Experimental and Finite Element Simulation of Polyolefin Elastomer Foams Using Real 3D Structures: Effect of Foaming Agent Content

In this study, polyolefin elastomer (POE) foams were prepared without any curing agent using a single-step foaming technique. The effect of azodicarbonamide (ADC) content as a chemical foaming agent on the foams’ morphology and mechanical properties was studied using scanning electron microscopy (SEM), mechanical properties (tension and compression) and hardness. The results showed that increasing the ADC content from 2 to 3, 4 and 5 phr (parts per hundred rubber) decreased the foam density from 0.75 to 0.71, 0.65 and 0.61 g/cm3, respectively. The morphological analysis revealed that increasing the ADC content from 2 to 4 phr produced smaller cell sizes from 153 to 109 µm (29% lower), but a higher cell density from 103 to 591 cells/mm3 (470% higher). However, using 5 phr of ADC led to a larger cell size (148 µm) and lower cell density (483 cells/mm3) due to cell coalescence. The tensile modulus, strength at break, elongation and hardness properties continuously decreased by 28%, 21%, 16% and 14%, respectively, with increasing ADC content (2 to 5 phr). On the other hand, the compressive properties, including elastic modulus and compressive strength, increased by 20% and 64%, respectively, with increasing ADC content (2 to 5 phr). The tensile and compression tests revealed that the former is more dependent on foam density (foaming ratio), while the latter is mainly controlled by the cellular structure (cell size, cell density and internal gas pressure). In addition, 2D SEM images were used to simulate the foams’ real 3D structure, which was used in finite element methods (FEM) to simulate the stress–strain behavior of the samples at two levels: micro-scale and macro-scale. Finally, the FEM results were compared to the experimental data. Based on the information obtained, a good agreement between the macro-scale stress–strain behavior generated by the FEM simulations and experimental data was obtained. While the FEM results showed that the sample with 3 phr of ADC had the lowest micro-scale stress, the sample with 5 phr had the highest micro-scale stress due to smaller and larger cell sizes, respectively

Effects of body weight regain on leptin levels: a systematic review and meta-analysis

Background: There are different changes observed before and after diet therapy, and also after weight regain. However, there is not sufficient information regarding weight regain and hormonal changes. Purpose: The purpose of this study was to review the connection between weight regain and leptin concentration levels. Methods: MEDLINE, SCOPUS, Web of Science, and the Cochrane Library were searched for interventional articles published from January 1, 1980, to June 30, 2020. Randomized clinical trials with parallel or cross over design assessing leptin concentrations at the baseline and at the end of study were reviewed. Two independent reviewers extracted data related to study design, year of publication, country, age, gender, body mass index (BMI), duration of the following up period and mean +/- SD of other intended variables. Results: Four articles were included, published between 2004 and 2016. Three of them were conducted in the US and one of them in Netherland. Sample size of the studies ranged between 25 and 148 participants. The range of following up period was from13 to 48 weeks. The age range of participants was from 34 to 44 years. Our analysis shows that weight regain could reduce leptin levels, but this change is not statistically significant. Conclusion: This review suggests that weight regain may induce a non-significant reduction in leptin level. However, the limited number and great heterogeneity between the included studies may affect the presented results and there are still need to well-designed, large population studies to determine the relationship between weight regain and leptin levels.info:eu-repo/semantics/publishedVersio