An investigation of chemical crosslinking effect on properties of high-density polyethylene

The DC conductivity of polymer blends composed of poly(ethylene-co-vinyl acetate) (EVA) and high density polyethylene (HDPE), where a conductive carbon black (CB) had been preferentially blended into the HDPE, were investigated to establish the percolation characteristics. The blends exhibited reduced percolation thresholds and enhanced conductivities above that of the individually carbon filled HDPE and EVA. The percolation threshold of the EVA/HDPE/CB composites was between 3.6 and 4.2 wt % carbon black, where the volume resistivity changed by 8 orders of magnitude. This threshold is at a significantly lower carbon content than the individually filled HDPE or EVA. At a carbon black loading of 4.8 wt %, the EVA/HDPE/CB composite exhibits a volume resistivity which is approximately 14 and 11 orders of magnitude lower than the HDPE/CB and EVA/CB systems, respectively, at the same level of incorporated carbon black. The dielectric response of the ternary composites, at a temperature of 23°C and frequency of 1 kHz, exhibited an abrupt increase of ca. 252% at a carbon concentration of 4.8 wt %, suggesting that the percolation threshold is somewhat higher than the range predicted from DC conductivity measurements. Percolating composites with increasing levels of carbon black exhibit significantly greater relative permittivity and dielectric loss factors, with the composite containing 6 wt % of carbon black having a value of ∈' 79 and ∈ 14.

Reduced percolation thresholds of immiscible conductive blends

https://www.fpl.fs.fed.us/documnts/pdf2015/fpl_2015_wei001.pdf

Grafting of bacterial polyhydroxybutyrate (PHB) onto cellulose via in situ reactive extrusion with dicumyl peroxide.

Thermally recyclable polylactic acid/cellulose nanocrystal films through reactive extrusion process

Apart from the conventional polyethylene and the bio-based or mainly bio-based biodegradable in soil mulching films, polyethylene mulching films of controlled degradation in soil are already used in agriculture. The use of special pro-oxidants as additives is expected to accelerate the abiotic oxidation and the subsequent chain scission of the polymer under specific UV radiation or thermal degradation conditions, according to the literature. The role of pro-oxidants in the possible biodegradation of polyethylene has been theoretically supported through the use of controlled laboratory conditions. However, results obtained in real soil conditions, but also several laboratory test results, are not supporting these claims and the issue remains disputed. Mulching films made of linear low-density polyethylene (LLDPE) with pro-oxidants, after being used for one cultivation period in an experimental field with watermelon cultivation, were buried in the soil under real field conditions. This work presents the analysis of the degradation of the mulching films during the cultivation period as compared to the corresponding changes after a long soil burial period of 8.5 years. The combined effects of critical factors on the photochemical degradation of the degradable mulching LLDPE films with pro-oxidants under the cultivation conditions and their subsequent further degradation behaviour in the soil are analysed by testing their mechanical properties and through spectroscopic and thermal analysis.

Analysis of long-term degradation behaviour of polyethylene mulching films with pro-oxidants under real cultivation and soil burial conditions

Polyethylene containing large quantities of carbon black can be crosslinked by a chemical free radical mechanism utilizing the thermal decomposition of dicumyl peroxide as the source of free radicals. Crosslinking in the presence of different types of blacks was studied in a liquid paraffin model system. The information obtained from this study provides a logical basis for speculation on the mechanism of crosslinking of carbon-black-loaded polyethylene compositions. Evidence is presented that the grade of carbon black is an important factor and that the course of the peroxide decomposition is largely determined by the chemical nature of the carbon surface. Carbon blacks having a highly oxygenated surface cause an ionic decomposition which is not effective for crosslinking, while those of low oxygen content allow the decomposition to follow a free radical mechanism which results in the crosslinking of polyethylene. Marked changes in the physical characteristics of the polymer are brought about as a result of pigmentation and subsequent crosslinking. Such compositions display a surprising degree of flexibility and strength over a wide range of temperatures. Moreover, significant strength is observed at temperatures above the crystalline melting point of the polymer. These improved properties are attributed to the reinforcing ability of the carbon black and the crosslinked network of polymer chains.

Harvey M Cole

Papers

Peroxide crosslinked carbon black polyethylene compositions

Carbon black product and method of preparation thereof

Carbon black process and product

Carbon black process

Pigment peptizing agent and process