Analytical pyrolysis in the determination of the aging of polyethylene

https://doi.org/10.1016/j.jaap.2015.02.005Get rights and content

Highlights

  • Polyethylene was artificially aged under oxygen and then characterized.

  • Analytical pyrolysis was used to investigate the aging process of polyethylene.

  • FTIR, SEC and pyrolysis techniques of degraded polyethylene were compared.

  • α, ω-diacids were identified as main degradation products of PE with THM-GC–MS.

Abstract

Polyethylene is nowadays used in many applications with lifetimes of several decades. To predict the lifetime artificial aging is widely used, but only little is known of the chemical change of the polymer chains itself. We have used different types of unstabilized polyethylene and aged them in water at elevated temperature and a high pressure of oxygen to accelerate the natural aging processes and analyzed the materials by Py-GC–MS, infrared spectroscopy and size exclusion chromatography. With pyrolysis 2-oxoalkanes and 2-oxoalkenes as well as carboxylic acids were identified as the degradation products. Thermally assisted hydrolysis and methylation was successfully applied to assess the degree of oxidation, especially targeting the more polar compounds. With this technique alkane dioic acids have been identified as valuable marker compounds for the oxidative degradation of polyethylene. The mechanism of formation of fatty acids in thermally assisted hydrolysis and methylation has been elucidated using two different alkanones. With size exclusion chromatography it could be shown that the oxidation occurs randomly along the polymer chain and that the final degradation products are in the range of a few thousand g mol−1, irrespective of the original molar mass.

Introduction

Polyethylene (PE) is the most commonly used plastic, covering wide areas of application. Particularly for PE products with a shelf life in the order of several decades, such as pipes, degradation caused by different environmental conditions e.g., heat, humidity or UV irradiation can severely impact optical and physical properties.

In order to predict product lifetimes and to understand degradation processes of polymers, accelerated aging tests are commonly employed [1], [2], [3], [4], [5], [6], [7]. Thereby the polymer material is exposed to harsh environmental conditions such as elevated temperatures, intense UV irradiation, mechanical load or acidic, alkaline or corrosive surroundings [3]. Particularly a combination of high temperatures and oxygen atmosphere is reported in the literature to be the most commonly used acceleration condition [3].

Different techniques are available to monitor degradation processes, whereby physical properties, chemical changes but also a degradation of the polymer stabilizers can be investigated [4], [5], [6], [7], [8], [9], [10], [11]. These include methods such as tensile testing, infrared (IR) spectroscopy, differential scanning calorimetry (DSC) or photoluminescence spectroscopy [7]. Size exclusion chromatography (SEC) and pyrolysis gas chromatography mass spectrometry (Py-GC–MS) are also reported as suitable methods for aging characterization of polymers [12]. Even though Py-GC–MS has been used in the analysis of polymers for decades [12], [13], there is only a limited number of studies employing Py-GC–MS in life-time predictions and aging experiments of polyolefins [12], [13], [14], [15].

Pyrolysis of virgin PE and PP usually results in a series of triplets of alkanes, α-alkenes and α,ω-alkadienes [16]. According to the literature, detecting changes in the polymer backbone is difficult except for severely degraded materials. Aging-induced oxidation, chain scission and crosslinking account only for relatively small changes in comparison to the intact polymer backbone. As a matter of fact, distinguishing between differences due to aging or to material inhomogeneity is problematic [12]. However, instead of backbone analysis also volatile compounds such as chain scission products or additives can be analyzed to determine the aging status [12]. FTIR analysis after natural exposure of HDPE revealed significant photo-oxidation resulting in changes in the carbonyl index and a decrease of the molecular weight distribution. With Py-GC-–MS, only the above mentioned triplets, ranging from C14 to C29, and no additional oxidative species were detected, concluding that the oxidative groups are still connected to the polymer backbone [12]. Other aging studies of PE revealed that the relative intensities of the respective triplets are dependent on the aging level of the polymer, showing higher intensities at lower carbon numbers for an advanced stage of aging [13].

Whereas previous studies focused on the analysis of volatiles, the present research investigates changes of the polymer backbone of two PE samples with different molar mass under harsh degradation conditions in order to extend the knowledge about aging processes of polyolefins. The current work presents the characterization of PE samples after several periods of accelerated aging in the presence of oxygen (60 bar) and water at an elevated temperature (125 °C) combining Py-GC–MS, THM-GC–MS, FTIR and SEC measurements. Several marker compounds for the tracking of the aging process could be identified, such as 2-oxo hydrocarbons in conventional pyrolysis and 2-oxo fatty acids or diacids in thermally assisted hydrolysis and methylation.

Section snippets

Samples

Polyethylene with a molar mass of Mw 64,000 g mol−1 (sample 1) was synthesized with a Ziegler–Natta catalyst as described elsewhere [17]. Polyethylene with a molar mass of Mw 460,000 g mol−1 (sample 2) was obtained from Borealis, both polymers were not stabilized. The polymer powders were pressed in a mold at a temperature of 160 °C with 100 bar for 20 min to yield PE rods with a dimension of 10 × 8 mm and a length of 40–50 mm. 2-Dodecanone, 5-dodecanone and tetramethylammonium hydroxide were purchased

Results and discussion

In order to monitor the effects of oxygen diffusion into the polymer samples, the rods were cut in the middle and all analyses were performed on these newly formed positions. A first interesting result was that in all samples the formation of a darker colored core was visible, whereas the outer part of the polymer, which has been in direct contact with water and/or oxygen, remained white but showed surface cracks (Fig. 1). In some cases a thin layer of greenish material was found on the surface

Conclusions

Polyethylene has been aged under high pressure/temperature conditions and was analyzed with Py-GC–MS and thermally assisted hydrolysis and methylation to monitor the chemical changes in the polymer chain. 2-Alkaneones and related products which can be traced by using the m/z = 58 ion have been found to be good indicators for degraded PE in conventional analytical pyrolysis, however, their abundance is typically rather low. A dramatic improvement can be made when using THM where fatty acids and

Acknowledgement

Parts of this work have been funded by the Austrian Science Fund (FFG) under Grant number 844061/1463.

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