Surface resistivity, mechanical and thermal properties of rotationally moulded polyethylene/graphite composites

Please read the abstract in the dissertation CopyrightDissertation (MSc)--University of Pretoria, 2013.Chemical Engineeringunrestricte

Surface resistivity and mechanical properties of rotationally molded polyethylene/graphite composites

Antistatic polymers are required to dissipate static charges safely from component surfaces. Our overall objective has been to develop cost-effective flame-retarded and antistatic polyethylene compounds suitable for rotomolding. This communication considers the surface resistivity and mechanical properties of rotationally molded linear low-density polyethylene (LLDPE)/graphite composites containing natural Zimbabwean graphite, expandable graphite, or expanded graphite. Dry blending and melt compounding were employed to obtain antistatic composites at the lowest graphite contents. Dry blending was found to be an effective mixing method for rotomolding antistatic LLDPE/graphite composites, thereby eliminating an expensive compounding step. Dry-blended Zimbabwean graphite composites showed the lowest surface resistivity at all graphite contents, with a surface resistivity of 105 Ω/square at 10 wt% loading. Although rotomolded powders obtained following the melt compounding of Zimbabwean graphite exhibited higher resistivity values, the variability was much lower. Injection molding resulted in surface resistivity values above 1014 Ω/square for all compositions used. The rotomolded composites exhibited poor mechanical properties, in contrast to injection-molded composites. The Halpin-Tsai model showed good fits to the tensile modulus data for injection-molded Zimbabwean and expandable graphite.Institutional Research Development Programme (IRDP) of the National Research Foundation of South Africa and Xyris Technology CC.http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1548-0585hb201

Evaluation of polycarboxylate ethers that can mitigate the detrimental effects of clay content of sand on concrete workability and strength

Clay tolerant superplasticizers can enable the use of low quality, clay containing aggregates and also alleviate the need for washing clay containing sand used in concrete fabrication, thereby promoting sustainable concrete production. Previous studies considered specific aspects of polycarboxylic ether (PCE) superplasticizer molecular design which influence their clay tolerance. In this study, the effect of a combination of various aspects of PCE molecular design on their clay tolerance was investigated. The aim of the study was to determine PCE structural differences that are more tolerant to clay contamination of sands used in concrete making. The objective was to evaluate the effectiveness of six recently developed proprietary PCEs with varying molecular designs in concretes fabricated with clay containing sands sampled from three South African quarries. The PCEs were evaluated by their ability to retain good workability and compressive strength in concretes fabricated using a standard laboratory clay free sand and high clay content sands from the three quarries. Workability was evaluated through slump flow tests and the compressive strength was evaluated through 24 h and 28 d compressive strength tests. Incorporation of an acrylic based PCE with low side chain graft ratio and long side chain lengths in the concretes resulted in a higher initial slump flow of at least 200 mm at 5 min, indicating relatively good workability, which however deteriorated rapidly in concretes made with the clay contaminated sands. Unexpectedly, the 28 d compressive strengths of concretes fabricated with higher clay content sands were found to be comparable to the compressive strengths of concretes made with the clay free standard sand when three of the PCEs were used. High compressive strength in the concretes fabricated with the sand containing the most clay was accomplished by incorporating a PCE with very high molecular weights, very long side chain lengths and a low graft ratio.http://www.aidic.it/cetam2020Chemical Engineerin

Adsorption of phenol and chromium (VI) pollutants in wastewater using exfoliated graphite

Phenol and Cr(VI) are two of the most common organic and heavy metal-based pollutants found in industrial effluents. Both pollutants pose considerable health risks if left untreated. Activated carbon adsorption is generally used for the physical removal of these types of pollutants during wastewater treatment. This work investigates the use of thermally exfoliated graphite as an alternative adsorbent material for the removal of phenol and Cr(VI) in wastewater. The well-developed surface pore structure and high adsorption capacities reported in literature make this material an ideal candidate for investigation. The effectiveness of the exfoliation process was characterised using x-ray diffraction whilst the particle morphology, surface structure and adsorption surface area were determined using scanning electron microscopy and Brunauer-Emmett-Teller (BET) specific surface area measurements. Upon expansion, the particle morphology of expandable graphite changed from flakes to worm-like, accordion structures. This change was accompanied by an increase in BET surface area from 2.4 to 22.4 g/m2. Batch experiments using simulated wastewater revealed that expandable graphite had negligible adsorption affinity towards both pollutants. However, exfoliated graphite had adsorption capacities of 0.73 mg/g and 0.55 mg/g for Cr(VI) and phenol. The equilibrium adsorption isotherms for both pollutants were best described by the Langmuir adsorption model and had adsorption constants of 0.84 and 0.32 L/mg for the two pollutants. The adsorption capacities obtained were much lower than those reported for the same pollutants when using activated carbon as an adsorbent. This is most likely due to the high BET surface areas recorded for activated carbon. Additional modification of the exfoliated graphite is required before satisfactory adsorption capacities can be achieved for large-scale wastewater treatment applications.http://www.aidic.it/cetam2019Chemical Engineerin

Removal of chromium (VI) from aqueous solution using exfoliated graphite/polyaniline composite

Exfoliated graphite/polyaniline (EG/PANI) composites were successfully prepared via in situ polymerization of aniline monomer in the presence of exfoliated graphite. The PANI precipitated into the interlayer space of the graphite flake significantly improving physiochemical properties of EG. The particle morphology and interaction of the EG and PANI was determined using the scanning electron microscopy (SEM). The SEM images clearly showed the formation of PANI and EG/PANI composite with PANI greatly covering the EG flake. The presence of polyaniline and its major functional groups were confirmed using Fourier Transform Infrared Spectroscopy (FTIR) between 3,500 – 3,000 cm-1 and 1,500 - 500 cm-1 . A 68 % change in Brunauer- Emmett-Teller (BET) surface area was observed in the 0.35 g PANI/g EG after the in situ polymerization of the PANI. The effectiveness of the composite adsorbent on the removal of Cr (VI) from aqueous solutions was evaluated by evaluating the effects of parameters such as EG to PANI ratio and initial concentration. The optimum composite loading was found to be 0.35 g PANI/g EG and removed more than 99 % of Cr (VI) from an initial concentration of 100 ppm was removed at 20 g/L loading within 4 h. The Freundlich adsorption isotherm model best described the adsorption of Cr (VI) with a regression coefficient, r 2 of 0.99 showing the heterogeneous adsorption at the surface of the composite. The maximum capacity of the composite was 38.20 mg/g.The National Research Foundation of South Africahttp://www.cetjournal.itpm2021Chemical Engineerin

Removal of organic dyes from water and wastewater using magnetic ferrite-based titanium oxide and zinc oxide nanocomposites : a review

Heterogeneous photocatalysis using titanium dioxide (TiO2) and zinc oxide (ZnO) has been widely studied in various applications, including organic pollutant remediation in aqueous systems. The popularity of these materials is based on their high photocatalytic activity, strong photosensitivity, and relatively low cost. However, their commercial application has been limited by their wide bandgaps, inability to absorb visible light, fast electron/hole recombination, and limited recyclability since the nanomaterial is difficult to recover. Researchers have developed several strategies to overcome these limitations. Chief amongst these is the coupling of different semi-conductor materials to produce heterojunction nanocomposite materials, which are both visible-light-active and easily recoverable. This review focuses on the advances made in the development of magnetic ferrite-based titanium oxide and zinc oxide nanocomposites. The physical and magnetic properties of the most widely used ferrite compounds are discussed. The spinel structured material had superior catalytic and magnetic performance when coupled to TiO2 and ZnO. An assessment of the range of synthesis methods is also presented. A comprehensive review of the photocatalytic degradation of various priority organic pollutants using the ferrite-based nanocomposites revealed that degradation efficiency and magnetic recovery potential are dependent on factors such as the chemical composition of the heterojunction material, synthesis method, irradiation source, and structure of pollutant. It should be noted that very few studies have gone beyond the degradation efficiency studies. Very little information is available on the extent of mineralization and the subsequent formation of intermediate compounds when these composite catalysts are used. Additionally, potential degradation mechanisms have not been adequately reported.https://www.mdpi.com/journal/catalystsam2022Chemical Engineerin

Flexible PVC flame retarded with expandable graphite

The utility of expandable graphite as a flame retardant for PVC, plasticized with 60 phr of a phosphate ester, was investigated. Cone calorimeter results, at a radiant flux of 35 kW m-2, revealed that adding only 5 wt.% expandable graphite lowered the peak heat release rate from 325 ± 11 kW m-2 to 63 ± 23 kW m-2 and the total heat release from 55 ± 11 MJ m-2 to only 10.7 ± 0.3 MJ m-2. All samples containing expandable graphite ignited and burned only very briefly before flame out. The remarkable effectiveness of the expandable graphite is attributed to an excellent match between the exfoliation onset temperature of the graphite and the onset of decomposition of the PVC. This means that the exfoliation of the graphite forms a protective barrier layer at the right place at the right time. In addition, the simultaneous release of halogen species by the polymer matrix and the exfoliating graphite prevents the formation of a flammable air fuel mixture.The South African Research Chairs Initiative of the Department of Science and Technology (DST) and the National Research Foundation (NRF)http://www.elsevier.com/locate/polydegstabhb201

Flame retarding effect of graphite in rotationally molded polyethlene/graphite composites

Linear low density polyethylene (LLDPE) compounds containing 10 wt.% graphite fillers were rotationally molded into flat sheets. Flame retardancy was studied using cone calorimeter tests conducted at a radiative heat flux of 35 kW m2. Only the expandable graphite, an established flame retardant for polyethylene, significantly reduced the peak heat release rate. Compared to the neat polyethylene, it was easier to ignite the LLDPE composites containing carbon black, expandable graphite and exfoliated graphite. However, rather unexpectedly, the inclusion of flake graphite increased the time to ignition by up to 80%. Simulations conducted with the ThermaKin numerical pyrolysis software suggest that increased reflectivity was mainly responsible for this effect.The South African Research Chairs Initiative of the Department of Science and Technology (DST) and the National Research Foundation (NRF) as well as Xyris Technology CC.http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1097-46282016-02-20hb201

Polyethylene flame retarded with expandable graphite and a novel intumescent additive

A novel intumescent additive was synthesized by neutralizing 3,5-diaminobenzoic acid hydrochloride salt with ammonium dihydrogen phosphate. This compound, which melts at 257 °C, decomposes concurrently to release carbon dioxide gas. The flame retardant performance of this compound as a primary fire retardant and in combination with expandable graphite was evaluated by cone calorimetry. Cone calorimeter results revealed that addition of 10 wt.% expandable graphite alone lowers peak heat release rate of carbon black-pigmented polyethylene from 710 ± 109 kW m-2 to 342 ± 15 kW m-2, while addition of 27 wt.% of the novel intumescent lowered it to 400 ± 16 kW m-2. Combinations of these two additives were able to decrease the peak heat release rate even further. Furthermore, the novel intumescent additive reduced the flame out time from 773 ± 307 s to 537 ± 69 s although all other expandable graphite containing samples increased it.South African Research Chairs Initiative of the Department of Science and Technology (DST) and the National Research Foundation (NRF)http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1097-46282015-07-31hb201

Cone calorimeter study of polyethylene flame retarded with expandable graphite and intumescent fire-retardant additives

Polyethylene was flame retarded with two intumescent-type additives, 3,5-diaminobenzoic acid phosphate and ethylenediamine phosphate, which differ with respect to their decomposition onset temperatures, along with varying ratios of two grades of expandable graphite, also differing with respect to their onset temperatures for exfoliation. Hot-pressed sheet specimens were subjected to evaluation in a cone calorimeter. Although the best char yields were obtained with formulations containing the higher decomposition temperature intumescent, 3,5-diaminobenzoic acid phosphate, the overall best performance was realized using the lower decomposition temperature intumescent, ethylenediamine phosphate, when compounded together with the low exfoliation temperature expandable graphite. These results are attributed to the formation, at the burning surface, of a more cohesive char with better thermal and mass transfer barrier properties.South African Research Chairs Initiative of the Department of Science and Technology (DST) and the National Research Foundation (NRF).http://jfs.sagepub.comhb201