Quasi-continuous melting of model polymer monolayers prompts reinterpretation of polymer melting

Condensed matter textbooks teach us that melting cannot be continuous and indeed experience, including with polymers and other long-chain compounds, tells us that it is a strongly first-order transition. However, here we report nearly continuous melting of monolayers of ultralong n-alkane C390H782 on graphite, observed by AFM and reproduced by mean-field theory and MD simulation. On heating, the crystal-melt interface moves steadily and reversibly from chain ends inward. Remarkably, the final melting point is 80 K above that of the bulk, and equilibrium crystallinity decreases continuously from ~100% to <50% prior to final melting. We show that the similarity in melting behavior of polymers and non-polymers is coincidental. In the bulk, the intermediate melting stages of long-chain crystals are forbidden by steric overcrowding at the crystal-liquid interface. However, there is no crowding in a monolayer as chain segments can escape to the third dimension.

Tuning the Ablation, Thermal and Mechanical Characteristics of Phenolic Resin Reinforced EPDM Ultra-High Temperature Insulation

The present research reports the influences of variant phenolic resin concentrations on the thermo-mechanical and ablation characteristics of ethylene propylene diene monomer (EPDM) elastomer. Backface temperature acclivity (BTA), charring rates, and insulation indexes were executed for the fabricated composite specimens. It was noticed that BTA was enhanced while linear/radial/mass ablation rates were significantly diminished with increasing concentration of phenolic resin (PR) in base matrix (elastomeric polymer). The composite (30wt%PR/EPDM) has 25% high thermal endurance compared to virgin EPDM composite. Thermal conductivity was increased with increasing PR to EPDM ratio. PR incorporation has remarkably enhanced the ultimate tensile strength of the EPDM elastomer. An efficient improvement in elastomeric hardness was also observed with increasing PR contents in EPDM matrix. Scanning Electron Microscopy (SEM) results showed the porosity generation and polymer melting during ablation.

CFD MODELING OF POLYETHYLENE GAS PHASE FLUIDIZED BED REACTOR WITH INTERNAL CHILLER

Polyethylene production in a gas phase fluidized bed reactor is exposed to unstable temperature behavior, and if not controlled properly the temperature oscillation can cause polymer melting and potential plant shutdown (Ghasem, 1999). Accordingly, the present work improves the previous simple mathematical model by the CFD modelling that addresses the prediction of ethylene concentration and temperature profile in entire reactor of the fluidized bed reactor employed for polyethylene production. The CFD model considers gas phase molecular diffusion in the axial and radial directions. Results revealed that the size of the internal heat exchanger mainly the exchanger heat transfer interface area has strong impact on the temperature contour inside the reactor. Ethylene feed rate and ethylene concentration, catalyst feed rate effect the temperature profile inside the reactor considerably. The increase in ethylene concentration and catalyst feed rate strongly influences the fluidized bed temperature. As ethylene concentration increases reactor temperature increases. Proper temperature control inside the polyethylene gas phase is essential fluidized reactor to maintain the reactor temperature below polymer melting point and hence long term operation of fluidized bed reactor without reactor shutdown and temperature excursion above the polymer melting point.

Effects of Plastic Waste on the Heat-Induced Spalling Performance and Mechanical Properties of High Strength Concrete

This paper investigates a potential application of hard-to-recycle plastic waste as polymeric addition in high strength concrete, with a focus on the potential to mitigate heat-induced concrete spalling and the consequent effects on the mechanical properties. The waste corresponds to soft and hard plastic, including household polymers vastly disposed of in landfills, although technically recyclable. Mechanical and physical properties, cracking, mass loss, and the occurrence of spalling were assessed in high strength concrete samples produced with either plastic waste or polypropylene fibers after 2-h exposure to 600 °C. The analysis was supported by Scanning Electron Microscopy and X-Ray Computed Tomography images. The plastic waste is composed of different polymers with a thermal degradation between 250 to 500 °C. Polypropylene (PP) fibers and plastic waste dispersed in concrete have proved to play an essential role in mitigating heat-induced concrete spalling, contributing to the release of internal pressure after the polymer melting. The different morphology of plastic waste and polypropylene fibers leads to distinct mechanisms of action. While the vapor pressure dissipation network originated by polypropylene fibers is related to the formation of continuous channels, the plastic waste seems to cause discontinuous reservoirs and fewer damages into the concrete matrix. The incorporation of plastic waste improved heat-induced concrete spalling performance. While 6 kg/m3 of plastic increased the mechanical performance after exposure to high temperature, the incorporation of 3 kg/m3 resulted in mechanical properties comparable to the reference concrete.

Environmental control methods under anthropogenic influence of additive production

The use of additive technologies ensures the individualization of products, reduces the consumption of raw materials, improves the economic performance of production, allows the use of new materials and composites, as well as improves the quality of products. However, it is necessary to take into account the factors of the anthropogenic impact of additive production on the environment. In additive manufacturing, various types of polymers are used that undergo certification. The environmental certificate ensures that, under normal operating conditions, the polymer product is safe. However, the greatest impact of additive technologies on the environment occurs in the process of polymer melting. The article proposes a method for assessing the environmental friendliness of additive production by analyzing air samples taken at the time of polymer extrusion. A gas chromatographic analysis of air samples obtained by melting several samples of different polymers used in additive production was carried out.