Polyethylene containing antioxidant moieties exhibiting high thermal-oxidative stability for high temperature applications
Graphical abstract
PE-bonded antioxidant exhibits high thermal-oxidative stability.
Introduction
Polyethylene (PE) is the most inexpensive and widely used thermoplastic in the world. However, its applications under elevated temperatures are limited due to relatively low melting temperature and liability to oxygen induced oxidation reactions that result in a mixture of chain degradation and crosslinking structures [[1], [2], [3], [4]]. High density PE (HDPE) also shows the tendency to crack when stressed, exposed to UV radiation, or attacked by strong oxidizers, as well as some solubility (swellable) in hydrocarbons [[5], [6], [7]]. It is a scientific challenge and technologically important to simultaneously overcome both chemical and physical limitations to expand the PE application to high temperatures and under severe environmental conditions [[8], [9], [10], [11]]. As illustrated in Scheme 1, the PE polymer chain involves an oxidative reaction mechanism that starts with the formation of alkyl radical (I) through cleavage of the covalent bond. The formed C* radicals may engage in radical coupling reaction to form the branched and crosslinked PE structures. However, with the presence of oxygen, the facile spontaneous oxidation reaction with oxygen takes place to form peroxyl radical (II) that then extract hydrogen atom (H*) from any adjacent PE chain to form hydroperoxide (III) and new polymeric C* radical [[12], [13], [14], [15]]. The subsequent thermal decomposition of the hydroperoxides degrades the polymer chain and generates oxygenated products containing various identified polar groups, such as aldehydes (IV), ketones, alcohols, acids, and esters [16,17]. In the presence of UV radiation, photolysis of ketone groups by Norrish reaction takes place to cleave the polymer chain and to form terminal ketone group (VI) and chain end vinyl group (VII). To prevent this catalytic mechanism, it is essential to have antioxidants that can donate hydrogen atoms (H*) and are located adjacent to the polymer chain to neutralize polymeric C* radicals (I) and inhibit (or slow down) the oxidative-degradation cycles [18,19].
In the polyolefin industry, it is a common practice to add a small amount (<1 wt%) of hindered phenol (HP) antioxidant, such as octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)-propionate, into the HDPE products immediately after the polymerization [20,21]. They are essential during melt processes under elevated temperatures (>180 °C), as well as during the applications in which the HDPE products are exposed to severe environmental conditions. The commercial HP antioxidants are highly effective during melt processing, with homogeneous mixing and short processing times (a few minutes). However, it is difficult to maintain this homogeneous mixture between polar HP antioxidant and highly crystalline non-polar HDPE during applications. This incompatibility also contributes to the constant HP diffusion from the bulk to the surface region, which accelerates the loss when exposed to solvents, heat, or strong electric fields [[22], [23], [24], [25]]. As expected, it is difficult to maintain a minimum effective antioxidant concentration throughout the PE matrix during applications.
To tackle the physical limitations under elevated temperature conditions, a crosslinking reaction between polymer chains is commonly applied to form a network structure that raises HDPE operating temperatures and improving its mechanical properties [[26], [27], [28], [29]]. Some crosslinked x-PE products show 5 times the tensile and impact strength and 20 times the environmental stress crack resistance of the corresponding HDPE products. Several methods have been applied to crosslink PE chains, involving peroxides [[30], [31], [32]], UV irradiation [[33], [34], [35], [36]], and silane groups [[37], [38], [39]]. The most common method is a peroxide mediated crosslinking reaction, which is based on similar free radical mediated reactions (Scheme 1) in forming a complex mixture. Some degraded PE chains are difficult to be incorporated in the network structure, and the resulting PE products also exhibit surface tackiness due to the presence of oxygenated groups. Crosslinking by UV radiation is a very slow process and the suitable photo-initiator is needed during the process. This method is commonly used for crosslinking of thin parts. The silane grafting reaction is usually performed by means of reactive extrusion where polymer is melt-grafting with vinyl alkoxysilanes. However, the curing time is very long and extra stream equipment is required during the sol-gel process. Overall, an efficient and effective crosslinking method is still needed to form the well-controlled and fully crosslinked PE products.
In our previous study [40,41], we reported a new class of polypropylene-bonded hindered phenol (PP-HP) copolymers with a specific concentration of HP groups homogenously distributed along the polymer chain. This PP-HP copolymer shows a dramatic increase PP thermal-oxidative stability in air under elevated temperatures. In addition, the HP antioxidant groups in PP chain simultaneously involve a facile crosslinking reaction to form a stable polymer network. It is very interesting to extend this approach to PE polymers by developing a suitable chemical route to prepare the corresponding well-defined PE-HP copolymers and examining their antioxidative performance and the simultaneous crosslinking reactions. Our objective is to increase the PE application temperature to a much higher temperature range (>150 °C), as well as providing safe and stable PE products (without leaking antioxidant additives).
Section snippets
Materials and structure chracterization
All oxygen and moisture sensitive chemicals were handled inside an argon-filled Vacuum Atmosphere dry box. [(η5-C5Me4)SiMe2(η1-NCMe3)]TiCl2 catalyst was prepared using the published procedures [42]. Several chemicals, including methylaluminoxane (5 wt% in toluene), chlorotrimethylsilane, 10-undecen-1-ol, triethylamine (Sigma-Aldrich), 3,5-bis(tert-butyl)-4-hydroxyphenylpropionic acid (Ciba), 1-(3-dimethylaminopropyl)-3 ethylcarbodiimide, 4-N,N-dimethylaminopyridine, calcium hydride (VWR
Result and discussion
As discussed, a small amount (<1 wt%) of HP antioxidants are commonly added into the polyolefin material immediately after the polymerization to prevent polymer chains from thermal-oxidative reactions during melt processing and applications. It is essential to maintain an effective antioxidant concentration throughout the polymer matrix, especially when the polyolefin products are exposed to severe environmental conditions. However, the poor compatibility between polar HP antioxidant and
Conclusion
A new class of polyethylene-bonded hindered phenol (PE-HP) polymers have been synthesized and investigated to understand their high temperature thermal-oxidative stability in air and the potential applications under severe environmental conditions. The combination of the CGC catalysis and the subsequent estification reaction successfully prepares the PE-HP copolymers with 1.1–5.4 mol% HP group content, high polymer molecular weight, and narrow molecular weight and composition distributions.
Acknowledgment
The authors gratefully acknowledge the financial support of this work from CSC scholarship.
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