Helmets are the most effective protective item for motorcyclists. The liner of the helmet is the part of the helmet which dissipates most of the impact energy and mitigates the risk of head injuries. It has been proposed that the helmet test standards should include assessment of the helmets for oblique impacts that is not currently addressed in the standards. A conventional uniaxial compression test method is still used for characterization of the helmet liner material. However, compressive tests of EPS foams provide reliable results for normal loading on EPS, but do not provide a realistic result for oblique impacts. Therefore, we carried out experimental tests to measure the response of EPS foams, which are commonly used for helmet liners, under biaxial loading. The result of our experiments show that the shear response of EPS foams is a function of axial compression, and increasing the axial strain leads to increased shear stiffness, and thus higher levels of shear stress. We also showed that including shear-stiffening of EPS in the FE assessment of helmets may change the headform rotational acceleration by 25%. Therefore, such behavior of EPS foams should be included in FE analysis of helmets in the case of oblique impacts for a more realistic assessment of their performance.
Nanoplastics (NPs), which we define in this paper as solid plastic particles with the size <1 μm, unintentionally produced from the degradation and fragmentation of larger plastic objects are probably the least known area of plastic litter but are suspected to pose the greatest risk to the environment. However, no NPs have been detected in natural environments to date. This review attempts to provide a critical overview from the polymer science perspective of the relevant scientific literature, which could facilitate finding secondary NPs in natural environments. The information on secondary NPs has been scarce due to the big challenges in sampling, separation, and detection of these nanoscale particles. This review highlights the most important challenges and obstacles and discusses the mechanisms of generation of secondary NPs. It provides also a critical overview on modern instrumentation, newly developed workflows, promising techniques for sampling and sample preparation, and detection methods including spectroscopies (Raman and FT-IR), microscopies (SEM and TEM) and mass spectrometry (GC–MS and ToF–SIMS). We conclude that finding NPs in natural environments is plausible yet uncertain, which drives towards the development of a methodology for collection, separation and identification of NPs in environmental matrices along with a thorough evaluation of the process of formation of secondary NPs, their fate and effects on living organisms and the environment. To find nanoplastics in natural environments it is important to know the process of their formation, their fate, and experimental constraints.
A NBR membrane containing carbon black (36 wt.%) and di(2-ethylhexyl) phthalate (DEHP; 11 wt.%) that had been used at temperatures up to 45 C in pressurised air showed cracking after 2 years in service. Samples were aged in air at elevated temperatures and their mechanical properties were assessed by tensile testing, the glass transition temperature was obtained by DSC, and the DEHP content was determined by liquid chromatography. The loss of DEHP was controlled by the boundary conditions at low temperatures and the loss rate was constant at 90 C within a certain DEHP concentration range (8 to 11 wt.%). The presence of carbon black and DEHP made it impossible to determine oxidation products by infrared spectroscopy. Strain-at-break data were analysed in a way that enabled the effect of DEHP migration to be separated from the effect of thermal oxidation. This allowed extrapolation in both temperature and oxygen pressure domains of high temperature/low oxygen pressure data to the service conditions. The analysis showed that both DEHP evaporation and thermal oxidation had a significant impact on the strain-at-break, but that the latter was the more important. Data for the mechanical properties and the glass transition temperature indicated that oxidation was non-uniform with increasing depth in the specimens. This condition of the 4.5 mm thick samples meant that it was inappropriate to use the specimen Young's modulus for extrapolation purposes. © 2013 Elsevier Ltd. All rights reserved.
The mass transport of four different aroma compounds (ethyl butyrate, 1-hexanol, heptanal and limonene) through styrene-acrylate latex dispersion coatings with different amounts of vinyl acid groups has been studied using a permeation cell. The permeation test was studied above and below the glass transition temperature of the styrene-acrylate latex films in order to study the influence of changes in free volume and polymer rigidity on the aroma permeation induced by glassy-rubbery transition. An increase in the carboxylic acid content of the latices reduced the diffusivity. The solubility of all the aroma compounds tended to increase with increasing carboxylic acid groups content in the latex films, including the non-polar limonene. Other factors than polarity and chemical attraction forces including volatility and molar volume of the aroma compounds thus affected the mass transfer. Since solubility and diffusion oppose each other, the differences in permeability towards aroma compounds between the latex films were small. Below the glass transition temperature, the free volume is less, resulting in a lower mobility of the permeants and, therefore, a lower diffusivity than above the glass transition. A small increase in solubility was seen at the lower temperature, probably due to the lower volatility of the aroma compounds. © 2007 Elsevier Ltd. All rights reserved.
FTIR-ATR (Focal Plane Array) FPA spectroscopic imaging was used to obtain depth resolved information about degradation of a polyester melamine coating by performing measurements on drilled conical holes. The results of the FTIR-FPA imaging show that combined effect of UV and moisture resulted in larger degradation effects in the outermost parts of the coatings but loss of melamine functionality was detected to the depth of approximately 8–10 μm after 2163 h weathering in QUVA accelerated test and after 4 years exposure in a marine environment. Longer periods of accelerated weathering, up to 4000 h, resulted in degradation through the whole coating outer layer.
Accelerated thermal ageing (ATA) coupled to mechanical testing is widely used to predict the lifetime of polymeric products. ATA implies that the mechanisms of ageing are the same at accelerated and service conditions, which may often not be the case. Hence, ageing closer to service conditions is of high importance, but require very sensitive tools. Therefore, a high sensitivity microcalorimetry (MC) method was applied here to assess if it can be a possible tool for lifetime/ageing prediction closer to service conditions. We chose to focus on a complex, yet commonly used, ethylene-propylene-diene terpolymer (EPDM) rubber. Arrhenius extrapolation of the heat flow data indicated two regimes at low and high temperature, with the former having the lower activation energy. The heat flow values measured by the MC revealed contributions from processes such as the melting of the antioxidant, its consumption at low temperature and the breakdown of residual peroxide. MC tests on the EPDM indicated a very low degree of oxidation appearing above 100 °C, too low to be observed with infra-red spectroscopy (FTIR), but noticeable with MC. The high sensitivity of the MC techniques enabled detection of early signs of polymer degradation/ageing and other thermally activated processes that take place at or close to service temperatures (such as those in nuclear power plants). The MC tests were combined with other techniques, such as scanning electron microscopy/energy dispersive X-ray spectroscopy, gas chromatography techniques, differential scanning calorimetry and FTIR to further understand the degradation mechanisms. © 2023 The Authors