Enhanced thermo-oxidative stability through covalent attachment of hindered phenolic antioxidant on surface functionalized polypropylene
Graphical abstract
Introduction
Polyolefins, especially polypropylene, are the materials of choice for wide range of applications due to their low cost and efficient physical and mechanical properties [1]. However, pristine polypropylene (without antioxidant) is prone to thermo-oxidative degradation, which causes intrinsic properties deterioration [2,3]. To prevent PP from changing chemical structure and degradation reactions when subjected to long-term heat, UV and oxygen exposure, it is mandatory to add small amounts of antioxidant to polyolefin matrix [2,4]. Although, due to incompatibility and low molecular weight, the effective concentration of antioxidant, which is needed for long-term stability, tends to decrease as a result of migration to surface and extraction [1,4]. Thus, expanding life span of polypropylene by restriction of antioxidant volatility and diffusion rate have attracted considerable scientific interests and a number of approaches have been developed [1,5]. These approaches include copolymerization of monomers bearing hindered phenolic moieties with olefin or vinyl monomers [6,7], increasing molecular weight of antioxidant molecule [4,8] and coupling stabilizer group on functional groups of nanoparticles [[9], [10], [11], [12]]. However, harsh service conditions may still cause physical loss of the high molecular weight antioxidants prepared based on the above mentioned techniques. Several studies have been performed on melt radical grafting of functional groups bearing stabilizer moieties onto commercially available polyolefins [[13], [14], [15]]. This approach accompanies with side reactions including homopolymerization of functional groups, low yield and unexpected changes in physical properties of polymer [[16], [17], [18]]. Thus, the efficient approach to provide homogeneous distribution, compatibility and high concentration of stabilizer groups, is to direct functionalization of polyolefins with hindered phenol antioxidant as side chain pendant branches (see Scheme 1).
A few studies have reported the synthesis of hyperbranched polymer-bound antioxidants [19] and grafting of stabilizer molecules onto modified polyolefin chain [17], which exhibit migration prevention, improved thermal stabilization and compatibility with matrix. In our previous works [16,20] we focused on preparation of high molecular weight antioxidants via covalent attachment of long-chain structures containing antioxidant moieties onto ethylene copolymers as side pendants. Using this approach it is possible to significantly enhance thermal stability of polyolefin blends. Since functional polypropylene reaps the benefit of compatibility and cocrystallization with PP homopolymer, in this work, we focused on preparation of polypropylene-bound antioxidant through direct surface functionalization of PP. The modified PP obtained using this approach, either film or powder, would be more beneficial as provides homogeneous properties throughout the surface due to comparability with pristine PP [17]. Taking into account the advantages of plasma treatment including facility, environmental safety and variety of functional groups created based on utilized settings [[21], [22], [23]], this technique was used to prepare surface modified polypropylene. This study details preparation of amine functionalized polypropylene followed by immobilization of 3,5-di-tert-butyl-4-hydroxybenzoic acid as active antioxidant group on amine side chains to offer long-term thermal stability in PP matrix. The polypropylene-bound phenolic amide derivative have the benefits of versatile stability toward extreme environments and compatibility with various functionalities due to amide linkage.
Section snippets
Materials and instrumentation
The unstabilized polypropylene homopolymer (MFI = 6 g/min, 230 °C, 2.16 kg) was provided by Polymer Co. Triethylamine (TEA, ≥99%), Hexamethylenediamine (HMDA, ≥99%) and solvents dichloromethane (DCM, 99.8%) and THF (99%) were purchased from Merck. 3, 5-di-tert-butyl-4-hydroxybenzoic acid (>98%), 4-(Dimethylamino) pyrirdine (DMAP, ≥99%) and Cyanuric chloride (99%) were purchased from Sigma-Aldrich.
ATR-FTIR measurements were performed using Bruker EQUINOX55 FTIR on films with average thickness of
Polypropylene functionalization
The changes in structure of polypropylene surface after amine functionalization and immobilization of antioxidant group were determined using ATR-FTIR spectroscopy. The ATR-FTIR spectra of pristine PP, plasma treated PP (E-PP), amine functionalized PP (PPNH2) and antioxidant coupled PP (PPNA) are depicted in Fig. 1. The pristine polymer exhibits characteristic peaks in 2700–2950 cm−1 corresponding to C-H stretching vibration of CH2 and CH3 groups. In addition, peaks at 1451 cm−1 and 1385 cm−1
Conclusions
In the present work, long-chain pendant molecules containing hindered phenolic stabilizer were used to improve surface characteristic and thermo-oxidative stability of polypropylene. The functionalized films were prepared using three-step process via plasma assisted functionalization followed by introduction of amine (PPNH2) and phenolic antioxidant (PPNA) groups. The characterization of surface modified polypropylene was confirmed using ATR, AFM, XPS, DSC and TGA. The results exhibited
References (34)
- et al.
Synthesis and evaluation of hyperbranched phenolic antioxidants of three different generations
Polym. Degrad. Stabil.
(2002) - et al.
An overview of degradable and biodegradable polyolefins
Prog. Polym. Sci.
(2011) - et al.
Immobilization of antioxidants via ADMET polymerization for enhanced long-term stabilization of polyolefins
Eur. Polym. J.
(2013) - et al.
Macromolecular antioxidants via thiol-ene polyaddition and their synergistic effects
Polym. Degrad. Stabil.
(2014) - et al.
Synthesis of a novel sulfur-bearing secondary antioxidant with a high molecular weight and its comparative study on antioxidant behavior in polypropylene with two commercial sulfur-bearing secondary antioxidants having relatively low molecular weight
Polym. Degrad. Stabil.
(2013) - et al.
Antioxidant behaviour of a nanosilica-immobilized antioxidant in polypropylene
Polym. Degrad. Stabil.
(2008) - et al.
Synthesis, characterization, and description of influences on the stabilizing activity of antioxidant-functionalized multi-walled carbon nanotubes
Carbon
(2015) - et al.
Thermal-oxidative effect of a co-condensed nanosilica-based antioxidant in polypropylene
Polymer
(2017) - et al.
Significantly enhancing the thermal oxidative stability while remaining the excellent electrical insulating property of low density polyethylene by addition of antioxidant functionalized graphene oxide
Carbon
(2016) - et al.
Melt grafting of maleimides having hindered phenol antioxidant onto low molecular weight polyethylene
Polym. Degrad. Stabil.
(2004)