Optimization of Polyurethane Foams for Enhanced Stir Bar Sorptive Extraction of Triazinic Herbicides in Water Matrices

In this work, polyurethane foams (PU) were developed, characterized and applied as new generation polymeric phases for stir bar sorptive extraction (SBSE) using seven triazinic herbicides (simazine, atrazine, prometon, ametryn, propazine, prometryn and terbutryn) as model compounds in water matrices. Assays performed for PU synthesis and characterization demonstrated that seven formulations presented remarkable stability and excellent mechanical and chemical resistance, for which the P₆ formulation showed the best results. By performing systematic assays on 25 mL of water samples spiked at the 10 μg/L level, it was established that the best experimental conditions using stir bars coated with P₆ were an equilibrium time of 6 h (1250 rpm), 5% of methanol as organic modifier, followed by liquid desorption with methanol as back extraction solvent under ultrasonic treatment (20 min) and high performance liquid chromatography with diode array detection (SBSE(PU)-LD-HPLC-DAD). This methodology provided good recoveries (20.4-62.0%) and remarkable reproducibility (R.S.D. <7.0%). Furthermore, excellent linear dynamic ranges between 0.9 and 16.7 μg/L (r² > 0.9949) and detection limits (0.1-0.5 μg/L) at trace level were also achieved. The application of the proposed analytical approach to analyze triazinic herbicides in ground and superficial water matrices, showed remarkable performance and by using the standard addition methodology the matrix effects are negligible. By comparing the best PU formulation (P₆, 71 μL) with commercial stir bars coated with PDMS (126 μL), recoveries normalized to the polymeric volume up to five times higher (atrazine) were attained. The ability of PU foams to extract the more polar compounds rather than PDMS makes this polymer a very valuable contribution for SBSE.     

Study of the fracture behavior of flexible polypropylene foams using the Essential Work of Fracture (EWF)

Two types of commercial low density polypropylene based flexible foams produced by extrusion foaming were characterized in terms of their fracture behaviour using the concept of the Essential Work of Fracture (EWF), focusing on the influence of the foam’s relative density and cellular structure on the values of the fracture parameters. With that in mind, correction procedures based on the expansion ratio of the foams and their cellular structure were proposed, with the objective of taking the complexity of these materials into account in the obtained fracture parameters. Significant differences were found between the fracture parameters of the two foams related to differences in their cellular structure, particularly cell size, cell aspect ratio and preferential cell orientation. Generally speaking, the specific fracture elastic contribution in the two considered extrusion directions increased with increase of the cell aspect ratio, especially in the case of the foams with a marked cell orientation in the direction of the extrusion flow. In any case, the fracture parameters for all foams were considerably lower in the direction perpendicular to the extrusion flow, hence demonstrating the highly anisotropic fracture behaviour of these foams due to the anisotropic cellular structure induced during foaming.     

Synthesis and morphology of rigid polyurethane foams with POSS as pendant groups or chemical crosslinks

This work reports on the preparation of polyurethane foams (PUFs) chemically modified by functionalized 1,2‐propanediolisobutyl polyhedral oligosilsesquioxane (PHI‐POSS) as pendant groups and octa(3‐hydroxy‐3‐methylbutyldimethylsiloxy) POSS (OCTA‐POSS) as chemical crosslinks. The resulting foams, which contain 0 to 15 wt% POSS (versus polyol), were characterized in terms of their structure, morphology, density, thermal conductivity, compressive strength, and water absorption. Fourier transform infrared‐attenuated total reflectance revealed good reaction rate between POSS and PUF. PHI‐POSS suppresses the formation of the hydrogen bonds in the soft phase. The composite foams with OCTA‐POSS showed a reduced number of cells and increased average area of foam cells in comparison with the PUF, while the addition of PHI‐POSS causes an increase in the number of cells of the foam as compared with the reference, and thus a reduction in the average area of cells. Scanning electron microscopy–energy‐dispersive X‐ray spectroscopy analysis revealed that POSS moieties form lamellae‐shaped crystals of different sizes, distributed homogeneously in the bulk (PHI‐POSS) or close to the self surfaces (OCTA‐POSS). The compressive strength of PUF/POSS hybrids in the direction parallel and perpendicular to the direction of foam rise is greater than the strength of the reference foam. PHI‐POSS improves monotonically the compressive strength in the studied loading range. About 5 wt% OCTA‐POSS also provides reinforcement, but further loading reverses the phenomenon. PUF/POSS hybrids absorb less water than the pristine foam because of an increase of foam density. Copyright © 2015 John Wiley & Sons, Ltd.     

Thermal stability of flexible polyurethane foams containing modified layered double hydroxides and zinc borate

Abstract

Flexible polyurethane foams (FPUFs) have been modified to contain layered double hydroxides (LDHs) by dihydrogen phosphate (H₂PO₄ ^?). The thermal stability of the prepared foams has been characterized using thermogravimetric analysis (TGA) at 5, 10, 20, 30, and 40?°C/min heating rates. The experimental data indicate that the temperature range for the two pyrolysis stages of FPUF is about 212–350?°C and 350–565?°C, respectively. Integral programmed decomposition temperature (IPDT) has been calculated according to the measured data, which was found that the IPDT of the modified FPUF was increased to 526?°C. Additionally, the thermal stability of FPUF composite has been also evaluated by the activation energy (E) on the basis of the pyrolysis kinetics of FPUF composites during thermal decomposition using Coats–Redfern integral method. These results manifest that the presence of intercalated LDHs enhances the thermal stability of FPUF.     

Analytical models for the mechanical behavior of closed and open-cell Al foams

Two 3D analytical models are proposed for the determination of mechanical properties of Al closed and open-cell foams under compression load. The first model, referring to closed cell foams, is symmetrical, considering ellipsoid cells equally arranged in a rectangular plate, whereas the second one, related to open-cell foams, consists of a simple unit parallelepiped cell.The closed cell model produces much higher values of the plateau stress than comparable experimental results, mainly due to the associated conditions of symmetry, contrary to the open-cell model which yields values close to experimental and theoretical results of other investigators. Additionally, in the latter case, a unit cubic cell is also considered for comparison reasons. Both models are solved by the finite element method using a commercial program. The open-cell model is simple, time sparing and easy to use. Finally, a fracture analysis of the model is conducted based on the energy density concept and results are given for distortion and dilatational effects.     

含空心球涂层粒子复合材料界面的脱粘应力分析

在球对称拉伸载荷作用下针对空心球涂层复合材料分析了空心球涂层粒子增强复合材料的局部应力场,得到了界面临界脱粘应力的解析表达式.讨论了各相几何参数对非均匀涂层空心球粒子临界脱粘应力的影响,比较了均匀涂层和非均匀涂层的脱粘应力.结果表明:在球对称拉伸下界面脱粘更容易发生在涂层相与基体相界面间,空心球的壁厚和涂层厚度是影响界面临界脱粘应力的重要因素,因而选择适当的空心球、涂层厚度和提高界面粘结能将有利于提高界面的临界脱粘应力.     

采用改进的SHPB方法对泡沫铝动态力学性能的研究

本文改进了传统的分离式霍布金森压杆(SHPB)技术，采用夹在透射杆中的PVDF压电计直接测量透射杆中的应力时程．同时，采用输入波形整形技术，通过调整加载波形，使试样加载过程中保证均匀变形及应力平衡．利用此改进了的SHPB技术对泡沫铝进行了高应变率下的动态压缩实验．实验结果表明：泡沫铝的动态应力应变曲线具有泡沫材料的应力应变曲线的“三阶段”特征(elastic region，collapse region and densification region)，并且应变率对其力学性能影响明显．     

Acid character control of bioactive glass/polyvinyl alcohol hybrid foams produced by sol–gel

Bioactive glass/polymer hybrids are promising materials for biomedical applications because they combine the bioactivity of bioceramics with the flexibility of polymers. In previous work hybrid foams with 80% bioactive glass and 20% polyvinyl alcohol were prepared by the sol–gel method. The produced hybrids presented a high acidic character due to the catalysts added. In this work different methods to control the acidity and toxicity of the hybrids were also evaluated, through changes in the synthesis pH and use of different neutralization solutions. The hybrids were prepared with inorganic phase composition of 70%SiO₂–30%CaO and PVA fractions of 20–60% by the sol–gel method. The characterization of the obtained foams was done by FTIR, SEM, Raman Spectroscopy, Helium Picnometry and TGA. The immersion of hybrids in a calcium acetate solution was the most adequate neutralization method. The foams presented porosity of 60–85% and pore diameters of 100–500 μm with interconnected structure.