Haber-Weiss reactions and mechanisms, in which the main chemical species is hydroperoxide. On the other hand, primary antioxidants are hydrogen donors, mainly to peroxyl radicals to form hydroperoxides. Hence, the
formation of new alkyl macroradicals is reduced and, consequently,
HDPE degradation is delayed. Secondary antioxidants decompose hydroperoxide into more stable species, such as alcohols, through redox
reactions. Thus, both additives (metallic salts and secondary antioxidants) act on the hydroperoxide radical, but the secondary antioxidant stabilizes the auto-catalytic oxidation cycle of HDPE, while the
metallic salts promote the formation of new peroxyl radicals, intensifying the HDPE degradation reactions [24–31].
The addition of metal salts (Fe, Mn and Co), at levels considered
residual, may accelerate the thermo- and photooxidation processes of
the different polyethylene (PE) types. Due to chemical and
morphological characteristics, the different types of PE present relatively different susceptibilities to the oxidative degradation processes,
e.g., studies have shown that LDPE and LLDPE are more susceptible to
thermooxidative degradation than HDPE .

∗ Corresponding auth degrading performance, but Fe presented a significantly lower performance, whereas in photooxidation the pro-degrading performance of Fe
was very similar to that of Co and Mn [39,46,47]. According to Focke
et al. [20] the catalytic effect on the photooxidation of LDPE can be
reduced with the addition of relatively high concentrations of secondary antioxidants associated with a UV stabilizer, such as sterically
hindered amines (HALS). The addition of primary antioxidants was,
however, less effective in minimizing the effects of photooxidation of
LDPE. When degradation was conducted under thermooxidation, the
addition of primary antioxidant in both Fe-based and Mn-based prodegrading systems and the increase in their concentrations increased
the induction times for degradation .
Antunes et al.  studied the influence of manganese stearate
concentration (pro-degradant) on the thermooxidative degradation of
HDPE at three different temperatures (60, 70 and 80 °C), and the carbonyl index results showed that Mn concentration does not significantly
affect the maximum degradation levels of the polymer. However, the
results indicate that the time to reach the maximum degradation levels
is dependent on Mn concentration.
This study assesses the influence of primary and secondary antioxidants on the thermooxidative degradation of HDPE containing prodegradant manganese stearate (Mn), at three temperatures (60, 70 and
80 °C). Degradation was monitored by means of variations in carbonyl
index (CI) using infrared spectroscopy, molar mass distributions using
SEC, as well as tensile strain at break using mechanical testing. The aim
of this study was to evaluate the interaction between antioxidant and
pro-degradant in the thermooxidation of HDPE, and propose formulations that guarantee the service life of HDPE products without impairing the purpose of the pro-degradant