Immobilization of antioxidants via ADMET polymerization for enhanced long-term stabilization of polyolefins

Antioxidant Activity of Biogenic Cinnamic Acid Derivatives in Polypropylene

Antioxidants (AOs) from natural resources are an attractive research area, as petroleum-based products can be replaced in polymer stabilization. Therefore, novel esters based on the p-hydroxycinnamic acids p-coumaric acid, ferulic acid and sinapic acid were synthesized and their structure properties relationships were investigated. The structures of the novel bio-based antioxidants were verified using NMR and Fourier-transform infrared (FTIR) spectrometry. The high thermal stability above 280 °C and, therefore, their suitability as potential plastic stabilizers were shown using thermal gravimetric analysis (TGA). The radical scavenging activity of the synthesized esters was evaluated by using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. Stabilization performance was evaluated in polypropylene (PP) using extended extrusion experiments, oxidation induction time (OIT) measurements and accelerated heat aging. In particular, the sinapic acid derivative provides a processing stability of PP being superior to the commercial state-of-the-art stabilizer octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate.

The Effect of Thermal Annealing on the Structure and Gas Transport Properties of Poly(1-Trimethylsilyl-1-Propyne) Films with the Addition of Phenolic Antioxidants

The thermally activated relaxation of poly(1-trimethylsilyl-1-propyne) (PTMSP) samples of various cis-/trans-compositions (50-80% units of cis-configuration) in the presence of phenolic antioxidants of various structures was investigated. It was pointed out that polymers with a high content of cis-units exhibited greater thermal-oxidative stability due to the greater flexibility of the cis-enriched macrochains. The use of hindered phenols as antioxidants made it possible to prevent the process of thermally initiated oxidative degradation. At the same time, the most effective stabilizing agents were antioxidants with larger molecules such as Vulkanox BKF, Irganox 1010, and Irganox 1076. It was shown that the permeability coefficients of stabilized PTMSP during thermal treatment initially slightly decreased (by 20-30%), which, according to the X-ray diffraction data, was associated with an increase in the density of the macrochain packing, and during further heating remained practically unchanged. Note that for the cis-enriched samples, no signs of oxidation or decrease in the transport characteristics were observed during polymer heating for 240 h at 140 °C.

Thermal Aging Properties of 500 kV AC and DC XLPE Cable Insulation Materials

Despite similar material composition and insulation application, the alternating current (AC) cross-linked polyethylene (XLPE) and direct current (DC) XLPE materials cannot replace each other due to different voltage forms. Herein, this work presents a systematical investigation into the effects of thermal aging on the material composition and properties of 500 kV-level commercial AC XLPE and DC XLPE materials. A higher content of antioxidants in the AC XLPE than in the DC XLPE was experimentally demonstrated via thermal analysis technologies, such as oxidation-induced time and oxidation-induced temperature. Retarded thermal oxidation and suppression of space charge effects were observed in thermally aged AC XLPE samples. On the other hand, the carbonyl index of DC XLPE dramatically rose when thermal aging was up to 168 h. The newly generated oxygen-containing groups provided deep trapping sites (~0.95 eV) for space charges and caused severe electric field distortion (120%) under -50 kV/mm at room temperature in the aged DC XLPE samples. For the unaged XLPE materials, the positive space charge packets were attributed to the residue crosslinking byproducts, even after being treated in vacuum at 70 °C for 24 h. Thus, it was reasoned that the DC XLPE material had a lower crosslinking degree to guarantee fewer crosslinking byproducts. This work offers a simple but accurate method for evaluating thermal oxidation resistance and space charge properties crucial for developing high-performance HVDC cable insulation materials.

Grafting of Hindered Phenol Groups onto Ethylene/α-Olefin Copolymer by Nitroxide Radical Coupling

The covalent immobilization of hindered phenol groups, with potential antioxidant activity, onto an ethylene/α-olefin (EOC) copolymer was carried out by the nitroxide radical coupling (NRC) reaction performed in the melt with a peroxide and the 3,5-di-tert-butyl-4-hydroxybenzoyl-2,2,6,6-tetramethylpiperidine-1-oxyl radical (BHB-T). Functionalized EOC (EOC-g-(BHB-T)) was exposed to photo- and thermo-oxidation. By comparison with some model compounds bearing the (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) moiety or the hindered phenol unit, it was observed that the grafted BHB-T could effectively help the stabilization of the polymer matrix both under photo- and thermo-oxidation. In addition, the immobilization of BHB-T can effectively increase the service life of the functionalized polymers when polymer films were put in contact with ethanol solution thus simulating a possible application of the modified polymer.

Structure of a putative fluorinated natural product from Streptomyces sp. TC1

Fluorine-containing natural products are extremely rare. The recent report on the isolation and biological activity of the bacterial secondary metabolite 3-(3,5-di-tert-butyl-4-fluorophenyl)propionic acid was thus highly remarkable. The compound contained the first aromatic fluorine substituent known to date in any natural product. The promise to discover an enzyme capable of aromatic fluorination in the producing strain Streptomyces sp. TC1 prompted our immediate interest. A close inspection of the originally reported analytical data of the fluoro metabolite revealed inconsistencies that triggered us to validate the reported structure. The results of these efforts are presented in this communication.

Total synthesis of a reported fluorometabolite from Streptomyces sp. TC1 indicates an incorrect assignment. The isolated compound did not contain fluorine

3,5-Di-tert-butyl-4-fluorophenylpropionic acid (1) was recently reported as a natural product from Streptomyces sp. TC1. This was a notable disclosure because fluorinated natural products are exceedingly rare, and in this case it suggested that the bacterium had the capacity to mediate an enzymatic aryl fluorination reaction. However, a synthesis of the putative metabolite 1 demonstrates that the spectroscopic data are inconsistent with the proposed structure. There is no evidence that the isolated compound contained a fluorine atom.