Elsevier

Polymer

Volume 138, 28 February 2018, Pages 41-48
Polymer

Enhanced thermo-oxidative stability through covalent attachment of hindered phenolic antioxidant on surface functionalized polypropylene

https://doi.org/10.1016/j.polymer.2018.01.048Get rights and content

Highlights

  • Long-chain amine containing molecules covalently attached to PP surface.

  • Phenolic stabilizer immobilized on the surface of amine functionalized PP (PP-NH2).

  • Macromolecular PPNA significantly enhanced the thermo-oxidative stability of PP.

  • PPNA revealed excellent extraction resistance characteristic from PP matrix.

Abstract

In order to prolong the thermo-oxidative stability of polypropylene and prevent antioxidant loss, this research details an effective approach to prepare polymeric antioxidant via surface functionalization of polypropylene with long-chain macromolecules containing hindered phenolic stabilizers. The polymeric antioxidant (PPNA) is prepared by amide bond formation between phenolic stabilizer containing carboxylic acid moiety and amine functionalized polypropylene (PPNH2). Test results confirmed surface changes through plasma treatment, amine and antioxidant modification. The resultant PPNH2 and PPNA exhibit reduced crystallinity while the crystallization temperature shifts toward higher values, suggesting simultaneous effects of steric hindrance and nucleation of long-chain structures. Immobilization of phenolic stabilizer results in improving thermo-oxidative stability of polypropylene. Subsequently, PPNA was used to prepare PP/PPNA blends to evaluate free radical scavenging ability and resistance to extraction. The blends demonstrate superior thermal stability and low volatility compared to stabilized polypropylene with Irganox 1010, indicating uniform distribution due to comparability. The synthesized polymer-bound antioxidant have the potential to be the material of choice as stabilizer concentrate for packaging, coating and thin film applications.

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

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