Influence of hygrothermal conditioning on the chemical structure and thermal mechanical properties of aliphatic polyketone

Highlights

• Prolonged hygrothermal degradation of Polyketone results to chain scission phenomenon.

• Thermal stability declines with increased in hygrothermal degradation conditioning.

• Mechanical property and performance declined arising from changes in crystallinity.

• Prolonged degradation retained better mechanical properties at elevated temperatures from dynamic mechanical tests.

Abstract

Hygrothermal conditioning on the chemical, thermal and mechanical properties of aliphatic polyketone terpolymer, ethylene-propylene-carbon monoxide (EPCO), were investigated under high temperature fluid transport conditions. The DSC results suggest the effect of hygrothermal treatment and exposure time on the advancement of chain scission leading to an increase in crystallinity induced by chemi-crystallization, which was confirmed by FTIR results. TGA results of EPCO correspond to reductions of thermal stability with decreases in T_5% and T_max characteristic temperatures by 19% and 10%, respectively. The yield stress, stiffness and dynamic storage modulus increased with degradation time. However, decreases in the elongation at break were observed after prolonged periods of degradation due to the embrittlement resulting from polymer chain scission.

Introduction

Aliphatic polyketones (PK) are a relatively new class of semi-crystalline polymers known for their enhanced chemical, thermal and mechanical properties. Polyketones are high-performance thermoplastics obtained from perfectly alternating olefins with carbon monoxide (CO) to form a copolymer of ethylene-CO (ECO) or terpolymerization of ethylene-propylene-CO (EPCO). Polyketones have excellent barrier performance, heat resistance, friction and wear characteristics, and good impact behavior compared to semi-crystalline polyethylene and polypropylene [1,2]. The versatile combination of properties gives polyketone an advantage over other semi-crystalline polymers and makes it a promising candidate for use in engineering, fiber reinforcement, barrier and packaging applications [[3], [4], [5]].

Compared to other aliphatic polymers such as polyethylene with a melting temperature around 140 °C [6,7], polyketone copolymers of ethylene-CO origin have relatively higher melting point (up to 255 °C) [1]. This increase of around 115 °C in the melting temperature is a result of strong interactions between the highly polar carbonyl groups of adjacent polymer chains. It is known that during processing, polyketone copolymers are relatively sensitive to thermal degradation [8], which can lead to intermolecular and intramolecular reactions limiting the allowable processing time. Therefore, polymerisation of terpolymers, through introduction of propylene monomers, is often employed to lower the melting temperature, making it more stable for processing [9].

There are several papers which deal with aliphatic polyketones, most of which focus on permeation barrier applications and toughening of the material using microparticles. Cho et al. fabricated a blend of EPCO with 1 wt% graphene nanoplatelets using aminopyrene to significantly improve the barrier performance [10]. Nobile et al. found EPCO permeation properties, above the glass transition temperature, and to be comparable to nylon-6, poly (ethylene terephthalate) and poly carbonates [11]. Zuiderduin et al. demonstrated improvements to the toughness of polyketone after the incorporation of calcium carbonate and rubber particles, [12,13]. Additionally, studies on the crystallization behavior of polyketone have also been a subject of interest recently. Holt and Spruiell studied the melt crystallization of polyketone show that lamellae thickness increases with increasing crystallization temperature, and the value of lamellae thickness could be described by a linear relationship [[14], [15], [16]].

Despite the above-mentioned interest in investigating polyketones’ various characteristics, to our knowledge no papers have studied the degradation behavior of aliphatic polyketones, especially EPCO terpolymers, during prolonged exposure to hot fluids, such as those observed in the oil transportation through pipelines.

Therefore, the objective of this study was to investigate the effects of hygrothermal degradation of an EPCO terpolymer upon exposure to hot deionized water, on its mechanical, thermal and chemical properties. Thus, fluid immersion was performed in an autoclave capable of sustaining the degradative environment for the duration of the exposure. Thermal and chemical changes were studied by means of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR). Mechanical properties were evaluated by tensile testing and dynamic mechanical analysis (DMA).